Measurement of in-plane thermal conductivity of carbon paper diffusion media in the temperature range of −20 °C to +120 °C

Carbon paper is commonly used as the gas diffusion layer (GDL) in polymer electrolyte membrane (PEM) fuel cells as it exhibits high chemical and mechanical durability. This diffusion medium is also anisotropic, which directly affects its transport properties and specifically the thermal conductivity. In this study, the in-plane thermal conductivity of the carbon paper GDL was determined using thermal diffusivity measurements for a temperature range from −20 to +120 °C and four Teflon loadings (0, 5, 20 and 50 wt.%). It is important to understand the effect of temperature on the thermal conductivity since PEM fuel cells are designed to operate under various temperatures depending on the application of use. Further, Teflon is used to change the hydrophobic properties of the carbon paper GDL with 20 wt.% as the most widely used percentage. In this study, the Teflon loadings were chosen to gain a comprehensive understanding of the thermal resistance due to Teflon. In this study, a quasi-steady method was used to measure the thermal properties of the carbon paper; hence, the phase transformation in the presence of PTFE was investigated. The thermal conductivity decreases with an increase in temperature for all samples. The addition of as little as 5 wt.% Teflon resulted in high thermal resistance decreasing the overall thermal conductivity of the sample. Further addition of Teflon did not have major effects on the thermal conductivity. For all treated samples, the thermal conductivity lies in the range of 10.1–14.7 W/mK. Finally, empirical relations for the thermal diffusivity and conductivity with temperature were deduced.     

Effect of the anisotropic thermal conductivity of GDL on the performance of PEM fuel cells

Gas diffusion layers (GDLs) are one of the main components in proton exchange membrane (PEM) fuel cells. In this paper, the effect of anisotropic thermal conductivity of the GDL is numerically investigated under different operating temperatures. 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.     

Polybenzimidazole-membrane-based PEM fuel cell in the temperature range of 120–200 °C

Phosphoric acid-doped polybenzimidazole-membrane-based PEM fuel cells were tested in the temperature range of 120–200 °C, with ambient backpressure and 0% RH. AC impedance spectroscopy, surface cyclic voltammetry and fuel cell performance simulation were used to obtain the exchange current densities for the cathodic oxygen reduction reaction (ORR) and anodic hydrogen oxidation reaction (HOR) on platinum-based catalysts at such high temperatures. The activation energies for ORR, HOR and membrane conductivity were also obtained separately. The results showed that temperature significantly affects the charger transfer and gas (O₂ and H₂) diffusion resistances. The effect of O₂ stoichiometry (ST_air) on fuel cell performance was also investigated. Increasing ST_air can effectively increase the O₂ partial pressure in the feed air, leading to improvements in both the thermodynamics and the kinetics of the fuel cell reactions. In addition, it was observed that increasing ST_air could also improve the gas diffusion processes.     

Analytic determination of the effective thermal conductivity of PEM fuel cell gas diffusion layers

Accurate information on the temperature field and associated heat transfer rates are particularly important in devising appropriate heat and water management strategies in proton exchange membrane (PEM) fuel cells. An important parameter in fuel cell performance analysis is the effective thermal conductivity of the gas diffusion layer (GDL). Estimation of the effective thermal conductivity is complicated because of the random nature of the GDL micro structure. In the present study, a compact analytical model for evaluating the effective thermal conductivity of fibrous GDLs is developed. The model accounts for conduction in both the solid fibrous matrix and in the gas phase; the spreading resistance associated with the contact area between overlapping fibers; gas rarefaction effects in microgaps; and salient geometric and mechanical features including fiber orientation and compressive forces due to cell/stack clamping. The model predictions are in good agreement with existing experimental data over a wide range of porosities. Parametric studies are performed using the proposed model to investigate the effect of bipolar plate pressure, aspect ratio, fiber diameter, fiber angle, and operating temperature.     

Hierarchy carbon paper for the gas diffusion layer of proton exchange membrane fuel cells

This communication described the fabrication of a hierarchy carbon paper, and its application to the gas diffusion layer (GDL) of proton exchange membrane (PEM) fuel cells. The carbon paper was fabricated by growing carbon nanotubes (CNTs) on carbon fibers via covalently assembling metal nanocatalysts. Surface morphology observation revealed a highly uniform distribution of hydrophobic materials within the carbon paper. The contact angle to water of this carbon paper was not only very large but also particularly even. Polarization measurements verified that the hierarchy carbon paper facilitated the self-humidifying of PEM fuel cells, which could be mainly attributed to its higher hydrophobic property as diagnosed by electrochemical impedance spectroscopy (EIS).     

Effect of conductive carbon material content and structure in carbon fiber paper made from carbon felt on the performance of a proton exchange membrane fuel cell

This study concerns the use of conductive carbon material with different content and structure to produce carbon fiber paper for use in proton exchange membrane fuel cells, and investigates how changes in the content and structure of the conductive carbon material influence fuel cell performance.In this study, phenolic resin is used as a conductive carbon material, and is subjected to heat treatment at temperatures of 700 °C, 1000 °C, and 1400 °C, which changes its structure. Before carbon fiber paper is prepared from carbon felt, the felt is treated with phenolic resin solutions with resin content of 5, 10, 15, 20, 25, and 30 wt%. During fuel cell testing, torsion of 40, 60, 80, 100, and 120 kgf-cm is applied. The study found that when the phenolic resin content is 15 wt%, the heat treatment temperature 1400 °C, the test area 25 cm², and the test temperature 65 °C, a fuel cell can achieve a current density of 2020 mA cm⁻² at 0.5 V and torque of 120 kgf-cm.     

A study on the characteristics of carbon nanofluids at the room temperature (25 °C)

The proper mixture ratio of nanofluids was examined by measuring thermal conductivity via transient hot-wire method. Comparisons are made with the nanofluid prepared by dispersing oxidized Multi-Walled Carbon NanoTubes (MWCNTs) in distilled water (herein referred to as “oxidized nanofluid”). Viscosity measurements were also carried out for the PVP-added nanofluids and oxidized nanofluids by using a digital viscometer. The nanofluids with 300 wt.% PVP and oxidized MWCNTs exhibited better thermal conductivity than that reported in previous studies. The thermal conductivity of oxidized carbon nanofluids was the highest of those compared and the use of additives in the nanofluid preparation deems to increase viscosity. For industrial applications, the chemical dispersion method applied in the preparation of oxidized carbon nanofluids should be considered as it offers high thermal conductivity with a slight increase in viscosity.     

YSZ-based SOFC with modified electrode/electrolyte interfaces for operating at temperature lower than 650 °C

A NiO/Yttrium-stabilized zirconia (YSZ) transition layer and/or a SDC function layer were introduced into the anode/electrolyte and/or electrolyte/cathode interface to decrease the activation polarization resulted from the mass transfer at electrode/electrolyte interface. With a NiO/YSZ transition layer, the activation polarization simulated from I–V curves drops from 4.42 to 2.42 Ω cm² at 600 °C, about 45% less than that of cell I; with additional SDC function layer, no activation polarization is obviously observed. The cell performance was also remarkably improved with the introduction of both the transition layer and the SDC function layer. Peak power densities of 187 and 443 mW cm⁻² at 600 and 650 °C, respectively, were achieved for a single cell with both a transition layer and a function layer, with an increment of 87% and 95% compared to that of the cell without any structural improvement, and about 30% and 25% compared to that of the cell with only anode transition layer. The study by ac impedance spectroscopy technique also indicated that the interfacial polarization resistance, the main source of cell resistance, could be effectively reduced by interface improvement.     

An attempt to improve electrical conductivity of the pyrolysed carbon-LiMn2O4−ySy (0 ≤ y ≤ 0.5) composites

An attempt to obtain conductive carbon layer (CCL) on the LiMn₂O₄ and LiMn₂O_4−yS_y spinels, using the radical precipitation polymerization of acrylonitrile (AN) and styrene–acrylonitrile (SAN) in water suspension of the spinels followed, by pyrolysis of the composed precursors was made. In the case of PAN–spinel precursors the CCL on spinel particles was effectively formed. Phase transitions, as well as thermal and electrical properties of the carbon-sulphided spinel composites were characterized. An attempt with SAN copolymer failed due to the reaction between SAN and the spinels during pyrolysis, even in the argon stream.     

A novel approach to determine the in-plane thermal conductivity of gas diffusion layers in proton exchange membrane fuel cells

Heat transfer through the gas diffusion layer (GDL) is a key process in the design and operation of a proton exchange membrane (PEM) fuel cell. The analysis of this process requires determination of the effective thermal conductivity. This transport property differs significantly in the through-plane and in-plane directions due to the anisotropic micro-structure of the GDL.A novel test bed that allows separation of in-plane effective thermal conductivity and thermal contact resistance in GDLs is described in this paper. Measurements are performed using Toray carbon paper TGP-H-120 samples with varying polytetrafluoroethylene (PTFE) content at a mean temperature of 65-70 °C. The measurements are complemented by a compact analytical model that achieves good agreement with experimental data. The in-plane effective thermal conductivity is found to remain approximately constant, k ≈ 17.5 W m⁻¹ K⁻¹, over a wide range of PTFE content, and its value is about 12 times higher than that for through-plane conductivity.     

九水合硝酸铝和八水合氢氧化钡导热系数的实验研究

用热敏电阻作加热元件和测温元件,首次测定了适合用作相变储能材料的九水合硝酸铝、八水合氢氧化钡导热系数的测定在10℃~80℃温度范围的导热系数,其实验值的不准确度分别为2.8%和3.2%。同时,还报道了九水合硝酸铝、八水合氢氧化钡实验中观测到的熔点依次为71℃和76℃,这些测量值在1℃~2℃的误差范围内与文献值相吻合。     

Measurement of the thermal conductivities of 2-amino-2-methyl-1,3-propanediol (AMP), 2-amino-2-hydroxymethyl-1,3-propanediol (TRIS) and the mixture (AMP+TRIS, mole ratio 50:50) in the temperature range from 20°C to their supermelting temperatures

2-Amino-2-methyl-1,3-propanediol (AMP), 2-amino-2-hydroxymethyl-1,3-propanediol (TRIS) and the mixture (AMP+TRIS, mole ratio 50:50) are being considered as potential candidates for the thermal storage of energy. The thermal conductivities have been measured with an uncertainty of ±3% from 20°C to the supermelting points of these substances by means of a calorimeter equipped with a thermistor. The experimental thermal conductivities of the substances under investigation are smoothed by fitting for different values of temperature and reported at intervals of 10 K. In addition, the solid–solid transition and melting temperatures are also determined to be 80 and 108°C for AMP, 135 and 175°C for TRIS and 70 and 142°C for (AMP+TRIS), based on the thermal conductivity–temperature diagrams of these substances.