Nitrogen-doped graphene as a cathode material for dye-sensitized solar cells: effects of hydrothermal reaction and annealing on electrocatalytic performance

Abstract

We prepared nitrogen doped graphene (NG) by reacting pristine graphene oxide (GO) with urea hydrothermally and elucidated its usage as a Pt-free cathode material in the DSCs based on cobalt complex redox shuttles. In the process of hydrothermal reaction, the graphene oxide sheets are kept in a flocculating state. This experimental protocol prevents the graphene sheets from gelation and yields a maximum nitrogen content of 7.6 at%. The resultant NG sediments can be readily deposited onto conductive glass sheet for fabricating a porous cathode. The content ratio of the doped nitrogen atoms to the residual oxygen atoms is demonstrated as a determinant factor affecting the electrocatalytic activities of the as-prepared NG sheets. Annealing treatment to the NG cathodes gives rise to a remarkable increase of exchange current density. Moreover, the XPS results indicate that the electrochemically active pyridinic-N groups formed in the early stage of hydrothermal reaction are unstable to annealing. However, their thermal stability can be improved by extending the time of the hydrothermal reaction. By optimizing the composition of the embedded nitrogen species and oxygen containing groups, the DSCs with the NG cathode yield a maximal device efficiency of 8.2%.