Trends in the Catalytic Activity of Hydrogen Evolution during CO2 Electroreduction on Transition Metals

During CO₂ electroreduction (CO₂R), the hydrogen evolution reaction (HER) is a competing reaction. We present a combined experimental and theoretical investigation of the HER activity of transition metals under CO₂R conditions. Experimental HER polarization curves were measured for six polycrystalline metal surfaces (Au, Ag, Cu, Ni, Pt, and Fe) in the presence of CO₂ gas. We found that the HER activity of the transition metals is significantly shifted, relative to the CO₂-free case. Density functional theory (DFT) calculations suggest that this shift arises from adsorbate–adsorbate interactions between *CO and *H on intermediate and strong binding metals, which weakens the *H binding energy. Using a simple model for the effect of *CO on the *H binding energy, we construct an activity volcano for HER in the presence of CO₂ gas that is consistent with experimental trends. The significant changes in HER activity in the presence of CO₂ gas is an important consideration in catalyst design and could help develop catalysts that are more selective for CO₂R than the HER.