Electrochemical Investigation of the O-2(g),Ni/YSZ System Using Cyclic Voltammetry

Cyclic voltammetry was used for the electrochemical study of a Ni thin film (similar to 880 nm) deposited on an yttria-stabilized zirconia (YSZ) solid electrolyte pellet at temperatures between 350 and 450 degrees C at 20 kPa O-2 and atmospheric pressure. NiO formed electrochemically by the O2- species supplied from the electrolyte upon anodic polarization, while it grew according to the parabolic growth law, in agreement with Wagner's oxidation theory of metals at high temperatures. A model for the NiO formation was proposed, where NiO is formed at the Ni/YSZ interface and grows by the outward diffusion of Ni2+ species through NiO, which is determined as the rate-limiting step. This implies the autoinhibition of NiO formation by its continuous growth. The above result agrees with Wagner's theory; however, a difference between the latter and the current model is the means of supplying oxygen for NiO formation. Also, O-2 evolution reaction is not firmly affected by NiO formation because it is controlled by the electronic current. An apparent activation energy of the limiting Ni2+ diffusion step was calculated to be 35 kJ/mol under 400 mV and decreased with potential increase.