Analysis of Electrochemomechanical Coupling in Non-Stoichiometric Oxide Thin Films
Authors: Raja Shilpa N , Swallow Jessica G. , Bishop Sean R. , Chi Yen-Ting , Chen Ting , Perry Nicola H. , Tuller Harry L. , Van Vliet Krystyn J. 
Published: February 2022

Non-stoichiometric oxides are used in a wide variety of applications including solid oxide fuel cells (SOFCs), lithium ion batteries (LIBs), gas sensors, and catalysis. Through the capacity of such materials to support large point defect concentrations, these functional oxides can readily store, transport, and exchange ions. An important consequence of this non-stoichiometry is a tendency toward chemomechanical coupling, particularly in the form of chemical expansion, or the coupling between material volume and defect concentration. Thin films of non-stoichiometric oxides are of particular interest in such device designs, given the potential for strain engineering. For example, it has been shown for several materials that tensile strain can increase the ionic conductivity or gas exchange reactivity for oxygen by up to an order of magnitude, potentially enabling enhanced device efficiency or decreased operating temperatures1.

In electrochemical devices, chemical expansion can generate stress or strain that can lead to mechanical failure, and/or changes in mechanical properties including elastic moduli. Given the extreme environments and range of non-stoichiometric oxides in which chemical expansion can be expected, robust device design requires accurate, flexible, and rapid characterization of environmental conditions and materials that maximize (or minimize) chemical expansion in situ. However, methods used at present for characterizing chemomechanical expansion, such as dilatometry, synchrotron techniques, reflectometry, and others, are not amenable to thin films or are difficult to implement in standard laboratory settings.

Recently, Swallow et al. described an approach for characterizing thin film non-stoichiometric oxide chemical expansion at high temperatures by way of electrochemically induced actuation that addresses the above needs2. That work characterized volume change within a fluorite film of PrxCe1-xO2-δ (PCO) and structural deflection of the PCO/YSZ (yttria-stablized zirconia) bilayer during electrochemical pumping of oxygen ions into the PCO film. It also demonstrated a positive attribute of such chemical expansion in the form of high temperature oxide actuators, which harness electrochemically generated chemical strain to produce measurable, nanoscale device deflections. The actuation produced ranged between 5-15 nm of displacement amplitude depending on the experimental conditions2.

Here, we provide an extended and graphically rich analysis of electrical and mechanical response data from such experiments. We model the current and mechanical response of PCO to an electrochemical driving force using previously established defect equilibria and kinetic relationships for that oxide, demonstrating the contributions that material properties and sample geometries make to device deflection and electrochemical pumping. We also extend the measurement approach to an additional material system, the perovskite-structured oxide SrTi0.65Fe0,35O3-δ (STF) used as part of magnetic memory devices, gas transport membranes, and fuel cells. This case study demonstrates the broad applicability of this measurement method, as well as means to leverage chemical expansion effects at elevated temperatures for diverse actuating and functional devices.

Yildiz, B. ‘Stretching’ the energy landscape of oxides—Effects on electrocatalysis and diffusion. MRS Bull.
39, 147–156 (2014).

Swallow, J. G. et al. Dynamic chemical expansion of thin-film non-stoichiometric oxides at extreme temperatures. Nat Mater (2017). doi:10.1038/nmat4898