A novel membrane for DMFC - Na2Ti3O7 nanotubes/Nafion composite membrane

In this study, novel sodium titanate (Na₂Ti₃O₇) nanotube/Nafion^® composite membranes were prepared by a solution casting method. The properties of these composite membranes were studied using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Additionally, the water uptake, methanol permeability, proton conductivity, and selectivity of the composite membranes were measured to evaluate the applicability of these membranes in DMFCs. It was found that the addition of Na₂Ti₃O₇ nanotubes enhanced the water uptake and reduced the methanol permeability of the composite membranes. The proton conductivity and methanol permeability depend on the Na₂Ti₃O₇ nanotube content. Using the selectivity, the optimal nanotube contents was found to be 5 wt%. The new composite membrane was found to have significantly higher selectivity than a pure Nafion^® membrane and thus has good potential to outperform Nafion^® in DMFCs.     

Comparison of ionically and ionical-covalently cross-linked polyaromatic membranes for SO2 electrolysis

It has been shown that hydrogen, which can be used for energy storage, can be produced efficiently by the membrane based Hybrid Sulfur (HyS) process. During the HyS electrolysis step, SO₂ and H₂O are converted to H₂ and H₂SO₄, which implies that membranes to be used for this process should have both a high proton conductivity and acid stability. In this study ionic and ionic-covalently cross-linked polybenzimidazole (PBI) blended membranes were investigated and compared with Nafion^®212 in terms of their acid stability. Characterization of the membranes, which included monitoring the change in weight, swelling, SEM/EDX, TEM, TGA-MS, FTIR and IEC before and after H₂SO₄ treatment showed that all tested membranes were stable in 80 wt% H₂SO₄ at 80 °C for 120 h. Subsequent HyS electrolysis showed that the blend membranes performed better than Nafion^®115 at current densities below 0.3 A/cm², while performing similar above 0.3 A/cm².     

Comparison of homogeneously and heterogeneously sulfonated polyetheretherketone membranes in preparation,properties and cell performance

For application in direct methanol fuel cells, polyetheretherketone (PEEK) membranes with different degrees of sulfonation (40–80%) are prepared by both homogeneous and heterogeneous methods. Sulfonation and its degree are identified by means of Fourier transform infrared (FT-IR) spectroscopy and a back-titration method, respectively. Thermal analysis shows that an increase in the sulfonation degree increases the glass-transition temperature and enhances the thermal stability. The room-temperature ion conductivity of the homogeneously sulfonated PEEK (sPEEK) membrane with a 68% degree of sulfonation is higher than that of Nafion^® 117, while its methanol permeability is lower. The tensile strength of the sPEEK membrane is comparable with that of Nafion^® 117 at the equilibrium water swollen state. Various properties of sPEEK membranes prepared by two methods are compared. Overall, the ion conductivity of the homogeneous system is higher than that of the heterogeneous counterpart, but there is little difference in methanol permeability. The cell performance of the homogeneously sulfonated PEEK (homo-sPEEK) membrane is much better than that of the heterogeneously sulfonated PEEK (hetero-sPEEK) and Naifion^® 117 membranes.     

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.     

Electrochemical investigation of sulfonated poly(ether ether ketone)/clay nanocomposite membranes for moderate temperature fuel cell applications

In the present study, polyelectrolyte membranes based on partially sulfonated poly(ether ether ketone) (sPEEK) with various degrees of sulfonation are prepared. The optimum degree of sulfonation is determined according to the transport properties and hydrolytic stability of the membranes. Subsequently, various amounts of the organically modified montmorillonite (MMT) are introduced into the sPEEK matrices via the solution intercalation technique. The proton conductivity and methanol permeability measurements of the fabricated composite membranes reveal a high proton to methanol selectivity, even at elevated temperatures. Membrane based on sPEEK and 1 wt% of MMT, as the optimum nanoclay composition, exhibits a high selectivity and power density at the concentrated methanol feed. Moreover, it is found that the optimum nanocomposite membrane not only provides higher performance compared to the neat sPEEK and Nafion^®117 membranes, but also exhibits a high open circuit voltage (OCV) at the elevated methanol concentration. Owing to the high proton conductivity, reduced methanol permeability, high power density, convenient processability and low cost, sPEEK/MMT nanocomposite membranes could be considered as the alternative membranes for moderate temperature direct methanol fuel cell applications.     

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.     

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.     

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.     

Semi-interpenetrating polymer network electrolyte membranes composed of sulfonated poly(ether ether ketone) and organosiloxane-based hybrid network

A semi-interpenetrating polymer network (semi-IPN) proton exchange membrane is prepared from the sulfonated poly(ether ether ketone) (sPEEK) and organosiloxane-based organic/inorganic hybrid network (organosiloxane network). The organosiloxane network is synthesized from 3-glycidyloxypropyltrimethoxysiane and 1-hydroxyethane-1,1-diphosphonic acid. The semi-IPN membranes prepared were stable up to 300 °C without any degradation. The methanol permeability is much lower than Nafion^® 117 under addition of the organosiloxane network. The proton conductivity of semi-IPN membranes increases with an increase the organosiloxane network content; the membrane containing the 20-24 wt% organosiloxane network shows higher conductivity than Nafion^® 117. The power density of the MEA fabricated with the semi-IPN membrane with 24 wt% organosiloxane network is 135 mW cm⁻², much better than that of the pristine sPEEK membrane, 85 mW cm⁻². Chemical synthesis of the semi-IPN membranes is identified using FTIR, and its ion cluster dimension examined using SAXS. The dimensional stability associated with water swelling and dissolution is investigated at different temperatures, and the semi IPN membranes dimensionally stable in water at elevated temperature.     

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.     

H2SO4 stability of PBI-blend membranes for SO2 electrolysis

For hydrogen to become a serious contender for replacing fossil fuels, the manufacturing thereof has to be further investigated. One such process, the membrane based Hybrid Sulfur (HyS) process, where hydrogen is produced from the electrolysis of SO₂, has received considerable interest recently. Since H₂SO₄ is formed during SO₂ electrolysis, H₂SO₄ stability is a prerequisite for any membrane to be used in this process. In this study, pure as well as blended polybenzimidazole (PBI), partially fluorinated poly(arylene ether) (sFS) and nonfluorinated poly(arylene ethersulfone) (sPSU) membranes were investigated in terms of their acid stability as a function of acid concentration. Membranes were characterized using weight change, TGA, GPC, SEM/EDX and IEC. While a general stability was observed at 30 and 60 wt% H₂SO₄, the blended sFS-PBI and sPSU-PBI showed the highest stability throughout. According to the VI curve obtained for the SO₂ electrolysis, the sPSU-PBI blend membrane performed slightly better than Nafion^®117.     

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.     

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.     

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.     

Investigation of the effects of AMPS-modified nanoclay on fuel cell performance of sulfonated aromatic proton exchange membranes

Here we describe preparation and characterization of a series of nanocomposite polyelectrolytes based on partially sulfonated poly (ether ether ketone) (SPEEK) and organically modified montmorillonite (MMT). Optimum degree of sulfonation for SPEEK is selected based on its transport properties. MMT is modified via ion exchange reaction using a 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as a functional modifier. AMPS-MMT at different loadings is introduced into the SPEEK matrices via the solution intercalation technique. Also, the nanocomposite membranes are fabricated using SPEEK and commercially available nanoclays like Cloisite Na (Na-MMT) and Cloisite 15A. Transport properties, proton conductivity and methanol permeability of the fabricated composite membranes are evaluated. Presence of AMPS-MMT significantly decreases the activation energy needed for proton conductivity. A membrane based on SPEEK/AMPS-MMT-3 wt% is selected as an optimum formulation which exhibits a high selectivity and power density at the elevated methanol concentrations. Moreover, it is found that the optimum nanocomposite membrane not only provides higher power output compared to the neat SPEEK and Nafion^®117 membranes, but also exhibits a higher open circuit voltage (OCV) in comparison with pristine SPEEK and commercial Nafion^® 117 membranes. Owing to the desirable transport and electrochemical properties SPEEK/AMPS-MMT nanocomposite can be considered as an alternative membrane for direct methanol fuel cell applications.     

Dynamic conducting effect of WO3/PFSA membranes on the performance of proton exchange membrane fuel cells

The homogenous proton conducting WO₃/PFSA membranes are prepared based on a dynamic conducting concept, that is, the resistance of the membrane can be reduced during the fuel cell operation due to the formation of the conducting hydrogen tungsten bronzes. The novel membranes are characterized by different techniques. The results proved that the resistances of the WO₃-containing membranes in single fuel cells measured by in situ AC impedance spectroscopy during the operation are significantly lower than that of the single fuel cell using Nafion^® 112 membrane. It is revealed that the performances of the single fuel cells with WO₃/PFSA membranes are superior to that of the single cell with Nafion^® 112 membrane.     

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.     

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.     

Surface-modified Nafion membranes with mesoporous SiO2 layers via a facile dip-coating approach for direct methanol fuel cells

In this study, Nafion^® 117 membrane is surface-modified with mesoporous silica layers through in situ surfactant-templated sol–gel reaction. The reaction makes use of tetraethyl orthosilicate (TEOS) under acidic condition via dip-coating technique on both sides. Scanning electron microscopy (SEM), Fourier transformation infrared (FTIR), and thermogravimetric analysis (TGA) are employed to characterize the resultant membranes. Proton conductivity and methanol permeability of the membranes are also studied. It is determined that the aging time, along with the number of the silicon dioxide (SiO₂) layer, influence both proton conductivity and methanol permeability. Specifically, double-side modified membrane with 5 min interval of the second layer (S (5)) exhibits optimal properties on the combined criterion of conductivity and permeability. However, the application of mesoporous silica layer in modifying commercial Nafion membranes through dip-coating is proven to be a facile route in improving the said criteria simultaneously.     

Improvement of electrochemical performances of sulfonated poly(arylene ether sulfone) via incorporation of sulfonated poly(arylene ether benzimidazole)

In the present study, modified acid–base blend membranes were fabricated via incorporation of sulfonated poly(arylene ether benzimidazole) (SPAEBI) into sulfonated poly(arylene ether sulfone) (SPAES). These membranes had excellent methanol-barrier properties in addition to an ability to compensate for the loss of proton conductivity that typically occurs in general acid–base blend system. To fabricate the membranes, SPAEBIs, which served as amphiphilic polymers with different degrees of sulfonation (0–50 mol%), were synthesized by polycondensation and added to SPAES. It resulted in the formation of acid–amphiphilic complexes such as [PAES-SO₃]⁻⁺[H-SPAEBI] through the ionic crosslinking, which prevented SO₃H groups in the complex from transporting free protons in an aqueous medium, contributing to a reduction of ion exchange capacity values and water uptake in the blend membranes, and leading to lower methanol permeability in a water–methanol mixture. Unfortunately, the ionic bonding formation was accompanied by a decrease of bound water content and proton conductivity, although the latter problem was solved to some extent by the incorporation of additional SO₃H groups in SPAEBI. In the SPAES–SPAEBI blend membranes, enhancement of proton conductivity and methanol-barrier property was prominent at temperatures over 90 °C. The direct methanol fuel cell (DMFC) performance, which was based on SPAES–SPAEBI-50–5, was 1.2 times higher than that of Nafion^® 117 under the same operating condition.