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.     

Compact direct methanol fuel cells for portable application

Consumers’ demand for portable audio/video/ICT products has driven the development of advanced power technologies in recent years. Fuel cells are a clean technology with low emissions levels, suitable for operation with renewable fuels and capable, in a next future, of replacing conventional power systems meeting the targets of the Kyoto Protocol for a society based on sustainable energy systems. Within such a perspective, the objective of the European project MOREPOWER (compact direct methanol fuel cells for portable applications) is the development of a low-cost, low temperature, portable direct methanol fuel cell (DMFC; nominal power 250 W) with compact construction and modular design for the potential market area of weather stations, medical devices, signal units, gas sensors and security cameras. This investigation is focused on a conceptual study of the DMFC system carried out in the Matlab/Simulink^® platform: the proposed scheme arrangements lead to a simple equipment architecture and a efficient process.     

A transient two-phase mass transport model for liquid feed direct methanol fuel cells

A transient two-phase mass transport model for liquid feed direct methanol fuel cells (DMFCs) is developed. With this model, various processes that affect the DMFC transient behaviors are numerically studied. The results show that the cell voltage exhibits an overshoot behavior in response to a sudden change in the current density. The magnitude of the overshoot depends on the magnitudes of the change in the cell current density and the initial current density. It is found that the dynamic change in the methanol permeation through the membrane to the cathode results in a strong cathode overpotential overshoot, which is believed to be the predominant factor that leads to the cell voltage overshoot. In contrast, the anode overpotential is relatively insensitive to the changes in the methanol concentration as well as CO surface coverage in the anode catalyst layer. Moreover, the effect of the double layer capacitance (DLC) on the cell dynamic behavior is studied and the results show that the DLC can smoothen the change in the cell voltage in response to a change in the cell current density. Furthermore, the dynamic response of mass transport to a change in the cell current density is found to be rather slow. In particular, it is shown that the slow response in the mass transport of methanol is one of the key factors that influence the cell dynamic operation.     

Passive direct methanol fuel cells for portable electronic devices

Due to the increasing demand for electricity, clean, renewable energy resources must be developed. Thus, the objective of the present study was to develop a passive direct methanol fuel cell (DMFC) for portable electronic devices. The power output of six dual DMFCs connected in series with an active area of 4 cm² was approximately 600 mW, and the power density of the DMFCs was 25 mW cm⁻². The DMFCs were evaluated as a power source for mobile phone chargers and media players. The results indicated that the open circuit voltage of the DMFC was between 6.0 V and 6.5 V, and the voltage under operating conditions was 4.0 V. The fuel cell was tested on a variety of cell phone chargers, media players and PDAs. The cost of energy consumption by the proposed DMFC was estimated to be USD 20 W⁻¹, and the cost of methanol is USD 4 kW h. Alternatively, the local conventional electricity tariff is USD 2 kW h. However, for the large-scale production of electronic devices, the cost of methanol will be significantly lower. Moreover, the electricity tariff is expected to increase due to the constraints of fossil fuel resources and pollution. As a result, DMFCs will become competitive with conventional power sources.     

Exergy analysis of a passive direct methanol fuel cell

An analytical, one-dimensional, steady state model is employed to solve for overpotentials at the catalyst layers along with the liquid water and methanol distributions at the anode, and oxygen transport at the cathode. An iterative method is utilized to calculate the cell temperature at each cell current density. A comprehensive exergy analysis considering all possible species inside the cell during normal operation is presented. The contributions of different types of irreversibilities including overpotentials at the anode and cathode, methanol crossover, contact resistance, and proton conductivity of the membrane are investigated. Of all losses, overpotentials in conjunction with the methanol crossover are considered as the major exergy destruction sources inside the cell during the normal operation. While the exergy losses due to electrochemical reactions are more significant at higher current densities, exergy destruction by methanol crossover at the cathode plays more important role at lower currents. It is also found that the first-law efficiency of a passive direct methanol fuel cell increases as the methanol solution in the tank increases in concentration from 1 M to 3 M. However, this is not the case with the second-law efficiency which is always decreasing as the concentration of the methanol solution in the tank increases.     

A semi-empirical model for efficiency evaluation of a direct methanol fuel cell

Energy density and power density are two of the most significant performance indices of a fuel cell system. Both the indices are closely related to the operating conditions. Energy density, which can be derived from fuel cell efficiency, is especially important to small and portable applications. Generally speaking, power density can be easily obtained by acquiring the voltage and current density of an operating fuel cell. However, for a direct methanol fuel cell (DMFC), it is much more difficult to evaluate its efficiency due to fuel crossover and the complex architecture of fuel circulation. The present paper proposes a semi-empirical model for the efficiency evaluation of a DMFC under various operating conditions. The power density and the efficiency of a DMFC are depicted by explicit functions of operating temperature, fuel concentration and current density. It provides a good prediction and a clear insight into the relationship between the aforementioned performance indices and operating variables. Therefore, information including power density, efficiency, as well as remaining run-time about the status of an operating DMFC can be in situ evaluated and predicted. The resulting model can also serve as an important basis for developing real-time control strategies of a DMFC system.     

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.     

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.     

Effect of pressure for direct fuel cells using DME-based fuels

The performance of direct fuel cells using dimethyl ether(DME)-based fuels is presented at a relatively low temperature of 80 °C. DME is supplied to the fuel cells either by gas phase or aqueous phase for the operation of direct fuel cells. In order to keep DME in liquid phase during operation, fuel cells were operated at higher pressure up to 5 bar. For further increase of the power density from direct DME fuel cells, DME was mixed with methanol solution and fed into the fuel cells by the vapor pressure of DME itself without a liquid pump. In this study, we have obtained the highest power density of 210 mW cm⁻² at a temperature of 80 °C when the fuel cell is operated with the mixed fuel with 2 M methanol solution under 4 bar.     

Investigation on the durability of direct methanol fuel cells

This study addresses the durability of direct methanol fuel cells (DMFCs). Three performance indices including permanent degradation, temporary degradation and voltage fluctuation are proposed to qualify the durability of DMFC. The decay rate, associated with permanent degradation, follows from such failure mechanisms as dissolution, growth and poisoning of the catalyst, while temporary degradation reflects the elimination of the hydrophobic property of the gas diffusion layer (GDL). However, voltage fluctuations reveal different results which cannot stand for degradation phenomenon exactly. In this investigation, such methods of examination as scanning electron microscope (SEM), and X-ray diffraction (XRD) are employed to check the increase in the mean particle size in the anode and cathode catalysts, and the degree is higher in the cathode. The Ru content in the anode catalyst and the specific surface area (SSA) of the anode and cathode catalysts decrease after long-term operation. Moreover, the crossover of Ru from the anode side to the cathode side is revealed by energy dispersive X-ray (EDX) analysis. Electro-catalytic activity towards the methanol oxidation reaction (MOR) at the anode is verified to be weaker after durability test by cyclic voltammetry (CV). Also, the electrochemical areas (ECAs) of the anode and cathode catalysts are evaluated by hydrogen-desorption. SSA loss simply because of agglomeration and growth of the catalyst particles, of course, is lower than ECA loss. The observations will help to elucidate the failure mechanism of membrane electrode assembly (MEA) in durability tests, and thus help to prolong the lifetime of DMFC.     

Effects of cathode flow fields on direct methanol fuel cell-simulation study

Flow-field design of direct methanol fuel cell (DMFCs) plays an important role affecting the cell performance. Previous studies suggest that the combination of anode parallel flow field and cathode serpentine flow-field present the best and stable performance. Among these, cathode flow-field holds higher influence than that of anode. However, more detailed experiments needed to be done to find out the reasons. In this study, CFDRC half-cell models are adopted to simulate the flow phenomena within serpentine, parallel and grid flow field. We find that gas is well distributed within serpentine flow field while barren region are observed within parallel flow field. These factors contribute to the cell performance greatly. In addition, the durability test of DMFCs using parallel flow field is improved when the flow rate is increased or the current is uphold at inferior, so the barren region maintained at an acceptable level.     

Supported PtRu on mesoporous carbons for direct methanol fuel cells

We prepared and characterized several cryogel mesoporous carbons of different pore size distribution and report the catalytic activity of PtRu supported on mesoporous carbons of pore size >15 nm in passive and in active direct methanol fuel cells (DMFCs). At room temperature (RT), the specific maximum power of the passive DMFCs with mesoporous carbon/PtRu systems as anode was in the range 3–5 W g⁻¹. Passive DMFC assembly and RT tests limit the performance of the electrocatalytic systems and the anodes were thus tested in active DMFCs at 30, 60 and 80 °C. Their responses were also compared to those of commercial Vulcan carbon/PtRu. At 80 °C, the specific maximum power of the active DMFC with C656/PtRu was 37 W g⁻¹ and the required amount of Pt per kW estimated at 0.4 V cell voltage was 31 g kW⁻¹, a value less than half that of Vulcan carbon/PtRu.     

Systematic approach for modeling methanol mass transport on the anode side of direct methanol fuel cells

A systematic method for modeling direct methanol fuel cells, with a focus on the anode side of the system, is advanced for the purpose of quantifying the methanol crossover phenomenon and predicting the concentration of methanol in the anode catalyst layer of a direct methanol fuel cell. The model accounts for fundamental mass transfer phenomena at steady state, including convective transport in the anode flow channel, as well as diffusion and electro-osmotic drag transport across the polymer electrolyte membrane. Experimental measurements of methanol crossover current density are used to identify five modeling parameters according to a systematic parameter estimation methodology. A validation study shows that the model matches the experimental data well, and the usefulness of the model is illustrated through the analysis of effects such as the choice fuel flow rate in the anode flow channel and the presence of carbon-dioxide bubbles.     

Passively operated vapor-fed direct methanol fuel cells for portable applications

The impact of structural parameters and operating conditions has not been researched yet for vapor-fed operation of a DMFC at near-ambient conditions. Thus, a detailed parameter study that included reference cell measurements to assess anode and cathode losses separately was performed. Among other parameters like temperature or air stoichiometry, different opening ratios that controlled evaporation of methanol into the vapor chamber were examined.     

Three-dimensional anode engineering for the direct methanol fuel cell

Catalyzed graphite felt three-dimensional anodes were investigated in direct methanol fuel cells (DMFCs) operated with sulfuric acid supporting electrolyte. With a conventional serpentine channel flow field the preferred anode thickness was 100 μm, while a novel flow-by anode showed the best performance with a thickness of 200-300 μm. The effects of altering the methanol concentration, anolyte flow rate and operating temperature on the fuel cell superficial power density were studied by full (2³ + 1) factorial experiments on a cell with anode area of 5 cm² and excess oxidant O₂ at 200 kPa(abs). For operation in the flow-by mode with 2 M methanol at 2 cm³ min⁻¹ and 353 K the peak power density was 2380 W m⁻² with a PtRuMo anode catalyst, while a PtRu catalyst yielded 2240 W m⁻² under the same conditions.     

Sulfonated poly(arylene ether sulfone) membranes blended with hydrophobic polymers for direct methanol fuel cell applications

In this paper, proton exchange membranes for direct methanol fuel cells were prepared by blending sulfonated poly(arylene ether sulfone) with poly (vinylidene fluoride-co-hecafluoropropylene)(PVdF-HFP) and polyethersulfone (PES) to decrease methanol permeability while maintaining high proton conductivity. The content of the second polymer, such as PES and PVdF, in the blend membranes was controlled at 10–40 wt% based on SPAES. In order to investigate the effects of the second polymer content in the blended membranes, parameters of the prepared membranes related to fuel cell performance were characterized, including their morphology, mechanical properties, methanol permeability, and proton conductivity. Surface roughness of the blend membrane was increased by the introduction of a hydrophobic polymer. Mechanical properties of the PES/SPAES blend membrane were improved owing to interaction between the sulfonic acid groups in SPAES and PES. However, the tensile strength of the PVdF/SPAES blend membrane was decreased by due to the poor compatibility of SPAES and PVdF. The methanol permeability in the blended membranes decreased with increasing content of PES and PVdF. The SPAES/PES blend membranes exhibited good proton conductivity and lower methanol permeability than the SPAES membrane. The SVdF15 blend membrane showed the highest selectivity due to the absence of methanol crossover and a small decrease of proton conductivity. These blend membranes are suitable for DMFC applications.     

A self-regulated passive fuel-feed system for passive direct methanol fuel cells

Operating a passive direct methanol fuel cell (DMFC) with high methanol concentration is desired because this increases the energy density of the fuel cell system and hence results in a longer runtime. However, the increase in methanol concentration is limited by the adverse effect of methanol crossover in the conventional design. To overcome this problem, we propose a new self-regulated passive fuel-feed system that not only enables the passive DMFC to operate with high-concentration methanol solution without serious methanol crossover, but also allows a self-regulation of the feed rate of methanol solution in response to discharging current. The experimental results showed that with this fuel-feed system, the fuel cell fed with high methanol concentration of 12.0 M yielded the same performance as that of the conventional DMFC running with 4.0 M methanol solution. Moreover, as a result of the increased energy density, the runtime of the cell with this new system was as long as 10.1 h, doubling that of the conventional design (4.4 h) at a given fuel tank volume. It was also demonstrated that this passive fuel-feed system could successfully self-regulate the fuel-feed rate in response to the change in discharging currents.     

A prediction model of degradation rate for membrane electrode assemblies in direct methanol fuel cells

This article proposes a new prediction model to describe the nonlinear performance degradation paths of membrane electrode assemblies (MEAs) in direct methanol fuel cell (DMFC): a bi-exponential model with random coefficients. The bi-exponential model is constructed on a mathematical basis representing second-order kinetics. Performance variation between MEAs is incorporated by random coefficients in the proposed model. A likelihood ratio test is sequentially executed to select random effects in the nonlinear random-coefficients model. Analysis results indicate that the reliability estimation can be substantially improved by using the nonlinear random-coefficients model to incorporate two heterogeneous degradation characteristics of MEA performance during continuous operation of DMFC. Confidence intervals of failure-time distributions are obtained by the parametric bootstrap method.     

Electrode in direct methanol fuel cells

Nanotechnology has recently generated a lot of attention and high expectations not only in the academic community but also among investors, scientists and researchers in both government and industry sectors. Its unique capability to fabricate new structures at the atomic scale has already produced novel materials and devices with great potential applications in a wide number of fields. Up to now, the electrodes in direct methanol fuel cells (DMFCs) have generally been based on the porous carbon gas diffusion electrodes that are employed in proton exchange membrane fuel cells. Typically, the structure of such electrodes is comprised of a catalyst layer and a diffusion layer, the latter being carbon cloth or carbon paper. It is a challenge to develop an electrode with high surface area, good electrical conductivity and suitable porosity to allow good reactant flux and high stability in the fuel cell environment. This paper presents an overview of electrode structure in general and recent material developments, with particular attention paid to the application of nanotechnology in DMFCs.     

Development of an air-breathing direct methanol fuel cell with the cathode shutter current collectors

An air-breathing direct methanol fuel cell with a novel cathode shutter current collector is fabricated to develop the power sources for consumer electronic devices. Compared with the conventional circular cathode current collector, the shutter one improves the oxygen consumption and mass transport. The anode and cathode current collectors are made of stainless steel using thermal stamping die process. Moreover, an encapsulation method using the tailor-made clamps is designed to assemble the current collectors and MEA for distributing the stress of the edges and inside uniformly. It is observed that the maximum power density of the air-breathing DMFC operating with 1 M methanol solution achieves 19.7 mW/cm² at room temperature. Based on the individual DMFCs, the air-breathing stack consisting of 36 DMFC units is achieved and applied to power a notebook computer.