Electrocatalysts for formic acid-powered PEM fuel cells: challenges and prospects

In view of the drawbacks of rechargeable batteries, such as low mass and volumetric energy densities, as well as slow charging rate, proton exchange membrane fuel cells (PEMFCs) are reckoned to be promising alternative devices for energy conversion. Currently, commercial PEMFCs mainly use H2 as the fuel, but the challenges in generation, storage and handling of H2 limit their further development. Among the liquid fuels, formic acid possesses the merits of low flammability, low toxicity, slow crossover rate, faster reaction kinetics, and high volumetric H2 storage capacity, thus being considered as the most promising energy carrier. It can be used as the energy source for direct formic acid fuel cells (DFAFCs) and formic acid-based H2-PEMFCs, which are also called indirect formic acid fuel cells (IFAFCs). A common issue hindering their commercialization is lacking efficient electrocatalysts. In DFAFCs, the anodic electrocatalysts for formic acid oxidation are suffering from stability issue, whereas the cathodic electrocatalysts for oxygen reduction are prone to poisoning by the permeated formic acid. As for IFAFCs, CO and 2 CO2 impurities generated from formic acid dehydrogenation will cause rapid decay in the catalytic activity. High working temperature can improve the CO and CO2 tolerance of catalysts, but will accelerate catalyst degradation. This review will discuss the mitigation strategies and recent advances from the aspect of electrocatalysts to overcome the above challenges. Finally, some perspectives and future research directions to develop more efficient electrocatalysts will be provided for this promising field.