Porous Stainless Steel Substrate Research Articles

Intermediate-temperature operation of solid oxide fuel cells (IT-SOFCs) with thin film proton conductive electrolyte

A novel solid oxide fuel cell (SOFC) structure, which is fabricated on a Pd-plated porous stainless steel substrate, was proposed for low-temperature SOFC operation. The surface of the substrate was covered with Pd layer without any pores, which reduces the difficulty of depositing thin film electrolyte on the porous substrate. A 1.2-μm thick proton conductive Sr(Zr0.8Y0.2)O3-δ (SZYO) layer and the cathode of a 100-nm thick (La0.6Sr0.4)(Co0.2Fe0.8)O3-δ (LSCF) layer were deposited on the Pd-plated substrates by the pulsed laser deposition (PLD) method. The low temperature operations at 400 and 450 °C were demonstrated with proposed SOFC cells.

Development of a novel cell structure for low-temperature SOFC using porous stainless steel support combined with hydrogen permeable Pd layer and thin film proton conductor

A novel cell structure for low-temperature SOFC was proposed, using porous stainless steel support substrates combined with thin hydrogen permeable Pd layers, thin-film proton conductor, and thin-film cathode. Metallic supports consist of sintered ferritic stainless steel spherical powders whose thermal expansion coefficient is close to that of the hydrogen permeable layers and proton conductive electrolytes. To prepare the cell, both the 1.2-μm thick proton conductive Sr(Zr0.8Y0.2)O3−δ electrolyte layer and the 100-nm thick (La0.6Sr0.4)(Co0.2Fe0.8)O3−δ cathode layer were deposited by pulsed laser deposition (PLD) on a Pd-coated porous ferritic stainless steel substrate. The power generation performance at a low temperature of 400 °C was demonstrated in this proposed cell for the first time, while obtained power density is still low and improvements are required.

Novel Electroless Plating of Ruthenium for Fabrication of Palladium-Ruthenium Composite Membrane on PSS Substrate and Its Characterization

Article history: This paper focused on a novel method of electroless plating ruthenium (Ru) on solid or porous substrates like porous stainless steel (PSS) discs or ceramic tubes. A novel complexing plating bath of Ru was developed. It is proven that Ru can be deposited directly on these substrates by the bath at a temperature of 328 K and strong alkaline environment. TGA, SEM, EDX and XRD confirmed the successful deposition of Ru. Relationships among plating rate, plating time, plating bath temperature and concentrations were also investigated. The Ru plating procedure was optimized. Based on the optimized procedure, a Ru layer was deposited by electroless plating of Ru onto the surface of pretreated aluminum-oxide modified PSS substrate. A 7.6 µm thick Pd-Ru membrane could be fabricated by electroless plating with a palladium (Pd) layer overlying the PSS substrate and a Ru layer overlying the Pd layer. Helium leakage and hydrogen permeation tests confirmed that the membrane was free of defects. Hydrogen diffusion through the Pd-Ru membrane was found to be the rate-controlling step in permeation. A new Pd-Ru composite membrane had the same features and permeation behavior as a previous one, indicating that electroless plating of both Ru and Pd is reliable and repeatable.

Preparation and characterization of TiO<sub>2</sub> membrane on porous 316 L stainless steel substrate with high mechanical strength

In this work the preparation and characterization of a membrane containing a uniform mesoporous Titanium oxide top layer on a porous stainless steel substrate has been studied. The 316 L stainless steel substrate was prepared by powder metallurgy technique and modified by soaking-rolling and fast drying method. The mesoporous titania membrane was fabricated via the sol-gel method. Morphological studies were performed on both supported and unsupported membranes using scanning electron microscope (SEM) and field emission scanning microscope (FESEM). The membranes were also characterized using X-ray diffraction (XRD) and N₂ - adsorption / desorption measurement (BET analyses)- It was revealed that a defect-free anatase membrane with a thickness of 1.6 <i>μ</i>m and 4.3 nm average pore size can be produced. In order to evaluate the performance of the supported membrane, single - gas permeation experiments were carried out at room temperature with nitrogen gas. The permeability coefficient of the fabricated membrane was 4 × 10^-8 lit s^-1 Pa^-1 cm^-1.

Effect of Pd concentration on electroless dense Pd-PSS membrane fabrication

This work addresses the effect of palladium solution concentration on combinatorial plating characteristics of surfactant and sonication coupled electroless plating baths for the fabrication of dense palladium films on porous stainless steel substrates. All plating experiments were carried out using palladium solution concentration of 0·005 and 0·01M plating baths with cetyltrimethylammonium bromide surfactant at four critical micelle concentrations and loading ratio of 203 cm2 L−1. The evaluated combinatorial plating characteristics include selective conversion, plating efficiency, plating rate, Pd film thickness and per cent pore densification. The enhancement in palladium solution concentration was found to be insignificant to provide better combinatorial plating characteristics, and the plating bath consisting of 0·005M Pd solution concentration has provided 99·98% per cent pore densification with a Pd film thickness of 8·81 μm and plating efficiency of 80·69%.

Growth of nano-textured graphene coatings across highly porous stainless steel supports towards corrosion resistant coatings

In this paper, we demonstrated for the first time the growth of 3D networks of graphene nano-flakes across porous stainless steel substrates of micron sized metal fibres, and its anti-corrosion properties. The controlled formation of graphene-grade coatings in the form of single sheets to complex and homogeneously distributed 2–4μm long nano-pillars is demonstrated by Scanning Electron Microscopy. The morphology and stability of these structures are shown to be particularly related to the temperature and feed gas flow rate during the growth. The number of layers across the graphene materials was calculated from the Raman spectra and is shown to range between 3 and more than 15 depending on the growth conditions and to be particularly related to the time and flow rate of the experiment. The presence of the graphene was shown to massively enhance the specific surface area of the material and to contribute to the increased corrosion resistance and electrical conductivity of the material without compromising the properties or structure of the native stainless steel materials. This new approach opens up a new route to the facile fabrication of advanced surface coatings with potential applications in developing new thermal exchangers, separation and bio-compatible materials.

PdCuAu ternary alloy membranes: Hydrogen permeation properties in the presence of H2S

PdCuAu ternary alloy membranes with different component compositions were synthesized by sequential electroless deposition of components onto porous stainless steel substrates. The ternary with the highest Au content, Pd69Cu14Au17, exhibited the highest hydrogen permeation flux, comparable to that of a Pd91Au9 membrane. Upon exposure to 100ppm H2S/H2 at 673 K for 24h, all PdCuAu membranes experienced flux reductions of ~55%, followed by recovery to ~80% of the initial hydrogen flux upon reintroduction of pure hydrogen at 400°C. Complete flux recovery after H2S exposure required hydrogen treatment at 500°C. X-ray diffraction (XRD) analysis of the H2S-exposed PdCuAu membranes revealed fcc alloy structure with no evidence of bulk sulfide formation. In agreement with the XRD results, sulfur was not detected in the bulk of H2S-exposed samples by energy dispersive spectroscopy (EDS). However, analysis of H2S-exposed PdCuAu alloys by X-ray photoelectron spectroscopy (XPS) depth profiling revealed low, but measureable, amounts of sulfur in the near-surface region, about 10nm in depth. The depth profiles of samples after hydrogen recovery treatment showed significantly reduced sulfur content. These results indicate that H2S exposure causes flux loss in PdCuAu alloys through a surface-poisoning mechanism, and that the surface sulfide can be removed—and flux recovered—by high temperature treatment in hydrogen.

Microstructure, hardness, and fracture toughness of suspension plasma sprayed yttria-stabilized zirconia electrolytes on stainless steel substrates

Abstract Recent research interest in solid oxide fuel cells (SOFCs) has grown towards effective manufacturing processes that are compatible with low-cost materials such as stainless steel structural supports. Suspension plasma spraying is a promising technique that rapidly produces a fully consolidated, thin electrolyte layer with good microstructure control and without any post-deposition heat treatment. In the present study, suspension plasma spraying was used to deposit yttria-stabilized zirconia electrolyte coatings on porous stainless steel substrates using a range of spray conditions. Mechanical properties of the coatings, such as hardness and fracture toughness, were characterized. These properties can affect the durability of SOFC electrolytes, coupled with the thermal stresses generated during operation. Coating toughness and hardness were observed to be dependent on the torch power and stand-off distance during fabrication, which affect coating microstructural features such as porosity, microcracks, and segmentation cracks. The fracture toughness measured using an indentation technique of the coatings produced with a torch power of 133 kW and a stand-off distance of 90 mm was found to be 1.5 ± 0.15 MPa m1/2.

Novel architectured metal-supported solid oxide fuel cells with Mo-doped SrFeO3−δ electrocatalysts

Barriers to technological advancement of metal-supported SOFCs include nickel coarsening in the anode, metallic interdiffusion between the anode and the metal substrate, as well as poor cathode adhesion. Here we report a robust and novel architectured metal-supported SOFC that consists of a thin dense yttria-stabilized zirconia (YSZ) electrolyte layer sandwiched between a porous 430L stainless steel substrate and a porous YSZ thin layer. The key feature is simultaneous use of impregnated nano-scale SrFe0.75Mo0.25O3−δ coatings on the internal surfaces of the porous 430L and YSZ backbones respectively as the anode and cathode catalyst. Such a fuel cell exhibits power densities of 0.74 W cm−2 at 800 °C and 0.40 W cm−2 at 700 °C when operating on hydrogen fuels and air oxidants.

Aluminizing and oxidation treatments on the porous stainless steel substrate for preparation of H2-permeable composite palladium membranes

Composite palladium membranes based on porous stainless steel (PSS) substrate are idea hydrogen separators and purifiers for hydrogen energy systems, and the surface modification of the PSS is of key importance. In this work, the macroporous PSS tubes were aluminized through pack cementation at 850 °C in argon, followed by an oxidation with air at 600 °C. Palladium membranes were prepared by electroless plating. Their permeation performances were tested, and the hydrogen permeation kinetics was discussed. The substrate materials and the palladium membranes were characterized by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD). An Al2O3-enriched surface layer with small pore size was created through aluminizing and oxidation treatments, which greatly improves the membrane integrity. The intermetallic diffusion between the palladium membranes and the PSS substrate material was not observed after a heat-treatment at 500 °C under hydrogen for 200 h. However, the aluminizing and oxidation treatments still need to be further optimized in order to improve the membrane permeability and selectivity, and particularly, the high diffusion resistance of the substrate materials greatly limited the hydrogen flux.

Performance and Long-Term Stability of Pd/PSS and Pd/Al2O3 Membranes for Hydrogen Separation.

The present work is focused on the investigation of the performance and long-term stability of two composite palladium membranes under different operating conditions. One membrane (Pd/porous stainless steel (PSS)) is characterized by a ~10 µm-thick palladium layer on a porous stainless steel substrate, which is pretreated by means of surface modification and oxidation; the other membrane (Pd/Al2O3) is constituted by a ~7 µm-thick palladium layer on an asymmetric microporous Al2O3 substrate. The operating temperature and pressure ranges, used for studying the performance of these two kinds of membranes, are 350–450 °C and 200–800 kPa, respectively. The H2 permeances and the H2/N2 selectivities of both membranes were investigated and compared with literature data. At 400 °C and 200 kPa as pressure difference, Pd/PSS and Pd/Al2O3 membranes exhibited an H2/N2 ideal selectivity equal to 11700 and 6200, respectively, showing stability for 600 h. Thereafter, H2/N2 selectivity of both membranes progressively decreased and after around 2000 h, dropped dramatically to 55 and 310 for the Pd/PSS and Pd/Al2O3 membranes, respectively. As evidenced by Scanning Electron Microscope (SEM) analyses, the pinholes appear on the whole surface of the Pd/PSS membrane and this is probably due to release of sulphur from the graphite seal rings.

Metal-Supported Solid Oxide Fuel Cells with a Simple Structure

Metal-supported solid oxide fuel cells with porous 430L stainless steel substrates, YSZ electrolytes and porous YSZ cathode backbones are studied. The tri-layer structure is obtained by the tape casting, lamination and co-firing techniques. Nano Ni-Ce0.8Sm0.2O2-δ (SDC) and La0.6Sr0.4Fe0.9Sc0.1O3-δ (LSFSc) particles, which act as the anode and cathode catalysts, individually, are impregnated onto the internal surfaces of the porous 430L and YSZ, respectively. The maximum power densities of the single cell are 0.68, 0.92, 1.09 and 1.23 Wcm−2 at 650, 700, 750 and 800 °C, respectively. Initial 190 h stability test of the single cell at 600 °C under a constant voltage of 0.7 V shows that the current density is stable at about 0.6 A cm−2 and no obvious degradation is observed.

Novel metal-supported solid oxide fuel cells with impregnated symmetric La0.6Sr0.4Fe0.9Sc0.1O3−δ electrodes

This paper reports on the fabrication of novel metal-supported solid oxide fuel cells containing porous 430L stainless steel substrates, YSZ electrolytes and porous YSZ cathode backbones. Such tri-layer structures are obtained by the tape casting, lamination and co-firing techniques. Redox-stable La0.6Sr0.4Fe0.9Sc0.1O3−δ (LSFSc) oxides are introduced as symmetric electrode catalysts onto the internal surfaces of porous 430L substrates and YSZ backbones using the solution impregnation method. The maximum power density is 0.65 W cm−2 measured at 800 °C. Impedance analyses show that the anode polarizations are the largest losses while the cathode polarizations make negligible contribution to the total cell resistances.

Characterization of Aluminum Coatings Formed on Porous Ferritic Stainless Steels by Pack Cementation

Pack cementation is an attractive method for the formation of aluminum coatings on metal substrates. When applied to porous stainless steel substrates, the aluminum vapour species can diffuse into the pores and coat the interior surfaces. In this study, aluminum coatings were grown on commercial porous UNS 430 stainless steel and freeze cast UNS 440 stainless steel substrates by the pack cementation method. The effects of pack mixture composition, temperature, time, and the morphology of the substrates were characterized by scanning electron microscopy. It was shown that the degree of coverage of the aluminum coatings increased with porosity of the substrate. In addition, increased reaction time and temperature of the pack led to a more homogeneous distribution of aluminum across the substrate.

Progress in Metal-Supported SOFCs Using Hydrogen and Methane Fuels

Metal-supported SOFCs with power densities above 700 mW/cm2 at 750oC were fabricated by atmospheric plasma spraying, which can be readily automated and scaled for larger cell areas and production volumes, with no cell sintering requirement. Porous stainless steel substrates were dip-coated to protect against oxidation and chromium loss, followed by fabrication of the functional layers. Preliminary performance in hydrogen was found to be stable for 200 hours, with longer durability tests ongoing. Cells with modified anodes can achieve performances close to those of standard cells operating in hydrogen, with no evidence of coking in TGA methane experiments. Fabrication of anodes for hydrocarbon fuels and the results of scaling up from button cells to 5cm x 5cm cells are discussed.

Metal-supported solid oxide fuel cells with impregnated SrFe0.75Mo0.25O3 cathodes

This paper reports on the fabrication in reducing atmospheres of SrFe0.75Mo0.25O3 (SFMO)–8 mol%Y2O3-stabilized ZrO2 (YSZ) composites by impregnating Sr2+-, Fe3+- and Mo7O246−-containing solutions into the porous YSZ backbones, which would find important applications as cathodes for co-fired metal-supported solid oxide fuel cells. X-ray diffraction examination shows that as-synthesized infiltrates consist of perovskite SFMO oxides and metallic Fe. In situ oxidation during the fuel cell operation eliminates metallic Fe, and SFMO oxides become the predominant component with some minor SrMoO4 impurities. Impedance measurements on symmetric cathode fuel cells show that such impregnated SFMO–YSZ composites exhibit low polarization resistances in air, e.g., 0.06 Ω cm2 at 800 °C. Metal-supported solid oxide fuel cells, consisting of porous 430L stainless steel substrates, Ni–YSZ active anodes, YSZ electrolytes and impregnated SFMO–YSZ composite cathodes, are fabricated using tape casting, tape lamination, co-sintering and solution impregnation techniques, and show maximum power densities of 438 mW cm−2 at 800 °C and 221 mW cm−2 at 700 °C.

Preparation of a novel Pd/layered double hydroxide composite membrane for hydrogen filtration and characterization by thermal cycling

A layered double hydroxide (LDH) layer was grown directly on a porous stainless steel (PSS) surface to reduce the pore opening of the PSS and to be a middle layer retarding Pd/Fe interdiffusion. A thin Pd film (∼7.85 μm) was plated on the modified PSS tube by an electroless plating method. A helium leak test proved that the thin Pd on the LDH-modified PSS substrate was free of defects. The membrane had a H2 flux of 28–36 m3/(m2 h) and H2/He selectivity larger than 2000 at a pressure difference of 1 bar. Thermal cycling between room temperature and 673 K was performed and showed that the membrane exhibited good permeance and selectivity. Long-term evaluation (1500 h) of the membrane at 673 K showed static results of H2 flux (∼30 m3/(m2 h)) and H2/He selectivity (∼2000) over the 1500 h test period.

Improved pre‐treatment of porous stainless steel substrate for preparation of Pd‐based composite membrane

AbstractThe integrity and robustness of Pd‐based composite membranes depend heavily on the quality of the metallic substrate. This paper presents an improved method for pre‐treating the substrate. Detailed pre‐treatment steps of these porous stainless steel substrates included washing, polishing, etching, coating the first alumina layer, calcining the first alumina layer, coating the second alumina layer, calcining the second alumina layer and repair. The repair cycle was the most helpful step, greatly improving the membrane reproducibility and stability. Repeat coating with dilute suspensions of alumina particles of 0.3 µm mean diameter can repair defects on the top alumina layer. The hydrogen permeation flux was 268 × 10−4 N m3/(m2 h Pa) after one repair cycle, decreasing by 14% after an additional repair cycle. The hydrogen permeation flux of the pre‐treated porous stainless steel substrate was thus maintained at a level comparable with previous work. Surface topography revealed that the average roughness was reduced from 1.15 to 0.47 µm, and the surface Maximum Peak to Valley decreased from 17.5 to 6.0 µm/mm2 as a result of the pre‐treatment.

Silver coating on porous stainless steel substrate and preparation of H2-permeable palladium membranes

The H2-permeable palladium membranes based on porous stainless steel (PSS) substrate are important for development of various hydrogen energy systems. To improve the surface of the PSS, a microporous silver layer was deposited successively by a coating with a suspension of silver powder in polyvinyl alcohol (PVA) solution, a heating under nitrogen at 500 °C for carbonization of PVA, an air treatment and a hydrogen reduction. The formation of carbon from PVA helps to maintain the porosity and integrity of the silver layer. After an activation of the resulting Ag/PSS surface through galvanic-cell reaction, palladium membranes with a thickness around 4 μm were successfully prepared by a suction-assisted electroless plating. SEM, EDS, metallography and porometry analyzes were conducted for material characterizations. The prepared Pd/Ag/PSS membrane is permeable and selective as compared with similar those reported in literature. The permeation tests were carried out at 350, 400, 450 and 500 °C for 48, 48, 48 and 60 h, respectively, and the membrane was found to be unstable at 500 °C due to the presence of pinholes. No significant intermetallic diffusion between the silver and palladium layers was observed.

Influence of plasma heat flux on segmentation cracking and permeability of thin suspension plasma sprayed coatings

Solid oxide fuel cell electrolytes with thicknesses of approximately 30μm were deposited by suspension plasma spraying. The torch pass speed (the relative speed between torch and substrate) was adjusted to vary the amount of heat transferred from the plasma to the accumulating coating surface with each pass. With porous stainless steel substrates, no segmentation cracks were observed in electrolytes made with any spray conditions and torch pass speeds, and pass speed did not strongly affect electrolyte permeability. With substrates consisting of cathode or anode layers on porous steel supports, segmentation cracks were observed in most coatings, and the frequency of these cracks diminished with higher torch pass speeds. Electrolyte permeability with these substrates was also lower with higher pass speeds.To better understand the results, plasma-to-surface heat flux profiles were determined for a variety of spray conditions, and these data were used as boundary conditions to model coating and substrate temperatures during spraying. The model found that increasing the pass speed from 64 to 380cms−1 should reduce the peak temperature difference between the electrolyte surface and the steel support by 35 to 40%. It is believed that a reduction in this temperature difference was the primary reason for the observed reduction in segmentation cracking with higher torch pass speeds.