Performance characteristics of a vapor feed passive miniature direct methanol fuel cell

In this paper, a new vapor feed fuel delivery system for a passive direct methanol fuel cell (DMFC) is developed and tested. Anode hydrophilic layers, electrical heating power and carbon dioxide release are examined to find their effects on the power density, efficiency and average temperatures of the cell. The hydrophilic layers act as a buffer layer between the vapor chamber and the anode gas diffusion layer (GDL). This layer allows water and methanol to mix, as well as distribute uniformly across the anode surface. Measurement of several parameters such as current, voltage, power, internal resistance, vapor chamber pressure, relative humidity and carbon dioxide concentration are taken. A maximum power density of 33 mW cm^?2 is achieved as well as 120 h of continuous operation at a constant current of 50 mA cm^?2 using the vapor feed system. The fuel utilization efficiency during the 120 h test is 34.8% and the energy efficiency is 8.2%.     

Advanced air-breathing direct methanol fuel cells for portable applications

A novel MEA is fabricated to improve the performance of air-breathing direct methanol fuel cells. A diffusion barrier on the anode side is designed to control methanol transport to the anode catalyst layer and thus suppressing the methanol crossover. A catalyst coated membrane with a hydrophobic gas diffusion layer on the cathode side is employed to improve the oxygen mass transport. It is observed that the maximum power density of the advanced DMFC with 2 M methanol solution achieves 65 mW cm⁻² at 60 °C. The value is nearly two times more than that of a commercial MEA. At 40 °C, the power densities operating with 1 and 2 M methanol solutions are over 20 mW cm⁻² with a cell potential at 0.3 V.     

Performance of an air-breathing direct methanol fuel cell

An air-breathing direct methanol fuel cell (DMFC) is attractive for portable-power applications. There are, however, several barriers that must be overcome before DMFCs reach commercially viability. This study shows that the cell power density is strongly affected by the fabrication conditions of the membrane electrode assembly (MEA) and by the technique used for assembly of the cell components. The results indicate that reducing the pressure and the thickness of catalyst layer in the MEA fabrication process can significantly improve power density. The production of water at the cathode, especially at a high power density, is shown to have a strong impact on the operation of an air-breathing DMFC since water blocks the feeding of air to the cathode. The power density (≧20 mW cm⁻²) of an air-breathing DMFC is found to drop to nearly half of its initial value after 30–40 min of operation in a short-term stability test. This appears to be one of the major limitations for potable electronic applications. Despite the many practical difficulties associated with an air-breathing DMFC, an attempt is also made to highlight the importance of the component assembly technique using a small cell pack with four integrated unit cells.     

Effect of membrane thickness on the performance and efficiency of passive direct methanol fuel cells

The use of various Nafion membranes, including Nafion 117, 115 and 112 with respective thicknesses of 175 μm, 125 μm and 50 μm, in a passive direct methanol fuel cell (DMFC) was investigated experimentally. The results show that when the passive DMFC operated with a lower methanol concentration (2.0 M), a thicker membrane led to better performance at lower current densities, but exhibited lower performance at higher current densities. When the methanol concentration was increased to 4.0 M, however, the three membranes exhibited similar cell voltages over a wide range of current densities. In contrast, this work also shows the polarization behaviors in an active DMFC when the three membranes were substantially different. Finally, the test of fuel utilization indicates that the passive DMFC with a thicker membrane exhibited higher efficiency.     

Sulfonated poly(ether sulfone) for universal polymer electrolyte fuel cell operations

The performances of the proton exchange membrane fuel cell (PEMFC), direct formic acid fuel cell (DFAFC) and direct methanol fuel cell (DMFC) with sulfonated poly(ether sulfone) membrane are reported. Pt/C was coated on the membrane directly to fabricate a MEA for PEMFC operation. A single cell test was carried out using H₂/air as the fuel and oxidant. A current density of 730 mA cm⁻² at 0.60 V was obtained at 70 °C. Pt–Ru (anode) and Pt (cathode) were coated on the membrane for DMFC operations. It produced 83 mW cm⁻² maximum power density. The sulfonated poly(ether sulfone) membrane was also used for DFAFC operation under several different conditions. It showed good cell performances for several different kinds of polymer electrolyte fuel cell applications.     

Optimum ionic conductivity and diffusion coefficient of ion-exchange membranes at high methanol feed concentrations in a direct methanol fuel cell

In direct methanol fuel cells (DMFCs), the optimum characteristics of ion-exchange membranes are investigated at high concentrations of methanol feed up to 7 M by modifying the diffusion coefficient and the ionic conductivity of the polyelectrolyte material. A Nafion membrane is modified by the incorporation of layered double hydroxide (LDH) nanoplatelets with different Mg²⁺:Al³⁺ ratios. When the feed concentration of methanol is lower than 3 M, the DMFC is controlled by the ionic conductivity of the polyelectrolyte membrane because methanol cross-over is not relatively significant. When the feed concentration is high, however, the diffusion coefficient of methanol is the key factor that determines the performance of the fuel cell. This is due to a high concentration gradient of methanol across the polyelectrolyte membrane. The open-circuit voltage is increased by the decreased diffusion coefficient in LDH/Nafion nanocomposite membranes at methanol feed concentrations up to 7 M; apparently because methanol cross-over is suppressed by the incorporation of LDH. The maximum power density of the DMFC is determined by the two competing transport processes of ion conduction and methanol diffusion, especially at a relatively high methanol concentration, that can provide optimum operating conditions in the membrane.     

Optimization of water and air management systems for a passive direct methanol fuel cell

A multi-phase, multi-component, thermal and transient model is applied to simulate the operation of a passive direct methanol fuel cell and optimize the design. The model takes into consideration the thermal effects and the variation of methanol concentration at the feeding reservoir above the fuel cell. Polarization and constant current cases are numerically simulated and compared with experiments for liquid feed concentration, membrane thickness, water management and air management systems. Parameters considered when determining an optimal design include power density, fuel utilization and energy efficiencies and water balance coefficients. An optimal liquid feed concentration is determined to be 2.0 mol kg^?1, which achieved a maximum power density of 21 mW cm^?2 and a fuel utilization efficiency of 63.0%. An optimal design of a cell uses a thick membrane (Nafion 117) to reduce methanol crossover and two additional cathode GDLs to improve the water balance coefficient and efficiency of the cell. This combination results in a power density of 23.8 mW cm^?2 and a water balance coefficient of ?1.71. An air filter may also be added to improve the efficiency and water balance coefficient of the cell, however, a small loss in power density will also occur. Using an Oil Sorbents air filter the water balance coefficient is increased to ?0.85, the fuel utilization efficiency is improved by 27.35% and the maximum power density decreased to 21.6 mW cm^?2.     

Development of micro direct methanol fuel cells for high power applications

A micro direct methanol fuel cell (μDMFC) with active area of 1.625 cm² has been developed for high power portable applications and its electrochemical characterization carried out in this study. The fragility of the silicon wafer makes it difficult to compress the cell for good sealing and hence to reduce contact resistance in the Si-based μDMFC. We have instead used very thin stainless steel plates as bipolar plates with the flow field machined by photochemical etching technology. For both anode and cathode flow fields, widths of both the channel and rib were 750 μm, with a channel depth of 500 μm. A gold layer was deposited on the stainless steel plate to prevent corrosion. This study used an advanced MEA developed in-house featuring a modified anode backing structure with a compact microporous layer. Maximum power density of the micro DMFC reached 62.5 mW cm⁻² at 40 °C, and 100 mW cm⁻² at 60 °C at atmospheric pressure, which almost doubled the performance of our previous Si-based μDMFC.     

Study of membrane electrode assemblies for direct methanol fuel cells

At DLR, membrane electrode assemblies (MEA) for direct methanol fuel cells (DMFC), are produced with the company’s own dry production technique. For improving this production technique, the MEAs in fuel cells are characterized electrochemically in fuel cell test facilities as well as physically by scanning electron microscopy (SEM).In order to measure the local current densities in polymer electrolyte membrane fuel cells, a method has been developed at DLR and tested in fuel cells supplied with hydrogen as fuel. For the DMFC, a measuring cell with 16 segments was built for examining MEAs with an overall active electrode area of 25 cm². With a sufficient resolution of location and time, simultaneous measurement of different local current densities in the cell can be carried out thus accelerating and improving operating parameter studies. This new tool is used at DLR for characterizing and developing improved MEAs and for examining the cell design (e.g. flow fields) and operating conditions of DMFC. In the measuring cell with its segments, the local mass conversion rates in the DMFC for liquid methanol–water mixtures are examined.     

Methanol crossover in direct methanol fuel cells: a link between power and energy density

Direct methanol fuel cell performance curves were obtained as a function of three parameters, (1) temperature, (2) fuel flow-rate and (3) concentration. Methanol crossover was measured by gas chromatography as a function of these three parameters at 100 mA/cm² in the single-pass fuel delivery mode. The data was used to model a continuous loop mode where pure methanol is injected into a loop that circulates through the flow-field and recovers water from the cathode. The modeled loop composition is identical to the fuel stream used in the single-pass experiments (dilute aqueous methanol). The model results, presented in three-dimensional surfaces, elucidate the impact of parameter variations on the energy and power density of the direct methanol fuel cell (DMFC) and the link between those two figures of merit. In addition, a reasonable estimate of the contribution of mass transport effects due to the carbon fabric current collectors is made along with in situ CO stripping experiments on membrane electrode assembly (MEA) anode surfaces. The analysis shows that, at present, serious compromises are required if reasonable energy and power densities are to be simultaneously maintained in DMFCs using Nafion™ 117 as an electrolyte.     

Fabrication and properties of hybrid membranes based on salts of heteropolyacid,zirconium phosphate and polyvinyl alcohol

The fabrication and properties of a hybrid membrane based on cesium salt of heteropoly acid, zirconium phosphate and polyvinyl alcohol are described. The fabricated membranes were characterized for their intra molecular interaction, thermal stability, surface morphology, water content and surface-charge properties using Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), water uptake and ion-exchange capacity measurements. These membranes showed reduced methanol crossover (for possible application in DMFC) relative to that of Nafion^® 115. At 50% of relative humidity, the protonic conductivity of the hybrid membranes was in the range of 10⁻³ to 10⁻² S cm⁻¹. The feasibility of these hybrid membranes as proton conducting electrolyte in direct methanol fuel cell (DMFC) was investigated and preliminary results are compared with that of Nafion^® 115. A maximum power density of 6 mW cm⁻² with PVA–ZrP–Cs₂STA hybrid membrane was obtained with the cell operated in passive mode at 373 K and atmospheric pressure. Open circuit voltage of the cell operated with hybrid membranes are high compared to that of Nafion^® 115 indicating reduced methanol crossover.     

Performance of direct methanol fuel cell using carbon nanotube-supported Pt–Ru anode catalyst with controlled composition

The performance of a single-cell direct methanol fuel cell (DMFC) using carbon nanotube-supported Pt–Ru (Pt–Ru/CNT) as an anode catalyst has been investigated. In this study, the Pt–Ru/CNT electrocatalyst was successfully synthesized using a modified polyol approach with a controlled composition very close to 20 wt.%Pt–10 wt.%Ru, and the anode was prepared by coating Pt–Ru/CNT electrocatalyst on a wet-proof carbon cloth substrate with a metal loading of about 4 mg cm⁻². A commercial gas diffusion electrode (GDE) with a platinum black loading of 4 mg cm⁻² obtained from E-TEK was employed as the cathode. The membrane electrode assembly (MEA) was fabricated using Nafion^® 117 membrane and the single-cell DMFC was assembled with graphite endplates as current collectors. Experiments were carried out at moderate low temperatures using 1 M CH₃OH aqueous solution and pure oxygen as reactants. Excellent cell performance was observed. The tested cell significantly outperformed a comparison cell using a commercial anode coated with carbon-supported Pt–Ru (Pt–Ru/C) electrocatalyst of similar composition and loading. High conductivity of carbon nanotube, good catalyst morphology and suitable catalyst composition of the prepared Pt–Ru/CNT electrocatalyst are considered to be some of the key factors leading to enhanced cell performance.     

Double microporous layer cathode for membrane electrode assembly of passive direct methanol fuel cells

A novel membrane electrode assembly (MEA) is described that utilizes a double microporous layer (MPL) structure in the cathode of a passive direct methanol fuel cell (DMFC). The double MPL cathode uses Ketjen Black carbon as an inner-MPL and Vulcan XC-72R carbon as an outer-MPL. Experimental results indicate that this double MPL structure at the cathode provides not only a higher oxygen transfer rate, but enables more effective back diffusion of water; thus, leading to an improved power density and stability of the passive DMFC. The maximum power density of an MEA with a double MPL cathode was observed to be ca. 33.0 mW cm⁻², which is found to be a substantial improvement over that for a passive DMFC with a conventional MEA. A. C. impedance analysis suggests that the increased performance of a DMFC with the double MPL cathode might be attributable to a decreased charge transfer resistance for the cathode oxygen reduction reaction.     

Design of a MEA with multi-layer electrodes for high concentration methanol DMFCs

According to the conventional MEA test, methanol and water crossover are the main factors to determine performance of a passive DMFC. Thus, to ensure the high cell performance of a passive DMFC using high concentration methanol of 50–95 vol%, the MEA in this study introduces the barrier layer to limit the crossover of high concentration methanol, a hydrophobic layer to reduce water crossover, and a hydrophilic layer to enhance the water recovery from the cathode to the anode. The functional layers of the MEA have the effect of improving the performance of the passive DMFC by decreasing the methanol and water crossover. In spite of the operation with 95 vol% methanol, the MEA with multi-layer electrodes for high concentration methanol DMFCs shows a maximum power density of 35.1 mW cm⁻² and maintains a high power density of 30 mW cm⁻² (0.405 V) under constant current operation.     

Study of sintered stainless steel fiber felt as gas diffusion backing in air-breathing DMFC

Adoption of a sintered stainless steel fiber felt was evaluated as gas diffusion backing in air-breathing direct methanol fuel cell (DMFC). By using a sintered stainless steel fiber felt as an anodic gas diffusion backing, the peak power density of an air-breathing DMFC is 24 mW cm⁻², which is better than that of common carbon paper. A 30-h-life test indicates that the degraded performance of the air-breathing DMFC is primarily due to the water flooding of the cathode. Twelve unit cells with each has 6 cm² of active area are connected in series to supply the power to a mobile phone assisted by a constant voltage diode. The maximum power density of 26 mW cm⁻² was achieved in the stack, which is higher than that in single cell. The results show that the sintered stainless steel felt is a promising solution to gas diffusion backing in the air-breathing DMFC, especially in the anodic side because of its high electronical conductivity and hydrophilicity.     

Behavioral pattern of a monopolar passive direct methanol fuel cell stack

A passive, air-breathing, monopolar, liquid feed direct methanol fuel cell (DMFC) stack consisting of six unit cells with no external pump, fan or auxiliary devices to feed the reactants has been designed and fabricated for its possible employment as a portable power source. The configurations of the stack of monopolar passive feed DMFCs are different from those of bipolar active feed DMFCs and therefore its operational characteristics completely vary from the active ones. Our present investigation primarily focuses on understanding the unique behavioral patterns of monopolar stack under the influence of certain operating conditions, such as temperature, methanol concentration and reactants feeding methods. With passive reactants supply, the temperature of the stack and open circuit voltage (OCV) undergo changes over time due to a decrease in concentration of methanol in the reservoir as the reaction proceeds. Variations in performance and temperature of the stack are mainly influenced by the concentration of methanol. Continuous operation of the passive stack is influenced by the supply of methanol rather than air supply or water accumulation at the cathode. The monopolar stack made up of six unit cells exhibits a total power of 1000 mW (37 mW cm⁻²) with 4 M methanol under ambient conditions.     

Mass transport analysis of a passive vapor-feed direct methanol fuel cell

A two-dimensional, two-phase, non-isothermal model using the multi-fluid approach was developed for a passive vapor-feed direct methanol fuel cell (DMFC). The vapor generation through a membrane vaporizer and the vapor transport through a hydrophobic vapor transport layer were both considered in the model. The evaporation/condensation of methanol and water in the diffusion layers and catalyst layers was formulated considering non-equilibrium condition between phases. With this model, the mass transport in the passive vapor-feed DMFC, as well as the effects of various operating parameters and cell configurations on the mass transport and cell performance, were numerically investigated. The results showed that the passive vapor-feed DMFC supplied with concentrated methanol solutions or neat methanol can yield a similar performance with the liquid-feed DMFC fed with much diluted methanol solutions, while also showing a higher system energy density. It was also shown that the mass transport and cell performance of the passive vapor-feed DMFC depend highly on both the open area ratio of the vaporizer and the methanol concentration in the tank.     

NMR investigation of water and methanol transport in sulfonated polyareylenethioethersulfones for fuel cell applications

We report an investigation of water and methanol transport in polymer electrolyte membranes based on highly sulfonated polyarelenethioethersulfones (SPTES) for direct methanol fuel cell (DMFC) applications. Measurements of both water and methanol self-diffusion coefficients of SPTES polymer as well as in a reference sample of Nafion-117 equilibrated in 2 M methanol solution have been carried out, using the pulsed gradient spin echo technique, over a temperature range of 20–140 °C. The selectivity of the membrane, defined as (D_OH/D_CH3), decreased from 6 to 2.4 as temperature increased from 20 to 140 °C in SPTES sample while in Nafion, the value decreased from 3.2 to 1.4 as temperature increased from 20 to 100 °C. These results indicate significantly lower fuel molecular permeability in SPTES compared to that of Nafion. All results suggest high-temperature stability in these materials, offering the possibility of fuel cell operation at temperatures >120 °C. High pressure NMR diffusion measurements were also carried out for three different water contents (between 20 and 55 wt.%) in a static field gradient in order to get supplemental information regarding water transport in SPTES materials. The calculated activation volume increased from 1.54 to 8.40 cm³/mol as the water content decreased from 55 to 20%. This behavior is qualitatively similar to previously reported results for Nafion-117.     

Nano-TiO2-coated polymer electrolyte membranes for direct methanol fuel cells

Composite polymer electrolyte membranes with nano-TiO₂ films are fabricated by deposition of titania nanoparticles from a sol solution. Measurements of ion conductivity, methanol permeability and single-cell performance of the modified Nafion membranes are conducted. The TiO₂ films adhere well and are crack-free. The protonic conductivity of the composite membranes decreases with increasing titania content, but methanol permeability is reduced. Preliminary tests on a single-cell of a direct methanol fuel cell (DMFC) indicate that a titania-coated membrane with 0.009 mg cm⁻² content gives the highest cell voltage and maximum power density.     

Hydrogels in methanol fuel cartridge used as a diffusion-rate-controlling agent suppressing the methanol crossover in passively operated flat-pack type DMFCs

Hydrogels are used in methanol fuel cartridges to control fuel diffusion from the fuel reservoir to the anode electrode. This ability to control fuel diffusion rates enables the reduction of excess fuel supply that causes fuel crossover through the proton-exchange membrane. Cell performance was increased drastically from 14 to 21.5 m W cm⁻² with higher methanol concentrations using hydrogel fuel cartridges that fulfill the role of a diffusion-rate-controlling agent. This result implies that the hydrogel retards fuel diffusion, even at higher concentrations and suppresses the methanol crossover.