Nafion-impregnated polyethylene-terephthalate film used as the electrolyte for direct methanol fuel cells

An electrolyte system for direct methanol fuel cells (DMFC) was prepared by basing its design on combination of the existing concepts such as a composite membrane and a barrier against methanol cross-over. A composite membrane was prepared by impregnating Nafion ionomer into 6 μm thick hydrophilic PETE (polyethylene-terephthalate) film whose average pore diameter was 0.3 μm. Owing to some features of the film such as high mechanical strength and less than 30% of its surface being occupied by the openings for proton passage, it was far more effective to block the passage with a very thin palladium film with the aim of preventing methanol cross-over. The DMFCs utilizing such a thin composite membrane as the electrolyte exhibited improved performances over those using conventional Nafion in the possibility of ambient temperature operation, higher current densities, and substantial reduction of methanol cross-over.     

Performance comparison of portable direct methanol fuel cell mini-stacks based on a low-cost fluorine-free polymer electrolyte and Nafion membrane

A low-cost fluorine-free proton conducting polymer electrolyte was investigated for application in direct methanol fuel cell (DMFC) mini-stacks. The membrane consisted of a sulfonated polystyrene grafted onto a polyethylene backbone. DMFC operating conditions specifically addressing portable applications, i.e. passive mode, air breathing, high methanol concentration, room temperature, were selected. The device consisted of a passive DMFC monopolar three-cell stack. Two designs for flow-fields/current collectors based on open-flow or grid-like geometry were investigated. An optimization of the mini-stack structure was necessary to improve utilization of the fluorine-free membrane. Titanium-grid current collectors with proper mechanical stiffness allowed a significant increase of the performance by reducing contact resistance even in the case of significant swelling. A single cell maximum power density of about 18 mW cm⁻² was achieved with the fluorine-free membrane at room temperature under passive mode. As a comparison, the performance obtained with Nafion 117 membrane and Ti grids was 31 mW cm⁻². Despite the lower performance, the fluorine-free membrane showed good characteristics for application in portable DMFCs especially with regard to the perspectives of significant cost reduction.     

Operation characteristics of portable direct methanol fuel cell stack at sub-zero temperatures using hydrocarbon membrane and high concentration methanol

Cold start and operation of a direct methanol fuel cell (DMFC) are investigated at sub-zero temperatures by using a 10-cell stack. The stack is manufactured with a hydrocarbon membrane to minimize the methanol crossover problem, which can be caused by use of high concentration methanol solutions. The stack is heated up for the cold start and operation only by heat of the exothermic reactions without any heating device and additional insulation means, to examine operation characteristics of the DMFC stack at low temperatures. The concentration of methanol solutions is selected in the range of 3-8 M, considering the freezing points of the solution for corresponding operation temperatures (−5 to −15 °C). Although the DMFC stack undergoes a sharp voltage drop and a significant performance decrease at the initial stage of the frozen condition, the self-heating DMFC are successfully operated at −5 and −10 °C in both constant current or constant voltage modes. The cold start-up time also is nearly independent of the operating modes. In contrast, the stack at −15 °C is barely started up only by a constant voltage mode with some voltage fluctuation. The DMFC stack after the cold operation exhibits the performance loss of about 45%. Such performance loss is mainly caused by degradation of the electrocatalysts.     

Nafion-TiO2 composite DMFC membranes: physico-chemical properties of the filler versus electrochemical performance

TiO₂ nanometric powders were prepared via a sol-gel procedure and calcined at various temperatures to obtain different surface and bulk properties. The calcined powders were used as fillers in composite Nafion membranes for application in high temperature direct methanol fuel cells (DMFCs). The powder physico-chemical properties were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and pH measurements. The observed characteristics were correlated to the DMFC electrochemical behaviour. Analysis of the high temperature conductivity and DMFC performance reveals a significant influence of the surface characteristics of the ceramic oxide, such as oxygen functional groups and surface area, on the membrane electrochemical behaviour. A maximum DMFC power density of 350 mW cm⁻² was achieved under oxygen feed at 145 °C in a pressurized DMFC (2.5 bar, anode and cathode) equipped with TiO₂ nano-particles based composite membranes.     

Enhanced transport properties in polymer electrolyte composite membranes with graphene oxide sheets

Polymer electrolyte membranes have been widely investigated for high performance fuel cells. Here, we report the synthesis of ionic conductive Nafion/graphene oxide (GO) composite membranes for application in direct methanol fuel cells. GOs interact with both the non-polar backbone and the polar ionic clusters of Nafion because of their amphiphilic characteristics attributable to hydrophobic conjugation and hydrophilic functional groups. Accordingly, GO sheets serve to modify the microstructures of two domains of Nafion. In particular, the transport properties of Nafion are favorably manipulated by the incorporation of GO. This modulated the ionic channels of Nafion and decrease methanol crossover while preserving ionic conductivity. Furthermore, strong interfacial interactions due to the insertion of GO nanofillers into the Nafion matrix improve the thermal and mechanical properties of the material. In particular, we exploit Nafion/GO composite membrane as electrolyte material for direct methanol fuel cell (DMFC) in order to resolve current issue of methanol crossover. This composite membrane-based DMFC compared to the Nafion 112-based DMFC remarkably enhanced cell performance, especially in severe operating conditions.     

Mass transport of direct methanol fuel cell species in sulfonated poly(ether ether ketone) membranes

Homogeneous membranes based on sulfonated poly(ether ether ketone) (sPEEK) with different sulfonation degrees (SD) were prepared and characterized. In order to perform a critical analysis of the SD effect on the polymer barrier and mass transport properties towards direct methanol fuel cell species, proton conductivity, water/methanol pervaporation and nitrogen/oxygen/carbon dioxide pressure rise method experiments are proposed. This procedure allows the evaluation of the individual permeability coefficients in hydrated sPEEK membranes with different sulfonation degrees. Nafion^® 112 was used as reference material. DMFC tests were also performed at 50 °C. It was observed that the proton conductivity and the permeability towards water, methanol, oxygen and carbon dioxide increase with the sPEEK sulfonation degree. In contrast, the SD seems to not affect the nitrogen permeability coefficient. In terms of selectivity, it was observed that the carbon dioxide/oxygen selectivity increases with the sPEEK SD. In contrast, the nitrogen/oxygen selectivity decreases. In terms of barrier properties for preventing the DMFC reactants loss, the polymer electrolyte membrane based on the sulfonated poly(ether ether ketone) with SD lower or equal to 71%, although having slightly lower proton conductivity, presented much better characteristics for fuel cell applications compared with the well known Nafion^® 112. In terms of the DMFC tests of the studied membranes at low temperature, the sPEEK membrane with SD = 71% showed to have similar performance, or even better, as that of Nafion^® 112. However, the highest DMFC overall efficiency was achieved using sPEEK membrane with SD = 52%.     

电解重整式甲醇燃料电池系统

为解决直接甲醇燃料电池中甲醇氧化活性低及甲醇穿透问题,提出一种新型的电解重整式甲醇燃料电池系统。在系统中,高温电解重整器重整甲醇为常温燃料电池供氢,燃料电池的部分电能供给电解重整器使用。通过对系统的物料流、焓流、有效能流的分析,确定了系统中的不可逆因素。结果表明:电解重整器电压是影响系统效率的重要参数;升高重整器温度可以显著降低其电压,但必须采用合理的压力、甲醇溶液浓度以抑制甲醇溶液的蒸发,降低热量损失。与传统的直接甲醇燃料电池系统以及高温甲醇热重整联合质子交换膜燃料电池系统相比,该系统性能较高、结构紧凑。     

Investigation of passive DMFC mini-stacks at ambient temperature

Two designs of flow fields/current collectors for a passive direct methanol fuel cell (DMFC) monopolar three-cell stack were investigated. The first one (A) consisted of two plastic plates covered by thin gold film current collectors in the area of electrodes with a distribution of holes through which methanol (from a reservoir) and air (from ambient) could diffuse into the electrodes. The second design (B) consisted of thin gold film deposited on the external borders of the fuel and oxidant apertures where the electrodes were placed in contact. A big central hole allowed a direct exposure of electrodes to ambient air (for the cathodes) and methanol solution (for the anodes). An investigation of the performance and discharge behaviour of the two designs was carried out. The advantages and disadvantages of each configuration were analysed. Similar performances in terms of maximum power were recorded; whereas, better mass transport characteristics were obtained with the design B. On the contrary, open circuit voltage (OCV) and stack voltage at low current were higher for the design A as a consequence of lower methanol cross-over. A longer discharge time (17 h) with a unique MeOH charge was recorded with design B at 250 mA compared to the design A (5 h). This was attributed to an easier CO₂ removal from the anode and better mass transport properties.     

Optimization of the sputter-deposited platinum cathode for a direct methanol fuel cell

The electrodes prepared by a sputtering method were evaluated as the cathodes for direct methanol fuel cells (DMFCs). Pt loading below 0.25 mg cm⁻² achieved higher mass activities than that of 0.5 mg cm⁻² prepared by the paste method, which was general conventional method. However, an increase in Pt loading reduced the catalyst activity for the oxygen reduction reaction (ORR). This result may suggest an increase in only electrochemically inactive Pt. Pt utilization efficiency can be found about ten times higher at Pt loading of 0.04 mg cm⁻². Moreover, addition of Nafion to sputter-deposited Pt cathodes is found possible to improve the catalyst activity for the ORR, but the excess Nafion over the optimum condition reduces the active sites.     

Preparation and DMFC performance of a sulfophenylated poly(arylene ether ketone) polymer electrolyte membrane

A sulfonated poly(aryl ether ether ketone ketone) (PEEKK) having a well-defined rigid homopolymer-like chemical structure was synthesized from a readily prepared PEEKK by post-sulfonation with concentrated sulfuric acid at room temperature within several hours. The polymer electrolyte membrane (PEM) cast from the resulting polymer exhibited an excellent combination of thermal resistance, oxidative and dimensional stability, low methanol fuel permeability and high proton conductivity. Furthermore, membrane electrode assemblies (MEAs) were successfully fabricated and good direct methanol fuel cell (DMFC) performance was observed. At 2 M MeOH feed, the current density at 0.5 V reached 165 mA/cm, which outperformed our reported similarly structured analogues and MEAs derived from comparative Nafion^® membranes.     

Optimization of operating parameters of a direct methanol fuel cell and physico-chemical investigation of catalyst–electrolyte interface

A physico-chemical investigation of catalyst–Nafion® electrolyte interface of a direct methanol fuel cell (DMFC), based on a Pt–Ru/C anode catalyst, was carried out by XRD, SEM-EDAX and TEM. No interaction between catalyst and electrolyte was detected and no significant interconnected network of Nafion micelles inside the composite catalyst layer was observed. The influence of some operating parameters on the performance of the DMFC was investigated. Optimal conditions were 2 M methanol, 5 atm cathode pressure and 2–3 atm anode pressure. Power densities of 110 and 160 mW cm⁻² were obtained for operation with air and oxygen, respectively, at temperatures of 95–100°C and with 1 mg cm⁻² Pt loading.     

Synthesis and characterization of Nafion-stabilized Pt nanoparticles for polymer electrolyte fuel cells

Platinum nanoparticles are synthesized by alcohol reduction method using Nafion as a stabilizer under various conditions such as the Nafion/Pt molar ratio and reflux temperature. Nafion-Pt nanoparticles are characterized by agglomeration and the particle size is typically in the range of 2-4 nm. The electrocatalytic activity of Nafion-Pt nanoparticles for polymer electrolyte and direct methanol fuel cells (PEFCs and DMFCs) is investigated in comparison to that of unsupported Pt black and carbon-supported Pt/C electrocatalysts. Nafion-Pt nanoparticles prepared with low Nafion/Pt ratios show higher and/or comparable activities towards O₂ reduction reaction in the absence and presence of methanol in comparison to that of Pt black and Pt/C electrocatalysts. In contrast, the electrocatalytic activity of the Nafion-Pt nanoparticles for the methanol oxidation reaction is very low. The results indicate that Pt nanoparticles embedded in Nafion polyelectrolyte are potential methanol tolerant electrocatalysts for the O₂ reduction reaction in DMFCs.     

Electrokinetic transport of water and methanol in Nafion membranes as observed by NMR spectroscopy

Electrophoretic NMR (eNMR) and pulsed-field-gradient NMR (PFG-NMR) methods were used to study transport processes in situ and in a chemically resolved manner in the electrolyte of an experimental direct methanol fuel cell (DMFC) setup, constituted of several layers of Nafion 117. The measurements were conducted at room temperature for membranes fully swollen by methanol-water mixtures over a wide concentration interval. The experimental setup and the experimental protocol for the eNMR experiments are discussed in detail. The magnitude of the water and methanol self-diffusion coefficients show a good agreement with previously published data while the ratio of the two self-diffusion coefficients may indicate an imperfect mixing of the two solvent molecules. On the molecular level, the drag of water and methanol molecules by protons is roughly of the same magnitude, with the drag of methanol molecules increasing with increasing methanol content. The electro-osmotic drag defined on mass-flow basis increased for methanol from a low level with increasing methanol concentration while that of water remained roughly constant.     

Non-Fluorinated Membranes Thickness Effect on the DMFC Performance

Abstract

In order to study the influence of the proton exchange membrane thickness on the direct methanol fuel cell (DMFC) performance, sulfonated poly (ether ether ketone) (sPEEK) membranes with a sulfonation degree (SD) of 42% and thicknesses of 25, 40, and 55 µm were prepared, characterized, and tested in a DMFC. These polymeric membranes were tested in a DMFC at several temperatures by evaluating the current-voltage polarization curve, the open circuit voltage (OCV) and the constant voltage current (CV, 35 mV). The CO₂ concentration at the cathode outlet was also measured. The thinnest sPEEK membrane proved to have the best DMFC performance, although having lower Faraday efficiency (lower ohmic losses but higher methanol permeation). In contrast, the thickest membrane presented improved properties in terms of methanol permeation (lower methanol crossover). DMFC tests results for this membrane showed 30% global efficiency, obtained with pure oxygen at the cathode feed.     

A study on direct glucose and fructose alkaline fuel cell

Direct glucose fuel cell (DGFC) has huge potential as a power source in low power long term portable devices. Electro-oxidation of glucose and fructose on PtRu/C catalyst are studied using cyclic voltammetry in alkaline medium to study the reason for deactivation of glucose fuel cell. A simple direct glucose fuel cell with PtRu/C as anode and activated charcoal as cathode was constructed and operated to study the effect of different temperature and concentration of glucose and KOH. An open-circuit voltage (OCV) of 0.91 V is obtained using 0.3 M glucose in 1 M KOH solution. OCV increased with the increase in glucose concentration. The maximum peak power density of 1.38 mW cm⁻² is obtained using 0.2 M glucose in 1 M KOH at 30 °C and it decreases with further increase in glucose concentration and temperature. In order to determine the reason for decrease in performance of glucose fuel cell due to conversion of glucose to fructose, the fuel cell was operated using 0.2 M fructose in 1 M KOH. The peak power density delivered is 0.57 mW cm⁻². The DGFC is continuously operated for 260 h at constant load of 500 Ω produces final constant voltage of 0.21 V.     

The influence of sodium ion as a potential fuel impurity on the direct methanol fuel cells

Na⁺ is a likely intrinsic impurity in water and is a sort of common cation impurity in the direct methanol fuel cells (DMFCs). In this paper, the effect of Na⁺ on the DMFC electrochemical response is studied by adding Na⁺ into the methanol water solution fed in the anode of DMFC. The dynamic variation of cell voltage results shows that the DMFC performance degraded by the presence of Na⁺ impurity, and the higher concentration of Na⁺ impurity, the higher poisoning rate is observed. In the meantime, an external reference electrode is used to measure the potential and impedance of the cathode and anode. It is found that the dramatic decrease of the cell voltage is mainly ascribed to the increase of the cathode overpotential which is caused by Na⁺ exchange with protons in the cathode catalyst layer. The electrochemical impedance measurements suggest that the lack of available protons and low oxygen concentration at the cathode catalytic sites contributed to this degradation. Furthermore, the recovery strategy is introduced and it is found that the poisoned MEA could be partly recovered by immersing in 0.5 M H₂SO₄ solution for 4 h.     

Prediction of methanol and water fluxes through a direct methanol fuel cell polymer electrolyte membrane

Development of a direct methanol fuel cell (DMFC) mass flux model, using conventional transport theory, is presented and used to predict the fluid phase superficial velocity, methanol and water molar fluxes, and the chemical species (methanol and water) dimensionless concentration profiles in the polymer electrolyte membrane, Nafion^® 117, of a DMFC. Implementation of these equations is illustrated to generate the numerical data as functions of the variables such as the pressure difference across the membrane, methanol concentration at the cell anode, temperature, and position in the membrane.     

直接甲醇燃料电池溶胶-凝胶流动相的制备与表征

对直接甲醇燃料电池溶胶-凝胶流动相的制备工艺进行了分析,采用溶胶-凝胶法以正硅酸甲酯为前驱体制备出了溶胶-凝胶流动相.分别以溶胶-凝胶流动相和液体流动相为燃料对比研究了直接甲醇燃料电池的放电性能,测定了溶胶-凝胶流动相在Nation117膜中的甲醇渗透率,研究了溶胶-凝胶流动相的质子电导率.实验结果表明,使用溶胶-凝胶...     

PEO–PPO–PEO triblock copolymer/Nafion blend as membrane material for intermediate temperature DMFCs

This paper describes homogeneous triblock copolymer/Nafion blend membranes, which facilitate proton conduction in direct methanol fuel cells (DMFCs) at intermediate temperatures. The interaction between the two polymer components is investigated by FT-IR spectroscopy. The blend membranes show higher proton conductivity than recast Nafion under partially anhydrous conditions. Protons can be transported with the assistance of ether chain under such conditions at elevated temperature. In addition, the membranes exhibit more favourable methanol permeability and selectivity. This kind of blend membrane shows somewhat better performance in DMFC compared to bare recast Nafion at intermediate temperature (≥120 °C). This work is a first attempt in our group to design membrane materials with enhanced proton conductivity under conditions typical of intermediate temperature DMFCs.     

Nanometer-thick films of titanium oxide acting as electrolyte in the polymer electrolyte fuel cell

0-18 nm-thick titanium, zirconium and tantalum oxide films are thermally evaporated on Nafion 117 membranes, and used as thin spacer electrolyte layers between the Nafion and a 3 nm Pt catalyst film. Electrochemical characterisation of the films in terms of oxygen reduction activity, high frequency impedance and cyclic voltammetry in nitrogen is performed in a fuel cell at 80 °C and full humidification. Titanium oxide films with thicknesses up to 18 nm are shown to conduct protons, whereas zirconium oxide and tantalum oxide block proton transport already at a thickness of 1.5 nm. The performance for oxygen reduction is higher for a bi-layered film of 3 nm platinum on 1.5 or 18 nm titanium oxide, than for a pure 3 nm platinum film with no spacer layer. The improvement in oxygen reduction performance is ascribed to a higher active surface area of platinum, i.e. no beneficial effect of combining platinum with zirconium, tantalum or titanium oxides on the intrinsic oxygen reduction activity is seen. The results suggest that TiO₂ may be used as electrolyte in fuel cell electrodes, and that low-temperature proton exchange fuel cells could be possible using TiO₂ as electrolyte.