Orthorhombic α-NiOOH Nanosheet Arrays: Phase Conversion and Efficient Bifunctional Electrocatalysts for Full Water Splitting

The development of efficient and earth-abundant electrocatalysts is essential for water splitting against future renewable energy systems. Generally, NiOOH is considered to play a key role in OER processes in alkaline electrolytes in the Ni-based OER catalysts (such as NiO, NiS, NiSe, and etc.). However, there are rare reports of NiOOH for both OER and HER electrocatalysis. Here, we proposed a facile strategy to obtain the vertically aligned orthorhombic α-NiOOH nanosheet arrays on 3D Ni foam (α-NiOOH/NF). The catalyst exhibited excellent OER activity obtaining 10 mA cm^-2 just affording a low overpotential of 266 mV. The performance was superior to those of the compared samples including Ni₂Cl(OH)₃ nanosheets on NF, NiO nanosheets on NF, Ni microspheres on NF, Ni foam and even better than the commercial IrO₂ as well as most of the reported OER electrocatalysts in references. Furthermore, the α-NiOOH/NF electrode demonstrated robust durability over a long period with the increase of current density, which was attributed to the phase conversion from α-NiOOH phase to the distorted β-NiOOH phase. Additionally, the prepared orthorhombic α-NiOOH nanosheet arrays also showed the more active electrocatalysis for HER of water splitting than the above comparative samples. The α-NiOOH nanosheets therefore showed a high performance of bifunctional electrocatalysis for full water splitting with the onset potential of 1.66 V.