(639a) The Electrochemical Reaction Mechanism and Kinetics of CO2 Reduction on Graphene-Supported Nickel Single Atom Catalysts from Quantum Mechanics | AIChE

(639a) The Electrochemical Reaction Mechanism and Kinetics of CO2 Reduction on Graphene-Supported Nickel Single Atom Catalysts from Quantum Mechanics

Authors 

Luo, Z. - Presenter, The Hong Kong University of Science and Technology
The excessive use of fossil fuels as the primary source for different industrial applications and human activities resulted high concentration of CO2 in the atmosphere. A substantial progress is being made in the electrochemical transformation of CO2 into chemical fuels in recent years, but the poor selectivity and stability limit their use in practical applications. The single atom catalysts (SACs) have emerged as an effective strategy providing an atomically dispersed single site with maximal atomic efficiency and high selectivity. With the experimental development, Quantum Mechanics (QM) based methods provide new tools to determine the reaction mechanisms for heterogeneous electrochemical reactions theoretically. Experiments have shown that graphene-supported Ni-single atom catalysts (Ni-SAC) provide a promising strategy for the electrochemical reduction of CO2 to CO, but the nature of the Ni site (Ni-N2C2, Ni-N3C1, Ni-N4) in Ni-SAC has not been determined experimentally. Here we apply the 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 find that Ni-N2C2 leadsto the lowest onset potential of -0.84 V (vs RHE) 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 hr-1 per Ni site at neutral (pH 7) electrolyte conditions, showing best agreement with various experimental observations at lower overpotentials. We predict the onset potential for 10 mA cm-2 current density of -0.92 V for Ni-N3C1 and -1.03 V for Ni-N4 (which exhibits the highest saturation current for high applied potentials) while the later showing the highest current leading to 700 mA cm-2 at U= -1.12 V. Additionally, We use quantum mechanics to predict the binding energy (BE) shift for the N and C 1s X-ray photoelectron spectroscopy (XPS) and the CO vibrational frequencies to help interpret the experimental nitrogen coordinations in Ni-SACs. We predict that the N 1s BE shift ranges from +1.18 eV to+0.96 eV for Ni-N4 and Ni-N2C2 respectively and the adsorbed CO intermediate vibrations range from 1985 cm-1 (perpendicular) at -1.0 V on Ni-N2C2 sites to 1942 cm-1 in the xz plane at -1.25 V applied potential on the Ni-N4 site. We hope our work will provide a guideline to design highly efficient single atom catalysts for CO2 reduction.

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