Electrochemical Reduction of CO2: Catalyst and Catalytic System Development

Abstract

Electrochemical CO2 reduction to fuels and chemicals could improve industrial sustainability,as the process is readily powered by renewable energy sources. In aqueous solvents it is important to minimize parasitic water reduction. Both metallic indium and indium oxide electrocatalysts typically have high selectivity for producing formate via the electrochemical reduction of CO2 in aqueous media. It has been suggested that under highly negative potentials, i.e. potentials typically sufficient to reduce indium oxide to In0, the native oxide layer on metallic indium or indium oxide particles is not reduced to In0 when exposed to CO2-saturated electrolytes. This meta-stable oxide layer is crucial in the mechanism for producing formate via the two-electron,two-proton reduction of CO2, however it prevents the catalysis from occurring on In0. Herein, we report that by electrochemically reducing In2O3 nanocatalysts in Ar-saturated electrolytes in-situ,prior to CO2 exposure, will remove this metastable oxide layer and create a In0-In2O3 composite. This In0-In2O3 composite material changes the selectivity and is able to electrochemically reduce CO2 to CO with near 100% selectivity at relatively low overpotentials (c.a. -1.0 V vs Ag/AgCl).We attribute the change in selectivity to the direct exposure of In0 to CO2 in solution that typically does not exist to due to the native oxide layer that forms on In metal. In addition, we observed that the first electron-transfer step to form the surface adsorbed intermediates is highly reversible on the In0-In2O3 composite, however it is irreversible on an In foil electrode. We also report the utilization of Substrate Generation-Tip Collection Scanning Electrochemical Microscopy (SG-TC SECM) to measure the production of CO as function of applied potential. This technique allows for the collection of CO in-situ during the voltammetry experiment as it is produced on the catalytic electrode, which results in accurate potential dependent measurements of CO production.Despite the good selectivity achieved with the indium catalysts, limited CO2 solubility in conventional liquid phases starves active electrocatalysts of substrate and results in low conversion rates. In this dissertation, we show that multimolar CO2 concentrations can be achieved in an organic solvent containing supporting electrolyte at relatively mild CO2 pressures (2.8 MPa), electrocatalysis is significantly attenuated Taken together, these studies reveal that pressure-tunable CXE media could improve the performance of many known electrocatalysts by alleviating substrate starvation. Simultaneously, the non-monotonic enhancement of CO2 reduction with pressure suggests that pressure is a crucial variable in maximizing the efficiency of electrocatalytic CO2 conversion. Investigating the non-monotonic behavior for CO2 reduction in CO2 Expanded Electrolytes we utilize the ability to tunably solvate multi-molar amounts of CO2. By utilizing tunablity to generate cyclic voltammograms at widely separated concentrations of CO2 it was found that on multiple catalysts the catalytic rate decreased at the highest concentrations of CO2. Using COMSOL modeling of CO2 electroreduction on gold we show that the maximum value for catalysis is a result of a decrease in the rate of an elementary reaction step at high concentrations of CO2. Continuing the use of CO2 Expanded Electrolytes to understand the kinetics of CO2 reduction,the kinetics of a homogeneous catalyst, Re(CO)3(bpy)Cl, were investigated. Again the pressure tunablity of CXEs was exploited to generate cyclic voltammograms at widely separated concentrations of CO2. The observed rates increased until a plateauing rate with increasing concentration of CO2. These rates match well with Michealis-Menten kinetics for enzymatic catalysis. Using the Michealis-Menten equation, the kinetic rate constant for electrochemical reduction was found.After the plateau a reduction in the catalytic rate was observed at the highest degree of expansion consistent with what has previously been observed for heterogeneous catalysts. The use of CXEs to investigate the relationship between CO2 concentration and catalytic rate will guide the future development of electrochemical CO2 reduction systems