Transition Metal-Based Electrocatalysts for Highly Selective C02 Reduction

Abstract

The electrochemical CO2 reduction reaction (CRR) can combine carbon cycling with renewable energy to convert CO2 into high-value carbonaceous feedstocks. However, this process su ers from kinetically sluggish because of the complicated electron transfer and high energy barriers involved. Well-designed transition metal materials as promising electrocatalysts show remarkable catalytic activities for the CRR. Therefore, this Thesis is to study the catalytic activity and selectivity on these transition metal catalysts, and a fundamental understanding of the catalytic mechanism is given through a series of experimental and computational results using advanced synthesis methods, electrochemical measurements, material characterization including microscopy and spectroscopy, synchrotron-based X- ray spectroscopy, in situ spectroscopy, and density functional theory (DFT) calculations. The scope of this Thesis is narrowed to nanoscale and sub-nanoscale engineered 3d-block transition metal (mainly, Fe, Co, Ni, Cu) catalysts for the CRR process. In this Thesis, the rst section introduces research progress including catalytic performance and mechanisms on sub-nanoscale 3d-block transition metal catalysts for the CRR. The second section consists of published and submitted works: (1) The rst project starts with the investigation of the CRR on Ni catalysts. We engineered and alloyed Ni with Cu to obtain ultrasmall graphene-encapsulated Ni-Cu bimetallic catalysts. The Cu-lean catalyst exhibited signi cant activity and selectivity, and the highest Faradaic e ciency (FE) toward CO was 90% at -1.0 V vs. RHE. By coupling synchrotron-based X-ray absorption and in situ Raman spectroscopy studies, we found that there is a negative correlation with the Cu content in Ni-Cu catalyst and CO selectivity due to redistribution of the 3d electrons from Ni and Cu. (2) Because of the high catalytic activity was received on ultrasmall Ni-Cu particles, the second project aims to fabricate sub-nanoscale transition metal catalysts for the CRR. We synthesized atomically dispersed Fe immobilized within N-doped carbon nanosheets. The optimal Fe catalyst achieved FE of 90% toward CO at -0.58 V vs. RHE. A series of controlled tests revealed that there is a synergistic e ect between the Fe sites and the pyrrolic-N-framework which promotes the catalytic activity of CO evolution. (3) The third work is based on the previous Fe catalyst and investigates the unique single-atom Cu catalyst (Cu-N4-NG). The chemical structure and coordination environment of Cu-N4-NG were identi ed using synchrotron-based characterization. Compared to a traditional bulk Cu catalyst, Cu-N4-NG performed a FE of 80.6% towards CO at -1.0 V vs. RHE. The experimental results revealed that the presence of Cu-N4 moieties largely promotes CO2 activation and water dissociation, showing CO2 reduction is kinetically preferred on Cu-N4-NG. Also, the computational investigation suggested a thermodynamic explanation that CO2 reduction is less hindered on Cu-N4-NG compared to hydrogen evolution. (4) Although high FEs were obtained on single-atom transition metal catalyst shown in the previous two works, the two catalysts were not strictly single-atom catalysts with a uniform structure of M-N4, some coordination defects existed. Thus, graphene- supported metal phthalocyanine catalysts with M-N4 structure were reported in the fourth work, which achieved almost 100% CO2 conversion to CO on graphene- supported cobalt phthalocyanine. Further experimental studies showed that the phthalocyanines with graphene were signi cantly activated than the pure ones. A series of control tests uncovered that the graphene substrate facilitates electron transfer between the catalyst and CO2 molecules, which increased CO selectivity.