Anion exchange membranes for alkaline fuel cells: A review

Abstract

Recent years have seen extensive research on the preparation and properties of anion exchange membranes. Nevertheless, there is as yet no rigorous scientific classification of these membranes, and the methods of synthesis and characterization. The present review offers a practical classification based on the nature and the properties of anion exchange membranes for alkaline fuel cells, arrived at studying the relevant literature. This review also contains a description and assessment of all polymeric materials potentially suitable for use in alkaline fuel cells, and of their specific properties. Although there is ample literature on anion exchange membranes for various other applications, such as electrodialysis, the number of publications reporting alkaline fuel cell performance is still relatively low compared to their acidic homologues, the proton exchange membrane fuel cell. Two tables at the end of the manuscript offer the reader a comprehensive overview by listing all reviewed commercial and non-commercial anion exchange membranes. Suggestions for further research such as elucidation of the ionic transport mechanisms, AFC testing and important issues like the chemical stability and ionic conductivity are addressed as well.

Introduction

Fuel cells are widely thought of as the environmentally friendly energy sources for the 21st century. Their development has undergone successive cycles of interest and disinterest. Whenever there was a renewed interest in fuel cells, it was usually sparked by major scientific advances and discoveries, or political and economic imperatives [1]. The current growing concerns about global pollution and increasing use of electricity cause an increased drive to find less polluting energy sources of non-fossil origin. One attractive alternative is the conversion of chemical energy into electrical energy.

Fuel cells are one of the oldest energy conversion technologies, but the exact origin of their invention is not clear. According to the Department of Energy of the United States [2], Christian Friedrich Schönbein discovered the concept; his work was published in Philosophical Magazine in the January issue of 1839 [3]. According Grimes [4], Sir William Grove who also published his work in Philosophical Magazine in 1839 [5], but in February devised the fuel cell [6].

Nowadays, five major types of fuel cells can be distinguished by the type of electrolyte: alkaline fuel cells (AFCs), proton-exchange membrane fuel cells (PEMFCs), solid oxide fuel cells (SOFCs), phosphoric acid fuel cells (PACFs) and molten carbonate fuel cells (MCFCs). The AFC (using aqueous KOH as electrolyte) was the first type to be put into practical service, at the start of the 20th century [7], [8], [9]. The AFC made it possible to generate electricity from hydrogen. In the 1950s, the NASA Apollo space program started using AFC systems and this technology is still used for today's shuttle missions.

Many research groups started to focus on AFCs for other applications. By the 1970s, a car had been built, by Kordesch [10], [11], [12], [13], [14], [15], [16], [17], [18], which ran on alkaline fuel cells combined with a lead-acid battery. Despite its early success, interest in AFC technology then dropped owing to economic factors, material problems, and certain inadequacies in the operation of electrochemical devices [19]. Discoveries and major scientific advances (especially with regard to PEMFCs) in the past two decades have created renewed interest in AFCs. Some previous limiting requirements such as the use of essentially pure fuels have been overcome by using a polymer membrane as an electrolyte. Expectations are that, in a some years’ time, the alkaline polymer electrolyte fuel cell will be used in numerous power applications, ranging from portable power and vehicle propulsion to distributed power generation.

The AFC is an electrochemical device that can convert the chemical energy of H₂ directly into an electrical current. In principle, the direction of the reactions at the electrodes is the reverse of that in alkaline water electrolysis. AFCs use a liquid electrolyte solution of potassium hydroxide (KOH) because it is the most conducting of all alkaline hydroxides. The hydrogen charged on the anode reacts with hydroxyl anions generating water and electrons. The electrons are transferred through an external circuit to the cathode, where the oxygen reacts with water to generate hydroxyl ions. The overall reactions are given by:

$$\begin{array}{ll}\text{Anode reaction}\hfill & 2{\text{H}}_2+4{\text{OH}}^{-}\to 4{\text{H}}_2\text{O}+4{\text{e}}^{-}\hfill \\ \text{Cathode reaction}\hfill & {\text{O}}_2+2{\text{H}}_2\text{O}+4{\text{e}}^{-}\to 4{\text{OH}}^{-}\hfill \\ \text{Overall cell reaction}\hfill & 2{\text{H}}_2+{\text{O}}_2\to 2{\text{H}}_2\text{O}\hfill \\ \hfill & +\text{electrical energy}+\text{heat}\hfill \end{array}$$

AFCs offer some advantages over other fuel cells, such as that they are easier to handle as the operating temperature is relatively low (roughly 23–70 °C). Another advantage is the higher reaction kinetics at the electrodes than in acidic conditions, of for example the PEMFC, resulting in higher cell voltages. This high electrical efficiency permits the use of a lower quantity of a noble metal catalyst, like platinum which is expensive.

One of the main drawbacks of the AFC is related to the use of the liquid electrolyte [20], [21], [22]. The KOH solution is very sensitive to the presence of CO₂. A major operating constraint is therefore the requirement for low CO₂ concentrations in the feed oxidant stream. When air is used instead of oxygen, the hydroxyl ions may react with CO₂ contained in the air [20], [23] and form K₂CO₃ according to the following reaction:

CO₂ + 2OH⁻ → CO₃⁻ + H₂O

and/or

CO₂ + 2KOH → K₂CO₃ + H₂O

The main cause of the decreasing performance on carbonate formation is the precipitation of large metal carbonate crystals such as K₂CO₃ (equation above). First of all, this reaction decreases the number of hydroxyl ions available for reaction at the anode. Furthermore, it modifies the composition of the electrolyte and thus reduces its ionic conductivity; Gülzow [19], [20] has shown that the CO₂ does not cause any degradation of electrodes, however. If the electrolyte is highly concentrated, carbonate precipitation may block the pores of the gas diffusion layer [24]. Another disadvantage is related to the amount of liquid electrolyte: if the liquid is in excess or if there is a lack, it leads to electrode flooding or electrode drying.

Because of this requirement of pure fuels as feed oxidant stream to eliminate the presence of CO₂, terrestrial applications such as vehicle propulsion are limited. Some efforts have been made to rectify the problem of CO₂-poisoning such as the use of a circulating electrolyte or of liquid hydrogen to condense the CO₂ out of air; nevertheless, most of the current strategies for solving the CO₂ poisoning issue in AFC are inadequate for commercialization. Cheng et al. [25] worked with a PEMFC and described the possibility and efficiency of using a solid polymer electrolyte to replace the liquid electrolyte. Since then, research has focused on the promising development of an AFC based on anion-conducting polymer electrolytes to replace the KOH solution [26], [27], [28]. This led to an impressive growth in the number of publications related to AFCs over the last thirty years (see Fig. 1).

Fig. 2 shows a schematic view of an AFC fuel cell with a polymer electrolyte membrane. In this new design, the membrane plays the roles of separator and conductive support between the anode and cathode. This structure, the membrane electrode assembly (MEA), sandwiches the membrane between the two electrodes, which include the catalyst layer and the gas diffusion layer. The reaction scheme for the solid electrolyte fuel cell is the same as for the liquid cell; hydrogen oxidation and oxygen reduction take place at the anode and the cathode, respectively.

The most important advantage of using a membrane instead of a liquid electrolyte is, as mentioned, elimination of the negative effects of CO₂. The conducting species is now in a fixed solid polymer; therefore there will be some carbonates due to the reaction of the –OH with CO₂ but because there are no mobile cations (K⁺), solid crystals of metal carbonate will not be formed to block the gas diffusion electrodes. Furthermore, no liquid caustic is present; hence electrode weeping and corrosion are minimized. Additional benefits include leakproofness, volumetric stability, solvent-free conditions, and easy handling. The size and weight of the fuel cell are reduced which enlarges the domain of application. The main idea behind employing an anion exchange membrane (AEM) in an AFC is to improve the AFC's efficiency and life (slow down performance degradation with time).

This review is structured as follows. Section 2 follows the introduction and covers basic principles, membrane properties, transport mechanisms, requirements for the membranes, and characterization methods. Section 3 deals with preparation and properties of the AEMs for AFCs. The AEMs are presented in three categories: heterogeneous membranes, interpenetrating polymer networks and homogeneous membranes. Their structures, characterization, performance and encountered problems are discussed. AEMs have numerous other applications such as in electro-dialysis and desalination. As the research concerning AEMs for use in AFCs is less advanced, this review also covers AEMs currently used in other applications, if their chemical and thermal stability makes them potential candidates for application in AFCs.

Section 4 offers a summary and discussion, as well as two tables with an overview of AEMs. Table 2 lists non-commercial AEMs described in Section 3, whereas Table 3 lists all commercial AEMs. Finally, conclusions and some guidelines for future work are presented in Section 5.