Ultrasmall Nickel Nanoclusters Accelerating Protonation for Efficient CO₂ Electroreduction towards CO

Proton-coupled electron transfer (PCET), particularly the protonation step is widely recognized as the kinetic bottleneck in electrochemical CO₂ reduction (CO₂RR). Modulating catalyst microstructures to accelerate protonation has thus emerged as a promising strategy to boost from CO₂ to CO selectivity. Here, we report ultrasmall Ni nanocluster catalysts (denoted as Ni₃─N─C) prepared via one-step pyrolysis of Ni-containing precursors under H₂ atmosphere. Compared to conventional Ni─N─C with symmetric Ni─N₄ motifs, Ni₃─N─C displays similar physicochemical characteristics-Ni loading, defect density, surface area-yet exhibits distinct local Ni coordination environments. These sub-nanoclusters markedly enhance CO₂RR performance, delivering > 90% CO Faradaic efficiency (FE_CO) across -0.6 to -1.0 V versus RHE, with a peak FE_CO of ∼95% at -0.8 V. Density functional theory calculations reveal that Ni₃─N─C substantially lowers the energy barrier for *COOH formation owing to altered adsorption configurations, thereby facilitating the rate-limiting protonation step. In situ FTIR measurements further confirm the accelerated *COOH formation on Ni₃─N─C surfaces. This work highlights the critical role of Ni sub-nanoclusters in PCET modulation and establishes a rational design principle for nanocluster-based catalysts in CO₂RR.