The electroless preparation of a series of self-supported heterostructured cobalt boride-based electrodes is schematically shown in Fig. A-H (details in Fig. ). Highly open-porous and hollow nickel foam (NF) (Fig. G, H, Fig. ) is used as substrate as it allows for excellent liquid/gas transport, is chemically stable, provides high electrical conductivity and a suited surface for the catalyst fabrication/operation. X-ray micro-tomography (µCT) furthermore depicts the hollow nature and thus material-saving NF due to its thin walls of the foam struts (Fig. ). The facile synthesis employs an iteratively controlled chemical reduction reaction of cobalt cations (solution A: Co(NO3)2•6H2O) by mixtures of reducing agents in an aqueous solution (solution B: NaBH4 and NaH2PO3•H2O). Sodium borohydride acts as the strong reaction-controlling reducing agent, while sodium phosphite is a weaker reducing agent. This combination of reducing agents facilitates, both, the inherent formation of CoxB-O and immediate doping by phosphite, while at a later stage, the complex amorphous-crystalline CoxB-[0.2]P-O catalyst is formed. The cobalt source, i.e. solution A, is first adsorbed on the NF substrate (Fig. ) and subsequently immersed in the reducing solution B. This iterative process was repeated 5 times, whereby the cobalt ions are locally reduced on the surface. As a result, a dense, nanocrystalline and well-adhering seed layer on the NF is generated (intermediate state depicted in Fig. C, D, Fig. ) to assure mechanical long-term stability during catalyst operation (Fig. ), good electron transport and an enhanced surface area (cf. Surface-sensitive X-ray photoelectron spectroscopy (XPS) (Fig. ), confirms the successful incorporation of the B and P species and their impact on the electronic environment of the Co active sites within the catalyst layer. XPS survey revealed a surface composition of 23.17 at.%, 29.71 at.%, 29.73 at.%, 5.02 at.%, and 12.37 at.% for B, C, O, P, and Co respectively (cf. Fig. ). Deconvolution of the Co 2p3/2 spectrum clearly reveals the tailored Co-B interaction, resulting in a distinct sub-peak at 777.7 eV, which is 0.4 eV shifted to lower binding energy compared to metallic Co 0 (778.1 eV ) due to the electron transfer from boron to cobalt (Fig. ). , In addition there are two major peaks of Co 2p3/2 at 781.6 eV and Co 2p1/2 at 797.4 eV, respectively, with a spin-orbit splitting of ΔE = 15.8 eV.