Synthesis And Electrocatalytic Performance For Oxygen Reduction Of Nitrogen-doped And Iron-nitrogen-doped Amorphous Carbon Films

Fuel cells are highly promising energy conversion devices because of their lowenvironmental impact and high conversion efficiency, but the prohibitive cost of noblemetals such as platinum as catalyst in oxygen reduction reaction (ORR) at the cathodehas hampered the widespread commercialization of fuel cells. N-doped carboncatalysts have been considered as promising alternative electrocatalysts for ORR. Inrecent years, although great process has been made on N-doped carbon catalysts bytheoretical and experimental studies, some key problems still need to be solved.1.Pyridinic N, pyrrolic N and graphitic N are believed to be catalytic active sites forORR, however, it is very difficult to regulate the chemical bonding states of doped Natoms by tuning the preparation parameters of the reported methods for revealingwhich chemical bonding state of N atoms plays a key role for ORR.2. The synthesismethod for N-doped carbon catalysts is usually complicated and heat treatment athigh temperature is required, so the exploration of a facile, low-cost method is highlydesired.3. In the reported methods for obtaining N-doped carbon catalysts, metals areusually employed as catalysts, and it is still not clear that what role Fe plays in ORR.Many people have believed that FeN4site embedded in the carbon matrix is an activesite for ORR. Others have proposed that Fe itself does not play a crucial role in ORRbut it may promote the doping of pyridinic N and graphitic N active sites. And someeven have believed that Fe cannot give any contribution to the electrocatalytic activityfor ORR. To solve the above key problems, the main research contents and the importantresults of this dissertation are summarized as follows:1. Metal-free N-doped amorphous carbon film catalysts for ORR can be obtainedby using a facile reactive magnetron sputtering and annealing. In order to effectivelyregulate the chemical bonding states of N atoms in N-doped carbon films, differentsubstrate bias (12.3~200V) are applied on the substrate and annealing (600~900oC) is performed. Via designing the preparation method and regulation of chemicalbonding states of N atoms, it demonstrates that the catalytic activity of the N-dopedcarbon films is highly dependent on pyridinic N, and pyridinic N is an active site forORR, which is important to the preparation of the ORR electrocatalysts with low costand high performance. The optimized N-doped carbon film with the highest activityexhibits not only excellent ORR electrocatalytic activity, but also much better stabilityand methanol tolerance than the commercial Pt/C catalyst in an alkaline solution. Thisinvestigation indicates that the metal-free amorphous N-doped carbon film enrichedpyridinic N is a potential alternative to Pt-based catalysts.2. In order to reduce the production cost of the catalyst, metal-free amorphousN-doped carbon films are synthesized using simple one-step magnetron sputtering atrelatively low temperature. The chemical bonding states of doped N atoms can betuned by adjusting substrate temperature (70~500oC) during deposition, and thisallows us to easily optimize the catalytic activity of N-doped carbon films for ORR.Through revealing a dependence of catalytic activity on the chemical bonding state ofdoped N atoms, we find that pyridinic N in N-doped carbon films plays a key role inelectrocatalytic activity. Via adjusting deposition time, the effect of the loading forcatalysts on ORR catalytic activity is studied. Although the catalytic activiy ofN-doped carbon film synthesized by a one-step approach is slightly lower thancommercial Pt/C catalyst, its stability and methanol tolerant are much better thancommercial Pt/C catalyst in an alkaline environment. Therefore, this N-doped carbonfilm catalyst is a cost-effective non-noble metal catalyst.3. Aiming at revealing the role of Fe in ORR, we have developed a facile strategyfor the synthesis of Fe-N-doped carbon catalysts by dripping FeAc(II) solution into N-doped carbon film enriched with pyridinic N and then annealing at800oC. Viatuning the Fe content and annealing temperature, the chemical bonding states ofdoped N atoms are effectively controlled and the catalytic activity of the films istailored. The result shows that Fe-N-doped carbon with an Fe content of0.2at.%annealed at800oC, in which Fe atom favorably forms high active FeN4sites withpyridinic N, shows a superior performance for ORR. The Fe-N-doped carbon catalystexhibits an excellent catalytic activity, stability and methanol tolerant via afour-electron pathway for ORR in alkaline media, which outperforms commercialPt/C catalyst. So the Fe-N-doped carbon film catalyst is a novel and efficientnon-noble metal catalyst.It is anticipated that the N-doped amorphous carbon and Fe-N-doped amorphouscarbon film catalysts will find wide application in alkaline fuel cells and directmethanol fuel cells because of their low price and high performance.