# Restructuring‐Regulated Bismuth Catalyst Promotes Electrochemical CO2 Reduction to Formic Acid in Acidic Electrolyte

## TL;DR

A new bismuth catalyst improves electrochemical CO2 reduction to formic acid in acidic conditions by maintaining structural stability during the reaction.

## Contribution

The study reveals distinct restructuring mechanisms in Bi-based oxysulfides and identifies a high-performance catalyst for acidic CO2 reduction.

## Key findings

- Bi9O7.5S6 shows high Faraday efficiency and stability for formic acid production in acidic electrolytes.
- The Bi9O7.5S6 structure resists decomposition and forms an active Bi/Bi2O2CO3 interface during electrocatalysis.
- Bi2O2S performs poorly due to over-reduction and lack of structural stability.

## Abstract

Electrochemical CO2 reduction (eCO2R) in acidic electrolytes is appealing due to its high CO2 utilization efficiency. For this reaction, bismuth (Bi)‐based catalysts have drawn considerable attention for their potential in producing formate/formic acid. However, the presynthesized materials for Bi‐based catalysts often undergo restructuring during electrocatalysis, resulting in altered electrochemical performance. Furthermore, the mechanisms underlying the restructuring of Bi‐based catalysts in acidic environments have not yet been clearly elucidated. Herein, distinct restructuring mechanisms are revealed in structurally different Bi‐based compounds, such as Bi9O7.5S6 and Bi2O2S. Among them, the Bi9O7.5S6 precatalyst exhibits high selectivity and activity for formic acid production, attributed to its unique structure, featuring stacking of [Bi2O2]2+ and [BiS2]− layers. In contrast, the conventional Bi2O2S catalyst, characterized by alternating [Bi2O2]2+ layers with S2− ions, delivers inferior eCO2R performances. Quasi‐in situ X‐ray diffraction and in situ Raman spectra results reveal that metal elements situated between two [Bi2O2]2+ layers can resist decomposition and prevent the over‐reduction of catalysts, leading to the restructuring in Bi/Bi2O2CO3 composite material with active Bi‐Bi2O2CO3 interface for formic acid production. As a result, the Bi9O7.5S6 precatalyst achieves a high Faraday efficiency above 95% at 100 mA cm−2 and remarkable stability of 117 h in a flow cell.

Two Bi‐based oxysulfides, Bi9O7.5S6 and Bi2O2S, undergo distinct restructuring under acidic eCO2R. Bi within the [BiS2]− layers of Bi9O7.5S6 guides the formation of an active Bi/Bi2O2CO3 interface, whereas Bi2O2S mainly reduces to Bi metal, giving lower activity for formic acid production.© 2025 WILEY‐VCH GmbH