Materials science aspects of zinc–air batteries: a review
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Metal–air batteries are becoming of particular interest, from both fundamental and industrial viewpoints, for their high specific energy density compared to other energy storage devices, in particular the Li-ion systems. Among metal–air batteries, the zinc–air option represents a safe, environmentally friendly and potentially cheap and simple way to store and deliver electrical energy for both portable and stationary devices as well as for electric vehicles. Zinc–air batteries can be classified into primary (including also the mechanically rechargeable), electrically rechargeable (secondary), and fuel cells. Research on primary zinc–air batteries is well consolidated since many years. On the contrary, research on the electrically rechargeable ones still requires further efforts to overcome materials science and electrochemical issues related to charge and discharge processes. In addition, zinc–air fuel cells are also of great potential interest for smart grid energy storage and production. This review aims to report on the latest progresses and state-of-the-art of primary, secondary and mechanically rechargeable zinc–air batteries, and zinc–air fuel cells. In particular, this review focuses on the critical aspects of materials science, engineering, electrochemistry and mathematical modeling related to all zinc–air systems.
KeywordsZinc air batteries Primary batteries Secondary batteries Fuel cells Zinc Air cathode
In the modern industrialized society, the electrical energy demand is increasing exponentially ; but environmental pollution, due to the usage of fossil fuels for power generation, is a very well known and urgent problem [1, 2]. Renewable and sustainable energy, such as solar and wind [3, 4, 5, 6], could replace hydrocarbons, but it is also important to find a safe, reliable and efficient way to store such energy and use it in transportation systems and large-scale applications, for instance. For this reason, researchers and industry are looking for new strategies for better electrical energy storage devices. Among these, the batteries represent the right key for the next generation of green vehicles and grid energy storage, due to their relatively high energy density compared to supercapacitors that have, instead, a higher power density [7, 8, 9].
Nowadays, the need for energy storage in a robust and reliable electric grid is increasing as a result of the growing and worldwide use of renewable power generation . In Japan, for instance, due to the potential decommissioning of nuclear fleet, it is becoming crucial the use of alternative and smart ways to generate, store and distribute electric energy . Italy, instead, has substantial renewable capacity relative to grid size, and the grid is currently struggling with reliability issues; moreover, in the USA, Canada and Germany there is a keen interest to invest much more in renewable power sources (wind, solar, thermal, etc.,) [10, 11, 12, 13]. For these reasons, it is important to develop more efficient grid storage systems. The currently available mature and reliable technologies for high system power ratings (100 MW–1 GW) are pumped hydro-storage (PHS) and compressed air energy storage. However, for smaller and smarter grids, electrochemical devices, chiefly batteries, play an important role for power rating in the range from c.a. 1 kW to 10 MW . Among batteries, the following are the most investigated ones: the well known and widely used lead–acid ones, that have also recently undergone notable technical improvements; the more recent lithium ion-based technology concept; the redox-flow batteries (e.g., Zn/Br; Zn/Cl, Vanadium-redox), and finally the sodium–sulfur and metal–air devices that are being actively studied by several industrial and academic institutions [14, 15, 16, 17, 18, 19]. In this framework, metal–air batteries potentially represent the method of choice for smart and green grid storage, since their peculiarity is the high theoretical energy density and the low environmental impact of their components [12, 20]. Recently (2012), the EOS Energy company began its mission to employ zinc–air batteries for grid storage, believing in their potentialities over other types of batteries where less safe, scarce and expensive materials, such as lithium, are employed .
Even if the technology of lithium ion batteries (LIBs) is well established for portable applications such as mobile telephones, digital cameras, etc., their energy density is still not entirely satisfactory for electric vehicles (EVs) applications and driving ranges comparable to those typically obtained with fossil fuels. Nevertheless, electric vehicles totally based on Li-ion technology are commercially available with a full charge autonomy up to c.a. 200 km, such as: Renault ZEO and Kangoo , BMW 1 series ActiveE , Ford Focus Electric , Mercedes Benz Smart Electric Drive , Mitsubishi Innovative Electric Vehicle (“I-MiEV”) , Nissan Leaf , Tesla Model S , Toyota RAV4 EV , and Volkswagen e-up . Furthermore, some automobile companies are selling vehicles with a hybrid technology; Toyota hybrid system has been in the market since 1997 using Ni–MH batteries, and since 2011 using Li-ion technology. Apart from Toyota, also Opel (Opel Ampera) , Ford  and Volkswagen  are in the market of hybrid electric vehicles powered by a Li-ion technology and by an internal combustion engine.
Characteristics of metal–air cells
Electrochemical equivalent of metal (Ah/g)
Theoretical cell voltagea (V)
Theoretical specific energy (of metal) (kWh/kg)
Practical operating voltage (V)
Zinc–air batteries can be classified into primary and electrically rechargeable. The former ones are commonly employed in hearing aid devices since the 1960s and represent a very well established technology without the need of further developments in terms of research on new materials and engineering improvements. The latter ones, on the contrary, still require research efforts to improve the efficiency and minimize obnoxious side effects occurring during the charging process , such as, for instance, zinc dendrites formation, limited number of charge–discharge cycles due to deterioration of the air cathode and carbonation of KOH electrolyte. In addition, in an electrically rechargeable zinc–air system, it is also important to deeply study and understand the chemistry of zincate solubility in the alkaline electrolyte as well as to develop new catalyst materials .
This review provides a comprehensive summary of the latest developments in zinc–air battery and fuel cell science and technology, covering, in particular, the materials used for the anode, the cathode, and the electrolyte as well as all the problems currently limiting the widespread success of electrically rechargeable zinc–air batteries.
Primary zinc–air batteries
This section concentrates on anodic, electrolytic and cathodic materials employed in primary zinc–air battery. From the point of view of applications, primary Zn–air batteries are well known for use in hearing aid devices (button-type cells), as hinted at in the introduction. Nevertheless, large primary Zn–air batteries have been also used to provide low rate and long-life power for applications such as seismic telemetry, railroad signaling and navigational buoys as well as remote communications. The theoretical specific energy density of Zn–air batteries is 1,084 Wh/kg. The theoretical voltage of a zinc–air cell is 1.667 V, but, in practice, the open circuit voltage is about 1.35 V. In addition, the discharge curve of a Zn–air cell is flat with minimum potential decay—compared to Li–air systems—when a constant current density (50–100 mA cm−2) is applied at the battery, and the voltage is measured and monitored over discharge time.
In particular, during discharge, the oxidation of the zinc electrode can involve several processes including oxidation of surface zinc atoms, ion solvation in the solution, ion diffusion in the electrolyte and precipitation into a solid phase when the solubility limit is reached.
Properties of zinc relevant to battery applications (at room temperature)
Closed packed hexagonal
5.96 μΩ cm
Stable dissolved form in KOH
Zn(OH) 4 2−
Reversible potential in 35 % KOH, 8.2 mol/L
Alloying zinc with other metals (Pb, Cd, Ni) with high hydrogen evolution overvoltage (a potentiodynamic polarization analysis is used to measure the relevant overpotential). However, all these elements are either highly toxic or not environmental friendly. For this reason, Zhang et al.  suggested the introduction of metallic bismuth to a pasted zinc electrode. In addition, alloys of Zn and Al are also used not only to inhibit corrosion of Zn, but also to increase the capacity of the anode material and decrease its weight  by harvesting both the reactivity of Al in alkaline solutions and its density. Special attention has been devoted to the more environmentally acceptable Zn–Ni alloy system. The comparative corrosion of Zn and Zn–Ni alloys in different conditions (temperature and concentration of the KOH electrolyte) has been studied in  by potentiodynamic and impedance methods. By plotting the corrosion current vs. concentration of KOH electrolyte (log[KOH]) for Zn and Zn–Ni alloys, a linear trend is observed for each alloy. Of course, by alloying Zn with Ni, the corrosion resistance of the pure metal is enhanced.
- (ii)Addition of Al2O3 either by mixing of Zn and Al2O3 particles or by surface modification of the Zn powders with an Al2O3 coating, deposited via a chemical solution process (see also Fig. 7 for details on the respective effects) . The effectiveness of the approach was tested by comparing the volumetric amount of hydrogen spontaneously evolved on the surface of bare Zn, Al2O3 mixed with Zn, and Zn powders coated with Al2O3 in a 9 M KOH electrolyte at 60 °C. The hydrogen evolution is almost suppressed in the presence of the alumina coating, owing to the formation of a passivation layer that prevents direct contact of Zn with the KOH electrolyte.
Coating the zinc metal particles with other materials such as Li2O–2B2O3 in a core–shell structure. This configuration prevents the zinc particles from contacting directly the alkaline electrolyte, thus hindering corrosion side-reactions.
Mixing the alkaline electrolyte with organic corrosion inhibitors such as (a) anions of organic acids , (b) HCO2–CH2–(OCH2CH2)–CH2–CO2H dicarboxylic acid-modified poly(ethylene glycol)  and phosphoric acid esters (e.g., GAFAC RA600) .
Since an alkaline solution is typically employed in Zn–air batteries, Eq. (5) represents the typically favored mechanism [63, 64]. As far as the catalysts are concerned, Zn–air batteries do not require precious metal catalysts, such as platinum, ruthenium or palladium, for the relevant chemistry; moreover, it has also been shown that noble catalysts tend to decrease the electrochemical performance since noble metal traces, released from the cathode, tend to diffuse to the zinc anode, lower its hydrogen overvoltage thus enhancing corrosion and gas production. References [33, 65] review the electrochemistry of the oxygen reduction reaction in metal–air batteries (including Zn–air ones) as well as the relevant catalysts. Moreover, Ref.  comprehensively compiles the published catalyst options and cathode configurations, with special attention to the patent literature. Among air cathode materials, Fe- and Co-based catalysts obtained by pyrolysis of N- and C-containing precursors (such as polypyrrole and polyaniline)  as well as graphene loaded with Mn3O4 nanoparticles  and CNT mixed with CoO , have been shown to be efficient for primary Zn–air batteries and to exhibit better performance than noble metals. The role of the carbonaceous materials is to support the catalyst and to create a porous and electronically conductive path. However, to avoid leakage of the electrolyte from the air cathode (see, e.g., ), superhydrophobic materials, such as polytetrafluoroethylene (PTFE), are also employed as fillers in the fabrication of air cathodes .
References [71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90] provide a comprehensive description of the available air cathode fabrication concepts that can be summarized as follows: (i) Teflon® membrane on the air side with a high air permeability of 2,000–4,000 s (Gurley method). This membrane protects the air cathode against possible leakage of electrolyte to the current collector. (ii) Current collector, typically a Ni mesh, a cheaper woven copper mesh with Ni coating, or an expensive Ni foam. Ni foam has been reported to exhibit a typically higher surface area, ensuring a better performance of the air cathode. (iii) Gas diffusion layer (GDL) chiefly composed of acetylene blacks (AB) with low Brunauer–Emmett–Teller (BET) surface area and wettability mixed with polytetrafluoroethylene (PTFE). The use of acetylene blacks with higher hydrophobic properties promotes more hydrophobic GDL properties. (iv) Catalyst layer: composed by the catalyst mixed with carbon blacks exhibiting high BET surface area and PTFE for hydrophobization.
Properties of KOH electrolyte relevant to battery applications (at room temperature)
Density of 35 % KOH, 8.2 mol/L
Resistivity of 8 mol/L KOH
2.3 Ω cm
Solubility of ZnO in 35 % KOH, 8.2 mol/L
Supersaturation of zincate in KOH
2–4 times of solubility
Ratio of diffusion coefficient of Zn(OH) 4 2− /OH−
Ratio of diffusion coefficient of Zn(OH) 4 2− /K+
Mechanically rechargeable Zn–air batteries
Electrically rechargeable Zn–air batteries
In this section, a discussion of the anode, cathode and electrolyte materials employed in electrically rechargeable Zn–air batteries (ERZAB) will be provided. In addition, the main problems, limiting their commercialization and widespread application, will be discussed. It is worth underlining that most of the materials employed in primary Zn–air batteries are also used in ERZAB, especially the anode materials, the electrolyte and the catalysts for the oxygen reduction reaction (ORR). Of course, the main research challenges and difficulties in materials selection stem from the requirement of reversibility and efficiency in both discharge and charge processes. Li et al. , in a very recent paper (May 2013) demonstrated an excellent bifunctional electrocatalyst, exhibiting optimal ORR and OER activities during discharge and charging processes, respectively.
As in the primary Zn–air battery, in the electrically rechargeable one, the anode material is zinc. However, in a secondary battery, this component changes structure and shape during repeated charge/discharge cycles, corresponding to zinc dissolution and re-deposition for several reasons, chiefly inhomogeneous current distribution and presence of concentration gradients in the electrolyte. As far as the shape changes are concerned, the typical shape change problems that have been reported to take place during charging are dendrite formation (leads to the loss of active material and to short-circuiting) and electrode densification (causing the originally porous zinc to leave active locations and agglomerate into self-screening compacts) . It is worth noting that the different types of shape change phenomena can be strongly interconnected. A comprehensive account of shape change phenomena can be found in Ref. . In the quest for reversible electrical recharging processes, effective dendrite suppression was achieved by applying a zeolite film over the zinc electrode . This film keeps the Zn2+ discharge product close to the electrode, preventing it from being flushed away into the electrolyte.
As hinted at above, at the cathode of a rechargeable Zn–air battery, both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are key factors for the efficiency of the system. Presently, a cathode material that is efficient and durable for both ORR and OER is not available, but research in the field seems active. Reference  reports on a novel core-corona bifunctional catalyst (CCBC) consisting of lanthanum nickelate centers supporting nitrogen-doped carbon nanotubes (NCNT). Reference  reports on the preparation of MnO2 nanotubes functionalized with Co3O4 nanoparticles for bifunctional air cathodes. These hybrid MnO2/Co3O4 nanomaterials exhibit enhanced catalytic reactivity toward oxygen evolution reaction under alkaline conditions compared with MnO2 nanotubes or Co3O4 nanoparticles alone. Reference  reports on novel silver nanoparticles-decorated MnO2 nanorods as an air electrode bifunctional catalyst. An alternative to the use of bifunctional catalysts is the approach of employing two specialized air cathodes for ORR and OER, respectively .
The electrolyte most commonly employed in an electrically rechargeable Zn–air battery is again KOH. However, in the oxidation–reduction cycling, the electrochemistry of zinc in potassium hydroxide gives some problems because the precipitation of zinc oxide is irreversible and reduces the availability of Zn2+ ions upon cycling. The use of chelating ionic liquids has been proposed to circumvent precipitation issues .
Mathematical modeling of electrically rechargeable Zn–air batteries
Zinc–air fuel cell
In this review, the different types of zinc–air batteries described in the literature and their functional components are examined from the point of view of electrochemical materials science. Among all metal–air batteries, the zinc–air ones are of particular interest since in principle they can be safer, cheaper and more environmentally friendly than other competing technologies. Furthermore, zinc–air batteries, both primary and electrically rechargeable, can meet the requirements of the whole range of applications: portable electronics, medium-scale energy production and storage and eventually grid storage. Fully engineered secondary zinc–air batteries are not yet available: research and development is still needed, especially in the fields of: (i) shape changes of the Zn electrode during charge/discharge cycles, (ii) durable and dual air cathode catalysts, (iii) KOH-based electrolyte chemistry. As far as high-power applications are concerned, the mechanically rechargeable and the flowing anode concepts are particularly appealing for the possibility of achieving essentially continuous operation as well as of implementing an off-line electrical generation process.
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