Preparation Of Porous Organic Polymer And Study On Its ORR Electrocatalytic Properties

Fuel cells convert chemical energy directly into electrical energy without the need for the carnot cycle.They are the most promising electrochemical devices with high energy conversion efficiency,low noise and zero emission.In this process,the electrocatalyst of fuel cell cathode oxygen reduction reaction?ORR?plays an important role.It is well known that platinum catalyst is an efficient catalyst with high ORR activity.However,platinum catalyst has many defects,including high cost,limited reserves,poor stability and low tolerance to methanol,which seriously restricts the large-scale commercial application of fuel cells.Therefore,it is of great significance to develop non-noble metal ORR catalysts with low cost,high activity,abundant resources and good stability.In recent years,more and more attention has been paid to porous organic polymers?POPs?as a precursor of carbon-based ORR catalysts due to their excellent pore properties,controllable chemical composition and structure.In this paper,two kinds of N-rich porous organic polymers?HCP?as precursors were prepared by sonogashira-hagihara alkylation reaction of conjugated microporous polymer?CMP?and one-step friedel-crafts alkylation reaction by selecting N-containing block unit,and two kinds of N-C carbon-based ORR catalysts were prepared by carbonization treatment.On this basis,cobalt metal was doped in HCP and M-N-C ORR catalyst was prepared by heat treatment.Their morphology and structure were characterized,and the oxygen reduction and other electrochemical properties of the three catalysts were systematically studied.The main research contents are as follows:?1?In this paper,conjugated microporous polymer?CMPs?was firstly selected as the precursor of carbon material.Using toluene and triethylamine as solvent,5,10,15,20-4'-bromophenyl porphyrins?TPP?and 1,4-diacetylenyl benzene as monomers,and Pd?0?/CuI as catalyst,two kinds of conjugated microporous polymers were generated by typical Suzuki cross-coupling polycondensation reaction,and heat treatment was conducted on them to obtain TPP-CMP-900 and CMP-900.The morphology of the catalyst was characterized by SEM and TEM,and the structure of the catalyst was characterized by means of XRD,XPS,Raman and BET.The results showed that the TPP-CMP-900 had a large BET surface area(624 m² g^-1),good porosity and high nitrogen atom content,which was conducive to the exposure of the active sites and the rapid transport of oxygen atoms.In addition,porphyrins acted as nitrogen sources to form nitrogen-doped carbon nanotubes.Due to the high content of pyridine N in TPP-CMP-900,the lone pair electrons of pyridine N had a strong affinity for oxygen atoms,thus improving the performance of ORR.Furthermore,as a non-metallic electrocatalyst,TPP-CMP-900 showed a great potential for ORR catalytic activity in the alkaline electrolyte,that was,the half-wave potential was 0.83 V?vs.RHE?,the limiting current density was 4.5 mA cm^-2,the electron transfer number was 3.95,indicating a direct four-electron reaction pathway,which was similar to the performance of commercial Pt/C.TPP-CMP-900 also showed an excellent stability and good ORR methanol resistance,which was better than the commercial Pt/C?20%?.The TPP-CMP-900 has great potential to be a candidate of the commercial Pt/C as efficient metal free ORR electrocatalysts.?2?Based on the mild reaction conditions and low reagent cost,HCP was selected as the ORR electrocatalyst in chapter 2.With pyrrole as monomer,formaldehyde dimethyl acetal?FDA?as the crosslinking agent,ferric chloride?FeCl₃?as the catalyst,1,2-dichloroethane?DCE?as the solvent,a one-step friedel-fashion alkylation reaction was used to synthesize N-containing HCPs.Three ORR catalysts at different temperatures were obtained by carbonization treatment at 800?,900? and 1000?.After a series of characterization methods and electrochemical performance tests,we found that the synthesized HCP had the best ORR catalytic performance at 900?,and the BET surface area could be as high as445m² g^-1.By calculation,PCF-HCP-900 had the highest pyridine N ratio?29.72%?.In addition,we used CV,LSV and other electrochemical methods to test the catalytic performance of the catalyst for ORR.The results were as follows:the half-wave potential was up to 0.84V?vs.RHE?,which was 10 mV higher than the commercial Pt/C catalyst?0.83V?,the onset potential was 0.95 V?vs.RHE?,the limit current density was 4.8 mA cm^-2,the electron transfer number was up to 3.96,and the yield of H₂O₂ was lower than 5%.In addition,the resistance of PCF-HCP-900 to methanol was tested.After an addition of 0.1M methanol into O_2-saturated 0.1M KOH solution,no noticeable decay was observed in the ORR current density at the PCF-HCP-900 electrode.In contrast,the Pt/C catalyst emerged a remarkable drop under the same conditions,indicating the much advantage of PCF-HCP-900 than the Pt/C catalyst to methanol crossover.In addition,by testing the LSV curve before and after3000 cycles,for PCF-HCP-900,there was only loss a 10 mV negative shift on half-wave potential without any obvious shifts of onset potential.While,the commercial Pt/C catalyst showed a 35mV onset potential shift,and the current density had over 30%attenuation.Based on these findings,this work may provide new opportunities for the development of low-cost,efficient and superior nonmetallic catalysts in the future.?3?Based on second chapter HCP materials as ORR catalyst research,we used PCF-HCP as precursor,we synthesized ORR catalyst derived from the fabrication of cobalt metal doped carbon skeleton?named as Co-HCP-900?,After high temperature carbonization,TEM and EDS characterization showed that Co was evenly miserably into carbon skeleton,and XPS proved the existence of Co.In 0.1 M KOH electrolyte electrochemical performance test,we found that the onset potential of the Co-HCP-900 was 0.91 V,limiting current density was 4.6 mA cm^-2,n was 3.84,and platinum carbon catalytic performance.In summary,the new Co-HCP-900 has a good prospect in the future development of fuel cells for ORR.