Competition between CO2 Reduction and H2 Evolution on Transition-Metal Electrocatalysts

The well-known hydrogen evolution reaction (HER) volcano plot describes the relationship between H binding energy and the corresponding hydrogen evolution catalytic activity, which depends on the species of metal. Under CO2/CO reduction conditions or in cases where CO impurities enter electrodes, the catalyst may exist under a high coverage of coadsorbed CO. We present DFT calculations that suggest that coadsorbed CO during hydrogen evolution will weaken the binding strength between H and the catalyst surface. For metals on the right-hand side (too weak of hydrogen binding) this should lead to a suppression of the HER, as has been reported for metals such as Cu and Pt. However, for metals on the left-hand side of the volcano (too strong of hydrogen binding), this may actually enhance the kinetics of the hydrogen evolution reaction, although this effect will be countered by a decreased availability of sites for HER, which are blocked by CO. We performed experiments in Ar and CO2 environments of two representative metals that bind CO on the far right- and left-hand side of the volcano, namely, Cu and Mo (respectively). On Cu, we find that the CO2 environment suppresses HER, which is consistent with previous findings. However, on Mo we find that the CO2 environment enhances HER in the kinetically active region. This helps to explain the outstanding performance of copper in CO2 reduction and suggests that searches for high-selectivity CO2/CO reduction catalysts may benefit from focusing on the right-hand side of the HER volcano. This also suggests principles for assessing the activity of catalysts for fuel cell and electrolysis reactions in which impurities such as CO may be present.