Proceedings of the International Workshop "Advanced Techniques for Energy Sources Investigation and Testing" 4 – 9 Sept. 2004, Sofia, Bulgaria TECHNIQUES FOR CHARACTERISATION OF HYDROGEN ABSORBTION/DESORBTION IN METAL HYDRIDE ALLOYS S. Bliznakov 1* , E. Lefterova 1 , L. Bozukov 2 , A. Popov 1 , P. Andreev 1 1 Institute for Electrochemistry and Energy Systems, Akad. G. Bonchev str., bl. 10, 1113 Sofia, Bulgaria 2 LABTech Int. Ltd. alloys Research, Tzarigradsko Shosse Blvd., bl.25, ent. A, Mladost-1, Sofia, BG-1784, Bulgaria *Corresponding autor:sbliznakov@mail.bg Abstract In this work two techniques for pressure-composition-temperature (PCT) isotherms measurements are utilised at different constant temperatures followed by an evaluation of the hydrogen desorption enthalpy and entropy derived from the van’t Hoff’s plots. The PCT isotherms from the gas phase are measured with an AB 5 type metal hydride alloy with MmNi 4 Co 0.6 Al 0.4 composition (Mm denotes mischmetal). Furthermore, the electrochemical PCT isotherms are measured with electrodes prepared from the same type of alloy with MmNi 3.6 Co 0.6 Al 0.8 composition in alkaline solution. Both techniques are proper for characterisation of hydrogen absorption/desorption on metal hydride alloys. 1. Introduction For application purposes in batteries and battery-related fields hydrogen storage alloys must be characterised by a high hydrogen capacity and moderate hydride stability as well as by a nearly constant equilibrium pressure during the solid phase (MH α to MH β ) conversion and a low sorption-desorption hysteresis. The information about these properties is generally inferred from PCT isotherms describing the dependence of the hydrogen equilibrium pressure to the amount of hydrogen dissolved and/or incorporated into the solid phase at different constant temperatures [1]. Furthermore, the PCT isotherms are used to reveal various hydride phases of alloy compounds. The enthalpy and the entropy of the hydride formation/decomposition process can be obtained from the variation of the logarithm of the equilibrium pressure with temperature in the van’t Hoff plots [2, 3, 4]. The enthalpy (∆H) of hydride formation is an important parameter characterising the alloy as a proper hydrogen absorber for various applications. Kleperis et al. [1] have stated that if the ∆H value ranges between - 25 kJ mol -1 and - 50 kJ mol -1 , the alloy is suitable for battery applications. Hong [5] has postulated that the heat of alloy hydride formation should be between -15 kJ mol -1 and - 40 kJ mol -1 . When it is lower than - 15 kJ mol -1 , the alloy hydride is not stable enough for charging the MH electrode at room temperature. On the other hand, the alloy hydride is too stable for room-temperature discharge when the ∆H value exceeds - 50 kJ mol -1 . In general, the difference in standard entropy (∆S) between a metal hydride alloy and its hydride is minute and is on the order of 10 J mol -1 K -1 . The change in entropy with hydride formation is primarily provided by the loss of the standard entropy of hydrogen gas (130.858 J mol -1 K -1 at 298K), which means that ∆S can be assumed to be a constant and does not depend on the nature of the metal hydride alloy [3]. The thermodynamic characterisation of the hydrogen absorption/desorption process may be determined by: P19-1