Stable operation of air-blowing direct methanol fuel cells with high performance

A membrane electrode assembly (MEA) that is a combination of a catalyst-coated membrane (CCM) for the anode and a catalyst-coated substrate (CCS) for the cathode is studied under air-blower conditions for direct methanol fuel cells (DMFCs). Compared with MEAs prepared by only the CCS method, the performance of DMFC MEAs employing the combination method is significantly improved by 30% with less methanol crossover. This feature can be attributed to an enhanced electrode|membrane interface in the anode side and significantly higher catalyst efficiency. Furthermore, DMFC MEAs designed by the combination method retain high power density without any degradation, while the CCM-type cell shows a downward tendency in electrochemical performance under air-blower conditions. This may be due to MEAs with CCM have a much more difficult structure of catalytic active sites in the cathode to eliminate the water produced by electrochemical reaction. In addition, DMFCs produced via combination methods exhibit a lower water crossover flux than CCS alternatives, due to the comparatively dense structure of the CCM anode. Hence, DMFCs with a combination MEA structure demonstrate the feasibility of a small fuel cell system employing the low noise of a fan, instead of a noisy and large capacity air pump, for portable electronic devices.     

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.     

The influences of multi-walled carbon nanotube addition to the anode on the performance of direct methanol fuel cells

Different amounts of multi-walled carbon nanotubes (MWCNTs) are added to anode catalyst layer in the membrane electrode assemblies (MEAs) of direct methanol fuel cells (DMFCs). The MEA with 0.5 wt.% carbon nanotubes (CNTs) shows the best performance in DMFC. In the protonic conductivity tests, a 0.5 wt.% amount of MWCNTs results in the highest protonic conductivity. SEM and TEM observations show that a continuous and uniform distribution of Nafion ionomer layer is formed on the MWCNT surface. Therefore, the dispersed MWCNTs in the catalyst layer are considered to be helpful for developing the pathways of protons transport.     

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.     

Comparison of numerical simulation results with experimental current density and methanol-crossover data for direct methanol fuel cells

The simulation results of a one-dimensional (1D) direct methanol fuel cell (DMFC) model are compared with the current density and methanol-crossover data that are experimentally measured under several different cell designs and operating conditions. No fitting parameters are employed for the comparison and model input parameters obtained from the literature are consistently used for all the cases of comparison. The numerical predictions agree well with the experimental data and the 1D DMFC model successfully captures key experimental trends that are observed in the cell current density and methanol-crossover data. This clearly illustrates that the present DMFC model can be applicable for optimizing DMFC component designs and operating conditions. In addition, the model simulations further indicate that the reduction of the methanol concentration in the anode catalyst layer is critical to simultaneously suppress both the electro-osmotic drag (EOD) and the diffusion aspects of methanol crossover.     

Effects of MEA preparation on the performance of a direct methanol fuel cell

The effects of membrane electrode assemblies (MEAs) fabrication methods (spraying and scraping methods) and the hot-pressing pretreatment of anode electrodes on the performance of direct methanol fuel cells (DMFCs) were investigated. The MEA prepared with scraped anode catalyst layer without the hot-pressing pretreatment showed the highest power density of 67 mW cm⁻² at 80 °C and ambient pressure. The scraping method proved to be a little more profitable for improving the cell performance than the spraying method. Atomic force microscopy (AFM) analysis revealed relatively smooth surface of the scraped anode catalyst layer compared with that of sprayed anode catalyst layer. Scanning electron microscopy (SEM) images showed that a suitable number of cracks which were uniformly distributed on the surface of scraped catalyst layer formed a porous structure. It was demonstrated that the surface structure and roughness of the anode catalyst layer had less effect on the performance of the anode electrode in a DMFC. The hot-pressing pretreatment of the anode electrode decreased the performance of the MEA due to the difficulty for electrons and mass transport in the anode electrode, namely the increase of internal cell resistance.     

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

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

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

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

Materials,morphologies and structures of MEAs in DMFCs

Even though direct methanol fuel cells (DMFCs) were developed to support power needs, there are still unresolved drawbacks. The main focus of this research is the membrane electrode assembly (MEA), which is the most important component of the fuel cell. The MEA consists of a combination of the diffusion layer, the electrode layer and a membrane. Several obstacles may interfere with the functioning of the MEA, including its relatively low anode electro-catalytic activity, methanol crossover and mass transport. In this paper, an overview of the layer and engineering challenges associated with the MEA will be presented in detail. This paper will also discuss the materials while focusing on the current problems, structure layers, morphologies and fabrication methods of MEAs in DMFCs.     

A global transient,one-dimensional,two-phase model for direct methanol fuel cells (DMFCs) – Part II: Analysis of the time-dependent thermal behavior of DMFCs

A transient-thermal model based on a lumped system is newly developed and implemented in a one-dimensional (1D), two-phase rigorous direct methanol fuel cell (DMFC) model presented in Part I. In this model, the main focus lies on the investigation of the transient thermal behavior of DMFCs and its influence on methanol crossover, cell performance, and efficiency. 1D simulations are carried out and the time-dependent thermal behaviors of DMFCs are analyzed for various methanol-feed concentrations and external heat-transfer conditions. Predicting the close interactions between the evolution of the transient temperature, methanol crossover, cell voltage, and efficiency during DMFC operations, the simulations of transient behavior indicate that the insufficient cooling of DMFCs finally lead to thermal runaway, particularly under high methanol-feed concentrations. Therefore, it is concluded that an efficient cooling system is greatly needed to safeguard DMFC operations and enhance the performance of DMFCs. The present 1D DMFC model is a useful tool for attaining a better understanding of complicated physical phenomena in DMFCs, which assists in optimizing the operating conditions of such cells and material/design parameters.     

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

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

Effect of 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.     

High performance membrane electrode assemblies by optimization of coating process and catalyst layer structure in direct methanol fuel cells

High performance membrane electrode assemblies (MEAs) for direct methanol fuel cells (DMFCs) are developed by changing the coating process, optimizing the structure of the catalyst layer, adding a pore forming agent to the cathode catalyst layer, and adjusting the hot-pressing conditions, such as pressure and temperature. The effects of these MEA fabrication methods on the DMFC performance are examined using a range of physicochemical and electrochemical analysis tools, such as FE-SEM, electrochemical impedance spectroscopy (EIS), polarization curves, and differential scanning calorimetry (DSC) of the membrane. EIS and polarization curve analysis show that an increase in the thickness and porosity of the cathode catalyst layer plays a key role in improving the cell performance with reduced cathode reaction resistance, whereas the MEA preparation methods have no significant effects on the anode impedance. In addition, the addition of magnesium sulfate as a pore former reduces the cathode reaction transfer resistance by approximately 30 wt%, resulting in improved cell performance.     

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.     

A one-dimensional,two-phase model for direct methanol fuel cells – Part I: Model development and parametric study

A one-dimensional, steady-state, two-phase direct methanol fuel cell (DMFC) model is developed to precisely investigate complex physiochemical phenomena inside DMFCs. In this model, two-phase species transport through the porous components of a DMFC is formulated based on Maxwell–Stefan multi-component diffusion equations, while capillary-induced liquid flow in the porous media is described by Darcy's equation. In addition, the model fully accounts for water and methanol crossover through the membrane, which is driven by the effects of electro-osmotic drag, diffusion, and the hydraulic pressure gradient. The developed model is validated against readily available experimental data in the literature. Then, a parametric study is carried out to investigate the effects of the operating temperature, methanol feed concentration, and properties of the backing layer. The results of the numerical simulation clarify the detailed influence of these key designs and operating parameters on the methanol crossover rate as well as cell performance and efficiency. The results emphasize that the material properties and design of the anode backing layer play a critical role in the use of highly concentrated methanol fuel in DMFCs. The present study forms a theoretical background for optimizing the DMFC's components and operating conditions.     

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.     

Non-isothermal dynamic modelling and optimization of a direct methanol fuel cell

A non-isothermal dynamic optimization model of direct methanol fuel cells (DMFCs) is developed to predict their performance with an effective optimum-operating strategy. After investigating the sensitivities of the transient behaviour (the outlet temperature, crossovers of methanol and water, and cell voltage) to operating conditions (the inlet flow rates into anode and cathode compartments, and feed concentration) through dynamic simulations, we find that anode feed concentration has a significantly larger impact on methanol crossover, temperature, and cell voltage than the anode and cathode flow rates. Also, optimum transient conditions to satisfy the desired fuel efficiency are obtained by dynamic optimization. In the developed model, the significant influence of temperature on DMFC behaviour is described in detail with successful estimation of its model parameters.     

Flexible graphite-based integrated anode plate for direct methanol fuel cells at high methanol feed concentration

An integrated anode plate suitable for operating direct methanol fuel cells (DMFCs) at a high methanol feed concentration is reported. This anode structure which was made of flexible graphite materials not only provides dual role of liquid diffusion layer and flow field plate, but also serves as a methanol blocker by decreasing methanol flux to the interface of catalyst and membrane electrolyte. DMFCs incorporating this new anode structure exhibited a much higher open circuit voltage (OCV) (∼0.51 V) than that (∼0.42 V) of a conventional DMFC at a 10 M methanol feed. Cell polarization data show that this new anode structure significantly improves the cell performance at high methanol concentration scenarios (e.g. 12 M or above). Moreover, this new design greatly simplifies the anode structure and offers a promising approach in running passive-mode DMFC at high methanol feed concentrations.     

Applications of graphene nano-sheets as anode diffusion layers in passive direct methanol fuel cells (DMFC)

The diffusion layer is an important structure in the membrane electrode assembly (MEA) of direct methanol fuel cells (DMFCs) that provide a support layer for catalysts, electronic channels, and gas–liquid mass transport channels. In this study, three types of carbon-based materials were used to fabricate anode diffusion layers – carbon black Vulcan^® (CBV), M-15 grade graphene nanosheets (GM-15) and C-500 grade graphene nanosheets (GC-500). The microporous layers of cathodes were constructed with CBV. A carbon-based microporous layer with a 2 mg cm^?2 loading was coated onto a PTFE-pretreated carbon cloth, while a Nafion-117 membrane was applied as the electrolyte to the DMFCs. Pt–Ru black and Pt black were used as anode and cathode electrode catalysts, each with loadings of 8 mg cm^?2 and 4 mg cm^?2, respectively. All tests were conducted using MEAs with active areas of 4 cm² and air was supplied to single cells by passive modes. Surface morphology was studied using scanning electron microscopy (SEM), which produced pictures of complex network formations within the structures. CBV consists of nanosized carbon particles, while both GM-15 and GC-500 are made of stacks of graphene sheets with flaky structures that increase catalyst utilization. Performance tests of the DMFCs were conducted using a potentiostat that generated polarization curves. The highest peak power density of 13.7 mW cm^?2 was obtained by the GC-500 anode diffusion layer using 3 M methanol as fuel. The energy efficiency of the passive DMFCs was approximately 10% with a specific energy of approximately 610 Wh kg^?1, which is higher than that of conventional lithium-ion batteries, portraying the bright future of alternative energy sources for use in power applications for portable devices. The high power densities obtained by both graphene-based materials, GM-15 and GC-500, demonstrate that graphene is a material other than state of the art carbon black that has the potential to be used as a DMFC anode support material.     

Comparative study of three different catalyst coating methods for direct methanol fuel cells

The performance of direct methanol fuel cells (DMFCs) with membrane–electrode assemblies (MEAs) made separately by three different catalyst coating methods, namely, air-spray, electro-spray and dual-mode spray, is evaluated. Platinum–ruthenium (PtRu) is incorporated as a catalyst for the anode. Several techniques (XRD, FE-SEM, and TEM) are used to examine whether the coating method affects the morphological features of the PtRu catalyst, whereas cyclic voltammetry is used to evaluate the active surface area. The cell polarization curves attained for the three coating methods that use different methanol concentrations are compared to determine the best method. It is found that the PtRu catalyst coated by the dual-mode spray shows the most uniform nanoparticle distribution and the highest active surface area. The DMFC performance is best when the dual-mode spray is employed (165 mW cm⁻² at 2 M methanol).