Study On Graphene As Supporting Materials For Proton Exchange Membrane Fuel Cell Electrocatalysts

Proton exchange membrane fuel cells have drawn great attention since the last century eighty’s due to its high energy density, high conversion efficiency, low operating temperature and quick response. However, in the vehicle power supply field, the performance of fuel cell need to be further enhanced to contest against the specific energy density of internal combustion engine. Synthesizing and searching for electrocatalyst with high activity is one of the important approaches to improve fuel cell performance. And the supporting materials are the key to the performance of catalysts, which have a direct effect on the dispersion and stability of catalysts. Carbon black (Vulcan XC-72, Cobat) is widely used as supporting materials for Pt/C, but the performance of Pt/C as well as long-term stability of carbon black under fuel cell operation is insufficient.Graphene, a two-dimensional sheet composed of sp2-bonded carbon atoms, has unique physical properties such as high surface areas (theoretical specific surface area of2620m2g-1) and superior electric conductivities. Therefore, Pt supported on graphene is gradually becoming a kind of promising fuel cell electrocatalyst. Moreover, the doping of nitrogen into graphene can further improve the properties of graphene, which can enhance the electrical conductivity and enrich free charge-carrier density. Besides, the nitrogen functional groups can serve as active sites for anchoring metal particles, strengthening the metal-graphene interaction. Thus, we have studied graphene as supporting materials for proton exchange membrane fuel cell electrocatalysts. The paper mainly focuses on:(1) Pt nanoparticles were deposited onto graphene sheets via synchronous reduction of H2PtCl6and graphene oxide (GO) suspension using NaBH4. Lyophilization was introduced to avoid irreversible aggregation of graphene (G) sheets, which happens during conventional drying process. Pt/G catalysts revealed a high catalytic activity for both methanol oxidation and oxygen reduction reaction compared to Pt/C. The performance of Pt/G catalysts was further improved after heat treatment in N2atmosphere at300℃for2h, and the peak current density of methanol oxidation for Pt/G after heat treatment was almost3.4times higher than Pt/C. Transmission electron microscope (TEM) images showed that the Pt particles were uniformly distributed on graphene sheets. X-ray photoelectron spectroscopy (XPS) results demonstrated that the interaction between Pt and graphene was enhanced during annealing.(2) A quick and efficient approach to prepare nitrogen doped graphene (NG) was proposed in this paper via microwave heating in NH3atmosphere. Results showed that graphene, as an allotrope of carbon, was a good microwave-absorbing material and can reach a high temperature in minutes, facilitating nitrogen incorporation into the structure under NH3. Pt/G and Pt/NG were prepared by microwave-assisted ethylene glycol. Transmission electron microscope (TEM) images showed that the NG improved the distribution of Pt particles. Themogravimetry (TG) and differential scanning calorimeters (DSC) revealed that better thermal stability of the Pt/NG can be obtained than that of the Pt/G. Furthermore, the electrochemical active surface area of Pt/NG catalysts was almost2.14times higher than Pt/G, which also exhibited higher methanol catalytic activity, better tolerance to CO poisoning and better electrochemical stability than those of the Pt/G under electrochemical test.Base on the above analysis, it suggests that graphene and N-doped graphene prepared by intermittent microwave have provided a new way to improve electrocatalytic activity in fuel cells.