title: CO2 to X: electrochemical CO2 reduction on metal foam catalysts
Creators: name: Kiran, Kiran
Publisher: name: Universität Bern
Year: 2021
Subjects: subject: 540 Chemistry

description: Societal, economic, and technological advancements have collectively led to a constant increase in the atmospheric CO2 concentration, thus giving rise to concerns about its harmful influence on the global climate. Against this backdrop, electrochemical CO2 reduction (CO2RR) is one particularly promising approach to alleviate the high CO2 concentration, thereby contributing to the closing of the anthropogenic carbon cycle. This process is considered appealing as it can be operated with renewable energy sources (wind, solar, etc.). However, one of the most challenging aspects of CO2RR is the design and fabrication of stable catalysts capable of selectively forming a specific target product at low over-potentials while remaining energetically efficient.

This thesis focused on the development of novel monometallic and binary foam catalysts. In addition to developing robust catalysts, the objective was to investigate the transformation of the catalysts under reaction conditions through operando techniques complemented by ex-situ techniques. These catalysts were prepared by adopting the so-called dynamic hydrogen bubble template (DHBT) deposition approach yielding metal foams and revealing porosity on various length scales. An open-cell structure of interconnected pores of various sizes on the μm length results in a primary porosity, whereas a secondary porosity was found on a nm length scale at the dendritic sidewalls of the macro-pores. In this thesis, synthesis protocols for monometallic Ag, Cu, and Bi-foams were developed and are presented. A key aspect of improved catalyst performances is the catalyst activation via thermal annealing and the subsequent reduction of the formed oxidic precursor under operando CO2RR conditions. A variety of advanced operando techniques (XAS, XRD, and Raman) were applied to examine the oxidic precursors’ potential-dependent stability and to answer the question about the extent to which the oxidic species actively participate in the electrocatalytic reaction. The Cu and the Bi foam were identified as model systems (i) for a complete oxide->metal transition prior to the CO2RR (Cu) and (ii) for a partial oxide->metal transition during initial CO2RR, thus indicating an active role of the oxides.

The preparation of metal foam catalysts could be further extended to bimetallic and binary alloy systems with a view to further improving the product selectivity of particular CO2RR products targeted in this Ph.D. project (e.g. ethanol, n-propanol, formate, and carbon monoxide). Specifically, CuPd alloy and bimetallic CuAg foams were identified as superior catalysts for selective n-propanol and ethanol formation, respectively. A detailed study was undertaken on the performance of CuIn and CuSn foams in dependence on their chemical composition demonstrating characteristic concentration-dependent transitions in the resulting product distribution (e.g. from CO to formate).

Finally, the concept of metal foam electrodeposition was successfully transferred to technical support electrodes (carbon papers) which find use as gas diffusion electrodes (GDEs). This is considered an important step toward the application of this novel type of catalysts under more realistic experimental conditions.