Conductivity Of Proton Exchange Membranes Research Articles

An enhanced proton conductivity of proton exchange membranes by constructing proton-conducting nanopores using PVP-UiO-66-NH-SO3H nanoparticles

The performance of proton exchange membranes (PEMs) can be enhanced by creating continuous proton conduction channels. In this work, PVP-UiO-66-NH-SO3H nanoparticles (PUNSNPs) were prepared and filled into sulfonated poly (ether ether ketone) (SPEEK) to fabricate PUNSNPs/SPEEK composite PEMs. To further improve the performance of the PEMs, phosphoric acid (PA) was doped and a continuous nanopore network was formed in PA/PUNSNPs/SPEEK porous PEMs due to the dissolution of PUNSNPs. The PA/PUNSNPs/SPEEK porous PEMs have high PA uptake ratio (96%–117 %) and dimensional stability thanks to the anchoring effect of the nitrogen-containing heterocyclic ring on the PVP chains. The acid-base pairing interaction between the chains (residual ligands from PUNSNPs) inside the unique continuous nanopore structure allows PA/PUNSNPs/SPEEK = 20 % porous PEMs to have the highest proton conductivity of 0.350 S cm−1 (80 °C/100 % RH). Furthermore, the PA/PUNSNPs/SPEEK porous PEMs retained PA up to more than 80 % (80 °C, 40 % RH, 7 days). Similarly, PA/PUNSNPs/SPEEK = 20 % porous PEMs immersed in deionized water for 45 days were able to maintain a proton conductivity of more than 83 %. Finally, PA/PUNSNPs/SPEEK porous PEMs have low methanol permeability. Thus, this study provides a new method for the preparation of porous PEMs with a continuous nanopore network structure and excellent performance.

Bioinspired supramolecular macrocycle hybrid membranes with enhanced proton conductivity

AbstractEnhancing the proton conductivity of proton exchange membranes (PEMs) is essential to expand the applications of proton exchange membrane fuel cells (PEMFCs). Inspired by the proton conduction mechanism of bacteriorhodopsin, cucurbit[n]urils (CB[n], where n is the number of glycoluril units, n = 6, 7, or 8) are introduced into sulfonated poly(ether ether ketone) (SPEEK) matrix to fabricate hybrid PEMs, employing a nature-inspired chemical engineering (NICE) methodology. The carbonyl groups of CB[n] act as proton-conducting sites, while the host–guest interaction between CB[n] and water molecules offers extra proton-conducting pathways. Additionally, the molecular size of CB[n] aids in their dispersion within the SPEEK matrix, effectively bridging the unconnected proton-conducting sulfonic group domains within the SPEEK membrane. Consequently, all hybrid membranes exhibit significantly enhanced proton conductivity. Notably, the SPEEK membrane incorporating 1 wt.% CB[8] (CB[8]/SPEEK-1%) demonstrates the highest proton conductivity of 198.0 mS·cm−1 at 60 °C and 100% relative humidity (RH), which is 228% greater than that of the pure SPEEK membrane under the same conditions. Moreover, hybrid membranes exhibit superior fuel cell performance. The CB[8]/SPEEK-1% membrane achieves a maximum power density of 214 mW·cm−2, representing a 140% improvement over the pure SPEEK membrane (89 mW·cm−2) at 50 °C and 100% RH. These findings serve as a foundation for constructing continuous proton-conducting pathways within membranes by utilizing supramolecular macrocycles as fuel cell electrolytes and in other applications.

Constructing a long-range proton conduction bridge in sulfonated polyetheretherketone membranes with low DS by incorporating acid-base bi-functionalized metal organic frameworks

Functionalized metal-organic frameworks (MOFs) are being extensively developed as viable fillers to enhance the proton conductivity of proton exchange membranes. Herein, an amino-pendant sulfonic acid bi-functionalized MOFs material (UNCS)-doped SPEEK membrane with low degree of sulfonation (DS) can improve the proton conductivity as well as maintain the membrane dimensional stability. UNCS can act as bridges of proton donors and acceptors to reduce the activation energy barrier and shorten the distance of long-range proton conduction. Among all as-prepared membranes, SPEEK/UNCS-3 exhibited the highest proton conductivity of 186.4 mS·cm−1 at 75 °C and 100% relative humidity (RH), which is much greater than that of pristine SPEEK and Nafion 117. Benefiting from the acid-base pair interaction between the amino groups of UNCS and the sulfonic acid groups of SPEEK, the dimensional stability and mechanical properties of the composite membranes were enhanced. More interestingly, STEM-HAADF and SAXS characterization consistently revealed that UNCS served as bridges among proton channels in the composite membranes for continuous proton transport.

Analysis of Ionic Domain Evolution on a Nafion-Sulfonated Silica Composite Membrane Using a Numerical Approximation Model Based on Electrostatic Force Microscopy.

It is important to characterize the proton transport mechanisms of proton exchange membranes (PEMs). Electrostatic force microscopy (EFM) is used to characterize the ionic structures of membranes. In this study, we attempted to quantitatively analyze the proton conductivity enhancement of Nafion-sulfonated silica (SSA) composite membranes with variations in the ionic channel distribution. This study involved several steps. The morphology and surface charge distribution of both membranes were measured using EFM. The measured data were analyzed using a numerical approximation model (NAM) that was capable of providing the magnitude and classification of the surface charges. There were several findings of ionic channel distribution variations in Nafion-SSA. First, the mean local ionic channel density of Nafion-SSA was twice as large as that of the pristine Nafion. The local ionic channel density was non-uniform and the distribution of the ionic channel density of Nafion-SSA was 23.5 times larger than that of pristine Nafion. Second, local agglomerations due to SSA were presumed by using the NAM, appearing in approximately 10% of the scanned area. These findings are meaningful in characterizing the proton conductivity of PEMs and imply that the NAM is a suitable tool for the quantitative assessment of PEMs.

Enhancing proton conductivity of proton exchange membranes via anchoring imidazole‐loaded MIL‐101‐NH 2 onto sulfonated poly (arylene ether ketone sulfone) by chemical bonding

Enhancing proton conductivity of proton exchange membranes via anchoring imidazole‐loaded <scp> MIL‐101‐NH ₂ </scp> onto sulfonated poly (arylene ether ketone sulfone) by chemical bonding

Constructing High-Density Hydrogen Bonding Networks via Introducing the Bipyridine Group for High-Performance Fuel Cell Proton Exchange Membranes

Constructing high-density hydrogen bonding networks is crucial to improve the proton conductivity of proton exchange membranes (PEMs) and the single-cell output power of high-temperature fuel cells (HTFCs). In this work, a series of benzimidazole polymers containing a pyridine group in the backbone are successfully synthesized via copolymerization. The high-density hydrogen network is constructed via blending the polyether polybenzimidazole (OPBI) with the bipyridine polybenzimidazole copolymer, and the 1,3,5-triglycidyl isocyanurate that contains nitrogen atoms and hydroxyl groups is used as a cross-linking agent. As a result, the proton conductivity and the output power density of the single cell are significantly enhanced by the high-density hydrogen bonding network. The single-cell performance of 693 mW cm–2 is achieved in the cross-linked OPBI/copolymer blend membranes containing pyridine group under a saturated phosphoric acid (PA) adsorption (284%). Even under the low PA uptake (178%), the proton conductivity (0.050 S cm–1) is 2.1 times that of the OPBI membrane (0.024 S cm–1), and the output power density of the single-cell performance (501 mW cm–2) is 1.4 times that of the OPBI membrane (358 mW cm–2). The results demonstrate that introducing nitrogen sites into polybenzimidazole cross-linked membranes is an effective strategy for preparing high-performance fuel cell PEMs.

A comprehensive review on the proton conductivity of proton exchange membranes (PEMs) under anhydrous conditions: Proton conductivity upper bound

The present study aims to assess the proton conductivities of the most investigated proton exchange membranes (PEMs) used in PEM fuel cells (PEMFCs). Specifically, PEMs are analyzed for their use in anhydrous fuel cells and proton conductivity upper bounds were provided for them. Considering the direct relationship between proton conductivity and temperature, an upper bound is presented. Based on the obtained upper bounds, suitable membranes for high-temperature performance are determined, and the average range of proton conductivity for each polymer group is discussed. By comparing the available proton conductivity data with upper bound, it was demonstrated that some of poly (ionic liquid)s have provided the highest proton conductivities, however aromatic polymers such as polybenzimidazole (PBI) are found more suitable choices for application at anhydrous conditions and high temperatures. The proton conductivity upper bound for anhydrous PEMs demonstrates the availability of promising polymer options for the deployment of anhydrous fuel cells.

Enhanced proton conductivity of Nafion membrane with electrically aligned sulfonated graphene nanoplates

To increase the conductivity of proton exchange membranes in the membrane thickness direction, a novel mixed matrix membrane of Nafion ionomer and sulfonated graphene nanoplates (sGNP) with aligned proton channels is prepared with the assistance of an electric field. A high voltage alternative electric field is applied to a casting solution consisting of Nafion ionomer, sGNP, N, N-dimethylformamide (DMF), and p-xylene (PX) while evaporating the solvents. The sGNP, naturally attracting the ionic groups of Nafion ionomer to their vicinity, is polarized and rotated under the electric field, leaving aligned proton channels in the solidified membrane. The trans-plane proton conductivity of the mixed matrix membrane can reach 0.155 S cm−1 at 80 °C and 100% RH, an increase of 48% as compared with the conventional cast Nafion membrane. Accordingly, the peak power output of H2/O2 fuel cell with the mixed matrix membrane increases by 15%, reaching 440 mW cm−2 at 80 °C.

Synthesis of 2,2′-hindered pyridine containing semifluorinated polytriazoles and investigation for low-temperature proton exchange membrane application with enhanced oxidative stability

A series of new sulfonated copolytriazoles were prepared from a novel fluorinated diazide monomer with pyridine moiety to enhance both the oxidative stability and proton conductivity of proton exchange membranes. These proton donor–acceptor based copolymers and homopolymers were synthesized by click cycloaddition polymerization of the dialkyne monomer 1,2,4,5-tetrafluoro-3,6-bis(prop-2-yn-1-yloxy)benzene (TFAK) with or without the pyridine containing diazide monomer 2,2′-bis(4-(4-azidophenoxy)-3-(trifluoromethyl)phenyl)pyridine (PYFAZ) and the sulfonated diazide monomer 4,4′-diazido-2,2′-stilbenedisulfonic acid disodium salt (DSSAZ). 1H, 13C, and 19F NMR, in addition to FTIR analyses, confirm the formation of polytriazoles with a graded degree of sulfonation between 70% and 100%. The solution cast membranes were flexible and showed high chemical, thermal and mechanical properties. Inclusion of pyridine moiety proved to play a significant role in increasing peroxide stability by quenching hydroxyl and hydroperoxide radicals and forming pyridine-N-oxide, which was analyzed by EPR and FTIR spectroscopy. The presence of pyridine moiety also enhanced proton conductivity owing to the presence of the weakly basic nitrogen in pyridine that acts as a proton hopping site. AFM and HR-TEM images of the membranes indicated the formation of phase-separated morphology and that became more pronounced with the increase in the degree of sulfonation. The membranes obtained from PYFTSH-90 copolymer exhibited proton conductivity of 125 mS cm−1 at 80 °C, which is comparable to Nafion-117. The proton conductivity of PYFTSH-90 was further increased to 139 mS cm−1 at 90 °C. Thus, three significant objectives have been fulfilled in this work, namely (i) high proton conductivity, (ii) high hydrolytic stability, and (iii) high peroxide stability. These properties demonstrate that these copolymer membranes are potentially highly suitable for application in low-temperature proton exchange membrane fuel cell (PEMFC).

Side Chain Engineering of Sulfonated Poly(arylene ether)s for Proton Exchange Membranes

Proton conductivity of proton exchange membranes (PEMs) strongly relies on microscopic morphology, which can be modulated by engineering the distribution of ionic groups. Herein, poly(arylene ether)s with densely distributed allyl functionalities are polymerized from a tetra-allyl bisphenol A monomer. The subsequent thiol-ene addition with sodium 3-mercapto-1-propanesulfonate yields comb-shaped sulfonated fluorinated poly(arylene ether)s (SFPAEs) with ion exchange capacities (IECs) ranging from 1.29 mmol·g−1 to 1.78 mmol·g−1. These SFPAEs exhibit superior proton conductivity over the whole temperature range, which is attributed to the enhanced hydrophilic/hydrophobic phase separation as evidenced by small angle X-ray scattering characterizations. The SFPAE-4-40 with an IEC of 1.78 mmol·g−1 shows the largest proton conductivity of 93 mS·cm−1 at room temperature under fully hydrated condition, higher than that of Nafion 212. Furthermore, the vanadium redox flow battery (VRFB) assembled with SFPAE-4-40 separator exhibits higher energy efficiency than the VRFB assembled with Nafion 212.

Sandwiching h-BN Monolayer Films between Sulfonated Poly(ether ether ketone) and Nafion for Proton Exchange Membranes with Improved Ion Selectivity.

Two-dimensional (2D) hexagonal boron nitride (h-BN) has attracted great interest due to its excellent chemical and thermal stability, electrical insulating property, high proton conductivity, and good flexibility. Integration of 2D h-BN into commercial proton exchange membranes (PEMs) has the potential to improve ion selectivity while maintaining the proton conductivity of PEMs simultaneously, which has been a longstanding challenge in membrane separation technology. Until now, such attempts are only limited in mechanically exfoliated small area h-BN and in proof-of-concept devices, due to the difficulty of growing and transferring large area uniform h-BN monolayers. Here, we develop a space-confined chemical vapor deposition approach and achieve the growth of wafer-scale uniform h-BN monolayer films on Cu rolls. We further develop a Nafion functional layer assisted transfer method which effectively transfers as-grown h-BN monolayer films from the Cu roll to sulfonated poly(ether ether ketone) (SPEEK) membrane. The as-fabricated Nafion/h-BN/SPEEK sandwich structure is used as the membrane and compared with the pure SPEEK membrane for flow batteries. Results show that the sandwich membrane exhibits ion selectivity 3-fold greater than that of a pure SPEEK membrane ( i.e., 32.1 × 104 vs 9.7 × 104 S min cm-3). In addition, we fabricate vanadium flow batteries using the Nafion/h-BN/SPEEK sandwich membrane and find that the sandwich structure does not affect the proton transport but inhibits vanadium crossover at low current density (<120 mA cm-2) due to the selective blocking of vanadium ions by 2D h-BN. As a result, the sandwich membrane exhibits a significantly improved Coulombic efficiency and energy efficiency, ∼95% and ∼91%, respectively. Our results suggest that a functional layer/2D film/target substrate-based sandwich structure shows clear potential for future 2D material-based membranes in separation technologies.

Enhanced Proton Conductivity in Sulfonated Poly(ether ether ketone) Membranes by Incorporating Sodium Dodecyl Benzene Sulfonate.

It is of great importance to improve the proton conductivity of proton exchange membranes by easy-handling and cost-efficient approaches. In this work, we incorporated a commercially obtained surfactant, sodium dodecyl benzene sulfonate (SDBS), into sulfonated poly(ether ether ketone) (SPEEK) through solution casting to prepare SPEEK/SDBS membranes. When no more than 10 wt % SDBS was added, the SDBS was well dissolved into the SPEEK matrix, and the activation energy for the proton transfer in the SPEEK/SDBS membranes was greatly reduced, leading to significant enhancement of the membrane proton conductivity. Compared with the SPEEK control membrane, the SPEEK/SDBS membrane with 10 wt % SDBS showed a 78% increase in proton conductivity, up from 0.051 S cm−1 to 0.091 S cm−1, while the water uptake increased from 38% to 62%. Moreover, the SPEEK/SDBS membrane exhibited constant proton conductivity under a long-term water immersion test.

High Performance Ion Exchange Membranes Prepared via Direct Polyacylation of Racemic and (S)‐1,1′‐Binaphthyl‐Based Cationic/Anionic Monomers

AbstractSide‐chain‐type sulfonated/quaternized aromatic polyelectrolytes with precisely controlled contents of ionized groups are successfully synthesized via direct polyacylation of sulfonated/quaternized monomers based on 2,2′‐dihydroxy‐1,1′‐binaphthyl (DHBN). Both proton exchange membranes (PEMs) and anion exchange membranes (AEMs) of the corresponding polyelectrolytes exhibit outstanding properties. Proton conductivity (116 mS cm−1 at 30 °C) higher than Nafion 115 for the PEMs and OH– conductivity (28.5–53.7 mS cm−1 at 30 °C) comparable to Tokuyama A901 for the AEMs are accomplished. In addition, the AEMs can withstand 60 days’ aging in 1 mol L−1 NaOH at 60 °C without degradation, as proved by 1H NMR. More intriguingly, when starting from optically active (S)‐DHBN instead of racemic DHBN, an enhancement in proton conductivity of PEMs is observed for the first time, which opens a new door to optically active ion exchange membranes.

Improving the proton conductivity of proton exchange membranes via incorporation of HPW-functionalized mesoporous silica nanospheres into SPEEK

A highly stable composite proton exchange membrane (PEM) was developed by loading phosphotungstic acid in mesoporous silica nanospheres (HPW@MSNs) and blending with sulfonated poly (ether ether ketone) (SPEEK). The SPEEK/HPW@MSNs-0.5 membrane exhibits enhanced comprehensive performance, such as improved and stable proton conductivity and increased methanol barrier property. The proton conductivity decreased by 15.10% after 240 h at 60 °C and was 1.9 times lower than that of the SPEEK/HPW membrane. The selectivity of the SPEEK/HPW@MSNs-0.5 membrane was about 2.0 times that of the pure SPEEK membrane and 3.4 times that of the SPEEK/HPW membrane.

Cellulose nanofiber-embedded sulfonated poly (ether sulfone) membranes for proton exchange membrane fuel cells

Cellulose nanofibers were embedded into sulfonated poly (ether sulfone) matrix to heighten the water retention and proton conductivity of proton exchange membranes (PEMs). Cellulose nanofibers were obtained by hydrolyzing cellulose acetate nanofibers, which were prepared via electrostatic-induction-assisted solution blow spinning. Morphology, thermal stability, and mechanical properties of the PEMs were investigated. The results showed that proton conductivity, water uptake, and methanol permeability of the composite membranes were improved. Hydrophilicity of the composite membranes was gradually improved with the addition of nanofibers. When the content of nanofibers was 5 wt%, the highest proton conductivity was 0.13 S/cm (80 °C, 100% RH). Therefore, the cellulose nanofiber could be used as support materials to enhance the performance of proton exchange membranes, the composite membranes have potential application in Direct methanol fuel cells (DMFCs).

Preparation and properties of ion exchange membranes for PEMFC with sulfonic and carboxylic acid groups based on polynorbornenes

Bifunctional groups proton exchange membranes (PEMs) with sulfonic and carboxylic acid groups were prepared via ring-opening metathesis polymerization (ROMP) based on norbornene derivatives. The sulfonic acid groups and the carboxylic acid groups were introduced into the polymer chains in the form of sulfonyl chloride and ester separately. Because of the introduction of these two groups, not only greatly improve the proton conductivity of PEMs, but also improve their mechanical performance and thermal stability. When the highest proton conductivity of the bifunctional groups PEMs is 123.96 mS/cm, its proton conductivity is higher than that of Nafion® 117 at 25 °C.

Effect of glycidyl methacrylate (GMA) incorporation on water uptake and conductivity of proton exchange membranes

The aim of this work was to investigate how hygroscopic moieties like hydrolyzed glycidyl methacrylate (GMA) influence the properties of sulfonated polysytrene based proton exchange membranes (PEM). Therefore, several membranes were synthesized by electron beam treatment of the ETFE (ethylene-alt-tetrafluoroethylene) base film with a subsequent co-grafting of styrene and GMA at different ratios. The obtained membranes were sulfonated to introduce proton conducting groups and the epoxide moiety of the GMA unit was hydrolyzed for a better water absorption. The PEM was investigated regarding its structural composition, water uptake and through-plane conductivity. It could be shown that the density of sulfonic acid groups has a higher influence on the proton conductivity of the PEM than an increased water uptake.

Engineering mesoporosity promoting high-performance polymer electrolyte fuel cells

Proton exchange membranes (PEMs) are a vital component in fuel cells (FCs) that attract significant research interest for the present hydrogen energy use. High proton conductivity of PEMs under various operation conditions highly influences the integrated performance of FCs that determines their commercial applications. Hence mesoporous superacidic sulfated zirconia (S-ZrO2) is fabricated and introduced into Nafion matrix to construct hybrid PEMs. The mesoporosity of S-ZrO2 is demonstrated highly controllable. High mesoporosity leads to increased amount of sulfonic groups (SO3H) aggregating on S-ZrO2 surface. When introduced in PEMs, the highly mesoporous S-ZrO2 chemically enhances the amount of proton-containing groups, structurally improves the density of ion channels, and reserves water as effective reservoirs, which resultantly maintains high proton conductivity under variable conditions, and thus the performance of assembled FCs. The S-ZrO2 exhibits the highest surface area of 181 m2 g−1. The hybrid PEMs loaded with 10 wt% such S-ZrO2 achieve a highest proton conductivity of 0.83 S cm−1 that is ∼7 time of that for pristine Nafion® membranes. The power density at 0.6 V of FCs with the hybrid PEMs is 786 mW cm−2, much higher than that for commercial Nafion 211.

Proton Conductivity of Proton Exchange Membrane Synergistically Promoted by Different Functionalized Metal-Organic Frameworks.

In this study, two functionalized metal-organic frameworks (MOFs), UiO-66-SO3H and UiO-66-NH2, were synthesized. Then, different composite proton exchange membranes (PEMs) were prepared by single doping and codoping of these two MOFs, respectively. It was found that codoping of these two MOFs with suitable sizes was more conducive to the proton conductivity enhancement of the composite PEM. A synergistic effect between these two MOFs led to the the formation of more consecutive hydration channels in the composite PEM. It further greatly promoted the proton conductivity of the composite PEM. The proton conductivity of the codoped PEM reached up to 0.256 S/cm under 90 °C, 95% RH, which was ∼1.17 times higher than that of the recast Nafion (0.118 S/cm). Besides, the methanol permeability of the codoped PEM was prominently decreased owing to the methanol trapping effect of the pores of these two MOFs. Meanwhile, the high water and thermal stabilities of these two MOFs were beneficial to the high proton conductivity stability of the codoped PEM under high humidity and high temperature. The proton conductivity of the codoped PEM was almost unchanged throughout 3000 min of testing under 90 °C, 95% RH. This work provides a valuable reference for designing different functionalized MOFs to synergistically promote the proton conductivities of PEMs.

Modeling and optimization of sulfonated poly (ether ether ketone) nanofibrous proton exchange membranes with response surface methodology

The morphological control of proton conductive nanofibrous mat used in proton exchange membranes (PEMs) is a key issue to improve proton conductivity of PEMs. In this study, response surface methodology (RSM) was utilized to investigate the morphological characterizations of electrospun sulfonated poly (ether ether ketone) (SPEEK) nanofibrous membranes. The effects of electrospinning parameters namely solution concentration, applied voltage and tip to collector distance were studied on the average diameter, its standard deviation and the degree of alignment of SPEEK nanofibers. The adequacy of the response surface models was verified by the validation experiments. Optimized conditions were obtained to meet the desired status of collecting more uniform and finer nanofibers with improved alignment. After obtaining the significant factors and optimal test level, an additional optimization was conducted at different take‐up speeds to fabricate high‐grade aligned nanofibers. The resultant nanofibers had the diameter of 107 ± 22 nm at the optimum point which was in good agreement with the predicted response values. The highest degree of alignment was yielded at the take‐up speed of 8.8 m/s with the maximum second‐order orientation parameter of 0.869. Electrochemical spectroscopy analysis showed that the aligning of proton conductive nanofibers improved in‐plane proton conductivity about 67%. POLYM. ENG. SCI., 2017. © 2017 Society of Plastics Engineers