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-N 4 -C moieties. DFT calculations reveal that the enhanced intrinsic catalytic activity ( i k = 60.16 mA cm -2 for Co/CNH Air NH 3 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-N 4 sites. In a single-cell experiment, a power density of 742 mW cm -2 was achieved, which is the remarkable high value among M-N-C catalysts to the best of our knowledge. • We synthesized a hierarchical porous single-wall carbon nanohorns with Co-N x sites. • Annealing played a critical role in the formation of hierarchical porous structures. • A superior activity was achieved, which is 7.3 times higher than commercial Pt/C. • The high catalytic activity originates from ligand push effect of the water to Co-N 4 . • A power density of 742 mW cm -2 was achieved, which is remarkable high performance among SACs.