Unconventional noble metal-free catalysts for oxygen evolution in aqueous systems

The oxygen evolution reaction (OER) is a known impediment in the development of electrochemical energy conversion and storage devices such as water-splitting electrolyzers and rechargeable metal–air batteries. The intrinsically slow OER kinetics can only be mitigated by effective catalysis. The search for low-cost alternatives to the conventional noble metal-based catalysts is a research priority and has thus far been focused mostly on metal oxides. It is the purpose of this review to outline the opportunities and available options besides the noble metals and metal oxides. These unconventional catalysts include transition metal phosphates, borates, chalcogenides, phosphides, nitrides and borides as well as metal-free carbon-based materials. They are all based on earth-abundant elements with some of them showing higher catalytic performance than the common metal oxides in aqueous solution. The review begins with the introduction of the evaluation criteria for OER catalysts. The development and breakthroughs in the unconventional catalysts are then succinctly summarized with discussion of some current scientific issues. We then present our perspectives on these issues and suggest some areas of further work. We hope this review can raise the interest in less common options to broaden the search for practical OER catalysts to beyond the metal oxides.