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Open Access Publications from the University of California

Scholarly Works (10 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2013)

Recent environmental factors have triggered a strong interest towards the development of scalable technologies that can increase the share of renewable sources into our energy mix. Artificial photosynthesis systems are a promising alternative as they can simultaneously capture and store solar energy in the form of a fuel. These systems are based on photoelectrochemical (PEC) cells that can take low energy density reactants such as water and/or carbon dioxide and transform them into energy dense hydrogen or carbon containing molecules via light-driven processes. Deployable solar-fuel generators need to be able to produce fuels in a robust, scalable, and efficient manner. Despite the large number of studies focusing on this technology since its inception in the early 1970's, a system that can satisfy those three requirements does not exist. Significant innovation is required to develop cost-effective components that can perform the light-absorption, catalytic redox reaction, ion transport and product separation requirements. Additionally, understanding of component performance in integrated devices is crucial for developing high efficiency solar-fuel generators.

This dissertation focusses on several aspects of component integration in solar-hydrogen generators. The initial focus involves the development of self-assembly techniques of nanometer scale units to obtain architectures necessary for solar-fuel devices. Starting with solutions of semiconducting nanorods and polymers, this work demonstrates that by controlling the evaporation rate during solvent casting, arrays of vertically aligned nanorods embedded in polymer films can be obtained over large areas (> 1 cm2). This architecture is desirable for the integration of hydrogen generating nanorods into integrated water splitting membranes, where H2 and O2 are evolved at physically separated sites. This work also describes how the structure of proton conducting membranes (Nafion®) is affected at inorganic interfaces such as the ones present in solar-fuels devices. The effects of thin-film confinement and wetting interactions are studied in Nafion thin-film model systems using a combination of X-ray scattering and mass transport characterization techniques. These studies show how confinement of Nafion films to thicknesses below 10 nm results in significant limitations to self-assembly, disruption of phase separation in the material and ultimately decrease in ionic conductivity. Wetting interaction also play a role in the orientation of conducting domains in the material. Hydrophobic surfaces results in a parallel orientation of ionic domains while films cast on hydrophilic substrates result in an isotropic orientation of domains. The differences in domain orientation also impact the mass transport behavior of the material.

Additionally, this dissertation covers several topics related with the integration of components for the fabrication of practical hydrogen generators. Here we describe the development of a microfluidic test-bed for the incorporation of catalytic and membrane components in scales amenable for research. This tool allows for the simple exchange and quantitatively assessment of the performance of integrated electrochemical fuel generating devices as well as each of the individual components that participate in the process. Lastly, this work also describes engineering solutions that allow both membrane-separated water electrolyzers and solar hydrogen generators to operate under buffered electrolytes. This is achieved by using supporting electrolytes to carry the ionic current through the membrane while controlled convective streams around the membranes are implemented to avoid the formation of large concentration gradients between reaction sides. This development opens up the space of operating electrolytes for the incorporation of wide range of components that are not stable under strong basic or acidic conditions.

    Cover page: Self-Assembly and Mass Transport in Membranes for Artificial Photosynthesis
    • Article
    • Peer Reviewed
    LBL Publications (2015)
    Nafion is an ion-containing random copolymer used as a solid electrolyte in many electrochemical applications thanks to its remarkable ionic conductivity and mechanical stability. Understanding the mechanism of ion transport in Nafion, which depends strongly on hydration, therefore requires a complete picture of its morphology in dry and hydrated form. Here we report on a nanoscale study of dry versus hydrated as-cast 100 nm Nafion membranes using analytical transmission electron microscopy (TEM) and cryogenic TEM tomography, respectively. For the dry membrane, spherical clusters ∼3.5 nm in diameter corresponding to the hydrophilic sulfonic-acid-containing phase are identified. In contrast, cryo TEM tomography of the hydrated membrane reveals an interconnected channel-type network, with a domain spacing of ∼5 nm, and presents the first nanoscale 3D views of the internal structure of hydrated Nafion obtained by a direct-imaging approach.
      • Article
      • Peer Reviewed
      LBL Publications (2019)
      Rapid improvements in polymer-electrolyte fuel-cell (PEFC) performance have been driven by the development of commercially available ion-conducting polymers (ionomers) that are employed as membranes and catalyst binders in membrane-electrode assemblies. Commercially available ionomers are based on a perfluorinated chemistry comprised of a polytetrafluoroethylene (PTFE) matrix that imparts low gas permeability and high mechanical strength but introduces significant mass-transport losses in the electrodes. These transport losses currently limit PEFC performance, especially for low Pt loadings. In this study, we present a novel ionomer incorporating a glassy amorphous matrix based on a perfluoro(2-methylene-4-methyl-1,3-dioxolane) (PFMMD) backbone. The novel backbone chemistry induces structural changes in the ionomer, restricting ionomer domain swelling under hydration while disrupting matrix crystallinity. These structural changes slightly reduce proton conductivity while significantly improving gas permeability. The performance implications of this tradeoff are assessed, which reveal the potential for substantial performance improvement by incorporation of highly permeable ionomers as the functional catalyst binder. These results underscore the significance of tailoring material chemistry to specific device requirements, where ionomer chemistry should be rationally designed to match the local transport requirements of the device architecture.
        • Article
        • Peer Reviewed
        LBL Publications (2020)
        Rapid improvements in polymer-electrolyte fuel-cell (PEFC) performance have been driven by the development of commercially available ion-conducting polymers (ionomers) that are employed as membranes and catalyst binders in membrane-electrode assemblies. Commercially available ionomers are based on a perfluorinated chemistry comprised of a polytetrafluoroethylene (PTFE) matrix that imparts low gas permeability and high mechanical strength but introduces significant mass-transport losses in the electrodes. These transport losses currently limit PEFC performance, especially for low Pt loadings. In this study, we present a novel ionomer incorporating a glassy amorphous matrix based on a perfluoro(2-methylene-4-methyl-1,3-dioxolane) (PFMMD) backbone. The novel backbone chemistry induces structural changes in the ionomer, restricting ionomer domain swelling under hydration while disrupting matrix crystallinity. These structural changes slightly reduce proton conductivity while significantly improving gas permeability. The performance implications of this trade-off are assessed, which reveal the potential for substantial performance improvement by incorporation of highly permeable ionomers as the functional catalyst binder. These results underscore the significance of tailoring material chemistry to specific device requirements, where ionomer chemistry should be rationally designed to match the local transport requirements of the device architecture.
          Cover page: Highly Permeable Perfluorinated Sulfonic Acid Ionomers for Improved Electrochemical Devices: Insights into Structure–Property Relationships
          • Article
          LBL Publications (2019)
          Rapid improvements in polymer-electrolyte fuel-cell (PEFC) performance have been driven by the development of commercially available ion-conducting polymers (ionomers) that are employed as membranes and catalyst binders in membrane-electrode assemblies. Commercially available ionomers are based on a perfluorinated chemistry comprised of a polytetrafluoroethylene (PTFE) matrix that imparts low gas permeability and high mechanical strength but introduces significant mass-transport losses in the electrodes. These transport losses currently limit PEFC performance, especially for low Pt loadings. In this study, we present a novel ionomer incorporating a glassy amorphous matrix based on a perfluoro(2-methylene-4-methyl-1,3-dioxolane) (PFMMD) backbone. The novel backbone chemistry induces structural changes in the ionomer, restricting ionomer domain swelling under hydration while disrupting matrix crystallinity. These structural changes slightly reduce proton conductivity while significantly improving gas permeability. The performance implications of this tradeoff are assessed, which reveal the potential for substantial performance improvement by incorporation of highly permeable ionomers as the functional catalyst binder. These results underscore the significance of tailoring material chemistry to specific device requirements, where ionomer chemistry should be rationally designed to match the local transport requirements of the device architecture.
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2014)
            Poly[(styrene)-block-((2-acryloxy)ethyltributylphosphonium bromide)] diblock copolymers (STBP) were synthesized in two steps. First, reversible addition-fragmentation chain transfer polymerization was used to synthesize the diblock copolymer precursors poly[(styrene)-block-(bromoethyl acrylate)] (SBEA), followed by functionalization with tributylphosphine. Copolymers with overall molecular weights ranging from 31 to 87 kg/mol were synthesized. The volume fraction of the ion-containing monomers in the copolymers was fixed at about 0.57. Self-assembly of these copolymers into ordered morphologies with tunable domain sizes was demonstrated by small-angle X-ray scattering. The effect of morphology on water uptake and bromide ion conductivity was explored in samples equilibrated in liquid water. The use of the pendant tributylphosphonium cations, which have some hydrophobic character, results in low water uptake and high anionic conductivity. The conductivity increases with increasing domain size while water uptake is unaffected by domain size.
              Cover page: High Anion Conductivity and Low Water Uptake of Phosphonium Containing Diblock Copolymer Membranes
              • Article
              • Peer Reviewed
              LBL Publications (2020)
              Commercially available perfluorinated sulfonic acid ionomers (PFSAs), utilized as polymer electrolytes in membrane electrode assemblies (MEAs), have driven rapid improvements in fuel cell performance. These materials have a polytetrafluoroethylene (PTFE) backbone and semicrystalline matrix which imparts mechanical integrity and low gas permeability, making them attractive membrane materials. However, their low gas permeability introduces significant mass-transport limitations in catalyst layers, especially severe in oxygen reduction at the cathode. In this study, we present the synthesis of an amorphous PFSA incorporating perfluoro(2-methylene 4-methyl-1,3-dioxolane) (PFMMD) in the backbone. This impacted the material nanostructure at multiple length scales, simultaneously increasing gas permeability (>3x oxygen permeability of Nafion) via reduced crystallinity and increased fractional free volume while reducing proton conductivity via changes in matrix mechanical properties which inhibited phase separation of ionomer domains. When integrated in a fuel cell MEA, this trade-off yielded significant improvements; specifically, current density per cm2 platinum increased up to 22% upon substituting the PFMMD based ionomer for Nafion in the cathode binder. In this presentation, I will discuss the facile synthesis of PFMMD based ionomers with tunable PFSA content. Furthermore, I will discuss structure-property relationships resolved from transport measurements and morphological characterization. Finally, I will discuss the implementation of these materials in fuel cells and the potential for meaningful performance improvements. These results demonstrate the value of rational ionomer design toward better performing electrochemical devices.
                Cover page: Elucidating Structure-Property Relationships in Highly Permeable Perfluorinated Sulfonic Acid Ionomers
                • Article
                • Peer Reviewed
                UC Irvine Previously Published Works (2016)
                An integrated cell for the solar-driven splitting of water consists of multiple functional components and couples various photoelectrochemical (PEC) processes at different length and time scales. The overall solar-to-hydrogen (STH) conversion efficiency of such a system depends on the performance and materials properties of the individual components as well as on the component integration, overall device architecture, and system operating conditions. This Review focuses on the modeling- and simulation-guided development and implementation of solar-driven water-splitting prototypes from a holistic viewpoint that explores the various interplays between the components. The underlying physics and interactions at the cell level is are reviewed and discussed, followed by an overview of the use of the cell model to provide target properties of materials and guide the design of a range of traditional and unique device architectures.
                  Cover page: Modeling, Simulation, and Implementation of Solar‐Driven Water‐Splitting Devices
                  • Article
                  • Peer Reviewed
                  LBL Publications (2020)
                  This article addresses the challenges presented by photoelectrochemical solar fuels technology in a discussion that begins with a functioning device and proceeds to the more fundamental science of its component parts. In this flow of discussion issues are addressed that frame the discussion for the next, increasingly more fundamental topic. The analysis begins with a discussion of the need for an analytical facility for confirmation of reported efficiencies of solar fuels device prototypes and then progressively narrows its scope to prototype design, the discovery of novel materials and the design of durable interfacial structures for fuels evolution. Molecular hydrogen will be considered first as the target fuel since many of the challenges with hydrogen production are general and applicable to the more complex CO2 reduction, which will be treated as a supplementary subject.
                    Cover page: Practical challenges in the development of photoelectrochemical solar fuels production
                    • Article
                    • Peer Reviewed
                    UC Irvine Previously Published Works (2018)
                    Solar-powered electrochemical production of hydrogen through water electrolysis is an active and important research endeavor. However, technologies and roadmaps for implementation of this process do not exist. In this perspective paper, we describe potential pathways for solar-hydrogen technologies into the marketplace in the form of photoelectrochemical or photovoltaic-driven electrolysis devices and systems. We detail technical approaches for device and system architectures, economic drivers, societal perceptions, political impacts, technological challenges, and research opportunities. Implementation scenarios are broken down into short-term and long-term markets, and a specific technology roadmap is defined. In the short term, the only plausible economical option will be photovoltaic-driven electrolysis systems for niche applications. In the long term, electrochemical solar-hydrogen technologies could be deployed more broadly in energy markets but will require advances in the technology, significant cost reductions, and/or policy changes. Ultimately, a transition to a society that significantly relies on solar-hydrogen technologies will benefit from continued creativity and influence from the scientific community.
                      Cover page: Pathways to electrochemical solar-hydrogen technologies
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