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Open Access Publications from the University of California

Scholarly Works (11 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2016)

Renewable energy is becoming an increasingly important component of the world’s energy supply as the threat of global warming continues to rise. There is a need to reduce the cost of this renewable energy and a future challenge to deal with the strain intermittent power sources like renewables place on the power grid. In this dissertation, electrochemistry is harnessed to address possible solutions to both of these issues. First, it is used to develop a low cost alternative photovoltaic material. Then, it is used to investigate the production of chemical fuel stocks which can be used for energy storage.

In chapter 2, advances are made in the electrochemical deposition of indium (In) on molybdenum foil which enables the deposition of electronic-grade purity, continuous films with thicknesses in the micron range. As an example application, the electrodeposited In films are phosphorized via the thin-film vapor-liquid-solid growth method. The resulting poly-crystalline InP films display excellent optoelectronic quality, comparable to films grown from more standard vacuum deposition techniques. This demonstrates the versatility of the developed electrochemical deposition procedure.

In the remaining chapters, renewable fuel production is investigated. First in chapter 3, molybdenum disulfide (MoS2) is examined as a catalyst for the hydrogen evolution reaction (HER). Typically, high-cost synthesized MoS2 is used as the catalyst because the pristine MoS2 mineral is known to be a poor catalyst. The fundamental challenge with pristine MoS2 is the inert HER activity of the predominant (0001) basal surface plane. Here, we report a general thermal process in which the basal plane is texturized to increase the density of HER-active edge sites. The process generates high HER catalytic performance in pristine MoS2 across various morphologies such as the bulk mineral, films composed of micron-scale flakes, and even films of a commercially-available spray of nanoflake MoS2.

In chapters 4-6, the electrochemical reduction of carbon dioxide (CO2R) is investigated as this reaction can produce hydrocarbons and alcohols as opposed to just hydrogen. First in chapter 4, the electrochemical cell, which is used to test the activity of CO2R catalysts, is scrutinized. The electrochemical cell is a mini-chemical reactor and it is important to monitor the reaction conditions within the reactor to ensure that they are constant throughout the study. I show that operating cells with high catalyst surface area to electrolyte volume ratios (S/V) at high current densities can have subtle consequences during CO2R, particularly as they relate to the bulk electrolyte CO2 concentration. By using the pH in the cell to measure the CO2 concentration, significant undersaturation of CO2 is observed in the bulk electrolyte, even at modest current densities of 10 mA cm-2. Undersaturation of CO2 produces large changes in the faradaic efficiency observed on copper electrodes, with hydrogen production becoming increasingly favored. I show that the size of the CO2 bubbles being introduced into the cell is critical for maintaining the equilibrium CO2 concentration in the electrolyte and I design an electrochemical cell that is able to maintain near-equilibrium CO2 concentrations for catalyst testing.

Then in chapter 5, the use of selected-ion flow-tube mass spectrometry (SIFT-MS) as an analytical tool to detect the products of CO2R is investigated. The real-time analysis of the products of CO2R is especially of interest to enable the study of how and when the liquid products of the reaction are generated. This is currently only possible in some limited situations and an analytical tool that can do quantitative analysis of all the products of the CO2R reaction in real-time does not exist. I show that SIFT-MS is a promising technique which can uniquely detect the hydrocarbon, alcohols, and aldehydes produced during CO2R on copper. Furthermore, SIFT-MS should be able to provide quantitative results; however, further study is needed to rigorously prove this.

Finally in chapter 6, a CO2R catalyst platform is developed based on templated electrochemically deposited nanowires. This platform is used to investigate the role of high surface area in catalyst activity and selectivity. It is found that high surface area Cu nanowires can be made that still produce hydrocarbons, in contrast to several other reports in the literature. This platform is also used to investigate the sequential reduction of CO2. Here silver nanowires are deposited on top of a planar Cu substrate. The Ag can convert the CO2 to CO in close proximity to the Cu catalyst which can further upgrade the CO to hydrocarbons and alcohols. It is found that the sequential catalysis approach successfully selects for the production of hydrocarbons through the 2 electron CO intermediate and shuts down the production of the competing 2 electron product formic acid which is a dead-end reaction pathway.

    Cover page: Electrochemical Approaches to Renewable Energy
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2017)
    The production of liquid fuel products via electrochemical reduction of CO2 is a potential path to produce sustainable fuels. However, to be practical, a separation strategy is required to isolate the fuel-containing electrolyte produced at the cathode from the anode and also prevent the oxidation products (i.e. O2) from reaching the cathode. Ion-conducting membranes have been applied in CO2 reduction reactors to achieve this separation, but they represent an efficiency loss and can be permeable to some product species. An alternative membraneless approach is developed here to maintain product separation through the use of a laminar flow cell. Computational modelling shows that near-unity separation efficiencies are possible at current densities achievable now with metal cathodes via optimization of the spacing between the electrodes and the electrolyte flow rate. Laminar flow reactor prototypes were fabricated with a range of channel widths by 3D printing. CO2 reduction to formic acid on Sn electrodes was used as the liquid product forming reaction, and the separation efficiency for the dissolved product was evaluated with high performance liquid chromatography. Trends in product separation efficiency with channel width and flow rate were in qualitative agreement with the model, but the separation efficiency was lower, with a maximum value of 90% achieved.
      Cover page: Membraneless laminar flow cell for electrocatalytic CO2 reduction with liquid product separation
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2016)
      Favoring the CO2 reduction reaction (CO2RR) over the hydrogen evolution reaction and controlling the selectivity towards multicarbon products are currently major scientific challenges in sustainable energy research. It is known that the morphology of the catalyst can modulate catalytic activity and selectivity, yet this remains a relatively underexplored area in electrochemical CO2 reduction. Here, we exploit the material tunability afforded by colloidal chemistry to establish unambiguous structure/property relations between Cu nanocrystals and their behavior as electrocatalysts for CO2 reduction. Our study reveals a non-monotonic size-dependence of the selectivity in cube-shaped copper nanocrystals. Among 24 nm, 44 nm and 63 nm cubes tested, the cubes with 44 nm edge length exhibited the highest selectivity towards CO2RR (80 %) and faradaic efficiency for ethylene (41 %). Statistical analysis of the surface atom density suggests the key role played by edge sites in CO2RR.
        Cover page: Tailoring Copper Nanocrystals towards C2 Products in Electrochemical CO2 Reduction
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2016)
        In the last few years, there has been increased interest in electrochemical CO2 reduction (CO2R). Many experimental studies employ a membrane separated, electrochemical cell with a mini H-cell geometry to characterize CO2R catalysts in aqueous solution. This type of electrochemical cell is a mini-chemical reactor and it is important to monitor the reaction conditions within the reactor to ensure that they are constant throughout the study. We show that operating cells with high catalyst surface area to electrolyte volume ratios (S/V) at high current densities can have subtle consequences due to the complexity of the physical phenomena taking place on electrode surfaces during CO2R, particularly as they relate to the cell temperature and bulk electrolyte CO2 concentration. Both effects were evaluated quantitatively in high S/V cells using Cu electrodes and a bicarbonate buffer electrolyte. Electrolyte temperature is a function of the current/total voltage passed through the cell and the cell geometry. Even at a very high current density, 20 mA cm-2, the temperature increase was less than 4 °C and a decrease of <10% in the dissolved CO2 concentration is predicted. In contrast, limits on the CO2 gas-liquid mass transfer into the cells produce much larger effects. By using the pH in the cell to measure the CO2 concentration, significant undersaturation of CO2 is observed in the bulk electrolyte, even at more modest current densities of 10 mA cm-2. Undersaturation of CO2 produces large changes in the faradaic efficiency observed on Cu electrodes, with H2 production becoming increasingly favored. We show that the size of the CO2 bubbles being introduced into the cell is critical for maintaining the equilibrium CO2 concentration in the electrolyte, and we have designed a high S/V cell that is able to maintain the near-equilibrium CO2 concentration at current densities up to 15 mA cm-2.
          Cover page: Effects of temperature and gas–liquid mass transfer on the operation of small electrochemical cells for the quantitative evaluation of CO 2 reduction electrocatalysts
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2018)
          Electrochemical CO2 reduction (EC-CO2R) has seen a resurgence in interest over the past several years; however, the means of analyzing catalytically produced products continues to rely on decades-old methods such as gas chromatography, high-performance liquid chromatography, and nuclear magnetic resonance. Real-time analysis of the gaseous and liquid products of this reaction is highly desirable; however, few analytical techniques have been developed thus far to meet this need. Here, we demonstrate the first use of selected-ion flow-tube mass spectrometry (SIFT-MS) as an analytical tool capable of measuring in real time both the gas- and liquid-phase products of EC-CO2R in aqueous solution. SIFT-MS uses well-understood ion–molecule reactions to enable the analysis of similar multicomponent mixtures by preventing substantial fragmentation of the analyte. We lay out the framework in which to evaluate the tool's capabilities and show that the C1–C3 hydrocarbon, alcohol, and aldehyde products of CO2R should be quantitatively detectable.
            Cover page: Initial Application of Selected‐Ion Flow‐Tube Mass Spectrometry to Real‐Time Product Detection in Electrochemical CO2 Reduction
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2017)
            Copper electrodes, prepared by reduction of oxidized metallic copper, have been reported to exhibit higher activity for the electrochemical reduction of CO2 and better selectivity toward C2 and C3 (C2+) products than metallic copper that has not been preoxidized. We report here an investigation of the effects of four different preparations of oxide-derived electrocatalysts on their activity and selectivity for CO2 reduction, with particular attention given to the selectivity to C2+ products. All catalysts were tested for CO2 reduction in 0.1 M KHCO3 and 0.1 M CsHCO3 at applied voltages in the range from -0.7 to -1.0 V vs RHE. The best performing oxide-derived catalysts show up to ∼70% selectivity to C2+ products and only ∼3% selectivity to C1 products at -1.0 V vs RHE when CsHCO3 is used as the electrolyte. In contrast, the selectivity to C2+ products decreases to ∼56% for the same catalysts tested in KHCO3. By studying all catalysts under identical conditions, the key factors affecting product selectivity could be discerned. These efforts reveal that the surface area of the oxide-derived layer is a critical parameter affecting selectivity. A high selectivity to C2+ products is attained at an overpotential of -1 V vs RHE by operating at a current density sufficiently high to achieve a moderately high pH near the catalyst surface but not so high as to cause a significant reduction in the local concentration of CO2. On the basis of recent theoretical studies, a high pH suppresses the formation of C1 relative to C2+ products. At the same time, however, a high local CO2 concentration is necessary for the formation of C2+ products. (Graph Presented).
              Cover page: Optimizing C–C Coupling on Oxide-Derived Copper Catalysts for Electrochemical CO2 Reduction
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2016)
              Carbon materials are frequently used as supports for electrocatalysts because they are conductive and have high surface area. However, recent studies have shown that these materials can contain significant levels of metallic impurities that can dramatically alter their electrochemical properties. Here, the electrocatalytic activity of pure graphite (PG), graphene oxide (GO), and carbon nanotubes (CNT) dispersed on glassy carbon (GC) are investigated for the electrochemical CO2 reduction reaction (CO2RR) in aqueous solution. It was observed that GO and CNT dispersed on GC all exhibit significant electrochemical activity that can be ascribed to impurities of Ni, Fe, Mn, and Cu. The level of Cu in GO can be particularly high and is the cause for the appearance of methane in the products produced over this material when it is used for the CO2RR. Washing these supports in ultrapure nitric acid is effective in removing the metal impurities and results in a reduction in the electrochemical activity of these forms of carbon. In particular, for GO, nearly all of the catalytically relevant metals can be removed. Electrochemical deposition of Cu on GO and PG supported on GC, and on GC itself, increased both the electrochemical activity of these materials and the production of methane via the CO2RR. Particularly high rates of methane formation per unit of Cu mass were obtained for Cu electrodeposited on GO and PG supported on GC. We suggest that this high activity may be due to the preferential deposition of Cu onto defects present in the graphene sheets comprising these materials.
                Cover page: Trace Levels of Copper in Carbon Materials Show Significant Electrochemical CO2 Reduction Activity
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2013)
                III-V photovoltaics (PVs) have demonstrated the highest power conversion efficiencies for both single- and multi-junction cells. However, expensive epitaxial growth substrates, low precursor utilization rates, long growth times, and large equipment investments restrict applications to concentrated and space photovoltaics (PVs). Here, we demonstrate the first vapor-liquid-solid (VLS) growth of high-quality III-V thin-films on metal foils as a promising platform for large-area terrestrial PVs overcoming the above obstacles. We demonstrate 1-3 μm thick InP thin-films on Mo foils with ultra-large grain size up to 100 μm, which is ~100 times larger than those obtained by conventional growth processes. The films exhibit electron mobilities as high as 500 cm²/V-s and minority carrier lifetimes as long as 2.5 ns. Furthermore, under 1-sun equivalent illumination, photoluminescence efficiency measurements indicate that an open circuit voltage of up to 930 mV can be achieved, only 40 mV lower than measured on a single crystal reference wafer.
                  Cover page: A direct thin-film path towards low-cost large-area III-V photovoltaics.
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2020)
                  Conversion of CO2 to reduced products is a promising route to alleviate irreversible climate change. Here we report the synthesis of a Co-based phthalocyanine with pyridine moieties (CoPc-Pyr), which is supported on a carbon electrode and shows Faradaic efficiency ∼90% for CO at 490 mV of overpotential (-0.6 V vs reversible hydrogen electrode (RHE)). In addition, its catalytic activity at -0.7 V versus RHE surpasses other Co-based molecular and metal-organic framework catalysts for CO2 reduction at this bias. Density functional theory calculations show that pyridine moieties enhance CO2 adsorption and electron affinity of the Co center by an inductive effect, thus lowering the overpotential necessary for CO2 conversion. Our study shows that CoPc-Pyr reduces CO2 at lower overpotential and with higher activity than noble metal electrodes, such as silver.
                    Cover page: Heterogenized Pyridine-Substituted Cobalt(II) Phthalocyanine Yields Reduction of CO2 by Tuning the Electron Affinity of the Co Center
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2018)
                    Copper oxides have been of considerable interest as electrocatalysts for CO2 reduction (CO2R) in aqueous electrolytes. However, their role as an active catalyst in reducing the required overpotential and improving the selectivity of reaction compared with that of polycrystalline copper remains controversial. Here, we introduce the use of selected-ion flow tube mass spectrometry, in concert with chronopotentiometry, in situ Raman spectroscopy, and computational modeling, to investigate CO2R on Cu2O nanoneedles, Cu2O nanocrystals, and Cu2O nanoparticles. We show experimentally that the selective formation of gaseous C2 products (i.e., ethylene) in CO2R is preceded by the reduction of the copper oxide (Cu2OR) surface to metallic copper. On the basis of density functional theory modeling, CO2R products are not formed as long as Cu2O is present at the surface because Cu2OR is kinetically and energetically more favorable than CO2R.
                      Cover page: Investigating the Role of Copper Oxide in Electrochemical CO2 Reduction in Real Time
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