Porous metal oxide electrocatalytic nanomaterials for energy conversion: Oxygen defects and selection techniques

Energy conversion technologies including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and carbon dioxide reduction reaction (CO2RR) remain cleaner and sustainable approaches to replace the finite fossil fuels and its consequences in a global world. Accelerating the conversion efficiencies of these systems requires the development of highly efficient electrocatalysts and the appropriate selection of electrolytes and conversion parameters. Porous metal oxides are a novel emergent class of electrocatalysts that are currently attracting wider attention for energy conversion applications. This study presents recent advancements in porous metal oxide (MO) electrocatalysts for different energy conversion technologies. Distinctly, it begins with discussions of the evolution of porous material sciences and atomic arrangements followed by the importance of oxides in electrocatalysis. A brief overview of the synthesis and different analytical techniques for porous metal oxides, and various experimental parameters for evaluating the performance of these electrocatalysts are provided. Recent progress in HER, OER, ORR, and CO2RR with their corresponding fundamental concepts, mechanisms, electrocatalytic performances, key challenges, and prospects were given for each category of energy conversion unlike the previous studies. This study further highlighted some critical roles of oxygen defects in metal oxide electrocatalysts and to bridge the knowledge gaps of previous reviews, the commendations on selecting techniques for energy conversion are also discussed. Conclusively, some vital outlooks for the advancement of the field toward industrial realization were discussed.