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Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Santa Cruz Electronic Theses and Dissertations (2019)

Electrochemical water splitting is a catalytic process in which water molecule can be catalytically reduced to dihydrogen in cathode and oxidized to oxygen molecule in anode. The electrical energy that is used for water splitting can be renewable when the energy source is sunlight, geothermal heat, tide, etc., and the hydrogen gas can thus be generated sustainably. Nonetheless, the hydrogen produced through electrochemical water splitting is less than 10% of the total amount.

The top limitation of its wide applications is the low activity of catalysts in both cathodic reaction (hydrogen evolution reaction, HER) and anodic reaction (oxygen evolution reaction, OER). The dissertation covers heteroatoms doping strategy in activating HER and OER catalysts that I have developed in my PhD study. For instance, N doping has been found to create new facets and active sites of Ni3S2 that are able to improve the hydrogen adsorption. As a result, the overpotential of Ni3S2 can be decreased from 240 mV to 155 mV at 10 mA/cm2, while the TOF of N doped Ni3S2 can be increased as much as 2 times of the pristine Ni3S2. A carbon doping strategy was adopted to activate NiO water-alkali HER catalyst. Combined our experimental and theoreticality study, carbon doping has created under-coordinated Ni sites that are favorable for hydrogen adsorption. Meanwhile, the carbon dopant also serves as the “hot-spot” in water dissociation that contributes to the improved kinetics of HER. The carbon doped NiO showed an ultralow overpotential of 29 mV at 10 mA/cm2, even comparable with the benchmark Pt/C catalysts. On the other hand, Fe-doped β-Ni(OH)2 nanosheets supported on Ni foam were developed for OER and able to achieve a low overpotential of 219 mV at geometric area current density of 10 mA cm-2, and a high electrochemical surface area current density of 6.25 mA cm-2 at the overpotential of 300 mV. This high intrinsic catalytic activity should be due to the strong electron-withdrawing ability of Fe dopant that makes the adjacent Ni active in OER, as well as the unique mixed amorphous/crystalline heterogeneous structure as preferential adsorption sites towards OER intermediates.

    Cover page: TRANSITION METAL BASED ELECTROCATALYSTS FOR WATER SPLITTING
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2019)
    The increasing demand for rare earth elements (REEs) in the modern economy motivates the development of novel strategies for cost-effective REE recovery from nontraditional feedstocks. We previously engineered E. coli to express lanthanide binding tags on the cell surface, which increased the REE biosorption capacity and selectivity. Here we examined how REE adsorption by the engineered E. coli is affected by various geochemical factors relevant to geothermal fluids, including total dissolved solids (TDS), temperature, pH, and the presence of specific competing metals. REE biosorption is robust to TDS, with high REE recovery efficiency and selectivity observed with TDS as high as 165,000 ppm. Among several metals tested, U, Al, and Pb were found to be the most competitive, causing >25% reduction in REE biosorption when present at concentrations ∼3- to 11-fold higher than the REEs. Optimal REE biosorption occurred between pH 5-6, and sorption capacity was reduced by ∼65% at pH 2. REE recovery efficiency and selectivity increased as a function of temperature up to ∼70 °C due to the thermodynamic properties of metal complexation on the bacterial surface. Together, these data define the optimal and boundary conditions for biosorption and demonstrate its potential utility for selective REE recovery from geofluids.
      Cover page: Recovery of Rare Earth Elements from Geothermal Fluids through Bacterial Cell Surface Adsorption
      • Article
      • Peer Reviewed
      UC Santa Cruz Previously Published Works (2019)
      Oxygen evolution reaction (OER) involves multiple electron-transfer processes, resulting in a high activation barrier. Developing catalysts with low overpotential and high intrinsic activity toward OER is critical but challenging. Here we report a major advancement in decreasing the overpotential for oxygen evolution reaction. Ni foam-supported Fe-doped β-Ni(OH) 2 nanosheets achieve an overpotential of 219 mV at the geometric current density of 10 mA cm -2 . To our knowledge, this is the best value reported for Ni- or Fe hydroxide-based OER catalysts. In addition, the catalyst yields a current density of 6.25 mA cm -2 at the overpotential of 300 mV when it is normalized to the electrochemical surface area of the catalyst. This intrinsic catalytic activity is also better than the values reported for most state-of-the-art OER catalysts at the same overpotential.
        Cover page: Ni Foam-Supported Fe-Doped β‑Ni(OH)2 Nanosheets Show Ultralow Overpotential for Oxygen Evolution Reaction
        • Article
        • Peer Reviewed
        UC Santa Cruz Previously Published Works (2023)
        Abstract: The significant amount of gas bubbles generated during high‐rate alkaline water splitting (AWS) can be detrimental to the process. The accumulation of bubbles will block the active catalytic sites and hinder the ion and electrolyte diffusion, limiting the maximum current density. Furthermore, the detachment of large bubbles can also damage the electrode's surface layer. Here, a general strategy for facilitating bubble detachment is demonstrated by modifying the nickel electrode surface with nickel nanocone nanostructures, which turns the surface into underwater superaerophobic. Simulation and experimental data show that bubbles take a considerably shorter time to detach from the nanocone‐modified nickel foil than the unmodified foil. As a result, these bubbles also have a smaller detachment size and less chance for bubble coalescence. The nanocone‐modified electrodes, including nickel foil, nickel foam, and 3D‐printed nickel lattice, all show substantially reduced overpotentials at 1000 mA cm−2 compared to their pristine counterpart. The electrolyzer assembled with two nanocone‐modified nickel lattice electrodes retains >95% of the performance after testing at ≈900 mA cm−2 for 100 h. The surface NC structure is also well preserved. The findings offer an exciting and simple strategy for enhancing the bubble detachment and, thus, the electrode activity for high‐rate AWS.
          Cover page: Nanocone‐Modified Surface Facilitates Gas Bubble Detachment for High‐Rate Alkaline Water SplittingCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC Santa Cruz Previously Published Works (2020)
          Hydrogen evolution reaction (HER) is more sluggish in alkaline than in acidic media because of the additional energy required for water dissociation. Numerous catalysts, including NiO, that offer active sites for water dissociation have been extensively investigated. Yet, the overall HER performance of NiO is still limited by lacking favorable H adsorption sites. Here we show a strategy to activate NiO through carbon doping, which creates under-coordinated Ni sites favorable for H adsorption. DFT calculations reveal that carbon dopant decreases the energy barrier of Heyrovsky step from 1.17 eV to 0.81 eV, suggesting the carbon also serves as a hot-spot for the dissociation of water molecules in water-alkali HER. As a result, the carbon doped NiO catalyst achieves an ultralow overpotential of 27 mV at 10 mA cm-2, and a low Tafel slope of 36 mV dec-1, representing the best performance among the state-of-the-art NiO catalysts.
            Cover page: Carbon doping switching on the hydrogen adsorption activity of NiO for hydrogen evolution reaction
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