Abstract
Experiments have shown that graphene-supported Ni-single atom catalysts (Ni-SACs) provide a promising strategy for the electrochemical reduction of CO2 to CO, but the nature of the Ni sites (Ni-N2C2, Ni-N3C1, Ni-N4) in Ni-SACs has not been determined experimentally. Here, we apply the recently developed grand canonical potential kinetics (GCP-K) formulation of quantum mechanics to predict the kinetics as a function of applied potential (U) to determine faradic efficiency, turn over frequency, and Tafel slope for CO and H2 production for all three sites. We predict an onset potential (at 10 mA cm−2) Uonset = −0.84 V (vs. RHE) for Ni-N2C2 site and Uonset = −0.92 V for Ni-N3C1 site in agreement with experiments, and Uonset = −1.03 V for Ni-N4. We predict that the highest current is for Ni-N4, leading to 700 mA cm−2 at U = −1.12 V. To help determine the actual sites in the experiments, we predict the XPS binding energy shift and CO vibrational frequency for each site.
Highlights
Experiments have shown that graphene-supported Ni-single atom catalysts (Ni-SACs) provide a promising strategy for the electrochemical reduction of CO2 to CO, but the nature of the Ni sites (Ni-N2C2, Ni-N3C1, Ni-N4) in Ni-SACs has not been determined experimentally
We find that Ni–N2C2 leads to the lowest onset potential of −0.84 V to achieve 10 mA cm−2 current density, leading to a Tafel slope of 52 mV dec−1 and a turn-over frequency (TOF) of 3903 h−1 per Ni site at neutral electrolyte conditions, showing best agreement with various experimental observations at lower overpotentials
grand canonical potential kinetics (GCP-K) uses a Legendre transformation to convert from fixed charge free energy, F (n), to grand canonical, G (n; U), allowing the thermodynamic free energy for heterogeneous electrochemical reactions to depend on the applied potential (U)
Summary
Experiments have shown that graphene-supported Ni-single atom catalysts (Ni-SACs) provide a promising strategy for the electrochemical reduction of CO2 to CO, but the nature of the Ni sites (Ni-N2C2, Ni-N3C1, Ni-N4) in Ni-SACs has not been determined experimentally. Several experimental studies have been published recently on the CO2 reduction reaction (CO2RR) for the nickel single atom catalysts (Ni–SACs) on graphene[20,21,25,26,27,28,29], but the performance varies markedly perhaps because of differences in the number of carbon or nitrogen bonds to Ni. Recent developments in quantum mechanics (QM) based methods provide new tools determine the reaction mechanisms for heterogeneous electrochemical reactions. We recently developed the grand canonical potential kinetics (GCP-K) method to combine fixed charge and fixed potential QM to allow the reaction barriers to change continuously as the applied potential is changed, leading directly to current versus potential relation (Tafel slope)[31]
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