COMPARISON OF ELECTROCHEMICAL-BEHAVIOR OF THE PD-NAOD AND PD-NAOH SYSTEMS

A comparison has been made between the electrolysis of 0.1 M NaOD and that of 0.1 M NaOH with platinum as the anode and palladium as the cat hode. Steady state galvanostatic polarization of palladium in 0.1 M Na OD and 0.1 M NaOH indicates greater cathodic polarization of palladium in heavy water. Steady state permeation currents for the cathodically generated D and H through palladium membranes of thickness 0.025 mm i ndicate lower values in the case of D than in the case of H. Polarizat ion curves reveal the possibility of the stepwise formation of alpha a nd beta phases of palladium deuteride and palladium hydride. This obse rvation is more pronounced in the case of the Pd-NaOD system, as indic ated by the Tafel slope and exchange current values. The final limitin g current density corresponding to H-2 evolution is about 90 mA cm-2 a nd that of D2 evolution is 30 mA cm-2. These limiting current values i ncidentally serve as the critical current densities for the formation of the beta-(Pd deuteride) or beta-(Pd hydride) phases. Photomicrograp hs of palladium cathodes (rods 0.025 mm thick and 4 mm in diameter) af ter electrolysis in NaOD revealed cluster formation of D around Pd ato ms in the lattice, indicating the possible formation of Pd deuteride d ue to the supersaturation of the Pd lattice with deuterium. Similar ph otomicrographs for the loading of palladium with hydrogen from NaOH in dicates no cluster formation and shows only particulates of hydrogen i n palladium, widely distributed. The large cathodic polarization of pa lladium in NaOD solution, the high diffusion coefficient of D in Pd, t he low steady state permeation current of D in Pd, and the clear indic ation of cluster formation only in the case of deuterium around Pd ato ms in the lattice jointly indicate the large amount of solid state con finement of D rather than hydrogen in the Pd lattice, thereby increasi ng the probability of tunnelling of deuterium atoms. This may be the c ause for some of or all the anomalous behaviour of the Pd-NaOD system during electrolysis observed by Fleischmann et al. and by others.