MEASUREMENT OF THE ABSORPTION-BASED HYDROGEN STORAGE CAPACITY OF Ti0.95Zr0.05Cr0.8Mn0.8V0.2Ni0.2 ALLOY

The article describes potential application of the BCC alloy Ti0.95Zr0.05Cr0.8Mn0.8V0.2Ni0.2 in the hydrogen absorption process. It deals with the measurements of the PCI (Pressure Concentration Isotherm) characteristics applying the volumetric method. With regard to the fact that it is an indirect measurement method, the thermodynamic description was analysed in details and subsequently used to identify the weight capacity of the stored hydrogen. The article presents a detailed description of the procedure for measuring the free volume of the measuring equipment using helium which is able to penetrate into pores and apertures with the size of only a few nanometres but does not participate in the absorption into the intermetallic structure of the alloy. The hydrogen absorption into the tested alloy was initiated by repeatedly performed activation of the alloy surface. The final section of the article presents the examination of the cyclic stability of the absorption-desorption cycle within the repeated monitoring of the maximum weight capacity of the stored hydrogen.


INTRODUCTION
The complete implementation of the use of hydrogen as an energy carrier requires efficient, safe, and cheap storage thereof [1][2][3]. Compared to gaseous and liquid storage systems, storage of hydrogen in metal hydrides (MH) exhibits several advantages [4][5][6][7][8]. The most important ones include mainly high volumetric density and low operating pressure; this represents significant improvement of the safety of applications thereof in the transportation segment, in various industries, or in households [9,10]. The development of novel MH materials requires the identification of their PCI (Pressure Concentration Isotherms) curves indicating the changes in the pressure depending on hydrogen concentration at a constant temperature [11][12][13]. The PCI curve shows the hydrogen absorption at the equilibrium pressure until the alloy is fully saturated, as indicated by a sharp pressure increase at the maximum concentration. The kinetics of alloy activation and storage capacity are also affected by the chemical composition of the alloy [14][15][16]. The impact of elementary composition on the hydrogenation characteristics of the BCC alloys was subjected to multiple investigations. As for the Ti0.95Zr0.05Cr0.8Mn0.8V0.2Ni0.2 alloy, the addition of Zr and Ni has a significant effect on the activation process. Without these components, the activation was required, at the temperatures of 300 to 750 °C. After the addition of the above mentioned elements, there is no need for the thermal activation of the alloy, unless it was exposed to oxygen; therefore, it may be assumed that the hydride surface will not be covered with the layer of oxides. The alloy with a similar composition was also investigated by Yu et al. [17].

DESCRIPTION OF THE SPECIMEN PREPARATION AND THE MEASURING SYSTEM ASSEMBLY
For the purpose of the PCI measurements, 6.177 g of the alloy were produced applying the arc welding method. In order to ensure higher homogeneity of the specimen, it was subjected to three cycles of melting and cooling to the ambient temperature. The resulting powder product was ground in the inert argon atmosphere and subsequently subjected to the X-ray diffraction. The results of the final chemical composition of the specimen are listed in Table 1. The measurements of the desorption PCI curve were carried out applying the volumetric method based on the principle of the gradual release of hydrogen into a volumetric cylinder [18][19][20]. After releasing a small amount of hydrogen, it was necessary to stabilise the pressure in the storage system. Subsequently, the amount of the released hydrogen was monitored and calculated for normal conditions with concurrent monitoring of the pressure in the hermetically sealed equipment and the hydrogen temperature. The scheme of the measuring apparatus is shown in Fig. 1.   Fig. 1 Scheme of the measurement apparatus A substantial part of the system is a hermetically sealed chamber containing the alloy specimen; the chamber is located upstream the V4 valve. The specimen was immersed in the water bath where it was possible to measure and regulate the temperature within the range from 0 to 60 °C. The system facilitated independent supplies of helium and hydrogen at the maximum pressure of 7 MPa. Gases were removed from the system using the Edwards E2M1.5 two-stage oil vacuum pump. The pressure measurements were carried out using the BD SENSORS DX9-DMP 331i pressure sensor with the pressure range from 0 to 6 MPa. The temperature was measured using the ALMEMO FPA32P1 temperature sensor. The sensors were attached to the ALMEMO 2890-9 datalogger by the ALMEMO ZA 9020-FS connector. The assembled apparatus contained the BOSS-ML6-05 spherical valves. The measured data were indirectly used to calculate the weight capacity of the stored hydrogen; it was necessary to apply the mathematical procedure described below.
The amount of hydrogen absorbed in the alloy specimen was identified using the following formula: abs,n, tot, FV,n, VMS,n, During the measurements of different specimens, the free volume of the hermetically sealed system VFV was changing due to changes in the porosity and the amount of the powder alloy. It was therefore necessary to identify this parameter prior to the measurement of the next specimen. For this purpose, the secondary gas, helium, was used prior to hydrogen absorption. The system was vacuumed, using a vacuum pump, to the absolute pressure of 5 Pa and then filled with helium with which absorption into the alloy structure does not occur; this facilitated measuring the amount of the gas released into the volumetric measurement system in order to identify the free volume between the grains and use such data to make a simple calculation of the missing parameter VFV. The calculation was based on the differential form of the equation of state; a pressure decrease in the hermetic chamber was adequate to the amount of the released hydrogen: By subsequent solving of the equation, it was possible to calculate the ratio of the free volume and the average temperature; the ratio was identical to that observed during the measurements with hydrogen and helium (while maintaining the same thermal field of the tank). Therefore, it was possible to substitute the ratio of the free volume to the average temperature directly to formula (2.) obtaining the following formula:

SPECIMEN ACTIVATION
The specimen of the Ti0.95Zr0.05Cr0.8Mn0.8V0.2Ni0.2 alloy was activated in order to eliminate the oxidation layer which might generate during the production of and manipulation with the specimen. The specimen was exposed to three cycles of heating and cooling at the temperatures from 3 to 100 °C and at the pressures from 0 to 4.3 MPa. The specimen was first heated to 100 °C at the absolute pressure below 5 Pa and then left in these conditions for the period of 30 minutes. Subsequently, the pressure tank of the hermetically sealed system was cooled to the temperature of 3 °C and the system was filled with hydrogen to the pressure of 4.3 MPa and then left in these conditions for the period of 5 hours. During hydrogen absorption, the pressure in the chamber slowly decreased (Fig. 2); this facilitated the calculation of the increase in the mass concentration of the stored hydrogen over time.

Fig. 2 Pressure decrease in the tank during the first activation
An increase in the mass concentration may be defined as the ratio of the change in the mass concentration of the stored hydrogen per time unit in the initial linear area of the pressure decrease (between 4,000 and 8,000 s) using the following formula: where: The second activation cycle was carried out in identical conditions, but the kinetics of the process of the initial absorption was as much as 117 times faster (Ψ2 = 5.8•10 -5 s -1 ). The curve of the pressure in the hermetic chamber during the second activation of the alloy is shown in Fig. 3. During the third activation, the value of the increase in the mass concentration Ψ3 was 1.66•10 -5 s -1 .

DISCUSSION
The measurements of the decrease in the pressure in the hermetically sealed measurement chamber during the activation indicate that the initiation of hydrogen absorption into the alloy structure requires at least three consecutive cycles of the activation process. This will initiate the process of hydrogen storage with acceptable process kinetics. Following the alloy activation and the measurement of the free volume of the hermetically sealed system, the system was vacuumed using a vacuum pump; this resulted in an increase in hydrogen pressure to 5 MPa. Through the gradual release of hydrogen into the volumetric measurement system and a subsequent calculation, as described in the mathematical apparatus, the PCI characteristics of the alloy were identified (Fig.  4). The relevant curve indicates that the plateau pressure for the tested alloy was 1.4 MPa and after the peak mass concentration of 0.52 % was achieved, the pressure sharply increased. This was caused by the alloy saturation and further supplies of hydrogen into the storage system caused the gas accumulation in the free volume VFV. In order to confirm the measurement accuracy and determine the cyclic stability, the measurements of the PCI characteristics were repeated 6 times while maintaining the same thermodynamic conditions. Fig. 5 shows the curve of the peak mass concentrations of the stored hydrogen at individual measurements.

Fig. 5
Curve of the peak weight capacity of the alloy under the periodic load A stable value of the mass concentration, i.e., 0.25 wt. %, was reached after the third absorption-desorption cycle. From cycle 4 onwards, the alloy appeared to be stable for further hydrogen storage.

CONCLUSION
Certain disadvantages of the pressure and cryogenic storage of hydrogen urge the necessity to search for alternative methods of storing this gas [21]. An option that seems to be appropriate is the use of metal hydride alloys which facilitate the storage of hydrogen in crystal lattices of metals. In the case of the tested alloy Ti0.95Zr0.05Cr0.8Mn0.8V0.2Ni0.2, hydrogen was stored in the cubic lattice. This alloy may be used for hydrogen storage, but the use thereof requires multiple activation of its surface. The measurements of the PCI characteristics were made applying the volumetric method which is, compared to the high-pressure thermogravimetric analysis, more cost-effective, provided that the obtained results are comparable. The problem associated with the practical application of the tested alloy in hydrogen storage is insufficient cyclic stability with a 50 % decrease in the maximum value of the mass concentration after three consequent storage processes. After the mass concentration stabilises, its value is only 0.25 wt. %, representing 27.8 litres of the stored hydrogen (in normal conditions: 101,325 Pa; 273.15 K) in 1 kg of the alloy. The measurements of the PCI characteristics clearly show that this alloy exhibits low absorption pressure of 1.4 MPa; it is therefore suitable for the direct absorption of hydrogen produced through the electrolysis of water in high-pressure electrolysers which achieve the pressures of up to 2 MPa. This eliminates the necessity of using a compressor.