Ultrathin-walled Co₉S₈ nanotube/reduced graphene oxide composite as an efficient electrocatalyst for the reduction of triiodide

A novel ultrathin-walled Co₉S₈ nanotube/reduced graphene oxide electrocatalyst, for the first time, is successfully prepared by a simple hydrothermal process coupling with an ion exchange process for the reduction of triiodide in dye-sensitized solar cells (DSSC). Ultrathin-walled Co₉S₈ nanotubes have an average diameter of 20–30 nm and a wall thickness of 3–4 nm, and the reduced graphene oxide possessing high conductivity is well dispersed in the Co₉S₈ nanotubes simultaneously, which contributed to the high specific surface area, well exposed active sites and excellent electric conductivity. The electrochemical performances of ultrathin-walled Co₉S₈ nanotube/reduced graphene oxide are evaluated by the EIS, Tafel polarization and CV measurements, exhibiting the significant improvement of electrocatalytic performance for the triiodide reduction. Optimizing the film thickness of Co₉S₈ nanotube/reduced graphene oxide counter electrode, the optimum photovoltaic conversion efficiency of 7.58% is obtained, which is even higher than that of the DSSC with Pt counter electrode (7.45%). In addition, the DSSC with Co₉S₈/reduced graphene oxide electrode exhibits a good repeatability and long-term electrochemical stability. Therefore, the ultrathin-walled Co₉S₈ nanotube/reduced graphene oxide is a reliable material to replace Pt.