Synergistic Ni-Nx Single-Atom and Ultrasmall Cluster Hybrid Sites Enable Tandem CO2 Activation and Proton-Coupled...

Single-atom Ni catalysts on nitrogen-doped carbon are promising for CO2 electroreduction to CO, yet simultaneously achieving high selectivity at industrial current densities and understanding the synergistic role of hybrid active phases remain challenging. Herein, a self-supporting carbon aerogel electrode (NCA) featuring atomically dispersed Ni-Nx sites coexisting with ultrasmall Ni clusters is reported, constructed via a urea-mediated in situ coordination strategy. The Ni-Nx single-atom sites serve as the primary centers for CO2 activation and *COOH intermediate stabilization, while the adjacent ultrasmall Ni clusters facilitate electron transfer and water dissociation for the proton supply. In situ FTIR spectroscopy reveals that pyridinic N-H+ species act as proton relays, accelerating the protonation of *COOH intermediates via a tandem proton-coupled electron transfer pathway. X-ray absorption fine structure analysis confirms the Ni-N2-C2 coordination with second-shell Ni-Ni bonding characteristic of coexisting clusters. The resulting electrode achieves a CO Faradaic efficiency of 97% at -0.73 V vs RHE, maintains >85% FEco over a 0.5 V potential window, and retains >90% even at 200 mA cm-2 in a flow cell. These results demonstrate how the electronic synergy between single atoms and clusters, combined with a proton relay mechanism, cooperatively enhances CO2 electroreduction.