Impacts of processing, storage, and operation on thin-film perfluorosulfonic acid (PFSA) ionomer coatings used in electrodes of electrochemical devices remains unestablished. In this work, alteration of structure-function relationship in ionomers is achieved via exposure to elevated temperature and humidity (hygrothermal aging). Findings reflect a strong inverse correlation between aging-induced ionomer thin-film domain orientation and water-transport kinetics evaluated from swelling. Impact of aging is shown to be more pronounced on platinum due to interactions with PFSA, as evidenced by greater increase in nanodomain orientation parallel to substrate accompanied by reduced water transport, in contrast to silicon support.

Alkaline anion-conducting polymer-based CO2 electrolysis and water electrolysis are among two emerging renewable energy conversion technologies. Their system design and integration offer promise of lower capital cost due to utilization of low-cost catalysts, in contrast to platinum group metal catalysts required for cation-conducting polymer-based devices. However, a critical component, the polymer electrolyte membrane, remains an obstacle hampering system performance and durability. In this study, commercially-available Sustainion® membranes with and without PTFE-reinforcement were investigated to understand previously unreported origins of improved device performance when compared to alternative membrane chemistries. We report critical membrane properties, such as morphology, thermal stability, as well as temperature-, hydration-, and counter-ion dependent ion conductivity. Moveover, the changes in uptake and conductivity of membranes in supporting electrolytes of K2CO3 and KOH investigated as a function of their concentration. Presence of reinforcement and supporting electrolyte type alter the membrane's transport functionality, which could help guide device design for improved performance. The obtained results not only show how Sustainion® properties change with operating environment for CO2 and water electrolysis applications, but also provide understanding for internal and external factors controlling anion-exhcnage membrane functionality in electrochemical devices.

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.

One of the most critical issues limiting widespread commercialization of polymer-electrolyte fuel cells is catalyst cost, which is directly related to dominant unexplained local gas and ion transport resistances at low catalyst loading (1). These resistances are believed to be highly correlated to the ionomer thin films in the catalyst layer. Unfortunately, current understanding of transport resistances in the catalyst layer is largely inferred from limiting current experiments employing heterogeneous, complex and multi-interfaced membrane electrode assemblies. An alternative study providing a more localized probe and promising fundamental insight is the microelectrode method (2). Ionomer thin film-coated microelectrodes can simulate an idealized thin film operating condition similar to that found at the platinum/ionomer interface in the catalyst layer of a typical fuel cell. In this work, an in-house, custom-made microelectrode setup with good humidity, temperature and potential control is used to determine the transport coefficient of hydrogen and oxygen gas across the ionomer thin film layer. The set-up is also capable of examining gas transport and evaluating the impact of ion activity on electrochemical kinetics in both acid and alkaline conditions, making it ideal for expanded understanding of gas and ion transport in both polymer electrolyte membrane and alkaline anion exchange fuel cells, respectively. A. Z. Weber and A. Kusoglu, J. Mater. Chem. A., 2, 17207 (2014). D. Novitski and S. Holdcroft, ACS Appl. Mater. Interfaces, 7, 27314 (2015). Acknowledgements: This study was partially 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.

Ionomer thin-films (i.e., 20–100 nm) on supports serve as model systems to understand ionomer-catalyst interfacial behavior as well as the confinement-driven deviation in properties from bulk membranes. While ionomer thin-films have been examined for proton exchange ionomers, the thin-film properties of anion exchange ionomers (AEIs) remain largely unexplored. More importantly, delineating the convoluted impact of chemistry and confinement on thin-film morphology and hydration is of interest to advancing the field on functional ionic interfaces. In this work, these aspects are studied by using AEIs of different backbones (perfluorinated, aliphatic, and aromatic) and side chains (various lengths, and single versus dual functional groups). Quartz-crystal microbalance and spectroscopic ellipsometry are used to analyze density and coupled with calculated free volume fraction of thin-films to provide insights on their gas transport properties. AEI side-chain's chemical character plays a key role in how confinement modulates hydration (in thin-film versus bulk). The results elucidate the effects of backbone, side-chain chemistry versus anion/cation type in the confinement-driven changes in thin-film morphology and swelling. This study also provides new insights for tuning AEI transport functionalities at interfaces via chemistry, which can benefit the design and development of electrode-ionomers for alkaline membrane-based energy systems.

Transport through vanadium redox-flow-battery membranes strongly influences cell performance. In this work, we use a multicomponent concentrated-solution model of transport and thermodynamics in phase-separated cation-exchange membranes, the most common separator type, to develop structure-performance relationships. The model incorporates species partitioning into the membrane, thermodynamic nonidealities, and Stefan-Maxwell-Onsager frictions between species. Molecular-thermodynamics and -transport theories parameterize the model. We validate the calculations against measured Coulombic and voltage efficiencies of a vanadium flow battery as a function of current density. Our model shows that species transport is the result of collective interactions between all species present in the system. The magnitude of coupling suggests that predictions made using dilute-solution theory for transport in these systems will be misleading in many situations. As a demonstration of the capabilities of the model, we predict cell performance, incorporating these interactions, as a function of electrolyte concentration and composition and membrane equivalent weight and backbone modulus. We find that electrolytes with high sulfuric acid concentrations provide the greatest cell performance (quantified by maximizing power density at a target energy efficiency). In the case of membrane properties, low equivalent-weight polymers perform better; at high equivalent weights, a low membrane modulus is preferred.

Thin perfluorosulfonated ion-conducting polymers (PFSI ionomers) in energy-conversion devices have limitations in functionality attributed to confinement-driven and surface-dependent interactions. This study highlights the effects of confinement and interface-dependent interactions of PFSI thin-films by exploring thin-film thermal transition temperature (T_T). Change in T_T in polymers is an indicator for chain relaxation and mobility with implications on properties like gas transport. This work demonstrates an increase in T_T with decreasing PFSI film thickness in acid (H⁺) form (from 70 to 130 °C for 400 to 10 nm, respectively). In metal cation (M⁺) exchanged PFSI, T_T remained constant with thickness. Results point to an interplay between increased chain mobility at the free surface and hindered motion near the rigid substrate interface, which is amplified upon further confinement. This balance is additionally impacted by ionomer intermolecular forces, as strong electrostatic networks within the PFSI-M⁺ matrix raises T_T above the mainly hydrogen-bonded PFSI-H⁺ ionomer.

Anion-exchange membranes (AEMs) are a possible replacement for perfluorosulfonic-acid membranes in energy-conversion devices, primarily due to the hydroxide mobile ion allowing the devices to operate in alkaline conditions with less expensive electrocatalysts. However, the transport properties of AEMs remain understudied, especially electro-osmosis. In this work, an electrochemical technique, where the open-circuit voltage is measured between two ends of a membrane maintained at different relative humidities, is used to determine the water transport number of various ionomers, including Versogen and Sustainion AEMs and Nafion cation-exchange membrane (CEM), as a function of water content and temperature. In addition, the CEMs and AEMs are examined in differing single-ion forms, specifically proton and sodium (CEM) and hydroxide and carbonate (AEM). Carbonate-form AEMs have the highest transport number (∼11), followed by sodium-form CEMs (∼8), hydroxide-form AEMs (∼6), and proton-form CEMs (∼3). Finally, a multicomponent transport model based on the Stefan-Maxwell-Onsager framework of binary interactions is used to develop a link between water transport number and water-transport properties, extracting a range for the unmeasured membrane water permeability of Versogen as a function of water content.

Solid polymer electrolytes are an emerging technology in electrochemistry driven by their use in energy applications such as fuel cells, electrolyzers, and solid-state batteries. Compared to traditional liquid electrolytes, solid polymer electrolytes provide safer, cheaper, and potentially improved device performance. However, there is a lack of standard experimental methods for studying solid electrolytes. Microelectrodes have inherent benefits capable of filling this experimental gap due primarily to their integration into model electrochemical cells with solid electrolytes that represent complex interfaces, enabling additional insight into reaction processes. In this tutorial review, we explore the use of microelectrodes to study solid polymer electrolytes, beginning with a brief history of the field including common experimental cell designs and their benefits and drawbacks. Methods of evaluating essential kinetic and mass-transport parameters are then examined. In addition, the key studies of the past 30 years utilizing microelectrode cells and solid polymer electrolytes are summarized, with important results highlighted and compared. Finally, future studies of solid polymer electrolytes with microelectrodes and potential new avenues of research are commented on.