Pyrolysis Productsof Fe/Co Doped PANI/PPYR Composites As Electrocatalysts For Oxygen Reduction Reaction

Fuel cell is a “green device for generating electricity”, which is clean, efficientand environmental friendly. Oxygen reduction reaction (ORR) is an importantcathodic process of the ORR. But the cathode catalysts are mostly Pt or Pt-basedcatalysts. Although they show the best electrocatalytic activity for ORR, the high costand low methanol tolerance limit their practical appliction. Much attention has beenpaid to the study on metal macromolecules containing nitrogen used aselectrocatalysts for ORR since1964. In this thesis, the main work is todevelopnitrogen-doped non-precious catalysts, which are low cost and present highelectroactivity for ORR.The structure and particle size distribution of the catalystsarestudiedwith various electrochemical methods and materials characterizationtechniques.The study on the non-precious catalysts for ORR shows important strategicsignifiance. The main contents of this thesis are as follows.(1)The transition metal has an important effect on the electroactivity ofnon-noble metal catalysts for ORR. We firstly synthesized the precursor containingMWCNTs,polyaniline,Fe and Co,and then the precursor was heated at900℃under N2to obtain the C-N catalysts with different Fe/Co mass ratios. Results show that theprepared catalyst with the Fe/Co mass ratio of6:1presents the highest electrocatalyticactivity for ORR. The ORR current density at2000r·min-1is12.5mA·mg-1@-0.3V(vs SCE) in acidic solution and7.8mA·mg-1@-0.8V(vs SCE) in alkaline solution,and the onset potential of ORR is0.52V(vs SCE) in acidic solution and-0.09V(vsSCE) in alkaline solution. Fe/Co mass ratio plays a great role on the electroactivity ofthese non-noble metal catalysts for ORR.(2)MWCNTs doped polyaniline (PANI) were used as a precursor and theprecursor added Fe salt was heated at900℃to obtain the Fe-containing catalyst FeNC.Cobalt salt was then added to theFeNC and the mixture was subjected to heattreatment at different temperatures to prepare the various composites FeCoNC.Resultsshow that the catalyst structure, N and Fe, Co contents display significant effect onthe electroactivity of the catalysts for ORR. The catalystFeCoNC obtained by theheating treatment at500℃, presents the highest electrocatalytic activity for ORR. Thecurrent density at2000r·min-1andthe onset potential for ORRare 11.67mA·mg-1@-0.3V and0.63Vin acidic solution, respectively, andthey are9.83mA·mg-1@-0.8V and-0.12V in alkaline solution, respectively.(3)we prepared the C-N composites containing Fe and/or Co (C-PANI,Fe/C-PANI, Co/C-PANI and FeCo/C-PANI) by directly calcining the Fe and/or Codoped polyaniline (PANI) precursors, and they show efficient electroactivity for ORRboth in acidic and in alkaline media. Among the prepared catalysts, Fe/C-PANI andCo/C-PANI present the most positive ORR onset potential that is0.61V (vs SCE) in0.5mol L-1H2SO4solution or-0.1V (vs SCE) in1mol L-1NaOH solution. Inaddition, the Fe/C-PANI catalyst shows the largest limiting-diffusion current densityfor ORR both in acidic and in alkaline media. A four-electron reaction of ORR on theFe/C-PANI and Co/C-PANI catalysts is more dominant than a two-electron reaction.Fe/C-PANI catalyst also presents good electrocatalytic activity stability for ORR bothin acidic and in alkaline media.(4)Polymerization reaction of pyrrole in the presence of Fe3+and Co2+wascarried out to synthesize the metal doped polypyrrole. The metal doped polypyrrolewas then calcined at700℃to obtain the carbonized products. Finally, the carbonizedproducts were heated to900℃to synthesize the catalysts PPY-M(M is Fe or Co). Thecatalyst PPY-Co presents the highest electrocatalytic activity for ORR. The ORRcurrent density at2000r·min-1is7.5mA·mg-1@-0.3V(vs SCE) in acidic solution and5.7mA·mg-1@-0.8V(vs SCE) in alkaline solution, and the onset potential of ORR is0.54V(vs SCE) in acidic solution and-0.11V(vs SCE) in alkaline solution.(5)We synthesized cathode catalysts with high performance oxygen reduction byusing a simple heat treatment process. These catalysts were fabricated by directlycalcining the Fe and/or Co doped polyaniline-polypyrrole composites. Among theprepared catalysts, Co/PANI-PPYR presents the most positive ORR onset potential of0.62V (vs SCE) in0.5mol L-1H2SO4solution or-0.09V (vs SCE) in1mol L-1NaOH solution. In addition, the Co/PANI-PPYR catalyst shows the largestlimiting-diffusion current density for ORR, which is4.3mA cm-2@0.2V(vs SCE) inacidic and2.3mA cm-2@-0.3V(vs SCE)in alkaline media. In acidic media,afour-electron reaction of ORR on the Co/PANI-PPYR and Fe/PANI-PPYR catalystsis more dominant than a two-electron reaction. In alkaline media however, afour-electron and a two-electron mechanisms are co-present for the ORR on all the prepared catalysts. Co/PANI-PPYR catalyst also presents good electrocatalyticactivity stability for ORR both in acidic and in alkaline media.