Solvothermal synthesis and photocatalytic performance of Mn4+-doped anatase nanoplates with exposed {001} facets

The photocatalytic activity of TiO2 and manganese doped TiO2 nanoplates with various manganese atomic percentages, in the range of 2-7%, was studied. The undoped and doped nanoplates with exposed {001} facets were produced by a solvothermal method. The crystal structure as well as the shape of the TiO2 and Mn4+/TiO2 anatase nanoparticles was determined with X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). Both techniques revealed that the nanocrystals are in the form of plates. Moreover, the anisotropic peak broadening of the X-ray diffraction patterns was studied using the Rietveld refining method. Chemical analysis of the photocatalyst that was carried out with X-ray photoelectron spectroscopy (XPS) showed the presence of manganese ions in the TiO2 anatase matrix. The Density Functional Theory (DFT) calculations exhibited a decrease in the energy gap and an increase in the density of the electronic stated inside the gap for the doped TiO2. These observations were in agreement with the results of the UV-visible diffuse reflectance spectroscopy (DRS) that demonstrated an adsorption shift towards the visible region for the same samples. The photocatalytic activity of the synthesized catalysts was investigated by the photocatalytic oxidation of the gaseous nitric oxide (NO) and decomposition of the gaseous acetaldehyde (CH3CHO) under visible light irradiation. The optimal concentration of dopant that improves the photocatalytic activity of the nanoplates was determined. © 2014 Elsevier B.V

Materials for hydrogen-based energy storage - past, recent progress and future outlook

Globally, the accelerating use of renewable energy sources, enabled by increased efficiencies and reduced costs, and driven by the need to mitigate the effects of climate change, has significantly increased research in the areas of renewable energy production, storage, distribution and end-use. Central to this discussion is the use of hydrogen, as a clean, efficient energy vector for energy storage. This review, by experts of Task 32, “Hydrogen-based Energy Storage” of the International Energy Agency, Hydrogen TCP, reports on the development over the last 6 years of hydrogen storage materials, methods and techniques, including electrochemical and thermal storage systems. An overview is given on the background to the various methods, the current state of development and the future prospects. The following areas are covered; porous materials, liquid hydrogen carriers, complex hydrides, intermetallic hydrides, electrochemical storage of energy, thermal energy storage, hydrogen energy systems and an outlook is presented for future prospects and research on hydrogen-based energy storage

Thermochemical energy storage performance of zinc destabilized calcium hydride at high-temperatures

© the Owner Societies. CaH2 has 20 times the energy density of molten salts and was patented in 2010 as a potential solar thermal energy storage material. Unfortunately, its high operating temperature (>1000 °C) and corrosivity at that temperature make it challenging to use as a thermal energy storage (TES) material in concentrating solar power (CSP) plants. To overcome these practical limitations, here we propose the thermodynamic destabilization of CaH2 with Zn metal. It is a unique approach that reduces the decomposition temperature of pure CaH2 (1100 °C at 1 bar of H2 pressure) to 597 °C at 1 bar of H2 pressure. Its new decomposition temperature is closer to the required target temperature range for TES materials used in proposed third-generation high-temperature CSP plants. A three-step dehydrogenation reaction between CaH2 and Zn (1 : 3 molar ratio) was identified from mass spectrometry, temperature-programmed desorption and in situ X-ray diffraction studies. Three reaction products, CaZn13, CaZn11 and CaZn5, were confirmed from in situ X-ray diffraction studies at 190 °C, 390 °C and 590 °C, respectively. The experimental enthalpy and entropy of the second hydrogen release reaction were determined by pressure composition isotherm measurements, conducted between 565 and 614 °C, as ΔHdes = 131 ± 4 kJ mol-1 H2 and ΔSdes = 151 ± 4 J K-1 mol-1 H2. Hydrogen cycling studies of CaZn11 at 580 °C showed sufficient cycling capacity with no significant sintering occurring during heating, as confirmed by scanning electron microscopy, demonstrating its great potential as a TES material for CSP applications. Finally, a cost comparison study of known destabilized CaH2 systems was carried out to assess the commercial potential

Hydroxylated closo-Dodecaborates M2B12(OH)12 (M = Li, Na, K, and Cs); Structural Analysis, Thermal Properties, and Solid-State Ionic Conductivity

Copyright © 2020 American Chemical Society. Closo-borates and derivatives thereof have shown great potential as electrolyte materials for all-solid-state batteries owing to their exceptional ionic conductivity and high thermal and chemical stability. However, because of the myriad of possible chemical modifications of the large, complex anion, only a fraction of closo-borate derivatives has so far been investigated as electrolyte materials. Here, the crystal structures, thermal properties, and ionic conductivities of M2B12(OH)12 (M = Li, Na, K, and Cs) are investigated with a focus on their possible utilization as new solid-state ion conductors for solid-state batteries. The compounds generally show rich thermal polymorphism, with eight identified polymorphs among the four dehydrated compounds. Both Li2B12(OH)12 and Na2B12(OH)12 undergo a first-order transition, in which the cation sublattices become disordered, resulting in an order of magnitude jump in ionic conductivity for Na2B12(OH)12. K2B12(OH)12 undergoes a second-order polymorphic transition driven by a change in the anion-cation interaction, with no evidence of dynamic disorder. The ionic conductivities of M2B12(OH)12 range from 1.60 × 10-8 to 5.97 × 10-5 S cm-1 at 250 °C for M = Cs and Li, respectively, showing decreasing conductivity with increasing cation size. Compared with the analogous M2B12H12 compounds, such relatively low conductivities are suggested to be a consequence of strong and directional anion-cation interactions, resulting in a more static anion framework

Enhancing oxygen reduction reaction activity of perovskite oxides cathode for solid oxide fuel cells using a novel anion doping strategy

Possessing a high oxygen reduction reaction (ORR) activity is one of the most important prerequisites for the cathode to ensure an efficient solid oxide fuel cell. Herein, a highly active cathode is developed by doping the fluorine anion in anion sites of perovskite oxides (ABO3). The electrocatalytic activities of three different cathode samples including the original perovskite La0.6Sr0.4Co0.2Fe0.8O3-d(LSCF), the doped La0.6Sr0.4Co0.2Fe0.8O2.95-dF0.05(LSCFF0.05) and La0.6Sr0.4Co0.2Fe0.8O2.9-dF0.1(LSCFF0.1) are comparatively investigated. The fluorine doped perovskites reveal higher electrochemical performance than the original perovskite. Based on three cathodes of LSCF, LSCFF0.05 and LSCFF0.1 operated at 850 °C, the measured area specific resistance was 0.018, 0.017 and 0.91 O cm2, respectively; and the respective maximum power density of the single fuel cell using the 9-µm-thick YSZ electrolyte was 754, 1005, and 737 mW cm-2. Such performance results vividly indicate that, the obtained perovskite oxyfluoride by doping an optimum amount of F ions can efficiently improve ORR activity and thus is a feasible strategy to develop cathode for high-performance solid oxide fuel cells

Exploring halide destabilised calcium hydride as a high-temperature thermal battery

© 2019 CaH2 is a metal hydride with a high energy density that decomposes around 1100 °C at 1 bar of H2 pressure. Due to this high decomposition temperature, it is difficult to utilise this material as a thermal battery for the next generation of concentrated solar power plants, where the currently targeted operational temperature is between 600 and 800 °C. In this study, CaH2 has been mixed with calcium halide salts (CaCl2, CaBr2 and CaI2) and annealed at 450 °C under 100 bar of H2 pressure to form CaHCl, CaHBr and CaHI. These hydride-halide salts incur a thermodynamic destabilisation of their hydrogen release, compared to CaH2, so that they can operate between 600 and 800 °C within practical operating pressures (1–10 bar H2) for thermochemical energy storage. The as-synthesised metal hydrides were studied by in-situ synchrotron X-ray diffraction, temperature programmed desorption and pseudo pressure composition isothermal analysis. Each of the calcium hydride-halide salts decomposed to form calcium metal and a calcium halide salt after hydrogen release. In comparison to pure CaH2, their decomposition reactions were faster when heated up to 850 °C, and the experimental values of the desorbed hydrogen gas were very close to the theoretical ones. All samples after their decomposition showed signs of sintering, which hindered their rehydrogenation reaction

Physicochemical Characterization of a Na-H-F Thermal Battery Material

Copyright © 2020 American Chemical Society. Fluorine-substituted sodium hydride is investigated for application as a thermal energy storage material inside thermal batteries. A range of compositions of NaHxF1-x (x = 0, 0.5, 0.7, 0.85, 0.95, 1) have been studied using synchrotron radiation powder X-ray diffraction (SR-XRD), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and nuclear magnetic resonance spectroscopy (NMR), with the thermal conductivity and melting points also being determined. SR-XRD and NMR spectroscopy studies identified that the solid solutions formed during synthesis contain multiple phases rather than a single stoichiometric compound, despite the materials exhibiting a single melting point. As the fluorine content of the materials increases, the Na-H(F) bond length decreases, increasing the stability of the compound. This trend is also observed during melting point analysis where increasing the fluorine content increases the melting point of the material; that is, x 0.7) enables melting at temperatures above 750 °C

Thermodynamics and performance of the Mg-H-F system for thermochemical energy storage applications

© 2018 the Owner Societies. Magnesium hydride (MgH 2 ) is a hydrogen storage material that operates at temperatures above 300 °C. Unfortunately, magnesium sintering occurs above 420 °C, inhibiting its application as a thermal energy storage material. In this study, the substitution of fluorine for hydrogen in MgH 2 to form a range of Mg(H x F 1-x ) 2 (x = 1, 0.95, 0.85, 0.70, 0.50, 0) composites has been utilised to thermodynamically stabilise the material, so it can be used as a thermochemical energy storage material that can replace molten salts in concentrating solar thermal plants. These materials have been studied by in situ synchrotron X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, temperature-programmed-desorption mass spectrometry and Pressure-Composition-Isothermal (PCI) analysis. Thermal analysis has determined that the thermal stability of Mg-H-F solid solutions increases proportionally with fluorine content, with Mg(H 0.85 F 0.15 ) 2 having a maximum rate of H 2 desorption at 434 °C, with a practical hydrogen capacity of 4.6 ± 0.2 wt% H 2 (theoretical 5.4 wt% H 2 ). An extremely stable Mg(H 0.43 F 0.57 ) 2 phase is formed upon the decomposition of each Mg-H-F composition of which the remaining H 2 is not released until above 505 °C. PCI measurements of Mg(H 0.85 F 0.15 ) 2 have determined the enthalpy (?H des ) to be 73.6 ± 0.2 kJ mol -1 H 2 and entropy (?S des ) to be 131.2 ± 0.2 J K -1 mol -1 H 2 , which is slightly lower than MgH 2 with ?H des of 74.06 kJ mol -1 H 2 and ?S des = 133.4 J K -1 mol -1 H 2 . Cycling studies of Mg(H 0.85 F 0.15 ) 2 over six absorption/desorption cycles between 425 and 480 °C show an increased usable cycling temperature of ~80 °C compared to bulk MgH 2 , increasing the thermal operating temperatures for technological applications

Fluorine Substitution in Magnesium Hydride as a Tool for Thermodynamic Control

© 2020 American Chemical Society. Metal hydrides continue to vie for attention as materials in multiple technological applications including hydrogen storage media, thermal energy storage (TES) materials, and hydrogen compressors. These applications depend on the temperature at which the materials desorb and reabsorb hydrogen. Magnesium hydride is ideal as a TES material, although its practical operating temperature is capped at ∼450 °C because of material degradation and high operating pressure. Fluorine substitution for hydrogen in magnesium hydride has previously been shown to increase the operating temperature of the metal hydride while limiting degradation, although full characterization is required before technological application can be ensured. The present study characterizes Mg(HxF1-x)2 solid solutions (x = 1, 0.95, 0.70, 0.85, 0.50, and 0) by inelastic neutron spectroscopy, powder X-ray diffraction, and thermal conductivity measurements, with the results being verified by density functional theory. For each experiment, a clear trend is observed throughout a series of solid solutions, showing the possibility of tuning the properties of MgH2. As F- substitution increases, the average Mg-H(F) bond distance elongates along the axial positions of the Mg-H(F) octahedra. Overall, this leads to an increase in Mg-H bond strength and thermal stability, improving the viability of Mg-H-F as potential TES materials