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 tip1. 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 composition2.
We are currently applying these methods to two model systems: Gd-doped CeO2 , a common solid oxide fuel cell electrolyte, and Li1-xCoO2 ,often used as a positive electrode material in solid state lithium-ion batteries.
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).