Anion Exchange Membrane Fuel Cells

Much of the work on Anion Exchange Membrane Fuel Cells (AEMFCs) in recent years has focused on the development of new catalysts and membranes. Though this work is important, it has overlooked mass transport in these systems, which is equally critical to achieving high performance. This chapter provides an overview of aspects related to AEMFC water management and carbonation upon exposure to carbon dioxide. managing both of these are needed in order to achieve high performing fuel cells.

The development of the first practical alkaline anion exchange membrane approximately 10 years ago opened up direct liquid fuel cell research to alkaline media, allowing the use of a wider variety of less noble metal catalysts due to the decreased (or eliminated) corrosion compared to acid media. While Pt-based catalysts are used in many acid fuel cells, Pd has been found to efficiently oxidize various small organic molecules in alkaline media. Much recent progress has been made in the addition of various non-noble admetals and supports in order to improve the oxidation rates of important molecules such as: formate, ethanol, propanol, ethylene glycol, propylene glycol, and glycerol. Evidence of significant contributions from the electronic effect and bifunctional effect on catalyst efficiency will be explored, and practical comparisons of each molecule will be made.

The anion exchange membrane fuel cell (AEM-FC) can potentially be much cheaper than the state of the art proton exchange membrane fuel cells (PEM-FC) for two main reasons. Firstly, the alkaline electrolyte enables the use of non-platinum electrocatalysts and secondly ultra-acid resistant fuel cell components (e.g. current collectors and bipolar plates) are not required. Some scientific and technological challenges must be overcome before AEM-FCs can compete with PEM-FCs. One of the most difficult is the poor kinetics of the hydrogen oxidation reaction (HOR) at high pHs. Consequently, developing non Pt HOR catalysts with high activity is key for improving the power densities of Pt-free fuel cells. In this chapter, we start by considering the mechanisms of the HOR in alkaline media and then review performance data recently reported for both PGM and non PGM HOR electrocatalysts. Emphasis is given to materials that have been used in complete AEM-FC tests.

The direct oxidation of hydrocarbons in the fuel cell has attracted increasing interest as a power source for portable applications as compared to that fed with hydrogen fuel. Hydrocarbons such as methanol, ethanol, ethylene glycol, and glucose exhibit high volumetric energy density, easy storing and delivery system, renewable in nature and are economically and environmentally friendly as compared to hydrogen fuel. Based on the oxidation of the hydrocarbons directly in the fuel cell, they are termed as the direct methanol fuel cell (DMFC), direct ethanol fuel cell (DEFC), direct ethylene glycol fuel cell (DEGFC), and direct glucose fuel cell (DGFC), which are discussed in the chapter. The performance of these fuel cell is better in alkaline electrolyte than that in the acid electrolyte at low temperature (25 °C) and are fascinating owing to use of low-cost alkaline exchange membrane, non-platinum catalyst, no fuel crossover and low CO poisoning. The anode catalysts such as noble (Pt, Ru, Pd) and non-noble metals (Co, Ni), binary and ternary alloys (PtRu, PtPdRu, PdBiRu, PdPtCo), oxides (PdCeO₂, PdNiO), different nanostructures (Pd@Pt) developed for direct oxidation of the hydrocarbons in the presence of different forms of support such as functionalised carbon nanotubes, graphene, metal (N, P, B) doped graphene, ordered mesoporous carbon in alkaline medium at low temperature are discussed. Recent development in the alkaline fuel cell using methanol, ethanol, ethylene glycol and glucose directly as fuel and various anode catalysts with the corresponding reaction mechanism in fuel oxidation in alkaline medium is elaborated. The best performance is achieved in a typical DMFC using PtRu as anode catalyst with maximum peak power density of 168 mW cm⁻² in alkaline medium. The performance of Pd based binary and ternary catalysts are much superior to Pt-based catalysts in DEFC, DEGFC, and DGFC in alkaline medium. A DEFC using PdNi/C as anode catalyst, a cation exchange membrane as electrolyte membrane shows peak power density of 360 mW cm⁻², a DEGFC using PdNi/C as anode catalyst, KOH doped PBI membrane as electrolyte membrane gives peak power density of 112 mW cm⁻² and a DGFC using PdNi/C as anode catalyst, Tokuyama A201 as anion exchange membrane electrolyte demonstrates peak power density of 38 mW.cm⁻². The chapter also includes the detailed comparison of cell parameters (fuel concentration, fuel flow rate, catalyst loading, operating temperature etc.), cell performance, current-voltage characteristics, stability and durability of the alkaline fuel cell using methanol, ethanol, ethylene glycol and glucose as fuel.

Fuel cells technology has attracted much attention owing to their high conversion efficiency in chemical energy to electrical energy with simple structures, clean emissions, insignificant scale effect, etc. Various types of fuel cells were constructed by employing metal, metal oxide and metal complexes as cathodes, where electrochemical H₂O₂ reduction proceeds. Molecular oxygen (O₂) often used as an oxidant for contracting fuel cells is abundant and free of charge, however, oxygen reduction involves four electrons and four protons to form two water molecules that is hard reaction from the kinetic point of view. H₂O₂ produced by two-electron reduction of O₂ is more easily reduced to water by further two-electron reduction (H₂O₂ + 2H⁺ + 2e^– = 2H₂O; E ^o = 1.78 V vs NHE) than O₂ by four-electron reduction (O₂ + 4H⁺ + 4e^– = 2H₂O; E ^o = 1.23 V vs NHE) from both kinetic and thermodynamic points of views. Not only active but also selective cathodes for two-electron reduction of H₂O₂ should be developed to achieve high power fuel cells using H₂O₂ as an oxidant. Herein, suitable catalysts, which are made of metal, metal oxide and metal complexes, for two-electron reduction of H₂O₂ are reviewed.

The modeling work on the alkaline anion exchange membrane (AEM) fuel cell has been greatly facilitated by the rapid development of AEM fuel cell in recent years. Mathematical modeling has been widely recognized as a powerful tool to quantify the physical and electrochemical processes inside the fuel cells. In this study, modeling researches on the AEM fuel cell fed by various fuels have been summarized and discussed. General modeling formulation for AEM fuel cell has been comprehensively introduced. The relevant modeling results with various cell design parameters and operational conditions are revealed and analyzed accordingly. Moreover, the comparison of the operating characteristics of AEM fuel cells fueled by hydrogen (H) and liquid alcohols is also carried out in this study.

Fuel cells that convert the chemical energy stored in a fuel into an electrical energy by electrochemical reactions have been recognized as one of the most promising technologies for the clean energy industry of the future, especially for alkaline direct ethanol fuel cells (DEFCs), because ethanol is less toxic than methanol and can be massively produced from agricultural products or biomass, in addition to the advantage of high specific energy and quicker electro-kinetics of both the ethanol oxidation reaction (EOR) and oxygen reduction reaction (ORR) in alkaline media. A considerable amount of effort has been devoted to the development of alkaline membranes and electro-catalystsin alkaline DEFCs, including synthesis of anion-exchange membrane and electro-catalysts, and the mechanism study of both the anodic EOR and cathodic ORR. For given materials, the improvement of the cell performance, however, depends mainly on the system design. This chapter provides a brief review of the development of alkaline DEFCs from the point of view of the system.

Fuel cells using borohydride as the fuel will be reviewed in this chapter. A direct borohydride fuel cell (DBFC) is a device that converts chemical energy stored in borohydride ion (\({\text{BH}}_{4}^{ - }\)) and an oxidant directly into electricity by redox processes. DBFC has some attractive features such as high open circuit potential, low operational temperature, and high power density. Both electro-oxidation of \({\text{BH}}_{4}^{ - }\) and electro-reduction of oxidant take place on a large variety of precious and non-precious materials. DBFCs share similarities in terms of electrode preparation methods, fuel cell system design, etc. with PEFCs, which have been developed more extensively. Therefore, in this chapter, fuel cell technology, particularly PEFC, will be first reviewed to better understand materials and components of DBFC. Then the chapter continues to discuss prominent features of DBFC, and finally points out potential future direction of DBFC research.

Metal-air fuel cells (MAFCs) are a kind of electrochemical devices that can directly convert the chemical energy stored in metals fuels (e.g., Mg, Al or Zn) or their alloys into electricity. Strictly, MAFCs and metal-air batteries are different, that is, the former one can continue to produce electricity by the metal fuels replacement, and the latter one is only one-time use.

Alkaline direct ethanol fuel cells (DEFCs) that have the quicker electro-kinetics of both the ethanol oxidation reaction and oxygen reduction reaction can yield much better performance than acid direct ethanol fuel cells, even with low-cost non-Pt metals as the electro-catalysts. The liquid-feed alkaline DEFC also possesses the advantages that a direct methanol fuel cell (DMFC) has, including simpler system structures, and fast refueling. Although appealing, many challenges in both material synthesis and system design have to be overcome before extensively commercializing the alkaline DEFCs. In view of these facts, there exists the need to improve the materials and design the novel systems, achieving the high cell performance and durability. This chapter emphasizes on challenges and perspectives in the ethanol electro-oxidation, anion-exchange membrane and the system designs.