Study On The Preparation And Catalytic Performance Of Nanotubes Derived From Conjugated Microporous Polymers For Oxygen Reduction Reaction

Fuel cells are new generation devices for clean energy conversion and storage with safe and reliable.They possess a number of advantages such as high energy efficiency and zero emission.At present,noble metals such as platinum?Pt?are mainly used as cathodic oxygen reduction reaction?ORR?catalysts in this field.However,Pt has a lot of inherent disadvantages such as high cost,scarcity,insufficient stability and poor tolerance to CO and methanol,which have severely impeded the development of fuel cells.Therefore,exploring non-precious metal even metal-free ORR electrocatalysts with low-cost and high-stability has become an urgent issue for fuel cells.Conjugated microporous polymers?CMPs?,composed of three-dimensional reticulated macromolecules of?-extended conjugated system,exhibits excellent application value in adsorption,separation and catalysis on account of their rich synthetic routes,adjustable pore size and chemical composition,easy functional modification and large specific surface area.Based on the characteristics of CMPs,aiming at the problems of low activity and poor stability of ORR electrocatalysts,this paper intends to design and synthesize conjugated microporous polymer nanotubes with good pore structure and high specific area.And then,the non-noble metal ORR catalysts were obtained by further heat treatment and heteroatom doping.The structure-activity relationship between the surface structure and electronic structure of the catalysts and the catalytic activity for ORR was studied.This work provides a new idea for the preparation of metal-free ORR catalysts with good activity,anti-poisoning and high stability,as well as the design and synthesis of transition metal doped carbon catalysts based on CMPs.And the design concept has research continuity.The research contents and main conclusions are as follows:?1?In the first part of the experiment,we prepared two CMP nanotubes denoted as CMP-1 and CMP-2 by the Sonogashira-Hagihara cross-coupling reaction with1,5-dibromonaphthalene,2-amino-3,5-dibromopyridine and 1,3,5-triethynylbenzene.Six metal-free ORR catalysts were synthesized by carbonization at various temperatures,and the tubular morphology did not change obviously before and after heat treatment.HCNT-1-900 obtained by carbonization of CMP-1 at 900?for 2 h has good catalytic activity for ORR.?2?In order to further improve the catalytic activity and stability of CMPs-based catalysts for ORR,we combined graphitic carbon nitride?g-C₃N₄?with CMP-1 to synthesize a new N-doped carbon nanotube denoted as NHCNT-1 in the second part of the experiment.Rotating ring-disk electrode?RRDE?techniques were carried out to explore the catalytic activity for ORR in alkaline and acidic electrolyte.Half-wave potential(E_1/2)of NHCNT-1 reaches to 0.77 V,only 20 mV less than the value of Pt/C in 0.1 M KOH solution.The average electron transferred number(n_avg)of the catalyst is 3.83,while the average hydrogen peroxide yield is only 8.3%,indicating excellent catalytic activity.The stability analysis of NHCNT-1 was evaluated by chronoamperometric experiments at a rotation speed of 1225 rpm with a constant potential?⁰.7 V vs.RHE?.Normalized current plot suggests that the loss of current density on NHCNT-1 is only 5%after 15,000 s polarization,implying compared with Pt/C,it is more stable.In addition,there is no noticeable change for the CV curves and LSV curves during the anti-poisoning on methanol experiments,which means NHCNT-1 also possesses a good resistance to poisoning.In 0.1 M HClO₄,the average electron transferred number of NHCNT-1 reaches to a staggering value of 3.94,and the average hydrogen peroxide yield is only 3.2%,while the stability and half-wave potential are slightly insufficient.NHCNT-1 also exhibits a competitive value for application in acid condition except the stability and E_1/2.Our work provides a good inspiration on the design anti-poisoning,sustainable metal-free ORR catalysts in alkaline and acidic electrolyte at the cathode of fuel cells.?3?Based on the two previous chapters on the ORR application of CMPs,we prepared a cobalt-doped carbon nanotube catalyst denoted as Co-HCNT by using CMP-1 as the precursor in the third part of the experiment.ORR catalytic performance data were collected in alkaline electrolyte.TEM-EDS mapping shows that Co is uniformly doped into the synthesized CMP nanotubes.The nitrogen isothermal adsorption-desorption test indicates that the introduction of Co atoms brings about great changes in the surface structure of samples,and the specific surface area reaches to 888 m² g^-1.The results of ORR catalytic performance for Co-HCNT as followed:the E_1/2 reaches to 0.78 V,the value of n_avg and peroxide yield are 3.84 and8.0%,respectively,indicating an excellent ORR catalytic activity the same as Pt/C.