Highly-dispersed and high-metal-density electrocatalysts on carbon supports for the oxygen reduction reaction: from nanoparticles to atomic-level architectures

Abstract

Electrocatalysts for the oxygen reduction reaction (ORR) are crucial for a variety of renewable energy applications (e.g., proton exchange membrane fuel cells, PEMFCs). The synthesis of highly-dispersed and high-metal-density ORR electrocatalysts (e.g., nanoscale and atomic-level structures) on carbon supports with strong durability is extremely desirable but remains challenging. Carbon-supported high-loading noble metal catalysts with nanoscale structures (e.g., Pt-based nanoparticles) are the most widely used catalysts with the best catalytic performances. Single atom catalysts (SACs) that integrate the merits of homogeneous and heterogeneous catalysts have attracted considerable attention in recent years. Aside from the manipulation of the geometric and electronic structures of active metal sites, another key challenge in this field is the development of strategies for preparing high-metal-density SACs, thus rendering atomic-level ORR electrocatalysts dramatically reactive, selective, and stable compared to their nanoscale counterparts. This review summarizes the recent advancements in carbon-supported nanoscale and atomic-level ORR electrocatalysts with high metal density (namely high loading) for fuel cells. Special emphasis is placed on the basic principles, preparation strategies and catalytic applications of these highly-dispersed and high-metal-density ORR electrocatalysts on carbon supports from nanoparticles to atomic-level architectures.