Effect of nonuniformity of the contact pressure distribution on the electrical contact resistance in proton exchange membrane fuel cells

Electrical contact resistance (ECR) is one of the most important factors affecting the ohmic loss in proton exchange membrane (PEM) fuel cells. Dominated by the contact pressure at the interface of two neighboring components, the ECR can be reduced by increasing the clamping force applied on fuel cell stack. However, too large a clamping force will result in excessive resistance to the transport of reactants in the gas diffusion layer (GDL) and even damage to the fuel cell components. Therefore, for a given clamping force, the minimum ECR is expected by making the pressure distribution as uniform as possible. This paper investigates two questions: (a) how to evaluate the distribution of non-uniform pressure based on the ECR, and (b) in what situation will a uniform pressure distribution reduce the ECR obviously, i.e., the sensitivity of the contact resistance to the pressure distribution.     

An experimentally validated thermo-mechanical model for the prediction of thermal contact conductance

A predictive model for estimating thermal contact conductance between two nominally flat metallic rough surfaces has been developed and experimentally validated. The predictive model consists of two complementary parts, the first of which is a surface deformation analysis to calculate the actual area of contact for each contact spot, while the second accounts for the effects of constriction resistance and gas gap conductance between the contacting surfaces. A surface characterization technique is developed which generates an equivalent 3-D surface profile from multiple 2-D profiles and determines the unique wavelengths of importance for the surface deformation and constriction resistance models. For given surface profiles and material properties of two contacting surfaces, and a specified contact pressure, the surface characterization technique filters out non-essential wavelengths on the surface, after which the surface deformation analysis calculates the deformation and contact area of each contacting asperity by considering three different modes of deformation, namely, elastic, elastic–plastic, and plastic. The constriction resistance model is then used to calculate the constriction resistance for each contacting asperity based on the area of contact and radius of curvature of the asperity. The constriction resistance values for all the contacting asperities are then used to calculate the total thermal contact conductance. An experimental facility has also been constructed to measure thermal contact conductance of interfaces to verify the results of the predictive model. Good agreement has been found between the model predictions and experimental measurements, validating the modeling approach.     

Effect of plastic deformation at room temperature on hydrogen diffusion of S30408

Understanding the influence of plastic deformation on diffusion is critical for hydrogen embrittlement (HE) study. In this work, thermal desorption spectroscope (TDS), slow strain rate test (SSRT), feritscope, transmission electron microscope (TEM) and TDS model were used to study the relation between plastic deformation and hydrogen diffusion, aiming at unambiguously elucidating the effect of plastic deformation on hydrogen diffusion of austenitic stainless steel, S30408. An effective method was developed to deduce apparent hydrogen diffusion coefficient of austenitic stainless steel in this paper. Results indicate apparent hydrogen diffusion coefficient decreases firstly and then increases with increasing plastic deformation at room temperature. Hydrogen diffusion effected by plastic deformation is a complicated process which is suggested to be divided into two processes controlled by dislocation and strain-induced martensite, respectively, and the transition point is about 20% strain demonstrated by experiments in this case.     

Numerical study of PEMFC heat and mass transfer characteristics based on roughness interface thermal resistance model

The thermal contact resistance (TCR) is the main component of proton exchange membrane fuel cell (PEMFC) thermal resistance due to the existence of surface roughness between the components of PEMFC, and the influence of TCR is often ignored in traditional three dimensional PEMFC simulations. In this paper, the heat and mass transfer characteristics including polarization curve, power density curve, temperature distribution, membrane water content distribution, membrane current density are studied under different component surface roughness conditions, and finally the effect of each TCR on the PEMFC performance is studied. It is found that under the same operating conditions, the TCR makes the radial heat transfer of the PEMFC decrease, and the temperature of the membrane electrode and the temperature difference of each component of the PEMFC is higher than that of the model without TCR. When the surface roughness of components in the PEMFC equals 1 μm, 2 μm, 3 μm, the cell current density decreases by 6.56%, 12.46% and 17.17% respectively when the output cell voltage equals 0.3 V, and the cell power density decreases by 3.64%, 7.54%, 13.14% respectively when the cell current density equals 1.2 A·cm^?2. When the TCR between the CL and PEM equals 0.003 K·m²·W^?1, 0.005 K·m²·W^?1, 0.01 K·m²·W^?1, the cell current density is increased by 2.30%, 3.65%, 6.74% respectively under the condition that the output cell voltage equals 0.3 V, and the cell power density is increased by 1.24%, 1.85%, 3.10% respectively when the cell current density equals 1.2 A·cm^?2. The results show that the numerical simulation of PEMFC cannot ignore the effect of TCR.     

Effect of surface roughness of composite bipolar plates on the contact resistance of a proton exchange membrane fuel cell

Polymer electrolyte membrane fuel cell performance strongly depends on properties of the fuel cell stack bipolar plates. Composite bipolar plates, though low cost and convenient in manufacturing, raise a major concern due to their high interfacial contact resistance caused by the mechanical treatment used to remove the polymer-rich layer on the surface. It is observed that most of this contact resistance is governed by electrical properties of the interface layer between the contacting surfaces. Measurements of contact resistance of mechanically polished composite bipolar plate/gas diffusion layer interface reveal a substantial influence of surface topography on the contact resistance, which varies significantly depending on the substrate surface treatment and roughness of composite bipolar plates.     

Modelling of thermal contact resistance within the framework of the thermal lattice Boltzmann method

A novel numerical approach, termed the partial bounce back scheme, is introduced within the framework of the thermal lattice Boltzmann method to account for thermal contact resistance between contacting surfaces. The correlation between thermal contact resistance and the partial bounce back parameter is established. A special case of the scheme leads to a new approach that can be directly applied for the treatment of adiabatic thermal boundary conditions in the thermal lattice Boltzmann method. Numerical examples are provided to validate and demonstrate the accuracy and effectiveness of the proposed methodology.     

Estimation of thermal contact resistance and temperature distributions in the pad/disc tribosystem

In this study, an inverse algorithm based on the conjugate gradient method and the discrepancy principle is applied to estimate the unknown time-dependent thermal contact resistance for the tribosystem consisting of a semi-infinite foundation (disc) and a plane-parallel strip (pad) sliding over its surface, from the knowledge of temperature measurements taken within the foundation. Subsequently, the temperature distributions in the medium can be determined as well. The temperature data obtained from the direct problem are used to simulate the temperature measurements, and the effect of the measurement errors upon the precision of the estimated results is also considered. Results show that an excellent estimation on the time-dependent thermal contact resistance can be obtained for the test case considered in this study. The current methodology can be applied to the prediction of thermal contact resistance in engineering problems involving sliding-contact elements.     

Effect of plastic deformation on the corrosion resistance of ferritic stainless steel as a bipolar plate for polymer electrolyte membrane fuel cells

Stainless steel is one of the best candidate materials for bipolar plate of polymer electrolyte membrane fuel cell (PEMFC) and there have been several manufacturing techniques for stainless steel bipolar plate. The deformation from manufacturing process for bipolar plate can induce the corrosion problem of bipolar plate. The deformed and the stamped stainless steels were examined by evaluating the corrosion resistance to understand the effect of the deformation on the stainless steel as a bipolar plate. The deformation of the stainless steel can significantly affect the corrosion resistance and the deformation from the shaping process for bipolar plate can induce the local anodic sites on the bipolar plate. Therefore, from the corrosion point of view, the shaping process for the bipolar plate is an important factor and the corrosion possibility by shaping process should be considered when selecting the optimum shaping method.     

A new method for numerical simulation of thermal contact resistance in cylindrical coordinates

In this paper, a random numbers model using an innovative equiperipheral grid in cylindrical coordinates has been proposed to predict the contact spot distribution of two rough surfaces at various loads. The ability of this method to predict the contact spot distribution has been proven through comparison with results using a conventional equiangular grid. Further, a network method using such an equiperipheral grid has been developed in order to solve a three-dimensional heat conduction problem where two cylindrical specimens were connected to each other along the longitudinal direction. A uniform heat flux is given at the bottom surface of specimen I, a uniform temperature is maintained at the top surface of specimen II, and thermal insulation is assumed at the outer radius of the two specimens. The present numerical results have been compared to calculations using conventional equiangular grids and to experimental results obtained for cylindrical brass specimens. The present results are shown to compare much more closely with experimental measurements than previous calculations using conventional numerical models.     

Thermal behavior of a copolymer PV/Th solar system in low flow rate conditions

A simulation model of finite differences describing a water heating system using a Hybrid Photovoltaic-Thermal collector manufactured in a copolymer material and running in low flow rate conditions has been developed. It includes the essential thermal transfers. The thermal and electrical performances of this solar system have been studied. The choice of the material and the structure of the solar collector are described. The temperatures evolution is modeled in various parts of the solar system and the stratification of the tank is shown. Average electrical, thermal and global efficiencies are calculated each month. We note the importance of thermal recuperation which can catalyze the development of such systems. The utilization of a copolymer for the total design of the solar collector has numerous advantages as reducing the weight, facilitating the manufacturing and reducing the cost.     

Laminar burning velocities and flame stability analysis of hydrogen/air premixed flames at low pressure

An experimental and numerical study on laminar burning velocities of hydrogen/air flames was performed at low pressure, room temperature, and different equivalence ratios. Flames were generated using a small contoured slot-type nozzle burner (5 mm × 13.8  mm). Measurements of laminar burning velocity were conducted using the angle method combined with Schlieren photography. Numerical calculations were also conducted using existing detailed reaction mechanisms and transport properties. Additionally, an analysis of the intrinsic flame instabilities of hydrogen/air flames at low pressure was performed. Results show that the behavior of the laminar burning velocity is not regular when decreasing pressure and that it depends on the equivalence ratio range. The behavior of the laminar burning velocity with decreasing pressure can be reasonably predicted using existing reaction mechanisms; however changes in the magnitude of the laminar burning velocity are underestimated. Finally, it has been found experimentally and proved analytically that the intrinsic flame instabilities are reduced when decreasing the pressure at sub-atmospheric conditions.     

重型电控共轨柴油机高海拔(低气压)热流量分配试验研究

针对柴油机在高原环境条件下工作时存在热负荷增大、冷却系统散热能力不足等热平衡问题,利用自行设计的重型电控共轨柴油机高海拔(低气压)热平衡模拟试验系统,开展了不同海拔(大气压力)下柴油机全负荷工况热平衡试验研究,分析了不同海拔(大气压力)对柴油机热流量分配的影响规律。结果表明:随着海拔的升高,转化为有效功的热量和排气带走的热量均不断下降,而冷却液带走的热量和余项损失均不断增加。海拔每升高1km,转化为有效功的热量平均下降了11.1kW(约3.8%),且低转速区下降幅度比高转速区大;排气带走的热量平均下降了13.3kW(约4.6%),且高转速区下降幅度比低转速区大;冷却液带走的热量平均增加了8.0kW(约5.3%);余项损失平均增加了16.4kW(约40.1%)。当海拔超过3km时,热流量分配的变化幅度更加明显。     

Thermal gap conductance at low contact pressures (<1 MPa): Effect of gold plating and plating thickness

Thermal contact conductance (TCC) measurements are made on bare and gold plated (?0.5 μm) oxygen free high conductivity (OFHC) Cu and brass contacts in vacuum, nitrogen, and argon environments. It is observed that the TCC in gaseous environment is significantly higher than that in vacuum due to the enhanced thermal gap conductance. It is found that for a given contact load and gas pressure, the thermal gap conductance for bare OFHC Cu contacts is higher than that for gold plated contacts. It is due to the difference in the molecular weights of copper and gold, which influences the exchange of kinetic energy between the gas molecules and contact surfaces. Furthermore, the gap conductance is found to increase with increasing thickness of gold plating. Topography measurements and scanning electron microscopy (SEM) analysis of contact surfaces revealed that surfaces become smoother with increasing gold plating thickness, thus resulting in smaller gaps and consequently higher gap conductance.     

Effect of conducting composite polypyrrole/polyaniline coatings on the corrosion resistance of type 304 stainless steel for bipolar plates of proton-exchange membrane fuel cells

A bilayer conducting polymer coating composed of an inner layer of polypyrrole (Ppy) with large dodecylsulfate ionic groups obtained by galvanostatic deposition, and an external polyaniline (Pani) layer with small SO₄²⁻ groups obtained by cyclic voltammetric deposition was prepared to protect type 304 stainless steel used for bipolar plates of a proton-exchange membrane fuel cell. The corrosion performance of the bare and coated steel in 0.3 M HCl was examined by electrochemical impedance spectroscopy, polarization and open-circuit potential measurements. The experimental results indicated that both the composite Ppy/Pani coatings and the single Ppy coatings increased the corrosion potential of the bare steel by more than 400 mV (saturated calomel electrode), and increased the pitting corrosion potential by more than 500 mV (saturated calomel electrode). The bilayer coatings could reduce the corrosion of the alloy much more effectively than the single Ppy coatings, serving as a physical barrier and providing passivity protection, with acceptable contact resistance.     

Steam reforming of CH4 at low temperature on Ni/ZrO2 catalyst: Effect of H2O/CH4 ratio on carbon deposition

In present work, the H₂O/CH₄ on carbon deposition in SRM reaction over Ni/ZrO₂ was studied. Prepared by impregnation, the catalysts were characterized by TEM, XPS, H₂-TPR, TG-DSC-MS, Raman, and XRD. The results showed that when H₂O/CH₄ = 2 with GHSV of 14460 h⁻¹, the highest CH₄ conversion (46.8%) was achieved on the Ni/ZrO₂ catalyst at 550 °C. This was due to the relatively high surface content of Ni⁰ (27%) species and the formation of easy removal polymer carbon (C₁). When the H₂O/CH₄ was 1, a large amount of difficult to remove fiber carbon (C₂) formed and polymer carbon would wrap around the catalyst to reduce its activity. However, excess water might promote surface reconstruction by converting Ni⁰ to Ni(OH)₂, which reduced the surface Ni⁰ content and then inhibited the catalytic activity, while the amount of carbon deposited, especially C₂, reduced significantly.     

ORR activity and stability of PtCr/C catalysts in a low temperature/pressure PEM fuel cell: Effect of heat treatment temperature

A series of PtCr/C catalysts was prepared by chemical reduction via the seeding/impregnation technique using sodium borohydride (NaBH₄) as the reducing agent and then treated at different temperatures in the range of 500–900 °C under a N₂ atmosphere for 2 h at ambient pressure. The effect of heat treatment on the catalyst morphology, oxygen reduction reaction activity and stability in a low temperature/pressure proton exchange membrane (PEM) fuel cell was explored. Increasing the heat treatment temperature within this range increased the degree of alloying, particle sizes, Cr content and in-plane conductivity, but diminished the catalyst dispersion and the electrochemical surface area of all the PtCr/C catalysts. Among all the prepared PtCr/C catalysts, that heat-treated catalyst at 500 °C (PtCr/C-500) was the most promising catalyst for the ORR in a PEM fuel cell under a H₂/O₂ environment, considered in terms of the catalytic activity and stability.