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Open Access Publications from the University of California

Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2022)

Fuel cells are devices that can efficiently convert fuels into electricity without the limitation of the Carnot cycle. Particularly, hydrogen fuel cells have attracted enormous interest due to their high energy density, high efficiency, and low environmental impacts. The most widely adopted proton-exchange-membrane fuel cells (PEMFCs) use platinum (Pt) catalysts to drive both anode and cathode reactions. To date, the most significant roadblock to the broad dissemination of this green energy technology remains the acceleration and retention of the reaction rate of oxygen reduction reaction (ORR) at the cathode. Developing robust, active, and cost-effective ORR catalysts is the solution to this challenge.In my first work, I demonstrated a Cu doping strategy to enhance the intrinsic activity and durability of octahedral PtNi nanoparticles in the rotating-disk-electrode (RDE) setting. By a novel integration of simulations and growth tracking experiments, we uncover the beneficial role of increased surface Pt composition, which reduces the generation of surface vacancies and subsequent dissolution of sub-surface Cu and Ni atoms. Unlike the well-controlled RDE system, the critical role of mass transports in a practical membrane-electrode-assembly (MEA) system demands additional consideration in stability-enhancing strategies. Therefore, in my subsequent work, I recognized the critical challenge in MEA with ultralow Pt loading and designed a graphene-nanopocket-encaged platinum cobalt nanocatalyst with good electrochemical accessibility and exceptional durability in practical MEA testing. With the greatly improved rated power and durability, a 6.8 gram Pt loading is projected for a 90-kW PEMFC light-duty vehicle, approaching that used in a typical catalytic converter. Furthermore, I presented a unique design of ultrafine Pt nanocatalysts with embedded cobalt oxide clusters, which further improve the catalyst stability for durable PEMFCs. This endohedral-oxide design exploits the strong Pt/oxide interaction, which grants the catalyst its exceptional structural and chemical durability, without sacrificing activity. The developed nanocatalyst exhibits exceptional durability promising an outstanding projected lifetime of 15,000 hours.

    Cover page: Durable Pt-based Catalysts for Oxygen Reduction Reaction in Fuel Cell
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2023)
    Extensive macroscale two-dimensional (2-D) platinum (Pt) nanowire network (NWN) sheets are created through a hierarchical self-assembly process with the aid of biomolecular ligands. The Pt NWN sheet is assembled from the attachment growth of 1.9 nm-sized 0-D nanocrystals into 1-D nanowires featuring a high density of grain boundaries, which then interconnect to form monolayer network structures extending into centimeter-scale size. Further investigation into the formation mechanism reveals that the initial emergence of NWN sheets occurs at the gas/liquid interfaces of the bubbles produced by sodium borohydride (NaBH4) during the synthesis process. Upon the rupture of these bubbles, an exocytosis-like process releases the Pt NWN sheets at the gas/liquid surface, which subsequently merge into a continuous monolayer Pt NWN sheet. The Pt NWN sheets exhibit outstanding oxygen reduction reaction (ORR) activities, with specific and mass activities 12.0 times and 21.2 times greater, respectively, than those of current state-of-the-art commercial Pt/C electrocatalysts.
      Cover page: Bubble-Mediated Large-Scale Hierarchical Assembly of Ultrathin Pt Nanowire Network Monolayer at Gas/Liquid Interfaces.
      • 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.
        • Article
        • Peer Reviewed
        UC Santa Cruz Previously Published Works (2020)
        Despite tremendous progress in catalyst development for rate-limiting cathodic oxygen reduction reaction (ORR), reducing Pt usage while meeting performance requirements in practical proton exchange membrane fuel cells (PEMFCs) remains a challenge. The ORR in PEMFCs occurs at a catalyst–electrolyte–gas three-phase interface. A desirable interface should exhibit highly active and available catalytic sites, as well as allow efficient oxygen and proton feeding to the catalytic sites and timely removal of water to avoid interface flooding. Here, we report the design of a three-phase microenvironment in PEFMCs, showing that carbon surface chemistry can be tuned to modulate its interaction with the ionomers and create favorable transport paths for rapid delivery of both reactants and products. With such an elaborate interfacial design, for the first time we have demonstrated PEMFCs with all key ORR catalyst performance metrics, including mass activity, rated power, and durability, surpassing the US Department of Energy targets.
          Cover page: Tailoring a Three-Phase Microenvironment for High-Performance Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells
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