Evaluation of porous 430L stainless steel for SOFC operation at intermediate temperatures

In this paper a 430L porous stainless steel is evaluated for possible SOFC applications. Recently, there are extensive studies related to dense stainless steels for fuel cell purposes, but only very few publications deal with porous stainless steel. In this report porous substrates, which are prepared by die-pressing and sintering in hydrogen of commercially available 430L stainless steel powders, are investigated. Prepared samples are characterized by scanning electron microscopy, X-ray diffractometry and cyclic thermogravimetry in air and humidified hydrogen at 400 °C and 800 °C. The electrical properties of steel and oxide scale measured in air are investigated as well. The results show that at high temperatures porous steel in comparison to dense steel behaves differently. It was found that porous 430L has reduced oxidation resistance both in air and in humidified hydrogen. This is connected to its high surface area and grain boundaries, which after sintering are prone to oxidation. Formed oxide scale is mainly composed of iron oxide after the oxidation in air and chromium oxide after the oxidation in humidified hydrogen. In case of dense substrates only chromium oxide scale usually occurs. Iron oxide is also a cause of relatively high area-specific resistance, which reaches the literature limit of 100 mΩ cm² when oxidizing in air only after about 70 h at 800 °C.     

Efficient hydrogen production from aqueous methanol in a PEM electrolyzer with porous metal flow field: Influence of change in grain diameter and material of porous metal flow field

Previous research has shown that hydrogen production performance of a PEM methanol electrolyzer was largely improved with a porous flow field made of sintered spherical metal powder compared with a conventional groove type flow field. In this study, we experimentally investigated the effect of the change in grain diameter and material of the porous metal flow field on hydrogen production performance in a PEM methanol electrolyzer cell. The experimental results indicated that the hydrogen production performance of the electrolyzer cell was improved by reducing the grain diameter. This could be mainly attributed to the lower interfacial contact resistance by reducing the grain diameter of the porous metal flow field. For investigating the influence of material, cell performances with a stainless steel and a nickel base alloy were compared.     

Modeling and prediction of the effective thermal conductivity of random open-cell porous foams

Although highly desirable, accurate prediction of the effective thermal conductivity of high-porosity open-cell porous foam materials has remained to be a challenging problem. Aiming at this thorny obstacle, we have developed a random generation-growth method to reproduce the microstructures of open-cell foam materials via computer modeling, and then solve the energy transport equations through the complex structure by using a high-efficiency lattice Boltzmann method in this contribution. The effective thermal conductivities of open-cell foam materials are thus numerically calculated and the predictions are compared with the existing experimental data. Since the porosity is high, the predicted thermal conductivity caused by thermal conduction is lower than the measured data when the thermal conductivity of either component is very low and the radiation heat transfer is non-negligible. After considering the radiation effect, the numerical predictions agree rather well with the experimental data. The radiation influence is diminishing as the material porosity decreases. In general the effective thermal conductivity of open-cell foam materials is much higher than that of granular materials of the same components due to the enhanced heat transfer by the inner netlike morphology of the foam materials.     

Optimization of Thermal Insulation Performance for the Porous Materials

ABSTRACT

Porous materials are widely used in porous media filtration, membrane separation, catalyst substrates, solid fuel cells, insulation, and other fields. When the porous material used in the field of insulation, heat transfer characteristics become its most important performance parameters. The heat transfer characteristics of porous material is a complex issue affected not only by solid elements and porosity, it is also affected by composite structures. Therefore, how to optimize the heat transfer properties of porous materials is a problem to be urgently solved. In this paper, the numerical method is used to study the effects of pore size, pore shape, pore connectivity, porosity and so on. It is found that pore shape, pore connectivity and gas conductivity have great impacts on the heat transfer of porous materials. The effect of pore arrangement is very little. The design optimization of porosity is affected by porous material mechanical property.     

Synthesis of mesoporous carbon-silica-polyaniline and nitrogen-containing carbon-silica films and their corrosion behavior in simulated proton exchange membrane fuel cells environment

In this study, polyaniline is deposited onto mesoporous carbon-silica-coated 304 stainless steel using electropolymerization method. Variation of the electropolymerization time and applied potential can affect the growth of polyaniline, and lead to different structural and electrochemical properties of the films. Nitrogen-containing groups are successfully introduced onto the mesoporous carbon-silica film by pyrolyzing treatment under N₂ atmosphere and the electrical conductivity is improved observably compared with the carbon-silica film. The electrochemical properties of the mesoporous carbon-silica-polyaniline films and nitrogen-containing carbon-silica composite films are examined by using potentiodynamic polarization, potentiostatic polarization and electrochemical impedance spectroscopy. The corrosion tests in 0.5 M H₂SO₄ system display that the carbon-silica-polyaniline films show the optimal protective performance. However, according to potentiostatic polarization process, nitrogen-containing carbon-silica film with a water contact angle 95° is extremely stable and better for the protection of stainless steel in simulated fuel cell environment compared to carbon-silica-polyaniline film. Therefore, the nitrogen-containing carbon-silica-coated 304 stainless steel is a promising candidate for bipolar plate materials in PEMFCs.     

Effects of thermal oxi-nitridation on the corrosion resistance and electrical conductivity of 446M stainless steel for PEMFC bipolar plates

Stainless steel has attracted interest as a bipolar plate material for polymer electrolyte membrane fuel cells due to its excellent mechanical properties, good corrosion resistance, and low cost. However, the application of thermal nitridation for the improvement of electrical conductivity deteriorates the corrosion resistance under PEMFC operating conditions due to the discontinuous formation of external Cr-nitride. In this study, nitridation with pre-oxidation of 446M stainless steel was performed in order to improve both the corrosion resistance and the electrical conductivity. 446M stainless steels with oxide and nitride on the surface were evaluated to assess their feasibility as a bipolar plate material for PEMFCs. The results were compared with those obtained using as-received and only nitrided 446M stainless steels. The oxide formed by the pre-oxidation protects the surface of 446M stainless steel from corrosion in corrosive environments, especially under cathode conditions, and the Cr-nitride formed by the subsequent nitridation serves as an electro-conductive channel. As a result, the pre-oxidized, nitrided 446M stainless steel exhibits improved corrosion properties and electrical conductivity under PEMFC operating environments.     

Examination of different lattice structures in porous electrodes using a three-dimensional conductivity model

In this paper a three-dimensional (3D) conductivity model previously developed for a porous electrode having a simple cubic lattice structure is expanded to include other lattice structure types. The other structures analyzed are the extended simple cubic (XSC), body centered cubic (BCC), and face centered cubic (FCC) lattices. As in the previous paper, we model the 3D lattice of nodes as identical spheres in electrical contact with each of its neighbors, and calculate the micro-porosity (i.e. the space between the spheres) for each of the three lattice structures. We use these theoretical derivations to analyze positive electrodes in lead acid batteries. We calculate the macro-porosity (i.e. porosity created by vacant node elements) needed to achieve a 50% total porosity for a lead acid positive electrode whose porosity distributions had been experimentally determined previously by other researchers. When the theoretical micro-porosities are compared to the experimentally determined porosity distributions, the sphere size for each of the lattice arrangements can be estimated. In addition, we estimate the critical volume faction (CVF) which is the maximum utilization of the positive electrode's active material for each lattice type. For these different lattice structures, the estimated node sizes vary from 1 to 10 μm and the critical volume fraction vary from 59% (SC) to 67% (XSC). An important result of this work is that the critical volume fractions for the physically realizable structures (i.e. SC, BCC, and FCC) are all very close to 60% which is the CVF for the 2D conductivity model.     

Synthesis, sintering and impedance spectroscopy of 8 mol% yttria-doped ceria solid electrolyte

CeO₂–8 mol% Y₂O₃ solid solution was prepared by the coprecipitation technique. Cerium nitrate and yttrium oxide were used as precursors. The main purpose of this work was to evaluate some precipitation parameters, the densification behaviour of powder compacts, and the electrical resistivity of sintered ceramics. Results on thermal analyses show that the decomposition of the precipitated gel is almost complete at 400 °C. The specific surface area of calcined powders decreases for increasing precipitation temperature. For powders precipitated at room temperature, the BET surface area is also found to be dependent on the gel storage time before calcination. The densification of powder compacts increases sharply for sintering temperatures higher than 1400 °C. However, a few minutes at 1500 °C is sufficient for the compacts to attain a high densification. Impedance spectroscopy experiments reveal that the ionic resistivity is not dependent on most of the synthesis parameters, although the grain size play a key role in the intergranular component of the resistivity.     

Preparation of Cr-based multilayer coating on stainless steel as bipolar plate for PEMFCs by magnetron sputtering

In the present study, a multilayer composing of Cr₃Ni₂/Cr₂N/CrN is sputtered onto stainless steel. The potential of using the coated stainless steel as the bipolar plate for polymer electrolyte membrane fuel cell (PEMFC) is evaluated. The coated stainless steel exhibits improved corrosion resistance and higher electrical conductivity. The coated surface also demonstrates a hydrophobic characteristic. By using single cell test, the multilayer-coated SS304 plate exhibits an improved performance in terms of I-V properties.     

Accelerated melting of PCM in a multitube annulus‐type thermal storage unit using lattice Boltzmann simulation

Melting of the ice/water as the phase change material in a horizontal single‐tube annulus is sluggish when the stable stratification exists at the bottom of the configuration. To obviate this problem, three heat transfer enhancement techniques could be implemented using the enthalpy‐based lattice Boltzmann method with the double distribution function method to accelerate the process. The multifarious arrangements of the tubes in this horizontal annulus are investigated to expand the region affected by the natural convection. Also, the dispersion of the Cu nanoparticles in the base PCM could boost the thermal conductivity and melting rate. Finally, the metallic porous matrix made of nickel–steel alloys and saturated with the base PCM could be used to enhance the thermal conductivity of the base PCM. The solid–liquid phase change process is defined as the constrained melting of ice‐water in the tube heating mode. There is a thermal equilibrium between ice/water and the nickel–steel porous matrix and the Cu nanoparticles. The Prandtl number, Stefan number, Rayleigh number, and Darcy number are 6.2, 1, 10⁴–10⁵, and 10^?3, respectively. The volumetric concentric of the nanoparticles is between 0 and 0.02 and the porosity ranging from 1 to 0.9 in the representative elementary volume scale.     

Effect of plasma nitriding on behavior of austenitic stainless steel 304L bipolar plate in proton exchange membrane fuel cell

A dense and supersaturated nitrogen layer with higher conductivity is obtained on the surface of austenitic stainless steel 304L by the low temperature plasma nitriding. The effect of plasma nitriding on the corrosion behavior and interfacial contact resistance (ICR) for the austenitic stainless steel 304L was investigated in 0.05 M H₂SO₄ + 2 ppm F⁻ simulating proton exchange membrane fuel cell (PEMFC) environment using electrochemical and electric resistance measurements. The experiment results show that the stable passive film is formed after the potentiostatic polarization at the specified anodic or cathodic potentials under PEMFC operation condition, and the plasma nitriding improves slightly the corrosion resistance and decreases markedly the ICR of 304L. The ICR of the plasma nitrided 304L increases after the potentiostatic polarizations for 4 h, and lower than 100 mΩ cm² at the compaction force of 150 N cm⁻².     

Highly dense (Mn,Co)3O4 spinel protective coating derived from Mn–Co metal precursors for SOFC interconnect applications

The major degradation issues of solid oxide fuel cells (SOFC) are associated with the Cr₂O₃ scale growth and Cr diffusion of the Cr-based ferritic stainless steel (FSS) interconnects. Although (Mn,Co)₃O₄ has been proved as a suitable material for protecting FSS interconnects, the porous structure of the coatings prepared with the pre-synthesized spinel weakens the protective capability of the coatings. In this paper, the widely-used pre-synthesized spinel is replaced with metal precursors (Mn and Co powders). Due to the low melting point (≤1290 °C) and the volume expansion during oxidation, the metal precursors, can be effectively sintered at 900 °C in a reducing atmosphere and form dense, well-protective coatings at 850 °C in the air. The samples are characterized with X-ray diffraction (XRD), scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and a 4-probe area-specific resistance (ASR) test. Compared with the coatings derived from pre-synthesized spinel, the metal-derived coatings present denser structures with better electrical conductivity (ASR = 5.76 mΩ cm²). The weight gain and ASR measurement results indicate that the metal-derived coatings significantly mitigate the increase of weight gain and ASR by inhibiting scale formation and growth, showing better protective capability for SOFC applications.     

基于激光快速加热的金属材料热扩散率及导热系数快速测量方法的研究

基于激光高能快速加热和多点温度计算机自动采集技术,以一维不稳定导热模型为核心,设计了金属材料导热系数的实验测量方案,并实测了304不锈钢的导热系数,测试结果具有较高的工程精度。     

Design optimization of a tubular solar receiver with a porous medium

The main objective of this research is to find the optimal design point of the proposed solar receiver concept to heat up compressed air. Within a tubular receiver made of stainless steel, a porous medium is filled to enhance the heat transfer via the large contact area and thereby to increase the system efficiency. Due to the low melting point associated with the selected material, a numerical simulation is conducted to pre-evaluate the effects of various controlling parameters on the maximum temperature and pressure loss of the system. The design factors expected to influence the system performance were the length, porosity, and thermal conductivity of the porous medium as well as the number of inlet pipes. The effect of each variable on the maximum temperature and pressure drop of the system is numerically investigated and the optimal design point is selected. The results of this study offer a valuable design guideline for future manufacturing processes.     

含湿非饱和多孔介质的完全分形传热研究

为了探究在含湿情况下多孔介质有效导热率的变化,基于分形理论,考虑多孔介质在含湿时加热过程中相变的影响,结合加热过程中的热量守恒方程和傅里叶导热定律推导出计算有效导热率的新公式。将该模型相关数据代入进行计算,分析了孔隙率、含湿率、面积分形维数和迂曲分形维数对有效导热率的影响。研究发现,孔隙率与有效导热率呈负相关,含湿率与有效导热率呈正相关,分形维数与有效导热率呈负相关。该研究能够反映多孔介质内的传热进程,对于探究微孔结构物质的传热具有一定的指导意义。     

Effects of niobium and titanium addition and surface treatment on electrical conductivity of 316 stainless steel as bipolar plates for proton-exchange membrane fuel cells

Niobium and titanium are added to 316 stainless steel, and then heat treatment and surface treatment are performed on the 316 stainless steel and the Nb- and Ti-added alloys. All samples exhibit enhanced electrical conductivity after surface treatment but have low electrical conductivity before surface treatment due to the existence of non-conductive passive films on the alloy surfaces. In particular, the Nb- and Ti-added alloys experience a remarkable enhancement of electrical conductivity and cell performance compared with the original 316 stainless steel. Surface characterization reveals the presence of small carbide particles on the alloy surface after treatment, whereas the untreated alloys have a flat surface structure. Cr₂₃C₆ forms on the 316 stainless steel, and NbC and TiC forms on the Nb- and Ti-added alloys, respectively. The enhanced electrical conductivity after surface treatment is attributed to the formation of these carbide particles, which possibly act as electro-conductive channels through the passive film. Furthermore, NbC and TiC are considered to be more effective carbides than Cr₂₃C₆ as electro-conductive channels for stainless steel.     

Effect of morphology and space charge on conduction through porous doped ceria

Porous samples of samaria (Sm₂O₃)-doped ceria (CeO₂) (SDC) were fabricated using two methods: (1) conventional sintering of powder compacts; and (2) fabrication of sintered NiO + SDC two-phase samples, reduction of NiO to Ni, and acid leaching of Ni. Electrical conductivities of as-fabricated bar samples were measured using a four-probe DC technique. The conductivity of samples made by acid leaching of Ni was up to two orders of magnitude higher than that of conventionally fabricated samples of the same porosity. This difference was attributed to differences in relative inter-particle neck sizes. A simple analytical model was developed to describe conductivity of porous bodies in terms of inter-particle neck sizes. The total electrical conductivity of samples fabricated by the leaching process was measured as a function of oxygen pressure and temperature. The effect of space charge was included in the model describing electrical conduction through porous bodies. The adverse effects of space charge on conductivity are more pronounced in porous ceria as compared to dense ceria.     

Influence of machining parameters on surface roughness and susceptibility to hydrogen embrittlement of austenitic stainless steels

In the present work, an investigation on the susceptibility to hydrogen embrittlement of AISI 304 and 310 austenitic stainless steels was performed. The hydrogen embrittlement process leads to degradation of mechanical properties and can be accelerated by the presence of surface defects combined with elevated surface hardness. Tensile test specimens of the selected materials were machined by turning with different cutting parameters in order to create variations in surface finish conditions. The samples thus prepared were submitted to tensile tests before and after hydrogen permeation by cathodic charging. Regarding the AISI 304 steel, it was possible to notice that the presence of strain-induced martensite on the material surface led to severe hydrogen embrittlement. In the case of the AISI 310 steel, due to its higher nickel amount, no martensite formation could be detected, and this steel was found to be less susceptible to embrittlement in the tested conditions.     

Fabrication and characterisation of the graphite bi-polar plates used in A 0.25 kW PAFC stack

The graphite bi-polar plates were fabricated using lamination technique with polyether sulfone (PES) films (50 μm) and graphite foils (400 μn) in between the two porous graphite plates (CBC) by keeping in a specially designed and fabricated fixture with stainless steel plates at the top and bottom. The fixture was then kept in an hydraulic hot press, at loads of 10–20 tons, and heat treated at 410 °C for 30 min. Then these graphite plates were sized to 30 cm × 20 cm × 0.64 cm, leaving 0.4 mm thick graphite foil at the centre of the plate, to avoid the intermixing of the hydrogen and oxygen/air. While, the gas permeability (cm²/sec) of the plates was determined, with N₂ gas using differential pressure method, their electrical resistivity (mΩm) was measured using milliohmmeter and passing DC current to the graphite plates, at loads from 1–5 kgs. The values of permeability and electrical resistivity of the plates are found to be lower than 0.01 cm²/sec and 4–14 mΩm respectively. A stack with 6 cells was assembled using the in house developed graphite bi-polar plates, anodes and cathodes with matrix, to generate a DC power of 0.25 kW (3.6 V × 71.0 amps). It was operated for 300 h successfully using H₂ and Air, 1 bar, at 175 °C. In this paper, the detailed fabrication method of graphite bi-polar plates and their characteristics of gas permeability, electrical resistivity and the results of the 0.25 kW PAFC stack operation are presented.     

Sucrose-nitrate auto combustion synthesis of Ce0.85Ln0.10Sr0.05O2-δ (Ln = La and Gd) electrolytes for solid oxide fuel cells

Nanocrystalline powders of co-doped ceria oxides Ce_0.85La_0.10Sr_0.05O_2-δ (CLSO) and Ce_0.85Gda_0.10Sr_0.05O_2-δ (CGSO) have been synthesized by auto combustion method at 100°C using sucrose as fuel. Thermal analysis (TGA/DSC) of as-prepared powders indicated calcination above 400°C to remove organic residue. The average grain size of the pellets sintered at 1200°C for 4 hours is 436 and 683 nm for CLSO and CGSO, respectively. The electrical conductivity of the sintered samples was determined by impedance measurements in the temperature range 300°C to 600°C and the frequency range 20 Hz to 2 MHz. At 600°C, the total electrical conductivity (σ_t) of CGSO is 6.78 × 10⁻³ S cm⁻¹, 2.5 times higher than 2.72 × 10⁻³ S cm⁻¹ of CLSO. Further, it is found that the value of grain boundaries blocking factor (α_gb) of CGSO is 0.47 which is 30% lesser than 0.68 of CLSO at 600°C. The higher value of electrical conductivity of CGSO as compared to CLSO is attributed to the lesser blocking effect of grain boundaries, smaller lattice distortion and denser microstructure of CGSO as compared to CLSO. The electrical conductivity of synthesized samples has been compared with the electrical conductivity of similar compositions of co-doped CeO₂ oxides. Our study indicated that the sintering temperature, and hence, the morphology of sintered samples has a significant role in determining the electrical conductivity. The presence of oxygen vacancies in the synthesized samples is experimentally supported by using UV-visible spectroscopy, Raman spectroscopy, and thermal analysis techniques.