Preparation Of The Doped Carbon-based Catalysts And Their Catalysis Towards Oxygen Reduction Reaction

Proton exchange membrane fuel cells(PEMFCs) have been recognized as a kind of advanced energy technologies, due to their high energy conversion efficiency, low/zero emission, operation convenient and friendly, and fast response etc. In today’s energy crisis and environmental pollution, it meets the requirement of the ideal power sources for the mobile applications. Generally, the cathodes oxygen reduction reaction(ORR) is sluggish; and a lot of platinum catalysts had to be used to accelerate ORR in order to improve the reaction rate and efficiency. To date, platinum is the first-rate catalyst for PEMFCs both activity and stability. However, platinum is expensive and scarce. The use of a lot of Pt-based catalysts has are becoming one of the most important factors seriously blocks the large-scale commercialization of fuel cells. And the catalyst technology has been the bottleneck in the development of PEMFCs. Therefore, developments of low-platinum or platinum-free catalysts are very urgent issue for the development of a fuel cell. Recently, a lot of research show that the doping carbon catalyst, especially nitrogen and/or transition metal doped carbon catalyst, exhibited excellent catalytic ORR activity, high stability, and impressive immunity towards methanol.In this thesis, we have prepared a series of doped carbon catalyst with melamine and some nitrogen rich materials as precursors. And the catalyst structure and performance of N-doped carbon metal-free catalyst, the catalyst structure and performance of N- and Feco-doped carbon catalyst, the probable roles of Fe and melamine to N- and Fe- co-doped carbon catalyst, and the effect of late transition metals(Mn, Fe, Co, Ni, Cu) on the structure and performance of the doped carbon catalysts have been intensively investigated in both experimental and theoretical aspects.Firstly, an N-doped carbon metal-free catalyst for ORR is prepared by the pyrolysis of the hybrid precursor of polyaniline and melamine in Ar atmosphere. The effects of pyrolysing temperature, heat treatment method on the performance are investigated. And the catalysts are characterized by means of XRD, TEM, Raman, FTIR and rotating disk electrode. The results show that 900 oC is the optimal pyrolysis temperature. It was found that the performance of the catalyst could be enhanced greatly through twice treating(pyrolysis and annealing). And the oxygen reduction on the catalyst follows a four-electron transfer mechanism.Secondly, we prepare a Fe- and N- doped carbon catalyst Fe-PANI/C-Mela. And we observe that Fe-PANI/C-Mela has a graphene structure and its BET surface area is up to 702 m2 g–1. Interestingly, the ORR onset potential of Fe-PANI/C-Mela is high up to 0.98 V in 0.1 M HCl O4 electrolyte; and the half-wave potential is ca. 60 m V less than that of the commercial Pt/C catalyst(Loadings: 51 μg Pt cm-2). The catalyst also shows high stability after 10,000 cyclic voltammetry cycles tests. And a membrane electrode assembly made with Fe-PANI/C-Mela catalyst as a cathode is tested in a H2-air single cell; the maximum power density is reached ca. 0.33 W cm2 at 0.47 V.Thirdly, inspired by high performance of Fe-PANI/C-Mela and as a continuation of our research interests in Fe- and N- co-doped graphene-like structure carbon catalyst, we prepare a doped carbon catalyst Fe/o PD-Mela by pyrolyzing hybrid precursor of poly o-phenylenediamine(o PD), melamine and iron in an Ar atmosphere. The results show that the Fe/o PD-Mela has a high surface area, high graphitic and pyridinic nitrogen content. And Fe/o PD-Mela also exhibit high performance towards the ORR in both alkaline and acid mediums; actually its activity surpassed that of commercial Pt/C catalyst in an alkaline medium. In 0.1 M KOH, the half-wave potential is high up to 0.91 V, which is about 50 m V higher than that of Pt/C, and the current density at 0.90 V is about 2.4 times to that of the Pt/C catalyst. And in an acidic medium, its activity approaches that of a Pt/C catalyst. Furthermore, this catalyst also showed high stability and impressive immunity towards methanol. The probable roles of added iron and melamine are investigated intensively. We find that the addition of melamine plays an important role of increasing the content of active nitrogen species, changes the ORR reaction mechanism form the 2-electron transfer mechanism of Fe/o PD to 4-electron transfer mechanism of Fe/o PD-Mela and facilitating the incorporation of N atom into the matrix of graphitized carbon. Furthermore, the melamine and Fe co-doped will improve the formation of graphene structure, increase BET surface area of the final product, and facilitate to form more Fe-N4 and CFe N2 species and both of them are playing an important role in the ORR activity of catalyst Fe/o PD-Mela.Finally, the effects of dopoed transition metals non-precious metal on the structure and performance of the doped carbon catalysts- M-PANI/C-Mela(M=Mn, Fe, Co, Ni, Cu), are intensively investigated in both experimental and theoretical aspects. It is important to note that doping of various transition metals will result in the different catalyst’s structure and performance. Doping with Fe or Mn forms a graphene-like structure, and doping with Co, Ni and Cu forms a disordered carbon or graphite structure. Furthermore, we find that the transition metals type also affects the distribution of various N statuses, surface area and micropores volume. The order of ORR activity of the catalysts doped with various metals is Fe>Co>Cu>Mn>Ni. We found that in co-doped carbon catalysts, the performance enhancements are contributed by surface area, nitrogen contents, active nitrogen proportion, and co-doping of transition metals etc, it seems that no factor play a dominant role; all of the factors affect the catalyst activity synergistically.