The hybrid nanostructure of vertically aligned cobalt sulfide nanoneedles on three-dimensional graphene decorated nickel foam for high performance methanol oxidation

In this report, vertically aligned one-dimensional (1D) cobalt sulfide nanoneedles (CS-NNs) were directly synthesized onto the three-dimensional graphene (3DG) as the CS-NNs/3DG hybrid nanostructure and applied to methanol oxidation reaction (MOR). The 3DG and CS-NNs were prepared by chemical vapor deposition and two-step hydrothermal method, respectively. The 1D cobalt carbonate hydroxide nanoneedles (CCH-NNs) were initially synthesized onto the graphene surface of 3DG via the hydrothermal method (Step I). Finally, the CCH-NNs were converted into CS-NNs via the hydrothermal treatment using Na2S as the sulfur source with the help of the Kirkendall effect (Step II). The structures and morphologies of 3DG, CCH-NNs/3DG, and CS-NNs/3DG were investigated by Raman spectroscopy, scanning electron microscopy and X-ray diffraction. The electrocatalytic performance was investigated by cyclic voltammetry measurements. The CS-NNs/3DG electrode showed higher electrocatalytic activity for MOR compared with CCH-NNs/3DG and 3DG. The excellent electrocatalytic performances of 3DG supported CS-NNs come from the unique 3D porous architecture, which can provide sufficient open spaces and shorter ion diffusion paths to facilitate rapid ionic transportation. Direct synthesis of 1D CS-NNs on graphene can result in a low resistance for fast electron transfer. The vertically aligned binder-free 1D CS-NNs not only avoid the aggregation to enhance the utilization efficiency of catalytic active sites but also serve as an electron highway in the axial direction for electron collection. The prepared CS-NNs/3DG is a promising non-platinum electrocatalyst for direct methanol fuel cells.