Strain-Based Scanning Probe Techniques for Imaging Space Charge Regions in Sm-Doped Ceria

Widespread implementation of renewable, but intermittent, energy sources is contingent upon a new generation of high performance batteries and electrolysis devices to store and convert energy chemically. However, development efforts are hindered by limited knowledge of kinetic and transport properties at the nanoscale, which often differ substantially from bulk properties. A deeper understanding of localized properties and their impact on material and device performance would facilitate further advancements in electrode materials which could lead to more cost effective energy storage and conversion devices. Traditional characterization techniques, such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), can probe kinetic and transport rates as functions of applied potential and timescale, but cannot resolve spatial variations. To elucidate localized response under dynamic polarization, we are currently developing scanning probe techniques that will act as a nanoscale witness of local response in operando during EIS measurements. These techniques include electrochemical strain microscopy (ESM) and, more recently, scanning thermionic microscopy (STIM). In ESM a potential is applied to a conductive atomic force microscopy (AFM) tip, thus generating an electric field in a sample to drive ionic motion. This causes a strain in the material, often interpreted as Vegard strain, which is detected as a deflection of the AFM tip 1 . STIM functions by applying a periodic temperature perturbation from a heating element located on top of the AFM tip, inducing ionic motion by thermal stress. Again, material strain is detected by deflection of the tip, from which the fourth harmonic of the input frequency is related to shifts in chemical composition 2 . We are currently applying these methods to two model systems: Gd-doped CeO 2 , a common solid oxide fuel cell electrolyte, and Li 1-x CoO 2 ,often used as a positive electrode material in solid state lithium-ion batteries. References: 1. Balke, N. et al. Nanoscale mapping of ion diffusion in a lithium-ion battery cathode. Nat. Nanotechnol. 5, 749–754 (2010). 2. Eshghinejad, A. et al. Scanning thermo-ionic microscopy for probing local electrochemistry at the nanoscale. J. Appl. Phys. 119, 205110 (2016).