Two-step conditioning method for high power-generating direct methanol fuel cells

Power-generation improvement of a direct methanol fuel cell (DMFC) has been investigated through the enhancements of the anode and cathode characteristics using a newly developed “two-step conditioning method”, in which the conditioning is conducted by supplying H₂ gas before the conventional DMFC conditioning. By using the two-step conditioning, the DMFC performances are enhanced. From the polarization curves measured during the DMFC operation using a single cell having an Ag/Ag₂SO₄ reference electrode, the methanol oxidation performance of the anode is proven to be improved with an increase in the methanol concentration. In addition, a decline in the O₂ reduction performance at the cathode due to the methanol crossover is suppressed by our original conditioning method. These results are also supported by the linear sweep voltammetry, and the superior DMFC performances after the two-step conditioning are related to the high speed cleaning of the electrocatalysts. Note that the optimum two-step conditioning leads to 1.4–2.2 times higher maximum power densities than those for the conventional DMFC conditioning even for a shorter conditioning time.     

An impedance study of an operating direct methanol fuel cell

An electrochemical impedance spectroscopy (EIS) technique was developed to characterize a direct methanol fuel cell (DMFC) under various operating conditions. A silver/silver chloride electrode was used as an external reference electrode to probe the anode and cathode during fuel cell operation and the results were compared to the conventional anode or cathode half-cell performance measurement using a hydrogen electrode as both the counter and reference electrode. The external reference was sensitive to the anode and the cathode as current was passed in a working DMFC. The impedance spectra and DMFC polarization curves were systematically investigated as a function of air and methanol flow rates, methanol concentration, temperature, and current density. Water flooding in the cathode was also examined.     

Evaluation of silver-coated stainless steel bipolar plates for fuel cell applications

In this study, computer-aided design and manufacturing (CAD/CAM) technology were applied to develop and produce stainless steel bipolar plates for DMFC (direct methanol fuel cell). Effect of surface modification on the cell performance of DMFC was investigated. Surface modifications of the stainless steel bipolar plates were made by the electroless plating method. A DMFC consisting of silver coated stainless steel as anode and uncoated stainless steel as cathode was assembled and evaluated. The methanol crossover rate (R_c) of the proton exchange membrane (PEM) was decreased by about 52.8%, the efficiency (E_f) of DMFC increased about 7.1% and amounts of methanol electro-oxidation at the cathode side (M_co) were decreased by about 28.6%, as compared to uncoated anode polar plates. These measurements were determined by the transient current and mathematical analysis.     

Enhancement of water retention in the membrane electrode assembly for direct methanol fuel cells operating with neat methanol

It is desirable to operate a direct methanol fuel cell (DMFC) with neat methanol to maximize the specific energy of the DMFC system, and hence increasing its runtime. A way to achieve the neat-methanol operation is to passively transport the water produced at the cathode through the membrane to the anode to facilitate the methanol oxidation reaction (MOR). To achieve a performance of the MOR similar to that under the conventional diluted methanol operation, both the water transport rate and the local water concentration in the anode catalyst layer (CL) are required to be sufficiently high. In this work, a thin layer consisting of nanosized SiO₂ particles and Nafion ionomer (referred to as a water retention layer hereafter) is coated onto each side of the membrane. Taking advantage of the hygroscopic nature of SiO₂, the cathode water retention layer can help maintain the water produced from the cathode at a higher concentration level to enhance the water transport to the anode, while the anode retention layer can retain the water that is transported from the cathode. As a result, a higher water transport rate and a higher water concentration at the anode CL can be achieved. The formed membrane electrode assembly (MEA) with the added water retention layers is tested in a passive DMFC and the results show that this MEA design yields a much higher power density than the MEA without water retention layers does.     

Analysis of the polarization of a direct methanol fuel cell using a pseudo-reversible hydrogen reference electrode

In order to observe the performance of the anode and cathode during actual direct methanol fuel cell (DMFC) operating conditions and to minimize the polarization of the reference electrode, we used a reversible hydrogen reference electrode (RHE) with its instability minimized. For analysis of the I–V polarization curve of each electrode, Tafel plots were used as the diagnostic tool. According to the slopes in the Tafel plot, the I–V polarization curves of each electrode were divided into the several regions. The effects of operating parameters on the performance of each electrode were interpreted in terms of mass transfer and electrode activation. The methanol and oxygen crossover through the membrane significantly affected the performance of the cell.     

New RuO2 and carbon–RuO2 composite diffusion layer for use in direct methanol fuel cells

A RuO₂ diffusion layer is examined for use in direct methanol fuel cells (DMFC) by comparison with acetylene black and Vulcan XC-72R. In the test with a DMFC unit cell, the RuO₂ diffusion layer is superior to the other two materials. The difference in performance is interpreted in terms of structural and electrical properties which are evaluated by porosity, scanning electron microscopy and resistance measurements. The RuO₂ diffusion layer displays different behaviors at the anode and cathode sides. These characteristics can be attributed to a reduced loss of catalyst in the active catalyst layer, which leads to increased methanol diffusion at the anode and prevention of water flooding in the cathode. The effect of the RuO₂ diffusion layer on cell performance becomes more pronounced at lower temperatures and during operation in the presence of air. Finally, a carbon–RuO₂ composite is evaluated as a diffusion layer material for a DMFC.     

Effect of design of multilayer electrodes in direct methanol fuel cells (DMFCs)

In a direct methanol fuel cell (DMFC), optimized multilayer electrode design is critical to mitigate methanol crossover and improve cell performance. In this paper, we present a one-dimensional (1-D) two-phase model based on the saturation jump theory in order to explore the methanol and water transport characteristics using various multilayer electrode configurations. To experimentally validate the 1-D model, two different membrane electrode assemblies (MEAs) with and without an anode microporous layer (MPL) are fabricated and tested under various cell current density and methanol feed concentration conditions. Then, 1-D DMFC simulations are performed and the results compared to the experimental data. In general, the numerical predictions are in good agreement with the experimental data; thus, the 1-D DMFC simulations successfully model the effects of the anode MPL that were observed experimentally. In addition to the comparison study, additional numerical simulations are carried out to precisely examine the role of the anode and cathode MPLs and the effect of the hydrophobicity of the anode catalyst layer on the water and liquid saturation distributions inside the DMFCs. This paper demonstrates the quantitative accuracy of the saturation jump model for simulating multilayer DMFC MEAs and also provides greater insight into the operational characteristics of DMFCs incorporating multilayer electrodes.     

Operational condition analysis for vapor-fed direct methanol fuel cells

This paper investigates the analysis and design of optimal operational conditions for vapor-fed direct methanol fuel cells (DMFCs). Methanol vapor at a temperature of 35 °C is carried with nitrogen gas together with water vapor at 75 °C. In this experimental condition, stoichiometry of 10 is maintained for each fuel gas. The results show that the optimal operational concentration was 25–30 wt.% under methanol vapor feeding at the anode. The peak power was 14 mW cm² in polarization curves. To analyze major losses, the activation losses of the anode and cathode were measured by an in situ reference electrode and a working electrode. The activation loss of the anode is proportional to the water content and the high methanol concentration caused the activation loss of the cathode to increase due to methanol crossover. In the vapor-fed DMFC, the activation loss of the anode is higher than that of the cathode. Also, depending on the variation of the methanol concentration, the IR loss and Faradaic impedance is measured via impedance analysis. The methanol concentration significantly affects the IR loss and kinetics. Although the IR loss was more than the desired value at the optimal condition (25–30 wt.%), it did not significantly affect the cell’s performance. The cell operated at room temperature and ambient pressure that is a typical operation environment of air-breathing fuel cells.     

Influence of cathode oxygen transport on the discharging time of passive DMFC

In this paper, the long-term discharge performances of passive DMFC at different currents with different cell orientations were investigated. Water produced in the cathode was observed from the photographs taken by a digital camera. The results revealed that the passive DMFC with anode facing upward showed the best long-term discharge performance at high current. A few independent water droplets accumulated in cathode when the anode faced upward. Instead, in the passive DMFC with vertical orientation, a large amount of produced water flowed down along the surface of current collector. The passive DMFC with vertical orientation showed relatively good performance at low current. It was concluded that the cathode produced less water in a certain period of time at smaller current. In addition, the rate of methanol crossover in the passive DMFC with anode facing upward was relatively high, which leaded to a more rapid decrease of the methanol concentration in anode. The passive DMFC with anode facing downward resulted in the worst performance because it was very difficult to remove CO₂ bubbles produced in the anode.     

A study on the overall efficiency of direct methanol fuel cell by methanol crossover current

This paper was presented to determine the methanol crossover and efficiency of a direct methanol fuel cell (DMFC) under various operating conditions such as cell temperature, methanol concentration, methanol flow rate, cathode flow rate, and cathode backpressure. The methanol crossover measurements were performed by measuring crossover current density at an open circuit using humidified nitrogen instead of air at the cathode and applied voltage with a power supply. The membrane electrode assembly (MEA) with an active area of 5 cm² was composed of a Nafion 117 membrane, a Pt–Ru (4 mg/cm²) anode catalyst, and a Pt (4 mg/cm²) cathode catalyst. It was shown that methanol crossover increased by increasing cell temperature, methanol concentration, methanol flow rate, cathode flow rate and decreasing cathode backpressure. Also, it was revealed that the efficiency of the DMFC was closely related with methanol crossover, and significantly improved as the cell temperature and cathode backpressure increased and methanol concentration decreased.     

Water management in a passive direct methanol fuel cell

Passive direct methanol fuel cells (DMFCs) are under development for use in portable applications because of their enhanced energy density in comparison with other fuel cell types. The most significant obstacles for DMFC development are methanol and water crossover because methanol diffuses through the membrane generating heat but no power. The presence of a large amount of water floods the cathode and reduces cell performance. The present study was carried out to understand the performance of passive DMFCs, focused on the water crossover through the membrane from the anode to the cathode side. The water crossover behaviour in passive DMFCs was studied analytically with the results of a developed model for passive DMFCs. The model was validated with an in‐house designed passive DMFC. The effect of methanol concentration, membrane thickness, gas diffusion layer material and thickness and catalyst loading on fuel cell performance and water crossover is presented. Water crossover was lowered with reduction on methanol concentration, reduction of membrane thickness and increase on anode diffusion layer thickness and anode and cathode catalyst layer thickness. It was found that these conditions also reduced methanol crossover rate. A membrane electrode assembly was proposed to achieve low methanol and water crossover and high power density, operating at high methanol concentrations. The results presented provide very useful and actual information for future passive DMFC systems using high concentration or pure methanol. Copyright © 2012 John Wiley & Sons, Ltd.     

Development of an advanced MEA to use high-concentration methanol fuel in a direct methanol fuel cell system

Despite serious methanol crossover issues in Direct Methanol Fuel Cells (DMFCs), the use of high-concentration methanol fuel is highly demanded to improve the energy density of passive fuel DMFC systems for portable applications. In this paper, the effects of a hydrophobic anode micro-porous layer (MPL) and cathode air humidification are experimentally studied as a function of the methanol-feed concentration. It is found in polarization tests that the anode MPL dramatically influences cell performance, positively under high-concentration methanol-feed but negatively under low-concentration methanol-feed, which indicates that methanol transport in the anode is considerably altered by the presence of the anode MPL. In addition, the experimental data show that cathode air humidification has a beneficial effect on cell performance due to the enhanced backflow of water from the cathode to the anode and the subsequent dilution of the methanol concentration in the anode catalyst layer. Using an advanced membrane electrode assembly (MEA) with the anode MPL and cathode air humidification, we report that the maximum power density of 78 mW/cm² is achieved at a methanol-feed concentration of 8 M and cell operating temperature of 60 °C. This paper illustrates that the anode MPL and cathode air humidification are key factors to successfully operate a DMFC with high-concentration methanol fuel.     

Performance of a direct methanol alkaline membrane fuel cell

A study of a direct methanol fuel cell (DMFC) operating with hydroxide ion conducting membranes is reported. Evaluation of the fuel cell was performed using membrane electrode assemblies incorporating carbon-supported platinum/ruthenium anode and platinum cathode catalysts and ADP alkaline membranes. Catalyst loadings used were 1 mg cm⁻² Pt for both anode and cathode. The effect of temperature, oxidant (air or oxygen) and methanol concentration on cell performance is reported. The cell achieved a power density of 16 mW cm⁻², at 60 °C using oxygen. The performance under near ambient conditions with air gave a peak power density of approximately 6 mW cm⁻².     

Effect of RuO2·xH2O in anode on the performance of direct methanol fuel cells

The effect of hydrous RuO₂ (RuO₂·xH₂O) in anode on the performance of direct methanol fuel cells (DMFCs) was examined by voltammetry, methanol stripping analysis, electrochemical impedance spectroscopy, polarization measurement and chronopotentiometry. The results showed that, compared with the DMFC with conventional structures, the dynamic response and quasi-steady state performance of the RuO₂·xH₂O-introduced DMFCs were significantly improved. The DMFC with RuO₂·xH₂O layer (ROL) sandwiched between anode catalyst layer and gas diffusion layer exhibited better quasi-steady state performance than those either with ROL sandwiched between anode catalyst layer and electrolyte membrane or with RuO₂·xH₂O uniformly distributed in anode catalyst layer. The maximum power density of the DMFC with this novel structure was 16% higher than the DMFC with the conventional structure. Moreover, the dynamic response of this RuO₂·xH₂O-introduced cell was more stable during 250-hour of operation when compared with that of the conventional cell.     

Performance test and degradation analysis of direct methanol fuel cell membrane electrode assembly during freeze/thaw cycles

Performance and degradation of direct methanol fuel cell (DMFC) membrane electrode assembly (MEA) are analyzed after repeated freeze/thaw cycles. Three different MEAs stored at −20 °C for 8 h with the anode side full of methanol solution are selected to test the effects of low temperatures on performance. After the cell heated to 60 °C within 30 min, they are inspected to determine the degradation mechanism. The resistance R obtained by the polarization curve is essential for identifying the main component affecting cell performance. The electrochemical impedance spectroscopy (EIS) technique is used to characterize the DMFC after freeze/thaw cycles. Thus, deterioration is assessed by measuring the high-frequency resistance (HFR) and the charge-transfer resistance (CTR). The electrochemical surface area (ECA) is employed to investigate not only the actual chemical degradation but also membrane status since sudden loss of ECA on the cathode side can result from a broken membrane. Moreover, a strategy is designed to simulate actual conditions that may prevent the membrane from being broken. A DMFC stack without any heating or heat-insulation devices shall avoid to be stored at subzero temperatures since the membrane will be useless due to frozen of methanol solution.     

High concentration methanol fuel cells: Design and theory

Use of highly concentrated methanol fuel is required for direct methanol fuel cells (DMFCs) to compete with the energy density of Li-ion batteries. Because one mole of H₂O is needed to oxidize one mole of methanol (CH₃OH) in the anode, low water crossover to the cathode or even water back flow from the cathode into the anode is a prerequisite for using highly concentrated methanol. It has previously been demonstrated that low or negative water crossover can be realized by the incorporation of a low-α membrane electrode assembly (MEA), which is essentially an MEA designed for optimal water management, using, e.g. hydrophobic anode and cathode microporous layers (aMPL and cMPL). In this paper we extend the low-α MEA concept to include an anode transport barrier (aTB) between the backing layer and hydrophobic aMPL. The main role of the aTB is to act as a barrier to CH₃OH and H₂O diffusion between a water-rich anode catalyst layer (aCL) and a methanol-rich fuel feed. The primary role of the hydrophobic aMPL in this MEA is to facilitate a low (or negative) water crossover to the cathode. Using a previously developed 1D, two-phase DMFC model, we show that this novel design yields a cell with low methanol crossover (i.e. high fuel efficiency, ∼80%, at a typical operating current density of ∼80-90% of the cell limiting current density), while directly feeding high concentration methanol fuel into the anode. The physics of how the aTB and aMPL work together to accomplish this is fully elucidated. We further show that a thicker, more hydrophilic, more permeable aTB, and thicker, more hydrophobic, and less permeable aMPL are most effective in accomplishing low CH₃OH and H₂O crossover.     

Catalyst failure analysis of a direct methanol fuel cell membrane electrode assembly

Lifetime testing of a single cell direct methanol fuel cell (DMFC) was carried out at 100 mA cm⁻², ambient pressure and 60 °C. X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS) were used to characterize the anode and cathode catalysts before and after lifetime testing. The XRD results showed that the particle sizes and lattice parameters of anode catalyst increased from 2.8 to 3.2 nm and from 3.8761 to 3.8871 Å; the cathode catalyst increased from 7.3 to 8.9 nm and from 3.9188 to 3.9204 Å before and after the lifetime test, respectively. The XPS results indicated that during the lifetime period, the extent of oxidation of the anode Pt and Ru components increased, and Ru appears in the XPS of the cathode. Polarization curves, power density curves, and in situ cyclic voltammetry were employed to test the performance of fuel cell and electrochemically active specific surface areas (S_EAS) of the anode and cathode catalysts before and after the lifetime test. The overall findings are that the cathode suffers the greatest degradation over the test period and that subtle changes at the anode can have substantial adverse effects on the cathode.     

Effect of cell orientation on the performance of passive direct methanol fuel cells

A passive liquid feed direct methanol fuel cell (DMFC) with neither liquid pump nor a gas compressor was tested at different orientations. The experimental results showed that the vertical operation always yielded better performance than did the horizontal operation. It was further demonstrated that the improved performance in the vertical orientation was caused by the increased operating temperature as a result of a higher rate of methanol crossover, which resulted from the stronger natural convection in the vertical orientation. The constant current discharging tests showed that, although the vertical operation of the passive DMFC can yield better performance, the fuel utilization at this orientation is lower as a result of the increased rate of methanol crossover. It was also shown that the horizontal orientation with the anode facing upward rendered an effective removal of both CO₂ bubbles on the anode and liquid water on the cathode and thereby a relative stable operation. Finally, it was revealed that the horizontal orientation with the anode facing downward exhibited rather unstable and short discharging duration because of the difficulties in removing CO₂ bubbles from the anode and the liquid water from the cathode at this particular orientation.     

Development and performance analysis of a metallic micro-direct methanol fuel cell for high-performance applications

As a promising candidate for conventional micro-power sources, the micro-direct methanol fuel cell (μDMFC) is currently attracting increased attention due to its various advantages and prospective suitability for portable applications. This paper reports the design, fabrication and analysis of a high-performance μDMFC with two metal current collectors. Employing micro-stamping technology, the current collectors are fabricated on 300-μm-thick stainless steel plates. The flow fields for both cathode and anode are uniform in shape and size. Two sheets of stainless steel mesh are added between the membrane electrode assembly (MEA) and current collectors in order to improve cell performance. To avoid electrochemical corrosion, titanium nitride (TiN) layers with thickness of 500 nm are deposited onto the surface of current collectors and stainless steel mesh. The performance of this metallic μDMFC is thoroughly studied by both simulation and experimental methods. The results show that all the parameters investigated, including current collector material, stainless steel mesh, anode feeding mode, methanol concentration, anode flow rate, and operating temperature have significant effects on cell performance. Moreover, the results show that under optimal operating conditions, the metallic μDMFC exhibits promising performance, yielding a maximum power density of 65.66 mW cm⁻² at 40 °C and 115.0 mW cm⁻² at 80 °C.     

Utilization and positive effects of produced CO2 on the performance of a passive direct methanol fuel cell with a composite anode structure

This study investigates the CO₂ behavior and its effect on the performance of a passive direct methanol fuel cell (DMFC) by use of visualization method. Different flow field designs are introduced to the anode of the DMFC. The parallel-fence structure and the new composite structure with a sintered porous metal fiber felt (SPMFF) are experimentally compared in terms of the cell performance and mechanisms of reactant and product managements. Results show that the cell based on the composite structure yields the best performance during high-concentration operation. The vertical parallel-fence structure is more suitable at a lower methanol concentration. The visualization tests indicate that the use of a composite structure can help enhance convection due to CO₂ self-promoting behavior and also enhance gas storage in the channel to control the methanol concentration. The presence of CO₂ helps to restrain methanol crossover, which should be actively controlled to enhance internal convection and thereby improve the cell performance. The use of a composite structure enables the cell to perform stably for continuous operation at a higher methanol concentration. The open-circuit and dynamic characteristics of this composite-structure-based DMFC are also evaluated in this study.