1 EXPERIMENTAL TECHNIQUES TO MEASURE OF THE EQUILIBRIUM PLATEAU PRESSURES OF METAL HYDRIDES ANDREAS BORGSCHULTE, SHUNSUKE KATO, MICHAEL BIELMANN, ANDREAS ZÜTTEL EMPA, Materials Science and Technology, Laboratory 138, Hydrogen & Energy, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland Various experimental techniques to measure the equilibrium pressures of metal hydrides are reviewed, that is gravimetric, volumetric pcT measurements, hydrogenography and resistance measurements on thin films. The agreement of the data for the archetypical example MgH 2 is very good. Differences are explained by kinetic effects of the sorption process. 1. Introduction The standard approach for the search of new hydrogen- storage materials is to synthesize bulk samples and to use gravimetric [1,2] or volumetric [1,3] techniques to follow their hydrogenation reaction and to record pressure–concentration isotherms (pcT). The equilibrium pressure of the metal-to-hydride transition is determined from the plateau of the pressure composition isotherms. The enthalpy of hydride formation is extracted from the temperature dependence of the equilibrium pressure, by means of the Van ‘t Hoff relation: R S RT H p p eq Δ − Δ = ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ 0 ln , (1) where ΔH is the enthalpy of formation in J/(mol H 2 ), ΔS is the entropy of formation in J/(K mol H 2 ) at standard pressure, R the gas constant, T the absolute temperature, p 0 = 1.013 bar the standard pressure, and p eq the H 2 equilibrium plateau pressure of the p–c isotherm. The technically challenging problem is the measurement of such equilibrium isotherms. Using gravimetric techniques, the mass of the sample is a measure of the exchanged hydrogen. ‘Equilibrium’ is practically defined, i.e. when the sample mass does not change anymore after applying a particular hydrogen pressure to the sample. Due to the slow kinetics of most systems, such measurements can take days to weeks. Accordingly, most frequently used techniques are scanning methods, in which one thermodynamic parameter (either pressure or temperature) is linearly varied and the response of the sample is recorded. As these measurements are by definition dynamic measurements, the equilibrium values have to be extrapolated. On the other hand, the scanning method allows the use of other physical properties than the direct measurement of the hydrogen content in the sample, e.g. the pressure change in the sample container accompanied with the amount of hydrogen applied (volumetric methods), the heat flux into the sample by high-pressure DSC, [4], or any other measurable change connected to hydrogen in the sample. The fact that hydrogen absorption in a metal leads to large optical changes is the basis of a new combinatorial method ‘hydrogenography’[5]. Hydrogenography provides a high-throughput method to measure optically pcTs and determine the enthalpy of hydride formation. Similarly, the electric conductivity can be used to probe the electronic changes accompanied with hydrogen absorption [6]. In this paper, we will compare the various methods and discuss their particular applicability. 2. Methods In what follows is a short description of the physical principles of the measurement of thermodynamic properties of metal hydrides. The interested reader is referred to literature references to obtain more information. 2.1. Gravimetry For pressure composition isotherms one measures the concentration of hydrogen inside the sample by measuring its weight as a function of the hydrogen gas pressure around the sample. [1,2] The procedure is as follows. First, the reaction chamber is evacuated. Then hydrogen gas is added to reach a pre-set pressure and maintain it until the concentration inside the sample has