New Insights on the Formation Process and Thermodynamics of the α-Phase PdH(D)x through Direct Enthalpy Measurement of H(D) Dissolution

The palladium (Pd)-hydrogen (H) system is one of the most famous hydrogen-storage systems. However, the formation process and thermodynamics at the initial stage of hydrogen storage in Pd remain unclear. Here we report the formation process and thermodynamics of the alpha-phase PdH(D)(x) (x < 0.03) for the first time through direct measurements of the enthalpy of H and D dissolution, -Delta H-sol, at around 303 K, with an accurate adiabatic calorimeter under precisely controllable H-2(D-2) gas pressure. This is the first observation that three steps are involved in the H(D) dissolution in the alpha-phase PdH(D)(x): the absorption of H(D) into trapping sites, the formation of the alpha phase, and the beta-phase nucleation. The determined values of -Delta H-sol were significantly larger than the indirectly estimated previous values. With the accuracy and the wide applicability of our methodology, these new insights may pave the way for future developments in hydrogen chemistry.