Study Of Electrocatalysts For Oxygen Reduction Reaction In Fuel Cell

Oxygen reduction reaction(ORR)is of utmost importance in fuel cell technologies.Nevertheless,the ORR is kinetically very sluggish and strongly relies on precious and scarce platinum-based catalysts,which serious hampers the large scale production of hydrogen-powered fuel cell vehicles.At this juncture,design and fabrication of low-cost but highly efficient catalysts for ORR turns out to be an important research subject in fuel cell technologies.Current studies on ORR catalysts have been devided into two mainlines aiming at weakening or even fully eliminating the dependency on noble Pt: developing catalysts that contain only tiny amounts of platinum(low-Pt)or do not contain platinum(Pt-free).Based on above two principal lines,and taking the primary issues exsisted in low-Pt and Pt-free research field into account,four works were carried out in this thesis as followed:(1)In order to tackle both durability and activity issues of traditional Pt/C catalyst,this work elaborately introduced a nitrogen-doped graphitic carbon(NGC)layer onto Pt/C surface.The results of electrochemical tests clearly showed that the introduced NGC shell would not affect the transfer of oxygen into the inter Pt sites due to its porous nature.And when compared with the un-modified Pt/C catalyst,the as-constructed Pt/C@NGC catalyst exhibited an evidently enhanced activity toward ORR as its Pt mass and specific activity turned out to be nearly 1.7 and 2.5 times higher than those of Pt/C catalysts.Pt/C@NGC also demonstrated a significantly improved stability compared with that of the unmodified Pt/C catalyst.After 1500 cycles,the Pt/C@NGC catalyst showed 8% diminution in the Pt ECSA and only a 16 mV degradation in half-wave potential,whereas the loss of Pt ECSA was as high as 41% and the shift in the half-wave potential was 61 m V for Pt/C catalyst.The high stability and activity of Pt/C@NGC catalyst can be ascribed to its novel NGC decorated core-shell structure,which induces a special electronic interation between Pt and NGC layer as disclosed by Pt4 f peak analysis.(2)We then applied above “N-doped carbon layer modification”strategy into PtNi alloy system in order to further promote the electrochemical performance of supported Pt Ni nanoalloy under ORR working condition.Electrochemical experiments demonstrated that ORR activity of as-prepared PtNi/C@NC catalyst strongly depends on the content of PANI precursor,and the greatest enhancement in ORR catalytic activty occurs at PANI wt.% of 60%.The half wave potential for PtNi/C@NC-60% was 23 mV and 45 mV higher than that of PtNi/C and JM-Pt/C,respectively.Accelerated durability tests(ADT)results showed that the ECSA of PtNi/C@NC-60% after 1500 cycles drops only 6% and its half-wave potential only degrads 22 mV,which is more stable compared with un-decorated PtNi/C and commercial JM-Pt/C under the same reaction conditions.The result obtaind from comparision of Pt4 f peaks of PtNi/C and PtNi/C@NC is similar as that in above single Pt system,namely,the NC decoration can induce an electronic interaction between Pt and outer NC,implying that the developed “N-doped carbon layer modification”strategy is universal.(3)The nitrogen-doped carbon catalysts are often suffered from low nitrogen utilization and insufficient electronic conductivity.Taking these issues into account,this work developed a core-shell nanostructured CNT@NC composite with an N-doped carbon shell well-wrapped around a carbon nanotube(CNT)core based on a “MnO2 sacrificed template”method.HRTEM analysis clearly evidenced the product has a core-shell nanostructure and the thickness of the NC shell is in the range of 6-11 nm.The prepared CNT@NC presented a superior ORR activity in alkaline media and its value of the half-wave potential was only 25 mV lower than that of commercial JM-Pt/C.Combined with electrochemical impedance spectroscopy,we ascribed the high ORR activity of the CNT@NC to the well-contacted CNT electron highway,which assures the rapid electron transport to outer N-containing catalytic active sites and alleviates low electronic conductivity issue casued by high doped ratio of graphitic N.Additionally,the introduction of CNT can lift the surface area of CNT@NC and promote the dispersion of N-containing active sites,which also favors for the enhancement of ORR activity.(4)In order to further boost ORR activity of the most promising candidate,i.e.,Fe/N/C catalyst,and provide deep insights into its nature of active sites,we synthesized high active Fe,N,S tri-doped Fe/N/S/C catalysts and investigate the influence of Fe,N,S content on their catalytic activity.ORR tests showed that with the increase of Fe content,the ORR activity of Fe/N/S/C rose first and then fell,and the greatest enhancement in ORR activty was o bserved with weight ratio of Fe3+ to PmPDA at 0.70.The half-wave potential difference between 0.70Fe/N/S/C and commercial JM-Pt/C catalyst in acid medium was only 41 mV,and the maximum outputting power of MEA using 0.70Fe/N/S/C as cathode catalyst reached 530mWcm-2 which is very approaching to that of JM-Pt/C.Integrated with the results acquired by ORR tests,XRD and XPS,we deeply analyzed the relationship between ORR activity and total doping content of N,S,Fe element,as well as the content of each controversial active site such as Nx-Fe site,pyridinic N,graphitic N and C-S-C structure.We found that Nx-Fe sites contribute to ORR activity dominatly and its conetent is consistent with the order of ORR activity.We further investigated the influence of iron salts on ORR activity and speculated that the content of Nx-Fe active sites may strongly associated with the type of coordination anion in iron salts.