Science and engineering for non-noble-metal-based electrocatalysts to boost their ORR performance: A critical review

Metal-air batteries (MABs) and fuel cells (FCs) critically rely on electrocatalytic O₂ activation, and O₂ reduction reaction (ORR), with noble metal-free materials. However, the inception of their synergist reactivity is still unclear due to several electronic and structural limitations. Therefore, the correlation between their science and engineering and their experimental as well as theoretical activity descriptors can pave the way for the development of novel cheap, and efficient catalysts. Moreover, with this framework, several volcanic correlations were established, indicating that catalyst activity increases linearly with increasing binding energy of ORR intermediates up to a certain point, but after that, the activity decreases as binding energy increases. The motivation of this review is to highlight (i) recent designs and developments on non-noble-metal-containing electrocatalysts for ORR, (ii) correlations between science and engineering and existing activity descriptors to improve the electrocatalyst's ORR performance, and (iii) prospects and challenges with non-noble-metal-based electrocatalysts. The “science and engineering” of the electrode materials discussed in this review will aid researchers in selecting and designing ORR electrocatalysts for energy conversion processes.