Porous N,S co-doped Carbon Mateirals from Natural Resources as Counter Electrode for Dye-Sensitized Solar Cells

Abstract

In the face of global energy and environmental crisis, renewable energy sources have become crucial as alternatives to fossil fuels. Among many energy sources, dye-sensitized solar cells (DSSCs) are attractive for their simple fabrication, low cost, and high power conversion efficiency. However, expensive Pt electrocatalyst, which has been conventionally used for triiodide reduction, should be substituted for more inexpensive alternative catalysts for large scale fabrication of DSSCs. Alternative catalysts are required to be highly electrocatalytically active and conductive as well as inexpensive and Earth-abundant. Many researchers have investigated alternative electrocatalysts, such as Pt-based alloys, transition metal alloys, transition metal compounds, conducting polymer and carbonaceous materials. Among these alternatives, carbonaceous materials have merits of lower cost and abundance. Whereas, intrinsic electrocatalytic activity of carbonaceous materials for triiodide reduction reaction is quite low. To overcome the low intrinsic activity of carbonaceous materials, various approaches like increasing roughness by tuning morphologies of the electrodes or increasing active sites by doping heteroatoms into carbonaceous materials have been tried. However, these approaches require expensive instruments or harsh and complex experimental conditions. In this research, we introduced waste coffee-grounds as a green electrocatalyst, especially for the triiodide reduction reaction. The coffee-grounds served as heteroatom-containing carbon precursors and were transformed into highly porous N,S co-doped carbon catalysts by one-step carbonization and activation process using ZnCl2 salt. The counter electrodes (CEs) were fabricated by spraying the obtained coffee catalyst ink onto the FTO glass. The activated coffee (AC) electrode exhibited superior electrocatalytic activity compared to non-activated coffee (NC) electrode due to tenfold increase in roughness of the electrode without significant loss of intrinsic catalytic activity. Also, the AC-CE exhibited better electrocatalytic activity than Pt-CE with charge transfer resistance (Rct) values at the interface of CE/electrolyte being 0.46 Ω cm2 and 1.46 Ω cm2 for AC-CE and Pt-CE. DSSC with AC-CE showed higher photovoltaic performance with a fill factor (FF, 73.2 %), a short circuit current density (Jsc, 15.07 mA/cm2), and a power conversion efficiency (η, 8.37 %), while those of the DSSC with Pt-CE were 71.0 %, 14.93 mA/cm2, and 8.06 %, respectively. In this research, carbon CE made from cheap and abundant coffee-grounds was proved to be a successful alternative to the Pt-CE.