Active Site Regulation Of ZIFs Derived Co-N-C Electrocatalysts For Oxygen Reduction Reaction

Fuel cells are efficient and clean energy conversion device.However,fuel cells with sluggish cathode oxygen reduction reaction(ORR)kinetic process.It is necessary to find excellent catalyst to promote ORR.Pt-based catalysts are effective ORR electrocatalysts.But their prohibitive cost,scarcity,low stability and poor methanol tolerance limit the performance and commercialization of fuel cells.Therefore,it is important to explore high-performance and cost-efficient non-precious metal ORR electrocatalysts.ZIFs-derived transition metal nitrogen-doped carbon(M-N-C)are effective ORR electrocatalyst.However,the M-N-C catalysts have weak binding force with oxygenated intermediates relative to a promising catalyst.Herein,We combine guest molecules and heteroatom sulfur with ZIFs-derived Co-N-C to produce new active sites and regulate chemical environment and electron distribution of Co-N-C to improved the ORR activity of Co-N-C in alkaline or acidic conditions.The main research contents and results are as follows:(1)Cobalt sulfide/nitrogen,sulfur co-doped hollow carbon nanocube(CoS_x/N,S-HCNC)for ORRZIF-67 as precursor to introduce guest molecules such as m-aminophenol,formaldehyde and sodium bicarbonate to obtain ZIF-67/CCH-PmAP core-shell structure.The shell is composed of cobalt carbonate hydroxide(CCH)and poly m-aminophenol(PmAP).The CoS_x/N,S-HCNC with hollow structure were obtained by two-step pyrolysis due to heterogeneous composition of ZIF-67/CCH-Pm AP.It is concluded from physical characterization and electrochemical test that:(1)CoS_x and heteroatom sulfur synergistic promote the ORR catalytic activity of Co-N-C;(2)There are additional active sites inside the hollow structure and the reactants diffuse rapidly,thus accelerate the mass transfer of ORR;(3)The CoS_x nanoparticles are coated with carbon layer,which increases the stability of the catalyst.Benefit from the superiority of composition and structure for CoS_x/N,S-HCNC,it's ORR activity and stability in alkaline media is superior to commercial Pt/C.(2)S-doped pyrrole/graphite nitrogen-rich cobalt-nitrogen-carbon(S-P/G N-rich Co-N-C)for ORRIn the previous work,Co nanocrystals exist in CoS_x/N,S-HCNC,which lead to poor ORR activity in acidic medium.Therefore,we proposed a one-step method for the in-situ immobilization of poly(p-phenylenediamine-formaldehyde)on the Co,Zn-zeolitic imidazo-late frameworks to construct pyrrolic/graphitic N-rich Co-N-C with monodisperse cobalt.It is concluded from physical characterization and electrochemical test that:on the one hand,the pyrrolic/graphitic N species derived from poly(p-phenylenediamine-formaldehyde)reduce the electron localization around the Co centers,leading to strengthening the adsorption of oxygenated species and improving the intrinsic activity.What's more,pyrrolic/graphitic N-rich Co-N-C structures with large surface area,high pore volume,and atomically dispersed Co to fully expose efficient Co-N-C catalytic sites.Hence,pyrrolic/graphitic N-rich Co-N-C with the half-wave potential of 0.755 V in acidic media,and are even superior to that of the state-of-the-art Pt/C in alkaline media with the half-wave potential of 0.889 V.On the other hand,doping heteroatoms sulfur into carbon lattice induces the spin densities of adjacent carbon atoms for providing more oxygen adsorption sites and facilitating the formation of the OOH species,resulting in synergistically boosting the ORR activity in an acidic environment with efficient Co-N-C sites,the half-wave potential is achieved to 0.772 V.