The FC-PAD Consortium: Advancing Fuel Cell Performance and Durability
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Energy Sciences↗
Engineering topics
Weber, Adam Z.
Publications and source records attributed to Weber, Adam Z..
Transport and Morphology of a Proton Exchange Membrane Based on a Doubly Functionalized Perfluorosulfonic Imide Side Chain Perflourinated Polymer
There is a critical need for higher performing proton exchange membranes for electrochemical energy conversion devices that would enable higher temperature and drier operating conditions to be utilized. A novel approach is to utilize multiacid side chains in a perfluorinated polymer, maintaining the mechanical properties of the material, while dramatically increasing the ion-exchange capacity; however, as we show in this paper, the more complex side chain gives rise to unexpected physical phenomena in the material. In this work we have thoroughly investigated a doubly functionalized perfluorosulfonic imide acid side chain perfluorinated polymer (PFIA), the simplest of many possible multiacid side chains currently being developed. The material is compared to its simpler perfluorosulfonic acid (PFSA) analogue via a battery of characterization and modeling investigations. The doubly functional side chain profoundly influences the properties of the PFIA polymer as it gives rise to both inter- and intraside chain interactions. These affect the nature of thermal decomposition of the material but, more importantly, force the backbone of the polymer into an unusually highly ordered more crystalline configuration. Under water saturated conditions, the PFIA has the same proton conductivity as the PFSA material, indicating that the additional proton does not contribute to the ionic conductivity, but the PFIA shows higher proton conductivity at lower RH conditions owing to dynamic changes in its local molecular environment. A transition is observed between 30 and 60 °C, indicating an order/disorder transition that is not present in the PFSA analogue. The mechanism of proton transport in the PFIA is due to more delocalized protons and more flexible side chains with better-dispersed, smaller water clusters forming the hydrophilic domains than in the PFSA analogue.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Mesoscopic modeling of transport resistances in a polymer-electrolyte fuel-cell catalyst layer: Analysis of hydrogen limiting currents
Understanding transport resistances in a polymer-electrolyte fuel cell (PEFC) catalyst layer (CL) is essential to mitigate the unexpected voltage loss when using low loadings of precious metals. In this paper, we explore through mesoscopic modeling the quantification analyses of the transport resistances in CL as derived using hydrogen-pump limiting current. Numerical treatments on the conjugated interfacial conditions at interfaces of ionomer/pore and Pt/ionomer are proposed to describe the mesoscopic transport processes of hydrogen and proton. Characterizations of the reconstructed microstructure of CL are performed. Parameter analyses on the influences of the critical transport properties such as the permeation coefficient and the dissolution and adsorption reaction rates at the surfaces of ionomer/pore and Pt/ionomer on the local transport resistance are presented. It is found that the local transport resistance is mainly originated from the diffusion resistance of the ionomer thin-film, which is more resistive than its bulk analogue with its permeation coefficient fitted to be 5.9% of the bulk one. The interfacial transport resistances and the diffusion resistance are coupled. The local transport resistance increases with I/C ratio due to thicker ionomer coated on the particles. Higher Pt/C ratio and bare carbon fraction lead to higher local transport resistance since the ionomer loading relative to Pt roughness factor decreases. The local transport resistance decreases with the porosity. Furthermore, the contribution of pores to the CL resistance, which decreases with the porosity, is comparatively small at low loadings.
25 ENERGY STORAGE↗
Electric Field Intensity Modulated Scattering as a Thin-Film Depth Probe
Grazing incidence x-ray scattering provides nanostructural information for thin film samples, but single images generally do not provide information on film thickness or the full complex index of refraction. Additionally, for thin films that possess stratification of scatterers vertically within a sample, it can be difficult to determine where those scatterers are positioned. We present an in-situ method to extract film thickness, the index of refraction, and depth information using scattering images taken across a range of incident angles. The underlying theory is presented, and we validate the technique using two sets of polymer thin films. Finally, we discuss how it can be implemented as a general beamline procedure. This technique is applicable to any thin-film material and has potentially far-reaching impact by enabling depth-sensitive information in situ at any grazing incidence-capable beamline.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Dictating Pt-Based Electrocatalyst Performance in Polymer Electrolyte Fuel Cells, from Formulation to Application
In this study, in situ electrochemical diagnostics designed to probe ionomer interactions with platinum and carbon were applied to relate ionomer coverage and conformation, gleaned from anion adsorption data, with O 2 transport resistance for low-loaded (0.05 mg Pt cm -2 ) platinum-supported Vulcan carbon (Pt/Vu)-based electrodes in a polymer electrolyte fuel cell. Coupling the in situ diagnostic data with ex situ characterization of catalyst inks and electrode structures, the effect of ink composition is explained by both ink-level interactions that dictate the electrode microstructure during fabrication and the resulting local ionomer distribution near catalyst sites. Electrochemical techniques (CO displacement and ac impedance) show that catalyst inks with higher water content increase ionomer (sulfonate) interactions with Pt sites without significantly affecting ionomer coverage on the carbon support. Surprisingly, the higher anion adsorption is shown to have a minor impact on specific activity, while exhibiting a complex relationship with oxygen transport. Ex situ characterization of ionomer suspensions and catalyst/ionomer inks indicates that the lower ionomer coverage can be correlated with the formation of large ionomer aggregates and weaker ionomer/catalyst interactions in low-water content inks. These larger ionomer aggregates resulted in increased local oxygen transport resistance, namely, through the ionomer film, and reduced performance at high current density. In the water-rich inks, the ionomer aggregate size decreases, while stronger ionomer/Pt interactions are observed. The reduced ionomer aggregation improves transport resistance through the ionomer film, while the increased adsorption leads to the emergence of resistance at the ionomer/Pt interface. Overall, the high current density performance is shown to be a nonmonotonic function of ink water content, scaling with the local gas (H 2 , O 2 ) transport resistance resulting from pore, thin film, and interfacial phenomena.
30 DIRECT ENERGY CONVERSION↗
Modeling Water Uptake and Pt Utilization in High Surface Area Carbon
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.
08 HYDROGEN↗
Along-the-channel modeling and analysis of PEFCs at low stoichiometry: Development of a 1+2D model
Water management remains a key challenge in polymer-electrolyte fuel cells. In this paper, a pseudo 3-D (1+2D) model is developed to account better for changes of water management along the channel, as well as verify the possibilities of using differential cells for data capture and translation to integral cell performance. An accurate 2-D membrane-electrode-assembly model is developed for differential cell modeling, which is combined with an along-the-channel stepping algorithm to account for down the channel changes in pressure, temperature, reactant concentration, and relative humidity. Variations in cell performance along the channel due to changes in operating conditions are characterized quantitatively and optimized, where drier feed conditions demonstratively require such an approach. In conclusion, the study identifies gaps between differential and integral cells including those related to flow velocity and highlights the need for better models to understand and link integral cell performance and water management.
30 DIRECT ENERGY CONVERSION↗
Optimization of the cerium-hydrogen redox flow cell
The Ce—H2 redox flow cell is optimized using commercially-available cell materials. Cell performance is found to be sensitive to the upper charge cutoff voltage, membrane boiling pretreatment, methanesulfonic-acid concentration, (+) electrode surface area and flow pattern, and operating temperature. Performance is relatively insensitive to membrane thickness, Cerium concentration, and all features of the (−) electrode including hydrogen flow. Cell performance appears to be limited by mass transport and kinetics in the cerium (+) electrode. Maximum discharge power of 895 mW cm−2 was observed at 60° C.; an energy efficiency of 90% was achieved at 50° C. The Ce—H2 cell is promising for energy storage assuming one can optimize Ce reaction kinetics and electrolyte.
Tucker, Michael C.↗
Mass-Transport Resistances of Acid and Alkaline Ionomer Layers: A Microelectrode Study Part 1 - Microelectrode Development
The use of microelectrodes to study localized mass-transport phenomena in fuel-cell catalyst layers is an increasingly valuable tool. However, existing microelectrode cells have been used in static, equilibrated environment modes with poorly controlled interfaces. In this work, we present a microelectrode cell design that expands the experimental space addressable by microelectrodes to include mechanical pressure, gas flow and ionomer medium, and experimental throughput. The feasibility of the design is examined for fuel-cell reactions, with oxygen reduction currents independent of mechanical pressure and gas flowrate. Finally, cell equilibration time and IR drop across the electrolyte are estimated. The new cell design is robust and provides a consistent base from which to perform more complicated studies examining mass-transport properties of ionomers and/or the electrochemical reaction kinetics of hydrogen oxidation and oxygen reduction.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Imaging and Modeling of Passive Water Management in a Miniature Fuel Cell
A small microwatt fuel cell stack with 8 cells was designed, fabricated, and tested for passive operation with pure H 2 and O 2 to provide continuous power for multiple decades with uncontrolled fluctuating ambient conditions. The stack was designed to operate with dead ended gas flows with water removal via passive membrane diffusion to notches in the bipolar plates. Two stacks with active areas of 0.2 cm 2 , one with 200 nm of membrane and the other with 400 urn, were assembled and tested at varying temperatures and current conditions. Temperatures ranged -55 °C to 70 °C, with focus on the lower temperatures. Factors affecting water production and removal included temperature, gas crossover, and membrane thickness. Currents were applied from 16 uA to 10 mA after the cells were stabilized at temperature, followed by a period of drying to observe water transport to the external peripheries of the cell. Finally, a two-dimensional multiphysics model was developed to further explore the water management system in varying conditions.
25 ENERGY STORAGE↗
Modeling Electrolyte Composition Effects on Anion-Exchange-Membrane Water Electrolyzer Performance
Anion-exchange-membrane (AEM) water electrolysis could allow inexpensive and greener hydrogen production than other alternatives, such as steam methane reforming. To increase performance, hydroxide salts are often added to the water feed, with the tradeoff of corrosivity and complexity. Recently, carbonate salts that are less corrosive have shown promise, but their specific functionality remains unknown. In this paper, we use a mathematical model to compare an AEM electrolyzer with added potassium carbonate to an AEM electrolyzer with added potassium hydroxide. Here, we show that the conductivity of the carbonate-form membrane has little impact on the performance of the device, but that carbonate ions replace hydroxide in the ionomer, which creates a Nernstian voltage difference across the membrane. The replacement of hydroxide anions with carbonate also reduces utilization of the catalyst in the anode, resulting in an additional voltage loss.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
System and method for returning material from the Br.sub.2 side of an H.sub.2/Br.sub.2 flow battery back after crossover
A flow battery system includes a first tank having a hydrogen reactant, a second tank having a bromine electrolyte, at least one cell including a hydrogen reactant side operably connected to the first tank through an ¾ feed and return system and a bromine electrolyte side operably connected to the second tank, and a crossover return system. The crossover return system includes a vessel operably connected to the ¾ feed and return system and configured to receive an effluent containing a first portion of the hydrogen reactant and a second portion of the bromine electrolyte, the vessel configured to separate the first portion from the second portion. A first return line returns the first portion of the hydrogen reactant to the first tank and a second return line returns the bromine electrolyte to the second tank.
Knudsen, Edward↗
Mass-transport properties of electrosprayed Pt/C catalyst layers for polymer-electrolyte fuel cells
Mass-transport properties of electrosprayed catalyst-layers based on Pt/C and ionomer (Nafion) are investigated with hydrogen limiting-current technique, water-vapor-uptake, scanning transmission microscopy (STEM), single-cell testing, and impedance spectroscopy. The hydrogen limiting-current technique provides the transport resistance of the layers ( R CL m t ), which demonstrates to be lower in electrosprayed layers compared with conventional layers, especially at very low platinum loadings (0.025 mg Pt ·cm -2 ) and low cell temperature, denoting superior mass-transport properties. Images of the distribution of Pt, F, and C elements reveal the ionomer preferentially interacting with the Pt nanoparticles. Water-vapor-uptake experiments show larger vapor absorption for electrosprayed than conventional catalyst layers. Such large water-vapor uptake capability is combined with superhydrophobicity, ie. very low interaction with water in liquid phase (wettability). Both apparently contradictory properties result from a particular configuration of the amphiphilic ionomer in the electrosprayed layers, and provide ideal conditions for high mass transport and ionic conductivity in a catalyst layer. Electrosprayed layers as cathode catalyst layers reflect peak response at a loading of 0.17 mg Pt ·cm -2 (18 μm layer thickness when using Pt/C 20 wt% catalyst) where they provide minimal mass-transport and polarization resistances.
08 HYDROGEN↗
Towards membrane-electrode assembly systems for CO 2 reduction: a modeling study
Membrane-electrode assemblies (MEAs) are an attractive cell design for the electrochemical reduction of CO 2 because they exhibit low ohmic loss and high energy efficiency. We describe here the development and application of a multiphysics model to investigate the fundamental limitations of two MEA designs: one with gaseous feeds at both the anode and cathode (full-MEA), and the other with an aqueous anode feed (KHCO 3 or KOH exchange solution) and a gaseous cathode feed (exchange-MEA). The total current density for the three cases follows the order: KOH-MEA > KHCO 3 -MEA > full-MEA. This trend is established by examining the distribution of the applied voltage. We show that the main charge-carrying species are carbonate anions for an MEA that uses an anion-exchange membrane (AEM). The amount of CO 2 consumed but not converted to CO decreases with increasing current densities above 100 mA cm –2 for a full-MEA, but converges to 50% for exchange-MEAs. The full-MEA becomes limited by ohmic resistance as the membrane dehydrates with increasing cell temperature, and eventually becomes limited due to water mass transport. The exchange-MEAs can maintain membrane hydration and the local ion concentration at the anode, but are limited by salt precipitation at the cathode, as well as a higher tendency to flood. Lastly, we explore the effects of temperature and discuss the possibility of increasing water supply to the full-MEA to improve its performance at elevated temperatures. The MEA model and the understanding of MEA performance for the electrochemical reduction of CO 2 presented in this study should help guide the design of next-generation CO 2 reduction cells.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Modeling Coupled Durability and Performance in Polymer-Electrolyte Fuel Cells: Membrane Effects
During operation, proton-exchange-membrane fuel cells (PEMFCs) are subjected to mechanical and chemical stressors that contribute to membrane degradation, performance loss, and eventual failure. Together, synergistic effects between mechanical and chemical degradation mechanisms lead to accelerated degradation. A physics-based model is developed to understand the synergistic effects of chemical and mechanical degradation and the coupled nature of performance and durability in PEMFCs. The model includes pinhole existence and growth in the membrane, which increases crossover of reactant gases as well as subsequent formation of chemical degradation agents that impact both transport and mechanical properties of the membrane. The fuel-cell model is fully coupled with a mechanical model to determine the stresses on the membrane and subsequent growth of pinholes during transient operation. Simulation results demonstrate pinhole growth under relative-humidity cycling and the resultant increased gas-crossover fluxes and decrease in polarization performance. Furthermore, the model results highlight nonlinearities and the importance of coupling mechanical and chemical degradation models in order to explain membrane degradation under various cycles, and serves as a foundation for examining coupled durability and performance.
25 ENERGY STORAGE↗
“The development of a through-plane reactive excitation technique for detection of pinholes in membrane-containing MEA sub-assemblies”
As markets for polymer-electrolyte-membrane fuel cells (PEMFC) expand, there continues to be a need for the development and implementation of in-line quality inspection techniques to enable high-volume manufacturing methods for membrane-electrode assemblies (MEAs). These techniques mitigate the potential impact of poor quality on the cost of MEA components by real-time identification of process-induced irregularities, which can result from the manufacturing process or subsequent handling of the material during assembly into a full MEA. A particular type of irregularity known to impact MEA lifetime is loss of membrane integrity, e.g. via a pinhole. In this work, we focus on development of a technique to detect pinholes in membrane-containing MEA sub-assemblies - as opposed to in just the membrane - using a reactive excitation strategy coupled with infrared thermography for detection. A specialized device is introduced and, using various MEA sub-assemblies, is shown to enable detection of pinholes in stationary samples. Multi-physics modeling is utilized to understand and predict the impact of various design and operational parameters of this technique. The feasibility of the technique is demonstrated through both modeling and in-situ results. For example, a detectable (≥1 degree C) thermal response for a membrane pinhole of approximately 90 um in diameter is measured in a fraction of a second. The observed detection time is considered to be appropriate for further exploration of the technique in an in-line configuration.
30 DIRECT ENERGY CONVERSION↗
Implications of inherent inhomogeneities in thin carbon fiber-based gas diffusion layers: A comparative modeling study
Not Available
García-Salaberri, Pablo A.↗
Gas-Diffusion Electrodes for Carbon Dioxide Reduction: A New Paradigm
Here significant advances have been made in recent years discovering new electrocatalysts and developing a fundamental understanding of electrochemical CO 2 reduction processes. This field has progressed to the point that efforts can now focus on translating this knowledge toward the development of practical CO 2 electrolyzers, which have the potential to replace conventional petrochemical processes as a sustainable route to produce fuels and chemicals. In this Perspective, we take a critical look at the progress in incorporating electrochemical CO 2 reduction catalysts into practical device architectures that operate using vapor-phase CO 2 reactants, thereby overcoming intrinsic limitations of aqueous-based systems. Performance comparison is made between state-of-the-art CO 2 electrolyzers and commercial H 2 O electrolyzers—a well-established technology that provides realistic performance targets. Beyond just higher rates, vapor-fed reactors represent new paradigms for unprecedented control of local reaction conditions, and we provide a perspective on the challenges and opportunities for generating fundamental knowledge and achieving technological progress toward the development of practical CO 2 electrolyzers.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗