UC Berkeley

Electrochemical carbon dioxide (CO2) reduction is one possible component of the effort to mitigate atmospheric CO2 concentration and develop a reliable renewable fuel source. However, CO2 reduction suffers from the high overpotentials needed to overcome sluggish kinetics, low selectivity for desired products, and unfavorable competition with water reduction (hydrogen evolution reaction (HER)). Challenges limiting energy efficiency and selective product formation must be addressed to make CO2 reduction a viable energy storage solution.

Plasmonic catalysis may bypass the shortcomings in traditional "dark" catalysis for multi-electron reductions because of its ability to couple light energy into surface chemistry. Surface plasmon resonance (SPR) in nanostructured metals has been shown to increase

reaction rates or alter selectivity in a variety of reactions, including gas-phase CO2 reduction. However, the effect of SPR on the coulombic efficiency and selectivity of electrochemical CO2 reduction product(s) has not been extensively explored despite the promising improvements in selectivity seen in purely plasmonic photocatalytic (without voltage bias) CO2 reduction.

In this dissertation, I explore the effect of SPR on aqueous electrochemical CO2 reduction. Incorporating illumination into any electrochemical system poses electrochemical cell geometry and temperature regulation challenges. In order to promote experiment reproducibility and enable direct comparison of photoelectrochemistry (PEC) studies across various laboratories, I outline a few best practices. The temperature of the electrochemical cell affects not only the reaction rate but also reaction selectivity in reactions with multiple products, and illumination may increase the temperature of the electrochemical cell components dramatically. Thus, the temperature of the electrochemical cell must either be regulated or reported. The materials, electrolyte, and path length from the surface to the photoelectrode vary between PEC cells. Each of these factors influences the light attenuation, necessitating that the light flux be measured at the surface of the photoelectrode so that conditions can be repeated in different cells. Additionally, a careful comparison of the photoelectrode before and after PEC is required for adequate stability characterization because both photoexcitation and applied voltage can affect the composition and morphology of materials.

Incorporating illumination into an electrochemical carbon dioxide reduction (CO2R) system poses additional challenges due to the requirements of supplying a gas (CO2) to the reaction vessel and detecting trace amounts of both gaseous and soluble products. Parallel electrodes reduce the potential gradients across their surfaces, front illumination of the photoelectrode allows more photoexcited charge carriers to reach the electrtrochemically active surface, small catholyte and anolyte volumes allows decreased electrolysis times necessary to detect trace soluble products, continuous CO2 sparging maintains CO2 saturation of the electrolyte and carries evolved gaseous products to an in-line gas chromatograph (GC), a temperature control system maintains a constant catholyte temperature so that the effects of temperature and illumination can be decoupled, a membrane separating the catholyte and anolyte to prevent soluble CO2R products from being oxidized at the anode. The CO2R product distribution and current density of silver foil electrodes is indistinguishable between this PEC cell and a conventional compression electrochemical CO2R cell.

The newly developed front-illumination electrochemical CO2R cell allows for the full characterization of the electrochemical CO2R performance of an illuminated and unilluminated electron-beam-deposited thin film polycrystalline silver electrode. The as-deposited electrode increases in absorption, decreases in elctrochemical surface area (ECSA), and broadens in particle size distribution over 45 min of elctrochemical CO2R, but these properties are then stable for hours of CO2R. The electrode exhibits a SPR absorption peak at 351 nm and was illuminated with a 365 nm light-emitting diode (LED). At small applied overpotentials (−0.8 to −0.6 Volts versus the reversible hydrogen electrode (VRHE)), illuminating the thin film Ag electrode increases the production rate of carbon monoxide (CO) and decreases the hydrogen (H2) production rate. This may be the first time that one reaction is enhanced while another is simultaneously suppressed by a plasmonic effect. Illuminating the Ag electrode decreases the overpotential required to generate CO selectively. The plasmonic effect suppresses the total reaction rate when the electrolyte is saturated with inert Ar rather than CO2. This

may be the first electrochemical reaction that is suppressed through a photoeffect. The CO2 reduction product that is promoted with plasmon excitation depends on the applied voltage. At larger cathodic potentials (−1.1 to −0.8 VRHE), methanol and formate production

rates are increased while CO and H2 production rates remain the same. This is the first demonstration of potential-dependent selectivity in any plasmon-enhanced reaction. The photoeffect is proportional to absorption and scales linearly with light intensity, two strong

indicators that the activity is photonic rather than thermal in nature.

The nanofeatures on the silver thin film electrode are not intentionally controlled, but it still exhibited remarkable photoactivity and selectivity for CO2R. Perhaps if the nanofeatures were rationally designed with specific sizes, shapes, and compositions, the photoactivity could be increased or the selectivity altered. Solution-based, electrochemical, and lithographic approaches to nanosynthesis are explored for creating nanopatterned electrodes. Though nanofeature-modified electrode stability has limited the investigation of these well-defined photoelectrodes, there may be additional strategies that have not yet been explored for

stabilization.

There is still more work needed to understand the mechanism for plasmon-enhanced electrochemical CO2R and increase the energy efficiency of plasmonic CO2R, but this work clearly demonstrates the unique selectivity achievable through the combination of plasmonics and electrochemistry. Plasmonics may also be a valuable tool in other multi-electron, multiproduct electrochemical reactions such as nitrogen (N2) reduction.