A novel membrane for DMFC – Na2Ti3O7 Nanotubes/Nafion composite membrane: Performances studies

Systematic experiments have been carried out to study the performance of the novel sodium titanate (Na₂Ti₃O₇) nanotube/Nafion^® composite membrane (5 wt% Na₂Ti₃O₇) in a single direct methanol fuel cell (DMFC) at different operating temperatures, methanol concentrations, air flow rates, methanol flow rates, and cathode humidification temperatures. The experimental results showed that the composite membrane outperform pure Nafion^® membranes with the same thickness, Nafion^®112, under all the operating conditions. Furthermore, under some operating conditions, the new composite membranes even outperform Nafion^®117, a much thicker membrane. These experimental results have proved that the new composite membrane is superior to pure Nafion^® membrane in DMFCs and the addition of Na₂Ti₃O₇ nanotubes into Nafion^® is an effective way to improve membrane performances.     

Sol–gel derived sulfonated-silica/Nafion composite membrane for direct methanol fuel cell

Sulfonated-silica/Nafion^® composite membranes were prepared in a sol–gel reaction of (3-Mercaptopropyl)trimethoxysilane (SH-silane) followed by solution casting, and then oxidated using 10 wt% H₂O₂ solution. The chemical and physical properties of the composite membranes were characterized by using FT-IR, XPS, ²⁹Si NMR and SEM analyses. Experimental results indicated that the optimum oxidation condition was 60 °C for 1 h. The performance of the silica–SO₃H/Nafion^® composite membranes was evaluated in terms of methanol permeability, proton conductivity and cell performance. The silica–SO₃H/Nafion^® composite membranes have a higher selectivity (C/P ratio = 26,653) than that of pristine Nafion^® (22,795), perhaps because of their higher proton conductivity and lower methanol permeability. The composite membrane with 0.6 wt% silica–SO₃H/Nafion^® performed better than pristine Nafion^®. The current densities were measured as 62.5 and 70 mA cm⁻² at a potential of 0.2 V with a composite membrane that contained 0 and 0.6 wt% silica–SO₃H, respectively. The cell performance of the DMFC was improved by introducing silica–SO₃H. The composite membrane with 0.6 wt% of silica–SO₃H yielded the maximum power density of 15.18 mW cm⁻². The composite membranes are suitable for DMFC applications with high selectivity.     

Nano-sized Fe2O3–SO4 solid superacid composite Nafion membranes for direct methanol fuel cells

In this paper, Fe₂O₃–SO₄²⁻/Nafion^® composite membranes were prepared by a solution casting method. The physico-chemical properties of composite membranes were characterized by X-ray diffraction (XRD), SEM–EDX and thermogravimetric analysis (TGA). The water uptake ability, proton conductivity, and methanol permeability of the composite membranes were evaluated and compared with the recast Nafion^® membrane. The results showed that the proton conductivity and the water uptake of the composite membranes were slightly higher than that of the recast Nafion^® membrane. The composite membrane containing 5 wt.% Fe₂O₃–SO₄^2- showed superior ability to suppress methanol crossover, and it further improved the direct methanol fuel cell (DMFC) performances with both 1 M and 5 M methanol feeding, compared with the recast Nafion^® membrane. The preliminary 30 h lifetime test of the DMFC with the composite membrane with 5% Fe₂O₃–SO₄²⁻ indicated that the composite membrane is stable working at the real DMFC operating conditions at least during the test. These results suggest the applicability of the composite membranes in DMFCs.     

Preparation and properties of high performance nanocomposite proton exchange membrane for fuel cell

Various spatially enlarged organoclays were prepared by using poly(oxyproplene)-backboned quaternary ammonium salts of various molecular weights M_w 230, 400 and 2000 as the intercalating agents for Na⁺-montmorillonite. The modified MMT was utilized to improve the compatibility with Nafion^®. Sufficient interaction of the modified MMT with Nafion^® was studied by using X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS). The performance of the Nafion^®/m-MMT composite membranes for direct methanol fuel cell (DMFCs) was evaluated in terms of water uptake, ion exchange capacity (IEC), methanol permeability, proton conductivity, and cell performance. The methanol permeability of the composite membrane decreased with the increasing of m-MMT content. The proton conductivity of the membrane was lowered slightly from that of pristine Nafion^® membrane. These results led to an essential improvement in the single-cell performance of DMFCs.     

A novel composite membranes based on sulfonated montmorillonite modified Nafion for DMFCs

A novel functional organoclay was prepared using POP-backboned quaternary ammonium salts that contained sulfonic acid (–SO₃H) to improve the performance of Nafion^® membranes used in direct methanol fuel cells. Modified layered silicate clays were cast with Nafion^®. The performance of the Nafion^®/MMT-POPD400-PS composite membranes was evaluated in terms of methanol permeability, proton conductivity and cell performance. The methanol permeability of the composite membrane declined as the MMT-POPD400-PS content increased. The MMT was functionalized using organic sulfonic acid to enhance proton conductivity. The proton conductivity of the composite membrane exceeded that of pristine Nafion^®. These effects essentially improved the single-cell performance of DMFC.     

Nafion/polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene composite membranes with electric field-aligned domains for improved direct methanol fuel cell performance

Composite membranes were fabricated consisting of aligned domains of Nafion^® 1100 surrounded by a supporting matrix of polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene (SEBS). The structure of the composite was controlled via the application of an electric field during solvent casting. Nafion^® domains were aligned across the membrane thickness to provide paths for proton conduction. The surrounding SEBS domain limits methanol permeability. Compared to randomly structured Nafion^®/SEBS composites, the membranes with field-aligned domains display significantly enhanced performance in direct methanol fuel cells (DMFCs). The membranes with field-aligned domains display DMFC performance better than commercial Nafion^® 117 membranes under high methanol fuel concentration.     

High proton-conducting Nafion/–SO3H functionalized mesoporous silica composite membranes

A novel organic–inorganic mesoporous silica (L64 copolymer-templated mesoporous SiO₂), functionalized with perfluoroalkylsulfonic acid groups analogous to that of Nafion^®, was prepared. A condensation reaction between the surface silanol groups of the mesoporous silicas and 1,2,2-trifluoro-2-hydroxy-1-trifluoromethylethane sulfonic acid Beta-sultone was conducted. High proton-conducting Nafion^®/functionalized mesoporous silica composite membranes were prepared via homogeneous dispersive mixing and the solvent casting method. In this investigation, the proton conductivity (σ) of the composite membrane is increased from 0.10 to 0.12 (S cm⁻¹) as the modified mesoporous silica content is increased from 0 to 3 wt%. The methanol permeability of the composite membrane declined as the sulfonic mesoporous silica content increased. The methanol permeability of the composite membrane that contained 3 wt% M–SiO₂–SO₃H was 4.5 × 10⁻⁶ cm² S⁻¹—30% lower than that of pristine Nafion^®. Results of this study demonstrate a significant improvement in the performance of DMFCs.     

Sulfonated poly(propylene oxide) oligomers/Nafion acid-base blend membranes for DMFC

A novel functional poly(propylene oxide)-backboned diamine of M_w 400 (abbreviated as D400) was grafted with sulfonic acid (abbreviated as D400-PS) to improve the performance of Nafion^® membranes in direct methanol fuel cells (DMFCs). The interaction of the D400-PS with Nafion^® was studied by Fourier transform infrared spectroscopy (FT-IR) and differential scanning calorimetry (DSC). The performance of the blend Nafion^®/D400-PS membranes was evaluated in terms of methanol permeability, proton conductivity and cell performance. The proton conductivity of the blend membrane was slightly reduced by rendering proton conductivity to D400 by functionalized with an organic sulfonic acid. The methanol permeability of the blend membrane decreased with increasing of D400-PS content. The methanol permeability of the blend Nafion^®/D400-PS with the composition 3/1 (–SO₃H/–NH₂) was 1.02 × 10⁻⁶ cm² S⁻¹, which was reduced 50% compared to that of pristine Nafion^®. The current densities that were measured with Nafion^®/D400-PS blend membranes in the ratio 1/0 and 5/1 (–SO₃H/–NH₂), were 51 and 72 mA cm⁻², respectively, at a potential of 0.2 V. Consequently, the blend Nafion^®/D400-PS membranes critically improved the single-cell performance of DMFC.     

Sulfonated poly(ether ether ketone) membranes with sulfonated graphene oxide fillers for direct methanol fuel cells

Sulfonated organosilane functionalized graphene oxides (SSi-GO) synthesized through the grafting of graphene oxide (GO) with 3-mercaptopropyl trimethoxysilane and subsequent oxidation have been used as a filler in sulfonated poly(ether ether ketone) (SPEEK) membranes. The incorporation of SSi-GOs greatly increases the ion-exchange capacity (IEC), water uptake, and proton conductivity of the membrane. With well-controlled contents of SSi-GOs, the composite membranes exhibit higher proton conductivity and lower methanol permeability than Nafion^® 112 and Nafion^® 115, making them particularly attractive as proton exchange membranes (PEMs) for direct methanol fuel cells (DMFC). The composite membrane with optimal SSi-GOs content exhibit over 38 and 17% higher power densities, respectively, than Nafion^® 112 and Nafion^® 115 membranes in DMFCs, offering the possibilities to reduce the DMFC membrane cost significantly while keeping high-performance.     

Implantation of Nafion ionomer into polyvinyl alcohol/chitosan composites to form novel proton-conducting membranes for direct methanol fuel cells

A series of cost-effective, proton-conducting composite membranes, comprising of Nafion^® ionomer, chitosan (CS), and polyvinyl alcohol (PVA), is successfully prepared. By taking advantage of the strong electrostatic interactions between Nafion^® ionomer and CS component, Nafion ionomer is effectively implanted into the PVA/CS composite membranes, and improves proton conductivity of the PVA/CS composite membranes. Furthermore, this effect dramatically depends on the composition ratio of PVA/CS, and the optimum conductivity is obtained at the PVA/CS ratio of 1:1. The developed composite membranes exhibit much lower methanol permeability compared with the widely used Nafion^® membrane, indicating that these novel membranes have great potential for direct methanol fuel cells (DMFCs).     

Effects and properties of various molecular weights of poly(propylene oxide) oligomers/Nafion acid–base blend membranes for direct methanol fuel cells

Various molecular weights of poly(propylene oxide) diamines oligomers/Nafion^® acid–base blend membranes were prepared to improve the performance of Nafion^® membranes in direct methanol fuel cells (DMFCs). The acid–base interactions were studied by Fourier transform infrared spectroscopy (FT-IR) and differential scanning calorimetry (DSC). The performance of the blend membranes was evaluated in terms of methanol permeability, proton conductivity and cell performance. The proton conductivity was slightly reduced by acid–base interaction. The methanol permeability of the blend D2000/Nafion^® was 8.61 × 10⁻⁷ cm² S⁻¹, which was reduced 60% compared to that of pristine Nafion^®. The cell performance of D2000/Nafion^® blend membranes was enhanced significantly compared to pristine Nafion^®. The current densities that were measured with Nafion^® and 3.5 wt% D2000/Nafion^® blend membranes were 62.5 and 103.5 mA cm⁻², respectively, at a potential of 0.2 V. Consequently, the blend poly(propylene oxide) diamines oligomers/Nafion^® membranes critically improved the single-cell performance of DMFC.     

SO2 permeability and proton conductivity of sPEEK membranes for SO2-depolarized electrolyzer

The SO₂ transport properties of Nafion^® and sPEEK membranes were measured using an electrochemical reaction cell to investigate their application in the electrochemical hybrid sulfur process. The permeability of SO₂ in the membranes was determined from a combined theory based on Faraday's law and Fick's law where the electrochemical reaction rate of SO₂ in the downstream membrane is the same as its diffusion flux through the membrane. Both Nafion^® and sPEEK membranes show higher SO₂ diffusion coefficients at higher temperatures. For sPEEK membranes, increasing the degree of sulfonation resulted in increasing permeability, as more water was imbibed in the membranes with higher degrees of sulfonation. Activation energy was extracted from the temperature-dependence of the diffusion coefficients for both membranes. The sPEEK membranes exhibited similar diffusion coefficients to those of Nafion^®, even at high sulfonation degrees of 70%. Besides SO₂ permeability, proton conductivity and mechanical properties were measured for comparison between the 2 polymer membranes. Although the proton conductivity of the sPEEK was slightly lower than the Nafion^® membrane, it was very competitive considering its higher mechanical strength and much lower cost.     

Fabrication and investigation of SiO2 supported sulfated zirconia/Nafion self-humidifying membrane for proton exchange membrane fuel cell applications

A self-humidifying composite membrane based on Nafion^® hybrid with SiO₂ supported sulfated zirconia particles (SiO₂–SZ) was fabricated and investigated for fuel cell applications. The bi-functional SiO₂–SZ particles, possessing hygroscopic property and high proton conductivity, were homemade and as the additive incorporated into our composite membrane. X-ray diffraction (XRD) and Fourier infrared spectrum (FT-IR) techniques were employed to characterize the structure of SiO₂–SZ particles. Scanning electronic microscopy (SEM) and energy dispersive spectroscopy (EDS) measurements were conducted to study the morphology of composite membrane. To verify the advantages of Nafion^®/SiO₂–SZ composite membrane, the IEC value, water uptake, proton conductivity, single cell performance and areal resistance were compared with Nafion^®/SiO₂ membrane and recast Nafion^® membrane. The single cell employing our Nafion^®/SiO₂–SZ membrane exhibited the highest peak power density of 0.98 W cm⁻² under dry operation condition in comparison with 0.74 W cm⁻² of Nafion^®/SiO₂ membrane and 0.64 W cm⁻² of recast Nafion^® membrane, respectively. The improved performance was attributed to the introduction of SiO₂–SZ particles, whose high proton conductivity and good water adsorbing/retaining function under dry operation condition, could facilitate proton transfer and water balance in the membrane.     

Nafion/histidine functionalized carbon nanotube: High-performance fuel cell membranes

Here we show preparation and characterization of a new type of composite membrane based on Nafion^®/histidine modified carbon nanotube by imidazole groups (Im-CNT), for direct methanol fuel cell (DMFC) applications. Due to the presence of this imidazole-based amino acid on the surface of CNT, new electrostatic interactions can be formed in the interface of Nafion^® and Im-CNT. Physical characteristics of these nanocomposite membranes are investigated by water uptake, methanol permeability, ion exchange capacity, proton conductivity, as well as fuel cell performance results.     

Use of in situ polymerized phenol-formaldehyde resin to modify a Nafion membrane for the direct methanol fuel cell

Commercial Nafion^®-115 (trademark registered to DuPont) membranes were modified by in situ polymerized phenol formaldehyde resin (PFR) to suppress methanol crossover, and SO₃⁻ groups were introduced to PFR by post-sulfonatation. A series of membranes with different sulfonated phenol formaldehyde resin (sPFR) loadings have been fabricated and investigated. SEM-EDX characterization shows that the PFR was well dispersed throughout the Nafion^® membrane. The composite membranes have a similar or slightly lower proton conductivity compared with a native Nafion^® membrane, but show a significant reduction in methanol crossover (the methanol permeability of sPFR/Nafion^® composite membrane with 2.3 wt.% sPFR loading was 1.5 × 10⁻⁶ cm² s⁻¹, compared with the 2.5 × 10⁻⁶ cm² s⁻¹ for the native Nafion^® membrane). In direct methanol fuel cell (DMFC) evaluation, the membrane electrode assembly (MEA) using a composite membrane with a 2.3 wt.% sPFR loading shows a higher performance than that of a native Nafion^® membrane with 1 M methanol feed, and at higher methanol concentrations (5 M), the composite membrane achieved a 114 mW cm⁻² maximum power density, while the maximum power density of the native Nafion^® was only 78 mW cm⁻².     

Proton conducting CS/P(AA-AMPS) membrane with reduced methanol permeability for DMFCs

A novel polyelectrolyte complex (PEC) membrane for direct methanol fuel cells (DMFCs) was prepared by blending a cationic polyelectrolyte, chitosan (CS), with an anionic polyelectrolyte, acrylic acid-2-acrylamido-2-methylpropane sulfonic acid copolymer (P(AA-AMPS)). The presence of –NH₃⁺ species detected by X-ray photoelectron spectroscopy (XPS) revealed that an ionic cross-linked interpenetrating polymer network (IPN) was formed between the two polyelectrolyte polymers. Methanol permeability and proton conductivity were measured and compared with the Nafion^®117 membrane. The dual function of P(AA-AMPS) as both an ionic crosslinker and a proton conductor led to not only a notable reduction in methanol permeability but also an increase in proton conductivity. The CS/P(AA-AMPS) membrane with a P(AA-AMPS) content of 41 wt.% exhibited a methanol permeability (P) of 2.41 × 10⁻⁷ cm² s⁻¹ which was fifteen times lower than that of the Nafion^®117 membrane, whereas its proton conductivity (σ) was comparatively high (3.59 × 10⁻² S cm⁻¹). In terms of the overall selectivity index (β = σ/P), the PEC membrane showed a remarkably higher selectivity than the Nafion^®117 membrane, and, furthermore, the overall selectivity index increased with the increase of P(AA-AMPS) content. The mechanism of proton transfer was tentatively discussed based on the activation energy of conductivity.     

Performance of composite Nafion/PVA membranes for direct methanol fuel cells

This work has been focused on the characterization of the methanol permeability and fuel cell performance of composite Nafion/PVA membranes in function of their thickness, which ranged from 19 to 97 μm. The composite membranes were made up of Nafion^® polymer deposited between polyvinyl alcohol (PVA) nanofibers. The resistance to methanol permeation of the Nafion/PVA membranes shows a linear variation with the thickness. The separation between apparent and true permeability permits to give an estimated value of 4.0 × 10⁻⁷ cm² s⁻¹ for the intrinsic or true permeability of the bulk phase at the composite membranes. The incorporation of PVA nanofibers causes a remarkable reduction of one order of magnitude in the methanol permeability as compared with pristine Nafion^® membranes. The DMFC performances of membrane-electrode assemblies prepared from Nafion/PVA and pristine Nafion^® membranes were tested at 45, 70 and 95 °C under various methanol concentrations, i.e., 1, 2 and 3 M. The nanocomposite membranes with thicknesses of 19 μm and 47 μm reached power densities of 211 mW cm⁻² and 184 mW cm⁻² at 95 °C and 2 M methanol concentration. These results are comparable to those found for Nafion^® membranes with similar thickness at the same conditions, which were 210 mW cm⁻² and 204 mW cm⁻² respectively. Due to the lower amount of Nafion^® polymer present within the composite membranes, it is suggested a high degree of utilization of Nafion^® as proton conductive material within the Nafion/PVA membranes, and therefore, significant savings in the consumed amount of Nafion^® are potentially able to be achieved. In addition, the reinforcement effect caused by the PVA nanofibers offers the possibility of preparing membranes with very low thickness and good mechanical properties, while on the other hand, pristine Nafion^® membranes are unpractical below a thickness of 50 μm.     

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).     

Evaluation of composite membranes for direct methanol fuel cells

The performance of direct methanol fuel cells (DMFCs) can be significantly affected by the transport of methanol through the membrane, depolarising the cathode. In this paper, the literature on composite membranes that have been developed for reduction of methanol crossover in DMFCs is reviewed. While such membranes can be effective in reducing methanol permeability, this is usually combined with a reduction in proton conductivity. Measurements of methanol permeability and proton conductivity are relatively straightforward, and these parameters (or a membrane ‘selectivity’ based on the ratio between them) are often used to characterize DMFC membranes. However, we have carried out one-dimensional simulations of DMFC performance for a wide range of membrane properties, and the results indicate that DMFC performance is normally either limited by methanol permeability or proton conductivity. Thus use of a ‘selectivity’ is not appropriate for comparison of membrane materials, and results from the model can be used to compare different membranes. The results also show that Nafion^® 117 has an optimum thickness, where DMFC performance is equally limited by both methanol permeability and proton conductivity. The model also indicates that new composite membranes based on Nafion^® can only offer significant improvement in DMFC performance by enabling operation with increased methanol concentration in the fuel. A number of composite membrane materials that have been reported in the literature are shown to deliver significant reduction in DMFC performance due to reduced proton conductivity, although improved performance at high methanol concentration may be possible.     

Layer-by-layer self-assembly of in situ polymerized polypyrrole on sulfonated poly(arylene ether ketone) membrane with extremely low methanol crossover

The surface of sulfonated poly(arylene ether ketone) bearing carboxyl groups (SPAEK-C) was modified by alternating deposition of oppositely charged polypyrrole (PPY) and phosphotungstic acid (PWA) via the layer-by-layer (LBL) method in order to prevent the crossover of methanol in the direct methanol fuel cell (DMFC). FT-IR confirms that PPY and PWA are assembled in the multilayers successfully. The morphology of the membranes studied in detail by SEM shows the presence and stability of thin PPY/PWA layers coated on SPAEK-C membranes. Methanol permeability was determined and was shown to be effectively reduced. The selectivity of SPAEK-C-(PPY/PWA)_n is 1 order more than Nafion^® 117, which is attractive in DMFCs. Thermal stability, water uptake, water swelling and proton conductivity of the SPAEK-C and SPAEK-C-(PPY/PWA)_n membranes were also investigated.