Production of Ni(OH)2 Nanosheets By Liquid Phase Exfoliation: High Performance Oxygen Evolution Catalysts and Supercapacitor Electrodes
Authors: Godwin Ian , Harvey Andrew , He Xiaoyun , McAteer David , Lyons Micheal E.G , Coleman Jonathan N 
Published: February 2020

2D materials have generated much excitement in recent years.1-2 Although these can now be produced in a number of ways, many applications, particularly in the energy arena, require liquid dispersions of 2D nanosheets. Liquid phase exfoliation3-4(LPE) is a very simple processing method which allows layered crystals to be exfoliated in liquids to give suspensions of good quality nanosheets. One strength of this process is its versatility. It has been used to exfoliate graphene and a number of inorganic 2D materials such as BN, MoS2 and GaS. Another is its simplicity – it is straightforward and easy to replicate. Here we bring together these two strengths by demonstrating that LPE can be used to exfoliate a new family of 2D materials, the layered double hydroxides (LDHs). This family of materials have been exfoliated previously using chemical exfoliation techniques. These techniques work well but are time consuming, take multiple steps and use harsh chemicals. Here, LPE method method involves sonication of commercially available powders in aqueous surfactant solutions and so is mild and potentially scalable.

More specifically, we demonstrate that LPE can be used to convert layered crystals of nickel hydroxide into Ni(OH)2 nanosheets in relatively large quantities. TEM, XPS and Raman spectroscopy show the exfoliated nanosheets to be relatively thin and of good quality. Size selection allowed the production of samples with mean nanosheet lengths ranging from 55 to 195 nm. Optical measurements showed the optical absorption coefficient spectra to be relatively invariant with nanosheet size while the scattering coefficient spectra varied strongly with size. The resulting strong size dependence allows the extinction spectra to be used to estimate nanosheet size as well as concentration. We used the exfoliated nanosheets to prepare thin film electrodes for use in supercapacitors and as oxygen evolution catalysts. Prior to use we perfomed an activation procedure of polarising the electrode at 10 mA/cm2 in 1 M NaOH which significantly increased both the capacitance and catalytic activity towards the oxygen evolution reaction (OER). While the resultant capacitance was reasonably high at ~1200 F/cm3 (20 mV/s), the catalytic performance was exceptional. We observed currents of 10 mA/cm2 at overpotentials as low as 297 mV for the OER, which is close to the state of the art.

Fig 1 (a) A) Structure of nickel hydroxide, Ni(OH)2. Blue, Ni; yellow, O; silver, H. B) Photograph of typical Ni(OH)2 dispersion in surfactant solution (H2O + sodium cholate) showing distinctive green colour. C) Representative low resolution TEM image of Ni(OH)2 nanosheets (b) Polarisation curves, 1 M NaOH (1 mV/s), for OER from Ni(OH)2 electrodes fabricated from Ni(OH)2films on Ni foam current collector before and after activation with the equivalent curve for the bare Ni foam shown for comparison.

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Figure 1