Preparation Of The Biomass-derived Doped Carbon Catalysts And Their Electrocatalytic Performance Towards Oxygen Reduction Reaction

Low-temperature proton exchange membrane fuel cells(PEMFC) is widly recognized as a promising energy technology due to their advantages, such as high energy conversion efficiency, no/low emission, quick startup, mild operation temperature etc. However, the high cost, mainly caused by the using of Pt catalyst, is becoming the biggest obstacle to obstruct commercialization of the low temperature PEM fuel cells. Especially, high loading of Pt-based catalysts on the cathode is needed to facilitate the sluggish oxygen reduction reaction(ORR) at the current technology status, and the high cost and unsustainability of low temperature fuel cells resulting from the expensive and scarce Pt are becoming one of the most important factors. Thus, researching and developing the inexpensive non-Pt cathodic catalysts with both high catalytic activity and durability to substitute the expensive Pt-based catalysts has been regarded as a feasible way to reduce the cost of the fuel cells, and it is also a key technology difficulty for the scientific community to overcome. Among all the non-Pt cathodic catalysts, heteroatom doped carbon materials have been received extensive attention of researchers due to their high catalytic performance, less sensitivity to methanol and excellent stability.In this thesis, drawing lessons from other disciplines, a series of biomass-derived doped carbon materials with high specific surface area and various morphologies have been prepared by the appropriate pretreatments, followed by high-temperature pyrolysis, with the new cheap natural biomass as precursors materials. Combinated with catalyst structure parameters such as morphology and chemical composition et al., effects of some factors on the performance of the catalyst are investigated and discussed, based on the the evaluation and the characterizations results. Further, the catalytic mechanism of this biomass-derived doped carbon materials is also explored preliminarily.Firstly, biomass derived nitrogen doped carbon catalyst has been prepared by a chemical activity-pyrolysis method, using raw soybean as the carbon and nitrogen precursor, using Zn Cl2 as activator. The catalyst has high specific surface area high up to 1000 m2 g-1, with the self-doped nitrogen content of 3 wt%. As a new type of doped carbon catalyst in alkaline medium( 0.1 M KOH solution), its ORR catalystic activity is compared to commercial platinum catalyst with the half-wave potential difference only 30 m V. The experimental results show that the catalyst has such good catalytic performance mainly depends on the cause of its high specific surface area after activation and the self-doped of nitrogen element inherent in raw biomass in carbon lattice. It is found that the pyridinic nitrogen and graphitic nitrogen play important role for the catalyst, it may be the most important components to the catalytic active site.Secondly, we select a marine algae-nori, which has special thin layer structure, as the precursor, and have prepared nitrogen doped graphene-like carbon catalyst with BET surface area of 550 m2 g-1 via hydrothermal carbonization, adding melanine to dope nitrogen element and pyrolysis process following by acid leaching with H2SO4 and graphitization. The existence of micro-, meso- and macroporous is in favors of the mass transmission during the ORR process. The addition of melamine not only increases the total nitrogen contents in the catalyst(about 3 wt%), but also improves the percentage of active nitrogen, including graphitic nitrogen, pyrrole nitrogen and pyridine nitrogen, which are conductive to promote the catalytic activity of the catalyst. The catalyst exhibits excellent catalytic activity towards oxygen reduction reaction comparable to commercial 20 wt% Pt/C, with the onset potential and half-wave potential reach to 0.97 V and 0.85 V(vs. RHE) in 0.1 M KOH. Meanwhile, furthermore, the catalyst exhibits excellent tolerance towards methanol, as well as excellent stability. It is suggested that the high catalytic activity of the catalyst results from its high surface area of graphene-like structure and high active center density, which may be caused by the doping of heteroatom in carbon lattice.Thirdly, N, F co-doped carbon catalyst with micron fiber structure has been prepared by hydrothermal treatment the mixture of silk cocoon powder and PTFE firstly, following by high temperature pyrolysis. The content of self-doped nitrogen in the doped carbon is about 3.0 wt%, and fluorine content is up to 7.5 wt%, and very little sulfur is detected. It is found that the addition of PTFE could enhance the surface area and performance significantly. Its surface area(BET) could be increased from 516 m2 g-1 to 1033 m2 g-1 for the samples without/with addition of PTFE. We suggest that the doping of fluorine may cause the change of the original carbon structure, and generate abandent micro- and mesoporous structure. The catalyst exhibits excellent activity towards the ORR as well as the excellent tolerane towards methhol and stability. In 0.1 M KOH solution, its onset potential, half-wave potential and limiting current density for ORR are 0.99 V, 0.84 V(vs.RHE) and 5.80 m A cm-2, respectively, which are comparable to those of commercial 20 wt% Pt/C catalyst. To understand the ORR mechanism on our catalyst, Tafel curve and K-L equation measurements and calculation have been conducted. It is found that the ORR follows four electrons transfer mechanism, similar to that of the commercial Pt/C catalyst. It suggested that the high ORR catalytic activity of the catalyst could be attributed to the co-doping of nitrogen and fluorine, as well as the large surface area and special feature of this catalyst.Finally, we attempte to use natural spirulina as biomass precuorsor to prepare a new type biomass derived nitrogen self-doped porous carbon materials with addition of glucose as additive. The mixture of spirulina and glucose was firstly hydrothermal carbonized in a autoclave, then pyrolyzed at 900 oC for 0.5 h hours under NH3 atmosphere, followed by acid leaching and graphitized at 900 oC again. It is found that the addition of glucose can further increase the nitrogen and carbon contents in the catalyst, enhancing its catalytic activity. For a optimal sample, the self-doped nitrogen content reaches to 5.3 wt% and the specific surface area is up to 1610 m2 g-1, along with abundance micro- and mesoporous structures. The onset potential of the catalyst is about 0.01 V(vs. Ag/Ag Cl) in 0.1 M KOH, which is superior to commercial Pt/C catalyst. The catalyst also shows excellent tolerance to methanol and stability. It is suggested that the high catalytic performance can be attributed to the self-doping of nitrogen inherent in biomass, as well as the high specific surface area and abundance porous structures generated by the escaping effect with NH3 atmosphere during pyrolysis process. Meanwhile, the catalyst also exhibits good catalytic activity in acid medium(0.1 M HCl O4).