Electrochemical CO2 Reduction with Well-Defined Electrodes

Abstract

Global climate change and its adverse effects can be combated with a societal shift to the consumption of abundant renewable energy resources such as solar and wind. Electrochemical reduction of CO2 (CO2RR) has emerged as a popular strategy to aid in the storage of renewable energy. Among heterogeneous catalysts, nanostructured gold-, silver-, and copper-based electrocatalysts have exhibited high selectivity and activity for CO2RR, demonstrating conversion of CO2 into single and multicarbon oxygenates and hydrocarbons. This thesis encompasses work to develop and utilize well-defined nanostructured electrodes to better understand catalytic behavior exhibited by CO2RR electrocatalysts and electrochemical cells overall. Efforts begin with the development of a highly versatile and modular platform to prepare well-defined nanostructured electrodes. Traditional silicon nanofabrication methodologies are integrated with an adaptable and high-throughput electrodeposition method to prepare tunable functionalized silicon nanowire array-based electrodes. The geometric parameters are readily controlled during nanofabrication and electrode deposition provides access to a range of electrode material(s). Furthermore, novel synthetic strategies allow for site-specific location of active materials within the array architecture. CO2RR is investigated on well-defined gold nanowire array electrodes and compared to gold “leaf” nanoparticulate and gold thin film electrodes. The overall potential-selectivity relationship exhibited by these electrocatalysts is investigated through analytical modeling and the roles of intrinsic catalyst activity, mass transport of CO2 in the system, and homogeneous reactions involving CO2 in determining CO2RR selectivity on gold electrocatalysts are disentangled. The reaction between CO2 and electrochemically generated hydroxide is found to be particularly important in limiting CO2RR faradaic efficiency in the system and presents critical implications for the industrial application of CO2RR. Stemming from this work, an investigation of the rate-limiting interfacial electron transfer between gold and CO2 is conducted on gold thin films. Development of a Marcus-type model provides understanding of the activation-driving force relationship exhibited for CO2RR and allows a first approximation of the reorganization energy and standard potential for CO2 reduction to carbon monoxide on gold electrodes. Gold/silver and copper bimetallic layered nanowire array and foam tandem electrodes are utilized to study cascade CO2RR, an emergent strategy to overcome kinetic limitations for the generation of multicarbon CO2RR products on copper-based catalysts. Uniquely, the catalyst architectures developed allow for the interrogation of the effect of the catalyst order in the overall tandem catalyst on the cascade catalysis process. In the foam architecture, incorporation of a secondary catalyst in the appropriate configuration results in a significant increase in the production of multicarbon alcohols relative to a copper-only catalyst, whereas incorrect ordering of the tandem catalyst system yields severely hampered generation of the same products. The effect of catalyst order within the tandem catalyst is attributed to mass transport of primary and secondary substrates in the catalytic cascade. Finally, this work on bimetallic electrodes is generalized through the development of a novel nanowire array-based generator-collector electrode. As an analogue to the rotating ring-disk electrode, the nanowire array electrode is designed to utilize two individually actuated electrodes to probe electrochemical reaction intermediates, kinetics, and mechanisms.