MxOy/M/graphene coated multi-shelled nano-sphere as Bi-functional electrocatalysts for hydrogen and oxygen evolution

Abstract

Multi-shelled NiO/Ni/Graphene and Co3O4/Co/Graphene nano-spheres for high performance electrocatalytic activity in water electrolysis have been synthesized using multi-step solvothermal process. The synthesis process was derived by Metal-Organic-Framework (MOF) with the successive carbonization and oxidation treatments, which results into the formation of NiO/Ni and Co3O4/Co nano-crystalline particles with a coverage of graphene shell. In the present work, the effect of variation in concentration of polyvinyl pyrrole (PVP) during synthesis has also been executed. XRD studies were confirmed the formation of Co0 and Co3O4 in Co3O4/Co/Graphene sample and Ni0 and NiO in NiO/Ni/Graphene. In both cases, the surface area values showed inclining nature with increase in the amount of PVP, till 1.21 g and 1.31 g during their synthesis for nickel and cobalt, respectively. Moreover, the nano-materials could not sustain their spherical shape above the optimal concentration of PVP during synthesis. The optimal concentration of PVP has been determined as 1.21 and 1.31 g in the nickel and cobalt nano-sphere formation, respectively. The SEM and TEM images have affirmed the morphological transformations from the MOFs to MxOy/M/graphene structures. The average diameter of MOFs got shrank down from 60 – 70 nm to 20–30 nm in MxOy/M/graphene nanospheres. It get decreased above the optimum concentration of PVP. The highest surface area value reached to 151.3 m2/g in the Co3O4/Co/graphene prepared with 1.31 g of PVP, whereas same with nickel it touched to maximum value of 146.5 m2/g with PVP concentration of 1.21 g. Both the materials were exhibited excellent electrocatalytic activity in HER and OER during water electrolysis in acidic media, however, Cobalt catalysts performed better in hydrogen production. The anodic and cathodic peak current density has reached maximum value of 50 and 30 A cm−2 in presence of cobalt catalysts (with 1.31 g of PVP). The performance of Co3O4/Co/graphene nano-sphere (1.31 g PVP) is evaluated up to 10 A cm−2, whereas this value is only analyzed as 3 A cm−2 in the case of cobalt non-sphere electrocatalysts during anodic polarization. All the hollow spherical nano-electrocatalysts have shown the electrocatalytic stability during 84 h long potentiometric study.