Discover the Evolution: A Comprehensive Review of Transition and Rare Earth Metals for Oxygen Reduction Reaction, from Mono to High-Entropy Catalysts.

Green energy, including metal-air batteries and fuel cells, is the key solution to climate change. The efficiency of these energy technologies depends on the oxygen reduction reaction (ORR) at the cathode, which is a slow process requiring expensive noble metal catalysts, like platinum, for improvement. The high cost of this catalyst restricts its widespread use in producing metal-air batteries and fuel cells. An alternative approach is to utilize non-noble metals, such as transition and rare earth metal catalysts, which are more cost-effective and demonstrate comparable durability and effectiveness to noble metals. With their affordability and distinct electronic structure, these non-noble metals have the potential to revolutionize the industry. Transition and rare earth metals can enhance the effectiveness of ORR catalysts by manipulating the electronic and surface molecular makeup through 'doping' and 'synergistic effects'. This article discusses the roles of various non-noble metals in the ORR process, covering fundamental to advanced levels, as well as the progression from mono to high-entropy systems (systems with increasing complexity and potential for improved performance), including bi-, tri-, and tetra-metallic catalysts in a comprehensive manner, and emphasizes opportunities for researchers to propose innovative strategies for optimizing the ORR process.