Experimental investigation of the thermal response of open-cathode proton exchange membrane fuel cell stack

The aim of this study is to investigate the thermal response characteristics of the proton exchange membrane fuel cell stack. In order to find out the regularities of temperature variation under rapidly increasing load change, a home-made 500 W open-cathode stack embedded with 30 thermocouples was made and tested. The result shows that the local temperature dominates the thermal response at the initial stage while the membrane hydration is the crucial impact factor at low power stage. Further, the anode flooding strongly affects the stability of the output performance and the change of temperature at the overloaded stage. The maximum temperature difference within one cell can reach a steady state faster than that of the temperature. At normal operation, there is little difference between the defined surfaces. The exergy analysis shows that the reaction air will have higher exergy if the temperature variation is more smooth. This experimental study contributes to the optimization of the cooling strategy and thermal management of the open-cathode stack in application.     

Experimental study on the endplate effect on the cold-start performance of an open-cathode air-cooled proton exchange membrane fuel cell stack

The temperature gradient inside an open-cathode air-cooled fuel cell is large because it uses air as its reaction and cooling media; moreover, the temperature of single cells near the endplates is low because of the high heat capacity of the endplate compared to single cells. Therefore, the cold start of open-cathode air-cooled fuel cells is difficult. In this work, the cold-start performance of an open-cathode air-cooled fuel cell stack, including the stack voltage, single-cell voltage and temperature distribution, are tested in a climatic chamber. The results show that the endplate effect has a significant adverse influence on the cold-start performance. Due to the existence of the endplate effect, the voltages of the single cells near the endplate decrease significantly. The stack can be successfully started at −5 °C without any external heating; however, when the temperature decreases below −10 °C, it cannot be started. At this time, if a certain power of endplate heating is adopted, successful cold-start can be achieved. However, if the temperature continues to decrease, the stack cannot be successfully started only through endplate heating because both the endplates and cold air affect the cold-start performance. Combining endplate and air heating may be a feasible cold-start method.     

Air and H2 feed systems optimization for open-cathode proton exchange membrane fuel cells

In this study, air and H₂ feed systems optimization for open-cathode proton exchange membrane fuel cells (PEMFCs) has been evaluated. For air feed system, a spoiler was introduced. The air velocity distribution, polarization curve, single-cell voltage distribution, and temperature distribution of the 11-cell open-cathode fuel cell stack with blowing, blowing-spoiler, and drawing air feed system were assessed. On this basis, the influences of the distance between the fan and stack with different air feed systems were investigated. The results show that the application of the spoiler could solve the problem of low air velocity in the middle of the stack and increase stack performance by 7.3%. And drawing air feed system could enhance the heat dissipation capacity of the stack and the uniformity of temperature distribution, resulting in the 7.9% stack performance increase. Optimization of the distance between the fan and stack enhances the full development of turbulence and the rate of heat transfer. In addition, the effects of four different H₂ feed systems and the flow direction between air and hydrogen on the fuel cell performance were also investigated. It is beneficial for open-cathode PEMFC to be operated with the location of the H₂ inlet and outlet staggered in two different endplates for better stack performance and single-cell voltage uniformity. Evidence also shows that the higher performance also could be obtained when the flow direction of air and hydrogen is vertical with lower ohmic resistance, charge and mass transfer resistance. The study contributes to the design of the open-cathode fuel cell stack to get better performance and reliability.     

质子交换膜燃料电池堆的热力学分析

建立了质子交换膜燃料电池(PEMFC)堆的热力学分析模型,研究了运行温度、气体分压和阳极流量等工作参数对燃料电池堆能量效率和火用效率的影响。结果表明:对气体加压,能提高热力学能效率和火用效率;温度升高时,系统性能无明显变化;阳极流量增加时,系统的热力学能效率和火用效率有所降低。     

Unsteady 2D PEM fuel cell modeling for a stack emphasizing thermal effects

Models currently used for analyses of thermal and water behavior of a PEM fuel cell are based 3D computational fluid dynamics (CFD). However, the analyses are limited to a single cell with static behavior. Thus, these models cannot be used for analyses of dynamic behavior of a stack that continuously varies according to operating conditions. The model proposed describes dynamic behavior of a stack with two adjoining cells and endplate assembly, and work as a current controlled voltage source that can be used for optimization of BOPs and the associated controls. Simulations have been conducted to analyze start-up behaviors and the performance of the stack. Our analyses deliver following results: (1) dynamic temperature distribution in both the through-plane direction and the along channel direction of the fuel cell stack, (2) effects influencing the source terms of current density, and (3) dynamic oxygen concentration distribution. The temperature profile and its variation propensity are comparable to the previous results [Y. Shan, S.Y. Choe, J. Power Sources, 145 (1) (2005) 30–39; Y. Shan, S.Y. Choe, J. Power Sources, in press].     

Dynamic modeling and analysis of a 20-cell PEM fuel cell stack considering temperature and two-phase effects

Dynamic characteristics and performance of a PEM fuel cell stack are crucial factors to ensure safe, effective and efficient operation. In particular, water and heat at varying loads are important factors that directly influence the stack performance and reliability. Herein, we present a new dynamic model that considers temperature and two-phase effects and analyze these effects on the characteristics of a stack.     

Membrane electrode assembly degradation by dry/wet gas on a PEM fuel cell

The vehicle that use a Polymer Electrolyte Membrane Fuel Cell (PEMFC) as a power source frequently experiences start up and shut down. Membrane Electrode Assembly (MEA) degradation by wet/dry gas repetition was studied for vehicle start up and shut down. The time of the wet/dry equilibrium state on the PEMFC was measured with High Frequency Resistance (HFR). The gas injection time was 20 min and 5 min for dry gas and wet gas, respectively. An experiment was carried out using electrochemical methods and a cross-section of the MEA was visualized with a Field Emission Scanning Electron Microscope (FE-SEM). After 1200 wet/dry cycles, the performance of the cell decreased by 45.7% to its current density of 800 mA/cm². Ohmic and charge transfer resistances of the cell increased in the Electrochemical Impedance Spectroscopy (EIS). The crossover current of hydrogen also increased in the linear sweep voltammetry (LSV). The reduction of the electrochemical active surface area (ECSA) was confirmed through cyclic voltammetry (CV). The interface among the membrane, catalyst layer, and gas diffusion layer was separated and significantly deteriorated compared with fresh MEA.     

Simulation and experimental analysis of the clamping pressure distribution in a PEM fuel cell stack

High performance and efficiency are often reported in single-cell polymer electrolyte membrane (PEM) fuel cell (FC) experiments. This however, can reduce substantially when moving from single-cell experiments to multiple cells. Fuel cell performance is degraded for many reasons when adding cells, but; possibly the most important, is contact resistance between the bipolar plate and gas diffusion layer (GDL). Contact resistance is in direct relation to the clamping configuration and clamping pressure applied to a FC stack. Simulation of a single cell and 16-cell FC was performed at various clamping pressures resulting in detailed 3D plots of stress and deformation. The stress on the GDL, for any value of clamping pressure simulated in this study, is around 1.5 MPa for the 16-cell stack and around 4 MPa in single cell simulations. Experimental testing of clamping pressure effects was performed on a 16-cell stack by placing a thin pressure-sensitive film between GDL and bipolar plate. Clamping pressure was applied using various loads, durations, and two types of GDLs. The results from experimental testing show that pressure on the GDL is in the range of 0–2.5 MPa. When using rectangular cells, experimental results show nearly zero pressure in the center of each cell and the center cells of the stack, regardless of clamping method.     

Electrode structure effects on the performance of open-cathode proton exchange membrane fuel cells: A multiscale modeling approach

In this paper we present a new dynamic multiscale model of an open-cathode Polymer Electrolyte Membrane Fuel Cell (PEMFC). The model describes two-phase water transport, electrochemistry and thermal management within a framework that combines a Computational Fluid Dynamics (CFD) approach with a micro-structurally-resolved model predicting the water filling dynamics of the electrode pores and the impact of these dynamics on the evolution of the electrochemically active surface area (ECSA). The model allows relating for the first time the cathode electrode structure to the cell voltage transient behavior during experimental changes in fuel cell temperature. The effect of evaporation rates, desorption rates and temperature changes on the performance of four different electrode pore size distributions are explored using steady-state and transient numerical simulations. The results are discussed with respect to water management and temperature control.     

Improving open-cathode polymer electrolyte membrane fuel cell performance using multi-hole separators

We developed a new separator with a multi-hole structure (MHS) in the rib region for open-cathode polymer electrolyte membrane fuel cell (OC-PEMFC) stack to improve performance. The electrochemical current–voltage performance results clearly demonstrate that the performance of the OC-PEMFC stack using the MHS design was higher than that using the conventional parallel design at high current regions (i.e., over 7 A). The current increased by 11.24% at 12 V (i.e., 0.6 V/cell). The effects of supplying additional oxygen and removing generated water were identified as factors improving the performance. The individual cell voltages demonstrate that the initial value of standard deviation for the OC-PEMFC stack using MHS was somewhat high, but it exhibited better uniformity at higher current regions.     

Turbulent flow in the distribution header of a PEM fuel cell stack

A numerical investigation of the flowfield in a model distribution header manifold of a polymer electrolyte membrane fuel cell stack is conducted. The computational model simulates two segments of an experimental setup of a pair of model headers which replicate the headers of a fuel cell stack. The model headers consist of an inlet and outlet sections connected with a plate containing an array of holes that replicate the unit cells. The flow structures in the outlet header are rather complex and are the result of the superposition of a series of impinging jets in a confined space in the presence of crossflow. The flow from each hole, which represents an individual cell outlet, enters the outlet header as a jet stream and is subjected to a crossflow. Large Eddy Simulations (LES) are performed for a portion of the outlet header to investigate the complex turbulent flow and related structures under different crossflow conditions, and are complemented by Particle Image Velocimetry (PIV) measurements. The LES results show that two large vortical structures are formed in the header cross-section, with a high-speed round jet from the cell outlet holes forcing a diversion of the crossflow, dividing it into two separate branches. Investigation of the flow restructuring after a blockage of one of the jets is performed. Simulation results using a slot opening for the jet show flow instabilities. The results of this study highlight the unsteady and highly turbulent nature of the flow in the header and provide a characterization of the complex three-dimensional structure of the flow. The flowfield and flow structures may impact the overall pressure drop along the header and the effective cross-sectional area for the flow leaving the header. The observations and insights obtained from the LES simulation and PIV measurements point to the need to further investigate the impact on flow sharing in a stack of the flowfield development in the outlet header.     

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.     

Microcontroller-based emulation of a PEM fuel cell

This paper proposes an accurate and easy-to-implement emulator which is able to track the characteristic curve of a Proton Exchange Membrane Fuel Cell (PEMFC). Such an emulator is based on a low-cost microcontroller , isolated voltage and current sensors. The proposed emulator takes advantages of the flexibility and robustness. The sensed voltage and current provide to the microcontroller the accurate information to compute the output voltage of an actual PEMFC. The obtained voltage is sent to a digital-to-analog converter in order to command the continuous control voltage. The simplified electrochemistry model is presented and validated through simulation and then corroborated via experimentation. The proposed emulator is subjected to two load conditions: fixed load resistor and power electronic converter. The employed power converter is controlled by a variable duty cycle which is adjusted to a value at which the power extracted from the emulator is maximum.     

Performance evaluation of PEM fuel cell stack on hydrogen produced in the oil refinery

The global progress in pushing the clean energy initiatives can be seen both as a challenge and opportunity for energy companies. Inclusion of “Hydrogen” as a value-added product from the refineries and use of fuel cells in the energy mix of an oil company, presents such an opportunity. However, the refinery hydrogen is not a fuel cell grade due to inherent impurities like carbon monoxide, carbon dioxide and methane are associated owing to the hydrocarbon source of production. In this study, a Low-temperature PEM fuel cell stack with 30 cells and active area of 50 cm² was optimized on neat hydrogen in terms of its operating parameters including temperature, pressure and relative humidity. Upon optimization, the contamination tests were performed by doping various concentrations contaminants for simulating the refinery hydrogen composition. The results of performance evaluation with neat hydrogen and the impact of different contaminants are discussed in this study.     

Computational study of forced air-convection in open-cathode polymer electrolyte fuel cell stacks

A mathematical model for a polymer electrolyte fuel cell (PEFC) stack with an open-cathode manifold, where a fan provides the oxidant as well as cooling, is derived and studied. In short, the model considers two-phase flow and conservation of mass, momentum, species and energy in the ambient and PEFC stack, as well as conservation of charge and a phenomenological membrane and agglomerate model for the PEFC stack. The fan is resolved as an interfacial condition with a polynomial expression for the static pressure increase over the fan as a function of the fan velocity. The results suggest that there is strong correlation between fan power rating, the height of cathode flow-field and stack performance. Further, the placement of the fan - either in blowing or suction mode - does not give rise to a discernable difference in stack performance for the flow-field considered (metal mesh). Finally, it is noted that the model can be extended to incorporate other types of flow-fields and, most importantly, be employed for design and optimization of forced air-convection open-cathode PEFC stacks and adjacent fans.     

Computational analysis of fuel saving by using porous-end configuration for a PEM fuel cell

In the present study, a novel porous-end PEMFC inspired by the characteristics of open-end and dead-end PEMFCs is proposed for fuel saving. For this purpose, a porous media region with a certain thickness is added to the outlet region of the anode channel of an open-end PEMFC. The effect of porous media thickness at the anode channel on the current density and hydrogen mass flow was numerically analyzed. Results indicate that in comparison to the base model PEMFC, the presence of porous media at the end of the anode channel of porous-end PEMFC leads to an increase in the pressure and a decrease in the velocity magnitude in the anode channel. Results illustrate that the porous-end PEMFC with t = 1 mm thickness can be an adequate choice to gain an optimum design for the porous-end configuration. This conclusion becomes more highlighted when the results give the 66.17% reduction in fuel consumption.     

Improvement of thermal management of proton exchange membrane fuel cell stack used for portable devices by integrating the ultrathin vapor chamber

UVC (ultrathin vapor chamber) simultaneously has a high heat-conducting property, excellent temperature uniformity and simple structure. These advantages are very suitable for thermal management of the open-cathode PEMFC (proton exchange membrane fuel cell) stack. In this work, two-type UVCs with different appearances are integrated into a conventional PEMFC stack respectively. The effect of UVC on the output performance, thermal management and operating stability is investigated by the experiment combined with simulation. The results show that UVC can significantly increase the output voltage under high current density. In 35 A, the output voltage of the stack integrated the vertical UVC increases by 20.25% relative to the conventional stack. Thermal management is also improved by UVC. The highest temperature inside the stack decrease by 9 °C in 35 A, and the membrane temperature is decreased obviously. But it still exceeds the optimal operating temperature of open-cathode PEMFC stack due to the poor cooling type in the condensation side of UVC. UVC improves the operation stability of the stack and slows the deteriorative speed of output performance. This work hopes to attract more attention to the application of UVC on the thermal management of portable power sources used open-cathode PEMFC stack.     

Degradation analysis of the core components of metal plate proton exchange membrane fuel cell stack under dynamic load cycles

The durability of metal plate proton exchange membrane fuel cell (PEMFC) stack is still an important factor that hinders its large-scale commercial application. In this paper, we have conducted a 1000 h durability test on a 1 kW metal plate PEMFC stack, and explored the degradation of the core components. After 1000 h of dynamic load cycles, the voltage decay percentage of the stack under the current densities of 1000 mA cm^?2 is 5.67%. By analyzing the scanning electron microscopy (SEM) images, the surfaces of the metal plates are contaminated locally by organic matter precipitated from the membrane electrode assembly (MEA). The SEM images of the catalyst coated membrane (CCM) cross section indicate that the MEA has undergone severe degradation, including the agglomeration of the catalyst layer, and the thinning and perforation of the PEM. These are the main factors that cause the rapid increase in hydrogen crossover flow rate and performance decay of the PEMFC stack.     

AC impedance characteristics of a 2 kW PEM fuel cell stack under different operating conditions and load changes

This paper mainly presents the AC impedance characteristics of a 2 kW PEMFC stack under different operating conditions and load changes. The AC impedances of the fuel cell stack are examined by a fuel cell impedance meter. Air stoichiometry, air humidity, and operation temperature are shown to have significant effects on the AC impedance of stack. When air stoichiometry decreases, the mass transfer resistance of stack increases obviously, but the influences on other resistances are very slight. The air humidity and operation temperature mainly influence the charge transfer resistance of stack. The influences of load changes on the AC impedance of stack are also investigated, and the results of which show that it is quite necessary to adjust the humidity of reactant gas according to the fuel cell load changes during fuel cell running. The AC impedance diagnosis of stack can provide some useful information for the running of fuel cell stack.     

Experimental analysis of pulsing techniques in a proton exchange fuel cell

The purpose of this study is to investigate the impact of pulsing reactant flows on the performance of a PEMFC at low current density. This study considers a full range of pulsing flows and their effect in voltage over time. The factors evaluated were voltage, pressure, and flow rates of each reactant flow over time. A specific current density was set for the experiments. The experiments were performed at lower flow rates and temperatures of reactants than in standard operating conditions. The experiments used constant temperature of reactants as well as constant relative humidity. Comparison made between continuous flow and several sets of pulsing flows for hydrogen and air were developed. Pulsing of reactants opens an opportunity as a practical water management procedure. In addition, this technique helps extending performance range on PEMFC when a limited amount of reactants is supplied. The data collected was presented in graphical form.