Pressure-composition Isotherms Research Articles

Exploring the Hydrogen Sorption Capabilities of a Novel Ti-V-Mn-Zr-Nb High-Entropy Alloy

Hydrogen is considered as a clean energy carrier able to achieve the decarbonization of the economy, but its compact, safe, and efficient storage represents an important challenge. Among many materials forming hydrides, this work reports the study of hydrogen sorption properties of a novel bcc high-entropy alloy, Ti0.30V0.25Mn0.10Zr0.10Nb0.25, synthesized by arc melting. In less than 60 s, the alloy fully absorbs hydrogen at room temperature, reaching a capacity of 2.0 H/M (2.98 wt.%). A two-step reaction with hydrogen is confirmed by pressure-composition isotherms, synchrotron X-ray and neutron diffraction: bcc solid solution ↔ bcc monohydride ↔ fcc dihydride. For the second step transformation, the calculated thermodynamic values indicate the formation of a very stable dihydride, with ΔHabs = −97 kJ/molH2. Moreover, the pair distribution function analysis of high-energy synchrotron X-ray scattering data validates a completely random distribution of metal atoms in the fcc dihydride phase, without noticeable lattice strain nor elemental segregation. In situ synchrotron X-ray and neutron diffraction, performed during hydrogen desorption by heating under vacuum, demonstrated full reversibility of the reaction with hydrogen. On the basis of these results, tuning the chemical composition of high-entropy alloys may have great implications in terms of hydrogen sorption properties.

Hydrogen Compression Materials with Output Hydrogen Pressure in a Wide Range of Pressures Using a Low-Potential Heat-Transfer Agent

In order to meet the demand of metal hydride–hydrogen compressors (MHHC) and their hydrogen compression materials for high-pressure hydrogen filling in a hydrogen energy field, four kinds of hydrogen storage alloys with low-grade heat source (<373 K) heating outputs and different hydrogen pressures (up to 80 MPa) were developed as hydrogen compression materials. The preliminary compositions of the hydrogen storage alloys were determined by using a statistical model and research experience. The rare earth series AB5 and Ti/Zr base AB2 hydrogen storage alloys were prepared using a high-temperature melting method. The composition, structure, and hydrogenation/dehydrogenation plateau characteristics of the alloys were tested by an inductively coupled plasma mass spectrometer (ICP-MAS), X-ray diffractometer (XRD), and pressure–composition isothermal (PCT) tester. The median output pressures of the four-stage hydrogen storage alloys at 363 K were 8.90 MPa, 25.04 MPa, 42.97 MPa, and 84.73 MPa, respectively, which met the requirements of the 20 MPa, 35 MPa, and 70 MPa high-pressure hydrogen injections for the MHHCs. In fact, due to the tilted pressure plateau of the PCT curve, the synergy between the adjacent two alloys still needed to be adjusted.

Phase formation behavior and hydrogen sorption characteristics of TiFe0.8Mn0.2 powders prepared by gas atomization

In this article we have investigated the phase formation behavior and hydrogen storage characteristics of TiFe0.8Mn0.2 samples produced by electrode induction melting gas atomization (EIGA) as well as conventional vacuum arc-melting (VAM). TiFe (46Ti–46Fe–8Mn (wt%) with cubic, CsCl-type structure) and C14 Laves (37Ti–51Fe–12Mn (wt%) with hexagonal, MgZn2-type structure) phases were detected in both atomized powders and crushed ingots, the latter possessing a lower fraction of Laves phase. Microstructural characterization included electron probe micro-analysis (EPMA) and transmission electron microscopy (TEM), which evidenced the existence of a non-equilibrium Ti2Fe phase in the atomized powders. The formation of this additional secondary phase was attributed to the rapid cooling rate during powder manufacturing, based on earlier findings and phase formation thermodynamic calculations combined with Scheil solidification simulations. In addition, kinetics and pressure-composition isotherms (PCIs) were acquired to determine both absorption and desorption enthalpy/entropy, as well as to study the influence of the synthesis route (and hence the resulting microstructure) on kinetics, thermodynamics and reaction mechanisms of the TiFe0.8Mn0.2 alloy.

Effect of Mn substitution on plateau pressure regulation of the Ti0.92Zr0.1Cr1.6−xMnxFe0.4 alloys for primary hydrogen compressor

The Ti0.92Zr0.1Cr1.6−xMnxFe0.4 (x = 0.15, 0.3, 0.45, 0.6 at%) alloys were synthesized by vacuum electric arc furnace. The effect of Mn substitution on plateau pressure regulation of these alloys was investigated for the primary hydrogen compressor. With increasing Mn content, the hydrogen absorption/desorption plateau pressures of the Ti0.92Zr0.1Cr1.6−xMnxFe0.4 (x = 0.15, 0.3,0.45,0.6 at%) alloys increased, the plateau slope factor decreased, meanwhile, both enthalpy and entropy change values decreased. The Ti0.92Zr0.1Cr1.0Mn0.6Fe0.4 alloy had the best comprehensive performance. Based on the pressure composition isotherms measurement, the plateau pressure and hydrogen storage capacity were obtained at 298–363 K. The hydrogen absorption/desorption curves of the Ti0.92Zr0.1Cr1.0Mn0.6Fe0.4 alloy at 353 and 363 K were firstly tested by experiment method. At 298 K, the hydrogen absorption plateau pressure and maximum capacity were 7.82 MPa and 1.79 wt%. At 363 K, the hydrogen desorption plateau pressure and maximum capacity were 27.21 MPa and 1.54 wt%. Hydrogen pressure from 8 MPa to 25 MPa was successfully obtained at 298–363 K, satisfying operating temperature and pressure requirements for the primary hydrogen compression materials.

Impact of yttrium hydride formation on multi-isotopic hydrogen retention by a getter trap for the DONES lithium loop

Compliance with imposed hydrogen concentration limits in the lithium loop of the DEMO-Oriented Neutron Source (DONES) requires the installation of an yttrium-based hydrogen trap. To determine an appropriate H-trap design, it is essential to have access to a numerical tool capable of simulating hydrogen transport in the DONES lithium loop connected to an yttrium pebble-bed. In the past, a simplified model was created that allows such calculations when hydrogen concentrations in the lithium are low. However, in certain DONES operating phases, the concentration in the lithium is high and in a range where yttrium dihydride (YH2) formation is likely. Due to the anticipated great impact of YH2 formation on the H-trap performance a new model is developed that includes the mechanism of hydride formation. It is based on a mathematical reproduction of complete pressure-composition isotherms of the Li–H and Y–H systems. Thus, the conditions that trigger YH2 formation are determined and the variation of hydrogen solubility in different yttrium hydride phases is deduced. An approximate concentration-dependent relationship of hydrogen diffusivity in yttrium is derived and incorporated into the model. Simulations are performed to analyze the dynamics of the concentration decrease during purification of the lithium circuit prior to the experimental DONES phase by varying design parameters of the trap. It is found that hydride formation greatly increases the hydrogen gettering capacity of the H-trap and limits the maximum concentration in the lithium. Indeed, YH2 formation may be purposefully triggered to exploit its beneficial properties for DONES. Simulations of the hydrogen purification process during the experimental phase of DONES show that the H-trap must be replaced at least every 28 days to meet tritium limits. This work sets the conditions for the required pebble-bed mass of the H-trap at a given temperature to comply with the DONES safety requirements. Finally, the model is validated by numerical reproduction of experimental results.

A novel method towards improving the hydrogen storage properties of hypoeutectic Mg-Ni alloy via ultrasonic treatment

Ultrasonic treatment has great contributions on modifying the morphology, dimension and distribution of constituent phases during solidification, which serve as dominate factors influencing the hydrogen storage performance of Mg-based alloys. In this research, ultrasonic treatment is utilized as a novel method to enhance the de-/hydriding properties of Mg-2Ni (at.%) alloy. Due to ultrasonic treatment, the microstructure of as-cast alloy is significantly refined and homogenized. Ascribing to the increased eutectic boundaries and shortened distance inside α-Mg for hydrogen atoms diffusion, the hydrogen uptake capacities and isothermal de-/hydriding rates improve effectively, especially at lower temperature. The peak desorption temperature reduces from 392.99 °C to 345.56 °C, and the dehydriding activation energy decreases from 101.93 kJ mol−1 to 88.65 kJ mol−1. Weakened hysteresis of plateau pressures and slightly optimized thermodynamics are determined from the pressure-composition isotherms. Owing to the refined primary Mg, a larger amount of hydrogen with the higher hydriding proportion is absorbed in the first stage when hydrides nucleate in eutectic region and grow on primary Mg periphery subsequently before MgH2 colonies impinging, resulting in the enhancement of hydrogenation rates and capacities.

Remarkable enhancement in durability of ZrCo alloy against hydrogen induced disproportionation by Ti and Nb co-substitution

Considering the thermodynamic stability of various hydrides, a strategy has been employed to improve the hydrogen isotope storage properties of ZrCo alloy which involves partial co-substitution of Zr with Ti and Nb. Herein, alloys of composition Zr0.8Ti0.2-xNbxCo (x = 0.05, 0.1, 0.15) is prepared, characterized and the effect of Ti and Nb doping on hydrogen storage properties of parent ZrCo alloy is investigated. XRD analysis confirmed the formation of desired pure cubic phase of all the synthesized alloys similar to ZrCo phase. The presence of a single plateau in hydrogen desorption pressure-composition isotherms confirms single step hydrogen absorption-desorption behavior in Zr0.8Ti0.2-xNbxCo alloys. The equilibrium pressure of hydrogen desorption decreases marginally with increasing Nb content in Zr0.8Ti0.2-xNbxCo alloys which is further corroborated by differential scanning calorimetry measurements. Investigation of hydrogen induced disproportionation behavior in ITER-simulating condition revealed substantial impact of co-substitution of Ti and Nb on anti-disproportionation properties of ZrCo alloy. These remarkable properties make the Ti and Nb co-substituted quaternary alloys a desirable material for hydrogen isotope storage and delivery application.

First-principles study on the crystal structure stability and hydrogen storage mechanism of hydrogen occupancy in interstitial sites in ZrCo compounds

To study the properties of Zr 8 Co 8 H in the α-phase process of hydrogen storage, the effects of three types of H occupy sites in the ZrCo alloy on the crystal structure characteristics and their stability were investigated under the framework of a first-principles study. Two types of octahedral gaps were determined after crystal structure optimization, where the O1 octahedron was composed of two Co atoms and four Zr atoms, while O2 had two Zr atoms at its symmetry axis and four Co atoms in the middle vertical plane. The lattice constant, gap volume, and formation enthalpy results showed that Zr 8 Co 8 H O1 hydrides easily formed preferentially compared with Zr 8 Co 8 H O2 . By substituting the enthalpy changes into the pressure-composition isotherm (PCT) curve during the hydrogen absorption process, Zr 8 Co 8 H O2 facilitated the following hydrogen absorption process. In addition, the partial density of states (PDOS), charge difference density, the Mulliken population, the magnetic moment, and mechanical constants of three compounds were investigated to further verify that the O1 octahedral hydrogen placeholder had priority with superior lattice stability of the Zr 8 Co 8 H O1 structure.

In-situ neutron diffraction during reversible deuterium loading in Ti-rich and Mn-substituted Ti(Fe,Mn)0.90 alloys

Hydrogen is an efficient energy carrier that can be produced from renewable sources, enabling the transition towards CO2-free energy. Hydrogen can be stored for a long period in the solid-state, with suitable alloys. Ti-rich TiFe0.90 compound exhibits a mild activation process for the first hydrogenation, and Ti(Fe,Mn)0.90 substituted alloys can lead to the fine tuning of equilibrium pressure as a function of the final application. In this study, the crystal structure of TiFe(0.90-x)Mnx alloys (x = 0, 0.05 and 0.10) and their deuterides has been determined by in-situ neutron diffraction, while recording Pressure-Composition Isotherms at room temperature. The investigation aims at analysing the influence of Mn for Fe substitution in Ti-rich Ti(Fe,Mn)0.90 alloys on structural properties during reversible deuterium loading, which is still unsolved and seldom explored. After activation, samples have been transferred into custom-made stainless-steel and aluminium alloy cells used for in-situ neutron diffraction experiments during deuterium loading at ILL and ISIS neutron facilities, respectively. The study enables remarkable understanding on hydrogen storage, basic structural knowledge, and support to the industrial application of TiFe-type alloys for integrated hydrogen tank in energy storage systems by determining the volume expansion during deuteration. Furthermore, the study demonstrates that different contents of Mn do not significantly change the volumetric expansion during phase transitions, affecting only the deuterium content for the γ phase and the cell evolution for the β phase. The study confirms that the deuterated structures of the γ phase upon absorption, β and α phase upon desorption, correspond to S.G. Cmmm, P2221 and Pm-3m, respectively.

A comparative study of hydrogen storage properties of AZ31 and AZ91 magnesium alloys processed by different methods

We studied the hydrogen storage properties of commercial AZ31 and AZ91 magnesium alloys with refined microstructure. Various processing techniques, such as equal channel angular pressing (ECAP) and high energy ball milling (HEBM), were utilized to modify the microstructure and phase composition of the alloys. X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical digital microscopy (ODM) were utilized to characterise the microstructure and phase transformations during the hydrogen absorption/desorption processes. The isothermal kinetics are measured by a Sieverts’-type apparatus at the temperature of 350 ℃. In the ECAP process, the AZ31 alloy had a slightly higher storage capacity and faster absorption kinetics, absorbing 6.0 wt% (6.6 wt% - maximum) of hydrogen in less than 12 min. In the HEBM process, AZ91 absorbed 6.5 wt% of hydrogen and desorbed it in 347 ± 13 s, a faster rate than AZ31. The pressure–composition isotherms of all studied alloys at a temperature of 300 °C were determined. The processed alloys exhibited no pressure hysteresis. The microstructure of bulk samples and the morphology of prepared powders were shown to affect their hydrogenation behaviour. The activation energies for hydrogen desorption were 119 ± 2 kJ/mol for AZ31 alloy processed by ECAP and 128 ± 3 kJ/mol for AZ91 alloy processed by HEBM, which are significantly lower than the activation energy values of pure Mg hydride.

An investigation into the multi-parameter identification and model optimization strategy of the PCT curves of hydrogen storage alloys by multiple intelligent algorithms

A multi-parameter sensitivity analysis method was proposed based on Latin hypercube sampling and the rank correlation method. The general genetic algorithm was improved by incorporating a niche parallel algorithm, suspicious peak point judgment, and a local search strategy, and both the improved genetic algorithm and multi-parameter sensitivity analysis method were implemented through C++ programming. The hydrogen pressure-composition isotherm (PCT curve) was found to play an important role in the study of hydrogen storage properties of metallic materials, and its reaction enthalpy change term was widely replaced by a power series form of smoothing. Based on the improved multi-parameter sensitivity analysis method, the effects of the range of parameter values and the number of samples on sensitivity were investigated, where the sensitivity ranking of the 10 parameters in the classical model was obtained. In addition, the equilibrium hydrogen pressure expression models of the highest order i of the power series from 3 to 9 were developed. The influence of the highest order on the optimization of multi-parameter identification was compared using an improved genetic algorithm, and we concluded that the eighth order polynomial model had the optimal convergence and fitting effect. Based on the inspiration of the function trend shape, two innovative forms of the PCT model power series were proposed in this work. Combined with typical examples, three high-order models were fully evolved, and a better power series form was obtained compared to the traditional model.

(Digital Presentation) Assessment of the Cyclic Stability of a Bench-Scale TiFe-Based Hydrogen Storage Tank

Hydrogen storage devices comprising of low-cost and flexible metal hydrides have recently regained vivid interest as basic elements in energy conversion systems. Among many intermetallic compounds, TiFe-based alloys are one of the most promising candidates to hold atomic hydrogen because of their significant storage capacity of about 1.9 wt.% and moderate operating conditions. However, the alloy degradation in repeated hydriding- and dehydriding cycles inevitably affects the performance of the hydrogen storage system. In this work, the intrinsic degradation behavior of a bench-scale TiFe-based alloy tank has been investigated. Thermal cyclic life tests were performed between 5°C and 45°C, and the heat transfer properties of the storage device were tested over repeated cycles by obtaining temperature histories. A thermal imaging device was used for the internal heat transfer mapping and the charging time and pressure-composition isotherms were recorded for the hydriding- and dehydriding reactions. In Figure 1 thermal images of the discharge reaction at Begin of Life (BoL) and End of Life (EoL) of the storage device are shown. The hydrogen storage capacity was observed to decrease by almost 90% after 100 cycles. Heat transfer mapping revealed that the excess portion of degradation occurs in the middle of the storage device and is attributed to the formation of inactive TiFe clusters based on the measurements. Acknowledgment: Financial support by the Austrian Research Promotion Agency (FFG) through the 8th Call of “COMET-Projects” is gratefully acknowledged. Figure 1 . Thermal image of the TiFe storage device at Begin of Life (BoL) a) and End of Life (EoL) b) after discharging for 240s respectively. Figure 1

Hydrogen adsorption and interactions in self-reducing shell hosted palladium nanoparticles on magnetite support

The nanoparticles (NP), consisting of hydrazine grafted organo-silica with PdNPs embedded shell on the Fe3O4 core, were prepared to study the adsorption and interactions of hydrogen in PdNPs and their support matrix. This material is expected to find the applications in the hydrogen technology including catalysis. The PdNPs were formed spontaneously in the organo-silica shell on magnetite nanoparticles by the reduction of Pd2+ ions with grafted hydrazine in the organo-silica shell. Thus formed NPs, termed as Fe3O4-GTEOS@PdNPs, were also thermally treated at 1033 K in Ar atmosphere to convert organic components to carbon. The chemical composition, physical structure, and magnetic properties were studied by high resolution transmission electron microscopy, X-rays diffraction, Mössbauer spectroscopy and X-ray photoelectron spectroscopy for the characterizations of physical, chemical and magnetic changes occurred in the Fe3O4-GTEOS@PdNPs after hydrogen adsorption-desorption at varying temperatures with respect to that in unused one. The hydrogen adsorption pressure-composition (PC) isotherms in Fe3O4-GTEOS@PdNPs followed the expected trend from 173 to 303 K as expected from PdNPs. However, thermally treated Fe3O4-GTEOS@PdNPs were found to adsorb lower amount of hydrogen due to oxidation of Pd0 to PdO and morphological changes during heating in Ar atmosphere. The comparison of nH/nPd value (0.49) obtained for the PdNPs in Fe3O4-GTEOS@PdNPs with the values those reported in the literature for different Pd materials showed the decrease in nH/nPd value with decrease in the size of Pd particles. This was attributed to stronger Pd–H bond in a nanoscale palladium, which prevented hydrogen transfer to interior matrix as compared to bigger Pd particles. The hydrogen adsorption PC isotherm at 373 K in Fe3O4-GTEOS@PdNPs could not be obtained as the unknown chemical reaction happened in the sample during the experiment. The considerably higher H2 consumption in the Fe3O4-GTEOS@PdNPs occurred at 373 K than that expected from the hydrogen adsorption in the PdNPs alone.

Design of multicomponent alloys with C14 laves phase structure for hydrogen storage assisted by computational thermodynamic

Design methods with predictive properties modeling are paramount tools to explore the vast compositional field of multicomponent alloys. The applicability of an alloy as a hydrogen storage media is governed by its thermodynamic properties, which can be represented by pressure-composition-temperature (PCT) diagrams. Therefore, the prediction of PCT diagrams for multicomponent alloys is fundamental to design alloys with optimized properties for hydrogen storage applications. In this work, a strategy to design C14-type Laves phase multicomponent alloys for hydrogen storage assisted by computational thermodynamic is presented. Since electronic and geometrical factors play an important role in the formation and stability of multicomponent Laves phase, valence electron concentration (VEC), atomic radius ratio (rA/rB), and atomic size mismatch (δ) are initially considered to screen a high number of compositions and find alloy systems prone to form Laves phase structure. Then, CALPHAD method was employed to investigate the phase stability of alloys of the (Ti, Zr or Nb)(V, Cr, Mn, Fe, Co, Ni, Cu, or Zn)2 system, resulting in 440 alloys prone to solidify as C14 Laves phase structure. In addition, we present a thermodynamic model, which allowed calculating the PCT diagrams of the suggested C14 Laves phase alloys based solely on the alloy's composition. For these calculations, the entropy and enthalpy of hydrogen solution in the C14 Laves phase were modelled considering that hydrogen solid solution occurs only at the A2B2-type interstitial sites of the C14 Laves phase structure. Therefore, the room temperature PCI diagrams of 440 C14 Laves phase multicomponent alloys were calculated. Experimental pressure-composition-isotherm (PCI) diagrams of six C14 Laves phase alloys were compared against the calculated ones. Although the PCI curve shapes were not perfectly predicted for some alloys, the order of magnitude of the equilibrium pressure for all the tested alloys were well predicted. The results show that C14 Laves phase multicomponent alloys within a wide range of equilibrium pressure at room temperature can be obtained, being promising candidates for different hydrogen storage applications, such as room temperature tanks, hybrid tanks and Ni-metal hydrides batteries.

Nanoconfinement of lithium alanate for hydrogen storage

LiAlH4 is considered as a promising material for solid state hydrogen storage. However, the lack of reversibility along with sluggish kinetics hinders its practical application. In this paper, hollow carbon nanospheres (HCNs) were used as a porous scaffold to confine LiAlH4 via solvent impregnation method. Nanoconfined LiAlH4 (LiAlH4@HCNs) exhibited significant improvements in hydrogen sorption compared to its bulk counterpart. LiAlH4@HCNs releases hydrogen sharply at 146 °C with full conversion to LiH within 1.5 h. The desorbed material can also be regenerated back to some extent into LiAlH4 under 8 MPa H2 at 150 °C. Measurement of the pressure-composition isotherm suggests an alteration in the equilibrium state upon confinement of LiAlH4 in voids of a few nanometres and thus altered hydrogen thermodynamic paths.

Structure and hydrogen sorption properties of Mg-Mg2Ni nanoparticles prepared by gas phase condensation

The aim of this work is to investigate the hydrogen sorption kinetics and thermodynamics of Mg-Ni nanoparticles at relatively low temperature in relation to their microstructure. To this purpose, Mg-Ni nanoparticles (20 at% Ni) were prepared by gas phase condensation employing two thermal vapour sources. In the as-prepared state, Mg and Ni are mixed within individual nanoparticles, but the intermetallic Mg2Ni compound is not fully formed. After keeping the nanoparticles at 150 °C for two hours under high vacuum or at a mild hydrogen pressure of 0.15 bar, the formation of a Mg-Mg2Ni or MgH2-Mg2NiH0.3 nanocomposite is observed. Subsequently, fast kinetics of hydrogen sorption are recorded at 150 °C with activation energy of 80±8 kJ/mol (absorption) and 60±6 kJ/mol (desorption). However, the maximum hydrogen storage capacity is limited to 2.5 wt% because the transformation from Mg2NiH0.3 to Mg2NiH4 does not take place at 150 °C even at pressures well above the expected thermodynamic equilibrium. Therefore, only the transformation Mg↔MgH2 contributes to the reversible storage capacity. The corresponding equilibrium pressure determined by pressure-composition isotherms of absorption and desorption at 150 °C is 7.5 mbar, very close to the extrapolated value for bulk Mg. The partial replacement of Ni with Fe does not significantly alter the thermodynamics and kinetics of hydrogen sorption. The structure and hydrogen sorption properties of Mg-Ni nanoparticles are compared to those of Mg-Ti nanoparticles prepared by a similar procedure.

Methods to Improve the First Hydrogenation of the Vanadium-Rich BCC Alloy Ti16V60Cr24

In this paper we report the effect of three different preparation methods on the first hydrogenation of the vanadium-rich BCC alloy Ti16V60Cr24: one-time cold rolling, 5 min ball milling and addition of 4 wt.% of Zr. All samples were synthesized by arc melting. Without Zr addition the alloy was single phase, but when 4 wt.% Zr was added, a secondary zirconium-rich phase was present. However, X-ray diffraction patterns only showed a single-body-centred cubic phase before hydrogenation for all samples. The crystal structure of the fully hydrogenated samples was body-centred tetragonal. The highest hydrogen capacity (3.8 wt.%) was measured for the Zr-doped alloy. The ball-milled alloy also exhibited a high storage capacity and fast kinetics. However, the maximum hydrogen storage capacity slightly decreased after cold rolling. It was found that air exposure increases incubation time for the first hydrogenation. The incubation time was shortened by cold rolling which, however, reduced the hydrogen storage capacity. The Pressure-Composition isotherms of Ti16V60Cr24 + 4 wt.% Zr at 297, 303 and 323 K were determined. The determined enthalpy and entropy of hydrides formation were −41 ± 5 kJ/mol and −134 ± 14 J/mol/K, respectively.

Thermodynamic and kinetic properties of Li3BO3 modified by the alkali fluorides addition in the CO2 capture process

Tri-lithium borate (Li3BO3) is a high-capacity CO2 sorbent recently proposed for the intermediate temperature range of 500 °C to 650 °C. Up to date, only a study of CO2 sorption kinetic behavior of Li3BO3 modified by nitrite salts has been carried out. The equilibrium properties that relate the dependence of the partial pressure of CO2 and temperature are also unknown. In this work, a novel two-step mechano-thermal method was optimized to produce both pure and modified Li3BO3 by addition of 0.1 mol of Na- and K-fluorides. Fast reaction by milling of Li2CO3 and H3BO3 forms the intermediate LiB5O8·5H2O compound at room temperature, which transforms to Li3BO3 as single phase by heating at 600 °C. Pressure-composition isotherms allow to determine the existence of two plateaus, each of them ascribed to different phases: Li6B4O9 at low CO2 partial pressure (ΔH = 58 ± 5 kJ/mol) and LiBO2 at high CO2 partial pressure (ΔH = 106 ± 6 kJ/mol). The mathematical expression that relates the equilibrium CO2 absorption partial pressure and the temperature of modified and unmodified Li3BO3 was reported. Na- and K-modified Li3BO3 show remarkably fast kinetics with high CO2 capture capacity (e.g. greater than 45 wt% at 500 °C) and excellent regenerability, with minor deterioration in performance over 30 cycles of CO2 sorption. The kinetic behavior of modified Li3BO3 by alkali fluorides is enhanced by the formation of an eutectic melt phase during carbonation, as confirmed by differential scanning calorimetry and the simultaneous detection of LiF. In addition, alkali fluorides could generate oxygen vacancies and modify the surface composition by enrichment of Li-O sites, also favoring the CO2 sorption kinetics.

Hydrogen storage properties of V0.3Ti0.3Cr0.25Mn0.1Nb0.05 high entropy alloy

In this paper, we present the synthesis, first hydrogenation kinetics, thermodynamics and effect of cycling on the hydrogen storage properties of a V0.3Ti0.3Cr0.25Mn0.1Nb0.05 high entropy alloy. It was found that the V0.3Ti0.3Cr0.25Mn0.1Nb0.05 alloy crystallizes in body-centred cubic (BCC) phase with a small amount of secondary phase. The first hydrogenation is possible at room temperature without incubation time and reaches a maximum hydrogen storage capacity of 3.45 wt%. The pressure composition isotherm (P–C–I) at 298 K shows a reversible hydrogen desorption capacity of 1.78 wt% and a desorption plateau pressure of 80.2 kPa. The capacity loss is mainly due to the stable hydride with the desorption enthalpy of 31.1 kJ/mol and entropy of 101.8 J/K/mol. The hydrogen absorption capacity decreases with cycling due to incomplete desorption at room temperature. The hydrogen absorption kinetics increases with cycling and the rate-limiting step is diffusion-controlled for hydrogen absorption.

Effect of Mn Element on the Structures and Properties of A2B7-Type La–Y–Ni-Based Hydrogen Storage Alloys

The structures, hydrogen storage behaviors and electrochemical properties of Y0.75La0.25Ni3.5−xMnx (x = 0–0.3) alloys were analyzed by X-ray diffraction, Neutron powder diffraction, pressure–composition isotherms and electrochemical tests. All alloys have a multiphase structure. With the increase in Mn content, the Gd2Co7-type phase of the alloys gradually transforms into the Ce2Ni7-type phase; the Mn atom mainly occupies the Ni sites in the [AB5] subunit and the interface between the [AB5] and [A2B4] subunits; the V[A2B4]/V[AB5] continuously decreases from 1.045 (x = 0) to 1.019 (x = 0.3), which reduces the volume mismatch between [A2B4] and [AB5] subunits. The maximum hydrogen absorption of the series alloys increases first and then decreases, and the addition of Mn effectively promotes the hydrogen absorption/desorption performance of the alloys. The maximum discharge capacity of the alloy electrodes is closely related to their hydrogen storage capacity at 0.1 MPa and hydrogen absorption/desorption plateau pressure. The cyclic stability of all the Mn-containing alloy electrodes is improved clearly compared to that of Mn-free alloy electrodes, because the volume mismatch between the [AB5] and [A2B4] subunits of the alloys becomes smaller after the addition of Mn, which can improve the structural stability and reduce the corrosion of alloys during hydrogen absorption/desorption cycles. When the Mn content is between 0.1 and 0.15, the Ce2Ni7-type phase of the alloys has high abundance and the alloy electrodes exhibit excellent overall performance.