Tuning of NiO into an Efficient Electrocatalyst for Water Splitting

• Designing a highly efficient and cost-effective catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is a significant element for the development of solar hydrogen into a competitive sustainable energy source.  In this context, nickel, cobalt, and iron oxides and their double hydroxides have been extensively investigated as electrocatalysts for the OER reaction with a large variation in crystal extensions and exposed lattice faces and thus also in the resulting overpotential.  In this work, we show that  the surface energies of exposed lattice planes of the growing NiO nanostructures can be altered and an enhanced crystal growth in different crystalline direction can be controlled by a hydrothermal method with a variation of the pH conditions of the precursor solution. The growth direction in between the [111] and [220] directions in cubic NiO with respect to the [200] direction can be altered resulting in NiO nanoflakes (NFs) with controllable extensions. On the basis of the different pH condition during synthesis of the NiO NFs, we find that the morphology of NiO nanostructure is also able to be changed according to the different surface charge, which results in different catalytic performance. By incorporating a redox active dopant (Fe), NiO can be tuned into higher OER efficiency as well as making the electrocatalyst bi-functional with respect to the OER and the HER processes under alkaline conditions. The Fe-NiO system is engineered into a 3D electrode by chemical bath deposition (CBD) method onto a nickel foam framework.  In order to identify the composition of the active phase on the surface of Fe-NiO/Ni foam, in situ Raman spectroscopic investigations are carried out during both the OER and HER reactions under water. The results show that the Fe doping plays a critical role for the catalytic property. In support of this, density functional theory (DFT) calculations show that Fe changes the local electron density, shifting the energetically preferable absorption site of H from oxygen in NiO onto Ni in Fe-NiO in the hydrogen evolution reaction. 

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