Role of Bi Adatoms in Bi@Pd Nanoparticles for Electrochemical CO2 Reduction

Surface modification of Pd by foreign metals is an appealing approach to achieve electrochemical CO2 reduction to formate (CO2RR-formate) with an overpotential close to zero. However, the exact role of surface dopants is still under debate. Here, we provide direct experimental proof that Bi adatoms on Pd nanoparticle surface are electrochemically active and geometrically assist electrochemical CO2RR, by utilizing element-selective in situ X-ray absorption spectroscopy (XAS), which reveals that the partly oxidized Bi adatoms as prepared are fully reduced and form a surface single-atom coordinated only with Pd nanoparticles at 0 VRHE, and are oxidized to Bi3+ valence state at 1.1 VRHE. Electrochemical measurements show that the onset potential of the formate formation for Bi@Pd/C (0.05 VRHE) is 0.1 V higher than that of Pd/C (-0.05 VRHE), with the local concentration of formate at -0.15 VRHE increasing from 1.6 to 4.6 mM. An oxygenated species chemisorbed on the Bi adatoms during CO2RR is identified by operando XAS and DFT calculations and is tentatively attributed to an active HC(O*-Bi)2 intermediate with direct Bi-O bonds. Our results clarify the long-term mysterious role of Bi adatoms in the CO2RR process.