Efficent Oxygen Reduction Over Snassisting S,N-doped Carbons Derived From New Covalent Organic Frameworks

Oxygen reduction reaction（ORR）is the key reaction that takes place in the cathodes of proton exchange membrane fuel cells（PEMFC）and zinc-air batteries.It is difficult to increase the power density of the two batteries significantly because of the slow kinetics of the ORR reaction.The high-performance materials for speeding up the rate of cathode ORR efficiently are the keys in PEMFCs,but high cost hamper their scale utilization.Transition metal-modified carbon materials have large potential to replace platinum-based precious metal metrials,which have exhibited the comparable activities for ORR in alkaline solution.However,its catalysis abilities is not enough in acidic media,especially in the practical application of PEMFC and zinc-air batteries.To developing efficient and stable electrocatalysts for ORR is the primary task to promote the commercial development of PEMFC and zinc-air batteries.Carbons modified by transition metals and their alloys have been widely reported to dedicate effective ORR activity.In particular,the studies have shown that the synergistic effect produced on two metals significantly enhances the catalytic activity and stability of ORR.However,there was a little research on whether the main group metals or their alloys about catalying the ORR process.The oxygen affinity of the tin assists to adsorbing the oxygen.The stable Sn-C bonds formed between Sn and C could contribute to the ORR catalytic activity.Sn was first detected in ancient bronzes,where the Sn species exsited in form of alloys.It is easy to form alloys with various metals in a high-temperature,which is facilitated to regulatintg the center of d-band in the alloy.These characteristics indicates that Sn as the main group metal and its alloy have the potential to become the efficient ORR electrocatalytic.The thesis systematically based on Sn and its alloys as ORR electrocatalytic materials.The main research results as follows:1)A new two-dimensional porous porphyrin was successfully synthesized using Shiff-base coupling reaction using 5,10,15,20-tetrakis（tetraphenylamino）porphyrin and 2,4-pyridinedicarbaldehyde as building blocks（Sn-COFs）.SnCl₂ 2H₂O as reducing agents and Sn（OH）_x species are left in COFs.By optimizing the heat treatment,Sn particles were successfully supported on the S-N co-doped porous carbons（Sn@Sn-S-N-Cs）.Sn@S-N-Cs material has a sheet-like morphology and a large specific surface area.The ORR half-wave potential of the Sn@S-N-C-900 in alkaline medium reached0.82 V,and 0.69 V in acidic medium.The ORR catalytic performance of Sn@S-N-C-900 is obviously improved compared with the S-N-C-900.It is confirmed that the presence of Sn could promote the ORR activity.2)Cu-Sn-COFs was constructed with 5,10,15,20-tetrakis（tetraphenylamino）porphyrin and 2,4-pyridinedicarbaldehyde blocks.Cu₆Sn₅ alloys with about 2-10 nm nanoparticle size supported on porous S,N-doped carbons（Cu₆Sn₅@S-N-Cs）were derived after pyrolysis.When compared with Sn-S-N-C-900 catalyst,Cu₆Sn₅@S-N-C-900 have the more positive half-wave potential for ORR in alkaline solution,increased by 40 mV,competitive to the commercial Pt/C,with the improvement stability.3)Fe,Sn co-doped porous covalent organic polymer（Fe-Sn-COFs）was prepared by using 5,10,15,20-tetrakis（tetraphenyl amino）porphyrin and 2,4-pyridine dicarbaldehyde.The formation of Sn-based core-shell nanostructures（FeSn₂@FeSnO_x）by thin carbon been facilitated during pyrolyzation.The optimized FeSn₂@FeSnO_x@S-N-C-900 material exhibited a remarkable performance for ORR,where the ORR half-wave potential in KOH reaches 0.88V,40 mV higher than the commercial Pt/C.And also,it showed a half-wave potential value of 0.79 V in HClO₄,which is close to Pt/C.4)A new porphyrinic Mn-COF was synthesized by reacting 5,10,15,20-tetrakis（tetraphenyl amino）porphyrin and 2,4,6-trihydroxy-1,3,5-benzenetrialdehyde with presence of Sn（OH）_x.Through thermally converting Mn-COFs,the mesoporous Mn-S-N-C with highly dispersed Mn-N_x units could be prepared.The generated of large Sn particles are etched by KOH and act as a hard template.Mn-S-N-C-900 catalyst demonstrated outstanding activitys for ORR.The ORR half-wave potential reached0.86V in 0.1 M KOH and 0.60 V in 0.1 M HClO₄,with very lower HO₂^-yield（4.02%）.