Review—Recent Progress in Electrocatalysts for Oxygen Reduction Suitable for Alkaline Anion Exchange Membrane Fuel Cells

Alkaline fuel cell technology has been reinvigorated since the recent rapid development and deployment of anion exchange membranes. Without the "acid-stability" requirement in low pH environments such as that of proton exchange membrane fuel cells, a much wider range of materials including noble metals, non-noble transition metals, and even metal-free electrocatalysts for the oxygen reduction reaction (ORR) in alkaline media have been developed due to both thermodynamic and kinetic reasons. As compared to the rapidly increasing number of reports on the development of novel catalyst materials, the understanding of the reaction mechanisms of the various ORR electrocatalysts is quite insufficient, and the application and investigation in real alkaline anion exchange membrane fuel cells (AAEMFCs) is even scarcer. By reviewing the compositions, preparation methods, physiochemical properties and ORR performance of different categories of cathodic electrocatalysts that have emerged in the past few years, some common and intrinsic properties and factors that account for the superior activity of these materials may be extracted and summarized, which may further help to identify the reasons for the kinetic facility of the ORR in alkaline media. Some practical issues of utilization of the promising novel replacement materials for the state-of-the-art Pt-based cathodic electrocatalysts in AAEMFCs are pointed out. In addition to the progress on the development of novel materials with outstanding ORR activity, many and varied compositions and morphologies in one, two and three dimensions, scalable preparation technologies, low cost, and other unique properties, some feedback on the performance and especially the problems of their use as cathodes in AAEMFCs is urgently needed. Such feedback should provide guidelines for the design and manufacture of next-generation electrocatalysts and accelerate the application of AAEMFCs.