Stabilization effects in binary colloidal Cu and Ag nanoparticle electrodes under electrochemical CO2 reduction conditions

Abstract

Nanoparticle modified electrodes constitute an attractive way to tailor-make efficient carbon dioxide (CO₂) reduction catalysts. However, the restructuring and sintering processes of nanoparticles under electrochemical reaction conditions not only impedes the widespread application of nanoparticle catalysts, but also misleads the interpretation of the selectivity of the nanocatalysts. Here, we colloidally synthesized metallic copper (Cu) and silver (Ag) nanoparticles with a narrow size distribution (<10%) and utilized them in electrochemical CO₂ reduction reactions. Monometallic Cu and Ag nanoparticle electrodes showed severe nanoparticle sintering already at low overpotential of −0.8 V vs. RHE, as evidenced by ex situ SEM investigations, and potential-dependent variations in product selectivity that resemble bulk Cu (14% for ethylene at −1.3 V vs. RHE) and Ag (69% for carbon monoxide at −1.0 V vs. RHE). However, by co-deposition of Cu and Ag nanoparticles, a nanoparticle stabilization effect was observed between Cu and Ag, and the sintering process was greatly suppressed at CO₂ reducing potentials (−0.8 V vs. RHE). Furthermore, by varying the Cu/Ag nanoparticle ratio, the CO₂ reduction reaction (CO₂RR) selectivity towards methane (maximum of 20.6% for dense Cu_2.5–Ag₁ electrodes) and C₂ products (maximum of 15.7% for dense Cu₁–Ag₁ electrodes) can be tuned, which is attributed to a synergistic effect between neighbouring Ag and Cu nanoparticles. We attribute the stabilization of the nanoparticles to the positive enthalpies of Cu–Ag solid solutions, which prevents the dissolution-redeposition induced particle growth under CO₂RR conditions. The observed nanoparticle stabilization effect enables the design and fabrication of active CO₂ reduction nanocatalysts with high durability.