Preparation And Oxygen Reduction Performance Of Multi-dimensional Carbon-based Catalysts Based On Fe/heteroatom Codoping

As a green energy conversion device,fuel cell has high efficiency and zero emission.In fuel cells,the oxygen reduction reaction（ORR）catalytic layer of its cathode is the key structure that determines the fuel cell performance,lifetime and cost.So far,Pt-based noble metals and their composites are considered to be the most promising electrocatalysts for ORR,but the characteristics of scarce resources and high prices have limited their large-scale commercialization to some extent.Therefore,alternative catalysts with high catalytic activity and long-term stability as well as low cost need to be explored.It is worth mentioning that the introduction of transition metals（Fe,Co,Mn,etc.）or heteroatoms（S,P,B,etc.）into nitrogen-doped carbon groups can significantly improve the ORR performance.Therefore,a series of non-precious metal and heteroatom co-doped carbon-based oxygen reduction catalysts were prepared in this thesis,which consisted of the following three main elements.（1）Iron-nitrogen co-doped carbon（Fe-N-C）nanosheet electrocatalysts with homogeneous micro-mesoporous structures were prepared by one-step pyrolysis using ascorbic acid as the carbon source,melamine as the nitrogen source,iron nitrate nine-hydrate（Fe（NO₃）₃·9H₂O）as the metal source,and zinc nitrate hexahydrate（Zn（NO₃）₂·6H₂O）as the pore-forming agent through a simple combination of self-jet vapor phase growth method and template method.The effect of iron content in the obtained Fe-N-C samples on their physical properties and catalytic performance for oxygen reduction reaction was systematically investigated.The results showed that the best catalytic performance was obtained for the sample with the addition of 19.9 mg Fe（NO₃）₃·9H₂O,i.e.Fe-N-C-19.9.In alkaline solution,Fe-N-C-19.9 had a relatively more positive onset potential of 1.024 V,a higher half-wave potential of 0.885 V and a current density of 6.264 m A·cm^-2 compared with the commercial 20%Pt/C catalyst,which were 52 m V,69m V and 0.831 m A·cm^-2,respectively.Meanwhile,in 0.1 M KOH solution,the sample Fe-N-C-19.9 also showed excellent stability and methanol resistance.The total current loss rate of Fe-N-C-19.9 catalyst only decreased by 2.4%after 10,000 s stability test.The excellent ORR catalytic performance makes Fe-N-C-19.9 catalyst a strong candidate to replace expensive Pt-based cathode catalysts.（2）Based on the metal-organic skeleton ZIF-8@ZIF-67 core-shell structure,the ZIF-8@ZIF-67@Fe polyhedral structure was synthesized by in situ addition of iron nitrate ninhydride（Fe（NO₃）₃·9H₂O）as the precursor,and the Fe Co-N-C catalyst was successfully prepared by roasting,which exhibited excellent ORR performance.The prepared catalysts were characterized by BET,SEM and TEM and found to have large specific surface area and micro mesoporous structure.Most importantly,among the prepared samples,the Fe Co-N-C-40sample showed relatively higher initial potential and half-wave potential,which were 182 m V and 45 m V higher,respectively,compared with the commercial 20%Pt/C catalyst.It also exhibited superior stability and methanol tolerance than the commercial Pt/C catalyst,which provides a reference for the preparation of efficient and stable ORR electrocatalysts.The preparation of the electrocatalysts was also demonstrated.（3）Hollow mesoporous carbon materials co-doped with Fe,B and N were synthesized by one-step pyrolysis based on the metal-organic skeleton ZIF-8 structure with precursors consisting of phenylboronic acid,melamine,ZIF-8 and iron nitrate nonahydrate.The physical characteristics and ORR activity of the samples with different iron additions（iron nitrate ninhydride as precursor）were also compared,and the optimum iron nitrate ninhydride addition of 20 mg was found.The results showed that Fe-BN-C-20 has a high specific surface area(435.8m²·g^-1),a relatively more positive onset potential（0.998 V）,a higher half-wave potential（0.854V）and a larger current density(7.175 m A·cm^-2),26 m V,38 m V,and 1.742 m A·cm^-2 higher than that of commercial Pt/C,respectively,all of which favor the 4e^-reduction pathway of ORR and have high stability and methanol tolerance in 0.1 M KOH solution.After 10,000 s testing,the current of Fe-BN-C-20 catalyst still maintained 93.4%of its initial current,while the current of Pt/C remained only about 78.1%.This excellent ORR performance is mainly attributed to its unique hollow mesoporous structure,large specific surface area and abundant defects.Therefore,the prepared Fe-BN-C catalyst is a facile and efficient ORR catalyst that is expected to be applied in the fuel cell field.