Thermal Conductivity Analysis of Carbon Paper – Optimizing Fuel-Cell Gas Diffusion Layers

Proton Exchange Fuel Cell (PEMFC)

Proton Exchange Membrane Fuel Cell (PEMFC), as an emerging low-temperature fuel cell, has the advantages of high efficiency, low operating temperature and zero emission, which is one of the main development directions of new green energy.

The core component of PEMFC is the Membrane Electrode Assembly (MEA), which consists of two Gas Diffusion Layers (GDL), two catalytic layers and a proton exchange membrane.

The reaction principle of a PEMFC fuel cell is shown in figure 1. The PEMFC single cell consists of an EMA (anode, cathode and proton exchange membrane) and bipolar plates. The anode is the place where the OxidationOxidation can describe different processes in the context of thermal analysis.oxidation of hydrogen fuel occurs, and the cathode is the place where the redox occurs. Both poles contain catalysts to accelerate the electrochemical reaction of the electrodes, and platinum/ carbon or platinum/ruthenium are generally used as the electrocatalysts. The proton exchange membrane acts as the electrolyte; hydrogen or purified reformed gas is the fuel; air or pure oxygen is the oxidant; and the graphite or the surface-modified metal plate with the gas flow channel is the bipolar plate. Hydrogen and oxygen with a certain humidity and pressure enter the anode and cathode, respectively, and reach the interface between the catalyst layer and the proton exchange membrane through the gas diffusion layer (carbon paper in the figure), where OxidationOxidation can describe different processes in the context of thermal analysis.oxidation and reduction reactions take place under the action of the catalyst.

Anode:_H2 → ^2H+ +^2e–

Cathode: ½_O2 + ^2H+ + 2^e– →_H2O

Total battery reaction:_H2 + ½_O2 →_H2O

At the anode, hydrogen gas reacts electrochemically to form hydrogen ions and electrons. The hydrogen ions are then conducted to the cathode through a proton exchange membrane (the unique properties of the proton exchange membrane allow only hydrogen ions to pass through) and the electrons reach the cathode through an external circuit, where the hydrogen ions, electrons and oxygen react to form water. The generated water is discharged from the cathode outlet as water vapor or condensate along with excess oxygen.

Gas Diffusion Layer (GDL)

The Gas Diffusion Layer (GDL) is located at both ends of the membrane electrode, which is one of the important components of the fuel cell; its role includes supporting the proton exchange membrane, coating the catalyst, connecting the membrane electrode with the bipolar plate, etc.

The GDL material needs to have the following points in terms of performance:

1. Because the GDL is between the bipolar plate and the catalyst layer, the electrochemical reaction (i.e., the current DensityThe mass density is defined as the ratio between mass and volume. density) is very high – there is a high degree of galvanic corrosion – so the GDL material must have corrosion resistance.
2. The GDL material – as hydrogen /oxygen or methanol/air diffusion to the catalyst layer reaction medium – must be a porous, breathable material.
3. The GDL material plays the role of a current conductor and must be highly conductive material.
4. The battery reaction is ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermic; the GDL material must be a high Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity material; heat dissipation must be timely to avoid local overheating caused by the proton exchange membrane breakage.
5. The GDL material should have high hydrophobicity to avoid damage to the catalyst layer caused by the water generated by the battery reaction

Carbon Fiber Paper

Carbon fiber paper (referred to as carbon paper) is manufactured from short-cut carbon fibers as a raw material; this has a fiber porous structure in microscopic, which can establish effective channels for gas and water conduction. At the same time, carbon paper has the advantages of light weight, a flat surface, corrosion resistance and uniform porosity. In addition, the high strength of carbon paper can bring protection for the installation and use of PEMFC batteries, stabilize electrode structure and improve battery life. The carbon paper manufacturing process is mature, with stable performance; therefore, carbon paper has become the mainstream choice for gas diffusion layer materials in the membrane electrode. The membrane electrode with carbon paper as the gas diffusion layer is shown in Figure 1. Due to fiber orientation arrangement in the preparation process for carbon paper, the carbon paper itself has various anisotropies.

Given that Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity is one of the important indexes of GDL materials, in this work, Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity tests were carried out on a carbon paper sample by means of the NETZSCH LFA HyperFlash^®®. In this test, the LFA 467 was used to test the Thermal DiffusivityThermal diffusivity (a with the unit mm2/s) is a material-specific property for characterizing unsteady heat conduction. This value describes how quickly a material reacts to a change in temperature.thermal diffusivity of the carbon paper sample in horizontal and vertical directions respectively, and DSC was used to test the Specific Heat Capacity (cp)Heat capacity is a material-specific physical quantity, determined by the amount of heat supplied to specimen, divided by the resulting temperature increase. The specific heat capacity is related to a unit mass of the specimen.specific heat capacity of the carbon paper sample. The Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity of the sample was obtained by multiplying the Thermal DiffusivityThermal diffusivity (a with the unit mm2/s) is a material-specific property for characterizing unsteady heat conduction. This value describes how quickly a material reacts to a change in temperature.thermal diffusivity, the Specific Heat Capacity (cp)Heat capacity is a material-specific physical quantity, determined by the amount of heat supplied to specimen, divided by the resulting temperature increase. The specific heat capacity is related to a unit mass of the specimen.specific heat capacity and the DensityThe mass density is defined as the ratio between mass and volume. density (at room temperature) of the sample.

Applications

Table 1 shows the results of the Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity test in the horizontal direction for this carbon paper sample (figure 2). The support used for this test is an in-plane sample holder (figure 3), which can be used to test the Thermal DiffusivityThermal diffusivity (a with the unit mm2/s) is a material-specific property for characterizing unsteady heat conduction. This value describes how quickly a material reacts to a change in temperature.thermal diffusivity of high Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity thin film materials in the horizontal direction. It can be seen that the Thermal DiffusivityThermal diffusivity (a with the unit mm2/s) is a material-specific property for characterizing unsteady heat conduction. This value describes how quickly a material reacts to a change in temperature.thermal diffusivity in the horizontal direction of the sample at 25°C and 100°C is 58.610^mm2/s and 50.122^mm2/s, respectively, and the Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity is 20.568 W/(m*K) and 21.794 W/(m*K), respectively.

Figure 4 shows the tested temperature rise curve, and it can be seen that the test curves (raw signal – blue) and the fitted curve (model evaluation – red) are in very good agreement.

Table 2 shows the results of the Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity test for this carbon paper sample in the vertical direction.

The support used for this test was a foil sample holder (figure 5) which can be used to test the Thermal DiffusivityThermal diffusivity (a with the unit mm2/s) is a material-specific property for characterizing unsteady heat conduction. This value describes how quickly a material reacts to a change in temperature.thermal diffusivity of thin film samples in the vertical direction. From the results, it can be seen that the Thermal DiffusivityThermal diffusivity (a with the unit mm2/s) is a material-specific property for characterizing unsteady heat conduction. This value describes how quickly a material reacts to a change in temperature.thermal diffusivity in the vertical direction of the sample is 7.463^mm2/s and 6.408^mm2/s at 25°C and 100°C, respectively, and the Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity is 2.619 W/(m*K) and 2.786 W/(m*K), respectively. The thermal conductivity of the samples in the horizontal direction is significantly higher than that in the vertical direction, with obvious individual anisotropy. Because the sample has a porous fiber structure, there is a certain degree of light transmission when testing in the vertical direction.

Summary

In proton exchange membrane fuel cells, the gas diffusion layer serves as an important component of the membrane electrode, and its cost usually accounts for 20-25% of the cost of the membrane electrode.

Industry analysis predicted that the market size for global gas diffusion layer materials will reach USD 3.34 billion by 2024. Carbon paper, as the preferred material for the gas diffusion layer, has a very promising future for industry development in China. Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature. Thermal conductivity is one of the important indicators for carbon papers. With the NETZSCH Flash Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.Thermal Conductivity Analyzer LFA 467 and its in-plane holder and foil sample holder, the thermal conductivity of carbon paper samples in the horizontal and vertical directions can be tested accurately and conveniently.