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Open Access Publications from the University of California

Scholarly Works (15 results)

  • Article
  • Peer Reviewed
UC Berkeley Previously Published Works (2017)
Catalyst layers (CLs) in polymer-electrolyte fuel cells have garnered the attention of researchers due to their association with high local mass-transport resistance at low catalyst loadings [1, 2]. Nevertheless, the genesis and nature of CL resistance(s) is still under debate. CLs are only a few micrometers thick with complex geometry and heterogeneous phases making in-situ examination of CL transport processes extremely difficult. In this study, we probe the nature of CL resistances experimentally and theoretically. Using a hydrogen-pump limiting-current modality, we deconvolute the various gas-transport-related resistances without the impact of those associated with the oxygen-reduction reaction (e.g., water production) [3]. An analytical model to quantify the experimental measurements is developed to analyze the data and to elucidate limiting regimes. Our new results are in agreement with literature limiting-current measurements. Similar to previously reported studies [4], the model demonstrates that the transport of the reactant to the surface through the ionomer film tends to be the dominant resistance in CLs with a linear dependence on the inverse of Pt loading (i.e., roughness factor) [5]. Further, we study the dependence of this resistance on operational parameters including pressure, RH, temperature, gas molecular mass, and ionomer type. The garnered information exposes the controlling resistances, especially in comparison to ex-situ ionomer thin-film properties. Our efforts can be utilized to understand and develop mitigation strategies for local CL resistances. Acknowledgements We would like to thank helpful discussions and data provided by KC Neyerlin at NREL. This work was funded under the Fuel Cell Performance and Durability Consortium (FC PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231. References: A. Weber and A. Kusoglu, Journal of Material Chemistry A, 2,17207–17211 (2014). A. Kongkanand and M. F. Mathias, The Journal of Physical Chemistry Letters, 7, 1127 (2016). F. Spingler, A. Phillips, T. Schuler, M. Tucker and A. Weber, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.036 N. Nonoyama, S. Okazaki, A. Weber, Y. Ikogi and T. Yoshida, Journal of The Electrochemical Society, 158 (5), B416 (2011). T. Greszler, D. Caulk and P. Sinha, Journal of The Electrochemical Society, 159, F831 (2012).
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2023)
    Polymer-electrolyte fuel cells (PEFCs) utilize porous catalyst layers (CLs) formed of carbon supports on which Pt particles are deposited and ionomer films are distributed. Carbon supports themselves have varying degrees of porosity, where high-surface-area carbon (HSC) supports possess nanometer-sized interior pores that are suitable for Pt nanoparticle deposition but prevent deleterious ionomer penetration. However, this requires protons to transport through water pathways inside the pores. To understand the generation of such pathways, we examine the various mechanisms of water uptake by PEFC CLs, and the subsequent impact of water uptake on Pt utilization through developing a multiphysics model of the water wetting phenomena as a function of relative humidity. The model details water uptake via ionomer absorption, capillary condensation in the hydrophilic pores, and surface adsorption using molecular potential that account for various water and surface dipole interactions. The results quantify how mesoporous carbons with highly hydrophilic pores increase Pt utilization through the development of wetted layers, which at the same time enable optimized gas-transport pathways. It also demonstrates the impact of pore-size distribution (PSD) and physical and chemical parameters on the water uptake phenomena, allowing for future CL particle and structure optimization.
      Creative Commons 'BY-NC-ND' version 4.0 license
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2017)
      Mass-transport resistances observed with low platinum (Pt) loadings in polymer-electrolyte fuel-cell (PEFC) catalyst layers (CLs) limit performance and thus, present challenges for necessary further Pt reduction to enable commercialization. It is believed that this resistance is due to mass-transport resistance near the Pt surface, probably due to transport through the ionomer thin-film layer on the Pt/C particles; the nature and genesis of this resistance needs to be explored further. In this study, a hydrogen-pump configuration operated at limiting-current is used to study local mass-transport limitations in CLs. Using an agglomerate model, analytical expressions for the total resistance at limiting current are developed to identify critical parameters controlling this resistance. The dependence of CL resistance on these parameters is examined.
        Cover page: Transport Resistances in Fuel-Cell Catalyst Layers
        • Thesis
        • Peer Reviewed
        UC Berkeley Electronic Theses and Dissertations (2021)

        Multiphase porous catalyst layers (CLs) find applications in a wide variety of electrochemical energy-conversion devices such as fuel cells, electrolysis, electrosynthesis, and batteries. The various phases enable transport of different species, while the porous structure exhibits a large interfacial area between the various phases in a small volume, thus enabling interfacial electrochemical reactions. In polymer-electrolyte fuel cells (PEFCs), the CL is composed of carbon agglomerates dispersed with Pt nanoparticles, which are the primary reaction sites. The carbon agglomerates are coated by an ion-conducting polymer (ionomer) that also acts as a binder. The three phases (solid carbon, ionomer, and pores between the agglomerates) allow the transport of electrons, ions, and reactant/product gases, respectively. A typical PEFC electrode is usually a few micrometers thick, while the carbon agglomerates are 50 to 300 nm in diameter and the ionomer thin film is only 5 to 15 nm thick. Overall, the structure is extremely heterogeneous with various length scales and multiple intercalating phases. Thus, it is extremely difficult to isolate and quantify the various transport processes.

        Gas transport is of particular interest since it determines the maximum power that can be derived from PEFC systems under typical operating conditions. Within the CL, the gas diffuses through the pores, followed by transport through the ionomer thin films to reach the Pt reaction sites. Several prior studies establish the presence of a large gas-transport resistance close to the Pt reaction site, commonly referred to as the “local” resistance. Further, the local resistance is known to increase with a decrease in Pt loading due to higher local fluxes. This finding necessitates the use of high-Pt mass loadings in PEFC CLs, thus driving up cost and posing a major barrier to the large-scale commercialization of PEFC systems. The primary aim of this dissertation is to understand the origin of the “local” resistance and propose mechanisms to ameliorate it.

        Traditionally, CL transport resistance is quantified from O2-limiting-current measurements, where the mass-transport limit is achieved by using a low-concentration O2 feed and applying high overpotential. In chapter 2, we study the CL transport resistance as a function of CL Pt loading and ionomer content. By combining multi-scale continuum modeling and experiment, the various gas-transport sub-resistances are isolated and quantified. The local resistance close to the Pt particles is shown to dominate the overall gas-transport resistance in PEFC CLs. Our results further expose the ionomer thin films as the origin of the local resistance, which can be sub-divided into: (i) a transport component due to diffusion through the ionomer thin film, and (ii) an interfacial component due to sulfonate adsorption on Pt.

        Having identified the ionomer as the origin of the local resistance to gas transport in CLs, subsequent chapters (chapters 3, 4, and 5) explore methods to ameliorate this resistance. In chapter 3, the impact of ionomer content and chemistry on overall and component CL gas-transport resistance is investigated. CL transport resistance increases with ionomer content, potentially due to the formation of thicker ionomer films. Low-equivalent-weight ionomers allow better gas and proton transport due to greater water uptake and low crystallinity but also cause significant interfacial resistance due to the high density of sulfonic-acid groups. Conversely, high-equivalent-weight ionomers provide poor gas and ion transport, but result in low interfacial resistance. These observed effects of equivalent weight are validated via in-situ ionic conductivity and CO-displacement measurements. Of critical importance, the results are supported by ex-situ ellipsometry and scattering of model thin-film systems, thereby providing direct linkages and applicability of model studies to probe complex heterogeneous structures. Lastly, structural, and resultant performance changes in the electrode are observed above a threshold sulfonic-group concentration, thus highlighting the significance of ink-based interactions.

        Next, we investigate the impact of carbon-support type. Carbon supports in CLs form the primary porous structure on which Pt particles are deposited and ionomer films are distributed. Unlike non-porous carbon supports, high-surface-area carbon supports possess interior micropores that are suitable for Pt nanoparticles deposition but prevent ionomer penetration. The micropores thus mitigate interfacial resistance by spatially segregating the ionomer and Pt particles, but they increase reactant transport resistance to the Pt reaction sites due to their small size and tortuosity. Additionally, the H+transport is forced through water pathways in the absence of ionomer, resulting in a humidity-dependent Pt utilization and power output. In chapter 4, we model the various mechanisms of water uptake by PEFC CLs, and the subsequent impact of water uptake on Pt utilization. The model results are in good agreement with literature data collected from various sources and highlight the role of wetted pores with surface adsorbed water. In addition to flooded pores that contain capillary condensed water, wetted pores significantly enhance Pt utilization at low to middle RHs. Additionally, Pt particles are exposed to very different local environments in wetted pores as compared to flooded pores. Until H+ transport is not limiting, wetted pores can significantly improve performance by enabling high gas transport through the pore gas phase as compared to flooded pores (which transport only gas dissolved in the liquid phase). Our results agree with recent studies, thereby suggesting that mesoporous carbons with highly hydrophilic surface are most suited to optimize both gas and H+ transport, and that should provide superior performance compared to traditional carbon-black supports.

        Lastly in Chapter 5, we examine the impact of ink properties on gas-transport resistance in terms of variations in the primary catalyst particle Pt loading (PPL). By investigating CL gas-transport resistance and zeta potential (ζ) of corresponding inks as a function of PPL, a direct correlation between CL high-current-density performance and ink zeta potential (ζ) is observed. We further explore ink behavior and find that ζ and pH are strong functions of PPL, indicating changes in ionomer-film distribution as a function of PPL. Resulting information is critical to unraveling ionomer-distribution heterogeneity in CLs to enable enhanced Pt utilization and improved device performance.In summary, this dissertation studies gas transport in PEFC CLs, the associated losses, and the impact of material properties on identified component gas-transport resistances. Most importantly, the results provide guidelines to improve existing material design to enhance gas transport, thus improving performance at high current densities.

          • 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
            UC Berkeley Previously Published Works (2019)
            Local transport losses limit high-current density performance of fuel-cells, especially at low Pt-loadings. Optimizing catalyst-layer structure and material properties to mitigate these losses is critical for wide-scale fuel-cell commercialization. Carbon supports form the primary porous structure of catalyst layers in which Pt particles are deposited and ionomer films are distributed. Unlike non-porous carbon supports, high surface area carbon support is unique in its relative humidity dependent Pt utilization and performance due to presence of Pt particles in the micropores interior of carbon particles. We model water uptake and Pt utilization in catalyst layers with high-surface-area carbon support. Interactions between functional groups on the carbon pore walls and water play a vital role in controlling water uptake from water vapor by catalyst layers. Ionomer is shown to influence proton access in micropores, even though it may not deposit into the micropores.
              • 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 (2019)
                  Reducing the loading of expensive Pt-based catalysts in polymer-electrolyte fuel-cell (PEFC) catalyst layers (CLs) is imperative for wide-scale commercialization. However, a local resistance close to the Pt reaction sites becomes significant at low Pt loadings and hinders adequate performance.1,2 In our previous study, we demonstrated that the local resistance is primarily related to the proton-conducting ionomer thin-film within the CL and consists of: (i) reactant transport resistance due to low permeability of ionomer thin-film (compared to bulk ionomer) and (ii) interfacial resistance due to Pt/sulfonic-moiety interactions.3 Thus, there is need to optimize ionomer design to minimize CL resistance. In this talk, we investigate the impact of ionomer chemistry, content, and equivalent weight (EW) on CL resistance. Using H2 limiting-current measurements, the total CL resistance is segregated into transport and interfacial components. CL resistance increases monotonically with ionomer content, primarily due to thicker ionomer films. Low EW ionomers demonstrate higher interfacial resistance due to high density of sulfonic groups, whereas higher EW ionomers exhibit higher transport resistance due to low-reactant permeability associated with low-water uptake. These findings are rationalized through comparison of properties and structure/function relationships elucidated by ionomer thin-film studies on planar substrates, thus bringing together model and in-situ studies. Further, time-permitting, the impact of carbon support (low versus high surface area carbon) on local CL resistance will also be discussed, mainly via modeling water uptake and proton pathways. Overall, the knowledge gained demonstrate pathways towards ameliorating the local resistance and enabling high performing low Pt-loaded PEFCs. Acknowledgements We would like to thank Sarah A. Berlinger and Robert Darling for helpful discussions. This work was funded under the Fuel Cell Performance and Durability Consortium (FC-PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231. References: 1. T. A. Greszler, D. Caulk and P. Sinha, J Electrochem Soc, 159, F831 (2012). 2. A. Z. Weber and A. Kusoglu, J Mater Chem A, 2, 17207 (2014). 3. T. Schuler, A. Chowdhury, A. T. S. Freiberg, B. T. Sneed, F. B. Spingler, M. C. Tucker, K. More, C. J. Radke and A. Z. Weber, J Electrochem Soc, 166, F3020 (2019).
                    • Article
                    • Peer Reviewed
                    LBL Publications (2022)
                    Proton-exchange-membrane fuel cells (PEMFC) are a clean energy conversion alternative to traditional fossil-fuel combustion; however, transport resistances in the electrode pose a lower-limit to catalyst loading and commercialization of PEMFCs. PEMFCs consist of simultaneous hydrogen (H2) oxidation and oxygen (O2) reduction at the anode and cathode, respectively. While oxygen transport resistances in PEMFCs have been widely studied both experimentally and analytically, hydrogen transport resistances are less understood. Herein, we present a physics-based model that encompasses multi-scale transport within the anode side of the PEMFC. The O2 in the cathode here is omitted and replaced with H2 to deconvolute O2 transport resistance contributions, similar to that of a hydrogen pump. Replication of the hydrogen pump setup allows for comparison of the model against experimental analysis of H2 gas-transport resistance in H2 limiting-current experiments, which can also inform gas transport (including oxygen) in general. Herein, we present a multi-scale analytical model of the porous anode catalyst layers and individual catalyst agglomerates that enables determination of the effects of electrode morphology such as agglomerate size, catalyst loading, etc. on H2 transport resistance through the porous electrode to complement and better understand H2 limiting current experiments and deconvolute local H2 transport resistances. Electrodes within PEMFCs are heterogeneous porous structures formed by agglomerates of carbon particles containing Pt nanoparticles, held together by an ion-conducting polymer (ionomer). The gas transport hence involves transport through multiple phases and length-scales: (i) gas-phase transport through electrode pores at the length scale of electrode thickness (micrometer) and (ii) diffusion into the agglomerate through an ionomer film at the length scale of agglomerate radius (nanometer). Based on previous modeling efforts of varying length-scales of the catalyst layer, our analytical model adequately and simultaneously captures reaction-diffusion at all of the aforementioned scales (i.e., the electrode thickness and agglomerate radius) to better describe porous anode performance. A sensitivity analysis is subsequently performed to compare current generated against transport parameters for both scales. This work ultimately provides insight that will guide the design and engineering of future porous anodes for applications in hydrogen pumping and PEMFCs.
                      Cover page: Multi-Scale Modeling of Hydrogen Transport in a Porous Fuel Cell Anode
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