A critical review of cooling techniques in proton exchange membrane fuel cell stacks

Effective cooling is critical for safe and efficient operation of proton exchange membrane fuel cell (PEMFC) stacks with high power. The narrow range of operating temperature and the small temperature differences between the stack and the ambient introduce significant challenges in the design of a cooling system. To promote the development of effective cooling strategies, cooling techniques reported in technical research publications and patents are reviewed in this paper. Firstly, the characteristics of the heat generation and cooling requirements in a PEMFC stack are introduced. Then the advantages, challenges and progress of various cooling techniques, including (i) cooling with heat spreaders (using high thermal conductivity materials or heat pipes), (ii) cooling with separate air flow, (iii) cooling with liquid (water or antifreeze coolant), and (iv) cooling with phase change (evaporative cooling and cooling through boiling), are systematically reviewed. Finally, further research needs in this area are identified.     

The study on transient characteristic of proton exchange membrane fuel cell stack during dynamic loading

The operations of fuel cell stacks in fuel cell vehicle are dynamic. During dynamic loading, the oxidant starvation often occurs, due to the gas response rate lagging the loading rate. To study the transient behavior of the fuel cell stack at load changes, the measuring methods of current and temperature distribution are developed. In this paper, the current distribution and temperature distribution as well as their dynamic changes in fuel cell stack have been evaluated in situ. The experimental results show that the local current and temperature rise when load rapidly. The extent of temperature fluctuation during dynamic loading is significantly influenced by air stoichiometries, loading rates, and air relative humidities. When air stoichiometry is very low, the temperature of cathode inlet rises sharply. The quicker the loading rate is, the bigger the extent of temperature fluctuation is. With increasing air relative humidity, the transient temperature of cathode inlet rises, while the transient temperature of cathode outlet decreases. This paper will provide reference for durability researches on fuel cell vehicles (FCVs).     

Experimental and modeling study on water dynamic transport of the proton exchange membrane fuel cell under transient air flow and load change

Water management is important in the proton exchange membrane fuel cell (PEMFC) operations, especially for those cells based on sulfonic acid polymers due to the depending of the conductivity on water. This paper aims at illustrating the effect of the change in membrane water content on cell potential response. For this purpose, the cell potential response has been investigated experimentally and computationally under transient air flow and load change of a PEMFC. From the experimental and computational results, an undershoot behavior of cell potential as well as the great influence of the relative humidity on the magnitude of undershoot is observed. It is found that the magnitude of cell potential undershoot increases as the relative humidity decreases. By carrying out a transient simulation on the water content of the membrane, the undershoot phenomena could be well explained. It is also found from the computational prediction that the time scale for the cell potential to reach its steady state is about 20 s, in agreement with experimental results. The model prediction also suggests that the dynamic behavior of PEMFC is critically dependent on the water content in the membrane.     

Simultaneous measurement of current and temperature distributions in a proton exchange membrane fuel cell

Using a specially designed current distribution measurement gasket in anode and thin thermocouples between the catalyst layer and gas diffusion layer (GDL) in cathode, in-plane current and temperature distributions in a proton exchange membrane fuel cell (PEMFC) have been simultaneously measured. Such simultaneous measurements are realized in a commercially available experimental PEMFC. Experiments have been conducted under different air flow rates, different hydrogen flow rates and different operating voltages, and measurement results show that there is a very good correlation between local temperature rise and local current density. Such correlations can be explained and agree well with basic thermodynamic analysis. Measurement results also show that significant difference exists between the temperatures at cathode catalyst layer/GDL interface and that in the center of cathode endplate, which is often taken as the cell operating temperature. Compared with separate measurement of local current density or temperature, simultaneous measurements of both can reveal additional information on reaction irreversibility and various transport phenomena in fuel cells.     

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.     

Effect of gas composition and gas utilisation on the dynamic response of a proton exchange membrane fuel cell

The transient response of a proton exchange membrane fuel cell (PEMFC) was measured for various cathode gas compositions and gas utilisations (fraction of supplied reactant gas which is consumed in the fuel cell reaction). For a PEMFC operated on pure hydrogen and oxygen, the cell voltage response to current steps was fast, with response times in the range 0.01–1 s, depending on the applied current. For a PEMFC supplied with air as cathode gas, an additional relaxation process related to oxygen transport caused a slower response (approximately 0.1–2 s depending on the applied current). Response curves up to approximately 0.01 s were apparently unaffected by gas composition and utilisation and were most likely dominated by capacitive discharge of the double layer and reaction with surplus oxygen residing in the cathode. The utilisation of hydrogen had only a minor effect on the response curves, while the utilisation of air severely influenced the PEMFC dynamics. Results suggested that air flow rates should be high to obtain rapid PEMFC response.     

Sliding-mode-based temperature regulation of a proton exchange membrane fuel cell test bench

The temperature regulation of a cooling system of a PEMFC (Proton Exchange Membrane Fuel Cell) test bench is studied in this paper. Because of the unique configuration which is dedicated for cold start experiments, the operation at nominal temperature is unstable with a simple PI controller. A sliding-based control strategy is applied to suppress the temperature fluctuation. Firstly the structure of the cooling system is demonstrated and the cause of temperature fluctuation is analyzed. Then, a physics-based model of the cooling system is proposed on the Matlab/Simulink platform and validated with experimental data. Based on the model, a Sliding-mode controller with Extended Kalman Filter (EKF) is designed to regulate the temperature. The simulation results showed that the controlled system performed satisfactorily. Furthermore, when applied to the real system, the controller's real-time performance fulfills the test bench criterion. Experimental data show that the coolant temperature at the outlet of the fuel cell stack is kept in a range within ±1 °C, disregarding the heat generated at various working condition.     

A review on water balance in the membrane electrode assembly of proton exchange membrane fuel cells

Water balance has been proven to be critical not only for the performance but also for the durability of proton exchange membrane fuel cells (PEMFCs). This paper reviews experimental investigations and modeling works on water transport and balance in different constituents of the membrane electrode assembly (MEA), which is the most important component determining the performance and durability of a PEMFC. Major water transport mechanisms in the membrane and porous layers of MEA are summarized and the strategies to balance water in these components are also discussed. However, the experimental water transport data for different components under varied operating conditions are still insufficient and the understanding of transport mechanisms is still limited. To obtain better water management in PEMFCs, the design of the key components requires refinements. For future investigations more attention should be paid to the fundamental understanding and systematic data of water transport in each component of the MEA under varied operating conditions.     

Effect of operating parameters on dynamic response of water-to-gas membrane humidifier for proton exchange membrane fuel cell vehicle

An analytic multi-dimensional dynamic model of a membrane type humidifier has been developed for the study of transient responses of the humidifier under proton exchange membrane fuel cell vehicle operating conditions. The dynamic responses of heat and mass transfer and fluid flow in a membrane humidifier are mathematically formulated and modeled with a newly developed pseudo-multi-dimensional concept. The model is used to analyze the performance of the humidifier under various operating conditions and the dynamic response of the humidifier under transient operating conditions. The simulation results show that, in the case of the water-to-gas type membrane humidifier modeled in this study, the time constant of water diffusion in the membrane is less than 1 s. Thus, the delay of the response of the humidifier induced by the vapor diffusion in the membrane is not significant in vehicle operation. However, it is also found that the dynamic behavior is mainly due to the thermal resistance and heat capacity of the membrane humidifier.     

Transient behavior of water generation in a proton exchange membrane fuel cell

The effect of water generation on the performance of proton exchange membrane fuel cell (PEMFC) was investigated by using a periodical linear sweep method. Three different kinds of I–V curves were obtained, which reflected different amount of water uptake in the fuel cell. The maximum water uptake that could avoid flooding in the fuel cell and the hysteresis of water diffusion were also discussed. Quantitative analysis of water uptake and water transport phenomena in this study were conducted both experimentally and theoretically. Results showed that the water uptake capacity for the fuel cell under no severe flooding was 27.837 mg cm⁻². The transient response of the internal resistance indicated that the high frequency resistance (HFR) lagged the current with a value of about 20 s. The effect of purging operation on the internal resistance of the fuel cell was also explored. Experimental data showed that the cell experienced a continuous 8-min purging process can maintain at a relatively steady and dry state.     

3D non-isothermal dynamic simulation of high temperature proton exchange membrane fuel cell in the start-up process

High temperature proton exchange membrane fuel cell (HT-PEMFC) with phosphoric acid doped polybenzimidazole (PBI) electrolyte shows multiple advantages over conventional PEMFC working at below 373 K, such as faster electrochemical kinetics, simpler water management, higher carbon monoxide tolerance. However, starting HT-PEMFC from room temperature to the optimal operating temperature range (433.15 K–453.15 K) is still a serious challenge. In present work, the start-up strategy is proposed and evaluated and a three-dimensional non-isothermal dynamic model is developed to investigate start-up time and temperature distribution during the start-up process. The HT-PEMFC is preheated by gas to 393.15 K, followed by discharging a current from the cell for electrochemical heat generation. Firstly, different current loads are applied when the average temperature of membrane reaches 393.15 K. Then, the start-up time and temperature distribution of co-flow and counter-flow are compared at different current loads. Finally, the effect of inlet velocity and temperature on the start-up process are explored in the case of counter-flow. Numerical results clearly show that applied current load is necessary to reduce start-up time and just 0.1 A/cm² current load can reduce startup time by 45%. It is also found that co-flow takes 18.8% less time than counter-flow to heat membrane temperature to 393.15 K, but the maximum temperature difference of membrane is 39% higher than the counter-flow. Increasing the inlet gas flow velocity and temperature can shorten the start-up time but increases the temperature difference of the membrane.     

Demonstration of a residence time distribution method for proton exchange membrane fuel cell evaluation

The volume sensitive residence time distribution method is ideally suited for the study of liquid water and ice formation within operating proton exchange membrane fuel cells. Sensitivity was demonstrated with the use of simulated water drops within the flow field channel (machined obstructions) yielding a linear correlation in the 0–20% volume obstruction range between measured and theoretical hydraulic volumes. The correlation was independent of obstruction spatial distribution but dependent on gas flow rate. Sensitivity was also demonstrated by varying the amount of liquid water within a gas diffusion electrode resulting in a linear correlation in the 7–44% void volume obstruction range between normalized time difference between the points at which the tracer concentration has decayed by 20 and 90% of the steady-state value prior to the tracer injection interruption and measured gas diffusion electrode liquid water content. Sensitivity to liquid water obstructions was maintained using an operating fuel cell and two different gas diffusion media with relatively similar transport properties but further work is needed to separate flow field from gas diffusion electrode contributions. The usefulness of the residence time distribution is also demonstrated for other applications, including gas crossover through the proton exchange membrane, flow distribution uniformity and gas diffusion electrode compressibility/deformation.     

Water management in a single cell proton exchange membrane fuel cells with a serpentine flow field

Gas and water management is the key to achieving good performance from a polymer electrolyte membrane fuel cell (PEMFC) stack. Imbalance between production and evaporation rates can result in either flooding of the electrodes or membrane dehydration, both of which severely limit fuel cell performance. In the present study, a mathematical model was developed to evaluate moisture profiles of hydrogen and air flows in the flow field channels of both the anode and the cathode. For model validation, a single fuel cell was designed with an active area of 200 cm². Six humidity sensors were installed in the flow fields of both the anode and the cathode at 457 mm, 1266 mm and 2532 mm from the inlets. The experiment was performed using an Arbin Fuel Cell Test Station. The temperature was varied (25 °C, 40 °C, 50 °C and 60 °C), while hydrogen and air velocities were fixed at 3 L min⁻¹ and 6 L min⁻¹, respectively, during the operation of the single cell. The feed relative humidity at the anode was fixed at 1.0, while the feed relative humidity at the cathode was fixed at 0.005 (dry air). All humidity sensor readings were taken at steady state after 2 h of operation. Model predictions were then compared with experimental results by using the least squares algorithm. The moisture content was found to decrease along the flow field at the anode, but to increase at the cathode. The moisture content profile at the anode was shown to depend on the moisture Peclet number, which decreased with temperature. On the other hand, the moisture profile at the cathode was shown to depend on both the Peclet number and the Damkohler number. The trend of the Peclet number in the cathode followed closely that of the anode. The Damkohler number decreased with temperature, indicating increasing moisture mass transfer with temperature. The moisture profile models were successfully validated by the published data of the estimated overall mass transfer coefficient and moisture effective diffusivity of the same order of magnitude. The strategy of saturating the hydrogen feed and using dry air, as in the present work, has been shown to successfully prevent water droplet formation in the cathode, and hence prevent flooding.     

Effects of operating parameters on the transient response of proton exchange membrane fuel cells subject to load changes

Water management is critical to the steady state and transient performance of proton exchange membrane (PEM) fuel cell systems. The dynamic behavior of PEM fuel cells is profoundly related to the dynamics of water transport in the fuel cell system, which include electro-osmotic drag and back-diffusion of water through membrane and rate at which water is generated, supplied and removed from the electrodes. In particular, for low humidity operations, water transport dynamics plays a dominant role in determining the time taken to reach steady state. Toward an understanding of the fuel cell dynamics for dry operations, numerical simulations are carried out for a single channel polymer electrolyte (PEM) fuel cell undergoing step change in cell voltage over wide range of operating conditions. A detailed model-based parametric study is carried out to analyze the effects of operating conditions on membrane water content and the time taken by the single cell system to reach steady state. Based on the studies, design windows are presented that limit the time taken to reach steady state to a desired value. Optimum parameter combinations that minimize the transient time are also identified from the studies.     

Numerical study on heat transfer enhancement of a proton exchange membrane fuel cell with the dimpled cooling channel

As the power density of a proton exchange membrane fuel cell (PEMFC) increases, the problems of internal heat accumulation and non-uniform temperature distribution are becoming significant. In this paper, a novel cooling channel with dimple structures is designed and a three-dimensional PEMFC numerical model is established. When comparing to the conventional channels, the heat transfer performance of dimpled channel is 10% higher than the smooth one, and the pressure loss is almost 13% lower than that of wavy channel. In addition, the optimization of dimple structure parameters is investigated based on the index of uniformity temperature (IUT) and performance evaluation criteria (PEC) of heat transfer. It is found that a diameter-to-depth ratio of 4 is recommended when the dimple diameter is less than 0.80 mm. Furthermore, the clock-wise vortex observed inside the dimple is considered to be the main reason affecting heat exchange. This study will contribute to the design of cooling channels for high-power density PEMFCs in the future.     

Degradation study of high temperature proton exchange membrane fuel cell under start/stop and load cycling conditions

The unreliable durability of high temperature proton exchange membrane fuel cells (HT-PEMFCs) is one of the restriction factors upon the commercialization process. In this paper, 600-h accelerated stress tests (ASTs) were performed on HT-PEMFCs to investigate the performance degradation under start/stop, load cycling (0.2–0.8 A cm^?2 and 0.04–0.2 A cm^?2) conditions. The activation, ohmic and mass transport polarization losses were determined in combination with the electrochemical impedance spectroscopy (EIS) and Tafel slope analysis, and the degradation modes such as carbon corrosion, catalyst degradation, acid loss and membrane degradation were analyzed by linear sweep voltammetry (LSV), cyclic voltammetry (CV) and transmission electron microscopy (TEM) characterization. The polarization curves of ASTs show that load cycling between 0.04 and 0.2 A cm^?2 extensively aggravates performance degradation, indicating that high-potential operation has the greatest detrimental impact on the durability. By comparing the changes of the activation, ohmic and mass transport polarization losses, it can be found that the activation polarization is the dominant factor leading to performance degradation in the 600-h test, and the ohmic polarization has little effect on the performance degradation. After the test at load cycling between 0.2 and 0.8 A cm^?2, the membrane is seriously degraded and the ohmic resistance increases significantly, and the severe acid loss becomes critical issue in the long-term durability of HT-PEMFC.     

Water transport study in a high temperature proton exchange membrane fuel cell stack

A study of water transport in a high temperature phosphoric acid doped polybenzimidazole (PBI) membrane fuel cell stack is reported. Tests with different stoichiometries of dry cathode and different humidity levels of anode are performed. It is found that water transport across the membrane electrode assembly (MEA) is noteworthy and that water vapor partial pressure on the anode outlet is almost always higher than on the cathode outlet, even when using dry hydrogen. The water transport is a strong function of current density but it also depends on stoichiometry and humidity level. In a series of tests with dry nitrogen on one side and humid nitrogen on the other side, the membrane's water permeability coefficient is determined to be 2.4 × 10⁻¹³ mol s⁻¹ cm⁻¹ Pa⁻¹ at 160 °C which is more than an order of magnitude higher than the values previously reported in the literature. Also, the results indicate that the permeability coefficient might be relative humidity dependent and could even be somewhat higher than the value reported here, but further investigation is needed. The experimental findings are reproduced and explained with a 2D steady state computational fluid dynamics (CFD) model. Internal water transport profiles across the membrane and along the gas flow channels are presented and discussed.     

Design of an integrated power system using a proton exchange membrane fuel cell

Integrated power systems could be a solution to provide energy to remote communities based on the use of renewable energies (such as wind or sun). This work proposed the design of one of those systems including alkaline water electrolysers, storage tanks and a proton exchange membrane fuel cell for generating of 53 kW (working at 60% of its maximum power). Electrode sizes and the quantity of unit cells proposed in this work were the same as those suggested in the research work by Yang et al., where a phosphoric acid fuel cell was built and studied. The results obtained in that research allowed comparing energy efficiency by scaling a laboratory prototype. The dimensions of the alkaline water electrolysers are the result of satisfying the necessity of fuel and oxidant. The energy consumption results from extrapolating laboratory devices. The integrated power system has a storage tank capacity of 16 h.     

Effect of surface dynamic wettability in proton exchange membrane fuel cells

It is known that the static contact angle reflecting the “contact area” between liquid and solid is insufficient to represent the dynamic wettability of a solid surface, and another parameter called the sliding angle is needed to describe the relative easiness of liquid moving on a solid surface. However, sliding angle has been largely neglected in the previous studies for proton exchange membrane fuel cell (PEMFC). In this study, three-dimensional multiphase simulations are carried out for a PEMFC with single straight flow channels considering both the static contact angles and sliding angles of gas diffusion layer (GDL) and catalyst layer (CL). The results show that the liquid water volume fraction in cathode CL (CCL) and GDL (CGDL) can be increased by several times when the sliding angle is increased while the static contact angle is kept constant. This could have significant implication on the water management strategy due to the considerable changes in the water transport and removal processes. Since GDL is much thicker than CL, changing the surface dynamic wettability of GDL has more significant effect on liquid water transport than changing the surface dynamic wettability of CL.     

Effect of the micro porous layer design on the dynamic performance of a proton exchange membrane fuel cell

The mass transport characteristics of a gas diffusion layer (GDL) predominantly affect the performance of a proton exchange membrane (PEM) fuel cell. However, studies examining the transient response related to the GDL are insufficient, although the dynamic behavior of a PEM fuel cell is an important issue. In this study, the effects of the design of a micro porous layer (MPL) on the transient response of a PEM fuel cell are investigated. The MPL slurry density and multiple functional layers are treated as the variable design parameter. The results show that the transient response is determined by the capillary pressure gradient through the GDL. The trade-off relation for the PEM fuel cell performance under low and high humidity conditions due to the hydrophobic GDL is mitigated by designing a reverse capillary pressure gradient in the MPL.