Erosion of the porous film by electrolyte during a long CV test may decrease the capacitance value. Fig. 8 shows TEM and mapping images of manganese oxide with 1.0 at.% Fe3O4 addition at 300 °C annealing. Fig. 8(a–c) show the material before CV testing: (a) TEM bright field image, (b) Mn element mapping image, and (c) Fe element mapping image. Fig. 8(d–f) show the material after the 50th cycle of CV testing: (d) TEM bright field image, (e) Mn element mapping image, and (f) Fe element mapping image.