In the current study, the nanocomposite of molybdenum disulfide and multi-walled carbon nanotubes (MWCNT@MoS 2 ) was proposed for the first time as a counter electrode (CE) catalyst in dye-sensitized solar cells (DSSCs) to speed up the reduction of triiodide (I 3 À ) to iodide (I À ). This novel catalyst was synthesized by simply mixing MWCNTs and MoS 2 in an acidic solution and then converting the solid intermediate into the MWCNT@MoS 2 nanocomposite in a H 2 flow at 650 C. X-ray powder diffraction, Raman and X-ray photoemission spectroscopy confirmed the composition and the structure of the MWCNT@MoS 2 nanocomposite. The microstructure details of the nanocomposite were studied by transmission electron microscopy, showing that only a few-layers of the MoS 2 nanosheets were formed on the MWCNT surface. This unique structure is beneficial to the improvement of the catalytic activity of MWCNT@MoS 2 towards the reduction of I 3 À. The extensive cyclic voltammograms (CV) showed that the cathodic current density of the MWCNT@MoS 2 CE was higher than those of MoS 2 , MWCNT and sputtered Pt CEs due to the increased active surface area of the former. Moreover, the peak current densities of the MWCNT@MoS 2 CE showed no sign of degradation after consecutive 100 CV tests, suggesting the great electrochemical stability of the MWCNT@MoS 2 CE. Furthermore, the MWCNT@MoS 2 CE demonstrated an impressive low chargetransfer resistance (1.69 U cm 2 ) for I 3 À reduction. Finally, the DSSC assembled with the MWCNT@MoS 2 CE showed a high power conversion efficiency of 6.45%, which is comparable to the DSSC with Pt CE (6.41%).

In the current study, a nanocomposite of molybdenum disulfide and graphene (MoS 2 /RGO) was proposed for the first time as the counter electrode (CE) catalyst in dye-sensitized solar cells (DSSCs) to speed up the reduction of triiodide (I 3 À ) to iodide (I À ). This novel catalyst was synthesized by simply mixing graphene oxide nanosheets with a solution of ammonium tetrathiomolybdate and then converting the solid intermediate into MoS 2 /RGO nanocomposite in a H 2 flow at 650 C. Atomic force microscopy, X-ray powder diffraction and X-ray photoemission spectroscopy confirmed that MoS 2 nanoparticles were deposited onto the graphene surface. The extensive cyclic voltammograms (CV) showed that the cathodic current density of the MoS 2 /RGO CE was higher than those of MoS 2 , RGO and sputtered Pt CEs, due to the increased active surface area of the former. Moreover, the peak current densities of the MoS 2 /RGO CE showed no sign of degradation after 100 consecutive CV tests, suggesting the great electrochemical stability of the MoS 2 /RGO CE. Furthermore, the MoS 2 /RGO CE demonstrated an impressively low charge-transfer resistance (0.57 U cm 2 ) for I 3 À reduction. Finally, the DSSC assembled with the MoS 2 /RGO CE showed a high power conversion efficiency of 6.04%, which is comparable to the DSSC with a Pt CE (6.38%).

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