Recent advances in the rational design of electrocatalysts towards the oxygen reduction reaction

The quest for low-cost yet efficient non-Pt electrocatalysts for the oxygen reduction reaction (ORR) has become one of the main focuses of research in the field of catalysis, which has implications for the development of the next generation of greener fuel cells. Here, we comprehensively describe the ‘big picture’ of recent advances made in the rational design of ORR electrocatalysts, including molecule-based, metal-oxide-based, metal-nanomaterial-based and two-dimensional electrocatalysts. Transition metals can fabricate molecular electrocatalysts with N₄-macrocycles such as porphyrin-class compounds and the so-formed M–N–C active centre plays a crucial role in determining the catalytic performances towards the ORR. Group-IV and -V Transition metal oxides represent another class of promising alternative of Pt-based catalysts for the ORR which catalytic activity largely depends on the surface structure and the introduction of surface defects. Recent advances in synthesis of metallic nanoparticles (NPs) allow for precise control over particle sizes and shapes and the crystalline facets exposed to enhance the ORR performance of electrocatalysts. Two-dimensional materials such as functionalized grapheme or MoS₂ are emerging as novel electrocatalysts for the ORR. This review covers various aspects towards the design of future ORR electrocatalysts, including the catalytic performance, stability, durability and cost. Some novel electrocatalysts even surpass commercial Pt/C systems, demonstrating their potential to be alternatives in industrial applications. Despite the encouraging progress, challenges, which are also described, remain to be overcome before the real-world application of novel ORR electrocatalysts.