General synthesis of NiCo alloy nanochain arrays with thin oxide coating: a highly efficient bifunctional electrocatalyst for overall water splitting

Bifunctional electrocatalysts that are cost effective and naturally abundant and have high electroactivities in both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline electrolytes are very desirable. Herein, we describe a general technique to fabricate thin oxide layer coated NiCo alloy nanochain arrays (NCAs) on Ni foam by a hydrothermal reaction and subsequent low-temperature reduction. The hybrid NiCo-300 electrocatalyst shows excellent bifunctional HER and OER activities in overall water electrolysis in an alkaline medium. It shows lower overportential and better electrocatalytic kinetics than pristine NiCo2O4 nanorod arrays (NRAs) and bare NiCo alloy, reaching 10 mA cm−2 and 100 mA cm−2 at low overpotentials of 156 mV and 245 mV in HER and 320 mV and 390 mV in OER, respectively. The thin oxide layer on the NiCo-300 surface produces synergistic effects to enhance the HER and OER activities. In a two-electrode system, the symmetrical electrolyzer needs a cell voltage of 1.688 V for a 10 mA cm−2 water-splitting current and exhibits good long-term stability. The results reveal a facile way to fabricate nanostructured transition metal alloys with thin oxide coatings as highly efficient bifunctional electrocatalysts promising for overall water electrolysis.