Deformation Rate-Adaptive Conducting Polymers and Composites.

Materials are more easily damaged during accidents that involve rapid deformation. Here, a design strategy is described for electronic materials comprised of conducting polymers that defies this orthodox property, making their extensibility and toughness dynamically adaptive to deformation rates. This counterintuitive property is achieved through a morphology of interconnected nanoscopic core-shell micelles, where the chemical interactions are stronger within the shells than the cores. As a result, the interlinked shells retain material integrity under strain, while the rate of dissociation of the cores controls the extent of micelle elongation, which is a process that adapts to deformation rates. A prototype based on polyaniline shows a 7.5-fold increase in ultimate elongation and a 163-fold increase in toughness when deformed at increasing rates from 2.5 to 10 000% min-1 . This concept can be generalized to other conducting polymers and highly conductive composites to create "self-protective" soft electronic materials with enhanced durability under dynamic movement or deformation.

Engineering Catalyst-Electrolyte Microenvironments to Optimize the Activity and Selectivity for the Electrochemical Reduction of CO2 on Cu and Ag.

The electrochemical reduction of carbon dioxide (CO2R) driven by renewably generated electricity (e.g., solar and wind) offers a promising means for reusing the CO2 released during the production of cement, steel, and aluminum as well as the production of ammonia and methanol. If CO2 could be removed from the atmosphere at acceptable costs (i.e., <$100/t of CO2), then CO2R could be used to produce carbon-containing chemicals and fuels in a fully sustainable manner. Economic considerations dictate that CO2R current densities must be in the range of 0.1 to 1 A/cm2 and selectivity toward the targeted product must be high in order to minimize separation costs. Industrially relevant operating conditions can be achieved by using gas diffusion electrodes (GDEs) to maximize the transport of species to and from the cathode and combining such electrodes with a solid-electrolyte membrane by eliminating the ohmic losses associated with liquid electrolytes. Additionally, high product selectivity can be attained by careful tuning of the microenvironment near the catalyst surface (e.g., the pH, the concentrations of CO2 and H2O, and the identities of the cations in the double layer adjacent to the catalyst surface).We begin this Account with a discussion of our experimental and theoretical work aimed at optimizing catalyst microenvironments for CO2R. We first examine the effects of catalyst morphology on the production of multicarbon (C2+) products over Cu-based catalysts and then explore the role of mass transfer combined with the kinetics of buffer reactions in the local concentration of CO2 and pH at the catalyst surface. This is followed by a discussion of the dependence of the local CO2 concentration and pH on the dynamics of CO2R and the formation of specific products over both Cu and Ag catalysts. Next, we explore the impact of electrolyte cation identity on the rate of CO2R and the distribution of products. Subsequently, we look at utilizing pulsed electrolysis to tune the local pH and CO2 concentration at the catalyst surface. The last part of the discussion demonstrates that ionomer-coated catalysts in combination with pulsed electrolysis can enable the attainment of very high (>90%) selectivity to C2+ products over Cu in an aqueous electrolyte. This part of the Account is then extended to consider the difference in the catalyst-nanoparticle microenvironment, present in the catalyst layer of a membrane electrode assembly (MEA), with respect to that of a planar electrode immersed in an aqueous electrolyte.

Influence of Mesoscale Interactions on Proton, Water, and Electrokinetic Transport in Solvent-Filled Membranes: Theory and Simulation.

Transport of protons and water through water-filled, phase-separated cation-exchange membranes occurs through a network of interconnected nanoscale hydrophilic aqueous domains. This paper uses numerical simulations and theory to explore the role of the mesoscale network on water, proton, and electrokinetic transport in perfluorinated sulfonic acid (PFSA) membranes, pertinent to electrochemical energy-conversion devices. Concentrated-solution theory describes microscale transport. Network simulations model mesoscale effects and ascertain macroscopic properties. An experimentally consistent 3D Voronoi-network topology characterizes the interconnected channels in the membrane. Measured water, proton, and electrokinetic transport properties from literature validate calculations of macroscopic properties from network simulations and from effective-medium theory. The results demonstrate that the hydrophilic domain size affects the various microscale, domain-level transport modes dissimilarly, resulting in different distributions of microscale coefficients for each mode of transport. As a result, the network mediates the transport of species nonuniformly with dissimilar calculated tortuosities for water, proton, and electrokinetic transport coefficients (i.e., 4.7, 3.0, and 6.1, respectively, at a water content of 8 H2O molecules per polymer charge equivalent). The dominant water-transport pathways across the membrane are different than those taken by the proton cation. Finally, the distribution of transport properties across the network induces local electrokinetic flows that couple water and proton transport; specifically, local electrokinetic transport induces water chemical-potential gradients that decrease macroscopic conductivity by up to a factor of 3. Macroscopic proton, water, and electrokinetic transport coefficients depend on the collective microscale transport properties of all modes of transport and their distribution across the hydrophilic domain network.

Highly Permeable Perfluorinated Sulfonic Acid Ionomers for Improved Electrochemical Devices: Insights into Structure-Property Relationships.

Rapid improvements in polymer-electrolyte fuel-cell (PEFC) performance have been driven by the development of commercially available ion-conducting polymers (ionomers) that are employed as membranes and catalyst binders in membrane-electrode assemblies. Commercially available ionomers are based on a perfluorinated chemistry comprised of a polytetrafluoroethylene (PTFE) matrix that imparts low gas permeability and high mechanical strength but introduces significant mass-transport losses in the electrodes. These transport losses currently limit PEFC performance, especially for low Pt loadings. In this study, we present a novel ionomer incorporating a glassy amorphous matrix based on a perfluoro(2-methylene-4-methyl-1,3-dioxolane) (PFMMD) backbone. The novel backbone chemistry induces structural changes in the ionomer, restricting ionomer domain swelling under hydration while disrupting matrix crystallinity. These structural changes slightly reduce proton conductivity while significantly improving gas permeability. The performance implications of this trade-off are assessed, which reveal the potential for substantial performance improvement by incorporation of highly permeable ionomers as the functional catalyst binder. These results underscore the significance of tailoring material chemistry to specific device requirements, where ionomer chemistry should be rationally designed to match the local transport requirements of the device architecture.

Role of ionic interactions in the deformation and fracture behavior of perfluorosulfonic-acid membranes.

The stability of ion-conductive membranes, such as perfluorosulfonic-acid (PFSA) membranes, as a solid-electrolyte separator in energy devices is strongly linked to their mechanical properties, the characterization of which presents challenges, especially in the presence of ionic interactions. Ionomer membranes' elastic properties are affected by cations; however, their influence on deformation at small and large strains is relatively unexplored. In this paper, we report the stress-strain response and fracture behavior of Nafion membranes exchanged with various cations examined in three deformation regimes. In the small-strain regime, the Young's modulus is strongly dependent on cation size, due to the reduced mobility and local stiffening of polymer chains. The Young's modulus, yield limit and strain-hardening modulus all increase with monovalent cation size in the order H+ < Li+ < Na+ < K+ < Cs+, but with varying dependence. In the failure regime, however, the break strain and fracture energy of the membrane decrease in the presence of larger cations, which promote deformation instability while decreasing plastic dissipation energy during crack propagation, thereby leading to more brittle fracture. These results not only demonstrate the trade-off between strength and fracture toughness, but also reveal how it is altered by the ionic interactions, which also dictate the inverse relationship between stretchability and stiffness. Moreover, the measured stress-strain data are reproduced by the constitutive relations to extract parameters that are correlated to the fracture energy through craze instability. Such relationships provide insight into how parameters extracted from tensile testing can be used to assess membrane stability and the role of ionic interactions.

Chemical and Morphological Origins of Improved Ion Conductivity in Perfluoro Ionene Chain Extended Ionomers.

The performance of ion-conducting polymer membranes is complicated by an intricate interplay between chemistry and morphology that is challenging to understand. Here, we report on perfuoro ionene chain extended (PFICE) ionomers that contain either one or two bis(sulfonyl)imide groups on the side-chain in addition to a terminal sulfonic acid group. PFICE ionomers exhibit greater water uptake and conductivity compared to prototypical perfluorinated sulfonic acid ionomers. Advanced in situ synchrotron characterization reveals insights into the connections between molecular structure and morphology that dictate performance. Guided by first-principles calculations, X-ray absorption spectroscopy at the sulfur K-edge can discern distinct protogenic groups and be sensitive to hydration level and configurations that dictate proton dissociation. In situ resonant X-ray scattering at the sulfur K-edge reveals that PFICE ionomers have a phase-separated morphology with enhanced short-range order that persists in both dry and hydrated states. The enhanced conductivity of PFICE ionomers is attributed to a unique multi-acid side-chain chemistry and structure that facilitates proton dissociation at low water content in combination with a well-ordered phase-separated morphology with nanoscale transport pathways. Overall, these results provide insights for the design of new ionomers with tunable phase separation and improved transport properties as well as demonstrating the efficacy of X-rays with elemental sensitivity for unraveling structural features in chemically heterogeneous functional materials for electrochemical energy applications.

New Insights into Perfluorinated Sulfonic-Acid 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. 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.

Hydrophobic Inorganic Oxide Pigments via Polymethylhydrosiloxane Grafting: Dispersion in Aqueous Solution at Extraordinarily High Solids Concentrations.

Building on the recent demonstration of aqueous-dispersible hydrophobic pigments that retain their surface hydrophobicity even after drying, we demonstrate the synthesis of surface-modified Ti-Pure R-706 (denoted R706) titanium dioxide-based pigments, consisting of a thin (one to three monolayers) grafted polymethylhydrosiloxane (PMHS) coating, which (i) are hydrophobic in the dry state according to capillary rise and dynamic vapor sorption measurements and (ii) form stable aqueous dispersions at solid contents exceeding 75 wt % (43 vol %), without added dispersant, displaying similar rheology to R706 native oxide pigments at 70 wt % (37 vol %) consisting of an optimal amount of conventional polyanionic dispersant (0.3 wt % on pigment basis). The surface-modified pigments have been characterized via 29Si and 13C cross-polarization/magic angle spinning solid-state NMR spectroscopy; infrared spectroscopy; thermogravimetric and elemental analyses; and ζ potential measurements. On the basis of these data, the stability of the surface-modified PMHS-R706 aqueous dispersions is attributed to steric effects, as a result of grafted PMHS strands on the R706 surface, and depends on the chaotropic nature of the base used during PMHS condensation to the pigment/polysiloxane interface. The lack of water wettability of the surface-modified oxide particles in their dry state translates to improved water-barrier properties in coatings produced with these surface-modified pigment particles. The synthetic approach appears general as demonstrated by its application to various inorganic-oxide pigment particles.

Stable Aqueous Dispersions of Hydrophobically Modified Titanium Dioxide Pigments through Polyanion Adsorption: Synthesis, Characterization, and Application in Coatings.

Polyanion dispersants stabilize aqueous dispersions of hydrophilic (native) inorganic oxide particles, including pigments currently used in paints, which are used at an annual scale of 3 million metric tons. While obtaining stable aqueous dispersions of hydrophobically modified particles has been desired for the promise of improved film performance and water barrier properties, it has until now required either prohibitively complex polyanions, which represent a departure from conventional dispersants, or multistep syntheses based on hybrid-material constructs. Here, we demonstrate the aqueous dispersion of alkylsilane-capped inorganic oxide pigments with conventional polycarboxylate dispersants, such as carboxymethylcellulose (CMC) and polyacrylate, as well as a commercial anionic copolymer. Contact-angle measurements demonstrate that the hydrophobically modified pigments retain significant hydrophobic character even after adsorbing polyanion dispersants. CMC adsorption isotherms demonstrate 92% greater polyanion loading on trimethylsilyl modified hydrophobic particles relative to native oxide at pH 8. However, consistent with prior literature, hydrophobically modified silica particles adsorb polyanions very weakly under these conditions. These data suggest that Lewis acidic heteroatoms such as Al(3+) sites on the pigment surface are necessary for polyanion adsorption. The adsorbed polyanions increase the dispersion stability and zeta potential of the particles. Based on particle sedimentation under centrifugal force, the hydrophobically modified pigments possess greater dispersion stability with polyanions than the corresponding native hydroxylated particles. The polyanions also assist in the aqueous wetting of the hydrophobic particles, facilitating the transition from a dry powder into an aqueous dispersion of primary particles using less agitation than the native hydroxylated pigment. The application of aqueous dispersions of hydrophobically modified oxide particles to waterborne coatings leads to films that display lower water uptake at high relative humidities and greater hydrophilic stain resistances. This improved film performance with hydrophobically modified pigments is the result of better association between latex polymer and pigment in the dry film.

Confinement-driven increase in ionomer thin-film modulus.

Ion-conductive polymers, or ionomers, are critical materials for a wide range of electrochemical technologies. For optimizing the complex heterogeneous structures in which they occur, there is a need to elucidate the governing structure-property relationships, especially at nanoscale dimensions where interfacial interactions dominate the overall materials response due to confinement effects. It is widely acknowledged that polymer physical behavior can be drastically altered from the bulk when under confinement and the literature is replete with examples thereof. However, there is a deficit in the understanding of ionomers when confined to the nanoscale, although it is apparent from literature that confinement can influence ionomer properties. Herein we show that as one particular ionomer, Nafion, is confined to thin films, there is a drastic increase in the modulus over the bulk value, and we demonstrate that this stiffening can explain previously observed deviations in materials properties such as water transport and uptake upon confinement. Moreover, we provide insight into the underlying confinement-induced stiffening through the application of a simple theoretical framework based on self-consistent micromechanics. This framework can be applied to other polymer systems and assumes that as the polymer is confined the mechanical response becomes dominated by the modulus of individual polymer chains.

Toward a Comprehensive Understanding of Cation Effects in Proton Exchange Membrane Fuel Cells.

Metal alloy catalysts (e.g., Pt-Co) are widely used in fuel cells for improving the oxygen reduction reaction kinetics. Despite the promise, the leaching of the alloying element contaminates the ionomer/membrane, leading to poor durability. However, the underlying mechanisms by which cation contamination affects fuel cell performance remain poorly understood. Here, we provide a comprehensive understanding of cation contamination effects through the controlled doping of electrodes. We couple electrochemical testing results with membrane conductivity/water uptake measurements and impedance modeling to pinpoint where and how the losses in performance occur. We identify that (1) ∼44% of Co2+ exchange of the ionomer can be tolerated in the electrode, (2) loss in performance is predominantly induced by O2 and proton transport losses, and (3) Co2+ preferentially resides in the electrode under wet operating conditions. Our results provide a first-of-its-kind mechanistic explanation for cation effects and inform strategies for mitigating these undesired effects when using alloy catalysts.

Tunable Anion Exchange Membrane Conductivity and Permselectivity via Non-Covalent, Hydrogen Bond Cross-Linking.

Ion exchange membranes (IEMs) are a key component of electrochemical processes that purify water, generate clean energy, and treat waste. Most conventional polymer IEMs are covalently cross-linked, which results in a challenging tradeoff relationship between two desirable properties─high permselectivity and high conductivity─in which one property cannot be changed without negatively affecting the other. In an attempt to overcome this limitation, in this work we synthesized a series of anion exchange membranes containing non-covalent cross-links formed by a hydrogen bond donor (methacrylic acid) and a hydrogen bond acceptor (dimethylacrylamide). We show that these monomers act synergistically to improve both membrane permselectivity and conductivity relative to a control membrane without non-covalent cross-links. Furthermore, we show that the hydrogen bond donor and acceptor loading can be used to tune permselectivity and conductivity relatively independently of one another, escaping the tradeoff observed in conventional membranes.

Linking Perfluorosulfonic Acid Ionomer Chemistry and High-Current Density Performance in Fuel-Cell Electrodes.

Transport phenomena are key in controlling the performance of electrochemical energy-conversion technologies and can be highly complex, involving multiple length scales and materials/phases. Material designs optimized for one reactant species transport however may inhibit other transport processes. We explore such trade-offs in the context of polymer-electrolyte fuel-cell electrodes, where ionomer thin films provide the necessary proton conductivity but retard oxygen transport to the Pt reaction site and cause interfacial resistance due to sulfonate/Pt interactions. We examine the electrode overall gas-transport resistance and its components as a function of ionomer content and chemistry. Low-equivalent-weight ionomers allow better dissolved-gas and proton transport due to greater water uptake and low crystallinity but also cause significant interfacial resistance due to the high density of sulfonic acid groups. These effects of equivalent weight are also observed via in situ ionic conductivity and CO displacement measurements. Of critical importance, the results are supported by ex situ ellipsometry and X-ray scattering of model thin-film systems, thereby providing direct linkages and applicability of model studies to probe complex heterogeneous structures. Structural and resultant performance changes in the electrode are shown to occur above a threshold sulfonic-group loading, highlighting the significance of ink-based interactions. Our findings and methodologies are applicable to a variety of solid-state energy-conversion devices and material designs.

Controlling the Distribution of Perfluorinated Sulfonic Acid 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.

Thermal Transitions in Perfluorosulfonated Ionomer Thin-Films.

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 (TT). Change in TT in polymers is an indicator for chain relaxation and mobility with implications on properties like gas transport. This work demonstrates an increase in TT 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, TT 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 TT above the mainly hydrogen-bonded PFSI-H+ ionomer.

Electrochemical/mechanical coupling in ion-conducting soft matter.

Mechanical and electrochemical phenomena exhibit many interesting multidirectional couplings in ion-exchange soft matter due to their intrinsic material physiochemical states and responses to environmental stressors. In this Perspective, such coupling is explored in terms of recent studies with a focus on the degradation of polymer-electrolyte fuel-cell membranes. In addition, (electro)chemical-mechanical coupling of ion-conducting polymers in other applications is also introduced, as there is a research need to explore the interactions between these often wrongly assumed disparate fields in order to optimize, exploit, and discover new technologies and applications.

In Situ Method for Measuring the Mechanical Properties of Nafion Thin Films during Hydration Cycles.

Perfluorinated ionomers, in particular Nafion, are an essential component in hydrogen fuel cells, as both the proton exchange membrane and the binder within the catalyst layer. During normal operation of a hydrogen fuel cell, the ionomer will progressively swell and deswell in response to the changes in hydration, resulting in mechanical fatigue and ultimately failure over time. In this study, we have developed and implemented a cantilever bending technique in order to investigate the swelling-induced stresses in biaxially constrained Nafion thin films. When the deflection of a cantilever beam coated with a polymer film is monitored as it is exposed to varying humidity environments, the swelling induced stress-thickness product of the polymer film is measured. By combining the stress-thickness results with a measurement of the swelling strain as a function of humidity, as measured by quartz crystal microbalance (QCM) and X-ray reflectivity (XR), the swelling stress can be determined. An estimate of the Young's modulus of thin Nafion films as a function of relative humidity is obtained. The Young's modulus values indicate orientation of the ionic domains within the polymer films, which were confirmed by grazing incidence small-angle X-ray scattering (GISAXS). This study represents a measurement platform that can be expanded to incorporate novel ionomer systems and fuel cell components to mimic the stress state of a working hydrogen fuel cell.

Morphology of Hydrated As-Cast Nafion Revealed through Cryo Electron Tomography.

Nafion is an ion-containing random copolymer used as a solid electrolyte in many electrochemical applications thanks to its remarkable ionic conductivity and mechanical stability. Understanding the mechanism of ion transport in Nafion, which depends strongly on hydration, therefore requires a complete picture of its morphology in dry and hydrated form. Here we report on a nanoscale study of dry versus hydrated as-cast 100 nm Nafion membranes using analytical transmission electron microscopy (TEM) and cryogenic TEM tomography, respectively. For the dry membrane, spherical clusters ∼3.5 nm in diameter corresponding to the hydrophilic sulfonic-acid-containing phase are identified. In contrast, cryo TEM tomography of the hydrated membrane reveals an interconnected channel-type network, with a domain spacing of ∼5 nm, and presents the first nanoscale 3D views of the internal structure of hydrated Nafion obtained by a direct-imaging approach.

Understanding Water Uptake and Transport in Nafion Using X-ray Microtomography.

To develop new ionomers and optimize existing ones, there is a need to understand their structure/function relationships experimentally. In this letter, synchrotron X-ray microtomography is used to examine water distributions within Nafion, the most commonly used ionomer. Simultaneous high spatial (∼1 µm) and temporal (∼10 min) resolutions, previously unattained by other techniques, clearly show the nonlinear water profile across the membrane thickness, with a continuous transition from dynamic to steady-state transport coefficients with the requisite water-content dependence. The data also demonstrate the importance of the interfacial condition in controlling the water profile and help to answer some long-standing debates in the literature.

Subsecond Morphological Changes in Nafion during Water Uptake Detected by Small-Angle X-ray Scattering.

The ability of the Nafion membrane to absorb water rapidly and create a network of hydrated interconnected water domains provides this material with an unmatched ability to conduct ions through a chemically and mechanically robust membrane. The morphology and composition of these hydrated membranes significantly affects their transport properties and performance. This work demonstrates that differences in interfacial interactions between the membranes exposed to vapor or liquid water can cause significant changes in kinetics of water uptake. In situ small-angle X-ray scattering (SAXS) experiments captured the rapid swelling of the membrane in liquid water with a nanostructure rearrangement on the order of seconds. For membranes in contact with water vapor, morphological changes are four orders-of-magnitude slower than in liquid water, suggesting that interfacial resistance limits the penetration of water into the membrane. Also, upon water absorption from liquid water, a structural rearrangement from a distribution of spherical and cylindrical domains to exclusively cylindrical-like domains is suggested. These differences in water-uptake kinetics and morphology provide a new perspective into Schroeder's paradox, which dictates a different water content for vapor- and liquid-equilibrated ionomers at unit activity. The findings of this work provide critical insights into the fast kinetics of water absorption of the Nafion membrane, which can aid in the design of energy conversion devices that operate under frequent changes in environmental conditions.