Hydrogen and oxygen generation with polymer electrolyte membrane (PEM)-based electrolytic technology
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- Badwal, S.P.S., Giddey, S. & Ciacchi, F.T. Ionics (2006) 12: 7. doi:10.1007/s11581-006-0002-x
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With dwindling liquid fuel resources, hydrogen offers a credible alternative. The use of hydrogen in a fuel cell offers the highest fuel conversion efficiency compared with all other technologies and it also has the potential to substantially reduce greenhouse gas and particulate emissions at least at the end-user sites. One of the major barriers to the introduction of the hydrogen economy and its wider acceptance is the lack of the rather costly hydrogen generation, transportation and distribution infrastructure to meet the local transport fuel demands. On-site or distributed hydrogen generation would remove the need for this up-front infrastructure requirements and assist with the early large-scale trials of the fuel cell technology for both transport and stationary applications and also introduction of the hydrogen economy. In this paper, the development of polymer electrolyte membrane electrolysis technology for on-site, on-demand hydrogen generation has been discussed. The major emphasis is given on reducing catalyst cost; interface design and modifications; interconnect materials, design and fabrication; and investigation of the sources of degradation. Stacks to 2 kWH2 capacity have been constructed and tested and show initial efficiencies of >87% at 1 A cm−2.
KeywordsHydrogen production Electrolysis Renewable energy Polymer electrolyte membrane
As fossil fuel and especially liquid fuel reserves start declining and the gap between demand and supply widens, the cost of transport fuels would increase at an alarming rate and there will be an increasing need to find alternative transport fuels. Hydrogen is the cleanest fuel available to mankind but it has to be generated from fossil fuels with inherent embedded energy or from water by supplying energy. Linking of hydrogen generation with renewable energy would provide a totally sustainable energy cycle. However, most renewable sources are intermittent sources of power and require energy storage for load levelling and to reduce overall plant cost. Hydrogen is arguably the best energy storage medium and is an excellent alternative to carbon-based transport fuels.
Over half of the total hydrogen produced is utilised for ammonia (fertilizer and explosive) production
About one third in oil refineries—for impurity removal and upgrading of heavy oil fractions into lighter and more valuable products
Remaining in methanol production, chemical and metallurgical industries and space programs
The use of hydrogen as a power generation source is minimal at this stage due to its high cost of production, lack of hydrogen transport, storage and distribution infrastructure and technological challenges related to production, storage and end-use of hydrogen. The current total global hydrogen production is barely enough to feed the US transport fleet. Global oil production is likely to peak in the next 5–10 years (it has already peaked in Australia, USA and Europe) and soon the world will run short of oil [2, 3, 4]. The demand for transport fuels and energy is increasing at an alarming rate especially in developing and heavily populated countries such as India and China. As the gap between supply and demand for liquid fuels increases, oil prices will increase and there will be increasing requirements to use alternative clean and low- or zero-emission fuels such as hydrogen.
Hydrogen is considered to be the best energy storage media due to its flexibility for end-use and regenerative and zero-emission properties. Hydrogen combusted in a fuel cell can give combined heat and power efficiencies of more than 80%, with zero greenhouse gas and pollutant emissions at end-use sites with water as the only by-product. Hydrogen can be alternatively used as a fuel in an internal combustion engine, for example, in a car to power the mechanical drive train.
Reforming fossil fuels (natural gas, liquified petroleum gas, gasoline, diesel, coal, methanol, etc.) with appropriate fuel processing/cleaning
- Electrolysis of water (energy source: nuclear, fossil, renewable)
Low-temperature (alkaline and polymer electrolyte membrane, PEM) and
High-temperature ceramic membranes
Hybrid fossil fuel/renewable technologies (e.g. solar thermal reforming)
Photo-electrochemical/photo-catalytic splitting of water
Thermolysis of water
Photo-electrolysis is considered to be an ideal prospect for hydrogen generation using only sunlight. However, the technology is in the early stages of research on materials development. There are substantial technical challenges related to efficiency, cost and lifetime. It is a long way away from meeting the collective technical and commercial targets for hydrogen production [5, 6]. There are other issues such as the logistics of hydrogen collection from large surface areas (small current densities for hydrogen production). Other proposed technologies such as thermolysis and thermochemical processes suffer from severe material-related problems [5, 6].
Although in the short to medium term, increased hydrogen demand is likely to be met by fossil fuels (e.g. natural gas reforming, coal gasification with CO2 separation/sequestration), in the medium to long term, renewable energy offers the best possible solution. With time, the use of renewable energy (solar, wind, tidal, wave, etc.) as a clean alternative power is likely to increase as the cost of production declines. These renewable sources are intermittent sources of power. In the absence of energy storage, these renewable systems have to be grossly over-designed in terms of capacity. Hydrogen offers the flexibility of energy storage for long duration.
Hydrogen can be easily produced from water by electrolysis using intermittent renewable energy and off-peak electricity from nuclear, hydro or thermal power plants. Water electrolysis is considered to be one of the key technologies for hydrogen generation as it is compatible with existing and future power generation technologies and a large number of renewable technologies (solar, biomass, hydro, wind, tidal, wave, geothermal, etc.). In particular, hydrogen enables the long-term storage of energy for load levelling.
The major advantages of producing hydrogen via this route are that the technology is relatively more mature compared to other technologies such as direct water splitting (photo-electrochemical), biological routes, etc., and the electrical/water infrastructure already exists. On-site hydrogen generation can reduce the need for up-front expensive hydrogen transmission/distribution infrastructure. The major issues associated with large-scale adoption of the electrolysis technology are the high capital cost of the electrolyser (>A$5,000/kW), low system efficiencies, lifetime performance and integration with renewable sources [5, 6].
In this paper, developmental work on solid state water electrolysis technology based on polymer electrolyte membrane for small-scale distributed hydrogen generation has been described. Small-scale distributed production of hydrogen will enhance its availability at consumer level (transport, distributed generation, fuelling of micro devices, etc.) without the cost of large hydrogen distribution and storage facilities. It will, therefore, accelerate the hydrogen economy by providing local hydrogen at more attractive investment costs. PEM-based electrolysis systems offer a number of attributes such as modular aspect (there is minimal penalty on efficiency due to unit size), all solid state system (no alkaline liquid electrolytes or its recycling involved and water and electricity are the only inputs required), pure hydrogen and oxygen generation (due to physical separation by solid electrolyte membrane), ability to produce hydrogen at a pressure (electrochemical compression), small footprint due to high current densities achievable (>1 A cm−2 as compared to ∼0.4 A cm−2 by alkaline system) [7, 8, 9]. PEM electrolysis technology due to its fast response time and start-up/shut-down characteristics (hydrogen generation starts immediately at ambient conditions) and ability to accept large variations in load is ideal for integration with intermittently available sources of electricity (renewable and off-peak grid). In addition, due to the similar aspects of the PEM electrolysis and fuel cell technologies, the impact on development and system cost reduction can be enormous.
Electrolysis cell/stack construction
Membrane electrode assemblies (MEAs) for water electrolysis varying in dimension from 9 up to 150 cm2 active electrode area and stacks up to multi-kilowatts equivalent hydrogen generation capacity were constructed for evaluation as described below. The hydrogen electrode of the cells was prepared using Pt/C catalyst with platinum loadings of up to 0.4 mg.cm−2 as detailed elsewhere . The oxygen electrode was prepared using various noble metal catalysts with loading in the 0.2–0.4 mg.cm−2 range on a metallic support. The polymer electrolyte membranes used were Nafion N112 and N115. Membrane electrode assemblies for electrolysis cells were prepared by hot pressing the membrane between the hydrogen and oxygen electrodes. Metallic plates with parallel cross channel flow fields were used as interconnects. The electrolysis cells with interconnect plates were stacked and assembled with the end interconnects on both sides of the stack, serving dual functions—as interconnects as well as current collectors for the stack.
Several single cells or stacks with 50, 100 and 150 cm2 active area per cell were fabricated and assembled to investigate the issues related to cell scale-up, stacking, performance, degradation and lifetime. Stack modules ranging in hydrogen generation capacity from 100 WH2 to 2 kWH2 were constructed and evaluated. For the 9 cm2 cells and 50 cm2 stacks, Nafion membrane N112 (50 μm thick) was used whereas for the 100 cm2 stacks, thicker Nafion membrane N115 (125 μm thick) was used.
Cell and stack evaluation
Test stations with built-in diagnostic tools were constructed in-house with multiple levels of operator and stack safety redundancy and capability to test electrolysis cells/stacks up to 5 kWH2 capacity. One test station was constructed to operate stacks to 20 barA pressure. The system design allows for a continuous unattended operation of the electrolysis cells/stacks for extended periods of time with continuous monitoring of voltages of individual cells and stacks, stack current and water temperatures in different parts of the flow circuit. All key operator safety and stack safety system components (vent pressure, gas and temperature sensors, low water level and flow switches) are interlocked with the power source to stop the hydrogen/oxygen generation in the event of hydrogen leak or any other safety-related issues.
For screening and evaluation of electrolysis cell materials, design parameters and operating conditions, cells of 9 cm2 active area, were used. Cells of this size were also used for electrochemical diagnostic analysis as even a small current signal can produce reasonably high current densities. The small cells (9 cm2) were operated at different temperatures between room temperature and 80°C to determine the optimum operating temperature. The cell temperature was maintained with external heating. The effect of catalyst loading, membrane thickness and interface modification on the cell performance was studied at a typical cell operating temperature in the 75–85°C range.
Electrochemical investigations included current–voltage characteristics, electrochemical impedance spectroscopy on selected cells, efficiency calculations and performance degradation as a function of time at current density of 1 A cm−2. The hydrogen generation efficiency was calculated based upon thermo-neutral voltage, Vthn, of 1.48 V . Current efficiency was determined by measuring the hydrogen gas flow exiting and that expected for a given charge flow.
Stack testing involved determining current–voltage curves at different operating times of its operation, current efficiency, overall stack hydrogen generation efficiency, lifetime testing, operation in a thermally self-sustaining mode (no supplementary heating of water) and evaluation of the degradation mechanisms.
Impedance spectroscopy was performed with the Zahner model IM6e over the frequency range of 0.1 Hz to 1 kHz at 75°C on selected 9 cm2 active area cells at the start and after operation of the cell for different times at a current density of 1 A cm−2. The software package Thales provided by Zahner was used for data acquisition and fitting of the data to a simple equivalent circuit consisting of a series resistor representing all ohmic losses in the cell in series with constant phase angle element in parallel with a resistor representing the electrode/electrolyte interface, after correcting the data for lead resistance and measuring circuit parameters.
Results and discussion
In general, higher performance was achieved with thinner membranes (50 μm). However, such membranes are found to be more fragile, have short lifetime and need to be handled very carefully. There is also a complex relationship between catalyst loading, the method of catalyst deposition, performance and lifetime. The three phase boundary area between the reactant species, electrode and electrolyte need to be optimised to reduce catalyst loading, attain high hydrogen generation efficiency and increase lifetime.
Scale-up and stack evaluation
Current or Faradaic efficiency
The current efficiency was measured by relating the flow rate measurements (the amount of hydrogen generated per unit time) with the current flow per unit time. Most of the cells showed near 100% Faradaic efficiency during operation. However, over a period of operation, some cells from time to time developed internal electronic short circuits which led to a drop in the current efficiency, as discussed in the next section.
Lifetime performance and degradation mechanism
Another catastrophic degradation has also been observed for cells that have been operated for long periods of time (typically >1500 h). This, in fact, is related to internal electronic short circuiting through the electrolyte membrane and loss of Faradaic efficiency as discussed below.
For the case when there is an electronic short through the membrane, circuit 2 in Fig. 8 can be used to describe the observed behaviour (△, Fig. 7). When the cell voltage is below 1.48 V, the current initially flows through electronic short circuit paths in the membrane and there is no ionic current flow and, thus no hydrogen generation occurs. With increasing applied voltage, more and more current flow through these short circuit paths represented collectively by Rshort [consisting of a number of resistors (Rs1, Rs2–Rsn) in parallel]. Once the applied voltage exceeds 1.48 V, part of the total current starts flowing as ionic current and it goes towards hydrogen and oxygen generation. We have fitted the data in Fig. 7 to the equivalent circuit models and equations given in Fig. 8 (○, circuit 1 and △, circuit 2) and derived the values of 0.0125 Ω for Rp (the polarisation resistance at the electrode/electrolyte interface) plus Rel (representing the total ohmic losses in the cell) for both sets of data, and the value of 0.165 Ω for Rshort for the data represented by △ in Fig. 7. The solid curves in Fig. 7 are the fitted data and show an excellent fit to the experimental data.
For fitting the data to these circuit models, we have made the following assumptions: The major voltages and efficiency losses across the cell are ohmic in nature (electrolyte membrane, support materials, current collection plates and contact resistances) with relatively low contribution from polarisation losses at interfaces between electrodes and the electrolyte as obvious from the near-linear voltage–current relationship above 1.48 V. It should be noted that, in general, the major efficiency losses in electrolysis cells occur as a result of activation polarisation and ohmic resistance, with insignificant contribution from concentration polarisation effects. However, it has been reported and as indeed also observed in the present work that ohmic losses are much higher compared to activation polarisation losses especially for higher current densities [12, 13].
We have also assumed that, at least during the collection of V–I data, Rshort remains reasonably constant. This assumption may, however, be somewhat flawed as it will be shown later that, once there is an electronic short through the membrane, the cell degradation at higher current densities increases more rapidly with time and it is possible that during collection of the current voltage curve, Rshort may have changed slightly.
Electrolysis cells have been scaled from 9 to 150 cm2 active area with very little loss in hydrogen generation efficiency. Test stations to test stacks to 5 kWH2 capacity and pressures to 20 barA have been constructed with multiple levels of safety redundancy. The system design allows for an unattended safe operation of stacks for extended periods. Stacks of varying dimensions and hydrogen generation capacity have been built and tested, some for periods exceeding 1,500 h at a current density of 1 A cm−2. Efficiency of up to 87% initially at 1 A cm−2 has been achieved through optimisation of interface, catalyst, MEA fabrication, fluid/current flow and stack design. The largest size stack tested to date has hydrogen generation capacity of 11 l/min and oxygen generation capacity of 5.5 l/min. The operation of this stack has been demonstrated in a thermally self-sustaining mode from cold start with water and electricity as the only inputs. Temperatures exceeding 80°C can be easily achieved at a current density of 0.8 A cm−2 or higher. Causes of degradation have been discussed and the major cause of hydrogen generation efficiency loss has been established to be the electronic short circuiting through the polymer membrane. The results have been explained in terms of electrical equivalent circuit models. Future work, apart from optimisation of stack components and system design, is to reduce cost and performance degradation. It would also involve demonstration of the technology initially as a remote area power supply by mapping the total available solar or wind power for a site on a yearly basis and matching this with daily, weekly and yearly power demand and hydrogen generation and storage requirements.
The authors would like to thank Kristine Giampietro and Pon Kao for their assistance with the experimental work. The manuscript was reviewed by Robin Clarke and Pon Kao. The work described in this paper forms part of CSIRO’s Energy Transformed Flagship research program into positioning Australia for a future hydrogen economy.