Amino acid-functionalized metal organic framework with excellent proton conductivity for proton exchange membranes

Proton conduction is a ubiquitous fundamental phenomenon in biological systems. Biology has solved the problem of energy-related proton conduction, in which the exposed amino acids in the protein ordered channels play a key role in proton conduction. In this contribution, amino acids (glutamate, lysine, and threonine) were attached to metal organic framework (MOF) nanoparticles, which were incorporated into sulfonated polysulfone matrix forming bio-proton channels for the nanocomposite membranes. It is shown in the results that the amino acid-functionalized MOF can help to enhance the proton conductivity and the proton exchange membrane with glutamate-functionalized MOF demonstrate excellent proton conductivity of 0.212 S cm^?1 at 80 °C and 100% relative humidity. Meanwhile, the methanol permeability coefficient of the nanocomposite membrane is reduced to 5.8 × 10^?7 cm² s^?1. The nanocomposite membranes demonstrate excellent proton conductivity, dimensional stabilities, water absorption, and resistance to methanol performance. Therefore, this material is promising for fuel cells.     

Enhancement of proton conductivity of polymer electrolyte membrane enabled by sulfonated nanotubes

Proton exchange membrane (PEM) with high proton conductivity is crucial to the commercial application of PEM fuel cell. Herein, sulfonated halloysite nanotubes (SHNTs) with tunable sulfonic acid group loading were synthesized and incorporated into sulfonated poly(ether ether ketone) (SPEEK) matrix to prepare nanocomposite membranes. Physicochemical characterization suggests that the well-dispersed SHNTs enhance the thermal and mechanical stabilities of nanocomposite membranes. The results of water uptake, ionic exchange capacity, and proton conductivity corroborate that the embedded SHNTs interconnect the ionic channels in SPEEK matrix and donate more continuous ionic networks. These networks then serve as proton pathways and allow efficient proton transfer with low resistance, affording enhanced proton conductivity. Particularly, incorporating 10% SHNTs affords the membrane a 61% increase in conductivity from 0.0152 to 0.0245 S cm⁻¹. This study may provide new insights into the structure-properties relationships of nanotube-embedded conducting membranes for PEM fuel cell.     

Double cross-linked polyetheretherketone proton exchange membrane for fuel cell

The proton exchange membrane based on polyetheretherketone was prepared via two steps of cross-linking. The properties of the double cross-linked membrane (water uptake, proton conductivity, methanol permeability and thermal stability) have been investigated for fuel cell applications. The prepared membrane exhibited relatively high proton conductivity, 3.2 × 10⁻² S cm⁻¹ at room temperature and 5.8 × 10⁻² S cm⁻¹ at 80 °C. The second cross-linking significantly decreased the water uptake of the membrane. The performance of direct methanol fuel cell was slightly improved as compared to Nafion^® 117 due to its low methanol permeability. The results indicated that the double cross-linked membrane is a promising candidate for the polymer electrolyte membrane fuel cell, especially for the direct methanol fuel cell due to its low methanol permeability and high stability in a methanol solution.     

An enhanced proton conductivity and reduced methanol permeability composite membrane prepared by sulfonated covalent organic nanosheets/Nafion

Sulfonated covalent organic nanosheets (SCONs) with a functional group (−SO₃H) are effective at reducing ion channels length and facilitating proton diffusion, indicating the potential advantage of SCONs in application for proton exchange membranes (PEMs). In this study, Nafion-SCONs composite membranes were prepared by introducing SCONs into a Nafion membrane. The incorporation of SCONs not only improved proton conductivity, but also suppressed methanol permeability. This was due to the even distribution of ion channels, formed by strong electrostatic interaction between the well dispersed SCONs and Nafion polymer molecules. Notably, Nafion-SCONs-0.6 was the best choice of composite membranes. It exhibited enhanced performance, such as high conductivity and low methanol permeability. The direct methanol fuel cell (DMFC) with Nafion-SCONs-0.6 membrane also showed higher power density (118.2 mW cm⁻²), which was 44% higher than the cell comprised of Nafion membrane (81.9 mW cm⁻²) in 2 M methanol at 60 °C. These results enabled us to work on building composite membranes with enhanced properties, made from nanomaterials and polymer molecules.     

Constructing interconnected nanofibers@ZIF-67 superstructure in nafion membranes for accelerating proton transport and confining methanol permeation

A high-performance proton exchange membrane was successfully prepared by incorporating an interconnected PMIA nanofiber@ZIF-8 network (ZHNFs) into Nafion solution, in which the unique ZHNFs were fabricated via in-situ growth of ZIF-67 on the surface of hierarchical PMIA nanofibers (HNFs) with multiscale nanofibers and high hydrophilicity. The hybrid membrane presents an outstanding performance with a peak proton conductivity of 0.277 S cm⁻¹ at 80 °C, 100RH% and a decreased methanol permeability of 1.415 × 10⁻⁷ cm² s⁻¹, implying a promising application in direct methanol fuel cells. The superior performance of the membranes could be due to the interconnected structure, high specific surface area of 122.637 m² g⁻¹ and active chemical bond of (−N–H) of ZHNFs. Specifically, the 3D interconnected structure of ZHNFs provides consecutive conduction path for protons, ensuring the improvement of proton conductivity. The effective interfacial acid-base pairs (−N–H ^… −SO₃H) formed via the tight interactions between the –N–H bonds in ZHNFs and –SO₃H groups in Nafion matrix could effectively ameliorate the compatibility of the nanofiber fillers and Nafion matrix, further promoting the methanol barrier of the hybrid membranes. Moreover, the generated acid-base pairs are beneficial for the efficient and rapid proton transfer via providing abundant proton conducting sites.     

Self-assembly of metal-organic framework onto nanofibrous mats to enhance proton conductivity for proton exchange membrane

Constructing consecutive proton-conducting nanochannels and optimizing nanophase-separation within proton exchange membrane (PEM) was of guiding significance for improving proton transfer. Metal organic framework (MOF), as a novel and functional material had drawn increasing attention in the research of proton PEM because of its flexible tunability and designability. Herein, a novel MOF-based nanofibrous mats (NFMs) were prepared by the self-assembly of zeolitic imidazole framework-67 (ZIF-67) onto polyacrylonitrile (PAN) NFMs. Subsequently, the ZIF-67 NFMs were incorporated into Nafion matrix to prepare ZIF-67@Nafion composite membrane which aimed at constructing consecutive proton-conducting channels. Especially, the acid–base pairs between N–H (ZIF-67 NFMs) and –SO₃H (Nafion) could promote the protonation/deprotonation and subsequent proton leaping via Grotthuss mechanisms. As expected, the ZIF-67@Nafion-5 composite membrane showed a promising proton conductivity of 288 mS/cm at 80 °C and 100% RH, low methanol permeability of 7.98 × 10⁻⁷ cm²s⁻¹, and superior power density of 298.68 mW/cm² at 80 °C and 100% RH. In addition, the resulting composite membrane exhibited considerable enhancement in thermal stability and dimensional stability. This promising strategy provided a valuable reference for designing high-performance PEMs.     

UiO-66-NH2 functionalized cellulose nanofibers embedded in sulfonated polysulfone as proton exchange membrane

Metal–organic frameworks (MOFs) exhibit high proton conductivity, thermal stability, and offer immense flexibility in terms of tailoring their size. Owing to their unique characteristics, they are desirable candidates for proton conductors. Nevertheless, constructing ordered MOF proton channels in proton exchange membranes (PEMs) remains a formidable challenge. Herein, blend nanofibers of cellulose and UiO-66-NH₂ (Cell–UiO-66-NH₂) obtained via the electrospinning process were embedded in a sulfonated polysulfone matrix to obtain high-performance composite PEMs with an orderly arrangement of UiO-66-NH₂. Comprehensive characterization and membrane performance tests reveal that composite membrane with 5 wt% (nominal) UiO-66-NH₂ have revealed high proton conductivity of 0.196 S cm^?1 at 80 °C and 100% relative humidity. Meantime, the composite membrane exhibits a low methanol permeability coefficient (~5.5 × 10^?7 cm² s^?1). Moreover, the composite membrane exhibits a low swelling ratio (17.3%) even at 80 °C. The Cell–UiO-66-NH₂ nanofibers exhibit strong potential for use as a proton-conducting nanofiller in fuel-cell PEMs.     

Pore size tuning of Nafion membranes by UV irradiation for enhanced proton conductivity for fuel cell applications

The influence of optimal ultraviolet irradiation of Nafion membranes in enhancing proton conductivity and performance of passive micro-direct methanol fuel cells with silicon micro-flow channels is investigated for the first time. Initially, Nafion membranes are irradiated with different doses of ultraviolet radiation ranging within 0–400 mJ cm⁻² and their water uptake, swelling-ratios, porosity, and proton conductivities are measured using standard procedure. Results show that there is an enhancement in proton conductivity with an optimal dose of 198 mJ cm⁻² ultraviolet radiation. This enhancement is due to optimum photo-crosslinking of –SO₃H species resulting in maximum pore-size which facilitates enhanced proton-hopping from one –SO₃H site to another in the hydrophilic channel. Nafion membranes with three different thicknesses (50 μm, 90 μm and 183 μm) are irradiated with ultraviolet radiation with 198 mJ cm⁻² dose and passive micro-direct methanol fuel cells are assembled with irradiated Nafion proton exchange membranes. The polarization plots are obtained for the assembled devices. Results show an enhancement of power density of devices nearly by a factor of 1.2–1.5 with optimally irradiated membranes indicating that optimum dose of ultraviolet irradiation of Nafion membranes is an effective technique for power enhancement of proton exchange membrane fuel cells which use fuels like methanol, ethanol and hydrogen.     

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.     

Increases in the proton conductivity and selectivity of proton exchange membranes for direct methanol fuel cells by formation of nanocomposites having proton conducting channels

We explore an approach to effectively enhance the properties of cost-effective hydrocarbon proton-exchange membranes for application in the direct methanol fuel cell (DMFC). This approach utilizes sulfonated silica nanoparticles (SA-SNP) as additives to modify sulfonated poly(arylene ether ether ketone ketone) (SPAEEKK). The interaction between the sulfonic acid groups of SA-SNP and those of SPAEEKK combined with hydrophilic-hydrophobic phase separation induce the formation of proton conducting channels, as evidenced by TEM images, which contribute to increases in the proton conductivity of the SPAEEKK/SA-SNP nanocomposite membrane. The presence of SA-SNP nanoparticles also reduces methanol crossover in the membrane. Therefore, the SPAEEKK/SA-SNP nanocomposite membrane shows a high selectivity, which is 2.79-fold the selectivity of Nafion^®117. The improved selectivity of the SPAEEKK/SNP nanocomposite membrane demonstrates potential of this approach in providing hydrocarbon-based PEMs as alternatives to Nafion in direct methanol fuel cells.     

Construction of a new continuous proton transport channel through a covalent crosslinking reaction between carboxyl and amino groups

Sulfonated poly(arylene ether ketone sulfone) bearing pendant carboxylic acid groups (C-SPAEKS) and sulfonated poly(arylene ether ketone sulfone) containing amino groups (Am-SPAEKS) were used to prepare C-SPAEKS/Am-SPAEKS crosslinked membranes. ¹H NMR and Fourier transform infrared spectra proved that C-SPAEKS and Am-SPAEKS copolymers, as well as C-SPAEKS/Am-SPAEKS crosslinked membrane, were successfully synthesized. TEM images showed that a continuous proton transport channel formed after crosslinking. Thermogravimetric analysis curves demonstrated that the thermal property of the crosslinked membranes improved. The crosslinked membranes exhibited suitable mechanical properties at 25 and 80 °C. The methanol permeability of C-SPAEKS/Am-SPAEKS-40 was 2.35 × 10⁻⁷ cm² s⁻¹ at 60 °C, which was lower than that of C-SPAEKS (24.12 × 10⁻⁷ cm² s⁻¹) and Am-SPAEKS (17.91 × 10⁻⁷ cm² s⁻¹). The proton conductivity of C-SPAEKS/Am-SPAEKS-40 was 0.089 S cm⁻¹, which was higher than that of C-SPAEKS and Am-SPAEKS at 80 °C. The results proved that C-SPAEKS/Am-SPAEKS crosslinked membranes were potential proton exchange membranes for direct methanol fuel cell applications.     

Transport properties and fuel cell performance of sulfonated poly(imide) proton exchange membranes

Selective sulfonated poly(imide)s with high proton conductivity and low methanol permeability were tested for their performance as proton exchange membranes in direct methanol fuel cells (DMFC). The proton to methanol transport selectivity of the poly(imide) membranes correlated well with the self-diffusion coefficients of water in the membranes as determined by pulsed-field gradient nuclear magnetic resonance. The poly(imide) membranes showed improved fuel cell device performance, however high interfacial resistance between the membranes and electrodes decreased the membrane electrode assembly (MEA) conductivity to methanol crossover selectivity, likely due to the use of NAFION^®-based electrodes. The maximum power densities of SPI-50, SPI-75, and NR-212 based MEAs were 75, 72, and 67 mW cm⁻², respectively, with a methanol feed concentration of 2 M at a cell temperature of 60 °C.     

Enhanced proton conductivities of chitosan-based membranes by inorganic solid superacid SO42−–TiO2 coated carbon nanotubes

To increase proton conductivity of chitosan (CS) based polymer electrolyte membranes, a novel nanofiller-solid superacide SO₄^2--TiO₂ (STi) coated carbon nanotubes (STi@CNTs) are introduced into CS matrix to fabricate membranes for polymer electrolyte membrane fuel cells (PEMFCs). Owing to the STi coating, the dispersion ability of CNTs and interfacial bonding are obviously improved, hence, CNTs can more fully play their reinforcing role, which makes the CS/STi@CNTs composite membranes exhibit better mechanical properties than that of pure CS membrane. More importantly, STi possesses excellent proton transport ability and may create facile proton transport channels in the membranes with the help of high aspect ratio of CNTs. Particularly, the CS/STi@CNTs-1 membrane (1 wt% STi@CNTs loading) obtains the highest proton conductivity of 4.2 × 10⁻² S cm^–1 at 80 °C, enhancing by 80% when compared with that of pure CS membrane. In addition, the STi@CNTs also confer the composite membranes low methanol crossover and outstanding cell performance. The maximum power density of the CS/STi@CNTs-1 membrane is 60.7 mW cm⁻² (5 M methanol concentration, 70 °C), while pure CS membrane produces the peak power density of only 39.8 mW cm⁻².     

Fabrication of novel proton exchange membranes for DMFC via UV curing

The radiation hardening of various UV curable resins provides a simple but powerful method to fabricate thin films or membranes with desirable physical and chemical properties. In this study, we proposed to use this method to fabricate a novel proton exchange membrane (PEM) for direct methanol fuel cells (DMFC) with good mechanical, transport and stability properties. The PEM was prepared by crosslinking a mixture of a photoinitiator, a bifunctional aliphatic urethane acrylate resin (UAR), a trifunctional triallyl isocyanate (TAIC) crosslinker and tertrabutylammonium styrenesulfonate (SSTBA) to form a uniform network structure for proton transport. Key PEM parameters such as ion exchange capacity (IEC), water uptake, proton conductivity, and methanol permeability were controlled by adjusting the chemical composition of the membranes. The IEC value of the membrane was found to be an important parameter in affecting water uptake, conductivity as well as the permeability of the resulting membrane. Plots of the water uptake, conductivity, and methanol permeability vs. IEC of the membranes show a distinct change in the slope of their curves at roughly the same IEC value which suggests a transition of structural changes in the network. It is demonstrated that below the critical IEC value, the membrane exhibits a closed structure where hydrophilic segments form isolated domains while above the critical IEC value, it shows an open structure where hydrophilic segments are interconnected and form channels in the membrane. The transition from a closed to an open proton conduction network was verified by the measurement of the activation energy of membrane conductivity. The activation energy in the closed structure regime was found to be around 16.5 kJ mol⁻¹ which is higher than that of the open structure region of 9.6 kJ mol⁻¹. The membranes also display an excellent oxidative stability, which suggests a good lifetime usage of the membranes. The proton conductivities and the methanol permeabilities of all membranes are in the range of 10⁻⁴ to 10⁻² S cm⁻¹ and 10⁻⁸ to 10⁻⁷ cm² s⁻¹, respectively, depending on their crosslinking density. The membranes show great selectivity compared with those of Nafion^®. The possibility of using this PEM for DMFC devices is suggested.     

Nafion/Pd–SiO2 nanofiber composite membranes for direct methanol fuel cell applications

One of the major challenges for direct methanol fuel cells is the problem of methanol crossover. With the aim of solving this problem without adverse effects on the membrane conductivity, Nafion/Palladium–silica nanofiber (N/Pd–SiO₂) composite membranes with various fiber loadings were prepared by a solution casting method. The silica-supported palladium nanofibers had diameters ranging from 100 nm to 200 nm and were synthesized by a facile electro-spinning method. The thermal properties, ionic exchange capacities, water uptake, proton conductivities, methanol permeabilities, chemical structures, and micro-structural morphologies were determined for the prepared membranes. It was found that the transport properties of the membranes were affected by the fiber loading. All of the composite membranes showed higher water uptake and ion exchange capacities compared to commercial Nafion 117 and proved to be thermally stable for use as proton exchange membranes. The composite membranes with optimum fiber content (3 wt%) showed an improved proton conductivity of 0.1292 S cm⁻¹ and a reduced methanol permeability of 8.36 × 10⁻⁷ cm² s⁻¹. In single cell tests, it was observed that, the maximum power density measured with composite membrane is higher than those of commercial Nafion 117.     

Covalent-ionically cross-linked polyetheretherketone proton exchange membrane for direct methanol fuel cell

In this paper, the proton exchange membrane prepared by covalent-ionically cross-linking water soluble sulfonated-sulfinated poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phenylene-oxy-phenylene) (SsPEEK-WC) is reported. Compared with covalent cross-linked PEEK-WC membrane, this covalent-ionically cross-linked PEEK-WC membrane exhibits extremely reduced water uptake and methanol permeability, but just slightly sacrificed proton conductivity. The proton conductivity of the covalent-ionically cross-linked PEEK-WC membrane reaches to 2.1 × 10⁻² S cm⁻¹ at room temperature and 4.1 × 10⁻² S cm⁻¹ at 80 °C. The methanol permeability is 1.3 × 10⁻⁷ cm² s⁻¹, 10 times lower than that of Nafion^® 117 membrane. The results suggest that the covalent-ionically cross-linked PEEK-WC membrane is a promising candidate for direct methanol fuel cell because of low methanol permeability and adequate proton conductivity.     

Cross-linked proton exchange membranes for direct methanol fuel cells: Effects of the cross-linker structure on the performances

A highly sulfonated poly (ether ether ketone) with pendant amino groups (Am-SPEEK) has been synthesized. The resulting copolymer showed good solubility in common organic solvents. Then the pendant amino groups of the Am-SPEEK were utilized to react with two kinds of cross-linker, epoxy resin and dibromide, to yield various cross-linked membranes. After cross-linking, the membranes could not dissolve in common solvents, just became swollen. In generally, all the cross-linked membranes showed improved mechanical properties and high dimensional stabilities, whereas the uncross-linked membranes highly swollen or even dissolved in water at high temperature. The proton conductivity of the membranes increased with an increase in temperature. At 80 °C, all the cross-linked membranes showed high proton conductivity, in the range of 0.111–0.140 S cm⁻¹. Especially, the TMBP cross-linked membrane showed a proton conductivity of 0.140 S cm⁻¹, which was higher than that of Nafion 117 (0.125 S cm⁻¹). The membranes with high proton conductivity, good oxidative stability, and improved methanol residence have been successfully developed. Furthermore, the influence of different main chain of the cross-linker on the performance of the cross-linked membranes was also investigated.     

Organic-inorganic backbone with high contents of proton conducting groups: A newly designed high performance proton conductor

To prepare new high temperature organic-inorganic proton conductor for applications in proton exchange membrane fuel cells (PEMFC), 2,4,6-triphosphono-1,3,5-triazine (TPT) was synthesized and reacted with three different types of metal ions (Ce, Zr and Fe) in varied molar ratios. In each TPT molecule, three phosphonic acid groups were introduced into the triazine ring to obtain an organic compound with high content of proton conducting groups, which was then reacted with metal ions to ensure the insolubility in water aiming to avoid leaking during PEMFC operation. CeTPT(1:2) exhibited good thermal stability up to 200 °C and showed crystalline phase. MTPT exhibited high ion exchange capacity (IEC, 1.53–2.12 meq. g⁻¹). CeTPT(1:2) exhibited highest proton conductivity among all samples, which reached 0.116, 0.070 and 0.034 S cm⁻¹ at 100% relative humidity (RH), 50% RH and anhydrous conditions at 180 °C, respectively. The corresponding activation energy for proton conduction was 14.5, 16.0 and 21.5 kJ mol⁻¹ at 100% RH, 50% RH and anhydrous conditions, respectively. The mechanism for proton conduction was proposed according to the activation energy. The proton conductor can find promising applications in fuel cells, corrosion inhibition and water desalination due to its good thermal stability and high IEC.     

SPEEK/sulfonated cyclodextrin blend membranes for direct methanol fuel cell

In this paper, the blend membranes based on sulfonated poly(ether ether ketone) and sulfonated cyclodextrin as the proton conducting membranes for DMFCs usage are prepared and investigated. The incorporation of sulfonated cyclodextrin in SPEEK membranes is evaluated by the characteristic absorptions of FT-IR spectra. Thermal stability and micro-morphology of the blend membranes are determined by thermogravimetry analysis and scanning electron microscope tests. The properties of the blend membranes are investigated such as swelling behavior, methanol permeability and proton conduction as function of the fraction of sulfonated cyclodextrin. The methanol crossover could be suppressed by the incorporation of sulfonated cyclodextrin and the methanol permeability decreases when the methanol concentration increases from 2.5 M to 20 M. Proton conduction is also promoted by the introduction of sulfonated cyclodextrin and the proton conductivity increases with the increase of sulfonated cyclodextrin content. The calculated activation energy for proton conduction of the blend membranes is very low and the maximum value is 4.20 kJ mol⁻¹, which is much lower than that of Nafion 115 (9.15 kJ mol⁻¹, measured in our experiments). These data indicate that proton can transport easily through the blend membranes. The selectivity of the blend membranes, a compromise between proton conductivity and methanol permeability, is much higher than that of Nafion 115 at the sulfonated cyclodextrin content above 15 wt.%. The blend membranes with 15, 20, and 25 wt.% of sulfonated cyclodextrin are assembled in the practical DMFCs and their polarization curves with 2.5 M and 8.0 M methanol solution are determined, respectively. The membrane with 20 wt.% sulfonated cyclodextrin reaches the highest power density of 29.52 mW cm⁻² at 120 mA cm⁻² and 8.0 M methanol solution. These results suggest the potential usage of the SPEEK membranes incorporating with sulfonated cyclodextrin in DMFCs.     

Preparation and characterization of self-crosslinked organic/inorganic proton exchange membranes

A series of silicon-containing sulfonated polystyrene/acrylate (Si-sPS/A) nanoparticles are successfully synthesized via simple emulsion polymerization method. The Si-sPS/A latexes show good film-forming capability and the self-crosslinked organic/inorganic proton exchange membranes are prepared by pouring the Si-sPS/A nanoparticle latexes into glass plates and drying at 60 °C for 10 h and 120 °C for 2 h. The potential of the membranes in direct methanol fuel cells (DMFCs) is characterized preliminarily by studying their thermal stability, ion-exchange capacity, water uptake, methanol diffusion coefficient, proton conductivity and selectivity (proton conductivity/methanol diffusion coefficient). The results indicate that these membranes possess excellent thermal stability and methanol barrier due to the existence of self-crosslinked silica network. In addition, the proton conductivity of the membranes is in the range of 10⁻³-10⁻² S cm⁻¹ and all the membranes show much higher selectivity in comparison with Nafion^® 117. These results suggest that the self-crosslinked organic/inorganic proton exchange membranes are particularly promising in DMFC applications.