Fig. 5: Morphology measurements of Ba_0.75Sr_0.25NiO₃ before and after OER testing.

a HRTEM before a stability test. b HRTEM after a stability test. Right side: STEM atomic mapping (scale bar: 500 nm). The labelled lattice spacing is around 0.3 nm, which corresponded to the (110) lattice planes of Ba_0.75Sr_0.25NiO₃, in good agreement with the PXRD measurements. The insets of a, b show the fast Fourier transform image of the corresponding HRTEM image. The well-regulated arrayed spots indicated that the grown crystal had high crystallinity. HRTEM of Ba_0.75Sr_0.25NiO₃ before and after OER testing clearly showed the same lattice spacing and very similar fast Fourier transform images, suggesting outstanding stability of the Ba_0.75Sr_0.25NiO₃ sample under OER conditions. The maintenance of good crystallinity indicates that Ba_0.75Sr_0.25NiO₃ is a stable OER electrocatalyst. In order to verify the atomic distribution, STEM mapping was conducted; the even distribution of the atoms over the analyzed area further demonstrates the excellent stability of the sample.