UC Berkeley

Using inputs of only sunlight, electricity, carbon dioxide (CO2), and water, photoelectrochemical CO2 reduction could help mitigate climate change while producing valuable fuels or chemicals. Current CO2 reduction technologies suffer from high overpotentials and low selectivity, producing a mixture of carbon monoxide (CO), methane, ethylene, formate, methanol, and other products. Plasmonic hot carriers and the strong local electric fields produced by plasmon excitation may open new mechanistic pathways for electrochemical reactions. Plasmon decay generates hot electrons that can be transferred selectively to an unoccupied electronic state of a surface molecule. Simultaneously, the enhanced electric fields can alter the electronic coupling with surface adsorbed molecules, thereby changing the binding energy of these species and the catalytic properties of plasmonic metals.

This dissertation aims to understand the mechanism of plasmon-enhanced electrocatalysis and how it can be used to direct the selectivity of CO2 reduction at voltage-biased cathodes. First, the relatively new field of plasmon-enhanced electrochemical conversion (PEEC) is compared with the more established field of photoelectrochemistry (PEC). This chapter illustrates the best practices of PEC that can be applied to PEEC and the additional factors that must be considered. Special emphasis is placed on temperature control, light flux, and changes to the surface under reaction conditions.

With these considerations in mind, I developed the first front-illuminated, temperature- controlled electrochemical cell with features that allow for precise gas and liquid product analysis. The cell design maximizes the electrode surface area to electrolyte volume ratio to increase liquid product concentration for enhanced detection and quantification. Gas is bubbled through the catholyte during operation to maintain a saturated reactant concentration and to continuously mix the electrolyte. This cell was used for all of the product analysis experiments in the following chapters.

The first application of the temperature-controlled photoelectrochemical cell was to investigate CO2 reduction at a plasmonic silver cathode. The silver thin film was electrochemically roughened to enhance the photocurrent and prevent further surface changes during electrochemical experiments. Illumination of this cathode selectively enhanced CO2 reduction products (CO, formate, and methanol) while simultaneously suppressing undesired hydrogen evolution. Strikingly, methanol was produced only upon illumination, representing an improvement in both selectivity and efficiency. CO2 partial pressure experiments revealed that the reduction of CO2 to CO has first-order behavior with respect to PCO2 at all applied potentials in both the dark and the light, likely indicating no change in the rate-determining step upon illumination. The investigation of product distribution with temperature in both the dark and the light demonstrated that the selectivity changes observed upon illumination are not caused by local heating of the cathode surface.

To further understand the plasmonic mechanisms responsible for the enhanced CO2 reduction and suppressed hydrogen evolution at the silver cathode, I conducted an in situ ATR–SEIRAS (attenuated total reflectance–surface-enhanced infrared absorption spectroscopy) study under both dark and illuminated conditions. The onset potential of CO2 reduction to adsorbed CO on the silver surface was found to be −0.25 V versus the reversible hydrogen electrode (VRHE) in both the light and the dark. As the production of gaseous CO was detected in the light near this onset potential but was not observed in the dark until −0.5 VRHE, the light must be assisting the desorption of CO from the surface. This can be understood through a desorption induced by electronic transitions (DIET) mechanism, where a plasmonically excited hot electron temporarily transfers to an adsorbed species and increases the adsorbate energy level above the energy of desorption before decaying back to the metal Fermi level. The bicarbonate wavenumber and peak area were observed to increase immediately upon illumination, precluding a thermal effect. The enhanced local electric field that results from the localized surface plasmon resonance (LSPR) could be strengthening the bicarbonate bond, further increasing the local pH. This would account for the decrease of hydrogen formation and increase of CO2 reduction products in the light.

Finally, I sought to combine the plasmonic properties of silver with the catalytic properties of copper, which is well known for its ability to form many two- and three-carbon CO2 reduction products. Unlike the pure silver cathode where all CO2 reduction products were enhanced in the light, the copper–silver cathode selectively promoted 5 of 15 products. At low overpotentials CO was promoted in the light while hydrogen was suppressed, and at high overpotentials ethylene, methane, formate, and allyl alcohol were enhanced upon illumination; generally C1 products and C2/C3 products containing a double carbon bond were selectively promoted under illumination.