Low Methanol Crossover Research Articles

Comprehensive one-dimensional, semi-analytical, mathematical model for liquid-feed polymer electrolyte membrane direct methanol fuel cells

Polymer electrolyte membrane direct methanol fuel cells (PEM-DMFCs) have several advantages over hydrogen-fuelled PEM fuel cells; but sluggish methanol electrochemical oxidation and methanol crossover from the anode to the cathode through the PEM are two major problems with these cells. In the present work, a comprehensive one-dimensional, single phase, isothermal mathematical model is developed for a liquid-feed PEM-DMFC, taking into account all the necessary mass transport and electrochemical phenomena. Diffusion and convective effects are considered for methanol transport on the anode side and in the PEM, whereas only diffusional transport of species is considered on the cathode side. A multi-step reaction mechanism is used to describe the electrochemical oxidation of methanol at the anode. Stefan–Maxwell equations are used to describe multi-component diffusion on the cathode side and Tafel type of kinetics is used to describe the simultaneous methanol oxidation and oxygen reduction reactions at the cathode. The model fully accounts for the mixed potential effect caused by methanol crossover at the cathode. It shows excellent agreement with literature data of the limiting current density for different low methanol feed concentrations at different operating temperatures. At high methanol feed concentrations, oxygen depletion on the cathode side, due to excessive methanol crossover, results in mass-transport limitations. The model can be used to optimize the geometric and physical parameters with a view to extracting the highest current density while still keeping a tolerably low methanol crossover.

Blend Membranes Consisting of Sulfonated Poly(ether ether ketone) and Polysulfone Bearing 4-Nitrobenzimidazole for Direct Methanol Fuel Cells

Low-cost blend membranes consisting of sulfonated poly(ether ether ketone) (SPEEK) (an acidic polymer) and polysulfone-4-nitrobenzimidazole (PSf-NBIm) (a basic polymer) have been prepared with various PSf–NBIm contents and characterized by ion-exchange capacity, proton conductivity, and water uptake measurements. The blend membranes with PSf–NBIm contents of 2.5 and show higher proton conductivity and lower water uptake in water and methanol/water solutions at a given temperature than plain SPEEK. The blend membranes also exhibit better electrochemical performance and lower methanol crossover in a direct methanol fuel cell than plain SPEEK and Nafion 115 membranes due to an enhancement in proton conductivity through acid-base interactions and an insertion of the benzimidazole side groups into the ionic clusters of SPEEK, as indicated by small-angle X-ray scattering. The maximum power density of the blend membrane with a PSf–NBIm content of is 1.5 times higher than that of the Nafion 115 in methanol solution at .

A novel inorganic/organic composite membrane tailored by various organic silane coupling agents for use in direct methanol fuel cells

A series of organic silica/Nafion composite membranes has been prepared by using organic silane coupling agents (SCA) bearing different hydrophilic functional groups. The physico-chemical properties of the composite membranes have been characterized by electrochemical techniques, scanning electron microscopy (SEM), diffuse-reflection Fourier-transform infrared spectroscopy (DRFTIR), wide-angle X-ray diffraction (WAXRD), thermogravimetric analysis (TGA), and thermogravimetric mass spectrometry (TG-MS). It has been found that some organic silica/Nafion composite membranes modified by organic silane agents bearing amino groups exhibit extremely low methanol crossover and proton conductivity values, e.g., a composite membrane shows a proton conductivity that is about five orders of magnitude lower and a methanol permeability that is about three orders of magnitude lower than those of a Nafion117 membrane. However, under optimized conditions for controlling the basicity of the amino groups, we also obtained a composite membrane with 89% lower methanol permeability and 49% lower proton conductivity compared with Nafion117 membrane. The results clearly demonstrate that the diffusion of methanol and protons through the membrane can be controlled by adjusting the functional groups on the organic silica.

A novel MEA architecture for improving the performance of a DMFC

Direct methanol fuel cell (DMFC) consisting of a double-catalytic layered membrane electrode assembly (MEA) provide higher performance than that with the traditional MEA. This novel structured MEA includes a hydrophilic inner catalyst layer and a traditional electrode with an outer catalyst layer, which was made using both catalyst coated membrane (CCM) and gas diffusion electrode (GDE) methods. The inner catalyst was PtRu black on anode and Pt black on cathode. The outer catalyst was carbon supported Pt–Ru/Pt on anode and cathode, respectively. Thus in the double-catalytic layered electrodes three gradients were formed: catalyst concentration gradient, hydrophilicity gradient and porosity gradient, resulting in good mass transfer, proton and electron conducting and low methanol crossover. The peak density of DMFC with such MEA was 19 mW cm −2, operated at 2 M CH 3OH, 2 atm oxygen at room temperature, which was much higher than DMFC with traditional MEA.

Direct methanol fuel cell operation with pre-stretched recast Nafion ®

Direct methanol fuel cell operation with uniaxially pre-stretched recast Nafion ® membranes (draw ratio of 4) was investigated and compared to that with commercial (un-stretched) Nafion ®. The effects of membrane thickness (60–250 μm) and methanol feed concentration (0.5–10.0 M) on fuel cell power output were quantified for a cell temperature of 60 °C, ambient pressure air, and anode/cathode catalyst loadings of 4.0 mg cm −2. Pre-stretched recast Nafion ® in the 130–180 μm thickness range produced the highest power at 0.4 V (84 mW cm −2), as compared to 58 mW cm −2 for Nafion ® 117. MEAs with pre-stretched recast Nafion ® consistently out-performed Nafion ® 117 at all methanol feed concentrations, with 33–48% higher power densities at 0.4 V, due to a combination of low area-specific resistance (the use of a thinner pre-stretched membrane, where the conductivity was the same as that for commercial Nafion ®) and low methanol crossover (due to low methanol solubility in the membrane). Very high power was generated with a 180-μm thick pre-stretched recast Nafion ® membrane by increasing the cell temperature to 80 °C, increasing the anode/cathode catalyst loading to 8.0 mg cm −2, and increasing the cathode air pressure to 25 psig. Under these conditions the power density at 0.4 V for a 1.0-M methanol feed solution was 240 mW cm −2 and the maximum power density was 252 mW cm −2.

Non-Fluorinated Membranes Thickness Effect on the DMFC Performance

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.

Inorganic–organic hybrid polymers with pendent sulfonated cyclic phosphazene side groups as potential proton conductive materials for direct methanol fuel cells

A synthetic method is described to produce a proton conductive polymer membrane with a polynorbornane backbone and inorganic–organic cyclic phosphazene pendent groups that bear sulfonic acid units. This hybrid polymer combines the inherent hydrophobicity and flexibility of the organic polymer with the tuning advantages of the cyclic phosphazene to produce a membrane with high proton conductivity and low methanol crossover at room temperature. The ion exchange capacity (IEC), the water swelling behavior of the polymer, and the effect of gamma radiation crosslinking were studied, together with the proton conductivity and methanol permeability of these materials. A typical membrane had an IEC of 0.329 mmol g −1 and had water swelling of 50 wt%. The maximum proton conductivity of 1.13 × 10 −4 S cm −1 at room temperature is less than values reported for some commercially available materials such as Nafion. However the average methanol permeability was around 10 −9 cm s −1, which is one hundred times smaller than the value for Nafion. Thus, the new polymers are candidates for low-temperature direct methanol fuel cell membranes.

Sulfonated polysulfone with 1,3-1 H-dibenzimidazole-benzene additive as a membrane for direct methanol fuel cells

1,3-1 H-Dibenzimidazole-benzene (DBImBenzene) has been synthesized using phosphorus pentoxide-methanesulfonic acid (PPMA) as a solvent and dehydration agent and investigated as an additive (up to 2.0 wt.%) in sulfonated polysulfone (SPSf) membranes to promote proton conduction via acid–base interactions. The SPSf/DBImBenzene blend membranes with various DBImBenzene contents (0–2.0 wt.%) have been prepared and characterized by proton conductivity measurement and electrochemical polarization and methanol crossover measurements in direct methanol fuel cells (DMFCs). The blend membranes with DBImBenzene content of 0.5 and 1.0 wt.% show higher proton conductivities (3.4 and 2.9 × 10 −4 S/cm, respectively) than plain SPSf (2.4 × 10 −4 S/cm) even though the blend membranes have lower ion exchange capacity (0.81 and 0.75 mequiv./g, respectively) than plain SPSf (0.86 mequiv./g). The blend membranes exhibit better electrochemical performance in DMFC than plain SPSf membrane due to an enhancement in proton conductivity through acid–base interactions and lower methanol crossover.

Basic model for membrane electrode assembly design for direct methanol fuel cells

This research proposes a model that predicts the effect of the anode diffusion layer and membrane properties on the electrochemical performance and methanol crossover of a direct methanol fuel cell (DMFC) membrane electrode assembly (MEA). It is an easily extensible, lumped DMFC model. Parameters used in this design model are experimentally obtainable, and some of the parameters are indicative of material characteristics. The quantification of these material parameters builds up a material database. Model parameters for various membranes and diffusion layers are determined by using various techniques such as polarization, mass balance, electrochemical impedance spectroscopy (EIS), and interpretation of the response of the cell to step changes in current. Since the investigation techniques cover different response times of the DMFC, processes in the cell such as transport, reaction and charge processes can be investigated separately. Properties of single layers of the MEA are systematically varied, and subsequent analysis enables identification of the influence of the layer's properties on the electrochemical performance and methanol crossover. Finally, a case study indicates that the use of a membrane with lower methanol diffusivity and a thicker anode micro-porous layer (MPL) yields MEAs with lower methanol crossover but similar power density.

Low methanol crossover and high performance of DMFCs achieved with a pore-filling polymer electrolyte membrane

We compared the performance of the membrane electrode assembly for direct methanol fuel cells (DMFCs) composed of a pore-filling polymer electrolyte membrane (PF membrane) with that composed of a commercial Nafion-117 membrane. In DMFC tests, the methanol crossover flux was 23% lower in the PF membrane than in the Nafion-117 membrane even though the thickness of the PF membrane was 43% that of Nafion-117. This led to a higher DMFC performance and the lower overpotential of the cathode of the PF membrane. Feeding an aqueous 10 M methanol solution at 50 °C produced a low cathode overpotential, as low as 0.40 V at 0.2 A in the PF membrane, whereas the potential was 0.65 V at 0.2 A in the Nafion-117 membrane. In contrast, the ohmic loss and anode overpotential were almost the same in the two membranes. We confirmed that a reduction in methanol crossover using the PF membrane results in lower cathode overpotential and higher DMFC performance. In addition, the electro-osmotic coefficient was estimated as 1.3 in the PF membrane and 2.6 in Nafion-117, based on a water mass-balance model and values showing that the PF membrane prevents the flooding of the cathode at a low gas flow rate using. A highly concentrated methanol solution can be applied as a fuel without decreasing DMFC performance using PF membranes.

Acid–gel-immobilized, nanoporous composite, protonic membranes as low cost system for Direct Methanol Fuel Cells

An alternative, low cost, proton conducting electrolyte, designed for low and intermediate temperature DMCF and formed by new type of nanoporous, composite membrane in which sulphuric acid is immobilized by gelification, is here reported. The membrane is based on a polyvinylidene fluoride polymer matrix containing dispersed SiO 2 ceramic powder at nanoparticles size. The stability of the immobilized sulphuric acid gel combined with the favorable swelling effect of the extended membrane porosity give to the membrane a high proton conductivity, a low methanol crossover and a satisfactory thermal stability. Good performances both at room and intermediate temperatures are showed by Direct Methanol Fuel Cells which use this system as separator.

Sulfonated peek ion exchange membranes for direct methanol fuel cell applications

Sulfonation of polyetheretherketones (PEEK) was carried out in order to fabricate commercial perfluorosulfonic acid membrane alternatives, which were characterized in terms of their ion exchange capacity, ionic conductivity, water swelling, methanol crossover and electrochemical performance in their direct application as a methanol fuel cell. A high ion exchange capacity, 1.88, was achieved with a sulfonation reaction time of 8 h, with a significantly low methanol crossover low compared to that of Nafion. However, the morphological stability was found to deteriorate for membranes with sulfonation reaction times exceeding 8 h. Electrochemical cell tests suggested that the fabrication parameters of the membrane electrode assembly based on the sulfonated PEEK membranes should be optimized with respect to the physicochemical properties of the newly prepared membranes.

Evaluation of Methanol Crossover through SPEEK/TPA/Y-zeolite Composite Membranes by Electrochemical Method

This research work reports the evaluation of methanol crossover through newly developed composite membranes prepared from inorganic proton conducting material (tungstophosphoric acid (TPA)/Y-zeolite) with sulfonated polyether ether ketone (SPEEK) polymer. The composite membranes were prepared by embedding different proportions (10-40 wt %) of conducting material (TPA/Y-zeolite) into SPEEK polymer matrix. Methanol crossover through these membranes was studied using potentiometric technique. It was found that membranes with low loading of inorganic conducting material show better reduction in methanol crossover. The membranes are thermally stable up to around 160 ºC. These membranes have potential to be considered for use in low power methanol fuel cells due to their low methanol crossover, adequate stability and low cost.

Proton exchange membranes modified with sulfonated silica nanoparticles for direct methanol fuel cells

Nanocomposite proton exchange membranes were prepared from sulfonated poly(phthalazinone ether ketone) (sPPEK) and various amounts of sulfonated silica nanoparticles (silica-SO 3H). The use of silica-SO 3H compensates for the decrease in ion exchange capacity of membranes observed when non-sulfonated nano-fillers are utilized. The strong –SO 3H/–SO 3H interaction between sPPEK chains and silica-SO 3H particles leads to ionic cross-linking in the membrane structure, which increases both the thermal stability and methanol resistance of the membranes. The membrane with 7.5 phr of silica-SO 3H (phr = g of silica-SO 3H/100 g of sPPEK in membranes) exhibits low methanol crossover, high bound-water content, and a proton conductivity of 3.6 fold increase to that of the pristine sPPEK membrane.

An Extremely Low Methanol Crossover and Highly Durable Aromatic Pore‐Filling Electrolyte Membrane for Direct Methanol Fuel Cells

A highly durable pore-filling-type electrolyte membrane for direct methanol fuel cells has been developed from a porous polyimide substrate and a wholly aromatic hydrocarbon (see figure). The membrane succeeds in reducing the methanol crossover to several hundred times lower than that of a commercial Nafion membrane.

High Power Direct Methanol Fuel Cells Running on 15M to Neat Methanol

Recent fundamental studies have indicated the key requirement of low water crossover through the electrolyte membrane in order to enable the use of highly concentrated methanol in direct methanol fuel cells for portable electronics applications. In this paper, we shall demonstrate a single cell operated at 60oC with direct feed of methanol concentration ranging from 15M to pure methanol, while still achieving the power density of 60 mW/cm2 at 0.4V. Results of surprisingly low methanol and water crossover through a thin membrane, Nafion 112, will be reported and the interplay of water and methanol crossover will be discussed. Various components enabling such high cell performance and low crossover of water and methanol will be described, including fabrication and characterization of membrane-electrode assembly based on thin membranes (e.g. N112), methanol resistant anode structures, and cathode structures that control water crossover from the DMFC anode to the cathode.

Blend Membranes Based on Sulfonated Poly(ether ether ketone) and Polysulfone Bearing Benzimidazole Side Groups for DMFCs

Blend membranes consisting of sulfonated poly(ether ether ketone) (SPEEK), which is an acidic polymer, and polysulfone bearing benzimidazole side groups (PSf-BIm), which is a basic polymer, have been prepared with various PSf-BIm contents (0-10 wt %). The blend membranes have been characterized by ion exchange capacity, proton conductivity, liquid uptake, methanol permeability, and polarization measurements in direct methanol fuel cell (DMFC). The membranes with an optimum PSf-BIm content of ∼ 8 wt % exhibit electrochemical performance in DMFC better than that of plain SPEEK membrane due to an enhancement in proton conductivity through acid-base interactions and lower methanol crossover.

Fluorenyl-containing sulfonated poly(aryl ether ether ketone ketone)s (SPFEEKK) for fuel cell applications

A series of fluorenyl-containing sulfonated poly(aryl ether ether ketone ketone)s (SPFEEKK) were synthesized via aromatic nucleophilic substitution polymerization. The sulfonation content (SC) was controlled by the feed ratios of sulfonated and nonsulfonated monomers. Flexible and strong membranes in the sulfonic acid form were obtained from cast membranes in the sodium salt forms by treatment with acid. The thermal properties, water uptake, swelling ratio, water state, oxidative stability, proton conductivity and methanol permeability were investigated. All the polymers had proton conductivities greater than 1 × 10 −2 S/cm at room temperature, and the conductivity values of m-SPFEEKK-80 and p-SPFEEKK-80 were up to 1.86 × 10 −1 and 1.78 × 10 −1 S/cm at 100 °C. This series of polymers also possessed good dimensional stability in water and low methanol crossover.

Low Crossover of Methanol and Water Through Thin Membranes in Direct Methanol Fuel Cells

Low crossover of both methanol and water through a polymer membrane in a direct methanol fuel cell (DMFC) is essential for using high concentration methanol in portable power applications. A novel design of the membrane-electrolyte assembly (MEA) has been developed in this work to attain low methanol crossover, low water crossover, and high cell performance simultaneously. The anode catalyst layer, in the form of a catalyzed diffusion medium (CDM), serves as a methanol diffusion barrier to reduce methanol crossover. In addition, a highly hydrophobic cathode microporous layer (MPL) is employed to build up the hydraulic pressure at the cathode and hence drive the product water from the cathode into the anode to offset the water dragged by electro-osmosis. The new MEA, consisting of a CDM anode, a thin Nafion membrane, and a carbon cloth precoated with an MPL on the cathode, is shown to attain: (i) a net water transport coefficient through the membrane smaller than 0.8 at and 0.4 at ; (ii) fuel efficiency of ; and (iii) a steady-state power density of at ca. and with low stoichiometric flow rates of ambient dry air and methanol solution.

Sulfonated poly(phthalazinone ether ketone) for proton exchange membranes in direct methanol fuel cells

Sulfonated poly(phthalazinone ether ketone) (SPPEK) membranes were cast from N, N-dimethylacetamide (DMAc) solution and tested for their application as proton exchange membranes (PEMs) in direct methanol fuel cells (DMFCs). The methanol and water swelling uptake and several permeation tests indicated that SPPEK had a lower affinity to methanol than Nafion and was less permeable than Nafion. In a single cell DMFC test at 70 °C with the membrane electrode assembly (MEA) made of SPPEK, the maximum power density was 55 mW/cm 2 as the current density was 276 mA/cm 2 and the ultimate (limiting) current density was 360 mA/cm 2. The lower permeability of SPPEK compared with Nafion resulted in lower methanol crossover. Consequently, the optimal concentration of aqueous feed methanol (3 M) for the SPPEK MEA was higher than that of Nafion (2 M) under the same operation conditions.