Enhancement of diffusion kinetics in porous MoN nanorods-based counter electrode in a dye-sensitized solar cell

Abstract

Developing low-cost materials to replace platinum as a counter electrode in dye-sensitized solar cells (DSSCs) is very important towards commercialization of DSSCs. For replacing platinum, it is found that transition metal nitrides are promising candidates to efficiently catalyze the reduction of triiodide ions in DSSCs. However, mass transport limitations in the metal nitride electrode are one of the problems that require solutions. Herein, we show enhancement of diffusion kinetics for the active electrochemical process on the counter electrode of DSSCs with MoN fabricated into a porous nanorod morphology. A thin film composed of porous MoN nanorods on a metallic Ti foil substrate has been prepared for the first time by nitridation of a nanowire-like precursor of Mo₃O₁₀(C₆H₈N)₂·2H₂O, which was prepared hydrothermally by reacting ammonium heptamolybdate and aniline. The diameter and the length of the MoN nanorods are in the range of 40–100 nm and 0.5–2 μm, respectively. Compared with a MoN electrode composed of sphere-like nanoparticles typically with a diameter from 50 to 100 nm, the porous MoN nanorod counter electrode provides a higher electrochemical performance in DSSCs. The corresponding DSSC shows an energy conversion efficiency of 7.29%, which is superior to the 6.48% of the device using the MoN electrode with sphere-like particles. Even more significantly, this value is comparable with the conversion efficiency of 7.42% that was achieved using the conventional precious metal-based Pt-FTO (fluorine-doped tin oxide) electrode in a DSSC. These exciting results, which are comparable to those obtained by precious metal electro-catalysis, are attributed to enhancement of diffusion kinetics of the MoN counter electrode as a result of favorable morphology. The electrochemical impedance spectra (EIS) demonstrate that the diffusion resistance in the case of the porous MoN nanorod electrode is much smaller than that in the MoN nanoparticle electrode due to a larger porosity and interconnected channels for electrolyte in the porous nanorod-structured electrode. The results are helpful for developing highly efficient and low-cost electrodes through the understanding of the kinetics of the porous MoN nanorod counter electrode.