(389f) The Effect of Electrode Potential on the Stability of Intermediates Involved in Both Electrochemical CO2 Reduction and Hydrogen Evolution

The CO₂ electrochemical reduction (CO₂ER) has attracted much attention due to the potential to reduce the carbon footprint under mild conditions.¹ The key to the success of a commercially feasible CO₂ER process is to discover stable, efficient and selective catalysts composed of inexpensive materials. To date, only copper exhibits the ability to produce hydrocarbons and alcohols with a moderate current density (5~10 mA cm^-2) and efficiency (up to 69%).^{2, 3} However, poor selectivity and high overpotentials prevent the implementation of CO₂ER in practical applications. A main barrier in developing efficient catalysts is the lacking of fundamental understandings at molecular level. Due to the dynamic nature of the electrocatalytic environments, experimental approaches were proved to be very challenging. ^{4, 5}

Here we employed grand-canonical quantum mechanics (GC-QM) based the density functional theory (DFT) to investigate the effect of U on the adsorption free energies (ΔGs) of 14 CO₂ER intermediates on Cu(111) as well as the intermediate of the competing hydrogen evolution reaction (HER).⁶ In contrast to many previous theoretical studies in which the ΔGs were calculated under constant charge, we calculated the ΔGs under constant potentials. By comparing the ΔGs calculated under constant potential to those calculated under constant charge, significant differences were observed, especially when the adsorption of intermediate resulted in a significant work function change. In addition, the effect of U on the reaction free energy (ΔG) of the crucial elementary steps for CO₂ER and HER on a series of metallic catalysts were investigated at pH = 0. Our results indicate that the influence of U is metal-dependent for CO₂ER. However, for HER, ΔG_*H was barely affected by U for the studied metals. By analysing all the free energies obtained in this study, we found substantial differences (> 0.22 eV in average) between the ΔGs calculated under constant charge and those calculated under U = -1.0 V_SHE, indicating the importance of using GC-QM based DFT calculations in computational study of CO₂ER.

1 Q. Lu and F. Jiao, Nano Energy, 2016, 29, 439-456.

2 Y. Hori, K. Kikuchi and S. Suzuki, Chem. Lett., 1985, 14, 1695-1698.

3 Y. Hori, A. Murata and R. Takahashi, Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 1989, 85, 2309-2326.

4 A. A. Peterson, F. Abild-Pedersen, F. Studt, J. Rossmeisl and J. K. Nørskov, Energy Environ. Sci., 2010, 3, 1311.

5 A. A. Peterson and J. K. Nørskov, J. Phys. Chem. Lett., 2012, 3, 251-258.

6 H. Zhang, W. A. Goddard, Q. Lu and M. J. Cheng, Phys. Chem. Chem. Phys., 2018, 20, 2549-2557.