Atomically Dispersed Pentacoordinated-Zirconium Catalyst with Axial Oxygen Ligand for Oxygen Reduction Reaction

Single-atom catalysts (SACs), affording extraordinary catalytic activity per site, represent promising alternatives to the commercial platinum-based electrocatalysts towards oxygen reduction reaction (ORR). Yet, the common in-plane coordination configuration limits platinum-competitive SACs to few transition metal elements with low metal loading (< 5 wt%). Here, we report the discovery of an ORR-efficient Zr-based SAC (denoted O-Zr-N-C), which consists of unique pentacoordinated Zr sites with nontrivial axial O ligands. The axial O ligand with the desirable electron-withdrawing effect downshifts the d-band center of Zr and confers single-atom Zr sites with stable local structure and proper adsorption capability for O-containing intermediates. Consequently, the ORR performance of O-Zr-N-C electrocatalyst in alkaline condition prominently surpasses that of commercial Pt/C, achieving a half-wave potential of 0.91 V vs. reversible hydrogen electrode, a kinetic current density of 76.0 mA cm–2, and outstanding durability (92% current retention after 130-hour operation). Moreover, the pentacoordinated Zr site shows good resistance towards aggregation, enabling the synthesis of O-Zr-N-C with high single atom Zr loading (9.1 wt%). With the high loading catalyst, the assembled zinc-air battery (ZAB) delivers a record-high power density of 324 mW cm−2 among those of SAC-based ZABs. This work not only provides a new member of ORR-active SACs, but also sheds light on the atomic-level design of advanced electrocatalysts.