Hydrogen intercalation within transition metal oxides: entropy, enthalpy, and charge transfer

Abstract

Intercalation isotherms of hydrogen within transition metal oxides, through H spillover, are quantitatively accounted for by a statistical model where the average enthalpy is the weighted sum of the energy of occupied, prevented, and vacant sites of an idealized network of interstitial sites ({open_quotes}interstitial lattice{close_quotes}) and where the entropy contains two contributions. The first corresponds to the number of configurations of occupied, prevented, and vacant sites within the interstitial lattice, whereas the second contribution contains the nonconfigurational contributions, including a charge-transfer term. This term can be approximated from the volume of the intercalated hydrogen. The charge transfer consists in the donation of H 1s electron to the oxide lattice. The enthalpy of formation, obtained from calorimetric measurements, correlates with the charge transfer as well. Filling the interstitial lattice predicts nonstoichiometric H contents, as observed experimentally. The observed stepwise intercalation isotherms can be fitted rather nicely using the calculated energy and entropy terms. It is also shown that the calculated intercalation free energy predicts nicely the observed order of stability. In the appendix it is suggested that this treatment can be extended to transition metal hydrides. 32 refs., 6 figs., 3 tabs.