Preparation And ORR Catalytic Properties Of N/C And Fe/N-C Catalysts Derived From Tetrazine Ring-Containing Polymers

Nowadays,environmental pollution and energy crisis brought by fossil fuel depletion have seriously restricted the sustainable social development,which have become the two major difficult problems that needed to be solved urgently.At present,fuel cell(FC)is one of the most promising technologies for sustainable and clean energy due to that FC can convert the chemical energy from a fuel into electric energy directly with very few emissions and even zero emissions.Meanwhile,FC can achieve high energy efficiency because there is no limitation of Carnot cycle.However,the high cost and instability of the state-of-the-art platinum(Pt)or Pt-based catalysts that used for cathodic oxygen reduction reaction(ORR)in FC seriously hamper the large-scale industrial application of FC.Therefore,the development of novel ORR catalysts with high catalytic activity,low-cost,and high electrochemical stability is becoming one of the research hotspots.In this thesis,two types of catalysts including metal-free N-doped carbon(N/C)and transition-metal(Fe)incorporated N/C(Fe/N-C)have been prepared by separate pyrolysis of tetrazine-based polymers as well as their mixtures with Fe(OAc)2.In addition,the ORR performances of N/C and Fe/N-C catalysts are also systematically investigated in both alkaline and acidic electrolytes.The main results are shown as follows:(1)Preparation and ORR performances of spherical N/C and Fe/N-C catalysts derived from tetrazine-based polypyrrole.In this chapter,tetrazine-based,cross-linked polypyrrole spheres(PTPy)were first synthesized by methanesulfonic acid-catalyzed Friedel-Crafts reaction with bis(N-pyrroiyl)-1,2,4,5-tetrazine(TPy)as the monomer and dimethoxymethane(DMM)as the cross-linker,respectively.The as-synthesized PTPy sample possesses a diameter of 100-300 nm.Subsequently,metal-free N-doped carbon(Py-N/C-900)and Fe/N-codoped carbon(Py-Fe/N-C-900)spherical catalysts with a diameter of 90-250 nm were prepared by directly pyrolyzing PTPy as well as PTPy/Fe(OAc)2 mixture at 900 °C,respectively.The N content of the Py-N/C-900 catalyst was found to be 7.05 at%,while the BET surface area(SBET)and total pore volume(Vtotai)of this catalyst were found to be 584 m2 g-1 and 0.29 cm3 g-1,respectively.As for the Py-Fe/N-C-900 catalyst,the N and Fe contents were found to be 5.39 and 1.11 at%,and the SBET and Vtotai values were found to be 610 m2 g-1 and 0.36 cm3 g-1,respectively.In alkaline electrolytes,the Py-N/C-900 catalyst exhibited a better ORR activity with an onset potential(E0),a half-wave potential(E1/2),and a limiting current density(J1)of 0.948 V(vs.RHE,the same hereinafter),0.823 V,and 5.42 mA cm-2,respectively,which is nearly comparable to the Pt/C catalyst(E0 =0.964 V,E1/2 = 0.829 V,J1 = 5.30 mA cm-22),while the Py-Fe/N-C-900 catalyst showed much more superior ORR activity(E0 = 0.986 V,E1/2 = 0.872 V,J1 = 5.89 mA cm-2)in alkaline media.In acidic media,the Py-Fe/N-C-900 catalyst also showed high ORR activity with E0,E1/2,and J1 values of 0.845 V,0.717 V,and 5.89 mA cm-2,which is close to the Pt/C catalyst(E0 = 0.891 V,E1/2 = 0.738 V,J1 = 4.74 mA cm-2).Moreover,the Py-Fe/N-C-900 catalyst still exhibited excellent stability together with superior tolerance toward poisons in acidic media.(2)Preparation and ORR performances of N/C and Fe/N-C spherical catalysts derived from tetrazine-based polyindole.In this chapter,tetrazine-based,cross-linked polyindole spheres(PTId)with a diameter of 100-200 nm were also synthesized by protonic acid-catalyzed Friedel-Crafts reaction of bis(N-indolyl)-1,2,4,5-tetrazine(TId)monomer.Subsequently,spherical In-N/C-900 and In-Fe/N-C-900 catalysts with a diameter of 100-200 nm were prepared by separately pyrolyzing PTId and the mixture of PTId with Fe(OAc)2 at 900 ℃.The N content and SBET and Vtotai values of the In-N/C-900 catalyst were 4.73 at%,662 m2 g-1 and 0.38 cm3 g11,respectively,while the N,Fe contents and SBET and Vtotai values of the In-Fe/N-C-900 catalyst were found to be 5.98 and 0.64 at%,and 735 m2 g-1 and 0.43 cm3 g-1,respectively.In alkaline electrolytes,both In-N/C-900 and In-Fe/N-C-900 catalysts exhibited higher ORR activity as compared to the Pt/C catalyst with the E0,E1/2,and J1 values of 0.986 V,0.846 V,and 5.76 mA cm-2 for In-N/C-900 catalyst,respectively,and 1.044 V,0.916 V,and 6.14 mA cm-2 for In-Fe/N-C-900 catalyst.In acidic media,the In-Fe/N-C-900 catalyst also showed superior ORR activity together with excellent stability and fuel tolerance as compared to the Pt/C catalyst with E0,E1/2,and J1 values of 0.895 V,0.768 V,and 5.34 mA cm-2.