Adjusting The Local Electronic Structure Of Fe-N_x Active Sites To Prepare Efficient Precious-Metal-Free Oxygen Reduction Electrocatalysts

Fuel cells have attracted more attention because of their high energy density,high energy conversion efficiency,and safety and environmental protection.In recent studies,great progress has been made in the development of fuel cells such as zinc-air batteries and hydrogen-oxygen fuel cells.However,there are still a series of key issues such as low power density,short life,and high price need to be solved urgently,so that fuel cells cannot be large-scale application.The main bottleneck of this type of battery is the slow cathode oxygen reduction reaction(ORR),which is largely due to the high overpotential of the cathode ORR and the slow kinetic process of the multi-step reaction.Although platinum and its alloys have been used as effective ORR electrocatalysts,the price of precious platinum is too high and its reserves are scarce.In addition,platinum-based materials are prone to the effects of precious metals dissolution and CO poisoning leading to poor cycle stability during use,which makes them difficult to apply on a large scale.Therefore,it very important to develop efficient,stable,and economical precious metal-free ORR catalysts.This thesis focuses on how to design and develop a new type of precious metal-free ORR electrocatalyst,and prepare two-dimensional nanosheets,ultraporous nanospheres and other materials with high specific surface area.The morphological characteristics and electrochemical performance of the electrocatalysts were studied systematically.The as-obtained catalytic materials were applied to liquid and all-solid flexible zinc-air batteries and hydrogen-oxygen fuel cells.The details are as follows:1.The hybrid carbon nanosheets with Fe3O4 nanoparticles encapsulated in N-doped porous carbon were developed by the pyrrole monomers in situ polymerized on graphene oxide.The as-prepared 2D-Fe3O4@FeNC-700 catalyst exhibits excellent ORR activity and with much better methanol resistance and durability than benchmark Pt/C.Moreover,the Zn-air batteries assembled with 2D-Fe3O4@FeNC-700 in liquid and flexible all-solid-state configurations show the maximum power density measured to be 138 mW cm-2 and 24.5 mW cm-2,respectively,both were higher than Pt/C.2.Fe-N2S2 active sites immobilized on nitrogen and sulfur co-doped carbon(Fe-N/S-C)as highly efficient catalysts for ORR by using porous ZnS nanospheres as multifunctional structure-directing agent.The as-prepared Fe-N/S-C exhibits high BET surface area(2539 m2 g-1),open hierarchical porous structure and good conductivity.The interactions of above merits resulting in the fabricated catalyst exhibits excellent ORR electrocatalytic activity(a half-wave potential of 0.882 vs 0.872 V for Pt/C),and with much better methanol resistance and durability than benchmark Pt/C in alkaline media.Moreover,H2-O2 proton membrane exchange fuel cell assembled with Fe-N/S-C as a cathode catalyst produced a high peak power density of 512 mW cm-2.