Electrochemical devices using a solid electrolyte are expected as next-generation energy conversion and storage devices. Typical examples are solid oxide cells using a solid-state oxide-ion (SOC) or proton conductor (PCC) and all-solid-state batteries using a solid-state lithium-ion conductor (ASSB). In such devices, the difference in chemical potential of reaction component, which is maintained across the electrolyte between the cathode and the anode, is the driving force for ionic transport and reaction. The chemical potential in the electrolyte also has a great impact on thermodynamic stability of the electrolyte, thus the device performance and durability [1, 2]. Therefore, in order to optimize designs and operating conditions of the devices or to ensure stability and durability of the devices, it is important to understand the chemical potential distribution in solid electrolyte under operation.

In this study, we aimed to experimentally evaluate the chemical potential distribution in solid state ionics devices. So far, a lot of numerical studies have been carried to estimate the chemical potential distribution in solid state ionics devices [1-3]. However, only a few studies have been reported to directly observe the distribution of chemical potential in an experimental manner [4-6]. And as far as the authors know, there exist no studies for its operando observation under device operation.

Challenges for the direct observation of chemical potential in solid state ionics devices are (i) how to detect the chemical potential change in the solid electrolyte and (ii) how to achieve operando measurement with high positional and temporal resolutions at desired temperature in controlled atmosphere under polarization. For the detection of chemical potential change, we embedded a potential probe into or on the electrolyte. In addition, as an operando analytical technique with high positional and temporal resolutions, micro-beam X-ray absorption fine structure (XAFS) was employed. In this presentation, our recent results on the direct observation of chemical potential distribution in SOC and ASSB will be given. The distribution of oxygen chemical potential in the former and that of lithium chemical potential in the latter, respectively, were evaluated under device operation. The obtained results were compared with the simulated results based on the ambipolar diffusion of ion and electron in the electrolyte.