Role of Lattice Strain vs. Solid Solution Doping on Atomistic near Order and Oxygen Ionic Transport for Ceria-based Micro-Energy Conversion Membranes

Abstract

Increasing the efficiencies for micro energy conversion devices based on oxygen ion conducting solid state membranes are the prerequisite toward next generations’ electrolysers and fuel cells. Here, gadolinium-doped ceria free-standing thin films are a key element i.e. for micro solid oxide fuel cells (µSOFCs). In this work, we investigate the different factors which influence ionic transport for doped ceria films with various strain states. Despite the knowledge it remains unclear what implicates more the fast oxygen ionic conductivity for ceria-based thin film membranes: How strongly does doping vs. lattice strain affect the oxygen ionic conduction and directions in free-standing film membranes for micro-energy convertors? For this, a model experiment is reported for which undoped and 20 mol%-doped ceria-based films deposited as substrate-supported and free-standing are prepared with Pt microelectrodes. We unequivocally show, that the activation energy of ionic transport is substantially increased by opposing a compressive in-plane strain on the free standing membranes (up to +Δ0.14 eV) when compared to the flat films independent on the doping level tested. The effect of lattice straining to alter the oxygen ionic transfer is more significant than the variations due to changes in the solid solution series of up to 20 mol% gadolinia in ceria for the membrane structures. Analysis of Raman shift on the F2g cationic-oxygen anionic vibration modes of pure and doped ceria confirmed a compressive strain introduced by doping. The interaction between compressive strain, dopant concentration and activation energy suggests new insights on the optimization of the micro-energy convertors.