Composition and Structure Regulation of Doped Carbon-based Electrocatalysts for Oxygen Reduction
Author:Luo Er Gui
Supervisor:liu chang peng xing wei
Fuel cells have been recognized as one representative of the most promising clean energy conversion devices contributing to address the climate and energy issues.However,the electrode reactions rely heavily on the costly Pt-based catalysts.Especially,the catalyst to drive the kinetically sluggish oxygen reduction reaction(ORR)constitutes nearly 40-50%of the overall cost.Moreover,the poor long-term durability of Pt-based catalysts shortens the life of fuel cells,further making the practical application of fuel cells hindered.Thus,exploiting low-cost,efficient and robust catalysts for ORR remains a critical step for the popularization of fuel cells.In this regard,in virtue of the rational design on composition and structure,a series of carbon-based non-noble metal and metal-free ORR catalysts were prepared through facile approaches,deriving from cheap precursors.The active sites,performances and structure-activity relations of these catalysts were also studied.More specific key points from our study are:1.Carbon-based oxygen reduction catalysts with dense active sites(1)Considering pyridinic N and pyrrolic N that located on exposed edge planes are more active towards ORR.We present the design and fabrication of selectively pyridiruc and pyrrolic N doped carbon by a CO2 activation method using nano-CaCO3 as template.The released CO2 reacts with carbon atoms and serves as an activating agent to produce exposed edges,which promotes the edge-N doping.Meanwhile,the materials possess 3D structure with hierarchical pores.The catalyst exhibits significantly enhanced ORR activity compared with the template-free sample in alkaline media with half-wave potential of 0.853 V.This work opens a new avenue to fabrieate high-performance earbon一based eatalysts.(2)To realize the high and effective doping of Fe in Fe/N/C catalyst derived from ZIF,we explore the influence of Fe source added at different stages and the dosage on the final activity of Fe/N/C catalysts.It is found that when FeCl3 is introduced directly in the ZIF forming process,the doping level is quite low.Only if the amount of Fe source is very high,Fe can be incorporated into the precursor in the form of hydrolyzate,which results in more nanoparticles in the products.When Fe is introduced by post-adsorption method after synthesis of ZIF-8,the hydrolysis issue is alleviated and the Fe doping proceeds easily.This is ascribed to that such a method excludes the effect of the crystal water of zinc nitrate and the occupying effect of Zn ions in the micropores.The final catalyst performs more efficiently in catalyzing ORR.Another method is that ZIF-8 is firstly carbonized and then mixed with FeCl3 and urea,which is followed by second carbonization.Interestingly,this strategy yields a catalyst with further improved performance compared with the post-adsorption strategy.The half wave potential of the optimal catalyst in acidic medium reached 0.825 V,along with high selectivity of 4e’ pathway and excellent durability.The reason may be that in this method the formation of high-density Fe-Nx active sites was promoted by sufficient N as well as the strong confinement effect of stable carbon skeleton.2.Fe/N/C oxygen reduction catalyst with high availability of active sitesWe present an in-situ delaminating method to tune the nanostructure of Fe/N/C catalyst,employing trithiocyanuric acid as structure-directing agent.In the pyrolysis stage,the polycondensation of trithiocyanunc acid generates layered S-doped carbon nitride which acts as a sacrificial template to direct the formation of carbon nanosheets.Meanwhile,the big-SH leaving groups exert an exfoliation eftect on the forming process of final micromorphology,resulting in highly uniform nanostructure with high specific surface area and large pore volume.These structural features improve the availability of active sites.The obtained S-doped Fe/N/C catalyst exhibits significantly enhanced ORR activity compared with S-free sample with the elevated half-wave potentials in both acid(0.76 V vs.0.68 V)and alkaline(0.87 V vs.0.84 V)media.This work not only expands our understanding of how the sulfur influences the performance of Fe/N/C catalyst,but also would open a new avenue to fabricate other carbon-based materials through this structure design.3.Novel Cr/N/C catalyst for oxygen reduction.To avoid the stability issue of conventional Fe/N/C catalysts induced by Fenton reaction,we for the first time obtained a pyrolyzed Cr SAC for ORR.The precursor is designed using a post-adsorption strategy and Cr ions are confined in the micropores of imidazolate framework.This strategy ensures that Cr single atoms are concentrated in the superficial layer and further pyrolysis induce the formation of hollow architecture which favors ORR catalysis.Combined HAADF-STEM and XAFS analyses confirm that Cr-N4 moieties rooted in carbon texture act as active sites and endow the final catalyst with an encouraging performance in acidic.electrolyte.The optimal Cr/N/C catalyst exhibits a half-wave potential of 0.773V in 0.1 M HClO4.More excitingly,the Fenton reactivity is 23 times lower in Cr SAC compared to the Fe counterpart,thus,Cr SAC demonstrates superb stability with E1/2 negatively shifts by only 15 mV after 20000 cycle tests,even more stable than the commercial Pt/C.In spite of the inactive nature of its macrocyclic counterpart,the highly active Cr/N/C catalyst firmly confirms the value in searching for SACs that initially claimed inert in macrocyclic surroundings.