Hexagonal NiO nanosheets on Ni-foam as an electrocatalyst for high-performance water splitting application

•NiO electrocatalyst displays excellent electrocatalytic activities.•Low overpotential of 83 mV during hydrogen evolution reaction.•Low overpotential of 380 mV during oxygen evolution reaction.•Low Tafel slope of 209 mV dec−1 during hydrogen evolution reaction.•Low Tafel slope of 299 mV dec−1 during oxygen evolution reaction.