The complex architecture of the developed electrocatalyst is further confirmed by threedimensional (3D) tomographic reconstruction (Fig. ). The reconstruction delineates the nanostructures showing agglomerated nanoparticles (seed layer) at the bottom of the electrode covered by porous nanosheets at the top exposing a large surface area. This hierarchical morphology is necessary for efficient mass transport during the catalytic reaction with the large surface area providing a high number of active sites for surface chemical reactions. Cross-sectional TEM samples, prepared by ultramicrotomy, enabled the detailed characterization of the local composition and bonding by STEM electron energy loss spectroscopy (EELS) (Fig. ). In Fig. , both, the seed layer and the active catalyst region are differentiated (Fig. ). The dense CoxB-[0.2]P-O particles with active catalyst coating appear bright in the HAADF-STEM images in contrast to the faint nanosheets (Fig. ).