Direct methanol fuel cells : Methanol crossover and its influence on single DMFC performance

In the present investigation, the methanol crossover rate through Nafion®-115 membrane at different temperatures and different concentrations had been investigated in a fuel cell test apparatus by using gas chromatography analysis. The singledirect methanol fuel cell (DMFC) tests were carried out to investigate the effect of the concentration of methanol aqueous solutions and cell temperature on methanol crossover and consequently, on the open circuit voltage and the cell performance of DMFC. It can be found that the methanol crossover rate through Nafion® membrane increases as methanol concentration and temperature increase. It can also be found that methanol crossover presented a negative effect on the open circuit voltage and the single DMFC performance. Single DMFC test results showed that an improved cell performance was obtained as temperature increased although the methanol crossover rate increased with temperature increment.     

Polyelectrolyte Nanocomposite Membranes Using Surface Modified Nanosilica for Fuel Cell Applications

The preparation and characterization of a new type of nanocomposite polyelectrolyte membrane (PEM), based on Nafion® (E. I. du Pont de Nemours and Co., Ltd., for its copolymer of tetrafluoroethylene and perfluorinated vinyl ether) and sulfonic acid (-SO₃H) or phosphotungstic acid (PWA) modified nanosilica (Si-SO₃H or Si-PWA, respectively), for direct methanol fuel cell (DMFC) applications are described. Physical characteristics of these manufactured nanocomposite membranes were investigated by scanning electron microscopy (SEM), water uptake, methanol permeability and ion exchange capacity, as well as proton conductivity. The Nafion®/Si-PWA and Nafion®/Si-SO₃H membranes showed higher proton conductivity, lower methanol permeability and, as a consequence, a higher selectivity parameter, in comparison to the neat Nafion® or Nafion®/pristine nanosilica membranes. The obtained results indicated that both the Nafion®/Si-PWA and Nafion®/Si-SO₃H membranes could be utilized as promising polyelectrolyte membranes for direct methanol fuel cell applications.     

Polyelectrolyte Nanocomposite Membranes with Imidazole-Functionalized Multi-Walled Carbon Nanotubes for Use in Fuel Cell Applications

The preparation and characterization of a new type of nanocomposite polyelectrolyte membrane (PEM), based on Nafion® and imidazole modified multi-walled carbon nanotubes (MWCNT-Im), for direct methanol fuel cell (DMFC) applications is described. Related to the interactions between the protonated imidazole groups, grafted on the surface of multi-walled carbon nanotubes (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 also higher proton conductivity. The physical characteristics of these manufactured nanocomposite membranes were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), water uptake, methanol permeability and ion exchange capacity, as well as proton conductivity. The Nafion®/MWCNT-Im membranes showed higher proton conductivity, lower methanol permeability and, as a consequence, a higher selectivity parameter in comparison to neat Nafion® or Nafion® containing –OH functionalized multi-walled carbon nanotubes (MWCNT-OH) membranes. The obtained results indicated that the Nafion®/MWCNT-Im membranes could be utilized as efficient polyelectrolyte membranes for direct methanol fuel cell applications.     

Design of novel SPEEK-based proton exchange membranes by self-assembly method for fuel cells

Fuel cell represents a new energy conversion device, which promises to provide clean source of power. Fuel cell [particularly proton exchange membrane fuel cell and direct methanol fuel cell (DMFC)] is a promising candidate for transportation and portable power source applications. In DMFC, there is a problem of methanol crossover. In order to reduce such a problem, there has been an intensive research activity in the modification of Nafion. In the present investigation, self-assembled membranes were fabricated with sulfonated polyether ether ketone as the core part of the membrane. Aminated polysulfone and sulfonated polysulfone were used as the layers in order to prevent the crossover of methanol. The assembled membranes were characterized by ion exchange capacity, water and methanol absorption, and durability. The methanol permeability and selectivity ratio proved a strong influence on DMFC application. Scanning electron microscopy proved smooth surface, which established strong cohesive force for the polymer chains. Among the synthesized self-assembled membranes, the membrane with two bilayers was the best in terms of power density in DMFC. The membrane electrode assembly with two bilayers showed higher performance (~61.05 mW/cm²) than sulfonated poly(ether ether ketone) and Nafion in DMFC.     

Effect of solvent and crystal size on the selectivity of ZSM-5/Nafion composite membranes fabricated by solution-casting method

Zeolite/Nafion composite membranes with high proton selectivity were successfully fabricated using the solution-casting method. The types of zeolites are nano-sized and large sized Na-ZSM-5, H-ZSM-5, and their ball-milled ones. Two different schemes of experiments were conducted depending on the type of solvent. In case of using as-received Nafion® ionomer dispersions, the experimental results clearly show that the proton conductivity of zeolite composite membrane using either H-type or Na-type ZSM-5 depends on the type of solvent. It is thought that when propanol and water as the solvents were used, more hydrophilic H-type ZSM-5 seems to have been more randomly dispersed into hydrophobic region rather than hydrophilic ionic clustered channels within Nafion. Therefore, H-type ZSM-5 existing near hydrophobic region seems to provide additional path for proton migration but weakening the mechanical strength. These composite membranes show higher water uptake than commercial Nafion® 115, strongly suggesting better water retention ability of zeolite. The most interesting result is that the methanol permeability has decreased with increasing zeolite contents even when the proton conductivity increased, and the proton selectivities of these composite membranes expressed as characteristic factor were higher than that of Nafion® 115. In case of using a mixture of high boiling point DMF and ethanol as the solvent, unlike the previous case where no DMF was used, the proton conductivity slightly dropped with increasing zeolite contents. These results should have been attributed to a blocking effect of zeolite particles surrounded by inversely oriented hydrophilic micelles of Nafion. However, the values of proton conductivity of most composite membranes were significantly higher than that of Nafion® 115, and methanol permeability also decreased with increasing zeolite contents. The significantly lower methanol permeability of the composite membrane fabricated with DMF as the solvent is probably due to the more effective blocking effect of H-ZSM-5 for ionic clustered channels as well as difficult transport of methanol through zeolite pores.In case of the composite membranes containing ZSM-5 with large crystal size, it is found that the methanol permeability has increased considerably with the increasing of zeolite contents due to void fractions between polymer phases and zeolite particles. In case of using ball-milled ZSM-5 with small crystal size, however, the value of characteristic factor tends to increase with increasing zeolite contents. Consequently, it is seen that the characteristic factor of Zeolite/Nafion composite membranes was much higher than Nafion® 115. The results obtained throughout this study strongly suggest that zeolites with small crystal size and high hydrophilicity are very prospective for composite membrane for direct methanol fuel cells in the future.     

Improving the Performance of Nafion®-Based Fuel Cell Membranes by Introducing Histidine Functionalized Carbon Nanotubes

A new polymer nanocomposite membrane based on Nafion and functionalized carbon nanotubes (CNTs) was developed for proton exchange membrane fuel cell (PEMFC) applications. Histidine, an imidazole-based amino acid, was used for modifying the surface of CNTs. The modification of CNTs was characterized by means of Fourier transform infrared spectroscopy (FTIR) and their Zeta potential. The imidazole groups, due to forming and breaking of hydrogen bonding, can facilitate proton transport across the polymer matrix by the Grotthuss mechanism. The final structure of the Nafion/CNT nanocomposites was investigated by small angle X-ray scattering (SAXS). The results confirm that the transport properties of the fabricated new membranes were significantly improved in comparison with unmodified and conventional Nafion® membranes. The power density of the imidazole-CNT (Im-CNT) Nafion® composite membranes was about three times more than Nafion® membranes. Also, the experimental results showed that the proton conductivity for the conventional Nafion® membranes decreased over 100°C but the conductivity for the Nafion®/Im-CNT remained at a nearly constant value above 100°C up to 120°C. Thus, the nanocomposite based on Nafion/imidazole functionalized CNT can be considered as an anhydrous PEMFC membrane for high-temperature applications.     

Pore-filling electrolyte membranes based on plasma-activated microporous PE matrices and sulfonated hydrogenated styrene butadiene block copolymer (SHSBS): Single cell test and impedance spectroscopy in symmetrical mode

In this research the performance of proton exchange membrane fuel cells (PEMFC's) was studied, using pore-filling electrolyte membranes based on plasma-activated microporous polyethylene (PE) matrices coated with sulfonated hydrogenated butadiene–styrene block copolymer (SHSBS). The voltage–current and power density curves were recorded under different experimental temperature and pressure conditions. In addition, an electrochemical study was completed by means of electrochemical impedance spectroscopy (EIS) in the symmetrical mode, adjusting the electrical response obtained to an equivalent circuit, which allows for isolation of the different processes occurring within the system. Two parameters were taken into account in the study: the membrane's proton transport or ion resistance (R₁) and its charge transfer resistance (R₂).The results obtained indicate that SHSBS shows a single cell behaviour which is comparable to that of the commercial membrane Nafion®. In contrast, the performance of the PE–SHSBS pore-filling electrolyte membranes was lower than that of Nafion®. Likewise it was found that the different plasma treatments applied to the microporous PE matrix have an effect on the proton exchange capacity of the pore-filling electrolyte membrane. EIS allowed to determine the ion resistance of the proton exchange membrane, and it was demonstrated that the kinetics of the cathodic reaction and the cathode itself are decisive elements in membrane performance and hence prime objectives to be optimized, when the reduction of the overvoltages is at stake, which are currently observed in the polarization curve at low and high power densities.     

Ion-beam formation of electrocatalysts for fuel cells with polymer membrane electrolyte

Active layers of electrocatalysts are prepared by the ion-beam assisted deposition (IBAD) of platinum onto carbon-based AVCarb® Carbon Fiber Paper P50 and Toray Carbon Fiber Paper TGP-H-060 T supports and Nafion® N 115 polymer membrane electrolyte in the mode where the deposited metal ions are used as ions assisting the deposition process. Metal deposition and mixing of the deposited layer with the substrate under an accelerating voltage of 10 kV by the same metal ions are carried out from a neutral fraction of metal vapor and the ionized plasma of a pulsed vacuum-arc discharge, respectively. The composition and microstructure of the surface layers obtained are studied by Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM), electron-probe microanalysis (EPMA), and X-ray fluorescence (XRF) analysis. The platinum concentration in the layers is (0.5–1.5) × 10¹⁶ at/cm². The prepared electrocatalysts exhibit activity in the process of the electrochemical oxidation of methanol and ethanol, which form the basis for the principle of operation of low temperature fuel cells (direct methanol fuel cells (DMFC) and direct ethanol fuel cells (DEFC)).     

Sulfonated Aromatic Polymers and Organically Modified Montmorillonite Nanocomposite Membranes for Fuel Cells Applications

Aromatic polymers, such as sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO), sulfonated poly(ether ether ketone) (SPEEK), and sulfonated poly(ether sulfone) (SPES), at the optimum degrees of sulfonation (DS), are suggested and evaluated as alternatives to Nafion for direct methanol fuel cells (DMFCs) applications. To reduce the methanol cross-over, which decreases the efficiency of the cell, organically modified montmorillonite nanoclays (OMMT) were added at 1 wt% to the sulfonated matrices with the optimum DS. The X-ray diffraction (XRD) patterns of nanocomposite membranes proved that the nanoclay layers were exfoliated. The proton conductivity and methanol permeability of the membranes, as well as the ion-exchange capacity (IEC), were measured. The selectivity parameter, ratio of proton conductivity to methanol permeability, was identified at 25°C for the nanocomposite membranes and the results were compared with Nafion117. Finally, the DMFC performance tests were investigated at 70°C and 5 M methanol feed for the manufactured nanocomposite polyelectrolyte membranes (PEMs). The SPEEK-based nanocomposite membrane showed the highest maximum power density in comparison with Nafion 117 and SPES and SPPO nanocomposite membranes. The results indicated that the nanocomposite membranes were promising PEMs for DMFC applications.     

Surface-modified Nafion membrane by trioctylphosphine-stabilized palladium nanoparticles for DMFC applications

Trioctylphosphine (TOP)/Pd composites have been synthesized and used as a methanol-barrier material to modify the surface of Nafion 115. The TOP/Pd composites have been applied to the surface of Nafion instead of being incorporated into the Nafion matrix, to provide the best chance of maintaining the inherent proton conductivity of Nafion. The properties of the TOP/Pd-modified membrane, in terms of its conductivity and methanol permeability, as well as the performance of the membrane electrode assembly (MEA) in direct methanol fuel cell (DMFC), have been analyzed and compared with those of bare Nafion. The DMFC performance of the TOP/Pd-modified membrane is somewhat better than that of the bare Nafion one at methanol concentration of 2 M and significantly better at a high concentration of 5 M. The TOP/Pd-modified membrane is able to operate the DMFC using a high concentration of methanol, which can satisfy the requirement to reduce the reactant volumes for portable applications as well as to achieve high performance. In contrast to bare Nafion, the TOP/Pd-modified membrane with its well-adhering and crack-free modified surface shows effect on reducing the methanol loss.     

Improvement of the barrier properties of Nafion® by fluoro-modified montmorillonite

Montmorillonite (MMT) was modified by two types of cationic fluorosurfactants in order to improve compatibility with and dispersion within Nafion® membranes. Perfluoropolyether-containing cationic surfactant improved efficiency, by improving the barrier properties of Nafion® towards methanol. Moreover, the fluoro-modified MMT had no deleterious impact on the ionic conductivity of the membranes in contrast to conventional organo-modified MMT. The performances of a small size fuel cell were accordingly improved.     

直接甲醇燃料电池

文章简要介绍了直接甲醇燃料电池(DMFC)的主要特点、工作原理、应用前景、技术状态及其商业化面临的挑战，开发能够提高甲醇氧化和氧还原动力学速率的低成本电催化剂，制备具有高质子电导率、低甲醇渗透率的电解质膜，实现高性能、长寿命的电极、膜电极制备技术以及系统集成技术的突破对DMFC商业化至关重要。     

Proton conduction in oxyacid salts at intermediate temperatures for fuel cell applications

Materials that are proton conducting at intermediate temperatures are of prime importance for fuel cell applications. For example, by lowering the operating temperature of a solid oxide fuel cell the severe material problems encountered at high temperatures may be avoided while fast kinetics at the electrodes may still be obtained. In this article a number of proton conducting solid materials based on oxyacid salts are discussed, materials with and without structural protons, as well as composites. Paper presented at the 97th Xiangshan Science Conference on New Solid State Fuel Cells, Xiangshan, Beijing, China, June 14–17, 1998.     

The effect of cell orientations and environmental conditions on the performance of a passive DMFC single cell

A single cell passive air-breathing liquid feed direct methanol fuel cell (DMFC) is designed and fabricated. Furthermore, the effects of cell orientation and environmental conditions such as temperature and relative humidity on the performance of such passive DMFC are tested experimentally. The obtained results indicate that both environmental temperature and relative humidity have significant effects on the performance of fabricated fuel cell. The experimental data contained within this work shows that under lower relative humidity and higher temperature, the passive air-breathing direct methanol fuel cell has higher power output and better performance. According to experimental results, flooding has a vital role on the cell performance in various relative humidity and temperatures. The results also show that cell orientation has a strong effect on the performance of passive DMFC. The best power output and performance were achieved under vertical orientation.     

Does power ultrasound affect Nafion® dispersions?

The effect of low frequency power ultrasound on Nafion® ionomer used for fabricating proton exchange membrane fuel cell (PEMFC) and water electrolyzer (PEMWE) catalyst inks was investigated. In this study, a series of Nafion® dispersions having three concentrations (10, 5, and 2.5% w/v) were studied under various irradiation durations (t_us), at fixed ultrasonic frequency (f = 42 kHz) and ultrasonic power (P > 2 W), under either controlled or unregulated bulk solution temperature conditions using a laboratory ultrasonic cleaning bath. Viscosity (η), thermal degradation, and glass transition temperature (T_g) for all Nafion® dispersion samples was measured and compared to untreated Nafion® samples. In our conditions, it was found that power ultrasound lowered the viscosity of all tested Nafion® dispersion samples; whilst thermogravimetric and differential scanning calorimetry analyses showed that for all ultrasonically irradiated samples, a negligible overall polymer degradation and no obvious change in T_g was observed under controlled and unregulated bulk temperature conditions. It was found that it is possible that acoustic cavitation causes depolymerisation followed by a polymerisation initiation step during ultrasonication. By comparing the ultrasonically treated and high-shear mixed samples, it was also observed that acoustic and hydrodynamic cavitation played an important role in the reduction of dispersion viscosity.     

直接甲醇燃料电池膜及阴极模拟

本文提出一个针对直接甲醇燃料电池膜及阴极的二维、多组分稳态数学模型．模型根据直接甲醇膜燃料电池膜及阴极运行工况特性,考虑质量、动量、组分守恒以及电池中的电化学过程而建立,并应用了计算流体动力学(CFD)技术．模拟结果表明传质对直接甲醇燃料电池的性能影响很大;本文还进行了直接甲醇燃料电池阴极水管理的初步探讨．     

Overview on grain-boundary and transport problems in solid oxide fuel cell

An overview is given on the present state of development of fuel cells based on solid oxygen ion conductors. The performance is compared to other types of fuel cells. The employed electrolyte and electrode materials and current problems are described. The possibilities of reduced temperature and the reduction of internal resistances is discussed. Paper presented at the 97th Xiangshan Science Conference on New Solid State Fuel Cells, Xiangshan, Beijing, China, June 14–17, 1998.     

Polyelectrolyte Nanocomposite Membranes,Based on Chitosan-phosphotungstic Acid Complex and Montmorillonite for Fuel Cells Applications

A polyelectrolyte complex (PEC) of chitosan and phosphotungstic acid (PWA) was prepared and characterized as a proton-conducting membrane for direct methanol fuel cell (DMFC) applications. Fourier transform infrared spectroscopy showed the presence of stable PWA in PEC. To reduce the methanol permeability, several amounts of montmorilonite (MMT) nanoclays (trade name: Cloisite Na) were introduced to the system. The X-ray diffraction patterns of nanocomposite membranes proved the nanoclay layers were exfoliated in the membranes at loading weights of MMT lower than 3 wt%. Proton conductivity and methanol permeability were measured. According to the selectivity parameter—ratio of proton conductivity to methanol permeability—PEC containing 2 wt% MMT (PEC/2 wt% MMT) was identified as the optimum composition. Finally, DMFC performance tests were investigated at 70°C and 5 M methanol feed and the optimum membrane showed higher maximum power density in comparison with Nafion 117. The results indicated the optimum nanocomposite membrane is a promising polyelectrolyte membrane (PEM) for DMFC applications.     

Strengthen the performance of sulfonated poly(ether ether ketone) as proton exchange membranes with phosphonic acid functionalized carbon nanotubes

Nanocomposite polymers based on phosphonic acid functionalized carbon nanotubes (CNT-POH) and sulfonated poly(ether ether ketone) (SPEEK) have been fabricated and employed as highly efficient proton exchange membranes. CNT-POH were synthesized through the grafting of carbon nanotubes (CNT) with diethylphosphatoethyl triethoxysilane and subsequent acidification of phosphate to phosphonic acid ligands. Incorporating CNT-POH into SPEEK matrix improves the proton conductivity at different temperatures and relative humidity, which can be attributed to the homogeneous dispersion of highly hydrophilic phosphonic acid groups and the formation of proton transport channels in the membrane. The methanol permeability of the composite membranes is also decreased, owing to the increased tortuosity of the methanol transport channel. The CNT in SPEEK matrix also enhance the dimensional stability and mechanical property remarkably. Consequently, this phosphonic acid functionalized CNT/SPEEK composite membrane (SPEEK-POH) is a potential candidate for application in direct methanol fuel cells (DMFC).     

Study on Electrical Conductivity and Mechanical Performance of Polymeric Composite Flow Field Plates for Fuel Cell Applications

Fuel cell technology has received a great deal of attention as a candidate for an alternative power source due to its environmental benefits compared to conventional sources. Flow-field plates are one of the most important components of the Proton Exchange Membrane (PEM) and Direct Methanol (DM) fuel cells technology, which is expected to possess high electrical and thermal conductivity, low hydrogen permeability, and good mechanical performance. In this study, a systematic investigation was carried out to formulate and fabricate electrically conductive polymer composites to be used in production of flow field plates using a simple low temperature, single step moulding process. It focused on the careful selection of both materials and processing methodology. ElectroPhen^? binders, various concentrations of graphite powder and enhanced dispersion agents were employed to make composite plates with high electrical conductivity and satisfactory mechanical performance. Electrical conductivity measurements were carried out using purpose-built in-plane and through-plane electrical conductivity test kits. Flexural tests were carried out to evaluate the mechanical performance of the composites. It has been found that polymer composite flow field plates can attain the electrical conductivity and mechanical performance requirements for commercial fuel cell applications without requiring a high temperature multi-step production process.