Effect of variation of hydrophobicity of anode diffusion media along the through-plane direction in direct methanol fuel cells

Reducing methanol crossover from the anode to cathode in direct methanol fuel cells (DMFCs) is critical for attaining high cell performance and fuel utilization, particularly when highly concentrated methanol fuel is fed into DMFCs. In this study, we present a novel design of anode diffusion media (DM) wherein spatial variation of hydrophobicity along the through-plane direction is realized by special polytetrafluoroethylene (PTFE) coating procedure. According to the capillary transport theory for porous media, the anode DM design can significantly affect both methanol and water transport processes in DMFCs. To examine its influence, three different membrane-electrode assemblies are fabricated and tested for various methanol feed concentrations. Polarization curves show that cell performance at high methanol feed concentration conditions is greatly improved with the anode DM design with increasing hydrophobicity toward the anode catalyst layer. In addition, we investigate the influence of the wettability of the anode microporous layer (MPL) on cell performance and show that for DMFC operation at high methanol feed concentration, the hydrophilic anode MPL fabricated with an ionomer binder is more beneficial than conventional hydrophobic MPLs fabricated with PTFE. This paper highlights that controlling wetting characteristics of the anode DM and MPL is of paramount importance for mitigating methanol crossover in DMFCs.     

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.     

Mass balance research for high electrochemical performance direct methanol fuel cells with reduced methanol crossover at various operating conditions

Mass balance research in direct methanol fuel cells (DMFCs) provides a more practical method in characterizing the mass transport phenomena in a membrane electrode assembly (MEA). This method can be used to measure methanol utilization efficiency, water transport coefficient (WTC), and methanol to electricity conversion rate of a MEA in DMFCs. First, the vital design parameters of a MEA are recognized for achieving high methanol utilization efficiency with increased power density. In particular, the structural adjustment of anode diffusion layer by adding microporous layer (MPL) is a very effective way to decrease WTC with reduced methanol crossover due to the mass transfer limitation in the anode. On the other hand, the cathode MPL in the MEA design can contribute in decreasing methanol crossover. The change of structure of cathode diffusion layer is also found to be a very effective way in improving power density. In contrast, the WTC of DMFC MEAs remains virtually constant in the range of 3.4 and 3.6 irrespective of the change of the cathode GDL. The influence of operating condition on the methanol utilization efficiency, WTC, and methanol to electricity conversion rate is also presented and it is found that these mass balance properties are strongly affected by temperature, current density, methanol concentration, and the stoichiometry of fuel and air.     

An impedance study for the anode micro-porous layer in an operating direct methanol fuel cell

This study presents the benefit to an operating direct methanol fuel cell (DMFC) by coating a micro-porous layer (MPL) on the surface of anode gas diffusion layer (GDL). Taking the membrane electrode assembly (MEA) with and without the anodic MPL structure into account, the performances of the two types of MEA are evaluated by measuring the polarization curves together with the specific power density at a constant current density. Regarding the cell performances, the comparisons between the average power performances of the two different MEAs at low and high current density, various methanol concentrations and air flow rates are carried out by using the electrochemical impedance spectroscopy (EIS) technique. In contrast to conventional half cell EIS measurements, both the anode and cathode impedance spectra are measured in real-time during the discharge regime of the DMFC. As comparing each anode and cathode EIS between the two different MEAs, the influences of the anodic MPL on the anode and cathode reactions are systematically discussed and analyzed. Furthermore, the results are used to infer complete and reasonable interpretations of the combined effects caused by the anodic MPL on the full cell impedance, which correspond with the practical cell performance.     

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.     

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.     

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.     

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.     

Study on operational aspects of a passive direct methanol fuel cell incorporating an anodic methanol barrier

This paper presents an investigation concerning the effects of operating conditions on the performance of a passive direct methanol fuel cell (DMFC). A self-developed porous metal fiber sintered plate (PMFSP) is used as the methanol barrier between the fuel reservoir and current collector at the anode in order to alleviate the effect of methanol crossover. The effectiveness of using this method is validated. A series of operating conditions such as operating orientation, methanol concentration, ambient temperature, forced air convection and dynamic load are evaluated. Results show that the use of a PMFSP promotes a higher cell performance during vertical operation than horizontal orientation. The effect of methanol concentration depends on the PMFSP porosity. A relatively lower porosity is favorable for high-concentration operation. The cell performance gets improved when increasing the ambient temperature and adopting forced air supply at the cathode. Compared with the traditional structure, the use of a PMFSP makes the fuel cell insensitive to the change of blowing intensity. In addition, the dynamic characteristics of the PMFSP-based passive DMFC are also reported.     

Effects of anode microporous layers made of carbon powder and nanotubes on water transport in direct methanol fuel cells

The effects of the design parameters of the anode diffusion layer (DL), including the PTFE loading in the backing layer (BL), and the carbon and PTFE loading in the microporous layer (MPL), on water transport through the membrane and the performance of a liquid-feed direct methanol fuel cell (DMFC) are experimentally investigated. The results indicate that increasing the PTFE loading in the BL and introducing a MPL could decrease water crossover through the membrane without sacrificing cell performance when the feed methanol concentration is increased. It is also found that changing the PTFE loading in the MPL has little effect on water crossover, whereas increasing the carbon loading in the MPL could noticeably decrease the water-crossover flux. Nevertheless, the ability of the MPL to reduce water crossover is limited by the presence of a number of mud cracks. To reduce further the water-crossover flux, a crack-free MPL made of multi-walled carbon nanotubes (MWCNTs) and PTFE is proposed. Tests indicate that the DMFC with the nanotube MPL results in a much lower water-crossover flux than a conventional carbon-powder MPL. More importantly, the use of the nanotube MPL allows the DMFC to be operated with a higher methanol concentration, and thereby increases the fuel cell system energy density.     

Construction of gradient catalyst layer anode by incorporating covalent organic framework to improve performance of direct methanol fuel cells

Improving cell performance is the most demanded task in direct methanol fuel cell (DMFC) research, although this fuel cell has several intrinsic features like high energy density, moderate operating temperature and environmentally friendly operation. The catalyst layer (CL) is the site of electrochemical reactions directly affecting the cell performance. Accordingly, the structure and preparation of the CL are crucial in optimizing performance. In this study, a novel gradient catalyst layer (G-CL) for the anode of DMFC is developed and better performance is obtained compared with that of single catalyst layer (S-CL) anode. A G-CL anode is composed of an outer CL of covalent organic framework (COF) materials-mixed catalyst near the microporous layer (MPL) and a conventional inner CL near the membrane side. Different loading of Pt catalyst in the two layers. Therefore, in the G-CL structure, there existed a catalyst concentration gradient and porosity gradient. Anode electrodes are characterized morphologically and electrochemically and the performance of individual cells containing such G-CL designs is measured. The results indicate that incorporation of the appropriate amount of COF materials enables the outer CL not only to have a larger electrochemical surface area (ECSA) and expose more catalytic active sites but also holds a strong proton transfer ability to improve the methanol oxidation reaction (MOR) performance. Due to the presence of the inner layer of the CL formed the methanol gradient oxidation process. With the same platinum-ruthenium (Pt–Ru) catalyst loading, the 5 wt.% COF G-CL anode structure exhibited lower methanol crossover and higher power density (nearly 11% increment) compared with that of the S-CL anode with high methanol concentration (8 M) at 60 °C, showing the promising potential in further applications.     

Water management of the DMFC passively fed with a high-concentration methanol solution

The methanol barrier layer adopted for high-concentration direct methanol fuel cells (HC-DMFCs) increases water transport resistance, and makes water management in HC-DMFCs more challenging and critical than that in the conventional direct methanol fuel cell (DMFC) without a methanol barrier layer. In the semi-passive HC-DMFC used in this work, oxygen was actively supplied to the cathode side while various concentrated methanol solutions, 4 M, 8 M, 16 M, and neat methanol, were passively supplied from the anode fuel reservoir. The effects of the cathode relative humidity, cathode pressure, and oxygen flow rate on the water crossover coefficient, fuel efficiency, and overall performance of the fuel cell were studied. Results showed that electrolyte membrane resistance, which was determined by its water content, was the predominant factor that determined the performance of a HC-DMFC, especially at a high current density. A negative water crossover coefficient, which indicated that water flowed back from the cathode through the electrolyte membrane to the anode, was measured when the methanol concentration was 8 M or higher. The back flow of water from the cathode is a very important water supply source to hydrate the electrolyte membrane. The water crossover coefficient was decreased by increasing the cathode relative humidity and back pressure. Water flooding at the cathode was not severe in the HC-DMFC, and a low oxygen flow rate was preferred to decrease water loss and yield a better performance. The peak power density generated from the HC-DMFC fed with 16 M methanol solution was 75.9 mW cm⁻² at 70 °C.     

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.     

Characteristics of water transport through the membrane in direct methanol fuel cells operating with neat methanol

The water required for the methanol oxidation reaction in a direct methanol fuel cell (DMFC) operating with neat methanol can be supplied by diffusion from the cathode to the anode through the membrane. In this work, we present a method that allows the water transport rate through the membrane to be in-situ determined. With this method, the effects of the design parameters of the membrane electrode assembly (MEA) and operating conditions on the water transport through the membrane are investigated. The experimental data show that the water flux by diffusion from the cathode to the anode is higher than the opposite flow flux of water due to electro-osmotic drag (EOD) at a given current density, resulting in a net water transport from the cathode to the anode. The results also show that thinning the anode gas diffusion layer (GDL) and the membrane as well as thickening the cathode GDL can enhance the water transport flux from the cathode to the anode. However, a too thin anode GDL or a too thick cathode GDL will lower the cell performance due to the increases in the water concentration loss at the anode catalyst layer (CL) and the oxygen concentration loss at the cathode CL, respectively.     

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.     

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.     

Effects of the microstructure and powder compositions of a micro-porous layer for the anode on the performance of high concentration methanol fuel cell

To investigate the effects of the microstructure and powder compositions for the micro-porous layer (MPL) of an anode on the cell performance of a direct methanol fuel cell (DMFC) using a highly concentrated methanol solution up to 7 M, various powders and their compositions were applied as a filler of the MPL in the membrane electrode assembly (MEA). Several nano- and microstructured carbons such as commercial carbon black (CB), spherical activated carbon (AC), multi-walled carbon nanotube (MWCNT), and platelet carbon nanofiber (PCNF) were selected with different morphology and surface properties, and a meso-porous silica (one of SBA series) was also included for its porous and hydrophilic properties. The coating morphology and physical properties such as porosity and gas permeability were measured, and electrochemical properties of MEA with the MPL were examined by using current–voltage polarization, electrochemical impedance spectroscopy, and voltammetric analyses. A mixture of different carbons was found to be effective for lowering methanol crossover with sustaining electrical conductivity and gas permeability. A MEA with modified-anode MPLs made of CB (50 vol%) and PCNF (50 vol%) powders showed a maximum power density of 67.7 mW cm⁻² under operation with a 7 M concentration of methanol.     

Modeling and optimizing the performance of a passive direct methanol fuel cell

Passive direct methanol fuel cells (DMFCs) are promising energy sources for portable electronic devices. Different from DMFCs with active fuel feeding systems, passive DMFCs with nearly stagnant fuel and air tend to bear comparatively less power densities. In the aspect of cell performance optimization, there could be significant differences in cell design parameters between active and passive DMFCs. A numerical model that could simulate methanol permeation and the pertinent mixed potential effect in a DMFC was used to help seek for possibilities of optimizing the cell performance of a passive DMFC by studying impacts from variations of cell design. The subjects studied include catalysis of the anode and the cathode, membrane thickness, membrane conductivity, and methanol concentration. In contrast to general understandings on a DMFC with active fuel and reactant gas, our simulation results for a passive DMFC used in this study indicated that the catalysis of the cathode appeared to be the most important parameter. The maximum power density was predicted to improve by 38% with the thickness of the cathodic catalyst layer doubled and by 36% with the catalyst loading doubled. The improvement on cell performance would multiply if we simultaneously adopted the most optimal parameters during the simulation study.     

Performance of air-breathing direct methanol fuel cell with Au-coated aluminum current collectors

Current collectors of the direct methanol fuel cell (DMFC) are of significant importance for portable power sources, and greatly determine the weight energy density and cost of the cell. In this paper, the air-breathing aluminum (Al) current collectors have been developed for powering portable applications. The anode and cathode current collectors with the area of 4.5 cm² were fabricated on the Al substrates utilizing Computer Numerical Control (CNC) technology. To obtain strong anti-corrosion resistance, a 3-μm-Au layer was deposited on the current collectors using chemical plating. Compared with the graphite and stainless steel, the characterization of the Au-coated Al current collector was investigated to exhibit superior characteristics in electric conductivity, weight and electrochemical corrosion resistance. The current collector was applied to a DMFC and the cell performance was experimentally investigated under different operating conditions. The measured maximum power density of the DMFC could reach 19.8 mW cm⁻² at current density of 98 mA cm⁻² with 2 M methanol solutions. The results indicated that the Au-coated Al current collectors presented in this paper might be helpful for the development of portable power sources applied in future commercial applications.     

Lightweight current collector based on printed-circuit-board technology and its structural effects on the passive air-breathing direct methanol fuel cell

To realize lightweight design of the fuel cell system is a critical issue before it is put into practical use. The printed-circuit-board (PCB) technology can be potentially used for production of current collectors or flow distributors. This study develops prototypes of a single passive air-breathing direct methanol fuel cell (DMFC) and also an 8-cell mono-polar DMFC stack based on PCB current collectors. The effects of diverse structural and operational factors on the cell performance are explored. Results show that the methanol concentration of 6 M promotes a higher cell performance with a peak power density of 18.3 mW cm⁻². The combination of current collectors using a relatively higher anode open ratio and inversely a lower cathode open ratio helps enhance the cell performance. Dynamic tests are also conducted to reveal transient behaviors and its dependence on the operating conditions. To validate the real working status of the DMFC stack, it is coupled with an LED lightening system. The performance of this hybrid system is also reported in this study.