Investigation of carbon nanotubes decorated with cobalt sulfides of different phases as nanocomposite catalysts in dye-sensitized solar cells

•CNT@CoS1.097 and CNT@Co9S8 were prepared via annealing at different temperatures.•CNT@Co9S8 showed the superior electrocatalytic activity to CNT@CoS1.097.•The cell efficiency of the Pt-free DSC reached 7.78% using the CoS CNT@Co9S8 CE.

In this study, nanocomposites of cobalt sulfide and carbon nanotube (CNT) in the forms of CNT@CoS1.097 and CNT@Co9S8 were deposited on fluorine-doped SnO2 (FTO)-coated glasses by using a facile spray-coating approach, followed by annealing under N2 atmosphere at 400 and 600 °C, respectively; and employed as counter electrodes (CEs) for dye-sensitized solar cells (DSCs). According to the electrochemical properties of CEs quantified by cyclic voltammetry, electrochemical impedance spectroscopy, and Tafel polarization measurements, the CNT@Co9S8 CE showed a superior electrocatalytic activity towards triiodide (I3) reduction to that of the CNT@CoS1.097 CE. As a result, the DSC based on the CNT@Co9S8 CE achieved an impressive photovoltaic conversion efficiency of 7.78%, which was even higher than those of the DSCs assembled with the CNT@CoS1.097 CE (7.29%) and Pt CE (7.46%). This signifies that the CNT@Co9S8 was indeed an efficient and cost-effective Pt-like alternative CE for I3 reduction reaction in DSCs.