Reconstructed Bi‐Sn Interfacial Sites Coupled with a Supramolecular Cation Cage Enable Low K ⁺ Acidic CO ₂ Electroreduction to Formate

ABSTRACT

Here, we demonstrate selective carbon dioxide reduction reaction (CO ₂ RR) to formic acid (HCOOH) conversion under highly acidic conditions and low K ⁺ concentration using a carbon‐supported Bi‐Sn bimetallic electrocatalyst (Bi‐Sn/C) in combination with 2.2.2‐cryptand for cation regulation. Thermal reduction of a Bi ₂ Sn ₂ O ₇ precursor yields an interface‐rich Bi‐Sn architecture with strong electronic coupling between Bi and Sn domains. Density Functional Theory (DFT) calculations reveal that the Bi−Sn synergy arises from a bidirectional electronic interplay: Sn enhances the activity of Bi for ^* OCHO generation, while Bi regulates Sn for the conversion of ^* OCHO to ^* HCOOH. This cooperative mechanism enables CO ₂ RR to outcompete the hydrogen evolution reaction under highly acidic conditions. We further introduce supramolecular cation regulation as a complementary strategy to enhance interfacial alkali‐metal ion effects. DFT calculations reveal that molecular cages such as 2.2.2. cryptand and 18‐crown ether‐6 selectively bind K ⁺ ions at the catalyst‐electrolyte interface, stabilizing CO ₂ ‐derived intermediates while suppressing proton adsorption. As a result, cryptand‐modified Bi‐Sn/C achieves a HCOOH faradaic efficiency of 94% at pH = 1, with only 50 mM K ₂ SO ₄ concentration. Response surface methodology further identifies cryptand loading as the dominant parameter governing selectivity, with a greater influence than applied potential and pH.