Bio- and Electrocatalysts for Oxygen Reduction Reaction in Neutral Media: From Mechanisms to Practical Applications

Oxygen reduction reaction (ORR) is a key electrochemical process with significant implications at the industrial level from energy conversion/storage to corrosion protection and production of valuable chemicals. Oxygen is also critically involved in biological systems at the level of the respiratory chain, allowing the development of several biomimicking bioelectrochemical devices encompassing microbial fuel cells (MFCs) and enzymatic fuel cells (EFCs) exploited for power generation, organics transformation, water desalination, biosensing and other applications. Many studies on ORR mechanisms in near-neutral environments for potential integration with bioprocesses are currently ongoing with many phenomena still to be explained, especially concerning biotic and abiotic electrocatalysts. This comprehensive review aims at summarizing the state-of-the-art for each electrocatalyst category, namely: noble-metal/transition-metal-based/carbonaceous electrocatalysts, enzymes, and bacterial cells. In particular, the performances are compared based on their ORR mechanisms, quantitatively discussing the practical limitations, and addressing the technological challenges of their integration in sustainable electronics.