H2 H2O Research Articles

Thickness-induced band-gap engineering in lead-free double perovskite Cs2AgBiBr6 for highly efficient photocatalysis.

In recent years, two-dimensional (2D) lead-free double perovskites have been attracting much attention because of their unique performance in photovoltaic solar cells and photocatalysis. Nonetheless, how thickness affects the photoelectric properties of lead-free double perovskite remains unclear. In this work, by means of density functional theory (DFT) with a spin orbit coupling (SOC) effect, we have investigated the electronic and optical properties systemically, including band structures, carrier mobility, optical absorption spectra, exciton-binding energies, band edges alignment and molecule adsorption performance of Cs2AgBiBr6 with different thicknesses. The calculated results revealed the thickness-induced band gap and optical performance for Cs2AgBiBr6. It shows a low band gap and outstanding optical absorption of visible and ultraviolet light. When the thickness is reduced to a monolayer, Cs2AgBiBr6 moves from an indirect band gap to a direct band gap. Moreover, the carrier mobility of Cs2AgBiBr6 is excellent and the exciton-binding energy increases with the decreased thickness. Importantly, an analysis of molecule adsorption and band edge alignment indicates that Cs2AgBiBr6 is prone to H2O adsorption and H2 desorption theoretically, which is conducive to the photocatalytic water splitting for hydrogen generation and other photovatalytic reactions. Our work suggests that Cs2AgBiBr6 is a potential candidate as a solar cell or a photocatalyst, and we provide theoretical explorations into reducing the layers of lead-free double perovskite materials to 2D atomic thickness for a better photocatalytic application, which can serve as guidelines for the design of excellent photocatalysts.

In operando x-ray photoelectron spectroscopy studies of H2 oxidation and H2O electrolysis on gadolinia-doped ceria electrodes

In operando, ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) has been used to evaluate surface states of gadolinia-doped ceria (GDC) thin-film electrodes during H2 oxidation and H2O electrolysis, on yttria-stabilized zirconia (YSZ)-supported solid oxide electrochemical cells (SOCs). Porous nickel (Ni) and gold (Au) overlayers deposited on separate GDC thin films served as current collectors and potential electrocatalysts to facilitate heterogeneous chemistry for H2 oxidation or H2O electrolysis. Electrochemical characterization of the GDC thin-film electrodes complemented in operando XPS measurements of the O 1s spectra to correlate electrochemical overpotentials with surface chemistry near the Ni/GDC and Au/GDC interfaces. Shifts in O 1s binding energies across the metal/GDC/YSZ interfaces signified changes of local surface potential and provided a means of estimating kinetic parameters associated with charge transfer reactions. Effective oxygen partial pressure and surface potential impacted oxide vacancy and ceria polaron concentrations in the GDC, resulting in different reactivities of the GDC under the tested conditions. Both the Ni/GDC and Au/GDC demonstrated much higher currents for H2O electrolysis vs. H2 oxidation for comparable metal/GDC overpotentials due to increased electronic conductivity of the GDC under positive potentials and associated spreading of the electrochemically active region away from the triple-phase boundary. Higher electrochemical activity of the Ni/GDC electrode is attributed to the increased H2 activation on Ni in promoting charge transfer reactions (particularly for H2 oxidation). These results provide a basis for developing more informed reaction mechanisms for both H2 oxidation and H2O electrolysis of GDC-based composite electrodes in SOCs.

Coke Gasification in Blast Furnace Shaft Conditions with H2 and H2O Containing Atmospheres

Most of world steel in 2018 has been produced from virgin iron ore via the blast furnace (BF)—basic oxygen furnace route. Therewith the BF is one of the most important unit processes in worldwide steel making. Among others, energy efficiency of the BF is dependent on coke reactivity. Coke gasification occurs via solution‐loss reactions either with CO2 or H2O. Herein, the gasification of coke is studied in CO–CO2–H2–H2O–N2 atmosphere simulating the BF conditions. The simulated atmospheres are based on the measurements from an actual BF. This research studies the effect of the composition of the BF atmosphere representing conditions near the wall as well as at the center and influence of the H2O content on coke gasification under simulated BF conditions. It is found that the location plays a role in coke gasification: wall atmosphere yields higher coke gasification compared with center. Dynamic tests show that introduction of H2 and H2O in gas atmosphere fastens coke gasification by +119% at temperature range between 800 and 1200 °C in wall conditions. When H2O is present in gas atmosphere, the mass loss of coke is also greater in both wall and center conditions.

Injection of H2-rich carbonaceous materials into the blast furnace: devolatilization, gasification and combustion characteristics and effect of increased H2–H2O on iron ore pellets reducibility

Increasing the share of hydrogen in reduction of iron oxide in the blast furnace iron making will directly reduce the share of blast furnace greenhouse gas emissions. In the present study, injection of H2-rich biomass and plastic materials was studied in terms of its devolatilization, gasification and combustion characteristics. The released gases were identified using mass spectroscopy attached to a thermogravimetric analyzer and the corresponding kinetics parameters were estimated.The devolatilization was found to occur through two or more steps. The first step is always associated with the release of CO2, CO, H2, H2O and hydrocarbons while only CO and H2 were detected during the later steps. Combustion and gasification starting temperatures of char of H2-rich carbonaceous materials were lower than that of pulverized coal char by ≥ 100 °C. The estimated activation energies suggested that, under the present conditions, devolatilization, gasification and combustion were chemically controlled. Carbon reactivity of the char of the studied H2-rich carbonaceous materials were higher than that of pulverized coal. Moreover, increased H2–H2O content in the blast furnace gas, due to injected H2-rich carbonaceous materials, was found to improve the iron ore pellets reduction kinetics.

Understanding electrochemical switchability of perovskite-type exsolution catalysts

Exsolution of metal nanoparticles from perovskite-type oxides is a very promising approach to obtain catalysts with superior properties. One particularly interesting property of exsolution catalysts is the possibility of electrochemical switching between different activity states. In this work, synchrotron-based in-situ X-ray diffraction experiments on electrochemically polarized La0.6Sr0.4FeO3-δ thin film electrodes are performed, in order to simultaneously obtain insights into the phase composition and the catalytic activity of the electrode surface. This shows that reversible electrochemical switching between a high and low activity state is accompanied by a phase change of exsolved particles between metallic α-­Fe and Fe-oxides. Reintegration of iron into the perovskite lattice is thus not required for obtaining a switchable catalyst, making this process especially interesting for intermediate temperature applications. These measurements also reveal how metallic particles on La0.6Sr0.4FeO3-δ electrodes affect the H2 oxidation and H2O splitting mechanism and why the particle size plays a minor role.

Measurements and predictive models of high-pressure H2 solubility in brine (H2O+NaCl) for underground hydrogen storage application

In the context of Underground Hydrogen Storage (UHS), the stored gas is in direct contact with brine (residual brine from the cavern or formation water of deep aquifers). Therefore, knowledge of the phase equilibria (solubility of hydrogen in brine and water content in the hydrogen-rich phase) in the geological reservoir is necessary for the study of hydrogen mobility and reactivity, as well as the control, monitoring and optimization of the storage. The absence of measured data of high-pressure H2 solubility in brine has recently led scientists to develop predictive models or to generate pseudo-data using molecular simulation. However, experimental measurements are needed for model evaluation and validation. In this work, an experimental apparatus based on the “static-analytic” method developed and used in our previous work for the measurement of gas solubility in brine was used. New solubility data of H2 in H2O+NaCl were measured more or less under the geological conditions of the storage, at temperatures between 323 and 373 K, NaCl molalities between 0 and 5m, and pressures up to 230 bar. These data were used to parameterize and evaluate three models (Geochemical, SW, and e-PR-CPA models) tested in this work. Solubility and water content tables were generated by the e-PR-CPA model, as well as a simple formulation (Setschenow-type relationship) for quick and accurate calculations (in the fitting range) of H2 solubility in water and brine was proposed. Finally, the developed models estimate very well the water content in hydrogen-rich phase and capture and calculate precisely the salting-out effect on H2 solubility.

Experimental investigation of a non-catalytic cold plasma water-gas shift reaction

This work explores the use of an atmospheric pressure, low temperature, cold non-thermal plasma (obtained by dielectric barrier discharge (DBD)) to achieve a water-gas shift (WGS) reaction (CO + H2O H2 + CO2). This work establishes the use of a DBD to generate hydroxyl radicals that initiate and enhance the WGS reaction at low temperatures. The effect of the steam to CO molar ratio (MR) and the gas residence time on the CO conversion (XCO) to H2 is studied. The results show that, at an MR of 20, with 2600 ms of gas residence time and a plasma power of 70 W, a maximum CO conversion of 63 ± 4% can be achieved with an H2 concentration of 48 ± 2 mol% in the product. Preliminary studies of reaction pathways for the enhanced hydrogen formation confirm the role of C formed from the CO2 dissociation. A reaction mechanism for the plasma WGS reaction is proposed and the hydrogen yield is calculated.

Disclosure of the internal transport phenomena in an air-cooled proton exchange membrane fuel cell—Part I: Model development and base case study

In this study, the internal transport phenomena and mechanism inside an air-cooled proton exchange membrane fuel cell (PEMFC) are investigated. It helps to understand the factors that affect the performance of an air-cooled PEMFC and optimize the design of Membrane Electrode Assembly (MEA) and the flow field. This series article contains two parts. In this paper, i.e., Part Ⅰ of this series, a three-dimensional, two-phase flow, non-isothermal, steady-state Computational Fluid Dynamics (CFD) model is established to investigate the liquid water generation mechanism and the species distributions inside an air-cooled PEMFC single cell with a Base Case flow field design. Dry hydrogen and ambient air (the relative humidity and the stoichiometry are 60% and 150 separately) are considered for the simulation and validation. It is found that the liquid water appears mostly inside the cathode electrode underneath the cathode rib. Inside the anode gas diffusion layer (GDL), the mass fraction of H2 underneath the cathode ribs is lower than that underneath the cathode channels, while the mass fraction of H2O shows the opposite. The distributions of O2 mass fraction and H2O mass fraction inside the cathode GDL have the same trend as those of H2 mass fraction and H2O mass fraction inside the anode GDL. The membrane water content is periodically distributed from channel to channel and its value underneath the cathode rib is much larger than that underneath the cathode channel. The current density distribution is affected by the distribution of water content, i.e., the part underneath the cathode rib shows a larger current density than that underneath the cathode channel.

Ti-Magnetite Crystallization in Melt Inclusions of Trachytic Rocks from the Dokdo and Ulleung Islands, South Korea: Implications for Hydrous and Oxidized Magmatism

The Dokdo and Ulleung islands (Korea) are volcanic islands in the East Sea (Sea of Japan), formed in the late Cenozoic. These volcanic islands, in the back-arc basin of the Japanese archipelago, provide important information about magma characteristics in the eastern margin of the Eurasian plate. The origin of the Dokdo and Ulleung intraplate volcanism is still controversial, and the role of fluids, especially water, in the magmatism is poorly understood. Here, we comprehensively analyzed the melt inclusions (10–100 m in diameter) hosted in clinopyroxene phenocrysts of trachyte, trachyandesite, and trachybasalt. In particular, we observed Ti-magnetite and amphibole which were crystallized as daughter mineral phases within melt inclusions, suggesting that Ti-magnetite was formed in an oxidized condition due to H2O dissociation and H2 diffusion. The Ti-magnetite exhibited compositional heterogeneities of MgO (average of 8.28 wt %), Al2O3 (average of 8.68 wt %), and TiO2 (average of 8.04 wt %). The positive correlation of TiO2 with Cr2O3 is probably attributed to evolutionary Fe–Ti-rich parent magma. Correspondingly, our results suggested hydrous and oxidized magmatism for the Dokdo and Ulleung volcanic islands.

Highly stable La0.5Sr0.5Fe0.9Mo0.1O3-δ electrode for reversible symmetric solid oxide cells

Reversible solid oxide cells (RSOCs) using identical material as both fuel electrode and air electrode have received extensive attentions due to their simplified fabrication process, increased compatibility between electrolyte and electrode. In this work, Molybdenum doped La0.5Sr0.5Fe0.9Mo0.1O3-δ (LSFMo) symmetric electrode based on RSOCs is firstly designed and synthesized via sol-gel method. The effect of Molybdenum substitution at Fe-site on crystal structure, chemical stability, conductivity and electrochemical performance of La0.5Sr0.5FeO3-δ oxide is thoroughly investigated. The structural stability of La0.5Sr0.5FeO3-δ (LSF) in reducing condition is significantly enhanced after the incorporation of Mo5+/6+ and the conductivity in 5% H2 for LSFMo is ~7 times higher than that of undoped LSF. In addition, the polarization resistance value at 850 °C based on LSFMo/LSGM/LSFMo is 0.08 and 0.09 Ω cm2 in air and wet H2, respectively. At 850 °C and 20%H2O–H2, a peak power density of 640 mW cm−2 is obtained in fuel cell mode, while a current density of −1000 mA cm−2 is attained at 1.3 V in electrolysis mode. Finally, the symmetric cell exhibits an excellent cycling reversible operation in both SOFCs mode and SOECs mode without detectable degradation.

Performance and mechanism study of LaFeO3 for biomass chemical looping gasification

Perovskite oxide is widely used in many important technological fields due to the excellent characteristics. In this work, the performances of the LaFeO3 as oxygen carrier in biomass chemical looping gasification were investigated. It is found that there is no significant decrease in CO and H2 yield with the increase in oxygen carrier cycle number. No obvious change of syngas yield with the increase in LaFeO3 mass is observed, indicating the stability of LaFeO3. The volume fraction of water vapor should be controlled as 39.9%. The DFT calculation results show that CO desorption is identified to be the rate-limiting step with an activation barrier of 0.648 eV for CO molecule formation. For CO oxidation, formation of COO* complex is the rate-limiting step with energy barrier of 0.368 eV. During the process of H2 formation, the pathway of H2 production over surface Fe site is more favorable with the calculated activation energy of 0.428 eV. Formations of H2O and H2 are competing reactions during chemical looping process. However, the relatively low activation energy barriers of H2O dissociation (0.141 eV or 0.081 eV) and H2 formation (0.428 eV) suggest that H2 formation is more competitive. For comparison, performances of Fe2O3 as oxygen carrier were also investigated. The calculation results show a relatively high energy barrier (1.314 eV) of CO formation and relatively low energy barrier (0.434 eV) of CO oxidation.

Hydrocarbon transformations on Co–zeolite in catalytic environment of different redox properties at 170–260 °C

Redox properties of catalytic environment, namely He as neutral, H2 as reducing and H2–H2O as reducing–oxidative one, determine the conversion of hydrocarbons on Co–zeolite catalysts at 170–260 °C as well as product distribution. The highest yield of isomerized products was achieved in reducing medium, while the methane formation was suppressed most efficiently in neutral and reducing–oxidative environments. Two distinct ranges can be identified for temperature dependencies of reaction parameters, namely 170–210 and 220–260 °C.

Photothermal Catalytic Gel Featuring Spectral and Thermal Management for Parallel Freshwater and Hydrogen Production

AbstractDesalination processes often require large amounts of energy to create clean water, and vice versa for the generation of energy. This interdependence creates a tension between the two essential resources. Current research focuses on one or the other, which exacerbates water‐energy stress, while few tackle both issues jointly. Herein, a low‐carbon technology, H2O–H2 co‐generation system that enables concurrent steady freshwater and clean energy output is reported. The water‐energy coupled technology features a spectrally and thermally managed solar harvesting gel for photoredox and photoheating effects. This photothermal catalytic gel exploits interfacial solar heating for heat confinement, and localized plasmonic heating at the catalyst active sites to remarkably improve water and hydrogen production, thus maximizing energy value per area. To this end, a stand‐alone renewable solar desalination system is successfully demonstrated for parallel production of freshwater and hydrogen under natural sunlight. By doing so, the water–energy nexus is transformed into a synergistic bond that offers opportunities to better meet expected demand rather than acting in competition.

DFT study of the effect of platinum on the H2 gas sensing performance of ZnO nanotube: Explaining the experimental observations

An experimental report displays that the Pt-decoration meaningfully increases the ZnO nanostructure sensitivity toward H2 gas. Here, DFT calculations are employed to explain the origin of Pt effect on the sensitivity of a ZnO nanotube (ZnONT) to H2. An H2 molecule weakly interacts with the ZnONT with adsorption energy of −2.1 kcal/mol) and cannot be detected. The Pt atom locates above the center of a hexagonal ring of ZnONT, releasing an energy of −39.7 kcal/mol. We showed that there is a relation between the experimental change of the sensor sensitivity and the change of the HOMO-LUMO gap of the ZnONT. Based on our results, the Pt metal first catalyzes the dissociation of O2 gas of air on the ZnONT which causes a charge transfer from the tube to the O atoms, forming O– anions and decreasing the gap. Then the H2 reacts with the pre-chemisorbed O ions and generates H2O molecules, releasing some of the trapped electrons back into the Pt-ZnONT and increasing the gap. This process needs to overcome an energy barrier of 18.9 kcal/mol, which can be accomplished at room temperature. After the H2 adsorption and H2O production, the gap of Pt-ZnONT somewhat back toward its initial value because of an electron back-donation. The sensing mechanism is based on the gap change, which is related to the change of electrical conductivity. We predicted that by Pt-decoration, the sensor response increases from 1.12 to 115.24 in agreement with experiment. The calculated recovery time is 13.5 ms for ZnONT sensor.

The Behavior of H2 in Aqueous Fluids under High Temperature and Pressure

The presence of H2 and H2O in planetary interiors prompts the need for fundamental studies on these compounds under corresponding conditions. Here, we summarize data on H2 properties in aqueous systems under conditions of high temperature and pressure. We explain how to measure important H2 fugacities in hydrothermal systems. We present available experimental data and thermodynamic models for H2 solubility and vapor–liquid partitioning under hydrothermal conditions. In addition, we introduce the fascinating world of H2–H2O clathrate hydrates under extreme temperatures and pressures. The properties of the H2–H2O system are well established below the critical point of water (374 °C and 22.06 MPa), but far less is known under higher temperatures and pressures, or the effect of salt.

Thermodynamic models for H2O–CO2–H2 mixtures in near-critical and supercritical regions of water

The PVTx properties of the H2O–CO2–H2 mixtures have significant applications in the technology of supercritical water gasification of coal. Here, we first carry out the molecular dynamics simulations of the PVTx properties of the H2O–CO2–H2 mixtures in the near-critical and supercritical regions of water to generate 600 datasets at 750–1150 K and 4.0–443.5 MPa. The molar fraction of each composition in the ternary mixtures ranges from 10% to 80%. Later we investigate the applicability of a well-known thermodynamic model for the ternary mixtures, namely the Duan-Møller-Weare equation of state (DMW EOS). It is observed that the DMW EOS shows great potential in the prediction of the PVTx properties of the ternary mixtures. However, it is noted that the mixing parameters describing the binary interactions of H2O–H2 and CO2–H2 are still unknown in the DMW EOS. By determining the missing mixing parameters using the Levenberg-Marquardt algorithm, the accuracy of the original DMW EOS is improved for the ternary mixtures. Moreover, optimizing the coefficients in the DMW EOS further promotes the accuracy of the model for the H2O–CO2–H2 mixtures. The results from this work may facilitate the development of supercritical water gasification of coal.

State-to-state inelastic rotational cross sections in five-atom systems with the multiconfiguration time dependent Hartree method.

We present a MultiConfiguration Time Dependent Hartree (MCTDH) method as an attractive alternative approach to the usual quantum close-coupling method that approaches some computational limits in the calculation of rotational excitation (and de-excitation) between polyatomic molecules (here collisions between triatomic and diatomic rigid molecules). We have performed a computational investigation of the rotational (de-)excitation of the benchmark rigid rotor H2O-H2 system on a recently developed Potential Energy Surface of the complex using the MCTDH method. We focus here on excitations and de-excitations from the 000, 111, and 110 states of H2O with H2 in its ground rotational state, looking at all the potential transitions in the energy range 1-200 cm-1. This work follows a recently completed study on the H2O-H2 cluster where we characterized its spectroscopy and more generally serves a broader goal to describe inelastic collision processes of high dimensional systems using the MCTDH method. We find that the cross sections obtained from the MCTDH calculations are in excellent agreement with time independent calculations from previous studies but does become challenging for the lower kinetic energy range of the de-excitation process: that is, below approximately 20 cm-1 of collision energy, calculations with a relative modest basis become unreliable. The MCTDH method therefore appears to be a useful complement to standard approaches to study inelastic collision for various collision partners, even at low energy, though performing better for rotational excitation than for de-excitation.

Intramolecular stretching vibrational states and frequency shifts of (H2)2 confined inside the large cage of clathrate hydrate from an eight-dimensional quantum treatment using small basis sets.

We report the results of calculations pertaining to the HH intramolecular stretching fundamentals of (p-H2)2 encapsulated in the large cage of structure II clathrate hydrate. The eight-dimensional (8D) quantum treatment assumes rotationless (j = 0) H2 moieties and a rigid clathrate structure but is otherwise fully coupled. The (H2)2-clathrate interaction is constructed in a pairwise-additive fashion, by combining the ab initio H2-H2O pair potential for flexible H2 and rigid H2O [D. Lauvergnat et al., J. Chem. Phys. 150, 154303 (2019)] and the six-dimensional (6D) H2-H2 potential energy surface [R. J. Hinde, J. Chem. Phys. 128, 154308 (2008)]. The calculations are performed by first solving for the eigenstates of a reduced-dimension 6D "intermolecular" Hamiltonian extracted from the full 8D Hamiltonian by taking the H2 moieties to be rigid. An 8D contracted product basis for the solution of the full problem is then constructed from a small number of the lowest-energy 6D intermolecular eigenstates and two discrete variable representations covering the H2-monomer internuclear distances. Converged results are obtained already by including just the two lowest intermolecular eigenstates in the final 8D basis of dimension 128. The two HH vibrational stretching fundamentals are computed for three hydrate domains having an increasing number of H2O molecules. For the largest domain, the two fundamentals are found to be site-split by ∼0.5 cm-1 and to be redshifted by about 24 cm-1 from the free-H2 monomer stretch frequency, in excellent agreement with the experimental value of 26 cm-1. A first-order perturbation theory treatment gives results that are nearly identical to those of the 8D quantum calculations.

Effective yttrium based coating for steel interconnects of solid oxide cells: Corrosion evaluation in steam-hydrogen atmosphere

This work describes manufacture, analysis and test of a new well conducting corrosion-protection coating that can be applied on steel types with high chromium content. Electrolytic deposition of yttrium salts is used to form thin (<100 nm) coatings on both flat steel sheets (material: Crofer 22 APU) and its properties are proven on woven wire-meshes (materials from two different sources: SUS316 and SUS316L). The oxide scale on the coated Crofer 22 APU sheet remains intact after 2000 h operation at 750 °C in H2 with 90% H2O. The corrosion rate at 750 °C of the coated Crofer 22 APU sheets is compared with that of uncoated samples and of samples with a commercial magneton sputtered CGO coating revealing that the coatings reduce the parabolic rate constant characteristic of the corrosion by a factor of 10 and 20 for the CGO and the Y, respectively.The coated meshes also exhibit high corrosion resistance and long-term-stable low electrical resistance at 650 °C, both in 90% H2O in H2, and in air. In addition to the experimental work, Y–Cr–O2 phase diagrams are constructed illustrating the high stability of the YCrO3 phase (that forms during corrosion) over a wide temperature and pO2 range.

Study on solid state steelmaking from thin cast iron sheets through decarburization in H2O–H2

ABSTRACT To study the decarburization behavior of cast iron sheets, a series of gas-solid experiments were carried out. The treatment was carried out under Ar-H2-H2O mixed gas atmosphere. Results show that P H2O/P H2 in the gas mixture determines the oxidation of iron at a given temperature and the critical value of P H2O/P H2 to produce ferric oxide rises with temperature. The oxidation of iron take place when the ratio of P H2O/P H2 > 0.62, 0.66, 0.7 at 1293 K, 1353 K, 1413 K, respectively. The gas-solid decarburization reaction of cast iron sheets follows first-order kinetics. Cross-sectional microstructure images indicate that graphitization of cast iron sheets occurs at 1353 and 1413 K. The depth of the decarburized layer increases gradually with temperature and time, the square of depth has a near linear relationship with time. The activation energies of the decarburization reaction calculated by two different ways are similar to that for internal carbon diffusion.