Characterization of hydrogen storage materials by means of pressure concentration isotherms based on the mass flow method

The determination of the equilibrium thermodynamic parameters of hydrogen storage materials from quasiequilibrium pressure data using the mass flow pressure concentration isotherm (pcI) method is presented. The method bases on the acquisition of pcI curves at different flow rates using a thermal mass flow controller to determine the amount of ad/desorbed hydrogen. These measurements provide a set of corresponding quasiequilibrium pressure functions from, which the true equilibrium pressure of the hydride is calculated by extrapolation to zero flow. The governing thermodynamic parameters can then be determined to characterize the material by the construction of a van't Hoff plot, extracting enthalpy of reaction Delta H(r) and entropy of reaction Delta S(r) from the equilibrium pressure p(eq) as a function of temperature. Naturally, true equilibrium can never be reached and therefore can only be approximated by measurement-a drawback that all experimental techniques share. This complication is alleviated by the flow-pcI approach at different flow rates. The compilation of the p(eq)(T) data from pcI-measurements can be performed by different methods, whereas the so called Sieverts apparatus is most commonly used. In this paper, we elaborate the differences and advantages of the mass flow-pcI over the Sieverts Apparatus and present measurements and results on LaNi(5) as a benchmark. Measurements at different flow rates are presented and equilibrium pressures at zero flow are achieved by extrapolation. The obtained results of Delta H(d)=32.5 kJ mol(-1) H(2) and Delta S(d)= 115 J K(-1) mol(-1) H(2) (desorption process) perfectly match literature values, emphasizing the excellent quality of the measurements and the performance of this measurement apparatus. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3186731]