Radiation-grafted membranes based on polyethylene for direct methanol fuel cells

Styrene was grafted onto ultrahigh molecular weight polyethylene powder (UHMWPE) by gamma irradiation using a ⁶⁰Co source. Compression moulded films of selected pre-irradiated styrene-grafted ultrahigh molecular weight polyethylene (UHMWPE-g-PS) were post-sulfonated to the sulfonic acid derivative (UHMWPE-g-PSSA) for use as proton exchange membranes (PEMs). The sulfonation was confirmed by X-ray photoelectron spectroscopy (XPS). The melting and flow properties of UHMWPE and UHMWPE-g-PS are conducive to forming homogeneous pore-free membranes. Both the ion conductivity and methanol permeability coefficient increased with degree of grafting, but the grafted membranes showed comparable or higher ion conductivity and lower methanol permeability than Nafion^® 117 membrane. One UHMWPE-g-PS membrane was fabricated into a membrane–electrode assembly (MEA) and tested as a single cell direct methanol fuel cell (DMFC). Low membrane cost and acceptable fuel cell performance indicate that UHMWPE-g-PSSA membranes could offer an alternative approach to perfluorosulfonic acid-type membranes for DMFC.     

Sulfonated poly(arylene ether sulfone)s membranes with distinct microphase-separated morphology for PEMFCs

Copoly (arylene ether sulfone)s was employed for proton exchange membrane preparation via atom transfer radical polymerization followed by mild sulfonation, enhanced phase-separated morphology and favorable proton conductivity were achieved. The comprehensive ex-situ properties of a range of membranes with different ion exchange capacities were characterized alongside the fuel cell performances investigation. The membranes exhibit higher water uptake, which is beneficial to the proton conduction, compared to Nafion® 211 while maintaining similar swelling ratio. The prepared membranes exhibit reasonably high proton conductivity (0.16 S/cm at 85 °C) benefitting from the well-defined microstructure and high connectivity of the hydrophilic domains. Considering the comprehensive property, membrane with moderate ion exchange capacity (1.39 mmol/g) was employed to fabricate the membrane electrode assembly and peak power density of 0.65 W/cm² at 80 °C, 60% relative humidity was achieved for a H₂/O₂ fuel cell, these hydrocarbon membranes can therefore be implemented in PEMFCs.     

Sulfonated poly(ether ether ketone)-based composite membrane for polymer electrolyte membrane fuel cells

The solid proton conductor zirconium phosphate sulfophenylenphosphonate of composition Zr(HPO₄)_0.65(SPP)_1.35 where SPP denotes metasulfophenylenphosphonate was prepared in the amorphous gel form in dimethyl formamide (DMF) and characterized by ³¹P NMR. The composite membranes of SPEEK up to 50 wt.% of zirconium phosphate sulfophenylenphosphonate content were prepared by introducing the solid proton conductor from the gel. The composite membranes were characterized using FT-IR, powder X-ray diffraction, SEM, DSC/TGA. The proton conductivity of the membranes was measured under 100% relative humidity up to 70 °C. The composite membranes had better thermal stability when compared with that of SPEEK. A three-fold increase in proton conductivity at 70 °C was observed for the composite membrane with 50 wt.% of solid proton conductor. Furthermore, the conductivity results imply that a critical percentage of proton conductor is needed to establish conduction pathways in the polymer matrix.     

Ultrathin membranes formation via the layer by layer self-assembly of carbon nanotubes-based inorganics as high temperature proton exchange membranes

Layered membranes could be prepared with the layer by layer (LBL) self-assembly technique and they showed the wide application in the field of energy storage and transformation. The aim of this research is to develop high temperature proton exchange membranes (PEMs) through introducing carbon nanotube (CNT) and CNT-based inorganics into polymer systems of polyurethane (PU) and chitosan (CS). Successful formation of the ultrathin and conductive membranes through the alternate deposition of PU as polycations, CS and CNT-based inorganics as polyanions has been demonstrated by the identification of components and structural characterizations of fourier transform infrared spectra, scanning electron microscope, etc. Phosphoric acid (PA) molecules were combined with the formation of PA doped membranes while the prepared LBL membranes were immersed into PA solutions through the formed intermolecular hydrogen bonds. Besides PA conducting protons, the decreased proton conduction resistance owing to the multilayered structure could contribute to proton conductivity of PA doped membranes. Specifically, (PU/CNT-CdTe/PU/CS)₁₅₀/60%PA membranes exhibited the maximum proton conductivity of 6.82 × 10⁻² S/cm at 150 °C with an activation energy of 22.9 kJ/mol. The results revealed that CNT-based inorganics showed the potential to function as efficient proton carriers for the preparation of high temperature PEMs. Furthermore, the LBL self-assembly technique could provide a promising strategy to construct the ultrathin and layered membrane electrolytes.     

Water and proton transport properties of hexafluorinated sulfonated poly(arylenethioethersulfone) copolymers for applications to proton exchange membrane fuel cells

In the present study, we examine the water and proton transport properties of hexafluorinated sulfonated poly(arylenethioethersulfone) (6F-SPTES) copolymer membranes for applications to proton exchange membrane fuel cells (PEMFCs). The 6F-SPTES copolymer membranes build upon the structures of previously studied sulfonated poly(arylenethioethersulfone) (SPTES) copolymer membranes to include CF₃ functional groups in efforts to strengthen water retention and extend membrane performance at elevated temperatures (above 120 °C). The 6F-SPTES copolymer membranes sustain higher water self-diffusion and greater proton conductivities than the commercial Nafion^® membrane. Water diffusion studies of the 6F-SPTES copolymer membranes using the pulsed-field gradient spin-echo NMR technique reveal, however, the fluorinated membranes to be somewhat unfavorable over their non-fluorinated counterparts as high temperature membranes. In addition, proton conductivity measurements of the fluorinated membranes up to 85 °C show comparable results with the non-fluorinated SPTES membranes.     

High-temperature water-free proton conducting membranes based on poly(arylene ether ketone) containing pendant quaternary ammonium groups with enhanced proton transport

Poly(arylene ether ketone) containing pendant quaternary ammonium groups (QPAEKs) are anion-conducting polymers synthesized from benzylmethyl-containing poly(arylene ether ketone)s (PAEK-TM). Then QPAEK membranes doped with different concentrations of H₃PO₄ are prepared and evaluated as high temperature proton exchange membranes. The H₃PO₄ doping ability of quaternary ammonium groups in QPAEK system is found to be stronger than that of imidazole groups in polybenzimidazole system. The doping level of resulting QPAEK/H₃PO₄ composite membranes increases with both the concentration level of soaking H₃PO₄ solution and the ion exchange capacity. For example, the highest doping level of composite membranes is 28.6, which is derived from QPAEK-5 with an ion exchange capacity of 2.02 mmol g⁻¹ saturated with concentrated phosphoric acid. A strong correlation between the doping level and the proton conductivity is observed for all the membranes. Besides their low cost, novel high temperature proton exchange membranes, QPAEK/H₃PO₄, show really high proton conductivity and possess excellent thermal and mechanical stability, suggesting a bright future for applications in high temperature fuel cell.     

Sulfonated poly(ether sulfone) electrolytes structured with mesonaphthobifluorene graphene moiety for PEMFC

Sulfonated poly(ether sulfone)s containing a mixture of cis and trans mesonaphthobifluorene moiety were synthesized, and their properties were characterized. The mesonaphthobifluorene graphene moiety contained 6 phenyl rings and was conjugated together to form planar sheets of sp²-bonded carbon. Poly(arylene ether sulfone)s containing a mixture of cis and trans tetraphenyl ethylene units were synthesized by polycondensation, and converted into graphene by intramolecular Friedel–Craft cyclization with Lewis acid (FeCl₃). The sulfonation was taken selectively on mesonaphthobifluorene units with concentrated sulfuric acid. The structural properties of the sulfonated polymers were investigated by ¹H NMR spectroscopy. The membranes were studied with regard to ion exchange capacity (IEC), water uptake, and proton conductivity.     

Improving the proton conductivity and water uptake of polybenzimidazole-based proton exchange nanocomposite membranes with TiO2 and SiO2 nanoparticles chemically modified surfaces

Poly [2,2′-(m-pyrazolidene)-5,5′-bibenzimidazole] (PPBI) was synthesized from pyrazole-3,5-dicarboxylic acid and 3,3′,4,4′-tetraaminobiphenyle (TAB) through polycondensation reaction in polyphosphoric acid (PPA) as reaction solvent. And polymer-grafted SiO₂ and TiO₂ nanoparticles were prepared through radical polymerization of 1-vinylimidazole and sulfonated vinylbenzene on the surface-vinylated nanoparticles. The polymer-grafted SiO₂ and TiO₂ nanoparticles were utilized as a functional additive to prepare PPBI/polymer-grafted SiO₂ and TiO₂ nanocomposite membranes. Imidazole and sulfonated vinylbenzene groups on the surface of modified nanoparticles forming linkages with PPBI chains, improved the compatibility between PPBI and nanoparticles, and enhanced the mechanical strength of the prepared nanocomposite membranes. The prepared nanocomposite membranes showed higher water uptake and acid doping levels comparing to PPBI. Also, after acid doping with phosphoric acid, nanocomposite membranes exhibited enhanced proton conductivity in comparison to the pristine PPBI and PPBI/un-modified SiO₂ and TiO₂ nanocomposite membranes. The enhancement in proton conductivity of nanocomposite membranes resulted from modified SiO₂ nanoparticles showed higher conductivity than modified TiO₂ nanoparticles. The above results indicated that the PPBI/modified SiO₂ and TiO₂ nanocomposite membranes could be utilized as proton exchange membranes for medium temperature fuel cells.     

Proton electrolyte membrane properties and direct methanol fuel cell performance: II. Fuel cell performance and membrane properties effects

In order to study the relationship between the properties of proton electrolyte membranes (PEMs), obtained through standard characterization methods, and the direct methanol fuel cell (DMFC) performance, inorganic–organic hybrid membranes, modified via in situ hydrolysis, were used in a membrane electrolyte assembly (MEA) for DMFC application. The membranes, the characterization of which was performed in the previous paper of this series, were based on sulfonated poly(ether ether ketone) (sPEEK) with a sulfonation degree (SD) of 87% and were loaded with different amounts of zirconium oxide (5.0, 7.5, 10.0, 12.5 wt.%). The standard characterization methods applied were impedance spectroscopy (proton conductivity), water uptake, and pervaporation (permeability to methanol). The MEAs were characterized investigating the DMFC current–voltage polarization curves, constant voltage current (CV, 35 mV), and open-circuit voltage (OCV). The fuel cell ohmic resistance (null phase angle impedance, NPAI) and CO₂ concentration in the cathode outlet were also measured. The characterization results show that the incorporation of the inorganic oxide in the polymer network decreases the DMFC current density for CV experiments, CO₂ concentration in the cathode outlet for both OCV and CV experiments and, finally, the maximum power density output. The opposite effect was verified in terms of the NPAI (ohmic resistance) for both OCV and CV experiments. A good agreement was found between the studied DMFC performance parameters and the characterization results evaluated by impedance spectroscopy, water uptake and pervaporation experiments.     

Enhanced conductivity and stability of composite membranes based on poly (2,5-benzimidazole) and zirconium oxide nanoparticles for fuel cells

Poly (2,5-benzimidazole) (ABPBI) and zirconium oxide (ZrO₂) nanoparticles composite membranes were synthesized. These membranes can be fabricated into tough, dense membranes by blending Poly (2,5-benzimidazole) (ABPBI) with zirconium oxide (ZrO₂) nanoparticles, which were characterized by using FTIR, XRD, SEM, TGA, DSC and tensile test. These composite membranes showed increased conductivity compared with original ABPBI membrane. Maximum proton conductivity at 100 °C was found to be 0.069 S cm⁻¹ on 10% ZrO₂ incorporated ABPBI composite membrane, almost four times as high as the 0.018 S cm⁻¹ obtained in the case of the ABPBI membrane. The conductivity was 0.0325 S cm⁻¹ at 180 °C in dry condition for ABPBI with 10% ZrO₂ nanoparticles composite membrane, higher than the conductivity 0.011 S cm⁻¹ of the ABPBI membrane at same condition. Furthermore, the composite membranes were shown to have high thermal and mechanical stability. These results suggest that ABPBI/ZrO₂ composite membranes may be a promising polymer electrolyte for fuel cells at medium or high temperature, due to their strong physical properties.     

Nanocomposite membranes of surface-sulfonated titanate and Nafion® for direct methanol fuel cells

Various thiol and sultone groups were grafted onto the surface of titanate nanosheets to render organic sulfonic acid (HSO₃–) functionality. The nanocomposite membranes were cast together with Nafion^® using these materials as inorganic fillers. Nanocomposite membranes containing surface-sulfonated titanates showed higher proton conductivity than composite membranes containing untreated TiO₂ P25 particles. They showed better mechanical and thermal stability than Nafion alone. The methanol permeability of nanocomposite membranes decreased with increasing the content of the sulfonated titanate in the nanocomposite membranes. The relative permeability of methanol through these composite membranes with 2 and 5 M methanol solutions was reduced by up to 38 and 26%, respectively, relative to pristine Nafion 115 membranes. The membrane electrode assembly using Nafion/sulfonated titanate nanocomposite membranes exhibited up to 57% higher power density than the assembly containing a pristine Nafion membrane under typical operating conditions of direct methanol fuel cells.     

Sulfonated poly(ether ether ketone)/clay-SO3H hybrid proton exchange membranes for direct methanol fuel cells

A new type of sulfonated clay (clay-SO₃H) was prepared by the ion exchange method with the sulfanilic acid as the surfactant agent. The grafted amount of sulfanilic acid in clay-SO₃H was 51.8 mequiv. (100 g)⁻¹, which was measured by thermogravimetric analysis (TGA). Sulfonated poly(ether ether ketone) (SPEEK)/clay-SO₃H hybrid membranes which composed of SPEEK and different weight contents of clay-SO₃H, were prepared by a solution casting and evaporation method. For comparison, the SPEEK/clay hybrid membranes were produced with the same method. The performances of hybrid membranes for direct methanol fuel cells (DMFCs) in terms of mechanical and thermal properties, water uptake, water retention, methanol permeability and proton conductivity were investigated. The mechanical and thermal properties of the SPEEK membranes had been improved by introduction of clay and clay-SO₃H, obviously. The water desorption coefficients of the SPEEK and hybrid membranes were studied at 80 °C. The results showed that the addition of the inorganic part into SPEEK membrane enhanced the water retention of the membrane. Both methanol permeability and proton conductivity of the hybrid membranes decreased in comparison to the pristine SPEEK membrane. However, it was worth noting that higher selectivity defined as ratio of proton conductivity to methanol permeability of the SPEEK/clay-SO₃H-1 hybrid membrane with 1 wt.% clay-SO₃H was obtained than that of the pristine SPEEK membrane. These results showed that the SPEEK/clay-SO₃H hybrid membrane with 1 wt.% clay-SO₃H had potential usage of a proton exchange membrane (PEM) for DMFCs.     

Investigation of physical properties and cell performance of Nafion/TiO2 nanocomposite membranes for high temperature PEM fuel cells

Synthesis and characterization of Nafion/TiO₂ membranes for proton exchange membrane fuel cell (PEMFC) operating at high temperatures were investigated in this study. Nafion/TiO₂ nanocomposite membranes have been prepared by in-situ sol–gel and casting methods. In the sol–gel method, preformed Nafion membranes were soaked in tetrabutylortotitanate (TBT) and methanol solution. In order to compare synthesis methods, a Nafion/TiO₂ composite membrane was fabricated with 3 wt.% of TiO₂ particles by the solution casting method. The structures of membranes were investigated by Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM), and Energy Dispersive X-Ray Analysis (EDXA). Also, water uptake and proton conductivity of modified membranes were measured. Furthermore, the membranes were tested in a real PEMFC. X-Ray spectra of the composite membranes indicate the presence of TiO₂ in the modified membranes. In case of the same doping level, sol–gel method produces more uniform distribution of Ti particles in Nafion/TiO₂ composite membrane than the ones produced by casting method. Water uptake of Nafion/TiO₂ membrane with 3 wt.% of doping level was found to be 51% higher than that of the pure Nafion membrane. EIS measurements showed that the conductivity of modified membranes decreases with increasing the amount of doped TiO₂. Finally, the membrane electrode assembly (MEA) prepared from Nafion/Titania nanocomposite membrane shows the highest PEMFC performance in terms of voltage vs. current density (V–I) at high temperature (110 °C) which is the main goal of this study.     

Novel ultrathin composite membranes of Nafion/PVA for PEMFCs

The electrospinning approach is an easy and useful method to fabricate porous supports with tailored properties for the preparation of impregnated membranes with enhanced characteristics. Therein, this technique was used to obtain polyvinyl alcohol (PVA) nanofiber mats in which Nafion^® polymer was infiltrated. These Nafion/PVA membranes were characterized in their mechanical properties, proton conductivity and fuel cell performance. Conductivity of the composite membranes was below the showed by pristine Nafion^® due to the non-ionic conducting behaviour of the PVA phase, although the incorporation of the PVA nanofibers strongly reinforced the mechanical properties of the membranes. Measurements carried out in a single cell fed with H₂/Air confirmed the high performance exhibited by a 19 μm thick nanofiber reinforced membrane owing to its low ionic resistance. These reasons make ultrathin (<20 μm) Nafion/PVA composite membranes promising candidates as low cost ion-exchange membranes for fuel cell applications.     

Inorganic–organic membranes based on Nafion, [(ZrO2)·(HfO2)0.25] and [(SiO2)·(HfO2)0.28]. Part I: Synthesis,thermal stability and performance in a single PEMFC

This work reports the preparation, characterization and test in a single fuel cell of two families of hybrid inorganic-organic proton-conducting membranes, each based on Nafion and a different “core-shell” nanofiller. Nanofillers, based on either a ZrO₂ “core” covered with a HfO₂ “shell” (ZrHf) or a HfO₂ “core” solvated by a “shell” of SiO₂ nanoparticles (SiHf), are considered. The two families of membranes are labelled [Nafion/(ZrHf)_x] and [Nafion/(SiHf)_x], respectively. The morphology of the nanofillers is investigated with high-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray spectroscopy (EDX) and electron diffraction (ED) measurements. The mass fractions of nanofiller x used for both families are 0.05, 0.10 or 0.15. The proton exchange capacity (PEC) and the water uptake (WU) of the hybrid membranes are determined. The thermal stability is investigated by high-resolution thermogravimetric measurements (TGA). Each membrane is used in the fabrication of a membrane-electrode assembly (MEA) that is tested in single-cell configuration under operating conditions. The polarization curves are determined by varying the activity of the water vapour (a_H2O) and the back pressure of the reagent streams. A coherent model is proposed to correlate the water uptake and proton conduction of the hybrid membranes with the microscopic interactions between the Nafion host polymer and the particles of the different “core–shell” nanofillers.     

Nafion/sPOSS hybrid membranes for PEMFC. Single cell performance and electrochemical characterization at different humidity conditions

This work describes the preparation of hybrid membranes using octaphenyl polyhedral oligomeric silsesquioxane Ph₈Si₈O₁₂ (POSS), either partially sulfonated (30%) or non-sulfonated as a filler and Nafion as polymer matrix. The membranes have been evaluated in terms of thermal stability (TGA, DSC), water uptake and ex situ in-plane proton conductivity. Morphology has been investigated using SEM and AFM. The hybrid membranes were electrochemically characterized into single cells in order to determine in situ through-plane proton conductivity, hydrogen crossover and cell performance. The analysis was made at 80 °C cell temperature, atmospheric pressure and different levels of relative humidity. The results showed the potential applicability of these membrane materials as electrolytes in PEMFC at medium/low humidity conditions.     

Experimental Investigation of boron phosphate Incorporated speek/pvdf blend membrane for proton exchange membrane fuel cells

The recent studies focused on the blend membranes to a great extent due to their capability of gathering some important polymer characteristic features. In this survey, boron phosphate (BP) doped sulfonated poly (ether ether ketone)/Poly (vinylidene fluoride) (SPEEK/PVDF) blend membrane having high ionic conduction capability was synthesized. The boron phosphate doping to the membrane matrix enhanced the membrane properties in terms of proton exchange membrane conditions. The sol-gel and casting method was used to synthesise the SPEEK/PVDF blend membrane. The characterization tests to observe the structure of the membrane, such as X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), FT-IR and mechanical/thermal stability tests were conducted. The membrane ionic transportation and water retention were improved directly by the addition of boron phosphate. The highest power density (242 mW cm^?2) and current density (400 mA cm^?2) at 0.6 V were obtained by SPEEK/PVDF/10BP, respectively. Additionally, the proton conductivity value of 39 mS cm^?1 was obtained for SPEEK/PVDF/10BP sample at 80 °C. The authors concluded that both boron phosphate additive and SPEEK/PVDF blend membrane have promising results for fuel cell future operations.     

Preparation and properties of proton exchange membranes based-on Nafion and phosphonic acid-functionalized hollow silica spheres

Vinyl functionalized hollow silica spheres (HSSs) were prepared via a template method and surface modification thereafter. Poly(vinylbenzyl phosphonic acid) (PVBPA) grafted HSSs (HPSSs) were prepared via emulsion polymerization of diisopropyl p-vinylbenzyl phosphonate (DIPVBP) on the surface of HSSs, and hydrolysis thereafter. The chemical structure and morphology of HPSSs were characterized by FTIR and TEM. A series of proton exchange membranes based-on Nafion^®212 and HPSSs were prepared via solution casting. The water uptake, swelling ratio, mechanical properties, thermal behavior, proton conductivity, and chemical oxidative stability of the composite membranes were investigated. The addition of HPSSs in Nafion^® membranes can improve the water retentivity of the composite membranes. The composite membranes with HPSSs exhibit higher water uptake and proton conductivity than that of the recast Nafion^® membranes. The water uptake and the proton conductivity of the composite membranes increase with increasing HPSSs loading. With the higher water retentivity, the membranes exhibit high proton conductivity at high temperature (1.6 × 10⁻¹ S cm⁻¹ at 125 °C).     

Enhancing proton conductivity of phosphoric acid-doped Kevlar nanofibers membranes by incorporating polyacrylamide and 1-butyl-3-methylimidazolium chloride

Freeze-drying (fd) technique is an effective and facile method to accumulate nanofibers for membrane preparation, and it has been frequently reported in the development of energy materials. Kevlar as amide nanofibers (ANFs) could serve as support materials for proton exchange membranes (PEMs) owing to the merits of exceptional stiffness and strength, etc. The aim of this research is to enhance the proton conductivity of phosphoric acid (PA)-doped Kevlar membranes by incorporating polyacrylamide (PAM) and 1-butyl-3-methylimidazolium chloride (bmimCl) with freeze-drying technique. The components of PAM and bmimCl could provide the binding sites to combine PA molecules with the formation of Kevlar/PAM/bmimCl/PA membranes. Furthermore, the prepared membranes with the freeze-drying posttreatment are substantially more effective at enhancing proton conductivity owing to the combination of more PA molecules. Specifically, Kevlar/PAM/bmimCl(fd)/PA membranes showed the anhydrous proton conductivity of 2.64 × 10⁻¹ S/cm at 180°C and 1.04 × 10⁻¹ S/cm at 140°C in a 270-hour non-stop test. As regard to the prepared PA-doped Kevlar/CdTe-based membranes, the mechanical strength was far from what was expected. Comparing to the solution casting method, the freeze-drying technique was a feasible strategy to deal with ANFs for exploiting high-temperature PEMs with high performance.     

Improvement of PEMFC performance with Nafion/inorganic nanocomposite membrane electrode assembly prepared by ultrasonic coating technique

Membrane electrode assemblies with Nafion/nanosize titanium silicon dioxide (TiSiO₄) composite membranes were manufactured with a novel ultrasonic-spray technique and tested in proton exchange membrane fuel cell (PEMFC). Nafion/TiO₂ and Nafion/SiO₂ nanocomposite membranes were also fabricated by the same technique and their characteristics and performances in PEMFC were compared with Nafion/TiSiO₄ mixed oxide membrane. The composite membranes have been characterized by thermogravimetric analysis, scanning electron microscopy, X-ray diffraction, water uptake, and proton conductivity. The composite membranes gained good thermal resistance with insertion of inorganic oxides. Uniform and homogeneous distribution of inorganic oxides enhanced crystalline character of these membranes. Gas diffusion electrodes (GDE) were fabricated by Ultrasonic Coating Technique. Catalyst loading was 0.4 mg Pt/cm² for both anode and cathode sides. Fuel cell performances of Nafion/TiSiO₄ composite membrane were better than that of other membranes. The power density obtained at 0.5 V at 75 °C was 0.456 W cm⁻², 0.547 W cm⁻², 0.477 W cm⁻² and 0.803 W cm⁻² for Nafion, Nafion/TiO₂, Nafion/SiO₂, and Nafion/TiSiO₄ composite membranes, respectively.