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Anion-Exchange Membrane Direct Ethanol Fuel Cells: Status and Perspective

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Anion-Exchange Membrane Direct Ethanol Fuel Cells: Status and Perspective Front. Energy Power Eng. China 2010, 4(4): 443–458 DOI 10.1007/s11708-010-0127-5 FEATURE ARTICLE T.S. Zhao, Y.S. Li, S.Y. Shen Anion-exchange membrane direct ethanol fuel cells: Status and perspective © Higher Education Press and Springer-Verlag Berlin Heidelberg 2010 Abstract Direct ethanol fuel cells (DEFCs) are a Currently, hydrogen is commonly used as fuel to energize promising carbon-neutral and sustainable power source fuel cells, particularly proton exchange membrane fuel for portable, mobile, and stationary applications. However, cells (PEMFCs). However, the production of pure conventional DEFCs that use acid proton-exchange hydrogen is currently expensive, and there are tremendous membranes (typically Nafion type) and platinum-based challenges in transporting, storing, and handling gaseous catalysts exhibit low performance (i.e., the state-of-the-art hydrogen. For this reason, liquid hydrogen-rich alcohol peak power density is 79.5 mW/cm2 at 90°C). Anion- fuels, which have a much higher energy density and are exchange membrane (AEM) DEFCs that use low-cost easier to transport, store, and handle, have become an AEM and non-platinum catalysts have recently been attractive alternative to hydrogen for direct oxidation fuel demonstrated to yield a much better performance (i.e., cells. Among various alcohol fuels, methanol has been the state-of-the-art peak power density is 160 mW/cm2 at considered the most promising primarily because it has the 80°C). This paper provides a comprehensive review of past simplest molecular structure without carbon bonds and is research on the development of AEM DEFCs, including thus easier to oxidize than other alcohol fuels. Hence, the aspects of catalysts, AEMs, and single-cell design and direct methanol fuel cells (DMFCs) have been extensively performance. Current and future research challenges are studied over the past few decades [1–5]. However, in identified along with potential strategies to overcome them. addition to the low performance problem associated with sluggish anode reaction kinetics [6–8] and methanol Keywords fuel cell, direct ethanol fuel cells, anion- crossover [9–11] in DMFCs, the inherent disadvantage of exchange membrane, ethanol oxidation reaction, oxygen methanol is its toxicity, which makes the fuel unsustainable reduction reaction, cell performance and unsafe for consumer use. In contrast, ethanol is much less toxic than methanol; it has higher energy density (8.0 kW$h/kg) than methanol (6.1 kW$h/kg) and can be 1 Introduction produced in great quantity from biomass through a fermentation process from renewable resources, such as The growing global energy demand and the large-scale use sugar cane, wheat, corn, or even straw. More importantly, of CO2-emitting fossil fuels pose a great threat to our the CO2 emitted during the energy production process already damaged planet. Hence, developing renewable from ethanol can be soaked up by growing plants from energy sources at a global scale is urgent for the sustainable which the fuel is produced. Hence, ethanol is a sustainable, development of our society. Fuel cells have been identified carbon-neutral fuel source, and direct ethanol fuel cells as one of the most promising technologies for the clean (DEFCs) are clearly the best choice for producing energy industry of the future. Fuel cells convert the sustainable energy in the future. chemical energy stored in fuel, such as hydrogen, into Past research on the development of DEFCs has focused electrical energy output through electrochemical reactions. mainly on the so-called PEM DEFCs that use PEM as the electrolyte, a Pt-based catalyst on the anode, and a pure Pt catalyst on the cathode [12–18]. The advantage of PEM Received August 3, 2010; accepted September 14, 2010 DEFCs is that the entire system setup and all the ✉ components can be the same as those in current DMFC T.S. Zhao ( ), Y.S. Li, S.Y. Shen technologies that also use PEMs. With regard to the Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Kowloon, Hong Kong SAR, China catalysts in PEM DEFCs, previous studies have demon- E-mail: [email protected] strated that the catalytic activity of PtRu- and PtSn-based 444 Front. Energy Power Eng. China 2010, 4(4): 443–458 catalysts, both binary and ternary, on the ethanol oxidation 2 General setup and working principle reaction (EOR) in an acid media is higher than that of pure Pt catalysts due to the bifunctional mechanism and Figure 1 illustrates a typical liquid-feed AEM DEFC setup electronic effect [12]. By combining Pt with Ru, the that consists of a membrane electrode assembly (MEA) oxidation of strongly bound adsorbed intermediates is sandwiched by an anode and a cathode bipolar plate. The enhanced to give a relatively high yield of CO2 [13]. MEA, which is regarded as an integrated multi-layered Recently, Basu et al. [14] systematically studied the effect structure, is composed sequentially of an anode diffusion of operating parameters on the performance of a PEM layer (DL), an anode catalyst layer (CL), an AEM, a DEFC with PtRu as the anode catalyst and showed a peak cathode CL, and a cathode DL. The function of the power density of 10.3 mW/cm2 at 90°C. Zhou et al. [15] membrane is to conduct hydroxyl ions from the cathode to showed that the sequence of catalytic activity for the EOR the anode and act as a separator between the anode and is PtSn > PtRu > PtW > PtPd > Pt, indicating that PtSn- cathode electrodes. Typically, quaternized hydrocarbon based catalysts are better than the PtRu based, which is the AEMs (e.g. A201 by Tokuyama) are used in AEM DEFCs state-of-the-art anode catalyst for DMFCs. However, the [27]. The CLs at both the anode and cathode are usually catalytic activity of PtSn was found to vary with the comprised of catalysts mixed with ionomer to provide preparation method [16] and the content of both alloyed triple-phase boundaries for the EOR and ORR. The DLs and non-alloyed Sn [17]. Léger et al. [18] synthesized PtSn consist of two layers, a backing layer (BL) made of carbon catalysts and determined the optimum composition of Sn cloth or carbon paper, and a micro-porous layer (MPL) to be in the range of 10%–20% (atom fraction). These composed of a hydrophobic polymer and carbon powder. catalysts achieved a peak power density of about 28 mW/ The function of each DL is to provide support for the cm2 at 90°C. corresponding CL, to distribute evenly the reactants over In summary, although tremendous efforts have been the CL, and to conduct electricity to the current-collector. expended on developing EOR catalysts for PEM DEFCs, On the anode, the ethanol solution flowing into the anode the performance of PEM DEFCs remains low even at flow field is transported through the anode DL to the anode relatively high operating temperatures, such as at 90°C. CL, where ethanol is oxidized to generate electrons, water, The low performance of this type of fuel cell is mainly and CO2 according to because the kinetics of the EOR in acid media is slow, þ – ↕ ↓ þ þ – leading to a large activation loss. Another critical obstacle CH3CH2OH 12OH 2CO2 9H2O 12e (1) that limits the wide application of PEM DEFCs is the cost The water in ethanol solution, along with that produced of the system, not only because a considerable amount of from the EOR, diffuses through the membrane to the Pt-based catalysts at both the anode and cathode is required cathode CL, while the electrons travel through an external but also because acid electrolyte membranes (typically circuit to the cathode. On the cathode, the oxygen/air fi Na on) are expensive. provided by the cathode flow field is transported through Unlike in acid media, the kinetics of both the EOR and the cathode DL to the cathode CL, where the oxygen reacts oxygen reduction reaction (ORR) in alkaline media is with water from the anode to produce hydroxide ions much faster and allows the use of non-precious metal according to catalysts to reduce the cost of the fuel cell. Similar to the þ þ – ↕ ↓ – use of PEM in acid fuel cells, the use of an anion-exchange 3O2 6H2O 12e 12OH (2) membrane (AEM) instead of a caustic alkaline liquid Subsequently, the generated hydroxide ions are conducted electrolyte in an alkaline fuel cell can solve carbonation to the anode for the EOR, as indicated by Eq. (1). The ideal and other related problems, such as electrolyte leakage and overall reaction in the AEM DEFC is precipitation of carbonate salts. AEM DEFCs have þ ↕ ↓ þ recently attracted increasing attention due to the reasons CH3CH2OH 3O2 2CO2 3H2O (3) cited above [19–26]. This article provides a comprehensive review of the past research on the development of AEM However, it should be noted that with the state-of-the-art DEFCs, including the aspects of catalysts, AEMs, and anode catalysts, the complete oxidation of ethanol single cell systems. Current and future research challenges expressed by Eq. (1) is not achieved. The main product are identified along with the potential strategies to of the reaction is acetic acid rather than CO2 [19], i.e., overcome them. The remainder of this paper is organized þ – ↕ ↓ þ þ – CH3CH2OH 4OH CH3COOH 3H2O 4e (4) as follows: Section 2 describes the general setup and working principle of the AEM DEFC; Sections 3–6 review In the present AEM DEFC system, to obtain appreciable the current status of this type of fuel cell, including AEMs, kinetics of the EOR and ionic conductivity of AEMs, catalysts for both the EOR and ORR, and the single-cell additional alkali needs to be added to the ethanol solution.
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