News | Kusoglu Research Lab

A new Compressive Creep Procedure for testing Polymer Membranes in Water

In water-splitting electrolyzers, the polymer electrolyte membrane is expected to exhibit stability under compressive loads for long operational times. To monitor the compressive-creep response of a membrane in liquid water, we have designed a new setup and developed a testing procedure.

Our results show that PEMs exhibit continuous creep response under compression over 24 h, with a dependence on the applied pressure and hydration state. This work was performed under the HydroGEN and H2NEW consortia funded by DOE.

Arthurs, Claire R, and Ahmet Kusoglu."Compressive Creep of Polymer Electrolyte Membranes: A Case Study for Electrolyzers."ACS Applied Energy Materials 4.4 (2021) 3249 - 3254. DOI

Anion-Exchange Ionomer Thin Films for Electrodes of Alkaline Devices

AEI thin films In collaboration with Yu Seung Kim's group in Los Alamos National Lab, we have investigated how anion-exchange ionomers (AEIs) change their behavior when cast as thin-films on supports compared to the bulk membranes. We have examined AEIs with different backbone chemistry as well as side-chain compositions to develop a better picture for the chemistry-anion effect on ionomer film properties for understanding their function in electrodes of alkaline devices. This work was carried out under the HydroGEN consortium of DOE.

Luo, Xiaoyan, Douglas I Kushner, Jonathan Li, Eun Joo Park, Yu Seung Kim, and Ahmet Kusoglu."Anion Exchange Ionomers: Impact of Chemistry on Thin‐Film Properties."Advanced Functional Materials (2021) 2008778. DOI

Effect of Cerium Ions on Structure-Property Relationship of Ionomer Membranes

In collaboration with Los Alamos National Lab, we have published a paper that systematically investigates and explains how the incorporation of Cerium (III) and Cerium(IV) cations into a proton-exchange ionomer membrane alters the internal structure, solvation, and hydration properties, as well as ion conductivity. The combined experimental and theoretical analysis reveals a nonlinear change in conductivity driven by cation solvation at the molecular level and morphological changes at longer length scales. The results are used to construct various property-maps for the ionomer membrane.

Baker, Andrew M, Andrew R Crothers, Kavitha Chintam, Xiaoyan Luo, Adam Z Weber, Rodney L Borup, and Ahmet Kusoglu."Morphology and Transport of Multivalent Cation-Exchanged Ionomer Membranes Using Perfluorosulfonic Acid–Ce^Z+as a Model System."ACS Applied Polymer Materials 2.8 (2020) 3642 - 3656. DOI

Transport phenomena in flow battery ion-conducting membranes

In collaboration with Weber group, we have published an Opinion Article in Current Opinion in Electrochemistry that discusses various contributing factors of transport phenomena, how they influence cell performance, and the performance tradeoffs inherent in membrane design.

Kushner, Douglas I, Andrew R Crothers, Ahmet Kusoglu, and Adam Z Weber."Transport phenomena in flow battery ion-conducting membranes."Current Opinion in Electrochemistry (2020). DOI

ALS Highlight: Multimodal Study of Ion-Conducting Membranes

Our work on multi-modal characterization of ionomers has been selected as a science highlight by the Advanced Light Source. This study uses multiple x-ray characterization tools to elucidate the chemical and structural origins of the improved performance of novel ionomers from 3M Company. The work provides insight into factors impacting the proton conductivity of ionomers used as solid-electrolyte membrane in fuel cells and water-splitting devices.

Download pdf.

Journal Publication: Characterization of Anion-Exchange Membranes

AEM A systematic study on the effects of anions and hydration on properties of AEMs based on commerically available polymers as well as a PAP-based polymer, provided by Yushan Yan Group at University of Delaware.

Luo, Xiaoyan, Santiago Rojas-Carbonell, Yushan Yan, and Ahmet Kusoglu."Structure-transport relationships of poly(aryl piperidinium) anion-exchange membranes: Effect of anions and hydration."Journal of Membrane Science (2019) 117680. DOI

Abstract: Structure-Transport Relationship of AEMs

Hydroxide-exchange membrane (HEM) fuel cells are emerging energy conversion technologies. A significant effort has been expended to develop new HEMs with enhanced transport functionality, which has driven the need for understanding how transport of hydroxide and other anions in these membranes is related to hydration and nano-morphology. In this work, we report the results of a systematic study on poly(aryl piperidinium)-based on terphenyl (PAP-TP-85), a HEM that previously showed promising fuel cell performance and durability. Membrane water uptake and anion conductivity in liquid and vapor water, as well as the impact of counter-anion forms on these properties, are investigated and compared with a commercial anion exchange membrane (AEM), Fumasep FAA3, and proton-exchange membranes (PEMs), Nafion and sPEEK. Different water uptake in liquid vs. saturated vapor is observed for both AEMs (i.e., PAP and FAA3), indicating Schroeder's paradox, regardless of anion form. Morphology of AEMs examined via small-angle X-ray scattering (SAXS) shows weak phase-separation, regardless of hydration level and anion type, which is attributed to the reduced chemical dissimilarity between the backbone and ionic moieties. Despite both AEMs' amorphous nanostructure, PAP-AEM has a higher ion conductivity than FAA3. Water content plays a more significant role than does temperature in controlling the anion conductivity in water vapor. In liquid water, normalized conductivity shows a universal dependence on hydration, regardless of the anion form. Moreover, in water vapor, conductivity is influenced more by ion mobility than ion concentration, and depends mainly on hydration. Thus, ion transport in disordered AEMs is governed primarily by hydration, in contrast to phase-separated ionomers, where ion transport is governed by nanostructure-hydration interplay.

Journal Publication: Tracking Evolution of Ionomer Structure during Film Formation

Dudenas, Peter, and Ahmet Kusoglu."Evolution of Ionomer Morphology from Dispersion to Film: An in Situ X-ray Study."Macromolecules 52.20 (2019) 7779 - 7785. DOI

Abstract: Evolution of Ionomer Morphology from Dispersion to Film

Ion-conducting polymers (ionomers) have been extensively studied in solution, as membranes and substrate-supported thin films for various electrochemical energy-conversion devices, including fuel cells and electrolyzers. Formation of an ionomer film from a solution, however, is not well understood, despite its importance for fabrication of electrodes in energy devices. Here, the evolution of the perfluorinated sulfonic acid morphology upon casting from a solution is observed using in situ grazing-incidence small- and wide-angle X-ray scattering. Aggregate interactions in dispersion directly impact the hydrophilic-domain network of the cast film and the onset of crystallization occurs simultaneously with the solution-to-film transition but continues to evolve on different time scales. In addition, confinement is shown to induce anisotropic morphology at multiple length scales. These results show promise for elucidating the role of casting parameters, drying protocols, and ionomer–solvent interactions in governing film morphology and open new avenues for establishing structure/processing/property relationships for ionomer films and modifying their transport functionality at catalytic interfaces.

Review Paper: New Insights into PFSA Ionomers

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.

This review covers and summarizes ~1000 papers on PFSAs in the open literature.

This paper is published under open-access and can be downloaded from the journal webpage.

Journal Publication: Probing Morphology of Sulfonated Ionomers via Tender X-rays

In this paper, our group and researchers at the Advanced Light Source (ALS) collaborated to examine the morphological features of sulfonated ionomers, including novel multi-acid side-chain chemistries developed by 3M, using state-of-the-art in-situ x-ray scattering and spectroscopy technqiues with tunable energy (eV) range, such as Sulfur-edge.

Our work has been featured on the cover of JACS.

Su, Gregory M, Isvar A Cordova, Michael A Yandrasits, Matthew Lindell, Jun Feng, Cheng Wang, and Ahmet Kusoglu."Chemical and Morphological Origins of Improved Ion Conductivity in Perfluoro Ionene Chain Extended Ionomers."Journal of the American Chemical Society 141.34 (2019) 13547 - 13561. DOI

Journal Publication: Multi-acid Side-Chain Ionomers with High Proton Conductivity

We have published a comprehensive paper exploring and elucidating the effect of complex side-chains on the properties of polymer-electrolyte membranes, in collaboration with Herring Group, Vito Di Noto, and 3M. This contribution is a result of a unique collaboration of researchers working on polymers, structural characterization, transport phenomena and theoretical modeling. Our results show how the side-chain modification of an ionomer can have unintended consequences on membrane properties.

Journal Publication: Characterization of PFSA Ionomer with Elastin-like Polypeptide

Tuning the properties of PFSA ionomer thin films on catalystic surfaces such as Pt is an important step toward controlling their functionality in electrodes of fuel cells and electrolyers. Renner Group (CaseWestern) works on altering the Ionomer-Pt interface by modulating the morphology of Nafion ionomer by incorporating an elastin-like polypeptide (ELP). We have collaborated to accomplish the structural characterization of these ionomers using x-ray techniques at the ALS, which have helped elucidate the origins of the observed functionality changes at the ionomer interface.

Pramounmat, Nuttanit, Charles N Loney, ChulOong Kim, Luke Wiles, Katherine E Ayers, Ahmet Kusoglu, and Julie N Renner."Controlling the Distribution of Perfluorinated Sulfonic Acid Ionomer with Elastin-like Polypeptide."ACS Applied Materials & Interfaces 11.46 (2019) 43649 - 43658. DOI

Abstract: Controlling the Distribution of PFSA Ionomer with Elastin-like Polypeptide

Proton-exchange-membrane (PEM)-based devices are promising technologies for hydrogen production and electricity generation. Currently, the amount of expensive platinum catalyst used in these devices must be reduced to be cost-competitive with other technologies. These devices typically contain Nafion ionomer thin films in the catalyst layers, which are responsible for transporting protons and gaseous species to and from electrochemically active sites. The morphology of the Nafion ionomer thin films in the catalyst layers with reduced platinum loading is impacted by interactions with the catalyst and the confinement to nanometer thicknesses, which leads to performance losses in PEM-based devices. In this study, an elastin-like polypeptide (ELP) is designed to modulate the morphology of Nafion ionomer on platinum surfaces. The ELP shows an ability to assemble into a monolayer on platinum and change the ionomer interaction with platinum, thereby modifying its thin-film structure and improving the Nafion ionomer coverage. As a proof of concept, an ELP-modified catalyst ink was prepared and morphological differences were observed. Overall, we discovered an engineered ELP that can modulate the ionomer–catalyst interface in the electrodes of PEM-based devices.

Journal Publication: Molecular and Nanostructural Orientation of Nafion Thin Films

In collaboration with Hickner Group (PennState), we have investigated the confinement-driven orientation in Nafion thin films to delineate the molecular orientation and nanostructural orientation (from GISAXS). The degree of orientation at these lengthscales differ as the ionomer gets thinner and depends on the nature of the substrate composition.

Kushner, Douglas I, Ahmet Kusoglu, Nikolas J Podraza, and Michael A Hickner."Substrate‐Dependent Molecular and Nanostructural Orientation of Nafion Thin Films."Advanced Functional Materials (2019) 1902699. DOI

Book Chapter: Ionomer Thin Films in Fuel Cells

This chapter on the role of ionomers as thin films in the electrodes of fuel cells is published in Encyclopedia of Sustainability Science and Technology.

In polymer-electrolyte fuel cells (PEFCs), ionomers play a key role not only as a proton exchange membrane (PEM) but also as nanometer-thick electrolyte “thin films” within porous catalyst layer (CL) structures, where they bind and cover the catalytic particles and provide transport pathways for the ions and reactant species. As the ionomer is confined to nanometer thicknesses in the CLs, its intrinsic nano-morphology and resulting transport properties deviate from bulk membrane behavior. In thin-film form, the ionomer forms dynamic interfaces with the air and substrate, which impose stronger impact on ionomer’s structure/functionality. This interplay between the confinement and dynamic interactions controls the catalyst ionomer’s properties, which affects the local transport resistances in CLs, and ultimately PEFC performance.

Journal Publication: Simulating crack growth in fuel-cell membranes

The role of cyclic hygrothermal stressors in generating an alternating stress-state in fuel-cell membranes has been inferred in many studies. Yet, no study has demonstrated how an existing crack in a fuel cell membrane grows under cyclic loads. In this study, in collaboration with researchers from University of Delaware, a computation mechanics model of a fuel cells is developed based on the theory of plastically-dissipated energy during crack growth, which is simulated using finite element model. The model is capable of not only capturing the growth of an exsiting crack as a function of humidity cycles, but also demonstrating the role of mechanical reinforcement in slowing down the crack growth and extending membrane lifetime, in line with experimental observations.

Ding, Guoliang, Michael H Santare, Anette M Karlsson, and Ahmet Kusoglu."Numerical evaluation of crack growth in polymer electrolyte fuel cell membranes based on plastically dissipated energy."Journal of Power Sources 316 (2016) 114 - 123. DOI

Confinement-Driven Increase in Ionomer Thin-Film Modulus

Our group, in collaboration with NIST, demonstrated the applicability of the buckling methodology to the ionomer thin films for measuring their moduli. In this work, we present a compelling evidence for the confinement-driven increase in stiffness of Nafion thin film due to nanostructural changes. As the ionomer is confined onto nanometer thicknesses, the increased orientation of its nano-domains results in a higher resistance to deformation in the plane of the film, thereby increasing its modulus compared to bulk values.

Abstract: Effect of Cations on PFSA Membranes

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 brought upon depends significantly on the membrane water content, with a master curve in terms of water volume fraction realized. Moreover, nanostructure and dynamical-mechanical behavior of the membrane 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 decreases. 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.

Page, Kirt A, Ahmet Kusoglu, Christopher M Stafford, Sangcheol Kim, R. R Joseph Kline, and Adam Z Weber."Confinement-Driven Increase in Ionomer Thin-Film Modulus."Nano Letters 14.5 (2014) 2299 - 2304. DOI

Journal Publication: Effect of Cation-Exchange on PFSA Structure-Functionality

Shi, Shouwen, Adam Z Weber, and Ahmet Kusoglu."Structure-Transport Relationship of Perfluorosulfonic-Acid Membranes in Different Cationic Forms."Electrochimica Acta 220 (2016) 517 - 528. DOI

Journal Publication: Characterization of Nafion XL Membranes

Shi, Shouwen, Adam Z Weber, and Ahmet Kusoglu."Structure/property relationship of Nafion XL composite membranes."Journal of Membrane Science 516 (2016) 123 - 134. DOI

Abstract: Characterization of Nafion XL Membranes

Perfluorosulfonic-acid (PFSA) ionomer membranes (most commonly Nafion (R)) 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.