Single atom catalysts for water electrolysis: from catalyst-coated substrate to catalyst-coated membrane

Green hydrogen production through water electrolysis is considered the next-generation technology capable of industrial-scale hydrogen production to achieve carbon neutrality. The core of constructing a water electrolyzer lies in designing the membrane electrode assembly (MEA) with optimal integration of the membrane, electrocatalysts, and gas diffusion layer. Among the two representative MEA fabrication methods, catalyst-coated substrates (CCS) and catalyst-coated membranes (CCM), CCM shows great promise due to its catalyst layer/membrane interface contact and scalability. The key factor in the CCM method is the effective application of the powdered catalyst onto the membrane. In this respect, the utilization of single-atom catalysts (SACs) has emerged as a noteworthy focus due to their unprecedented catalytic activity resulting from unique electronic/atomic configurations and high atomic utilization efficiency. Incorporating SACs into CCM–MEA has the potential to be a cutting-edge water electrolysis technology. However, it is still in its infancy due to the instability of the components (SACs, membranes, ionomers, supports) and degradation during the SACs–CCM–MEA fabrication and cell operation. Herein, we outline the representative fabrication method of MEA and provide a comprehensive analysis of SACs applicable to MEA. Then, we discuss the advantages of SACs–CCM–MEA and the challenges for industrial hydrogen production. Finally, this review concludes with future perspectives on the development of single-atom catalyst-coated membranes and the expected achievements.