CO2 reduction conducted in electrochemical cells with planar electrodes immersed in an aqueous electrolyte is severely limited by mass transport across the hydrodynamic boundary layer. This ...limitation can be minimized by use of vapor-fed, gas-diffusion electrodes (GDEs), enabling current densities that are almost two orders of magnitude greater at the same applied cathode overpotential than what is achievable with planar electrodes in an aqueous electrolyte. The addition of porous cathode layers, however, introduces a number of parameters that need to be tuned in order to optimize the performance of the GDE cell. In this work, we develop a multiphysics model for gas diffusion electrodes for CO2 reduction and used it to investigate the interplay between species transport and electrochemical reaction kinetics. The model demonstrates how the local environment near the catalyst layer, which is a function of the operating conditions, affects cell performance. We also examine the effects of catalyst layer hydrophobicity, loading, porosity, and electrolyte flowrate to help guide experimental design of vapor-fed CO2 reduction cells.
For next-generation polymer-electrolyte fuel cells, material solutions are being sought to decrease the cost of the cell components, and, in particular, the amount of catalyst, without sacrificing ...performance and lifetime. However, as recently shown, this cannot be achieved in practice due most likely to limitations caused by the ionomer thin-film surrounding the catalyst sites, where confinement and substrate interactions dominate and result in increased mass-transport limitations. Mitigation of this issue is paramount to the future commercial viability of polymer-electrolyte fuel cells.
Perfluorosulfonic-acid (PFSA) ionomer membranes (most commonly Nafion registered ) are currently the prototypical proton-exchange-membrane in polymer-electrolyte fuel cells (PEFCs), for which ...durability still represents a technical barrier to their commercialization. In an effort to address the durability demands, PFSA membranes with reinforcement and/or stabilizers have become of great interest as they have demonstrated superior durability in PEFCs compared to their unreinforced analogues. One such particular membrane that is tailored for enhanced durability and commonly employed in PEFCs is Nafion XL, a Nafion-based ionomer membrane with mechanical reinforcement and chemical stabilizers. Despite an increasing number of recent studies demonstrating its improved lifetime in accelerated stress testing (AST), its structure and transport properties have not been investigated in a systematic fashion. In this paper, we report water uptake, dimensional change, conductivity, and mechanical properties of Nafion XL membrane, as well as its strong anisotropy, in comparison to (unreinforced) Nafion 212 membrane. Moreover, water-domain spacing and crystallinity of Nafion XL membrane, determined from small- and wide-angle X-ray scattering (SAXS/WAXS) experiments, are correlated with the measured properties to establish a structure/property relationship, and discussed within the context of composite materials. It is also found that (pre)conditioning of the membrane by heating in water at different temperatures could have significant impacts on its structure/property relationship, in particular, the mechanical stability and conductivity, and their anisotropy, which were related to morphological changes observed from microscopy studies. The findings reported here not only provide a new dataset that can be used for PEFC performance and durability modeling but also benefit the efforts on developing composite ion-conductive membranes.
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Perfluorosulfonic-acid (PFSA) ionomers are widely used as solid electrolytes and ion-exchange membranes in electrochemical devices, wherein their properties are impacted by the ...interactions among the anionic sulfonate groups, mobile counter-ions (cations), and hydration levels. Cation-form and humidity collectively affect the structure/transport-property relationship, yet their interplay is still not well known. In this paper, we report changes in water uptake and conductivity of cation-exchanged PFSA in both vapor and liquid water, which are then correlated with changes in mechanical properties and nanostructure (hydrophilic-domain spacing and phase-separation). It is found that the magnitude of changes depends significantly on the membrane water content, with master curves in terms of water volume fraction and water per charge realized. Moreover, membrane nanostructure and dynamical-mechanical behavior is examined to establish structure/transport and transport/stability relationships. It is found that with increasing cation size (radius) and valence, the storage modulus increases, while the water uptake and conductivity decrease. In addition, regardless of the cation type, a universal relationship is found between the conductivity and modulus, indicative of a transport/stability tradeoff. The extent to which the cations impact the transport properties depends on the water content: at low hydration levels the controlling factor is the cation (and its interaction with the sulfonate sites), at increasing hydration the dominant factor becomes water volume fraction, although it is also controlled by the cations. Similarly, the decrease in hydrophilic domain spacing of PFSA exchanged with larger cations scales with cation radius at low water contents, but with Lewis acid strength (LAS) at higher hydration levels. The findings reported here not only provide valuable insights into the interaction between sulfonate groups, cations, and water surrounding these ionic groups, but also for understanding cation contamination in fuel cells and redox flow batteries.
Perfluorinated sulfonic acid (PFSA) ionomers are the most widely used solid electrolyte in electrochemical technologies due to their remarkable ionic conductivity with simultanous mechanical ...stability, imparted by their phase‐separated morphology. In this work, the morphology and swelling of PFSA ionomers (Nafion and 3M) as bulk membranes (>10 μm) and dispersion‐cast thin films (<100 nm) are investigated to identify the roles of equivalent weight (EW) and side‐chain length across lengthscales. Humidity‐dependent structural changes as well as different PFSA chemistries are explored in the thin‐film regime, allowing for the development of thickness‐EW phase diagrams. The ratio of macroscopic (thickness) to nanoscopic (domain spacing) swelling during hydration is found to be affine (1:1) in thin films, but increases as the thickness approaches bulk values, revealing the existence of a mesoscale organization governing the multiscale swelling in PFSAs. Ionomer chemistry, in particular EW, is found to play a key role in altering the confinement‐driven structural changes, including thin‐film anisotropy, with phase separation becoming weaker as the film thickness is reduced below 25 nm or as EW is increased. For the lower‐EW 3M PFSA ionomers, confinement appears to induce even stronger phase separation accompanied by domain alignment parallel to the substrate.
Phase separation in ion‐conducting polymers drives their functionality. Here, the impact of chemistry, including equivalent weight and side‐chain length, is systematically explored to elucidate the underlying fundamental drivers and results of phase separation across critical lengthscales found in devices from micrometer bulk membranes to nanometer thin films. The origin and results of hydration‐induced swelling are revealed.
Perfluorosulfonic acid (PFSA) dispersions are used as components in a variety of electrochemical technologies, particularly in fuel-cell catalyst-layer inks. In this study, we characterize ...dispersions of a common PFSA, Nafion, as well as inks of Nafion and carbon. It is shown that solvent choice affects a dispersion’s measured pH, which is found to scale linearly with Nafion loading. Dispersions in water-rich solvents are more acidic than those in propanol-rich solvents: a 90% water versus 30% water dispersion can have up to a 55% measured proton deviation. Furthermore, because electrostatic interactions are a function of pH, these differences affect how particles aggregate in solution. Despite having different water contents, all inks studied demonstrate the same particle size and surface charge trends as a function of pH, thus providing insights into the relative influence of solvent and pH effects on these properties.
Significant advances have been made in recent years discovering new electrocatalysts and developing a fundamental understanding of electrochemical CO2 reduction processes. This field has progressed ...to the point that efforts can now focus on translating this knowledge toward the development of practical CO2 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 CO2 reduction catalysts into practical device architectures that operate using vapor-phase CO2 reactants, thereby overcoming intrinsic limitations of aqueous-based systems. Performance comparison is made between state-of-the-art CO2 electrolyzers and commercial H2O electrolyzersa 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 CO2 electrolyzers.
In this comprehensive review, recent progress and developments on perfluorinated sulfonic-acid (PFSA) membranes have been summarized on many key topics. Although quite well investigated for decades, ...PFSA ionomers’ complex behavior, along with their key role in many emerging technologies, have presented significant scientific challenges but also helped create a unique cross-disciplinary research field to overcome such challenges. Research and progress on PFSAs, especially when considered with their applications, are at the forefront of bridging electrochemistry and polymer (physics), which have also opened up development of state-of-the-art in situ characterization techniques as well as multiphysics computation models. Topics reviewed stem from correlating the various physical (e.g., mechanical) and transport properties with morphology and structure across time and length scales. In addition, topics of recent interest such as structure/transport correlations and modeling, composite PFSA membranes, degradation phenomena, and PFSA thin films are presented. Throughout, the impact of PFSA chemistry and side-chain is also discussed to present a broader perspective.
Our previous models and a catalyst-layer model are combined to study the effects of microporous layers in polymer electrolyte fuel cells. The combined sandwich model is used to fit data both with and ...without a microporous layer with saturated feed conditions. In terms of water management, the simulations clearly show that the effect of a microporous layer is to keep water out of the cathode gas diffusion layer and move it through the anode. Additional effects of microporous layers include better ohmic behavior and perhaps better catalyst utilization, among other things. Optimizations for different structural parameters are investigated, as are the effects of microporous layers under different operating conditions. The discussion is geared toward how microporous layers increase performance and their effect on fuel cell water management.
Thin films of ion‐conducting polymers are an important area of study due to their function in many electrochemical devices and as analogues for interfacial phenomena that occur in bulk films. In this ...paper, the properties of Nafion, a prototypical ionomer, are investigated as thin films (4 to 300 nm) on carbon, gold, and platinum substrates that are fabricated using different casting methods and thermal histories. Specifically, water uptake, swelling, and morphology are investigated by quartz‐crystal microbalance, ellipsometry, and grazing‐incidence X‐ray scattering to develop structure/property/processing relationships. For all substrates, as the films' thickness decreased, there is an initial decrease in swelling followed by a subsequent increase for film thicknesses below ≈20 nm due to a disordering of the film hydrophilic/hydrophobic structure. Decreased swelling and less structural order is observed on gold for spin‐cast films compared to self‐assembled films; the opposite effect is observed for films on carbon. The presented systematic data set and analyses represent a thorough study of the behavior of Nafion thin films on model substrates of interest in metal catalyst/carbon electrodes, and these insights help to elucidate the underlying polymer physics and confinement effects in these and related systems.
Structure‐property‐processing behavior of Nafion thin films is controlled by a complex interplay between substrate/film interactions, thickness, and casting method. Self‐assembled and spin‐cast films demonstrate different behavior depending on the substrate. Swelling decreases from the bulk polymer values or films between ≈20 to 100 nm thick and then increases for films thinner than 20 nm, for which phase‐separation is weak.
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