First-principles microkinetics simulations of electrochemical reduction of CO2 over Cu catalysts.

Electrochemical reduction of CO 2 can contribute to the storage of excess renewable electricity in chemical bonds. Here we incorporate reaction energetics for CO 2 reduction on Cu(111) and Cu(211) determined by DFT calculations in microkinetics simulations to predict the influence of surface topology, the presence of water and possible diffusion limitations on current density-potential curves and Faradaic efficiencies. A reaction-diffusion model was used that takes into account the effect of electrochemical potential on the stability of intermediates and associated activation barriers in proton-coupled electron transfer steps as well as diffusion of protons and CO 2 from the bulk electrolyte to the electrode surface. The basic model can well reproduce hydrogen evolution including the effect of proton diffusion limitations and a shift of proton reduction (low potential) to water reduction (high potential). Considering CO 2 electro-reduction, the stepped Cu(211) surface is more active than the Cu(111) terrace towards HCOO(H), CO and CH 4. The presence of a catalytic H 2 O molecule increases the overall rate and selectivity to products (CO and CH 4) derived from dissociated CO 2. A catalytic H 2 O molecule facilitates the difficult electrochemical CO 2 activation step to COOH and suppresses the competing activation step towards HCOO, which mainly yields HCOO(H). In general, the current densities increase at higher negative potential and the products follow the sequence CO 2 → CO → CH 4. That is to say, CO 2 is converted to CO via COOH dissociation, followed by CO hydrogenation. Trendwise, the simulated product distribution follows the potential-dependent distribution observed in experiment. The low selectivity to CH 3 OH can be understood from the fast electrochemical steps that lead to CH x -OH dissociation. At high overpotentials the hydrogenation step from CO 2 to COOH controls both activity and selectivity towards CH 4. At high potential CO 2 reduction becomes increasingly diffusion-limited, thus limiting the selectivity of CO 2 reduction vs. hydrogen evolution. This aspect supports the need for better design of mass transfer in electrochemical reactors, which operate at high current density. Image 1 • We simulated electrochemical CO 2 reduction on Cu(111) and Cu(211) surfaces. • The model includes water reduction, a diffusion layer for CO 2 , and pH gradients. • The simulations predict realistic current densities and Faradaic efficiencies. • Water molecules stabilize essential transition states for CO 2 activation. • Step-edge sites show high methane production via the CO 2 → COOH → CO → CH 4 pathway. [ABSTRACT FROM AUTHOR]