Microwave-Assisted Rapidly Synthesis Graphene-based Oxygen Reduction Reaction Catalysts And Their Electrocatalytic Performance

Owning to the advantages of high efficiency and zero pollution, fuel cells are the potential alternatives in the future. The oxygen reduction reaction in cathode is a slow and complex process, even reacting on the surface of the commercialized Pt-based catalyst, which resulting in a low exchange current density. At present, there are still some challenges for Pt-based catalysts, such as poor durability, easily poisoned by carbon monoxide(CO), and the high price. Heteroatom-doped graphene shows excellent catalytic activity, superior durability and the resistance to poisoning, which can greatly reduce the cost of fuel cells. The synthesis methods for doped graphene generaly is based on the treatment in the present of heteroatom sources and pristine graphene, and during the heating process, the oxygen groups on graphene are removed and the heteroatoms are doped into the framework of graphene. In this work, double-layer and porous reduced graphene oxide(r GO) was mixed with the heteroatom sources(melamine or p-toluene sulfonic acid), and the mixture was under a microwave-assisted heating process to synthesize nitrogen- or sulfur-doped graphene. The doped graphenes were emoloyed as catalysts and supports of platinum nanoparticles, and the catalytic properties of graphene-based catalysts were studied.The double-layer and porous r GO was rapidly obtained by thermal expansion of uniform graphene oxides powder in the microwave-assisted heating system. And the graphene oxide was prepared by a modified Hummers method, with ultrasonically promoted intercalating reactions. In this work, ultrasonic dispersion process was applied again after the neutral graphene oxide solution was obtained, therefore, producing uniform graphene oxide particles, and the solution was dryed by the small spray drying apparatus. Then the microwave-assisted heating rapidly removed the oxygen-containing groups on the surface of the graphene oxide and the double layer and porous r GO was syntheszied.By the research of catalytic properties of nitrogen-, sulfur-doped graphene, it was found that when the mass ratio for double-layer and porous r GO and melamine is 5:1, the nitrogen-doped graphene(NG-1) showed the best catalytic activity in the alkaline medium, its transfer electron number is about 3.84, which is very close to the four electronic efficient catalytic process. For nitrogen doped-graphene, the greater the defect degree, the greater the transfer electron number, and the degree of the defect in NG-1 was the largest with the analysised by Raman spectrum, therefore, exhibiting the best performance. while among the sulfur-doped graphene, the maximum transfer electron number was 1.87(SG-5), which was less than the number of r GO(2.33). And by the Raman analysis, the minimum degrees of the defect of SG-5 is 1.65, resulting in lower catalytic performance.After loading with Pt nanoparticles, the transfer electron numbers of Pt/graphene catalysts were increased, and the most influented sample was SG-1, increasing from 1.31 to 3.33, while the least is NG-1, only increased from 3.84 to 4.02. It was shown that the catalytic performance for NG-1 was comparable to Pt-based catalysts, indicating that NG-1 is a potential alternative for Pt-based catalysts as metal-free catalyst for oxygen reduction reaction.