Photo-Enhanced Grain Boundary Ionic Conductivity in Gadolinium Doped Ceria

Abstract

Oxide-based ionic and mixed ionic-electronic conductors (MIEC) play strategic roles in applications ranging from energy conversion and storage to thin film nano-ionic devices such as fuel cells, batteries, permeation membranes and nano-electronic memory and sensing devices. Most cost-effective production methods for fabricating both bulk and thin films samples generally result in the formation of polycrystalline microstructures with grain boundaries, where the crystallographic periodicity is interrupted.Grain boundaries possess significantly different properties compared to the bulk and often block ion migration. Due to the high resistance and large activation energies, they represent a ubiquitous problem to overcome for a variety of electrochemical energy storage and conversion devices especially for low temperature applications. Space-charge potential barriers, featuring depletion zones and band bending, forming at the interface between adjacent grains, have been used to explain their blocking nature in polycrystalline semiconductors.In the field of photoelectrochemistry, above band-gap light is well-known to contribute to reduce band bending at interfaces by providing additional charge carriers to screen potential barriers. Here, we show that the same principle can be applied to ionic conductors as well, and that we can significantly decrease the grain boundary impedance contribution in such systems.We illustrate photo-enhanced ionic conduction in a thin film of 3% Gd-doped ceria upon illumination by UV light, whose photon energy is slightly above ceria’s band-gap. In fact, we demonstrate the ability to decrease the grain boundary resistance by 72% at 250°C and reduce its activation energy nearly in half from 1.12 eV in the dark to 0.68 eV under UV light illumination. We further demonstrate that this effect is not caused by heat, electronic conductivity, surface catalysis or other effects. Our findings are based on electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent spectroscopy (IMPS) measurements, performed on polycrystalline and epitaxial samples, and backed by theoretical considerations of grain boundary potential height and space charge regions.This work was supported by JSPS Core-to-Core Program, A. Advanced Research Networks: “Solid Oxide Interfaces for Faster Ion Transport”, Department of Energy, Basic Energy Sciences and the Swiss National Science Foundation. Figure 1