Effect of anisotropic transport properties of porous layers on the dynamic performance of proton exchange membrane fuel cell

To study the influence of anisotropic transport properties of porous layers (PLs) on the dynamic performance (DP) of fuel cells (FC), a numerical model was developed. The model uses self-compiled codes to couple the actual structure of PLs with its transport capability, while introducing a non-homogeneous model that reflects the effect of the actual agglomerate structure of the catalyst layer on the electrochemical reaction, based on the consideration of the internal heat transport and external heat exchange of FC, the three-phase transformation of water, the change of momentum, energy, species, and water transport with time, and finally realizes the transient changes of complete transport and reaction process inside the FC. Using the current density as the input load, the law of the anisotropic transport properties of PLs on the FC's DP was explored. The results show that among all properties, the proton conductivity has the most significant influence on DP indexes such as steady-state transition time, over/undershoot amplitude, the electrical conductivity and diffusivity have similar and significant effects, the thermal conductivity and permeability have minor and most negligible effects, respectively. When designing the material, structure and composition of the PLs, the focus should be on protons, electron conductivity and diffusivity.     

Investigation and analysis of proton exchange membrane fuel cell dynamic response characteristics on hydrogen consumption of fuel cell vehicle

The number of working points and response speed are two essential characteristics of proton exchange membrane fuel cell (PEMFC). The improper setting of the number of working points and response speed may reduce the life of PEMFC and increase the hydrogen consumption of the vehicle. This paper explores the impact of the response speed as well as the working points of the PEMFC on the hydrogen consumption in the real-system level. In this paper a dynamic model of the PEMFC system is established and verified by experiments. The model is able to reflect the dynamic response process of PEMFC under a series different number of working points and different response speed. Based on the proposed model, the influence of working points and the response speed of PEMFC on the hydrogen consumption in the vehicle under different driving cycles is analyzed and summarized, for the first time, in the open literature. The results highlight that the hydrogen consumption will decreases in both cases that with the increase of working point number and increase of response speed. However, the reduction range of hydrogen consumption trends to smaller and may reach to an optimal level considering the trade-off between the hydrogen saving and the other costs, for example the control cost. Also, with a more complex driving cycle, the working points and response speed have a greater the impact on the hydrogen consumption in the vehicle applications.     

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.     

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.     

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.     

Parametric analysis of the steady state and dynamic performance of proton exchange membrane fuel cell models

Proton exchange membrane fuel cells (PEMFCs) are devices that attract the interest for a variety of applications including portable devices, transportation and stationary power. Several models are available in the literature concerning PEMFCs with different modelling approaches. In this paper, two representative dynamic models are examined, one using an electrical equivalent and one based on semi-empirical equations. Moreover, an enhanced model based on semi – empirical equations and a simplified transfer function representation for the dynamic response is proposed. All models can be easily incorporated in power system simulation software. Scope of this paper is to present a parametric analysis method in order to determine the ability of each model to represent accurately the steady – state as well as the slow and fast dynamics of a PEMFC. The influence of each specific parameter is investigated and the tuning procedure is described. Finally, simulation results are presented and the adaptability of all models is evaluated.     

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.     

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.     

Quasi-three dimensional dynamic model of a proton exchange membrane fuel cell for system and controls development

A first principles dynamic model of the physical, chemical, and electrochemical processes at work in a proton exchange membrane fuel cell has been developed. The model solves the dynamic equations that govern the physics, chemistry and electrochemistry for time scales greater than about 10 ms. The dynamic equations are solved for a typical but simplified quasi-three dimensional geometric representation of a single cell repeat unit of a fuel cell stack. The current approach captures spatial and temporal variations in the important physics of heat transfer and water transport in a manner that is simple enough to make the model amenable to PEMFC system simulations and controls development. Comparisons of model results to experimental data indicate that the model can well predict steady state voltage–current relationships as well as the oxygen, water, and nitrogen spatial distribution within the fuel cell. In addition, the model gives dynamic insight into the distribution of current, water flux, species mole fractions, and temperatures within the fuel cell. Finally, a control system test is demonstrated using the simplified dynamic model.     

Development of a proton exchange membrane fuel cell cogeneration system

A proton exchange membrane fuel cell (PEMFC) cogeneration system that provides high-quality electricity and hot water has been developed. A specially designed thermal management system together with a microcontroller embedded with appropriate control algorithm is integrated into a PEM fuel cell system. The thermal management system does not only control the fuel cell operation temperature but also recover the heat dissipated by FC stack. The dynamic behaviors of thermal and electrical characteristics are presented to verify the stability of the fuel cell cogeneration system. In addition, the reliability of the fuel cell cogeneration system is proved by one-day demonstration that deals with the daily power demand in a typical family. Finally, the effects of external loads on the efficiencies of the fuel cell cogeneration system are examined. Results reveal that the maximum system efficiency was as high as 81% when combining heat and power.     

Enhancement of proton exchange membrane fuel cell performance by titanium-coated anode gas diffusion layer

Titanium was coated onto an anode gas diffusion layer (GDL) by direct current sputtering to improve the performance and durability of a proton exchange membrane fuel cell (PEMFC). Scanning electron microscopy (SEM) images showed that the GDLs were thoroughly coated with titanium, which showed angular protrusion. Single-cell performance of the PEMFCs with titanium-coated GDLs as anodes was investigated at operating temperatures of 25 °C, 45 °C, and 65 °C. Cell performances of all membrane electrode assemblies (MEAs) with titanium-coated GDLs were superior to that of the MEA without titanium coating. The MEA with titanium-coated GDL, with 10 min sputtering time, demonstrated the best performance at 25 °C, 45 °C, and 65 °C with corresponding power densities 58.26%, 32.10%, and 37.45% higher than that of MEA without titanium coating.     

Study on the performance of a proton exchange membrane fuel cell related to the structure design of a gas diffusion layer substrate

Although characteristics of the gas diffusion layer (GDL) affect the performance of a proton exchange membrane fuel cell (PEMFC), mass transfer mechanisms inside the GDL and the performance of the PEMFC have not been directly correlated. To determine the design parameters of the GDL, the effects of substrate design of the GDL on performance of a PEMFC are investigated. By adding an active carbon fiber (ACF), which has a high surface area, the substrate is designed to have a different pore size structure. The results show that steady-state and transient responses are determined by capillary pressure gradient characteristics of the GDL made by pore size distribution of the substrate. The small macro-pore functions as water-retaining passage and the large macro-pore functions as water-removal passage. It is concluded that both small and large macro-pore must be present on the substrate to facilitate its function in a wide range of operating conditions.     

Experimental study on the dynamic performance of an air-cooled proton exchange membrane fuel cell stack with ultra-thin metal bipolar plate

In this paper, a compact 3 kW air-cooled fuel cell stack consists of 95 single cells with metallic bipolar plate is designed. Compared with graphite bipolar plates, metal stamping bipolar plates are lighter in weight, smaller in size and faster in heat conduction, therefore the transient behaviors of the voltage and temperature of each cell are analyzed. The results show that the heat distribution of the air-cooled fuel cell is very uniform, and the temperature difference between the inlet and outlet of cathode air of the fuel cell is lower than 15 °C. The individual cell voltage uniformity percentage variation value reaches 7% when the drop in the loading current is over 25 A. Moreover, the voltage uniformity variation value is higher than 4% when the loading current output exceeds 35A. Thus, a large drop in loading and a high loading current easily increase the voltage uniformity variation value. Long-term continuous operation has a negative influence on the performance of the stack, especially the last fuel cell near the anode outlet. Anode purging can effectively alleviate the uniformity percentage variation in the voltages. The designed air-cooled fuel cell exhibits good performance and strong environmental adaptability.     

Transient computation fluid dynamics modeling of a single proton exchange membrane fuel cell with serpentine channel

The proton exchange membrane fuel cell (PEMFC) has become a promising candidate for the power source of electrical vehicles because of its low pollution, low noise and especially fast startup and transient responses at low temperatures. A transient, three-dimensional, non-isothermal and single-phase mathematical model based on computation fluid dynamics has been developed to describe the transient process and the dynamic characteristics of a PEMFC with a serpentine fluid channel. The effects of water phase change and heat transfer, as well as electrochemical kinetics and multicomponent transport on the cell performance are taken into account simultaneously in this comprehensive model. The developed model was employed to simulate a single laboratory-scale PEMFC with an electrode area about 20 cm². The dynamic behavior of the characteristic parameters such as reactant concentration, pressure loss, temperature on the membrane surface of cathode side and current density during start-up process were computed and are discussed in detail. Furthermore, transient responses of the fuel cell characteristics during step changes and sinusoidal changes in the stoichiometric flow ratio of the cathode inlet stream, cathode inlet stream humidity and cell voltage are also studied and analyzed and interesting undershoot/overshoot behavior of some variables was found. It was also found that the startup and transient response time of a PEM fuel cell is of the order of a second, which is similar to the simulation results predicted by most models. The result is an important guide for the optimization of PEMFC designs and dynamic operation.     

Dynamic modeling of a high-temperature proton exchange membrane fuel cell with a fuel processor

A dynamic model of a high-temperature proton exchange membrane fuel cell with a fuel processor is developed in this study. In the model, a fuel processing system, a fuel cell stack, and an exhaust gas burner are modeled and integrated. The model can predict the characteristics of the overall system and each component at the steady and transient states. Specifically, a unit fuel cell model is discretized in a simplified quasi-three-dimensional geometry; therefore, the model can rapidly predict the distribution of fuel cell characteristics. Various operating conditions such as the steam-to-carbon ratio, oxygen-to-carbon ratio, and autothermal reforming inlet temperature are varied and investigated in this study. In addition, the dynamic characteristics exhibited during the transient state are investigated, and an efficiency controller is developed and implemented in the model to maintain the electrical efficiency. The simulation results demonstrate that the steam-to-carbon ratio and the oxygen-to-carbon ratio affect the electrical and system efficiency and that controlling the fuel flow rate maintains the electrical efficiency in the transient state. The model may be a useful tool for investigating the characteristics of the overall system as well as for developing optimal control strategies for enhancing the system performance.     

Two dimensional dynamic modeling of a shell-and-tube water-to-gas membrane humidifier for proton exchange membrane fuel cell

Water management is a crucial factor in determining the performance of proton exchange membrane fuel cell (PEMFC) for automotive application. The shell-and-tube water-to-gas membrane humidifier is useful for humidifying the PEMFC due to its good performance. Shell-and-tube water-to-gas membrane humidifiers have liquid water on one side of the tube wall and a dry gas on the other. In order to investigate humidifier performance, a two-dimensional dynamic model of a shell-and-tube water-to-gas membrane humidifier is developed. The model is discretized into three control volumes – shell, tube and membrane – in the cross-sectional direction to resolve the temperature and species concentration of the humidifier. For validation, the dew point temperature of the simulation result is compared with that of experimental data and shows good agreement with only a slight difference. The distribution of humidification characteristics can be captured using the discretization along the air-flow direction. The humidification performance of two different flow configurations, counter and parallel, are compared under various operating conditions and geometric parameters. Finally, the dynamic response of the humidifier at the step-change of various air flow rates is investigated. These results suggest that the model can be used to optimize the inlet flow humidity of a PEMFC.     

The effects of the gravity on transient responses and cathode flooding in a proton exchange membrane fuel cell

The work presented in this paper includes contributions that provide insight into liquid water transport in the proton exchange membrane fuel cell (PEMFC).     

Dynamic modeling of proton exchange membrane fuel cell using non-integer derivatives

The modeling of proton exchange membrane fuel cells (PEMFC) may work as a powerful tool in the development and widespread testing of alternative energy sources in the next decade. In order to obtain a suitable PEMFC model, which can be used in the analysis of fuel cell-based power generation systems, it is necessary to define the values of a specific group of modeling parameters. In this paper, the authors propose a dynamic model of PEMFC, the originality of which lays on the use of non-integer derivatives to model diffusion phenomena. This model has the advantage of having least number of parameters while being valid on a wide frequency range and allows simulating an accurate dynamic response of the PEMFC.

In this model, the fuel cell is represented by an equivalent circuit, whose components are identified with the experimental technique of electrochemical impedance spectroscopy (EIS). This identification process is applied to a commercially available air-breathing PEMFC and its relevance is validated by comparing model simulations and laboratory experiments. Finally, the dynamic response derived from this fractional model is studied and validated experimentally.     

Effect of water distribution in gas diffusion layer on proton exchange membrane fuel cell performance

Water management of proton exchange membrane fuel cells remains a prominent issue in research concerning fuel cells. In this study, the gas diffusion layer (GDL) of a fuel cell is partially treated with a hydrophobic agent, and the effect of GDL hydrophobicity on the water distribution in the fuel cell is examined. First, the effect of the position of the cathode GDL hydrophobic area relative to the channel on the fuel cell performance is investigated. Then, the water distribution in the fuel cell cathode GDL is observed using X-ray imaging. The experimental results indicate that when the hybrid GDL's hydrophobic area lies on the channel, water tends to accumulate under the rib, and the water content in the channel is low; this improves the fuel cell performance. When the hydrophobic area is under the rib, the water distribution is more uniform, but the performance deteriorates.