Pergamon ht. J. Hydrogen Energy, Vol. 20, No. 6, pp. 441447, 1995 Copyright 0 1995International Association for Hydrogen Energy Elsevier Science Ltd OS3199(94)ooo54-9 Printed in Great Britain. Ali rights reserved 036u-3199/95 $9.50+ 0.00 HYDROGEN ABSORPTION-DESORPTION STUDIES WITH HYDROGEN STORAGE UNIT ST-9@ CONTAINING THE ALLOY MmNi,.,Al,~, S. DEVANATHAN, B. LAKSHMI BRINDA, K. S. DHATHATHREYAN, S. PARTHASARATHY* and V. SRINIVASANT Centre for Electrochemical & Energy Research, SPIC Science Foundation, 110 Mount Road, Guindy, Madras 600 032, India (Received for publication 9 May 1994) Abstract-Hydrogen absorptiondesorption studies werecarried out with the ST-90 unit containing u 18.6 kg of thealloy MmNi,.,Al,., in thetemperature range 303-343K. Two plateaux were observed in theabsorptiondesorption branches of the isotherms except at 343K.In addition to the above conventional methods, “alternative” experiments of absorption in the temperature range 303-323K followedby desorption at 343K showinflection at the same H/M values corresponding to the second plateauat lower temperature. INTRODUCTION Intermetallic alloys capable of absorbing and desorbing hydrogen reversibly are gaining importance in recent years.Among the AB, type alloys, absorption-desorption isotherms over a wide range of temperatures and press- ures have been reported for the alloy LaNi, [1,2]. Alloys with mischmetal (Mm) in place of lanthanum have been developed to reduce the cost of the alloy. However, the MmNi, alloy has a high plateau pressure compared with LaNi, [3]. The plateau pressure could be reduced by introducing iron or aluminium in the nickel site in MmNi, [4]. Unlike the case of LaNi,, data for MmNi,, MmNi, -x Al, and MmNi, -xFe, are scarce[3-lo]. Even with the available literature only desorption P-C-T curves [ll], for small quantities of alloy are dealt with. Absorptiondesorption isotherms and hysteresis phe- nomena play a crucial factor for a successful deployment of the alloy system in various applications. In such an event it would be prudent to have the data derived from a large scale unit and a comparison made with data from a limited quantity of alloy. In continuation of our work on the hydrogen storage unit ST-90R, we present here the absorption-desorption behaviour, hysteresis phenomena and also report results derived from “alternative” experi- ments. EXPERIMENTAL Material The ST-90Runit (hereinafter referred to as “the unit”) containing MmNi,,,Al,., alloy (18.62 kg) was supplied *To whom all communications should be addressed. t Formerly of Indian Institnte of Technology, Madras, India. by M/s Ergenics Inc., U.S.A. The description of the ST-90 unit and the system used for carrying out absorption- desorption studies have been described elsewhere [12]. Method The absorption&sorption studies were carried out in the temperature range 303-343K. The absorption experi- ments were carried out in the following manner-the unit was evacuated at 343K for 1 h, after which it was cooled down to the required temperature. The charging pressure from the cylinder was fixed at 30 atm. A massflow meter (Matheson, U.S.A., model no. 8170)was used to measure the rate of flow of gas. An X-Y recorder (Yokogawa, Japan, model-3023) was connected to the massflow meter to record the gas flow profile, from which the volume of the gas was calculated. Typically, the hydrogen gas was passedfor a minute and the volume noted from the gas flow profile in the X-Y recorder. Pressure was noted (using a pressuretransducer) after equilibration for 5 min. The procedure was repeated until the maximum pressure in the unit was reached. During desorption, the discharge pressure was fixed at 1 kg cm-‘. The gas was desorbed for 1 min through the mass flow meter. The unit was equilibrated for 5 min and then the pressure was noted from the pressuretransducer. Desorption was carried out at regular intervals in the manner described. The volume was calculated from the flow profile recorded in the X-Y recorder. In addition to these classical methods, experi- ments were also carried out by an “alternative” method. In this method, the unit was charged to a particular pressure at a definite temperature. This was followed by heating it to a higher temperature with the valves closed. After equilibration for 5 min the gas was desorbed isothermally. This method is of interest as it simulates the reaction route which is realised during hydrogen 441