One-step synthesis of graphene hollow nanoballs with various nitrogen-doped states for electrocatalysis in dye-sensitized solar cells

Abstract

Nitrogen-doped graphene hollow nanoballs (N-GHBs) were synthesized in chemical vapor deposition (CVD) reaction using melamine as a chemical precursor via an in situ nitrogen-doping approach. In the CVD reaction, N-GHBs were deposited directly on carbon cloth (CC) to be used as an efficient metal-free electrocatalyst for dye-sensitized solar cell (DSSC) applications. The highly curved N-GHBs could avoid the self-assembly restacking of planar graphene sheets, which usually occurred during the film preparation. Keeping oxygen contaminations from N-GHBs, the characteristic electrical conductivity of graphene was preserved in the as-synthesized N-GHBs. By controlling the evaporation temperature of melamine, the nitrogen-doping content of 8.7–14.0% and different nitrogen-doped configurations in N-GHBs could be adjusted. The catalytic activities of different nitrogen-doped states in N-GHBs toward the triiodide (I3−) reduction in DSSCs were investigated, revealing that the pyridinic and quaternary nitrogens, rather than the total nitrogen doping level, in N-GHBs are mainly responsible for their catalytic activities in DSSCs. For solar cell applications, the high surface area and heteroatomic nitrogens of GHBs can remarkably improve the catalytic activity toward the triiodide reduction, lower the charge-transfer resistance, and enhance the corresponding photovoltaic performance (7.53%), which is comparable to that (7.70%) of a standard sputtered Pt counter electrode-based cell. These exceptional properties allow N-GHBs/CC to act as a promising electrocatalytic electrode for DSSC and other electrochemical energy applications.