Exploring a novel ceramic (Ti,W)3SiC2 for interconnect of intermediate temperature solid oxide fuel cell

A solid solution (Ti,W)₃SiC₂ possessing good oxidation resistance and low area-specific resistance (ASR) after oxidation has been synthesized by an in-situ hot pressing process. The oxidation rate constant at 800 °C in air is 6.29 × 10^?14 g² cm^?4 s^?1 for (Ti,W)₃SiC₂. The formed single-layer oxide is composed of W doped rutile TiO₂ and amorphous SiO₂. SiO₂ is evenly inlaid in the communicative body frame of TiO₂. W doped in TiO₂ mainly exists as W⁶⁺. W doping not only hinders the outward diffusion of Ti by decreasing the concentration of native Ti interstitials in TiO₂, but also restrains the inward diffusion of oxygen by decreasing the concentration of O vacancies. Furthermore, W dopant in TiO₂ enhances the electrical conductivity of TiO₂ by increasing the concentration of semi-free electron. Therefore, the low ASR of (Ti,W)₃SiC₂ after oxidation owes to high electrical conductivity of TiO₂ as well as the reduced thickness of oxide scale. All the results render (Ti_1-xW_x)₃SiC₂ promising as interconnects for the intermediate temperature solid oxide fuel cell.     

Controlled thermal pre-treatment of ZMG232G10® for corrosion mitigation under simulated SOFC interconnect exposure conditions

The IT-SOFC candidate interconnect material ZMG232G10® was thermally pre-treated in air and in H₂ – 3% H₂O to mitigate dual atmosphere corrosion. Bare and pre-treated steel samples were exposed to a dual atmosphere exposure condition (dry air/metal/4% H₂ – N₂ (bal.) + 3% H₂O) for 500 h at 600 °C. Corrosion products and oxide morphology developed on the air-exposed surfaces were analyzed. Pre-treatments of the metal in oxidizing (air) and reducing (H₂ – 3% H₂O) gas atmospheres provide improved passivation against dual atmosphere corrosion compared to bare steel. The uniform and iron-free oxide scale developed during the pre-treatment of alloy in the low pO₂ H₂ – 3% H₂O gas atmosphere provides an effective diffusion barrier against the outward transport of cations and oxidation of iron observed under dual atmosphere conditions.     

Performance evaluation of several commercial alloys in a reducing environment

Several commercial alloys including Ebrite, Crofer 22 APU, Haynes 230 and Haynes 242, which are candidates for intermediate-temperature solid oxide fuel cell (SOFC) interconnect materials, were isothermally and cyclically oxidized at 900 °C in the reducing atmosphere of Ar + 5 vol.% H₂ + 3 vol.% H₂O corresponding to the SOFC anode environment. Results indicate that these alloys exhibited good scale spallation resistance with the Ni-base alloys possessing better oxidation resistance over the Fe-base alloys. Both Mn–Cr spinel and Cr₂O₃ were formed in the oxide scales of these alloys. For Crofer 22 APU and Haynes 242, a continuous protective MnO and Mn–Cr spinel layer formed outside on the inner layer of Cr₂O₃. The increase in scale ASR after longer-term thermal exposure in the reducing environment was relatively slower for the Ni-base alloys than for the Fe-base alloys.     

Effect of coatings on long term behaviour of a commercial stainless steel for solid oxide electrolyser cell interconnect application in H2/H2O atmosphere

K41X (AISI 441) stainless steel evidenced a high electrical conductivity after 3000 h ageing in H₂/H₂O side when used as interconnect for solid oxide electrolyser cells (SOEC) working at 800 °C. Perovskite (La_1 − xSr_xMnO_3 − δ) and spinel (Co₃O₄) oxides coatings were applied on the surface of the ferritic steel for ageing at 800 °C for 3000 h. Both coatings improved the behaviour of the steel and give interesting opportunities to use the K41X steel as interconnect for hydrogen production via high temperature steam electrolysis. Co₃O₄ reduced into Co leading to a very good Area Specific Resistance (ASR) parameter, 0.038 Ω cm². Despite a good ASR (0.06 Ω cm²), La_1 − xSr_xMnO_3 − δ was less promising because it partially decomposed into MnO and La₂O₃ during ageing in H₂/H₂O atmosphere.     

H2 and CH4 oxidation on Gd0.2Ce0.8O1.9 infiltrated SrMoO3–yttria-stabilized zirconia anode for solid oxide fuel cells

Strontium molybdate (SrMoO₃) as an electronic conductor was incorporated with yttria-stabilized zirconia (YSZ) to form an anode scaffold for solid oxide fuel cells. Gd_0.2Ce_0.8O_1.9 (GDC) nanoparticles were introduced by wet impregnation to complete the Ni-free GDC infiltrated SrMoO₃–YSZ anode fabrication. The effects of SrMoO₃ on the electrode conductivity and GDC infiltration on the catalytic activity were examined. A pronounced performance improvement was observed both on wet H₂ and CH₄ oxidation for the 56 wt.% GDC infiltrated SrMoO₃–YSZ. In particular, the polarization resistance decreased from 8 Ω cm² to 0.5 Ω cm² under wet H₂ (3% H₂O) at 800 °C with the introduction of GDC. Under wet CH₄ at 900 °C, a maximum power density of 160 mW cm⁻² was obtained and no carbon deposition was observed on the anode. It was found that the addition of H₂O in the anode caused an increase of electrode ohmic resistance and a decrease of open circuit voltage. Redox cycling stability was investigated and only a slight drop in cell performance was observed after 5 cycles. These results suggest that GDC infiltrated SrMoO₃–YSZ is a promising anode material for solid oxide fuel cells.     

The preparation and properties of Mn–Co–O spinel coating for SOFC metallic interconnect

To meet the performance requirements of solid oxide fuel cell (SOFC) metallic interconnect, the Mn–Co–O spinel coating is prepared on the surface of AISI430 by pack cementation method to reduce the growth kinetics of oxides and inhibit the outward diffusion of Cr. The microstructural characterization shows that a dense, uniform, defect-free spinel coating is successfully fabricated on the surface of AISI430. Under the simulated SOFC cathode environment, the weight gain of coated steel (0.608 mg cm⁻²) after oxidation at 800 °C for 800 h is significantly lower than that of uncoated (1.586 mg cm⁻²). In addition, the area specific resistance (ASR) of the coated steel oxidized for 500 h is 17.69 mΩ cm², much smaller than that of the bare steel, indicating that the oxidation resistance and electrical conductivity of AISI430 are significantly improved by Mn–Co–O spinel coating. Cross-sectional observations of the Mn–Co–O spinel coating are conducted to assess the compatibility of substrate with the adjacent coating and its effectiveness in reducing the growth of the Cr₂O₃ layer.     

The effect of different atmosphere treatments on the performance of Ni/Nb–Al2O3 catalysts for methane steam reforming

Methane steam reforming is currently the most widely used hydrogen production reaction in industry today. Ni/Nb–Al₂O₃ catalysts were prepared by treatment under H₂, N_2, and air atmosphere prior to reduction and applied for methane steam reforming reaction at low temperature (400–600 °C). The hydrogen-treated catalysts increased catalytic activity, with 55.74% methane conversion at S/C = 2, GSVH of 14400 mL g^?1 h^?1 and 550 °C. The H₂ atmosphere treatment enhanced the Ni–Nb interaction and the formation of stable, tiny, homogeneous Ni particles (6 nm), contributing to good activity and stability. In contrast, the catalysts treated with nitrogen and air showed weaker interactions between Ni and Nb species, whereas the added Nb covered the active sites, which caused the decrease in activity. Meanwhile, carbon accumulation was also observed. This work is informative for preserving small nano-sized nickel particles to enhance catalytic performance.     

Thin films of La0.7Sr0.3MnO3−δ dip-coated on Fe–Cr alloys for SOFC metallic interconnect

La_0.7Sr_0.3MnO_3−δ (LSM) porous films were deposited on different ferritic stainless steels (SS) (430: Cr-16.0%; 439: Cr-16.6%; 444: Cr-17.4%) by sol–gel/dip-coating process. The structure, morphology and composition profiles of investigated assemblies were examined using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. The area specific resistance (ASR) was measured during long term oxidation in air at 800 °C for 200 h by DC measurements. ASR values lower than 10 mΩ cm² were recorded after 200 h for LSM-coated SS439 and SS444. This is likely to be due to the high Cr content and to Nb, Ti and Mo elements used to stabilize the stainless steel against oxidation. This paper shows that LSM coatings provide an enhanced stability of the alloy at high temperature and the formation of an interfacial Cr–Mn spinel layer hinders the oxide scale growth.     

BaZr0.1Ce0.7Y0.1Yb0.1O3−δ as highly active and carbon tolerant anode for direct hydrocarbon solid oxide fuel cells

BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) perovskite is synthesized and examined as an alternative anode material for intermediate temperature solid oxide fuel cells (IT-SOFCs) based on direct hydrocarbon fuels, using polarization and electrochemical impedance spectroscopy techniques. Single-phased BZCYYb anode shows an excellent activity for both hydrogen and methane oxidation reactions, achieving a polarization resistance of 0.25 and 0.93 Ω cm², and overpotential of 20 and 202 mV at 100 mA cm⁻² and 750 °C in wet H₂ (3% H₂O/97% H₂) and wet CH₄ (3% H₂O/97% CH₄), respectively. The electrocatalytic activity of BZCYYb anodes is significantly higher than that of the (La,Sr)(Cr,Mn)O₃ anodes as reported in the literature. Furthermore, BZCYYb exhibits excellent resistance to carbon deposition. The present study demonstrates that BZCYYb perovskite is a promising alternative anode material for direct hydrocarbon fuels based SOFCs.     

Adhesion of oxide scales grown on ferritic stainless steels in solid oxide fuel cells temperature and atmosphere conditions

Adhesion of thermal oxide scales grown at 800 °C on ferritic stainless steels F18TNb (AISI 441) and F18MT (AISI 444) proposed as interconnectors in solid oxide fuel cells (SOFCs) was investigated. The effect of oxidising atmosphere – synthetic air or 2% H₂O in H₂ as the representative cathode and anode atmosphere respectively – was considered. Using a room temperature tensile test sitting in the SEM chamber, thermally grown oxide scales were forced to spall and their adhesion energy was derived. Adhesion energy, considered as the elastic energy per unit area stored in oxide was determined at the strain of first spallation or at the strain where the derivative of spallation versus strain was maximum. Adhesion energies were shown to lie in the range 10–100 J cm⁻². Adhesion values exhibited decreasing values with increasing oxide thickness, with higher values for oxidation in 2% H₂O/H₂ compared to oxidation in synthetic air. The adhesion energy of scales on F18MT was lower than that on F18TNb due to the presence of Mo-containing intermetallic compounds at the metal/scale interface.     

Fabrication using sequence wet-chemical coating and electrochemical performance of Ni–Fe-foam-supported solid oxide electrolysis cell for hydrogen production from steam

Ni–Fe-alloy-foam supported solid oxide electrolysis cell with an arrangement of nickle and Sc_0.1Ce_0.005Gd_0.005Zr_0.89O₂ (Ni-SCGZ) cathode, SCGZ electrolyte and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) anode is successfully fabricated by the sequence wet-chemical coating. The multi-layer cathode with a gradient of thermal expansion coefficient (TEC) is deposited on the alloy-foam support. Two-step firing processes are applied including cathode pre-firing (1373 K, 2 h) and electrolyte sintering (1623 K, 4 h) using slow heating rate enhanced with compressive loading. The fabricated cell shows current density of ?0.95 Acm^?2 at 1.1 V with H₂O:H₂ = 70:30 and 1073 K, providing hydrogen production rate at 4.95 × 10^?6 mol s^?1. However, performance degradation was observed with the rate of 0.08 V h^?1, which can be ascribed to the delamination of BSCF anode under operating at high current density.     

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.     

Evaluation of pulse electroplated cobalt/yttrium oxide composite coating on the Crofer 22 APU stainless steel interconnect

Ferritic stainless steels have been evaluated as favorable materials for utilization in SOFC interconnects. However, there are difficulties in utilizing these metallic interconnects, including the quick decrease of their electrical conductivity and cathode poisoning due to the evaporation of Cr species. In this work, Co and Co/Y₂O₃ composite coatings have been deposited onto Crofer 22 APU stainless steels by the pulse current electrodeposition method and the oxidation and electrical properties of uncoated and coated steels have been evaluated. Cyclic oxidation was performed in air at 800 °C for 500 h, oxidation rates were calculated, and oxide layer microstructures were examined. SEM–EDS and XRD investigations exhibited the created oxide layer on both coated samples made up of two scale after oxidation. The internal thin scale was composed of Cr and O and the external scale comprised of Co, Mn, Cr, Fe, and O. Y₂O₃ was observed as dispersed particles in the external oxide scale after the cyclic oxidation test. The thicknesses of internal oxide scale were reduced and oxidations rates also were meaningfully decreased for Co/Y₂O₃-coated steels relative to uncoated and Co-coated steels. Finally the ASR values of coated and uncoated substrates was also tested as a function of temperature and time in air. Results showed that the ASR value of the Co/Y₂O₃-coated steel was 13.1 mΩ cm² after 500 h of cyclic oxidation at 800 °C, which was significantly lower than that of bare steel and the Co-coated sample.     

Syngas production through CH4-assisted co-electrolysis of H2O and CO2 in La0.8Sr0.2Cr0.5Fe0.5O3-δ-Zr0.84Y0.16O2-δ electrode-supported solid oxide electrolysis cells

High temperature co-electrolysis of H₂O/CO₂ allows for clean production of syngas using renewable energy, and the novel fuel-assisted electrolysis can effectively reduce consumption of electricity. Here, we report on symmetric cells YSZ-LSCrF | YSZ | YSZ-LSCrF, impregnated with Ni-SDC catalysts, for CH₄-assisted co-electrolysis of H₂O/CO₂. The required voltages to achieve an electrolysis current density of ?400 mA·cm^?2 at 850 °C are 1.0 V for the conventional co-electrolysis and 0.3 V for the CH₄-assisted co-electrolysis, indicative of a 70% reduction in the electricity consumption. For an inlet of H₂O/CO₂ (50/50 vol), syngas with a H₂:CO ratio of ≈2 can be always produced from the cathode under different current densities. In contrast, the anode effluent strongly depends upon the electrolysis current density and the operating temperature, with syngas favorably produced under moderate current densities at higher temperatures. It is demonstrated that syngas with a H₂:CO ratio of ≈2 can be produced from the anode at a formation rate of 6.5·mL min^?1·cm^?2 when operated at 850 °C with an electrolysis current density of ?450 mA·cm^?2.     

Exploring MnCr2O4–Gd0.1Ce0.9O2-δ as a composite electrode material for solid oxide fuel cell

Symmetrical solid oxide fuel cell (SOFC) adopting the same material at both electrodes is potentially capable of promoting thermomechanical compatibility between near components and lowering stack costs. In this paper, MnCr₂O₄–Gd_0.1Ce_0.9O_2-δ (MCO-GDC) composite electrodes prepared by co-infiltration method for symmetrical electrolyte supported and anode supported solid oxide fuel cells are evaluated at a temperature range of 650–800 °C in wet (3% H₂O) hydrogen and air atmospheres. Without any alkaline earth elements and cobalt, the co-infiltrated MCO-GDC composite electrode shows excellent activity for oxygen reduction reaction but mediocre activity for hydrogen oxidation reaction. With MCO-GDC as the cathode, the Ni-YSZ (Y₂O₃ stabilized ZrO₂) anode supported asymmetrical cell demonstrates a peak power density of 665 mW cm⁻² at 800 °C. The above results suggest MCO-GDC is a promising candidate cathode material for solid oxide fuel cells.     

Fine interface contact and high activity of nickel-based anode modified by gadolinium for hydrogen and methane fuel

In this work, gadolinium is used to modify nickel catalyst, which can improve the properties of nickel oxide particle and inhibit its sintering and grain growth. Interface contact between nickel catalyst and YSZ is significantly improved and fine anode microstructure can be obtained when gadolinium is used to modify Ni-YSZ anode. Fine interface contact of GdNi-YSZ anode can accelerate charge transfer process and steam formation process, which leads to high activity for electrochemical oxidation of hydrogen and low impedance resistance. The remarkable characteristic of GdNi-YSZ anode cell is that the cell performance for humidified methane fuel is greatly improved due to the high anode activity for methane reforming and electrochemical oxidation of hydrogen. The maximum power density of GdNi-YSZ anode cell with humidified methane as fuel can reach 1.59 W/cm² at 800 °C and 0.46 W/cm² at 650 °C. High performance of GdNi-YSZ anode cell with humidified methane as fuel leads to much H₂O produced during the electrochemical oxidation process, which can depress carbon deposition and improve the cell stability for humidified methane fuel.     

Coated interconnects development for high temperature water vapour electrolysis: Study in anode atmosphere

High temperature water vapour electrolysis (HTE) is an efficient technology for hydrogen production. In this context, a commercial stainless steel, K41X (AISI 441), was chosen as interconnect. In a previous paper, the high temperature corrosion and the electrical conductivity were evaluated in both anode (O₂–H₂O) and cathode (H₂–H₂O) atmosphere at 800 °C. In O₂–H₂O atmosphere, the formation of a thin chromia protective layer was observed. Nevertheless, the ASR parameter measured was higher than the maximum accepted value. These results, in addition with chromium evaporation measurements, proved that the K41X alloy is not suitable for HTE interconnect application. In this study, two perovskite-type oxides La_0.8Sr_0.2MnO_3−δ and LaNi_0.6Fe_0.4O_3−δ were tested as coatings in O₂–H₂O atmosphere at 800 °C. Screen-printing and physical vapour deposition were used as coating processes. The high temperature corrosion resistance and the electrical conductivity were improved, especially with the LaNi_0.6Fe_0.4O_3−δ coating. Cr specie volatility was also reduced.     

Oxidation behavior of ferritic stainless steel containing Nb,Nb–Si and Nb–Ti for SOFC interconnect

The effect of Nb on the oxidation kinetics, electrical conductivity and Cr evaporation behavior of FSS has been discussed depending on the Nb content and oxygen active element such as Ti and Si. Nb in ferritic stainless steel is saturated during heat treatment as NbO₂ at the outermost oxide scale and as both Nb₂O₅ and Laves phase near the oxide scale/alloy interface. Excess Nb (>4.7 wt%) suppresses precipitation of Nb₂O₅, because of rapid Laves phase growth. Nb enhances selective Ti oxidation, whereas Ti retards Nb₂O₅ precipitation near the scale/alloy interface. On the other hand, Si suppresses Nb enrichment near the scale/alloy interface and it reduces the precipitation of both Nb₂O₅ and Laves phase. Nb also suppresses Si enrichment and the formation of continuous Si oxide at the scale/alloy interface. Co-addition of Nb and Ti is effective to decrease the electrical resistance and Cr evaporation rate of oxide scale.     

Precursor of Pr2NiO4+δ as a highly effective catalyst for the simultaneous promotion of oxygen reduction and hydrogen oxidation reactions in solid oxide electrochemical devices

The electrochemical behaviour of a Sr₂Fe_1.5Mo_0.5O_6–δ based double-layer electrode decorated with a “symmetric” catalyst by the wet impregnation technique for the simultaneous acceleration of cathodic and anodic reactions was investigated for the first time. As a “symmetric” catalyst, a solution of precursor for the synthesis of praseodymium nickelate (Pr₂NiO_4+δ) was considered. Since the catalyst consists of NiO and Pr₆O₁₁ in an oxidizing atmosphere and of Ni and Pr₂O₃ in a reducing atmosphere, it effectively accelerates the rate of oxygen reduction at the cathode and hydrogen oxidation at the anode of solid oxide fuel cells with symmetric electrodes. It was shown that the rate of oxygen reduction after introduction of the catalyst into the electrode increased due to an increase in the rate of oxygen interfacial exchange between the electrode and the gas phase. The rate of hydrogen oxidation increased due to an increase in the rate of dissociation of adsorbed hydrogen. During tests of the fuel cell with a 300 μm LaGaO₃-based supporting electrolyte and decorated electrodes, a maximum power density of about 0.83 W cm^?2 at 800 °C under wet hydrogen/air condition was obtained.     

Electrochemical impedance study of the poisoning behaviour of Ni-based anodes at low concentrations of H2S in an MCFC

The effect was investigated of low H₂S concentrations, simulating biogas impurity, on the poisoning behaviour of a Ni-based, molten carbonate fuel cell anode. A conventional Ni–Cr anode was coated with ceria using dip coating to form a rare earth metal oxide thin layer on the surface of the anode. Electrochemical studies of the Ni-based samples were performed in symmetric cells under anode atmosphere (H₂, CO₂, H₂O and N₂ as balance) with 2, 6, 12, and 24 ppm of H₂S by means of electrochemical impedance spectroscopy.