Detailed analysis of the Co 2p3/2 peak reveals two contributions with typical shifts to higher binding energies (781.1 eV and 782.9 eV) owing to cobalt (II) cations bound to highly electronegative species such as hydroxides and phosphites, respectively. The electron transfer between Co and B is further indicated by the positive 0.6 eV shift relative to B 0 (187.1 eV) in the B 1s peak (187.7 eV), attributed to the boride species (Fig. ). Phosphite incorporation into the catalyst was evidenced by the deconvoluted P 2p spectrum (Fig. ) into two major sub-peaks at 133.6 eV and 134 eV (Δ = 0.87 eV) characteristic for the presence of phosphorous (III) species and an extra peak at higher energy for phosphorous (V) in oxidized P-O molecules. Like boron, the phosphorous inclusion into the Co-B system directly modulates the electron density around cobalt atoms, which impacts electron transfer and the hydrogen interaction with the active sites. The O 1s spectrum (Fig. L) shows three subpeaks at 531.5 eV, 532.5 eV, and 533.9 eV. The signals at 531.5 eV and 532.5 eV consistently reveals the presence of mixed metal-oxide/hydroxide (e.g. Co(OH)2/ CoO) and the P/B-O (in oxidized P species and B-O in boron-oxo species), respectively. A comprehensive scale-bridging investigation of the microstructure, crystal structure and chemistry of the electrodes with emphasis on the active catalyst is indispensable to understand catalyst functionality, performance and long-term stability. The continuous coating of the NF surface is confirmed by SEM top-view and cross-sectional imaging ( Fig. F, G, Fig. ) as corroborated by the Co mapping. The coating roughness (Fig. ), governed by the seed layer with a mean grain size of the polycrystalline CoxB-[0.2]P-O of around 100 nm (Fig. ), enhances active surface area and thus promotes HER. The tailored two-step reduction reaction resulted in the formation of the targeted sequence of seed layer and active catalyst, with the latter exhibiting amorphous-crystalline morphology surpassing conventional reduction strategies with typical amorphous structure. Transmission electron microscopy (TEM) confirmed the continuous coverage of the crystalline CoxB-[0.2]P-O seed layer (and partly tiny particles generated during the second coating step) by the amorphous-crystalline catalyst heterostructure (Fig. ).