Synthesis Of Nitrogen/Sulfur Co-doped Graphene For Efficient ORR Electrocatalysis

The sluggish kinetics of electrochemical oxygen reduction reaction(ORR) possesses as a major challenge for the development of various high-performance electrochemical energy conversion and storage devices such as fuel cells and metal-air batteries in response to the ever-increasing energy crisis. The platinum(Pt) has been recognized as the most active ORR electrocatalyst due to its low overpotential and high current density. However, the scarcity, high cost, and poor durability of precious Pt hinder its widespread application in these devices. In this regard, considerable efforts have been devoted to seeking alternative ORR electrocatalysts for sustainable energy solution. Heteroatom doped carbon materials as promising metal-free ORR electrocatalysts have encouraged intensive research, but the relation between precursor-doping pattern-ORR activity remains unclear. In this thesis, we developed two facile and eco-friendly methods to synthesize mesoporous nitrogen/sulfur(N/S) co-doped graphene for efficient ORR electrocatalysis. The main contents are as follows:(1) A bioinspired method was developed to synthesize N/S co-doped graphene as an efficient ORR electrocatalyst via self-polymerization of dopamine thin layer on graphene oxide(GO) sheets, followed by reacting with cysteine and finally thermal annealing in Argon(Ar) atmosphere. Heteroatom N and S were sucessfully doped into GO framework. It is found that as-prepared N/S co-doped graphene nanosheets form porous 3D network with high surface area. As-prepared N/S co-doped graphene exhibits significantly enhanced ORR catalytic activity in alkaline solution compared with pristine graphene or N-doped graphene. It also displays long-term operation stability and strong tolerance to methanol poison effect, indicating that it is a promising ORR electrocatalyst.(2) Cysteine was used as nontoxic N and S sources to synthesize N/S co-doped graphene and its ORR activity was optimized by adjusting the reaction time of GO and cysteine, pH of the solution, and the annealing temperature in argon(Ar) atmosphere. It is found that the catalytic activity of the N/S co-doped graphene prepared under optimal condition is significantly improved compared to pristine graphene and even superior to the commercial Pt/C catalyst in term of onset potential in alkaline solution. It also exhibits excellent long-term stability and strong resistance to methanol interference.This thesis not only reports two methods for synthesis of N/S co-doped graphene as high-performance ORR electrocatalyst, but also provides useful clues to rationally design and synthesize efficient ORR electrocatalysts by carefully choosing precursor and reaction condition.