Perfluorosulfonic acids (PFSAs), are commonly used as solid polymer electrolyte membranes (PEMs) in electrochemical energy devices, where they are vulnerable to attack by hydroxyl radical species during operation, which reduces their effectiveness. A popular strategy to combat this problem is to introduce radical scavengers like cerium (Ce) ions that neutralize these species before they attack the PFSA. Such cation doping creates a multi-ion system, in which understanding the mechanisms of cation solvation and transport becomes important for effective design and utilization of PFSA-cation systems. Ce ions also provide a representative model system for multication-exchanged ionomers in electrochemical systems. In this study, hydration and conductivity measurements, along with X-ray fluorescence and scattering, are employed to elucidate how Ce ion exchange alters PFSA's ionic solvation as well as nano-and mesoscale morphology which ultimately control its ion transport properties. A molecular transport model is used to delineate the impact of Ce ions on the local solvation structure of water in the membrane from mesoscale changes of the transport pathways. The combined experimental and theoretical analysis reveals a nonlinear decrease in conductivity driven by cation solvation at the molecular level and morphological changes at larger length scales. Migration-diffusion coupling, its nonlinear dependence on ion-exchange and hydration, and its overall implications for ionomer performance are also discussed in order to provide an applicable case study. These findings have the potential to be translated into other mixed cation-ionomer systems for a wide range of energy and environmental devices.
A hydrogen pump experiment was utilized to simultaneously determine the migration and diffusivity of Ce ions in perfluorosulfonic acid (PFSA) ionomer membranes over a range of temperatures and relative humidities. Ce ion migration profiles were quantified as a function of charge transfer through the cell using X-ray fluorescence (XRF). Competing transport phenomena were decoupled by fitting XRF profile data with our previously-developed one-dimensional model, which was updated with improved conductivity and water uptake relations. Measured transport values showed good agreement with the Einstein relation and Okada transport theory, implying that conductivity can be used to estimate migration and diffusivity of cations in other cation/PFSA systems. Results presented here may be used to populate device-level models in order to further understand the effects of cation transport on fuel cell performance and durability to determine the mitigation controls for cation stabilization.
Atomic force microscopy (AFM) is used as a tool to analyze agglomerates in proton exchange membrane fuel cell electrodes. Catalyst layers were prepared with catalyst inks with varying ionomer/carbon (I/C) ratios. AFM is used to image the height and adhesion on the electrode surface and in cross-section to study the relationship between the I/C, carbon particle size, and agglomerate size. A positive correlation between ionomer coverage and particle diameter and I/C was observed. There was no identifiable trend observed between ionomer thickness in agglomerates and I/C, suggesting that ionomer coats the surface in the z direction at higher I/C ratios.
A small microwatt fuel cell stack and system was designed, fabricated, and tested for passive operation with pure H2 and O2 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. Additional requirements for decades long operation is minimizing H2 and O2 consumption, specifically membrane cross-over; to minimize cross-over, stacks were built with layered membranes of 100 µm and 400 µm. The stack spatial water concentration level was monitored during operation by in situ neutron imaging to evaluate the passive water management design. Operation was verified by testing at temperatures ranging from -55 °C to 80 °C, including periodic pulses in power.
Neutron radiography was used to measure water concentrations through the cross-sections of sprayed gas diffusion electrodes (GDE) and electrospun perfluorosulfonic acid-platinum nanofiber (NF) electrodes during fuel cell operation. The performance of Generation 1 poly(acrylic acid)-based NF electrodes is improved at high current densities compared to the baseline given by a GDE cell. Through-thickness water profiles reveal that at high current densities, the water concentrations within the membrane electrode assembly (MEA) and gas diffusion layers are around 2x lower in the nanofiber-containing MEA compared to the baseline GDE, commensurate with the observed polarization performance. In Generation 2 electrospun nanofiber electrodes, poly(ethylene oxide)-based electrodes show higher performance than poly(acrylic acid) electrodes, while retaining less water in the MEA. These results imply that the electrospun nanofiber electrodes have an improved ability to maintain hydration without flooding, which leads to improved performance over a range of relative humidities.
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