The Preparation Of Three Dimensional N Or S Doped Graphene Materials And Their Application In Dye-sensitized Solar Cells

Dye-sensitized solar cells have attracted much attention because of their advanced properties. The main function of the counter electrode, which is a significant part of the DSSC, is to catalytic the reduction of I₃^-. In this paper, we investigate the preparation of N-doped and S-doped three dimensional graphene via one-step hydrothermal process as well as study its application in dye-sensitized solar cell. During the preparation, we vary the reaction conditions, such as types and concentration of precursors, the hydrothermal reaction time and temperature to achieve better morphology. In additional, we characterize the prepared samples with scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis, N₂ isothermal adsorption desorption tests, X-ray photoelectron energy spectrum and so on. Furthermore, we proved that N-doped and S-doped three dimensional graphene have remarkable electrocatalytic properties through electrochemical characterization. Finally, we successfully fabricate dye-sensitized solar cells based on doped graphene counter electrode and C106 dye, and study its electrochemical properties. The achievable efficiency of the SG electrode device can be as high as 7.19%. The main conclusions are as following:（1） Three dimensional nitrogen doped graphene materials are synthesized by one-step hydrothermal method using graphene oxide（GO） and nitrogen sources. The influence of the type and concentration of nitrogen sources, hydrothermal reaction time and temperature on the morphology and electrochemical properties of the NG are studied. When ammonium acetate is used as the nitrogen source, at low concentration the NG material’s specific surface area is larger. With the increasing of the concentration, NG material will aggregate, leading to reducing in specific surface area. Experimental results show that all the prepared NG materials have better catalytic ability than the reduced graphene oxide（RGO）, especially when the hydrothermal reaction is carried out at 180 ℃ for 12 h, with 3 mM ammonium acetate as the nitrogen source. That NG material exhibits highest catalytic activity in the reduction of I₃^-, which is similar with the platinum, and the most optimal three dimensional morphology. That is caused by the introduction of more lone electron pairs by nitrogen atoms to graphene lattice, which leads to the additional electrocataytic activity. Meanwhile, the three dimensional morphology can increase the specific surface area of the material, so that the electrolyte can rapidly diffuse to the active site. In the characterization of devices, under simulated AM 1.5G irradiation（100 cm-2 MW）, the short-circuit current density（Jsc）, open-circuit voltage（Voc）, FF, PCE of NG device are in turns 15.53 mA cm-2, 0.68 V 0.61, 6.47%. The NG device efficiency is much higher than that of the RGO device（4.08%）, and is similar with the platinum device（7.57%）.（2） Three dimensional porous sulfur doped graphene materials are synthesized by one-step hydrothermal method using GO and sulfur sources. The influence of the type and concentration of sulfur sources, hydrothermal reaction time and temperature on the morphology and electrochemical properties of the SG are studied. When 3.2 mM 3-mercaptopropionic acid is used as the sulfur source, the SG material reveals web-like structure. Mesoporous and macroporous structures can be easily observed from the scanning electronic microscopy. When the concentration is smaller than 3.2 mM, the SG material structure changes from sheet structure to web structure with the increasing of concentration. While when the concentration is larger than 3.2 mM, the increase in concentration will lead to the aggregation of SG material, reducing the specific surface area. Experimental results show that all the prepared SG materials have better catalytic ability than the RGO, especially when the hydrothermal reaction is carried out at 180 ℃ for 12 h, with 3.2 mM 3-mercaptopropionic acid as the sulfur source. That SG material exhibits highest catalytic activity in the reduction of I₃^-, which is also higher than the platinum, and the most optimal three dimensional morphology. This is because of the additional electrochemical activity of the SG material due to the lone pair electrons of the sulfur atoms embedded in the graphene lattice and three-dimensional porous structure can increase the material specific surface area and is conducive for the electrolyte to rapidly diffuse to the active sites of SG. Under simulated AM 1.5G irradiation（100 cm-2 MW）, the Jsc, Voc, FF, PCE of SG device are in turn 15.95 mA cm-2,0.69 V, 0.65, 7.19% and the device efficiency achieved can be comparable with Pt（7.51%）. its Jsc higher than the Jsc of Pt device（15.40 mA cm-2）.