Cobalt-Doped Bismuth Nanosheet Catalyst for Enhanced Electrochemical CO₂ Reduction to Electrolyte-Free Formic Acid

Electrochemical carbon dioxide (CO₂) reduction reaction (CO₂RR) to valuable liquid fuels, such as formic acid/formate (HCOOH/HCOO^-) is a promising strategy for carbon neutrality. Enhancing CO₂RR activity while retaining high selectivity is critical for commercialization. To address this, we developed metal-doped bismuth (Bi) nanosheets via a facile hydrolysis method. These doped nanosheets efficiently generated high-purity HCOOH using a porous solid electrolyte (PSE) layer. Among the evaluated metal-doped Bi catalysts, Co-doped Bi demonstrated improved CO₂RR performance compared to pristine Bi, achieving ~90 % HCOO^- selectivity and boosted activity with a low overpotential of ~1.0 V at a current density of 200 mA cm^-2. In a solid electrolyte reactor, Co-doped Bi maintained HCOOH Faradaic efficiency of ~72 % after a 100-hour operation under a current density of 100 mA cm^-2, generating 0.1 M HCOOH at 3.2 V. Density functional theory (DFT) results revealed that Co-doped Bi required a lower applied potential for HCOOH generation from CO₂, due to stronger binding energy to the key intermediates OCHO* compared to pure Bi. This study shows that metal doping in Bi nanosheets modifies the chemical composition, element distribution, and morphology, improving CO₂RR catalytic activity performance by tuning surface adsorption affinity and reactivity.