Theoretical Investigations on Carbon-Supported Single Atom Catalysts for Electrocatalytic Carbon Dioxide Reduction Reaction

Abstract

Electrochemical CO₂ reduction reaction (CO₂RR) has the potential to mitigate excessive carbon emission and achieve carbon neutrality eventually. Single atom catalysts (SACs) display unprecedented activity and selectivity toward the CO₂RR. Carbon-supported SACs have been widely used due to their excellent conductivity and porous structures. This thesis uses density functional theory (DFT) to explore a series of carbon-supported SACs for CO₂RR in relation to the structural design, reaction activity, product selectivity, and intrinsic essence.

Initially, first-row transition metal embedded pyrazine-based graphynes (TM-pyGYs) are evaluated as potential SACs for the CO₂RR by using DFT. The computational results show that TM-pyGYs exhibit large cohesive energies ranging from 6.67 to 6.78 eV/atom and metal binding energies ranging from 0.79 to 5.48 eV. Electronic structure analyses demonstrate the strong covalent bond, large charge transfer, and distinct orbital overlap between the TM atoms and pyGY, which prove the high stability of TM-pyGYs in the CO2RR. Most TM-pyGYs exhibit preferred CO₂RR selectivity over the hydrogen evolution reaction (HER). The most favorable reaction pathways of the CO₂RR to CO, HCOOH, CH₃OH, and CH₄ on TM-pyGYs are systematically explored. Among all the TM-pyGYs, Mn/Fe/Ni-pyGYs are determined to be outstanding electrocatalysts in the CO₂RR for producing HCOOH with low limiting potentials of −0.21 to −0.36 V, and Co-pyGY exhibit high-performance CO₂RR to CH₄ with a low limiting potential of −0.35 V, which surpass the performance of the vast majority of electrocatalysts. With applied potential, Fe/Co/Ni-pyGYs exhibit great advantages in the CO₂RR to CH₄.

Then, N and B doped Fe-embedded graphyne (Fe-GY), Fe-nXGYm (n = 1, 2, 3; X = N, B; m = 1, 2, 3), are employed as probes to reveal the effect of the coordination environment engineering on CO₂RR performance via heteroatom doping in SACs. The results show that the doping position and number of N or B in Fe-GY significantly affect catalyst activity and CO₂RR product selectivity. In comparison, Fe-1NGY exhibits high-performance CO₂RR to CH₄ with a low limiting potential of −0.17 V, and Fe-2NGY3 is demonstrated as an excellent electrocatalyst in the CO₂RR for producing HCOOH with a low limiting potential of −0.16 V. With applied potential, Fe-GY, Fe-1NGY, and Fe-2NGY3 exhibit significant advantages in the CO₂RR to CH₄ while the HER is inhibited. The intrinsic essence analysis illustrates that heteroatom doping modulates the electronic structure of active sites and regulates the adsorption strength of the intermediates, thereby rendering a favorable coordination environment for CO₂RR.

Finally, 26 transition metal anchored pyrrole nitrogen doped carbon catalysts (TM-NCs) are proposed as high-performance SACs for CO₂RR via a multi-level screening. The results reveal that 23 out of the 26 TM-NCs possess superior thermodynamic and electrochemical stability, and 11 TM-NCs exhibit preferred CO₂RR selectivity over the side reactions. The reaction pathways of CO₂RR to C1 products, including CO, HCOOH, CH₃OH, and CH₄, on TM-NCs are explored. Considering the activity and product selectivity along CO₂RR, Cr/Mn/Co-NCs are screened out as outstanding SACs in CO₂RR to HCOOH with low limiting potentials of −0.18 to −0.10 V, which surpass most previously reported SACs. With the applied potentials over −0.6 V, Cr/Mn/Co-NCs exhibit the advantages of yielding high-throughput reaction products of CH₄. The coordinated N-type analyses demonstrate that compared with pyridine-type TM-NCs, pyrrole-type TM-NCs possess weaker binding strengths with intermediates but better or comparable CO₂RR activity.

Date of Award	30 Apr 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Lawrence WU (Supervisor)