Porous Metal Substrate Research Articles

Double Columnar Interlayer for Increased Cathodic Activity of Intermediate Temperature Solid Oxide Fuel Cells

Effects of double columnar structure interlayer film, in which tensile strain were introduced, on cathodic activity of SmCoO3 was investigated. Double columnar film of Pr2NiO4(PNCG)-Sm doped CeO2 (SDC), PrBaCo2O5.5-SDC, Sm0.6Sr0.4CoO3(SSC)- SDC, and La0.6Sr0.4FeO3-SDC were successfully prepared by PLD method and it was found that insertion of PNCG-SDC or SSC-SDC double columnar film decreased the overpotential of SSC cathode. The power density of the cell deposited on Ni-Fe porous metal substrate was higher than 3W/cm2 at 773K and impedance results suggested that the increased power density was explained by decreased cathodic overpotential. Increased diffusivity of oxide ion in double columnar film is effective for expanding reaction area resulting in the increased cathodic performance for oxide ion dissociation.

Proton-Conducting Ceramics for Metal-Supported Solid Oxide Fuel Cells and Electrolysis Cells

Proton conducting oxide electrolyte materials could potentially lower the operating temperature of metal-supported solid oxide cells (MS-SOCs) to the intermediate range 400 to 600 °C. The porous metal substrate provides the advantages of MS-SOCs such as high thermal and redox cycling tolerance, low-cost of structural materials, and mechanical ruggedness. The cell structure for fuel cell and electrolysis operation is shown in Figure 1. To incorporate proton-conductors into metal-supported cell configuration through co-sintering fabrication, a key factor is that the proton conductors must be compatible with reducing atmosphere sintering, as the porous metal substrate can not be fired in oxidizing atmosphere. In this work, viability of co-sintering fabrication of metal-supported proton conducting solid oxide cells is investigated. Candidate proton-conducting ceramics including perovskite-type oxides BaZr0.7Ce0.2Y0.1O3-δ (BZCY), SrZr0.5Ce0.4Y0.1O3-δ (SZCY), and Ba3Ca1.18Nb1.82O9-δ (BCN), pyrochlore-type oxides La1.95Ca0.05Zr2O7-δ (LCZ) and La2Ce2O7 (LCO), and acceptor doped rare-earth ortho-niobates La0.99Ca0.01NbO4 (LCN) were synthesized via solid state reactive synthesis or sol-gel synthesis. These ceramics are sintered at 1450°C in reducing environment alone and supported on Fe-Cr alloy metal support, and their key characteristics such as phase formation, sintering property, and chemical compatibility with metal support are determined. Most electrolyte candidates suffer from one or more challenges identified for this fabrication approach, including: phase decomposition in reducing atmosphere, evaporation of electrolyte constituents, contamination of the electrolyte with Si and Cr from the metal support, and incomplete electrolyte sintering. BZCY is particularly interesting because of its high proton conductivity, but it is found to suffer from reaction with Si and Cr present in the metal support, and loss of Ba via evaporation during sintering. Various approaches to overcome these limitations are proposed, and preliminary assessment indicates that the use of barrier layers, low-Si-content stainless steel, and sintering aids warrant further development. In contrast, La0.99Ca0.01NbO4 is found to be highly compatible with the metal support and co-sintering processing in reducing atmosphere, allowing dense electrolyte and porous electrode made of LCN to be co-sintered on porous stainless steel, Figure 2. A metal-supported cell is fabricated with La0.99Ca0.01NbO4 electrolyte, ferritic stainless steel support, Pt air electrode and nanoparticulate ceria-Ni hydrogen electrocatalyst. The total resistance is 50 Ω∙cm2 at 600 °C. This work clearly demonstrates the challenges, opportunities, and breakthrough of metal-supported proton-conducting solid oxide cells by co-sintering fabrication. Figure 1

Approaches for co-sintering metal-supported proton-conducting solid oxide cells with Ba(Zr,Ce,Y,Yb)O3-δ electrolyte

Abstract Proton conducting oxide electrolyte materials could potentially lower the operating temperature of metal-supported solid oxide cells (MS-SOCs) to the intermediate range 400 to 600 °C. The porous metal substrate provides the advantages of MS-SOCs such as high thermal and redox cycling tolerance, low-cost of structural materials, and mechanical ruggedness. In this work, the viability of co-sintering fabrication of metal-supported proton conducting solid oxide cells using BaZr1-x-yCexYyO3-δ (BZCY) is investigated. BZCY ceramics are sintered at 1450 °C in reducing environment alone and supported on Fe Cr alloy metal support, and key characteristics such as Ba loss, sintering behavior, and chemical compatibility with metal support are determined. Critical challenges are identified for this fabrication approach, including: Contamination of the electrolyte with Si and Cr from the metal support, incomplete electrolyte sintering, and evaporation of electrolyte constituents. Various approaches to overcome these limitations are proposed, and preliminary assessment indicates that the use of barrier layers, low-Si-content stainless steel, and sintering aids warrant further development.

Lanthanum tungstate membranes for H2 extraction and CO2 utilization: Fabrication strategies based on sequential tape casting and plasma-spray physical vapor deposition

In the context of energy conversion efficiency and decreasing greenhouse gas emissions from power generation and energy-intensive industries, membrane technologies for H2 extraction and CO2 capture and utilization become pronouncedly important. Mixed protonic-electronic conducting ceramic membranes are hence attractive for the pre-combustion integrated gasification combined cycle, specifically in the water gas shift and H2 separation process, and also for designing catalytic membrane reactors. This work presents the fabrication, microstructure and functional properties of Lanthanum tungstates (La28−xW4+xO54+δ, LaWO) asymmetric membranes supported on porous ceramic and porous metallic substrates fabricated by means of the sequential tape casting route and plasma spray-physical vapor deposition (PS-PVD). Pure LaWO and W site substituted LaWO were employed as membrane materials due to the promising combination of properties: appreciable mixed protonic-electronic conductivity at intermediate temperatures and reducing atmospheres, good sinterability and noticeable chemical stability under harsh operating conditions. As substrate materials porous LaWO (non-substituted), MgO and Crofer22APU stainless steel were used to support various LaWO membrane layers. The effect of fabrication parameters and material combinations on the assemblies’ microstructure, LaWO phase formation and gas tightness of the functional layers was explored along with the related fabrication challenges for shaping LaWO layers with sufficient quality for further practical application. The two different fabrication strategies used in the present work allow for preparing all-ceramic and ceramic-metallic assemblies with LaWO membrane layers with thicknesses between 25 and 60 μm and H2 flux of ca. 0.4 ml/min cm2 measured at 825 °C in 50 vol% H2 in He dry feed and humid Ar sweep configuration. Such a performance is an exceptional achievement for the LaWO based H2 separation membranes and it is well comparable with the H2 flux reported for other newly developed dual phase cer-cer and cer-met membranes.

Assessment of co-sintering as a fabrication approach for metal-supported proton-conducting solid oxide cells

Proton conducting oxide electrolyte materials could potentially lower the operating temperature of metal-supported solid oxide cells (MS-SOCs) to the intermediate range 400 to 600 °C. The porous metal substrate provides the advantages of MS-SOCs such as high thermal and redox cycling tolerance, low-cost of structural materials, and mechanical ruggedness. In this work, viability of co-sintering fabrication of metal-supported proton conducting solid oxide cells is investigated. Candidate proton conducting oxides including perovskite oxides BaZr0.7Ce0.2Y0.1O3−δ, SrZr0.5Ce0.4Y0.1O3−δ, and Ba3Ca1.18Nb1.82O9−δ, pyrochlore oxides La1.95Ca0.05Zr2O7−δ and La2Ce2O7, and acceptor doped rare-earth ortho-niobate La0.99Ca0.01NbO4 are synthesized via solid state reactive or sol-gel methods. These ceramics are sintered at 1450 °C in reducing environment alone and supported on Fe-Cr alloy metal support, and their key characteristics such as phase formation, sintering property, and chemical compatibility with metal support are determined. Most electrolyte candidates suffer from one or more challenges identified for this fabrication approach, including: phase decomposition in reducing atmosphere, evaporation of electrolyte constituents, contamination of the electrolyte with Si and Cr from the metal support, and incomplete electrolyte sintering. In contrast, La0.99Ca0.01NbO4 is found to be highly compatible with the metal support and co-sintering processing in reducing atmosphere. A metal-supported cell is fabricated with La0.99Ca0.01NbO4 electrolyte, ferritic stainless steel support, Pt air electrode and nanoparticulate ceria-Ni hydrogen electrocatalyst. The total resistance is 50 Ω·cm2 at 600 °C. This work clearly demonstrates the challenges, opportunities, and breakthrough of metal-supported proton-conducting solid oxide cells by co-sintering fabrication.

Continuous Crystalline Membranes of a Ni(II)-Based Pillared-Layer Metal-Organic Framework In Situ Grown on Nickel Foam with Two Orientations

The membranes of a pillared-layer structure Metal-Organic Framework (MOF), [Ni(HBTC)(4,4′-bipy)] (HBTC = 1,3,5-Benzenetricarboxylic acid, 4,4′-bipy = 4,4′-bipyridine), have been in situ fabricated on Nickel foam substrate. The orientations of MOF crystals in the membranes can be controlled by the molar ratio of ligand H3BTC to 4,4′-bipyridine. Scanning electron microscope images and powder X-ray diffraction patterns were used to characterize the membranes and confirm the orientations of their MOF layers. Control experiments have revealed that the presence of homologous metal element Nickel in both the MOF and the substrate and the presence of the neutral 4,4′-bipyridine in the reaction system are necessary for in situ growth of the well-intergrown MOF membranes. This work provides a successful example of directly growing continuous MOF layers on porous metallic substrate with desired orientations by a facile approach.

Uniformly coated highly porous graphene/MnO2 foams for flexible asymmetric supercapacitors

Supercapacitors are called to play a prominent role in the newly emerging markets of electric vehicles, flexible displays and sensors, and wearable electronics. In order to compete with current battery technology, supercapacitors have to be designed with highly conductive current collectors exhibiting high surface area per unit volume and uniformly coated with pseudocapacitive materials, which is crucial to boost the energy density while maintaining a high power density. Here, we present a versatile technique to prepare thickness-controlled thin-film micro graphene foams (μGFs) with pores in the lower micrometer range grown by chemical vapor deposition which can be used as highly conductive current collectors in flexible supercapacitors. To fabricate the μGF, we use porous metallic catalytic substrates consisting of nickel/copper alloy synthesized on nickel foil by electrodeposition in an electrolytic solution. Changing the duration of the electrodeposition allows the control of the thickness of the metal foam, and thus of the μGF, ranging from a few micrometers to the millimeter scale. The resulting μGF with a thickness and pores in the micrometer regime exhibits high structural quality which leads to a very low intrinsic resistance of the devices. Transferred onto flexible substrates, we demonstrate a uniform coating of the μGFs with manganese oxide, a pseudocapacitively active material. Considering the porous structure and the thickness of the μGFs, square wave potential pulses are used to ensure uniform coverage by the oxide material boosting the volumetric and areal capacitance to 14 F cm−3 and 0.16 F cm−2. The μGF with a thickness and pores in the micrometer regime in combination with a coating technique tuned to the porosity of the μGF is of great relevance for the development of supercapacitors based on state-of-the-art graphene foams.

One millimeter per minute growth rates for single wall carbon nanotube forests enabled by porous metal substrates.

We report an exceptionally high-efficiency synthesis of long single-wall carbon nanotube (SWCNT) forests using porous substrates (metal meshes) in place of nonporous flat substrates. This study examined the dependence of the growth efficiency on various mesh structures, including wire diameter, aperture size, and total surface area. We demonstrated that the synthesis of SWCNT forests is highly dependent on the initial porosity as well as maintaining the open pores throughout the duration of the growth. Our results show that carbon nanotubes (CNTs) can be grown on all surfaces of the mesh in high efficiency with the optimum growth efficiency observed for a mesh porosity of ∼30%. Based on these results, we demonstrated the high efficiency synthesis of SWCNT forests (height: >3.47 mm, average growth rate: 301 μm min−1, and yield: 12.7 mg cm−2 in 10 min growth time). Furthermore, we showed that the initial growth rates exceeded 1 millimeter per minute (1000 μm min−1). Our results further indicate that metal meshes represent a viable alternative to nonporous flat substrates for the efficient synthesis of tall and high yielding SWCNTs.

Sol-gel deposition of hydroxyapatite coatings on porous titanium for biomedical applications

The stress shielding and the poor osseointegration in titanium implants are still problems to be resolved. In this context, this work proposes a balanced solution. Titanium samples were fabricated, with a porosity of 100–200μm of pore size employing space-holder technique (50vol% NH4HCO3, 800MPa at 1250°C during 2h under high vacuum conditions), obtaining a good equilibrium between stiffness and mechanical resistance. The porous titanium substrates were coated with hydroxyapatite, obtained by sol-gel technique: immersion, dried at 80°C and heat treatment at 450°C during 5h under vacuum conditions. Phases, surface morphology and interfacial microstructure of the transverse section were analyzed by Micro-Computed Tomography, SEM and confocal laser, as well as the infiltration capability of the coating into the metallic substrate pores. The FTIR and XRD showed the crystallinity of the phases and the chemical composition homogeneity of the coating. The size and interconnected pores obtained allow the infiltration of hydroxyapatite (HA), possible bone ingrowth and osseointegration. The scratch resistance of the coating corroborated a good adherence to the porous metallic substrate. The coated titanium samples have a biomechanical and biofunctional equilibrium, as well as a potential use in biomedical applications (partial substitution of bone tissue).

RAMMS - A High-Throughput Membrane Measurement System for Fuel Cell Manufacturing

The objective of this work was to develop a robust, high-throughput polymer electrolyte membrane (PEM) resistance measurement system for fuel cell manufacturing quality control (QC). The principal outcome was the Rapid Membrane Measurement System (RAMMS), a fully-automated, off-line device that can be used by membrane manufacturers during production for QC and by membrane users for lot acceptance of a critical component of PEM fuel cell systems. The prototype RAMMS, shown in Figure 1(a), was designed and developed with the following functions and features in mind: (1) rapid and simple sample loading and unloading of bare membrane, (2) safe, long-term, unattended operation, (3) stable control of the cell environment (temperature, RH), and (4) reproducible electrode clamping pressure, (5) user-selectable operating parameters (electrode pressure, temperature, RH, conditioning time, etc.) to facilitate evaluation of membranes of various properties (thickness, conditioning kinetics) and/or with different resistance specification conditions. After developing a detailed specification for the RAMMS in collaboration with an industrial stakeholder, we designed, fabricated and tested a prototype unit. The system includes a novel humidification and environmental control system; integrated sample chamber, electrode and actuator system; impedance measurement for through-thickness membrane resistance; and application software. Studies with the prototype revealed that electrode durability, in particular adhesion of platinum deposit on a porous metallic substrate, was the most significant technical challenge and performance limitation of the device. It was concluded that acceptable durability could not be obtained with any of the deposition methods and post-deposition treatments designed to improve coating adhesion. Custom-fabricated porous platinum electrodes exhibited acceptable durability. In addition to conditioning time, the electrode squeeze pressure was identified as a primary test variable. Quantification of the effect of electrode squeeze pressure on the area-normalized HFR is shown in Figure 1(b). As hypothesized, lower HFR is strongly correlated with greater squeeze pressure. This is rationalized as being primarily driven by decreasing interfacial resistance with increasing electrode contact pressure, and secondarily, some compression and thinning of the membrane. In addition to the decrease in mean HFR, the range and standard deviation also decrease with increasing squeeze pressure. At the highest pressure investigated (2.36 MPa), the relative standard deviation (= standard deviation / mean) was 2.6% (n = 9). The dependence of the HFR on the electrode contact pressure demonstrated by the data in Figure 1(b) is not inconsistent with the statements that variation in the pressure is not the primary source of variability in HFR. We observed that the effect of contact pressure on the HFR was -0.044 mΩ/kPa (-0.3 mΩ/PSI). That is, the resistance decreased approximately 0.044 mΩ per kPa (0.3 mΩ per PSI) increase in squeeze pressure. Other results indicated that the test-to-test variation in squeeze pressure was < 6.9 kPa (< 1 PSI). Therefore, < 0.3 mΩ of variation could be attributed to test-to-test variability in the electrode contact pressure, which is smaller than the observed variability. Thus, the bulk of the test-to-test variation in observed resistance was not due to variation in the electrode squeeze pressure. Acknowledgement: This work was supported by a U.S. Department of Energy Small Business Innovation Research grant (DE-FG02-06ER84574). The support of Will Johnson (W.L. Gore & Associates) and the Gore Fuel Cell Group for helpful discussions and technical assistance is gratefully acknowledged. Figure 1

Metal Supported SOFCs: Electrochemical Performance under Various Testing Conditions

Since many years solid oxide fuel cells (SOFCs) are used in stationary systems, in a range from 0.1 up to 1,000 kW, to generate electric power or combined heat and power (CHP). For these applications, electrolyte or anode supported cells (ESCs, ASCs) are the preferred SOFCs and have been demonstrated in different installation environments. In the last years lots of research has been done on metal supported solid oxide fuel cells (MSCs) for mobile applications e.g. auxiliary power units (APUs) or range extenders. For this type of cell, a powder-metallurgically manufactured metal substrate (e.g. Fe26Cr) works as backbone of the cell. This substrate is highly porous and its coefficient of thermal expansion (CTE) matches quite well with the commonly used 8YSZ (8 mol% Y2O3 fully stabilized ZrO2) ceramic electrolyte material. This adopted CTE avoids cracks within the electrolyte layer during heating-up and cooling-down procedures as well as the operation itself. Besides, the ductile substrate together with the thin electrolyte is very robust against vibrations. The anode is applied onto the metal substrate via screen printing and sintering and consists of porous Ni/8YSZ, which is a very well-known SOFC anode material. The anode reduces the pore size of the coarse porous metal substrate by more than one order of magnitude and enables the thin electrolyte to gas-tightly separate the porous anode from the cathodic side. The electrolyte (8YSZ) is rather thin being applied by a special PVD-process, a gas flow sputtering process. Compared to ESCs or ASCs, this thinness enables lower operation temperatures, which is advantageous for mobile applications. Lastly, the cathode is applied via screen printing and consists of porous LSCF (La0.6Sr0.4Co0.8Fe0.2O3-δ), which is also well-known in the SOFC community.To show the performances and reliabilities of MSCs, a series of electrochemical characterizations on button cells were performed. The cell conditioning during heat-up will be described as it is slightly different to that of ASCs and ESCs. The main focus of this proceeding is to show the influence of gas-flow rates, gas compositions and operational temperatures on recorded i-V-curves. To do so, some of the testing parameters work at high fuel utilization rates, which is not commonly used for button cell experiments. However, with this parameters, it was interesting to see, how the anode and the metal substrate handle high humidity. Furthermore, significant performance improvements with recently developed anodes and cathodes will be presented. To sum up, this conference proceeding gives an overview on (i) manufacturing, (ii) appropriate test procedures, and (iii) electrochemical testing of MSCs.

Recent developments in plasma spray processes for applications in energy technology

This work focuses on recent developments of plasma spray processes with respect to specific demands in energy technology. High Velocity Atmospheric Plasma Spraying (HV-APS) is a novel variant of plasma spraying devoted to materials which are prone to oxidation or decomposition. It is shown how this process can be used for metallic bondcoats in thermal barrier coating systems. Furthermore, Suspension Plasma Spraying (SPS) is a new method to process submicron-sized feedstock powders which are not sufficiently flowable to feed them in dry state. SPS is presently promoted by the development of novel torch concepts with axial feedstock injection. An example for a columnar structured double layer thermal barrier coating is given. Finally, Plasma Spray-Physical Vapor Deposition (PS-PVD) is a novel technology operating in controlled atmosphere at low pressure and high plasma power. At such condition, vaporization even of high-melting oxide ceramics is possible enabling the formation of columnar structured, strain tolerant coatings with low thermal conductivity. Applying different conditions, the deposition is still dominated by liquid splats. Such process is termed Low Pressure Plasma Spraying-Thin Film (LPPS-TF). Two examples of applications are gas-tight and highly ionic and electronic conductive electrolyte and membrane layers which were deposited on porous metallic substrates.

Spinel-based coatings for metal supported solid oxide fuel cells

Metal supports and metal supported half cells developed at DTU are used for the study of a solution infiltration approach to form protective coatings on porous metal scaffolds. The metal particles in the anode layer, and sometimes even in the support may undergo oxidation in realistic operating conditions leading to severe cell degradation. Here, a controlled oxidation of the porous metal substrate and infiltration of Mn and/or Ce nitrate solutions are applied for in situ formation of protective coatings. Our approach consists of scavenging the FeCr oxides formed during the controlled oxidation into a continuous and well adhered coating. The effectiveness of coatings is the result of composition and structure, but also of the microstructure and surface characteristics of the metal scaffolds.

A comparative study of La0.6Sr0.4Co0.2Fe0.8O3−δ oxygen transport membranes deposited on porous metal supports prepared by supersonic air-gas plasma spraying (SAPS) and low pressure plasma spraying-physical vapor deposition (PS-PVD)

Abstract Perovskite-type La0.6Sr0.4Co0.8Fe0.2O3 − δ (LSCF) oxygen transport membranes were successfully deposited on porous austenitic stainless steel by SAPS. A performance comparison between SAPS membrane and the one prepared by the reported PS-PVD was carried out. X-ray diffraction (XRD) result demonstrates that the degree of perovskite crystallinity of the PS-PVD membrane is better than that of the SAPS membrane. However, scanning electron microscope (SEM) and gas-tightness measurement results indicate that the SAPS membrane shows a much denser microstructure than that of the PS-PVD membrane. The thermal stability result reveals that some vertical through cracks appears on the PS-PVD membrane, while no obvious vertical cracks can be observed on the SAPS membrane. Then the oxygen permeation flux through the membranes was measured under high temperature (700 °C and 900 °C). It is found that the oxygen permeation flux through the PS-PVD membrane is higher than that of the SAPS membrane. Further studies demonstrate that the oxidation of porous metallic substrates greatly decrease the oxygen permeation flux.

High surface area electrodes by template-free self-assembled hierarchical porous gold architecture

The electrode active surface area is a crucial determinant in many electrochemical applications and devices. Porous metal substrates have been employed in electrode design, however construction of such materials generally involves multistep processes, generating in many instances electrodes exhibiting incomplete access to internal pore surfaces. Here we describe fabrication of electrodes comprising hierarchical, nano-to-microscale porous gold matrix, synthesized through spontaneous crystallization of gold thiocyanate in water. Cyclic voltammetry analysis revealed that the specific surface area of the conductive nanoporous Au microwires was very high and depended only upon the amount of gold used, not electrode areas or geometries. Application of the electrode in a pseudo-capacitor device is presented.

中温作動型固体酸化物燃料電池用Ni-Feアノード材料の機械的特性評価に関する基礎的研究

Solid Oxide Fuel Cells (SOFCs) have high power generation efficiency, but their low durability is a big issue. Because Intermediate-Temperature SOFCs (IT-SOFCs) operate at lower temperatures than those for SOFCs, the improvement of durability is expected. In recent years, many candidate materials of IT-SOFCs have been studied. A highly porous Ni-Fe metal substrate is expected to be used as anodes of IT-SOFCs. In this study, we evaluated mechanical properties of Ni-Fe by using environmental-controlled (Small-Punch : SP) testing equipment that can control temperatures and oxygen partial pressures. The results of SP testing, both oxidizing and reducing atmosphere at 800°C, load-displacement curves showed ductile behavior compared with 25°C. In addition, the load under oxidizing atmosphere showed higher than that of reducing atmosphere. The results of EDX analysis showed that oxygen was mainly located at the surface of Ni-Fe. It indicated that dense Fe2O3 layer prevents further oxidation under oxidizing atmosphere. Thus, in spite of oxidizing atmosphere, load-displacement curves showed ductile behavior.

Deposition of Lanthanum Strontium Cobalt Ferrite (LSCF) Using Suspension Plasma Spraying for Oxygen Transport Membrane Applications

Suspension plasma spray deposition was utilized to fabricate dense lanthanum strontium cobalt ferrite oxygen separation membranes (OSMs) on porous metal substrates for mechanical support. The as-sprayed membranes had negligible and/or reversible material decomposition. At the longer stand-off distance (80 mm), smooth and dense membranes could be manufactured using a plasma with power below approximately 81 kW. Moreover, a membrane of 55 μm was observed to have very low gas leakage rates desirable for OSM applications. This thickness could potentially be decreased further to improve oxygen diffusion by using metal substrates with finer surface pores.

(Invited) Formation of Self-Organized Porous Anodic Films on Iron and Stainless Steels

Porous anodic films formed on a range of metals and alloys have attracted much attention because of fundamental interest in self-ordering process of nanopore and nanotube arrays as well as their potential applications. Anodizing of aluminum have been studied since 1920s, but the intensive fundamental and applied research on anodizing of metals have been conducted in the last two decades. It has been believed that thick anodic films could not be formed on iron and iron base alloys because of dissolution and gas generation in aqueous electrolytes. However, nanoporous and nanotubular anodic films on iron were first reported in 2007 and their formation was achieved by using organic electrolytes containing fluoride and a small amount of water.(1) Here, our recent studies on formation and characterization of the porous anodic films on iron and stainless steels will be summarized.(2-4) When iron was anodized in ethylene glycol (EG) electrolyte containing 0.3 mol dm-3 or less water and 0.1 mol dm-3ammonium fluoride, only nanoporous anodic films were formed. The film growth appeared to be different from the usual porous anodic alumina; the interpore distance of nanoporous anodic films on iron formed under this condition is much smaller than the thickness of the barrier layer located between the porous layer and metal substrate, while the ratio of the interpore distance to the thickness of barrier layer for aluminum is ~2.5. The latter porous film growth is recently explained by field-assisted flow of film materials from pore bottom to cell wall. This model might not be applicable to the anodic films on iron at the low water concentrations from geometrical point of view. When water concentration was increased to 1.5 mol dm-­3in the EG electrolyte, a scalloped metal/film interface, which was typical of porous anodic alumina growth, was developed on iron. In addition, the film morphology changed apparently from nanoporous to nanotube by increasing the anodizing voltages. Thus, we can control the film morphology by anodizing voltage and electrolyte composition. For the formation of relatively thick porous anodic films on stainless steel, the EG electrolytes with lower water concentrations were needed in comparison with the film formation on iron. When austenitic Type 304 stainless steel was anodized, nanoporous anodic films with scalloped alloy/film interface were developed even at a low water concentration of 0.1 mol dm-3. In contrast, the ferritic Type 430 stainless steel, free from nickel, formed anodic films with more irregular and highly branched pore morphology. The alloy/film interface was also flat. The steel dependent growth behavior may arise from the presence of nickel in the alloy, since the anodizing behavior of magnetron-sputtered Type 304 stainless steel, which had a bcc structure, was similar to that of bulk Type 304 stainless steel.

Sintering of NiO/YSZ Anode Layers for Metal-Supported Solid Oxide Fuel Cell

The construction of a metal-supported fuel cell has been recently generating a growing interest among the designers of solid oxide fuel cells. The present work is aimed at solving the problem related to high-temperature sintering of fuel cell NiO/YSZ anode on the porous metal substrate functioning both as a supporting structure and a reliable current collector. Hence, its irreversible oxidation during high-temperature sintering should be avoided. NiO/YSZ layers were applied on porous metal samples by a screen-printing technique and sintered in reducing and inert atmospheres, as well as in vacuum. The obtained layers were studied by means of scanning electron microscopy and X-ray phase analysis. It was shown that a temperature of 1000°С does not ensure a substantial sintering of Ni and YSZ granules in a reducing atmosphere. Under the sintering temperature above 1230°С in an inert atmosphere and vacuum, the nickel oxide dissociation and its massive agglomeration are observed. The conditions of NiO/YSZ layer vacuum sintering were experimentally determined which provide a high-grade sintering of nickel cermet granules without Ni agglomeration, disturbance of homogeneity in the formed anode layer, and the metal substrate oxidation.

Development of metal supported solid oxide fuel cells based on powder metallurgical manufacturing route

In the past few decades, stationary solid oxide fuel cell (SOFC) systems have been developed that can generate electricity and heat from the energy stored in hydrogen or hydrocarbons with total efficiencies up to 95%. While the mechanical cell support of stationary systems is commonly supplied by thick ceramic cell components (i.e. anode and electrolyte supported concepts), mobile systems demand a more robust design. This is ensured by a strong yet porous metallic substrate which serves as the mechanical backbone of thin film membrane electrode assemblies [metal supported cell (MSC) concept]. Porous PM Fe–Cr oxide dispersion strengthened alloys for use as MSC supports have recently been developed. These materials provide mechanical and chemical long term stability in typical SOFC atmospheres at operation temperatures up to 850°C. The substrates support a multilayer anode–electrolyte–cathode thin film assembly, constituting a high performance MSC repeat unit. These units are the building blocks for MSC stacks with superior properties for mobile applications.