Electrodeposited bismuth nanomaterials as binder free and scalable catalysts for highly selective CO₂ to formate conversion

The design of binder-free catalyst materials that can be manufactured in a scalable manner and utilized in gas diffusion electrodes (GDEs) remains a critical bottleneck for further advancement of CO₂-to-formate electrolysis. Here, direct chronopotentiometric electrodeposition from a choline chloride:ethylene glycol deep eutectic solvent (DES) was applied to manufacture conformal, nanowire-like bismuth catalyst layers on commercial GDE substrates (Bi/GDE). These materials demonstrate a significantly improved catalytic activity per mass, compared with the conventional aqueous deposition method, as they showed uncompromised catalytic activity at a 6-times lower mass loading. Insight into the electrochemical surface restructuring mechanism was gained by combining ICP-MS dissolution analysis with in situ Raman spectroscopy. It could be shown that both DES and aqueous-derived catalysts undergo an identical Bi₂O₃ → Bi₂O₂CO₃ → Bi⁰ restructuring sequence, converging to a common hierarchical nanoflower active phase morphology with equivalent charge-transfer resistance, demonstrating that the activated state, not the initial morphology, governs selectivity. Both catalysts deliver high formate faradaic efficiencies at 100 mA cm^-2 (96.8% for the DES- and 93.7% for the aqueous-derived electrode), with the DES electrode achieving a mass-normalised activity for formate production of 166.9 mA mg_Bi ^-1. The electrodeposition process could be scaled to 150 cm² substrates while maintaining a formate faradaic efficiency of 95.5%. Operation at 100 mA cm^-2 is extended to 48 h of total elapsed time, corresponding to 32 h of cumulative electrolysis, through a periodic electrolysis swing protocol that mitigates GDE flooding, identified as the primary degradation pathway, while intrinsic catalyst stability showed no decline. This work establishes DES electrodeposition directly on GDEs as a material-efficient and inherently scalable route to high-performance Bi/GDE catalysts, and reveals electrochemical activation history as the key design parameter for CO₂-to-formate conversion.