| The porous carbon materials with high specific surface area, developed pore structure and good electric and thermal conductivity were widely used in energy conversion and storage, industrial catalysis, gas adsorption and separation. In recent years, hierarchically porous carbon with three-dimensional connected micro-meso-macro pore structure can greatly improve the mass transfer of reactants, which were beneficial for the enhancement of catalytic activity for oxygen reduction reaction, thus possessing bright prospects in fuel cell applications. The biomass, which is enviromently friendly and widely spread, is coming up to be an ideal precurors to prepare hierarchically porous carbon. Here, we selected cheap and availble porphyra tenera as precuror to prepare series of hierarchically porous carbon by tuning pyrolysis conditions, and disclosed the carbonization mechanism for porphyra tenera.Firstly, the hierarchically porous nitrogen-doped carbon with high specific surface area and three-dimensional honey comb-like pore structure was prepared after pre-carbonization and activation processes. It was found that the abundant protein and polysaccharide were the origin of carbon and nitrogen elements, what's more, calcium alginate in porphyra tenera could be adopted as a natural templete to tune pore structure during activation process. When the ratio of activating agent of potassium hydroxide to pre-carbide was 2 and the activation temperature was 900 0C, the biggest specific surface area of 2152.5 m2 g-1 was obtained, and pore volume was 1.03 cm3 g-1 in which micro-pore was dominant (91.6 %), and nitrogen content was 1.8 at.% in the hierarchically porous nitrogen-doped carbon.Secondly, we used the porphyra tenera-derived hierarchically porous nitrogen-doped carbon as support to carry phthalocyanine iron molecules with Fe (?)-N4 centers through adsorption method, and then after low temperature heat treatment, molecular structured Fe and N co-doped hierarchically porous carbon (Fe-N-C) catalyst was obtained. We studied the effects of carbon supports and phthalocyanine iron loading on catalytic performance. It was found that the porous sturcture and N-doping of porphyra tenera-derived carbon were beneficial to improve phthalocyanine iron loading and strengthen the bonding between phthalocyanine iron and carbon supports, thus enhancing ORR performance. The derived Fe-N-C catalyst whose pyrolysis temperature was 400 ?, possessed the best ORR performance with a half-wave potential of 0.88 V, which was 40 mV more positive than that of commercial Pt/C, and it's electron transfer number was 4, suggesting an ideal 4 electron process.What's more, after 3000 continuous potential cycles, It's half wave potential only shifted 6 mV negatively, the results demonstrated that the derived Fe-N-C catalyst possessed much stronger durability than that of Pt/C.According to the restriction impacts of abundant micro-pore structure and surface heteroatom doping on activity center at atomic level, we adopted hemin as natural Fe and N sources, porphyra tenera-derived porous carbon as supports, and tranferred the original Cl-Fe(?)-N4 in hemin with low catalytic activity into high activity Fe(?)-N4 structure which was firmly bonded with carbon supports by adsorption and subsequent high temperature heat treatment at 800 ?. It was found that Fe atoms were uniformly dispersed in the derived single atom-like Fe-N-C catalyst. Electrochemical studies showed that the derived single atom-like Fe-N-C catalyst possessed a half wave potential of 0.87 V,30 mV more positive than that of Pt/C, and it's kinetic current density at 0.88 V was up to 4.1 mA cm-2. what's more, the methanol tolerance and durability were also much better than those of Pt/C. Due to the carbonization of organic phase in hemin, the bonding between Fe and C atoms was much stronger than that of previous molecular structured Fe-N-C catalyst, leading to a more excellent durability, the half wave potential of which was just shifted 1 mV negatively. |
PDF Full Text Request