Host–Guest Metal Interaction in Cu‐In Single Atom Alloy Switching Electrocatalytic CO₂ Reduction Pathway

Abstract The catalytic behavior of alloy electrocatalyst is strongly influenced by host–guest metal interaction, which governs adsorption energy and product selectivity. However, in conventional bimetallic alloy systems, the catalyst composition and the geometric configuration often obscure the identification of critical active sites. Here, we investigate the host–guest metal interaction in Cu–In single atom alloy (SAA) catalysts, demonstrating a remarkable switching of electrochemical CO 2 reduction reaction (CO 2 RR) pathway. Doping 1% Indium into a Cu matrix forms isolated In‐Cu interfaces, enabling efficient CO 2 ‐to‐CO conversion with a Faradaic efficiency exceeding 90%. Conversely, doping 1% Cu into an Indium matrix leads to the formation of a CuIn alloy phase, shifting the product selectivity to HCOOH with a Faradaic efficiency exceeding 90%. In situ spectroscopic measurements and density functional theory (DFT) simulations reveal that Cu serves as the active site on both Cu–In SAA catalysts. The adsorption energy of host Cu atoms is affected by doped Indium at the In‐Cu interface, which promotes CO 2 adsorption and activation while weakening the binding strength of linearly bonded *CO, thereby enhancing CO selectivity. Conversely, the rigid matrix of the CuIn alloy stabilizes the bridge‐bonded *CO, favoring the production of HCOOH.