Designing Novel Molecular Catalyst Systems for Electrochemical CO2 Reduction with High Activity at Low Effective Overpotentials

Abstract

The electrocatalytic CO2 reduction reaction (CO2RR) is a promising strategy of converting CO2 to fuels and value-added chemicals by renewable energy sources. However, designing active catalysts for selective CO2 reduction at low effective overpotentials remains a challenge. Compared to most state-of-the-art solid-state catalysts such as Cu, which reduce CO2 with high activity but usually suffer from poor selectivity, molecular catalysts show promise for the selective conversion of CO2 to single products with intrinsic catalytic ability that can be tuned through synthetic structure modifications.

For most traditional molecular catalysts, beneficial decreases in overpotentials are usually correlated with detrimental decreases in catalytic activity, which is referred to as the “molecular scaling relationship”. The main reason for the “molecular scaling relationship” is that the catalytic reaction is initiated by the redox activation of the metal center where the substrate is coordinated and reduced, so that both the kinetic reactivity and the effective overpotential scale with the nucleophilicity of metal sites, which correlates to the redox potential of the metal center. The goal of this thesis is to design novel and efficient molecular catalyst systems to break the typical “molecular scaling relationship”, showing high activity at low effective overpotentials for electrocatalytic CO2 reduction.

In this thesis, two series of transition metal complexes with redox-active ligands have been designed and investigated as promising molecular catalysts for the CO2RR—cobalt bis(pyridylmonoimine) complexes ([Co(BPMI)]) and cobalt pyridyldiimine complexes ([Co(PDI-R)]). For both molecular catalyst systems, catalytic onset is preceded by the formation of a ligand radical through a ligand-based redox process instead of a metal-based process. This unique redox feature provides an opportunity to break “molecular scaling relationships” by modulating the kinetic reactivity and the effective overpotential of the catalyst independently through rational ligand design and modification.

In Chapter 2, a planar Co-bis(pyridylmonoimine) [Co(L-L)] was prepared and studied, which shows catalytic activity for the CO2RR in acetonitrile. Addition of a proton source such as water or trifluoroethanol dramatically improves the activity and stability of the catalyst. The further electrochemical kinetic studies of [Co(L-L)] in Chapter 3 reveal that [Co(L-L)] undergoes a reductive dimerization upon reduction to the CoI complex, delaying the catalytic onset for the CO2RR. To facilitate the CO2RR, a series of [Co(L-R-L)] complexes were designed and prepared by modulating ligand flexibility and changing the complex’s planarity. This not only prevents catalyst dimerization but also leads to both a positive shift in catalyst onset and an increase in initial catalytic activity for CO2RR, thus breaking the typical “scaling relationship.” In Chapter 4, for [Co(PDI-R)] complexes, three substituent effects are sequentially integrated into the structure to facilitate the ligand reduction, which leads to the inverse “molecular scaling relationship” achieved in Chapter 3. The resulting [Co(PDI-PyCH3+I−)] catalyst shows one of the highest TOFcat (~ 4.1 × 104 s-1) reported for CO2RR. In Chapter 5, a binuclear bi-[MCo(PDI)] complex (M = Co and/or Zn) represents another efficient system which breaks the “molecular scaling relationship,” operating with ~4 orders of magnitude higher activity at 0.15 V lower magnitude overpotential for the CO2RR compared to the mononuclear [Co(PDI)] analog.

The work in this thesis highlights several molecular catalyst designs, where a ligand-based redox event precedes catalytic onset, to break the “scaling relationship” for the CO2RR. As discussed in Chapters, these designs are believed applicable for other catalytic reactions as well.