Heteroatom-doped graphene synthesis, characterization and investigation of energy storage and conversion applications

The rapid increasing energy demand around the world increases the importance of studies on energy production systems. Therefore, electrochemical energy storage and conversion systems have become the most effective means of meeting the energy needs of people. Among the alternative systems, fuel cells have high energy density, and supercapacitors have high power density. Heteroatom doped graphene materials are promising in increasing the energy obtained by using it as electrode material in both systems due to their exceptional physical and chemical properties. In this study, the synthesis of heteroatom-doped graphene materials were carried out in one step using the solvothermal method. The study was devided into two parts; in the first part, the reaction time, the amount of metallic reducing agent used and the pH conditions of the liquid mixture were optimized for the case of the production of nitrogen-doped graphene (N-GN). Structural characterizations of the products obtained from the optimization studies were carried out. The morphological structures of the samples were determined by SEM and TEM. The surface area values were calculated by processing the N2 adsorption/desorption data in the Brunauer-Emmet-Teller (BET) equation. X-ray diffraction technique (XRD) was used to determine the crystalline and/or amorphous structures of the samples, and Raman spectroscopy was used to observe the graphene structure and defective regions resulting from doping. The chemical structures of the products, the bonding patterns of the doped atoms to graphene and the relative ratios of the elements were determined by X-ray photoelectron spectroscopy (XPS). As a result of optimization studies, controllable nitrogen content (0.55%-4.04%), controllable surface area (1289.1 m2/g-2.12 m2/g) and high conjugated carbon atom content (C=C 54.6%) was obtained. Moreover, the wrinkled leaf structure specific to graphene was observed with SEM and TEM images, the presence of defective regions in the graphene structure from Raman spectrum data, and the amorphous structure of the materials from XRD patterns. Electrocatalyst activities of N-GN samples were tested by electrochemical characterization in 0.1 M HCIO4 and 0.1 M KOH electrolyte conditions. In this context, it was observed that the samples showed weak electrocatalyst activity in acidic conditions and significant electrocatalyst activity in alkaline conditions. In addition, voltammograms and charge/discharge curves were obtained in 6.0 M KOH and 3.0 M H2SO4 electrolyte solutions to determine the usability of the samples as electrode material in supercapacitors. From these data, except for the sample with the mixture pH adjusted as acidic, other samples have high current density, high specific capacitance (333.9 F/g), high energy (46.6 Wh/kg) and power density (26.8 kW/kg) values were found. In the second part of the study, the synthesis of silicon-nitrogen-doped (Si-N-GN) and iron-nitrogen-doped (Fe-N-GN) graphene products was carried out under the optimum conditions determined in the first part in order to increase the type of doped heteroatoms. Structural characterization techniques were applied to elucidate the structures of the samples. It was determined that Si-N-GN had silicon content and Fe-N-GN had iron content from the SEM-EDX data. Surface area values were calculated to as 523.2 m2/g and 482.5 m2/g for Si-N-GN and Fe-N-GN, respectively, and D and 2D bands were obtained for both samples in Raman spectrum data pointing out the defective structure. In the XRD patterns, on the other hand, the doping of silicon to the N-GN structure, the diffraction angle shifted to the left, and the diffraction peaks of FexOy compounds were observed in Fe-N-GN. From XPS spectrum data, the relative ratio of silicon atoms in Si-N-GN structure was calculated as 2.28% (at.%), and in Fe-N-GN as 1.8% (at.%). Electrochemical characterization processes applied to N-GN samples were also used for Si-N-GN and Fe-N-GN. In this context, electrocatalytic activity of both samples was tested in 0.1 M KOH electrolyte solution, and it was found that Si-N-GN showed oxygen reduction reactivity (ORR) close to commercial Pt/C and followed the highly desired 4 e- pathway for the reduction of oxygen. In addition, the capacitive behavior of Si-N-GN and Fe-N-GN were investigated at 0.2 A/g current density, specific capacitance values of 239.6 F/g (6.0 M KOH) and 267.5 F/g (3.0 M H2SO4) were obtained for Si-N-GN, and 130.2 F/g (6.0 M KOH) and 116.8 F/g (3.0 M H2SO4) were obtained for Fe-N-GN. It was determined that Si-N-GN largely preserved the specific capacitance and energy density in different current density ranges in all samples.