Effect of cathode design on the performance of an air-breathing PEM fuel cell

Effect of cathode design on the performance of an air-breathing fuel cell is studied using a three dimensional, non-isothermal, steady state and single phase model developed using commercial CFD software FLUENT (version 6.3). Performances of ducted (channel) and ribbed (planar) cathode designs are compared and the cell characteristics such as current density, species, temperature distributions, velocity and net water transport coefficient are presented. Peak power density obtained for the cell with ducted cathode is 205 m W/cm², whereas with ribbed cathode it is 232 m W/cm². The limiting current density of the cell with ribbed cathode (690 m A/cm²) is much higher than that of the cell with ducted cathode (430 m A/cm²). The performance curves as well as the cell characteristics show that the ribbed cathode design is a better configuration compared to ducted design. Cell orientation has a significant effect on the cell performance. Best performance is obtained when the cell is oriented vertically for both ribbed and ducted cathode designs.     

Development of a novel radial cathode flow field for PEMFC

This paper presents an innovative radial flow field design for PEMFC cathode flow plates. This new design, which is in the form of a radial flow field, replaces the standard rectangular flow channels in exchange for a set of flow control rings. The control rings allow for better flow distribution and use of the active area. The radial field constructed of aluminum and plated with gold for superior surface and conductive properties. This material was selected based on the results obtained from the performance of the standard flow channels of serpentine and parallel designs constructed of hydrophilic gold and typical hydrophobic graphite materials. It is shown that the new flow field design performs significantly better compared to the current standard parallel channels in a dry-air-flow environment. The polarization curves for a dry flow, however, show excessive membrane drying with the radial design. Humidifying the air flow improves the membrane hydration, and in the meantime, the fuel cell with the innovative radial flow field produces higher current compared to other channel designs, even the serpentine flow field. The water removal and mass transport capacity of the radial flow field was proven to be better than parallel and serpentine designs. This performance increase was achieved while maintaining the pressure drop nearly half of the pressure drop measured in the serpentine flow field.     

高温空气燃烧器换向过程非稳态流场和温度场的仿真分析

采用FLUENT软件和燃烧模型,对烧嘴交错布置的高温空气燃烧器换向后的非稳态过程进行了数值研究,换向后炉内的流场、温度场变化的计算结果表明,在换向后的前3S内燃烧炉的流场和温度场变化很大,但是经过3s的变化后,燃烧逐步稳定,最后重新形成稳定的燃烧,直至下一个换向前保持稳定燃烧。     

A new cathode structure for air-breathing DMFCs operated with pure methanol

In this paper, a new cathode structure for air-breathing DMFCs operated with pure methanol was proposed to push the water produced at the cathode to the anode through the membrane. The open-circuit voltage (OCV) measurement showed that the single cell with this new structure attained its stable value within less time and reached a higher OCV than that of traditional cathode structure. The measurement of the amount of water collected in the cathode showed that the new cathode structure could push more water to the anode. The I–V performance test proved that MEA with new cathode structure had better performance than that of the traditional one, and reached a peak power density of 29.4 mW/cm², 100% higher than that of the traditional one.     

Specific approaches to dramatic reduction in stack activation time and perfect long-term storage for high-performance air-breathing polymer electrolyte membrane fuel cell

Air-breathing polymer electrolyte membrane fuel cell (PEMFC) systems without the humidifier and air blower have been developed to overcome the cost and complexity of balance of plants (BOPs). Until now, there has been no specific way to improve the stack's initial performance through the specific activation protocol and maintain the initial performance for a very long time. Herein, we studied a technique for finishing the total activation within 1 h by using a pre-activation process (i.e., soaking the stack in a DI-water reservoir) and applying current at 0.65 V. The pre-activation procedure significantly increased the swelling of the polymer membrane and the Nafion binder in the catalyst layer, reducing the total activation time. Also, we showed that long-term storage using humidified N₂ gas in a closed box did not hinder the electrocatalytic activity of the Pt and the drying of the polymer membrane for 60 days.     

Portable PEMFC stack using sulfonated poly (fluorenyl ether ketone) ionomer as membrane

Portable polymer electrolyte membrane fuel cells (PEMFCs) stack was assembled with sulfonated poly(fluorenyl ether ketone) (SPFEK) ionomer membranes. The portable PEMFC stack was studied by means of cell performance tests at high temperatures under low relatively humidity (RH). The experimental results showed that the output power of the stack increased from 28.74 W to 37.11 W with increasing operating temperature from 30 to 90 °C under 100% RH. When the operating temperature was over 100 °C, the output power decreased with further increasing temperature from 27.68 W (100 °C, 85% RH) to 19.87 W (120 °C, 50% RH). The output at 120 °C and under 50% RH was 69% output power of the stack at 30 °C and under 100% RH. These results demonstrated that the self-prepared SPFEK ionomer membrane was a promising PEM for the application in high-temperature PEMFC.     

The development of air-breathing proton exchange membrane fuel cell (PEMFC) with a cylindrical configuration

A small air-breathing proton exchange membrane fuel cell with a cylindrical configuration (Cy-PEMFC) and a helical flow-channel was developed to provide a uniform contact pressure to the membrane electrode assembly (MEA) with a thin cathode current collector. A comparison of the contact pressure and performance of the Cy-PEMFC and general planar PEMFC was performed to determine the effect of the cylindrical configuration. For the contact pressure comparison, numerical analysis was performed using commercial software. Numerical analysis showed that the Cy-PEMFC has its own structural advantage of changing the applied clamping pressure to a uniformly distributed contact pressure. The actual pressure measurements were carried out with pressure-sensitive film to support results of numerical analysis. These results also showed that the Cy-PEMFC had a uniformly distributed contact pressure, whereas the planar PEMFC did not. The polarization curves of both PEMFCs were measured to determine the performance variations caused by the uniform contact pressure and better mass transfer. The maximum power density of the Cy-PEMFC was 220 mW/cm², which was approximately 24% higher than the planar PEMFC.     

不同型式流场的PEMFC内部传质分析

对采用不同型式流场的PEMFC进行建模,并用控制容积法对控制方程进行离散,求解得到PEMFC内部各物理量的分布以及综合水拖带系数、质子交换膜平均电导率等。分析了采用交趾型流场和常规流场时PEMFC的内部传质以及阴极性能、电池性能和膜性能,结果认为采用交趾型流场时,PEMFC阴极性能高于采用常规流场的PEMFC阴极性能,但质子交换膜的平均电导率低于采用常规流场时。在没有液态水产生时常规流场PEMFC性能高于交趾型流场PEMFC。     

Study of membrane electrode assemblies for PEMFC,with cathodes prepared by the electrospray method

The electrospray deposition of platinum supported on carbon (Pt/C) particles has been used for the preparation of electrodes for proton exchange membrane fuel cells (PEMFCs). The departing suspensions contain the Pt/C electrocatalyst together with an ionomer (Nafion^®) and a solvent. Two types of solvent have been used, isopropanol and a mixture of butylacetate, ethanol and glycerol (BEG). The microscopic characterisation of electrosprayed films shows the electrospray deposited Pt/C films with a dendritic morphology. XPS analysis of the films reflects changes in the ionomer component after electrospray deposition. A decrease in the signal corresponding to backbone chain (CF₂) is observed on the films deposited with the low evaporation temperature solvent (isopropanol), indicating some disruption of ionomer chains during the electrospray process. With high evaporation temperature solvent (BEG), the disruption effect seems less acute. Membrane electrode assemblies were prepared with the electrosprayed electrodes as cathodes. Good general performance is encountered, comparable with standard commercial cathodes. Electrosprayed electrodes prepared from high evaporation temperature solvent (BEG) show a higher surface specific area. The internal resistance is something higher for MEAs with electrosprayed cathodes. The long term stability test shows a performance loss of about 10 μV h⁻¹ over 700 h continuous testing.     

Evaluation of Palladium-based electrocatalyst for oxygen reduction and hydrogen oxidation in intermediate temperature polymer electrolyte fuel cells

A carbon-supported Palladium electrocatalyst was investigated for oxygen reduction and hydrogen oxidation in a polymer electrolyte fuel cell operating at intermediate temperatures (80–110 °C) and with low relative humidity (33%). A 30% Pd/C was synthesized by a colloidal method and subsequent carbothermal reduction. A mean particle size of 4.0 nm and a homogeneous dispersion of Pd particles on the support were obtained. The performance of the Pd catalyst was compared to those obtained with a 50% Pt/C catalyst and a 50% Pt₃Co₁/C as anode and cathode, respectively. The Pd/C catalyst showed low overpotential for hydrogen oxidation whereas its performance as cathode was significantly lower than the benchmark Pt₃Co₁ catalyst. The main limiting effects for the Pd-based electrocatalyst appeared to be associated to a larger mean particle size compared to the benchmark Pt catalysts and to the modification of the carbon support during the synthesis procedure. These effects led to a stronger activation control, a slight increase of the series resistance and some diffusion constraints.     

Effects of operating parameters on transport phenomena and cell performance of PEM fuel cells with conventional and contracted flow field designs

A contracted parallel flow field design was developed to improve fuel cell performance compared with the conventional parallel flow field design. A three-dimensional model was used to compare the cell performance for both designs. The effects of the cathode reactant inlet velocity and cathode reactant inlet relative humidity on the cell performance for both designs were also investigated. For operating voltages greater than 0.7 V because the electrochemical reaction rates are lower with less oxygen consumption and less liquid water production, the cell performance is independent of the flow field designs and operating parameters. However, for lower operating voltages where the electrochemical reaction rates gradually increase, the oxygen transport and the liquid water removal efficiency differ for the various flow field designs and operating parameters; therefore, the cell performance is strongly dependent on both the design and operating parameters. For lower operating voltages, the cell performance for the contracted design is better than for the conventional design because the reactant flow velocities in the contracted region significantly increase, which enhances liquid water removal and reduces the oxygen transport resistance. For lower operating voltages, as the cathode reactant inlet velocity increases and the cathode reactant inlet relative humidity decreases, the cell performance for both designs improves.     

Study of the influence of the amount of PBI–H3PO4 in the catalytic layer of a high temperature PEMFC

The influence of the amount of polybenzimidazole (PBI)-H₃PO₄ (normalized with respect to the PBI loading, which expressed as C/PBI weight ratio) content in both the anode and cathode has been studied for a PBI-based high temperature proton exchange membrane (PEM) fuel cell. The electrodes prepared with different amounts of PBI have been characterized physically, by measuring the pore size distribution, and visualizing the surface microstructure. Afterwards, the electrochemical behaviour of the electrodes has been evaluated. The catalytic electrochemical activity has been measured by voltamperometry for each electrode prepared with a different PBI content, and the cell performance results have been studied, supported by the impedance spectra, in order to determine the influence of the PBI loading in each electrode. The best results have been achieved with a C/PBI weight ratio of 20, for both the anode and the cathode. A lower C/PBI weight ratio (larger amount of PBI in the catalytic layer) reduced the electrocatalytic activity, and impaired the mass transport processes, due to the large amount of polymer covering the catalyst particle, lowering the cell performance. A higher C/PBI weight ratio (lower amount of PBI in the catalytic layer) reduced the electrocatalytic activity, and slightly increased the ohmic resistance. The low amount of the polymeric ionic carrier PBI–H₃PO₄ limited the proton mobility, despite of the presence of large amounts of “free” H₃PO₄ in the catalytic layer.     

Effect of GDL permeability on water and thermal management in PEMFCs—I. Isotropic and anisotropic permeability

The performance of proton exchange membrane fuel cells (PEMFCs) with various isotropic and anisotropic permeabilities of the gas diffusion layer (GDL) was investigated using computational fluid dynamics analysis. A three-dimensional, non-isothermal model was employed with a single straight channel; both humidification and phase transportation were included in the model. The total water and thermal management for systems operating at high current densities was obtained. The results showed that the cell performance deteriorated for low isotropic permeability of the GDL. Water removal from the cathode GDL was significantly reduced in systems with low isotropic permeability or anisotropic systems with low permeabilities in both the in-plane and through-plane directions. Moreover, both the in-plane and through-plane permeabilities were found to affect water and thermal management in PEMFCs, especially in the low permeability ranges. Variations in GDL permeability had a greater influence on ohmic losses than on cathode overpotentials because the former losses depend on water and thermal management. In addition, the results showed that water and thermal management was good in systems in which the permeability in at least one direction (in-plane or through-plane) was high, whereas systems with low permeability in both the in-plane and through-plane directions exhibited poor water and thermal management. However, heat removal in PEMFCs was negatively affected by low permeability, leading to higher temperatures in the cell. The present numerical results suggested that modeling with isotropic permeability conditions may overpredict the cell performance, and inaccurately predict the water and thermal management in PEMFCs.     

1.5 kWe HT-PEFC stack with composite MEA for CHP application

In this work, the performance of a High Temperature (HT) Polymer Electrolyte Fuel Cell (PEFC) stack for co-generation application was investigated. A 3 kW power unit composed of two 1.5 kW modules was designed, manufactured and tested. The module was composed of 40 composite graphite cell with an active area of 150 cm². Composite Membrane Electrode Assemblies (MEAs) based on Nafion/Zirconia membranes were used to explore the behavior of the stack at high temperature (120 °C). Tests were performed in both pure Hydrogen and H₂/CO₂/CO mixture at different humidification grade, simulating the exit gas from a methane fuel processor. The fuel cells stack has generated a maximum power of 2400 W at 105 A with pure hydrogen and fully hydrated gases and 1700 W at 90 A by operating at low humidity grade (95/49 RH% for H₂/Air). In case the stack was fed with reformate simulated stream fully saturated, a maximum power of 2290 W at 105 A was reached: only a power loss of 5% was recorded by using reformate stream instead of pure hydrogen. The humidification grade of Nafion membrane was indicated as the main factor affecting the proton conductivity of Nafion while the addition of the inert compound like YSZ, did not affectthe electrochemical properties of the membrane but, rather has enhanced mechanical resistance at high temperature.     

The neural networks based modeling of a polybenzimidazole-based polymer electrolyte membrane fuel cell: Effect of temperature

Neural network models represent an important tool of Artificial Intelligence for fuel cell researchers in order to help them to elucidate the processes within the cells, by allowing optimization of materials, cells, stacks, and systems and support control systems. In this work three types of neural networks, that have as common characteristic the supervised learning control (Multilayer Perceptron, Generalized Feedforward Network and Jordan and Elman Network), have been designed to model the performance of a polybenzimidazole-polymer electrolyte membrane fuel cells operating upon a temperature range of 100-175 °C. The influence of temperature of two periods was studied: the temperature in the conditioning period and temperature when the fuel cell was operating. Three inputs variables: the conditioning temperature, the operating temperature and current density were taken into account in order to evaluate their influence upon the potential, the cathode resistance and the ohmic resistance. The Multilayer Perceptron model provides good predictions for different values of operating temperatures and potential and, hence, it is the best choice among the study models, recommended to investigate the influence of process variables of PEMFCs.     

Study of flow channel geometry using current distribution measurement in a high temperature polymer electrolyte membrane fuel cell

To improve fuel cell design and performance, research studies supported by a wide variety of physical and electrochemical methods have to be carried out. Among the different techniques, current distribution measurement owns the desired feature that can be performed during operation, revealing information about internal phenomena when the fuel cell is working. Moreover, short durability is one of the main problems that is hindering fuel cell wide implementation and it is known to be related to current density heterogeneities over the electrode surface. A good flow channel geometry design can favor a uniform current density profile, hence hypothetically extending fuel cell life. With this, it was thought that a study on the influence of flow channel geometry on the performance of a high temperature polymer electrolyte membrane (PEM) fuel cell using current distribution measurement should be a very solid work to optimize flow field design. Results demonstrate that the 4 step serpentine and pin-type geometries distribute the reactants more effectively, obtaining a relatively flat current density map at higher current densities than parallel or interdigitated ones and yielding maximum powers up to 25% higher when using oxygen as comburent. If air is the oxidant chosen, interdigitated flow channels perform almost as well as serpentine or pin-type due to that the flow conditions are very important for this geometry.