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.     

Effect of helium implantation on the hydrogen retention of Cr2O3 films formed in an ultra-low oxygen environment

To simulate and investigate the irradiation damage of neutron and transmutation effect (He production) on the hydrogen isotope trapping behavior, in the present study, a new monoclinic hydrogen permeation barrier composed of Cr₂O₃ was fabricated and helium implantation with different fluences was tentatively employed. First, pure chromium samples were oxidized in an ultra-low oxygen partial pressure (1.7 × 10⁻²³ Pa) environment to obtain a single Cr₂O₃ layer. Then a dense layer of helium bubbles was formed in a substrate using a helium ion implantation method. Finally, the samples were treated in a hydrogen plasma environment at 500 °C. The damage, vacancy, and helium distribution in these samples were then simulated by SRIM. The morphology, phase, surface characteristics, thermal desorption, and electrochemical properties were subsequently characterized and evaluated. Thermal desorption spectrum analysis (TDS) was used to study the thermal desorption of hydrogen and helium at different temperatures. Our results showed that the inhibitory effect of the composite hydrogen barrier layer on the hydrogen diffusion in the substrate first increased and then decreased with the increase of the helium ion implantation fluence.     

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.     

Coating the porous Al2O3 substrate with a natural mineral of Nontronite-15A for fabrication of hydrogen-permeable palladium membranes

Substrate surface modification is a key pretreatment during fabrication of composite palladium membranes for hydrogen purification in hydrogen energy applications. The suspension of a natural porous material, Nontronite-15A mineral, without any organic additives was employed in dip-coating of the porous Al₂O₃ substrate. The Nontronite-15A mineral was characterized by SEM, XRD, TG−DSC and granulometry analysis. The surface and cross-section of the coated porous Al₂O₃ tubes were observed by SEM, and their pore size distribution and nitrogen flux were also measured. Palladium membranes were fabricated over the coated Al₂O₃ tubes by a suction-assisted electroless plating. The optimal loading amount of the Nontronite-15A mineral is just to fill in and level up the surface cavities of the Al₂O₃ substrate rather than to form an extra continuous layer. A thin and selective palladium membrane was successfully obtained, and its permeation performances were tested. The kinetic analyses on the hydrogen flux indicate that the hydrogen permeation behavior exhibits typical characteristics for most of the palladium membranes. During the stability test at 450 °C for 192 h, no membrane damage was detected, and the hydrogen flux increased slightly.     

Improved hydrogen permeation through thin Pd/Al2O3 composite membranes with graphene oxide as intermediate layer

Here we proposed the decreasing in the roughness of asymmetric alumina (Al₂O₃) hollow fibers by the deposition of a thin graphene oxide (GO) layer. GO coated substrates were then used for palladium (Pd) depositions and the composite membranes were evaluated for hydrogen permeation and hydrogen/nitrogen selectivity. Dip coating of alumina substrates for 45, 75 and 120 s under vacuum reduced the surface mean roughness from 112.6 to 94.0, 87.1 and 62.9 nm, respectively. However, the thicker GO layer (deposited for 120 s) caused membrane peel off from the substrate after Pd deposition. A single Pd layer was properly deposited on the GO coated substrates for 45 s with superior hydrogen permeance of 24 × 10⁻⁷ mol s⁻¹m⁻² Pa⁻¹ at 450 °C and infinite hydrogen/nitrogen selectivity. Activation energy for hydrogen permeation through the Al₂O₃/GO/Pd composite membrane was of 43 kJ mol⁻¹, evidencing predominance of surface rate-limiting mechanisms in hydrogen transport through the submicron-thick Pd membrane.     

Synthesis of Cr2O3–Al2O3 powders with various Cr2O3/Al2O3 molar ratios and their applications as support for the preparation of nickel catalysts in CO2 methanation reaction

The Methanation of CO₂ to CH₄ is a significant route to save energy and reduce CO₂ emission. In this work, a series of Cr₂O₃–Al₂O₃ powders were synthesized by a novel and simple solid-state method and considered as the carrier for the nickel catalysts in CO₂ methanation. The BET area and pore volume of the supports decreased with the decrease in Al₂O₃/Cr₂O₃ molar ratio. The results indicated that the increase in Cr₂O₃ content improved the catalytic performance and 15 wt%Ni/Cr₂O₃ catalyst exhibited the highest CO₂ conversion of 80.51%, and 100% CH₄ selectivity at 350 °C. The results indicated that the CO₂ conversion improved with the increment in H₂/CO₂ molar ratio from 2 to 5. The improvement in CO₂ conversion was also observed with decreasing GHSV due to the longer residence time of the reactants on the catalyst surface. Also, the results showed that increasing calcination temperature led to a decrease in CO₂ conversion. The 15 wt%Ni/Cr₂O₃ catalyst exhibited high stability in carbon dioxide methanation reaction.     

Hydrogen permeation on Al2O3-based nickel/cobalt composite membranes

Al₂O₃ was synthesized using the sol-gel process with aluminum isopropoxide as the precursor and primary distilled water as the solvent. Nickel and cobalt metal powders were used to increase the strength of the membranes. The Al₂O₃-based membranes were prepared using HPS following a mechanical alloying process. The phase transformation, thermal evolution, surface and cross-section morphology of Al₂O₃ and Al₂O₃-based membranes were characterized by XRD, TG-DTA and FE-SEM. The hydrogen permeation of Al₂O₃-based membranes was examined at 300–473 K under increasing pressure. Hydrogen permeation flux through an Al₂O₃-20wt%Co membrane was obtained to 2.36 mol m⁻² s⁻¹. Reaction enthalpy was calculated to 4.5 kJ/mol using a Van’t Hoff’s plot.     

Influence of the method of synthesis on hydrogen adsorption properties of mesoporous binary B2O3/Al2O3 gel systems

Three series of binary oxide systems B₂O₃/Al₂O₃ were prepared and the effect of alumina on dispersion of boron (B₂O₃) component was investigated. The aim of the study was to achieve a maximum dispersion of B₂O₃ in the Al₂O₃ a gel matrix that would lead to increased sorption capacity on boron oxide. Many attempts were made to establish the preparation conditions that would lead to a maximum dispersion of B₂O₃ in the Al₂O₃ gel matrix needed to increase the hydrogen sorption capacity on boron oxide. The systems were characterized by X-ray diffraction, SEM, TEM and low temperature nitrogen adsorption. Hydrogen adsorption was tested in the volumetric system.Results of the study showed that the amount of hydrogen adsorbed on B₂O₃ depended not only on the surface area of the system but also on the separation of B₂O₃ domains in Al₂O₃ gel network. Irrespective of the method of synthesis of the binary oxide system, the dispersion of B₂O₃ phase reflected in the amount of hydrogen adsorbed was the highest for the systems of the lowest B/Al molar ratios studied, i.e. for B/Al = 0.25.     

Plasma-enhanced catalytic ammonia decomposition over ruthenium (Ru/Al2O3) and soda glass (SiO2) materials

Catalytic ammonia (NH₃) decomposition has been identified as a COx-free, sustainable hydrogen production method for fuel cell applications. In this study, the performance of plasma–catalyst-based NH₃ decomposition over ruthenium (Ru/Al₂O₃) and soda glass (SiO₂) catalytic materials at atmospheric pressure and ambient temperature was investigated. NH₃ decomposition reactions were conducted in a dielectric barrier discharge plasma plate-type reactor. NH₃ was fed into the plate catalytic microreactor at flow rates of 0.1–1 L/min and plasma voltages of 12–18 kV. Compared to plasma NH₃ decomposition without a catalyst, plasma–catalyst-based NH₃ decomposition showed a significant enhancement of the hydrogen production rate and energy efficiency. Furthermore, the hydrogen concentration results obtained over the Ru/Al₂O₃ catalyst were higher than those over the SiO₂ catalyst because Ru/Al₂O₃ possesses good electronic properties and exhibits high sensitivity to NH₃ decomposition. In addition, the resulting plasma heat enhanced the activation of the catalytic material, subsequently leading to an increase in the hydrogen production rate from NH₃. The maximum conversion rates were 85.65% and 84.39% for Ru/Al₂O₃ and SiO₂, respectively. Moreover, the energy efficiency of NH₃ decomposition over the Ru-based catalyst material was higher than that over the SiO₂ material. The presence of the catalyst active sites and plasma enhanced the mean electron energy, which could enhance the dissociation of NH₃. It can be concluded that the SiO₂ material can be utilised as a catalyst and that its combination with plasma accelerates the decomposition process of NH₃ and incurs a lower cost compared to Ru materials.     

Protective coatings for Cr2O3-forming interconnects of solid oxide fuel cells

Cr₂O₃ evaporation from Cr₂O₃-forming metallic interconnects during operation of the solid oxide fuel cells (SOFC) can poison other cell components and cause degradation. Protective NiFe₂O₄ spinel coatings on interconnect alloys were developed by electroplating and screen printing, respectively. Results indicate that NiFe₂O₄ coatings can significantly improve the oxidation resistance of the alloy while providing effective conducting path with inherent low resistance, and also are expected to serve as a diffusion barrier to effectively reduce the Cr₂O₃ evaporation. Two coating techniques were evaluated in terms of the performances of the coatings. A very interesting and smart coating structure was reported.     

The strain and transition metal doping effects on monolayer Cr2O3 for hydrogen evolution reaction: The first principle calculations

Chromic oxide (Cr₂O₃) monolayer is a promising alternative hydrogen evolution reaction (HER) catalyst compared with expensive platinum (Pt) due to its advantages such as low cost, large specific surface area, high reserves, and designability. In this study, the two practical strategies, strain engineering and transition metal (TM) doping (Mn, Fe, Zn, etc.), are proposed to activate the catalytic sites of Cr₂O₃ monolayer for the HER. The density functional theory (DFT) calculations demonstrate that the strained Cr₂O₃ monolayer can stimulate the HER activity with the Gibbs free energy of hydrogen adsorption (ΔG_H1) close to 0.09eV, which can be considered as a performable strategy to tune the HER catalytic behavior of Cr₂O₃ monolayer. For the TM doping, it also plays a role in the performance adjustment. These results provide a guideline to optimize the HER performance of Cr₂O₃ monolayer.     

Facile synthesis of M-CuFe2O4 (M= Al2O3, CeO2, La2O3, ZrO2, Mn2O3) catalysts using a one-pot method and their applications in high-temperature water gas shift reaction

Water gas shift reaction is an essential process of hydrogen production and carbon monoxide removal from syngas. In this study, the promotional effect of ZrO₂, CeO₂, La₂O₃, Al₂O₃, and Mn₂O₃ was investigated on the CO conversion and thermal stability of the copper ferrite in high-temperature water gas shift reaction (HTSR) and hydrogen purification. The powders were synthesized by a simple solid-state route and characterized by XRD, H2-TPR, SEM, FT-IR, TG-DTA, and BET analyses. Promoters (ZrO₂, CeO₂, La₂O₃, Al₂O3, and Mn₂O₃) could affect the WGSR performance in activity and stability. In the M-CuFe2O4 catalyst, alumina acts as a texture promoter and aids in the fine dispersion of copper ferrite. The results indicated that the surface area of the Al₂O₃–CuFe₂O₄ (210 m²/g) catalyst was higher than the other samples. This catalyst presented higher CO conversion in HTSR and had higher stability at 1000 min on stream. It was found that the incorporation of different contents of alumina had a significant influence on the textural and catalytic properties of the CuFe₂O₄-based catalysts. The 30%Al₂O₃–70%CuFe₂O₄ catalyst exhibited the highest CO conversion of 65% at 350 °C, uniform pore size distribution, and intense interaction between copper ferrite and alumina, causing the effective stabilization of the active phase in the catalyst structure. The findings of this study represent that the solid-state method, due to its simplicity and creation of a mesoporous structure, can also be applied for the preparation of many heterogeneous metal oxide catalysts.     

Influence of the grain structure of yttria thin films on the hydrogen isotope permeation

Steel components are required in the infrastructure and the facilities of the hydrogen economy. The high hydrogen pressures in the hydrogen economy lead to embrittlement and surface corrosion of the steels. For the functionality of the facilities it is necessary to suppress the embrittlement and the surface corrosion of the steels by protective layers, e.g. ceramic thin films. With regard to fusion power plants ceramic thin films on the structural steel materials are also required. These thin films work as a tritium permeation barrier that is necessary to prevent the loss of the radioactive fuel inventory. Oxide thin films, e.g. Al₂O₃, Er₂O₃, and Y₂O₃, are promising candidates as tritium permeation barrier layers. In terms of the application in the first wall, this is especially true for yttrium due to its favorably short decay time after neutron activation compared to the other candidates. The Y₂O₃ layers with thicknesses of 0.5 μm–1 μm are deposited on both substrate sides by RF magnetron sputter deposition. Since the microstructure of the barrier layer plays an important role for the permeation reduction, layers with three different magnetron process modes and thus three different microstructures are prepared. After annealing the cubic crystal structure of all thin films is verified by X-ray diffraction and the different microstructures are investigated by scanning electron microscopy and transmission electron microscopy. The Y₂O₃ stoichiometry of all thin films and a chromium oxide material segregation at the interface are verified by analysis methods such as X-ray photoelectron spectroscopy. The permeation reduction factors of all thin films are determined in gas-driven deuterium permeation experiments. Corresponding to the three different microstructures, reduction factors of 25, 45, and 1100 are identified. Thus, the permeation reduction is strongly dependent on the Y₂O₃ microstructure. The measurement results suggest that a high density of grain boundaries leads to a high hydrogen permeation.     

Spontaneous dissociation of H2 and high energy barrier of H invasion on W doped Al2O3(0001) surface

In view of the wide use of tungsten in fusion experimental devices and the importance of hydrogen isotopes permeation, here we studied the adsorption, dissociation, diffusion and invasion behavior of hydrogen on W doped α-Al₂O₃ (0001) surface. Based on the first-principle approaches, we found the W substitution for a top surface Al atom is the most energetically favorable. H₂ molecule prefers to be adsorbed on the surface W and spontaneously dissociates into two H anions. Near the W defects, H atoms favor to be adsorbed at the W and Al sites rather than O sites on the surface, and within the subsurface layer H can only bond to W stably. As a result, H migration to subsurface should occur around W with an energy barrier as large as 4.22 eV which is much larger than the 1.91 eV around the O atom on undoped α-Al₂O₃ (0001) surface. These findings suggest that W surface doping is beneficial to α-Al₂O₃ as tritium permeation barrier.     

Fabrications and evaluations of hydrogen permeation on Al2O3/CeO2/graphene (ACG) composites membrane by Hot Press Sintering (HPS)

Ceramic membrane has high permeation rate of hydrogen and chemical stability. Al₂O₃ indicates stable at high temperature and a relatively large surface area. In addition, Al₂O₃ of porous is used as hydrogen separation membranes support, because of the high hydrogen permeability based on Knudsen diffusion mechanism.     

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.     

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.     

Eutectic structure evolution of Al2O3-ZrO2-Y2O3 system for potential hybrid solar cell application

Abstract

Ternary Al₂O₃-ZrO₂-Y₂O₃ samples with a eutectic composition were prepared by slow cooling. The microstructural evolution was observed with X-ray diffraction (XRD), scanning electron microscopy (SEM).

The SEM observation of the ternary samples agreed with the XRD with a completion of crystallisation by slow cooling. The target materials commonly have ‘cantaloupe skin’ microstructures as shown in the previous studies by Han et al. The nanocomposite may have experienced different cooling rates with two different microstructures, near the surface having experienced optimal conditions for the eutectic reaction during their cooling and thus formed the eutectic microstructure, near the centre having experienced a slower cooling rate. The crystallised eutectic ternary Al₂O₃-ZrO₂-Y₂O₃ system had three different phases with a 3Y₂O₃-5Al₂O₃ (yttrium-aluminium garnet phase), an alumina phase formed by the eutectic reaction, and a solid solution of ZrO₂ and Y₂O₃.     

Effect of metal additives on the catalytic performance of Ni/Al2O3 catalyst in thermocatalytic decomposition of methane

Thermocatalytic decomposition of methane is proposed to be an economical and green method to produce CO_x-free hydrogen and carbon nanomaterial. In present work, 60 wt% Ni/Al₂O₃ catalysts with different additives (Cu, Mn, Pd, Co, Zn, Fe, Mg) were prepared by co-impregnation method to investigate promotional effects of these metal additives on the activity and stability of 60 wt% Ni/Al₂O₃ and find out a really effective promoter for decomposition of methane. The catalyst was characterized by N₂ adsorption/desorption, X-ray diffraction, scanning electron microscopy, inductively coupled plasma optical emission spectrometer and hydrogen temperature programmed reduction. While metal additives (5 wt%) were added into 60 wt% Ni/Al₂O₃, the activity stability of 60 wt% Ni/Al₂O₃ was improved and the CH₄ conversion of 60 wt% Ni/Al₂O₃ was also improved except Zn addition. Mn addition was found to improve the catalytic activity of 60 wt% Ni/Al₂O₃ significantly and the CH₄ conversion of 5 wt% Mn-60 wt% Ni/Al₂O₃ was ∼80%. Cu addition was found to remarkably improve the catalytic stability of 60 wt% Ni/Al₂O₃ and the CH₄ conversion of 5 wt% Cu-60 wt% Ni/Al₂O₃ decreased from 61% to 45% after 250 min of reaction time. Carbon nanomaterials formed in the thermocatalytic decomposition process were characterized by X-ray diffraction, scanning electron microscopy, thermal gravimetric analyzer and Raman spectroscopy. Carbon deposits consist of amorphous carbon and carbon nanofibers.