We investigated the effect of Co atoms, doped into ZnO lattice, on ionic conductivity in the internal grains or through the grain boundaries. Influence of Co amount on the conductivity was associated with enhanced activation energies of the ionic conductivity through the grain boundaries. The change in activation energy implies that the mechanism of ionic conduction through the boundaries can be modified by the Co amount in the lattice. Three conductance mechanisms were identified by the Cole Cole plots in order to understand the relaxation mechanism and activation energies of ionic transportations. Newly formed activation energy, 395 meV, by increasing Co amount up to 10 mol% was attributed to the ionic conductivity through the enhanced (or increased) (Zn,Co)O/ZnO) interface at the grain boundaries. In addition, the activation energy was also enhanced by the electronic stability at high temperatures due to decrease in electronic conductivity in the Co-doped ZnO compared to that in undoped ZnO. (C) 2017 Elsevier Ltd. All rights reserved.