The present study is focused on the extension of electrochemically active sites in oxygen electrodes for solid oxide fuel cells (SOFC) or electrolyser cells (SOEC), at the same time as preventing degradation, by the introduction of thin mixed ionic electronic conductive buffer layer materials between electrode and electrolyte. The performance of a Nd2NiO4 electrode material with YSZ electrolyte was studied with a mixed conducting, Ce0.8Tb0.2O2-delta, buffer layer material and compared to that of a more typical approach using a predominately ionic conducting buffer layer, Ce0.8Gd0.2O2-delta. Each buffer layer and oxygen electrode were coated on YSZ electrolytes by spin-coating. The chemical reactivity of oxygen electrode, buffer layer and electrolyte sintered powders were analysed by the X-ray diffraction technique. Scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy analysis (EDS) revealed that spin coated layers have good adhesion, are continuous and offer very good chemical compatibility. Impedance spectroscopy under a range of applied DC bias was used to analyze the contribution of the buffer layer materials under SOFC and SOEC operational modes. The cell with the buffer layer that offers mixed conduction significantly reduces the total electrode polarization resistance across the whole of the studied temperature range (600-850 degrees C) by around an order of magnitude when compared to an otherwise identical cell without the buffer layer. In comparison, only half of this performance increase can be obtained for a predominantly ionic buffer layer. The critical nature of mixed conductivity in the buffer layer to maximize performance is further reinforced by comparison of current results to the literature performance of another mixed conducting buffer layer, Ce0.8Pr0.2O2-delta. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.