CO₂ reduction efficiency through electrolyte immersion in hierarchical bismuth-nickel catalysts

Nanostructures are critical for improving the contact area with an electrolyte and catalytic efficiency for the CO₂ reduction reaction (CO₂RR). However, their hydrophobicity conflicts with the intended increase in the contact area and complicates the determination of the active contact area. Here, bismuth-nickel (BiNi) micro-nano hierarchical catalysts for the CO₂RR were studied to understand the effects of electrolyte-catalyst contact area variation with the immersion duration in an aqueous electrolyte. The immersed BiNi samples showed about 13.4-fold higher formate production compared to the pristine BiNi sample. The 2-day pre-immersed BiNi sample exhibited faradaic efficiencies (FE%) of ∼80.1% for formate and ∼10% for H₂ with a current density of 10.2 mA cm^-2 at -1.5 V vs. Ag/AgCl. In contrast, the pristine BiNi catalysts exhibited an FE% of ∼12.9% for formate and ∼76.3% for H₂ with a current density of 5.38 mA cm^-2. Our experimental results reveal that the improved contact between the electrolyte and the catalyst surface through pre-immersion can lead to enhanced CO₂RR efficiency for formate production using hierarchical BiNi catalysts.