The hydrogen transport through the Pd/$TiO_2$ film bilayer has been investigated in 0.1M NaOH as a function of applied potential by using hydrogen permeation and AC-impedance methods. The $TiO_2$ films were deposited on Pd by using Plasma Enhanced Chemical Vapor Deposition (PECVD) method. $N_2O$, titanium isopropoxide and Ar gas were used as an oxygen source, a titanium source and a carrier gas, respectively. The deposition temperature and deposition time amounted to 250℃ and 1800 s, respectively. The permeation experiments were carried out on the Pd/$TiO_2$ and $TiO_2$/Pd bilayer systems. Cathodic charging current and potentials into Pd and $TiO_2$ sides respectively amounted to 80 μA/㎠ and -750 to -900 $mV_{SCE}$. Anodic potentials of -300 to +300 $mV_{SCE}$and 50 $mV_{SCE}$ were applied to $TiO_2$ and Pd sides respectively. AC-impedance measurements were made on the both systems without and with concurrent hydrogen permeation. The impedance values were determined during the permeation after the permeation transient attained a steady state. Time lag determined from the permeation experiments was considerably reduced by applied potential and film resistance from the AC-impedance was diminished by hydrogen charging. These results suggested that hydrogen in the $TiO_2$ film exists as a proton and its transport is greatly influenced by the electric field strength. Warburg impedance appeared in low frequency range due to diffusional mass transfer of hydrogen through the $TiO_2$ film. The diffusivities of proton in the $TiO_2$ film were determined to be an order of $10^{-14}cm^2s and $10^{-11}cm^2/s, respectively, from the time lag equation derived from the Pd/$TiO_2$ system and the Warburg impedance equation derived from the $TiO_2$/Pd bilayer system. Large difference in diffusivity determined from the two methods indicated the occurrence of hydrogen trapping in the $TiO_2$ film.
