Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (12 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2025)

Energy is crucial for the operation of the modern world. The depletion of fossil fuels and rising global warming challenges urges us to sort alternative energy sources and viable energy storage and delivery technologies. In this regard, hydrogen, an attractive energy carrier with high energy density (140 MJ/kg) has been recognized as a promising alternative to achieve fossil fuels phase-out and limit climate change. The hydrogen economy, consisting of the generation, storage, and utilization of the hydrogen, aims to replace fossil fuels with hydrogen alongside renewable energy to reduce emissions of greenhouse gases. While Pt-based materials overwhelm other catalysts in those reactions, the mechanisms, activities, and stabilities remain challenges for large-scale application. Therefore, my research focuses on investigating and developing Pt-based nanomaterials for various catalysis in hydrogen economy. In Chapter 2, we presented a systematic study into the effects of Na⁺ and OH⁻ on hydrogen evolution reaction/hydrogen oxidation reaction (HER/HOR) activities and mechanisms using twenty-eight electrolytes with independently varied Na⁺ and OH⁻ concentrations ranging from 0.001 M to 1 M. Our study reveals a strong correlation between the electrical double layer (EDL) thickness and HER/HOR rates. It is found that OH⁻ has a positive effect on reducing the EDL thickness, strengthening interfacial electric field, facilitating water dissociation during HER and Had/OH⁻ recombination during HOR, and boosting the corresponding reaction rates. When Na⁺ concentration is below 0.1 M, the Na⁺ initially reduces the EDL thickness and enhances HER/HOR activity. However, further increasing Na⁺ concentration beyond 0.1 M paradoxically increases the EDL thickness and suppresses HER/HOR rates due to the formation of ion pairs at the outer Helmholtz plane under high Na⁺ concentration condition, which weakens the surface electric field and slows the reaction kinetics. This study elucidates the intricate effects of Na⁺ and OH⁻ concentrations on EDL thickness and establishes the critical role of EDL thickness and surface electric field in modifying the surface water structure and thus the HER/HOR kinetics, providing valuable insights for the design of next-generation electrochemical systems. In Chapter 3, using single-atom Rh-tailored Pt nanowires as a model system, we demonstrate that hydroxyl groups adsorbed on the Rh sites (Rh-OHad) can profoundly reorganize the Pt surface water structure to deliver record-setting alkaline HOR performance. In-situ surface characterizations, together with theoretical studies, reveal that surface Rh-OHad could promote oxygen-down H2O↓ configuration that favors more hydrogen bond with Pt surface adsorbed hydrogen (H2O↓···Had-Pt) than that of hydrogen-down configuration (OH2↓). The H2O↓ further serves as the bridge to facilitate the formation of an energetically favorable six-membered-ring transition state structure with neighboring Pt-Had and Rh-OHad, thus reducing the Volmer step activation energy and boosting HOR kinetics, achieving the highest reported specific activity (9.7±0.3 mA/cmPt²) and mass activity (7.3±0.2 A/mgPt) at 0.05 VRHE in 1 M KOH. In Chapter 4, we reported an in-situ synthesized Pt-Cuboctahedra/Al2O3 catalyst with superior catalytic activity for the dehydrogenation of 12H-NEC. By leveraging the unique properties of Pt (100) and Pt (111) facets, our catalyst demonstrated significantly enhanced performance compared to Pt-Cubes/Al2O3, Pt-Octahedra/Al2O3, and commercial Pt/Al2O3. Kinetic analyses confirmed its superior hydrogen generation rate of 5690 mol H2 molPt-1 h-1, alongside an efficient hydrogen release of 5.79 wt% at an ultra-low Pt loading. These findings provide new insights into the structure-activity relationship of Pt-based catalysts and contribute to the development of high-performance liquid organic hydrogen carriers (LOHCs) systems for practical hydrogen storage applications. During my PhD career, I have explored mechanism studies and novel catalysts design to understand and improve the catalysis in reactions involved in hydrogen economy. Theses works can provide invaluable insights in materials design and catalysis optimization for prospective hydrogen applications.

    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2024)
    Microbial fuel cells utilize exoelectrogenic microorganisms to directly convert organic matter into electricity, offering a compelling approach for simultaneous power generation and wastewater treatment. However, conventional microbial fuel cells typically require thick biofilms for sufficient metabolic electron production rate, which inevitably compromises mass and charge transport, posing a fundamental tradeoff that limits the achievable power density (<1 mW cm-2). Herein, we report a concept for redox-mediated microbial flow fuel cells that utilizes artificial redox mediators in a flowing medium to efficiently transfer metabolic electrons from planktonic bacteria to electrodes. This approach effectively overcomes mass and charge transport limitations, substantially reducing internal resistance. The biofilm-free microbial flow fuel cell thus breaks the inherent tradeoff in dense biofilms, resulting in a maximum current density surpassing 40 mA cm-2 and a highest power density exceeding 10 mW cm-2, approximately one order of magnitude higher than those of state-of-the-art microbial fuel cells.
      Cover page: High power density redox-mediated Shewanella microbial flow fuel cells.
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2024)
      The Tafel slope represents a critical kinetic parameter for mechanistic studies of electrochemical reactions, including the hydrogen evolution reaction (HER). Linear fitting of the polarization curve in a N2-saturated electrolyte is commonly used to determine Tafel slopes, which is, however, frequently plagued with inconsistencies. Our systematic studies reveal that the Tafel slopes derived from this approach are loading- and potential-dependent, and could substantially exceed the theoretical limits. Our analyses indicate that this discrepancy is largely attributed to the locally trapped HER-generated H2 in the catalyst layer. A non-negligible hydrogen oxidation reaction (HOR) current more prominently offsets the HER current at the smaller HER overpotential regime, resulting in an artificially smaller Tafel slope. On the other hand, at the higher overpotential where the HOR current becomes negligible, the locally trapped H2 substantially suppresses further HER current growth, leading to an artificially larger Tafel slope. The Butler-Volmer method accounts for both the HER and HOR currents in the fitting, which offers a more reliable method for pure Pt catalysts but is less applicable to transition-metal decorated Pt surfaces with distinct HER/HOR kinetics. Our studies underscore the challenges in Tafel slope analysis and the need for strict controls for reliable comparisons among different catalyst systems.
        Cover page: Unraveling and Resolving the Inconsistencies in Tafel Analysis for Hydrogen Evolution Reactions.Creative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2024)
        Hydrazine assisted water electrolysis provides an attractive pathway for low-voltage hydrogen production while at the same time mitigating the hazardous hydrazine environmental pollutants. Herein we report the design and synthesis of Ru decorated Ag nanoparticles (NPs) where the Ag-Ru interfaces act as highly effective bifunctional electrocatalysts for the hydrazine oxidation reaction (HzOR) and the hydrogen evolution reaction (HER). The electrocatalysts with Ag-Ru interfaces demonstrate improved HzOR performance with lower overpotential, enhanced mass activity (MA) and highly selective oxidation of hydrazine into N2. Density functional theory (DFT) computations reveal the Ag-Ru interfaces feature higher barrier for N-N bond cleavage and easier N2 desorption, contributing to the electrocatalytic activity and selectivity. At the same time, improved HER performance is also observed due to the more favourable hydrogen desorption. Together, by employing the Ru decorated Ag NPs as electrocatalysts for both HzOR and HER, the hydrazine assisted water electrolyser delivers record-high performance with a current density of 100 mA cm−2 at an ultralow cell voltage of 16 mV and a high current density of 983 ± 30 mA cm−2 at a cell voltage of 0.45 V without any IR compensation.
          Cover page: Ag–Ru interface for highly efficient hydrazine assisted water electrolysisCreative Commons 'BY-NC-ND' version 4.0 license
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2024)
          The fundamental understanding of sluggish hydrogen evolution reaction (HER) kinetics on a platinum (Pt) surface in alkaline media is a topic of considerable debate. Herein, we combine cyclic voltammetry (CV) and electrical transport spectroscopy (ETS) approaches to probe the Pt surface at different pH values and develop molecular-level insights into the pH-dependent HER kinetics in alkaline media. The change in HER Tafel slope from ∼110 mV/decade in pH 7-10 to ∼53 mV/decade in pH 11-13 suggests considerably enhanced kinetics at higher pH. The ETS studies reveal a similar pH-dependent switch in the ETS conductance signal at around pH 10, suggesting a notable change of surface adsorbates. Fixed-potential calculations and chemical bonding analysis suggest that this switch is attributed to a change in interfacial water orientation, shifting from primarily an O-down configuration below pH 10 to a H-down configuration above pH 10. This reorientation weakens the O-H bond in the interfacial water molecules and modifies the reaction pathway, leading to considerably accelerated HER kinetics at higher pH. Our integrated studies provide an unprecedented molecular-level understanding of the nontrivial pH-dependent HER kinetics in alkaline media.
            Cover page: Platinum Surface Water Orientation Dictates Hydrogen Evolution Reaction Kinetics in Alkaline Media.Creative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2022)
            The platinum-catalysed hydrogen evolution reaction (HER) generally shows poorer kinetics in alkaline electrolyte and represents a key challenge for alkaline water electrolysis. In the presence of alkali metal cations and hydroxyl anions, the electrode–electrolyte (platinum–water) interface in an alkaline electrolyte is far more complex than that in an acidic electrolyte. Here we combine electrochemical impedance spectroscopy and an electrical transport spectroscopy approach to probe and understand the fundamental role of different cations (Li+, Na+ and K+) in HER kinetics. Our integrated studies suggest that the alkali metal cations play an indirect role in modifying the HER kinetics, with the smaller cations being less destabilizing to the hydroxyl adsorbate (OHad) species in the HER potential window, which favours a higher coverage of OHad on the platinum surface. The surface OHad species are highly polar and act as both electronically favoured proton acceptors and geometrically favoured proton donors to promote water dissociation in alkaline media, thus boosting the Volmer-step kinetics and the HER activity. [Figure not available: see fulltext.]
              Cover page: The role of alkali metal cations and platinum-surface hydroxyl in the alkaline hydrogen evolution reaction
              • Article
              • Peer Reviewed
              UCLA Previously Published Works (2022)
              Electrocatalytic hydrogen evolution reaction (HER) is critical for green hydrogen generation and exhibits distinct pH-dependent kinetics that have been elusive to understand. A molecular-level understanding of the electrochemical interfaces is essential for developing more efficient electrochemical processes. Here we exploit an exclusively surface-specific electrical transport spectroscopy (ETS) approach to probe the Pt-surface water protonation status and experimentally determine the surface hydronium pKa [Formula: see text] 4.3. Quantum mechanics (QM) and reactive dynamics using a reactive force field (ReaxFF) molecular dynamics (RMD) calculations confirm the enrichment of hydroniums (H3O[Formula: see text]) near Pt surface and predict a surface hydronium pKa of 2.5 to 4.4, corroborating the experimental results. Importantly, the observed Pt-surface hydronium pKa correlates well with the pH-dependent HER kinetics, with the protonated surface state at lower pH favoring fast Tafel kinetics with a Tafel slope of 30 mV per decade and the deprotonated surface state at higher pH following Volmer-step limited kinetics with a much higher Tafel slope of 120 mV per decade, offering a robust and precise interpretation of the pH-dependent HER kinetics. These insights may help design improved electrocatalysts for renewable energy conversion.
                Cover page: Determining the hydronium pK[Formula: see text] at platinum surfaces and the effect on pH-dependent hydrogen evolution reaction kinetics.Creative Commons 'BY-NC-ND' version 4.0 license
                • Article
                • Peer Reviewed
                UCLA Previously Published Works (2024)
                The performance of electrocatalysts is critical for renewable energy technologies. While the electrocatalytic activity can be modulated through structural and compositional engineering following the Sabatier principle, the insufficiently explored catalyst-electrolyte interface is promising to promote microkinetic processes such as physisorption and desorption. By combining experimental designs and molecular dynamics simulations with explicit solvent in high accuracy, we demonstrated that dimethylformamide can work as an effective surface molecular pump to facilitate the entrapment of oxygen and outflux of water. Dimethylformamide disrupts the interfacial network of hydrogen bonds, leading to enhanced activity of the oxygen reduction reaction by a factor of 2 to 3. This strategy works generally for platinum-alloy catalysts, and we introduce an optimal model PtCuNi catalyst with an unprecedented specific activity of 21.8 ± 2.1 mA/cm2 at 0.9 V versus the reversible hydrogen electrode, nearly double the previous record, and an ultrahigh mass activity of 10.7 ± 1.1 A/mgPt.
                  Cover page: Surface molecular pump enables ultrahigh catalyst activity.Creative Commons 'BY-NC' version 4.0 license
                  • Article
                  • Peer Reviewed
                  UCLA Previously Published Works (2023)
                  Hydrazine-assisted water electrolysis offers a feasible path for low-voltage green hydrogen production. Herein, the design and synthesis of ultrathin RhRu0.5 -alloy wavy nanowires as bifunctional electrocatalysts for both the anodic hydrazine oxidation reaction (HzOR) and the cathodic hydrogen evolution reaction (HER) is reported. It is shown that the RhRu0.5 -alloy wavy nanowires can achieve complete electrooxidation of hydrazine with a low overpotential and high mass activity, as well as improved performance for the HER. The resulting RhRu0.5 bifunctional electrocatalysts enable, high performance hydrazine-assisted water electrolysis delivering a current density of 100 mA cm-2 at an ultralow cell voltage of 54 mV and a high current density of 853 mA cm-2 at a cell voltage of 0.6 V. The RhRu0.5  electrocatalysts further demonstrate a stable operation at a high current density of 100 mA cm-2 for 80 hours of testing period with little irreversible degradation. The overall performance greatly exceeds that of the previously reported hydrazine-assisted water electrolyzers, offering a pathway for efficiently converting hazardous hydrazine into molecular hydrogen.
                    Cover page: Bifunctional Ultrathin RhRu0.5‐Alloy Nanowire Electrocatalysts for Hydrazine‐Assisted Water SplittingCreative Commons 'BY-NC-ND' version 4.0 license
                    • Article
                    • Peer Reviewed
                    UCLA Previously Published Works (2023)
                    In analogy to natural enzymes, an elaborated design of catalytic systems with a specifically tailored local chemical environment could substantially improve reaction kinetics, effectively combat catalyst poisoning effect and boost catalyst lifetime under unfavourable reaction conditions. Here we report a unique design of 'Ni(OH)2-clothed Pt-tetrapods' with an amorphous Ni(OH)2 shell as a water dissociation catalyst and a proton conductive encapsulation layer to isolate the Pt core from bulk alkaline electrolyte while ensuring efficient proton supply to the active Pt sites. This design creates a favourable local chemical environment to result in acidic-like hydrogen evolution reaction kinetics with a lowest Tafel slope of 27 mV per decade and a record-high specific activity and mass activity in alkaline electrolyte. The proton conductive Ni(OH)2 shell can also effectively reject impurity ions and retard the Oswald ripening, endowing a high tolerance to solution impurities and exceptional long-term durability that is difficult to achieve in the naked Pt catalysts. The markedly improved hydrogen evolution reaction activity and durability in an alkaline medium promise an attractive catalyst material for alkaline water electrolysers and renewable chemical fuel generation.
                      Cover page: Amorphous nickel hydroxide shell tailors local chemical environment on platinum surface for alkaline hydrogen evolution reaction
                      Top