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.     

Performance improvement in direct methanol fuel cells using a highly porous corrosion-resisting stainless steel flow field

Power generation with direct methanol fuel cell (DMFC) systems requires only simple equipment, and has the important advantage of using a liquid fuel with higher energy density and easier handling characteristics than hydrogen. However, the power output of DMFC is lower than hydrogen fuel cells. To improve the power output of DMFC it is very important to reduce diffusion polarization at higher current density conditions. This research used a corrosion-resisting type porous stainless steel developed based on the technology for metal–hydride battery electrodes in the separator flow fields for reactants and products in a single cell DMFC and analyzed its influence on performance characteristics.     

Effect of water concentration in the anode catalyst layer on the performance of direct methanol fuel cells operating with neat methanol

This paper reports on a chromatography-based method for determining the water concentration in the anode catalyst layer (CL) of a direct methanol fuel cell (DMFC). By this method, the effect of the water concentration in the anode CL on the product distribution of the methanol oxidation reaction (MOR), the anode potential, and the cell internal resistance is experimentally investigated in a DMFC operating with neat methanol. Interestingly, it is found that the main product of the anode MOR is still carbon dioxide even when the water concentration in the anode CL is extremely low. The experimental data also show that an increase in the water concentration in the anode CL decreases the internal resistance, the production of by-products (methyl formate and methylal), and the anode potential. As the mole ratio of water to methanol increases beyond a critical value, however, both the internal resistance and the anode potential tend to be stabilized at the points under diluted methanol operating conditions.     

A novel design for a flow field configuration, of a direct methanol fuel cell

We proposed and tested a new and novel arrangement for a direct methanol fuel cell consisting of one inlet for a methanol solution and four outlets for oxidant gas (air), in both the anode and cathode flow fields. It utilizes different operating temperatures of 40 °C and 60 °C, and different methanol solution flow rates of 5 ml min⁻¹, 10 ml min⁻¹, and 20 ml min⁻¹. Test results indicate a significant reduction in produced CO₂ gas in the anode flow channels and product water in the cathode flow channels; consequently, cell performance can be greatly improved. Furthermore, methanol crossover can also be avoided and reduced.     

Characteristics of water transport through the membrane in direct methanol fuel cells operating with neat methanol

The water required for the methanol oxidation reaction in a direct methanol fuel cell (DMFC) operating with neat methanol can be supplied by diffusion from the cathode to the anode through the membrane. In this work, we present a method that allows the water transport rate through the membrane to be in-situ determined. With this method, the effects of the design parameters of the membrane electrode assembly (MEA) and operating conditions on the water transport through the membrane are investigated. The experimental data show that the water flux by diffusion from the cathode to the anode is higher than the opposite flow flux of water due to electro-osmotic drag (EOD) at a given current density, resulting in a net water transport from the cathode to the anode. The results also show that thinning the anode gas diffusion layer (GDL) and the membrane as well as thickening the cathode GDL can enhance the water transport flux from the cathode to the anode. However, a too thin anode GDL or a too thick cathode GDL will lower the cell performance due to the increases in the water concentration loss at the anode catalyst layer (CL) and the oxygen concentration loss at the cathode CL, respectively.     

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

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

An algorithm for sensor-less fuel control of direct methanol fuel cells

We report an algorithm for real-time control of the fuel of a DMFC. The MEA voltage decay coefficients [e₁, e₂], and I-V-T, M′-I-T, and W′-I-T curves (where I is the current, V the voltage, T the temperature, and M′ and W′ the methanol and water consumption rates, respectively) of n fuels with specified methanol concentrations C_M,k (k = 1, 2,…, n) are pre-established and form (I,V,T), (M′,I,T), and (W′,I,T) surfaces for each C_M,k. The in situ measured (I,V,T)_u after voltage decay correction is applied to the n preset (I,V,T) surfaces to estimate C_M,u (the C_M corresponding to (I,V,T)_u) using an interpolation procedure. The C_M,u is then applied to the n preset (M′,I,T) and (W′,I,T) surfaces to estimate cumulated “methanol” and “water” consumed quantities . Thus in a real-time system, the C_M and total quantity of fuel can be controlled using the estimated C_M,u and cumulated “methanol” and “water” consumed quantities.     

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.     

Enhancement of water retention in the membrane electrode assembly for direct methanol fuel cells operating with neat methanol

It is desirable to operate a direct methanol fuel cell (DMFC) with neat methanol to maximize the specific energy of the DMFC system, and hence increasing its runtime. A way to achieve the neat-methanol operation is to passively transport the water produced at the cathode through the membrane to the anode to facilitate the methanol oxidation reaction (MOR). To achieve a performance of the MOR similar to that under the conventional diluted methanol operation, both the water transport rate and the local water concentration in the anode catalyst layer (CL) are required to be sufficiently high. In this work, a thin layer consisting of nanosized SiO₂ particles and Nafion ionomer (referred to as a water retention layer hereafter) is coated onto each side of the membrane. Taking advantage of the hygroscopic nature of SiO₂, the cathode water retention layer can help maintain the water produced from the cathode at a higher concentration level to enhance the water transport to the anode, while the anode retention layer can retain the water that is transported from the cathode. As a result, a higher water transport rate and a higher water concentration at the anode CL can be achieved. The formed membrane electrode assembly (MEA) with the added water retention layers is tested in a passive DMFC and the results show that this MEA design yields a much higher power density than the MEA without water retention layers does.     

Metal foams as flow field and gas diffusion layer in direct methanol fuel cells

Metal foams are routinely used in structures to enhance stiffness and reduce weight over a range of platforms. In direct methanol fuel cells, the controlled porosity and high electrical conductivity of metal foams provide additional benefits. Performance studies were conducted with direct methanol fuel cells incorporating metal foams as the flow field. The influence of the foam pore size and density on cell performance was investigated. The performance of similar density metal foams but with different pore sizes was non-monotonic due to the opposing trends of electrical contact and CO₂ removal with pore size. In contrast, for metal foams with the same in-plane pore size, the performance improved with increasing density. Because the cell operates in a diffusion-dominated regime, its performance showed a strong dependence on methanol concentration and a moderate dependence on methanol flow rate. The feasibility of using metal foams as a gas diffusion layer (GDL) was also explored.     

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.     

Supported bimetallic nanoparticles as anode catalysts for direct methanol fuel cells: A review

Direct methanol fuel cell (DMFC) is an environment friendly energy source that transforms chemical energy of methanol oxidation into electrical energy. The Pt- and non-Pt based bimetallic nanoparticles (BMNPs) with electrocatalyst support materials are employed as anode electrocatalysts for methanol oxidation. These supported BMNPs have drawn prominent consideration due to their incredible physical and chemical properties. This article reviews the advancements in the field of supported BMNPs of varied structures, compositions and morphologies, using innumerable carbonaceous support materials such as carbon black, carbon nanotubes, carbon nanofibers, graphene, mesoporous carbon as well as non-carbonaceous supports like inorganic oxides, graphitic carbon nitride, metal nitrides, conducting polymers and hybrid support materials. The performance of electrocatalysts on the basis of support material, structure, composition and morphology of BMNPs, and pros and cons of various support materials have been discussed.     

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.     

Current density measurements in direct methanol fuel cells

The current density in the fuel cell is the direct consequence of reactions taking place over the active surface area. Thus, measurement of its distribution will lead to identification of the location and nature of reactions and will give opportunity to improve the overall efficiency of fuel cells. Within this study, the current density distribution in a direct methanol fuel cell was analyzed by segmenting the current collector into nine sections. Besides, the effect of the different operating parameters such as molarity, flow rate and reactant gas on the current density distribution was analyzed.     

Modeling of a passive DMFC operating with neat methanol

A mathematical model is developed to simulate the fundamental transport phenomena in a passive direct methanol fuel cell (DMFC) operating with neat methanol. The neat methanol operation is realized by using a ‘pervaporation’ membrane that allows the methanol concentration from the neat methanol in the fuel reservoir to be declined to an appropriate level in the anode catalyst layer (CL). The water required by the methanol oxidation reaction on the anode is passively obtained by diffusion from the cathode through the membrane. The numerical results indicate that the methanol delivery rate from the fuel reservoir to the anode CL is predominately controlled by the pervaporation process. It is also found that under the neat methanol operating condition, water distribution across the membrane electrode assembly is greatly influenced by the membrane thickness, the cathode design, the operating temperature, and the ambient relative humidity.     

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.     

Operating characteristics and performance stability of 5 W class direct methanol fuel cell stacks with different cathode flow patterns

In this study, 5 W class direct methanol fuel cell (DMFC) stacks using the flow field patterns of serpentine, parallel, and square spot are fabricated to compare how well they are capable of mass transport and water removal in the cathode. The stability of the stack is predicted through the simulation results of the flow field patterns on the pressure drop and the water mass fraction in the cathode of the stack. It is then estimated through the performance and the voltage distribution of the stack. According to the simulation results, although the square spot pattern shows the lowest pressure drop, the square spot pattern has much higher water mass fraction in the central region of the channel compared to the other flow field patterns. In accordance with the results, a square spot pattern for the stack-SSMA exhibits very poor water removal capabilities, leading to water flooding near the channel exit. In contrast, the performance stability of a stack-SPMA is comparable to the stack-SSMM.     

Regulating methanol feed concentration in direct methanol fuel cells using feedback from voltage measurements

Regulating methanol feed concentration in direct methanol fuel cells (DMFCs) is important for improving electrical performance and fuel utilization. Low methanol concentration reduces the reaction rate at the anode due to Nernstian effects resulting in a lower operating voltage. However, simply increasing the methanol concentration does not always lead to improved performance due to increased methanol crossover from the anode to the cathode resulting in mixed-potential losses and the associated fuel loss. Hence, there exists an optimal intermediate value of methanol concentration for each current density that will yield the highest electrical performance (V). In this paper, we describe the development of an in situ methodology which uses the measured cell voltage as the feedback to regulate the methanol feed concentration for maximum power density. This methodology is demonstrated at the current densities of 50, 100, and 250 mA cm⁻²and the results for optimal concentration are presented. Fuel loss as a function of methanol concentration is evaluated by oxidizing the crossover methanol at the cathode exhaust and measuring the CO₂ mass flux.     

Investigation of bipolar plate geometry on direct methanol fuel cell performance

The effect of bipolar plate (BPP) channel width on the performance of a single-cell direct methanol fuel cell (DMFC) stack was investigated. Three BPPs with a single-channel serpentine configuration were fabricated with three different channel widths having the same effective flow areas. Experimental tests showed that the fuel cell with the narrowest channel width had the best overall performance while the one with the widest channel width had the worst overall performance. A computational fluid dynamics (CFD) model was developed and simulated to investigate and understand the reasons for this change in performance. The CFD simulation showed that in the narrower channel the diffusion of the fuel within the diffusion layer was higher and the fuel distribution was more uniform. However, the pressure drop across the inlet and outlet was higher in the narrower channel, which increased the pumping power requirements.