Single-atom catalysts (SACs) attract significant attention owing to their high catalytic activity, high metal atom utilization efficiency, and well-defined and configurable active sites. However, achieving single-atom dispersion of active metals at high metal loadings remains challenging, limiting the performance of SACs in many practical applications. Herein, we provide a comprehensive review of recent methods developed for synthesizing high-loading SACs, critically exploring their advantages, limitations, and wider applicability. Additionally, we showcase the benefits of high-loading SACs in the oxygen reduction reaction (ORR), water electrolysis, photocatalytic hydrogen production and CO oxidation. Although great recent progress has been made in the synthesis of high loading SACs, simple and universal routes that allowed the pre-programmed preparation of single metal and multi-metal SACs with specific metal coordination need to be discovered.

At present, the catalysts commercially used for the oxygen reduction reaction of the cathode of proton exchange membrane fuel cells (PEMFCs) are carbon-supported platinum-based catalysts. However, the carbon supports are susceptible to corrosion under harsh working conditions, which greatly shortens the life of the catalysts. Highly stable carbon supports are urgently required for high-performance PEMFCs. In this work, we developed structure-stable and highly graphitized three-dimensional network carbon nanofibers (CNF) derived from polyaniline by heat treatment at 1200 °C. The CNF-1200-based catalyst (PtNi/CNF-1200) loaded with PtNi nanoparticles showed excellent stability. After 5000 cycles from 1.0 to 1.5 V in oxygen saturated 0.1 M HClO₄ electrolyte, the losses in the half-wave potential and mass activity were only 5 mV and 15%, respectively, far lower than those of commercial Pt/C. The high graphitization degree of CNF-1200 promotes the corrosion resistance of the catalyst. In addition, nitrogen doping effectively facilitates the catalyst–support interaction, stabilizes the highly dispersed PtNi nanoparticles, and improves the stability and activity of PtNi/CNF-1200.