A Study Of Fe/N/C Cathode Catalysts For Alkaline Polymer Electrolyte Fuel Cells

The widespread application of proton exchange membrane fuel cells(PEMFC)is hindered by its dependence on Pt-based catalysts,and the proposing of alkaline polymer electrolyte fuel cells(APEFC)is expected to overcome this obstacle.By.switching from the acidic media to alkaline media,the choice of catalysts is greatly expanded,especially for oxygen reduction reaction(ORR)that lots of nonprecious metal catalysts will able to be employed.Among those catalysts,Fe-containing nitrogen doped carbon(Fe/N/C)is one of the most promising Pt-free cathode catalysts that can be applied for APEFC due to its high catalytic activity and high stability in alkaline media.Nevertheless,Fe/N/C catalysts possessing excellent ORR catalytic activity in rotating-disk-electrode(RDE)tests doesn't necessarily mean it will perform superior and applicable for fuel cell.In this thesis,in order to study the applicability of Fe/N/C in APEFC,.we 'synthesized several types of Fe/N/C catalysts and compared their intrinsic ORR activity,physicochemical properties and single cell performance.A guidance of Fe/N/C design has been given based on the above studies and the contents/results are summarized as follows:1.The increase of catalytic active-site density is needed to improve the ORR catalytic activity and the APEFC cell performance of the catalysts.Adenosine has been used to synthesize C-FeN and C-FeN-solvo via hydrothermal and solvothermal carbonization methods,respectively.By changing the media from water to DMF/ethanol,the hydrolysis of Fe3+ and the bond-breaking process of adenosine can be effectively inhibited,which leading to a higher doping percentage of iron and nitrogen for C-FeN-solvo.Under the precondition that there's no obvious difference between the two morphologies,The C-FeN-solvo shows a remarkably higher ORR activity in 1M KOH(with its half-wave potential of 0.895 V vs.RHE)and a higher APEFC power density(130 mW/cm2)than that of C-FeN(0.875 V vs.RHE,.100 mW/cm2),indicating that the increase of active-site density may significantly increase the intrinsic ORR activity and the fuel cell performance for Fe/N/C.2.A novel composite catalyst,C-FeN-solvo/CNT,has been prepared by blending carbon nanotubes with the carbonized oligomer generated from the solvothermal processes.C-FeN-solvo/CNT possesses a unique structure that the graphitized carbon nanotubes are coated by ultrathin amorphous heteroatom-doped carbon layer.The composite catalyst shows a relatively low ORR activity due to its insufficient surface active sites,however,the CNT make it have a certain advantage in the electronic conductivity,bringing about a significant decrease in the high frequency resistance(HFR)and a further enhancement in the peak power density(170 mW/cm2).3.By introducing ZnCl2 to catalyze the polymerization of the adenosine,a unique nanotube-structure catalyst Fe/Fe3C@NCNT,which exhibited a high catalytic ORR activity as well as a high electronic conductivity,has been prepared.The Fe/Fe3C-encapsulated carbon structure facilitates the transfer of electrons along the nanotubes,the well dispersed Fe?N4/C active sites located on the carbon tubes promote the ORR processes.The volume-specific catalytic activity of the Fe/N/C catalyst toward the ORR is as good as that of the commercial 20 wt%Pt/C catalyst,and better in durability.APEFC with this advanced cathode catalyst exhibits a power density greater than 485 mW/cm2,which is thus far the highest record in the literature for APEFC using nonprecious metal cathode.4.Two novel Fe/N/C catalysts with high-density mono-dispersed FeNx/C sites,high N content and hierarchical pore structure have been obtained,namely MgCB-CFe-phen' and CFe-imizadole.By comparing with Pt/C,the two catalysts exhibit comparable or more excellent electro-catalytic activity toward ORR.Unfortunately,both of their APEFC performance were markedly attenuated in the ohmic polarization region and the mass transfer polarization region.While the high-level doping of heteroatom and microporous systems can enhance the density of active sites in the catalysts,they may also bring about some unfavorable factors,such as the low electronic conductivity,low hydrophilicity and low utilization of active sites5.The intrinsic ORR activity of the catalysts can be reflected by the RDE tests,but not for the fuel cell tests.As to the cell performance,it is also related to the physicochemical properties of Fe/N/C materials,such as the electronic conductivity,the active-site density,the porosity,the hydrophilicity,and the catalyst-ionomer interaction,in addition to the chemical property.By analyzing the seven types of Fe/N/C catalysts obtained in this thesis,we can conclude that it is difficult to univariate the change of a certain characteristic by controlling the synthesis conditions.Thus,the rational design of the Fe/N/C catalyst should be more pertinently,instead of pursuing the high ORR activity in the RDE test blindly.6.The ORR activity of Fe/N/C possesses a rapid decline when using crosslinking-network quaternary ammonia polysulfone C12 xaQAPS as the binder to replace Nafion during the RDE tests.By using a series of different polymer binders,it can be concluded that the hydrophilicity of the polymer is the key factor to the ORR activity for Fe/N/C.On the contrary,the hydrophilicity of the polymers is a favorable factor that facilitates the ORR occurred at the three-phase interfaces in APEFC.The specific mechanism and the relevant scientific model need to be studied in the future.