Single Atom Catalysts for Efficient Electroreduction of Carbon Dioxide and Carbon Monoxide

Abstract

Electrochemical reduction of CO2 (CO2RR), powered by renewable electricity, is a promising approach to convert CO2 into valuable chemicals and fuels, mitigating strong dependence on traditional fossil fuels and addressing the environmental issues. However, CO2RR suffers from sluggish kinetics, the side-reaction of hydrogen evolution reaction (HER) and high overpotential. Single atom catalysts (SACs) can overcome these problems and achieved excellent CO2RR performances, attributed to their well-defined active sites, strong atom-support interaction, and maximum metal utilization. Up to date, achievements have been made for CO2RR in strong alkaline electrolytes, especially for the production of high value C2+ species. However, CO2RR is unstable under alkaline conditions, due to the interaction between CO2 and OH-, which results in carbonate formation. There also exists a large energy penalty for CO2 regeneration from the generated carbonate. To address this problem, an alternative route is carrying out CO2RR in two steps: CO2-to-CO and then CO-to-C2+. Compared with CO2RR, electrochemical reduction of CO (CORR) can operate stably and achieve high selectivity to C2+ species. In this case, efficient catalysts with high activity and selectivity to C2+ species from CORR are required. This thesis focuses on designing efficient SACs for CO2RR and CORR, which exhibited high activity, selectivity and stability. Besides, their catalytic mechanism is explored, laying foundations for future developments of SACs for CO2RR. Firstly, we developed a facile synthetic strategy for fabricating metal-nitrogen-carbon nanotube (M-N-CNT, M=Ni, Co, Cu, Fe, Mn, Zn, Pt, or Ru) SACs at scale (> 1 g) by direct pyrolysis of metal cations, phenonathroline and CNT at high temperature. The pyrolysis leads to forming coordinated Ni-N active sites anchored on CNT. The prepared Ni-N-CNT catalyst with a remarkable Ni loading of 2 wt% determined by inductively coupled plasma optical emission spectrometry (ICP) exhibits the highest activity for CO2-to-CO conversion with a high faradaic efficiency of 94% and excellent stability. Aberration-corrected high-angle annular dark-field transmission electron microscopy (HAADF-STEM), X-ray photoelectron spectroscopy and X-ray absorption spectroscopy confirm the presence of isolated Ni single atoms in Ni-N-CNT, which act as the active centers for CO2 electroreduction while the CNT support offers fast pathways for electron and mass transports. This work laid foundations for practical applications of SACs in CO2 electroreduction and beyond. Secondly, nanoconfined ionic liquids are introduced into porous atomically dispersed nickel-nitrogen-carbon (Ni-N-C) catalysts to enrich local CO2 concentration and increase the CO2RR reaction kinetics. A series of high-CO2-solubility ionic liquids (ILs) were impregnated into the pores of the columnar Ni-N-C catalyst to alter the CO2-Ni sites interactions and create a solid/liquid interface with high CO2 concentration. The optimal Ni-N-C/[Bmim][PF6] composite outperforms the Ni-N-C catalyst for pure CO2 electroreduction with a maximum CO Faradaic efficiency (FECO) of 99.6% and 2.7-fold larger CO partial current density (jCO). The high solubility of CO2 in ILs compared to aqueous electrolyte enables direct electrolysis of CO2 at low concentrations. When fed with 5-10 % (v/v) CO2, the Ni-N-C/[Bmim][PF6] composite exhibited up to 1.5-fold higher FECO and a 68% increase of jCO, in comparison to Ni-N-C, and robust stability over 30 h. Thirdly, a Cu-Au alloy catalyst with abundant atomic Cu-Au interfaces was developed to drive efficient CORR for acetate production. The unique geometric and electronic structure of atomic Cu-Au interfaces affords improved acetate activity and selectivity, surpassing the metallic Cu nanoparticles and CuAu bulk alloys. A high Faradaic efficiency of 39% was achieved with a large partial current density of 217 mA cm-2 for acetate production in alkaline flow cells. Density functional theory calculation reveals that the introduced Au atoms into Cu support promotes C-C coupling and improve acetate formation by weakening the binding strength of *CO+*CO on catalyst surface. Fourthly, atomically dispersed Cu-Au alloy was functionalized with aromatic heterocycle such as thiadiazole derivate (N2SN) to improve the conversion of CO into C2+ species e.g. acetate as the main product. Theoretical calculation predicted that the N2SN molecule doping contributed to lower energy barrier for C-C coupling, improved activity and selectivity to CORR, and suppressed HER, as compared with the unmodified sample. The N2SN functional groups with electron withdrawing property could alternate the oxidization state of copper, as confirmed by XPS and XAS, thus orienting the CORR pathway to C2+/acetate. In situ Raman revealed that the N2SN treated sample exhibited stronger signal of *CO intermediate for further dimerization and the C-C-O intermediate related to acetate formation. As a result, we achieve high Faradaic efficiency (FEC2+, 78.8%) and maximum partial current density (jC2+, 422.82 mA cm-2) for C2+ formation, as well as that (57.42%, 307.92 mA cm-2) for acetate production in alkaline flow cell. Besides, the optimal FEC2+ (89%), jC2+ (397 mA cm-2), and energy efficiency for C2+ species (24%) were obtained in MEA. This thesis develops new strategies for SACs, including fabricating Ni SAC at gram-scale, introducing nanoconfined ILs into Ni SACs, structure control of atomic Cu-Au interfaces, and functionalizing atomic Cu-Au alloy with molecule doping, which demonstrate great potential of SACs for CO2RR and CORR.