Electrodeposited Bismuth on Copper for Enhanced Electrochemical Nitrogen Reduction to Ammonia through Surface Oxidation

The electrochemical nitrogen reduction reaction (eNRR) offers a sustainable route to ammonia synthesis under ambient conditions, but achieving high selectivity over the competing hydrogen evolution reaction remains a challenge. Here, we report the synthesis and electrochemical evaluation of bimetallic bismuth-on-copper (Cu/Bi) catalysts prepared by electrodeposition at three potentials (0.01 V, −0.29 V, and −0.59 V vs RHE) on Cu substrates, with and without a subsequent electrooxidation treatment. The Cu/Bi electrode deposited at −0.29 V and subsequently oxidized achieves the highest Faradaic efficiency (FE) of 25% and an NH 3 production rate of 5.7 μg cm –2 h –1 at 0 V vs RHE, nearly doubling the FE of its nonoxidized counterpart. X-ray diffraction, scanning electron microscopy, atomic force microscopy, and contact angle measurements collectively demonstrate that the electrooxidation step selectively converts metallic Bi into crystalline Bi 2 O 3 exclusively for the −0.29 V deposit, accompanied by pronounced surface restructuring into platelet-shaped oxide crystals and increased surface roughness. Electrochemically active surface area measurements confirm that enhanced performance arises from improvements in both site density and intrinsic site activity, rather than surface area alone. Copper-on-copper (Cu/Cu) control electrodes show more modest and uniform responses to oxidation, confirming that Bi incorporation and its conversion to the oxide is the primary driver of superior eNRR selectivity. These results establish electrodeposition potential and controlled surface oxidation as critical design variables for tuning the phase composition and morphology of Cu/Bi catalysts and identify Bi 2 O 3 formation as a key feature for enhanced eNRR activity.