Development of materials for mini DMFC working at room temperature for portable applications

Methanol permeability measurements and direct methanol fuel cell tests were performed at room temperature with different commercially available or recast Nafion^® membranes and sulfonated polyimide (SPI) membranes. Power densities as high as 20 mW cm⁻² could be obtained with Nafion^® 115. However, in order to meet the technological requirements for portable applications, thinner membranes have to be considered. As the MeOH crossover increases greatly (from (7 to 20) × 10⁻⁸ mol s⁻¹ cm⁻²) while Nafion^® membranes thickness decreases, non-perfluorinated polymers having high IEC are promising candidates for DMFC working at room temperature. The development catalysts tolerant to methanol is also relevant for this application. In spite of the low permeability to MeOH of SPI membranes, the obtained electrical performance with E-TEK electrodes based MEAs was lower than that obtained with Nafion^® membranes. No significant increase of performances was neither evidenced by using homemade PtCr(7:3)/C and PtRu(4:1)/C catalysts instead of E-TEK electrodes with recast Nafion^® based MEAs. However, MEAs composed with thin SPI membranes (50 μm) and homemade PtCr/C catalysts gave very promising results (18 mW cm⁻²). Based on experimental observations, a speculative explanation of this result is given.     

A review of thermoplastic composites for bipolar plate applications

Bipolar plates are responsible for functions of vital importance to the long‐term performance of fuel cells. They play crucial roles in water and gas management, mechanical strength and electrical conductivity. It also significantly contributes to the volume, weight and cost of fuel cell stacks. The properties of bipolar plates are affected by the materials and processes used in the manufacturing of the plates. The objective of this article is to review the use of thermoplastic materials as polymer matrices in bipolar plate applications. Conductive composites consisting of different types and blends of thermoplastic polymers are detailed discussed. The effects of filler types and processing conditions are given. Several thermoplastic blends consisting of carbon black, carbon nanotube and graphite are evaluated. The dispersion of conductive fillers, in particular, polymer composites and polymer blend composites is also given. Copyright © 2013 John Wiley & Sons, Ltd.     

Operational characteristics of a 50 W DMFC stack

The characteristics of a 50 W direct methanol fuel cell (DMFC) stack were investigated under various operating conditions in order to understand the behavior of the stack. The operating variables included the methanol concentration, the flow rate and the flow direction of the reactants (methanol and air) in the stack. The temperature of the stack was autonomously increased in proportion to the magnitude of the electric load, but it decreased with an increase in the flow rates of the reactants. Although the operation of the stack was initiated at room temperature, under a certain condition the internal temperature of the stack was higher than 80 °C. A uniform distribution of the reactants to all the cells was a key factor in determining the performance of the stack. With the supply of 2 M methanol, a maximum power of the stack was found to be 54 W (85 mW cm⁻²) in air and 98 W (154 mW cm⁻²) in oxygen. Further, the system with counter-flow reactants produced a power output that was 20% higher than that of co-flow system. A post-load behavior of the stack was also studied by varying the electric load at various operating conditions.     

Fabrication of high strength and a low weight composite bipolar plate for fuel cell applications

The bipolar plate is one of the most important components in a PEM fuel cell. A polymer composite bipolar plate possessing high strength (81 MPa) and high stiffness (20 GPa) has been developed by making use of carbon fiber network in a specific form as the filler component. Such high strength is very much desired, especially when the fuel cells are used for mobile applications, since it is the bipolar plate that provides mechanical support to all the other cell components. The addition of carbon black and the effect of particle size of the natural graphite flakes used as other reinforcements also play a crucial role in controlling the physical and electrical properties of the composite plates. The plate when used in the unit fuel cell assembly showed I–V performance comparable to that of the commercially available bipolar plates.     

Development of a 700 W anode-supported micro-tubular SOFC stack for APU applications

A 700 W anode-supported micro-tubular solid-oxide fuel cell (SOFC) stack for use as an auxiliary power unit (APU) for an automobile is fabricated and characterized in this study. For this purpose, a single cell was initially designed via optimization of the current collecting method, the brazing method and the length of the tubular cell. Following this, a high-power single cell was fabricated that showed a cell performance of at 0.7 V and using H₂ (fuel utilization=45%) and air as fuel and oxidant gas, respectively. Additionally, a fuel manifold was designed by adopting a simulation method to supply fuel gas uniformly into a single unit cell. Finally, a 700 W anode-supported micro-tubular SOFC stack was constructed by stacking bundles of the single cells in a series of electrical connections using H₂ (fuel utilization=49%) and air as fuel and oxidant gas, respectively. The SOFC stack showed a high power density of ; moreover, due to the good thermo-mechanical properties of the micro-tubular SOFC stack, the start-up time could be reduced by 2 h, which corresponds to 6/min.     

Development and characterization of a miniature PEM fuel cell stack with carbon bipolar plates

This paper details the fabrication and testing of a fuel cell stack using a novel manufacturing approach for creating carbon bipolar plates. The fundamental fabrication techniques have been described in a previous contribution [1]. The initial paper characterized a single cell. In this work, the fabrication techniques are utilized to fabricate a three-cell fuel cell stack. Operating data in different operating conditions is measured and presented. Information on cell performance at different operating temperatures and pressures is included. The methodology for the fuel cell characterization is presented.     

Development of a four‐cell DMFC stack with air‐breathing cathode and dendrite flow field

A four‐cell direct methanol fuel cell (DMFC) stack with an air‐breathing cathode with an active area of 0.48 cm² for each cell is designed, fabricated and tested. A pure copper sheet 300 µm thick with innovative perforated flow plates (dendrite type) is fabricated and used for the cathode. For the anode, conventional serpentine flow channels made of pure copper sheets 250 µm thick are used. An extensive parametric study is conducted to determine the optimum working conditions for the fuel flow rate (anode), methanol solution concentration, channel‐to‐land ratio and stack temperature. Comparisons are made with conventional serpentine flow channels. In addition, CO₂ (water) bubbles in the anode (cathode) channels are visualized, and the results are presented and discussed. It is found that the maximum stack power of the four‐cell μDMFC stack is up to 40 mW/cm² with a limiting current density of 335 mA/cm² at a maximum volumetric and gravimetric power density of 11.16 mW/cm³ and 3.13 W/kg, respectively. Copyright © 2014 John Wiley & Sons, Ltd.     

Development of high-performance planar borohydride fuel cell modules for portable applications

Direct borohydride fuel cell (DBFC) as a liquid type fuel cell is promising for portable applications. In this study, we report our recent progress in the micro-fuel cell development. A power density of 80 mW cm⁻² was achieved in passive mode at ambient conditions when using the anode containing nickel, carbon-supported Pd catalyst and Nafion ionomer. Current efficiency was also found to be greatly increased due to the use of Nafion rather than polytetrafluoroethylene (PTFE). Based on improvements on single cell performance, planar multi-cell power modules were assembled to study the feasibility of making high-performance and practical DBFC power units. A power of 2.5 W was achieved in a fully passive eight-cell module after significantly simplifying cell structure.     

Thermal analysis and optimization of a portable,edge-air-cooled PEFC stack

Internally humidified, edge-air-cooled PEFC stacks are promising for portable systems in terms of specific power and specific cost. However, their main drawbacks are thermal power limitations due to limited heat removal from inside the stack. The aim of this work is to minimize the cooling limitation with a simultaneous cost and weight reduction by optimization of the stack geometry. A steady-state, thermal FE-model was developed and validated against experimental temperature distributions. The model includes anisotropic heat conduction and heat convection by the cooling air. Cell voltage, liquid water fraction and limiting temperature were experimentally determined for improved accuracy. Complex flowfield structures were approximated with the numerical volume averaging method to reduce computational cost. As a result of the optimization study specific power was improved by +86% with simultaneous reduction of specific cost by −35$-35$%.     

The durability investigation of a 10‐cell metal bipolar plate proton exchange membrane fuel cell stack

The metal bipolar plates (BPs) have replaced the graphite BPs in vehicle‐used proton exchange membrane fuel cell (PEMFC) stack because of their high volume power density. To investigate the durability of metal BP stack, this paper performed a durability test of 2000 hours on a 10‐cell metal BP fuel cell stack. The degradations of the average voltage and individual cell voltage in fuel cell stack were analyzed. To investigate the degradation mechanism, the stack was disassembled and the morphologies and compositions of no. 1, no. 5, and no. 10 cells after 2000 hours were characterized by SEM, TEM, and ASS. The results indicated that at 800 mA/cm², the voltage decay rate is 42.303 μV/hour and the voltage decay percentage of the stack is 14.34% after 2000 hours according to the linearly fitting result. According to the US Department of Energy (DOE) definition of fuel cell stack life, only the voltage decay rate of OCV and the tenth cell is lower than the maximum voltage degradation rates of 10 000 hours. The decreases of homogeneity of stack were the main reason for its performance degradation. Especially for the tenth cell, its performance has almost no drop. The main failure reason of this metal BP stack is structural design rather than metal corrosion. The losses of Pt catalyst and C supporting are the main reason of performance degradation.     

Alkaline high power batteries in a bipolar stack design

For several applications, batteries with a high power/energy ratio are required. In order to meet these requirements, alkaline batteries based on fibre structures and hydrogen storage negative electrodes that are arranged in bipolar stacks, can be used. The bipolar stacks are designed to provide comparatively high voltages, e.g. 12, 36 and even 110 V. These stacks display electrochemical characteristics of a typical battery but are able to provide high power densities. This paper compiles data obtained from such storage units.     

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.     

Dynamic behavior of a PEM fuel cell stack for stationary applications

We discuss the behavior and performance of a proton exchange membrane fuel cell stack under fast load commutations. We present experimental results for the polarization curves, energy balance sheet, and time response of the fuel cells. Although load transients are present both in the voltage and current generated, it is found that the fuel cell system response is faster than 0.15 s to load commutations. The experimental results were also compared to the Amphlett et al. and Kim et al. models, which were found to describe the data well.     

Voltage loss in bipolar plates in a fuel cell stack

Voltage loss in the bipolar plate (BP) is induced by in-plane current in the BP, which arises when the distributions of local current density over the surfaces of adjacent cells are different. We show that potential of BP satisfies Poisson equation with the right side proportional to the difference of local current densities on both sides of BP. Solution to this equation is obtained for BP between two hydrogen cells with the single straight channels and ideally humidified membranes. The general relation for voltage loss in the BP is derived.     

Factors affecting methanol transport in a passive DMFC employing a porous carbon plate

The effect of the pore structure and thickness of the porous carbon plate, PCP, as well as the gas barrier thickness on the methanol transport and the performance of a passive DMFC under the different cell voltages of 0.1, 0.2 and 0.3 V using different methanol concentrations was investigated. As a result of the mass transfer restrictions by employing the PCP, high methanol concentrations over 20 M could be efficiently used to produce the relatively high power density of 30 mW cm⁻² for more than 10 h. The DMFC was operated under limiting current conditions in all the PCPs at 0.1 and 0.2 V to more than 20 M. The main factors for controlling the methanol transport were the barrier of the gas layer with CO₂, which was formed between the anode surface and the PCP and the properties of the PCP. At the low current densities of less than 60 mA cm⁻², when no CO₂ bubbles are emitted, both the pore structure and thickness of the PCP did not affect the methanol transport and the current voltage relationship. At the higher current densities, CO₂ bubbles were evolved through the PCP and different resistances to the methanol transport were observed depending on the PCP pore structure and thickness. The CO₂ gas layer between the MEA and the PCP caused a major resistivity for the methanol transport, and its resistivity increased with its thickness increasing. By using the PCP at 0.1 V, the energy density of the passive DMFC was significantly increased, e.g., more than seven times.     

Surface-modified Nafion membrane by oleylamine-stabilized Pd nanoparticles for DMFC applications

The surface of Nafion was modified by applying palladium nanoparticles as methanol barrier materials to decrease methanol crossover and improve the performance of fuel cells. The properties of the Pd-modified membrane, in terms of conductivity, methanol permeability, percentage of liquid uptake as well as the performance of its membrane electrode assembly (MEA) in the direct methanol fuel cell, were analyzed and compared with those using bare Nafion. The modified membrane showed considerable improvement on reducing methanol loss without decreasing proton conductivity. The DMFC performance of modified membrane was superior to that of bare Nafion both at a typical fuel state of 2 M and at high concentration of 5 M, implying that the palladium-modified Nafion can be a good alternative approach for DMFC applications.     

Performance of DMFC with SS 316 bipolar/end plates

This work mainly emphasizes the development of new materials and design for a bipolar/end plate in a direct methanol fuel cell (DMFC). According to the DOE requirements, preliminary studies show that SS 316 (Stainless Steel 316) is a suitable candidate. Several flow field designs were studied and a modified serpentine design was proposed. SS 316 end plates were fabricated with an intricate modified serpentine flow field design on it. The performance of a single stack DMFC with SS 316 end plates were studied with different operational parameters. A long-term test was carried out for 100 h with recycling the methanol and the contaminants in the MEA were characterized. The stack efficiency is found to be 51% and polarization losses are discussed. SS 316 with low permeability resulted in an increased pressure drop across the flow field, which increased the fuel cell performance. The use of SS 316 as bipolar plate material will reduce the machining cost as well as volume of the fuel cell stack.     

Development of a 1 W passive DMFC

In this paper, development techniques for a passive DMFC prototype in the 1 W range are described in detail. The prototype includes a fuel cell stack, a fuel tank and a passive ancillary system (termed “thermal-fluids management system” in this paper). The fuel cell stack in this study incorporates a window-frame structure that provides a large open area for more efficient mass transfer and is modular. Two stack units connected in series, with a total combined active area of 72.0 cm², are used in the prototype. The thermal-fluids management system utilizes passive approaches for fuel storage and delivery, air-breathing, water management, CO₂ release, and thermal management. The air filter also serves as a waterproof layer for the cathode in order to prevent water contamination. Water immersion tests are conducted to evaluate the air filter. The performance evaluation of the prototype is performed in two fuel feeding modes: dilute methanol solution and pure methanol. A peak power output of 1.5 W is achieved with the dilute methanol solution feed.     

Computationally efficient modeling of the dynamic behavior of a portable PEM fuel cell stack

A numerically efficient mathematical model of a proton exchange membrane fuel cell (PEMFC) stack is presented. The aim of this model is to study the dynamic response of a PEMFC stack subjected to load changes under the restriction of short computing time. This restriction was imposed in order for the model to be applicable for nonlinear model predictive control (NMPC). The dynamic, non-isothermal model is based on mass and energy balance equations, which are reduced to ordinary differential equations in time. The reduced equations are solved for a single cell and the results are upscaled to describe the fuel cell stack. This approach makes our calculations computationally efficient. We study the feasibility of capturing water balance effects with such a reduced model. Mass balance equations for water vapor and liquid water including the phase change as well as a steady-state membrane model accounting for the electro-osmotic drag and diffusion of water through the membrane are included. Based on this approach the model is successfully used to predict critical operating conditions by monitoring the amount of liquid water in the stack and the stack impedance. The model and the overall calculation method are validated using two different load profiles on realistic time scales of up to 30 min. The simulation results are used to clarify the measured characteristics of the stack temperature and the stack voltage, which has rarely been done on such long time scales. In addition, a discussion of the influence of flooding and dry-out on the stack voltage is included. The modeling approach proves to be computationally efficient: an operating time of 0.5 h is simulated in less than 1 s, while still showing sufficient accuracy.