Polymer Self-Assembly Derived Three-Dimensional Porous Carbon-based Materials And Their ORR Catalytic Performance

Oxygen reduction reaction(ORR),as the cathode reaction of the fuel cell,directly affects the performance of the fuel cell.However,due to the slow kinetic characteristics and high reaction potential,the ORR often requires catalyst assistance.The current commercial Pt/C is an ideal catalyst for ORR,but its extremely low reserves,high cost and unsatisfactory stability limit the large-scale application.It is of great significance to find non-noble metal-based electrocatalysts that can be comparable in performance to Pt/C.In this paper,a polymer self-assembly strategy is used to construct a three-dimensional sponge structure with abundant pores,and nitrogen-doped carbon sponges were obtained by the pyrolysis of the precursor.Due to the unique thermal properties of the polymer precursor poly(amic acid,PAA),the porous sponge structure of the precursor can be well-preserved during the carbonization process,and it exhibits excellent ORR catalytic performance.Furthermore,the sponge structure precursor is used as a universal support,and a variety of metal ions are adsorbed into the precursor through electrostatic adsorption.After carbonization,transition metal(oxide)-loaded nitrogen-doped carbon sponges are obtained,which further improve their ORR catalytic performance.The detailed research content is as follows:(1)The amphiphilic polymer poly(amic acid)(PAA)is prepared by condensation polymerization by using monomers with high nitrogen content,and the three-dimensional porous sponge is prepared through the "bottom-up" assembly process.A series of nitrogen-doped carbon sponge(NCS)ORR electrocatalysts were prepared by controlling the carbonization temperature of the three-dimensional porous sponge precursor.The characterization techniques such as XRD,BET.SEM,TEM,Raman,XPS were used to analyze the phase,pore structure,morphology,chemical composition,etc.of the NCS.Besides,the ring disk electrode and electrochemical workstation were also used to evaluate the ORR performance of NCS.The results showed that the three-dimensional porous sponge is carbonized into nitrogen-doped carbon sponge with wellpreserved morphological and porous structure.Among the NCSs,the NCS-800 has the best ORR performance,cycle stability and resistance to methanol toxicity in alkaline electrolyte.The onset potential,half-wave potential,and limiting current density are 0.940 V,0.816 V and-4.34 mA cm-2,respectively,comparable to Pt/C(20%).The ORR electron transfer number is 3.8,which is close to the four-electron transfer pathway.Moreover,the cycle stability and anti-methanol toxicity of NCS-800 are much better than commercial Pt/C.This self-assembly strategy provides a feasible method for the preparation of micro-/mesoporous structured carbon nanomaterials by template-free methods.(2)Using the PAA sponge as support,different metal ions M(Co,Fe,Cu)are in situ adsorbed onto the sponge.After pyrolysis at inert atmorsphere,different metal nanoparticles loaded NCS(M@NCS)were obtained with well-preserved pore structure.The characterization and performance test results showed that the degree of graphitization of M@NCS increased due to the catalytic activity of metal nanoparticles.The size of the metal nanoparticles of Co@NCS,FeCo@NCS were smaller and there was no obvious agglomeration.However,the particle size of Cu@NCS and CoCu@NCS were much larger due to the agglomeration.The prepared Co@NCS and FeCo@NCS showed excellent ORR catalytic activity.The onset potential,half-wave potential and limiting current density of Co@NCS were 0.950 V,0.830 V and-4.6 mA cm-2,respectively.And the onset potantial,half-wave potential and limiting current density of FeCo@NCS are 0.950 V,0.840 V and-4.5 mA cm-2,respectively,comparable to that of Pt/C(20%).Compared with NCS,the ORR performance is further improved.The ORR electron transfer numbers of Co@NCS and FeCo@NCS are 3.65 and 3.4,respectively,indicating that the four electron transfer pathways is predominant.The cycle stability of Co@NCS and FeCo@NCS are better than Pt/C electrocatalysts,demonstrating that the polymeric precursor can be used as a general platform to support a variety of transition metals and their oxide nanoparticles by in-situ adsorption.