Detecting Dynamic Manipulation of the Space Charge Region in Doped Ceria with Scanning Thermo-Ionic Microscopy

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A new generation of high performance energy storage and conversion devices is instrumental to commercial adoption of renewable, but intermittent, energy sources. Developing new materials for these devices often relies on bulk characterization techniques, such as electrochemical impedance spectroscopy, which are inherently unable to spatially resolve electrochemical phenomena and material properties. Conversely, scanning probe methods (SPM) have long been employed to probe local electrochemistry and material properties; however, relating these measurements to macroscopic performance can be challenging. We aim to bridge this gap by using SPM as a localized in operando witness during macroscopic measurements. Workers have recently employed Scanning Thermo-Ionic Microscopy (STIM), a strain-based SPM that leverages mechano-thermo-chemical coupled effects to detect local ionic concentrations. Briefly, STIM operates by inducing thermal stresses in the material from a periodic temperature perturbation at the scanning probe tip. The strain related to these thermal stresses drives ionic motion and is detected by deflection of the probe tip at a higher order harmonic of the temperature perturbation. This allows the ionic response to be distinguished from electrostatic and electromechanical responses, which occur at the temperature perturbation frequency. Previous ex-situ measurements demonstrated the potential of STIM to image grain boundary effects in doped ceria, possibly due to space charge regions. More recently, STIM was implemented point-wise to study manipulation of the space charge region under static polarization. Though promising, intensive hardware demands prevented imaging with STIM, thus ultimately reducing the impact of s-SPM implementation. Herein we demonstrate the use of data science enhanced SPM to overcome hardware limitations and extend our work to in operando, dynamic polarization studies. Furthermore, the effect of adsorbed surface species on STIM response is examined by controlling temperature, gas environment, and relative humidity.