Ultralow-iridium content NiIr alloy derivative nanochain arrays as bifunctional electrocatalysts for overall water splitting

The development of low-cost and high-durability bifunctional electrocatalysts is of considerable importance for overall water splitting (OWS). This work reports the controlled synthesis of nickel–iridium alloy derivative nanochain array electrodes (NiIr(x) NCs) with fully exposed active sites that facilitated mass transfer for efficient OWS. The nanochains have a self-supported three-dimensional core–shell structure, composed of a metallic NiIr(x) core and a thin (5–10 nm) amorphous (hydr)oxide film as the shell (e.g., IrO(2)/NiIr(x) and Ni(OH)(2)/NiIr(x)). Interestingly, NiIr(x) NCs have bifunctional properties. Particularly, the oxygen evolution reaction (OER) current density (electrode geometrical area) of NiIr(1) NCs is four times higher than that of IrO(2) at 1.6 V vs. RHE. Meanwhile, its hydrogen evolution reaction (HER) overpotential at 10 mA cm(−2) (η(10) = 63 mV) is comparable to that of 10 wt% Pt/C. These performances may originate from the interfacial effect between the surface (hydr)oxide shell and metallic NiIr(x) core, which facilitates the charge transfer, along with the synergistic effect between Ni(2+) and Ir(4+) in the (hydr)oxide shell. Furthermore, NiIr(1) NCs exhibits excellent OER durability (100 h @ 200 mA cm(−2)) and OWS durability (100 h @ 500 mA cm(−2)) with the nanochain array structure well preserved. This work provides a promising route for developing effective bifunctional electrocatalysts for OWS applications.