Novel proton-conductive nanochannel membranes with modified SiO2 nanospheres for direct methanol fuel cells

A novel approach is proposed to prepare a proton-conductive nanochannel membrane based on polyvinylidene difluoride (PVDF) porous membrane with modified SiO₂ nanospheres. The hydrophilic PVDF porous membrane with a 450-nm inner pore size was chosen as the supporting structure. Pristine SiO₂ with a uniform particle size of 95–110 nm was synthesized and functionalized with –NH₂ and –COOH, respectively. Through-plane channels of porous membrane and arranged functional nanoparticles in pores could contribute to constituting efficient proton transfer channels. The characteristics such as morphology, thermal stability, water uptake, dimensional swelling, proton conductivity and methanol permeability as proton exchange membranes, of the SiO₂ nanospheres, and the composite membrane were investigated. The formation of ionic channels in membrane enhanced the water uptakes and proton conduction abilities of the composite membranes. PVDF/Nafion/SiO₂–NH₂ exhibited superior proton conductivities (0.21 S cm^?1) over other samples due to several proton sites and the acid–base pairs formed between –NH₂ and –SO₃H. Furthermore, all the composite membranes exhibited improved methanol resistance compared with Nafion. Therefore, such a design based on porous membrane provided feasibility for high-performance proton exchange membrane in fuel cell applications.     

Synthesis and characterization of P2O5–ZrO2–SiO2 membranes doped with tungstophosphoric acid (PWA) for applications in PEMFC

Homogeneous, transparent and crack-free P₂O₅–ZrO₂ and P₂O₅–ZrO₂–SiO₂ membranes have been synthesized by the sol–gel process. A first step has been oriented to the optimization of the synthesis and characterization of different compositions by TGA, FE-SEM, FTIR and EIS to choose the best inorganic composition in terms of chemical and mechanical stability, and proton conductivity. The addition of SiO₂ improves the mechanical and chemical stability. On the other hand, compositions with higher content in P₂O₅ have demonstrated lower mechanical and chemical stability against water, but higher proton conductivity. The water retention and high porosity of inorganic membranes leads to high proton conductivity, 10⁻² S/cm, at 140 °C and 100% relative humidity. The second step has been focused in the study of doped inorganic membranes of molar composition 99.65(40P₂O₅–20ZrO₂–40SiO₂)–0.35PWA. The high homogeneity, transparency and SEM-EDX analysis of these membranes indicates no phase separation suggesting that PWA is well dispersed in the inorganic structure. The incorporation of PWA in sol–gel oxides provides an increase of the proton conductivity at low relative humidity due to the adequate distribution of PWA in the inorganic network. Conductivity increases in two orders of magnitude at low humidity (10⁻⁴ S/cm at 50 °C and 62% RH) compared with undoped sol–gel oxide membranes.     

In situ formation of platinum nanoparticles in Nafion recast film for catalyst-incorporated ion-exchange membrane in fuel cell applications

Platinum (Pt) nanoparticles were synthesized inside a Nafion polyelectrolyte membrane for use as a catalyst membrane integrated layer in fuel cells. The integrated membrane was prepared by making use of the cation exchange between the tetraammineplatinum (II) cations ([Pt(NH₃)₄]²⁺) and sulfonic groups in the Nafion molecules, followed by film casting and chemical reduction. The synthesized Pt nanoparticles, which had a cubic shape with diameters of 11.5–14.5 nm, dispersed in the recast Nafion film, increased its proton conductivity and open circuit voltage compared with the pristine Nafion membrane. The Pt-incorporated membrane provided a 29% increment of the maximum power density, seemingly by oxidizing the crossover methanol passing through the proton-exchange membrane. At a high loading of Pt (over 3 wt.% in this study), the Nafion clusters were likely squeezed by the synthesized Pt nanoparticles so as to decrease the water uptake and proton conductivity. This hypothesis was also supported by the increased Ohmic resistance in the I–V polarization curve.     

Composite Polymer Electrolytes for Fuel Cell Applications: Filler-Induced Effect on Water Sorption and Transport Properties

Nafion- and sulfonated polysulfone (SPS)- based composite membranes were prepared by incorporation of SnO₂ nanoparticles in a wide range of loading (0 35 wt. %). The composites were investigated by differential scanning calorimetry, dynamic vapor sorption and electrochemical impedance spectroscopy to study the filler effect on water sorption, water mobility, and proton conductivity. A detrimental effect of the filler was observed on water mobility and proton conductivity of Nafion-based membranes. An increase in water mobility and proton conductivity was instead observed in SPS-based samples, particularly at low hydration degree. Analysis of the water sorption isotherms and states of water revealed that the presence of SnO₂ in SPS enhances interconnectivity of hydrophilic domains, while not affecting the Nafion microstructure. These results enable the design of suitable electrolyte materials that operate in proton exchange membrane fuel cell conditions.     

Sol–Gel Derived Rh/TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> Nanocomposite Membranes

A porous borosilicate substrate has been coated with base catalysed SiO₂ sol–gel nanoparticles. Onto these were deposited the vapour of titanium isopropoxide, where it reacted with the surface OH groups to give a TiO₂-overcoat. This nanocomposite sol–gel derived TiO₂/SiO₂ membrane was then doped with 1%Rh giving Rh/TiO₂/SiO₂ membranes. These are shown to be coherent and crack-free, to have good permeability and activity in the isomerisation of butanes. It appears that sol–gel chemistry will allow such membranes to be totally engineered at a nm level.     

Relationships of Acid and water content to proton transport in statistically sulfonated proton exchange membranes: variation of water content via control of relative humidity

An in-depth analysis for proton exchange membranes to examine the effects of acid concentration and effective proton mobility upon proton conductivity as well as their relationship to water content was carried out on two main-chain, statistically sulfonated polymers at 25 degrees C. These polymer systems consisted of poly(ethylenetetrafluoroethylene-graft-polystyrenesulfonic acid) (1) and sulfonated trifluorostyrene (BAM) membrane (2). Nafion (3) was used for comparison. Water content (as represented by Xv, the water volume fraction, where Xv = volume of water in hydrated PEM / volume of hydrated PEM), for each sample was varied by adjusting the relative humidity (RH) of the membrane environment from 50% to 98%. It was found that, at low RH (RH < 70%), the major factor determining proton conductivity is proton mobility. In order to remove the differences in acid strength for the membranes, proton mobility values at infinite dilution (Xv = 1.0) and 25 degrees C were calculated and found to be 2.6 +/- 0.2 x 10-3 (average of 1a-c), 1.6 +/- 0.3 x 10-3 (average of 2a-e), and 2.32 +/- 0.01 x 10-3 cm2 s-1 V-1 (3). These were then compared to the theoretical value for the mobility of a free proton at infinite dilution and to previously reported data. Possible differences in tortuosity and the juxtaposition of acid groups are proposed in order to account for the significant deviations of all samples from the theoretical value.     

Preparation of Nafion/sulfonated poly(phenylsilsesquioxane) nanocomposite as high temperature proton exchange membranes

Nafion/sulfonated poly(phenylmethyl silsesquioxane) (sPPSQ) composite membranes are fabricated using homogeneous dispersive mixing and a solvent casting method for direct dimethyl ether fuel cell (DDMEFC) applications operated above 100 °C. The inorganic conducting filler, sPPSQ significantly affects the characteristics in the nanocomposite membranes by functionalization with an organic sulfonic acid to PPSQ. Moreover, sPPSQ content plays an important role in membrane properties such as microstructure, proton conductivity, fuel crossover, and single cell performance test. With increasing sPPSQ content in the nanocomposite membrane, the proton conductivity increased and fuel crossover decreased. However, in a higher temperature range above 110 °C, Nafion/sPPSQ 5 wt.% composite membrane has the highest proton conductivity. Also, the DME permeability for the composite membrane with higher sPPSQ content increased sharply. The excessive sPPSQ content caused a large aggregation of inorganic fillers, leading to the deterioration of membrane properties. In this study, the optimal sPPSQ content for maximizing the DDMEFC performance was 5 wt.%. Our nanocomposite membranes demonstrated proton conductivities as high as 1.57 × 10⁻¹ S/cm at 120 °C, which is higher than that of Nafion. The cell performances were compared to Nafion/sPPSQ composite membrane with Nafion 115, and the composite membrane with sPPSQ yielded better cell performance than Nafion 115 at temperatures ranging from 100 to 120 °C and at pressures from 1 to 2 bar.     

A novel composite Nafion membrane for direct alcohol fuel cells

Trifluoromethanesulfonic acid or triflate acid, chemical formula CF₃SO₃H, is regarded as one of the strongest acids and resembles Nafion^® in structure. Erbium triflate, a lanthanum salt of triflate, is thermally stable. This paper reports data on the formation of membranes by the fixation of erbium triflate salts (ErTfO) into the Nafion structure. Five different loadings of ErTfO were used to fabricate ErTfO/Nafion composite membranes and these were characterized, extensively for possible use in direct alcohol fuel cells. The membranes were characterized using XRD, TGA, FTIR, and for mechanical strength, water uptake, ion exchange capacity, alcohol uptake, swelling, proton conductivity, alcohol permeability and oxygen stability. The ErTfO/Nafion composite membranes reduced alcohol permeability by 77–80%. The proton conductivity of 3% ErTfO/Nafion composite membranes was 38% higher than that of a pure cast Nafion membrane. The oxygen stability of the ErTfO/Nafion composite membranes was higher than pure cast Nafion. However, the mechanical strength of 7% and 9% ErTfO/Nafion was lower than that of pure cast Nafion. The composite membrane was chemically stable and has potential for use in direct alcohol fuel cells.     

Properties of PWA/ZrO2-doped phosphosilicate glass composite membranes for low-temperature H2/O2 fuel cell applications

Phosphosilicate doped with a mixture of phosphotungstic acid and zirconium oxide (PWA/ZrO₂–P₂O₂–SiO₂) was investigated as potential glass composite membranes for use as H₂/O₂ fuel cell electrolytes. The glass membranes were studied with respect to their structural and thermal properties, proton conductivity, pore characteristics, hydrogen permeability, and performance in fuel cell tests. Thermal analysis including TG and DTA confirmed that the glass was thermally stable up to 400 °C. The dependence of the conductivity on the humidity was discussed based on the PWA content in the glass composite membranes. The proton transfer in the nanopores of the PWA/ZrO₂–P₂O₅–SiO₂ glasses was investigated and it was found that a glass with a pore size of ∼3 nm diameters was more appropriate for fast proton conduction. The hydrogen permeability rate was calculated at various temperatures, and was found to be comparatively higher than for membranes based on Nafion^®. The performance of a membrane electrolyte assembly (MEA) was influenced by its PWA content; a power density of 43 mW/cm² was obtained at 27 °C and 30% relative humidity for a PWA/ZrO₂–P₂O₅–SiO₂ glass membrane with a composition of 6–2–5–87 mol% and 0.2 mg/cm² of Pt/C loaded on the electrode.     

Self-assembled Nafion–silica nanoparticles for elevated-high temperature polymer electrolyte membrane fuel cells

In this paper, a novel Nafion/SiO₂ nanocomposite membrane based on the self-assembled Nafion–SiO₂ nanoparticles was developed. The average particle size of Nafion–SiO₂ nanoparticles prepared by self-assembly process was 2.8 ± 0.5 nm. The self-assembled Nafion–SiO₂ nanoparticles significantly enhance the durability of the Nafion/silica nanocomposite membrane as compared to that of conventional Nafion/silica composite and Nafion 212 membranes under wet/dry cyclic tests at 90 °C. With an addition of 5 wt% self-assembled Nafion–SiO₂ nanoparticles, the Nafion/SiO₂ nanocomposite membrane shows a significantly improved performance stability at cell/humidifying temperatures of 100 °C/60 °C under a current density of 600 mA/cm², and the degradation rate is 0.12 mV/min, almost 20 times lower than 2.33 mV/min measured on the pristine Nafion 212 membrane under the same conditions. The present results demonstrate the promises of the self-assembled Nafion/SiO₂ nanocomposite membrane for elevated-high temperature PEM fuel cells applications.     

Nano oxides incorporated sulfonated poly(ether ether ketone) membranes with improved selectivity and stability for vanadium redox flow battery

Three kinds of sulfonated poly(ether ether ketone) (SPEEK)/nano oxide (Al₂O₃, SiO₂, and TiO₂) composite membranes are fabricated for vanadium redox flow battery (VRFB) application. The composite membranes with 5 wt% of Al₂O₃, SiO₂, and TiO₂ (S/A-5 %, S/S-5 %, and S/T-5 %) exhibit excellent cell performance in VRFB. Incorporation of nano oxides (Al₂O₃, SiO₂, and TiO₂) in SPEEK membrane improves in aspect of thermal, mechanical, and chemical stabilities due to the hydrogen bonds’ interaction between SPEEK matrix and nano oxides. The energy efficiencies (EEs) of composite membranes are higher than that of Nafion 117 membrane, owing to the good balance between proton conductivity and vanadium ion permeability. The discharge–capacity retentions of composite membranes also overwhelm that of Nafion 117 membrane after 200 cycles, indicating their good stability in VRFB system. These low-cost SPEEK/nano oxide composite membranes exhibit great potential for the application in VRFB.     

Synthesis and Ex-situ Characterization of Nafion/TiO2 Composite Membranes for Direct Ethanol Fuel Cell

Nafion/TiO₂ composite membranes for different loadings of TiO₂ were prepared by casting method for the possible application in direct ethanol fuel cell (DEFC). The properties of the composite membranes were investigated by scanning electron microscopy (SEM), x-ray diffraction (XRD), thermogravimetric analyser (TGA), ion exchange capacity, water and alcohol uptake, swelling ratio, proton conductivity, and ethanol crossover. The observed characteristics of the membranes were evaluated for DEFC and compared with the direct methanol fuel cell (DMFC) membrane. The analysis reveales a significant influence on the TiO₂ surface characteristics, water and alcohol uptake, and swelling of the membrane. The TiO₂ composite membranes exhibited a sharp decrease in methanol and ethanol crossover for 5% TiO₂ and the proton conductivity was heighest for 1% TiO₂ loading. The best compromise between proton conductivity and crossover has been found out with the help of the characteristic factor ϕ. The optimum loading of 5% TiO₂ composite membrane has shown the maximum characteristic factor.     

Surface modified multi‐walled carbon nanotubes and Nafion nanocomposite membranes for use in fuel cell applications

The preparation and characterization of the nanocomposite polyelectrolyte membranes, based on Nafion, sulfonated multi‐walled carbon nanotubes (MWCNT‐SO₃H) and imidazole modified multi‐walled carbon nanotubes (MWCNT‐Im), for direct methanol fuel cell applications is described. The results showed that the modification of multi‐walled carbon nanotubes (MWCNT) with proton‐conducting groups (sulfonic acid groups or imidazole groups) could enhance the proton conductivity of the nanocomposite membranes in comparison to Nafion 117. Regarding the interactions between the protonated imidazole groups, grafted on the surface of MWCNT, and the negatively charged sulfonic acid groups of Nafion, new electrostatic interactions can be formed in the interface of the Nafion and MWCNT‐Im, which result in both lower methanol permeability and higher proton conductivity. The physical characteristics of these manufactured nanocomposite membranes were investigated by thermogravimetric analysis, differential scanning calorimetry, Fourier transform infrared spectroscopy, water uptake, methanol permeability, and ion exchange capacity, as well as proton conductivity. The Nafion/MWCNT‐Im membranes showed the higher proton conductivity, lower methanol permeability, and, as a consequence, a higher selectivity parameter in comparison to the neat Nafion or Nafion membrane containing MWCNT‐SO₃H or ─OH functionalized multi‐walled carbon nanotubes (MWCNT‐OH) membranes. The obtained results indicated that the Nafion/MWCNT‐Im membranes could be used as efficient polyelectrolyte membranes for direct methanol fuel cell applications.     

Hybridization of Nafion membranes with an acid functionalised polysiloxane: Effect of morphology on water sorption and proton conductivity

Polysiloxane-modified hybrid Nafion membranes were prepared by casting a mixture of Nafion solution and a precursor of acid functionalised polysiloxane based on tetraethoxysilane and a mercaptan-organoalkoxysilane.Scanning Electron Microscopy (SEM) and Atomic Force Microscopy analysis revealed that the functionalised polysiloxane was dispersed either as finely nanosized inclusions or as coarse domains depending on the rate of the solvent evaporation during the casting procedure. In particular the slower is the rate of solvent evaporation the more interpenetrated and homogenously dispersed at nanosized level is the polysiloxane inside the Nafion membrane.The hybridization process increases the thermal stability of the membranes of about 50 °C relatively to the unmodified Nafion. Small angle X-ray scattering (SAXS) analysis reveals that the hybrid membranes exhibited the typical morphology of Nafion consisting of distinct hydrophilic and hydrophobic domains.Water vapor sorption and proton conductivity were measured varying the temperature (up to 120 °C) and the water activity conditions (from 0.1 to 0.8). The polysiloxane network always increases the water vapor uptake of the membranes and increases significantly the proton conductivity at higher temperature depending on the type of morphology developed by the manufacturing method. In particular hybrid membranes exhibiting nanosized polysiloxane dispersion show a proton conductivity which is up to one-and-half time higher than Nafion recast membrane at high temperature and low water content.     

New PVDF organic–inorganic membranes: The effect of SiO2 nanoparticles content on the transport performance of anion-exchange membranes

Organic–inorganic anion-exchange membranes based on poly(vinylidene fluoride)/SiO₂ were prepared and quaternary ammonium groups were introduced by the reaction of the epoxy groups of glycidyl methacrylate with trimethylamine. Various membranes were prepared with different weight fraction of nano-sized SiO₂ particles. The effect of silica content on the performance of these anion-exchange membranes was extensively characterized in terms of transport properties (TSP). The hydrophilic nature, swelling and TSP of these anion-exchange membranes were dependent on nanoparticles content in the membrane matrix. Higher TSP values of these membranes were obtained, which increased with increasing silica content. The hydrophilic property of the membranes was improved in presence of silica. Results of the effect of silica content on the membrane conductivity were confirmed by the analysis of phenomenological coefficients using non-equilibrium thermodynamic principles. It can be concluded that these anion-exchange membranes prepared with 2% loading with nanoparticles exhibited better transport properties, which may be used for their application in electro-driven separation or for other electrochemical processes.     

Self-assembled polyelectrolyte multilayered films on Nafion with lowered methanol cross-over for DMFC applications

The paper is concerned with the deposition of self-assembled polyelectrolyte multilayer on Nafion membrane by layer-by-layer (LbL) technique with lowered methanol cross-over for direct methanol fuel cell (DMFC) applications. The formation of self-assembled multilayered film on Nafion was characterized by UV–vis spectroscopy and it was found that the polyelectrolyte layers growth on the Nafion surface regularly. Furthermore, the proton conductivity and methanol cross-over measurements were carried out for characterization of the LbL self-assembled composite membranes. The results showed that the concentration and pH of the polyelectrolytes significantly affect the proton conductivity and methanol barrier properties of the composite membranes. 10⁻¹ monomol polyelectrolyte concentration and pH 1.8 was found to be optimum deposition conditions considering proton conductivity and methanol permeation properties of the LbL self-assembled composite membranes. The methanol permeability of the 10 bi-layers of PAH1.8/PSS1.8 deposited LbL self-assembly composite membrane was significantly suppressed and found to be 4.41 × 10⁻⁷ cm²/s while the proton conductivity value is in acceptable range for fuel cell applications.     

Free-standing anion-exchange PEO–SiO2 hybrid membranes

Free-standing anion-exchange polyethylene oxide (PEO)–SiO₂ hybrid membranes with higher flexibility and good mechanical strength (tensile strength (TS) as high as 20.55 MPa) as well as high temperature tolerance (thermal degradation temperature in air, T_d, in the range of 220–240 °C) were prepared through sol–gel reaction of different precursors: charged alkoxysilane-functionalized PEO-1000 (PEO-[Si(OCH₃)₃]₂(+)), N-triethoxysilylpropyl-N,N,N-trimethylammonium iodine (A-1100(+)), monophenyltriethoxysilane (EPh) and in some cases also tetraethoxysilane (TEOS). Properties of the hybrid membranes, such as the thermal stability, tensile properties, hydrophilicity, and electrical performances, can be controlled by changing the feed ratio of the different sol–gel precursors. The results showed that some of the membranes have relatively good conductivity (∼0.003 S/cm) and so may find potential applications in alkaline membrane fuel cells.     

Mesoporous hollow silica spheres as micro-water-tanks in proton exchange membranes

In order to decrease the swelling of Nafion^® and reduce the dependency of proton conductivity on high relative humidity (RH), mesoporous hollow silica spheres were synthesized and dispersed in Nafion matrix as micro-water-tanks in the proton exchange membranes (PEM). The morphologies of MHSi and Nafion/MHSi composite membranes are characterized by SEM and TEM. The effects of MHSi on water uptake, swelling, dehydration rate and proton conductivity of the composite membranes were investigated. The results show that, with a suitable portion of MHSi in the membrane, composite PEMs with enhanced water uptake, reduced swelling and improved proton conductivity are obtained.     

Nafion/PTFE and zirconium phosphate modified Nafion/PTFE composite membranes for direct methanol fuel cells

We prepared Nafion/PTFE (NF) and zirconium phosphate (ZrP) hybridized Nafion/PTFE composite membranes (NF–ZrP). NF–ZrP composite membranes were prepared via two processes. One is impregnating sub-μm porous PTFE membrane directly in a Nafion/ZrOCl₂ solution (NF–Zr–d). The other is impregnating sub-μm porous PTFE membrane in a Nafion solution to prepare NF composite membrane, and then the NF membrane was impregnated in a ZrOCl₂ aqueous solution via in situ precipitation method (NF–Zr–I). The ZrOCl₂ inserted in NF composite membranes was then reacted with phosphoric acid to form ZrP and thus NF–ZrP–d and NF–ZrP–I composite membranes were obtained. The direct methanol fuel cell (DMFC) performances of membrane electrode assemblies prepared from Nafion-117, NF, NF–ZrP–d, and NF–ZrP–I composite membranes were investigated. The effects of introducing sub-μm porous PTFE film and ZrP particles into Nafion membranes on the DMFC performance were investigated. The influence of ZrP hybridizing process into NF membranes (the process of preparing NF–ZrP–I is inserting ZrOCl₂ into NF membranes after Nafion is annealed and the process of preparing NF–ZrP–d is mixing ZrOCl₂ into a Nafion solution before Nafion is annealed) on the morphology of NF–ZrP composite membranes and thus on the DMFC performance was also discussed.     

PPO-based acid–base polymer blend membranes for direct methanol fuel cells

New acid–base polymer blend membranes for direct methanol fuel cells (DMFC) have been designed using a very accessible commercial polymer, poly(2,6-dimethyl-1,4-phenylene oxide) (PPO). The preparation begins with the sulfonation and bromination of PPO to sulfonated PPO (SPPO) and bromomethylated PPO (BrPPO), respectively. Blend membranes are formed by mixing n-propylamine(PrNH₂)-neutralized SPPO and PrNH₂-aminated BrPPO solutions in N-methyl-2-pyrrolidone (NMP), and casting the mixed solution on glass petri dishes followed by acidification with aqueous hydrochloric acid. The compatibility between the acid and base components of the blend is assured by using acidic and basic polymers deriving from the same parent polymer (PPO). Ionic crosslinking is established between the sulfonic groups of SPPO and the amine groups of aminated BrPPO. The ionic crosslinking strengthens the membrane dimensional stability by reducing water uptake and membrane swelling up to temperatures as high as 80 °C. The membranes fabricated as such display good resistance to methanol crossover amidst some, but acceptable loss of proton conductivity. The characteristic factor (i.e. the ratio of proton conductivity to methanol permeability) increases noticeably with the BrPPO content, with the sample containing 30 wt.% BrPPO showing a 16-fold improvement over Nafion 117. The mechanical properties and oxidative stability of the blend membranes also satisfy the requirements for fuel cell assembly and operation.