Design Of Transition Metal-nitrogen-carbon For Efficient Oxygen Reduction Reaction

The increasing energy demand and environment issues have stimulated intensive research on clean and sustainable energy technologies,such as fuel cells and metal-air batteries.Oxygen reduction reaction（ORR）at the cathode plays a central role in these devices.The performance of those devices,unfortunately,is seriously limited by the sluggish kinetics associated with ORR because ORR involves in four-electron transfer with*OOH,*O,and*OH being the intermediates.Although Pt has been generally considered as one of the best ORR electrocatalysts because of its high activity,it has been suffering from high cost,low abundance,poor stability and poor resistance to crossover of fuels such as methanol.Developing efficient ORR electrocatalysts with earth abundant element as alternatives for Pt is highly desirable.Transition metalnitrogen-carbon（M-N-C）is one of the most promising alternative ORR electrocatalysts for Pt.However,the challenges associated with M-N-C are their relatively lower intrinsic activity than Pt,poor mass transfer,and limited active sites.This thesis will focus on the development of M-N-C materials for electrocatalytic ORR,including the following sections:1.A MOF-engaged self-templated strategy is developed for the synthesis of cobaltembedded hollow N-doped carbon spheres（Co/HNC）.The Co/HNC manifests high ORR activity with half-wave potential（E1/2）of 0.83 V and diffusion-limiting current density（jL）of 5.46 mA cm^-2,which is better than commercial Pt/C.In addition,Co/HNC shows much higher methanol tolerance and ORR stability than commercial Pt/C.It can be ascribed to the hollow structure which provides high electrochemically active surface area,the formation of abundant Co-N species,and the introduction of defects.2.We rationally design and synthesize hierarchical Zn-N-C microtubes via MOFbased self-templated method.In virtue of abundant Zn-N active sites,hierarchical porosity and reasonable graphitization degree,the optimized Zn-N-C shows a halfwave potential（E1/2）of 0.84 V vs RHE,with higher stability and methanol tolerance than commercial Pt.This work highlights the importance of rational engineering of ZnN-C based electrocatalysts for enhanced ORR performance.3.Hierarchical porous Fe-N4-C is successfully fabricated,and the electronic structure and porosity of atomically dispersed Fe-N-C can be precisely tailored by incorporation of carbon nanotubes（CNTs）.By combining density functional theory（DFT）calculations,X-ray adsorption spectroscopy（XAS）,electrochemical characterizations and nitrogen adsorption-desorption isotherms,we reveal that CNTs can not only draw electrons from Fe-N4 through Fe-C interaction,leading to more optimal electronic structure of Fe,but also enables the formation of more mesopores.In virtue of the optimized electronic structure with enhanced intrinsic ORR activity as well as the hieratical porosity that ensures the efficient exposure of active sites and accelerates the mass transfer,the optimal Fe-N-C/CNT sample manifests high ORR activity with half wave potential（E1/2）of 0.894 V vs RHE and kinetic current density（jK at 0.850 V）of 9.25 mA cm^-2,which is much better than Fe-N-C(E1/2=0.856 V and jK=3.67 mA cm^-2)and commercial Pt/C.In addition to the high ORR activity,FeNC/CNT also exhibits better durability and resistance to methanol crossover than Pt/C.