Controlling Electrocatalytic Interfaces using Thin Films and Nanostructures for Energy Applications
The release of greenhouse gases such as CO2 due to various human activities and the use of fossil fuels causes climate change and increases global temperature. For this reason, we must create new technologies that help shift energy production away from fossil fuels to renewable energy sources. Over the past few decades, researchers in academia and industry have focused on developing novel techniques for clean and renewable energy, which could in part be mediated by H2 fuel cells. The oxygen reduction reaction (ORR) occurs at the cathode of fuel cells and is the rate-limiting reaction. Water can be electrolyzed using electricity from renewable sources to generate H2 in a green manner. The oxygen evolution reaction (OER) is the rate-limiting reaction for water electrolysis. Unfortunately, catalysts based on Pt and Ir have the best performance for the ORR and OER, respectively. However, the widespread application of these catalysts is limited because of the high cost and scarcity of Pt- and Ir-based catalysts. Non-precious metal catalysts such as those based on Cu and Ni are promising alternatives. In this dissertation, I have developed a new electrochemical platform that allows for the study of the control of electron and proton transfer in the ORR. Specifically, I use Cu as one of the non-precious metal catalysts to study the ORR. I prepared a dinuclear Cu ORR catalyst that can be covalently attached to thiol-based self-assembled monolayers (SAMs) on Au electrodes using azide-alkyne click chemistry. Using this architecture, the electron transfer rate to the catalyst is modulated by changing the length of the SAM, and the proton transfer rate to the catalyst is controlled with an appended lipid membrane modified with proton carriers. By tuning the relative rates of proton and electron transfer, the current density of the lipid-covered catalyst was enhanced significantly without altering its core molecular structure. Also, I utilized designer small-molecule proton carriers bearing nitrile functional groups that mimic naturally occurring protonophores. These bio-inspired CN-based proton carriers with tailorable proton kinetics were used to turn on the ORR activity of a Cu-based non-precious metal electrocatalyst supported on a modular hybrid bilayer membrane platform under alkaline conditions. In addition, I designed and developed OER electrocatalysts using non-precious metals for energy conversion and storage processes. Hydrogen gas is an alternative fuel that is produced from the electrolysis of water, but technical challenges have heretofore limited the efficiency of water electrolyzers. In order for hydrogen gas to achieve widespread use, it is critical to develop electrocatalysts for the OER that are more cost-effective and widely available than the current state of the art. Thus, I prepared bimetallic electrocatalysts based on Ni and Cu for the OER. I used thin films of Cu2O modified with an overlayer of Ni to construct novel electrocatalysts and determined the optimal ratio of Ni to cuprous oxide for performing the OER in alkaline conditions by tuning the amount of Ni electrodeposited on the Cu2O. Moreover, I developed nanostructured Ni-Cu systems by synthesizing both metallic and bimetallic Ni-Cu nanoclusters and nanoparticles. I found that, for both nanoclusters and nanoparticles, the ratio of Ni to Cu is highly associated with OER electrocatalysis efficiency. Furthermore, I modified carbon electrodes using different compositions of alkyl amine SAMs with various chain lengths and diluent ratios. I investigated the role of defect sites in the SAMs to understand the electron-transfer properties of the appended ferrocene molecules by modifying the SAMs with ZnO electrodeposits. Interestingly, I found that there is a significant change in the electron-transfer rates as a function of SAM linker length when the SAM defect sites are blocked with ZnO electrodeposits. The surface modification protocols used in this study are important in a wide range of applications such as energy catalysis, electroanalysis, and biosensors.
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