Bandura K. The structure and electrochemical properties of nickel hydroxide / reduced graphene oxide composite materials.

In the paper, graphene oxide has been synthesized by the Hummers and Marcano-Tour methods with subsequent chemical reduction. The comparison of structural, morphological and electrical pro¬per¬ties of the obtained graphene oxide and reduced graphene oxide have been done. It has been found that RGO(H), which is characterized by a large number of structural defects, is formed from lame¬llar pa¬rticles with sizes of D = 15-35 nm, and RGO(MT) consists of randomly aggregated gra¬phene packages (thick-ness of 12 nm) that form mesoporous 3D-network. The electrical conductivity of both RGO samp¬les is due to charge migration in graphene fragments and charge transfer between lay¬ers of gra¬phene packages. The obtained different values of the activation energy of RGO(Н) and RGO(MT) – 0.60-0.70 eV and 0.06-0.08 eV, respectively, are the result of morphological differences bet¬ween the samples.The composite materials based on hydrotermally synthesized nickel hydroxide and reduced graphene oxide have been obtained by ultrasonic dispersion. It has been established that ultrasonic dispersion leads to the introduction of graphene fragments into the interlayer space of β-Ni(OH)2.The influence of the graphene component on the electrical and electrochemical properties of the β-Ni(OH)2/RGO composite material has been investigated. For composites based on β-Ni(OH)2 and redu¬ced graphene oxide synthesized according to Hummers and Marcano-Tour protocol (particle size in both cases is up to 13 nm at crystallographic orientation (001)) at temperatures < 100°C and low fre¬quencies the protonic conductivity is predominant, while the effect of the conductivity of the graphene compo¬nent increases at higher temperatures and frequencies.For a sample of an electrode based on β-Ni(OH)2/RGO(Н) composite material, the maxi¬mum obtai¬ned specific capacitance according to the cyclic voltammetry is 513 F•g–1 at 0.5 mV•s–1, and in galva¬nostatic measurements of 494 F•g–1 at 0.08 A•g–1. The maximum specific energy is about 17 W•h·kg–1 with a specific power of about 20 W•kg–1. It has been confirmed experimentally that for electrodes based on β-Ni(OH)2 / RGO(Н), the specific capacitance consists of double layer capacitance and pseudo¬capa¬citance with a contribution of 91 %. The value of the coefficient of electrostimulated protonic diffu¬sion at a zero bias potential for β-Ni(OH)2 / RGO(Н) of 1.69•10–12 cm2•s–1.The influence of the graphene component on the electrochemical properties of the β-Ni(OH)2/RGO(МТ) composite material has been investigated. For β-Ni(OH)2/RGO(МТ) composite with a mass ratio of components 1:2, the activation energy is 0.06 eV, which corresponds to the best ele¬ctrical conductivity of the composite material. The highest value of specific for electrodes based on β-Ni(OH)2/RGO(МТ) has been observed for a composite material with a component ratio of 2:1 – 145 F•g–1 at a potential scan rate of 0.5 mV•s–1.