Emerging noble metal-free Mo-based bifunctional catalysts for electrochemical energy conversion

The transition from a global economy dependent on fossil fuels to one based on sustainable energy conversion technologies presents the primary challenge of the day. Equipping water electrolyzers and metal-air batteries with earth-abundant bifunctional transition metal (TM) catalysts that efficiently catalyse the hydrogen and oxygen evolution reactions (HER and OER) and the oxygen reduction and evolution reactions (ORR and OER), respectively, reduces the cost and system complexity, while also providing prospects for accelerated scaling and sustainable material reuse. Among the TMs, earth-abundant molybdenum (Mo)-based multifunctional catalysts are especially promising and have attracted considerable attention in recent years. Starting with a brief introduction to HER, OER, and ORR mechanisms and parameters governing their bifunctionality, this comprehensive review focuses on such Mo-based multifunctional catalysts. We review and discuss recent progress achieved through the formation of Mo-based compounds, heterostructures, and nanoscale composites, as well as by doping, defect engineering, and nanoscale sculpting of Mo-based catalysts. The systems discussed in detail are based on Mo chalcogenides, carbides, oxides, nitrides, and phosphides, as well as Mo alloys, highlighting specific opportunities afforded by synergistic interactions of Mo with both non-metals and non-noble metals. Finally, we discuss the future of Mo-based multifunctional electrocatalysts for HER/OER, ORR/OER, and HER/ORR/OER, analysing emerging trends, new opportunities, and underexplored avenues in this promising materials space.