Preparation of Cr2O3/Al2O3 bipolar oxides as hydrogen permeation barriers by selective oxide removal on SS and atomic layer deposition

Iron-nickel based stainless steel (SS) applied in nuclear plants as a substrate material barely suppresses the permeation of hydrogen plasmas, which are mainly composed of positive and negative hydrogen ions with trace amounts of non-ionized hydrogen atoms. In this work, a new Cr₂O₃/Al₂O₃ bipolar oxide barrier was prepared using atomic layer deposition (ALD) of Al₂O₃ on a Cr₂O₃ layer that was generated by removing partial oxides with cyclic voltammetry (CV) of SS that had been pre-oxidized at 550 °C in air. We found that a small layer of α-Al₂O₃ was formed by the template effect of Cr₂O₃ at the interface of this composite film. The hydrogen permeation behavior of this bipolar oxide barrier in a fusion reactor was simulated with hydrogen-discharging plasma treatment. The results demonstrated that the hydrogen permeation resistance of this bipolar oxide was superior to the original oxide or a Cr₂O₃ film. Impressively, hydrogen plasma treatment repaired the bipolar oxide via reduction of the defective CrO₃, resulting in an improvement in the hydrogen permeation resistance. These findings demonstrate a novel method of hydrogen permeation barrier preparation on SS, providing insight into hydrogen barrier construction for future nuclear energy applications.     

Hydrogen interaction characteristics of a Cr2O3Y2O3 coating formed on stainless steel in an ultra-low oxygen environment

Ceramics are the most promising candidates for tritium permeation barriers for fusion reactors due to their high thermal and chemical stabilities and low hydrogen isotope permeation reduction factors. However, hydrogen embrittlement and a large number of defects in ceramic coatings are new challenges for first wall materials in nuclear reactors. To address this issue, a new Cr₂O₃Y₂O₃ coating with a thickness of about 100 nm was synthesized and placed in an ultra-low oxygen partial pressure (8 × 10⁻²⁰ Pa) environment, in which a compact CrY alloy coating was successfully deposited on the stainless-steel substrate by pulsed electrochemical deposition. The interactions between the coating and hydrogen plasma were comprehensively analyzed and compared via surface analysis techniques, including TEM, XPS and electrochemical impedance spectroscopy (EIS). The mechanical properties of the coating before and after hydrogen permeation were studied by tensile testing. It was found that this ceramic coating effectively reduced the defect concentration and retained a high protective performance upon hydrogen exposure. Therefore, this new Cr₂O₃Y₂O₃ coating has potential as a promising hydrogen permeation barrier.     

NiO/NiFe2O4 dual-layer coating on pre-oxidized SUS 430 steel interconnect

A Ni/NiFe₂ dual-layer coating is deposited on 50-h pre-oxidized SUS 430 steel by magnetron sputtering for solid oxide fuel cell (SOFC) interconnects application, followed by thermal exposure in air at 800 °C for 1680 h. The thermally grown oxide scales exhibit tri-layer structure with inner Cr₂O₃ layer, middle NiO layer and outer NiFe₂O₄ spinel layer. The oxide coating converted from Ni/NiFe₂ coating not only inhibit the growth of Cr₂O₃ and the outward diffusion of Cr species but also improve the electrical performance of the surface scale. In addition, pre-oxidation treatment for the steel before Ni/NiFe₂ coating deposition prevents the interdiffusion between steel substrate and coating in the oxidation process.     

Stabilized zirconia-based sensor utilizing SnO2-based sensing electrode with an integrated Cr2O3 catalyst layer for sensitive and selective detection of hydrogen

A highly selective hydrogen (H₂) sensor has been successfully developed by using an yttria-stabilized zirconia (YSZ)-based mixed-potential-type sensor utilizing SnO₂ (+30 wt.% YSZ) sensing electrode (SE) with an intermediate Al₂O₃ barrier layer which was coated with a catalyst layer of Cr₂O₃. The sensor utilizing SnO₂ (+30 wt.% YSZ)-SE was found to be capable of detecting H₂ and propene (C₃H₆) sensitively at 550 °C. In order to enhance the selectivity towards H₂, a selective C₃H₆ oxidation catalyst was employed to minimize unwanted responses caused by interfering gases. Among the examined metal oxides, Cr₂O₃ facilitated the selective oxidation of C₃H₆. However, the addition or lamination of Cr₂O₃ to SnO₂ (+30 wt.% YSZ)-SE was found to diminish the sensing responses to all examined gases. Therefore, an intermediate layer of Al₂O₃ was sandwiched between the SE layer and the catalyst layer to prevent the penetration of Cr₂O₃ particles into the SE layer. The sensor using SnO₂ (+30 wt.% YSZ)-SE coated with a catalyst layer of Cr₂O₃ as well as an intermediate layer of Al₂O₃ exhibited a sensitive response toward H₂, with only minor responses toward other examined gases at 550 °C under humid conditions (21 vol.% O₂ and 1.35 vol.% H₂O in N₂ balance). A linear relationship was observed between sensitivity and H₂ concentration in the range of 20–800 ppm on a logarithmic scale. The results of sensing performance evaluation and polarization curve measurements indicate that the sensing mechanism is based on the mixed-potential model.     

Investigation of Al2O3 diffusion barrier layer fabricated by atomic layer deposition for flexible Cu(In,Ga)Se2 solar cells

The use of Al₂O₃ fabricated by atomic layer deposition (ALD) as a metal diffusion barrier between the stainless steel substrate and the back contact layer in flexible Cu(In,Ga)Se₂ (CIGS) photovoltaic (PV) devices was found to reduce metal ion diffusion from the substrate and reduce the number of defects at the CIGS absorber layer, as determined from the secondary ion mass spectrometry (SIMS) depth profile and quantitative defect analysis using C–V measurements. Cells with Al₂O₃ barrier layers were found to show higher efficiency and uniformity compared to cells with ZnO barrier layers. XRD pattern analysis showed the Al₂O₃ barrier layer's amorphous characteristic which can form a complex diffusion path. In addition, quantum efficiency (QE) analysis of the cells showed that the main advantage of using an Al₂O₃ barrier layer is derived from the increase in the current density due to the decrease in the number of recombination sites resulting from the decrease in the number of defects due to the amorphous nature of the layer. Therefore, cells with an Al₂O₃ barrier layer fabricated by ALD showed better average conversion efficiency and uniformity (11.23 ± 1.86%) compared to cells with a ZnO barrier layer fabricated by sputtering. Ongoing advancements in ALD processes make the use of Al₂O₃ barrier layers promising for obtaining large-scale flexible solar cells.     

Preparation and hydrogen penetration performance of TiO2/TiCx composite coatings

Titanium carbide is a good candidate for tritium permeation barrier in a fusion reactor. However, its oxidation susceptibility and the mismatch between the ceramic coating and substrate are still a challenge. In this study, a promising candidate as a hydrogen permeation barrier, comprising a titanium-based ceramic TiO₂/TiC_x composite coating, was proposed. The preparation process of this TiO₂/TiC_x composite coating involves two steps of carbon ion implantation and oxidation under ultra-low oxygen partial pressure. According to the results, the optimal oxidation temperature for TiO₂ coating is 550 °C, with the increase of the oxidation temperature, the particles on the surface of the oxide layer become coarse and loosely arranged, and the protective performance of the oxide layer is greatly reduced. The hydrogen barrier permeation behavior of the composite coating in a fusion reactor was simulated via hydrogen plasma discharge environment, the results show that the hydrogen barrier permeation performance of the composite is significantly better than that of a single TiO₂ coating. In addition, the coatings treated with hydrogen plasma showed a certain self-repairing performance through the diffusion growth of the TiC_x layer. These findings illustrate a novel method for preparing composite coatings to restrain hydrogen permeation, providing insight into the development of hydrogen permeation barrier materials.     

Effect of catalysts on microstructure,hydrogen storage thermodynamics,and kinetics performance of La5Mg85Ni10 alloy

The effects of the type and amount of transition metal catalyst on the microstructure and hydrogen storage performance of La₅Mg₈₅Ni₁₀ + x wt.% M (x = 1, 3, 5, 7; M = TiF₃, NbF₅, Cr₂O₃) alloys milled for 10 h have been investigated. The evolution of microstructure and phase of catalyzed alloys in the absorption/desorption process have been characterized by XRD and HRTEM. The results show that the hydrogen storage capacity of the alloy decreases as the catalyst increases. On the one hand, the catalytic effects of different amount of catalyst TiF₃ were studied. TiF₃ exists in form of MgF₂ and TiH₂ phases and Ea decreases firstly and then increases as the amount of TiF₃ increases. When 5 wt.% TiF₃ is added, the hydrogen desorption activation energy shows the lowest Ea = 45.2 kJ/mol. On the other hand, the catalytic effects of TiF₃, Cr₂O₃ and NbF₅ are compared in detail. It was found that TiF₃ has better catalytic effect than Cr₂O₃ and NbF₅ due to TiF₃ nanoparticles can refine the grains better, provide hydrogen diffusion channels and reduce the nucleation driving force of the alloys.     

Hydrogen permeation behavior and mechanism of Cr2O3/Al2O3 bipolar oxide film under plasma discharging environment

To improve the thermal stability between aluminide and stainless steel substrate and obtain thermodynamically stable phase of alpha-Al₂O₃, a new Cr₂O₃/Al₂O₃ bipolar oxide barrier was proposed, in which metallic Al was sputtered on the preoxidation coating of electroplated chromium and then oxidizing by oxygen plasma. It was found that Cr₂O₃ film exhibits P-type semiconducting properties while Al₂O₃ acts as N-type. Hydrogen discharging plasma was used to simulate the in-pile hydrogen permeation. Raman spectra and atomic force microscopy (AFM) were employed to analyze phase structure and surface morphology. Electrochemical impedance spectroscopy and Mott–Schottky were utilized to qualitatively evaluate effective thickness and the integrity for the oxide film. The depth profile and surface chemical states of involving elements were analyzed by auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), respectively. The result shows that Cr₂O₃/Al₂O₃ bipolar oxides have improved hydrogen permeation resistance and would be a potential candidate for barrier application.     

Multiphase Nb–TiCo alloys: The significant impact of surface corrosion on the structural stability and hydrogen permeation behaviour

Multiphase Nb_xTi_(100-x)/2Co_(100-x)/2 (x = 30–60) alloys are a promising material for hydrogen separating membranes. These alloy membranes exhibit a rapid decline in hydrogen permeation flux within ∼12 h when operated at 773 K. To address this issue, a dense oxide (e.g. Nb₂O₅, TiO₂ and CoO) layer was prepared between a Pd coating layer and an Nb–TiCo substrate by surface corrosion for improving their thermal stability, and the corrosion resistance of Nb–TiCo alloys was investigated. An increase in the Nb content (x) lowers the corrosion resistance of these alloys, but makes it easier to form the above oxide layer. Substantial enhancement of hydrogen permeability and thermal stability at 773 K was observed for the alloys (x = 30 and 40) after corrosion, which can be ascribed to an increase in hydrogen diffusivity. This improved permeability and stability are closely related to the formation of the above surface oxide layer that impeded interdiffusion between the Pd film and Nb–TiCo substrates. This study demonstrates that insertion of a diffusion barrier between the Pd and Nb-based substrates by surface corrosion is a viable approach to enhance the high-temperature stability of Pd-coated Nb–TiCo alloys, an aspect not widely explored in Nb-based hydrogen separation and purification membranes.     

Improvement of cyclic durability of Ti-Cr-V alloy by Fe substitution

Cyclic durability of hydrogen storage Ti-Cr-V alloys has been investigated. After 100 absorption/desorption cycles, Ti₁₂Cr₂₃V₆₄Fe₁ desorbs 2.34 mass% of hydrogen while Ti₁₂Cr₂₃V₆₅ desorbs 2.19 mass% of hydrogen. These desorption capacities of Ti₁₂Cr₂₃V₆₄Fe₁ and Ti₁₂Cr₂₃V₆₅ correspond to 97% and 88% of their initial capacities, respectively. The X-ray diffraction profiles of the alloys suggest that Fe substitution inhibits the increase of the lattice strain and the decrease of the crystallite size accompanied by hydrogen absorption and desorption. This inhibition most likely relates to the improvement of cyclic durability by Fe substitution.     

Hydrogen storage properties of V0.3Ti0.3Cr0.25Mn0.1Nb0.05 high entropy alloy

In this paper, we present the synthesis, first hydrogenation kinetics, thermodynamics and effect of cycling on the hydrogen storage properties of a V_0.3Ti_0.3Cr_0.25Mn_0.1Nb_0.05 high entropy alloy. It was found that the V_0.3Ti_0.3Cr_0.25Mn_0.1Nb_0.05 alloy crystallizes in body-centred cubic (BCC) phase with a small amount of secondary phase. The first hydrogenation is possible at room temperature without incubation time and reaches a maximum hydrogen storage capacity of 3.45 wt%. The pressure composition isotherm (P–C–I) at 298 K shows a reversible hydrogen desorption capacity of 1.78 wt% and a desorption plateau pressure of 80.2 kPa. The capacity loss is mainly due to the stable hydride with the desorption enthalpy of 31.1 kJ/mol and entropy of 101.8 J/K/mol. The hydrogen absorption capacity decreases with cycling due to incomplete desorption at room temperature. The hydrogen absorption kinetics increases with cycling and the rate-limiting step is diffusion-controlled for hydrogen absorption.     

Electrical and microstructural characterization of spinel phases as potential coatings for SOFC metallic interconnects

Several spinel samples, i.e., Mn_xCr_3−xO₄ (0.5 ≤ x ≤ 2.5), NiCr₂O₄ and CoCr₂O₄, were synthesized and studied in terms of phase analysis, density, electrical resistivity and thermal expansion behaviour. The spinel samples were generally single phase; exceptions included MnCr₂O₄ and Mn_0.5Cr_2.5O₄ with significant amounts of Cr₂O₃ and NiCr₂O₄ with trace amounts of NiO.Porosity, in general, decreased with increasing sintering temperature, except for Mn_0.5Cr_2.5O₄, which showed increasing porosity with increasing sintering temperature. NiCr₂O₄, CoCr₂O₄ and MnCr₂O₄, all had similar thermal expansion behaviour, with thermal expansion coefficients (TEC) ranging from 7.2–7.6 × 10⁻⁶/°C. The TEC difference between the spinels and ferritic stainless steel was larger than the difference between the steel and chromia, which had a TEC of 9.6 × 10⁻⁶/°C. The spinels and chromia exhibited semiconductor-type behaviour, with electrical resistivities decreasing with increasing temperature. Only Mn₂CrO₄ and NiCr₂O₄ had resistivities lower than Cr₂O₃ over the entire temperature range of testing (20–900 °C). For Mn_xCr_3−xO₄, resistivity decreased with increasing Mn content.     

Hydrogen permeation properties of CrxCy@Cr2O3/Al2O3 composite coating derived from selective oxidation of a CrC alloy and atomic layer deposition

Metal oxides and carbides are promising tritium permeation barrier coatings for fusion reactors. However, the thermomechanical mismatch between the coating and substrate poses a threat to their interface's integrity during fabrication and operation. To address this issue, a metallic interlayer coating was introduced followed by selective oxidation in which a compact and uniform CrC amorphous alloy coating was successfully deposited on the stainless steel substrate by pulsed electrochemical deposition. A new composite coating of Cr_xC_y@Cr₂O₃/Al₂O₃ was formed by subsequent controlled oxidation conversion and atomic layer deposition. The phase, morphology, chemical state and defects of the films were analyzed and compared both before and after hydrogen exposure at 300 °C. The results show that this new kind of composite coating, based on the principles of grain boundary pinning of chromic oxide with carbide and defect healing of alumina, can remarkably improve the hydrogen permeation barrier performance of these materials.     

The effect of Fe2O3 sintering aid on Gd0.1Ce0.9O1.95 diffusion barrier layer and solid oxide fuel cell performance

Solid oxide fuel cells (SOFCs) with the Gd_0.1Ce_0.9O_1.95 (GDC) diffusion barrier layer require the densification of GDC to improve the performance of the cells. In this work, the addition of 0.5 mol% Fe₂O₃ in GDC diffusion barrier layer as sintering aid is studied. The symmetrical cell and the fuel cell are fabricated by hot-pressing, co-sintering and screen-printing technologies. It is found that the addition of Fe₂O₃ can make GDC barrier layer denser at a reduced sintering temperature of 1250 °C and prevent diffusion of Sr to form ionic insulating interface between YSZ (Y₂O₃ stabilized ZrO₂) and La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (LSCF). The fuel cell based on the GDC-Fe₂O₃ barrier layer achieves better maximum power density of 0.89 W cm⁻² at 800 °C than that of 0.68 W cm⁻² without Fe₂O₃ addition. No obvious degradation was observed on fuel cell based on the GDC-Fe₂O₃ diffusion barrier layer after a stability test at 750 °C for 100 h under 0.75 V and the thermal cycling between 750 and 400 °C. The results indicate that the addition of Fe₂O₃ sintering aid in GDC diffusion barrier layer can promote the densification of GDC and exhibit good long-term stability and thermal cycle stability.     

Development of Ti1.02Cr2-x-yFexMny (0.6≤x≤0.75, y=0.25, 0.3) alloys for high hydrogen pressure metal hydride system

To save compressor investment and promote operation efficiency of hydrogen refueling station, the hydrogen storage alloys for high-pressure hydrogen metal hydride tank is developed. Ti_1.02Cr_2-x-yFe_xMn_y (0.6 ≤ x ≤ 0.75, y = 0.25, 0.3) alloys with main structure of C14 type Laves phase and low dehydrogenation enthalpy were prepared by plasma arc melting and heat treatment. Pressure-composition-temperature measurements show that hydrogen desorption plateau pressures increase with Cr substituted by Fe increasing. The maximum and reversible hydrogen storage capacities are more than 1.85 and 1.65 wt% at 201 K respectively. The hydrogen desorption plateau slopes are all less than 0.5. The symmetry weakening of 2a sites may deteriorate the plateau slop characteristic. Ti_1.02Cr_0.95Fe_0.75Mn_0.3 and Ti_1.02Cr_1.0Fe_0.75Mn_0.25 alloys are suitable for high pressure hybrid tank which can supply the effective hydrogen (more than 70 MPa) about 40.0, 44.2, 46.9 kg/m³ with 45, 70, 90 MPa compressor, respectively.     

Hydrogen storage and cyclic properties of V60Ti(21.4+x)Cr(6.6−x)Fe12 (0 ≤ x ≤ 3) alloys

Hydrogen storage and cyclic properties of V₆₀Ti_(21.4+x)Cr_(6.6−x)Fe₁₂ (0 ≤ x ≤ 3) alloys were investigated systematically. All alloys were composed of single BCC phase and exhibited good activation performance. V₆₀Ti_22.4Cr_5.6Fe₁₂ showed the highest desorption capacity of 2.12 wt% with the plateau pressure of 0.061 MPa. In the absorption–desorption cycle tests, both the hydrogen desorption capacity and the micro-strain of V₆₀Ti_22.4Cr_5.6Fe₁₂ alloy showed exponential relationship with the increase of cycle numbers, which indicated that the micro-strain induced and thereafter accumulated during the absorption–desorption cycles might lead to the decrease of the desorption capacity.     

Effect of annealing conditions on in situ formation of Cr2O3 diffusion barrier

The previous investigation suggested the approach for an in situ formation of Cr₂O₃ diffusion barrier by annealing the cold-sprayed Ni coatings on 310SS. In this paper, the influences of annealing conditions on the growth kinetics of Cr₂O₃ and substrate microstructure were investigated. Results show that Cr₂O₃ formed at the selected annealing temperatures of 850, 900 and 950°C for different durations of 4, 8 and 20?h. Increasing temperature enhanced the growth kinetics of Cr₂O₃ and the Mn content in the oxide layer. The annealing process for the growth of Cr₂O₃ improves the coating adhesion compared to the as-deposited coating. However, annealing at 950°C resulted in the precipitation of chromium carbides and enhanced the element inter-diffusion across the substrate/coating interface.     

Effectiveness of a dual surface modification of metallic interconnects for application in energy conversion devices

A dual surface modification of an SOFC metallic interconnect with a Gd₂O₃ layer and an MnCo₂O₄ coating was evaluated. The tested samples were oxidized for 7000 h in air at 1073 K. Oxidation products were characterized using XRD, SEM-EDS, and confocal Raman imaging, and ASR was measured. The effect of gadolinium segregation at grain boundaries in Cr₂O₃ was evaluated using S/TEM-EDS. Area specific-resistance was measured and fuel cell tests investigating electrochemical performance and Cr contamination of electrodes were also performed. The results show that the proposed dual modification was more advantageous than either modification applied separately. The fuel cell tests performed after aging in humidified hydrogen at 1073 K and involving an actual interconnect made of this dual-modified material showed that after 250 h of aging its electrochemical parameters were nearly identical to those of the non-aged reference electrode. Moreover, the modification protected the electrodes from Cr poisoning.     

A potential interconnect material for solid oxide fuel cells: Nd0.75Ca0.25Cr0.98O3−δ

Chromium-deficient Nd_0.75Ca_0.25Cr_1−xO_3−δ (0.02 ≤ x ≤ 0.06) oxides are synthesized and assessed as a novel ceramic interconnect for solid oxide fuel cells (SOFCs). At room temperature, all the samples present single perovskite phase after sintering at 1600 °C for 10 h in air. Cr-deficiency significantly improves the electrical conductivity of Nd_0.75Ca_0.25Cr_1−xO_3−δ oxides. No structural transformation occurs in the Nd_0.75Ca_0.25Cr_1−xO_3−δ oxides in the temperature range studied. Among all the samples, the Nd_0.75Ca_0.25Cr_0.98O_3−δ sample with a relative density of 96.3% exhibits the best electrical conductivity of 39.0 and 1.6 S cm⁻¹ at 850 °C in air and hydrogen, respectively. The thermal expansion coefficient of Nd_0.75Ca_0.25Cr_0.98O_3−δ sample is 9.29 × 10⁻⁶ K⁻¹ in the temperature range from 30 to 1000 °C in air, which is close to that of 8 mol% yttria stabilized zirconia electrolyte (10.3 × 10⁻⁶ K⁻¹) and other cell components. The results indicate that Nd_0.75Ca_0.25Cr_0.98O_3−δ is a potential interconnect material for SOFCs.     

Microstructure and hydrogen storage properties of V48Fe12Ti15-xCr25Alx (x=0, 1) alloys

The microstructure and hydrogen storage characteristics of V₄₈Fe₁₂Ti_15-xCr₂₅Al_x (x = 0, 1) alloys prepared by vacuum arc melting were studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and pressure–composition isotherm measurements. It was confirmed that all of the alloys comprise a BCC phase, a Ti-rich phase, and a TiFe phase. Al as a substitute for part of the Ti content caused an increase of lattice parameters of the BCC phase and of the equilibrium pressures of hydrogen desorption, but decrease of the hydrogen storage capacities. The kinetic mechanism of the hydrogenation and dehydrogenation of the alloys was investigated by the classical Johnson–Mehl–Avrami equation. The reaction enthalpies (ΔH) for the dehydrogenation of alloys without and with Al were calculated by the Van't Hoff equation based on the PCI measurement data, which are 30.12 ± 0.14 kJ/mol and 28.02 ± 0.46 kJ/mol, respectively. The thermal stability of the metal hydride was measured by differential scanning calorimetry. The hydrogen desorption activation energies were calculated using the Kissinger method as 79.41 kJ/mol and 83.56 kJ/mol for x = 0 and 1, respectively. The results suggest that the substitution of titanium with aluminum improves the thermodynamic properties of hydrogen storage and reduces the kinetic performance of hydrogen desorption.