The invention relates to a proton exchange membrane fuel cell and a method of designing the same.
Proton exchange membrane (PEM) fuel cells have a rapid start-up due to their low operating temperatures, which make them suitable for portable applications. One of the most important issues that should be taken into account when operating PEM fuel cells is the heat management to keep the temperature distribution within the fuel cell components as uniform as possible, otherwise the fuel cell may experience a thermal failure due to dehydration of the membrane. This requires an investigation into the effective thermal conductivity, an important component being the thermal conductivity of the porous media, which has anisotropic properties.
Recently, researchers have shown an increased interest in the effect of the anisotropic properties of GDLs on the performance of PEM fuel cells [1-5]. Khandelwal and Mench [6] reported that the through-plane thermal conductivity of SIGRACET to be 0.22±0.04 W/(m·K), whereas Toray reported it to be 1.8±0.27 W/(m·K). Ramousse et al. [7] reported the through-plane thermal conductivity of the GDL under different pressures, obtaining values of about 0.2 and 0.27 W/(m·K) under pressures of 4.6 and 13.9 bar respectively. However, Karimi [8] found the through-plane thermal conductivity to be 0.2 to 0.7 W/(m·K) under pressures of 0.7 and 13.8 bar. It is clear from these results that the thermal conductivity of the GDLs has differed significantly from one GDL to another. Many numerical investigations have been performed to investigate the effect of the thermal conductivity of the GDL. However, most PEM fuel cell models assume that the GDLs are comprised of an isotropic material.
Pharaoah and Burheim [9] developed two-dimensional models to investigate the temperature distribution in PEM fuel cells. The effect of the thermal conductivity of the GDL and the change in the water phase leads to higher temperatures in the cathode side than in the anode side. Zamel et al. [10] numerically estimated the in-plane and through-plane thermal conductivity of carbon paper, which is typically used as a gas diffusion layer in PEM fuel cells. The thermal conductivity of the GDL was sensitive to the porosity of the carbon paper. The thermal conductivity of the carbon paper was found to increase with a decrease in the porosity of the carbon paper, and the in-plane thermal conductivity was much higher than the through-plane thermal conductivity of the carbon paper. Burlatsky et al. [11] developed a mathematical model to investigate the scenario of water removal in PEM fuel cells. The water transport was dependent on the thermal conductivity of GDL and the water diffusion coefficients. He et al. [12] investigated the effect of the thermal conductivity of the GDL on the temperature distribution in PEM fuel cells. Their results indicated that the anisotropic thermal conductivity of the GDL results in higher temperature gradients than for an isotropic GDL, which led to a decrease in the water saturation in the anisotropic case. According to Ju Hyunchul [24], the temperature differences in PEM fuel cells were higher when using anisotropic GDL than the isotropic GDL. Furthermore, the isotropic GDLs achieved a uniform current density better than anisotropic gas diffusion layers.
However, so far, no researchers have validated their model results with experimental data.
U.S. Pat. No. 7,785,748 B2 of University of Delaware discloses novel methods for producing a nano-porous gas diffusion media, compositions thereof and devices comprising the same. A porous metallic gas diffusion layer is disclosed. The nano-porous diffusion media of this disclosure are said to display superior electro- and thermal conductivity.
According to a first aspect of the invention, there is provided a method of designing a proton exchange membrane fuel cell comprising a gas diffusion layer, the method comprising:
Using said model to determine performance may comprise determining one or more of temperature distribution, water saturation, and/or current density of the fuel cell. The performance may be improved by providing a more uniform temperature distribution across the gas diffusion layer. The performance may be improved by maximising the water saturation of the fuel cell, e.g. at an interface between the gas diffusion layer and a catalyst layer.
Said fuel cell preferably comprises an anode and a cathode connected by a membrane. The model may comprise multiple zones defined within the fuel cell. Said multiple zones may comprise one or more of a current collector, a channel, a gas diffusion layer, a catalyst layer and said membrane. Separate zones may be defined for each of said anode and said cathode. Each of said zones may be subdivided into a plurality of cells whereby calculation time may be improved.
The method may further comprise making a fuel cell to said design whereby said results may be validated with the experimental data.
The plurality of parameters may include the material of the gas diffusion layer (GDL). For example, a conventional carbon-fibre-based GDL may be replaced with a metal-based GDL whose thermal and electrical conductivities are significantly higher than that of the conventional one. As an example, the thermal conductivity of copper and aluminium are about 400 and 240 W/(m·K) respectively.
The anisotropic properties may include one or more of the electrical conductivity, thermal conductivity, and/or permeability of the gas diffusion layer. Including such properties should enhance the prediction of the numerical model.
The thermal conductivity may include the in-plane thermal conductivity and/or the through plane thermal conductivity. The in-plane thermal conductivity may be adjusted to be at least 1 W(m·K), at least 10 W(m·K), at least 20 W(m·K) or at least 100 W(m·K). Whilst the in-plane thermal conductivity is being adjusted, the through plane thermal conductivity may be held constant, e.g. at 1 W(m·K). The through-plane thermal conductivity may be adjusted to be at least 0.1 W(m·K), at least 1 W(m·K), or at least 10 W(m·K). Whilst the through-plane thermal conductivity is being adjusted, the in-plane thermal conductivity may be held constant, e.g. at 10 W(m·K). The ratio of in-plane thermal conductivity to through plane thermal conductivity may be 10:1.
Adjusting the in-plane and through plane thermal conductivities separately allows the model to take account of the anisotropic thermal conductivity of the gas diffusion layer. A similar method could be applied to the electrical conductivity and/or permeability.
It is noted that as the thermal conductivity of the GDL increases, the rate of heat dissipation increases and therefore the temperature distribution become more uniform and the maximum temperature decreases. The heat is mainly generated as a result of exothermic electrochemical reaction taking place at the catalyst layer.
According to another aspect of the invention, there is provided a proton exchange membrane fuel cell comprising a gas diffusion layer, said proton exchange membrane fuel cell having a plurality of parameters, wherein said parameters are selected to provide substantially uniform temperature distribution across said gas diffusion layer.
The parameters may include the thermal conductivity of the gas diffusion layer. The thermal conductivity may comprise in-plane thermal conductivity and/or through-plane thermal conductivity of the gas diffusion layer is substantially isotropic.
The gas diffusion layer may have an in-plane thermal conductivity of at least 10 W/(m·K) or at least 100 W/(m·K). The through-plane thermal conductivity of the gas diffusion layer may be at least 1 W/(m·K) or at least 10 W/(m·K). The gas diffusion layer may have an in-plane thermal conductivity of at least 10 W/(m·K) and a through-plane thermal conductivity of at least 1 W/(m·K).
According to another aspect of the invention, there is provided a fuel cell comprising a proton exchange membrane having a gas diffusion layer, wherein the thermal conductivity of the gas diffusion layer is substantially isotropic.
According to another aspect of the invention, there is provided a fuel cell comprising a proton exchange membrane having a gas diffusion layer, wherein the in-plane thermal conductivity of the gas diffusion layer is substantially isotropic.
According to another aspect of the invention, there is provided a fuel cell comprising a proton exchange membrane having a gas diffusion layer, wherein the through-plane thermal conductivity of the gas diffusion layer is substantially isotropic.
According to another aspect of the invention, there is provided a fuel cell comprising a proton exchange membrane having a gas diffusion layer, wherein the gas diffusion layer has an in-plane thermal conductivity of at least 10 W/(m·K).
The in-plane thermal conductivity of the gas diffusion layer may be at least 100 W/(m·K), or at least 200 W(m·K) or at least 400 W(m·K).
The in-plane thermal conductivity of the gas diffusion layer may at least 1 W/(m·K) or at least 10 W/(m·K).
According to another aspect of the invention, there is provided a fuel cell comprising a proton exchange membrane having a gas diffusion layer, wherein the gas diffusion layer has an in-plane thermal conductivity of at least 10 W/(m·K) and a through-plane thermal conductivity of at least 1 W/(m·K).
The gas diffusion layer may be metallic.
According to another aspect of the invention, there is provided a fuel cell proton exchange membrane having a gas diffusion layer. According to another aspect of the invention, there is provided a fuel cell proton exchange membrane gas diffusion layer.
According to another aspect of the invention, there is provided a method of making a fuel cell comprising a proton exchange membrane having a gas diffusion layer, comprising the step of arranging the thermal conductivity of the gas diffusion layer in the in-plane and/or through-plane directions to be substantially isotropic.
The invention further provides processor control code to implement the above-described systems and methods, for example on a general purpose computer system or on a digital signal processor (DSP). The code is provided on a physical data carrier such as a disk, CD- or DVD-ROM, programmed memory such as non-volatile memory (eg Flash) or read-only memory (Firmware). Code (and/or data) to implement embodiments of the invention may comprise source, object or executable code in a conventional programming language (interpreted or compiled) such as C, or assembly code. As the skilled person will appreciate such code and/or data may be distributed between a plurality of coupled components in communication with one another.
The invention is diagrammatically illustrated, by way of example, in the accompanying drawings, in which:
a to 4c show the variation in temperature (K) distribution within the cathode GDL for the three different fuel cells;
a to 5c shows the variation in water saturation at the interface between the cathode GDL and the cathode catalyst layer for the three different fuel cells;
a to 8c show the variation in temperature (K) distribution within the cathode GDL for the three different fuel cells of
a to 9c show the variation in water saturation at the interface between the cathode GDL and the cathode catalyst layer for the three different fuel cells of
a to 10c show the variation in temperature (K) distribution within the PEM fuel cells for three theoretic fuel cells each having different in-plane thermal conductivities; and
Gas diffusion layers (GDLs) are one of the main components in proton exchange membrane (PEM) fuel cells. Proton exchange membrane (PEM) fuel cells are the most popular type of fuel cell due to their high efficiency, quick start-up and low operating temperature. In order to obtain effective thermal and water management in PEM fuel cells, the thermal conductivity of the porous media should be determined. In addition, the thermal conductivity of the gas diffusion layers (GDLs) has anisotropic properties such as electrical conductivity and permeability. However, most PEM fuel cell models assume that the GDLs comprise isotropic material.
As described in more detail below, the effect of anisotropic thermal conductivity of the GDL is numerically investigated under different operating temperatures. It is found that the output of the numerical model with realistic thermal conductivity values is in good agreement with the experimental data. Furthermore, the sensitivity of the PEM fuel cell performance to the thermal conductivity of the GDL is investigated for both in-plane and through-plane directions and the temperature distributions between the different GDL thermal conductivities are compared. The results show that increasing the in-plane and through-plane thermal conductivity of the GDL increases the power density of PEM fuel cells significantly. Moreover, the temperature gradients show a greater sensitivity to the in-plane thermal conductivity of the GDL as opposed to the through-plane thermal conductivity. In summary, the effects of anisotropic GDLs on temperature distribution, and current density were assessed and the results were validated with experimental data.
In this study, a three-dimensional (3-D) multiphase model was developed with the following assumptions:
Basically, the fluid flow in the fuel cell is governed by the following equations [13]:
∇·(ερ{right arrow over (u)})=0
∇·(ερ{right arrow over (u)})=0 (1)
∇·(ερ{right arrow over (u)}Yk)=∇·(ρDkeff∇Yk)+Sk (3)
where ρ is the fluid density, {right arrow over (u)} is the fluid velocity vector, p is the fluid pressure, μ is the mixture viscosity, Yk is the mass fraction for gas species k, ε is the porosity of the porous media, Sk is the source or sink term for species k, and Dkeff is the diffusion coefficient of species k and it can be calculated as follow:
D
k
eff=εξDk (4)
where is the ξ of the porous media and D is the ordinary diffusion coefficient.
where σsol is the electric conductivity of solid, σmem is the proton conductivity in membrane, φsol is the potential of solid phase, φmem is the potential of membrane phase, Ja is cathode catalyst reaction rate and Jc is cathode catalyst reaction rate.
∇·(ρLVLS)=rw (7)
where S is the liquid water saturation, L is the liquid water and rw is the mass transfer rate between the gas and liquid.
(ρcp)eff(v·∇T)=∇·(keff∇T)+Se (8)
where cp is the specific heat capacity of the gas mixture, T is the temperature, Se is the energy source term and keff is the effective thermal conductivity of the gas mixture which is defined as the follows:
where ks and kF are the thermal conductivities of the solid and fluid regions, respectively.
All the source terms in the above equations are listed in Table 1.
A schematic of the 11-channel serpentine flow field of the PEM fuel cell is shown in
The different components of a PEMFC are bipolar plates, electrodes, catalyst, membrane, and the necessary hardwares. The materials used for different parts of the fuel cells differ by type. The bipolar plates may be made of different types of materials, such as, metal, coated metal, graphite, flexible graphite, C—C composite, carbon-polymer composites etc. The membrane electrode assembly (MEA) is usually made of a proton exchange membrane sandwiched between two catalyst coated carbon papers. Platinum and/or similar type of noble metals are usually used as the catalyst for PEMFC. The electrolyte could be a polymer membrane.
Merely as an example, the PEM fuel cell dimensions were specified as 32×10.81×32 mm in the x, y and z directions, respectively. The 3-D model consisted of nine zones which are: cathode current collector, cathode channel, cathode gas diffusion layer, cathode catalyst layer, membrane, anode catalyst layer, anode gas diffusion layer, anode channel, and anode current collector. 5 meshes were built with different numbers of cells and the average current density at 0.55 V was calculated for these 5 meshes. For the purposes of the example, a mesh which has about 1,800,000 control volumes is used to save calculation time and the computing memory to investigate the effect of the anisotropy thermal conductivity of the GDL on the performance of PEM fuel cell. This simulation has been performed by using the fuel cell module in the FLUENT® software.
The fluid flow in the PEM fuel cell was generated under steady state conditions and all of the governing parameters, at the same values as the experimental parameters, are listed in Table 1. For the purposes of the examples, the velocity at the anode side was set to be 0.42 m/s with fully humidified hydrogen, while the velocity at the cathode channel was 1.06 m/s with humidified air. Isothermal constant temperature wall boundaries were defined for the cell sides and the current collectors. The operating temperatures were 303K, 313K, 323K, and 333K, respectively. The gauge pressure was set to be 2.5 bar at both the anode and cathode sides. All the physical, geometrical and operational parameters for the example are summarised in table 2 below:
In order to investigate the effect of the anisotropic thermal conductivity of the GDL in PEM fuel cells, nine different cases were developed. The first three cases investigated the effect of the in-plane thermal conductivity and the results are shown in
The first three cases are summarised below:
In these first three examples, the in-plane thermal conductivity of the GDL was increased from 1 to 10 to 100 W/(m·K). The in-plane thermal conductivity has been reported to be between 10-15 W/(m·K) [10] and based on this it has been decided to increase and decrease this value by a factor of 10. The through-plane thermal conductivity of the GDL was retained at a constant value of 1 W/(m·K), namely the reported experimental value [6, 10].
The effect of the thermal conductivity of the GDL on the power density was because of the decrease in the electrical resistance when the temperature decreases as a result of increasing the thermal conductivity [21]. Furthermore, the increased overall thermal conduction of the GDL assists in dissipating the heat from the MEA and consequently these results in a more uniform temperature distribution and having more liquid water to humidify the membrane, which enhances the ionic conductivity, and subsequently improves the performance of the cell [22].
The temperature distribution through the GDL is presented in
The low in-plane thermal conductivity causes regions of the fuel cell to remain relatively cold, thus increasing the likelihood of the formation of water pockets which may block the channels in the PEM fuel cell. This is illustrated in
It can be seen from
The second three cases are summarised below:
In these second three examples, the effect of the through-plane thermal conductivity was investigated. The through-plane thermal conductivity of the GDL increases from 0.1 to 1 to 10 W/(m·K), while the in-plane thermal conductivity of GDL was kept constant at 10 W/(m·K), the experimental value. The through-plane thermal conductivity was reported to be between 0.1-1 W/(m·K) [6, 10] and based on this it has been decided to increase and decrease this value by a factor of 10.
The effect of the through-plane thermal conductivities of the GDL on the temperature distribution in the PEM fuel cell is illustrated in
It can be seen from
Another three cases are summarised below:
In this study, as set out above, a three-dimensional (3-D) model was developed under steady state conditions. In this case; the velocity at the anode side was 0.24 m/s with fully humidified hydrogen, while the velocity at the cathode channel was 1.06 m/s with humidified air. The thermal wall boundaries were defined for the cell sides and the current collectors. The in-plane thermal conductivity of the GDL increased slightly, while the through-plane thermal conductivity of the GDL kept constant at 1 (W/(m·K).
As shown in
As shown in
A 3-D multiphase model has been developed to investigate the effect of the anisotropic thermal conductivity of the GDL on the performance of PEM fuel cells, and the results have been validated with an in-house PEM fuel cell. It has been found that the maximum temperature in the PEM fuel cell decreases when the thermal conductivity increases under the operating conditions investigated. In addition, the difference in the temperatures decreases when increasing the in-plane and through-plane thermal conductivities. The results show an increase in the current density of PEM fuel cells with an increase in the thermal conductivity of the GDL in both directions, namely the in-plane and the through-plane. This is the situation for all of the different operating temperatures that have been investigated (303K, 313K, 323K, and 333K). Moreover, increasing the thermal conductivity of the GDL increases the liquid water saturation as the maximum temperature decreases. This study has highlighted the need to accurately determine the thermal conductivity of the GDL.
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No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.
Number | Date | Country | Kind |
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1121394.9 | Dec 2011 | GB | national |
PCTGB2012053050 | Dec 2012 | WO | international |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/GB2012/053050 | 12/7/2012 | WO | 00 | 6/13/2014 |