Bi–O–S Catalyst for Enhanced CO2 Electroreduction to Formic Acid: Catalyst–Substrate Engineering and Multi-Objective Optimization

Electrochemical CO2 reduction to formic acid is a promising approach for carbon valorization, but achieving both high productivity and Faradaic efficiency remains a major challenge. In this study, a Bi–O–S electrocatalyst was synthesized and deposited onto an acid-treated Sn substrate to fabricate a Bi–O–S/acid-treated Sn electrode. Structural, chemical, morphological, and textural characterization confirmed successful formation and integration of the Bi–O–S catalytic phase with the Sn surface. Acid treatment of the electrode surface enhanced the formic acid concentration from 170 mg L−1 for untreated Sn to 750 mg L−1. Central composite design coupled with response surface methodology was subsequently employed to optimize applied current density, catholyte concentration, and catalyst loading. Multi-objective optimization experimentally achieved 1020 mg L−1 formic acid and 67% Faradaic efficiency, compared with predicted values of 950 mg L−1 and 61%, respectively, corresponding to a production rate of 41 mg h−1. The electrode maintained stable performance for 33 h, demonstrating the potential of Bi–O–S/acid-treated Sn electrode engineering for efficient and optimized CO2-to-formic-acid conversion.