Exergetic performance analysis of a PEM fuel cell

In this paper we investigate the effects of thermodynamic irreversibilities on the exergetic performance of proton exchange membrane (PEM) fuel cells as a function of cell operating temperature, pressures of anode and cathode, current density, and membrane thickness. The practical operating conditions are selected to be 3–5 atm for anode and cathode pressures, and 323–353 K for the cell temperatures, respectively. In addition, the membrane thicknesses are chosen as 0.016, 0.018 and 0.02 cm, respectively. Moreover, the current density range of the PEM fuel cell is selected to be 0.01–2.0 A cm^?2. It is concluded that exergy efficiency of PEM fuel cell decreases with a rise in membrane thickness and current density, and increases with a rise of cell operating pressure and with a decrease of current density for the same membrane thickness. Thus, it can be said that, in order to increase the exergetic performance of PEM fuel cell, the lower membrane thickness, the lower current density and the higher cell operating pressure should be selected in case PEM fuel cell is operated at constant cell temperature. Copyright © 2005 John Wiley & Sons, Ltd.     

Experimental factors that influence carbon monoxide tolerance of high-temperature proton-exchange membrane fuel cells

The poisoning effect of carbon monoxide (CO) on high-temperature proton-exchange membrane fuel cells (PEMFCs) is investigated with respect to CO concentration, operating temperature, fuel feed mode, and anode Pt loading. The loss in cell voltage when CO is added to pure hydrogen anode gas is a function of fuel utilization and anode Pt loading as well as obvious factors such as CO concentration, temperature and current density. The tolerance to CO can be varied significantly using a different experimental design of fuel utilization and anode Pt loading. A difference in cell performance with CO-containing hydrogen is observed when two cells with different flow channel geometries are used, although the two cells show similar cell performance with pure hydrogen. A different combination of fuel utilization, anode Pt loading and flow channel design can cause an order of magnitude difference in CO tolerance under identical experimental conditions of temperature and current density.     

Maximizing the efficiency of a HT-PEMFC system integrated with glycerol reformer

The efficiency and output power density of an integrated high temperature polymer electrolyte fuel cell system and glycerol reformer are studied. The effects of reformer temperature, steam to carbon ratio (S/C), fuel cell temperature, and anode stoichiometric ratio are examined. An increase in anode stoichiometric ratio will reduce CO poisoning effect at cell’s anode but cause lower fuel utilization towards energy generation. High S/C operation requires large amount of the energy available, however, it will increase anode tolerance to CO poisoning and therefore will lead to enhanced cell performance. Consequently, the optimum gas composition and flow rate is very dependent on cell operating current density and temperature. For example, at low current densities, similar efficiencies were obtained for all the S/C ratio studied range at cell temperature of 423.15 K, however, at cell temperature of 448.15 K, low S/C ratio provided higher efficiency in comparison to high S/C ratio. High S/C is essential when operating the cells at high current densities where CO has considerable impact on cell performance. Optimal conditions that provide the maximum power density at a given efficiency are reported.     

Two-phase flow behaviour and performance of polymer electrolyte membrane electrolysers: Electrochemical and optical characterisation

Understanding gas evolution and two-phase flow behaviour are critical for performance optimization of polymer electrolyte membrane water electrolysers (PEMWEs), particularly at high current densities. This study investigates the gas-bubble dynamics and two-phase flow behaviour in the anode flow-field of a PEMWE under different operating conditions using high-speed optical imaging and relates the results to the electrochemical performance. Two types of anode flow-field designs were investigated, the single serpentine flow-field (SSFF) and parallel flow-field (PFF). The results show that the PFF design yielded a higher cell performance than the SSFF design at identical operating conditions. Optical visualization shows a strong relationship between the flow path length and the length of gas slugs produced, which in turn influences the flow regime of operation. Longer flow path length in the SSFF results in annular flow regime at a high current density which degrades cell performance. The annular flow regime was absent in the PFF design. It was found the effect of flow rate on performance depends strongly on operating temperature in both flow patterns. Results of this study indicate that long channel length promotes gas accumulation and channel-blocking which degrades performance in PEMWEs.     

Effects of operating conditions on the performance of a micro-tubular solid oxide fuel cell (SOFC)

A parametric analysis is carried out to study the effects of the operating conditions on the performance and operation of a micro-tubular solid oxide fuel cell. The computational fluid dynamics model incorporates mass, momentum, species and energy balances along with ionic and electronic charge transfers. Effects of temperature, fuel flow rate, fuel composition, anode pressure and cathode pressure on fuel cell performance are investigated. Polarization curves are compared to allow an understanding of the effects of different operating conditions on the performance of the fuel cell. Effects of anode flow rate on fuel cell efficiency and fuel utilization are also investigated. Moreover, influence of operating temperature on the internal electronic current leaks is outlined. Temperature distributions, current density profiles and hydrogen mole fraction profiles are also utilized to have a better understanding of the spatial effects of operating parameters. It is predicted that at 550 °C, for an output current demand of 0.53 A cm⁻², fuel cell needs to generate 0.65 A cm⁻² ionic current density where the difference in these values is attributed to internal current leaks. On the other hand for temperatures lower than 500 °C, the effect of electronic leakage currents are not significant.     

Effects of channel geometrical configuration and shoulder width on PEMFC performance at high current density

Computational fluid dynamics analysis was employed to investigate the performance of proton exchange membrane fuel cells (PEMFCs) with different channel geometries at high operating current densities. A 3D, non-isothermal model was used with a single straight channel geometry. Both anode and cathode humidifications were included in the model. In addition, phase transportation was included in the model to obtain the total water management for systems operating at different current densities. The simulation results showed that a rectangular channel cross-section gave higher cell voltages compared with trapezoidal and parallelogram channel cross-sections. However, the trapezoidal channel cross-section facilitated reactant diffusion, leading to more uniform reactant and local current density distributions over the reacting area, and thus to a lower cathode overpotential of the cell. Simulations of the three different channel cross-sections using the same boundary conditions showed that among the cell geometrical parameters, the shoulder width is one of the most influential in terms of its impact on cell performance. Simulations using different channel–shoulder width ratios showed that at high operating current densities, Ohmic losses significantly increase with decreasing shoulder width. In contrast, a smaller shoulder width facilitates the distribution of reactants and helps to reduce concentration losses. The simulations disclosed the existence of an optimum channel–shoulder width ratio that gives the highest cell voltage under high current density operating conditions. Under such conditions, however, the cell performance deteriorated dramatically with decreasing shoulder width, even when higher reactants flow rates and inlet velocities were used.     

NUMERICAL analysis of the effects of current collector plate geometry on performance in a cylindrical PEM fuel cell

In this study, a numerical analysis was conducted to investigate the effects of current collector plate geometry on performance in a cylindrical PEM (Proton Exchange Membrane) fuel cell. For this purpose, 2 anode and cathode current collector plate geometries for each helix channel and straight channel were designed. Current collector plates with different geometries were combined with different sequences, and four different main model fuel cell geometries were created. Accordingly, anode and cathode current collector plates for Model-1, Model-2, Model-3, and Model-4 geometries were determined as straight-straight, helix-helix, straight-helix, and helix-straight, respectively. Using these model geometries, simulations were conducted for three different operating pressures, four different operating flow rates, and ten different operating voltages. It was observed that when helix flow channels were used instead of straight flow channels in current collection plate geometries, the flow density increased by approximately 63.18%. The results also revealed that the current density increased by approximately 206.9% when the fuel cell operating pressure increased. In addition, the power density increased as the operating pressure increased. As the gas flow to anode and cathode increased, a 19.05% increase in the current increase in the pressure difference was observed. As a result, the helix flow channel usage performed better than the straight flow channel for the parameters adopted in this study.     

Numerical investigation of the chemical and electrochemical characteristics of planar solid oxide fuel cell with direct internal reforming

A fully three-dimensional mathematical model of a planar solid oxide fuel cell (SOFC) with complete direct internal steam reforming was constructed to investigate the chemical and electrochemical characteristics of the porous-electrode-supported (PES)-SOFC developed by the Central Research Institute of Electric Power Industry of Japan. The effective kinetic models developed over the Ni/YSZ anode takes into account the heat transfer and species diffusion limitations in this porous anode. The models were used to simulate the methane steam reforming processes at the co- and counter-flow patterns. The results show that the flow patterns of gas and air have certain effects on cell performance. The cell at the counter-flow has a higher output voltage and output power density at the same operating conditions. At the counter-flow, however, a high hotspot temperature is observed in the anode with a non-fixed position, even when the air inlet flow rate is increased. This is disadvantageous to the cell. Both cell voltage and power density decrease with increased air flow rate.     

Configuration effects of air,fuel, and coolant inlets on the performance of a proton exchange membrane fuel cell for automotive applications

The configuration of fuel, air, and cooling water paths is one of the major factors that influence the performance of a proton exchange membrane fuel cell (PEMFC). In order to investigate the effects of these factors, a quasi-three-dimensional dynamic model of a PEMFC has been developed. For validation, simulation results are compared with experimental data in one-flow configuration case and show good agreement with the experimental cell performance data. Five different flow configurations are then simulated to systematically investigate the effects of fuel, air, and cooling channel configuration on the local current and species distribution. Voltage and power vs. current density for five different configurations are compared. The type 1 configuration, which has a fuel–air counter flow and an air-coolant co-flow, has the highest performance in all ranges of current density because the membrane remains the most hydrated. When the operating current density increases, the effects of temperature on membrane hydration slightly decrease. It is confirmed that fuel cell performance improves with increased humidity until flooding conditions appear. An interesting result shows that it is possible to lower the fuel cell operating temperature to improve fuel cell hydration, which in turn improves cell performance. In addition, the different flow configurations are shown to have an effect on the pressure losses and local current density, membrane hydration, and species mole fractions. These results suggest that the model can be used to optimize the flow configuration of a PEMFC.     

Reactants flow behavior and water management for different current densities in PEMFC

Computational fluid dynamics analysis was carried out to investigate the reactants flow behavior and water management for proton exchange membrane fuel cell (PEMFC). A complete three-dimensional model was chosen for single straight channel geometry considering both anode and cathode humidification. Phase transformation was included in the model to predict the water vapor and liquid water distributions and the overall performance of the cell for different current densities. The simulated results showed that for fully humidified conditions hydrogen mole fraction increases along the anode channel with increasing current density, however, at higher current densities it decreases monotonically. Different anode and cathode humidified conditions results showed that the cell performance was sufficiently influenced by anode humidification. The reactants and water distribution and membrane conductivity in the cell depended on anode humidification and the related water management. The cathode channel–GDL (Gas Diffusion Layer) interface experiences higher temperature and reduces the liquid water formation at the cathode channel. Indeed, at higher current densities the water accumulated in the shoulder area and exposed higher local current density than the channel area. Higher anode with lower cathode humidified combination showed that the cell had best performance based on water and thermal management and caused higher velocity in the cathode channel. The model was validated through the available literature.     

Simultaneous direct visualisation of liquid water in the cathode and anode serpentine flow channels of proton exchange membrane (PEM) fuel cells

Water flooding is detrimental to the performance of the proton exchange membrane fuel cell (PEMFC) and therefore it has to be addressed. To better understand how liquid water affects the fuel cell performance, direct visualisation of liquid water in the flow channels of a transparent PEMFC is performed under different operating conditions. Two high-resolution digital cameras were simultaneously used for recording and capturing the images at the anode and cathode flow channels. A new parameter extracted from the captured images, namely the wetted bend ratio, has been introduced as an indicator of the amount of liquid water present at the flow channel. This parameter, along with another previously used parameter (wetted area ratio), has been used to explain the variation in the fuel cell performance as the operating conditions of flow rates, operating pressure and relative humidity change. The results have shown that, except for hydrogen flow rate, the wetted bend ratio strongly linked to the operating condition of the fuel cell; namely: the wetted bend ratio was found to increase with decreasing air flow rate, increasing operating pressure and increasing relative humidity. Also, the status of liquid water at the anode was found to be similar to that at the cathode for most of the cases and therefore the water dynamics at the anode side can also be used to explain the relationships between the fuel cell performance and the investigated operating conditions.     

Experimental investigation of liquid water formation and transport in a transparent single-serpentine PEM fuel cell

Liquid water formation and transport were investigated by direct experimental visualization in an operational transparent single-serpentine PEM fuel cell. We examined the effectiveness of various gas diffusion layer (GDL) materials in removing water away from the cathode and through the flow field over a range of operating conditions. Complete polarization curves as well as time evolution studies after step changes in current draw were obtained with simultaneous liquid water visualization within the transparent cell. The level of cathode flow field flooding, under the same operating conditions and cell current, was recognized as a criterion for the water removal capacity of the GDL materials. When compared at the same current density (i.e. water production rate), higher amount of liquid water in the cathode channel indicated that water had been efficiently removed from the catalyst layer.

Visualization of the anode channel was used to investigate the influence of the microporous layer (MPL) on water transport. No liquid water was observed in the anode flow field unless cathode GDLs had an MPL. MPL on the cathode side creates a pressure barrier for water produced at the catalyst layer. Water is pushed across the membrane to the anode side, resulting in anode flow field flooding close to the H₂ exit.     

Development and performance analysis of a metallic micro-direct methanol fuel cell for high-performance applications

As a promising candidate for conventional micro-power sources, the micro-direct methanol fuel cell (μDMFC) is currently attracting increased attention due to its various advantages and prospective suitability for portable applications. This paper reports the design, fabrication and analysis of a high-performance μDMFC with two metal current collectors. Employing micro-stamping technology, the current collectors are fabricated on 300-μm-thick stainless steel plates. The flow fields for both cathode and anode are uniform in shape and size. Two sheets of stainless steel mesh are added between the membrane electrode assembly (MEA) and current collectors in order to improve cell performance. To avoid electrochemical corrosion, titanium nitride (TiN) layers with thickness of 500 nm are deposited onto the surface of current collectors and stainless steel mesh. The performance of this metallic μDMFC is thoroughly studied by both simulation and experimental methods. The results show that all the parameters investigated, including current collector material, stainless steel mesh, anode feeding mode, methanol concentration, anode flow rate, and operating temperature have significant effects on cell performance. Moreover, the results show that under optimal operating conditions, the metallic μDMFC exhibits promising performance, yielding a maximum power density of 65.66 mW cm⁻² at 40 °C and 115.0 mW cm⁻² at 80 °C.     

Detailed analysis of polymer electrolyte membrane fuel cell with enhanced cross‐flow split serpentine flow field design

Most generally used flow channel designs in polymer electrolyte membrane fuel cells (PEMFCs) are serpentine flow designs as single channels or as multiple channels due to their advantages over parallel flow field designs. But these flow fields have inherent problems of high pressure drop, improper reactant distribution, and poor water management, especially near the U‐bends. The problem of inadequate water evacuation and improper reactant distribution become more severe and these designs become worse at higher current loads (low voltages). In the current work, a detailed performance study of enhanced cross‐flow split serpentine flow field (ECSSFF) design for PEMFC has been conducted using a three‐dimensional (3‐D) multiphase computational fluid dynamic (CFD) model. ECSSFF design is used for cathode part of the cell and parallel flow field on anode part of the cell. The performance of PEMFC with ECSSFF has been compared with the performance of triple serpentine flow design on cathode side by keeping all other parameters and anode side flow field design similar. The performance is evaluated in terms of their polarization curves. A parametric study is carried out by varying operating conditions, viz, cell temperature and inlet humidity on air and fuel side. The ECSSFF has shown superior performance over the triple serpentine design under all these conditions.     

A two-phase flow and transport model for the cathode of PEM fuel cells

A unified two-phase flow mixture model has been developed to describe the flow and transport in the cathode for PEM fuel cells. The boundary condition at the gas diffuser/catalyst layer interface couples the flow, transport, electrical potential and current density in the anode, cathode catalyst layer and membrane. Fuel cell performance predicted by this model is compared with experimental results and reasonable agreements are achieved. Typical two-phase flow distributions in the cathode gas diffuser and gas channel are presented. The main parameters influencing water transport across the membrane are also discussed. By studying the influences of water and thermal management on two-phase flow, it is found that two-phase flow characteristics in the cathode depend on the current density, operating temperature, and cathode and anode humidification temperatures.     

Study on operational aspects of a passive direct methanol fuel cell incorporating an anodic methanol barrier

This paper presents an investigation concerning the effects of operating conditions on the performance of a passive direct methanol fuel cell (DMFC). A self-developed porous metal fiber sintered plate (PMFSP) is used as the methanol barrier between the fuel reservoir and current collector at the anode in order to alleviate the effect of methanol crossover. The effectiveness of using this method is validated. A series of operating conditions such as operating orientation, methanol concentration, ambient temperature, forced air convection and dynamic load are evaluated. Results show that the use of a PMFSP promotes a higher cell performance during vertical operation than horizontal orientation. The effect of methanol concentration depends on the PMFSP porosity. A relatively lower porosity is favorable for high-concentration operation. The cell performance gets improved when increasing the ambient temperature and adopting forced air supply at the cathode. Compared with the traditional structure, the use of a PMFSP makes the fuel cell insensitive to the change of blowing intensity. In addition, the dynamic characteristics of the PMFSP-based passive DMFC are also reported.     

Efficient hydrogen production from aqueous methanol in a PEM electrolyzer with porous metal flow field: Influence of PTFE treatment of the anode gas diffusion layer

In a proton exchange membrane (PEM) methanol electrolyzer, the even supply of reactant to and the smooth removal of carbon dioxide from the anode are very important in order to achieve a high hydrogen production performance. An appropriate design of flow field and gas diffusion layer (GDL) is a key factor in satisfying the above requirements. Previous research has shown that hydrogen production performance of the PEM methanol electrolyzer cell was largely improved with a porous flow field made of sintered spherical metal powder compared with a conventional groove type flow field. Based on this improvement, the current study investigated the influence of polytetrafluoroethylene (PTFE) treatment of the anode GDL on hydrogen production performance of the PEM methanol electrolyzer with porous metal flow fields. Influences of operating conditions such as methanol concentration and cell temperature with the flow field were also investigated.     

Effect of operating parameters and anode diffusion layer on the direct ethanol fuel cell performance

A parametric study was conducted on the performance of direct ethanol fuel cells. The membrane electrode assemblies employed were composed of a Nafion^® 117 membrane, a Pt/C cathode and a PtRu/C anode. The effect of cathode backpressure, cell temperature, ethanol solution flow rate, ethanol concentration, and oxygen flow rate were evaluated by measuring the cell voltage as a function of current density for each set of conditions. The effect of the anode diffusion media was also studied. It was found that the cell performance was enhanced by increasing the cell temperature and the cathode backpressure. On the contrary, the cell performance was virtually independent of oxygen and fuel solution flow rates. Performance variations were encountered only at very low flow rates. The effect of the ethanol concentration on the performance was as expected, mass transport loses observed at low concentrations and kinetic loses at high ethanol concentration due to fuel crossover. The open circuit voltage appeared to be independent of most operating parameters and was only significantly affected by the ethanol concentration. It was also established that the anode diffusion media had an important effect on the cell performance.     

Two-dimensional simulation of cold start processes for proton exchange membrane fuel cell with different hydrogen flow arrangements

Proton exchange membrane (PEM) fuel cells with an off-gas recirculation anode (ORA) or dead-ended anode (DEA) are widely adopted in engineering. However, those two hydrogen flow arrangements may cause anodic water and nitrogen accumulation in comparison with the flow-through anode (FTA) mode, which causes significant performance degradation. In this paper, a two-dimensional cold-start model is developed with detailed consideration of water phase changes and the nitrogen crossover phenomenon. A simplified electrochemical module is built to calculate the current density distribution in the model. The simulation results are consistent with the experimental data at both subzero temperatures and normal operating temperatures. The effects of hydrogen flow arrangements, flow configurations, and startup strategies are investigated during startup process from subzero to normal operating temperatures. Much less ice is generated in counter-flow cases than in co-flow cases during constant current operation. A relatively lower startup voltage can effectively shorten the cold-start process and enhance the cold-start capacity for the PEM fuel cell. The ORA mode has the best hydrogen flow arrangement due to its general abilities, including higher hydrogen utilization efficiency, higher anodic nitrogen tolerance, better output performance and better startup capability.     

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.