N- and S-codoped graphene hollow nanoballs as an efficient Pt-free electrocatalyst for dye-sensitized solar cells

We synthesize heteroatoms-doped graphene hollow nanoballs (GHBs) on flexible carbon cloth (CC) substrates via chemical vapor deposition (CVD) reaction to be used as an efficient non-noble electrocatalyst in dye-sensitized solar cells (DSSCs). The as-synthesized heteroatoms-doped GHBs/CC, including nitrogen-doped GHBs, sulfurdoped GHBs, and nitrogen and sulfur-codoped GHBs (denoted by N-GHBs/CC, S-GHBs/CC and N,S-GHBs/CC, respectively), are used as an efficient counter electrode (CE) in DSSCs. Unlike planar graphene sheets, the highly curved GHBs can avoid self-assembly restacking to provide high surface areas for electrocatalytic reactions. In addition, the heteroatomic incorporation in GHBs can reduce the charge-transfer resistance to enhance the electrocatalytic activity. Among these doped GHB samples, N,S-GHBs show the best catalytic performance due to the synergistic effect from both electronic and geometric changes, caused by the N- and Sdopings, respectively. The DSSC with a N,S-GHB CE exhibits the power conversion efficiency of 9.02%, comparable to that (8.90%) of a Pt-based counterpart.