Theoretical Insight into Design and Screening of High-Efficient Carbon Dioxide Reduction Electrocatalysts

高效二氧化碳電還原催化劑的理論設計與篩選

Student thesis: Doctoral Thesis

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Award date9 Aug 2021

Abstract

Excessive CO2 emission has caused increasingly severe greenhouse effect in the past few decades. CO2 conversion to value-added products has been regarded as one of the most effective strategies to solve the CO2 emission problem. Electrochemical CO2 reduction reaction (CO2RR) has attracted extensive attention due to its minor environmental pollution, mild reaction condition, and adjustable product selectivity. Benefited from the rapid increase of computational power, theoretical simulations, especially density functional theory (DFT), have been widely applied to explain experimental results and provide guidance for experiments in the field of electrocatalysis. Thus, in this thesis, DFT calculations were performed to design various catalysts, to reveal their reaction mechanism, and to screen for high-efficient CO2RR catalysts.

Firstly, single atom catalysts (SACs) composed of single Ni, Co, and Fe atom embedded in graphitic carbon nitride (g-C3N4) framework were designed. The mechanism of CO2RR to C1 products, including CO, HCOOH, CH3OH, and CH4, were investigated systematically. The results indicated that CO2 could be chemically adsorbed on Co-C3N4 and Fe-C3N4, but physically adsorbed on Ni-C3N4. The initial protonation intermediates on Ni-C3N4 and Co/Fe-C3N4 are COOH and OCHO respectively due to the adsorption difference. The thermodynamics analysis showed that Co-C3N4 performed more favorable CO2RR activity and selectivity for CH3OH production with unique potential-limiting step and lower limiting potential compared to Ni-C3N4 and Fe-C3N4.

Then, metal-nitrogen-doped carbon (M-N-C) materials with twelve types of transition metal centers (Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) and various amounts of adjacent nitrogen atoms were investigated as SACs for CO2RR. The adsorption energies of COOH and CO were found to be linearly related and could be used as a descriptor for CO production. Moderate adsorption strengths of CO and COOH were favorable for CO generation. Co-N4, Ni-N1, Pd-N1, Pt-N1, and Rh-N4 were the most active sites for CO2RR but showed low selectivity due to the HER interference. Considering both reaction selectivity and activity, the Ni-N4 and Fe-N4 were proved to be the most effective site for CO2RR, in which the former site was favorable for CO production while the latter one preferred to reduce CO2 to CH3OH and CH4.

Furthermore, the rule of adsorption strength and CO2RR mechanism on bimetallic alloy catalysts based on Au-Pd were investigated. The adsorption energy of COOH was observed as more ideal CO2RR reactivity descriptor than that of CO. The Au-based catalysts with core-shell structures or single active sites were then designed. The thermodynamic analysis showed that the monolayer Au over Pt, Cu, and Rh substrates could enhance the activity and selectivity for CO production than pure Au. The SAC with single Pd atom embedded on Au(111) could adjust the adsorption strength of CO, which provided an effective site to receive and further reduce CO to CH3OH and CH4 at a low limiting potential.

Finally, diatomic CO2RR catalysts with homonuclear and heteronuclear bimetallic centers embedded on nitrogen-doped carbon frameworks were designed. C2N carbon nitride monolayer and pyridine N-doped graphene were selected as the substrates for metallic dimer centers of Fe, Co, Ni, and Cu. The results indicated that the diatomic electrocatalysts based on nitrogen-doped carbon frameworks could provide potential active sites for both C1 and C2 production at the limiting potential lower than -0.90 V. *CH2 + *CO → *CH2CO was found as the dominating C−C coupling reaction on these diatomic catalysts when generating C2 products. The FeNi on C2N and FeCo dimers on pyridine N-doped graphene could reduce CO2 to the mixture of C1 and C2. However, the FeNi and FeFe on N-doped graphene are more CO2RR selective towards HCOOH and CH3OH respectively at low limiting potential.