Ultrathin nickel boride nanosheets anchored on functionalized carbon nanotubes as bifunctional electrocatalysts for overall water splitting

Transition metal boride (TMB) materials have recently gained vast interest as a new class of catalysts. However, their catalytic performance is still limited due to poor electrical conductivity and limited specific surface area. Here, we demonstrate a generalizable approach to overcome these limitations by anchoring ultrathin nickel boride (Ni_xB) sheets on the surfaces of functionalized small-diameter multi-walled carbon nanotubes (f-MWCNTs). The electrochemically active surface area and charge transfer resistance of the resulting hybrid materials (Ni_xB/f-MWCNT) is 3.4 and 0.24 times that of the Ni_xB nanosheets, respectively. And, Ni_xB/f-MWCNT exhibited superior catalytic activities and stability toward both oxygen evolution and hydrogen evolution reactions. For the overall water splitting, it requires a cell voltage of 1.60 V to reach the current density of 10 mA cm^-2, outperforming existing metal boride catalysts as well as commercial IrO₂/Pt/C catalysts. Further, X-ray photoelectron spectroscopy revealed the strong chemical coupling between Ni_xB and f-MWCNTs and the in situ formation of highly active NiOOH/Ni_xB and Ni(OH)₂/Ni_xB heterojunctions, which contributes to the superior activity. The developed design concept can serve as a general approach to improve other electrocatalysts with low electrical conductivity and specific surface area, such as metal oxides, metal hydroxides, and metal-organic framework-derived materials.