Design of Cluster-free Polymer Electrolyte Membranes and Implications on Proton Conductivity

Ion transport in polymerized ionic liquids: a comparison of polycation and polyanion systems

The correlation among chemical structure, mesoscale structure, and ion transport in 1,2,3-triazole-based polymerized ionic liquids (polyILs) featuring comparable polycation and polyanion backbones is investigated by wide-angle X-ray scattering (WAXS), differential scanning calorimetry, and broadband dielectric spectroscopy (BDS). Above the glass transition temperature, Tg, higher ionic conductivity is observed in polycation polyILs compared to their polyanion counterparts, and ion conduction is enhanced by increasing the counterion volume in both polycation or polyanion polyILs. Below Tg, polyanions show lower activation energy associated with ion conduction. However, the validity of the Barton-Nakajima-Namikawa relation indicates that hopping conduction is the dominant charge transport mechanism in all the polyILs studied. While a significant transition from a Vogel-Fulcher-Tammann to Arrhenius type of thermal activation is observed below Tg, the decoupling index, often used to quantify the extent to which segmental dynamics and ion conduction are correlated, remains unaltered for the polyILs studied, suggesting that this index may not be a general parameter to characterize charge transport in polymerized ionic liquids. Furthermore, detailed analyses of the WAXS results indicate that both the mobile ion type and the structure of the pendant groups control mesoscale organization. These findings are discussed within the framework of recent models, which account for the subtle interplay between electrostatic and elastic forces in determining ion transport in polyILs. The findings demonstrate the intricate balance between the chemical structure and interactions in polyILs that determine ion conduction in this class of polymer electrolytes.

Confined Nano-Channels Incorporated with Multi-Quaternized Cations for Highly Phosphoric Acid Retention HT-PEMs

Developing a new strategy to retain phosphoric acid (PA) to improve the performance and durability of high-temperature proton exchange membrane fuel cell (HT-PEMFC) remains a challenge. Here, a strategy for ion-restricted catcher microstructure that incorporates PA-doped multi-quaternized poly(fluorene alkylene-co-biphenyl alkylene) (PFBA) bearing confined nanochannels is reported. Dynamic analysis reveals strong interaction between side chains and PA molecules, confirming that the microstructure can improve PA retention. The PFBA linked with triquaternary ammonium side chain (PFBA-tQA) shows the highest PA retention rate of 95%. Its H2/O2 fuel cell operates within 0.6% voltage decay at 160 °C/0% RH, and it also runs over 100 h at 100 °C/49% RH under external humidification. This combination of high PA retention, and chemical and dimensional stability fills a gap in the HT-PEMFC field, which requires strict moisture control at 90-120 °C to prevent acid leaching, simplifying the start-up procedure of HT-PEMFC without preheating.

Ion channels in sulfonated copolymer-grafted nanoparticles in ionic liquids

The use of ionic liquids as solvents for polymers or polymer-grafted nanoparticles provides an exciting feature to explore electrolyte-polymer interactions. 1-Hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HMIm-TFSI) can have specific interactions with the polymer through ion-dipole forces or hydrogen bonding. In this work, poly(methyl methacrylate)-b-poly(styrene sulfonate) (PMMA-b-PSS) copolymer-grafted Fe₃O₄ nanoparticles with different sulfonation levels (∼4.9 to 10.9 mol% SS) were synthesized, and their concentration dependent ionic conductivities were reported in acetonitrile and HMIm-TFSI/acetonitrile mixtures. We found that conductivity enhancement with the particle concentration in acetonitrile was due to the aggregation of grafted particles, resulting in sulfonic domain connectivity. The ionic conductivity was found to be related to the effective hopping transfer within ionic channels. On the contrary, the conductivity decreased or remained constant with increasing particle concentration in HMIm-TFSI/acetonitrile. This result was attributed to the ion coupling between ionic liquids and copolymer domains.

Scalable Manufacturing of Solid Polymer Electrolytes with Superior Room-Temperature Ionic Conductivity

A scalable manufacturing protocol is developed to prepare polymer-based solvent-free all-solid flexible energy storage devices based on a two-roll mill and adapted rubber mixing technology. The as-prepared solid polymer electrolytes (SPEs) consisting of commercial poly(methyl methacrylate)-grafted natural rubber (MG30) and lithium bis(trifluoromethanesulfonyl)imide achieve a superior ionic conductivity of 2.7 × 10-3 S cm-1 at 30 °C. The superior ionic conductivity is attributed to the formation of an ionic cluster network in the composite as proved by small-angle X-ray scattering and infrared spectroscopy measurements. Moreover, the as-prepared SPEs show good mechanical stability over a broad temperature range, that is , a storage modulus above 1 × 104 Pa from 30 to 120 °C as indicated by the rheology data. Furthermore, the SPEs were assembled with the carbon black-filled MG30 (i.e., MG30C) electrode into a flexible supercapacitor cell, which had a wide voltage window of 3.5 V, good energy density of 28.4 μW h·cm-2 at 160 °C, and good temperature tolerance up to 160 °C. This scaling-up manufacture strategy shows tremendous potential to the advancing of SPEs in applications of flexible energy storage device.

Theory of ionic conductivity with morphological control in polymers

We present a general theory of ionic conductivity in polymeric materials consisting of percolated ionic pathways. Identifying two key length scales corresponding to inter-path permeation distance ξ and one-dimensional hopping conduction path length mλ, we have derived closed-form formulas in terms of the energy U required to unbind a conductive ion from its bound state and the partition ratio ξ/mλ between the three-dimensional permeation and one-dimensional hopping pathways. The results provide design strategies to significantly enhance ionic conductivity in single-ion conductors. For large barriers to dissociate an ion, corrections to the Arrhenius law are presented. The predicted dependence of ionic conductivity on the unbinding time is in agreement with results in the literature based on simulations and experiments. This theory is generally applicable to conductive systems where the two mechanisms of permeation and hopping occur concurrently.

Nanostructured high-performance electrolyte membranes based on polymer network post-assembly for high-temperature supercapacitors

The development of high-temperature supercapacitors highly relies on the explore of stable polymer electrolyte membranes (PEMs) with high ionic conductivities at high-temperature conditions. However, it is a challenge to achieve both high stability and high conductivity in a PEM at elevated temperatures. Herein, we report the fabrication of high-performance proton conductive PEMs suitable for high-temperature supercapacitors (HT-SCs), which is based on a post-assembly strategy to control the rearrangement of polymer networks in the PEMs. This strategy can create cross-linked PEMs with bicontinuous nanostructures, as well as highly stable and highly conductive features. Specifically, a series of bicontinuous PEMs are prepared by the controllable cross-linking of poly(ether-ether-ketone) and poly(4-vinylpyridine), followed by the inducement of phosphoric acid. These PEMs exhibit both a high proton conductivity of 70 mS cm^-1 and a high modulus of 39.3 MPa at 150 ℃, which can serve as high-performance electrolytes. The HT-SCs based on these PEMs display a specific capacitance of 138.0 F g^-1 and a high capacitance retention of 80.0% after 2500 galvanostatic charge-discharge cycles at 150 ℃, exhibiting excellent high-temperature capacitance and cycle stability. This post-assembly concept can provide a new route to design high-performance PEMs for HT-SC and other energy device applications.

Understanding the ionic activity and conductivity value differences between random copolymer electrolytes and block copolymer electrolytes of the same chemistry

Herein, a systematic study where the macromolecular architectures of poly(styrene-block-2-vinyl pyridine) block copolymer electrolytes (BCE) are varied and their activity coefficients and ionic conductivities are compared and rationalized versus a random copolymer electrolyte (RCE) of the same repeat unit chemistry. By performing quartz crystal microbalance, ion-sorption, and ionic conductivity measurements of the thin film copolymer electrolytes, it is found that the RCE has higher ionic activity coefficients. This observation is ascribed to the fact that the ionic groups in the RCE are more spaced out, reducing the overall chain charge density. However, the ionic conductivity of the BCE is 50% higher and 17% higher after the conductivity is normalized by their ion exchange capacity values on a volumetric basis. This is attributed to the presence of percolated pathways in the BCE. To complement the experimental findings, molecular dynamics (MD) simulations showed that the BCE has larger water cluster sizes, rotational dynamics, and diffusion coefficients, which are contributing factors to the higher ionic conductivity of the BCE variant. The findings herein motivate the design of new polymer electrolyte chemistries that exploit the advantages of both RCEs and BCEs.

Dynamics and Glass Transition of Supercooled Water Confined in Amphiphilic Polymer Films

The glass transition of supercooled water is not well understood yet. We have observed a clear glass transition of the supercooled water confined in channel of amphiphilic polymer films at 145 K. Using NMR, we probe two types of relaxations occurred in the glass former, e.g., a rapid local β-process and a slow α-process (most likely). It is found that slow α-relaxation follows the Arrhenius relationship, indicating the glass former is a strong liquid. We also find a dynamic crossover from low-temperature Arrhenius α-process to high-temperature VFT process at 198-208 K, accompanying with simultaneous disappearing of local β-relaxation.

Using Coupling Motion of Connecting Ions in Designing Telechelic Ionomers

Conventional telechelic ionomers have one ion fixed at each end, enabling the chains to form a physical network. Here we report a type of telechelic ionomers with a distribution of the number of ions at the chain ends, which endows them with very rich rheological properties. We synthesized the ionomer samples via a two-step polymerization. Namely, we synthesized a precursor chain first and then polymerized a few ion-containing monomers at its two ends. An average number of ion-containing monomers per chain end, m, varies from 0 to 3.0. Linear viscoelasticity of these samples can be well explained through considering the Poisson distribution of m, and the hierarchical relaxation of the chains ends according to the number of connecting ions that exhibit the coupling motion.

Alleviating the Mechanical and Thermal Degradations of Highly Sulfonated Poly(Ether Ether Ketone) Blocks via Copolymerization with Hydrophobic Unit for Intermediate Humidity Fuel Cells

In this contribution, sulfonated poly(ether ether ketone) (SPEEK) is inter-connected using a hydrophobic oligomer via poly-condensation reaction to produce SPEEK analogues as PEMs. Prior sulfonation is performed for SPEEK to avoid random sulfonation of multi-block copolymers that may destroy the mechanical toughness of polymer backbone. A greater local density of ionic moieties exist in SPEEK and good thermomechanical properties of hydrophobic unit offer an unique approach to promote the proton conductivity as well as thermomechanical stability of membrane, as verify from AC impedance and TGA. The morphological behavior and phase variation of membranes are explored using FE-SEM and AFM; the triblock (XYX) membranes exhibits a nano-phase separated morphology. Performance of PEFC integrated with blend and block copolymer membranes is determined at 60 °C under 60% RH. As a result, the triblock (XYX) membrane has a high power density than blend (2X1Y) membrane.

Imidazolium-based anion exchange membranes for alkaline anion fuel cells: (2) elucidation of the ionic structure and its impact on conducting properties

In our previous study (Soft Matter, 2016, 12, 1567), the relationship between the morphology and properties of graft-type imidazolium-based anion exchange membranes (AEMs) was revealed, in that the semi-crystalline features of the polymer matrix maintain its mechanical properties and the formation of interconnected hydrophilic domains promotes the membrane conductivity. Here, we report a novel ionic structure of the same graft-type AEMs with different grafting degrees, analyzed using a small-angle X-ray scattering method under different relative humidity (RH) conditions. The characteristic "ionomer peak" with a corresponding correlation distance of approximately 1.0 nm was observed at RH < 80%. This distance is much smaller than the literature-reported mean distance between two ionic clusters, but close to the Bjerrum length of water. Since the representative number of water molecules per cation, n_w, was small, we proposed that dissociated ion-pairs are distributed in the hydrophilic domains (ion-channels). At RH < 80%, ion-channels are disconnected, however in liquid water, they are well-connected as evidenced by the sharp increase in n_w. The disconnected ion-channels even under relatively high RH conditions should be a substantial factor for the low power generation efficiency of AEM-type fuel cells.

Acid-Group-Content-Dependent Proton Conductivity Mechanisms at the Interlayer of Poly(N-dodecylacrylamide-co-acrylic acid) Copolymer Multilayer Nanosheet Films

The effect of the content of acid groups on the proton conductivity at the interlayer of polymer-nanosheet assemblies was investigated. For that purpose, amphiphilic poly(N-dodecylacrylamide-co-acrylic acid) copolymers [p(DDA/AA)] with varying contents of AA were synthesized by free radical polymerization. Surface pressure (π)-area (A) isotherms of these copolymers indicated that stable polymer monolayers are formed at the air/water interface for AA mole fraction (n) ≤ 0.49. In all cases, a uniform dispersion of the AA groups in the polymer monolayer was observed. Subsequently, polymer monolayers were transferred onto solid substrates using the Langmuir-Blodgett (LB) technique. X-ray diffraction (XRD) analyses of the multilayer films showed strong Bragg diffraction peaks, suggesting a highly uniform lamellar structure for the multilayer films. The proton conductivity of the multilayer films parallel to the direction of the layer planes were measured by impedance spectroscopy, which revealed that the conductivity increased with increasing values of n. Activation energies for proton conduction of ∼0.3 and 0.42 eV were observed for n ≥ 0.32 and n = 0.07, respectively. Interestingly, the proton conductivity of a multilayer film with n = 0.19 did not follow the Arrhenius equation. These results were interpreted in terms of the average distance between the AA groups (l_AA), and it was concluded that, for n ≥ 0.32, an advanced 2D hydrogen bonding network was formed, while for n = 0.07, l_AA is too long to form such hydrogen bonding networks. The l_AA for n = 0.19 is intermediate to these extremes, resulting in the formation of hydrogen bonding networks at low temperatures, and disruption of these networks at high temperatures due to thermally induced motion. These results indicate that a high proton conductivity with low activation energy can be achieved, even under weakly acidic conditions, by arranging the acid groups at an optimal distance.

Nanoscale Aggregation in Acid- and Ion-Containing Polymers

In this review we summarize recent efforts in understanding nano-aggregation in acid- and ion-containing polymer systems. The acid and ionic groups have specific interactions that drive aggregation and alter polymer behavior at the nano-, micro-, and bulk length scales. Advancements in synthetic methods, characterization techniques, and computer simulations have enabled researchers to better understand the morphologies and dynamics, particularly at the nanoscale. This overview of recent advancements in nano-aggregated polymer systems highlights the current understanding of the field and presents promising directions for future investigations and new 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.

Charge Transport of Polyester Ether Ionomers in Unidirectional Silica Nanopores

Dielectric relaxation spectroscopy is employed to investigate charge transport properties of two polyester ether ionomers in the bulk state and when confined in unidirectional nanoporous membranes (average pore diameter = 7.5 nm). Under nanometric confinement in nonsilanized pores, the macroscopic transport quantities (dc conductivity and characteristic frequency rate) are lower by about 1.4 decades compared to the bulk. The remarkable decrease of transport quantities in nonsilanized nanoporous membranes can be quantitatively explained by considering the temperature dependence of the interfacial layer between the ionomer and the silica membrane surfaces. On the other hand, an enhancement of dc conductivity is observed when the surfaces of the pores are treated with a nonpolar organosilane. This effect becomes more pronounced at lower temperatures and is attributed to slight changes in molecular packing density caused by the two-dimensional geometrical constraint.

Polymerized ionic liquid diblock copolymers: impact of water/ion clustering on ion conductivity

Herein, we examine the synergistic impact of both ion clustering and block copolymer morphology on ion conductivity in two polymerized ionic liquid (PIL) diblock copolymers with similar chemistries but different side alkyl spacer chain lengths (ethyl versus undecyl). When saturated in liquid water, water/ion clusters were observed only in the PIL block copolymer with longer alkyl side chains (undecyl) as evidenced by both small-angle neutron scattering and intermediate-angle X-ray scattering, i.e., water/ion clusters form within the PIL microdomain under these conditions. The resulting bromide ion conductivity in the undecyl sample was higher than the ethyl sample (14.0 mS cm(-1)versus 6.1 mS cm(-1) at 50 °C in liquid water) even though both samples had the same block copolymer morphology (lamellar) and the undecyl sample had a lower ion exchange capacity (0.9 meq g(-1)versus 1.4 meq g(-1)). No water/ion clusters were observed in either sample under high humidity or dry conditions. The resulting ion conductivity in the undecyl sample with lamellar morphology was significantly higher in the liquid water saturated state compared to the high humidity state (14.0 mS cm(-1)versus 4.2 mS cm(-1)), whereas there was no difference in ion conductivity in the ethyl sample when comparing these two states. These results show that small chemical changes to ion-containing block copolymers can induce water/ion clusters within block copolymer microdomains and this can subsequently have a significant effect on ion transport.

Enhanced performance of poly(ether sulfone) based composite proton exchange membranes with sulfonated polymer brush functionalized graphene oxide

Novel poly(ether sulfone) based composite proton exchange membranes with enhanced performance were prepared by incorporating sulfonated polymer brush functionalized GO.

Highly efficient solid polymer electrolytes using ion containing polymer microgels

A solid polymer electrolyte fabricated using ion containing microgels manifests high ionic conductivity for potential use in lithium batteries.

Absence of Schroeder's paradox in a nanostructured block copolymer electrolyte membrane

This is a study of morphology, water uptake, and proton conductivity of a sulfonated polystyrene-block-polyethylene (PSS-PE) copolymer equilibrated in humid air with controlled relative humidity (RH), and in liquid water. Extrapolation of the domain size, water uptake, and conductivity obtained in humid air to RH = 100% allowed for an accurate comparison between the properties of PSS-PE hydrated in saturated vapor and in liquid water. We demonstrate that extrapolations of domain size and water uptake on samples equilibrated in humid air are consistent with measurements on samples equilibrated in liquid water. Small (5%) differences in proton conductivity were found in samples equilibrated in humid air and liquid water. We argue that differences in transport coefficients in disordered heterogeneous systems, particularly small differences, present no paradox whatsoever. Schroeder's Paradox, wherein properties of polymers measured in saturated water vapor are different from those obtained in liquid water, is thus not observed in the PSS-PE sample.

Morphology-Conductivity Relationship of Single-Ion-Conducting Block Copolymer Electrolytes for Lithium Batteries

A significant limitation of rechargeable lithium-ion batteries arises because most of the ionic current is carried by the anion, the ion that does not participate in energy-producing reactions. Single-ion-conducting block copolymer electrolytes, wherein all of the current is carried by the lithium cations, have the potential to dramatically improve battery performance. The relationship between ionic conductivity and morphology of single-ion-conducting poly(ethylene oxide)-b-polystyrenesulfonyllithium(trifluoromethylsulfonyl)imide (PEO-PSLiTFSI) diblock copolymers was studied by small-angle X-ray scattering and ac impedance spectroscopy. At low temperatures, an ordered lamellar phase is obtained, and the "mobile" lithium ions are trapped in the form of ionic clusters in the glassy polystyrene-rich microphase. An increase in temperature results in a thermodynamic transition to a disordered phase. Above this transition temperature, the lithium ions are released from the clusters, and ionic conductivity increases by several orders of magnitude. This morphology-conductivity relationship is very different from all previously published data on published electrolytes. The ability to design electrolytes wherein most of the current is carried by the lithium ions, to sequester them in nonconducting domains and release them when necessary, has the potential to enable new strategies for controlling the charge-discharge characteristics of rechargeable lithium batteries.

Design of a humidity controlled sample stage for simultaneous conductivity and synchrotron X-ray scattering measurements

We report on the design and operation of a novel sample stage, used to simultaneously measure X-ray scattering profiles and conductivity of a polymer electrolyte membrane (PEM) surrounded by humid air as a function of temperature and relative humidity. We present data obtained at the Advanced Light Source and Stanford Synchrotron Radiation Laboratory. We demonstrate precise humidity control and accurate determination of morphology and conductivity over a wide range of temperatures. The sample stage is used to study structure-property relationships of a semi-crystalline block copolymer PEM, sulfonated polystyrene-block-polyethylene.

Atomistic Simulations Predict a Surprising Variety of Morphologies in Precise Ionomers

The nature of ionic aggregates in ionomers remains an important open question, particularly considering its significance to their unique electrical and mechanical properties. We have carried out fully atomistic molecular dynamics simulations of melts of lithium-neutralized precise ionomers that reveal the structural features of ionic aggregates in unprecedented detail. In particular, we observe a rich variety of aggregate morphologies depending on neutralization level and ionic content, including string-like and percolated aggregates. The traditional assumption of spherical ionic aggregates with liquid-like ordering that is typically used to interpret experimental scattering data is too simplistic; a more rich and complex set of structures exist that also fit the scattering data.