Hierarchical porous single-wall carbon nanohorns with atomic-level designed single-atom Co sites toward oxygen reduction reaction

Hierarchical pore structure is crucial for effective mass transfer and utilization of a number of active sites in single-metal atom catalysts. Here, we present a strategy for developing a hierarchical porous structure in single-wall carbon nanohorns with Co-N_x sites (Co/CNH) and maximizing their oxygen reduction activity. Thermal annealing is effective for generating hierarchical pore by removing amorphous carbons and opening internal and interstitial pore channels. Ammonia annealing modifies coordination structure and relieves local strain around cobalt atoms to form more ideal Co-N₄-C moieties. DFT calculations reveal that the enhanced intrinsic catalytic activity (i_k = 60.16 mA cm⁻² for Co/CNH Air NH₃ vs. 8.24 mA cm⁻² for Pt/C) is attributed to the ligand-push effect of water molecules on the other side of Co-N₄ sites. In a single-cell experiment, a power density of 742 mW cm⁻² was achieved, which is the remarkably high value among M-N-C catalysts using commercialized membrane electrode assemblies (MEAs).