Preparation Of The N(S) Doped Graphene Using Amino Acids As Heteroatoms Sources And Their Electrochemical Performances

Graphene, as a unique two-dimensional 2D monolayer honeycomb structure of carbon material, has shown enormous potential applications in various fields due to its excellent optical, electrical, mechanical and thermal properties. Substitutional doping, which introduces heteroatoms into the carbon lattice of graphene can effectively tailor the physical and chemical properties of pristine graphene and expand remarkably the application of graphene. Among various doping sources, nitrogen is the most widely available and thus most commonly used. Environmental and toxicity concerns related to most of the nitrogen sources motivate us to find more environment-friendly ones. In this thesis, amino acids, which are similar in chemical structure, were used as environment-friendly nitrogen sources to synthesize doped graphene via a one-pot hydrothermal method. Several factors influencing on the morphology and structure, and ultimately the electrochemical performance were studied systematically, including the type of amino acid and the p H value of the solution. In addition, the reason for the differences in electrochemical performance was illustrated based on the mechanism of interactions between amino acid and GO. The main results are summarized as follows:(1). Three types of amino acid with different acidities were selected as nitrogen sources to synthesis the nitrogen doped graphene(N-G). The effect of the amino acid’s acidity on the morphology, structure and electrochemical performance of N-G were explored and the affecting patterns were discussed. The results show that acidic amino acid yielded N-G possess cross-linked 3D network, larger specific surface area but lower nitrogen content, while N-G arising from the use of neutral amino acid, especially basic one showed a tightly stacked structure with a much smaller surface area and higher nitrogen content. The specific capacitance for the prepared samples are following the order of acidic > neutral > basic amino acids. The interaction mechanism between amino acid with different acidity and GO offers satisfactory basis for explaining the differences in morphology and capacitive performance of N-G.(2). On the basis of understanding the interaction mechanism that above mentioned, we changed the pH value of the mixture solution of acidic, neutral and basic amino acids with GO to prepare N-G. The influence of pH value on the morphology, composition and capacitive performance of N-G was investigated. It has been found that N-G shows serious aggregation with lower surface area, nitrogen content and specific capacitance when pH<p I. The increased pH value weakened the aggregation, and the surface area, nitrogen content and specific capacitance of N-G samples are increased when pH=pI. When pH>pI, the obtained N-G displays cross-linked 3D network and the samples’ specific surface area, nitrogen content and specific capacitance are further increased. According to the different interaction between GO and amino acid with different net charge, the influence rule of pH values on the samples’ capacitive performance is explained reasonably. In short, the capacitive performance of N-G can be well tuned by adjusting the pH value.(3). The nitrogen and sulfur co-doping of graphene(NS-G) with superior capacitive performance was synthesized using cysteine as the doping agent. The results show that the specific capacitance of NS-G reached 566 F g-1 at 0.5 A g-1. At a constant power density of 10 Wh kg-1, the energy density obtained for NS-G was 29.4 KW kg-1, and the capacitance retention of 95% was obtained after 2000 cycles. Meanwhile, the synergy effect between N and S was also explored. It is found that single and co-doped samples possess 3D network structure and the specific surface areas are comparably. However, the co-doping, compared with single doping, increaced the doping concentration, enhanced or improved thespecific capacitance and electrocatalytic performance toward oxygen reduction. We believe that the simultaneous incorporation of N and S species with the presence of O significantly modified the surface property of carbon in a grapheme sheet, which leading to considerably higher doping levels. As a result, the synergetic effect occurred in the performance of capacitance and electrocatalytic reduction of oxygen. Moreover, the co-doping of graphene(NS-Gm) was also synthesized using methionine as the doping agent. The effect of the different doping ratio on the capacitance and reduction performance in electrocatalysts of NS-G was explored. The results shown that the specific capacity of NS-Gm05 is 431 F g-1 at 0.5 A g-1, and the electrocatalytic performance of the prepared NS-Gm samples toward oxygen reduction reaction were increased as the increasing of doping ratio.