Perfluorinated Sulfonic Acid Membranes Research Articles

Mercaptoethanol-assisted synthesis of sulfonic acid-functionalized polysiloxanes as inorganic fillers in composite membranes for energy-storage applications

Developing proton-exchange membranes (PEMs) for energy applications is necessary for fulfilling the increasing energy demand without environmental effects. In this study, we prepared composite membranes containing mercaptoethanol-assisted sulfonic acid-functionalized polysiloxanes (MASFPs) and perfluorinated sulfonic acid (PFSA). The MASFP-containing composite membrane (i.e., CMASFP-2) exhibited better membrane properties than the composite membrane prepared without mercaptoethanol (CMASFP-1) and the reference Nafion 212 membrane. Wide-angle X-ray diffraction (XRD) studies revealed a broad peak at 2θ = 17.4° and 39°, indicating that the composite membranes are semicrystalline. Scanning electron microscopy (SEM) analysis revealed that the prepared composite membranes had good morphology, whereas energy-dispersive X-ray spectroscopy analysis revealed that it had siloxane and more sulfonic acid than the pristine PFSA membrane. Thermogravimetric and mechanical strength analyses revealed that incorporating sulfonic acid-functionalized siloxanes improved the thermal stability and mechanical strain of the membrane. The prepared CMASFP-2 membrane had a high ionic conductivity of 0.106 mS cm−1 and an ion-exchange capacity of 1.09 meq·g−1. CMASFP-2 also exhibited lower hydrogen gas permeability than the Nafion 212 and PFSA membranes, demonstrating that it can be used for energy applications.

Exploring the development of natural biopolymer (chitosan)-based proton exchange membranes for fuel cells: A review

Fuel cells use proton exchange membranes (PEMs) to transform chemical energy into electricity. PEMs are selective barriers that permeates protons, obstructing gases and other species like electrons. A polymer electrolyte containing both positively and negatively charged ions forms a PEM. Nafion, a perfluorinated sulfonic acid membrane, is the most popular polymer membrane used as PEM. It exhibits good chemical stability, strong mechanical qualities, and high proton conductivity. However, it has several issues, one of which is its propensity to degrade with time, especially at high temperatures and in the presence of pollutants. The other draw backs associated with perfluorinated based membranes are that they are expensive and non-eco-friendly In order to address these issues, researchers are looking into new materials and methods to form a PEM so that the performance and durability are enhanced .Chitosan (biopolymer) based PEMs have demonstrated great potential to be used in fuel cells. They are environmentally friendly and economical. However, the key challenge in using chitosan in fuel cells is their relatively poor ionic conductivity. Researchers have developed various strategies to improve their conductivity, such as doping with conductive materials or incorporating functional groups that enhance charge transfer. Overall, chitosan has shown promise as renewable and sustainable material for use in fuel cells. The review summarizes the current development and evolution of chitosan-based PEMs.

Self-Enhancement of Perfluorinated Sulfonic Acid Proton Exchange Membrane with Its Own Nanofibers.

High-performance proton exchange membrane (PEM) is crucial for the proton exchange membrane fuel cell (PEMFC). Herein, a novel "self-enhanced" PEM is fabricated for the first time, which is composed of perfluorinated sulfonic acid (PFSA) resin and its own nanofibers as reinforcement. With this strategy, the interfacial compatibility issue of conventional fiber-reinforced membranes is fully addressed and up to 80 wt% loading of PFSA nanofibers can be incorporated. Furthermore, on account of chain orientation within the PFSA nanofiber, single fiber exhibits super-high conductivity of 1.45 S cm-1, leading to state-of-the-art proton conductivity (1.1 S cm-1) of the as-prepared "self-enhanced" PEM so far, which is an order of magnitude increase compared with the bulk PFSA membrane (0.29 S cm-1). It surpasses any commercial PEM including the popular GORE-SELECT and Nafion HP membranes and is the only PEM with conductivity at 100 S cm-1 level. In addition, the mechanical strength and swelling ratio of membranes are both substantially improved simultaneously. Based on the high-performance "self-enhanced" PEM, high peak power densities of up to 3.6W cm-2 and 1.7W cm-2 are achieved in H2-O2 and H2-Air fuel cells, respectively. This strategy can be applied in any polymeric electrolyte membrane.

Determination of Membrane Degradation in Polymer Electrolyte Fuel Cells with in-Situ and Ex-Situ Measurements

Fuel cells convert chemical energy directly into electrical energy with high efficiency and without pollutants. The decomposition products in the clean exhaust gas caused by the chemical degradation of the membrane of polymer electrolyte fuel cells (PEFCs) can be used to monitor the service life of the cell. For perfluorinated sulfonic acid (PFSA)-based membranes, chemical degradation is investigated via ex situ monitoring of the effluent water. The decomposition of the PFSA backbone, which is part of the polymer electrolyte membrane (PEM), can be detected by the degradation products such as fluoride or fluorinated compounds [1]. There are multiple methods for measuring fluoride in water. We have developed a new method for the simple and precise measurement of fluoride based on the optical change caused by the quenching of a zirconium complex. This method requires only 900 µL of the sample and 100 µL of the complex [2].We have extended the two fluoride measurement methods, namely fluoride sensitive electrode (FSE) and photometry (PM), to include ion chromatography (IC). This serves as an additional point of comparison with high accuracy and expands the measuring scope to include other ions like sulfate and their potential influence on photometric measurements.The electrochemical characterization of the materials has been improved by adapted electrochemical impedance spectroscopy (EIS) equivalent circuits, which include a more detailed analysis of the contribution of degradation processes. Linear sweep voltammetry (LSV) was used to measure the gas crossover through the membrane. Cyclic voltammetry provides information about the change in electrochemical surface area. Electrochemical characterizations and fluoride measurements are supplemented by ex situ imaging methods to determine membrane thinning.[1] M. A. Yandrasits, A. Komlev, K. Kalstabakken, M. J. Kurkowski, and M. J. Lindell, “Liquid Chromatography/Mass Spectrometry Analysis of Effluent Water from PFSA Membrane Fuel Cells Operated at OCV,” J. Electrochem. Soc., vol. 168, no. 2, p. 024517, Feb. 2021, doi: 10.1149/1945-7111/abe56a.[2] M. Heidinger, E. Kuhnert, K. Mayer, D. Sandu, V. Hacker, and M. Bodner, “Photometric Method to Determine Membrane Degradation in Polymer Electrolyte Fuel Cells,” Energies 2023, Vol. 16, Page 1957, vol. 16, no. 4, p. 1957, Feb. 2023, doi: 10.3390/EN16041957.

Modeling the viscoelastic-viscoplastic behavior of glassy polymer membrane with consideration of non-uniform sub-chains in entangled networks

Glassy polymers are common engineering materials and usually work in a linear elastic stage. However, some functional glassy polymers, such as perfluorinated sulfonic acid (PFSA) membranes, face more complicated working conditions: varied temperature, humidity, and long-term loading, where the viscoelastic behavior has a huge impact on their durability. The early studies on the constitutive model of glassy polymers focus on the elastic–plastic behavior, especially yielding and strain hardening, while showing limitations in predicting the time-dependent viscoelastic response of the PFSA membrane at complex conditions. This paper establishes a viscoelastic-viscoplastic (VE-VP) constitutive model of glassy polymers based on the non-uniform length distribution of entangled molecular chains. During loading, heating and hydration, the prior activation of short entangled chains is the precondition of the viscoelastic response of the material in the small deformation stage. To distinguish activated molecular chains from others, a calculation method of chain activation is established based on the energy conservation criterion and applied to monitor the activation state of the entangled chain network. For the verification of the model, uniaxial tensile, stress relaxation, and dynamic thermomechanical tests are conducted. The present model shows great accuracy in predicting the VE-VP behavior of the PFSA membrane, with the error less than 8% in the stress relaxation tests, providing a new perspective of constitutive modeling for glassy polymers.

Composite membranes for fuel cells

The current ecological situation attracts particular attention to alternative energy sources with no detrimental impact on the ecosystem. In comparison with conventional energy sources, fuel cells exhibit the following advantages: small and compact size, light weight, lack of noise when working, and cost-effectiveness in terms of fuel consumption. Most importantly, fuel cells are environmentally friendly, since no harmful substances are released into the atmosphere during their operation. Their goal is to convert chemical energy from various sources into environmentally friendly electric power. At present, chemical sources of energy are used everywhere, including batteries for mobile phones, laptops, as well as cars and uninterruptible power supplies, to name a few. The main components of solid polymer fuel cells are proton-exchange membranes, the main function of which is to ensure the transfer of protons from the anode to the cathode. The proton conductivity of such materials is determined by the presence of hydrophilic channels that transport mobile protons. The proton-exchange membrane must meet the following requirements: electrochemical and chemical stability in aggressive chemical environments, mechanical and thermal strength, low permeability to reagent gases (fuel and oxidizer), high ion exchange capacity and electrical conductivity, as well as a relatively low cost. This paper considers perfluorinated sulfonic acid membranes, organic–inorganic and acid–base composite membranes, as well as hybrid membranes obtained by sol-gel process, which can contribute to the development of technologies related to fuel cells in the future.

Investigation of mechanical damage in PFSA membrane via gas permeation rate test of membrane blister

The perfluorinated sulfonic-acid (PFSA) membrane is a key component in proton exchange membrane (PEM) fuel cells. The gas permeability of PFSA membrane, related to its deformation and damage, affects the performance and safety of the fuel cell. On the basis of constitutive model and solution-diffusion model of PFSA membrane, a numerical model was built to predict the deformation and gas permeation rate of membrane blister. The result indicates that damage occurs during the blister and accelerate the gas permeation.

High-performance ultrathin perfluorinated sulfonic acid membranes with thermo-morphology control for a vanadium redox flow battery

We demonstrate the morphology control of pre-aligned hydrophilic channels of ultrathin (∼30 nm) perfluorinated sulfonic acid membranes leading to enhancement of ion-selectivity and vanadium redox flow battery cell performance.

A comprehensive study on the ionomer properties of PFSA membranes with confocal Raman microscopy

The proton exchange membrane is the heart of many electrochemical energy systems, and their performance and longevity depend on a careful selection and processing of membrane materials. In this work, we use confocal Raman microscopy to investigate application-relevant properties of perfluorinated sulfonic acid (PFSA) membranes. Raman spectra of Nafion, Aquivion, and 3M Ionomer at different equivalent weights show differences in the spectral area and position of various Raman bands but reveal a clear trend of band intensities versus equivalent weight. Characteristics of these ionomers such as swelling and water uptake were determined with through-plane imaging. The technique offers advantages over conventional measurements, such as contact-free and non-destructive data acquisition. Calibration curves for the equivalent weight were derived for the different PFSAs with excellent linear fits that allow a quantification of the ionizable group content. The high spatial resolution of confocal Raman microscopy of less than 2 μm enables the local evaluation of all these ionomer properties. Furthermore, Raman imaging was employed to characterize composite membranes, which shows that it can resolve individual phases and features in multi-layered membranes. This study provides a comprehensive summary of confocal Raman microscopy as a powerful tool for the analysis of ionomer and membrane properties.

Analysis of Platinum Distribution within a Nafion 212 Membrane during Electroless Deposition

Ionic polymer metal composite (IPMC) is an electrically-responsive material made of perfluorinated sulfonic acid (PFSA) ionomers electroless plated with noble metal, such as platinum. Recently, interest in thin IPMC has been rising because of its potential applications in micro-electromechanical systems. However, little effort has been made so far on studying metal electroless deposition within thin PFSA membranes (thinner than 100 um). In this paper we study the platinum distribution within a thin Nafion membrane (Nafion 212, 50 µm-thick) using standard manufacturing recipe. We compare the obtained distribution with the distribution reported in literature based on thick Nafion membrane (Nafion 117, 180 µm-thick). We show that in Nafion 212 the maximum Pt concentration is situated in the bulk of the membrane, while in Nafion 117 the maximum peak is situated at the liquid-membrane boundary, suggesting a different rate determining deposition mechanism. Electroless deposition of platinum in Nafion 212 Ionic polymer metal composite (IPMC) is rising interest in the microelectromechanical system (MEMS) since smaller and thinner IPMC substrates would expand the applications. IPMCs are mostly made of a perfluorinated sulfonic acid ionomers (PFSA) membrane, such as commercially-available Nafion, coated with platinum. The prevalent way of manufacturing IPMC relies on the chemical reduction of platinum on the surface of the PFSA membrane, i.e. electroless deposition (1). Most of the reports on IPMC manufacturing rely on Nafion 117 (thickness of 0.180 mm, equivalent weight 1100 g/mol) while Nafion 212 has been poorly studied (thickness 0.050 mm, equivalent weight 1100 g/mol). This contribution aims at filling this gap.Materials and methodsA PFSA membrane (Nafion 212 membrane) has been coated with platinum using the standard recipe already describe elsewhere (1). An immersion-reduction step was performed in an immersion bath composed of tetraammineplatinum chloride (Pt(NH3)4Cl2, 0.2%), and reduction baths made of sodium borohydride (NaBH4, 5%). After the first immersion (30 min and 2 hours, respectively), the samples were immerged in the reduction bath until the end of the precipitation. Samples were then frozen in liquid nitrogen and cut, and analyzed with electrons dispersive x-ray spectroscopy (EDS) and processing of scanning electron microscopy (SEM) images (Fig 1a).Figure 1. a) EDS detection of platinum (yellow) overlapped with SEM picture of the cross-section of IPMC. Platinum distribution and Rayleigh fit for single reduction step (NaBH4) of platinum in a Nafion 212 membrane, after 30 minutes (b) and 2 hours (c) of immersion.Results and discussionThe distribution of platinum within Nafion 117 has already been studied by Millet et al. (2). The Pt distributions thereby reported strongly differs from the profiles we obtained (Fig 2). In the aforementioned profiles, the maximum Pt concentration is located at the liquid-substrate boundary of the Nafion membrane (2), whereas the Pt distribution in Nafion 212 reaches the maximum inside the Nafion membrane, between 0.5 µm and 1 µm below the surface (Fig 1b 1c).As reported by Millet et al., two limiting types of kinetics may be encountered in a dynamic exchange process, such that the deposition rate-determining mechanism is either diffusion in the liquid-film boundary (F-mechanism), or diffusion in the membrane (M-mechanism).The position of the peaks in our experimental data suggests that the mechanism determining the Pt precipitation rate inside Nafion 212 is diffusion inside the membrane while using Nafion 117 with the same reducing agent concentration leads to a precipitation rate determined by diffusion at the liquid/film boundary. It has been proven experimentally that ionic conductivity and water uptake of Nafion 212 is higher than for Nafion 117 (3). Thus we can hypothesize that the different behavior of thinner Nafion could explain the different empirical Pt distribution obtained using same reducing agent concentration. We could obtain a good fit of the Pt distribution (R2=0.919 and R2=0.945) within Nafion 212 with the Rayleigh distribution for immersion time of 30 minutes and 2 hours respectively (Eq 1.):f(x,σ 2) = (x/σ 2) exp(-x/2σ 2) [1]The scale parameter σ was found to be 4.19 and 4.58 for the sample immerged for 30 minutes and 2 hours, respectively. Further investigation of the precipitation mechanism inside Nafion 212 membrane and estimations of the phenomenological equations of mass transport correlated to Rayleigh distribution is reserved for future work.References K. Kim, M. Shahinpoor, Smart Mater. Struct., 12, 1, p. 65 (2003).P. Millet, R. Durand, E. Dartyge, G. Tourillon, and A. Fontaine J. Electrochem. Soc., 140, 5, pp. 1373-1380 (1993).A. Kusoglu, A.Z. Weber Chem. Rev. 117, 3 pp. 987-1104 (2017). Figure 1

Dynamic Water Transport through the Perfluorinated Sulfonic-Acid Membrane

Proton exchange membrane (PEM) is essential material for the proton transport in a polymer electrolyte fuel cell (PEFC). Since relative humidity (RH) in a cell affects the effective proton conductivity and the effective oxygen diffusivity through the catalyst layer as well as the proton conductivity, the electroosmotic drag coefficient, the effective diffusivity of water, and the gas permeances of the PEM, water behavior which determines relative humidity is important. To estimate the PEFC performance over time, the dynamic change in the moisture content of the PEM is needed to know. In this study, the PEM is considered to consist of a bulk layer and skin layers on both sides of the bulk layer. The bulk layer effective diffusion coefficient and the skin layer water transfer coefficient were formulated from the experimental results measured by the steady-state permeation method and the unsteady-state sorption and desorption method.

Origins of Water State and Ionic Cluster Morphology for High Proton Conductivity of Short Side-Chain Perfluorinated Sulfonic Acid Membranes

The property of perfluorinated sulfonic acid (PFSA) membranes depends not only on the ion exchange capacity (IEC), but also on the chemical structure of the functional side-chain and the phase-separation morphology. Two PFSA membranes, the long side-chain (LSC) and the short side-chain (SSC), have been investigated to study the structure–property relationship, covering the ionic domain structure and the proton transport. The proton conductivity of the SSC PFSA membrane is 143 and 209 mS/cm at 30 °C and 80 °C in water, which is 30–40% higher than that of the LSC PFSA membrane (103 and 161 mS/cm). The bound-to-free water ratio in the hydrated membranes was analyzed by differential scanning calorimetry and Raman spectroscopy, which show that a higher ratio accounts for the improved proton conductivity of the SSC PFSA membrane. The chain mobility was analyzed by solid-state nuclear magnetic resonance, which reveals that the side chain of the SSC membrane more readily self-assembles. This result was verified by the morphology from transmission electron microscopy. The small-angle X-ray scattering results show that the SSC PFSA membrane exhibits smaller domain spacing between the ionic clusters in dehydrated membranes. These observations, a larger ionic cluster and smaller domain spacing in the dehydrated SSC membrane, indicate a reduced size of the hydrophobic assembly feature domains, and the ionic channel connectivity is better in the SSC, which can be another key issue for its improved proton conductivity, in addition to the higher IEC and higher proton mobility.

Nanostructure‐property relationship of two perfluorinated sulfonic acid ( PFSA ) membranes

The physicochemical properties of perfluorinated sulfonic acid (PFSA) polymers are closely correlated with their nanostructure. However, their real nano-structural morphology is still controversial because it is difficult to observe their accurate morphology at the nanoscale. Moreover, studies on the nanostructures of the PFSA membranes have been mainly focused on the ionic domain. On this basis, here we describe the crystalline domain of two PFSA membranes as well as their ionic domain based on small-angle X-ray scattering results. Both ionic and crystalline domains showed significant alterations during hydration, and the different behaviors based on the side-chain length of the two PFSA membranes are also described. The short side chain-tethered PFSA membrane (higher ion exchange capacity (IEC)) showed a widespread ionic domain and lacking crystalline domain with their relatively temperature-dependent tendency compared to the flexible long side chain-tethered PFSA membrane (lower IEC). On this basis, the correlation between nanostructure and membrane properties is described from various perspectives.

Understanding the mechanism of nitrogen transport in the perfluorinated sulfonic-acid hydrated membranes via molecular dynamics simulations

In perfluorinated proton exchange membrane fuel cells (PEMFCs), the diffusion of inert nitrogen across the membranes can lead to its accumulation at the anode, which will lead to the failure of the cells. However, such a diffusion mechanism has not been clarified up to now. In this work, molecular dynamics simulations were performed to investigate the effect of water content and equivalent weight (EW) value on the diffusion of nitrogen molecules in perfluorinated sulfonic-acid (PFSA) hydrated membranes. It is found that the nitrogen molecules tend to distribute in the polymer phase and show the restricted regional motion at relatively low water content. However, with the increase of water content and the decrease of the EW value, the distribution and movement of nitrogen molecules show completely different behaviors. Nitrogen molecules are forced to migrate to the polymer/water interface and have a higher probability to diffuse along the water clusters. Meanwhile, the diffusion of nitrogen molecules is promoted and they show a jump-like motion. This study provides a fundamental and comprehensive understanding of nitrogen transport in PFSA hydrated membranes. • The distribution and diffusion dynamics of nitrogen have great differences for PFSA membranes with different water content and main chain length. • Nitrogen distributes in the polymer phase and is restricted in a specific region at low water content. • Nitrogen distributes at the polymer/water interface and diffuses along the polymer/water interface with high water content and low EW value.

Controlling hydrophilic channel alignment of perfluorinated sulfonic acid membranes via biaxial drawing for high performance and durable polymer electrolyte membrane water electrolysis

This study focuses on improving performance and durability of perfluorinated sulfonic acid (PFSA) membranes for polymer electrolyte membrane water electrolysis by controlling the membrane microstructure. To replace the highly gas permeable and expensive PFSA membranes, a membrane with low gas permeability and ohmic resistance is required for the high performing and safe water electrolyzers. Here, a thick PFSA membrane, Nafion 117 is biaxially stretched up to 6.4 times to reduce the membrane thickness, and increase the hydrophilic channels’ tortuosity. The thinned and planarly stretched Nafion shows better cell performance and durability at a constant current than the thick Nafion due to the reduced ohmic resistance and the enhanced interfacial stability between membrane and catalyst layer. With more tortuous hydrophilic pathways of the stretched Nafion, hydrogen permeability is reduced by a factor of 1.6, which is beneficial to prevent the membrane degradation and also to widen the operating current density range. The current density at 1.9 V increases from 1.5 to 3.2 A/cm 2 after stretching, and the biaxially stretched membrane shows 4 times lower enhancement in ohmic and cathodic charge transfer resistance than the similarly thick Nafion after the constant current experiment, yielding 2.5 times lower degradation rate. • Nafion 117 was biaxially stretched for PEMWE. • Planarly oriented hydrophilic channels reduced the hydrogen permeation. • Biaxially stretched Nafion showed higher cell performance. • Interfacial stability with the stretched Nafion was better than the un-stretched one.

The effect of ultrasonication of polymer solutions on the performance of hybrid perfluorinated sulfonic acid membranes with SiO2 nanoparticles

Perfluorinated sulfonic acid (PFSA) polymer membranes are widely used as ion-conducting electrolytes in energy-conversion devices. The development of novel hybrid materials containing inorganic dopants offers the route towards optimization of the performance of the PFSA membranes. In this work, the effect of ultrasonic (US) treatment of the PFSA solutions in the presence of SiO2 nanoparticles on the characteristics of the cast hybrid Nafion+SiO2 membranes was studied for the first time. Upon ultrasonication of polymer solutions, the length of macromolecules is reduced, and the number of sulfo groups decreases. When polymer solutions are ultrasonicated in the presence of SiO2, they experience additional crosslinking due to the interaction of SiO2 with sulfo groups of the PFSA polymer. As a result, up to 20% of -SO3H groups appears to be excluded from the ion-exchange process, and the temperature corresponding to destabilization of ionic clusters is reduced. When a hydrophilic dopant is incorporated within pores, the overall water uptake of the hybrid membranes increases, and their proton conductivity is improved. Maximum conductivity (78.6 mS/cm) at 40°С in the contact with water is observed for the Nafion+1 wt% SiO2 membrane cast from polymer solutions upon the ultrasonication for 10 min. The membranes preserve their high conductivity at low relative humidity (4.1 mS/cm at 30°С, 30% RH), and this value is 1.7 times higher than that of the pristine Nafion membrane. Hydrogen permeability of the hybrid Nafion+SiO2 membranes appears to be lower than that of the Nafion membranes by 15%. Hence, US-assisted dispersion of dopant nanoparticles in the PFSA solutions allows preparation of hybrid membranes with improved transport characteristics.

Hydrogen‐Bromine Redox‐Flow Battery Cycling with Bromine Complexing Agent: on the Benefits of Nanoporous Separator Versus Proton Exchange Membrane

The reversible and fast redox kinetics of bromine/bromide makes it a desirable couple as a catholyte in redox‐flow batteries (RFBs). In principle, the highest possible energy density is obtained with hydrogen‐bromine RFBs. Bromine sequestration agents, also called bromine complexing agents (BCAs), bind bromine in a non‐miscible phase and can, therefore, reduce the vapor pressure of bromine, mitigate its crossover, and result in higher practical range of electrolyte concentration. Therefore, BCAs can enhance the battery's safety and competitivity by significantly decreasing the cost of components. To date, BCAs are commonly used in membrane‐free bromine systems, which cannot provide the high current density demonstrated in hydrogen‐bromine RFBs. Herein, the drastic limitations encountered are shown while operating a hydrogen‐bromine RFB with a standard perfluorinated sulfonic acid membrane due to the strong BCA–perfluorinated sulfonic acid interaction. On the other hand, the benefits of using a polyvinylidene difluoride‐silica (PVDF–SiO2) nanoporous separator are demonstrated, which does not interact with the BCA. In this approach, the hydrogen‐bromine RFB can sustain cycling, albeit at a more moderate current than a BCA‐free battery.

Evaluation of Perfluorinated Sulfonic Acid Membranes for Vanadium Redox

The performance and cost of the membranes have always been crucial for the utilization of electrochemical energy devices. This article presents the properties and the performance of two commercially available and low-cost perfluorinated sulfonic acid membranes of GN115 and GN212C for vanadium redox flow battery (VRFB) application in comparison with Nafion117 (N117) membrane. The vanadium (IV) permeability of GN115 membrane was found to be close to N117, unlike GN212C membrane which showed 4 times higher vanadium (IV) permeability than N117 under similar conditions. Both GN115 and GN212C membranes showed isotropic conductivity and higher values than N117. The battery test results indicated that the high coulombic efficiency (CE) and high voltage efficiency (VE) followed the vanadium (IV) permeability and proton conductivity trends. Both membranes revealed outstanding stability in long-term charge-discharge testing under various current densities. The energy efficiency was found to be higher than N117 with values of 76.6% and 76.8% for GN115 and GN212C, respectively. The overall results suggest that both lower-cost membranes have a strong potential for VRFB application as the single-cell performance results were close to N117. The high value of proton conductivity of GN212C membrane compensates its vanadium (IV) permeability leaving reasonable battery performance.

Effect of the Side-Chain Length in Perfluorinated Sulfonic and Phosphoric Acid-Based Membranes on Nanophase Segregation and Transport: A Molecular Dynamics Simulation Approach.

The effect of side-chain length on the nanophase-segregated structure and transport in perfluorinated sulfonic acid (PFSA)-based and perfluorinated phosphoric acid (PFPA)-based membranes is investigated at 20 and 5 wt % water content conditions using a molecular dynamics simulation method. It is found using the pair correlation analysis that the longer side chain leads to more developed local water structures in the water phase at 20 wt % water content, observable in both membrane chemistries albeit more distinct in PFPA-based membranes. It is also confirmed from the structure factor analysis that large-scale nanophase segregation is enhanced with increasing side-chain length for PFPA membranes, whereas no significant change is observed at these scales for PFSA membranes. Next, it is revealed that the proton transport is increased by 0.004 S/cm in PFSA-based membranes with increasing side-chain length due to the enhanced vehicular and hopping mechanisms, whereas the proton transport in PFPA-based membranes is decreased by 0.002 S/cm despite improved nanophase segregation. As confirmed by the pair correlation function analysis, the diminished proton transport in PFPA-based membranes is attributed to the molecular association of phosphate groups with hydronium ions via hydrogen bond in the longer side-chain case, which is namely a hydronium-mediated bridge configuration. Such bridge configurations and correspondingly similar trends in proton transport are also observed at 5 wt % water content condition to a lesser extent. Our simulation study demonstrates that the proton transport is affected by the hydrogen-bonding network as well as by the nanophase segregation.

Quasi-solid single Zn-ion conductor with high conductivity enabling dendrite-free Zn metal anode

Aqueous Zn-ion batteries (ZIBs) have great potential in the field of large-scale energy storage. However, the dendrite formation on Zn anodes hinders the practical applications of ZIBs. Herein, a zincic perfluorinated sulfonic acid membrane (ZPSAM) is prepared as a quasi-solid single-ion conductor. In this membrane, Zn-ions move along with the negatively charged branched chains which possess a limited degree of freedom, forming fast Zn-ion transport channels. Consequently, ZPSAM exhibits high conductivity (~1.17 ​× ​10−3 ​S ​cm−1), enabling stable Zn plating and stripping processes at a current density of 2 ​mA ​cm−2 over 2000 cycles. In addition, due to the confined anion matrix in the membrane, dendrite formation and side reaction are both effectively suppressed. Finally, by applying the electrolyte in both aqueous super-capacitor and Zn-ion batteries, significantly improved cycling performance can be achieved. This work demonstrates a promising solution for the practical use of Zn metal anode in neutral aqueous media, and thus benefits the development of low-cost, high-performance aqueous energy storage devices.