Synergy of Ni Nanoclusters and Single Atom Site: Size Effect on the Performance of Electrochemical CO₂ Reduction Reaction and Rechargeable Zn−CO₂ Batteries

The design of bifunctional electrocatalysts toward reduction reaction of carbon dioxide (ECO₂RR) and oxygen evolution reaction (OER) in aqueous rechargeable Zn─CO₂ batteries (ZABs) still poses a significant challenge. Herein, Ni clusters (Ni_x) of 0.5 and 0.8 nm in diameter coupled with single Ni site (Ni−N₄−C), denoted as Ni−N₄/Ni₅ and Ni−N₄/Ni₈, respectively, are synthesized and the size effect of Ni nanoclusters are studied. Ni−N₄/Ni₅ exhibits an ≈100% Faradaic efficiency (FE_CO) toward ECO₂RR for CO from −0.4 to −0.8 V versus the reversible hydrogen electrode, superior to that of Ni−N₄−C (FE_CO = 55.0%) and Ni−N₄/Ni₈ (FE_CO = 80.0%). The OER performance of Ni−N₄/Ni₅ and Ni−N₄/Ni₈ are superior or comparable to that of commercial RuO₂ but outperform that of Ni−N₄−C. Theoretical calculation indicates that ^*COOH of ECO₂RR intermediates bond synergistically with Ni_x clusters and Ni−N₄−C single atom site, promoting the activation of CO₂ and reducing the energy barrier of the potential determining step of ECO₂RR. Such effect is strongly size‐dependent and larger Ni_x nanoclusters result in too strong binding of ^*COOH intermediates, impede the formation of ^*CO. As a bifunctional cathode electrocatalyst of rechargeable alkaline aqueous ZABs, Ni−N₄/Ni₅ exhibits a peak power density of 11.7 mW cm⁻² and cycling durability over 1200 cycles and 420 h.