Enhanced catalytic properties of bimetallic sulfides with the assistance of graphene oxide for accelerating triiodide reduction in dye-sensitized solar cells

•Bimetallic sulfides (Co8FeS8/N-C and (Co0.75Ni0.25)9S8/N-C anchored on graphene oxide (GO) were synthesized.•Bimetallic sulfides/GO composites exhibited improved electrocatalytic performance for triiodide reduction reaction.•Co8FeS8/N-C/GO and (Co0.75Ni0.25)9S8/N-C/GO were applied as low-cost Pt-free counter electrode for the DSSCs.•Co8FeS8/N-C/GO and (Co0.75Ni0.25)9S8/N-C/GO achieved high photo-electric conversion efficiency of 8.74% and 8.98%.