Electrochemical solution of hydrogen in palladium and dilation of electrode

The physicochemical properties of hydrogen in Pd have been studied by in-situ potentio-dilatometric measurement. A set of potential and dilation was recorded after the establishment of equilibration of hydrogen with Pd lattice. The measurement system was constructed by computer controlled potentiostat and dilatometer. The hydrogen electrode potential is converted to 'equivalent hydrogen pressure' which might signify gaseous pressure in a void or on a surface. Thermodynamic consideration showed that hydrogen atom behaves ideally as a solute, whereas it interacts with Pd lattice in the concentrate Pd-H alloys. The values of entropy and enthalpy change of solution of hydrogen are consistent with those obtained by a gas equilibrium method in a gas tight chamber. Further charge of hydrogen appeared in the structural phase change (α → β) and the concentrations of on-set and end of (α → β) transition region are well consistent with those predicted from Pd-H phase diagram. On the other hand, hydrogen in a solid matter causes lattice expansion, which resulted in the dilation of Pd electrode. The plot of the incremental increase of dilation vs. concentration shows a straight line, whose slope, the molar volume of hydrogen is consistent with that of gas equilibrium experiment. (author)