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Disputation Shan Jiang

05.11.2021 | 10:30
Thema der Dissertation:
Operando Raman investigation of electrochemical CO2 reduction reaction catalyzed by copper-based materials
Abstract: Anthropogenic activity causes excessive CO2 emission in the atmosphere. In the CO2 reduction reaction (CO2RR), CO2 is converted electrochemically into valuable products including methane, ethylene, and other hydrocarbons. Large-scale employment of efficient CO2RR could diminish the use of fossil-fuel resources and thereby counteract rising levels of atmospheric CO2. Although plenty of catalysts have been investigated, copper (Cu) is the only metal that can produce reasonable amount of hydrocarbon products with two or more carbon atoms (C2+ products). However, low selectivity and high overpotentials remain as two obstacles for practical application. The reactivity determinants and reaction mechanisms in CO2RR and their relation to formation of specific products are insufficiently understood only. Therefore, operando techniques that provide insight in reaction mechanism for the catalyst material during electrocatalytic operation are of great importance.
In this thesis, operando Raman spectroscopy was adopted to investigate Cu-based foam-like materials for CO2RR catalysis. These Cu foams, which have been characterized regarding CO2RR activity previously, are shown to exhibit surface-enhanced Raman scattering (SERS) activity, thereby facilitating the detection of surface-bound molecular species. A normalization procedure was developed for meaningful analysis of the electric potential dependence of the detected species. A comprehensive overview of various potential dependencies of Raman spectra are reported.
Special focus was put on the role of oxides and the bicarbonate electrolyte. Evidence for copper carbonate hydroxide (CuCarHyd), which resembles the mineral malachite, was obtained. Its carbonate ions can be directly converted to CO at low overpotentials. These and further experiments, including 13C isotope experiments and local pH detection, suggest a basic mode of CO2/carbonate reduction at Cu electrodes that contrasts previous mechanistic models: the starting point in carbon reduction is not CO2 but carbonate ions bound to the metallic Cu electrode in form of CuCarHyd structures. Cu oxides residues could enhance CO2RR indirectly by supporting formation of CuCarHyd motifs. The presence of CuCarHyd patches at catalytic potentials may result from alkalization in conjunction with local electrical potential gradients, enabling the formation of metastable CuCarHyd motifs over a large range of potentials. It is conceivable that the interaction of oxide and bicarbonate also may be the key for the enhanced C2+ production activity.
Operando Raman spectroscopy was also used to investigate a bimetallic catalyst CuAg foam that shows a primary selectivity for CO. The Raman spectra on CuAg foam shows an earlier departure of CO from the surface. Further 13C isotope experiments probed a surface intermediate that can be assigned to the symmetrical stretching vibration of bidentate.
Mass transport limitations of carbonate species may influence the CO2RR mechanism. Raman spectroscopy (without SERS) was used to detect the local pH at the Cu foam surface. Severe local alkalization was observed. The same process was analyzed in D2O electrolyte and using a flow-cell setup, persistent local alkalization was also observed. Further results of potential influences of local pH change on the CO2RR are presented.

Zeit & Ort

05.11.2021 | 10:30