Accelerating the Understanding and Development of Hydrogen Storage Materials: A Review of the Five-Year Efforts of the Three DOE Hydrogen Storage Materials Centers of Excellence
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A technical review of the progress achieved in hydrogen storage materials development through the U.S. Department of Energy’s (DOE) Fuel Cell Technologies Office and the three Hydrogen Storage Materials Centers of Excellence (CoEs), which ran from 2005 to 2010 is presented. The three CoEs were created to develop reversible metal hydrides, chemical hydrogen storage materials, and high-specific-surface-area (SSA) hydrogen sorbents. For each CoE, the approach taken is specified, key outcomes and accomplishments identified, and recommendations for future work are suggested. The Metal Hydride Center of Excellence addresses work on destabilized hydrides, including the LiBH/Mg2NiH4 system, borohydrides, amides, and alanes; and compares the best materials to DOE targets. The Chemical Hydrogen Storage Center of Excellence discusses the classes of materials studied for chemical hydrogen storage, focusing on ammonia borane and examines the progress in developing efficient regeneration schemes. The Hydrogen Sorption Center of Excellence describes the progress in developing high-SSA sorbents and pathways for developing improved materials capable of achieving DOE targets. The phenomenon of spillover is also observed and its importance to ensuring improved measurements is discussed. Through the five-year effort of the Hydrogen Storage Materials Centers of Excellence, significant progress was achieved in developing and understanding hydrogen storage materials.
KeywordsHydrogen Storage Metal Hydride Spend Fuel MgH2 Hydrogen Release
The use of hydrogen and fuel cells to power light-duty vehicles offers an effective pathway to reduce greenhouse gas emissions and petroleum usage. In addition to the challenges associated with improving the power density and durability of polymer electrolyte membrane (PEM) fuel cells while reducing their costs, there are difficulties in developing hydrogen storage technologies that offer high specific energy and energy density at low costs for use in onboard vehicles. Working with the U.S. automotive manufacturers through the FreedomCAR and Fuel Partnership (now U.S. DRIVE), the U.S. Department of Energy (DOE) produced in 2003 a comprehensive set of performance metrics for onboard hydrogen storage systems. The metrics included 2015 targets for specific energy and energy density of 3.0 kWh/kg (9 wt pct) and 2.7 kWh/L (81 g H2/L), respectively. In 2009, after re-evaluating the performance metrics with comparisons to available vehicle performance data, the DOE issued a revised set of performance targets that included “Ultimate Full Fleet” targets for specific energy and energy density of 2.5 kWh/kg (7.5 wt pct) and 2.3 kWh/L (70 g H2/L), respectively. Just considering the energy density target as an example, compressed hydrogen at ambient temperatures has a density of about 40 g H2/L; liquid hydrogen at its normal boiling point of 20 K (−253 °C) has a density of 71 g H2/L. Since the energy density targets are for the complete system, neither normal compressed hydrogen nor liquid hydrogen is theoretically able to meet all of the targets. Therefore, it was recognized that advanced hydrogen storage technologies would need to be developed if all the performance metrics were to be achieved.
When hydrogen interacts with other materials or elements, either as the dihydrogen molecule or as monoatomic hydrogen, hydrogen densities greater than that of compressed hydrogen gas or liquid hydrogen can be obtained. For instance, dihydrogen can be adsorbed onto high surface area, porous materials, where even at low pressures of a few bars the hydrogen density of the adsorbed layer can approach the density of liquid hydrogen. When atomic hydrogen bonds to other elements, either through metallic-type bonding as in interstitial metal hydrides, covalent bonding as in complex hydrides and compounds such as ammonia borane and water, or ionic bonding as in binary alkali metal hydrides such as sodium hydride, hydrogen densities double that of liquid hydrogen can be obtained. Therefore, materials might provide a pathway to high energy density storage of hydrogen with the potential to meet the DOE performance targets. The materials-based storage technologies can be roughly categorized into three groups: sorbents, reversible metal hydrides, and offboard regenerable chemical hydrogen storage. Prior to the 2003 timeframe, most materials-based hydrogen storage technology development had focused on reversible interstitial metal hydrides, hydrolysis of chemical hydrogen storage materials, specifically sodium borohydride (NaBH4), and investigation of activated carbon, carbon nanotubes, and nanofibers as hydrogen adsorbents.
Hydrogen Storage Research in the DOE Metal Hydride Center of Excellence (MHCoE)
Prior to the establishment of the MHCoE, there had been already a great deal of research directed toward metal hydrides, both interstitial and complex anionic materials. The interstitial metal hydrides had already received extensive research.[7, 8, 9] These interstitial materials satisfied virtually all of the requirements for automotive use (e.g., volumetric density, kinetic performance, and purity of hydrogen released among others). However, their gravimetric hydrogen capacity is typically 1 to 3 pct, which was viewed as too low by the DOE for fuel cell automotive applications with a range of at least 300 miles. Less extensively examined were the complex anionic materials, although the existing studies pointed to the potential of these materials. The catalytic promotion of reversibility had been demonstrated in NaAlH4. Nitrogen-based materials were found that could reversibly bind hydrogen with improved capacity and it had been shown that the thermodynamic properties of complex anionic materials could be modified through additives.[12,13] Furthermore, it was known that AlH3 had excellent hydrogen storage and release properties, but it was not possible to recharge the spent material (i.e., hydrogenate Al) at reasonable conditions. Interest had been steadily moving toward higher gravimetric capacity materials, in anticipation of automotive and other mobile applications. However, in many cases there were only a handful of examples for each material class, and the ability to theoretically account for the hydrogen desorption/absorption in these materials was limited. Thus, the purpose of the MHCoE was to perform exhaustive applied R&D in these and other topical areas of complex metal hydrides, working toward a material that could satisfy all of the DOE hydrogen storage targets for light-duty vehicles.
Metal Hydride Center of Excellence List of Partners
California Institute of Technology
Brookhaven National Laboratory
HRL Laboratories, LLC
Carnegie Mellon University
Jet Propulsion Laboratory
Georgia Institute of Technology
National Institute of Standards & Technology
United Technologies Research Center
Ohio State University
Oak Ridge National Laboratory
Sandia National Laboratories
University of Hawaii at Manoa
Savannah River National Laboratory
University of Illinois Urbana-Champaign
University of Nevada, Reno
University of New Brunswick
University of Pittsburgh
University of Utah
Destabilized hydrides, where the objective was to develop strategies for reducing hydrogen storage thermal requirements and improve kinetics by destabilizing metal hydride systems. This project also aimed to enhance kinetics by evaluating nanoengineering.
Complex anionic materials, where the objective was to predict and synthesize highly promising new anionic hydride materials, with a particular focus on borohydride materials.
Amide/imide storage materials, where the objective was to assess the viability of amides and imides (materials containing –NH2 and –NH moieties, respectively) for onboard hydrogen storage.
Alane, AlH 3 , where the objective was to understand the sorption and regeneration properties of alane (AlH3) for hydrogen storage. AlH3 is a nearly ideal hydrogen releasing material, but to regenerate AlH3 directly from the end product Al with gaseous H2 requires impractical high pressures.
In addition to these formal projects, the MHCoE established a Theory Group. The MHCoE Theory Group made use of first-principles methods to predict new materials and their thermodynamic properties, provide new directions for experimentalists, and assist in the interpretation of experimental results. In many ways, the MHCoE Theory Group set a new standard for how collaboration among theorists can be achieved, and how that theoretical activity can guide experimental work.
A full account of the MHCoE work can be found in the Final Report for the MHCoE.[16,17] The following are selected highlights from the MHCoE program, with an emphasis on the most promising materials that came out of the MHCoE.
Outcomes and Accomplishments
Significant progress was made in the MHCoE over its 5-year duration. Major accomplishments in the four different project areas are described below. The MHCoE R&D effort can be numerically summarized as follows. Ninety-four new material systems were explored, leading to 221 publications describing the MHCoE R&D activity. Approximately one-third of the publications were collaborative in nature, involving at least two different institutional partners in the MHCoE. Nine patents were submitted in the course of the MHCoE work. More details are now given for a selected number of these new results.
Many light element compounds are known that have high hydrogen capacities, as reviewed by Orimo and colleagues. However, most of these materials are thermodynamically too stable, and they release and store hydrogen much too slowly for practical use.
One of the approaches to improving the thermodynamic properties of metal hydride materials is called “destabilization.”[13,19, 20, 21] In destabilization, another material is added that modifies the final chemical state upon hydrogen release. The concept is that by varying the final state there is a potential to tune the reaction thermodynamics and, therefore, reduce the enthalpic barrier to hydrogen release. During the 5-year tenure of the MHCoE, approximately 20 new “destabilized” hydrogen storage systems were experimentally developed and tested including LiBH4/MgF2, LiBH4/MgCl2, LiBH4/MgI2, LiBH4/MgS, LiBH4/MgSe, LiBH4/Mg2Si, LiBH4/Mg2Cu, LiBH4/Mg2NiH4, LiH/B4C, LiBH4/Si, CaSiN2, MgSiN2, LiBC, Mg(BC)2, LiH/TiO2, LiH/SiO2, and LiBH4/SiO2. Although light element hydrides have high hydrogen contents, thus far no single compound has been identified that has the thermodynamic properties required to meet all of the DOE targets.
The initial destabilized systems investigated by Vajo et al. were LiH/Si and MgH2/Si. Both systems have 5 wt pct hydrogen capacity. While the release temperature for LiH/Si was too high for practical applications, thermodynamic modeling (CALPHAD) indicated that the decomposition of MgH2 in the presence of Si to form Mg2Si would reduce the reaction enthalpy from −75.3 kJ/mol H2 to only −36.4 kJ/mol and, therefore, might be of interest. However, in practice the system was found to have very slow kinetics and to be challenging to reverse the reaction and rehydrogenate Mg2Si. Efforts were, therefore, turned to destabilized systems with higher potential capacities and more promise to meet the challenging storage performance targets.
The LiBH4/Mg2NiH4 system was studied by Vajo et al. at HRL because of its remarkable features including: high capacity, reversibility, reaction through a direct low-temperature kinetic pathway, formation of a unique ternary boride phase, and low reaction enthalpy coupled with low entropy. This interesting system reveals in many ways the full power of the destabilization approach, and points the way to possible future courses of study.
This reaction step releases 2.6 wt pct hydrogen. As shown in Figure 2, dehydrogenation occurs at temperatures lower than the dehydrogenation temperature for either pure LiBH4 or pure Mg2NiH4. This is an important feature, as it indicates that a new hydrogen release pathway has been opened up by mixing the LiBH4 and Mg2NiH4 materials. This pathway likely involves direct reaction between LiBH4 and Mg2NiH4, rather than H2 release from either of the individual components. The reaction begins at temperatures as low as 523 K (250 °C), which is low for borohydride-based systems. The low reaction temperature may be due to the catalytic nature of Ni in the [NiH4]2− anion. Thus far, this system appears to be the only reversible destabilized system that reacts through a (new) direct reaction pathway. In contrast, the well-studied LiBH4/MgH2 system[13,21] reacts sequentially with initial dehydrogenation of MgH2 followed by reaction of Mg with LiBH4. It seems likely that to achieve the full benefit of the mixed hydride system destabilization strategy, reaction through new pathways is necessary, making this system an important demonstration that such methods are possible.
The good kinetics of the LiBH4/Mg2NiH4 reaction allows its equilibrium to be characterized over the temperature range 543 K to 633 K (270 °C to 360 °C). The pressure varies logarithmically with the inverse temperature characterized by a change in enthalpy (∆H) of 15 kJ/mol-H2 and a change in entropy (∆S) of 62 J/K mol-H2. This change in enthalpy is very low for a reversible system. A low enthalpy is advantageous for practical systems in which heat must be supplied to release hydrogen and dissipated during rehydrogenation. However, systems with low enthalpies (<~20 kJ/mol-H2) typically cannot be rehydrogenated at practical pressures because the equilibrium temperatures, given by T eq = ∆H/∆S, are too low. Remarkably, for this system ∆S is also very low, which raises the equilibrium temperature and enables reversibility. Vajo et al. speculated that the low ∆S originates from the relatively high entropy of two complex hydride anion species, [BH4]− and [NiH4]2−, in the hydrogenated phase.
Overall, the capacity for the direct low-temperature step shown above is too low for practical use for hydrogen storage onboard vehicles. However, Mg2NiH4 is a transition metal-based complex hydride of which there are numerous (literally hundreds of formulations) known examples. Therefore, the remarkable behavior of this system holds promise that other LiBH4/transition metal-based complex hydride systems could be found with higher hydrogen capacities.
Quite often the rates of hydrogen absorption and release from light metal hydrides are undesirably low. It may be that the slow hydrogen exchange rates can be attributed to slow rates of diffusion which, in turn, originate from the covalent and ionic bonding characteristic of light elements. Based on this hypothesis, the first project group sought to improve the hydrogen exchange rates by restricting the hydride particle size to the nanometer scale. Diffusion times vary as the square of the diffusion length; thus, decreasing particle size by a factor of 10 would decrease diffusion times by a factor of 100. Restricting the particle size of hydride materials is difficult considering that metal hydride materials often agglomerate as hydrogen is released and stored. To address this issue, a novel approach was employed where hydride materials were incorporated into the pores of nanoporous scaffolds to confine the hydride particles to nanoscale dimensions and to maintain these dimensions during cycling. The strategy is described by de Jongh and Adelhelm who has employed a similar approach.
Figure 3 shows how precursor MgBu2 can be infused in the aerogel scaffold using heptane as a solvent. Then the MgBu2 can be thermally decomposed (also driving off heptane), yielding nanoconfined Mg that can then be hydrogenated to give scaffold-incorporated MgH2.
Equilibrium studies of the plateau pressure of hydrogen at 523 K (250 °C) from MgH2 confined to 13 nm pores agree well with database plateau pressure for bulk MgH2 to within 10 pct accuracy. If there were a change in the thermodynamics, there would be a corresponding change in the equilibrium hydrogen pressure at a given temperature, which was not observed experimentally. Presumably if the MgH2 particles were made significantly smaller than 13-nm diameter there would be a significant change in the thermodynamics of H2 release. The effects of the nanoscaffold on the material’s gravimetric and volumetric hydrogen storage density have been discussed by Vajo.[20,24]
The second project group, complex anionic materials, focused on the synthesis and characterization of high-weight-capacity metal hydrides that contain well-defined chemical moieties, in particular the borohydrides containing BH4 − anions. A review of the major international effort to develop borohydride hydrogen storage materials has been published by Stavila et al., and within the MHCoE, over 50 borohydride materials were developed and studied by many of the MHCoE PI’s. A great deal of screening and characterization work was conducted at Sandia on Ca(BH4)2 and also Mg(BH4)2 by Majzoub, Rönnebro, and co-workers. The group II alkaline earth metal borohydrides were of primary interest over alkali metal borohydrides due to their high potential capacities for hydrogen storage.
Calcium borohydride has a theoretical hydrogen capacity of 11.6 wt pct, making it of interest to potentially meet the DOE onboard storage performance targets; however, prior to the work of Rönnebro, Majzoub, and co-workers at Sandia, there was little known regarding its fundamental properties as a potential hydrogen storage material. Ca(BH4)2-(THF)2 was commercially available and could be desolvated by heating under vacuum. Theoretical calculations by Majzoub indicated that Ca(BH4)2 could be obtained directly through a solid-state reaction of CaB6 with CaH2 under hydrogen pressure. The researchers demonstrated that a 1:2 ratio mixture of CaB2 with CaH2 reacted under 700 bar H2 at 673 K to 713 K (400 °C to 440 °C) directly produced Ca(BH4)2 in 60 pct yield with respect to starting materials and with 9.6 wt pct theoretical reversible hydrogen capacity. Cycle studies showed loss of reversible capacity on subsequent cycles, with only 3.5 to 5 wt pct reversible after several cycles, depending on conditions and additives used. Several polymorphs for Ca(BH4)2 with differing stability ranges were identified. Majzoub and Rönnebro used a combined method of searching the inorganic crystal structure database (ICSD) coupled with use of the prototype electrostatic ground state (PEGS) method to identify potential low-energy ground-state structures. Further work to refine the structures with in situ synchrotron data in collaboration with Filinchuk et al. identified three polymorphs: a low-temperature α-phase (F2dd) that undergoes a second-order α → α′ (I-42d) transition upon heating to ~495 K (222 °C). The α- and α′-phases were found to transform into a denser, stable β-phase (P-4). The β-phase fully decomposes between 655 K and 660 K (382 °C and 387 °C). While work on Ca(BH4)2 stimulated much international interest and research, due to high desorption temperatures, poor reversibility and thermodynamics, further work on Ca(BH4)2 within the MHCoE was discontinued.
As with many borohydrides, Mg(BH4)2 was attractive because of the exceptionally high theoretical hydrogen weight percent, 14.9 pct, for this material. Mg(BH4)2 was first synthesized by Konopolev, who reported on the synthetic procedures and first-order hydrogen desorption properties. Konopolev observed H2 desorption from Mg(BH4)2 above 593 K (320 °C), with ∆H ~53 kJ/mol H2 for hydrogen desorption. At that time, almost nothing was known about the reaction pathway, and nothing was known about the possible reversibility of the system.
Zhao and co-workers initiated MHCoE work on Mg(BH4)2. A great deal of effort was devoted to understanding the crystal structures of the material as it was heated, eventually releasing hydrogen. Using a unique in situ X-ray diffraction (XRD) method that also allowed full characterization of emitted volatiles, Soloveichik and colleagues at GE were able to directly correlate crystal structure with temperature and hydrogen release.
Thus, hydrogen is first released from Mg(BH4)2 to form MgH2 and Mg(B12H12). MgH2 then releases H2, followed by eventual release at high temperature from the highly stable MgB12H12 material. A full 12 wt pct of hydrogen is released from the material, and the low ∆H of desorption indicates that facile hydrogenation of MgB2 should be possible. Unfortunately, due to presumed kinetic limitations, the reaction was not reversible under the conditions used for these initial GE studies. However, reversibility was subsequently demonstrated at higher temperatures and pressures by Severa et al. No toxic gaseous species (such as diborane, B2H6) or other impurities were found in the desorbed gas in this study. An intermediate MgB12H12 was observed which turned out to have widespread implications for hydrogen storage from borohydride materials. These studies demonstrated an attractive ∆H for hydrogen desorption of ~40 kJ/mol H2, indicating a moderate chance for reversibility under reasonable conditions if no kinetic limitations existed.
This ~12 wt pct reversibility for Mg(BH4)2 makes it a record-breaking material for reversible hydrogen storage. Although the process conditions of pressure and temperature for both dehydrogenation and hydrogenation are higher than desired, Mg(BH4)2 has the highest reversible capacity for any metal hydride material discovered thus far. If the kinetic limitations could be overcome, then the thermodynamics of the system would allow facile hydrogen release and reversibility. As a result of this promise, further investigation of Mg(BH4)2 was recommended since this material has high promise, but more material R&D is still needed.
It should be noted that the appearance of these relatively unknown MB12H12 salts led to a great deal of MHCoE work. Stavila et al. at Sandia developed an efficient synthesis method to directly make MB12H12 materials in an unsolvated state, allowing the properties of pure CaB12H12, Li2B12H12, and NaB12H12 to be examined for the first time. Interestingly, it proved difficult to synthesize MgB12H12. These B12H12 salts were also investigated extensively theoretically by Ozolins and co-workers.
The third project group was inspired by the 2002 Nature article by Chen et al. and investigated nitrogen-containing materials for hydrogen storage. Chen observed that LiMgN took up a significant amount of hydrogen, and the resulting material (proposed to be LiNH2 + 2LiH) could also desorb H2, giving a reversible hydrogen storage system operating below ~523 K (~250 °C). This report spurred a great deal of international interest in N-based hydrogen storage materials, as reviewed recently. During the course of the MHCoE program, 15 different amide-related systems were investigated, including mixtures of amides and borohydrides, amides and alanates, and LiMgN itself.[16,17] Two prominent N-based systems (2LiNH2 + MgH2) were explored by Luo and co-workers[37,38] at Sandia, and LiMgN investigated by Fang and co-workers at Utah. Additional information on the LiMgN system follows.
This system was first investigated theoretically by the MHCoE Theory Group PIs Sholl, Johnson, and their colleagues. Theoretically, this reaction was predicted to have a very attractive hydrogen desorption enthalpy of ∆H = 32 kJ/mol H2 and a hydrogen capacity of 8.2 wt pct. The high predicted weight percent of hydrogen is due to the full dehydrogenation to LiMgN, bypassing the undesired imide intermediate. Since Reaction 5 represents the complete dehydrogenation of the system, LiMgN would be an important candidate material for hydrogen storage if the dehydrogenation took place at low temperature and if it was reversible.
Of course, the material must be reversible to be useful for automotive hydrogen storage. Further studies showed that LiMgN could be rehydrogenated at 138 bar H2 and 513 K (240 °C), and that 2 to 4 mol pct Ti significantly catalyzed the rehydrogenation. However, the reaction was observed to be more complex than the single-step process shown in Reaction 5 and subsequent theory by Akbarzadeh and co-workers predicted a multistep nature to the hydrogenation and dehydrogenation reactions governed by thermodynamic factors.
Extensive FTIR and XRD measurements support the assignment that upon hydrogenation, LiMgN converts to LiH + 1/2 Mg(NH2)2 + 1/2 MgH2 via the intermediate imide Li2Mg(NH)2. The assignment of the imide in this reaction as an intermediate is speculative, as its detection in the cycling has not been confirmed, although the overall data suggest a multi-step behavior as predicted by Akbarzadeh et al. The nature of the reaction pathway for the LiMgN system needs further investigation. Nonetheless, it is a material showing reversibility, with ~8 wt pct capacity and dehydrogenation and hydrogenation temperatures in the range 433 K to 493 K (160 °C to 220 °C). The long-term cycling behavior needs to be examined.
Chandra and co-workers at the University of Nevada-Reno investigated the long-term cycling and effects of impurities on cycling for various material systems. In the case of lithium amide/lithium hydride system (LiNH2 + LiH), Chandra et al. discovered that the hydrogen capacity losses during cycling could be mitigated through the addition of N2 to the hydrogen used to rehydrogenate the material. Nitrogen loss from the system through the formation of ammonia in the desorption products has been observed with most amide/imide systems. For the LiNH2 + LiH system, it was proposed that liquid Li formed at low absorbed hydrogen contents with which N2 could react to form Li3N and prevent further formation and accumulation of LiH.
Virtually all of the materials investigated in the MHCoE were considered for their potential as onboard reversible materials in which the spent material can be rehydrogenated without being removed from the vehicle. However, two materials out of the 94 investigated in the MHCoE were considered for their “off-board” regeneration potential. These materials were AlH3 and LiAlH4. A comprehensive review of the history and recent progress associated with AlH3 and LiAlH4 has been written by Graetz et al.
In many ways, aluminum hydride (AlH3), often referred to as “alane,” represents a nearly ideal hydrogen storage material from a desorption point of view. AlH3 has a gravimetric capacity of 10 wt pct and volumetric capacity of 149 g H2/L, along with a hydrogen desorption temperature of ~333 K to 448 K (~60 °C to 175 °C) (depending on particle size and the addition of catalysts). Due to aluminum hydride’s low temperature of decomposition and its ability to store 10 pct hydrogen by weight, this material has been the subject of study for decades, as reviewed by Sandrock et al. and more recently by Graetz et al.[43,45] The difficulty with AlH3 lies in its regeneration from the dehydrogenated state, namely Al metal; the pressure to regenerate AlH3 directly from aluminum and gaseous hydrogen is simply too high to be practical on a large scale. After the initial research phase,[43,45] characterizing the structure and hydrogen release properties of AlH3, attention turned quickly to the problem of regeneration. It was already known that AlH3 could be generated by reacting LiAlH4 with AlCl3 using organic solvents. This method produces stable salt by-products that are costly to recycle and, therefore, not useful for many applications.
In the MHCoE, two primary methods were discovered for the offboard regeneration of AlH3 from Al metal. The first method is regeneration of AlH3 using a two-step organometallic process developed by Graetz and colleagues at Brookhaven National Laboratory (BNL). The second method is the regeneration of AlH3 via electrochemical means developed by Zidan and co-workers at Savannah River National Laboratory (SRNL).
In the BNL work, the general approach was to use organometallic chemistry to try to form AlH3 in the presence of a stabilizing agent. Whereas forming AlH3 directly from Al and H2 in the gas phase is nearly impossible due to the instability of AlH3; through adduct formation, AlH3 could be more easily formed and stabilized, with the need to subsequently remove the stabilizing agent to obtain the final material.
As shown in Figure 8, direct formation of the stabilized AlH3 moiety has been achieved using dimethylethylamine (DMEA) and also trimethylamine (TMA) as the stabilizing agents. Both DMEA and TMA bind too strongly to AlH3 to allow facile removal of the stabilizing amine. However, both TMA-AlH3 and DMEA-AlH3 can be transaminated using triethylamine (TEA) to TEA-AlH3, which could then be easily thermally dissociated to produce pure AlH3 without inadvertent release of H2.
A major concern with such an “off-board” regeneration method is the energy required to execute these chemical steps. In 2011, an independent analysis of an AlH3 storage system was conducted by Hua and Ahluwalia of Argonne National Laboratory. Argonne’s analysis assumed 70-wt pct aluminum hydride slurry, and used TMA as the stabilizing agent. The Argonne analysis indicated a well-to-tank (WTT) efficiency of 40 pct assuming 75 pct yields in the transamination step and 75 pct in TEA decomposition and recovery step, along with losses in producing hydrogen (73 pct efficiency) and compressing and recycling hydrogen. This WTT value could be higher if the yields in the various synthetic steps increased, and full use was made of waste heat during the processing. The WTT DOE target is 60 pct.
Zidan and colleagues at SRNL investigated the second possibility of regenerating AlH3 electrochemically. The idea is to utilize electrolytic potential to increase hydrogen activity and, therefore, drive chemical reactions to regenerate AlH3. However, the use of electrochemistry has to take into account that Al and AlH3 oxidize in aqueous environments, thereby prohibiting the use of all protic solvents as electrolytes. For this reason, SRNL developed a novel route using a non-aqueous solvent system. A polar aprotic solvent such as tetrahydrofuran (THF) is used in the electrolytic cells which are operated at ambient pressure and temperature.
Details of the electrochemical regeneration route can be found in the publication by Zidan et al. However, the general idea is as follows: starting with spent Al (formed from the desorption of H2 from AlH3), the Al is removed from the vehicle, and processing begins. The Al is reacted with NaH and H2 to form NaAlH4. This NaAlH4 is dissolved in THF and placed in the cell. Alternatively, Al from the vehicle can enter the electrochemical environment as the Al anode. When the cell is polarized, Al metal can be converted to Al3+, which reacts with dissolved AlH4 − ions to produce AlH3 complexed with THF.
Once the AlH3-THF adduct is formed as a white solid, the THF must be removed from the AlH3. This proved to be problematic, as the temperatures required to remove the THF also led to some dehydrogenation of AlH3, leading to a mixture of both Al and AlH3 in the final product. Results improved dramatically when another proprietary adduct besides THF was introduced, which led to facile removal of the adduct, forming pure AlH3 in gram quantities.
Similar to AlH3, LiAlH4 does not form by direct hydrogenation at reasonable hydrogen pressures. Therefore, there is considerable interest in developing new routes to regenerate the material from the dehydrogenated products LiH and Al at low temperature to yield crystalline LiAlH4.
This process is remarkable for its mild conditions and convenience. The dehydrogenated products, LiH + Al + Ti catalyst, are dissolved in Me2O at a hydrogen pressure of 100 bar. At this pressure, Me2O is a liquid, and the LiH + Al [Ti] material dissolves. After 24 hours at room temperature, the hydrogen pressure is released. With venting of H2, Me2O also vents, and one is left with a quantitative conversion to catalyzed LiAlH4. Studies of the hydrogen release properties of the LiAlH4 (Ti catalyzed) thus formed shows that 7 wt pct of hydrogen is released from 353 K to 453 K (80 °C to 180 °C), with excellent kinetics, and only ~100 ppm of Ti is required for the catalysis. The material has been cycled up to 5 times with this process, but there is some cycle-induced degradation of that material that is being investigated further. A study by Argonne has suggested that as the relative amount of DME required for the process is minimized, the WTT efficiency of the process can approach the 60 pct DOE target.
Density functional theory (DFT) had been used to compute the thermodynamics of only a few individual metal hydride compounds. There had been little attempt to predict phase diagrams/van’t Hoff plots for metal hydrides.
Theory could not predict reaction complexities, for example metastable species and multi-step reactions, that can occur in solid-state H2 desorption/absorption reactions.
Theory could not predict the crystal structures of complex metal hydrides.
Virtually no theoretical work had been done on amorphous metal hydrides, or kinetics.
DFT can now be routinely used to predict the ∆H and ∆G of complex metal hydrides.
Reactions can now be predicted over wide ranges of pressure, temperature, and composition, thereby focusing experimental efforts on promising compounds. Linear search methods have been implemented allowing the scanning of literally millions of different reaction conditions (composition, temperature, and pressure), with predictions for promising materials finding confirmation in experiments. Their multistep character can now be predicted and understood as well.
Theory can now predict the existence of important and surprising reaction intermediates that are being confirmed by experiment (e.g., [B12H12]2− intermediates).
The prototype electrostatic ground-state (PEGS) method, developed by Majzoub and Ozolins, can predict ground-state crystal structures beyond the use of the ICSD database, thereby increasing accuracy and enabling thermodynamic predictions for new structural phases of materials.
Theory is addressing the problem of understanding metal hydride interactions with nanoconfined structures in order to better understand nanoconfined materials.
This list above represents a considerable advancement in the theoretical prediction of materials, phases, intermediates, and mechanisms of hydrogen storage reactions. The progress is attributable not only to the MHCoE Theory Group, but to hydrogen storage theorists worldwide. This review of the MHCoE theory efforts is too short given the considerable theoretical advancements that were made. The reader is referred to the recent review by Stavila et al., as well as to the MHCoE Final Report to the DOE[16,17] for more details of the MHCoE theoretical results.
One of the primary tasks of the MHCoE was to make sober assessments of the materials that were discovered, and to assess if further work was warranted on them, given the technical goals of the program. This process was termed “materials downselect.” If a material was “downselected” it was considered worthy of further study, and work on it continued. If a material was not “downselected,” due to lack of promise, work on it discontinued. Although all of the DOE requirements indicated previously are required for a hydrogen-fueled light-duty automobile, within the MHCoE program particular attention was paid to five technical targets because they are challenging and have a large influence on the successful engineered implementation of the technology. These included system gravimetric density, material reversibility, thermodynamic requirements, material stability and volatilization, and materials kinetics.
Over the course of the MHCoE work, 94 materials systems were investigated in the 4 materials Projects (A–D). Of these 94, research work on 84 materials was discontinued and not recommended for further investigation due to some shortcoming of the material. Mg(BH4)2, AlH3, LiAlH4, 2LiNH2 + MgH2, and LiMgN were recommended for further study. Some materials were investigated in the final days of the MHCoE, and needed further study before a downselect decision could be made. These other materials include Mg(BH4)2(NH3)2, (NH4)2B10H10, AlB4H11, and Al(BH4)3/NH3.
Comparison of Materials to DOE Targets
This section compares the best onboard reversible materials the MHCoE produced against the targets set forth by the DOE. These targets are for the entire storage system that would be placed inside a light-duty vehicle, including not only the metal hydride material but also the tank holding the metal hydride, the heat exchange hardware inside the tank to allow thermal management of the system, all associated plumbing, regulators, pressure relief devices, and other pieces of hardware. However, in our materials work, most of our information arrives in the form of materials properties, for example the material gravimetric capacity, material volumetric capacity, etc. In order to compare to the DOE 2010 targets, it is most convenient to translate the system “targets” into materials “goals.”
Measured Values of “Material Goals” for “Onboard Regenerated” Metal Hydride Systems LiBH4/MgH2, LiBH4/Mg2NiH4, 2LiNH2/MgH2, Mg(BH4)2, LiNH2/MgH2, NaAlH4, and AB2H3 (A=Ti, Zr; B=V, Cr, Mn)
2010 Materials “Goals”
AB2H3 A=Ti, Zr
B=V, Cr, Mn
Gravimetric density (wt pct)
Volumetric density (gH2/L)
Min. delivery pressure at 85 °C (PEMFC) (bar)
Minimum flow rate (gH2/s) at 85 °C
1/(Recharge time = 4.2 min), min−1
1/(Fuel impurities = 100 ppm), ppm−1
Improve our understanding of destabilized materials
Research in the MHCoE demonstrated that several advantages may accrue when incorporating a metal hydride into a nanoporous material. For example, the kinetics of hydrogen release can be enhanced greatly. The initial assumption of why the kinetics might be improved are based on the general idea that diffusion times follow the relationship t ~ L 2/D, where t is the diffusion time, D is the diffusion coefficient, and L is the required diffusion distance. If nanoporous materials reduce the required diffusion distances, the diffusion time should be reduced and the reaction rate should increase. This is only a very general concept, and in fact the real atomistic mechanism for why the kinetics of the studied systems is improved remains unknown. Fundamental studies need to be performed in order to understand the nature and origin of fast diffusing species presumed to exist in these nanoconfined systems.
Kinetics of Solid-State Reactions
Catalysis of Solid-State Reactions
The borohydrides offer the potential for 12 to 14 wt pct materials and their properties can vary widely. It is important to gain a fundamental understanding of the factors that control borohydride properties, such as reversibility, diborane release, ammonia release, and temperature for H2 release. With this understanding, a material that satisfies all DOE requirements might be found.
Offboard Materials AlH3 and LiAlH4
For AlH3, further work on the organometallic approach should emphasize increasing the yield of the different chemical steps, and the purity of the products obtained. In general, the yields for forming DMEA-AlH3 using a pressurized reactor were good. Difficulties occurred using a vacuum distillation transamination step, where trace amounts of metallic aluminum were observed. It is known that trace amounts of aluminum promotes the decomposition of AlH3. This fact restricts the window for cleanly separating AlH3 from TEA-AlH3. Hence after repeated attempts to recover pure AlH3 from TEA-AlH3, the final product always consisted of both AlH3 and aluminum. This problem needs to be resolved experimentally. As for LiAlH4, work needs to be conducted to investigate why the material loses hydrogen capacity with cycling and in particular what is happening to the Ti catalyst during the repeated hydrogenation/rehydrogenation cycles.
Hydrogen Storage Research in the DOE Chemical Hydrogen Storage Center of Excellence (CHSCoE)
Chemical hydrogen storage is defined as the release of hydrogen from covalent chemical compounds, and as in any materials-based approach, includes the recycling of the dehydrogenated “spent” fuel back to the original material to complete the storage cycle. The sections below will cover the approaches and strategies for release of hydrogen from covalent compounds as well as their regeneration from their respective “spent” fuels.
Hydrolysis, and particularly catalytic hydrolysis, was explored as a way to release hydrogen from covalent chemical compounds such as polyhedral boranes and sodium borohydride. Here, water is used to hydrolyze [element] –H bonds, forming [element] –O bonds and hydrogen. One of the drawbacks of hydrolysis is that it leads to the formation of very stable [element] –O bonds which, as will be discussed below, leads to thermodynamic costs associated with recycling of very stable compounds to high energy compounds—fuel. One of the positive attributes of hydrolysis is that water contributes hydrogen to the total hydrogen released, but the water must be carried onboard the vehicle, and represents a potentially more complex engineering problem to resolve.
Thermolysis of compounds that release hydrogen endothermically tends to require a high temperature (>423 K [150 °C]) to sustain hydrogen release. They are less desirable as the heat to sustain the reaction must come from burning some of the hydrogen released, diminishing the amount of hydrogen available. There are numerous materials that release hydrogen endothermically including simple hydrocarbons, alane, and the compounds described in the section on the MHCoE activities. Materials that release hydrogen exothermically should not in principle require an auxiliary source of energy to drive reaction; however, the existence of materials that release hydrogen exothermically owe their limited stability to a relatively small activation barrier to reaction that must be overcome to initiate the reaction. In exothermic release materials, once the reaction is initiated, the heat of reaction drives further release. A significant challenge for this type of material is that the reaction can runaway if there are no means to control the rate of thermolysis. Thus, an engineered system that utilizes an exothermic release material such as ammonia borane (to be described in more detail below) must utilize a method that can initiate reaction and enable subsequent control of the rate of reaction, as well as the rejection of the heat of reaction.
Chemical catalysis is the means by which rates of reaction may be increased at a given temperature or the temperature of reaction may be reduced. Many approaches involving a wide variety of storage materials have attempted to improve temperature or rates of release by finding an appropriate catalyst for the release reaction. For exothermic reactions, if the reaction can be initiated at a lower temperature in the presence of a catalyst and in an appropriate reactor, such as in a flow reactor, then the residence time in the catalytic zone may help to control the rate of release, and thus the temperature of release. This is, therefore, a means of controlling an exothermic reaction. Catalysts have been discovered that promote the rapid dehydrogenation of ammonia borane at room temperature, and in the process liberating heat. This then provides an example of a method to initiate and control the release of hydrogen from a material that releases hydrogen exothermically.
When the CHSCoE was in its formative stages in the 2004 to 2005 timeframe, little was known about the potential for storing hydrogen in covalent chemical bonds. Some research had been undertaken on “activated” hydrocarbons by Cooper at Air Products. The release of hydrogen from ammonia borane (AB) was known largely through the use of AB as a precursor to boron nitride, a high-temperature ceramic material resulting from the release of all of the hydrogen from AB at high temperature.
The potential of AB as a hydrogen storage material was subsequently recognized by T-Raissi at the University of Central Florida who provided much of the early literature on AB as a storage material, demonstrating that up to nearly 20 wt pct of hydrogen could be released. Another line of research on chemical hydrogen storage was focused on the generation of hydrogen upon hydrolysis of sodium borohydride (SBH). A substantial fraction of the U.S. literature was contributed by researchers at Millennium Cell, which, at the time, was developing an onboard hydrogen storage system demonstration with Daimler-Chrysler.[56,57]
In 2005, the interest in exploring AB and other related materials was driven by the high potential hydrogen capacity, the exothermicity of hydrogen release that could lead to high rates of hydrogen evolution, and the possibility of reducing the temperature of release into the range of what was recognized to be needed for vehicular applications. Major barriers to success were identified, which included the regeneration of spent fuel. Because of the exothermicity of releasing two moles of hydrogen from ammonia borane (ca. 58.6 kJ/mol [14 kcal/mol]), the release is irreversible at reasonable and practical pressures of hydrogen. The spent fuel must, therefore, be regenerated in some way other than simple rehydrogenation; it was recognized that this would likely have to be done chemically and thus offboard the vehicle. A second challenge resulting from offboarding the spent fuel and refueling the vehicle with AB then became one of materials handling—how to efficiently, reproducibly, reliably, and safely onboard/offboard a solid material. Thus, a further challenge for the CHSCoE was the search for liquid fuels and spent fuels that could provide an alternative to the difficulties in on- and off-boarding of solid materials.
Chemical Hydrogen Storage Center of Excellence List of Partners
Rohm and Haas (now Dow Chemical Co.)
University of Alabama
Los Alamos National University
Northern Arizona University
Pacific Northwest National Laboratory
University of California, Davis
University of California, Los Angeles
University of Missouri
Pennsylvania State University
University of Pennsylvania
The CHSCoE’s approach was to focus on light element compounds (B, C, and N) containing hydrogen, with a particular focus on those that might have the possibility of releasing hydrogen exothermically to be able to attain fast kinetics of hydrogen release. This approach also required the CHSCoE to address the development of simple, energetically inexpensive offboard regeneration schemes.
As the CHSCoE conducted its research, it became evident that the class of B–N–H compounds is promising owing to their electronic structure and bonding that made them particularly susceptible to losing hydrogen. Much of the research conducted by the CHSCoE focused on ammonia borane, NH3BH3, its derivatives, and other closely associated compounds. Other materials classes were also examined, but did not have the promise of the B–N–H or B–C–N–H containing compounds. Because of the CHSCoE’s principle focus on ammonia borane, the regeneration of spent fuel resulting from dehydrogenation of ammonia borane was necessary to demonstrate feasibility of closing the fuel cycle. At the start of the Center in 2005, very little was known or demonstrated within the storage community regarding the chemical regeneration of the BNH x spent fuel back to ammonia borane. At the time, the hydrogen storage community’s opinion was that the undeveloped notion of spent fuel regeneration represented a daunting and significant technical barrier to the acceptance of the concept of offboard regenerable storage materials.
In Figure 11, data for hydrogen release from some materials are shown at multiple temperatures to indicate the temperature range over which the release of hydrogen is possible. For example, catalyzed release of hydrogen from AB is shown as three distinct points (diamonds) ranging in temperature from 298 K to 383 K (25 °C to 110 °C). Above 343 K (70 °C), AB has the capacity and the rates of release to potentially meet DOE system targets. At 298 K (25 °C), catalyzed release of hydrogen from AB can be fast with the appropriate catalyst, and may be of interest for “cold start” applications even though the capacity is unlikely to meet DOE system targets. Similarly, mixtures of AB in ionic liquids (IL) also demonstrate a broad temperature range for fast hydrogen release in the temperature regime of practical application for vehicular applications. From this plot, it can be seen that a number of materials fall close to the gravimetric and temperature ranges considered necessary for vehicular applications. Others, such as silicon nanoparticles[58, 59, 60, 61] release hydrogen at too high a temperature, and do not release hydrogen with adequate gravimetric capacity. When the CHSCoE concluded its research, chemical regeneration of spent fuel from ammonia borane had been demonstrated in the laboratory with two major variants with over ten regeneration schemes having been partially or completely demonstrated, thereby showing potential to overcome regeneration as a barrier to the technological implementation of chemical hydrogen storage; however, more R&D is need to reduce cost and increase efficiencies.
The rest of this section provides some of the technical details of the findings of the CHSCoE, and a discussion of the most promising materials found to date.
Classes of Materials Examined by the CHSCoE Results and Accomplishments
The major findings and accomplishments for the materials classes investigated by the CHSCoE are described below.
Exothermic Hydrocarbons as Chemical Hydrogen Storage Materials
CHSCoE researcher Thorn and colleagues examined certain “activated” hydrocarbons as potential hydrogen storage materials, such as substituted dihydrobenzimidazoles. It was found that certain derivatives exhibited exothermic and rapid hydrogen release when reacted with protic acids in the presence of a catalyst. However, these materials were limited to <2 wt pct hydrogen, and so the CHSCoE moved on to other more promising compounds. The CHSCoE also made a cursory examination of C–N based carbene compounds; however these, as are most hydrocarbons, are limited to releasing 1H per CH2 unit or approximately 7 wt pct hydrogen. It was felt that these compounds had little hope of achieving the DOE system targets.
Hydrolysis of Boron Hydrogen Compound
As mentioned briefly above, one of the chemical hydrogen storage systems that pre-dated the CHSCoE was the hydrolysis of sodium borohydride, NaBH4 (SBH). Aqueous solutions of SBH can provide 4 to 7 wt pct hydrogen, with half the hydrogen coming from water, and the product being sodium metaborate. Additional research and analysis carried out by CHSCoE researchers Linehan and colleagues indicated that regeneration of the borate back to borohydride was energetically inefficient and costly. Also, hydrolysis of SBH at concentrations sufficient to meet DOE’s gravimetric target resulted in the crystallization of borate from solution; this was deemed to be unacceptable from an engineered systems perspective. Precipitation could be avoided by diluting the fuel with more water, but then the gravimetric hydrogen content fell below what is felt to be feasible for a storage system as described by the DOE technical targets. With these less than promising difficulties of SBH hydrolysis and regeneration, the DOE determined to forego additional research on hydrolysis of SBH.
While these hydrolysis reactions have a slightly higher gravimetric capacity than SBH they suffer in the same way as the product borates are insoluble and are energetically expensive to recycle to the borane fuels. The CHSCoE discontinued research on hydrolysis-based approaches early on, and moved on to more promising materials and processes.
Thermolytic Hydrogen Release from Ammonia Borane (AB)
One particular intermediate, borazine, B3N3H6, is volatile and reactive, and under certain conditions small amounts of the by-products ammonia and diborane are released. These gaseous impurities represent a challenge to not only avoid losing boron or nitrogen from the system, but also in poisoning downstream fuel cell membranes and electrocatalysts.
The release of hydrogen from pure solid AB exhibits complex kinetics that may be defined as a nucleation and growth process, described by an Avrami kinetics model. Such processes are characterized by an “incubation” or nucleation period, followed by a rapid net reaction phase or growth phase. Thus, there is a considerable delay in observable hydrogen release upon heating of pure solid AB until critical concentrations of reactive intermediates are achieved.
Another facet of the release of hydrogen from pure solid AB is the material foams. During the early stages of hydrogen release, the material undergoes a phase change to a somewhat fluid phase that is formed by the hydrogen released, undergoing a significant expansion in volume. Upon additional hydrogen release, the material resolidifies. The large change in volume is unacceptable in a fuel system where there is little volume to accept such expansion in volume of the spent fuel. CHSCoE research led by Autrey and colleagues led to the finding that there are a number of additives that mitigated the foaming of AB. One of those additives was methylcellulose; upon addition of as little as 15 wt pct methylcellulose the foaming was completely reduced. This comes with the compromise of a small fraction of the hydrogen capacity; however, it was also found that additives that mitigated the foaming also shortened or eliminated the “incubation” period. So the net impact of the additive was not only to reduce foaming, but to improve the overall kinetics of reaction by eliminating the incubation period.
In addition to the CHSCoE work on pure solid AB, partners worked on various approaches to improving the kinetics and temperature of release. Infiltrating AB from solution into the pores of mesoporous or nanoscale materials such as carbons, silicon oxides, or nanoscale boron nitrides was performed by Autrey, Karkamkar, and Kauzlarich[70, 71, 72, 73] and led to reduced temperatures of hydrogen release with improved kinetics and a reduction in the concentration of volatile intermediates or by-products. However, the mass of the mesoporous “scaffold” reduced the overall gravimetric capacity of the material to the point where the materials could no longer meet the gravimetric system targets for hydrogen capacity, and so this work was set aside in favor of more promising approaches and materials.
Other additives to AB also led to interesting results, some of which will be described in more detail below. Sneddon found that the addition of LiNH2 or LiH, both strong bases, increased the rate and extent of hydrogen release from AB. However, these additives promote the formation of LiBH4, a relatively unreactive compound that limits further hydrogen release at low temperature.
Catalytic Release of Hydrogen from Ammonia Borane
In order to perform the catalytic dehydrogenation of ammonia borane, it is necessary to make intimate contact of ammonia borane with the catalyst. This is most readily accomplished using a “homogeneous catalysis” approach, where the catalyst and the substrate ammonia borane are both dissolved in a liquid. Another method is the “heterogeneous catalysis” approach, where the substrate as a liquid is contacted with a solid catalyst. Each approach has their advantages and disadvantages depending on the nature of the desired reaction to be carried out. However, both require that the substrate, ammonia borane in this case, be in a liquid state. A number of approaches to developing liquid storage materials were examined ranging from solutions of ammonia borane in water and in conventional organic solvents to more concentrated solutions of ammonia borane in IL (low melting organic salts) as demonstrated by Sneddon and his colleagues,[88, 89, 90, 91, 92] to various organic amine derivatives of ammonia borane that had melting points at or below room temperature in a few cases. The CHSCoE’s research began by exploring the homogeneous catalyst approach owing to its experimental simplicity, and with the hope that this could guide future heterogeneous catalyst selection. Using the homogeneous catalysis approach, the CHSCoE gained valuable information on the catalytic and mechanistic details involved in the catalytic release of hydrogen from BNH compounds.
Strong Lewis and Bronsted acids were examined by Baker as catalysts for hydrogen release of aqueous solutions of AB. The formation of acyclic aminoborane oligomers by chain transfer is followed by facile dehydrocyclization at >333 K (>60 °C) to yield borazine and additional hydrogen. From this work, a proposed mechanism for initiation of hydrogen release via dehydropolymerization of ammonia borane for this acid catalyzed hydrogen release was supported by an examination of the reaction thermodynamics using DFT and experimental evidence.
The research groups of Baker, Heinekey, and Goldberg demonstrated a variety of other catalyst systems that promote dehydrooligomerization of ammonia borane in conventional organic solvents. The reaction products, temperatures of release, and extent of conversion are a function of the catalyst used. For example, N-heterocyclic carbene–nickel complexes form active catalysts that result in approximately 2.5 mol of hydrogen evolved per mole of AB at 333 K (60 °C), with the BNH product being what is referred to as polyborazylene or cross-linked borazylene, a product soluble under reaction conditions.
A variety of other homogeneous and heterogeneous catalysts also promote the release of >2 equivalents of hydrogen per mole of AB at good rates at temperatures above 343 K (70 °C). The so-called “Pincer” ligand complexes of Ir and less expensive Co were shown by Heinekey and Goldberg to form active catalysts that promote the release of hydrogen by way of an apparently different pathway. These pincer ligand catalysts catalyze the very rapid release (minutes) of only one mole of hydrogen/AB at room temperature in organic solvents, generating the BNH product described as the “cyclic pentamer” [BH2NH2]5. The cyclic pentamer is unreactive to further release of hydrogen under all reasonable conditions explored. This low-temperature pincer ligand catalyst system is of great interest not only because of its apparent mechanistic differences with most other catalysts, but also because it may represent a potential “cold start” catalyst, one that can quickly generate hydrogen and heat to help initiate higher-temperature catalysts to generate more hydrogen and heat to facilitate the startup of a cold fuel cell system.
There are several practical considerations and challenges to using a homogeneous metal catalyst in a hydrogen storage system for vehicular applications where regeneration of the spent fuel is a requirement. If a homogeneous catalyst is used, separation of the catalyst from the spent fuel mixture prior to regeneration must be accomplished. Also, if the homogeneous catalyst is mixed with ammonia borane to provide a fuel, then it may be a challenge to meet the 323 K (50 °C) shelf life target, as developing a catalyst that has essentially no dehydrogenation activity at 323 K (50 °C) and high activity at 353 K (80 °C) is a significant challenge. Developing an engineered system where ammonia borane in solution is mixed with a homogeneous catalyst may mitigate this concern, but this may be a highly complex system that does not resolve the problem of separation of the catalyst from the spent fuel.
Heterogeneous catalysts avoid the shelf life and catalyst separation challenges faced by homogeneous catalyst systems. Heterogeneous catalysts are generally employed in a tubular reactor that holds the solid catalyst in a “fixed bed.” The substrate, in this case a hydrogen storage material dissolved into a solution with a solvent or as a neat liquid material, is flowed through the bed of the solid catalyst, and the dehydrogenation catalysis begins upon mixing. This configuration mitigates the possibility of catalytic reactions release of hydrogen occurring until the fuel is contacted with the heterogeneous catalyst. The spent fuel flows out of the fixed bed reactor, and thus the separation of heterogeneous catalysts from the spent fuel is accomplished. There are several difficulties with the heterogeneous catalysis approach as well. The most pressing problems are difficulties that arise in engineering the reactor system to handle the large and rapid release of hydrogen gas and the concomitant amount of heat during the exothermic dehydrogenation of ammonia borane. Rejecting and controlling the heat transfer from the reactor are crucial to maintaining control over the reaction as it proceeds. There are also the typical problems associated with any heterogeneous catalyst—maintenance of activity over the lifetime of the catalyst and regeneration of catalysts that may become potentially poisoned with products or by-products, or otherwise deactivated. While these are potentially serious problems, the CHSCoE determined that a heterogeneous catalyst system had a higher probability of success in a vehicular application, and so the CHSCoE narrowed its focus to the development of heterogeneous catalysts and continued to focus on developing additional liquid phase ammonia borane compositions that are required to implement a heterogeneous catalyst reactor system potentially amenable to an onboard vehicular hydrogen storage system.
Subsequently, CHSCoE researchers—Burrell and Semelsberger and their colleagues found that heterogeneous catalysts such as platinum and other transition metals supported on alumina were effective for the dehydrogenation of ammonia borane. Dehydrogenation of AB dissolved in conventional organic solvent catalyzed by Pt/Al2O3 catalysts was found to be rapid at temperatures of 393 K (120 °C), with decreasing rates down to 323 K (50 °C). Non-precious metal heterogeneous catalysts containing metals such as Fe, Ni, Co, or Cu among others were also identified. Subsequent screening of these heterogeneous non-precious metal catalysts demonstrated that it was possible to generate high rates (up to twice the DOE target rate) of hydrogen release with a high yield of >9 wt pct H2 at temperatures as low as 343 K (70 °C). However, the spent fuel solutions generated insoluble solid products under these conditions that would foul the catalyst, making cyclability and catalyst lifetime problematic. While this type of catalyst fouling could be reversed, the inability to generate solvent/ammonia borane solutions of high enough concentration to meet the gravimetric target led the CHSCoE to discontinue the use of conventional organic solvents except for catalyst screening purposes. Better liquid formulations using either better solvents or novel liquid compositions are still required to maintain high concentrations of ammonia borane and the corresponding spent fuels in solution throughout the reaction cycle.
Ionic Liquid/AB Mixtures
ILs are ionic, organic salts that have melting points at around ambient temperature. ILs have found utility as useful solvents for a variety of chemical reaction types. Since the cation and the anion of the salt have chemical properties that can be “tuned” through a diversity of combinations of cation and anion, the physical properties of IL can be chosen to match the desired property most suitable for the application of interest.
Sneddon’s group first explored the utility of IL as a solvent and as an activator of AB toward dehydrogenation reactions. Subsequently, ILs were found to promote the rate and extent of H2 release from AB. Rates become very fast above 373 K (100 °C). For example, mixtures of AB with 1-butyl-3-methylimidazolium chloride (bmimCl) (50:50-wt pct) at 358 K (85 °C) exhibited no induction period and released 1.0 H2 equivalent in 67 minutes and 2.2 H2 equivalent in 330 minutes at 358 K (85 °C) as shown in Figure 15. At the same temperature, pure solid AB released 0.81 H2 equivalent after 360 minutes, and so the presence of the ionic liquid substantially increases the rate of hydrogen evolution. Significant rate enhancements for the ionic liquid mixtures were obtained with only moderate increases in temperature, with, for example, a 50:50-wt pct AB/bmimCl mixture releasing 1.0 H2 equivalent in 5 minutes and 2.2 H2 equivalents in only 20 minutes at 383 K (110 °C). Increasing the AB/bmimCl ratio to 80:20 still gave enhanced H2 release rates compared to the solid state, and produced a system that achieved 11.4 wt pct H2 release.
The hydrogen produced from the AB/IL mixtures contained only trace levels of borazine and no detectable diborane as determined by IR spectroscopy. Further analysis indicated that the IL interacts with borazine, impeding its release from the IL such that subsequent cross-linking reactions can occur, mitigating the loss of volatile borazine which is a known fuel cell catalyst poison.
In addition to IL, Baker and Davis and their colleagues within the CHSCoE attempted to find liquid compositions consisting of AB dissolved in liquid alkylamine boranes (RNH2BH3, where R is an alkyl or aryl substituent). Some promising preliminary results were found suggesting this to be a promising approach, but the CHSCoE did not have time at the end of the project to fully explore this possibly rich area from which to discover liquid compositions. More work is needed on this and related approaches to preparing liquid fuels having high gravimetric hydrogen contents with rapid rates of release at appropriate temperatures to generate the corresponding liquid spent fuels to enable simplified off- and on-boarding of material.
The Search for Less Exothermic Release Materials
Two approaches were investigated in which AB was combined with metallic hydrides or metal amides. In one approach, physical mixtures of AB with metal hydrides, amides, or borohydrides were prepared to develop new composite systems. The other approach used more reactive hydrides that reacted with AB to form new amidoborane (NH2BH3 −) complexes. One of the goals of this line of research was to search for materials that released hydrogen at near thermoneutrality. Such materials would then be likely to be directly regenerable onboard with hydrogen pressure. Thus, the potential benefit of discovering an onboard reversible storage material justified the risk of this approach.
This line of research led CHSCoE researchers Autrey and Burrell to recognize that mixtures containing ammonia borane and strongly basic or hydridic reagents such as lithium amide were resulting in compound formation via deprotonation of ammonia borane, subsequently leading to M-NH2BH3 compounds. Subsequent to these key observations, Center researchers prepared a number of these metal amidoborane (MAB) compounds by multiple routes and were well characterized, spectroscopically, structurally, and for their rates and thermodynamics of their hydrogen release reactions. Burrell and his colleagues made a unique contribution to this area by demonstrating that classical solution organometallic chemistry synthetic techniques utilizing reactions between metal-alkyls, hydrides, amides, or other bases strong enough to deprotonate ammonia borane could be used to prepare and isolate a variety of pure metal amidoboranes that could then be studied in detail.[97, 98, 99]
All of the monometallic MAB’s studied to date released hydrogen too exothermically to allow for subsequent direct rehydrogenation of the spent fuel with hydrogen gas. No safe path forward was identified for direct chemical regeneration of the spent fuel MNBH x so work in monometallic MAB’s was discontinued. However, the results from both solid-state and solution approaches demonstrate that the properties can possibly be tuned using a multimetallic approach so the search should continue with adjustments to the strategy to drive toward the goal of achieving a more readily regenerated spent fuel form.
In the instance of the reaction of AB with alkaline earth borohydrides, hydrogen evolution (Eqs.  and ) ensued indicating that compound formation was occurring. TG/DSC/MS analysis during thermolysis of these mixtures indicated that hydrogen release occurred less exothermically relative to pure AB, had lower gas-phase impurity levels, and these mixtures did not foam during hydrogen release. This is in contrast to mixtures of AB and alkali metal borohydrides where there did not appear to be any beneficial additive effects on the enthalpy of hydrogen release. The impurity levels of borazine and ammonia resulting from the alkali metal borohydride/AB mixtures were lower than but not as low as in the Ca- or Mg-borohydride/AB mixtures.
Combinations of AB with TiH2 (0.1 equivalent) resulted in a mixture that upon thermolysis demonstrated a remarkable drop in impurity levels of borazine and ammonia, <1 pct compared to neat AB. No foaming of the material is observed during hydrogen release, nor was there significant difference in the onset of hydrogen release as compared to pure AB. Addition of MgH2 to this mixture resulting in AB/MgH2/TiH2 releases hydrogen at a temperature of about 85 °C, 30 deg lower than pure AB; while maintaining the reduced levels of gaseous impurities observed in the AB/TiH2 mixtures. Hydrogen release from AB/MgH2/TiH2 is less exothermic than hydrogen release from pure AB.
Regeneration of Spent Fuel
The CHSCoE recognized early in its lifetime that if the spent fuel from a storage material could not be regenerated at low cost, then there was no path forward for additional research. It is important to note that no chemical regeneration scheme can be simpler or less costly than the straightforward rehydrogenation with hydrogen gas at moderate pressure. The compounds that the CHSCoE focused on were highly exothermic release materials, and had properties of capacity and rate (among others) that have led most to feel that the positive attributes of chemical hydrogen storage may be on par, or outweigh the more complex and costly offboard chemical reprocessing described below.
Discussion of chemical hydrogen storage regeneration schemes must start with a quantitative description of the free energy of hydrogen release from the storage material. The dehydrogenation of AB (2.5 equivalents of H2 released, generating polyborazylene) is exothermic by ca. −59.9 kJ/mol (−14.3 kcal/mol). While this is certainly less exothermic than hydrolysis of AB [ΔH = ca. −950.2 kJ/mol (−227 kcal/mol)] or sodium borohydride, for example these reactions are quite exergonic due to the increase in entropy contributed by the evolved H2 gas. The reverse, endergonic reactions require a tremendous hydrogen pressure, on the order of 100,000 atmospheres, to rehydrogenate the spent fuel. This pressure is of course too great to be feasible on an industrial scale. Instead, the CHSCoE researchers considered several regeneration schemes that involve stepwise reactions of spent fuel with proton (H+) and hydride (H−) sources. Addition of H+ and H− has the same net result as direct H2 addition, but the stepwise addition of protons and hydride is mechanistically and thermodynamically feasible as described in this review. Schemes of this sort are the best way to efficiently overcome the intrinsic energy barrier in chemical hydrogen storage regeneration by separating the large thermodynamic and entropic barriers into achievable steps. Because of this processing strategy, the spent fuels from chemical hydrogen storage materials generated to date by the Center must be regenerated offboard in a chemical processing facility.
The CHSCoE invested a substantial amount of effort in providing numerous proof-of-principle demonstrations of reasonable spent fuel resynthesis/regeneration chemistries. One of the most important aspects that we learned through doing multiple cost and energy efficiency analyses was that the most energetically efficient process may not be the least costly, as the most energetically efficient processes are likely to be equilibrium limited and invoke substantial capital and operating expense related to the many complex separations that must be accomplished along the regeneration chemistry pathway. There are many compromises that must be struck in any such complex chemical-reprocessing schemes.
Theory and modeling as contributed by the Dixon group and Camaioni were essential to the CHSCoE spent fuel regeneration efforts. There are thousands of potential combinations of reagents available to perform various reduction sequences, thousands of potential ligands to help drive various equilibrium processes, etc. Theory allowed us to select an experimentally tenable number of these possibilities, and attempt them in the laboratory. From among the dozens of regeneration schemes that were attempted out of thousands of the potential regeneration schemes, two are discussed here that were felt to be the most promising. Both of these processes were operated at small scale in a once-through mode in the laboratory to demonstrate that a spent fuel from ammonia borane could be chemically efficiently recycled back to ammonia borane at high yield at every step.
This scheme[102,103] demonstrated by the CHSCoE team of Davis, Stephens, and Gordon involved the dissolution of BNH x spent fuel with dithiocatechol (addition of H+) followed by reduction of the intermediate with a tin hydride (addition of H−). This required the use of an auxiliary ligand L to help speed the reaction and drive the reaction to completion. This ligand had to be replaced with ammonia to complete the regeneration. The tin hydride was regenerated using a formic acid loop that completed the tin cycle. This scheme, while very complicated in appearance, was demonstrated on a 50 milligram scale in the lab. Even at this small scale, the unoptimized yield was 65 pct. A cost analysis performed by the team of Linehan, Lipiecki, Chin, and Klawiter and their colleagues at Rohm and Haas (now Dow Chemical Co.) indicated that the total regeneration cost was on the order of $7 to $8/kg H2 equivalent. This cost analysis indicated that much of the cost was associated with moving very high molecular weight materials through the plant. To mitigate that, efforts began to look for lower molecular weight reductants and digesting agents.
From a purely chemical regeneration perspective, hydrazine regeneration appears to be very attractive because of the simplicity of operation and the near-quantitative yields of ammonia borane from spent fuel. One major roadblock that remains is to reduce the cost of hydrazine which currently dominates the overall cost of this regeneration process, driving the overall cost of this regeneration to more than $45/kg H2 equivalent! If AB is to be used on a large scale as a hydrogen storage material, then additional R&D would be required to improve the costs of the rather dated chemical processes by which hydrazine is currently produced.
While the CHSCoE successfully demonstrated a number of ammonia borane regeneration processes, still more R&D is required to demonstrate long-term cycling and maintenance of chemical efficiencies, reduce process material and energy inputs, and thus provide a better basis for cost estimation of chemical hydrogen storage materials that require offboard chemical regeneration of spent fuels.
The CHSCoE demonstrated many chemical hydrogen storage materials that liberate hydrogen from covalent compounds, and also demonstrated that the spent fuel may be efficiently recycled back to starting material using offboard chemical-reprocessing strategies. Together, these results provide substantial proof-of-principle that chemical hydrogen storage is technically feasible as an onboard storage mechanism. More work is required to demonstrate an engineered system, and gain a better understanding of the total cost of hydrogen that these approaches represent.
The CHSCoE demonstrated potential solutions to a number of key barriers to implementation of chemical hydrogen storage for onboard vehicular application. CHSCoE researchers demonstrated the utility of borane amine compounds as storage materials that can release hydrogen at rates that can meet vehicular requirements owing to the exothermicity of release. The CHSCoE’s research also demonstrated means to further increase the rate of evolution of hydrogen at moderate temperatures that are commensurate with vehicle operation. These means consisted of additives and catalysts, both homogeneous and heterogeneous. These approaches also uncovered strategies to reduce or eliminate the presence of volatile impurities that could negatively affect downstream fuel cell stack components either by changing the reaction pathway to avoid volatile intermediate formation (in the case of additives) or by the use of catalysis to more rapidly cross-link borazine before exiting the hydrogen generation reactor.
At the conclusion of the research program carried out by the CHSCoE, the materials that were demonstrated to have the most probability of success of achieving all of the DOE technical targets for vehicular storage were ammonia borane mixtures either in ionic liquid solution or as a pumpable slurry. Both materials could then readily address fueling/defueling requirements. Other areas of promising research are on the heterocyclic CBN compounds that are just now emerging, and the class of metal amidoboranes, where there is reason to think that materials may be found that release hydrogen less exothermically.
Discovery of high-capacity materials that release hydrogen less exothermically (nearly thermoneutral) is required to simplify onboard heat rejection, and to enable the material to be potentially onboard rechargeable. High-capacity materials that produce negligible gas-phase impurities under hydrogen release conditions are still required.
The Center has demonstrated that given a spent fuel from a chemical hydrogen system, efficient process chemistries can be devised and implemented for offboard regeneration. The chemistries that should be considered must have the minimal number of separations steps, and any separations should be of low energy intensity (e.g., crystallization). The reagent that transfers the hydrogen to the spent fuel should be inexpensive to recycle. Ideally, the incorporation of hydrogen into the spent fuel cycle should involve an efficient catalytic approach that can be directly coupled into a spent fuel regeneration scheme. Raw materials costs must be minimal, or there must be processes to make currently expensive reagents less expensive when practiced on a world scale.
First Fill of Storage Material
For any storage material, the cost of the material is central to the overall cost of the fuel. Many storage materials, whether they are a chemical hydrogen storage material, a sorbent, or a complex metal hydride, are derived from currently expensive starting materials. The CHSCoE’s work on sodium borohydride has shown that reducing raw materials’ costs is required to reduce the overall fuel cost. The CHSCoE’s work on first fill also has implications for the MHCoE, as many of the storage materials they have examined are borohydride based, and the cost of these fuels will also rely upon cost-effective synthesis of borohydride intermediates.
More emphasis on process analysis and process research is needed to determine whether there is a path forward for a given material type to achieve cost targets. If so, more emphasis is required on research to develop energy and chemically efficient syntheses of first fill hydrogen storage materials of all types.
When large-scale hydrogen storage materials needs are considered, resource availability studies should be performed at an early stage to ensure that there is enough U.S. and/or global resources available to accomplish the scale at which the implementation of the storage material is intended.
Hydrogen Sorption Center of Excellence (HSCoE)
The primary focus of the HSCoE was to develop sorbent materials that could be used to meet DOE 2010 and 2015 onboard hydrogen storage system targets due to the relatively low volumetric energy density of hydrogen compared to typical liquid transportation fuels. The hydrogen sorbents investigated by the HSCoE typically rely on non-covalent interactions (for example, typically 5 to ~30 kJ/mol). The optimal hydrogen interactions with sorbents could enable fast hydrogen on-vehicle fill and discharge rates, nominal thermal management requirements during fueling, lower pressure requirements for onboard storage and fueling, ease of engineering on the vehicle, and favorable “well-to-fuel cell” energy efficiencies that decrease vehicle and station costs. In general, sorbents increase hydrogen storage capacities compared to high-pressure compressed gas systems at a given pressure and temperature, thus enabling lower pressure to be used to achieve a comparable or higher capacity. At sufficiently high pressures where compressed H2 becomes very dense (typically 250 to 300 bar at ambient temperature), sorbents no longer improve hydrogen storage capacities. When the HSCoE was established, the main challenge for sorbents revolved around the low binding energies of H2 with interfaces, and thus the need to use cryogenic temperatures to achieve high capacity. Thus, from the outset, the HSCoE focused on adjusting the binding energies to achieve higher capacity at temperatures closer to ambient. Overall, the main issues for hydrogen storage with sorbents involve achieving required volumetric and gravimetric capacities as well as system cost. These issues are related, because system costs are directly addressed by increased capacities and storage at temperatures closer to ambient and at lower pressures.
Since detailed system analysis, which was outside the scope of the HSCoE, is needed to project actual H2 storage system capacities, the HSCoE typically reported what is termed “excess” H2 storage material values. Excess values represent the H2 actually stored on the sorbent surfaces and thus what the material is contributing to storage in the system/tank. Since sorbents tend to have additional pore and intraparticle volume where H2 gas also resides, a given material will typically “contain” more (i.e., material “total”) H2 than the reported material excess value. However, DOE-directed detailed analyses performed by Ahluwalia and colleagues indicate that systems using sorbents will have usable system capacities close to the excess values, and thus these excess values can be used to gauge differences between materials and what an actual system may store. Specifically, while the exact details will vary based on the storage pressure, temperature, and storage mechanism, the HSCoE focused on developing sorbents with excess capacities greater than ~6 wt pct and 40 g/L, and in reducing system and station costs by limiting storage pressures to <200 bar and temperatures to higher than ~77 K (−196 °C), with the ultimate goal of higher than 200 K (−73 °C). Note that material “total” capacities are only normalized to the sorbent weight and are often misleading because they typically translate to far lower system capacities when the weight of the entire system is used.
When the HSCoE was established, the main challenge for sorbents revolved around the low binding energies of H2 with interfaces, and thus the need to use cryogenic temperatures to achieve high capacity. Previous to the start of the center, a substantial amount of work had been done developing “activated” and “superactivated” carbons, with materials such as AX-21 representing the state-of-the-art in sorbents. These superactivated carbons are formed using pyrolysis and/or chemical-treatment techniques that create pores in bulk natural carbons. Kidnay and Hiza was one of the first to investigate the use of high-surface-area carbons for hydrogen sorption. In this early work, Kidnay and Hiza reported ~2 wt pct hydrogen storage at 76 K (−197 °C) and 25 atm using coconut-shell-derived charcoal. Since then, numerous investigations have demonstrated that activated carbon can store 4 to 6 wt pct hydrogen at moderately low pressures (~40 bar) and cryogenic temperatures (~77 K [~−196 °C]) (e.g., see the reviews by Dillon and Heben, Bénard and Chahine, Poirier et al., Carpetis and Peschka, Hynek et al., and the work by Schwarz et al.[114,115] However, work by Ströbel and colleagues show that only ~1.6 wt pct hydrogen storage has been measured for activated/porous carbons at ambient temperatures, even with pressures up to 125 atm. It should be noted that typical hydrogen storage values at ambient temperatures of activated carbons are significantly lower. Carpetis, and Peschka and others have suggested that activated carbons may be used to inexpensively store hydrogen at cryogenic temperatures. As discussed by Chahine and Bénard, for activated carbons, typically the maximum hydrogen uptake at 77 K (−196 °C) is ~1 wt pct for every 500 m2/g of specific surface area. However, the inclusion of the best available “conventional” activated carbons in a storage tank can only marginally enhance the overall energy storage density of a compressed hydrogen gas system under specific pressure and temperature conditions. Hynek et al. compared the hydrogen storage of several types of porous carbon materials to that of an empty compressed tank. The results indicated that of the ten materials they tested, only one showed marginal improvement over an empty tank at 190 K and 300 K (−83 °C and 26 °C). None of the materials were better than the empty tank at cryogenic temperatures (i.e., 80 K [−193 °C]). Chahine and Bénard formalized this concept by defining a “gain” parameter that provides a direct measure of the improvement a sorbent material provides in a compressed hydrogen system. Sorbents such as AX-21 have ~5 wt pct maximum excess hydrogen storage capacity at ~80 K (−193 °C) with ~30-bar pressures, but because of their relatively large pore sizes (1 to 4 nm), their bulk densities are relatively low (~0.3 g/mL as powders), and thus their volumetric hydrogen storage capacities are only ~15 g/L. Thus, from the outset, the HSCoE focused on adjusting the binding energies to achieve higher capacity at temperatures closer to ambient. Overall, the main issues for hydrogen storage with sorbents involve achieving required volumetric and gravimetric capacities as well as system cost. System costs are directly addressed by increased capacities and storage at temperatures closer to ambient and at lower pressures.
Hydrogen Sorption Center of Excellence List of Partners
Air Products & Chemicals Inc.
California Institute of Technology
Argonne National Laboratory
Lawrence Livermore National Laboratory
University of Michigan
National Institute of Standards and Technology
University of Missouri-Columbia
National Renewable Energy Laboratory
Pennsylvania State University
Oak Ridge National University
Texas A&M University
University of North Carolina, Chapel Hill
University of Pennsylvania
University of California, Los Angeles
Optimized Nanostructures primarily focused on increasing the specific surface area and optimizing the pore sizes to maximize physisorption of hydrogen.
Substitution focused on increasing dihydrogen binding through boron substitution in high specific surface area carbons. Typically, the boron concentration and specific surface area are inversely related.
Strong/Multiple H 2 Binding by Metal Centers used partially coordinated metal atoms and other electronically unsatisfied structures to create sorbent sites with strong binding to dihydrogen and/or the ability to sorb multiple dihydrogen molecules.
Weak Chemisorption/Spillover investigated materials with weaker than anticipated binding to atomic hydrogen such that storage could be accomplished via dihydrogen dissociation and spillover onto the receptor surfaces.
Specific partners with the right skill sets were assigned to each research group; these partners worked closely together on the most difficult issues for each focus area. Selection criteria were established for each focus area, and the HSCoE quickly selected specific sorbents/classes for focused development efforts based on their hydrogen storage potential and/or performance—along with material classes that should not be developed further.
Results and Accomplishments
During its 5-year tenure, the HSCoE made substantial progress in developing sorbents that can be used for light-duty vehicle and other applications. The HSCoE’s close interactions enabled substantially more development to occur more quickly than could have ever been done as independent projects. The HSCoE developed high specific surface area sorbents that could be used to construct systems that meet DOE’s 2010 system targets (i.e., 4.5 wt pct and 28 g/L). Furthermore, the HSCoE identified development paths for designing and synthesizing sorbents with the potential to meet DOE’s 2015 and DOE’s ultimate full-fleet system targets for light-duty vehicles. The HSCoE worked together to systematically develop or investigate hundreds of different materials and/or processes, resulting in hundreds of publications, with more than 25 pct of them involving multiple U.S. and international institutions as co-authors. Based on the huge number of framework [e.g., metal organic frameworks (MOF)] materials alone, as well as the large number of potential new materials identified, the exact number of materials developed/studied is impossible to quantify.
The HSCoE also determined more than a hundred pathways (e.g., synthesis routes) and materials that were down-selected as not being applicable to meeting current light-duty vehicle targets. For example, elements with molecular weights over ~16 grams per mol (g/mol) probably cannot be used to meet DOE 2015 storage targets at temperatures above ~80 K (~−193 °C) with physisorption. Furthermore, if stronger H2 bonding is produced, then elements with molecular weights higher than ~32 g/mol will require multiple H2 adsorptions per sorbent atom to be able to meet DOE targets. In addition, nearly all the atoms must be accessible to the hydrogen to have sufficient capacities. These simple criteria virtually eliminate many typical sorbents such as zeolites.
HSCoE principals identified four main mechanisms for adsorbent hydrogen storage and created specific development plans to solve the associated technical issues. Specific results are provided below.
Cryogenic Storage via Physisorption on High Specific Surface Area (SSA) Materials
The HSCoE interactively used theory developed by Ding et al. and experiments to design and synthesize different high-SSA sorbents to tune pore size distributions and understand their relationship to H2 storage capacity. Syntheses developed included precursor pyrolysis, polymer chemistry, aerogel chemistry, templating, chemical vapor nanotube and nanohorn processing, and scaffolding. For example, McNicholas and colleagues at Duke made pyrolyzed polyether ether ketone (PEEK) materials and worked with NREL, Caltech, NIST, and UNC to optimize its pore structures to produce carbon-based sorbents with tunable pore sizes that provide excellent gravimetric and volumetric hydrogen storage capacities. This and other synthesis techniques were employed by the HSCoE to develop a number of different high-SSA materials. Typically, materials made by these processes followed the general empirical rule of ~1 percent by weight (wt pct) maximum excess H2 storage capacity for every 500 meter square area per gram (m2/g) SSA at 77 K (−196 °C) and ~30 to 60 bar. Several materials were made with SSAs >2500 m2/g that had ~5 wt pct excess H2 storage capacities at 77 K (−196 °C). However, unlike commercially available activated carbons that typically have bulk densities of 0.1 to 0.3 g/mL, several of these materials developed by McNicholas and Yuan et al.[124,125] have pore sizes in the range of 0.7 to 1.5 nm, which enable bulk densities between 0.7 and 1.4 g/mL. With ~5 wt pct excess capacity, these materials have volumetric capacities over 35 g/L. In general, activated-carbon-based materials with SSAs substantially >3000 m2/g were not achieved; therefore, while optimization of pore sizes may still improve the volumetric capacities slightly, no significant additional improvement is anticipated with these materials. However, material optimization may still be needed if high-SSA sorbents are used for applications where substantially cooler storage temperatures are used. H2 excess storage capacities vary substantially with temperature, and ~14 wt pct and 90 g/L can be achieved at ~30 K (−243 °C).
To go beyond the SSA’s achievable with activated carbons, TAMU, Michigan, Caltech, NREL, UNC, and NIST developed (via molecular modeling and design) crystalline nanoporous materials based on coordination of multifunctional and multiple binding ligands to well-defined metal centers. With a vast library of ligand “building blocks” and different coordination chemistries, the HSCoE literally made hundreds of frameworks that were characterized by XRD, SSA analysis, and H2 capacities. The HSCoE efforts by Wong-Foy and colleagues was at the forefront of formulating and synthesizing frameworks for hydrogen storage and the first to make materials with >3800 m2/g SSA and 7 wt pct excess H2 storage capacities. Subsequently, frameworks with ~6500 m2/g and ~8.5 wt pct excess H2 storage have been reported by Farha and colleagues. The main issues with these framework materials are the need for cryogenic storage temperature and the trade-off between very high SSAs that achieve good gravimetric capacities and lower bulk densities (i.e., 0.1 to 0.5 g/mL) that have relatively low volumetric storage capacities (at best, less than ~30 g/L).
Reducing pore size with nanoscopic cages or catenation (interconnected frameworks).
Synthesizing stable frameworks with mesocavities (>2 nm) but microwindows (<2 nm).
Synthesizing anthracene-derivative frameworks to provide additional binding sites.
Using different coordinated metal centers to improve their affinities to hydrogen.
Constructing frameworks with “close-packing” alignment of open metal sites.
Frameworks offer a huge potential to continue increasing the SSA and gravimetric capacities of H2 storage materials past 9 wt pct. Furthermore, their ability to have higher binding energies and sub-nanometer-sized pores offers additional opportunities. For example, catenation or developing frameworks with smaller pore sizes has tended to result in decreased SSA and thus lower H2 capacities. Furthermore, the relatively small number of metal atoms and their limited access by H2 means that only one to two H2 could interact and have enhanced binding. Since metals make up only a small percentage of the frameworks, and due to the relatively high molecular weight of many metals typically used (i.e., copper, iron, manganese, and zinc), H2 storage by the metal centers may decrease the overall H2 storage capacity of the material. Thus, the enhanced binding from the metal centers must be carefully integrated to enhance storage capacities. The main focus for future framework-development efforts needs to be on optimizing the pore sizes for H2 adsorption with air- and water-stable materials, while still retaining suitable SSAs. In addition, future framework development should also investigate methods for improving binding energies of the high-SSA ligand components.
At the outset of the HSCoE’s activities, numerous publications including results from Zlotea et al. had reported extraordinary results for high-SSA materials in which the enhanced capacities were potentially a result of novel geometries or structures within the material. In general, Purewal and colleagues showed that heats of adsorption can be increased with multiple wall interactions, but this ultimately reduces capacities due to the decrease in space for the hydrogen molecules. The HSCoE did not validate any single-element material or any materials with unexceptional electronic states that have substantially higher capacities beyond that expected based on SSA and specific storage conditions. For example, at one time carbon nanotubes where thought to possibly have unique H2 storage properties; but after a dedicated focused effort, the HSCoE made a No-Go decision on using carbon nanotubes as an ambient-temperature hydrogen storage material. Ultimately, carbon nanotubes may still provide excellent hydrogen storage at cryogenic temperatures. Based on a vast amount of measured sorbent results and through thorough detailed investigations of numerous published results to the contrary, the HSCoE concluded that standard physisorption-based dihydrogen adsorption scales with specific surface area, and thus no substantial increase in capacity can be achieved with geometric structures alone. The main issue is that increases in H2 binding can be achieved with very small (<0.4 nm) pores due to multiple wall interactions, but less space is available for hydrogen and thus the capacities actually decrease.
Toward Ambient-Temperature Storage with Increased and Multiple H2 Binding
Boron substitution was achieved by either starting with chemical compounds with high concentrations of B and forming high-SSA materials, forming B-substituted activated and graphitic carbons (e.g., BC3), or substituting boron for carbon atoms in preformed materials accomplished by Tour and Kittrell. These multiple institution efforts culminated with Brown and colleagues obtaining neutron spectroscopy data showing, for the first time, a large rotational splitting, indicative of enhanced H2 interactions in a B-substituted carbon. Similarly, for the first time, HSCoE measurements from diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed reversible hydrogen interaction with the B:C material. Although different approaches were used, the ultimate boron concentration and SSA achieved were approximately the same: SSAs of ~800 m2/g with 10 to 15 pct B substitution. This is well short of the ~3000 square meters area per gram (m2/g) and ~25 pct B concentrations expected to be achievable. In addition, based on X-ray photoelectron spectroscopy (XPS), approximately 25 pct of the B in these materials is in the correct coordination for enhanced binding, indicating that only small fractions of the B in the materials were providing enhanced hydrogen binding. However, the BC x materials typically have approximately twice the hydrogen storage adsorption on a per SSA basis compared to activated carbon. In general, maximum excess gravimetric hydrogen adsorption capacities of ~3 wt pct are observed at 77 K (−196 °C) for materials with 600 to 800 m2/g SSAs. This compares to ~1.5 wt pct for activated carbons with similar surface areas at 77 K (−196 °C). In addition, ~65 pct of the 77 K (−196 °C) capacity is retained with some porous BC x materials at ~200 K (−73 °C). This compares to 25 to 30 pct with pure carbon materials at 200 K (−73 °C).
Future efforts must focus on increasing simultaneously both the boron concentration (in BC3 coordination) and SSAs of these substituted materials. Paths forward to do both include solution-phase synthesis and lower-energy chemical vapor synthesis in which the BC x surfaces with the stronger binding are not allowed to interact and collapse. If a BC3 material can be made, then more than half of the hydrogen storage capacity would be at the higher isosteric heats of adsorption needed to have substantially higher hydrogen storage at temperatures greater than ~200 K (−73 °C).
Prior to the HSCoE, little systematic work had succeeded in identifying ways of increasing H2 binding to well-coordinated chemically stable sorbents, especially for the majority of the material capacity. The HSCoE focused efforts by Kim et al.[139,140] on the use of unsaturated coordinated metal centers to increase H2 binding energies (predicted values of >20 kJ/mol) and to enable the potential of multiple H2-molecule binding at a single adsorption metal site. In these materials, the metal atoms interact with a lattice but are sufficiently configured to have enhanced interactions with H2 via both forward and back donation of electrons. Zhao et al.’s[139,140] seminal paper in this area was the first to predict unique structures that may have the potential to hold multiple (i.e., 2 or more) H2 molecules at a specific metal site. The results in this paper opened up new areas of investigation for hydrogen storage. The key findings on this topic since the inception of the HSCoE are that lightweight alkali, alkaline earth, and 3d-transition metals may be configured to enhance binding and have the potential to bind multiple H2 molecules to a single metal atom. More fundamental experimental work is needed to fully prove these concepts where configurations can be made to adsorb multiple dihydrogen molecules while appropriately controlling dissociation and provide experimental validation for the model predictions. However, the potential is promising: the HSCoE identified materials with improved synthesis pathways with the potential to store 10 wt pct and 100 g/L at ambient temperature.
Ambient-Temperature Storage via Weak Chemisorption/Spillover
Computations by Rice, APCI, and NREL identified that it is thermodynamically possible for hydrogen atoms to be stably stored in groups or clusters. The main step that is not well understood is that of hydrogen atom diffusion on the receptor. The team of Lee and colleagues was the first to identify that barriers to migration are lowered sufficiently via structural (e.g., hopping between closely spaced surfaces) and electronic features. Collaborative work from Mitchell et al. using inelastic neutron scattering spectroscopy observed spillover hydrogen on carbon supports. Deuterium tracer investigations demonstrated that, effectively, the spillover process is sequential with the first hydrogen adsorbed being the last desorbed. The results are direct evidence that (1) atomic species are formed during the spillover processes, as shown by hydrogen—deuterium formation, and (2) the desorption follows a reverse spillover process in which atoms migrate back onto the metal particle to recombine and desorb as molecules. In general, the size, dispersion, and type of catalyst affect the efficiency and thus the capacity of spillover.
In general, the conclusion from the different computational and mechanistic work by Lin and co-workers is that hydrogen atoms can interact with materials such as graphene in a way in which the graphene structures are not substantially (irreversibly) changed, and the hydrogen interaction is more like adsorption. The diffusion and storage of atomic hydrogen on the receptor surfaces must have substantially weaker bonding/barriers to migration than typically observed with chemical covalent bonding (i.e., weak chemisorption). Although the phenomenon of spillover has been known for decades for petrochemical and refining catalysis applications with ~0.01 wt pct hydrogen adsorption, the HSCoE partners Li and Yang reported that this process could be used to reversibly store substantially more (>30 g/L and >4 wt pct) hydrogen at ambient temperatures and nominal pressures (<200 bar). This is over 300 pct greater than H2 storage on a sorbent or as gas in a compressed tank at the same pressure and temperature. The HSCoE demonstrated spillover both experimentally and by confirming agreement with thermodynamic principles as a process that may be appropriate for ambient temperature, reversible hydrogen storage.
However, the materials tend to be very sensitive to processing conditions, the material synthesis procedures lack reproducibility, and the accuracy of the measurement techniques varied, all of which can lead to substantial variations in sorption capacities. Although theoretical predictions and non-reversible hydrogenation experiments demonstrate that capacities of close to one hydrogen atom per receptor atom (e.g., carbon) should be achievable via spillover. This translates to a potential for excess capacities >7 wt pct and 50 g/L at ambient temperatures and <200 bar. At the end of the HSCoE, DOE formed an international team led by NREL to validate the measurement and synthesis methods of spillover materials to improve reproducibility. In addition, the team is determining the specific hydrogen–receptor interactions using spectroscopic techniques to fully understand the mechanisms involved. This is important to understanding the significant difference between fill and discharge rates, as well as issues that limit capacities. Ultimately, the issues with slow fill rates and lower than expected capacities may be related, and once the mechanisms are fully understood, these issues should be addressable.
Overall, the HSCoE demonstrated that substantial increases in hydrogen storage capacity are achieved at ambient temperatures with weak chemisorption processes such as spillover. Future work must improve adsorption rates (e.g., refill kinetics) via improved catalyst dispersion and integration, and improved receptor properties. Care must be taken to ensure that irreversible chemical reactions with the receptor materials do not occur and that the measured uptake is truly representative of the amount of hydrogen that can be delivered to the fuel cell. Although a significant amount of work is still required (including fundamental surface science studies) to develop highly reproducible and robust materials that have high adsorption rates, kinetics, and capacities, the clear indication is that weak chemisorption is a potential path for on-vehicle hydrogen storage.
The major conclusions of the HSCoE include results for materials, measurements, and theory.
For cryogenic storage, new materials were developed that increased gravimetric (>60 pct, i.e., from ~5 to >8.5 wt pct at ~80 K [−193 °C]) and volumetric (~150 pct, i.e., from ~15 to >35 g/L at ~80 K [−193 °C]) hydrogen storage on high-SSA sorbents by optimizing pore size distributions (0.7 to 1.5 nm) to increase SSA and packing density—standard physisorption-based H2 gravimetric capacity scales with SSA. Thus, no substantial increase in capacity can be achieved with geometric structures alone. Although binding energies can be effectively doubled with very small pores that enable multiple wall interactions with the H2 molecules, effectively, the space for adsorption is decreased, thus decreasing the overall capacity.
To move toward more ambient-temperature storage, the isosteric heat of adsorption must be increased. Substitutional materials such as boron in carbon or metal–organic frameworks (MOFs) were developed that exhibit enhanced dihydrogen binding energy (i.e., 8 to 12 kJ/mol) that may increase capacities in some materials (e.g., doubles or triples) on a per SSA basis at near-ambient temperatures. Furthermore, reversible high-capacity sorbents that were designed and made via ambient-temperature hydrogenation techniques such as spillover store 1 to 4 wt pct at ambient temperatures, with the potential of 7 wt pct and 50 g/L at ambient temperatures and less than 200 bar. In addition, coordinated unsaturated metal centers are a novel class of H2 storage materials with the potential to store at ambient temperature >10 wt pct and >100 g/L. More fundamental experimental work is needed to fully prove these concepts and provide validation for the model predictions.
The HSCoE focused significant efforts on improving unique measurement capabilities to accurately and reproducibly characterize H2 storage properties of small laboratory-scale samples (1 to 200 mg). These measurement capabilities enhanced high-throughput, rapid-screening analyses. In addition to capacity measurements, a high-pressure nuclear magnetic resonance spectroscopy system was developed to help identify hydrogen interactions in micropores vs those in macropores. The HSCoE also used several different techniques including neutron scattering, Raman and Fourier transform infrared spectroscopies, as well as differential volumetric measurements to provide unique hydrogen storage materials’ characterization. In addition, the HSCoE helped with the publication of DOE’s “Best Practices” guide for hydrogen storage measurements—a reference guide for kinetics, capacity, thermodynamics, and cycling measurements.
Finally, research approaches used by the HSCoE, which combined iterative and coupled theory and experimental efforts, accelerated materials’ design and development. First-principles theorists designed synthesis pathways and accompanying materials with optimal hydrogen storage properties. These predictive approaches sped identification of materials with the potential to meet DOE hydrogen storage targets, including novel heterogeneous materials, paths to creating high-capacity, fast-filling spillover materials, and novel classes of sorbents with the potential of >100 g/L and 10 wt pct at ambient temperature. If these materials are reproducibly synthesized, they would have the potential of enabling systems that meet DOE’s 2017 system targets.
The HSCoE demonstrated that the onboard fueling capability of sorbent materials offers advantages that should be exploited for light-duty vehicle applications. Thus, the HSCoE recommends that future development efforts be performed that focus on reducing material and associated balance-of-plant system costs by improving material storage capacities and transient performance at near-ambient temperatures (i.e., between 193 K and 353 K [−80 °C and 80 °C]). Overall, the HSCoE recommends that development efforts for specific material classes be continued where viable reproducible routes exist for synthesizing sorbents that can be used to meet the appropriate set of application targets. In general, the closer to ambient conditions the system operates, the less expensive the system. This must be traded against overall system performance, which includes the potential need for added heat removal during fueling at the station. This need for balance led to four specific recommendations related to materials for associated hydrogen storage systems and classes. Ultimately, cryogenic storage of hydrogen gas may be able to meet DOE near term targets but will require materials with substantial specific surface areas. Thus, efforts should be limited to development of materials in which the storage mechanism is purely physisorption to only those with optimized pore structures and micropore volumes. However, the use of these materials for light-duty vehicles will have the highest hydrogen infrastructure costs and environmental impact, and these costs probably will not be reduced significantly even with anticipated material performance improvements. Thus, the HSCoE recommends that longer-term efforts need to be made to develop materials that store substantial amounts of hydrogen at or near ambient temperatures and relatively low pressures. This includes developing substituted/heterogeneous materials with demonstrated hydrogen binding energies of 10 to 25 kJ/mol over most of the capacity range, multiple dihydrogen sorbents, and possibly weak chemisorption materials. While a substantial amount of fundamental development will be needed, the latter two classes of materials provide the potential to meet DOE hydrogen storage system targets at ambient temperatures. Specifically, the development of materials for hydrogen storage by weak chemisorption needs to emphasize reproducible syntheses and performance, with improved hydrogen fill kinetics and overall net available capacity. Similarly, substantial fundamental experimental work needs to be performed to fully prove concepts of multiple dihydrogen adsorption on designed metal complexes and to provide experimental validation for the model predictions.
The recommended sorbent classes as a whole have common issues remaining that must be adequately addressed. These include the need to use fundamental and applied research to develop robust, reproducible, scalable, and cost-effective syntheses that manufacture materials in which all adsorption sites are accessible to the hydrogen. Efforts should be made to improve fundamental computational methods to more accurately predict the ability to synthesize designed materials and hydrogen storage capacity as a function of temperature and pressure. This could help develop a better fundamental and applied understanding of atomic hydrogen transport energetics and kinetics on receptor materials, as well as a better fundamental understanding of metal center coordination and how the processes can be directly applied to create high-capacity, low-cost hydrogen sorbents.
Based on systematic development efforts, the HSCoE also recommended that no additional research and development be performed on numerous materials and processes. Only a select few elements and materials used in sorbents will be able to meet DOE light-duty vehicle system targets. The key is arranging those elements and materials optimally for hydrogen storage. For more details, please see the full Final Report for the HSCoE. The Final Report also provides a bibliography of the publications and patents from HSCoE partners.
Furthermore, highly accurate hydrogen storage materials’ characterization capabilities, including sorbent material measurements’ standards and certifications, must be emphasized to quickly identify material properties and validate results so that minimal efforts are wasted on poor materials or erroneous results. This is critical to eliminating the large expense and years of effort often associated with rooting out what ultimately turns out to be erroneous results. Finally, future efforts need to also adequately address additional material properties related to hydrogen storage systems including thermal conductivity, heat dissipation, refill and discharge rates, durability, and other engineering issues.
Through the 5-year efforts of the three multi-partner collaborative Hydrogen Storage Materials Centers of Excellence, substantial progress was achieved in expanding the knowledge and understanding of the potential of materials for hydrogen storage applications. The progress was accelerated through inclusion of multiple institutional partners that brought theory and computational capabilities, various materials synthesis and preparation methods, and a wide range of materials characterization techniques for use in the efforts. While a comprehensive review of all of the hundreds of materials investigated, the conclusions made and the recommendations developed through these efforts are beyond the scope of this paper, interested readers are directed to the complete final reports from each of the Centers of Excellence, available through the DOE’s Fuel Cell Technologies Office’s website. To further aid in advancing development of hydrogen storage materials, the DOE has launched a searchable hydrogen storage materials’ properties database that includes data from the Centers of Excellence efforts, the DOE/IEA-HIA metal hydrides database, and individual research efforts.
This work was performed through the collaborations of the Metal Hydride, Chemical Hydrogen Storage and Hydrogen Sorption Centers of Excellence, funded through the Fuel Cell Technologies Office of the DOE’s Office of Energy Efficiency and Renewable Energy. The authors acknowledge and thank all of the Centers of Excellence partners who contributed to the work described in this review. L. E. Klebanoff wishes to acknowledge the contributions of all Principal Investigators within the MHCoE to the work summarized herein. L. E. Klebanoff also acknowledges the many DOE and U.S. DRIVE Hydrogen Storage Tech Team members who have assisted the MHCoE in its work. Special thanks to Dr. Jay Keller who served as the Deputy Director of the MHCoE and to Marcina Moreno who was the MHCoE business manager. L. J. Simpson would like to specifically acknowledge all the people both within the HSCoE and especially those throughout the world who contributed and provided wonderful collaborations to help advance our efforts. Special thanks should also be given to Michael Heben for starting these efforts and for the support of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, under Contract No. DE-AC36-08-GO28308 with the National Renewable Energy Laboratory. L. J. Simpson would also like to pay a special tribute to Anne Dillon, Peter Eklund, Alan MacDiarmid, and Richard Smalley. Kevin Ott would like to thank all of the excellent and hardworking partners who were involved in the Chemical Hydrogen Storage Center of Excellence.
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