Preparation And Investigation On Oxygen Reduction Performance Of Carbon-based Non-noble Metal Catalysts

Oxygen reduction reaction(ORR)cathodic catalyst plays a crucial role in determining the performance of PEMFC and metal-air batteries.Currently,noble metal-based catalysts are deemed to be the perfect ORR catalysts.However,their limited resources and huge expense,greatly obstruct their commercial-scale application.Therefore,many research efforts were taken to develop earth-abundant,efficient and cost-effective catalysts with competitive ORR performance to replace the traditional Pt based catalyst has become a hot topic in the field of fuel cell and metal air batterry electrocatalysis.In this thesis,we designed and prepared a series of high-performance Fe-Nx/C oxygen reduction catalysts by employing co-doping,organic iron sources are introduced in the self-assembly,chemical vapor deposition method and phase transition procedures.The results of the corresponding research are mainly included as follows:(1)We intelligently design a gas doping approach to prepare ZIF-8-derived N and atomic Fe dual-doped porous carbon nanocubes.Interestingly,the carbonized imidazole ligand of ZIF8 nanocubes confined the Fe precursor within its cubic structure and resulted in Fe ion bond with N,this eventually led to the formation Fe-Nx active center.In alkaline medium,the optimized catalyst(PCN-A@Fe SA)exhibits obviously enhanced ORR properties with halfwave potential(E1/2)of 0.905 V(vs.RHE),which is 145 and 55 mV higher than PCN-A(the catalyst without Fe doping)and commercial Pt/C.The limiting current density(jlimit)of PCNA@Fe SAis 1.43 and 1.20 times that of PCN-A and commercial Pt/C.In acid medium,PCNA@Fe SA catalyst demonstrates an E1/2 of 0.79 V(vs.RHE),which is 60 mV lower than commercial Pt/C.Its limiting current density(jlimit)is 2.18 and 1.31 times that of PCN-A and commercial Pt/C.The discharge power density of PCN-A@Fe SA reaches 185 mW cm-2 in ambient air in Zn-air battery.Furthermore,its power density of 514 mW cm-2 is realized with acidic PEM fuel cells.Finally,DFT calculations further explain that Fe2+N4 moieties are the main active center and favorable for oxygen affinity and OH*intermediate desorption.This work offers a universal approach to rationally design single-atomic/atomic non PGM catalysts with high catalytic properties for sustainable energy applications.(2)The coordination polymerization of Zn(C2H3O2)2 with 2,2-bisimidazole as nitrogen source,with Fe(C2H3O2)2 as iron source,a highly active iron atom anchoring bamboo like morphology constructing porous carbon nano-networks oxygen reduction catalyst were prepared follow by high temperature.The optimized catalysts exhibit superior oxygen reduction(ORR)activity with the half-wave potential(E1/2)of 0.905 V(vs.RHE)in the medium of alkaline.In the medium of acidic,the catalyst demonstrates an E1/2 of 0.79 V(vs.RHE).And the oxygen reduction process almost completely followed the four-electron mechanism.The results showed that:by adjusting the coordination polymerization of Zn/Fe and 2,2-biimidazole,the catalyst with bamboo like morphology and Fe-Nx activity was prepared.Benefiting from the high nitrogen content of 2,2-biimidazole,it can improve specific surface area of the catalyst and promote the iron loading content and mesoporous structure,prevent the appearance of large particles of iron species,meanwhile tune the content of nitrogen and the proportion of nitrogen species in the catalyst.The iron loading content is as high as 3.33%wt.(3)We developed Fe/N co-doped carbon based/nanotube nanocomposites with Fe-Nx active sites with 2,2-biimidazole iron as the organic iron source and ZIF-8 as the precursor,follow by high temperature pyrolysis.The optimum catalyst not only exhibits excellent oxygen reduction catalytic performance but also high stability in alkaline and acid medium:in an alkaline medium,the optimized catalyst(C-FeTZ 0.06)exhibits obviously enhanced ORR properties with the initial potential and half wave potential were 1.02 V(vs.RHE)and 0.90 V(vs.RHE),respectively.In an acidic medium,the optimized catalyst(C-FeTZ 0.06)demonstrates an Eonset of 0.97 V(vs.RHE),which its E1/2 is 60 mV lower than commercial Pt/C.(4)We attempted to construct an efficient ORR electrocatalyst with face-concaved morphology,derived from ZIF-8 framework and doped with coupling Fe-Fe atoms and cyclopentadienyliron dicarbonyl dimer containing specific Fe-Fe atomic pair as Fe precursor,A coupling Fe atoms doped carbon catalyst N-CC@Fe DA was successfully prepared with a gaseous doping process.Impressively,numerous coupling FeNx active sites are formed in the as-prepared N-CC@Fe DA catalyst.Moreover,it has increased specific surface area,meso porous architecture,and high density of active sites.Benefiting these advantages,the NCC@Fe DA displayed an outstanding ORR performance in both acidic and alkaline medium,even sueprior to the commercial Pt/C electrode.Especially the power density of N-CC@Fe DA reached 724 mW cm-2 when it is used as an air cathodic catalyst in a PEMFC.Finally,DFT calculations and XAS suggest the activating the O-O bond over the coupling Fe system,which are favorable for oxygen affinity and OH*intermediate desorption and thus enhances the efficiency of the oxygen reaction.