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.