Properties Of Proton Exchange Membranes Research Articles

(Invited) Mechanical Properties of Proton Exchange Membranes for Water Electrolysis Applications

Water electrolysers are critical to the advancement of sustainable hydrogen technologies. A key component of proton-exchange membrane water electrolysers (PEMWE) is the ionomer membrane which transports protons and water from the anode to the cathode and physically separates the H₂ and O₂ reaction products. Importantly, the barrier properties of the membrane must be maintained at high differential pressure over the lifetime of the PEMWE. To help predict the mechanical durability and operational safety of the PEMWE, it is important to understand the mechanical properties of the PEM as a function of temperature, differential pressure, and time.The main component of a typical modern PEM is a perfluorosulfonic acid (PFSA) ionomer. PFSAs have a hydrophobic perfluorinated backbone with pendant perfluorinated side chains terminating in sulfonic acid groups. The general chemical structure of PFSAs is understood to result in phase separation into hydrophilic, sulfonate-rich domains that act as transport pathways and hydrophobic, tetrafluoroethylene-rich domains that impart thermomechanical stability. In PEMWE applications, short-side chain PFSAs are a promising alternative to the benchmark Nafion, a long-side chain PFSA. At a given ion-exchange capacity (IEC), the ratio of perfluorinated backbone to sulfonic acid side chain increases as the side chain length decreases. Thus, short-side chain PFSAs may be more thermomechanically stable while maintaining good proton and water transport necessary for PEMWE efficiency.In this work, the mechanical properties of short-side chain PFSAs are investigated as a function of IEC, temperature, differential pressure, and time. Particular focus is given to compressive mechanical measurements to simulate the conditions experienced by the membrane in PEMWE applications. The findings are expected to help inform the design of next-generation membranes for hydrogen technology applications.

Confocal Raman Microscopy – Shedding Light on the Ionomer Properties of PFSA Membranes

Perfluorinated sulfonic acid (PFSA) polymers are the state-of-the-art material for the proton exchange membrane (PEM) in various electrochemical energy conversion devices, e.g., PEM fuel cells, PEM water electrolyzers, or redox flow batteries, combining chemical and mechanical durability with high proton conductivity.The most prominent example of a PFSA material is NafionTM, which is a so-called long side chain (LSC) ionomer due to its comparably long side chain between the polymer backbone and the charged sulfonyl end group. On the other hand, short side chain (SSC) ionomers like Aquivion® and 3M Ionomer or composite membranes are promising approaches to improve performance in harsh conditions, such as low humidity or high operating temperatures.1 SSC PEMs feature increased water uptake, proton conductivity,2 and crystallinity3 compared to LSC PEMs. Composite PEMs can be specifically tailored to the required operating conditions by carefully designing the interlayers or additives. As the performance and longevity of electrochemical cells are a delicate convolution of various (composite) membrane properties and parameters, a method for evaluating ionomer membrane properties is crucial for rationally engineering optimized PEMs.Here, we use confocal Raman microscopy (CRM) to investigate application-relevant properties of PFSA PEMs and for high-resolution imaging of composite membranes.4 Thus, the high spatial resolution of confocal optical microscopy and the chemical sensitivity of Raman scattering are combined in a single measurement approach. NafionTM, 3M Ionomer, and Aquivion® at varying equivalent weights (EW) feature distinctive Raman spectra (Fig. 1) that show characteristic spectral changes depending on the ionomer’s side chain structure and EW. We show that the EW of these PFSA types can be reliably measured using Raman spectroscopy. Further, parameters such as swelling, ionizable group content, and water uptake can be quantified by CRM non-destructively and contact-free. The diffraction-limited resolution of CRM of less than 2 µm enables a view of the local distribution of these properties and, therefore, allows to image and analyze composite membranes. In summary, we comprehensively study ionomer properties of (composite) membranes and establish CRM as a powerful platform for characterizing advanced PEMs for electrochemical energy deviceReferences A. S. Aricò, A. Di Blasi, G. Brunaccini, F. Sergi, G. Dispenza, L. Andaloro, M. Ferraro, V. Antonucci, P. Asher, S. Buche, D. Fongalland, G. A. Hards, J. D. B. Sharman, A. Bayer, G. Heinz, N. Zandonà, R. Zuber, M. Gebert, M. Corasaniti, A. Ghielmi and D. J. Jones, Fuel Cells, 10(6), 1013–1023 (2010).Y.-C. Park, K. Kakinuma, H. Uchida, M. Watanabe and M. Uchida, Journal of Power Sources, 275, 384–391 (2015).K. D. Kreuer, M. Schuster, B. Obliers, O. Diat, U. Traub, A. Fuchs, U. Klock, S. J. Paddison and J. Maier, Journal of Power Sources, 178(2), 499–509 (2008).M. Maier, D. Abbas, M. Komma, M. S. Mu'min, S. Thiele and T. Böhm, Journal of Membrane Science, 669, 121244 (2023). Figure 1

Nanostructure-transportation relation to PEMFCs activity and durability degradation

The Proton exchange membrane (PEM) is a crucial component in the membrane electrode assembly (MEA), playing a vital role in providing a channel for water transport and acting as a bridge for proton conduction. However, during the long-term operation of fuel cells, PEM is susceptible to attack from metal ions such as Cr3+ originating from the corrosion of metallic bipolar plates, leading to changes in its transport properties. Here, we report on the impact of Cr3+ on the microstructure and mechanical properties of PEM and then correlate it with changes in water uptake and proton conductivity, studying the effect of Cr3+ contamination on fuel cell performance and durability. The study reveals that Cr3+ forms a dense cross-linking structure with sulfonic acid groups inside the ionomer, resulting in an increase in storage modulus due to the larger ion radius and higher valence state, leading to a decrease in water uptake and proton conductivity. The membrane transport properties are primarily dependent on its water content. It is observed that Cr3+ contamination reduced the fuel cell voltage and maximum power density by 47.6 % and 57.1 %, respectively, with a voltage attenuation rate up to 2.9 mV/h, nearly 14 times that of the uncontaminated condition (0.21 mV/h). These findings provide valuable insights for developing high-transmission PEMs and aid in predicting the degradation and lifespan of fuel cells under actual operating conditions.

Enhanced properties of Nafion nanofibrous proton exchange membranes by altering the electrospinning solvents

Abstract Nanofibrous proton exchange membranes (PEMs) play an important role in improving the performance of the fuel cells. In this paper, two kinds of Nafion nanofibrous PEMs, Nafion-E/W and Nafion-DMF, were fabricated respectively by using ethanol/water (E/W) and N, N-dimethylformamide (DMF) as the solvent and their properties, such as the morphologies, water uptake, area swelling, ion exchange capabilities, conductivities, and mechanical properties were examined. Nafion-E/W nanofibers showed a thick diameter of 6,089 nm and Nafion-DMF nanofibers a thin diameter of 410 nm. Then the two Nafion nanofibers were annealed to provide the PEMs. Compared with Nafion 117 membranes and Nafion-DMF PEMs, Nafion-E/W PEMs showed the greatest water uptake and area swelling of respectively 59.75 % and 30.31 % and the conductivity increased to 0.1405 S/cm, more than twice as much as Nafion 117 membranes, but the broken stress decreased to 5.49 MPa, nearly half of Nafion 117 membranes. Nafion-DMF PEMs showed the lowest water uptake, area swelling, and conductivity of 22.67 %, 10.75 %, and 0.0410 S/cm, and the broken stress reached 14.20 MPa, greater than 11.0 MPa of Nafion 117 membranes. The obtained experimental results are instructive to improve the properties of Nafion PEMs.

The effect of the number of sulfonic acids in polysiloxanes as fillers for polymer electrolyte membranes in energy storage applications

A series of sulfonic acid-functionalized polysiloxanes (MASFPs) that can be used to reduce the cost and improve the properties of proton exchange membranes (PEMs) by acting as fillers have been synthesized. In this paper, we describe the synthesis of MASFPs with varying numbers of sulfonic acid groups with the aid of mercaptoethanol. An array of analytical techniques were employed to characterize the synthesized MASFPs. The broad peak observed in the X-ray diffraction pattern of MASFPs between 2θ = 15–25° signifies their amorphous characteristics. Analyses by infrared spectroscopy also identify MASFPs that contain stretching vibrations for –SO3H, –CH2, and Si–O– group. The thermogravimetric analysis demonstrates the thermal stability of these MASFPs. The surface morphology analysis of the MASFPs using FESEM and HRTEM revealed the presence of spherical particles that exhibited significant aggregation and distortion. By employing X-ray photoelectron spectroscopy (XPS) to determine the atomic weight percentage of sulfur present in the MASFPs, the number of sulfonic acids contained within was verified. The sulfur content of MASFPs is determined by XPS to be as follows: MASFP-4 > MASFP-3 > MASFP-2 > MASFP-1. The presence of Q4, Q3, T3, and T2 signals, as determined by 29Si MAS NMR analysis, confirms the polysiloxane formation. Preliminary studies of the composite membrane prepared using the MASFPs with perfluorosulfonic acid (PFSA:MASFP in 75:25 ratio) exhibited superior ionic conductivity (0.102 S/cm) and ion-exchange capacity (1.16 meq/g) in comparison to the commercial Nafion membrane. Overall, using a mercaptoethanol-assisted process to synthesize sulfonic acid functionalized polysiloxanes is an excellent way to increase the incorporation of sulfonic acids and it can be applied to the synthesis of various other inorganic nanofillers.

Next‐Generation Proton‐Exchange Membranes in Microbial Fuel Cells: Overcoming Nafion's Limitations

The adoption of microbial fuel cell (MFC) technology hinges on the development of efficient proton‐exchange membranes (PEMs), which significantly influences fuel cell performance and cost. PEMs have a critical role in preventing oxygen crossover, maintaining electrochemical neutrality, and supporting microorganisms within MFCs. Nafion, the current industry‐standard PEM, grapples with environmental, cost, and performance issues. Although improvements to Nafion have been reported using additives, immersion in heteropolyacids, different pretreatment methods, and UV irradiation, many of the challenges still remain. Herein, the recent developments in the area of alternative PEMs are reviewed and analyzed. Among them, sulfonated aromatic hydrocarbons, particularly sulfonated polyether ether ketone, have emerged as top contenders in terms of scale up and commercial viability. At the same time, membranes based on polyvinyl alcohol, ionic liquids, and natural materials are also being actively researched for various MFC applications. Since most studies are short term and lab scale, there is a need evaluate long‐term stability and economic cost of PEMs in terms of standardized parameters such as power‐to‐cost and normalized energy recovery. Additionally, for emerging low‐energy‐density MFC applications like biosensors and in vivo power sources, PEM properties and design need to be tailored carefully.

Next Generation Fluorine-Free Proton Exchange Membranes for Electrochemical Energy Applications

Most commercially available proton exchange membrane (PEM)-based fuel cell and electrolyser systems currently employ perfluorinated sulfonic acid (PFSA)-based PEMs, e.g., Nafion. The use of these fluorinated polymers presents serious health and environmental concerns as their production, degradation, and disposal results in the release of hazardous perfluorinted compounds that have been shown to accumulate in the environment and are toxic to living organisms.[1] Developing non-toxic, safe-by-design hydrocarbon PEMs is therefore crucial to ensure that renewable technologies that rely on proton exchange membranes remain an environmentally friendly solution. Additional shortcomings of PFSA-based PEMs include their expensive synthetic cost, inherently high hydrogen crossover, and limited operating temperature range due to poor thermo-mechanical properties above 90 °C. Hydrocarbon (HC)-based membranes offer advantages over their perfluorinated counterparts in these specific areas, however, the main drawback of HC-PEMs is that they require a much greater density of sulfonic acid groups to achieve similar proton conductivities as PFSA-based polymers.[2] This leads to greater water uptake and excessive swelling which ultimately leads to reduced mechanical stability of HC membranes.More recently there has been an increasing number of reports exploring the use of block copolymer strategies to limit the water uptake and swelling properties of hydrocarbon PEMs while maintaining good ionic conductivity.[3] As the ion, liquid, and gas transport properties of the membrane are largely attributed to the hydrophilic blocks (i.e., with sulfonic acid groups) and the mechanical, chemical, and thermal stability of the resulting membrane is determined by the hydrophobic blocks (i.e., no sulfonic acid groups), it is postulated that the resulting membrane properties can be tuned by varying the fraction of hydrophilic/hydrophobic blocks.The overall goal of this project is to synthesise hydrocarbon block copolymer PEMs with reduced gas crossover and enhanced the mechanical properties while maintaining sufficiently high ionic conductivity, ultimately leading to higher performance and greater durability inelectrochemical energy conversion and storage systems, e.g., fuel cells and electrolysers. In this project we have compared the ex-situ properties of novel hydrocarbon block copolymers with PFSA-based Nafion membranes, e.g., water uptake and swelling, conductivity measurements, oxidative stability via Fenton's test, etc., as well as in-situ performance tests in single cell fuel cell and electroylzers.[1]Feng, M.; Wang, Z. et al. Sci. Rep. 2015, 5, 9859.[2]Holdcroft, S. Chem. Mater. 2014, 26, 381.[3]Guiver, M. et al. Chem. Rev. 2017, 117, 4759.

Applicability of Phosponated Poly(pentafluorostyrene) Fiber Reinforced Nafion Composite Membranes in Low-Temperature Fuel Cells

Reducing the fuel crossover of proton exchange membranes (PEMs) is an important measure to improve the fuel efficiency and performance of fuel cells and electrolyzers 1, 2. Additionally, the long-term stability and safety of PEM-based systems increase with a reduction in the gas crossover 1, 3, 4. A possible approach for reducing the gas crossover of a PEM is the implementation of fiber meshes to form a composite membrane 5. Furthermore, fiber-reinforced composite membranes can also increase the mechanical properties of PEMs and therefore enable the utilization of thinner membranes without sacrificing the mechanical integrity of the membrane or causing safety risks 6.In this work, we demonstrate the successful fabrication of electrospun fibers of a partially phosophonated, conductive polymer (phosponated poly(pentafluorostyrene); PWN70) and its non-phosphonated and, therefore, non-conductive equivalent (poly(pentafluorostyrene); PPFSt). Two different loadings of both fiber meshes were successfully infiltrated with Nafion, analyzed mechanically and electrochemically, and compared to a non-reinforced Nafion reference. We show that the mechanical properties of both fiber-reinforced composite membranes are superior to the reference membrane. The electrochemical analysis of the membrane electrode assemblies (MEAs) shows a reduction in performance with the incorporation of the non-conductive PPFSt fibers. An increase in the high-frequency resistance (HFR) of the MEAs containing the non-conductive PPFSt fibers explains the reduced performance and scales with fiber loading. In contrast, the MEAs with the proton conductive PWN70 fiber reinforcement maintained the same HFR and performance as the reference, independently of the fiber loading. Additionally, hydrogen crossover measurements revealed a significantly reduced hydrogen crossover of the MEAs composed of the conductive PWN70 fiber-reinforced membranes (between 30 and 40 % less than the reference). We hypothesize that PWN70 shows a decreased diffusivity for hydrogen compared to Nafion, and therefore, the fibers in the Nafion matrix increase the tortuosity of the membrane for fuel crossover.In summary, we developed a fiber-reinforced Nafion membrane with improved mechanical and gas barrier properties while maintaining the membrane's proton conductivity compared to the same membrane without reinforcement.References Q. Tang, B. Li, D. Yang, P. Ming, C. Zhang and Y. Wang, Int. J. Hydrog. Energy, 46(42), 22040–22061 (2021).M. Schalenbach, M. Carmo, D. L. Fritz, J. Mergel and D. Stolten, Int. J. Hydrog. Energy, 38(35), 14921–14933 (2013).B. Wu, M. Zhao, W. Shi, W. Liu, J. Liu, D. Xing, Y. Yao, Z. Hou, P. Ming, J. Gu and Z. Zou, Int. J. Hydrog. Energy, 39(26), 14381–14390 (2014).C. Klose, P. Trinke, T. Böhm, B. Bensmann, S. Vierrath, R. Hanke-Rauschenbach and S. Thiele, J. Electrochem. Soc., 165(16), F1271-F1277 (2018).J. Choi, K. M. Lee, R. Wycisk, P. N. Pintauro and P. T. Mather, Macromolecules, 41(13), 4569–4572 (2008).J. B. Ballengee and P. N. Pintauro, Macromolecules, 44(18), 7307–7314 (2011).

Impact of deposited Pt particles on water channel connectivity and proton conductivity in proton exchange membranes: A molecular dynamics study

Deposition of platinum ions in the proton exchange membrane significantly affects the long-term stability and proton transport capacity of the membrane electrode assembly. In this study, the impacts of deposited Pt particles on the intrinsic properties of proton exchange membranes and its mechanisms are investigated using molecular dynamics simulation method. The effect of differently sized Pt particles on water channel connectivity and proton conductivity in proton exchange membranes under dry, normal, and saturated conditions was detailly discussed. The results reveal that Pt particles reduce water channel connectivity at low hydration level while increasing it at normal and high hydration levels. However, the enhancement mechanism is different. Moreover, proton conductivity was evaluated by examining the diffusion of hydronium ions and hydrogen bond dynamics. We found that the proton conduction performance of the membranes at low hydration levels is significantly reduced by Pt particles. We believe this study can promote the design of more robust and efficient proton exchange membranes.

Side chain sulfonic acid polymers with intrinsic pores in the main chain as proton exchange membranes for fuel cells and redox flow battery

The high proton conductivity, good dimensional stability and acceptable mechanical properties of proton exchange membranes (PEMs) are important indicators for the practical application of membrane materials in clean energy storage and conversion systems. In this work, we prepared SPDI-x, a membrane material with rigid and twisted main chains and intrinsic channels by an efficient and simple route. It combined a rigid and twisted skeleton with a flexible sulfonic acid side chain to achieve high conductivity and a limited swelling rate. The conductivity of SPDI-x at 80 °C was 233 mS cm−1, and the swelling rate was lower than 14%. SPDI-x demonstrated higher coulomb efficiency, energy efficiency and voltage efficiency than commercial Nafion212 in redox flow battery. In addition, it had more stable long-term cycle performance under the same test conditions. The H2–O2 fuel cell has a maximum peak power density of 823 mW cm−2, which is also higher than the battery performance of commercial PEMs under the same test conditions. The high mass transfer performance and good battery performance of SPDI-x provide a new idea for the structural design of high-performance proton exchange membranes.

A Critical Review of Electrolytes for Advanced Low- and High-Temperature Polymer Electrolyte Membrane Fuel Cells

In the 21st century, proton exchange membrane fuel cells (PEMFCs) represent a promising source of power generation due to their high efficiency compared with coal combustion engines and eco-friendly design. Proton exchange membranes (PEMs), being the critical component of PEMFCs, determine their overall performance. Perfluorosulfonic acid (PFSA) based Nafion and nonfluorinated-based polybenzimidazole (PBI) membranes are commonly used for low- and high-temperature PEMFCs, respectively. However, these membranes have some drawbacks such as high cost, fuel crossover, and reduction in proton conductivity at high temperatures for commercialization. Here, we report the requirements of functional properties of PEMs for PEMFCs, the proton conduction mechanism, and the challenges which hinder their commercial adaptation. Recent research efforts have been focused on the modifications of PEMs by composite materials to overcome their drawbacks such as stability and proton conductivity. We discuss some current developments in membranes for PEMFCs with special emphasis on hybrid membranes based on Nafion, PBI, and other nonfluorinated proton conducting membranes prepared through the incorporation of different inorganic, organic, and hybrid fillers.

Optimization of the ionomer-to-carbon ratio in the cathode catalyst layer of PtCoMn/C alloy catalysts

The optimization of membrane electrode assembly (MEA) structures based on alloy catalysts can further improve the properties of proton exchange membrane fuel cells and accelerate their commercialization. Herein, novel MEAs with ultra-low platinum loading and high performance were prepared by a decal transfer method using a highly active PtCoMn/C alloy catalyst. The effects of different I/C ratios on the performance of PtCoMn/C cathodes were investigated by polarization curves. The results suggested an optimal I/C ratio of 0.9, leading to a maximum power density of 1.51 W/cm2. The effects of the I/C ratio on catalyst activity and proton conductivity were studied by AC impedance and cyclic voltammetry, and the impact of the I/C ratio on local oxygen transport resistance was evaluated by the limiting current method. The data revealed that the MEA has the highest ECSA of CL combined with the lowest Rct and higher proton conductivity,when the I/C ratio is 0.9. An influencing mechanism of ionomer content on performance of low platinum loading MEA prepared by PtCoMn/C was then drawn, suggesting that The highest activity at the I/C ratio of 0.9 mainly depends on its excellent proton conductivity.

Preparation of Sulfonated Poly(arylene ether)/SiO2 Composite Membranes with Enhanced Proton Selectivity for Vanadium Redox Flow Batteries

Proton exchange membranes (PEMs) are an important type of vanadium redox flow battery (VRFB) separator that play the key role of separating positive and negative electrolytes while transporting protons. In order to lower the vanadium ion permeability and improve the proton selectivity of PEMs for enhancing the Coulombic efficiency of VRFBs, herein, various amounts of nano-sized SiO2 particles were introduced into a previously optimized sulfonated poly(arylene ether) (SPAE) PEMs through the acid-catalyzed sol-gel reaction of tetraethyl orthosilicate (TEOS). The successful incorporation of SiO2 was confirmed by FT-IR spectra. The scanning electron microscopy (SEM) images revealed that the SiO2 particles were well distributed in the SPAE membrane. The ion exchange capacity, water uptake, and swelling ratio of the PEMs were decreased with the increasing amount of SiO2, while the mechanical properties and thermal stability were improved significantly. The proton conductivity was reduced gradually from 93.4 to 76.9 mS cm-1 at room temperature as the loading amount of SiO2 was increased from 0 to 16 wt.%; however, the VO2+ permeability was decreased dramatically after the incorporation of SiO2 and reached a minimum value of 2.57 × 10-12 m2 s-1 at 12 wt.% of SiO2. As a result, the H+/VO2+ selectivity achieved a maximum value of 51.82 S min cm-3 for the composite PEM containing 12 wt.% of SiO2. This study demonstrates that the properties of PEMs can be largely tuned by the introduction of SiO2 with low cost for VRFB applications.

Sulfonated branched poly(arylene ether ketone sulfone) proton exchange membranes: Effects of degree of branching and ion exchange capacity

Sulfonated branched polymers exhibit substantial potential for application as proton exchange membranes (PEMs). Most studies develop branched polymer PEMs by regulating the degree of branching (DB), without considering another important factor, i.e., the ion exchange capacity (IEC). Herein, we report on a systematical comparison regarding the effects of DB and IEC on the branched polymer PEMs and a solution to advance the properties of PEMs. To accomplish this, two series of branched poly(arylene ether ketone sulfone)s containing densely sulfonated tetraarylmethane units were synthesized. The DB and IEC were controlled in the range of 5–10 % and 1.41–1.94 meq g−1, respectively. It was found that both increasing DB and IEC of membranes promoted water absorption, proton conductivity, single-cell performance, denser distribution of microscopic hydrophilic phase, and also reduced the mechanical property. Furthermore, the strengthening via increased DB maintained dimensional stability and improved oxidative stability, while the impact of IEC was the opposite. Ultimately, a polymer membrane bearing optimal DB (7.5 %) and IEC (1.94 meq g−1) exhibited the highest proton conductivity (134.5 mS cm−1) and maximum power density (426.8 mW cm−2), satisfactory dimensional change (18.8 % in-plane, 27.2 % through-plane, and 79.5 % volume) at 80 ℃, and sufficient thermal, oxidative, and mechanical stability. This work is expected to serve as a guide for designing and synthesizing the DB and IEC of sulfonated branched polymer PEMs.

Multi-component polymeric membranes based on acrylamide monomers for fuel cells

The preparation, characterization, and evaluation of properties of proton exchange membranes (PEMs) for application in fuel cells (FCs) based on non-fluorinated hydrocarbons are described in this paper. A new class of membranes was synthesized by radical copolymerization, in aqueous medium, using low-cost monomers. The copolymers were constituted of 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) as proton-conducting groups, hydroxyl ethylacrylamide (HEA) as crosslinking sites and styrene and N-isopropyl-acrylamide (NIPAM) as hydrophobic domains. The membranes were obtained by crosslinking the copolymers with glutaraldehyde in acidic medium under different conditions. The copolymers were characterized by infrared spectroscopy (FTIR), hydrogen nuclear magnetic resonance (1H NMR), size exclusion chromatography (SEC) and thermogravimetric analysis (TGA). The membranes were also characterized by FTIR, which pointed out the changes resulting from the reaction with glutaraldehyde. TGA analyzes allowed to evaluate the thermal stability of the membranes in temperature ranges consistent with the use in FCs. The water uptake tests showed greater water absorption for samples without or with a low percentage of hydrophobic component. In addition, higher crosslinking agent content, acidification and higher curing temperatures led to a reduction in the degree of swelling. For the conductivity measurements, only the samples with NIPAM showed adequate stability to be submitted to the previous hydration treatment. As a result, these membranes presented higher conductivity values, close to those of commercial membranes. In general, membranes with more conductive groups in the composition, higher hydration and less crosslinks showed better conductivities. Finally, the set of samples with the best results was evaluated in oxidative degradation tests. In summary, the results presented allowed an evaluation of the adjustment of the properties of PEMs through the control of crosslinking and the formulation of the membranes.

Dynamic Water Transport through the Perfluorinated Sulfonic-Acid Membrane

IntroductionThe proton exchange membrane (PEM) is indispensable material for proton transport in PEFC. Since RH 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 PEM, water behavior which determines relative humidity is most important.To estimate the PEFC performance over time, the dynamic change in transport properties of PEM should be reproduced over full range of operating temperature and moisture content. Though water diffusion in Nafion™ membrane was extensively investigated [1, 2], the transport properties have not been formulated as the function of both temperature and moisture content. In this study, the effective water diffusivity and the skin layer water transfer coefficient were formulated from the experimental results measured by the permeation method and the sorption/desorption method.Experimental–Permeation experimentsA Nafion™ membrane, NR-212, N115 or N117 with different thickness (at relative humidity, RH of 50 %), 51 μm, 127 μm, and 183 μm, respectively, was set in a Japan Automobile Research Institute (JARI) standard cell whose active area was reduced to 2.0 cm×2.0 cm. The cell was placed in the constant temperature and humidity oven (ESPEC, LHL-113). Nitrogen was supplied to both sides of the cell at 300 cm3/min (20 °C, 1 atm). The relative humidity of the supplied gas was controlled by the bubbler temperature. The water vapor flowing out of the cell was collected with an ice-cooled trap to estimate the water permeation flux of the membrane.–Sorption/desorption experimentsThe membrane was placed in a desiccator in which RH was controlled by saturated salt solution method at room temperature (22 °C). The weight of the membrane was recorded constantly. After reaching equilibrium, the membrane was placed in another RH condition and weighed.Results and discussionThe moisture content of the membrane, λ was calculated by a GAB sorption equation in which paraments were fit using reported water equilibrium experimental data [3, 4, 5, 6] as shown in Fig. 1. Schematic diagram of permeation mechanism is shown in Fig. 2. Water permeation flux through the membrane, N A (M) is expressed as follows: N A (M)=c (M) ρ (M) (λ -m -λm )/(1/k A++1/k A-+δ (M)/D eA (M)) (1)where c (M) is the ion-exchange capacity [mol/kg], ρ (M) is the density of dry membrane [kg/m3], λ is the moisture content, k A is the skin layer mass transfer coefficient [m/s], δ (M) is the membrane thickness [m] and D eA (M) is the effective diffusivity [m2/s]. The subscript A means absorbed water, m is the surface of the membrane, - is the supply side and + is the permeate side. Total resistances calculated by eq. (1) were plotted against the membrane thickness. D eA (M) and k A calculated from the trendlines are shown in Figs. 3 and 4, and Arrhenius plots of D eA (M) are shown in Fig. 5.Skin layer mass transfer coefficient, k A can be obtained directly from the sorption/desorption experiments at low temperature. k A obtained from the permeation and sorption/desorption experiments are summarized in Fig. 6 and they are located on an identical line. The transport properties at λ= 5 are expressed as follows as an example: D eA (M) = 3.35×10–5 m2 s–1 exp(–3760 K/T) k A = 1.27×105 ms–1 exp(–8330 K/T)Since these transport properties depend on the moisture content, the desorption is faster than sorption and vice versa even at completely same range of moisture content. This is because the moisture content in the skin layer is always different during sorption and desorption even with the same moisture content variation range.ConclusionsFrom the permeation experiments and the sorption/desorption experiments, the transport properties over wide range of temperature and moisture content of the perfluorinated sulfonic-acid membrane were formulated. Considering the change in k A and D eA depending on the moisture content, the sorption and desorption rate are unequal even in identical RH range.AcknowledgementThis work was supported by the FC Platform Program: Development of design-for-purpose numerical simulators for attaining long life and high-performance project (FY 2020–FY 2022) conducted by the New Energy and Industrial Technology Development Organization (NEDO), Japan.

Monitoring and Modeling of Compression Creep of Perfluorosulfonic Acid Membranes for Electrolyzers

Water-splitting electrolyzers are a key technology for enabling the hydrogen economy and reshaping the renewable energy landscape. Among the most viable candidates for the low-temperature electrolyzers is the proton-exchange membrane (PEM) water-splitting electrolyzers (PEMWEs), which uses an ion-conductive polymer solid-electrolyte. For PEMWEs to be commercially viable, PEMs must perform over long operational times in liquid environments under compression and therefore must exhibit good mechanical stability when hydrated. [1-2]. While a hydrated environment is an intrinsic result of the operation and membrane’s conductive function, it undermines PEM stability. [3] PEMs are under unique mechanical stresses due to differential pressure affecting the active area, and high sealing pressures to prevent off-board leaks, thus, high differential pressure in PEMWEs makes compressive creep a great concern. [4] However, mechanical stability of PEMs is commonly characterized by tensile testing [5], the applicability of which to electrolyzers or technologies with similar cell designs is unclear. Using compression and compression creep data of hydrated PEMs in situ and in controlled temperature [6], this work develops a material model to describe PEM under device-relevant operation.An important aspect of PEM design consideration is the stability of membrane, especially under conditions relevant to operation, such as, in a liquid environment with varying range of temperatures (50 to 80 °C). While hydration, pressure, and temperature are used as design parameters for optimizing electrolyzers performance, their role in durability is not established [5]. This creates a gap between performance and lifetime assessment of membranes for electrolyzers. Thus, there is need for a model that could capture membrane durability by accounting for performance operators. This study identifies compression creep as a potential material stability metric that can account for the operation-dependent variables, such as pressure, dehydration, and temperature, and provides guidance for better assessment of membrane lifetime in electrolyzers and similar electrochemical devices.Our results show compressive response of perfluorosulfonic acid (PFSA) membranes is significantly different than tensile behavior in both dry and hydrated states [6]. Moreover, PEMs exhibit creep response under compression with continuous decrease in its thickness over 24 hours, with a dependence on the applied pressure and temperature [6]. This work demonstrates the importance of studying the mechanical properties of PEMs under compression, which is more relevant to the stress-states the membrane undergoes during operation in an electrolyzer and similar electrochemical devices.

직접메탄올 연료전지를 위한 Poly(vinyl alcohol-co-styrenesulfonic acid) 기반의 양성자 교환막의 제법 및 특성 연구

Development of proton conducting membrane electrolyte is one of the most important research area within the domain of direct methanol fuel cells (DMFCs). This study reports the preparation of glutaraldehyde-crosslinked poly(vinyl alcohol-co-styrenesulfonic acid) proton exchange membrane (PEM) for DMFC. In order to understand the effect of the extent of crosslinking on the PEM properties, varying concentration of glutaraldehyde was used. Key properties of PEMs such as water uptake, swelling behavior, ion-exchange capacity, proton conductivity, and methanol permeability were thoroughly analyzed. PEM formed using 0.5 M glutaraldehyde solution resulted in a crosslinked PEM with the moderate properties, such as swelling, water uptake, proton conductivity and mechanical strength, along with the exhibition of high proton conductivity and lower methanol permeability compared to commercial Nafion-117. In addition, optimized PEM produced a peak power density of 58 W/㎡ at a current density of 300 A/㎡, when DMFC fed with 2 M methanol solution at 60 °C.

Evaluation of sulfonated chitosan-g-sulfonated polyvinyl alcohol/polyethylene oxide/sulfated zirconia composite polyelectrolyte membranes for direct borohydride fuel cells: Solution casting against the electrospun membrane fabrication technique

To improve the mechanical properties of proton exchange membranes, consequently improving the performance of direct borohydride fuel cells, the present study prepared sulfonated chitosan-g-sulfonated polyvinyl alcohol/polyethylene oxide doped with sulfated zirconia composite (SCS-g-SPVA/PEO/SZrO2) polyelectrolyte membranes. Two fabrication techniques were followed, solution casting and electrospinning, to have the membranes in film and fiber forms and study the effect of the different forms on the membrane’s physicochemical properties. For the casting technique, different concentrations of SZrO2 (1-3 wt%) were used, while the optimum concentration of SZrO2 (3 wt%) was used in the electrospun one (SCS-g-SPVA/PEO/SZrO2-CF). SCS-g-SPVA/PEO/SZrO2-C membranes were prepared in a single step. The grafting and the crosslinking were carried out using glutaraldehyde and sulfosuccinic acid as sulfonating agents for chitosan and PVA and coupling agents simultaneously using click chemistry. On the other hand, SCS-g-SPVA/PEO/SZrO2-CF membranes were prepared in two steps. They were fabricated with electrospinning and then dipped into the coupling and crosslinking solutions. The casting membranes’ physicochemical properties were improved by increasing the SZrO2 content. The experimental results further show that the fabrication procedure significantly influences the physicochemical properties of the membranes. For instance, the composite fiber membrane demonstrated higher selectivity and higher ion exchange capacity (IEC) than the casting membrane. Furthermore, by using the response surface methodology model, the effects of ion exchange capacity, water uptake, and oxidative stability were optimized as three independent variables that affected the ionic conductivity of SCS-g-SPVA/PEO/SZrO2-3C. The optimized ionic conductivity of the SCS-g-SPVA/PEO/SZrO2-3C membrane was 13.6 mS cm−1, achieved at the maximum point of the polynomial model, with an IEC of 0.74 meq g−1, ∼92% water uptake, and about 93% oxidative stability.

Multifunctional Sulfonated Polytriazoles: Proton-Exchange Membrane Properties, Molecular Logic Gates, and Modeling of Stimuli-Responsive Behaviors

Multifunctional Sulfonated Polytriazoles: Proton-Exchange Membrane Properties, Molecular Logic Gates, and Modeling of Stimuli-Responsive Behaviors