Investigation of the Cycling Stability of Room-Temperature Metal Hydrides (MHs)

Metal hydride (MH) hydrogen storage is a highly promising method, especially under room temperature conditions[1]. Compared to other hydrogen storage methods, no significant energy input for heating, cooling, or achieving high pressure is needed [2]. The process of hydrogen sorption in the MH is an exothermic reaction, which involves heat release, and the desorption process is endothermic and heat energy is consumed. For RT MHs this energy is emitted to environment by charging and taken from environment by discharging. Therefore, the reaction rate of these processes depends on the temperature conditions in which the charging- and discharging cycles are carried out. The cyclic stability is one of the most important properties for the evaluation and characterization of MH storage tanks. It is the ability to absorb and release hydrogen over a large number of charge- and discharge cycles without a loss in capacity. In this work, a method is developed to analyze the cyclic stability of low-temperature MHs. Accelerated stress cycling (ASC) is performed by controlling the charge- and discharge temperature of the MH storage, for this purpose a test rig was set up (Fig.1). The object of the study is a 20 nL TiFe Hydrogen Components, Inc. BL-18 MH storage tank . Thermal parameters of the charging and discharging process are measured by a thermocouple and a thermal imaging camera. The charge- and discharge rate, volumetric values as well as the flow rate (mL/min) are determined by a mass flow meter (max. = 1000 mL/min). The capacity values were additionally determined by gravimetric measurements. The desired temperature range was achieved using water circulators. In the process of experiments, it was found that by controlling and maintaining the targeted temperature range of sorption- and desorption, both the charging and discharging cycles are significantly accelerated. The increase in the flow rate in the first 15-20 minutes of the cycle allows the development of a set of ASC protocols. After the flow rate investigation of temperature-controlled cycles, it was decided to limit the ASCs with a lower flow rate of 100mL/min.
By using temperature-control at 100mL/min flow rate, the volume reaches a value of about 80% of full capacity. The usage of temperature-control decreases charging cycle time by 80% and discharging cycle time by 90%. An overall 8-fold decrease of cycle-time could be observed compared to RT.

ACKNOWLEDGMENT
Financial support by the Austrian Research Promotion Agency (FFG) through the 8th Call of “COMET-Project” is gratefully acknowledged.
REFERENCES
[1]. Modi Poojan, Aguey-Zinsou Kondo-Francois, Room Temperature Metal Hydrides for Stationary and Heat Storage Applications: A Review, Front. Energy Res., 09 April 2021 Sec. Hydrogen Storage and Production, ISSN=2296-598X, https://doi.org/10.3389/fenrg.2021.616115
[2]. I.A. Hassan, Haitham S. Ramadan, Mohamed A. Saleh, Daniel Hissel, Hydrogen storage technologies for stationary and mobile applications: Review, analysis and perspectives, Renewable and Sustainable Energy Reviews,Volume 149, 2021, 111311, ISSN 1364-0321, https://doi.org/10.1016/j.rser.2021.111311