Heteroatom-doped Carbon Materials As Metal-free Electrocatalyst For Oxygen Reduction Reaction

With the advantages of environment friendly,high power generation efficiency,low noise,high reliability,fuel cells have been considered as a promising alternative for energy sustainable development in the future in the transportation,aerospace,aviation communication and so on.The oxygen reduction reaction?ORR?of cathode is an important part of fuel cells.Up to now,the platinum-based catalyst is still adopted as fuel cells cathode catalyst for oxygen reduction reaction.However,shortage of resources,the high cost of platinum catalyst,poor resistance to CO poisoning and methanol interference and poor stability characteristics seriously hindered the development of fuel cell.Recently,free-metal heteroatom doping of graphene carbon nanomaterials are widely used in fuel cell cathode catalyst research.The introduction of heteroatoms?e.g.,nitrogen,boron,phosphorus,sulfur,fluorine,and so on?into carbon nanomaterials could cause electron modulation to change the charge distribution and electronic properties.This,together with the doping-induced defects could further change the chemical activity of carbon nanomaterials,and then improves the performance of the electric catalytic materials for ORR.We explore the excellent performance of heteroatom doping carbon nanomaterials based on environment friendly,low-cost electrocatalyst as the goal.The main work of this thesis as follows:?1?Study on synthesis of nitrogen-doped carbon nanomaterials and their oxygen reduction performance.Nitrogen-doped carbon nanomaterials?DLNDC?were synthesized by biomolecules as carbon and nitrogen source via an activation-/template-free and high temperature carbonization method.DLNDC was characterized by XPS,BET and so on.The results show the high specific surface area up to 700.9 m² g-1,the percentages of different nitrogen of DLNDC were estimated to 3% for oxidized N,16% for graphitic-N,37% for pyrrolic-N,and 44% for pyridinic-N,indicating that nitrogen atoms are arranged into the carbon structure in four different chemical states.Using the B3LYP/6-31G* level density functional theory?DFT?calculations in Gaussian 09,we also simulated the oxygen adsorption processes on the N-doped graphene.Based on the results of the DFT calculations,O-O bonding length can be stretched to 1.24 ? from 1.21 ? of free oxygen molecule and the dipole moment of absorbed O₂ molecule changes from 0 D to 3.20 D.The sample makes great contribution to its comparable ORR catalytic ability,superior stability,and tolerance to methanol and CO poisoning in alkaline to commercial Pt/C catalyst.Such biowaste-based high-value-added electrochemical-active materials can effectively relieve both fossil fuel and environment,which is beneficial to a sustainable society.?2?Study on synthesis of nitrogen,sulfur co-doped carbon nanomaterials and their oxygen reduction performance.China is the major producer of sweet potato and has an absolutely high proportion in the annual world's production.Sweet potatoes are generally rich in carbon,nitrogen,and sulfur?from threonine,lysine,amino acids?.Herein,we present the effective synthesis of nitrogen,sulfur co-doped carbon nanomaterials generated by carbonizing the wasted sweet potato vines as highly serviceable ORR electrocatalyst?In-situ nitrogen,sulfur self-doping,porous graphitic structure along with a high surface area?884.9 m² g-1?show a hopeful alternative for replacing commercial Pt/C catalyst in fuel cells in terms of electrocatalytic activity,selectivity,and especially durability.Density functional theory calculations also support these results.The peak potential and current density of the as-prepared nitrogen,sulfur co-doped porous carbon nanomaterial have no obvious change with a superior cycling stability after 10,000 cycles,suggesting that it has a great stability.This wisdom translating organic-rich natural discard biowaste into high activity ORR catalyst carbon nanomaterials not only solve the problem of waste disposal,but also create value-added.?3?Study on synthesis of nitrogen,fluorine co-doped carbon nanomaterials and their oxygen reduction performance.Nitrogen,fluorine co-doped carbon nanomaterials were synthesized via tableting and calcining at high temperature,using organic molecules as fluorine and nitrogen source.The acquired nanomaterial was characterized by Raman,XPS,BET and so on.The structural characterization shows that the fluorine and nitrogen elements are successfully doped in carbon structure.The percentage composition of F atom becomes lower and lower by XPS with the increase of temperature.The nitrogen,fluorine co-doped carbon nanomaterials at 700 oC has lower percentage composition?0.11%?of F atom and high specific surface area?932.15 m² g-1?.The electrochemical tests indicate that nitrogen,fluorine co-doped carbon nanomaterials at 700 oC has superior stability,tolerance to crossover effect of methanol and CO poisoning that Pt/C catalyst and shows a good application prospect.Superior electrical catalytic activity may mainly owing to N,F co-doped carbon materials with a large specific surface area and the content of fluorine.