Cu kα1

The dispersion ratio and particle diameter of Pd catalysts were characterized using CO pulse (BEL CAT II; Microtrac-Bel Japan Inc.). Before measurements, the catalyst sample was pre-treated under He flow at 473 K for 1 h. After the treatment, the temperature was decreased to 323 K with He, and 10% CO was pulsed. The results of CO pulse are presented in Supplementary Materials Table S9. TOF-s and TOF-p were calculated with the number of surface Pd atoms and that of interfacial Pd atoms. Detailed procedures are presented in Supplementary Materials. Pd and Ce K-edge in-situ X-ray adsorption fine structure (in-situ XAFS) spectra were recorded on BL14B2 in SPring-8 (Hyogo, Japan). The 3 wt% Pd/CeO₂ catalyst was pressed into a pellet. Then the pellet was attached to a cell prepared for in-situ measurements, as shown in Figure S18. The pellet sample was pre-treated with Ar flow at 723 K for 30 min. Measurements were conducted with 5 mA current and CH₄: H₂O: Ar = 1: 2: 117, total 120 SCCM flow at 473 K. Software (Athena ver. 0.8.056, Artemis ver. 0.8.012) was used to analyze the obtained XAFS spectra. Phase and morphology of CeO₂ disc for AC impedance measurements were analyzed by X-ray diffraction (XRD, MiniFlex 600, Cu K_α1, Rigaku) and scanning electron microscopy (FE-SEM, Quanta 200 FEG, FEI Company).

Crystallinity of the fabricated thin films were investigated by grazing incidence X-ray diffraction analyses (Cu Kα₁, ATX-G, Rigaku Co.), which revealed that all oxide layers were amorphous in nature expect for NiO polycrystalline film. Cross-sectional thin-film samples for TEM observations were prepared by focused-ion-beam (FIB) micro-sampling technique, in which the multilayer structure region of the TFTs was cutout and thinned by FIB (FB-2000A, HITACHI) to obtain samples for cross-sectional observation. The cross-sectional microstructure and electron diffraction pattern of the a-WO₃ devices were examined by high-resolution TEM and STEM (JEM-ARM200F, 200 kV, JEOL Ltd.).