Electrocatalytic Properties of MoS2 In-situ Grown on Graphene Oxide as Counter Electrode for Dye-sensitized Solar Cells

Abstract

Dye-sensitized solar cells (DSSCs) have attracted great attention as one of next-generation photovoltaic devices due to low cost and simple fabrication process. A conventional DSSC consists of TiO2 photoelectrode, counter electrode (CE) and liquid electrolytes containing iodide/triiodide (I/I3 -) redox couple sandwiched between the electrodes. The CE serves to collect electrons and catalyze redox couple regeneration; thus, its material and performance determine the cost and photoelectric conversion efficiency of DSSC. Generally, noble Pt is employed as a CE owing to its rapid electron transfer ability and superior electrocatalytic behavior toward I3 - species. However, Pt is expensive and limited in resource. Hence, it is vital to find inexpensive and readily available non-Pt catalysts for the development of DSSCs. Carbon materials, conductive polymers, transition-metal compounds, metal alloys, and their composites have been explored to replace Pt. Among these alternatives, transition-metal compounds including carbides, nitrides, chalcogenides and oxides have demonstrated some desirable performances. MoS2 consists of three atom layers: one Mo layer sandwiched between two S layers via weak Vander Waals interactions. Advantageously, MoS2 has abundant exposed active sulfur edges for catalytic activity. On the other hand, graphitic carbon nitride (g-C3N4) demonstrates excellent thermal and chemical stability as well as interesting electric properties, which make it have good catalytic activity for a variety of reactions. In addition, g-C3N4 can be economically prepared in large scale. Herein, we present the fabrication and characterization of porous g-C3N4 nanosheet/MoS2 nanoparticle (g-C3N4/MoS2) composite. Such an architecture can provide short diffusion distance for excellent charge transport as well as large contact area for fast interfacial charge separation and electrochemical reactions. As a result, the nanocomposite is anticipated to exhibit good electrochemical and catalytic activities. In this study, we first developed a solution-based coating method to prepare g-C3N4/MoS2 counter electrode. Through a facile hydrophilization process for g-C3N4 and sintering the Mo-S precursor coated g-C3N4 at 280 °C for 12 h, crystalline MoS2 nanoparticles were formed on g-C3N4 substrate, resulting in strong mechanical adhesion. In addition, low-cost solution-processed g-C3N4/MoS2 electrode had similar electrocatalytic behavior toward I3 - species to Pt. As a result, the DSSC based on g-C3N4/MoS2 demonstrates desirable photovoltaic performance.