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Open Access Publications from the University of California

Scholarly Works (8 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2020)

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.

    Cover page: Plasmon-Enhanced Electrochemical Reduction of Carbon Dioxide
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2020)
    In this perspective we compare plasmon-enhanced electrochemical conversion (PEEC) with photoelectrochemistry (PEC). PEEC is the oxidation or reduction of a reactant at the illuminated surface of a plasmonic metal (or other conductive material) while a potential bias is applied. PEC uses solar light to generate photoexcited electron-hole pairs to drive an electrochemical reaction at a biased or unbiased semiconductor photoelectrode. The mechanism of photoexcitation of charge carriers is different between PEEC and PEC. Here we explore how this difference affects the response of PEEC and PEC systems to changes in light, temperature, and surface morphology of the photoelectrode.
      Cover page: Important Considerations in Plasmon-Enhanced Electrochemical Conversion at Voltage-Biased Electrodes
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2020)
      Electrochemically deposited copper nanostructures were coated with silver to create a plasmonically active cathode for carbon dioxide (CO2) reduction. Illumination with 365 nm light, close to the peak plasmon resonance of silver, selectively enhanced 5 of the 14 typically observed copper CO2 reduction products while simultaneously suppressing hydrogen evolution. At low overpotentials, carbon monoxide was promoted in the light 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. Temperature-dependent product analysis in the dark showed that local heating is not the cause of these selectivity changes. While the exact plasmonic mechanism is still unknown, these results demonstrate the potential for enhancing CO2 reduction selectivity at copper electrodes using plasmonics.
        Cover page: Reduction of carbon dioxide at a plasmonically active copper–silver cathode
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2019)
        Catalysts for electrochemical carbon dioxide reduction in aqueous electrolytes suffer from high energy input requirements, competition with hydrogen evolution from water reduction, and low product selectivity. Theory suggests that plasmonic catalysts can be tuned to selectively lower the energy barrier for a specific reaction in a set of competitive reactions, but there has been little experimental evidence demonstrating plasmon-driven selectivity in complicated multielectron electrochemical processes. Here, the photoactivity at a plasmonically active silver thin film electrode at small cathodic potentials selectively generates carbon monoxide while simultaneously suppressing hydrogen production. At larger cathodic potentials, the photoactivity promotes production of methanol and formate. Methanol production is observed only under illumination, not in dark conditions. The preference of the plasmonic activity for carbon dioxide reduction over hydrogen evolution and the ability to tune plasmonic activity with voltage demonstrates that plasmonics provide a promising approach to promote complex electrochemical reactions over other competing reactions.
          Cover page: Directing Selectivity of Electrochemical Carbon Dioxide Reduction Using Plasmonics
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2021)
          Carbon dioxide (CO2) reduction at a plasmonically active silver cathode was investigated by varying the pressure and temperature at multiple applied potentials under both dark and illuminated conditions to understand the mechanism of selectivity changes driven by plasmon-enhanced electrochemical conversion. CO2 partial pressures (PCO2) from 0.2 to 1 atm were studied during linear sweep voltammetry and chronoamperometry at −0.7, −0.9, and −1.1 VRHE. At a given applied overpotential the total current density increased with increasing PCO2 in both the dark and the light, but there were significant differences in the Tafel behavior between dark and illuminated conditions. The reduction of CO2 to carbon monoxide (CO) was found to have 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. Activity for the hydrogen (H2) evolution reaction decreased with increasing PCO2 at slightly different rates in the dark and the light at each applied potential, making it unclear if light is influencing CO or H2 intermediate adsorbate coverage. Both formate and methanol production showed no dependence on PCO2 under any conditions, but the true reaction orders may be masked by the much higher activity for CO and H2 at the silver cathode. The investigation of product distribution with temperature at 14, 22, and 32∘C at −0.7, −0.9, and −1.1 VRHE 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.
            Cover page: Effect of pressure and temperature on carbon dioxide reduction at a plasmonically active silver cathode
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2018)
            In this study, we describe the design and operation of a temperature-controlled photoelectrochemical cell for analysis of gaseous and liquid products formed at an illuminated working electrode. This cell is specifically designed to quantitatively analyze photoelectrochemical processes that yield multiple gas and liquid products at low current densities and exhibit limiting reactant concentrations that prevent these processes from being studied in traditional single chamber electrolytic cells. The geometry of the cell presented in this paper enables front-illumination of the photoelectrode and maximizes the electrode surface area to electrolyte volume ratio to increase liquid product concentration and hence enhances ex situ spectroscopic sensitivity toward them. Gas is bubbled through the electrolyte in the working electrode chamber during operation to maintain a saturated reactant concentration and to continuously mix the electrolyte. Gaseous products are detected by an in-line gas chromatograph, and liquid products are analyzed ex situ by nuclear magnetic resonance. Cell performance was validated by examining carbon dioxide reduction on a silver foil electrode, showing comparable results both to those reported in the literature and identical experiments performed in a standard parallel-electrode electrochemical cell. To demonstrate a photoelectrochemical application of the cell, CO2 reduction experiments were carried out on a plasmonic nanostructured silver photocathode and showed different product distributions under dark and illuminated conditions.
              Cover page: A temperature-controlled photoelectrochemical cell for quantitative product analysis
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2018)
              Electrochemical reduction of carbon dioxide (CO2) typically suffers from low selectivity and poor reaction rates that necessitate high overpotentials, which impede its possible application for CO2 capture, sequestration, or carbon-based fuel production. New strategies to address these issues include the utilization of photoexcited charge carriers to overcome activation barriers for reactions that produce desirable products. This study demonstrates surface-plasmon-enhanced photoelectrochemical reduction of CO2 and nitrate (NO3−) on silver nanostructured electrodes. The observed photocurrent likely originates from a resonant charge transfer between the photogenerated plasmonic hot electrons and the lowest unoccupied molecular orbital (MO) acceptor energy levels of adsorbed CO2, NO3−, or their reductive intermediates. The observed differences in the resonant effects at the Ag electrode with respect to electrode potential and photon energy for CO2 versus NO3− reduction suggest that plasmonic hot-carriers interact selectively with specific MO acceptor energy levels of adsorbed surface species such as CO2, NO3−, or their reductive intermediates. This unique plasmon-assisted charge generation and transfer mechanism can be used to increase yield, efficiency, and selectivity of various photoelectrochemical processes.
                Cover page: Surface‐Plasmon‐Assisted Photoelectrochemical Reduction of CO2 and NO3− on Nanostructured Silver Electrodes
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2020)
                Illumination of a voltage-biased plasmonic Ag cathode during CO2 reduction results in a suppression of the H2 evolution reaction while enhancing CO2 reduction. This effect has been shown to be photonic rather than thermal, but the exact plasmonic mechanism is unknown. Here, we conduct an in situ ATR-SEIRAS (attenuated total reflectance-surface-enhanced infrared absorption spectroscopy) study of a sputtered thin film Ag cathode on a Ge ATR crystal in CO2-saturated 0.1 M KHCO3 over a range of potentials under both dark and illuminated (365 nm, 125 mW cm-2) conditions to elucidate the nature of this plasmonic enhancement. We find that the onset potential of CO2 reduction to adsorbed CO on the Ag surface is -0.25 VRHE and is identical in the light and the dark. As the production of gaseous CO is detected in the light near this onset potential but is not observed in the dark until -0.5 VRHE, we conclude that the light must be assisting the desorption of CO from the surface. Furthermore, the HCO3- wavenumber and peak area increase immediately upon illumination, precluding a thermal effect. We propose that the enhanced local electric field that results from the localized surface plasmon resonance (LSPR) is strengthening the HCO3- bond, further increasing the local pH. This would account for the decrease in H2 formation and increase the CO2 reduction products in the light.
                • 1 supplemental PDF
                Cover page: In Situ ATR–SEIRAS of Carbon Dioxide Reduction at a Plasmonic Silver Cathode
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