A small quantity of uncontrollable cobalt (Co) and cobalt hydroxide (Co(OH)(2)), remaining in the cobalt sulfide (CoS) film prepared by the cyclic voltammetry (CV) electrodeposition method, will negatively influence the catalytic activity and stability of the CoS counter electrode (CE). The residual Co and Co(OH)(2) compounds are dissolved after a sodium hydrosulfide (NaHS) hydro-thermal treatment. CV, electrochemical impedance spectroscopy (EIS), and Tafel polarization curves indicate that the treated CoS CE exhibits superior electrocatalytic activity and stability for the I-3(-) reduction. The platinum-free dye-sensitized solar cell (DSSC) using the treated CoS CE obtains an enhanced photovoltaic conversion efficiency of 7.16% compared to that using the untreated CoS CE (5.59%) under full sunlight illumination (100 mW cm(-2), AM1.5 G). (C) 2015 Elsevier Ltd. All rights reserved.