Local Structural Characterization of Metal Oxides Nanocomposites for Electro-Chemo-Mechanical (ECM) Devices

An Electro-Chemo-Mechanical (ECM) effect refers to the dimensional change in a solid due to electric field-driven compositional change. In this effect, the electric field drives compositional changes across the device resulting in a macroscopic strain, which is an attractive property for applying the ECM effect in micro-and nano-actuators. The structure of an actuator device comprises a micrometer thick solid electrolyte (SE) in the middle of two ECM working bodies (WBs) made by mixing ionic/electronic conductors (MIEC). In our work, 20% Gd-doped ceria (GDC20) is chosen as SE. Meanwhile, Ti-doped GDC20 (TiGDC20) and V-doped GDC20 (VGDC20) nanocomposites serve as WBs. These WBs are required to undergo an efficient redox reaction associated with massive volumetric expansion or contraction under bias. However, VGDC20 WBs are responsible for the longer response times by comparison with TiGDC20 WBs that produce a response time of a few seconds.<br/><br/>In this study, synchrotron-based X-ray absorption spectroscopy (XAS) is employed to probe structural changes in ECM devices at the atomic level. In TiGDC20 and VGDC20, XAS measurements demonstrated that Ti, V, and Ce were in mixed oxidation states (Ti³⁺/Ti⁴⁺, V⁴⁺/V⁵⁺, and Ce³⁺/Ce⁴⁺), which are essential for explaining the observed electromechanical effect, based on their volumetric changes during redox processes. In Ti-based nanocomposites, the local environment around the Ti atom is TiO₆ octahedron. The displacement of Ti atoms from the center of the TiO₆ octahedron, referring to off-center displacement, introduces the disorder in the nanocomposites. The amount of the disorder would correspond to the response to the external electric field, which plays an essential role in generating volumetric changes, resulting in generating strain in the working body. Meanwhile, V-based nanocomposites contain mixed phases characterized by tetrahedral and square pyramidal environments. The changes in proportions of each phase under the external electric field would be related to the volumetric changes. By comparison with the Ti-based nanocomposites, the strongly non-symmetric local environment of V in V-based nanocomposites may be non-conducive for applications requiring rapid oxidation-reduction.