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Zinc–air fuel cell, a potential candidate for alternative energy
Journal of Industrial and Engineering Chemistry 15 (2009) 445–450
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Review
Zinc–air fuel cell, a potential candidate for alternative energy Prabal Sapkota, Honggon Kim * Energy and Environment Research Division, Korea Institute of Science and Technology, Hawolgokdong 39-1, Seongbukgu, Seoul 136-791, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 8 October 2008 Accepted 6 January 2009
A zinc–air fuel cell (ZAFC), which generates electricity by the reaction between oxygen and zinc pallets in a liquid alkaline electrolyte, is a potential candidate for an alternative energy generator. It is efficient, completely renewable, and cheap in fabrication because precious metal catalysts are not necessary. In addition, it is environmentally benign because of producing solely recyclable zinc oxide without gas emission. It is applicable to portable, mobile, stationary, and military purposes. In spite of its high potential as an alternative power source, it is yet in a preliminary stage of commercialization because of a few uncertainties remained. This paper reviews the present status of the ZAFC technology and the problems to be overcome for further advancement toward the potential next-generation alternative energy. ß 2009 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Zinc–air fuel cell ZAFC Metal air fuel cell Power generator Electrochemical cell
Contents 1. 2.
3. 4.
5.
6.
7. 8.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamentals of voltage generation in a ZAFC 2.1. Thermodynamics . . . . . . . . . . . . . . . . . . 2.2. Standard electrode potentials. . . . . . . . . Design of a ZAFC . . . . . . . . . . . . . . . . . . . . . . . . Materials for ZAFC . . . . . . . . . . . . . . . . . . . . . . . 4.1. Anode . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Cathode . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Electrolyte and separator . . . . . . . . . . . . Factors affecting the performance of ZAFC . . . . 5.1. Ohmic losses . . . . . . . . . . . . . . . . . . . . . . 5.2. Activation losses . . . . . . . . . . . . . . . . . . . 5.3. Dendrite formation . . . . . . . . . . . . . . . . . 5.4. Carbon dioxide absorption . . . . . . . . . . . Cell performance evaluation . . . . . . . . . . . . . . . 6.1. Power density . . . . . . . . . . . . . . . . . . . . . 6.2. Polarization curves . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction The issue of global warming and immense thrust of energy that is growing on day by day have forced to search new sustainable
* Corresponding author. Tel.: +82 2 958 5858; fax: +82 2 958 5859. E-mail address: [email protected] (H. Kim).
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energies which can replace fossil fuels and nuclear energy. Various alternative energies such as solar, hydropower, wind, and geothermal energies are already in practice, and some are in the phase of commercialization. The fuel cell, a plausible nextgeneration power generating system which is known to efficiently convert fuels to electricity with producing environmentally benign byproducts, is also under development and nearly about to be commercialized. Various sizes of the fuel cell can extend its
1226-086X/$ – see front matter ß 2009 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. doi:10.1016/j.jiec.2009.01.002
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P. Sapkota, H. Kim / Journal of Industrial and Engineering Chemistry 15 (2009) 445–450
Table 1 Various types of fuel cells. Fuel cell type
Operating temperature
Fuel
Catalyst
Applications
Alkaline (AFC) Proton exchange membrane (PEMFC) Direct methanol (DMFC) Phosphoric acid (PAFC) Molten carbonate (MCFC) Solid oxide (SOFC) Metal–air, zinc–air (ZAFC)
80–90 8C 30–100 8C 20–90 8C 200 8C 650 8C 500–1000 8C 0–60 8C
H2 H2 Methanol H2 H2, CO2 H2 Metal, Zn
Pt/Pd, Pt/Au Pt Pt, Pt/Ru Pt Ni Perovskites MnO2/Pt
Space application Vehicles, mobile, low powered domestic use Low powered portable system Domestic application Power plant (MW) Power plant (MW) Mobile and stationary applications
application from micropower devices to industry purposes including mobile and stationary applications. The versatility of its application induces the most immense interest among the alternative energies presently being developed. Fuel cells are the electrochemical devices which directly convert chemical energy into electrical energy. A basic fuel cell structure consists of an electrolyte layer sandwiched between a porous anode and a cathode. Even though the fuel cell resembles quite similar to a battery in components, the former is an energy converting device where as the latter is an energy storage device. In other words, the fuel cell can continuously produce energy as long as the fuel, hydrogen in most cases, is supplied, while the battery cannot. Various fuel cells are being developed according to the operation temperature range, and the most representative types are exampled in Table 1 [1–4]. The recent research on lowtemperature fuel cells such as PEMFC (proton exchange membrane fuel cell) and DMFC (direct methanol fuel cell) for mobile and stationary applications is at a peak. But issues of hydrogen compression and storage and the methanol crossover have hindered their commercialization. A metal–air fuel cell, which uses the phenomenon that electrons are discharged when the metal is oxidized, can be a candidate for an environmentally benign alternative energy generator. It is efficient, environmentally safe, and completely renewable, but does not require precious metals such as Pt as its catalyst. In addition to its low fabrication cost because of cheap fuels, a metal and the air, it can also be widely used for portable, mobile and stationary applications. Various metals such as Li, Ca, Mg, Al, Zn and Fe can be adopted for metal–air fuel cells. As far as the energy density, the energy per unit mass, is concerned, Al exceeds other metals. However, it is easily corroded in the alkaline electrolyte. The corrosion rate of Zn is slower than Al in an aqueous or alkaline solution. In addition, Zn is highly electro-positive and abundantly available at low cost, and a cell with Zn can produce high specific energy and high power density. Therefore, Zn is so far considered as the most reasonable material for the anode in a metal–air fuel cell [5,6]. The zinc–air fuel cell (ZAFC) works on the basis of a reaction between the atmospheric oxygen and zinc pallets in a liquid alkaline electrolyte. Since the ZAFC produces only zinc oxide, which is entirely recyclable, without gas emission while generating electricity, it is considered environmentally friendly. Fig. 1 shows a general schematic diagram of a ZAFC. In an overall chemistry, zinc is converted to zinc oxide when zinc pellets are fed into the chamber. Firstly, when air or any oxygen source such as peroxide is supplied at the cathode side, oxygen is electrochemically reduced to hydroxide ions by reacting with water on the surface of the gas-permeable cathode. Meanwhile, the oxidation of Zn takes place at the surface of the anode current collector, where Zn is converted to zinc oxide by reacting with hydroxide ions. During the conversion of Zn to ZnO, electrons are discharged and transferred to the anodic current collector. The electrons on the anode pass across the external load and come back to the cathode current collector, where the reduction of oxygen takes place. Therefore, the overall chemical reaction in a ZAFC can
Fig. 1. Schematic of ZAFC.
be written as 2Zn þ O2 ¼ 2ZnO
(1)
Electricity, theoretically 1.65 V, can be generated from the above electrochemical reaction between zinc and oxygen. The zinc–air system in the form of a primary battery has already been in practice. However, in spite of having enormous potential as an alternative energy generator, the ZAFC system is yet in a preliminary stage of commercialization because there are some uncertainties for which further studies and development need to be addressed. The purpose of this paper is to review the present status of the ZAFC technology and the problems to be overcome in order to make the ZAFC a potential candidate for the nextgeneration alternative energy. 2. Fundamentals of voltage generation in a ZAFC 2.1. Thermodynamics For generating electricity from a ZAFC, the overall reaction (1) should be thermodynamically favorable. It is convenient to express the overall reaction in terms of the overall cell electromotive force, Eemf (V), which is defined as the potential difference between a cathode and an anode. The Eemf is related to the electrical work done in a cell. Electrical work done ðWÞ ¼ charge ðQ Þ voltage ðEemf Þ
(2)
where Q = nF, n is the number of electrons which take part in the chemical reaction, and F is the Faraday constant. W ¼ nFEemf
(3)
If there is no loss in the system, the electrical work done is equal to the Gibbs free energy change (DGr). Eemf ¼
DGr nF
(4)
P. Sapkota, H. Kim / Journal of Industrial and Engineering Chemistry 15 (2009) 445–450
In the case of a ZAFC, n = 2. So the equation becomes Eemf ¼
DGr 2F
(5)
2.2. Standard electrode potentials The overall electrochemical reaction taking place in the ZAFC can be analyzed by considering the anode and the cathode reactions in separate. Zn þ 4OH ¼ ZnðOHÞ4 2 þ 2e ZnðOHÞ4 2 ¼ ZnO þ 2OH þ H2 O
Anode :
ðEo ¼ 0:625 VÞ (6)
Cathode :
O2 þ 2H2 O þ 4e ¼ 4OH
ðEo ¼ þ0:40 VÞ
(7)
The overall reaction can be expresses as 2Zn þ O2 ¼ 2ZnO
ðEo ¼ 1:65 VÞ
(8)
In the above equations, Eo represents the standard electrode potential of each reaction with respect to the standard hydrogen electrode at the standard temperature and pressure. The overall cell electromotive force is calculated as Eemf ¼ Eo ðcathodeÞ Eo ðanodeÞ
(9)
so that the overall cell electromotive force of a ZAFC should be theoretically 1.65 V as shown in Eq. (8). The Eemf is a thermodynamic value which does not include internal losses. The open circuit voltage (OCV) is obtained at no load condition and should theoretically be equal to the Eemf. However, the actual open circuit voltage of a practical ZAFC is always less than the theoretical value due to various potential losses and found to be around 1.45 V [1]. Possible reasons for the potential loss will be discussed in Section 5. 3. Design of a ZAFC Various configurations were proposed for ZAFCs. In the beginning stage, a zinc–air system with mechanically refuelable anodes shown in Fig. 2 was proposed [7]. This system can be considered as a zinc–air refuelable battery rather than a ZAFC. A Zn plate which is replaceable is placed in the middle as an anode, and the surfaces of two sides face the oxygen-reduction cathodes, which are often called air-cathodes. Aqueous KOH is used as an electrolyte for transferring ions. The two electrically opposite sides, the anode and the cathode, are separated by a membrane
Fig. 2. Zinc–air system with mechanically refuelable anode.
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which is ionically conductive but electrically non-conductive. For being a fuel cell, a system should continuously produce the electrical energy as long as the fuel is supplied. So electrically discharging and mechanically refueling of the above system does not support the exact definition of a fuel cell. However the working principle and the concept are same as those of a fuel cell. Several approaches for ZAFCs have been proposed by different research groups. Since the solid Zn and zinc oxide particles are handled in a ZAFC, the clogging problem of unreacted zinc, solid products, and byproducts in the electrolyte is severe. Some of the research works to solve this problem were conducted in a fluidized bed system or a static bed system. The Metallic Power developed a 36 V fuel cell delivering approximately 6 kWh by adopting the concept of the fluidized bed system [8]. However, the clogging problem still remained in this case. The clogging problem by unreacted zinc pellets could be solved by a tapered-end structure [9]. As an example shown in Fig. 3, two non-parallel surfaces are placed with a small vertical angle which produces a difference between two ends, the lower and upper ends. In this approach, the preferable vertical angle is ca. 0.1–38. A continuous flow of electrolyte is maintained from the top to the bottom of the cell. The upper portion of a cell is electrochemically active and called a hopper which acts as a reservoir for the zinc pellets or particles. Once the cell starts to discharge the electricity, the pellets reduce their sizes by the chemical reaction and flow downwards by natural movement. Solid products and byproducts as well as smallsize unreacted zinc particles escape out from the cell along with the effluent of electrolyte. The major disadvantages of this taperedend approach are the presence of inactive volume in the hopper which unnecessarily increases the weight of a cell and makes it bulky, a relatively long filling time of zinc pellets in the hopper, and the high shunt current flowing between the cells during refueling via a feed tube. Pluto et al. suggested an advanced design of ZAFC to overcome the problems encountered in the tapered-end structure [10]. In their design, a slightly leaned anode current collector and a vertical permeable cathode which air can pass through are placed on the two opposite sides of the cell which cover the entire cell area. And the fuel is pumped from the adjacent tank so that almost all space in the cell frame actively works electrochemically. This arrange-
Fig. 3. ZAFC with a tapered-end structure.
P. Sapkota, H. Kim / Journal of Industrial and Engineering Chemistry 15 (2009) 445–450
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is mollified in the cases of mechanically refuelable zinc–air systems and continuous ZAFC systems. 4.2. Cathode
Fig. 4. ZAFC cell designed by Pluto et al.
ment seems useful in reducing the shunt current between unit cells. Furthermore, it maintains a constant level of zinc particles inside the cell. In Fig. 4, the liquid electrolyte containing zinc particles is pumped and supplied to the cell. The vertical baffles help to direct the pellet movement in the cell. Once the active space of the cell is full, the electrolyte containing zinc pellets flows along the paths and returns back to the fuel tank via an overflow exit line. As the cell electrically discharges, zinc pellets get smaller and move downwards. The electrolyte containing zinc oxide particles, the product, and small amount of unreacted zinc pellets passes through a grid mesh placed at the bottom of the cell and is pumped to the recycle tank. The vacant space in the upper part is occupied by new zinc pellets which are fed from the inlet stream. Smedley and Zhang used this concept in their research work with a 12 cellstack providing 1.8 kW [11]. 4. Materials for ZAFC 4.1. Anode In a ZAFC, the anode material is zinc. The shape and size of zinc depend on the cell design. In the mechanically refuelable zinc–air system, a single zinc plate with a current collecting element embedded in it is used as an anode. During refueling, the whole anode assembly is replaced by a new set with a current collecting element. According to the Electric Fuel Ltd., refueling of Zn for ZAFC-powered vehicles can be done in a short time by an automatic refueling machine in a refueling station just like that in a gasoline refueling station [12]. In the case of the cells with tapered ends, proper sizes of zinc pellets are required in order to continuously charge the solid fuel. Proper shape and size of zinc pellets depend on the shape and size of the cell components. The current collecting element on the anode side can be made of any material having high electrical conductivity and high anticorrosion to the electrolyte material. Nickel, copper, silver, and stainless steel are commonly used for the anode. Relatively few research works have been done for improving the anode. Furthermore, most research works on the anode focused on the electrically rechargeable zinc–air batteries and the mechanically refuelable zinc–air systems rather than ZAFCs. In the case of an electrically rechargeable system, irreversibility of the system and corrosion of the elements are the major problems. These could be alleviated by using a zinc oxide electrode or a modified electrode containing zinc oxide and lead oxide [13]. But the harmlessness of lead oxide on the environment is not yet proven. The performance of zinc anode can be further improved by adopting an alloy of Ni (7.5%), Zn (90%) and Ir (2.5%) [14]. Although the problem associated with the zinc anode seems to play a significant role in the case of electrically rechargeable batteries, it
One of the major factors determining the performance of a ZAFC is an air cathode where the oxygen reduction takes place. A typical ZAFC cathode consists of three layers: a current collecting layer, a diffusion layer, and a catalytically active layer. The current collecting layer is simply the metal mesh sandwiched between the diffusion layer and the catalytically active layer. There is no specific metal, but it should be non-corrosive and highly electrically conductive. Metals most commonly used are nickel, gold, silver, copper, silver-plated nickel, silver-plated or nickel-plated iron. The diffusion layer composed of carbon particles and hydrophobic particles needs to be air-permeable but water-impermeable. Graphite for carbon particles and polytetrafluoroethylene (PTFE) for hydrophobic particles are bonded together with or without epoxy resin [15]. A proper ratio can be chosen in a range of 30–70 wt.% PTFE and the balanced amount of conductive carbon [16]. The catalytically active layer is the most important part in the air cathode for a ZAFC. This is the place where the catalytic reduction of oxygen takes place so that a larger active area is desirable from the reaction point of view. The active area consists of catalysts mixed in carbon particles. One advantage of the ZAFC is that it does not require precious and costly metals for catalysts. Inexpensive nonnoble metal oxides such as MnO2 can be used as a ZAFC catalyst [17]. A perovskite type (La0.6Ca0.4CoO3) doped with metal oxides can be used for an oxygen reducing catalyst [18]. Carbon particles supporting the catalyst provide not only a high surface area for the reaction but also electrical paths to electrons. The cell performance depends on the type and the pore size of carbon. Activated carbon with a large number of macropores (>50 nm) or mesopores (2–50 nm) gives high performance [19]. Chao et al. suggested the addition of clay in the air cathode in order to enhance the dispersion of carbon particles and catalyst [20]. Shun and Lou suggested an advanced catalytic layer consisting of carbon particles, high surface area particulates such as molecular sieve or zeolite, hydrophobic particles, and metal hydroxide as catalyst such as nickel hydroxide, cobalt hydroxide, iron hydroxide, cerium hydroxide, manganese hydroxide, lanthanum hydroxide and chromium hydroxide [15]. The active layer can also be prepared by spraying a catalyst material on the commercially available carbon papers or carbon cloths which are used as gas diffusion layers. Zhu et al. proposed a thin air cathode of 0.13–0.50 mm thickness which is prepared by putting carbon particles in a sinter-locked metal fiber network [21]. This could reduce the cathode thickness by 30–75% of commercial air cathodes. Wang Chen developed an air cathode with multi-layers in order to minimize the moisture transfer from the inner electrolyte to the outside air and to preserve the constant water content of zinc anode [22]. This cathode was composed of a metallic mesh as a substrate which is sandwiched by two diffusion layers of carbon and PTFE binder. Different types of catalysts are coated on a diffusion layer one by one to form a multi-layered catalytic surface. For example, a three-layered active surface can be prepared by coating 1 mg/cm2 of MnO2, CoO and MnO2 in the first, the second and the third layers, respectively. Desirable thickness of the air cathode is chosen in the range of 0.01 and 20 mm according to the configuration and the number of layers. The noble Pt can be used as a catalyst material, but its use increases the cost of a cell. 4.3. Electrolyte and separator The electrolyte plays an important role in the transportation of ions. Liquid alkalines are used for ZAFC electrolytes. The most
P. Sapkota, H. Kim / Journal of Industrial and Engineering Chemistry 15 (2009) 445–450
commonly used ones are potassium hydroxide (KOH), sodium hydroxide (NaOH) and lithium hydroxide (LiOH). Some articles suggested to use the hydroponics gel as an electrolyte gelling agent due to its enormous capability of storing solution [23,24]. A separator or a membrane needs to be highly ionically conductive but electrically non-conductive. Organic polymers such as a copolymer of polyvinyl alcohol and polyvinyl acetate have been used for the separator [25]. Commercially available Nafion (DuPont Co.), a sulfonated perfluoropolymer, has been widely used as ion exchange membrane because of its high ion exchange capability and chemical stability [26,27]. However, because Nafions are far more expensive than hydrocarbon separators, Dewi et al. suggested the use of polysulfonium membrane, which can prevent the permeation of zinc cations and only allow hydroxide anions to pass through [28]. A hydrophilic polyestersulfone film with micropores of 0.45 mm pore size was also proposed [29]. Wu et al. suggested the polypropylene membrane (Celgard) treated with sulfuric acid, which is usually used in lithium batteries, as a separator for zinc–air electrochemical cells [30].
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5.4. Carbon dioxide absorption As the system is operated with air instead of pure oxygen, CO2 present in the air dissolves in the electrolyte forming carbonate. The formation of carbonate increases the viscosity of the electrolyte and decreases its ionic conductivity. In a ZAFC, this can be minimized by continuously supplying a fresh electrolyte solution. Use of pure oxygen instead of air can decrease the formation of carbonate but increase the operation cost. The problem of carbonate formation can possibly be reduced if an acidic electrolyte is adopted instead of an alkaline [33]. 6. Cell performance evaluation In most cases of electrochemical devices, the performance is usually expressed in terms of power density and a polarization curve even though there are diverse types of methods and bases. The evaluation of a cell normally starts from its open circuit voltage (OCV). The OCV of a ZAFC is so far known to be ca.1.45 V.
5. Factors affecting the performance of ZAFC
6.1. Power density
Even though the theoretically available voltage of a ZAFC is 1.65 V and its OCV is 1.45 V, the practically attainable value is always less than them. There are several factors responsible for the low voltages during operation.
The power density is a specific power normalized to the projected active area of an electrode or to the volume of a system. P P Pd ¼ or P d ¼ A V
5.1. Ohmic losses Ohmic losses occur due to the electrical resistances of electrodes and interconnections, and the resistance to the flow of ions in the electrolyte. The amount of voltage drop (V) depends on the current (i) and the resistances of components (R). This can be expressed as V ¼ iR The ohmic losses can be minimized by using the electrodes with high electrical conductivity and the electrolyte with high ionic conductivity. Aqueous NaOH, KOH or LiOH is preferred as a good electrolyte [31]. 5.2. Activation losses Activation losses result from the slowness of reactions taking place on the surface of the electrodes. In low-temperature fuel cells, the air cathode is primarily responsible for the activation loss [2]. The activation loss increases as the current density increases. And it can be reduced by increasing the active surface area of cathodes, the catalytic activity, or the roughness of the electrodes. Increasing the oxygen concentration by using pure O2 instead of air can also reduce the activation loss, but this is not favorable because of the high cost of O2 and the difficulty in oxygen compression and storage for small portable devices.
where Pd is the power density, P is the total power expressed in watt or milliwatt, A is the area of an electrode in cm2, and V is the volume of a system in cm3. For a ZAFC system, the air cathode area is usually taken as the active area where the electrochemical reaction seems to take place. 6.2. Polarization curves Polarization occurs when the electrical resistance around the electrodes abruptly increases. In a typical polarization curve which can represent the performance of the cell or can indicate certain change in the cell, the voltage change is expressed in terms of the current change or the current density change. The polarization curve exampled in Fig. 5 can be divided in to three zones as follows: (i) Activation loss zone ranging from the OCV at zero current to the initial steep decrease of voltage. (ii) Ohmic loss zone where the voltage slowly drops. (iii) Concentration loss zone where the mass transport effect is dominant and the voltage rapidly falls at high current densities.
5.3. Dendrite formation In the cell stacks where the electrolyte is shared, a shunt current appears. This shunt current is responsible for the formation of zinc dendrite. Even though it is slowly formed, it eventually blocks the flow of electrolyte and zinc particles into the cell. Colbon suggested the construction of dendrite elimination zone to prevent the accumulation of zinc particles in this zone [32]. Use of an acidic electrolyte instead of an alkaline could minimize the dendrite formation [33]. Addition of a small amount of cellulose (1–10% to zinc electrode) showed a positive effect to reduce the formation of zinc dendrite in the case of rechargeable zinc–air batteries [34,35].
Fig. 5. Typical polarization curve of ZAFC.
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In a ZAFC, the activation loss dominantly occurs in the initial stage, and the ohmic loss occurs moderately. The most significant loss occurs when a high current density is applied. If the ZAFC is operated in a middle range of current density, it steadily discharges a constant voltage. In a ZAFC well modified so far, a high current density of 360 mA/cm2 could be sustained for 1000 h [11]. Either a potentiostat or a variable resistor box can be used to set variable external loads. The voltage change is measured when the load decreases (or increases) periodically. The current or the current density is calculated using Ohms law. When the external resistance is varied, the changes of current and voltage should be counted after a temporary pseudo-steady state condition is established in a few minutes. 7. Applications Due to its high specific energy, high power density, cheap and abundantly available fuel, no use of precious metals as catalysts, and no issue of difficulty in fuel storage and transportation, the ZAFC is definitely one promising option for both stationary and mobile applications. Zinc–air batteries are already in practice as a primary battery in small devices like hearing aids. It can replace alkaline or mercury batteries because its energy density is up to five times of these batteries. As the zinc–air batteries, the ZAFC is suitably applied for the areas where a high energy density, the energy per weight, is continuously required. Recently, several companies are involved in development and commercialization of ZAFCs for electric vehicles, indoor power generators, industrial facilities, and military purposes. However, few research groups and companies are so far working on the development of Zn–air systems throughout the world. The Electric Fuel, Ltd. has worked on a zinc–air battery system for electric vehicles, demonstrated the systems for vans, and been developing the systems for buses. The company is also working on the primary and secondary batteries for military uses [12,36–41]. The Power Zinc has commercialized zinc–air batteries for military uses and electric vehicles, and developed first-stage models of ZAFC products [42]. The Power Air Co. has produced various ZAFC systems for mobile device power packs, backup power applications and indoor generators, and has currently demonstrated its ninth generation ZAFC system in 2007 [43]. The Metallic Power Inc. produces ZAFCs as alternative backup or emergency electrical sources for the generators which use internal combustion engines [6]. 8. Conclusions Even though the zinc–air fuel cell system is one of the potential candidates to fulfill the world’s energy requirements, several problems including full exploitation and modification of zinc as a source for the environmentally benign energy production need to be solved for its commercialization. The main problem associated with the ZAFC so far is the performance of an air cathode. A multilayered catalyst electrode can be a plausible option. Improvement of the cell performance can also be achieved by changing the configuration of a cell assembly in such a manner that OH ions can travel a short distance. The addition of ionically conductive materials into the electrolyte can hopefully help to solve the
problems of dendrite formation, carbonate formation and corrosion of electrodes. In order to improve the life-time of a cell, several factors need to be solved simultaneously. After all, the development and commercialization of a ZAFC is one of the interesting research fields for the scientists and engineers who wish to devote to the excavation of future energies. References [1] N.J. Cherepy, R. Krueger, J.F. Cooper, 14th annual Battery Conference on Applications and Advances, IEEE, 1999, 11–13. [2] (a) J. Larminie, A. Dicks, Fuel Cell Systems Explained, 2nd edn, Wiley, England, 2005; (b) M.C. Jo, G.H. Kwon, W. Li, A.M. Lane, J. Ind. Eng. Chem. 15 (2009) 336. [3] (a) EG and G Technical Services, Fuel Cell Handbook, 6th edn, US Department of Energy, West Virginia, 2002; (b) S. Uhm, J.K. Lee, S.T. Chung, J.Y. Lee, J. Ind. Eng. Chem. 14 (2008) 493. [4] Technical/OCM Primary System, Duracell, Procter & Gamble, 2005. [5] G. Koscher, K. Kordesch, J. Power Sources 136 (2004) 215. [6] M.J. Riezenman, IEEE Spectrum 38 (2001) 55. [7] J. Goldstein, I. Brown, B. Koretz, J. Power Sources 80 (1999) 171. [8] S. Smedley, IEEE Aerosp. Electron. Syst. Mag. 15 (2000) 19. [9] J.F. Cooper, US Patent 5,434,020 (1995). [10] M. Pinto, S. Smedley, J.A. Colborn, US Patent 6,706,433 B2 (2004). [11] S.I. Smedley, X.G. Zhang, J. Power Sources 165 (2007) 897. [12] J.R. Goldstein, I. Gektin, B. Koretz, Electric Fuel Ltd, Presented in the 1995 Annual Meeting of the Applied Electrochemistry Division of the German Chemical Society, Duisburg, Germany, 1995. [13] C.W. Lee, S.W. Eom, K. Sathiyanarayanan, M.S. Yun, Electochim. Acta 52 (2006) 1588. [14] C.W. Lee, K. Sathiyanarayanan, S.W. Eom, M.S. Yun, J. Power Sources 160 (2006) 1436. [15] Y.-K. Shun, C.-L. Lou, US Patent 6,127,061 (2000). [16] Y. Kiros, WO/2002/075827 (2002). [17] G.Q. Zhang, X.G. Zhang, Electrochim. Acta 49 (2004) 873. [18] X. Wang, P.J. Sebastian, M.A. Smit, H. Yang, S.A. Gamboa, J. Power Sources 124 (2003) 278. [19] S.W. Eoma, C.W. Lee, M.S. Yun, Y.K. Sun, Electrochim. Acta 52 (2006) 1592. [20] W.K. Chao, C.M. Leea, S.Y. Shieua, C.C. Chou, F.S. Shieu, J. Power Sources 177 (2008) 637. [21] W.H. Zhu, B.A. Poole, D.R. Cahela, B.J. Tatarchuk, J. Appl. Electrochem. 33 (2003) 29. [22] K.Y.W. Chen, US Patent 0,044,640 A1 (2008). [23] R. Othman, W.J. Basirun, A.H. Yahaya, A.K. Arof, J. Power Sources 103 (2001) 34. [24] A.A. Mohamad, J. Power Sources 159 (2006) 752. [25] J. Treger, S. Sargeant, J. Licata, US Patent 6,514,637 B2 (2003). [26] H. Kato, US Patent 7,125,626 B2 (2006). [27] W.K. Chao, C.M. Lee, S.Y. Shieu, C.C. Chou, F.S. Shieu, J. Power Sources 177 (2008) 637. [28] E.L. Dewi, K. Oyaizu, H. Nishide, E. Tsuchida, J. Power Sources 115 (2003) 149. [29] R. Jiang, Rev. Sci. Instrum. 78 (2007) 072209. [30] G.M. Wu, S.J. Lin, J.H. You, C.C. Yang, Mater. Chem. Phys. (in press). [31] T. Burchardt, WO 2007/144357 (2007). [32] J.A. Colbon, US Patent 6,153,328 (2000). [33] R. Clarke, WO 2004/001879 (2004). [34] S. Muller, F. Holzerm, O. Hass, J. Appl. Electrochem. 28 (1998) 895. [35] C.W. Lee, K. Sathiyanarayanan, S.W. Eom, H.S. Kim, M.S. Yun, J. Power Sources 160 (2006) 1436. [36] B. Koretz, Y. Harats, J. Goldstein, M. Korall, Electric Fuel Ltd, Presented in Batterien und Batteriemanagement Conference, Essen, Germany, 1995. [37] B. Koretz, Y. Harats, J. Goldstein, Electric Fuel Ltd, Presented in 12th International Seminar on Primary and Secondary Battery Technology and Application, Deerfield beach, Florida, USA, 1995. [38] B. Koretz, J.R. Goldstein, Electric Fuel Ltd, Presented in the 32nd Intersociety Energy Conversion Engineering Conference, Honolulu, Hawaii, 1997. [39] B. Koretz, N. Naimer, Electric Fuel Ltd, Presented in the 32nd Intersociety Energy Conversion Engineering Conference, Honolulu, Hawaii, 1997. [40] J. Wartman, I. Brown, Electric Fuel Ltd, Presented in the 15th International Electric Vehicle Symposium and Exhibition (EVS-15), Brussels, Belgium, 1998. [41] J. Goldstein, I. Brown, B. Koretz, Electric Fuel Ltd, Presented in the 21st International Power Sources Symposium, Brighton, UK, 1999. [42] Official website of Power Zinc, http://www.powerzinc.com. [43] Official website of Power Air Corporation, http://www.poweraircorp.com.
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Review
Zinc–air fuel cell, a potential candidate for alternative energy Prabal Sapkota, Honggon Kim * Energy and Environment Research Division, Korea Institute of Science and Technology, Hawolgokdong 39-1, Seongbukgu, Seoul 136-791, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 8 October 2008 Accepted 6 January 2009
A zinc–air fuel cell (ZAFC), which generates electricity by the reaction between oxygen and zinc pallets in a liquid alkaline electrolyte, is a potential candidate for an alternative energy generator. It is efficient, completely renewable, and cheap in fabrication because precious metal catalysts are not necessary. In addition, it is environmentally benign because of producing solely recyclable zinc oxide without gas emission. It is applicable to portable, mobile, stationary, and military purposes. In spite of its high potential as an alternative power source, it is yet in a preliminary stage of commercialization because of a few uncertainties remained. This paper reviews the present status of the ZAFC technology and the problems to be overcome for further advancement toward the potential next-generation alternative energy. ß 2009 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Zinc–air fuel cell ZAFC Metal air fuel cell Power generator Electrochemical cell
Contents 1. 2.
3. 4.
5.
6.
7. 8.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamentals of voltage generation in a ZAFC 2.1. Thermodynamics . . . . . . . . . . . . . . . . . . 2.2. Standard electrode potentials. . . . . . . . . Design of a ZAFC . . . . . . . . . . . . . . . . . . . . . . . . Materials for ZAFC . . . . . . . . . . . . . . . . . . . . . . . 4.1. Anode . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Cathode . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Electrolyte and separator . . . . . . . . . . . . Factors affecting the performance of ZAFC . . . . 5.1. Ohmic losses . . . . . . . . . . . . . . . . . . . . . . 5.2. Activation losses . . . . . . . . . . . . . . . . . . . 5.3. Dendrite formation . . . . . . . . . . . . . . . . . 5.4. Carbon dioxide absorption . . . . . . . . . . . Cell performance evaluation . . . . . . . . . . . . . . . 6.1. Power density . . . . . . . . . . . . . . . . . . . . . 6.2. Polarization curves . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction The issue of global warming and immense thrust of energy that is growing on day by day have forced to search new sustainable
* Corresponding author. Tel.: +82 2 958 5858; fax: +82 2 958 5859. E-mail address: [email protected] (H. Kim).
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energies which can replace fossil fuels and nuclear energy. Various alternative energies such as solar, hydropower, wind, and geothermal energies are already in practice, and some are in the phase of commercialization. The fuel cell, a plausible nextgeneration power generating system which is known to efficiently convert fuels to electricity with producing environmentally benign byproducts, is also under development and nearly about to be commercialized. Various sizes of the fuel cell can extend its
1226-086X/$ – see front matter ß 2009 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. doi:10.1016/j.jiec.2009.01.002
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P. Sapkota, H. Kim / Journal of Industrial and Engineering Chemistry 15 (2009) 445–450
Table 1 Various types of fuel cells. Fuel cell type
Operating temperature
Fuel
Catalyst
Applications
Alkaline (AFC) Proton exchange membrane (PEMFC) Direct methanol (DMFC) Phosphoric acid (PAFC) Molten carbonate (MCFC) Solid oxide (SOFC) Metal–air, zinc–air (ZAFC)
80–90 8C 30–100 8C 20–90 8C 200 8C 650 8C 500–1000 8C 0–60 8C
H2 H2 Methanol H2 H2, CO2 H2 Metal, Zn
Pt/Pd, Pt/Au Pt Pt, Pt/Ru Pt Ni Perovskites MnO2/Pt
Space application Vehicles, mobile, low powered domestic use Low powered portable system Domestic application Power plant (MW) Power plant (MW) Mobile and stationary applications
application from micropower devices to industry purposes including mobile and stationary applications. The versatility of its application induces the most immense interest among the alternative energies presently being developed. Fuel cells are the electrochemical devices which directly convert chemical energy into electrical energy. A basic fuel cell structure consists of an electrolyte layer sandwiched between a porous anode and a cathode. Even though the fuel cell resembles quite similar to a battery in components, the former is an energy converting device where as the latter is an energy storage device. In other words, the fuel cell can continuously produce energy as long as the fuel, hydrogen in most cases, is supplied, while the battery cannot. Various fuel cells are being developed according to the operation temperature range, and the most representative types are exampled in Table 1 [1–4]. The recent research on lowtemperature fuel cells such as PEMFC (proton exchange membrane fuel cell) and DMFC (direct methanol fuel cell) for mobile and stationary applications is at a peak. But issues of hydrogen compression and storage and the methanol crossover have hindered their commercialization. A metal–air fuel cell, which uses the phenomenon that electrons are discharged when the metal is oxidized, can be a candidate for an environmentally benign alternative energy generator. It is efficient, environmentally safe, and completely renewable, but does not require precious metals such as Pt as its catalyst. In addition to its low fabrication cost because of cheap fuels, a metal and the air, it can also be widely used for portable, mobile and stationary applications. Various metals such as Li, Ca, Mg, Al, Zn and Fe can be adopted for metal–air fuel cells. As far as the energy density, the energy per unit mass, is concerned, Al exceeds other metals. However, it is easily corroded in the alkaline electrolyte. The corrosion rate of Zn is slower than Al in an aqueous or alkaline solution. In addition, Zn is highly electro-positive and abundantly available at low cost, and a cell with Zn can produce high specific energy and high power density. Therefore, Zn is so far considered as the most reasonable material for the anode in a metal–air fuel cell [5,6]. The zinc–air fuel cell (ZAFC) works on the basis of a reaction between the atmospheric oxygen and zinc pallets in a liquid alkaline electrolyte. Since the ZAFC produces only zinc oxide, which is entirely recyclable, without gas emission while generating electricity, it is considered environmentally friendly. Fig. 1 shows a general schematic diagram of a ZAFC. In an overall chemistry, zinc is converted to zinc oxide when zinc pellets are fed into the chamber. Firstly, when air or any oxygen source such as peroxide is supplied at the cathode side, oxygen is electrochemically reduced to hydroxide ions by reacting with water on the surface of the gas-permeable cathode. Meanwhile, the oxidation of Zn takes place at the surface of the anode current collector, where Zn is converted to zinc oxide by reacting with hydroxide ions. During the conversion of Zn to ZnO, electrons are discharged and transferred to the anodic current collector. The electrons on the anode pass across the external load and come back to the cathode current collector, where the reduction of oxygen takes place. Therefore, the overall chemical reaction in a ZAFC can
Fig. 1. Schematic of ZAFC.
be written as 2Zn þ O2 ¼ 2ZnO
(1)
Electricity, theoretically 1.65 V, can be generated from the above electrochemical reaction between zinc and oxygen. The zinc–air system in the form of a primary battery has already been in practice. However, in spite of having enormous potential as an alternative energy generator, the ZAFC system is yet in a preliminary stage of commercialization because there are some uncertainties for which further studies and development need to be addressed. The purpose of this paper is to review the present status of the ZAFC technology and the problems to be overcome in order to make the ZAFC a potential candidate for the nextgeneration alternative energy. 2. Fundamentals of voltage generation in a ZAFC 2.1. Thermodynamics For generating electricity from a ZAFC, the overall reaction (1) should be thermodynamically favorable. It is convenient to express the overall reaction in terms of the overall cell electromotive force, Eemf (V), which is defined as the potential difference between a cathode and an anode. The Eemf is related to the electrical work done in a cell. Electrical work done ðWÞ ¼ charge ðQ Þ voltage ðEemf Þ
(2)
where Q = nF, n is the number of electrons which take part in the chemical reaction, and F is the Faraday constant. W ¼ nFEemf
(3)
If there is no loss in the system, the electrical work done is equal to the Gibbs free energy change (DGr). Eemf ¼
DGr nF
(4)
P. Sapkota, H. Kim / Journal of Industrial and Engineering Chemistry 15 (2009) 445–450
In the case of a ZAFC, n = 2. So the equation becomes Eemf ¼
DGr 2F
(5)
2.2. Standard electrode potentials The overall electrochemical reaction taking place in the ZAFC can be analyzed by considering the anode and the cathode reactions in separate. Zn þ 4OH ¼ ZnðOHÞ4 2 þ 2e ZnðOHÞ4 2 ¼ ZnO þ 2OH þ H2 O
Anode :
ðEo ¼ 0:625 VÞ (6)
Cathode :
O2 þ 2H2 O þ 4e ¼ 4OH
ðEo ¼ þ0:40 VÞ
(7)
The overall reaction can be expresses as 2Zn þ O2 ¼ 2ZnO
ðEo ¼ 1:65 VÞ
(8)
In the above equations, Eo represents the standard electrode potential of each reaction with respect to the standard hydrogen electrode at the standard temperature and pressure. The overall cell electromotive force is calculated as Eemf ¼ Eo ðcathodeÞ Eo ðanodeÞ
(9)
so that the overall cell electromotive force of a ZAFC should be theoretically 1.65 V as shown in Eq. (8). The Eemf is a thermodynamic value which does not include internal losses. The open circuit voltage (OCV) is obtained at no load condition and should theoretically be equal to the Eemf. However, the actual open circuit voltage of a practical ZAFC is always less than the theoretical value due to various potential losses and found to be around 1.45 V [1]. Possible reasons for the potential loss will be discussed in Section 5. 3. Design of a ZAFC Various configurations were proposed for ZAFCs. In the beginning stage, a zinc–air system with mechanically refuelable anodes shown in Fig. 2 was proposed [7]. This system can be considered as a zinc–air refuelable battery rather than a ZAFC. A Zn plate which is replaceable is placed in the middle as an anode, and the surfaces of two sides face the oxygen-reduction cathodes, which are often called air-cathodes. Aqueous KOH is used as an electrolyte for transferring ions. The two electrically opposite sides, the anode and the cathode, are separated by a membrane
Fig. 2. Zinc–air system with mechanically refuelable anode.
447
which is ionically conductive but electrically non-conductive. For being a fuel cell, a system should continuously produce the electrical energy as long as the fuel is supplied. So electrically discharging and mechanically refueling of the above system does not support the exact definition of a fuel cell. However the working principle and the concept are same as those of a fuel cell. Several approaches for ZAFCs have been proposed by different research groups. Since the solid Zn and zinc oxide particles are handled in a ZAFC, the clogging problem of unreacted zinc, solid products, and byproducts in the electrolyte is severe. Some of the research works to solve this problem were conducted in a fluidized bed system or a static bed system. The Metallic Power developed a 36 V fuel cell delivering approximately 6 kWh by adopting the concept of the fluidized bed system [8]. However, the clogging problem still remained in this case. The clogging problem by unreacted zinc pellets could be solved by a tapered-end structure [9]. As an example shown in Fig. 3, two non-parallel surfaces are placed with a small vertical angle which produces a difference between two ends, the lower and upper ends. In this approach, the preferable vertical angle is ca. 0.1–38. A continuous flow of electrolyte is maintained from the top to the bottom of the cell. The upper portion of a cell is electrochemically active and called a hopper which acts as a reservoir for the zinc pellets or particles. Once the cell starts to discharge the electricity, the pellets reduce their sizes by the chemical reaction and flow downwards by natural movement. Solid products and byproducts as well as smallsize unreacted zinc particles escape out from the cell along with the effluent of electrolyte. The major disadvantages of this taperedend approach are the presence of inactive volume in the hopper which unnecessarily increases the weight of a cell and makes it bulky, a relatively long filling time of zinc pellets in the hopper, and the high shunt current flowing between the cells during refueling via a feed tube. Pluto et al. suggested an advanced design of ZAFC to overcome the problems encountered in the tapered-end structure [10]. In their design, a slightly leaned anode current collector and a vertical permeable cathode which air can pass through are placed on the two opposite sides of the cell which cover the entire cell area. And the fuel is pumped from the adjacent tank so that almost all space in the cell frame actively works electrochemically. This arrange-
Fig. 3. ZAFC with a tapered-end structure.
P. Sapkota, H. Kim / Journal of Industrial and Engineering Chemistry 15 (2009) 445–450
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is mollified in the cases of mechanically refuelable zinc–air systems and continuous ZAFC systems. 4.2. Cathode
Fig. 4. ZAFC cell designed by Pluto et al.
ment seems useful in reducing the shunt current between unit cells. Furthermore, it maintains a constant level of zinc particles inside the cell. In Fig. 4, the liquid electrolyte containing zinc particles is pumped and supplied to the cell. The vertical baffles help to direct the pellet movement in the cell. Once the active space of the cell is full, the electrolyte containing zinc pellets flows along the paths and returns back to the fuel tank via an overflow exit line. As the cell electrically discharges, zinc pellets get smaller and move downwards. The electrolyte containing zinc oxide particles, the product, and small amount of unreacted zinc pellets passes through a grid mesh placed at the bottom of the cell and is pumped to the recycle tank. The vacant space in the upper part is occupied by new zinc pellets which are fed from the inlet stream. Smedley and Zhang used this concept in their research work with a 12 cellstack providing 1.8 kW [11]. 4. Materials for ZAFC 4.1. Anode In a ZAFC, the anode material is zinc. The shape and size of zinc depend on the cell design. In the mechanically refuelable zinc–air system, a single zinc plate with a current collecting element embedded in it is used as an anode. During refueling, the whole anode assembly is replaced by a new set with a current collecting element. According to the Electric Fuel Ltd., refueling of Zn for ZAFC-powered vehicles can be done in a short time by an automatic refueling machine in a refueling station just like that in a gasoline refueling station [12]. In the case of the cells with tapered ends, proper sizes of zinc pellets are required in order to continuously charge the solid fuel. Proper shape and size of zinc pellets depend on the shape and size of the cell components. The current collecting element on the anode side can be made of any material having high electrical conductivity and high anticorrosion to the electrolyte material. Nickel, copper, silver, and stainless steel are commonly used for the anode. Relatively few research works have been done for improving the anode. Furthermore, most research works on the anode focused on the electrically rechargeable zinc–air batteries and the mechanically refuelable zinc–air systems rather than ZAFCs. In the case of an electrically rechargeable system, irreversibility of the system and corrosion of the elements are the major problems. These could be alleviated by using a zinc oxide electrode or a modified electrode containing zinc oxide and lead oxide [13]. But the harmlessness of lead oxide on the environment is not yet proven. The performance of zinc anode can be further improved by adopting an alloy of Ni (7.5%), Zn (90%) and Ir (2.5%) [14]. Although the problem associated with the zinc anode seems to play a significant role in the case of electrically rechargeable batteries, it
One of the major factors determining the performance of a ZAFC is an air cathode where the oxygen reduction takes place. A typical ZAFC cathode consists of three layers: a current collecting layer, a diffusion layer, and a catalytically active layer. The current collecting layer is simply the metal mesh sandwiched between the diffusion layer and the catalytically active layer. There is no specific metal, but it should be non-corrosive and highly electrically conductive. Metals most commonly used are nickel, gold, silver, copper, silver-plated nickel, silver-plated or nickel-plated iron. The diffusion layer composed of carbon particles and hydrophobic particles needs to be air-permeable but water-impermeable. Graphite for carbon particles and polytetrafluoroethylene (PTFE) for hydrophobic particles are bonded together with or without epoxy resin [15]. A proper ratio can be chosen in a range of 30–70 wt.% PTFE and the balanced amount of conductive carbon [16]. The catalytically active layer is the most important part in the air cathode for a ZAFC. This is the place where the catalytic reduction of oxygen takes place so that a larger active area is desirable from the reaction point of view. The active area consists of catalysts mixed in carbon particles. One advantage of the ZAFC is that it does not require precious and costly metals for catalysts. Inexpensive nonnoble metal oxides such as MnO2 can be used as a ZAFC catalyst [17]. A perovskite type (La0.6Ca0.4CoO3) doped with metal oxides can be used for an oxygen reducing catalyst [18]. Carbon particles supporting the catalyst provide not only a high surface area for the reaction but also electrical paths to electrons. The cell performance depends on the type and the pore size of carbon. Activated carbon with a large number of macropores (>50 nm) or mesopores (2–50 nm) gives high performance [19]. Chao et al. suggested the addition of clay in the air cathode in order to enhance the dispersion of carbon particles and catalyst [20]. Shun and Lou suggested an advanced catalytic layer consisting of carbon particles, high surface area particulates such as molecular sieve or zeolite, hydrophobic particles, and metal hydroxide as catalyst such as nickel hydroxide, cobalt hydroxide, iron hydroxide, cerium hydroxide, manganese hydroxide, lanthanum hydroxide and chromium hydroxide [15]. The active layer can also be prepared by spraying a catalyst material on the commercially available carbon papers or carbon cloths which are used as gas diffusion layers. Zhu et al. proposed a thin air cathode of 0.13–0.50 mm thickness which is prepared by putting carbon particles in a sinter-locked metal fiber network [21]. This could reduce the cathode thickness by 30–75% of commercial air cathodes. Wang Chen developed an air cathode with multi-layers in order to minimize the moisture transfer from the inner electrolyte to the outside air and to preserve the constant water content of zinc anode [22]. This cathode was composed of a metallic mesh as a substrate which is sandwiched by two diffusion layers of carbon and PTFE binder. Different types of catalysts are coated on a diffusion layer one by one to form a multi-layered catalytic surface. For example, a three-layered active surface can be prepared by coating 1 mg/cm2 of MnO2, CoO and MnO2 in the first, the second and the third layers, respectively. Desirable thickness of the air cathode is chosen in the range of 0.01 and 20 mm according to the configuration and the number of layers. The noble Pt can be used as a catalyst material, but its use increases the cost of a cell. 4.3. Electrolyte and separator The electrolyte plays an important role in the transportation of ions. Liquid alkalines are used for ZAFC electrolytes. The most
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commonly used ones are potassium hydroxide (KOH), sodium hydroxide (NaOH) and lithium hydroxide (LiOH). Some articles suggested to use the hydroponics gel as an electrolyte gelling agent due to its enormous capability of storing solution [23,24]. A separator or a membrane needs to be highly ionically conductive but electrically non-conductive. Organic polymers such as a copolymer of polyvinyl alcohol and polyvinyl acetate have been used for the separator [25]. Commercially available Nafion (DuPont Co.), a sulfonated perfluoropolymer, has been widely used as ion exchange membrane because of its high ion exchange capability and chemical stability [26,27]. However, because Nafions are far more expensive than hydrocarbon separators, Dewi et al. suggested the use of polysulfonium membrane, which can prevent the permeation of zinc cations and only allow hydroxide anions to pass through [28]. A hydrophilic polyestersulfone film with micropores of 0.45 mm pore size was also proposed [29]. Wu et al. suggested the polypropylene membrane (Celgard) treated with sulfuric acid, which is usually used in lithium batteries, as a separator for zinc–air electrochemical cells [30].
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5.4. Carbon dioxide absorption As the system is operated with air instead of pure oxygen, CO2 present in the air dissolves in the electrolyte forming carbonate. The formation of carbonate increases the viscosity of the electrolyte and decreases its ionic conductivity. In a ZAFC, this can be minimized by continuously supplying a fresh electrolyte solution. Use of pure oxygen instead of air can decrease the formation of carbonate but increase the operation cost. The problem of carbonate formation can possibly be reduced if an acidic electrolyte is adopted instead of an alkaline [33]. 6. Cell performance evaluation In most cases of electrochemical devices, the performance is usually expressed in terms of power density and a polarization curve even though there are diverse types of methods and bases. The evaluation of a cell normally starts from its open circuit voltage (OCV). The OCV of a ZAFC is so far known to be ca.1.45 V.
5. Factors affecting the performance of ZAFC
6.1. Power density
Even though the theoretically available voltage of a ZAFC is 1.65 V and its OCV is 1.45 V, the practically attainable value is always less than them. There are several factors responsible for the low voltages during operation.
The power density is a specific power normalized to the projected active area of an electrode or to the volume of a system. P P Pd ¼ or P d ¼ A V
5.1. Ohmic losses Ohmic losses occur due to the electrical resistances of electrodes and interconnections, and the resistance to the flow of ions in the electrolyte. The amount of voltage drop (V) depends on the current (i) and the resistances of components (R). This can be expressed as V ¼ iR The ohmic losses can be minimized by using the electrodes with high electrical conductivity and the electrolyte with high ionic conductivity. Aqueous NaOH, KOH or LiOH is preferred as a good electrolyte [31]. 5.2. Activation losses Activation losses result from the slowness of reactions taking place on the surface of the electrodes. In low-temperature fuel cells, the air cathode is primarily responsible for the activation loss [2]. The activation loss increases as the current density increases. And it can be reduced by increasing the active surface area of cathodes, the catalytic activity, or the roughness of the electrodes. Increasing the oxygen concentration by using pure O2 instead of air can also reduce the activation loss, but this is not favorable because of the high cost of O2 and the difficulty in oxygen compression and storage for small portable devices.
where Pd is the power density, P is the total power expressed in watt or milliwatt, A is the area of an electrode in cm2, and V is the volume of a system in cm3. For a ZAFC system, the air cathode area is usually taken as the active area where the electrochemical reaction seems to take place. 6.2. Polarization curves Polarization occurs when the electrical resistance around the electrodes abruptly increases. In a typical polarization curve which can represent the performance of the cell or can indicate certain change in the cell, the voltage change is expressed in terms of the current change or the current density change. The polarization curve exampled in Fig. 5 can be divided in to three zones as follows: (i) Activation loss zone ranging from the OCV at zero current to the initial steep decrease of voltage. (ii) Ohmic loss zone where the voltage slowly drops. (iii) Concentration loss zone where the mass transport effect is dominant and the voltage rapidly falls at high current densities.
5.3. Dendrite formation In the cell stacks where the electrolyte is shared, a shunt current appears. This shunt current is responsible for the formation of zinc dendrite. Even though it is slowly formed, it eventually blocks the flow of electrolyte and zinc particles into the cell. Colbon suggested the construction of dendrite elimination zone to prevent the accumulation of zinc particles in this zone [32]. Use of an acidic electrolyte instead of an alkaline could minimize the dendrite formation [33]. Addition of a small amount of cellulose (1–10% to zinc electrode) showed a positive effect to reduce the formation of zinc dendrite in the case of rechargeable zinc–air batteries [34,35].
Fig. 5. Typical polarization curve of ZAFC.
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In a ZAFC, the activation loss dominantly occurs in the initial stage, and the ohmic loss occurs moderately. The most significant loss occurs when a high current density is applied. If the ZAFC is operated in a middle range of current density, it steadily discharges a constant voltage. In a ZAFC well modified so far, a high current density of 360 mA/cm2 could be sustained for 1000 h [11]. Either a potentiostat or a variable resistor box can be used to set variable external loads. The voltage change is measured when the load decreases (or increases) periodically. The current or the current density is calculated using Ohms law. When the external resistance is varied, the changes of current and voltage should be counted after a temporary pseudo-steady state condition is established in a few minutes. 7. Applications Due to its high specific energy, high power density, cheap and abundantly available fuel, no use of precious metals as catalysts, and no issue of difficulty in fuel storage and transportation, the ZAFC is definitely one promising option for both stationary and mobile applications. Zinc–air batteries are already in practice as a primary battery in small devices like hearing aids. It can replace alkaline or mercury batteries because its energy density is up to five times of these batteries. As the zinc–air batteries, the ZAFC is suitably applied for the areas where a high energy density, the energy per weight, is continuously required. Recently, several companies are involved in development and commercialization of ZAFCs for electric vehicles, indoor power generators, industrial facilities, and military purposes. However, few research groups and companies are so far working on the development of Zn–air systems throughout the world. The Electric Fuel, Ltd. has worked on a zinc–air battery system for electric vehicles, demonstrated the systems for vans, and been developing the systems for buses. The company is also working on the primary and secondary batteries for military uses [12,36–41]. The Power Zinc has commercialized zinc–air batteries for military uses and electric vehicles, and developed first-stage models of ZAFC products [42]. The Power Air Co. has produced various ZAFC systems for mobile device power packs, backup power applications and indoor generators, and has currently demonstrated its ninth generation ZAFC system in 2007 [43]. The Metallic Power Inc. produces ZAFCs as alternative backup or emergency electrical sources for the generators which use internal combustion engines [6]. 8. Conclusions Even though the zinc–air fuel cell system is one of the potential candidates to fulfill the world’s energy requirements, several problems including full exploitation and modification of zinc as a source for the environmentally benign energy production need to be solved for its commercialization. The main problem associated with the ZAFC so far is the performance of an air cathode. A multilayered catalyst electrode can be a plausible option. Improvement of the cell performance can also be achieved by changing the configuration of a cell assembly in such a manner that OH ions can travel a short distance. The addition of ionically conductive materials into the electrolyte can hopefully help to solve the
problems of dendrite formation, carbonate formation and corrosion of electrodes. In order to improve the life-time of a cell, several factors need to be solved simultaneously. After all, the development and commercialization of a ZAFC is one of the interesting research fields for the scientists and engineers who wish to devote to the excavation of future energies. References [1] N.J. Cherepy, R. Krueger, J.F. Cooper, 14th annual Battery Conference on Applications and Advances, IEEE, 1999, 11–13. [2] (a) J. Larminie, A. Dicks, Fuel Cell Systems Explained, 2nd edn, Wiley, England, 2005; (b) M.C. Jo, G.H. Kwon, W. Li, A.M. Lane, J. Ind. Eng. Chem. 15 (2009) 336. [3] (a) EG and G Technical Services, Fuel Cell Handbook, 6th edn, US Department of Energy, West Virginia, 2002; (b) S. Uhm, J.K. Lee, S.T. Chung, J.Y. Lee, J. Ind. Eng. Chem. 14 (2008) 493. [4] Technical/OCM Primary System, Duracell, Procter & Gamble, 2005. [5] G. Koscher, K. Kordesch, J. Power Sources 136 (2004) 215. [6] M.J. Riezenman, IEEE Spectrum 38 (2001) 55. [7] J. Goldstein, I. Brown, B. Koretz, J. Power Sources 80 (1999) 171. [8] S. Smedley, IEEE Aerosp. Electron. Syst. Mag. 15 (2000) 19. [9] J.F. Cooper, US Patent 5,434,020 (1995). [10] M. Pinto, S. Smedley, J.A. Colborn, US Patent 6,706,433 B2 (2004). [11] S.I. Smedley, X.G. Zhang, J. Power Sources 165 (2007) 897. [12] J.R. Goldstein, I. Gektin, B. Koretz, Electric Fuel Ltd, Presented in the 1995 Annual Meeting of the Applied Electrochemistry Division of the German Chemical Society, Duisburg, Germany, 1995. [13] C.W. Lee, S.W. Eom, K. Sathiyanarayanan, M.S. Yun, Electochim. Acta 52 (2006) 1588. [14] C.W. Lee, K. Sathiyanarayanan, S.W. Eom, M.S. Yun, J. Power Sources 160 (2006) 1436. [15] Y.-K. Shun, C.-L. Lou, US Patent 6,127,061 (2000). [16] Y. Kiros, WO/2002/075827 (2002). [17] G.Q. Zhang, X.G. Zhang, Electrochim. Acta 49 (2004) 873. [18] X. Wang, P.J. Sebastian, M.A. Smit, H. Yang, S.A. Gamboa, J. Power Sources 124 (2003) 278. [19] S.W. Eoma, C.W. Lee, M.S. Yun, Y.K. Sun, Electrochim. Acta 52 (2006) 1592. [20] W.K. Chao, C.M. Leea, S.Y. Shieua, C.C. Chou, F.S. Shieu, J. Power Sources 177 (2008) 637. [21] W.H. Zhu, B.A. Poole, D.R. Cahela, B.J. Tatarchuk, J. Appl. Electrochem. 33 (2003) 29. [22] K.Y.W. Chen, US Patent 0,044,640 A1 (2008). [23] R. Othman, W.J. Basirun, A.H. Yahaya, A.K. Arof, J. Power Sources 103 (2001) 34. [24] A.A. Mohamad, J. Power Sources 159 (2006) 752. [25] J. Treger, S. Sargeant, J. Licata, US Patent 6,514,637 B2 (2003). [26] H. Kato, US Patent 7,125,626 B2 (2006). [27] W.K. Chao, C.M. Lee, S.Y. Shieu, C.C. Chou, F.S. Shieu, J. Power Sources 177 (2008) 637. [28] E.L. Dewi, K. Oyaizu, H. Nishide, E. Tsuchida, J. Power Sources 115 (2003) 149. [29] R. Jiang, Rev. Sci. Instrum. 78 (2007) 072209. [30] G.M. Wu, S.J. Lin, J.H. You, C.C. Yang, Mater. Chem. Phys. (in press). [31] T. Burchardt, WO 2007/144357 (2007). [32] J.A. Colbon, US Patent 6,153,328 (2000). [33] R. Clarke, WO 2004/001879 (2004). [34] S. Muller, F. Holzerm, O. Hass, J. Appl. Electrochem. 28 (1998) 895. [35] C.W. Lee, K. Sathiyanarayanan, S.W. Eom, H.S. Kim, M.S. Yun, J. Power Sources 160 (2006) 1436. [36] B. Koretz, Y. Harats, J. Goldstein, M. Korall, Electric Fuel Ltd, Presented in Batterien und Batteriemanagement Conference, Essen, Germany, 1995. [37] B. Koretz, Y. Harats, J. Goldstein, Electric Fuel Ltd, Presented in 12th International Seminar on Primary and Secondary Battery Technology and Application, Deerfield beach, Florida, USA, 1995. [38] B. Koretz, J.R. Goldstein, Electric Fuel Ltd, Presented in the 32nd Intersociety Energy Conversion Engineering Conference, Honolulu, Hawaii, 1997. [39] B. Koretz, N. Naimer, Electric Fuel Ltd, Presented in the 32nd Intersociety Energy Conversion Engineering Conference, Honolulu, Hawaii, 1997. [40] J. Wartman, I. Brown, Electric Fuel Ltd, Presented in the 15th International Electric Vehicle Symposium and Exhibition (EVS-15), Brussels, Belgium, 1998. [41] J. Goldstein, I. Brown, B. Koretz, Electric Fuel Ltd, Presented in the 21st International Power Sources Symposium, Brighton, UK, 1999. [42] Official website of Power Zinc, http://www.powerzinc.com. [43] Official website of Power Air Corporation, http://www.poweraircorp.com.