Preparation And Electrochemical Performance Of Nitrogen Doped Graphene

Proton exchange membrane fuel cell (PEMFCs) is an ideal clean energy conversion device. PEMFCs use Pt based catalyst as catalysts of cathode and anode. But Pt resources are scarce, resulting in high cost of catalysts, limiting the development of PEMFC commercialization. Therefore, it is very important for the development of PEMFCs to use non-noble metal catalysts instead of Pt based catalysts. Among the many non-noble metal catalysts, nitrogen doped graphene (N-doped graphene) catalysts have a great of potential in the field of fuel cell technology because of their low cost, high catalytic activity and good stability.The improved Hummer oxidation was employed to prepare graphene oxide, and graphene oxide sol was obtained by ultrasonic stripping. N-doped graphene catalysts with different catalytic properties were prepared by low temperature hydrothermal method after uniformly mixing graphene oxide sol and the different added amounts of dicyandiamide. In addition, it was studied N-doped graphene catalysts made from the different chemical compositions and chemical structures of compounds (cyanamide, dicyandiamide, melamines) as nitrogen sources. TEM, HRTEM and XPS were employed to detect and characterize impacts of different added amounts of dicyandiamide as well as differe ntnitrogen sources (cyanamide, dicyandiamide and melamine) for morphology, chemical composition and valence states of N-doped graphenes. And the catalytic properties of N-doped graphene catalysts were researched by electrochemical method under alkaline conditions. The results were as follows:1. N-doped graphene sheets appear transparent gauze-like structure, sizes in hundreds of nanometer, and unique fold and crimp morphology of graphene materials. It is indicated that the prepared N-doped graphene catalysts maintain a good two-dimensional graphene flexible structure, and has a very high surface area.2. N-doped graphene prepared from dicyandiamides contains species of pyridinic N, pyrrolic N, and quaternary N. It indicates that the hydrothermal method makes nitrogen dope graphene lattice and surface, and keep the morphology of graphene, the proportion of graphite nitrogen in N-doped graphene catalysts have an important contribution for ORR activity.3. When the weight ratio of dicyandiamide to graphene oxide was 50:1, the stability of N-graphene-300 catalyst and the activity of ORR were the highest. Compared with the 20% of standard platinum carbon catalyst (JM-20%-Pt/C catalyst), ORR start reduction potential of N-graphene-300 catalyst has difference of only 50mV, and difference of only 70mV half wave potential, its limiting current density close to the limiting current density of JM-20%-Pt/C catalyst.4. The contrast of N-doped graphenes made from different nitrogen sources such as cyanamide, dicyandiamide, melamine and so on, it was found that the N-doped graphene made from dicyandiamide has the best catalytic performance, their electrocatalytic activity have no the corresponding relations with nitrogen content, but with nitrogen species (pyridinic N, pyrrolic N, and quaternary N) and their contents. The pyrrolic N formed in the doping process may hinder the oxygen reduction process, but the graphite nitrogen with high content can improve the ORR activity.