Hydrogen Electrocatalysis: From Mechanistic Study to Advanced Catalyst Development

Abstract

H2-based polymer membrane fuel cells, electrolyzers and solar hydrogen generators are key components in a sustainable hydrogen economy. In particular, alkaline membrane based devices are promising alternatives to their acidic counterparts owing to the availability of earth-abundant metal catalysts for the oxygen reactions, while such catalysts have not been reported for the hydrogen reactions. Moreover, previous research on the hydrogen oxidation/evolution reactions (HOR/HER) on the state-of-the-art Pt catalysts has shown significantly reduced kinetics in alkaline environment than in acids, which projects an increased Pt loading in alkaline membrane based devices, and impedes their large scale commercialization.[ 1 ] Developing highly efficient and cost-effective catalysts for the HOR/HER in alkaline media requires identification of the true reaction mechanism of the HOR/HER. However there has been a substantial debate regarding the HOR/HER mechanism in strong bases, which has led to the different approaches in the rational HOR/HER catalyst design. A proposed bi-functional mechanism indicates that the co-existence of adsorbed H and adsorbed OH is required for the HOR/HER in bases[ 2 ] while others attribute the slower HOR/HER kinetics in bases to the strengthened hydrogen binding energy (HBE).[ 3 ] Our previous investigation of the HER on a variety of monometallic surfaces in an alkaline electrolyte shows that the exchange current densities follow a volcano type of relationship with respect to the calculated HBE, suggesting that the HBE is the dominating factor for H2 electrocatalysis in bases.[ 4 ] Furthermore, by examining the HOR/HER on Pt in a wide range pH-buffered solutions from 0 to 13, we demonstrate a monotonic relation between the HOR/HER activities and experimentally measured HBEs, showing decreasing HOR/HER activities with increasing HBEs.[ 5 ] This correlation further confirms that HBE is the sole reaction descriptor, and has to take a leading role in the highly efficient and cost-effective catalyst design. Motivated by these findings and the fact that Ni exhibits certain HOR activity in bases, we have developed NiMo bi-metallic and CoNiMo multi-metallic thin film catalysts. These novel thin film catalysts exhibit 20 fold enhancement of HOR/HER activities over pure Ni, owing to their much weakened HBE values.[ 6 ] These promising materials will greatly alleviate the catalyst cost issue raised by higher Pt loading in alkaline electrolytes, and will facilitate the development of alkaline or alkaline membrane fuel cell technology.