Carbon-free fuels for direct liquid-feed fuel cells: Anodic electrocatalysts and influence of the experimental conditions on the reaction kinetics and mechanisms

Direct fuel cells fed with liquid carbon-free fuels (borohydride, ammonia-borane, hydrazine) present a number of benefits compared to state-of-the-art proton-exchange membrane fuel cells, among them ease of fuel transportation and distribution, high volumetric energy density, high theoretical cell voltage, and number of transferred electrons n > 2. However, taking full advantage of these benefits requires highly active anodic catalysts, which allow efficient fuel valorization at close-to-equilibrium potentials. This requires understanding the complex mechanisms of the multi-electron fuel oxidation reactions and the main factors affecting reaction rates and product selectivities. This review offers a state-of-the-art understanding of borohydride, ammonia-borane, and hydrazine oxidation on noble metal and noble metal-free catalysts both in half- and in full-cell configuration. Electrochemical data are complemented with coupled physicochemical techniques and numerical calculations to unveil the main intermediates and co-products and the influence of the different experimental factors on the reaction kinetics and mechanisms.