Facile synthesis of cobalt–nickel sulfide thin film as a promising counter electrode for triiodide reduction in dye-sensitized solar cells

In typical dye-sensitized solar cells, platinum metal-based counter electrode is employed to collect electrons from the external circuit and catalyze the triiodide (I₃⁻) reduction in the electrolyte. However, due to the high cost of platinum, development of inexpensive material based counter electrodes is highly desirable to lower the cost of production of the devices. Herein, we report a facile strategy to deposit a cobalt-nickel sulfide nanostructured thin film showing promising electrocatalytic characteristics, as a low-cost counter electrode material. A simple one-step solution-based fabrication method for the binder-free cobalt-nickel sulfide counter electrode at low-temperature is one of the key features of this work. The as-deposited cobalt-nickel sulfide film is highly crystalline even without thermal treatment. The results demonstrate that the as-obtained cobalt-nickel sulfide thin film on a conductive substrate can be employed as a counter electrode directly in a dye-sensitized solar cell. Cyclic voltammetry, Tafel plot and electrochemical impedance spectroscopy show that the as-synthesized cobalt-nickel sulfide counter electrode exhibits an excellent electrocatalytic property competing with the state-of-the-art platinum counter electrode. As a result, the dye-sensitized solar cell based on the cobalt-nickel sulfide counter electrode yields the best power conversion efficiency of 8.86%, which closely matches the performance of the dye-sensitized solar cell based on a platinum counter electrode (9.08%). The excellent electrocatalytic property obtained from the low cost and simple synthesis process could render the cobalt-nickel sulfide nanostructure standing out as a competitive alternative to the expensive platinum counter electrode. As such, this work overlays a path for the rational design of inexpensive, and so far achieved highly-efficient cobalt-nickel sulfide counter electrode for the advanced energy applications.