Experimental results on the direct electrochemical oxidation of methanol in PEM fuel cells

The cell performance of direct methanol fuel cells (DMFC) is 0.5 V at 0.5 A cm^–2 under high pressure oxygen operation (3 bar abs.) at 110 °C. However, high oxygen pressure operation at high temperatures is only useful in special market niches. Therefore, our work has now focused on air operation of a DMFC under low pressure (up to 1.5 bar abs.). At present, a power density of more than 100 mW cm^–2 can be achieved at 0.5 V on air operation at 110 °C. These measurements were carried out in single cells with an electrode area of 3 cm² and the air stoichiometry only amounted to 10. The effects of methanol concentration and temperature on the anode performance were studied by pseudo half cell measurements and the results are presented together with their impact on the cell voltage. A cell design with an electrode area of 550 cm², which is appropriate for assembling a DMFC stack, was tested. A three-celled stack based on this design revealed nearly the same power densities as in the small experimental cells at low air excess pressure and the voltage–current curves for the three cells were almost identical. At 110 °C a power output of 165 W at a stack voltage of 1.5 V can be obtained in the air mode.     

Development and characterization of a silicon-based micro direct methanol fuel cell

A silicon-based micro direct methanol fuel cell (μDMFC) for portable applications has been developed and its electrochemical characterization carried out in this study. Anode and cathode flowfields with channel and rib width of 750 μm and channel depth of 400 μm were fabricated on Si wafers using the microelectromechanical system (MEMS) technology. A membrane-electrode assembly (MEA) was specially fabricated to mitigate methanol crossover. This MEA features a modified anode backing structure in which a compact microporous layer is added to create an additional barrier to methanol transport thereby reducing the rate of methanol crossing over the polymer membrane. The cell with the active area of 1.625 cm² was assembled by sandwiching the MEA between two micro-fabricated Si wafers. Extensive cell polarization testing demonstrated a maximum power density of 50 mW/cm² using 2 M methanol feed at 60 °C. When the cell was operated at room temperature, the maximum power density was shown to be about 16 mW/cm² with both 2 and 4 M methanol feed. It was further found that the present μDMFC still produced reasonable performance under 8 M methanol solution at room temperature.     

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.     

Comparative studies of a single cell and a stack of direct methanol fuel cells

Comparative studies have been conducted to observe the characteristics of a single cell and a stack of direct methanol fuel cells (DMFC) at ambient conditions. The maximum power density of a single cell was about 70 mW/ cm² at 2M methanol (CH₃OH) of 3.75 cc/min and dry air of 250 cc/min at room temperature and atmospheric pressure. In a stack, on the other hand, the maximum power density of the stack was 85mW/cm² which was about a 20% higher value. This could be attributed to higher internal temperature than that of the single cell: the temperature of single cell increased up to 35 °C, while the highest temperature of the stack was 69 °C. This is because the cell temperature in DMFC was autonomously increased by exothermal reaction such as chemical oxidation of CH₃OH and oxygen reduction. The temperature was strongly dependent on the number of unit cells in a stack and the amount of electric load applied. In DMFC stacks, the performance of an individual cell showed uneven distribution when the electric load was increased and it was mostly influenced by different local concentration of reactants and non-uniform temperature     

Fuel cell performance of polymer electrolyte membrane based on hexafluorinated sulfonated poly(ether sulfone)

The potential-current fuel cell characteristics of membrane electrode assemblies (MEAs) using hexafluorinated sulfonated poly(ether sulfone) copolymer are compared to those of Nafion^® based MEAs in the case of proton exchange membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC). The hexafluorinated copolymer with 60 mol% of monosulfonated comonomer based acid form membrane is chosen for this study due to its high proton conductivity, high thermal stability, low methanol permeability, and its insolubility in boiling water. The catalyst powder is directly coated on the membrane and the catalyst coated membrane is used to fabricate MEAs for both fuel cells. A current density of 530 mA cm^?2 at 0.6 V is obtained at 70 °C with H₂/air as the fuel and oxidant. The peak power density of 110 mW cm^?2 is obtained at 80 °C under specific DMFC operating conditions. Other electrochemical characteristics such as electrochemical impedance spectroscopy, cyclic voltammetry, and linear sweep voltammetry are also studied.     

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.     

Direct methanol alkaline fuel cells with catalysed anion exchange membrane electrodes

Membrane electrodes prepared by chemical deposition of platinum directly onto the anion exchange membrane electrolyte were tested in direct methanol alkaline fuel cells. Data on the cell voltage against current density performance and anode potentials are reported. The relatively low fuel cell performance was probably due to the low active surface area of Pt deposits on the membrane comparing to other membrane electrode assembly (MEA) fabrication methods. However, the catalysed membrane electrode showed good performance for oxygen reduction. A reduction in cell internal resistance was also obtained for the catalysed membrane electrode. By combining the catalysed membrane electrodes with a catalysed mesh, maximum current density of 98 mA cm^–2 and peak power density of 18 mW cm^–2 were achieved.     

An improved-performance liquid-feed solid-polymer-electrolyte direct methanol fuel cell operating at near-ambient conditions

Results on the performance of a 25 cm² liquid-feed solid-polymer-electrolyte direct methanol fuel cell (SPE-DMFC), operating under near-ambient conditions, are reported. The SPE-DMFC can yield a maximum power density of c. 200 mW cm⁻² at 90 °C while operating with 1 M aqueous methanol and oxygen under ambient pressure. While operating the SPE-DMFC under similar conditions with air, a maximum power density of ca. 100 mW cm⁻² is achieved. Analysis of the electrode reaction kinetics parameters on the methanol electrode suggests that the reaction mechanism for methanol oxidation remains invariant with temperature. Durability data on the SPE-DMFC at an operational current density of 100 mA cm⁻² have also been obtained.     

Experimental and Numerical Study of Serpentine Flow Fields for Improving Direct Methanol Fuel Cell Performance

Methanol crossover is an important issue as it affects direct methanol fuel cell (DMFC) performance. But it may be controlled by selecting a proper flow field design. Experiments were carried out to investigate the effect of single, double and triple serpentine flow field configurations on a DMFC with a 25 cm² membrane electrode assembly (MEA) with a constant open ratio. A three dimensional model was also developed for the anode of the DMFC to predict methanol concentration and cell current density distributions. Experimental and model results show that at lower methanol concentrations (0.25–0.5M), single serpentine flow field (SSFF) provides high peak power density, while a double serpentine flow field (DSFF) gives high peak power density at a high methanol concentration (1–2M). Single and double serpentine flow fields exhibit the same peak power density (33 mW cm⁻²) at 1M. But the cell efficiency of double serpentine flow field is 12.5% which is 3.5% point greater than single serpentine flow field. This is attributed to reduced mixed potential. triple serpentine flow field (TSFF) shows the lowest peak power density and cell efficiency, which is attributed to high mass transfer resistance.     

A study on the dissymmetrical microporous layer structure of a direct methanol fuel cell

The effect of carbon type, carbon loading and microporous layer structure in the microporous layer on the performance of a direct methanol fuel cell (DMFC) at low temperature was investigated using electrochemical polarization techniques, electrochemical impedance spectroscopy, scanning electron microscope and other methods. Vulcan XC-72 carbon was found to be most suitable as a microporous layer for low temperature DMFC. Maximum fuel cell performance was obtained utilizing a microporous layer with carbon loading of 1.0 mg cm⁻² when air was used as an oxidant. A membrane electrode assembly with 1.0 mg cm⁻² Vulcan XC-72 carbon with 20 wt.% Teflon in the cathode and no microporous layer in the anode showed a maximum power density of 36.7 mW cm⁻² at 35 °C under atmospheric pressure. The AC impedance study proved that a cell with a dissymmetrical microporous layer structure had lower internal resistance and mass transfer resistance, thus obtaining better performance.     

Pore‐forming technology and performance of MEA for direct methanol fuel cells

BACKGROUND: The commercialization of DMFCs is seriously restricted by its relatively low power density. Lots of work has been concentrated on catalysts with high activity, the optimization of flow path design, development of new kinds of proton exchange membrane and modification of Nafion membrane. Meanwhile, very few reports have involved the structure optimization of the membrane electrode assembly (MEA). To improve the performance of direct methanol fuel cells (DMFCs), the catalyst layer (CL) structures of anode and cathode were optimized by utilizing ammonium carbonate as pore forming agent. RESULTS: The polarization curves showed that in catalyst slurry the optimal content of ammonium carbonate was 50 wt%, and the DMFC performance was enhanced from 75.65 mW cm^?2 to 167.42 mW cm^?2 at 55 °C and 0.2 MPa O₂. Electrochemical impedance spectroscopy and electrochemical active surface area (EASA) testing revealed that the improved performance of optimized MEAs could be mainly attributed to the increasing EASA and the enhanced mass transfer rate of CLs. But poor methanol crossover limited the performance enhancement of MEAs with porous anodes. CONCLUSION: With regard to improving cell performance, this pore‐forming technology is better applied to the cathode catalyst layer to improve its structure rather than the anode catalyst layer. © 2012 Society of Chemical Industry     

A direct methanol fuel cell using acid-doped polybenzimidazole as polymer electrolyte

A direct methanol/oxygen solid polymer electrolyte fuel cell was demonstrated. This fuel cell employed a 4 mg cm^–2 Pt-Ru alloy electrode as an anode, a 4 mg cm^–2 Pt black electrode as a cathode and an acid-doped polybenzimidazole membrane as the solid polymer electrolyte. The fuel cell is designed to operate at elevated temperature (200°C) to enhance the reaction kinetics and depress the electrode poisoning, and reduce the methanol crossover. This fuel cell demonstrated a maximum power density about 0.1 W cm^–2 in the current density range of 275–500 mA cm^–2 at 200°C with atmospheric pressure feed of methanol/water mixture and oxygen. Generally, increasing operating temperature and water/methanol mole ratio improves cell performance mainly due to the decrease of the methanol crossover. Using air instead of the pure oxygen results in approximately 120 mV voltage loss within the current density range of 200–400 mA cm^–2 .     

Effects of organically modified nanoclay on the transport properties and electrochemical performance of acid‐doped polybenzimidazole membranes

Nanocomposite polyelectrolyte membranes based on phosphoric acid (H₃PO₄) doped polybenzimidazoles (PBIs) with various loading weights of organically modified montmorillonite (OMMT) were prepared and characterized for direct methanol fuel cell (DMFC) applications. X‐ray diffraction analysis revealed the exfoliated structure of OMMT nanolayers in the polymeric matrices. An H₃PO₄–PBI/OMMT membrane composed of 500 mol % doped acid and 3.0 wt % OMMT showed a membrane selectivity of approximately 109,761 in comparison with 40,500 for Nafion 117 and also a higher power density (186 mW/cm²) than Nafion 117 (108 mW/cm²) for a single‐cell DMFC at a 5M methanol feed. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Tubular Cathode Prepared by a Dip‐Coating Method for Low Temperature DMFC

A novel tubular cathode for the direct methanol fuel cell (DMFC) is proposed, based on a tubular titanium mesh. A dip‐coating method has been developed for its fabrication. The tubular cathode is composed of titanium mesh, a cathode diffusion layer, a catalyst layer, and a recast Nafion® film. The titanium mesh is present at the inner circumference of the diffusion layer, while the recast Nafion® film is at the outer circumference of the catalyst layer. A DMFC single cell with a 3.5 mgPt cm^–2 tubular cathode was able to perform as well, in terms of power density, as a conventional planar DMFC. A peak power density of 9 mW cm^–2 was reached under atmospheric air at 25 °C.     

Cell/stack electrochemical performance and stability of cone-shaped tubular SOFCs for direct operation with methane

Cone-shaped tubular anode-supported solid oxide fuel cells (SOFCs) and two-cell-stack based on NiO-YSZ traditional anodes direct utilization methane as fuel were successfully developed in this study. The single cell exhibited maximum power densities of 1.255 W cm⁻² for hydrogen and 1.099 W cm⁻² for methane at 800 °C, respectively. A stability test of the single cell was performed with different constant current densities at 700 °C in methane. The results indicated that the single cell can be operated stable at high current density in methane. And EDX results showed that there is no measurable coking effect of operation in methane at relatively high current density.A two-cell-stack based on the above-mentioned SOFCs was fabricated and tested by direct utilization of methane. Its typical electrochemical performance was investigated. The two-cell-stack provided a maximum power output of about 3.5 W (350 mW cm⁻² calculated using effective cathode area) by directly using methane at 800 °C. The stack experienced 20 h durability testing. The results demonstrated that the stack was kept at around 1 V (J = 0.05 A cm⁻²) at 700 °C. The stack presented basically stably during the whole test, and the performance of the stack is acceptable for application.     

Development of a Direct Alcohol Alkaline Fuel Cell Stack

Direct alcohol alkaline fuel cells (DAAFC) are one of the potential fuel cell types in the category of low temperature fuel cells, which could become an energy source for portable electronic equipment in future. In the present study, a simple DAAFC stack has been developed and studied to evaluate the maximum performance for a given fuel (methanol or ethanol) and electrolyte (KOH) at various concentrations and temperatures. The open circuit voltage of the stack of four cells was nearly 4.0 V. A particular combination, 2 M fuel (methanol or ethanol) and 3 M KOH, results in maximum power density of the stack. The maximum power density obtained from the DAAFC stack (25 °C) was 50 mW cm^–2 at 20 mA cm^–2 for methanol and 17 mA cm^–2 for ethanol. The stack power density corroborated with that obtained from a single cell, indicating there was no further loss in the stack.     

Modified Sulfonated Poly(ether ether ketone) Based Mixed Matrix Membranes for Direct Methanol Fuel Cells

Mixed matrix membranes based on zeolite 4A‐methane sulfonic acid (MSA)‐sulfonated poly(ether ether ketone) (SPEEK) are evaluated as a potential polymer electrolyte membrane (PEM) for direct methanol fuel cells (DMFCs). Ion‐exchange capacity, sorption of water, and water–methanol mixture, proton conductivity, and methanol permeability for the mixed‐matrix membranes have been extensively investigated. The mixed‐matrix membranes are also characterized for their cross‐sectional morphology, mechanical, and thermal properties. DMFCs employing SPEEK‐MSA (20 wt.%) blend, zeolite 4A (4 wt.%)‐SPEEK‐MSA (20 wt.%) mixed matrix membranes deliver peak power densities of 130 and 159 mW cm^–2, respectively; while a peak power density of only 95 mW cm^–2 is obtained for the DMFC employing pristine SPEEK membrane at 70 °C. The results showed that these SPEEK based mixed matrix membranes exhibit higher DMFC performance and lower methanol permeability in comparison to Nafion‐117 membrane.     

A Solid-polymer Electrolyte Direct Methanol Fuel Cell with a Methanol-tolerant Cathode and its Mathematical Modelling

Solid-polymer electrolyte direct methanol fuel cells (SPE-DMFCs) employing carbon-supported Pt–Fe as oxygen-reduction catalyst to mitigate the effect of methanol on cathode performance while operating with oxygen or air have been assembled. These SPE-DMFCs provided maximum power densities of 250 and 120 mW cm^–2 at 85 °C on operating with oxygen and air, respectively. The polarization data for the SPE-DMFCs and their constituent electrodes have also been derived numerically employing a model based on phenomenological transport equations for the catalyst layer, diffusion layer and the membrane electrolyte.     

A portable photocatalytic fuel cell based on TiO2 nanorod photoanode for wastewater treatment and efficient electricity generation

A portable photocatalytic fuel cell (P-PFC) has been proposed in the present study, which has a simple structure and high performance. The photoanode of the P-PFC was composed of F-doped tin oxide (FTO)/TiO₂ seed layer/TiO₂ nanorods, and this nanostructure could expand the absorption range of the solar light. The cathode was a silver paste that was printed on a waterproof and breathable layer. It could effectively absorb oxygen from the air and avoid adding additional oxygen supply equipment. Two electrodes were encapsulated in a hot-melt film to form a small package with a quick-release port. This packaging model enabled a fuel replacement, and this P-PFC was universal to all types of fuels. When 1 mol/L methanol was used as fuel, the photocurrent could reach 2.8 mA/cm² under a UV light of 30 mW/cm², and the power density was 0.49 mW/cm². When the power density of the simulated solar light was as low as 2 mW/cm², the system could still work normally. The degradation performance of the P-PFC was also evaluated, and its maximum degradation efficiency for methylene blue (MB) reached 74.3%. The results indicated that the P-PFC had great potential in simultaneously degrading organic pollutants and generating electricity.     

Development of a Self‐Adaptive Direct Methanol Fuel Cell Fed with 20 M Methanol

A passive and self‐adaptive direct methanol fuel cell (DMFC) directly fed with 20 M of methanol is developed for a high energy density of the cell. By using a polypropylene based pervaporation film, methanol is supplied into the DMFC's anode in vapor form. The mass transport of methanol from the cartridge to the anodic catalyst layer can be controlled by varying the open ratio of the anodic bipolar plate and by tuning the hydrophobicity of anodic diffusion layer. An effective back diffusion of water from the cathode to the anode through Nafion film is carried out by using an additive microporous layer in the cathode that consists of 50 wt.% Teflon and KB‐600 carbon. Accordingly, the water back diffusion not only ensures the water requirement for the methanol oxidation reaction but also reduces water accumulation in the cathode and then avoids serious water flooding, thus improving the adaptability of the passive DMFC. Based on the optimized DMFC structure, a passive DMFC fed with 20 M methanol exhibits a peak power density of 42 mW cm^–2 at 25 °C, and no obvious performance degradation after over 90 h continuous operation at a constant current density of 40 mA cm^–2.