Influence of Co Doping on Copper Nanoclusters for CO2 Electroreduction

Abstract

Copper stands out as one of the few metals capable of reducing carbon dioxide (CO₂) beyond carbon monoxide (CO) and formic acid (HCOOH). Furthermore, substitutional doping in nanoclusters (NCs) has been expected to enhance their catalytic performance, even though our atomistic understanding of the influence of dopants is far from complete. Here, we investigate the effects induced by cobalt (Co) substitution doping in the Cu₅₅ NC on the electroreduction of CO₂ using density functional theory calculations combined with the computational hydrogen electrode model. We found that the replacement of a single copper atom in Cu₅₅ by Co is energetically favorable, and it induces a drastic change in the density of states, for example, the appearance of a sharp peak near the Fermi level. The presence of a dopant atom on the surface increases the adsorption strength for all reaction intermediates, while also changing the preference of the adsorption site for selected species. The presence of the dopant atom on the surface of the particle hinders the production of CO in favor of more reduced products such as methane and methanol. From our analysis, it was observed that the catalyst will not suffer from poisoning by the OH species. However, our calculations predict that the catalysts will also enhance the formation of hydrogen in a competing reaction.

Figure 1

Schematic pathways of the reactions involved in the CO₂ reduction process. (a) indicates the actual CO₂RR steps, with dashed arrows highlighting those considered in this study. Similarly, (b) illustrates the competing hydrogen evolution reaction, while (c) shows the hydrogenation of OH, which is important to account for OH poisoning.

Figure 2

Density of states and Local density of states (d-states contributions) for unary and doped clusters. The top panel compares the total density of states of the substrates. The middle panel compares the contribution of a subsurface Co atom in Cu₅₄Co with a subsurface Cu atom in Cu₅₅. The lower panel shows a similar plot, but for the atoms on the surface. Continuous and dashed lines represent different spin contributions, while the vertical dashed line indicates the Fermi energy.

Figure 3

Most stable configurations for each adsorption systems. The values in parentheses are the adsorption energies in eV. Cu is shown in blue, Co in green, H in white, O in red, and C in brown.

Figure 4

Free energy diagrams for selected processes involved in the electroreduction of CO₂. For comparison, we show the results obtained with the solvation energies obtained through the two methods reported in this work (SC1 and SC2).