Electrocatalysts For Oxygen Reduction Reaction Based On Nitrogen-doped Graphene

To develop high performance low and non-platinum cathode catalyst for oxygen reduction reaction (ORR) is the key to success in commercialization of the proton exchange membrane fuel cells (PEMFCs) technology. More recently, graphene exhibit its promising potential for applications in fuel cell catalysis, due to its ultra-high specific surface area, good electrical conductivity and rich doping characteristics. Theoretical and experimental studies have indicated that nitrogen-doped graphene (NG) or iron and nitrogen co-doped graphene (Fe/NG) have certain catalytic activity for oxygen reduction. In addition, nitrogen functional groups on NG can serve as anchoring site for noble metal particles, thus enhancing the interaction between precious metal particles and graphene slice layer. This paper has carried out research on these two goals of NG. One is directly serve as the non-platinum catalysts for oxygen reduction and the other is as carrier for platinum nanoparticles to prepare low-platinum cathode catalyst. The main research contents and results are summarized as follows:(1) Study on the preparation and electrochemical properties of nitrogen-doped graphene catalyst. NG was synthesized by the pyrolysis method with graphene oxide (GO) as raw material and urea as the reducing-doping agent, and its electrocatalytic characteristics towards oxygen reduction was studied. The results showed that:(Ⅰ) Compared to pure substance, the decomposition rate of urea in precursor mixture was accelerated and its complete thermal decomposition temperature was lowered, due to the fact that oxygen-containing functional groups in GO strongly react with urea decomposition intermediates in the vicinity of200℃;(Ⅱ) Nitrogen functional groups in NG were composed mainly of the amino-like nitrogen at low heat-treatment temperature, which gradually converting to pyridinc nitrogen with the temperature rise and finally transforming into more stable graphite nitrogen. This gradual thermal transformation of nitrogen bonding configurations indicated that nitrogen atom was gradually incorporated into graphene lattice;(Ⅲ) The ORR activity of NG depended strongly on the pyrolysis temperature. NG pyrolyzed at900℃exhibited the best catalytic activity, while graphitic nitrogen reach the max content. This illustrated that graphitic nitrogen played an important role in ORR catalysis.(2) Study on the preparation and electrochemical properties of iron and nitrogen co-doped graphene catalyst. Single step heat treatment of precursor mixture consisting of Fe salts, urea, carbon black (CB) and GO resulted in an efficient non- precious metal electrocatalyst (Fe/NG/C) for ORR. The results showed that:(Ⅰ) The introduction of CB as spacer inhibited the agglomeration of the NG sheets and increase the specific surface area of the as-prepared electrocatalyst, ensuring the extensive use of catalytic active sites located in the surface of graphene sheets;(Ⅱ) Compared to pure NG, the addition of Fe species led to boost the catalytic activity of the resulting material, which suggested that iron-contained active site was more efficient than metal-free one;(Ⅲ) Because the strong etching effect on carbon materials during the heat-treatment process was avoided by employment of the solid urea as a nitrogen source for the formation of NG-based catalyst, the iron-contained active sites on the surface of the prepared materials were high density and uniform distribution;(Ⅳ) The activity of this NG-based catalyst which containing Fe and CB was equivalent to Pt/C in alkaline medium and approaching to Pt/C in acid medium.(3) Study on the preparation and electrochemical properties of nitrogen-doped graphene supported platinum catalyst. Platinum nanoparticles were successfully loaded on NG-based non-precious metal catalyst as substrate by ethylene glycol reduction method in order to prepare a low platinum cathode catalyst (Pt(Fe)/NG/C) for oxygen reduction. The results showed that:(Ⅰ) Compared to home-made Pt/C followed by the same preparation procedure, this kind of Pt-loaded catalyst had more evenly-dispersed Pt nanoparticles and narrower particle size distribution, thus its electrochemical active area were much higher;(Ⅱ) Due to synergy effect between NG support and Pt nanoparticles, its quality activity for ORR was further enhanced;(Ⅲ) More importantly, its stability performed significantly better than commercial Pt/C. This was because nitrogen functionalities of NG anchored Pt nanoparticles and provide resistance to their agglomeration and coarsening.