Dynamically Stable Cu⁰Cu^δ+ Pair Sites Based on In Situ-Exsolved Cu Nanoclusters on CaCO₃ for Efficient CO₂ Electroreduction

Copper-based catalysts are the choice for producing multi-carbon products (C₂₊) during CO₂ electroreduction (CO₂RR), where the Cu⁰Cu^δ+ pair sites are proposed to be synergistic hotspots for C-C coupling. Maintaining their dynamic stability requires precise control over electron affinity and anion vacancy formation energy, posing significant challenges. Here, we present an in situ reconstruction strategy to create dynamically stable Cu⁰Cu^0.18+OCa motifs at the interface of exsolved Cu nanoclusters and CaCO₃ nanospheres (Cu/CaCO₃). In situ XAFS analysis confirmed the low-valency state of Cu^δ+ during CO₂RR. DFT calculations demonstrated that the nanocluster size arises from the balance between metal-support interactions and Cu-Cu cohesive energy, while the dynamic stability of rich interfacial Cu^δ+ sites is attributed to their low electron affinity and high CO₃ ^2- vacancy formation energy, which collectively contribute to reduced reducibility. The transformed Cu/CaCO₃ exhibits an impressive C₂₊ Faradaic efficiency of 83.7 % at a partial current density of 393 mA cm^-2, facilitated by adsorption of *CO with varying electronegativity at heterogeneous copper sites that lowers the C-C coupling energy barrier. Our findings establish insoluble carbonate as an effective anion pairing for Cu⁰Cu^δ+ sites, highlighting the effectiveness of the in situ reconstitution strategy in producing a high density of dynamically stable Cu⁰Cu^δ+ pair sites.