Design and structural engineering of single-atomic-site catalysts for acidic oxygen reduction reaction

Proton-exchange membrane fuel cells (PEMFCs) are a promising new energy-conversion technology due to their safe and environmentally friendly characteristics in a wide range of operating conditions. However, catalysis of the acidic oxygen reduction reaction (ORR) at the cathode is the main obstacle. In this regard, platinum-free single-atomic-site catalysts (SACs) with designable active sites are a potential solution. Unlike catalysts for the ORR in alkaline media that have been widely explored, there is still a gap between the performance of SACs and Pt-based catalysts in acidic media. In this review, we introduce the latest progress of SACs in acidic ORR. SACs consist of metal centers and substrates. We conclude controllable synthetic strategies for SACs from single-atomic sites and substrate design, discuss the relationship between the structure of single-atom sites and acidic ORR performance of SACs at the atomic level, and propose challenges and perspectives of SACs in acidic ORR.