Wet N2 Research Articles

Diode-Type Hydrogen Sensor Employing Pd-coated Pt Electrodes and Anodized Titania under Gaseous Atmosphere and Its Application to Oil-Deterioration Monitoring

The H2-sensing properties of the diode-type sensor using Pd-coated Pt electrodes and an anodized titania film (Pd/Pt/TiO2 sensor) were investigated in air and N2 at 250°C. The current (I)–voltage (V) characteristics of the Pd/Pt/TiO2 sensor were largely dependent on the composition of the gaseous atmospheres, and the sensor showed a large H2 response in wet air, which was comparable to that in wet N2. This indicates that the H2 response are hardly dependent on the oxygen concentration under a wet gaseous atmosphere. In addition, the oil-deterioration monitoring properties of the Pd/Pt/TiO2 sensor were also examined at 30°C. The sensor also exhibited nonlinear I–V characteristics in fresh and deteriorated oils, and the magnitudes of the current and resistance responses were largely dependent on the deterioration of the oils. These results clearly indicate that the Pd/Pt/TiO2 sensor can monitor the deterioration of oils.

Effects of ZnO additive on the electrical and densification properties of A-site deficient barium cerate perovskite

The dual strategies of A-site deficiency and addition of ZnO sintering aid have been employed to optimise stability and densify the high-temperature proton conductor Ba0.95Ce0.8Y0.2O3-δ at 1200 °C (B95CYZn-1200). Structural analysis performed by Rietveld refinement based on X-ray diffraction data indicated perovskite phase with monoclinic symmetry (space group, I2/m). Raman spectroscopy revealed the presence of ZnO for material sintered at 1200 °C, which most likely partially resides in grain-boundary regions, leading to enhanced electrical transport observed by impedance spectroscopy at low temperature. Anomalous and reproducible frequency-dependent impedance behaviour below 500 °C, observed in various atmospheres, is suggested to be attributable to the varistor-type properties of ZnO. The electrical conductivity of B95CYZn-1200 in the range 500–900 °C is typical of perovskite proton conductors but with greater conductivity than both Ba-stoichiometric BaCe0.8Y0.2O3-δ and ZnO-free Ba0.95Ce0.8Y0.2O3-δ in wet O2 and N2, reaching ∼ 1.1 S m−1 at 550 °C in wet O2. Resistance to carbonate formation and mechanical breakdown was demonstrated via prolonged impedance measurements in CO2- and H2-containing atmospheres in the range 600–700 °C.

Molecular and Dissociative Adsorbed Water Concentration and Surface Protonic Conduction in Nanocrystalline TiO2.

Surface protonic conduction in porous nanocrystalline oxides is commonly involved in catalytic processes. The configuration of surface adsorbed water on oxides plays a crucial role in surface protonic conduction. However, studies on the impact of complex surface adsorbed water configuration on the surface water concentration and diffusivity remain limited, and hinder an in-depth understanding of surface proton transport mechanisms, and the design of better surface proton conductors. Here, in situ Raman spectroscopy is utilized to quantitatively identify the contribution of dissociative and molecular adsorbed water components on porous nanocrystalline TiO2 surfaces between 25 and 200°C. The variations in molecular and dissociative adsorbed water concentration agree with the predominant surface proton conduction mechanisms at three different temperature stages. From 40 to 125°C, the reduced coverage of molecular adsorbed water layer results in the decreasing proton diffusivity. Water dissociation on the nanocrystalline TiO2 surface is easier in wet N2 than in wet O2, resulting in higher proton conductivity in wet N2; while the surface proton diffusivities in these two atmospheres are similar. The in situ spectroscopy technique enables the improvement of surface proton conducting oxides through quantitative evaluation and modulation of the surface proton concentration and diffusivity.

High Proton Conduction in the Octahedral Layers of Fully Hydrated Hexagonal Perovskite-Related Oxides.

Proton conductors have potential applications such as fuel cells, electrolysis cells, and sensors. These applications require new materials with high proton conductivity and high chemical stability at intermediate temperatures. Herein we report a series of new hexagonal perovskite-related oxides, Ba5R2Al2SnO13 (R = Gd, Dy, Ho, Y, Er, Tm, and Yb). Ba5Er2Al2SnO13 exhibited a high proton conductivity without chemical doping (e.g., 0.01 S cm-1 at 303 °C), which is attributed to its high proton concentration and diffusion coefficient. The high diffusion coefficient of Ba5Er2Al2SnO13 can be attributed to the fast proton migration in the octahedral layers. The high proton concentration is attributed to full hydration in hydrated Ba5Er2Al2SnO13 and the large amount of intrinsic oxygen vacancies in the dry sample, as evidenced by both neutron diffraction and thermogravimetric analysis. Ba5Er2Al2SnO13 was found to exhibit high chemical stability under wet atmospheres of O2, air, H2, and CO2. High proton conductivity and high chemical stability indicate that Ba5Er2Al2SnO13 is a superior proton conductor. Ba5R2Al2SnO13 (R = Gd, Dy, Ho, Y, Tm, and Yb) exhibited high electrical conductivity in wet N2, suggesting that these materials also exhibit high proton conductivity. These findings will open new avenues for proton conductors. The high proton conductivity via full hydration and fast proton migration in octahedral layers in highly oxygen-deficient hexagonal perovskite-related materials would be an effective strategy for developing next-generation proton conductors.

Multistep kinetics of the thermal dehydration/decomposition of metakaolin-based geopolymer paste

This study investigated the kinetics of the multistep thermal dehydration/decomposition of the metakaolin-based geopolymer paste. The component two reaction steps were characterized by the evolution of water vapor and the simultaneous evolution of water vapor and CO2, respectively. In a stream of dry N2, the kinetics of the first and second reaction steps were characterized by the apparent activation energy (Ea) values of 92 and 166 kJ mol−1, respectively. Both reaction steps exhibited a diffusion-controlled rate behavior. In a stream of wet N2, the mass loss curves systematically shifted to higher temperatures with an increase in the water vapor pressure (p(H2O)). The first reaction step was significantly influenced by p(H2O), and the apparent Ea increased to 175 kJ mol−1 at p(H2O) = 11.4 kPa. The second reaction step was less sensitive to the atmospheric water vapor, as characterized by its Ea of ∼165 kJ mol−1, irrespective of the p(H2O).

Passivation Improvement of Poly‐SiOx Based TOPCon Contacts by Thermal Annealing in a Water Vapor Atmosphere: Mechanism Exploration and Application Research

AbstractPost‐treatment techniques of tunnel oxide passivated contact (TOPCon) structure are universally implemented via executing an additional hydrogenation process to optimize the passivation performance. However, the underlying physical mechanism and which method is most applicable are still being investigated. Herein, the effectiveness of thermal annealing in water vapor and N2 atmosphere is studied, which is both environmentally friendly and easy to operate. It is demonstrated that compared to other common hydrogenation techniques, the wet N2 outperforms in improving the passivation performance, which can be attributed to the neutralization of internal defects in poly‐Si and the optimization of structural densities, and interestingly, this gain effect is amplified when this contact is doped with oxygen impurity. A power conversion efficiency of 22.62% is achieved using this technology which verifies its reliability and applicability. A loss analysis based on numerical simulations, outlining ways to achieve higher conversion efficiency and highlighting the great potential of this technology is also provided. Extensive experiments and first‐principles calculations based on density‐functional theory are conducted to clarify the underlying dynamics, including the surface adsorption process and the potentiation mechanisms, revealing that passivation and neutralization of hydrogen atoms couple with the compactness optimization of the structure.

Assessing boron doped Ba2In1.8Si0.2O5.1 as a electrolyte for potential use in ceramic electrolyte fuel cells

Doped perovskite oxides have been widely studied in recent years as potential proton conducting electrolytes for electrochemical devices. One such system, Ba2In2O5 transforms from an orthorhombic phase at room temperature to a cubic perovskite phase at high temperature, which results in a dramatic increase in ionic conductivity. In this paper, we report the incorporation of Boron (B) and Silicon (Si) at the Indium (In) cation site (Ba2In2-x-yBxSiyO5+y/2) to stabilize the highly conducting cubic phase at room temperature, as well as lower the sintering temperature required. X-ray powder diffraction data of the synthesized powders sintered at 1250 ​°C reveal successful incorporation of Si and B at the In site with maximum solubility of 10 ​mol% for both, and conductivity measurements show an enhancement upon dopant incorporation. Further enhancement in conductivity was observed in measurements performed under wet N2(g) atmosphere which is attributed to a protonic contribution to the overall ionic conductivity. In summary, the ionic conductivity of all the doped samples was shown to be at least an order of magnitude higher than that of undoped Ba2In2O5 with the best performance observed for the Ba2In1.7B0.1Si0.2O5.1 composition.

Experiments on the thermal activation of hydrogen release of NaBH4-sodalites characterized by IR- and MAS-NMR spectroscopy

New results on the reactions of the (BH4)-anion enclathrated in the cages of sodalites are reported. Hydrothermally synthesized NaBH4-sodalites (ǀNa8(BH4)2ǀ[SiAlO4]6) always contain hydro-sodalite type cages (ǀNa3(H2O)4ǀ[SiAlO4]3). With increasing temperature dehydration occurs. Above 250 °C a limited reaction of the residual water is going on with (BH4)-cage fillings releasing hydrogen and the appearance of certain borate specifications enclosed in the sodalite cages. The effect of a reaction of oxygen with the (BH4)-anions in the sodalite-cages at temperatures above 400 °C is also shown. The degree of (BH4)-conversion using wet and dry N2 stream is further followed by IR and MAS NMR investigations. External supply of water largely enhances the degree of reaction, e.g. at 400 °C from 16% to 44% loss of (BH4)-absorption intensity. However, 11B MAS NMR shows 8% and 22% of a conversion of (BH4)-cage fillings into new borate species in dry and wet N2 stream, respectively. These lower values are explained by a loss of B-ions via formation and evaporation of BH3 from the sodalite. Further evaluation of the 11B MAS NMR spectra could resolve the formation of (B(OH)3)-, (BO(OH)2)-, (B(OH)4)- along with unreacted (BH4)-species in the cages. 1H-MAS NMR shows a −3.8 ppm signal related to cage isolated (OH)-, which suggests an initial reaction step via (H+ + BH4−) to (BH3 + H2). The formation of (B(OH)3), however, also indicated sufficient water for a reaction of BH3 releasing further hydrogen. The formation of (BH3) could be observed in temperature dependent IR investigations at temperatures above 400 °C.

Low-temperature sintering and enhanced stability of fluorine-modified BaZr0.8Y0.2O3-δ synthesised by a sol-gel alkoxide route

Sol-gel synthesis using alkoxides is employed to obtain fluorine-modified BaZr0.8Y0.2O3-δ nanopowders with Ba excess at 750 °C. Possible effects of F loss at high temperature were limited on densifying at the low temperature of 1200 °C for 4 h on addition of ZnO as sintering agent. The presence of F in sintered samples was confirmed by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry. XPS and Raman spectroscopy indicated that F doping of BZY inhibits carbonate formation. Conductivity measurements for prolonged periods in wet H2-containing and CO2-containing atmospheres at 500 °C also demonstrated chemical stability. The temperature dependence of the electrical conductivity of sintered samples in wet and dry N2 and O2 indicates that the grain-boundary dominates the electrical behaviour to 500 °C, above which the bulk process dominates. Ionic conduction is prevalent at low temperature, with a transition to a mixed ionic-electronic conducting behaviour at 450 °C.

Effect of Ce/Zr Ratio on Thermal and Chemical Expansions and CO2 Resistance of Rare Earth-Doped Ba(Ce,Zr)O3 Perovskite-Type Proton Conductors

To make clear the effect of Ce/Zr ratio on the thermal and chemical expansions of rare earth-doped Ba(Ce,Zr)O3 perovskite-type proton conductors, we investigated the crystal structures (crystal phase, lattice constants, lattice volume, etc.) of Ba(Ce0.7Zr0.1Y0.1Yb0.1)O3- δ, Ba(Ce0.4Zr0.4Y0.1Yb0.1)O3-δ, etc. on the basis of Rietveld refinement results of high-temperature X-ray diffraction data measured between 298 and 1173 K under dry and wet N2 atmospheres. We will also report on the CO2 resistance of these proton conductors.

Effect of Ce/Zr Ratio on Thermal and Chemical Expansions and CO2 Resistance of Rare Earth-Doped Ba(Ce,Zr)O3 Perovskite-Type Proton Conductors

The crystal structures, thermal expansions, chemical expansions, and CO2 resistances of Ba(Ce0.7Zr0.1Y0.1Yb0.1)O3-δ (BCZYYb7111) and Ba(Ce0.4Zr0.4Y0.1Yb0.1)O3-δ (BCZYYb4411) have been investigated via high-temperature X-ray diffraction (HT-XRD) in controlled atmospheres (dry N2, wet N2, and dry 33.3 vol% CO2 - 66.6 vol% N2) on cooling from 1173 to 298 K. Two phase transitions, cubic to trigonal and trigonal to monoclinic, were observed for BCZYYb7111, whereas one phase transition, cubic to trigonal for BCZYYb4411. BCZYYb7111 and BCZYb4411 exhibited the linear thermal expansion coefficients of 10.6 × 10-6 K-1 and 9.8 × 10-6 K-1 between 773 and 1173 K in dry N2, respectively. The maximum chemical expansion rates of 0.33 % and 0.30 % were observed at 773 K for BCZYYb7111 and BCZYYb4411, respectively. BCZYYb4411 having lower Ce/Zr ratio and higher symmetry showed higher CO2 resistance than BCZYYb7111.

Ni-exsolved PrBaMn2-xNixO6-δ–based catalysts for high performance of ethanol steam reforming

PrBaMn2O6-δ (PBMO) and PrBaMn2-xNixO6-δ (PBMNO) with exsolved Ni nanoparticles (1.25 wt.%, Ni-PBMNO) are prepared for the development of high-performance catalysts for ethanol steam reforming (ESR). The activity of Ni-PBMNO increases with temperature and is higher than that of PBMO. At 600 °C, Ni-PBMNO demonstrates an ethanol conversion above 90%, an H2–CO yield about 80%, and a negligible amount of CH4. Both Ni-PBMNO and PBMO remain unchanged structurally in dry and wet N2. The former adsorbs more H2O than the latter by forming hydroxyl groups in the lattice because of its higher concentration of oxygen vacancy caused by Ni exsolution, rendering a coking resistance one to two orders higher than conventional catalysts. In the ESR atmosphere, the PBMNO substrate decomposes increasingly without lowering ESR activity. The co-existence of CO2 and H2O is necessary for the decomposition that formed BaCO3 and MnO.

Electrochemical sensors based on proton-conducting electrolytes for determination of concentration and diffusion coefficient of CO2 in inert gases

The monitoring of CO2 levels in gases has important environmental, technological and energy applications. In the present work, results of the fabrication and testing of an amperometric-type sensor based on a proton-conducting material for analysing the concentration of carbon dioxide (CO2) in inert gases at elevated temperatures are presented. The analysis consists in reading the limiting current values determined during preliminary tests, obtaining the calibration dependence on variation of CO2 concentration and further using it for the investigation of gas mixtures. Experimental results show that the sensor is able to precisely detect CO2 concentration ranging between ~2 and 14 vol% in a wet N2 atmosphere at an intermediate-temperature range (500–600 °C) with a response time of ~30 min. Moreover, the possibility of estimating diffusion coefficients for CO2–N2 mixtures is also demonstrated. These results open a new window for high-temperature electrochemical analysis of gas mixtures, when solid-state gas sensors and proton-conducting electrolyte membranes are jointly utilised.

Unravelling Crystal Superstructures and Transformations in the La6–xMoO12−δ (0.6 ≤ x ≤ 3.0) Series: A System with Tailored Ionic/Electronic Conductivity

Crystalline La6-xMoO12-δ materials with different lanthanum/molybdenum ratios (0.6 ≤ x ≤ 3.0) have been prepared via a freeze-drying precursor route. The influence of the lanthanum content, sintering temperature and cooling rate on the phase existence range and polymorphism was evaluated. Lanthanum rich compounds present three different polymorphs: a cubic (disordered) fluorite and two complex rhombohedral superstructures related to the fluorite. For the first time, the structural resolution of these rhombohedral superstructures, 7×7×1 and 5×5×1, has been successfully accomplished by neutron pow-der diffraction and transmission electron microscopy studies. As the La/Mo ratio decreases, the cubic symmetry is stabilized, although a phase transformation from cubic to monoclinic occurs at a low cooling rate. Impedance spectroscopy measure-ments under different atmospheres (dry and wet N2 and 5% H2-Ar) show that all materials exhibit mixed proton-electronic conductivity. The n-type electronic conductivity is attributed to Mo6+ reduction and increases for those phases with lower lanthanum content, i.e. for quenched samples, from 5 mS cm-1 for La5.4MoO11.1 to 9.5 mS cm-1 for La4MoO9 at 700 oC in very reducing and wet conditions, which are significantly better than the values published to date for mixed lanthanum tungstates/molybdates. This make these materials potential candidates for hydrogen separation membranes.

Effect of electric current on the microstructural evolution and tribological behavior of highly oriented pyrolytic graphite

The sliding electrical contact behavior on the perpendicular surface of highly oriented pyrolytic graphite has been systematically studied under various electric current densities and environmental atmospheres. The results indicated that the friction coefficient decreased from 0.30 to 0.23 in wet air, and it increased from 0.32 to 0.39 in dry N2 with increasing electric current density from 0 to 50 A/cm2. The opposite tribological behavior induced by electric current between wet air and dry N2 was attributed to the changes in the microstructure and carbon dangling bonds. Electric current caused the nanostructure defects and carbon dangling bonds to increase in both wet air and dry N2. In wet air, these carbon dangling bonds were passivated by –H and –OH, which reduced the sliding force and caused the debris parallel to the sliding direction, thereby decreasing the friction coefficient. In dry N2, however, the carbon dangling bonds could not be passivated for the lack of water molecules. As a result, electric current oriented the debris perpendicular to the sliding direction, and the friction coefficient increased. In addition, it provided a novel strategy for tuning the tribological behavior of carbon materials in situ by adjusting the environmental atmospheres and electric current density.

Synergic Effect of Metal and Fluorine Doping on the Structural and Electrical Properties of La5.4MoO11.1-Based Materials.

Cationic and anionic frameworks of La5.4MoO11.1 proton conductors have been modified by means of metal (Ti4+, Zr4+, and Nb5+) and fluorine (F-) doping. This synergic effect leads to the stabilization of high-symmetry and single-phase polymorphs. The materials have been fully characterized by structural techniques, such as X-ray and neutron powder diffraction and transmission electron microscopy. The fluorine content was determined by ion chromatography. Impedance spectroscopy analysis under different atmospheres (dry and wet N2 and O2 and wet 5% H2-Ar) showed an improvement in the electronic conductivity under reducing conditions, making these materials potential candidates for hydrogen separation membranes.

Carbon‐Bridged g‐C3N4 Nanosheets for High Hydrogen Evolution Rate by a Two‐Step Gaseous Treatment

AbstractCarbon nitride (g‐C3N4) possesses a bright prospect in transforming solar power into chemical energy by photocatalytic hydrogen production. However, low specific surface area and rapid charges recombination of bulk g‐C3N4 obtained by direct thermal polymerization restrict its photocatalytic water splitting efficiency. Herein, carbon‐bridged g‐C3N4 hybrid nanosheets were synthesized by a two‐step gaseous treatment composed of gaseous stripping in wet N2 ambience and carbon deposition in ethanol ambience. The obtained hybrids possess high optical absorption and low recombination rate at the same time. Furthermore, the deposited carbon acts as the “bridge” between g‐C3N4 and co‐catalyst Pt, which facilitates the electron transfer. With 3 wt% Pt as co‐catalyst, the optimized C/g‐C3N4 (5) hybrid nanosheets achieve the highest hydrogen evolution rate of 8536 μmol⋅h−1⋅g−1, which is 11 times that of bulk g‐C3N4.

Structural and proton conductivity study of BaZr1-xRExO3-δ(RE = Dy, Sm) ceramics for intermediate temperature solid oxide fuel cell electrolyte

Proton-conducting BaZr1-xRExO3-δ (RE = Dy, Sm) (x = 0.05, 0.10, 0.15, 0.20) ceramics were synthesized via a conventional mixed oxide reaction route. The X-ray diffraction patterns indicated that the studied compositions crystallized as a single phase in the cubic space group $$ Pm\overline{3}m $$. The site preference of the rare earth dopant has been proved by Rietveld analysis of the XRD profile, and the site occupancies have been derived for the studied compositions. Thermogravimetric study of the pre-hydrated samples showed a substantial mass loss, proving the oxygen vacancy filling by H2O after hydration of the samples. Dense microstructures of sintered ceramics are observed, with the Dy-doped compositions showing fairly larger grains (2–3 μm) as compared to Sm-doped barium zirconate. The electrical conductivity under wet N2 ($$ {P}_{{\mathrm{H}}_2\mathrm{O}} $$ = 0.031 atm) environment has been calculated using electrochemical impedance spectroscopy. The conductivity improved gradually with the increase in doping proportions, in case of both the dopants. However, the conductivity of Dy-doped BaZrO3 is found one order higher as compared to Sm-doped BaZrO3, for any fixed doping concentration. The total conductivity of 20% Dy-doped barium zirconate is found to be 4.15 × 10−3 S/cm at 600 °C which is the highest among the studied compositions. The high proton conductivity suggests the material suitable for solid oxide fuel cell electrolyte at intermediate temperatures.

Conduction and relaxation phenomena in barium zirconate ceramic in wet N2 environment

Proton conducting barium zirconate perovskite was prepared by conventional mixed oxide reaction route. The x-ray diffraction patterns proved the single phase cubic perovskite structure of the synthesized ceramic. SEM analysis of the ceramic pellets sintered at 1600 °C reveals the formation of dense microstructure with average grain size of 5–6 μm. Complex impedance spectroscopy study was performed over a wide range of frequency (10 Hz-5MHz) under wet N2 (pH2O = 0.031 atm) and dry N2 environment respectively varying the temperature from 300 °C to 600 °C. Two types of relaxation phenomena were evident from the impedance spectroscopic analysis, indicating the effect of grain and grain boundary to the overall resistance of the compositions. The conductivity due to grain and grain boundary contributions measured for the sample under wet N2 environment was found fairly one order higher in magnitude than that measured in dry N2 atmosphere respectively. The activation energy for conduction under wet N2 condition is found lower than that measured in dry conditions. The comparison of conductivity under two different environments reveals that additional charge species take part in conduction process under wet condition. These are attributed to the protonic defects which are incorporated into barium zirconate through extrinsic oxygen vacancies generated due to sintering at higher temperature.

Novel Perovskite Materials for Thermal Water Splitting at Moderate Temperature.

Materials with the formula Sr2 CoNb1-x Tix O6-δ (x=1.00, 0.70; δ=number of oxygen vacancies) present a cubic perovskite-like structure. They are easily and reversibly reduced in N2 or Ar and re-oxidized in air upon heating. Oxidation by water (wet N2 ), involving splitting of water at a temperature as low as 700 °C, produces hydrogen. Both compounds displayed outstanding H2 production in the first thermochemical cycle, the Sr2 CoNb0.30 Ti0.70 O6-δ material retaining its outstanding performance upon cycling, whereas the hydrogen yield of the x=1 oxide showed a continuous decay. The retention of the materials' ability to promote water splitting correlated with their structural, chemical, and redox reversibility upon cycling. On reduction/oxidation, Co ions reversibly changed their oxidation state to compensate the release/recovery of oxygen in both compounds. However, in Sr2 CoTiO6-δ , two phases with different oxygen contents segregated, whereas in Sr2 CoNb0.30 Ti0.70 O6-δ this effect was not evident. Therefore, this latter material displayed a hydrogen production as high as 410 μmol g-1 perovskite after eight thermochemical cycles at 700 °C, which is among the highest ever reported, making this perovskite a promising candidate for thermosolar water splitting in real devices.