Design,Preparation And Property Of Fe-N/C Oxygen Reduction Electrocatalysts

Electrochemical oxygen reduction reaction plays a crucial role in various energy storage and conversion systems,such as fuel cells and metal-air batteries.The sluggish oxygen reduction reaction requires catalysts,of which platinum-based materials are known to exhibit the best overall performance.However,the high price,unsustainable supply,inferior durability and susceptibility to methanol crossover effect and CO poisoning of Pt-based catalysts,yet hinder widespread commercialization of renewable energy technologies.To break through this bottleneck,enormous research efforts have been devoted to explore highly active and stable non-precious metal catalysts as alternatives of Pt-based materials.Among noble-metal-free alternatives,transition-metal-coordinating nitrogen-doped carbon catalysts labelled as M-Nx/C(M = Fe,Co,etc.)have been regarded as particularly promising platinum-free oxygen reduction electrocatalysts because of their high element abundance,low cost,lower environmental impact,and moderate activity.This thesis is mainly aimed at the synthesis and property of Fe-N/C and its composites as well as their application in the zinc-air battery.The content of this paper is divided into the following four chapters:The first chapter is the introduction.First,the route and mechanism of oxygen reduction reaction are decribed.Then,the selection and desigen principle of electrocatalysts for oxygen reduction reaction are briefly summarized.Subsequently,noble metals and non-noble metals electrocatalysts are introduced.Finally,a brief description of the research basis and contents of the dissertation is provided.In chapter 2,we have developed a highly active non-platinum self-supporting Fe-N/C catalyst through the pyrolysis of a new type precursor of iron coordination complex,1,4-bis(1H-1,3,7,8-tetraazacyclopenta(1)phenanthren-2-yl)benzene(btcpb)functions as a ligand complexing Fe(Ⅱ)ions.The optimal catalyst pyrolyzed at 700℃(Fe-N/C-700)shows the best ORR activity with a half-wave potential(E1/2)of 840 mV vs reversible hydrogen electrode(RHE)in 0.1 M KOH,which is more positive than that of commercial Pt/C(E1/2:835 mV vs.RHE).Additionally,the Fe-N/C-700 catalyst also exhibits high ORR activity in 0.1 M HClO4 with the onset potential and E1/2 comparable to those of the Pt/C catalyst.Notably,the Fe-N/C-700 catalyst displays superior durability(9.8 mV loss in 0.1 M KOH and 23.6 mV loss in 0.1 M HClO4 for E1/2 after 8000 cycles)and better tolerance to methanol than Pt/C.Furthermore,the Fe-N/C-700 catalyst can be used for fabricating the air electrode in Zn-air battery with a specific capacity of 727 mA hg-1 at 5 mA cm-2 and a negligible voltage loss after continuous operation for 110 h.In chapter 3,we report the effective and simple preparation of iron carbide-embedded Fe-N-doped carbon(Fe3C/Fe-N/C)composites using iron-phenanthroline(Fe-Phen)complex and dicyandiamide(DCA)as the precursors that are subsequently heat treated.The optimal catalyst pyrolyzed at 800 ℃(Fe-Phen-N-800)exhibits superior oxygen reduction activity with onset and half-wave potentials of 0.99 and 0.86 V in 0.1 M KOH,respectively,which are higher to those of Pt/C(onset and half-wave potentials of 0.98 and 0.84 V)at the same catalyst loading.Moreover,the obtained Fe-Phen-N-800 displays comparable activity to Pt/C in 0.1 M HClO4 solution.Notably,the well-developed Fe-Phen-N-800 catalyst shows much higher long-term stability and better methanol tolerance than Pt/C.The results suggest our catalyst is one of the most promising candidates to replace Pt catalysts toward oxygen reduction.Striking,a primary Zn-air battery using Fe-Phen-N-800 as the air cathode catalyst delivers higher voltages and gravimetric energy densities than those of Pt/C-based system at the discharge current densities of 10 and 25 mA cm-2,thus demonstrating the potential application of our catalyst in energy conversion devices.In chapter 4,we develop a novel iron-nitrogen on carbon(Fe-N/C)catalyst from pyrolysis of a well-designed Fe-based coordination polymer,which exhibits outstanding performance in both alkaline and acidic solution.A nitrogen-rich bridging ligand,tetrapyrido[3,2-a:2’,3’-c:3",2"-h:2"’-3’"-j]phenazine(tpphz)is employed for the formation of iron coordinated coordination polymer(Fe-tpphz),as well as the nitrogen and carbon sources.The catalyst pyrolyzed at 700 ℃(Fe-N/C-700)shows the highest ORR activity with more positive onset(992 mV vs.RHE)and half-wave potentials(826 mV vs.RHE)than commercial Pt/C in 0.1 M KOH.While in acidic media,the optimal Fe-N/C-700 catalyst shows an excellent catalytic behavior,which is comparable to commercial Pt/C.Besides,the Fe-N/C-700 catalyst displays great long-term stability(only 19 mV loss in 0.1 M KOH and 27 mV loss in 0.1 M HClO4 for half-wave potential after 8000 cycles).Importantly,a Zn-air battery with the Fe-N/C-700 catalysts as the cathode provides a high open circuit voltage(OCV)and gravimetric energy densities,which match those of a commercial Pt/C catalyst,thus indicating the practical implementation of our catalyst into clean renewable energy conversion devices.