Measurement of the through-plane thermal conductivity of carbon paper diffusion media for the temperature range from −50 to +120 °C

Carbon paper, a fibrous material, is often used as the gas diffusion layer in polymer electrolyte membrane (PEM) fuel cells, which are being vigorously developed as a zero-emission power source for transportation applications. The temperature field and heat transfer in this material is determined by its thermal conductivity and diffusivity, which are directly dependent on the operating temperature. In this work, we use a quasi-steady method known as the thermal capacitance (slug) method to experimentally measure the through-plane thermal conductivity of TORAY carbon paper for a temperature range from −50 to +120 °C. The effects of compression and PTFE loading on the overall thermal conductivity are also investigated. Compression leads to a decrease in thermal resistance between the carbon fibers; hence, an increase in the overall thermal conductivity. However, it is also found that this thermal resistance is highly dependent on the temperature and the PTFE loading. In contrast with our in-plane thermal conductivity measurements from a previous study, the through-plane thermal conductivity is found to increase with an increase in temperature in this study. This finding suggests that the thermal expansion of the carbon fibers is a direction dependent quantity.     

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.     

The impact of thermal conductivity and diffusion rates on water vapor transport through gas diffusion layers

Proper water management in a hydrogen-fueled polymer electrolyte membrane (PEM) fuel cell is critical for performance and durability. A mathematical model has been developed to elucidate the effect of thermal conductivity and water vapor diffusion coefficient in the gas diffusion layers (GDLs). The fraction of product water removed in the vapor phase through the GDL as a function of GDL properties/set of material and component parameters and operating conditions has been calculated. The current model enables identification of conditions wherein condensation occurs in each GDL component. The model predicts the temperature gradient across various components of a PEM fuel cell, providing insight into the overall mechanism of water transport in a given cell design. The water condensation conditions and transport mode in the GDL components depend on the combination of water vapor diffusion coefficients and thermal conductivities of the GDL components. Different types of GDLs and water transport scenarios are defined in this work, based on water condensation in the GDL and fraction of water that the GDL removes through the vapor phase, respectively.     

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.     

宽温域中碳纤维导热特性研究

碳纤维作为一种被广泛应用的微纳米材料,对其导热性能的测量研究一直被作为对碳纤维性能研究的重要内容。在利用氦气的气体液化基础上搭建的超低温实验环境中,基于瞬态电热法对处于290到10 K温度内的碳纤维样品的导热性能进行研究。实验发现,当实验温度低于某一特定温度后,材料的热扩散率表现出与声子散射分析相反的实验结果。通过引入热扩散系数倒数这一理论研究声子热阻在低温下的变化,分析得出,当实验环境温度低于某一特定温度后,低温会造成碳纤维材料内的石墨微晶体结构发生变化,从而造成材料热扩散率下降。     

Effect of anisotropic thermal conductivity of the GDL and current collector rib width on two-phase transport in a PEM fuel cell

A two-dimensional two-phase model based on the classical two-fluid model is used to analyze electrochemical and thermal transport in a PEMFC. The model is extended to account for the dependence of interfacial area density on liquid volume fraction. At a given fixed voltage, the fuel cell generates maximum current density for low through-plane and high in-plane thermal conductivities at high humidity operating conditions. It is also predicted that for low humidity operating conditions, the fuel cell generates maximum current density if the GDL is tailored to have high through-plane thermal conductivity near the inlet and progressively decreasing through-plane thermal conductivity at distances away from the inlet. At fully humidified cathode inlet conditions, narrower current collector ribs generate higher current densities at all voltages by reducing the resistance to diffusion of reactants and products through the GDL. In order to maximize the current density at low humidities, ribs must be wider near the inlet and narrower away from the inlet. The proposed methodology for tailoring GDL through-plane thermal conductivities and rib widths reduces the risk of membrane dehydration near inlet and also reduces the possibility of excessive liquid accumulation in the region away form the inlet.     

Effect of polytetrafluoroethylene-treatment and microporous layer-coating on the electrical conductivity of gas diffusion layers used in proton exchange membrane fuel cells

The purpose of this study is to investigate the effect of ploytetrafluoroethylene (PTFE)-treatment and microporous layer (MPL)-coating on the electrical conductivity of gas diffusion layers (GDLs), as used in proton exchange membrane fuel cells (PEMFCs). The results show that, for PTFE-treated GDLs, the electrical conductivity in orthogonal in-plane directions is almost invariant with the PTFE loading. On the other hand, the in-plane conductivity of the MPL-coated GDL SGL 10BE (50% PTFE) was found to be higher than that of the counterpart SGL 10BC (25% PTFE) and this was explained by the presence of more conductive carbon particles in the MPL of SGL 10BE. Further, the conductivity of each GDL sample was measured in two perpendicular in-plane directions in order to investigate the in-plane anisotropy. The results show that the electrical conductivity of the GDL sample in one direction is different to that in the other direction by a factor of about two. The contact resistance, the main factor affecting the through-plane conductivity, of PTFE-treated GDLs shows a different trend to the corresponding in-plane conductivity, namely it increases as the PTFE loading increases. On the other hand, the contact resistance of the MPL-coated GDL SGL 10BE (50% PTFE) was found to be lower than that of the counterpart SGL 10BC (25% PTFE) and again this was explained by the presence of more conductive carbon particles in the MPL of SGL 10BE. Also, it was noted that the MPL coating appears to have a positive effect in reducing the contact resistance between the GDL and the bipolar plate. This is most likely due to the compressibility of the MPL layers that allows them to fill in the ‘gaps’ that exist in the surface of the bipolar plates and therefore establishes a good contact between the latter plates and the GDLs. Finally, good curve fitting of the contact resistance as a function of the clamping pressure has been achieved.     

Thermal management of high temperature polymer electrolyte membrane fuel cell stacks in the power range of 1–10 kWe

Maintaining optimal temperature of the stack is critical for efficient operation of high temperature polymer electrolyte membrane fuel cells. While a number of possibilities of thermal management exist for small stacks, the problem becomes more complicated for larger stacks. In the present study, the thermal management of stacks in the power range of 1–10 kWe is considered through computational fluid dynamics simulations. It is shown that large stacks need to have dedicated cooling plates through which a coolant is circulated. Further, stacks of the size of 10 kWe can have reasonably low cell temperature variations (∼20 K) only by passing pre-heated liquid coolant through the coolant plates. Estimates show that the concomitant increase in the coolant flow rate induces large pressure drops, of the order of 30 bar, if a four-parallel serpentine is used on an active cell area of 30 cm × 30 cm. It is therefore necessary to use parallel channel flow fields with carefully designed feeder manifolds to maintain optimal cell temperatures and reasonably low coolant pressure drops in large stacks.     

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.     

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.     

An experimental and theoretical investigation of the thermal treatment of wood (Fagus sylvatica L.) in the range 200-260 °C

A comprehensive heat and mass transfer computational model is used to analyse the intricate two-way coupling arising from the activated chemical reactions involved in the heat treatment of wood. The 2D version of a drying code known as TransPore is used to simulate the coupled heat and mass transfer phenomena. This code accounts for the internal pressure in the porous medium. The pyrolysis model describing the chemical reactions occurring in the main constituents within the cell walls of wood (cellulose, hemicelluloses and lignins) is derived using data taken from the literature. Refined computational strategies were required to address the two-way coupling between the heat and mass transfer and chemical mechanisms, including the thermal activation of the chemical reactions, together with the treatment of heat sources (or sinks) and the production of volatiles. The experimental set-up allows the overall weight loss, and the internal temperature and pressure at specific locations within the board to be determined during processing. The reported simulations highlight that the model is able to capture two particular phenomena observed during the heat treatment of wood: the double pressure peak due to water evaporation and volatiles production; and the temperature overshoot during the heat treatment phase.     

Performance of a single nutating disk engine in the 2 to 500 kW power range

A new type of internal combustion engine with distinct advantages over conventional piston-engines and gas turbines in small power ranges is presented. The engine has analogies with piston engine operation, but like gas turbines it has dedicated spaces and devices for compression, burning and expansion. The engine operates on a modified limited-pressure thermodynamic cycle. The core of the engine is a nutating non-rotating disk, with the center of its hub mounted in the middle of a Z-shaped shaft. The two ends of the shaft rotate, while the disk nutates. The motion of the disk circumference prescribes a portion of a sphere. In the single-disk configuration a portion of the surface area of the disk is used for intake and compression, a portion is used to seal against a center casing, and the remaining portion is used for expansion and exhaust. The compressed air is admitted to an external accumulator, and then into an external combustion chamber before it is admitted to the power side of the disk. The external combustion chamber enables the engine to operate on a variable compression ratio cycle. Variations in cycle temperature ratio and compression ratio during normal operation enable the engine to effectively become a variable-cycle engine, allowing significant flexibility for optimizing efficiency or power output. The thermal efficiency is similar to that of medium sized diesel engines. For the same engine volume and weight this engine produces approximately twice the power of a two-stroke engine and four times the power of a four-stroke engine. The computed sea-level engine performance at design and off-design conditions in the 2 to 500 kW power range is presented.     

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。     

New results on the visco-elastic behaviour of ionomer membranes and relations between T–RH plots and proton conductivity decay of Nafion 117 in the range 50–140 °C

The colligative properties of acidic solution inside Nafion^® 117 membranes have been investigated, in a large temperature range, by two different methods.     

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.     

Activity,stability and deactivation behavior of Au/CeO2 catalysts in the water gas shift reaction at increased reaction temperature (300 °C)

The effect of increasing the reaction temperature to 300 °C on the activity, stability and deactivation behavior of a 4.5 wt.% Au/CeO₂ catalyst in the water gas shift (WGS) reaction in idealized reformate was studied by kinetic and spectroscopic measurements at 300 °C and comparison with previously reported data for reaction at 180 °C under similar reaction conditions [A. Karpenko, Y. Denkwitz, V. Plzak, J. Cai, R. Leppelt, B. Schumacher, R.J. Behm, Catal. Lett. 116 (2007) 105]. Different procedures for catalyst pretreatment were used, including annealing at 400 °C in oxidative, reductive or inert atmospheres as well as redox processing. The formation/removal of stable adsorbed reaction intermediates and side products (surface carbonates, formates, OH_ad, CO_ad) was followed by in situ IR spectroscopy (DRIFTS), the presence of differently oxidized surface species (Au⁰, Au^0′, Au³⁺, Ce³⁺) was evaluated by XPS. The reaction characteristics at 300 °C generally resemble those at 180 °C, including (i) significantly higher reaction rates, (ii) comparable apparent activation energies (44 ± 1/50 ± 1 kJ mol⁻¹ vs. 40 ± 1 kJ mol⁻¹ at 180 °C), (iii) a correlation between deactivation of the catalyst and the build-up of stable surface carbonates, and (iv) a decrease of the initially significant differences in activity after different pretreatment procedures with reaction time. Different than expected, the tendency for deactivation did not decrease with higher temperature, due to enhanced carbonate decomposition, but increases.