Facile fabrication of ZnCo2S4@MWCNT as Pt-free counter electrode for high performance dye-sensitized solar cells

Substitution of platinum as a counter electrode material in dye-sensitized solar cells (DSSCs) is critical because Pt is rare and expensive. In addition, Pt-based counter electrodes have problems with the long-term stability of DSSCs. Among Pt-free electrocatalysts, composite materials are of great interest because they can be advantageous due to the combined performance of their components. Here, we present a ternary ZnCo2S4 composite grown on multi-walled carbon nanotubes as a Pt-free counter electrode for the reduction of triiodide in DSSCs. The composite ZnCo2S4@MWCNT was synthesized in a simple two-step process: (i) spray coating of a MWCNT dispersion on a FTO glass substrate, followed by (ii) solvothermal growth of the ternary sulfide ZnCo2S4 on a MWCNT layer. Cyclic voltammetry analyzes showed superior electrocatalytic activity of the composite in the reduction of triiodide compared to Pt. Electrochemical impedance spectroscopy measurements of symmetric dummy cells showed lower charge transfer resistance of ZnCo2S4@MWCNT (RCT = 5.54 Ω × cm2) than the traditional Pt electrocatalyst (7.07 Ω × cm2) in catalyzing the triiodide-to-iodide conversion reaction. The photovoltaic measurements of dye-sensitized solar cells showed an improved conversion efficiency of ZnCo2S4@MWCNT (8.55%) with excellent stability compared to Pt (8.19%).