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.     

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.     

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.     

High-temperature oxidation process analysis of MnCo2O4 coating on Fe-21Cr alloy

Even though the operation temperature of solid oxide fuel cells (SOFCs) stacks has been reduced (∼750 °C), stainless steel interconnect within the stacks still requires protection by high conductive coatings to delay the growth of oxide scales and reduce chromium evaporation. Manganese cobaltite spinel protective coating with a nominal composition of MnCo₂O₄ was produced on Fe-21Cr stainless steel. Electrical, microstructural and compositional analysis were performed to investigate the interfacial reaction of MnCo₂O₄ protective coating with the stainless steel substrate during 750 °C oxidation process. The spinel coating not only acts as a barrier to Cr outward transport, but also improves the electrical conductivity of the alloy interconnect during long-term oxidation. The coated alloy demonstrates good electrical conductivity with an area specific resistance (ASR) of about 5 mOhm cm² after oxidation for 1000 h at 750 °C, which is about 1/4 of the ASR of bare Fe-21Cr alloy. The reduction of ASR might be caused by the fact that Cr migrated from the steel substrate interact with MnCo₂O₄ coating and generated Mn-Co-Cr spinel phase, which has higher electrical conductivity than that of Cr₂O₃.     

CuOCr2O3 core-shell structured co-catalysts on TiO2 for efficient photocatalytic water splitting using direct solar light

Synthesis of core-shell structured CuOCr₂O₃ nanoparticles as co-catalyst to improve the photocatalytic hydrogen evolution performance of TiO₂ was demonstrated. The effect of co-catalyst loading on TiO₂ and the nature of the reactor was found to be more significant for H₂ production under direct solar light. The formation of 9.3 nm Cr₂O₃ shell over CuO core in the CuOCr₂O₃ nanostructured co-catalyst was confirmed using transmission electron microscopy. A very high H₂ production rate of 82.39 and 70.4 mmol h^?1 g^?1_cat was observed with quartz and pyrex reactors under direct solar light of irradiation 96–100 mW/cm², respectively. This is almost three times higher than that of bare TiO₂ under similar experimental conditions. The core-shell co-catalyst loaded on TiO₂ by simple mechanical mixing method which is useful for bulk scale synthesis in practical applications. The observed high H₂ production was explained with plausible mechanism where the synergic effect of CuOCr₂O₃ co-catalyst loaded TiO₂ surface that reduces the effective charge carriers recombination and impeded backward reaction by the Cr₂O₃ thin layer. The presence of Cu²⁺ and absence of Cu⁺ and metallic Cu was confirmed using XPS analysis. The effect of co-catalyst loading and sacrificial agent concentration on the photocatalytic hydrogen production was also reported. The stability of the CuOCr₂O₃ core-shell NPs loaded TiO₂ photocatalyst under the direct solar light was examined by continuous cycling for three days and it was found to be 81 and 70% of photocatalyst activity is retained after 3 days in the quartz and pyrex reactor systems, respectively.     

Advance fuel cells using Al2O3-nNaAlO2 composite as ion-conducting membrane

In present work, we reported an novel oxide-salt Al₂O₃NaAlO₂ composite, which was prepared by mixing Al₂O₃ and Na₂CO₃ two phase materials in different weight ratio, and then sintering at 1100 °C. The X-ray diffraction pattern, scanning-electron microscope and impedance spectra are applied to characterize the crystal structure, morphology and electrical properties of the Al₂O₃NaAlO₂ composite. The Al₂O₃NaAlO₂ composite as electrolyte membrane was sandwiched by two pieces of Ni_0.8Co_0.15Al_0.05Li-oxide (NCAL) electrode layer to construct advanced fuel cell. Optimizing the weight ratio of Al₂O₃ and NaAlO₂, such cell delivered an highest power density of 789 mW/cm² and an open circuit voltage (V_oc) of 1.13 V at 575 °C. The superior performance is mainly due to the excellent ion-conducting of Al₂O₃NaAlO₂ composites and the outstanding catalysis activity of the NCAL eletrodes. The EIS results revealed that the Al₂O₃NaAlO₂ composite possessed superior ionic conductivity of 0.121 S/cm at 575 °C. The interfacial effects between oxide-salt two phase including space-charge and structural misfit at the interface region dominated the ion transport for Al₂O₃NaAlO₂ composite.     

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.     

Electrodeposited Ni-Cr2O3 nanocomposite anodes for ethanol electrooxidation

Nanocomposite coatings of Ni-Cr₂O₃ supported on carbon electrodes have been prepared by electrodeposition technique from nickel Watts bath in presence of Cr₂O₃ nanoparticles. Their electrochemical catalytic activities have been evaluated towards electrooxidation of ethanol in 1.0 M NaOH solution by using cyclic voltammetry, chronoamperometry and Tafel plots. The performance of the prepared anodes towards electrooxidation of ethanol as a function of co-deposited Cr₂O₃ content was studied. The catalytic activity of fabricated electrodes increases with increasing the volume fraction percent (V_f%) of Cr₂O₃ in the deposited film up to 7V_f%. The Ni-Cr₂O₃/C (7V_f%) electrode displayed significantly enhanced catalytic activity and stability towards electrooxidation of ethanol compared with Ni/C electrode. The kinetic parameters of Ni(OH)₂/NiOOH and ethanol oxidation at Ni/C and Ni-Cr₂O₃/C electrodes have been evaluated.     

Synthesis of nanocrystalline mesoporous Ni/Al2O3SiO2 catalysts for CO2 methanation reaction

A series of nanocrystalline mesoporous Ni/Al₂O₃SiO₂ catalysts with various SiO₂/Al₂O₃ molar ratios were prepared by the sol-gel method for the carbon dioxide methanation reaction. The synthesized catalysts were evaluated in terms of catalytic performance and stability. The catalysts were studied using XRD, BET, TPR and SEM. The BET results indicated that the specific surface area of the samples with composite oxide support changed from 254 to 163.3 m²/g, and an increase in the nickel crystallite size from 3.53 to 5.14 nm with an increment of Si/Al molar ratio was visible. The TPR results showed a shift towards lower temperatures, indicating a better reducibility and easier reduction of the nickel oxide phase into the nickel metallic phase. Furthermore, the catalyst with SiO₂/Al₂O₃ molar ratio of 0.5 was selected as the optimal catalyst, which showed 82.38% CO₂ conversion and 98.19% CH₄ selectivity at 350 °C, high stability, and resistivity toward sintering. Eventually, the optimal operation conditions were specified by investigating the effect of H₂/CO₂ molar ratio and gas hourly space velocity (GHSV) on the catalytic behavior of the denoted catalyst.     

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.     

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.     

MWCNTs and Cu2O sensitized TiFe2O3 photoanode for improved water splitting performance

Fe₂O₃ and Cu₂O, both earth abundant materials are used in functionalizing Ti doped Fe₂O₃ photoanodes with Cu₂O and MWCNTs for improving photoelectrochemical performance for hydrogen generation. Pristine Ti doped Fe₂O₃ are fabricated by spray pyrolysis deposition method on the conducting ITO coated glass substrate. Two different modifications are adopted to improve the photoelectrochemical performance of pristine sample by subsequent deposition of multi walled carbon nano tubes (MWCNTs) alone and also in combination with Cu₂O. Better photoresponse in modified samples is attributed to increase in conductivity and promotion of electron transport to Fe₂O₃ layer due to presence of MWCNTs while formation of heterojunction also promotes charge transfer kinetics by effective separation of charge carriers. Offering high photocurrent density of 5.17 mA cm^?2 at 1 V vs SCE, high open circuit voltage (V_oc), least resistance, higher negative flat band potential (V_fb), TiFe₂O₃/(MWCNTs + Cu₂O), emerges as the most photoactive sample. High applied bias photon to current conversion efficiency (ABPE) value of 4.6% is obtained for the modified sample against 0.07% ABPE for TiFe₂O₃ photoanodes.     

Influence of multiple oxide (Cr2O3/Nb2O5) addition on the sorption kinetics of MgH2

The influence of multiple additions of two oxides, Cr₂O₃ and Nb₂O₅, as additives on the hydrogen sorption kinetics of MgH₂ after milling was investigated. We found that the desorption kinetics of MgH₂ were improved more by multiple oxide addition than by single addition. Even for the milled MgH₂ micrometric size powders, the high hydrogen capacity with fast kinetics were achieved for the powders after addition of 0.2 mol% Cr₂O₃ + 1 mol% Nb₂O₅. For this composition, the hydride desorbed about 5 wt.% hydrogen within 20 min and absorbed about 6 wt.% in 5 min at 300 °C. Furthermore, the desorption temperature was decreased by 100 °C, compared to MgH₂ without any oxide addition, and the activation energy for the hydrogen desorption was estimated to be about 185 kJ mol⁻¹, while that for MgH₂ without oxide was about 206 kJ mol⁻¹.     

Numerical analysis of H2 formation during partial oxidation of H2SH2O upon activation of oxidizer by an electric discharge

The numerical analysis of H₂ production during partial oxidation of H₂SH₂O in a plug-flow reactor at atmospheric pressure and a rather low temperature (T₀ = 500 K) was conducted, when the oxidizer (oxygen or air) was preliminarily activated by an electrical discharge with different values of reduced electric field and input energy. It was shown that a significant hydrogen yield in flow reactor can be obtained only after ignition of the mixture. The ignition delay length depends on the reduced electric field E/N and input energy E_s in the discharge and is minimal at E/N～8–10 Td for the discharge in oxygen and at E/N～4–10 and 120–150 Td in air discharge, when O₂(a¹Δ_g) mole fraction in the discharge products is maximal. If the H₂SH₂OO₂(air) mixture ignites inside the flow reactor, the mole fraction of hydrogen and its relative yield do not depend on E/N. The relative hydrogen yield increases monotonically with an addition of water to H₂S. It was found, that the approach based on the partial oxidation of the H₂SH₂O mixture upon activation of oxygen by an electric discharge can ensure very low energy cost for H₂ production. The minimum specific energy requirement, obtained for the H₂SO₂ mixture, was found to be 0.83 eV/(molecule H₂) and 0.18 eV/(molecule H₂S) at atmospheric pressure and can be further decreased if the energy released during partial oxidation of H₂S is spent on heating the reagents. The use of air as an oxidizer requires higher energy costs and seems to be less promising.     

NiMn2O4 spinel as an alternative coating material for metallic interconnects of intermediate temperature solid oxide fuel cells

In an effort to improve the performance of SUS 430 alloy as a metallic interconnect material, a low cost and Cr-free spinel coating of NiMn₂O₄ is prepared on SUS 430 alloy substrate by the sol-gel method and evaluated in terms of the microstructure, oxidation resistance and electrical conductivity. A oxide scale of 3-4 μm thick is formed during cyclic oxidation at 750 °C in air for 1000 h, consisting of an inner layer of doped Cr₂O₃ and an outer layer of doped NiMn₂O₄ and Mn₂O₃; and the growth of Cr₂O₃ and formation of MnCr₂O₄ are depressed. The oxidation kinetics obeys the parabolic law with a rate constant as low as 4.59 × 10⁻¹⁵ g² cm⁻⁴ s⁻¹. The area specific resistance at temperatures between 600 and 800 °C is in the range of 6 and 17 mΩ cm². The above results indicate that NiMn₂O₄ is a promising coating material for metallic interconnects of the intermediate temperature solid oxide fuel cells.     

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.     

Photoelectrochemical behaviour of n-silicon photoanodes coated with chromium(III) oxide films and Cr2O3 containing composite layers

n-Type silicon single crystals coated with Cr₂O₃-containing composite and/or p-Cr₂O₃ thin films were studied as photoanodes for electrocatalytic photo-oxidation of isopropanol in aqueous media. Film deposition was by pyrolysis of organometallic compounds (i.e. chemical vapour deposition) for Cr₂O₃ and by a ceramic technique for the Cr₂O₃-containing composite. The photocurrent at the heterojunction electrodes illuminated with visible light is produced by holes which are photoexcited in n-silicon only. There is no energy barrier for minority carriers to be transferred across the n-p heterojunction, so considerable quantum efficiencies of photocurrent are observed. Heterogeneous redox catalysis is responsible for the oxidation of isopropanol with Cr₂O₃ as electrocatalyst. The use of a composite coating makes the photoanode highly stable, kinetics at the electrode/electrolyte interface being a key factor for the stability against photocorrosion.     

An improvement of the hydrogen permeability of C/Al2O3 membranes by palladium deposition into the pores

The development of compact hydrogen separator based on membrane technology is of key importance for hydrogen energy utilization, and the Pd-modified carbon membranes with enhanced hydrogen permeability were investigated in this work. The C/Al₂O₃ membranes were prepared by coating and carbonization of polyfurfuryl alcohol, then the palladium was introduced through impregnation–precipitation and colloid impregnation methods with a PdCl₂/HCl solution and a Pd(OH)₂ colloid as the palladium resources, and the reduction was carried out with a N₂H₄ solution. The resulting Pd/C/Al₂O₃ membranes were characterized by means of SEM, EDX, XRD, XPS and TEM, and their permeation performances were tested with H₂, CO₂, N₂ and CH₄ at 25 °C. Compared with the colloid impregnation method, the impregnation–precipitation is more effective in deposition of palladium clusters inside of the carbon layer, and this kind of Pd/C/Al₂O₃ membranes exhibits excellent hydrogen permeability and permselectivity. Best hydrogen permeance, 1.9 × 10⁻⁷ mol/m² s Pa, is observed at Pd/C = 0.1 wt/wt, and the corresponding H₂/N₂, H₂/CO₂ and H₂/CH₄ permselectivities are 275, 15 and 317, respectively.     

Analysis of H2 and CO production via solar thermochemical reacting system of NiFe2O4 redox cycles combined with CH4 partial oxidation

Thermal reduction of the partial oxidation of CH₄NiFe₂O₄ followed by oxidation with H₂O and CO₂ was numerically investigated for H₂ and CO production. P1 radiation model was used to account for radiative heat transfer. The synergistic effect of the reactivity of Fe/Ni exhibited a very promising strategy for producing 45% of syngas with 2.54 ratios of H₂:CO at the first step and 55% of syngas with 2.34 ratios of H₂:CO at the second step. The increase in incident radiation heat flux to 437.69 kW/m² resulted in higher reduction kinetics of species conversion until the formation of oxygen carriers consisting of 65% of FeO, 35% of NiFe and 2.6% of carbon deposition. However, during the reduction process, the decrease in total pressure to 0.05 MPa enhanced the species reactivity and the production of H₂ and CO while minimizing carbon deposition. Moreover, the oxidation temperature, operating pressure and the concentration of oxidizing species have strong impacts on the oxidation kinetics. Unlike high thermal reduction process, increasing the total pressure to 1 MPa has favorable effects on syngas production at oxidation step.