A three-dimensional two-phase flow model for a liquid-fed direct methanol fuel cell

A three-dimensional, two-phase, multi-component model has been developed for a liquid-fed DMFC. The modeling domain consists of the membrane, two catalyst layers, two diffusion layers, and two channels. Both liquid and gas phases are considered in the entire anode, including the channel, the diffusion layer and the catalyst layer; while at the cathode, two phases are considered in the gas diffusion layer and the catalyst layer but only single gas phase is considered in the channels. For electrochemical kinetics, the Tafel equation incorporating the effects of two phases is used at both the cathode and anode sides. At the anode side the presence of gas phase reduces the active catalyst areas, while at the cathode side the presence of liquid water reduces the active catalyst areas. The mixed potential effects due to methanol crossover are also included in the model. The results from the two-phase flow mode fit the experimental results better than those from the single-phase model. The modeling results show that the single-phase models over-predict methanol crossover. The modeling results also show that the porosity of the anode diffusion layer plays an important role in the DMFC performance. With low diffusion layer porosity, the produced carbon dioxide cannot be removed effectively from the catalyst layer, thus reducing the active catalyst area as well as blocking methanol from reaching the reaction zone. A similar effect exits in the cathode for the liquid water.     

Optimum operating strategies for liquid-fed direct methanol fuel cells

This study focuses on optimum operating strategies for liquid-fed direct methanol fuel cells (DMFCs) to minimize methanol consumption. A mathematical model is developed and verified with experimental data from the literature using the parameter estimation method. The model consists of a set of differential and algebraic equations and makes it possible to describe zero initial hold-up conditions. Based on the model, steady-state simulation results are obtained and explain the dependence on the feed concentration of key variables such as cell voltage, cell power density, overpotentials of both electrodes, and methanol crossover ratio. Dynamic simulation results are also presented to check the transient behaviour of a DMFC operated from start-up to shut-down. Dynamic optimization allows determination of the optimum transient strategies of feed concentration required to maximize the fuel efficiency. With six scenarios of power density load, it is demonstrated that the optimum transient strategies depend heavily on both the load of power density and the number of control actions. The main advantage of these approaches is to reduce fuel consumption and, ultimately, to enable DMFCs to be operated more efficiently.     

Comprehensive one-dimensional,semi-analytical,mathematical model for liquid-feed polymer electrolyte membrane direct methanol fuel cells

Polymer electrolyte membrane direct methanol fuel cells (PEM-DMFCs) have several advantages over hydrogen-fuelled PEM fuel cells; but sluggish methanol electrochemical oxidation and methanol crossover from the anode to the cathode through the PEM are two major problems with these cells. In the present work, a comprehensive one-dimensional, single phase, isothermal mathematical model is developed for a liquid-feed PEM-DMFC, taking into account all the necessary mass transport and electrochemical phenomena. Diffusion and convective effects are considered for methanol transport on the anode side and in the PEM, whereas only diffusional transport of species is considered on the cathode side. A multi-step reaction mechanism is used to describe the electrochemical oxidation of methanol at the anode. Stefan–Maxwell equations are used to describe multi-component diffusion on the cathode side and Tafel type of kinetics is used to describe the simultaneous methanol oxidation and oxygen reduction reactions at the cathode. The model fully accounts for the mixed potential effect caused by methanol crossover at the cathode. It shows excellent agreement with literature data of the limiting current density for different low methanol feed concentrations at different operating temperatures. At high methanol feed concentrations, oxygen depletion on the cathode side, due to excessive methanol crossover, results in mass-transport limitations. The model can be used to optimize the geometric and physical parameters with a view to extracting the highest current density while still keeping a tolerably low methanol crossover.     

A mathematical model for simulating methanol permeation and the mixed potential effect in a direct methanol fuel cell

A mathematical model for simulating methanol permeation and the pertinent mixed potential effect in a direct methanol fuel cell (DMFC) is presented. In this model a DMFC is divided into seven compartments namely the anodic flow channel, the anodic diffusion layer, the anodic catalyst layer, the proton exchange membrane (PEM), the cathodic catalyst layer, the cathodic diffusion layer and the cathodic flow channel. All compartments are considered to have finite thickness, and within every one of them a set of governing equations are given to stipulate methanol transport and oxygen transport. For the flow channels, fluid dynamics, which could substantially lower the local methanol concentration within catalyst layers is taken into account. With the knowledge of local concentrations of the species, the electrochemical reaction rates within both catalyst layers can be quantified by a kinetic Tafel expression. For the anodic catalyst layer the local external current generated by methanol oxidation is computed; for the cathodic catalyst layer, in addition to the local external current generated by oxygen reduction, the local internal current as a result of methanol permeation is also computed. With the information of the local internal current, the mixed potential effect, which is responsible for adversely lowering the cell voltage can be analyzed.     

Two-dimensional two-phase thermal model for passive direct methanol fuel cells

A two-dimensional two-phase thermal model is presented for direct methanol fuel cells (DMFC), in which the fuel and oxidant are fed in a passive manner. The inherently coupled heat and mass transport, along with the electrochemical reactions occurring in the passive DMFC is modeled based on the unsaturated flow theory in porous media. The model is solved numerically using a home-written computer code to investigate the effects of various operating and geometric design parameters, including methanol concentration as well as the open ratio and channel and rib width of the current collectors, on cell performance. The numerical results show that the cell performance increases with increasing methanol concentration from 1.0 to 4.0 M, due primarily to the increased operating temperature resulting from the exothermic reaction between the permeated methanol and oxygen on the cathode and the increased mass transfer rate of methanol. It is also shown that the cell performance upgrades with increasing the open ratio and with decreasing the rib width as the result of the increased mass transfer rate on both the anode and cathode.     

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.     

Structural diversity and orientation dependence of a liquid-fed passive air-breathing direct methanol fuel cell

This paper investigates the interesting effects of structural diversity and operating orientation on the performance of a liquid-fed passive air-breathing direct methanol fuel cell (PAB-DMFC). The results indicate that a higher thickness of the GEFC^®-10N membrane helps enhance the cell performance due to its ability in reducing methanol crossover (MCO). When the cell uses carbon cloth at the anode but carbon paper at the cathode as the diffusion media, it produces higher performances than other combinations. The work also confirms the merit of using a cathode diffusion layer since it improves water, methanol and heat management. As for the structural optimization of current collector, it is recommended to use the circular-hole-array pattern with a lower open ratio at the anode but the parallel-fence pattern with a higher open ratio at the cathode. It is further demonstrated that the vertical operation yields a higher cell performance at a lower methanol concentration while the horizontal operation performs better at a higher methanol concentration. Besides, the effects of opening pattern and working orientation on the CO₂ evolvement behaviors are analyzed by using visualized methods. Detailed mechanisms related to the resultant phenomena are comprehensively provided in this work.     

Fabrication of three-dimensional copper nanodomes as anode materials for direct methanol fuel cells

In this study, we demonstrate a novel approach for fabricating copper nanodomes (Cu-NDs) by combining of soft lithography, nanosphere lithography, physical vapor deposition (PVD) and electrochemical deposition methods. The 3D nano structures were characterized using surface microscopic techniques. The methanol oxidation activity of the Cu-NDs anode was tested by electrochemical methods in 0.1 M KOH +1 M CH₃OH solution and the results were compared with that of bulk Cu as a reference point. The results showed that very well-structured, uniformly and homogeneously distributed Cu-NDs could be fabricated using these combined methods. The peak current density related to methanol oxidation reaction increased and charge transfer resistance reduced almost three times at the Cu-NDs electrode with respect to the bulk Cu. Also, the Cu-NDs electrode has good time stability and high tolerance to CO_ads poisoning. The enhanced activity of the nanostructures was related to good intrinsic activity of Cu for this reaction and their larger available electrochemical active sites.     

Pyramidal direct methanol fuel cells

The main concept of this article is to allow for the optimized tree-like design of the flow distributors to actually define the shape of the fuel cell, thereby eliminating problems associated with the mismatch between a predecided rectangular shape and a functionally preferred channel distribution system. The work focuses on direct methanol fuel cells (DMFC). A one-dimensional across-the-cell model is extended to a two-dimensional along-the-channel model and numerically solved to predict the polarization curves of a direct methanol fuel cell with tree network channels and of a cell with traditional serpentine channels. For both flow configurations, pressure drop and pumping power are estimated. Net power densities are computed in terms of constructal parameters and operating conditions. In contrast to the traditional rectangular shape of fuel cells, the resulting “pyramidal” or “double-staircase” shape is based on the functionality of the fluid distribution system. It is found that tree network channels can provide substantially improved electric and net power densities compared to the traditional non-bifurcating single serpentine channels, as a result of their intrinsic advantage with respect to both mass transfer and pressure drop. For six (12) branching levels and inlet channel diameters of 0.05, 0.04, and 0.03 cm, the tree network channels allow for 14% (21%), 17% (26%), and 30% (46%) higher net power densities, respectively.     

New membranes for direct methanol fuel cells

The performance of direct methanol fuel cells (DMFC) is limited by the cross-over of methanol through the electrolyte. Electrolyte membranes prepared by blending of sulfonated arylene main-chain polymers like sulfonated PEEK Victrex (sPEEK) or sulfonated PSU Udel (sPSU) with basic polymers like poly(4-vinylpyridine) (P4VP) or polybenzimidazole (PBI) show excellent chemical and thermal stability, good proton-conductivity, and good performance in H₂ PEM fuel cells. Furthermore, these materials have potentially lower methanol cross-over when compared to standard Nafion-type membranes.In this work, membrane electrode assemblies (MEAs) have been prepared from such membranes according to the thin-film method. The catalyst layer was spray-coated directly on the heated membrane using an ink consisting of an aqueous suspension of catalyst powder and Nafion solution. Unsupported catalysts were used for anode and cathode. A rather high catalyst loading was chosen in order to minimize the effects of limited catalyst utilization due to flooding conditions at both electrodes.     

Basic model for membrane electrode assembly design for direct methanol fuel cells

This research proposes a model that predicts the effect of the anode diffusion layer and membrane properties on the electrochemical performance and methanol crossover of a direct methanol fuel cell (DMFC) membrane electrode assembly (MEA). It is an easily extensible, lumped DMFC model. Parameters used in this design model are experimentally obtainable, and some of the parameters are indicative of material characteristics. The quantification of these material parameters builds up a material database. Model parameters for various membranes and diffusion layers are determined by using various techniques such as polarization, mass balance, electrochemical impedance spectroscopy (EIS), and interpretation of the response of the cell to step changes in current. Since the investigation techniques cover different response times of the DMFC, processes in the cell such as transport, reaction and charge processes can be investigated separately. Properties of single layers of the MEA are systematically varied, and subsequent analysis enables identification of the influence of the layer's properties on the electrochemical performance and methanol crossover. Finally, a case study indicates that the use of a membrane with lower methanol diffusivity and a thicker anode micro-porous layer (MPL) yields MEAs with lower methanol crossover but similar power density.     

A novel hybrid Nafion-PBI-ZP membrane for direct methanol fuel cells

Methanol crossover through proton conducting membranes represents one of the main drawbacks in DMFCs. This study presented a novel organic-inorganic hybrid membrane with several different compositions by casting mixtures of zirconium phosphate (ZP), polybenzimidazole (PBI) and Nafion dispersion in dimethylacetamide. The presence of PBI and ZP in the membranes was demonstrated with energy dispersive X-ray (EDX) analysis. From the scanning electron microscopy (SEM) analysis, it was observed that the hybrid Nafion-PBI-ZP membrane had the finest structure. This is because the synthesized films were homogeneous and therefore formed a dense membrane. The water content was higher in the hybrid membrane: 39.91% compared with 35.52% in Nafion117. The water content is important for the ion transportation in the membrane; therefore, a higher water uptake rate will contribute to a better fuel cell performance. It was determined that the proton conductivity of the hybrid membrane was 0.020 S cm⁻¹, which was comparable with Nafion117, which had a proton-conductivity of 0.022 S cm⁻¹. The methanol permeability of the hybrid membrane was 2.34 × 10⁻⁷ cm² s⁻¹, while the value for Nafion117 was 8.91 × 10⁻⁷ cm² s⁻¹. This showed that the methanol permeability of the hybrid membrane was almost 4 times lower than that of Nafion117. The selectivity factor for the Nafion-PBI 1%-ZP 1% membrane was 8.64 × 10⁴ Scm⁻³, while that of Nafion117 was 2.48 × 10⁴ S scm⁻³. From a thermogravimetry analysis (TGA), the addition of PBI and zirconium phosphate was shown to improve the thermal durability in the temperature range from room temperature to 450 °C over that of Nafion117. This study proofed that the Nafion-PBI 1%-ZP 1% performed better than commercial Nafion117 and other type of membranes. The membrane was tested on as single cell of DMFC. It gave the highest power density as compared to other type of membrane and proofed that it has potential to be used in DMFCs.     

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.     

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.     

Carbon dioxide vent for direct methanol fuel cells

Passive, stand-alone, direct methanol fuel cells require a pressure management system that releases CO₂ produced in the anode chamber. However, this must be done without allowing the methanol fuel to escape. In this paper, two siloxane membranes are investigated and shown to selectively vent CO₂ from the anode chamber. The addition of hydrophobic additives, 1,6-divinylperfluorohexane and 1,9-decadiene, improved the selectivity of the siloxane membranes. The best performing CO₂ vent was obtained with 50:50 wt% poly(1-trimethyl silyl propyne) and 1,6-divinylperfluorohexane.     

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.     

Passive direct methanol fuel cells fed with methanol vapor

A vapor fed passive direct methanol fuel cell (DMFC) is proposed to achieve a high energy density by using pure methanol for mobile applications. Vapor is provided from a methanol reservoir to the membrane electrode assembly (MEA) through a vaporizer, barrier and buffer layer. With a composite membrane of lower methanol cross-over and diffusion layers of hydrophilic nanomaterials, the humidity of the MEA was enhanced by water back diffusion from the cathode to the anode through the membrane in these passive DMFCs. The humidity in the MEA due to water back diffusion results in the supply of water for an anodic electrochemical reaction with a low membrane resistance. The vapor fed passive DMFC with humidified MEA maintained 20–25 mW cm⁻² power density for 360 h and performed with a 70% higher fuel efficiency and 1.5 times higher energy density when compared with a liquid fed passive DMFC.     

Two-dimensional two-phase mass transport model for methanol and water crossover in air-breathing direct methanol fuel cells

A two-dimensional two-phase mass transport model has been developed to predict methanol and water crossover in a semi-passive direct methanol fuel cell with an air-breathing cathode. The mass transport in the catalyst layer and the discontinuity in liquid saturation at the interface between the diffusion layer and catalyst layer are particularly considered. The modeling results agree well with the experimental data of a home-assembled cell. Further studies on the typical two-phase flow and mass transport distributions including species, pressure and liquid saturation in the membrane electrode assembly are investigated. Finally, the methanol crossover flux, the net water transport coefficient, the water crossover flux, and the total water flux at the cathode as well as their contributors are predicted with the present model. The numerical results indicate that diffusion predominates the methanol crossover at low current densities, while electro-osmosis is the dominator at high current densities. The total water flux at the cathode is originated primarily from the water generated by the oxidation reaction of the permeated methanol at low current densities, while the water crossover flux is the main source of the total water flux at high current densities.     

A Nafion-silica cathode electrolyte for durable elevated-temperature direct methanol fuel cells

A novel catalyst layer assisted by a Nafion-silica electrolyte for elevated-temperature direct methanol fuel cells is fabricated through a self-assembly process. The catalyst layer demonstrates good water retention abilities and structural stability during the fuel cell operation. After a dehydration period of 30 min under 25% relative humidity at 100 °C, the proton conductivity of the novel catalyst layer is maintained at ∼0.014 S cm⁻¹, and the single cell assembled with the novel catalyst layer achieves a maximum power density of 108 mW cm⁻². Moreover, a stability operation test conducted under 20 ppm CO and a current density of 100 mA cm⁻² demonstrates the structural stability and water retention abilities of the catalyst layer. The cell voltage of a fuel cell featuring the novel catalyst layer decreases from 0.45 to 0.38 V at a slight degradation rate of 0.6 mV min⁻¹.