Electrocatalytic Oxygen Reduction Over Silver Activating N-Doped Graphene

The rapid increase in fossil fuels consumption and the urgent global environment issues call for clean and renewable energy supply.Fuel cells,which electrochemically convert chemical energy of hydrogen into electricity,are promising candidates for efficient and clean energy conversion.However,the sluggish four-electron oxygen reduction reduction(ORR)at the cathode limits the performance of fuel cell.Traditionally,Pt has been regarded as the best ORR catalyst,but it is too expensive for commercial purpose.Thus,developing non-platinum-based electrocatalysts such as transition metal based or metal-free catalysts has become the hot point in the catalytic field of ORR.Graphene based nanocarbons are considered as one class of promising ORR catalysts due to their unique physical chemical properties such as ultrahigh surface areas,high carrier mobility and adjustable Fermi level by element doping.Pristine graphene is a zero-overlap semimetal with low density of states(DOS)near the Fermi level,which is actually inert in ORR.Chemical modification with dopants such as nitrogen can increase the DOS and trigger the catalytic activity of Graphene.Metals with sufficient electron have also been proved to be electron donors for tuning the electronic state of graphene surface.Graphene layer encapsulating metal nanoparticles as a promising strategy,which can effectively protect the inside metals from being destroyed in harsh condition and improve the ORR activity of the graphene shell simultaneously.In this dissertation,we proposed a strategy to increase Fermi level of graphene by introducing metal such as silver as strong electron donor.Ag,a low-work-function metal(work function,4.2eV),was embedded by N-doped graphene(work function,4.5 eV)to narrow the energetic difference of graphene and the C-O*antibonding states.The glucose as the carbon source,the silver nitrate as the metal precursor were mixing with potassium chloride and sodium chloride.After the Maillard reaction,the flux synthetic andnitride,the N-doped graphene encapsulating silver(Ag@NG)was prepared.A series of characterizations indicated that the doping of N helps to anchor the silver and obtain a sample with uniform distribution of silver nanoparticles,while the presence of silver can catalyze the introduction of pyridinic-N during the nitride.The catalyst Ag@NG exhibited the ORR onset potential of 0.804 VRRHE and the half-wave potential of 0.638 VRRHE better than pristine graphene and N-doped graphene and excellent stability over 10000 electrochemical cycle than Pt/C in 0.1M HClO4 electrolyte.Rotating disk electrode(RDE)measurement indicated that the four-electron process dominates the oxygen reduction reaction,whether Ag@NG or NG.The doping of nitrogen introduced a high-efficiency active site,which completely reduce oxygen into water as efficient four-electron pathway.Spectroscopic analysis confirmed Ag-N coordination bond as an efficient electron transfer route significantly benefiting upshift of graphene Fermi level that enhances the ORR activity.Microscopic characterization confirmed the existence of graphene embedding metal structure that enhances the stability of the catalyst in acidic medium.