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.     

A three-dimensional mathematical model for liquid-fed direct methanol fuel cells

A three-dimensional, single-phase, multi-component mathematical model has been developed for a liquid-fed direct methanol fuel cell (DMFC). The traditional continuity, momentum, and species conservation equations are coupled with electrochemical kinetics in both the anode and cathode catalyst layer. At the anode side, the liquid phase is considered, and at the cathode side only the gas phase is considered. Methanol crossover due to both diffusion and electro-osmotic drag from the anode to the cathode is taken into consideration and the effect is incorporated into the model using a mixed-potential at the cathode. A finite-volume-based CFD technique is used to develop the in-house numerical code and the code is successfully used to simulate the fuel cell performance as well as the multi-component behavior in a DMFC. The modeling results of polarization curves compare well with our experimental data. Subsequently, the model is used to study the effects of methanol crossover, the effects of porosities of the diffusion layer and the catalyst layer, the effects of methanol flow rates, and the effects of the channel shoulder widths.     

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.     

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.     

Experimental validation of a methanol crossover model in DMFC applications

A design of experiments (DOEs) coupled with a mathematical model was used to quantify the factors affecting methanol crossover in a direct methanol fuel cell (DMFC). The design of experiments examined the effects of temperature, cathode stoichiometry, anode methanol flow rate, clamping force, anode catalyst loading, cathode catalyst loading (CCL), and membrane thickness as a function of current and it also considered the interaction between any two of these factors. The analysis showed that significant factors affecting methanol crossover were temperature, anode catalyst layer thickness, and methanol concentration. The analysis also showed how these variables influence the total methanol crossover in different ways due to the effects on diffusion of methanol through the membrane, electroosmotic drag, and reaction rate of methanol at the anode and cathode. For example, as expected analysis showed that diffusion was significantly affected by the anode and cathode interfacial concentration, by the thickness of the anode catalyst layer and membrane, and by the diffusion coefficient in the membrane. Less obvious was the decrease in methanol crossover at low cathode flow rates were due to the formation of a methanol film at the membrane/cathode catalyst layer interface. The relative proportions of diffusion and electroosmotic drag in the membrane changed significantly with the cell current of the cell.     

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.     

Water management in direct methanol fuel cells

Water management is an important challenge in portable direct methanol fuel cells. Reducing the water and methanol loss from the anode to the cathode enables the use of highly concentrated methanol solutions to achieve enhanced performances. In this work, the results of a simulation study using a previous developed model for DMFCs are presented. Particular attention is devoted to the water distribution across the cell. The influence of different parameters (such as the cathode relative humidity (RH), the methanol concentration and the membrane, catalyst layer and diffusion media thicknesses) over the water transport and on the cell performance is studied. The analytical solutions of the net water transport coefficient, for different values of the cathode relative humidity are successfully compared with recent published experimental data putting in evidence that humidified cathodes contribute to a decrease on the water crossover. As a result of the modelling results, a tailored MEA build-up with the common available commercial materials is proposed to achieve low methanol and water crossover and high power density, operating at relatively high methanol concentrations. A thick anode catalyst layer to promote methanol oxidation, a thin anode gas diffusion layer as methanol carrier to the catalyst layer and a thin polymer membrane to lower the water crossover coefficient between the anode and cathode are suggested.     

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.     

Influence of anode diffusion layer on the performance of a liquid feed direct methanol fuel cell by AC impedance spectroscopy

Effect of anode diffusion layer over the performance of the liquid feed direct methanol fuel cell has been studied by AC impedance spectroscopy. The anode employed comprises of the catalyst layer and diffusion layer. The latter comprises of backing layer and catalyst‐supporting layer. The supporting layer is present in between the backing layer and the catalyst layer. The composition of the supporting layer is optimized based on the information obtained from polarization and AC impedance measurements. Among the three types of carbons (Black pearl 2000, Vulcan XC‐72, Shawinigan acetylene black), Black pearls 2000 is found to be the ideal type of carbon used in the supporting layer. The optimized loading compositions of carbon, Nafion and PTFE in the supporting layer are reported to be 3 mg cm^?2, 10 wt%, and 0 wt%, respectively. These values are rationalized on the basis of the transport of methanol and carbon dioxide and the crossover of methanol from the anode to the cathode. Copyright © 2006 John Wiley & Sons, Ltd.     

A transient, multi-phase and multi-component model of a new passive DMFC

A 2-dimensional, transient multi-phase, multi-component fuel cell model is developed to model a passive fuel delivery system including the fuel cell itself for a direct methanol fuel cell (DMFC). This model captures evaporative effects, as water and fuel management are crucial issues. The evaporation/condensation rates are formulated in a manner to capture non-equilibrium effects between the phases. Also, the full kinetics are modeled at both the anode and cathode catalyst layers, along with the electric potential of the membrane, catalyst and gas diffusion layers. The fuel cell operation is examined by quantifying the fuel consumption due to chemical reaction and evaporation as a function of feed concentration. The passive delivery system utilizes a porous media to passively deliver methanol to the fuel cell while controlling the concentration of methanol at the anode side to limit the amount of methanol cross-over. The results illustrate the feasibility of the passive thermal-management system, and characterize the relevant transport phenomena.     

Influence of anion ionomer content and silver cathode catalyst on the performance of alkaline membrane electrode assemblies (MEAs) for direct methanol fuel cells (DMFCs)

Alkaline membrane electrode assemblies (MEAs) were fabricated by a dry spraying method in order to evaluate and improve their performance. I–V tests indicated that the performance of alkaline direct methanol fuel cells (DMFCs) deeply depends on the ionomer contents of MEAs. MEA with 45.4% mass ionomer content showed the highest performance when non-alkaline (MeOH (1 M)) and alkaline (MeOH (1 M), NaOH (0.5 M)) fuels were used. When alkaline fuel was used, the anode and cathode performances of MEAs were also measured. The ionomer content has been shown to contribute ohmic polarization of the anode and diffusion polarization of the cathode. Furthermore, the performance of MEA with an Ag cathode catalyst was characterized. The Ag cathode catalyst was demonstrated to be a promising alternative to a Pt cathode catalyst because of its tolerance for methanol crossover.     

Modeling of water transport through the membrane electrode assembly for direct methanol fuel cells

In this work, a one-dimensional, isothermal two-phase mass transport model is developed to investigate the water transport through the membrane electrode assembly (MEA) for liquid-feed direct methanol fuel cells (DMFCs). The liquid (methanol–water solution) and gas (carbon dioxide gas, methanol vapor and water vapor) two-phase mass transport in the porous anode and cathode is formulated based on classical multiphase flow theory in porous media. In the anode and cathode catalyst layers, the simultaneous three-phase (liquid and vapor in pores as well as dissolved phase in the electrolyte) water transport is considered and the phase exchange of water is modeled with finite-rate interfacial exchanges between different phases. This model enables quantification of the water flux corresponding to each of the three water transport mechanisms through the membrane for DMFCs, such as diffusion, electro-osmotic drag, and convection. Hence, with this model, the effects of MEA design parameters on water crossover and cell performance under various operating conditions can be numerically investigated.     

Investigations of direct methanol fuel cell (DMFC) fading mechanisms

In this report, we present the microscopic investigations on various fading mechanisms of a direct methanol fuel cell (DMFC). High energy X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), energy dispersive X-ray spectroscopy (EDX), and Raman spectroscopic analysis were applied to a membrane-electrode-assembly (MEA) before and after fuel cell operation to figure out the various factors causing its fading. High energy XRD analysis of the fresh and faded MEA revealed that the agglomeration of the catalyst particles in the cathode layer of the faded MEA was more significant than in the anode layer of the faded MEA. The XAS analysis demonstrated that the alloying extent of Pt (J_Pt) and Ru (J_Ru) in the anode catalyst was increased and decreased, respectively, from the fresh to the faded MEA, indicating that the Ru environment in the anode catalyst was significantly changed after the fuel cell operation. Based on the X-ray absorption edge jump measurements at the Ru K-edge on the anode catalyst of the fresh and the faded MEA it was found that Ru was dissolved from the Pt-Ru catalyst after the fuel cell operation. Both the Ru K-edge XAS and EDX analysis on the cathode catalyst layer of the faded MEA confirms the presence of Ru environment in the cathode catalyst due to the Ru crossover from the anode to the cathode side. The changes in the membrane and the gas diffusion layer (GDL) after the fuel cell operation were observed from the Raman spectroscopy analysis.     

直接液体甲醇燃料电池流场组织行为研究(Ⅰ) 传统对称流道电池的模型建立

建立了直接甲醇燃料电池垂直流道方向电池单元的二维稳态数学模型,考虑了电化学动力学、多组分传递和甲醇渗透影响.计算了流道布置密度、扩散层、催化层和质子交换膜等组件尺度对电池内物料传质特性、化学反应组织和电池输出性能的影响.研究发现,增加流道布置密度、增加催化层厚度能有效提高电极反应均匀性和电池性能.其中催化层和质子膜的厚度影响最为显著,在该文研究范围内分别可提高电池的平均电流密度131.0%和17.8%.而扩散层和质子交换膜厚度都存在一个最佳值,需要与以上流场板设计尺寸和膜电极尺寸匹配.     

Effect of anode MPL on water and methanol transport in DMFC: Experimental and modeling analyses

The regulation of mass transport through anode diffusion layer is one of the major issue of direct methanol fuel cell. In fact it is critical to maintain an adequate methanol concentration in the anode electrode such that both the rate of methanol crossover and the mass transport loss can be minimized. In the present work the effect of anode micro-porous layer on system operation is investigated both experimentally and theoretically. The developed 2D two-phase isothermal model is validated with respect to three different typologies of measure at the same time, increasing results reliability. Model simulations highlight that anode micro-porous layer can cause an inversion of water diffusion flux through the membrane and enhances methanol gas diffusion mechanism, reducing methanol crossover. Finally the developed model is used as a tool to design an optimized anode diffusion layer.     

直接液体甲醇燃料电池的数学模型

描述了一个用于模拟直接甲醇燃料电池特性的垂直于流道的二维数学模型。模型同时考虑了电化学动力学、水动力学和多组分传递。计算了电地内反应物浓度的分布、电流密度分布、甲醇窜流以及电压—电流特性曲线等。结果表明：集流板前的催化层内反应物浓度非常低；流道边缘附近电流密度比平均电流密度大许多倍。图5表l参9     

Direct ethanol fuel cell anode simulation model

The electrochemical behavior of a direct ethanol feed proton exchange membrane fuel cell (DEFC) operating under steady-state isothermal conditions at 1 atm at both anode and cathode sides is considered. A mathematical model that describes in one phase and one dimension the ethanol mass transport throughout the anode compartment and proton exchange membrane is developed. The influence of the operation parameters such as current density, temperature, catalyst layer thickness and ethanol feed concentration on both anode overpotential and ethanol crossover rate has been examined. According to the simulation results, it was found that the anode overpotential is more sensitive to the protonic conductivity than to the diffusion coefficient of ethanol in the catalyst layer. It was concluded that in the case of low current density values and high concentrations of ethanol aqueous solutions, ethanol crossover is a serious problem for a DEFC performance. Finally, it was found a good agreement between simulation and experimental results.     

Cell performance of Pd–Sn catalyst in passive direct methanol alkaline fuel cell using anion exchange membrane

Direct methanol alkaline fuel cell (DMAFC) using anion exchange membrane (AEM) was operated in passive condition. Cell with AEM exhibits a higher open circuit voltage (OCV) and superior cell performance than those in cell using Nafion. From the concentration dependences of methanol, KOH in fuel and ionomer in anode catalyst layer, it is found that the key factors are to improve the ionic conductivity at the anode and to form a favorable ion conductive path in catalyst layer in order to enhance the cell performance. In addition, by using home-made Pd–Sn/C catalyst as a cathode catalyst on DMAFC, the membrane electrode assembly (MEA) using Pd–Sn/C catalyst as cathode exhibits the higher performance than the usual commercially available Pt/C catalyst in high methanol concentration. Therefore, the Pd–Sn/C catalyst with high tolerance for methanol is expected as the promising oxygen reduction reaction (ORR) catalyst in DMAFC.     

Three-dimensional transport model of PEM fuel cell with straight flow channels

In this work, an isothermal, steady-state, three-dimensional (3D) multicomponent transport model is developed for proton exchange membrane (PEM) fuel cell with straight gas channels. The model computational domain, includes anode flow channel, membrane electrode assembly (MEA) and cathode flow channel. The catalyst layer within the domain has physical volume without simplification. A comprehensive set of 3D continuity equation, momentum equations and species conservation equations are formulated to describe the flow and species transport of the gas mixture in the coupled gas channels and the electrodes. The electrochemical reaction rate is modified by an agglomerate model to account for the effect of diffusion resistance through catalyst particle. The activation overpotential is predicted locally in the catalyst layer by separately solving electric potential equations of membrane phase and solid phase. The model is validated by comparison of the model prediction with experimental data of Ticianelli et al. The results indicate the detailed distribution characteristics of oxygen concentration, local current density and cathode activation overpotential at different current densities. The distribution patterns are relatively uniform at low average current density and are severely non-uniform at higher current density due to the mass transfer limitation. The local effectiveness factor in the catalyst layer can be obtained with this model, so the mass transport limitation is displayed from another point of view.