Equilibrium Hydrogen Pressure Research Articles

Amorphous TiCu-Based Additives for Improving Hydrogen Storage Properties of Magnesium Hydride.

Magnesium hydride has long been regarded as a promising candidate material for hydrogen and heat storage due to its high hydrogen capacity, reversibility, and low cost. Catalytic doping has been demonstrated as one of the most effective methods to improve hydrogen storage properties of MgH2. In this study, amorphous Ti45Cu41Ni9Zr5 and Ti40Cu47Zr10Sn3 alloys are used as additives for MgH2. Nanostructured MgH2 doped with amorphous or crystalline TiCu-based alloys are prepared by using a high-energy mechanochemical synthesis method. Results show that the amorphous TiCu additives provide enhanced catalytic effects compared to crystalline alloys of the same composition. Doping MgH2 using an amorphous Ti45Cu41Ni9Zr5 alloy yielded improved dehydrogenation kinetics compared to using crystalline Ti40Cu47Zr10Sn3 alloy. The analysis using transmission electron microscopy reveals that there are nanostructured catalytic particles uniformly distributed in the amorphous TiCu-catalyzed MgH2. The MgH2 system catalyzed by amorphous TiCu-based alloy shows little degradation during hydrogenation and dehydrogenation cycling at 300 °C. The amorphous TiCu-based catalysts are thermally stable at temperatures up to 360 °C. Heating the amorphous Ti45Cu41Ni9Zr5-catalyzed MgH2 to temperatures above 360 °C led to disproportionation of the catalyst alloy and the formation of MgCu2 and Ti2Cu. In addition, PCI analysis of the amorphous Ti45Cu41Ni9Zr5-catalyzed MgH2 shows a slight increase in hydrogen equilibrium pressure, resulting in a reaction enthalpy of -78.7 kJ/mol·H2 and an entropy of 145.0 J/K·mol·H2. The entropy calculated from this study is approximately 10 J/K·mol·H2 higher than values previously reported for undoped and catalyzed Mg-H systems.

The potential of metal hydrides paired with compressed hydrogen as thermal energy storage for concentrating solar power plants

Concentrating solar power (CSP) plants require thermal energy storage (TES) systems to produce electricity during the night and periods of cloud cover. The high energy density of high-temperature metal hydrides (HTMHs) compared to state-of-the-art two-tank molten salt systems has recently promoted their investigation as TES systems. A common challenge associated with high-temperature metal hydride thermal energy storage systems (HTMH TES systems) is storing the hydrogen gas until it is required by the HTMH to generate heat. Low-temperature metal hydrides can be used to store the hydrogen but can comprise a significant proportion of the overall system cost and they also require thermal management, which increases the engineering complexity. In this work, the potential of using a hydrogen compressor and large-scale underground hydrogen gas storage using either salt caverns or lined rock caverns has been assessed for a number of magnesium- and sodium-based hydrides: MgH2, Mg2FeH6, NaMgH3, NaMgH2F and NaH. Previous work has assumed that the sensible heat of the hydrogen released from the HTMH would be stored in a small, inexpensive regenerative material system. However, we show that storing the sensible heat of the hydrogen released would add between US$3.6 and US$7.5/kWhth to the total system cost for HTMHs operating at 565 °C. If the sensible heat of released hydrogen is instead exploited to perform work then there is a flow-on cost reduction for each component of the system. The HTMHs combined with underground hydrogen storage all have specific installed costs that range between US$13.7 and US$26.7/kWhth which is less than that for current state-of-the-art molten salt heat storage. Systems based on the HTMHs Mg2FeH6 or NaH have the most near term and long term potential to meet SunShot cost targets for CSP thermal energy storage. Increasing the operating temperature and hydrogen equilibrium pressure of the HTMH is the most effective means to reduce costs further.

Improved hydrogen sorption kinetics in Mg modified by chosen catalysts

Hydrogen sorption characteristics were investigated in Mg modified by ten chosen catalyzing additives X = Mg2Si, Mg2Ge, Mg17Al12, Mg5Ga2, NaCl, LiCl, NaF, LiF and by two combinations Ni + Mg17Al12 and Ni + Mg2Si. Amorphous carbon (CB) was used as an anti-sticking component. It was found that optimum content of both X and CB was between 10 and 14 wt %. To assess the effect of X itself upon the sorption kinetics, concentration of both X and CB was fixed to about 12 wt %. The shortest desorption times t09 (time to desorption of 90 pct of stored hydrogen) showed alloys with X = NaF, Mg2Si, Mg2Ge and NaCl. Careful measurements of equilibrium hydrogen pressure, peq, at 623 K showed an increase in peq for added chlorides, fluorides, Mg2Ge and also for Mg2Si. TPD experiments revealed desorption peaks for X = NaCl, LiF shifted down to temperatures as low as 597 K.

Hydrogen sorption properties of metal hydride beds: Effect of internal stresses caused by reactor geometry

A number of peculiarities in sorption behavior of metal hydride beds depending on the reactor geometry have been discovered. Comparison of PCT absorption and desorption isotherms was conducted for the same 500 g sample of La0·9Ce0·1Ni5-alloy and two different geometries: horizontal half-cylinder and vertical annular. This approach gives the way to directly investigate the influence of the bed geometry on the hydrogen sorption properties excluding the effect of other factors. The influence of the internal stresses in the fine dispersed beds of intermetallic compounds on their sorption properties is discussed in the present study. The occurrence of internal stresses in the vertical annular metal hydride bed results in the decrease of the hydrogen equilibrium pressure, increase of the enthalpy and entropy of hydrogen absorption and decrease of pressure hysteresis due to significantly stronger decrease of absorption pressure compared to desorption. The analysis of SEM images of the sample taken from the bottom of the working autoclave showed the presence of a macro particle which avoided the dispersion due to the agglomerated particles around.

Kinetics in Mg-based hydrogen storage materials: Enhancement and mechanism

Abstract Mg-based materials have been intensively studied for hydrogen storage applications due to their high energy density up to 2600 Wh/kg or 3700 Wh/L. However, the Mg-based materials with poor kinetics and the necessity for a high temperature to achieve 0.1 MPa hydrogen equilibrium pressure limit the applications in the onboard storage in Fuel cell vehicles (FCVs). Over the past decades, many methods have been applied to improve the hydriding/dehydriding (H/D) kinetics of Mg/MgH2 by forming amorphous or nanosized particles, adding catalysts and employing external energy field, etc. However, which method is more effective and the intrinsic mechanism they work are widely differing versions. The hydrogenation and dehydrogenation behaviors of Mg-based alloys analyzing by kinetic models is an efficient way to reveal the H/D kinetic mechanism. However, some recently proposed models with physical meaning and simple analysis method are not known intimately by researchers. Therefore, this review focuses on the enhancement method of kinetics in Mg-based hydrogen storage materials and introduces the new kinetic models.

Hydrogen storage behavior of Mg@Mg2Si and Mg@Mg17Al12 alloys with additions of carbon allotropes and talc

Mg based hydrogen storage alloys containing catalyzing particles are frequently studied because the catalysis is one of possible ways how to speed-up effectively the hydrogen sorption kinetics. Ball milled alloys Mg@Mg2Si and Mg@Mg17Al12 are studied in the present paper. They consist of Mg grains, which hydrogen storage performance is catalyzed by the Mg2Si and Mg17Al12 particles. Graphite, amorphous carbon (carbon black) and magnesium silicate monohydrate (talc) were added to facilitate the fine ball milling. It was found that addition 0.14 wt% of amorphous carbon to Mg@Mg2Si shows the best hydrogen sorption kinetics and the highest sorption capacity of about 6 wt% H2. A weak destabilization of MgH2 hydride by Mg2Si and Mg17Al12 was observed. New technique of equilibrium hydrogen pressure estimation was proposed.

Hydrogen sorption and corrosion properties of La2Ni9CoSn0.2alloy

AbstractThe hydrogenation and corrosion behaviour of La2Ni9 · CoSn0.2alloy was analysed in respect of its use in Ni–MH batteries. It has been proved that the presence of tin in the alloy causes a decrease in hydrogen equilibrium pressure. In the electrochemical studies several techniques, such as chronopotentiometry, multi-potential steps, linear sweep voltammetry and the potentiokinetic polarization were applied to characterize the electrochemical properties of a La2Ni9CoSn0.2powder composite electrode. During long cycling, powder particles undergo micro-cracking or other forms of surface development causing a progressive increase in the exchange current density of the H2O/H2system, but, on the other hand, this increase favours corrosion processes such as the electrode material's oxidation. This is also reflected in the capacity loss values.

Metal-hydrogen systems with an exceptionally large and tunable thermodynamic destabilization

Hydrogen is a key element in the energy transition. Hydrogen–metal systems have been studied for various energy-related applications, e.g., for their use in reversible hydrogen storage, catalysis, hydrogen sensing, and rechargeable batteries. These applications depend strongly on the thermodynamics of the metal–hydrogen system. Therefore, tailoring the thermodynamics of metal–hydrogen interactions is crucial for tuning the properties of metal hydrides. Here we present a case of large metal hydride destabilization by elastic strain. The addition of small amounts of zirconium to yttrium leads to a compression of the yttrium lattice, which is maintained during (de)hydrogenation cycles. As a result, the equilibrium hydrogen pressure of YH2 ↔ YH3 can be rationally and precisely tuned up to five orders of magnitude at room temperature. This allows us to realize a hydrogen sensor which indicates the ambient hydrogen pressure over four orders of magnitude by an eye-visible color change.

Interaction of hydrogen with the intermetallic compound Nd2Fe17 studied by calorimetry

Interaction of hydrogen with the intermetallic compound Nd2Fe17 has been studied for the first time by calorimetry using a differential heat conduction calorimeter coupled to a Sieverts apparatus. Hydrogen absorption and desorption reactions were run at 200°C, and two types of data were obtained: p–C–T and ΔH–C–T (where p is the equilibrium hydrogen pressure, C = H/Nd2Fe17, ΔH is the reaction enthalpy, and T is the measurement temperature). The p–C–T curves obtained for the hydrogen absorption and desorption processes have no plateau or two-phase region, in contrast to what is characteristic of the formation of a hydride phase. At the same time, the ΔH(C) curves have a few portions where the enthalpy of reaction between hydrogen and the intermetallic compound remains constant: 0 < C < 2.0, with ΔH abs =–85.05 ± 0.65 kJ/mol H 2; 2.0 < C < 2.7, with ΔH abs =–80.64 ± 1.00 kJ/mol H2; and 1.9 < C < 2.7, with ΔH des = 76.48 ± 0.85 kJ/mol H2. The data obtained in this study suggest that positions 9e and 18g in the intermetallic compound are occupied by hydrogen in a particular order.

Improvement in Hydrogention and Dehydrogenation Characteristics of Magnesium by Addition of Titanium (III) Chloride via Transformation-Accompanying Milling

A specimen consisting of 90 wt.% Mg and 10 wt.% TiCl3 (called 95 Mg + 10 TiCl3) was made by transformation-accompanying milling, and its hydrogenation and dehydrogenation features were checked. The activation of 95 Mg + 10 TiCl3 was not demanded. 95 Mg + 10 TiCl3 revealed an useful hydrogenation-dehydrogenation capacity (the quantity of hydrogen absorbed in 60 min) of about 5.6 wt.%. At n = 1, the specimen absorbed 4.06 wt.% of hydrogen in 5 min, 5.09 wt.% of hydrogen in 10 min, and 5.59 wt.% of hydrogen in 60 min at 593 K in 1.2 MPa H2. At n = 1, the specimen desorbed 0.47 wt.% of hydrogen in 2.5 min, 3.81 wt.% of hydrogen in 30 min, and 5.20 wt.% of hydrogen in 60 min at 593 K in 0.1 MPa H2. The XRD pattern of 95 Mg + 10TiCl3 dehydrogenated at the 4th hydrogenation-dehydrogenation cycle included Mg, β-MgH2, MgO, and a small amount of TiH1.924 phases. The P (equilibrium hydrogen pressure)-C (composition)-T (temperature) curve at 593 K revealed an equilibrium plateau pressure at about 0.25 MPa. 95 Mg + 10 TiCl3 had a higher early hydrogenation rate and a larger amount of hydrogen absorbed in 60 min than Mg, 90 Mg + 10 Fe2O3, 90 Mg + 10 MnO, and 90 Mg + 10 TaF5, the hydrogenation and dehydrogenation features of which were before informed of.DOI: http://dx.doi.org/10.5755/j01.ms.23.3.16375

Synthesis and electrochemical properties of binary MgTi and ternary MgTiX (X = Ni, Si) hydrogen storage alloys

Mg-based hydrogen storage alloys are promising candidates for many hydrogen storage applications because of the high gravimetric hydrogen storage capacity and favourable (de)hydrogenation kinetics. In the present study we have investigated the synthesis and electrochemical hydrogen storage properties of metastable binary MgyTi1−y (y = 0.80–0.60) and ternary Mg0.63Ti0.27X0.10 (X = Ni and Si) alloys. The preparation of crystalline, single-phase, materials has been accomplished by means of mechanical alloying under controlled atmospheric conditions. Electrodes made of ball-milled Mg0.80Ti0.20 powders show a reduced hydrogen storage capacity in comparison to thin films with the same composition. Interestingly, for a Ti content lower than 30 at.% the reversible storage capacity increases with increasing Ti content to reach a maximum at Mg0.70Ti0.30. The charge transfer coefficients (α) and the rate constants (K1 and K2) of the electrochemical (de)hydrogenation reaction have been obtained, using a theoretical model relating the equilibrium hydrogen pressure, electrochemically determined by Galvanostatic Intermittent Titration Technique (GITT), and the exchange current. The simulation results reveal improved values for Mg0.65Ti0.35 compared to those of Mg0.80Ti0.20. The addition of Ni even more positively affects the hydrogenation kinetics as is evident from the increase in exchange current and, consequently, the significant overpotential decrease.

Modelling and simulation of hydrogen redistribution in a heterogeneous alloy during the cooling down to 200 °C

Hydrogen embrittlement induced by internal hydrogen has been identified as a critical issue for heavy components manufactured from either plain or hollow ingots made of 16MnNIMo5 through 20MnNiMo5 alloys that have the particularity to be heterogeneous at various scales. Based on available data of solubilities, diffusivities and temperature dependent phase transformations for the matrix and segregated bands, a model of hydrogen distribution in such a heterogeneous microstructure has been developed. Additionally, penny-shaped thin cavities have been introduced to account for the presence of incoherent MnS inclusions leading to a potential decohesion at MnS/matrix interface. Pressure increase in such cavities has been evaluated using ideal gas behavior for hydrogen. A unidimensional Finite Difference numerical approach has been implemented in order to simulate hydrogen redistribution among matrix, segregation_band and cavity. This model has allowed to quantitatively evaluate the redistribution of hydrogen at the scale of the microstructural heterogeneities during the cooling down to 200 °C: first, close to the component surface where important kinetic effects have been demonstrated due to high cooling rate and second, in the bulk, where hydrogen redistribution was close to the thermodynamic equilibrium during the whole cooling period. It has been shown that the final equilibrium pressure of hydrogen in the cavity at 200 °C is negligible even for a high initial hydrogen content.

Hysteresis phenomena on the crystal lattice of Ti0.8Zr0.2Mn1.5 in the hydrogenation and dehydrogenation process

The crystal structure changes in the hydrogenation and dehydrogenation process have been investigated by the in-situ synchrotron X-ray diffraction measurement to elucidate the mechanism of hysteresis phenomena in equilibrium hydrogen pressure. The hysteresis phenomena were observed on the crystal lattice. The lattice constants and volume of α-phase (hydrogen solid solution) and β-phase (hydride) in the dehydrogenation process with lower equilibrium pressure were smaller than those in the hydrogenation process with higher equilibrium pressure. This phenomenon is opposite to the general relationship between the lattice volume and the equilibrium pressure. The hysteresis phenomenon was also observed in the diffraction peak widths. The peak widths of β-phase in the dehydrogenation process were wider than those in the hydrogenation process. The hysteresis phenomena may be related to lattice defects and the hydrogen content in crystalline regions.

Hydrogen uptake of cobalt and copper oxide-multiwalled carbon nanotube composites

Hydrogen storage capacity of a pristine multi-walled carbon nanotubes is increased 10-fold at 298 K and an equilibrium hydrogen pressure of ∼23 atm, upon addition of a hydrogen spillover catalyst cobalt- and copper oxide, from 0.09 to 0.9 wt.%. In situ reduction method is utilized to synthesize Co-oxide/MWCNTs and Cu-oxide/MWCNTs composite. Blocking of channels and pores of MWCNTs by oxide nanoparticles during preparation method is responsible for low BET specific surface area of composites compared to pristine sample. X-ray diffraction, scanning, and transmission electron microscopy demonstrates nanostructural characterization of MWCNTs and composites. Thermogravimetric analysis of two oxide/MWCNTs composites showed a single monotonous fall related to MWCNTs gasification. Enhancement of hydrogen storage of both composites is attributed to the spillover mechanism due to decoration of Co and Cu-oxide nanoparticles on the outer surface of MWCNTs.

Effect of hydrogen content on hydrogen desorption kinetics of titanium hydride

Abstract Thermal desorption behaviours of titanium hydride powders with different hydrogen contents were studied using simultaneous TG-TDS measurements at a heating rate of 10 °C/min. The as-received TiH 2 powders were pre-heated to different temperatures and subsequently cooled to room temperature in argon. The XPS depth profile analysis demonstrated that after heat treatments, the relative oxygen concentrations decreased slightly compared with that of as-received TiH 2 powder. The changes in the TDS spectra for the pre-heat treated TiH 2 powders were primarily associated with the residual hydrogen contents. The origins of the observed TDS peaks and shoulders were attributed to the equilibrium hydrogen pressures of the phase regions that the decomposition reaction passed through. The hydrogen content was demonstrated to be essential for the phase transformation sequence during titanium hydride decomposition.

Experimental determination of the enthalpies of formation of the lithium silicides Li7Si3 and Li12Si7 based on hydrogen sorption measurements

Abstract This work focuses on the determination of the standard molar enthalpies of formation Δ f H °(298) of the lithium silicides Li 7 Si 3 and Li 12 Si 7 . For this purpose the hydrogenation/dehydrogenation equilibria in the system Li-Si-H were investigated at three different temperatures (450 °C, 475 °C, 500 °C) using a Sieverts type apparatus. The measurements are based on two different sample types: (i) pure silicide Li 7 Si 3 (Li 2.33 Si) and (ii) a stoichiometric mixture of lithium hydride and silicon in the ratio of Li:Si = 2.33:1. The calculation of the standard enthalpies of formation by using the slopes of the classical Van't Hoff plot results in uncertainties of at least 10%. Due to this relatively large error, an alternative evaluation method based on the linking of the hydrogen equilibrium pressures p eq (H 2 ) from the sorption measurements with recently published precise heat capacity and entropy data of the lithium silicides was applied. The obtained values for the standard enthalpy of formation amount to Δ f H °(298) = (−25.3 ± 0.3) kJ/(mol atom) for Li 7 Si 3 and Δ f H °(298) = (−23.5 ± 0.3) kJ/(mol atom) for Li 12 Si 7 exhibiting strongly reduced uncertainties.

Estimating the state of charge of MH-Ni batteries by measuring their stable internal pressure

Abstract Nickel metal hydride (MH-Ni) batteries are widely used in hybrid electric vehicles (HEVs). Estimating a battery's state of charge (SOC) remains challenging in practical applications, and it is also the core technology. Because MH-Ni batteries exhibit high rates of self-discharge and have flat and broad charge-discharge voltage plateaus, the estimation of their SOC through their voltage, current, internal resistance, and temperature is not accurate and has a large cumulative error. In this study, a new method for estimating SOC based on battery's stable internal pressure is proposed using the one-to-one correspondence between the hydrogen equilibrium pressure and the reversible hydrogen-storage capacity described by the pressure–concentration–isotherm (P-C-T) curves of hydrogen storage alloys. The actual SOC and the stable internal pressure of the battery have a one-to-one correspondence after the battery was stored at different temperatures and SOCs, and this relationship is maintained after different cycling number and after four years of storage.

AB5-type intermetallic compounds for biohydrogen purification and storage

Intermetallic compounds (IMC) provide a promising solution for a biohydrogen purification problem due to their selective hydrogen absorption from gaseous mixtures produced during dark fermentation. A main IMC characteristic for use in biohydrogen purification is hydrogen equilibrium pressure, which has to be as low as possible during hydrogen sorption from a mixture. Intermetallic compounds LaNi5–xMx (M−Al, Sn, Fe; x = 0.1–0.3) were prepared and their PCT properties were investigated. Compositions LaNi4.6Fe0.2Al0.2 (0.049 MPa at 293 K) and LaNi4.6Sn0.2Fe0.2 (0.055 MPa at 293 K) are found to be the most promising for practical applications due to low equilibrium pressures.

Investigation of hydrogen interaction with magnetic materials of Nd–Fe–B type by calorimetry method

The interaction of hydrogen with the Nd–Fe–B system is studied via calorimetry with use of a Tian–Calvet differential calorimeter and reconstruction of the pressure–composition isotherms (P–C–T, where Р is the equilibrium pressure of hydrogen, С = H/Nd2Fe14B, and T is the reaction temperature). No hybrid phase coexistence is found on the P–C hydrogen sorption/desorption isotherms or on plots of the dependence of the enthalpy on the hydrogen concentration in the metallic matrix. Nevertheless, several segments with constant enthalpy values can be distinguished in the reaction enthalpy on the hydrogen content plot.

Thermodynamic and Kinetic Properties of La20.5MgNi78.5 and La15.5Mg6Ni78.5 Hydrogen Storage Alloys: The Theoretical Models and Their Verifications

In this paper, we present a statistic thermodynamic model quantitatively describing the pressurecomposition- isotherms (PCI) curve, which consists of hydrogen storage capacity, temperature, and equilibrium pressure in the hydriding and dehydriding (H/D) reactions, and a theoretical kinetic model to clarify the H/D kinetic mechanism of hydrogen storage alloys. The results of the calculations are well agreed with the experiments performed with La20.5MgNi78.5 and La15.5Mg6Ni78.5 alloys, their maximum hydrogen storage capacities and the hydride formation enthalpies at 303~333 K are 1.41 wt.% H2 and -31.64 kJ/mol H2 for La20.5MgNi78.5 and 1.31 wt.% H2 and -27.23 kJ/mol H2 for La15.5Mg6Ni78.5, respectively. A new diffusion kinetic model is proposed with the consideration of Pilling–Bedworth Ratio, i.e. hydrogen-induced volume change used for studying the hydrogen absorption reaction kinetics. The activation energies are calculated to be 31.25 kJ/mol H2 for La20.5MgNi78.5 and 24.24 kJ/mol H2 for La15.5Mg6Ni78.5, respectively.