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Preliminary study about the formation of Mg2NiH4 by SHS

Agote I.1, Vadchenko S 2, Lagos M.A 1 , Sargsyan A.R. 1.

1 Inasmet Foundation, San Sebastián, Spain; iagote@inasmet.es 2 Institute of Structural Macrokinetics and Materials Science (ISMAN), Chernogolovka, Russia ABSTRACT Metallic hydrides are a safe method for hydrogen storage compared to other hydrogen storing methods. Among different alternatives, magnesium hydrides are the most promising materials because of their lightweight, high specific storage capacity (7,6 wt% for MgH2 and 3,6 wt% for Mg2NiH4), abundant raw materials and low environmental impact. The aim of this work is to study several characteristics of the synthesis of pure Mg2NiH4 by SHS (Self-propagating High-temperature Synthesis) and some operating parameters (holding time, green density and cooling rate) are explained. In addition, absorption/desorption cycles and the effect of the pressure on the desorption process are also investigated. 1. INTRODUCTION 1.1. Hydrogen storage options Hydrogen storage [1-5] is a key enabling technology. None of the current technologies satisfy all of the hydrogen storage attributes sought by manufacturers and end users. Government-industry coordination on research and development is needed to lower costs, improve performance and develop advanced materials. Efforts should focus on improving existing commercial technologies, including compressed hydrogen gas and liquid hydrogen, and exploring new storage technologies involving advanced materials such as metal hydrides. One of the main drawbacks for the limited use of hydrogen as fuel, is the safety issue. Hydrogen is flammable when mixed with oxygen in the range from 4% to 75% of hydrogen. This fact generates an important social concern about the use of hydrogen (liquid or gas form). In this sense, it can be said that, hydrogen in the gaseous and liquid states is very combustible and the related law imposes strict regulations on its transportation and utilisation. In addition, although the storage of compressed hydrogen gas in tanks is a mature technology the design has some problems: inefficient use of the space, high weight and volume, low impact resistance and low safety. Liquid hydrogen takes up less storage volume than gas but requires cryogenic containers: the liquefaction of hydrogen is an energy-intensive process and results in large evaporative losses; about one-third of the energy content of the hydrogen is lost in the process.

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Metal hydrides offer an alterative way of overcoming this safety problem, presenting the advantages of lower pressure storage, conformable shapes, reasonable volumetric storage efficiency and safe storage (compared to gas and liquid state). 1.2. Methods to produce metallic hydrides There are several methods for obtaining metallic hydrides: The most widespread way for obtaining the hydrides of transition metals is the direct absorption of molecular hydrogen by whole mass of solid or liquid metal [6-10]. The hydrides of transition metals can also be obtained due to chemical reactions in solutions such as: reduction of transition metal compounds in the presence of metalorganic compounds, exchange reactions with other hydrides (simple or complex) and with other reducers in the medium of an aqueous or nonaqueous solvent. Promising materials, such as Mg2FeH6, Mg2CoH5, and Mg2NiH4, can be obtained by mechanical alloying. Reactants are milled during long periods of time (several days) in hydrogen atmosphere. Another innovative and cost-effective method for the synthesis of metallic hydrides is Self propagating high temperature synthesis (SHS). The SHS is based on the principle of maximum utilization of chemical energy of reacting substances (exothermicity) for obtaining inorganic compounds, materials, and items of various application purposes and also for organizing highly efficient technological processes. This technique besides obtaining binary hydrides, it also allows obtaining intermetallic hydrides, which can store greater hydrogen quantities. SHS offered good promises when applied to metal hydrogen (M-H) system [11-13]. The macroscopic characteristics of SHS [14-17] processes resemble those observed in conventional combustion processes. A variant of this scheme involves one gaseous reactant. This approach allows the synthesis of nitrides, hydrides and oxides. Main advantages of SHS for the synthesis of metallic hydrides are:

Safe way of obtaining metallic hydrides: due to the low reaction pressure required to obtain the hydrides, this methodology is much safer that current conventional manufacturing methods.

Low energy consumption. Simple technological equipment. Feasibility of production lines adaptable to production of different materials. Reduced processing time. Environmentally friendly technique.

A distinguishing feature of combustion of metal powders in hydrogen is the fact, that at small gas pressures the amount of gaseous reagent is sufficient to ensure the product formation. Low hydrogen pressures give place to the desired product. This is a great advantage, particularly from the process safety point of view, in comparison to conventional metallic

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hydride synthesis methods where high hydrogen pressures and long times are required to obtain the product. 1.3. Magnesium metal hydrides Magnesium alloys are promising storage materials because of their lightweight, high specific storage capacity (7,6 wt% for MgH2 and 3,6 wt% for Mg2NiH4), abundant raw materials and low environmental impact. Nevertheless, magnesium is inadequate to be used in hydrogen storage applications due to its high hydrogen desorption temperature and relatively slow hydrogen absorption/desorption kinetics. Storage capacity of Mg2NiH4 is smaller; however the kinetic is better and desoption temperature lower. Different efforts to substitute Mg or Ni by a third element to form ternary materials are being developed. Several methods are used for the preparation of Mg-based hydrogen storage alloys[18-19]: conventional melting, mechanical alloying , melting spinning, combustion synthesis (SHS)… It is difficult to obtain Mg2Ni by the conventional melting method because of the large difference in vapour pressure and melting point between magnesium and nickel. Re-melting processes (Mg addition) is needed; besides, the produced intermetallic can not absorb hydrogen without several cycles of absorption and desorption. Mechanical alloying usually takes a long time to prepare alloys and it is easy for the sample to be polluted and oxidized during the milling process. Moreover, products of MA can have problems of homogeneity. Combustion synthesis is an alternative process to produce high purity Mg2Ni [20], saving energy and shortening operating time due to exothermic reaction. Besides, production of Mg2NiH4, directly from magnesium and nickel powders, has been studied [21-26]. This work investigates some the Mg2NiH4 synthesis parameters, such as: thermal cycle holding time, cooling rate and green density. Absorption/desorption processes are also studied. Other interesting characteristic for future applications is the effect of the pressure on the desorption temperature. 2. EXPERIMENTAL 2.1. Raw materials The samples were prepared from commercially available magnesium and nickel powders. Purity is 99.9 wt% and particle size:

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• Reaction vessel: the reactor top has all the connections for temperature measuring

(two thermocouples) and power supply. An oven is placed inside the reactor. • Hydrogen installation: especial hydrogen installation which allows working with this

gas in the safest way possible. • Vacuum pump: the objective of this pump is to remove the oxygen from the reactor to

avoid any contact between hydrogen and oxygen. • Electrovalve • Digital Flowmeter • Control software: control software allows monitoring and storing all the relevant data

of the process: temperature (two thermocouples), gas volume flow along the time, reaction pressure during the process, gas consumption during the hydridation process.

• Computer: the control software is installed in the computer. This receives electrical signals from the electro valve, pressure gauge, flowmeter, thermocouples, and power supply device.

• Power supply device (with a changeable voltage from 3 to 12 volts). • Argon line: there is a system to supply argon to the reactor. • A safety valve • There are several high pressure manual valves to close or open the reactor inlets and

outlets.

Pressure programmed value

Meassured Pressure Value

Hydrogen Flowl

VACUUM

OUTLET

(H2)

Pressure Gauge

Sweeping Gas

(Ar)

Electro-valve

Flor-meter

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H2

valve

valve Valve

valve

ignition

ignition CONTROL

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VACUUM PUMP

REACTOR

HIGH PRESSURE LOW PRESSURE ELECTRIC SIGNALS

Figure 1: Scheme of the hydride synthesis facility.

The system is based on the flowmeter data acquisition at a constant reaction pressure: the reaction takes place at a constant pressure. This pressure is programmed on the PC and the valve controls the pressure inside the reactor (by means of the pressure gauge). Once the reaction begins, the hydrogen starts filtrating into the sample and reacting with the metallic powder. As a consequence of this, the pressure drops into the reactor. In order to maintain the pressure constant at the programmed value, the valve opens until it reaches the set-up value. As the hydrogen is passing by the pipe, the flowmeter measures the amount of gas introduced into the reactor. This way, it is possible to know the hydridation parameters (temperature of the system, instantaneous gas consumption, product stoichiometry, gas volume flow and reaction pressure), at every stage of the process.

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2.3. Experimental procedure Main steps are:

• Mixing process: powders were dry mixed in 2:1 Mg:Ni molar ratio (in some tests an excess of magnesium was introduced).

• Operating parameters: Powders were heated to 600 ºC. Reaction occurred in thermo-explosion mode afterwards the sample was cooled down to room temperature.

The phase formation study was performed using SIEMENS D-500 X ray difractometer with a cooper anode for the crystalline phase analysis. This diffractometer runs from a 2-theta angle of 10 and till an ending 2-theta of 90 degrees, at 40 Kv and 30 mA. Steps were in increments of 0.1 degrees, and counts were collected for 4 seconds at each step. Microstructure and chemical composition were analysed using light microscopy and a Jeol JSM 5910 LV Scanning Electron Microscope (SEM) with associated Oxford Inca 300 energy dispersive spectroscopy probe (EDS). 3. RESULS AND DISCUSSION 3.1. Thermograms and effect of the holding time

Figure 2: Thermogram of the Mg2NiH4 synthesis (free cooling)

In the figure 2 can be observed the thermogram for the formation of M2NiH4 by SHS. In the heating stage there is an exothermic peak. This is the formation of the intermetallic Mg2Ni (thermo-explosion regime). The reaction is completed during the holding time [21]. During the cooling down (free cooling) a plateau (isothermal zone) appears where Mg2NiH4 is formed. The duration of this is plateau is called “absorption time”. In order to explain the reaction process, several tests were carried out, the results are:

• Tests in argon atmosphere: the product is pure Mg2Ni. • Tests without holding time (hydrogen atmosphere): XRD pattern of the product is very

similar to Mg2Ni.

Mg2Ni reaction Mg2NiH4 formation

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• Tests with short holding time (hydrogen atmosphere): the product is Mg2NiH4 and Mg2Ni (XRD peaks of this component are slightly shifted).

• Tests with big holding time (hydrogen atmosphere): the obtained material is pure Mg2NiH4.

A deeper study is needed to explain the reaction mechanism with more detail. However, it seems that an increment of the holding time changes the structure of Mg2Ni. This fact allows hydrogen absorption during the cooling process.

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0 500 1000 1500 2000 2500 3000 3500

t, s

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Figure 3: Effect of the holding time

It is interesting to know the minimum holding time to produce Mg2NiH4. When the holding time is very short (see figure 4), the absorption time is very small and the hydride formation is not complete. If the holding time is longer, hydriding stage is bigger too. A holding time of one hour is enough to produce pure Mg2NiH4. In XRD patterns (figure 4), it can be observed:

• With short holding time, main phases are Mg2NiH4 and Mg2Ni. • With medium holding time: main phases are Mg2NiH4 and Mg2Ni, but the ratio

Mg2NiH4/Mg2Ni is higher. • With long holding time, only Mg2NiH4 is obtained.

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Figure 4. XRD patterns: short holding time (up), medium holding (medium) time and long holding time (down)

3.2. Effect of cooling rate

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t, s

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Figure 5: Thermogram of the Mg2NiH4 synthesis: controlled cooling (blue line) and free cooling (black line)

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A cooling rate of 5ºC/min was done in order to study the cooling process. In the figure 5 free and controlled cooling are represented. The formation of the hydride starts at the same temperature (around 300ºC) in both processes. Nevertheless, in the free cooling, the hydriding plateau is longer and finishes at lower temperature. This is an interesting characteristic, because it shows that cooling rate affects hydride formation. A minimum cooling rate is necessary in order to obtain a pure product. Besides, cooling rate could affect the formation of the different phases. Mg2NiH4 presents two phases: monoclinic and orthorhombic 3.3. Effect of the pressure on the desorption process

200210220230240250260270280290300

-2 -1 0 1

lgP (bar)

T des

orb., C

Figure 6: Desorption temperature vs pressure

The effect of the gas pressure on the desorption temperature is very important for the future applications of the hydride. As it can be seen in the figure, this effect is relevant. Temperature desorption can vary between 200 ºC (vacuum) and 300 ºC (pressure 10 bar). 3.4. Effect of several absorption/desorption cycles 3.4.1. Discontinuous cycles These cycles were done with the material synthesized according to section 3.1. Later, hydrogen is released in vacuum. Each cycle was carried out with the product of the previous one. After several absorption/desorption cycles, absorption time increases. In the first cycle, it is possible that during the first cycles not all the material is hydrogenated and some cycles are needed to reach the maximum hydrogen storage capacity. In the next picture, this effect can be observed for 4 cycles. tabs represents the duration of the absorption plateau (see figure 2) and n is the number of cycles (operating conditions of all the cycles were the same).

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160170180190200210220230240250

1 2 3 4

n

t abs,

s

Figure 7: Absorption time vs number of cycles

Figures 8.A and 8.B show scanning electron microscopy (SEM) images of Mg2NiH4 samples synthesized with one (A) and several (B) absorption/desorption cycles. The surfaces of the samples are smooth. It is possible that there is a liquid phase in the combustion synthesis of this material. .

A) B) Figure 8. A) Mg2NiH4 after one cycle B) Mg2NiH4 after several cycles It can be observed that after several cycles, particle size is smaller. Absorption/desorption processes can break the particles because; when hydrogen is released can generate fissures in the grains, leading to a particle size reduction.

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3.4.2. Continuous cycles (constant pressure)

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Figure 9: Several absorption cycles

In the figure 9, several absorption/desorption cycles can be observed. Starting material was Mg2NiH4 and the pressure of the process is constant. The blue line is the hydrogen flow absorbed by the material. In cycles with constant pressure, absorption and desorption curves are very similar. Absorption temperature decreases very slightly. Therefore, it seems that the absorption/desorption process remains constant in cycles carried out under constant pressure. 3.5. Effect of magnesium addition In first trials, the composition selected was Mg:Ni 2:1. In XRD patterns (see figure below) several phases were identified. The main phase was Mg2NiH4, but also MgNi2 was found. The reason could be that a small amount of magnesium is lost during the thermal cycle. In order to solve this problem, an excess of magnesium was used. The XRD patter shows that the product is pure Mg2NiH4. The material could be discriminated into two paterns of crystal structure, Orthorhombic and Monoclinic.

Hydrogen absorption

Hydrogen desorption

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Figure 10. XRD patterns A) Sample without Mg excess B) Sample with Mg excess

3.6. Effect of the green density Green density is another parameter that was be optimized. In order to study the effect of this parameter, different tests were carried ou. In one of them, powder was pressed in a cylindrical mould and the resultant green density was around 65%, the other sample was loose powder. XRD patterns of the two tests show that in the case of the loose powder, the product is pure Mg2NiH4, while in the dense sample, Mg2Ni is also present. Besides, as it can be observed in the figure 10, the product grain size is smaller when the reagents are loose powder.

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There are several possibilities to explain this fact: worse hydrogen diffusion, green density affects reaction temperature…, but it is necessary a deeper study.

Figure 11 . A) Dense sample B) Free powder

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4. CONCLUSIONS Main conclusions of this work are:

• Pure Mg2NiH4 can be obtained by SHS (Self-propagating High-temperature Synthesis).

• Process thermogram shows that Mg2Ni is formed around 600ºC and the hydride is formed during the cooling process. A holding time is necessary to obtain Mg2NiH4.

• Cooling rate modifies hydrogen absorption. The formation of different phases of Mg2NiH4 depends on the cooling rate. A more complete study could be interesting.

• After several cycles, grain size of the product decreases. Absorption/desorption processes can generate fissures in the particles.

• Pressure have a strong effect on the desorption temperature. Between 1 and 10 bar, the temperature changes 50 ºC.

• An excess of of Mg is needed to obtain pure Mg2NiH4. • When green density is higher, the product is not pure. The results with loose powder

are better. Further work is being carried out in order to improve the knowledge about the synthesis of Mg2NiH4 by SHS.

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