Electrochemical Characterization of Immobilized Organometallic Complexes on Metallic and Semiconducting Electrodes

Reducing carbon emissions is necessary to mitigate climate change effects. Intermittent renewable energy sources such as wind and solar are promising alternatives to fossil fuels; however, novel energy storage solutions are needed to be able to meet the continuous demands of the electric grid. An attractive option is to store renewable energy in chemical bonds by using renewable electricity to drive electrochemical fuel forming reactions such as CO2 reduction, O2 reduction reaction, water oxidation, methane oxidation and N2 reduction. However, these processes are thermodynamically and kinetically challenging, requiring efficient electrocatalysts to mediate electrochemical reactions. Many molecular electrocatalysts have been studied and optimized to tune the overpotential, kinetics and selectivity of these fuel forming processes; however, they are challenging to recycle, have poor atom economy and not easily integrated into practical devices. A promising alternative is to immobilize electrocatalysts on electrode surfaces to generate hybrid electrodes, with the optimized reactivity and selectivity of the molecular catalyst. Unfortunately, immobilization strategies are limited and can require harsh conditions that decompose sensitive molecular electrocatalysts. Further, it is poorly understood how immobilization changes the local environment around the catalyst and its impact on catalytic behavior. This work has sought to develop new strategies for generation of hybrid electrodes with immobilized electrocatalysts and to elucidate the differences in reactivity that occur upon immobilization of redox active complexes. First, the work discusses a sonochemical hydrosilylation strategy for generating mixed monolayers on doped silicon of a molecular electrocatalyst for CO2 reduction and an organic molecule that both passivates remaining Si atop sites and can modulate the local environment and band structure. This method can produce hybrid photoelectrodes competent for CO2 reduction to CO under 1 sun illumination and applied negative bias. Next, an improved Grignard strategy for preparation of redox active monolayers on carbon materials is discussed and the impacts of disorder in the monolayer on charge transfer are thoroughly analyzed. Finally, a molecular model system for a methane oxidation catalyst is thoroughly characterized via electroanalytical techniques, elucidating the competing bimolecular reaction pathways that are gated by axial ligation.