Cross-sectional preparation of the sensors was conducted by ultramicrotomy utilizing a Leica EM UC7 microtome. Individual sensor strips were separated by doctor-blade cutting and embedded in epoxy resin (Araldite 502, Science Services, Germany) to facilitate sectioning of the highly porous sensor heterostructures. Impregnation of the pores was supported by evacuation and subsequent pressurization of the uncured resin using a dedicated pressure vessel (miniclave steel, Büchi AG, Switzerland). Curing was carried out overnight at 60 °C. To create a trapezoid-shaped block face and to remove potentially damaged sample regions from cutting, trimming with a diamond knife (DiATOME 45° trim knife) was performed. A DiATOME 45° ultrasonic diamond knife was employed to obtain cross-sectional TEM samples with minimal compression. The floating sections (deionized water) were transferred onto lacey carbon TEM grids (Plano AG) to provide sufficient support at reduced impact during EELS measurements.