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Open Access Publications from the University of California

Scholarly Works (28 results)

  • Thesis
  • Peer Reviewed
UC Irvine Electronic Theses and Dissertations (2018)

Lithium oxygen battery has the potential to outdo the best battery system on the market due to its high theoretical specific energy of 11,400 Wh/kg (Li) which is comparable to that of gasolines. However, their practical implementation is still facing challenges, including low cyclic performance, high charging voltage, insoluble discharge product formation, and electrolyte degradation. Air cathodes, where oxygen reacts with Li ions and electrons to produce insoluble discharge oxides, are often considered as the most challenging component in nonaqueous Li-air batteries. To understand the voltage loss mechanisms in air cathodes due to insoluble discharge oxides, a mathematical model is developed, which incorporates the major thermodynamic, transport, and kinetic processes, and comprehensive analysis is conducted. Experiment is also designed and conducted for the model formulation and validation. Li battery components were fabricated, then assembled in an argon filled glove box. Electrochemical testing was conducted on the experimental Li air battery by using the PARSTAT MC Multichannel Potentiostat. An X-ray Diffraction, scanning electron microscope, and high-precision mass scale were employed to determine the composition, morphology, and spatial variation of Li oxide products. Through the model analysis, it is found that the electric passivation and oxygen transport blockage caused by the Li oxides precipitates reduce the battery voltage and energy capacity. The first stage of voltage drop is dominated by the electrode passivation and surface loss, while the latter stage of voltage drop is dominated by the oxygen blockage. In addition, several morphologies of oxides are identified in the cathode structure under various current densities, thus the proposed surface coverage model is superior to the traditional film-resistor approach. Further, it is found there exists spatial variations in discharge Li oxides in terms of mass and morphology, and experimental data show a good agreement with the theoretical analysis in term of the spatial variation of the Li oxides mass. The Da number is identified as a major parameter governing the spatial variation.

    Cover page: Li Oxide Deposit Formation and Impacts in Non-aqueous Li-air Battery
    • Article
    • Peer Reviewed
    UC Irvine Previously Published Works (2017)
    This paper presents a fundamental study on the precipitate formation/morphology and impact of discharge precipitates in Li-air batteries and compares the voltage loss with two Li-air battery models, namely a film-resistor model and surface coverage model. Toray carbon cloth is selected as cathode, which serves as large-porosity electrodes with an approximately planar reaction surface. Imaging analysis shows film formation of precipitates is observed in all the experiments. In addition, toroidal and aggregate morphologies are present under lower currents as well. Specially, toroidal or partially toroidal deposit is observed for 0.06 A/cm2. Aggregates, which consist of small particles with grain boundaries, are shown for 0.03 A/cm2. We found that the film-resistor model is unable to predict the discharge voltage behaviors under the two lower currents due to the presence of the deposit morphologies other than the film formation. The coverage model's prediction shows acceptable agreement with the experimental data because the model accounts for impacts of various morphologies of precipitates.
      Cover page: Discharge Precipitate's Impact in Li-Air Battery: Comparison of Experiment and Model Predictions
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2010)
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2019)
        In this paper, we present a study of Lithium (Li)-air battery capacity by accounting for the voltage loss associated with the electrode passivation and transport resistance caused by insoluble product formation. Two regimes are defined, in which approximate formulas are developed to explicitly evaluate the battery capacity, along with extensive validation against experimental data of various cathode properties and materials from our and several other groups. The dependence of battery capacity on the surface coverage factor, tortuosity, and Damköhler numbers (Da) is explicitly expressed and discussed. The formulas provide a guideline for experimentalists and practitioners in air cathode design, analysis, and control.
          Cover page: Capacity loss of non-aqueous Li-Air battery due to insoluble product formation: Approximate solution and experimental validation
          • Article
          • Peer Reviewed
          UC Irvine Previously Published Works (2016)
          This paper presents an experimental and computational study on laser-induced breakdown spectroscopy (LIBS) for both unconfined flat surface and confined cavity cases. An integrated LIBS system is employed to acquire the shockwave and plasma plume images. The computational model consists of the mass, momentum, and energy conservation equations, which are necessary to describe shockwave behaviors. The numerical predictions are validated against shadowgraphic images in terms of shockwave expansion and reflection. The three-dimensional (3D) shockwave morphology and velocity fields are displayed and discussed.
            Cover page: Experimental and computational investigation of confined laser-induced breakdown spectroscopy
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2015)
            Air cathodes, where oxygen reacts with Li ions and electrons with discharge oxide stored in their pore structure, are often considered as the most challenging component in nonaqueous Lithium-air batteries. In non-aqueous electrolytes, discharge oxides are usually insoluble and hence precipitate at local reaction site, raising the oxygen transport resistance in the pore network. Due to their low electric conductivity, their presence causes electrode passivation. This study aims to investigate the air cathode's performance through analytically obtaining oxygen profiles, modeling electrode passivation, evaluating the transport polarization raised by discharge oxide precipitate, and developing analytical formulas for insoluble Li oxides storage capacity. The variations of cathode quantities, including oxygen content and temperature, are evaluated and related to a single dimensionless parameter - the Damköhler Number (Da). An approximate model is developed to predict discharge voltage loss, along with validation against two sets of experimental data. Air cathode properties, including tortuosity, surface coverage factor and the Da number, and their effects on the cathode's capacity of storing Li oxides are formulated and discussed.
              Cover page: DISCHARGE OXIDE STORAGE CAPACITY AND VOLTAGE LOSS IN LI-AIR BATTERY
              • Article
              • Peer Reviewed
              UC Irvine Previously Published Works (2019)
              In this study, a new cooling strategy for a proton exchange membrane (PEM) fuel cell stack is investigated using a three-dimensional (3D) multiphase non-isothermal model. The new cooling strategy follows that of the Honda's Clarity design and further extends to a cooling unit every five cells in stacks. The stack consists of 5 fuel cells sharing the inlet and outlet manifolds for reactant gas flows. Each cell has 7-path serpentine flow fields with a counter-flow configuration arranged for hydrogen and air streams. The coolant flow fields are set at the two sides of the stack and are simplified as the convective heat transfer thermal boundary conditions. This study also compares two thermal boundary conditions, namely limited and infinite coolant flow rates, and their impacts on the distributions of oxygen, liquid water, current density and membrane hydration. The difference of local temperature between these two cooling conditions is as much as 6.9 K in the 5-cell stack, while it is only 1.7 K in a single cell. In addition, the increased vapor concentration at high temperature (and hence water saturation pressure) dilutes the oxygen content in the air flow, reducing local oxygen concentration. The higher temperature in the stack also causes low membrane hydration, and consequently poor cell performance and non-uniform current density distribution, as disclosed by the simulation. The work indicates the new cooling strategy can be optimized by increasing the heat transfer coefficient between the stack and coolant to mitigate local overheating and cell performance reduction.
                Cover page: Three-dimensional simulation of a new cooling strategy for proton exchange membrane fuel cell stack using a non-isothermal multiphase model
                • Article
                • Peer Reviewed
                UC San Diego Previously Published Works (2013)
                Lipoprotein-associated phospholipase A(2) (Lp-PLA(2)), specifically Group VIIA PLA(2), is a member of the phospholipase A(2) superfamily and is found mainly associated with LDL and HDL in human plasma. Lp-PLA(2) is considered as a risk factor, a potential biomarker, a target for therapy in the treatment of cardiovascular disease, and evidence suggests that the level of Lp-PLA(2) in plasma is associated with the risk of future cardiovascular and stroke events. The differential location of the enzyme in LDL/HDL lipoproteins has been suggested to affect Lp-PLA(2) function and/or its physiological role and an abnormal distribution of the enzyme may correlate with diseases. Although a mutagenesis study suggested that a surface helix (residues 362-369) mediates the association between Lp-PLA(2) and HDL, the molecular details and mechanism of association has remained unknown. We have now employed hydrogen deuterium exchange mass spectrometry to characterize the interaction between recombinant human Lp-PLA(2) and human HDL. We have found that specific residues 113-120, 192-204, and 360-368 likely mediate HDL binding. In a previous study, we showed that residues 113-120 are important for Lp-PLA(2)-liposome interactions. We now find that residues 192-204 show a decreased deuteration level when Lp-PLA(2) is exposed to apoA-I, but not apoA-II, the most abundant apoproteins in HDL, and additionally, residues 360-368 are only affected by HDL.The results suggest that apoA-I and phospholipid membranes play crucial roles in Lp-PLA(2) localization to HDL.
                  Cover page: Structural basis of specific interactions of Lp-PLA2 with HDL revealed by hydrogen deuterium exchange mass spectrometry[S]
                  • Article
                  • Peer Reviewed
                  UC San Diego Previously Published Works (2011)
                  Phospholipases represent one of the earliest enzyme activities to be identified and studied, and the phospholipase A2 superfamily traces its roots to the identification of lytic actions of snake venom at the end of the 19th century. Both electrostatic and hydrophobic interactions contribute to the interfacial binding of sPLA2 to anionic phospholipid membranes. The interaction between basic residues on the binding surface with anionic vesicles plays an important role in interfacial binding. The major functions will be summarized below and include the ability to kill Gram-positive and Gram-negative bacteria, thereby affecting host defense against bacterial infections. sPLA2 may be involved in the pathogensis of inflammatory bowel disease including Crohn's disease and ulcerative colitis. GIIA sPLA2 protein and mRNA were detected in Paneth cells of the small intestinal mucosa in the intestine in Crohn's disease patients.
                    Cover page: Phospholipase A2 Enzymes: Physical Structure, Biological Function, Disease Implication, Chemical Inhibition, and Therapeutic Intervention
                    • Article
                    • Peer Reviewed
                    UC San Diego Previously Published Works (2013)
                    Group VI Ca²⁺-independent phospholipase A₂ (iPLA₂) is a water-soluble enzyme that is active when associated with phospholipid membranes. Despite its clear pharmaceutical relevance, no X-ray or NMR structural information is currently available for the iPLA₂ or its membrane complex. In this paper, we combine homology modeling with coarse-grained (CG) and all-atom (AA) molecular dynamics (MD) simulations to build structural models of iPLA₂ in association with a phospholipid bilayer. CG-MD simulations of the membrane insertion process were employed to provide a starting point for an atomistic description. Six AA-MD simulations were then conducted for 60 ns, starting from different initial CG structures, to refine the membrane complex. The resulting structures are shown to be consistent with each other and with deuterium exchange mass spectrometry (DXMS) experiments, suggesting that our approach is suitable for the modeling of iPLA₂ at the membrane surface. The models show that an anchoring region (residues 710-724) forms an amphipathic helix that is stabilized by the membrane. In future studies, the proposed iPLA₂ models should provide a structural basis for understanding the mechanisms of lipid extraction and drug-inhibition. In addition, the dual-resolution approach discussed here should provide the means for the future exploration of the impact of lipid diversity and sequence mutations on the activity of iPLA₂ and related enzymes.
                      Cover page: Insertion of the Ca2+-Independent Phospholipase A2 into a Phospholipid Bilayer via Coarse-Grained and Atomistic Molecular Dynamics Simulations
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