Influence of the Cathodic Binder on CO2 Electroreduction to Formic Acid in a Three-Compartment Electrolyzer

Abstract

Restructuring synthetic processes is essential to enable decarbonization in industrial synthesis. The electrochemical reduction reaction of CO2 to value-added products, such as formic acid, integrates CO2 as a feedstock and can be coupled with renewable energy sources to enable a CO2-negative synthetic process. Techno-economic analyses (TEAs) present the direct formic acid production (DFAP) reactor as the most promising design for industrial competitiveness. Using this electrolyzer design, we fabricated custom-made cathodic Bi2O3-based gas diffusion electrodes (GDEs) with various ionomer materials and evaluated the performance of the reactor. Contrary to our initial expectations, the anionic polymer (PiperION) used as a binder exhibited low performance at higher current densities (350 and 500 mA cm─2) when paired with its corresponding anionic exchange membrane (AEM), suggesting that achieving a homogeneous GDE/AEM interface may not be critical as previously thought. Akin to the anionic binders, the cationic ionomer (Nafion) exhibited good performance only at 200 mA cm─2. The best performance was obtained with the non-ionic polymer (PVDF/HFP), achieving the highest reported current density (750 mA cm─2) with this electrolyzer design while maintaining stable operation with high faradaic efficiency (69%), high formic acid concentrations (3.8 M; ~16 wt%), and an optimized energy consumption of 8.3 kWh kg─1. We postulate that the higher hydrophobicity of PVDF/HFP suppresses water accumulation within the catalyst layer, thereby enhancing CO2 diffusion and supporting higher current densities, while reducing diffusion overpotential. Scanning electron microscopy (SEM) and X-ray computed tomography (XCT) helped to identify the migration of potassium and bismuth within the cathode. TEA studies indicate promising scalability potential based on the experimental data presented in this study, driven by the ability to produce high concentrations of formic acid at high current densities.