Understanding the electrocatalytic activity of counter electrode (CE) favors to tune the sluggish triiodide reduction kinetics and to compare their catalytic activities. We present here the synthesis of Co_XSn_1-X alloys (0 ≤ X ≤ 1) on reduced graphene oxide (RGO) under atmospheric pressure plasma without using any toxic chemical reagents. The developed materials are then applied as catalysts for the reduction of triiodide ions to iodide ions in CE of dye-sensitized solar cells (DSCs). It should be noted that the triiodide reduction reaction is one of the utmost significances in the advanced electrochemical energy conversion of DSCs. Therefore, insights into the relationship between alloy composition, electronic structure and catalytic activity should be carefully determined in this study. As the TEM measurements, CoSn nanoparticles with size in the range of 1-5 nm is successfully immobilized on the surface of RGO. Electrochemical catalytic activity performances indicate that the Co_0.9Sn_0.1/RGO gave the highest catalytic activity compared other electrodes which were determined by the lowest charge-transfer resistance and diffusion impedance values. Thus, the highest power conversion efficiency was obtained for the device using Co_0.9Sn_0.1/RGO CE. The strategy has a great promise in developing high-performance DSCs and the launched reduction rate constant having commonality in comparing the catalytic activity of CE electrocatalysts.