An operational high temperature thermal energy storage system using magnesium iron hydride

Metal hydrides have been demonstrated as energy storage materials for thermal battery applications. This is due to the high energy density associated with the reversible thermochemical reaction between metals and hydrogen. Magnesium iron hydride (Mg₂FeH₆) is one such material that has been identified as a thermal energy storage material due to its reversible hydrogenation reaction at temperatures between 400 and 600 °C. This study demonstates an automated thermal battery prototype containing 900 g of Mg₂FeH₆ as the thermal energy storage material with pressurised water acting as the heat transfer fluid to charge and discharge the battery. The operating conditions of the system were optimised by assessing the ideal operating temperature, flow rate of the heat transfer fluid, and hydrogen pressures. Overall, excellent cyclic energy storage reversibility was demonstrated between 410 and 450 °C with a maximum energy capacity of 1650 kJ which is 87% of the theoretical value (1890 kJ).     

Effect of powder characteristics for a magnesium based metal hydride store

The particle size and shape of commercially available MgH₂ have been characterised, together with those of ball milled MgH₂. These parameters have then been compared with those obtained for the hydrogenated and dehydrogenated state. In addition, thermal conductivity measurements of each specimen have been made. The results show that significant changes occur upon dehydrogenation, which are contrary to expectation. These changes include an increase in average particle size of 28% post dehydrogenation for as received MgH₂, rather than the theorised decrease, and an increase of approximately 300% post dehydrogenation in the ball milled MgH₂ sample. The change in thermal conductivity between the two material states is also higher than expected with an average variation of 14% between the samples in the hydrogenated and dehydrogenated states.     

Chemical equilibrium analysis for hydrolysis of magnesium hydride to generate hydrogen

Magnesium hydride is a promising hydrogen source because of its high mass density of hydrogen, 15.2%, when it is hydrolyzed; MgH₂ + 2H₂O = Mg(OH)₂ + 2H₂ + 277 kJ. However, a magnesium hydroxide, Mg(OH)₂, layer forms rapidly on the surface of the unreacted MgH₂ as the pH increases, hindering further reaction. The purpose of this study is to find acids that could effectively accelerate the reaction by using a chemical equilibrium analysis where the relationships of pH to concentration of ionized Mg were calculated. For the best performing acid, the calculated and measured relationships were compared, and the effects of acid concentration on hydrogen release were measured. The analysis revealed that citric acid and ethylenediamine-tetraacetic acid were good buffering agents. The calculated and measured relationships between pH and concentration of ionized Mg were in good accord. Hydrogen release improved considerably in a relatively dilute citric acid solution instead of pure distilled water. The maximum amount of hydrogen generated was 1.7 × 10³ cm³ g⁻¹ at STP after 30 min. We estimated the exact concentration of citric acid solution for complete MgH₂ hydrolysis by a chemical equilibrium analysis method.     

Thermal stability of Vale Inco nanonometric nickel as a catalytic additive for magnesium hydride (MgH2)

The structure stability of nanometric-Ni (n-Ni) produced by Vale Inco Ltd. Canada as a catalytic additive for MgH₂ has been investigated. Each n-Ni filament is composed of nearly spherical interconnected particles having a mean diameter of 42 ± 16 nm. After ball milling of the MgH₂ + 5 wt.%n-Ni mixture for 15 min the n-Ni particles are found to be uniformly embedded within the particles of MgH₂ and at their surfaces. Neither during ball milling of the MgH₂ + 5 wt.%n-Ni mixture nor its first decomposition at temperatures of 300, 325, 350 and 375 ^°C the elemental n-Ni reacts with the elemental Mg to form the Mg₂Ni intermetallic phase (and eventually the Mg₂NiH₄ hydride). The n-Ni additive acts as a strong catalyst accelerating the kinetics of desorption. From the Arrhenius and Johnson–Mehl–Avrami–Kolmogorov theory the activation energy for the first desorption is determined to be ∼94 kJ/mol. After cycling at 300 ^°C the activation energy for desorption is determined to be ∼99 kJ/mol. This is much lower than ∼160 kJ/mol observed for the undoped and ball milled MgH₂. During cycling at 275 and 300 ^°C the n-Ni additive is converted into Mg₂Ni (Mg₂NiH₄). The newly formed Mg₂NiH₄ has a nanosized grain on the order of 20 nm. Its catalytic potency seems to be similar to its n-Ni precursor. The formation of Mg₂Ni (Mg₂NiH₄) may be one of the factors responsible for the systematic decrease of hydrogen capacity observed upon cycling at 275 and 300 ^°C.     

Experimental and numerical study of a magnesium hydride tank

A small-scale experimental magnesium hydride tank was designed and tested to illustrate the feasibility of hydrogen storage in magnesium hydride. A prototype of the tank was filled with 123 g of previously ball-milled and doped MgH₂. About 80 nl of hydrogen can be reversibly stored at a pressure less than 1 MPa. However, owing to the fact that the heat of a reaction limits the absorption and desorption processes, these latter are slowed down. To have a better understanding of the heat and mass transfers in the tank, a numerical model was developed using the Fluent software. The numerical simulations of hydrogen sorption are found to be in good agreement with the experimental results. During desorption, as an example, the reaction occurs locally and progresses from the tank walls towards the core.     

Hydrogen absorption of catalyzed magnesium below room temperature

Hydrogen absorption of magnesium (Mg) catalyzed by 1 mol% niobium oxide (Nb₂O₅) was demonstrated under the low temperature condition even at −50 °C. The kinetic and thermodynamic properties were examined for MgH₂ with and without Nb₂O₅. By considering the remarkable absorption features at such low temperature, the essential hydrogen absorption properties were investigated under accurate isothermal conditions. As the results, the activation energy of hydrogen absorption for the catalyzed Mg was evaluated to be 38 kJ/mol, which was significantly smaller than that of MgH₂ without the catalyst. The kinetic improvement was also found on the hydrogen desorption process. On the other hand, thermodynamic properties were not changed by the catalyst as a matter of course. Therefore, the Nb₂O₅ addition mainly affects the reaction rates between Mg and hydrogen and shows the excellent catalytic effects.     

Coupling and thermal integration of a solid oxide fuel cell with a magnesium hydride tank

A solid oxide fuel cell (SOFC) based combined heat and power (CHP) system in the power range of 1 kWe fed by pure hydrogen stored in a MgH₂ tank thermally integrated with the SOFC is presented. Different system configurations were first simulated to compare the system performances in each case. An experimental setup specially designed to test the thermal integration of a magnesium hydride tank with a 1 kWe SOFC stack is fully described. The difficulties encountered during the coupling tests are useful to understand how to solve these technical issues.     

Ultrasonic irradiation on hydrolysis of magnesium hydride to enhance hydrogen generation

This paper describes the ultrasonic irradiation on the hydrolysis of magnesium hydride to enhance hydrogen generation; the effects of the ultrasonic frequency and the sample size on the hydrogen generation were mainly examined. In the experiments, three MgH₂ particle and nanofiber samples were soaked in distilled water and ultrasonically irradiated at frequencies of 28, 45, and 100 kHz. Then, the amount of hydrogen generated was measured. We found that the low frequency of ultrasonic irradiation and the relatively small sample size accelerated the hydrolysis reaction MgH₂ + 2H₂O = Mg(OH)₂ + 2H₂ + 277 kJ. In particular, the MgH₂ nanofibers exhibited the maximum hydrogen storage capacity of 14.4 mass% at room temperature at a frequency of 28 kHz (ultrasound irradiation). The results also experimentally validated that a combination of ultrasonic irradiation and MgH₂ hydrolysis is considerably effective for efficiently generating hydrogen without heating and adding any agent.     

Performance analysis of a high-temperature magnesium hydride reactor tank with a helical coil heat exchanger for thermal storage

Metal hydrides are regarded as one of the most attractive options for thermal energy storage (TES) materials for concentrated solar thermal applications. Improved thermal performance of such systems is vitally determined by the effectiveness of heat exchange between the metal hydride and the heat transfer fluid (HTF). This paper presents a numerical study supported by experimental validation on a magnesium hydride reactor fitted with a helical coil heat exchanger for enhanced thermal performance. The model incorporates hydrogen absorption kinetics of ball-milled magnesium hydride, with titanium boride and expanded natural graphite additives obtained by Sievert's apparatus measurements and considers thermal diffusion within the reactor to the heat transfer fluid for a realistic representation of its operation. A detailed parametric analysis is carried out, and the outcomes are discussed, examining the ramifications of hydrogen supply pressure and its flow rate. The study identifies that the enhancement of thermal conductivity in magnesium hydride has an insignificant impact on current reactor performance.     

A Round Robin Test exercise on hydrogen absorption/desorption properties of a magnesium hydride based material

A Round Robin Test exercise on magnesium hydride (MgH₂) was performed by 14 laboratories with the aim to compare experimental isothermal data such PCI curves, kinetics curves and formation enthalpies together with a basic statistical evaluation of the results.     

Preparation of Cr-based multilayer coating on stainless steel as bipolar plate for PEMFCs by magnetron sputtering

In the present study, a multilayer composing of Cr₃Ni₂/Cr₂N/CrN is sputtered onto stainless steel. The potential of using the coated stainless steel as the bipolar plate for polymer electrolyte membrane fuel cell (PEMFC) is evaluated. The coated stainless steel exhibits improved corrosion resistance and higher electrical conductivity. The coated surface also demonstrates a hydrophobic characteristic. By using single cell test, the multilayer-coated SS304 plate exhibits an improved performance in terms of I-V properties.     

Thermal conductivity measurements of magnesium hydride powder beds under operating conditions for heat storage applications

One of the major issues of the change in energy politics is the storage of renewable energy in order to facilitate a continuous energy supply to the grid. An efficient way to store energy (heat) is provided by the usage of Thermochemical Energy Storage (TES) in metal hydrides. Energy is stored in dehydrogenated metal hydrides and can be released by hydrogenation for consumption. One prominent candidate for high temperature (400 °C) heat storage is magnesium hydride. It is a well-known and investigated material which shows high cycling stability over hundreds of cycles. It is an abundant material, non-toxic and easy to prepare in bigger scales. One of the major drawbacks for heat storage applications is the low heat transfer capability of packed beds of magnesium hydrides. In this work we present results of effective thermal conductivity (ETC) which were measured under hydrogen pressure up to 25 bar and temperatures up to 410 °C in order to meet the operating conditions of magnesium hydride as a thermochemical heat storage material. We could show that the effective thermal conductivity of a magnesium hydride – hydrogen system at 410 °C and 25 bar hydrogen increases by 10% from 1.0 W m⁻¹ K⁻¹ to 1.1 W m⁻¹ K⁻¹ after 18 discharging and charging cycles. In dehydrogenated magnesium hydride this increase of the thermal conductivity was found to be at 50% from 1.20 W m⁻¹ K⁻¹ to 1.80 W m⁻¹ K⁻¹ at 21 bar hydrogen. These data are very important for the design and construction of heat storage tanks based on high temperature metal hydrides in the future.     

Examination of the catalytic effect of Ni,NiCr, and NiV catalysts prepared as thin films by magnetron sputtering process in the hydrolysis of sodium borohydride

In this study, nickel, nickel-chromium alloy, and nickel-vanadium alloy were coated to form a thin film on the slides prepared by magnetron sputtering process, which were used as a catalyst for the hydrolysis of alkaline sodium borohydride. Factors, such as the temperature of the solution, amount of the catalyst, initial pH of the solution and the performance of these catalysts on hydrogen generation rate were investigated using response surface methodology. Moreover, the catalysts were characterized using XRD and FE-SEM/EDS analyses. Utilizing the obtained optimum conditions of the response surface methodology estimation, the maximum hydrogen generation rate was 35,071 mL min⁻¹ g_NiV⁻¹ from NiV catalyst at 60 °C, pH 6, and 1.75 g catalyst conditions. Under the same experiment conditions, the maximum hydrogen generation rates of Ni and NiCr catalyst systems are 28,362 mL min⁻¹ g_Ni⁻¹, and 30,608 mL min⁻¹ gNiCr⁻¹, respectively.     

Synthesis and characterization of a highly stable amorphous silicon hydride as the product of a catalytic helium–hydrogen plasma reaction

A novel highly stable surface coating SiH(1/p) which comprised high-binding-energy hydride ions was synthesized by a microwave plasma reaction of a mixture of silane, hydrogen, and helium wherein it is proposed that He⁺ served as a catalyst with atomic hydrogen to form the highly stable hydride ions. Novel silicon hydride was identified by time of flight secondary ion mass spectroscopy (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). The ToF-SIMS identified the coatings as hydride by the large SiH⁺ peak in the positive spectrum and the dominant H⁻ in the negative spectrum. XPS identified the H content of the SiH coatings as hydride ions, H⁻(1/4), H⁻(1/9), and H⁻(1/11) corresponding to peaks at 11, 43, and 55 eV, respectively. The silicon hydride surface was remarkably stable to air as shown by XPS. The highly stable amorphous silicon hydride coating may advance the production of integrated circuits and microdevices by resisting the oxygen passivation of the surface and possibly altering the dielectric constant and band gap to increase device performance.

The plasma which formed SiH(1/p) showed a number of extraordinary features. Novel emission lines with energies of q·13.6 eV where q=1,2,3,4,6,7,8,9, or 11 were previously observed by extreme ultraviolet spectroscopy recorded on microwave discharges of helium with 2% hydrogen (Int. J. Hydrogen Energy 27 (3) 301–322). These lines matched H(1/p), fractional Rydberg states of atomic hydrogen where p is an integer, formed by a resonant nonradiative energy transfer to He⁺ acting as a catalyst. The average hydrogen atom temperature of the helium–hydrogen plasma was measured to be 180–210 eV versus ≈3 eV for pure hydrogen. Using water bath calorimetry, excess power was observed from the helium–hydrogen plasma compared to control krypton plasma. For example, for an input of 8.1 W, the total plasma power of the helium–hydrogen plasma measured by water bath calorimetry was 30.0 W corresponding to 21.9 W of excess power in 3 cm³. The excess power density and energy balance were high, 7.3 W/cm³ and −2.9×10⁴ kJ/molH₂, respectively. This catalytic plasma reaction may represent a new hydrogen energy source and a new field of hydrogen chemistry.     

Scaled-up production of a promising Mg-based hydride for hydrogen storage

The feasibility of scaling up the production of a Mg-based hydride as material for solid state hydrogen storage is demonstrated in the present work. Magnesium hydride, added with a Zr–Ni alloy as catalyst, was treated in an attritor-type ball mill, suitable to process a quantity of 0.5–1 kg of material. SEM–EDS examination showed that after milling the catalyst was well distributed among the magnesium hydride crystallites. Thermodynamic and kinetic properties determined by a Sievert's type apparatus showed that the semi-industrial product kept the main properties of the material prepared at the laboratory scale. The maximum amount of stored hydrogen reached values between 5.3 and 5.6 wt% and the hydriding and dehydriding times were of the order of few minutes at about 300 °C.     

High temperature metal hydrides for energy systems Part B: Comparison between high and low temperature metal hydride reservoirs

A dynamic analysis model aimed at describing the hydrogen absorption and desorption phases of a metal hydride has been calibrated for magnesium hydride in Part A of the present work. We can make use of the estimate of the main kinetic parameters associated to this kind of hydride in order to study the behaviour of a metal hydride-based energy system.A metal hydride becomes the basis of an energy system when the enthalpy related to its hydriding/dehydriding reactions is used by an applicator. Therefore, magnesium hydride, which is a high-temperature hydride, can be virtually placed in an energy system thanks to the model and its main energy-related characteristics can be calculated. This allows us to get a glimpse of the performances of magnesium hydride in the field of energy production and to compare them to those of well-established low-temperature hydrides, such as LaNi₅ hydride.     

New horizon in mechanochemistry—high-temperature,high-pressure mechanical synthesis in a planetary ball mill—with magnesium hydride synthesis as an example

A new route of materials synthesis, namely, high-temperature, high-pressure reactive planetary ball milling (HTPRM), is presented. HTPRM allows for the mechanosynthesis of materials at fully controlled temperatures of up to 450 °C and pressures of up to 100 bar of hydrogen. As an example of this application, a successful synthesis of magnesium hydride is presented. The synthesis was performed at controlled temperatures (room temperature (RT), 100, 150, 200, 250, 300, and 325 °C) while milling in a planetary ball mill under hydrogen pressure ($>$50 bar). Very mild milling conditions (250 rpm) were applied for a total milling time of 2 h, and a milling vial with a relatively small diameter (φ = 53 mm, V = ～0.06 dm³) was used. The effect of different temperatures on the synthesis kinetics and outcome were examined. The particle morphology, phase composition, reaction yield, and particle size were measured and analysed by scanning electron microscopy, X-ray diffraction, differential scanning calorimetry (DSC) techniques. The obtained results showed that increasing the temperature of the process significantly improved the reaction rate, which suggested the great potential of this technique for the mechanochemical synthesis of materials.     

Aluminum hydride coated single-walled carbon nanotube as a hydrogen storage medium

We report a first principle study on the hydrogen storage in Aluminum hydride (AlH₃) coated (5, 5) single-walled carbon nanotube (SWCNT). Our study indicates that a SWCNT coated with Aluminum hydride (Alane – AlH₃) can bind up to four hydrogen molecules. At half coverage of AlH₃, the hydrogen storage capacity of the SWCNT is 8.3 wt%. The system with full coverage is also studied and it is found that, even though the hydrogen storage capacity increases, the binding of H₂ is weak. All the H₂ adsorption is molecular with H–H bond length of 0.756 Å. Our result on a full molecular adsorption of hydrogen via light metal hydride is new and it leads to a practically viable storage process.     

Alternative method for generating hydrogen through high-energy mechanical milling using magnesium and methanol

This paper presents a new way to generate hydrogen through mechanical milling. Hydrogen was generated “in-situ” inside a stainless steel container where methanol and magnesium were utilized. The methanol atoms due to the high-impact process were broken to obtain hydrogen. In this same process, magnesium was selectively reacted with oxygen to form the corresponding oxide and hydrogen remain in gas form. Relatively short milling times were programmed. After the programmed times, hydrogen from the stainless steel container through liquid displacement was measured. Likewise, solids were analyzed before and after hydrogen production using XRD and SEM techniques. The volume of hydrogen was a function of the programmed milling time; the average values between 160 and 310 mL were obtained when the most appropriate amounts of magnesium and methanol were used. Only magnesium oxide was formed as by-product. XRD analyses demonstrated the transformation of magnesium and methanol to generate hydrogen.