Introducing amorphous and ultrathin nanosheets of transition bimetal phosphate arrays that are highly active in the oxygen evolution reaction (OER) as shells over an electronically modulated crystalline core with low hydrogen absorption energy for an excellent hydrogen evolution reaction (HER) can boost the sluggish kinetics of the OER and HER in alkaline electrolytes. Therefore, in this study, ultrathin and amorphous cobalt‐nickel‐phosphate (CoNiPO x ) nanosheet arrays are deposited over vanadium (V)‐doped cobalt‐nitride (V 3% ‐Co 4 N) crystalline core nanowires to obtain amorphous‐shell@crystalline‐core mesoporous 3D‐heterostructures (CoNiPO x @V‐Co 4 N/NF) as bifunctional electrocatalysts. The optimized electrocatalyst shows extremely low HER and OER overpotentials of 53 and 270 mV at 10 mA cm −2 , respectively. The CoNiPO x @V 3% ‐Co 4 N/NF (+/−) electrolyzer utilizing the electrocatalyst as both anode and cathode demonstrates remarkable overall water‐splitting activity, requiring a cell potential of only 1.52 V at 10 mA cm −2 , 30 mV lower than that of the RuO 2 /NF (+)/20%‐Pt/C/NF (−) electrolyzer. Such impressive bifunctional activities can be attributed to abundant active sites, adjusted electronic structure, lower charge‐transfer resistance, enhanced electrochemically active surface area (ECSA), and surface‐ and volume‐confined electrocatalysis resulting from the synergistic effects of the crystalline V 3% ‐Co 4 N core and amorphous CoNiPO x shells boosting water splitting in alkaline media.

Heteroatom‐doped transition metal‐oxides of high oxygen evolution reaction (OER) activities interfaced with metals of low hydrogen adsorption energy barrier for efficient hydrogen evolution reaction (HER) when uniformly embedded in a conductive nitrogen‐doped carbon (NC) matrix, can mitigate the low‐conductivity and high‐agglomeration of metal‐nanoparticles in carbon matrix and enhances their bifunctional activities. Thus, a 3D mesoporous heterostructure of boron (B)‐doped cobalt‐oxide/cobalt‐metal nanohybrids embedded in NC and grown on a Ni foam substrate (B‐CoO/Co@NC/NF) is developed as a binder‐free bifunctional electrocatalyst for alkaline water‐splitting via a post‐synthetic modification of the metal–organic framework and subsequent annealing in different Ar/H2 gas ratios. B‐CoO/Co@NC/NF prepared using 10% H2 gas (B‐CoO/Co@NC/NF [10% H2]) shows the lowest HER overpotential (196 mV) and B‐CoO/Co@NC/NF (Ar), developed in Ar, shows an OER overpotential of 307 mV at 10 mA cm−2 with excellent long‐term durability for 100 h. The best anode and cathode electrocatalyst‐based electrolyzer (B‐CoO/Co@NC/NF (Ar)(+)//B‐CoO/Co@NC/NF (10% H2)(−)) generates a current density of 10 mA cm−2 with only 1.62 V with long‐term stability. Further, density functional theory investigations demonstrate the effect of B‐doping on electronic structure and reaction mechanism of the electrocatalysts for optimal interaction with reaction intermediates for efficient alkaline water‐splitting which corroborates the experimental results.

Water‐Splitting In article number 2201311 , Seunghyun Lee and co‐workers demonstrate excellent water‐splitting activity of 3D mesoporous heterostructures assembled from CoNiPO x ‐nanosheets amorphous‐shells and V‐doped Co 4 N nanowires crystalline‐cores on Ni foam substrate. The synergistic effect between the crystalline‐core and amorphous‐shell results remarkable oxygen and hydrogen evolution reaction at low overpotentials.