A theoretical model of the membrane electrode assembly of liquid feed direct methanol fuel cell with consideration of water and methanol crossover

An algebraic model of the membrane electrode assembly of the direct methanol fuel cell is developed, which considers the simultaneous liquid water and methanol crossover effects, and the associated electrochemical reactions. The respective anodic and cathodic polarization curves can be predicted using this model. Methanol concentration profile and flux are correlated explicitly with the operating conditions and water transport rate. The cathode mixed potential effect induced by the methanol crossover is included and the subsequent cell voltage loss is identified. Water crossover is influenced by the capillary pressure equilibrium and hydrophobic property within the cathode gas diffusion layer. The model can be used to evaluate the cell performance at various working parameters such as membrane thickness, methanol feed concentration, and hydrophobicity of the cathode gas diffuser.     

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.     

Modeling of dynamic operating behaviors in a liquid-feed direct methanol fuel cell

A transient, two-dimensional two-phase mass transport model is applied to investigate the cell dynamic operating behaviors of a liquid-feed direct methanol fuel cell (DMFC). The influences of various processes on the cell dynamics in response to sudden change of cell current density, methanol feed concentration, oxygen feed concentration, and the transient gas-slug blocking in the anode channel are studied. The results reveal that in response to the sudden drop of cell current density and methanol concentration, the cell voltage exhibits overshooting behavior as a result of the interaction between cathode and anode overpotentials with different time responses. The dominant factor causing the long response of cell voltages is the methanol rebalance in the membrane electrode assembly, which usually takes tens of seconds because of the sluggish methanol transport process. Also, it is indicated that in response to temporary blocking of anode diffusion layer surface with gas slug, the cell can still operate normally for a while because the anode diffusion layer serves as the fuel reservoir. It takes over a minute for the cell to break down in this case studied, implying that the cell output can be maintained stable if the gas bubbles or slugs in the anode channel can be removed quickly. However, too long residence time of gas slug in the channel definitely degrades the cell performance.     

Towards operating direct methanol fuel cells with highly concentrated fuel

A significant advantage of direct methanol fuel cells (DMFCs) is the high specific energy of the liquid fuel, making it particularly suitable for portable and mobile applications. Nevertheless, conventional DMFCs have to be operated with excessively diluted methanol solutions to limit methanol crossover and the detrimental consequences. Operation with diluted methanol solutions significantly reduces the specific energy of the power pack and thereby prevents it from competing with advanced batteries. In view of this fact, there exists a need to improve conventional DMFC system designs, including membrane electrode assemblies and the subsystems for supplying/removing reactants/products, so that both the cell performance and the specific energy can be simultaneously maximized. This article provides a comprehensive review of past efforts on the optimization of DMFC systems that operate with concentrated methanol. Based on the discussion of the key issues associated with transport of the reactants/products, the strategies to manage the supply/removal of the reactants/products in DMFC operating with highly concentrated methanol are identified. With these strategies, the possible approaches to achieving the goal of concentrated fuel operation are then proposed. Past efforts in the management of the reactants/products for implementing each of the approaches are also summarized and reviewed.     

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.     

Anode optimization based on gradient porous control medium for passive liquid-feed direct methanol fuel cells

The direct methanol fuel cell (DMFC) is a potential candidate to be used as a portable power source which still faces great challenges in structure optimization because of complex interactions and even conflicts between the reactant and product managements. This work presents an effective method for the anode optimization by using a gradient porous medium to realize more active control of the anode mass transfer mechanisms of a passive liquid-feed DMFC. This functional medium is made of a self-developed metal fiber sintered felt based on multi-tooth cutting and high-temperature sintering. Its structural features and processing parameters can be adaptively controlled according to the application requirement. Results indicate that the porosity, assembly pattern and thickness of this gradient porous medium have great effects on the cell performance. The DMFC is insensitive to the change of sintering process. The use of a gradient porosity promotes a higher cell performance than the uniform structure, especially when a lower porosity is used inward. How the methanol concentration affects the cell performance is also discussed in this study.     

Simplified model for the direct methanol fuel cell anode

A kinetic model for the anode of the direct methanol fuel cell (DMFC) is presented. The model is based on the generally accepted dual site mechanism of methanol oxidation, in aqueous solution, on well characterized Pt–Ru catalyst and it can predict the performance of the electrode as a function of cell temperature, anode potential and methanol concentration. In addition the model also generates data regarding the surface coverage of significant adsorbates involved in methanol oxidation on the dual site catalyst.     

Two-phase,mass-transport model for direct methanol fuel cells with effect of non-equilibrium evaporation and condensation

A two-phase, mass-transport model for liquid-feed direct methanol fuel cells (DMFCs) is developed by taking into account the effect of non-equilibrium evaporation and condensation of methanol and water. The comparison between the present model and other models indicates that the present model yields more reasonable predictions of cell performance. Particularly, it is shown that the models that invoke a thermodynamic-equilibrium assumption between phases will overestimate mass-transport rates of methanol and water, thereby resulting in an inaccurate prediction of cell performance. The parametric study using the present model reveals that the gas coverage at the flow channel–diffusion-layer interface is directly related to the gas-void fraction inside the anode porous region; increasing the gas-void fraction will increase the mass-transfer resistance of methanol and thus lower cell performance. The effects of the geometric dimensions of the cell structure, such as channel width and rib width, on cell performance are also investigated with the model developed in this work.     

Multifunctional composite membrane based on a highly porous polyimide matrix for direct methanol fuel cells

A highly porous polyimide film with tunable pore size, porosity and thickness is synthesized and used as a matrix to construct a Nafion-infiltrated composite membrane. A very efficient way for an easy and complete infiltration of the proton-conducting polymer into this substrate is developed, which is usually a major problem for composite membranes. Due to the complete inertness to methanol and the very high mechanical strength of the polyimide matrix, the swelling of the composite membrane is greatly suppressed and the methanol crossover is also significantly reduced (80 times), where as while high proton conductivity (comparable with Nafion) and mechanical strength (4 times stronger than Nafion) is still maintained. This membrane demonstrates significantly improved cell performance compared with the Nafion membrane and is a promising candidate for use in direct methanol fuel cells.     

Design and simulation of a liquid electrolyte passive direct methanol fuel cell with low methanol crossover

This study investigates an aqueous solution of sulfuric acid that serves as the liquid electrolyte (LE) in a passive direct methanol fuel cell (DMFC). The addition of an LE can reduce methanol crossover and increase the fuel utilization significantly. To improve the performance of an LE-DMFC, a mathematical model is developed to optimize the thicknesses of both the LE layer and the Nafion membrane. The maximum power density of the LE-DMFC is improved by approximately 30% compared with a conventional DMFC (C-DMFC) when each is fed by methanol solutions of the same concentration. Due to the low methanol crossover of the LE-DMFC, a highly concentrated methanol solution can be directly fed into the LE-DMFC. The discharge time and volume energy density of the LE-DMFC are two times longer and three times greater than those of the C-DMFC, respectively. In addition, fuel utilization increases by approximately 100%.     

Exergy flow and energy utilization of direct methanol fuel cells based on a mathematic model

An exergetic analysis model for direct methanol fuel cell (DMFC) is established in the present paper. Expressions of electrical, thermal and total exergetic efficiencies have been deduced with consideration of methanol crossover and over potential in operation. Furthermore, energy utilization of a DMFC system is quantitatively calculated and changes of electrical efficiency and thermal efficiency at various current density, methanol concentration, operating temperature, and cathode pressure have been investigated. Some suggestions of optimal operating conditions of direct methanol fuel cell based on our findings are put forward. Results show that the thermal energy generated in a DMFC takes up a significant amount of exergy in total energy and should be sufficiently used to obtain high total efficiency in a DMFC, high methanol crossover rate is the predominant cause of energy loss when the fuel cell operates at low current density, and total exergetic efficiency of a DMFC reaches its peak value at relatively high current density.     

Nafion-impregnated electrospun polyvinylidene fluoride composite membranes for direct methanol fuel cells

Composite membranes consisting of polyvinylidene fluoride (PVdF) and Nafion have been prepared by impregnating various amounts of Nafion (0.3–0.5 g) into the pores of electrospun PVdF (5 cm × 5 cm) and characterized by scanning electron microscopy, differential scanning calorimetry, X-ray diffraction, and proton conductivity measurements. The characterization data suggest that the unique three-dimensional network structure of the electrospun PVdF membrane with fully interconnected fibers is maintained in the composite membranes, offering adequate mechanical properties. Although the composite membranes exhibit lower proton conductivity than Nafion 115, the composite membrane with 0.4 g Nafion exhibits better performance than Nafion 115 in direct methanol fuel cell (DMFC) due to smaller thickness and suppressed methanol crossover from the anode to the cathode through the membrane. With the composite membranes, the cell performance increases on going from 0.3 to 0.4 g Nafion and then decreases on going to 0.5 g Nafion due to the changes in proton conductivity.     

Molecular sieve as an effective barrier for methanol crossover in direct methanol fuel cells

Methanol crossover is still a significant barrier to the commercialization of direct methanol fuel cells with wide-used Nafion® membrane. Herein, molecular sieve is introduced into the design of polymer electrolyte membrane to alleviate methanol crossover. The UZM-9 zeolite with an intermediate window size of 0.42 nm can effectively separate hydrated methanol (ca. 1.10 nm) and hydrated proton (ca. 0.23 nm). The methanol diffusion rate through the membrane is effectively suppressed after modified with UZM-9, which is about four times lower than the origin Nafion® membrane. The resulted peak power density reached 80 mW cm⁻² with 2 mol L⁻¹ methanol solution feed, which is 2.5-fold higher than that of direct methanol fuel cell with commercial Nafion® membrane. These results open a promising route to alleviate methanol crossover in direct methanol fuel cells.     

Modelling and analysis of a direct methanol fuel cell with under-rib mass transport and two-phase flow at the anode

For the past decade, extensive mathematical modelling has been conducted on the design and optimization of liquid-feed direct methanol fuel cells (DMFCs). Detailed modelling of DMFC operations reveals that a two-phase flow phenomenon at the anode and under-rib convection due to the pressure difference between the adjacent channels both contribute significantly to mass-transfer in a DMFC and its output performance. In practice, comprehensive simulations based on the finite volume technique for two-phase flow require a high level of numerical complexity in computation. This study presents a complexity-reduced mathematical model that is developed to cover both phenomena for a realistic, but fast, in computation for the prediction and analysis of a DMFC prototype design. The simulation results are validated against experimental data with good agreement. Analysis of the DMFC mass-transfer is made to investigate methanol distribution at anode and its crossover through the proton-exchange membrane. From a comparison of the influence of two-phase flow and under-rib mass-transfer on DMFC performance, the significance of gas-phase methanol transport is established. Simulation results suggest that both the optimization of the flow-field structure and the fuel cell operating parameters (flow rate, methanol concentration and operating temperature) are important factors for competitive DMFC performance output.     

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

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

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.     

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.     

Noble metal nanowires incorporated Nafion membranes for reduction of methanol crossover in direct methanol fuel cells

We electrodeposited noble metal (palladium, platinum) nanowires into the hydrophilic pores of Nafion membrane for mitigating the problem of methanol crossover in direct methanol fuel cells (DMFCs). The DMFC performance result shows that the composite membranes yield lower rate of methanol crossover and better cell performance than the pure Nafion^® membrane. At low current densities, the Pd nanowire incorporated Nafion membrane shows the best performance. In comparison, the highest performance is achieved at higher current densities with the Pt nanowire modified Nafion membrane. Based on the above findings, we suggest that for the Pd nanowire incorporated Nafion membrane, the mechanism for the suppression of the methanol crossover is mainly the blocking effect due to the ‘narrowed’ hydrophilic channels in Nafion membrane. For the Pt nanowire modified Nafion membrane, the mechanism includes both increasing the membrane tortuosity and so-called ‘on-way consumption’ of methanol on the Pt nanowires deposited into the Nafion membrane when the fuel cell is discharging.     

Application of silica as a catalyst support at high concentrations of methanol for direct methanol fuel cells

Various silica particles were adopted as catalyst supports, and silica-supported PtRu catalysts were evaluated as catalysts for the anode of direct methanol fuel cells at methanol concentrations of 1–10 M through single cell tests. Compared to a carbon black supported Pt–Ru catalysts, the silica-supported Pt–Ru catalysts exhibited higher performance in MEA, especially with high concentration over 3 M, and the maximum power density reached to 90 mW cm⁻² and 60 mW cm⁻² with 5 M and 10 M, respectively, which were 1.5 and 3 times higher than the reference carbon black supported catalysts. It was found that the silica particles as a catalyst support have a significant effect on reduction of methanol crossover and control of fuel feeding. Such a high performance in the operation with high concentrations was confirmed in the long-term durability test.     

Sulfonated graphene oxide/Nafion composite membranes for high-performance direct methanol fuel cells

An easy and effective method for producing low methanol-crossover membranes is developed by dispersing sulfonated graphene oxide (SGO) into a Nafion matrix. A SGO/Nafion mixture with low SGO content exhibits unique viscosity behavior and allows for better SGO dispersion within the Nafion. After film casting, the composite membranes show lower methanol and water uptakes, a reduced swelling ratio, improved proton conductivity in low relative humidity, and extremely high methanol selectivity, which can be implemented in direct methanol fuel cells (DMFCs). The regular backbone of the composite membrane shows a higher storage modulus, increased α-relaxation (transition temperature), and improved tolerance to pressure during membrane electrode assembly (MEA). The small angle X-ray spectra indicate the shrinkage of the ionic clusters in the composite membranes, which thus reduce methanol crossover. The hybrid membranes applied to DMFCs demonstrate performances superior to that of the commercial Nafion 115 in 1 M and 5 M methanol solutions.