Preparation Of MOF-derived Multi-element Doped Carbon-based Non-noble Metal Catalysts And Their Electrocatalytic Properties For Oxygen Reduction Reaction

Proton exchange membrane fuel cells（PEMFCs）are considered to be one of the most important energy conversion technologies,which are widely used in automobiles,stationary power supplies and other fields.Because of the sluggish oxygen reduction reaction（ORR）kinetics of proton exchange membrane fuel cell（PEMFC）cathode,a large number of expensive platinum-based electrocatalysts are required,leading to high fuel cell costs.In addition,large-scale use of platinum catalysts is also restricted by the scarcity of platinum resources.Therefore,to explore low-cost,high-performance non-noble metal electrocatalysts to replace platinum-based catalysts is of great significance.Transition metal and nitrogen co-doped carbon catalyst（M-N/C）is a non-precious metal catalyst that has attracted the most attention in the past ten years,which has made great progress in both activity and stability through the long-term efforts of the scientific community.Therefore,it is considered to be one of the most promising non-precious metal catalysts.By virtue of high and adjustable specific surface area,controllable porosity and tunable structures,metal-organic frameworks（MOFs）has become important precursor to prepare M-N-C catalysts with uniform structure,controllable morphology and high density of active centers.In this paper,the ZIF-8 material is used as a precursor to prepare high performance non-precious metal catalyst,and the effects of nitrogen doping/non-nitrogen doping/transition metal doping on the structure and performance of the material have been intensively investigated.The research works mainly include:（1）As the secondary nitrogen source,g-C3N4 was mixed with ZIF-8,and then the mixture of ZIF-8 and g-C3N4 was pyrolyzed at 950℃,at the same time,ferrocene vapor was used for gaseous doping of iron on the mixture,and a new type of porous carbon catalyst with Fe and N co-doped is prepared successfully,in which the doped Fe is atomically dispersed and coordinated with N atoms to form Fe-Nx active moieties.It is found that the addition of g-C3N4into the precursor not only enhanced the N content of the catalyst,but also improved the morphology and porosity of the material.Our optimized catalyst（denoted as Fe@NPC-0.75）,exhibited excellent ORR activity and stability.In 0.1 M KOH solution,the half-wave potential of the catalyst reaches 0.87 V（vs.RHE）,which is 30 m V higher than 20wt%Pt/C,and the limiting current density is up to 6.93 m A·cm^-2;Meanwhile,in 0.1 M HCl O₄ solution,the catalyst’s The half-wave potential reaches 0.785 V,which is only 65 m V lower than 20wt%Pt/C,and the limiting current density is up to 6.67 m A·cm^-2.（2）In the bases of above prepared catalyst,the effect of the co-doping of iodine on the structure and performance of the catalyst was studied.KI is used as precursor of iodine,and the iodine co-doped catalyst is prepared by pyrolyzing the mixture of potassium iodide,ferric ammonium citrate and ZIF-8 at 950°C.It is found that the co-doping of iodine can greatly increase the specific surface area(S_BET=1831.2 m²·g^-1)and pyridine N content（27.46%）of the catalyst.It is showed by electrochemical test results that the ORR half-wave potential and limiting current density of Fe/N/I-PC catalyst in 0.1 M KOH solution are 0.865 V and 7.36m A·cm^-2,respectively,35 m V and 1.12 m A·cm^-2 higher than the catalyst without iodine;in 0.1M HCl O₄ solution are 0.81 V and 5.45 m A·cm^-2,respectively.In addition,the catalyst exhibited excellent stability,its current density could retain 95%of its initial values after running for20000 s,compared with only 92%of the catalyst without iodine co-doping.（3）The dual transition metal elements doping is investigated,a series of dual transition metal co-doped catalysts（denoted as M-Fe/NC）,were designed and synthesized by introducing different acetylacetonates during the self-assembly of ZIF-8 materials.It is found that the products prepared by doping Mn or Cu can well maintain the original polyhedral morphology of MOF,while doping Ni or Co results in the formation of a three-dimensional interconnected porous carbon structure.The ORR activities of the dual transition metal doped catalysts follows the order as:Mn-Fe/NC>Cu-Fe/NC>Ni-Fe/NC>Co-Fe/NC.the Mn-Fe/NC exhibited the best ORR activity,in 0.1 M KOH solution,its half wave potential can be up to 0.87 V（vs.RHE）.Moreover,the stability of the Mn-Fe/NC catalyst is better than that of commercial Pt/C catalyst and Fe/NC catalyst without second transition metal co-doping.After running for 20000 s,it still retains 94%of the its initial current value,compared to the 91%of the catalyst without Mn,and the 91%of the Pt/C catalyst.