The Performance Improvement And Active Center Exploration Of Oxygen Reduction Reaction Catalyst Prepared By Riboflavin

Fuel cell,as an environmentally friendly and efficient energy conversion device,is gradually applied in our life.The oxygen reduction reaction(ORR)at cathode is slow and complicated,requiring a large amount of Pt catalysts.However,the platinum-based catalysts cost highly and hinder the development of the fuel cells.In recent years,developing non-precious metal catalysts has become one of the main directions for fuel cell researches.Among non-precious metal catalysts,transition metal and nitrogen doped carbon catalysts(M-N-C)with high activity and stability have been extensively investigated.The electrocatalytic activity of M-N-C catalysts is still poor as compared to the platinum-based catalysts.Therefore,it is necessary to take some measures to improve the activity of M-N-C catalysts to the level of the platinum-based catalyst and ultimately replace it.In addition,the active sites of M-N-C catalysts have been still in debate,and whether the transition metal constitutes its active sites becomes one of the focuses of controversy.In our group previous work,riboflavin has firstly used to prepare Fe-N-C catalysts via one-step pyrolysis in the presence of anhydrous iron chloride.This thesis aims to further improve the performance of Fe-N-C catalysts and explore its possible active sites.Riboflavin is used as a cheap,nontoxic carbon and nitrogen precursor to prepare Fe-N-C catalysts in the presence of anhydrous iron chloride.Then,the catalyst is modified by acid treatment to change the properties of the catalyst surface,a template method is used to improve the pore size of the catalyst and then facilitate the electrochemical mass transfer,adding extra nitrogen source is to increase the total nitrogen content and change the proportion of different types of nitrogen to improve the ORR activities of the catalysts.Eventually the binary Fe-Co-N-C and Fe-Ni-N-C catalysts are prepared to study the synergy effects of the two transition metal elements.Combining the physicochemical properties and ORR results of the presence and absence of FcCl3,as well as adding extra nitrogen source,the possible active sites of the catalysts will be analyzed.The Fe-N-C catalysts were modified by acid then second heat treatment.The Fe-N-C catalysts were refluxed in concentrated sulfuric acid at 60 ? for 3 h,with the second heat treatment at the same pyrolysis temperature.The X-ray Diiffraction(XRD)patterns and Transmission Electron Microscopy(TEM)images show the Fe-N-C catalyst modified by acid then second heat treatment consists of graphitic carbon and Fe2O3 nanoparticles.The X-ray Photoelectron Spectroscopy(XPS)results show the types of nitrogen in the catalyst are pyridinic,pyrrolic,graphitic and oxidized nitrogen.The contents of pyridinic and graphitic nitrogen are higher.The Fourier Transform Infrared Spectroscopy(FTIR)spectrum shows that the acid treatment introduced some oxygen-containing groups.After the second heat treatment,partial oxygen-containing groups on the surface are removed,which can improve the conductivity of the catalysts and increase the surface defects.From Cyclic Voltammograms(CV)and Linear Sweep Voltammetry(LSV)results,the ORR performance of the modified Fe-N-C catalysts is significantly improved.The optimum catalyst is prepared at 850 ?,followed by acid treatment and second heat treatment,its half-wave potential is-0.08 V(vs.Hg/HgO)and limiting current density was-4.69 mA cm-2,while the half-wave potential of Fe-N-C catalyst is-0.18 V(vs.Hg/HgO)and limiting current density of Fe-N-C catalyst is-4.17 mA cm-2.As can be seen from chronoamperometry curve,the current density of the catalyst maintains at 97%of the initial current density after discharging 3 h at a constant potential of-0.2 V.From the LSV curves before and after the chronoamperometry,it showed that there is only a little difference in the ORR activity,indicating the catalyst with excellent stability.The template method is used to modify the Fe-N-C catalysts.The SiO2 nanospheres are spherical and stable under high temperature.The template is used to synthesize Fe-N-C catalysts at high temperature,then removed by NaOH.The catalysts formed an ordered porous structure that contributes to the mass transfer of the ORR,thus improves the ORR activity.NaCl is used as another template to modify the Fe-N-C catalysts.NaCl is a typical regular face-centered cubic crystal structure.However,its melting point is 800 0C and its crystal structure cannot be maintained at higher temperature.The Fe-N-C catalysts prepared with Si02 nanospheres as a template is not observed iron or iron compounds from their XRD pattern and TEM image.From Scanning Electron Microscope(SEM)spectra,the Fe-N-C catalysts prepared with SiO2 nanospheres as a template displays a granular morphology,but the Fe-N-C catalysts prepared with NaCl as a template shows a blocky and sheet morphology.From Energy Dispersive Spectrometer(EDS)spectra,the both catalysts contain iron,nitrogen and carbon.From pore-size distribution results,the both catalysts has mesoporous structure,and the Fe-N-C catalyst prepared with SiO2 nanospheres as a template has larger specific surface area and pore volume than the Fe-N-C catalysts prepared with NaCl as a template.The best catalyst prepared with the mass ratio of riboflavin to SiO2 of 2:1 demonstrates a half-wave potential of-0.07 V(vs.Hg/HgO)and limiting current density of-4.99 mA cm 2.The NaCl-modified Fe-N-C catalysts shows lower ORR activity than the Fe-N-C catalysts.When the mass ratio of riboflavin to NaCl is 1 to 4,its half-wave potential is-0.10 V(vs.Hg/HgO)and limiting current density is-4.01 mA cm-2.The nitrogen content in the catalysts is regulated by adding extra nitrogen source.Urea and dimethylformamide(DMF)are used as the nitrogen sources to increase the nitrogen content and change the ratio of different nitrogen types.The Fe-N-C catalyst prepared by adding urea has higher total nitrogen content,graphitic and pyridinic nitrogen content than the Fe-N-C catalyst prepared by adding DMF.The best catalyst is prepared when the urea amount is 7 wt%,its half-wave potential is-0.07 V(vs.Hg/HgO)and limiting current density is-4.2 mA cm-2.Similarly when the amount of DMF is 3 wt%,its half-wave potential is-0.10 V(vs.Hg/HgO)and limiting current density is-4.19 mA cm-2.The binary Fe-Co-N-C and Fe-Ni-N-C catalysts are prepared to study the synergy effects of the two transition metal elements.From the N2 adsorption-desorption and pore-size distribution results,the Fe-Co-N-C catalyst with mesoporous structure has larger specific surface area and pore volume than the Fe-Ni-N-C catalyst.From SEM spectra,the Fe-Co-N-C catalyst displays a granular and porous morphology,Fe-Ni-N-C catalyst shows a blocky and sheet morphology.The morphology of Fe-Co-N-C catalyst is more conducive to oxygen adsorption,reduction and desorption of reduced products.When the ratio of iron to cobalt is 3 to 1,the ORR activity of the Fe-Co-N-C catalyst reaches the best,with half-wave potential of-0.08 V(vs.Hg/HgO)and limiting current density of-4 mA cm-2.When the ratio of iron to nickel is 7 to 1,the half-wave potential is-0.10 V(vs.Hg/HgO)and limiting current density is-3.60 mA cm-2 of Fe-Ni-N-C catalyst.No synergic effects are found between Fe and Ni/Co.Explore the ORR active sites of the catalysts.The N-C catalyst prepared in absence of FeCl3 demonstrates the ORR activity,indicating that nitrogen-doped carbon material has the ORR active sites itself.By adding extra nitrogen source,the amount of nitrogen and the content of different types of nitrogen change,and correspondingly the ORR activities change,indicating that the active sites are related to the content and types of nitrogen.Combining the results of the presence and absence of FeCl3,as well as adding nitrogen sources,the possible N-C active sites of the catalysts are graphitic and pyridinic nitrogen.The ORR activity of Fe-N-C catalysts in the presence of anhydrous iron chloride is better than N-C catalyst,indicating that the transition metal can catalyze the formation of more ORR active sites during the preparation process or itself is included in the active sites.According to the XPS spectrum,iron exists as Fe3+,the Fe3+ should form a compound with N and/or C on the surface of the catalyst,because Fe3+ on the surface is not removed in the acid treatment.So it can be speculated that the active sites contain Fe,but whether it is combined with N or C still needs further study.