Recent Progress and Viability of PGM-Free Catalysts for Hydrogen Evolution Reaction and Hydrogen Oxidation Reaction

The field of precious metal group (PGM)-free hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) catalysts grew rapidly in recent years. There are several different approaches, from theoretical calculations and engineering of known materials. The mechanisms for HER and HOR can be broken down into distinct elemental steps. The difference between HER and HOR is the direction in which the reactions occur. In most PGM-free catalysts, the pathways may differ, the exact active site is not necessarily known and might consist of more than one atom or an edge site. Hence, it cannot be assumed that the mechanism will follow the exact same pathway, and more theoretical calculations and experiments are needed to determine the mechanism for each catalyst. PGM-free HOR catalysts in AEMFC can be divided into three classes, namely, metals and alloys; bioinspired macromolecules; and ceramics (mostly metal nitrides and oxides). A summary of PGM-free HER catalysts is provided. A true performance comparison of different catalysts is often difficult to attain especially when coming to ascertain the state-of-the-art, mostly due to lack of uniformity in reporting performance parameters and diversity in specific active site and site density estimation. Development of standard testing protocols, performance parameters, and benchmarks must be done to allow a fair comparison between PGM-free catalysts, which will surely help pave the way to their further development.