Experimental investigation on the voltage uniformity for a PEMFC stack with different dynamic loading strategies

The operating life of the proton exchange membrane fuel cell stack is mainly decided by performances of its weakest single cell because of the “Buckets effect”, thus high voltage uniformity during a dynamic loading process is key to the stack durability. In this work, a 3-kW stack is examined experimentally on its voltage uniformity (voltage coefficient variation (Cv)) under conditions of loading from open-circuit state (0 A) to nominal current (165 A) and stack temperatures of 30 °C, 45 °C and 65 °C. Different dynamic loading strategies, namely constant loading rate strategy, decreasing loading rate strategy, and increasing loading rate (square/cube increasing loading rate) strategy, are examined and compared. Results display that during the loading process, (a) the voltage uniformity rises abruptly and goes down quickly when the loading current is small (e.g. from 0 A to 22 A), (b) the voltage uniformity under a small loading current is better than that under the open-circuit state, and (c) voltage uniformity decreases as the loading current increases from a small value to the nominal current. Comparisons of different current loading strategies show that as the stack temperature rises from 30 °C to 65 °C, the stack Cv value under the open-circuit state increases from 1.12 to 1.84 and decreases from 3.85 to 2.45 in the nominal current state. The maximum Cv for the decreasing loading rate strategy decreases from 16.25 to 9.49 and that of the constant loading rate strategy also decreases from 5.85 to 4.96. Cv values of the square current increasing loading rate strategy keep below 3.85 under conditions of the three stack temperatures and display a slight fluctuation during the whole current loading process, which indicates that the strategy can effectively make the stack being of an excellent voltage uniformity during the instantaneous response process.     

Experimental investigation on statistical characteristics of cell voltage distribution for a PEMFC stack under dynamic driving cycle

The proton exchange membrane fuel cell (PEMFC) stack consists of individual cells in series. Its operating life is subjected to performance of the weakest cell because of the short-board effect, thus voltage uniformity during dynamic long-running process is significant to its durability. In this work, based on a 1044 h aging experiment on a 6.55 kW PEMFC stack under dynamic driving cycle, voltage uniformity is analyzed. In a single cycle, voltage uniformity becomes worse with the increase of loading current and there are some local maxima of voltage coefficient variation (C_v) at the moment of loading or unloading step. C_v value at higher current is greater and increases faster with cycles. At the end of experiment, C_v at 135 A is more than 6%. Besides, skewness (S_k) is used to evaluate the skew direction and degree of cell voltage data in a cycle. In most cycles, S_k values at 34.22 A are above 0 and S_k values at 59.70 A and 135 A are below 0. After the Box-Cox transformation, which is used to improve symmetry of data and reduce S_k, the cell voltage data have passed the verifications of normal fitting, probability-probability plot and quantile-quantile plot. Therefore, it is found that cell voltage data tend to obey skewed normal distribution, which is of positive significance for improving voltage uniformity and durability of PEMFC stack.     

Quantitative analysis on cold start process of a PEMFC stack with intake manifold

To systematically explore the low-temperature operating characteristics of polymer electrolyte membrane fuel cell (PEMFC) stack, a three-dimensional PEMFC stack model with intake manifold is developed in this study. The characteristics of different cold start modes in the stack are compared and analyzed. The distribution and transmission characteristics of water, ice, and heat in each cell of the stack are analyzed in detail. The location of water accumulation in each cell of the stack is also explored. Finally, finite difference sensitivity is calculated for the cumulated charge transfer density to quantify the effects of operating parameters on the cold start process at low temperature. And how these parameters affect the operation of the PEMFC stack at low temperature is investigated. The results show that inconsistency exists in stack operation due to the position particularity of the intermediate cell. Irreversible heat is the main heat source for the cold start of the stack, and the cathode catalyst layer is the main heat-generating component. The heat production proportion of cathode catalyst layer can reach 90%, which decreases with the increment of current density and the running time, especially for the edge cell. The initial ionomer water content is most sensitive to the cold start process of the stack, followed by the porosity of cathode catalyst layer. These parameters are sensitive to the cold start process mainly because of the change in volumetric exchange current density and oxygen concentration.     

High performance PEMFC stack with open-cathode at ambient pressure and temperature conditions

An open-air cathode proton exchange membrane fuel cell (PEMFC) was developed. This paper presents a study of the effect of several critical operating conditions on the performance of an 8-cell stack. The studied operating conditions such as cell temperature, air flow rate and hydrogen pressure and flow rate were varied in order to identify situations that could arise when the PEMFC stack is used in low-power portable PEMFC applications. The stack uses an air fan in the edge of the cathode manifolds, combining high stoichiometric oxidant supply and stack cooling purposes. In comparison with natural convection air-breathing stacks, the air dual-function approach brings higher stack performances, at the expense of having a lower use of the total stack power output. Although improving the electrochemical reactions kinetics and decreasing the polarization effects, the increase of the stack temperature lead to membrane excessive dehydration (loss of sorbed water), increasing the ohmic resistance of the stack (lower performance).     

Evidence of a non-dimensional parameter controlling the flooding of PEMFC stack

Water management is a key issue to get satisfactory and stable Polymer exchange membrane fuel cell (PEMFC) performances. The work reported in the present paper focuses on the determination of the operational conditions when using PEMFC stack working with ambient air without extra humidification. The objectives are to reduce as much as possible the auxiliaries consumptions. As far as the reaction air blower is concerned, the specific goal of the present tests is to find the minimum air flow rate to feed the PEMFC stack in order to prevent flooding. Our particular interest concerns the control of a PEMFC stack to power a prototype vehicle for the Shell Eco Marathon race.     

Experimental study on rapid cold start-up performance of PEMFC system

The cold start-up of a proton exchange membrane fuel cell is considered one of the main factors affecting the commercialization of fuel cell vehicles. In this study, an automotive fuel cell system was designed and tested for cold start-up at low temperatures. In the absence of PTC (Positive Temperature Coefficient) heating device, the stack was directly loaded to generate heat, which provided the cold start-up characteristics of system at low temperatures. Cold start-up process and purging control strategies were analyzed at −20 °C and −30 °C. It was found that the fuel cell system could produce 50% power in 25 s at −20 °C, the coolant temperature's heating rate was 0.78 °C/s, the coolant outlet temperature could reach 20 °C within 40 s and no apparent low voltage of single cell occurred. While, the cell close to the end plate had low cell voltage and reverse polar phenomena throughout the −30 °C cold start-up process. The heating rate of the coolant temperature was 0.44 °C/s, and the temperature of coolant outlet reached 20 °C within 90 s. The purging time ranged from 180 to 260 s according to the voltage drop value of stack and the ohmic resistance of stack was 360–470 mΩ after the high-volume air purging at different tests. After 30 cold start-up tests, the rated point performance of the stack declined by about 1%, and the consistency of cell voltages did not change significantly. Future work will focus on optimizing cold start-up strategy and speeding up purging time to minimize the performance impact of the cold start-up.     

Research on the long-term stability of a PEMFC stack: Analysis of pinhole evolution

In this paper, the structure of a two-cell PEM fuel cell stack is introduced. Results of a long-term stability test are presented. Pinholes are considered to be formed and grow up after the 357th hour because four oscillation stages are found in the temperature profile of each bipolar plate. Based on the data of Pinhole Growth Region, and considering the growth of a pinhole and water droplets formation and their growth and movement, four models are proposed to give reasonable explanations for the pinhole evolution during these four stages. The analysis shows that the pinhole growth increases the amplitude of the temperature oscillation of each bipolar plate. The pressure difference between the anode and the cathode is a key factor to cause deadly destruction to a membrane. The stack is broken after the 647.15th hour because the pinhole is considered to be huge. The dimensionless hydrogen concentration of the cathode exhaust and the temperature of each bipolar plate are closely related, which is a good support for the models.     

Experimental study on improving the dynamic characteristics of open-cathode PEMFC stack with dead-end anode by condensation and circulation of hydrogen

Water management is one of the most important issues for proton exchange membrane fuel cell stack. Liquid water accumulates in the stack may impede the transport of the reaction gas, resulting in unstable output performance and poor durability. In this study, Condensation mode and Condensation Circulation mode are proposed to reduce the accumulation of liquid water in the anode compartment, thus reducing the risk of flooding. Comparative research among the traditional dead-end anode (DEA) mode and presented modes are carried out on a ten-cell open-cathode PEMFC stack. The comparisons show that the proposed strategies can effectively alleviate the voltage decay caused by flooding and improve output stability. And the Condensation Circulation mode is more effective than the Condensation mode.     

Experimental analysis of performance degradation of 3-cell PEMFC stack under dynamic load cycle

A vehicular fuel cell is dynamically operated at the demand of the driver, so that the durability of the fuel cell quickly deteriorates. This study analyzes the durability of a 3-cell short stack under normal vehicle operation. An acceleration test is scheduled with operation temperatures of 55 °C and 70 °C at 50% relative humidity for 300 h. The dynamic load cycle (DLC) conditions are a repetition of the New European Driving Cycle (NEDC), which can allow a short stack to run on the vehicle operating load. At 100-hour intervals, recovery procedures are conducted to understand the order of performance retrieval. Significant stack degradation is observed at 75 °C operation for 300 h. Results show that the recovery protocol can return the performance of the fuel cell at a low and a middle current density regime, but it is hard to recover the performance at a very high current density regime. Performance recovery is very effective for lower temperature operation (55 °C), but the recovery procedures only returned about 4% of the performance at 300 h and 75 °C.     

Development,manufacture and validation of an open cathode LT-PEMFC stack at HySA systems

This work presents open cathode low temperature polymer electrolyte membrane fuel cell stack development and validation process project performed at HySA Systems as a part of a long-term programme funded by Department of Science and Innovation in South Africa. A detailed explanation of the stack design, manufacturing, assembly and validation is given as well as detailed analysis of results is presented. Prototype stack has an electrode active area of 50 cm², bipolar plates made of graphite composite material (Eisenhuth) and membrane electrode assemblies manufactured in South Africa - HyPlat (Pty) Ltd. A short 10-cell stack is validated using FuelCon Evaluator stack test station and custom designed stack control system integrated with complete balance of plant components. The stack maximum current and power densities are 1.2 Acm⁻² at 0.5 V and 0.6 Wcm⁻², respectively. Performed current hold (300 h) and open circuit voltage (60 h) durability tests resulted in degradation rates of 0.64 mVh⁻¹ and 3.83 mVh⁻¹, respectively.     

Parameter optimization of PEMFC stack under steady working condition using orthogonal experimental design

Operating parameters have a huge impact on the output characteristics of a proton exchange membrane fuel cell stack. In this study, to optimize the performance of proton exchange membrane fuel cell stack, 4 sets of operating parameters, which include working temperature, cathode stoichiometric, relative humidity, and backpressure, were optimized by means of the orthogonal experimental design. The experiment was developed with the help of 4‐factor and 3‐level orthogonal table. Nine orthogonal experiments were performed, and the polarization curve, local current density distribution, and electrochemical impedance spectroscopy of each experiment were obtained. It is observed that cathode stoichiometric and working temperature have much stronger effects on the output voltage and output consistency of stack than that of relative humidity and backpressure. Using comprehensive equilibrium method, the optimized combination of each parameter was achieved as follows: the working temperature was 75°C, cathode stoichiometric was 2.5, relative humidity was 50%, and backpressure was 1 bar. The on‐site test result showed that when the cathode stoichiometric was low, and some part of the stack would be in a starvation condition and when the temperature was low, it might cause mass transfer problems.     

Numerical analysis on the effects of manifold design on flow uniformity in a large proton exchange membrane fuel cell stack

The size and configuration of manifold can affect the flow characteristics and uniformity in proton exchange membrane fuel cell (PEMFC) stack; then its efficiency and service life. Based on the simulation results of a single fuel cell considering electrochemical reaction, a stack model with 300 porous media is established to numerically investigate the performances of a large commercial PEMFC stack. The effects of manifold width and configuration type on the pressure drop and species concentration are studied by computational fluid dynamics (CFD). The results show that the uniformity for most cases of U-type configuration is better than those of Z-type configuration. For U-type configuration, a very good uniformity can be obtained by selecting anode inlet manifold width of 20 mm and anode outlet manifold in range from 25 to 30 mm; the uniformity is bad for all cathode inlet manifold width, relatively better uniformity can be achieved by adjusting cathode outlet manifold width. For Z-type configuration, bad uniformity is found for cathode inlet and outlet manifold with all width; a relatively good uniformity can be obtained with suitable anode manifold width of 35 mm. The research can provide some references to improve gas distribution uniformity in large PEMFC stacks.     

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.     

Performance improvement of a PEMFC system controlling the cathode outlet air flow

This paper presents a stationary and dynamic study of the advantages of using a regulating valve for the cathode outlet flow in combination with the compressor motor voltage as manipulated variables in a fuel cell system. At a given load current, the cathode input and output flowrate determine the cathode pressure and stoichiometry, and consequently determine the oxygen partial pressure, the generated voltage and the compressor power consumption. In order to maintain a high efficiency during operation, the cathode output regulating valve has to be adjusted to the operating conditions, specially marked by the current drawn from the stack. Besides, the appropriate valve manipulation produces an improvement in the transient response of the system. The influence of this input variable is exploited by implementing a predictive control strategy based on dynamic matrix control (DMC), using the compressor voltage and the cathode output regulating valve as manipulated variables. The objectives of this control strategy are to regulate both the fuel cell voltage and oxygen excess ratio in the cathode, and thus, to improve the system performance. All the simulation results have been obtained using the MATLAB-Simulink environment.     

Planar array stack design aided by rapid prototyping in development of air-breathing PEMFC

The polymer electrolyte membrane fuel cell (PEMFC) is one of the most important research topics in the new and clean energy area. The middle or high power PEMFCs can be applied to the transportation or the distributed power system. But for the small power application, it is needed to match the power requirement of the product generally. On the other hand, the direct methanol fuel cell (DMFC) is one of the most common type that researchers are interested in, but recently the miniature or the micro-PEMFCs attract more attention due to their advantages of high open circuit voltage and high power density.     

Experimental and numerical investigation on effects of cathode flow field configurations in an air-breathing high-temperature PEMFC

Air-breathing high-temperature proton exchange membrane fuel cell (HT-PEMFC) gets rid of the cumbersome air supplying systems and avoids the water flooding problem by directly exposing the cathode to air and operating the fuel cell at elevated temperature. Performance of the air-breathing HT-PEMFC is dependent on many factors particularly the cathode flow field configurations. However, studies about air-breathing HT-PEMFCs are quite limited in the literature. In the present study, an experimental testing system was setup for the performance measurement of the air-breathing HT-PEMFC. A 3D numerical model was established and validated by the experimental data. Effects of the cathode flow field configurations including the opening shape, end plate thickness, open ratio and opening direction on performance of the air-breathing HT-PEMFC were experimentally and numerically investigated. It was found that the cathode end plate thickness and upward or sideways orientation have the least effect on the performance. The maximum power density of 160 mW/cm² at the current density of 394 mA/cm² can be achieved for the cathode flow field with slot holes and an open ratio of 75%.     

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.     

Performance and transient behavior of the kW-grade PEMFC stack with the PtRu catalyst under CO-contained diluted hydrogen

In the study, a self-made kW-class 40-cell proton exchange membrane fuel cell (PEMFC) stack, with an active area of 112.85 cm² for each membrane electrode assembly and with the anodic PtRu catalyst, was tested under different simulated reformate gases of different CO concentrations and different hydrogen concentrations. The performances and the transient voltages of the stack and the individual cells under different CO/N₂/H₂ mixtures were studied. The results show that increasing the CO concentration or decreasing the H₂ concentration of the CO-contained reformate gas negatively affects the performance of the PEMFC stack. Moreover, the PEMFC stack with the PtRu anodic catalyst can tolerate a CO concentration of up to 50 ppm under non-diluted H₂. However, it can only tolerate 10 ppm CO under diluted H₂. The CO tolerance decreases dramatically with an increase in the H₂ dilution level. In addition, increasing the CO concentration in diluted H₂ or decreasing the H₂ concentration in CO-contained H₂ accelerates the occurrence of potential oscillation. The potential oscillation is owing to the interactions of CO electro-oxidation and adsorption reactions on the catalyst. This work is also the first to report that the potential oscillation phenomenon initially occurs at the upstream cells of the stack.     

A numerical analysis of PEMFC stack assembly through a 3D finite element model

A finite element model is developed to investigate the influence of the assembly phase of proton exchange membrane fuel cell (PEMFC) stacks on the mechanical state of the active layer (MEAs). Validated by experimental measurements, this model offers the possibility to analyze the influence of different parameters through the use of a complete parametric set, such as the number of cells and their position in the stack. The simulations show that a better uniformity of the MEA compression is obtained with the greatest number of cells, and at the center of the stack. The finite element analysis (FEA) is finally found to be an effective tool to show the influence of the assembly phase on the performance of PEMFCs, and will help the designer to adapt the future generations of stack to ensure the uniformity of the MEA mechanical strain.     

A preliminary study of a miniature planar 6-cell PEMFC stack combined with a small hydrogen storage canister

The fabrication and performance evaluation of a miniature 6-cell PEMFC stack based on Micro-Electronic-Mechanical-System (MEMS) technology is presented in this paper. The stack with a planar configuration consists of 6-cells in serial interconnection by spot welding one cell anode with another cell cathode. Each cell was made by sandwiching a membrane-electrode-assembly (MEA) between two flow field plates fabricated by a classical MEMS wet etching method using silicon wafer as the original material. The plates were made electrically conductive by sputtering a Ti/Pt/Au composite metal layer on their surfaces. The 6-cells lie in the same plane with a fuel buffer/distributor as their support, which was fabricated by the MEMS silicon–glass bonding technology. A small hydrogen storage canister was used as fuel source. Operating on dry H₂ at a 40 ml min⁻¹ flow rate and air-breathing conditions at room temperature and atmospheric pressure, the linear polarization experiment gave a measured peak power of 0.9 W at 250 mA cm⁻² for the stack and average power density of 104 mW cm⁻² for each cell. The results suggested that the stack has reasonable performance benefiting from an even fuel supply. But its performance tended to deteriorate with power increase, which became obvious at 600 mW. This suggests that the stack may need some power assistance, from say supercapacitors to maintain its stability when operated at higher power.