Synthesis Of Carbon-based Nonmetal Oxygen Reduction Electrocatalysts And Active Site Study

The cost and scarcity of platinum-group-metal?PGM?catalysts severely limit the commercialization of fuel cells.Therefore,considerable efforts have been made to develop earth-abundant material as oxygen reduction reaction?ORR?catalysts.Benefiting from the advantages of carbon materials,such as diversified construction,chemical stability,high conductivity and extremely high carbon surface area,nanostructured carbon material doped with heteroatoms,so-called as metal-free carbon-based catalysts,are considered as one of most promising ORR catalysts.Although notable progress has been achieved in ORR activity,the chemical nature of the active sites and ORR process on metal-free carbon-based catalysts are not clear yet.Controversies are on the role of pyridinic N,graphitic N,defect structure and even the trace metal impurity.However,the difficulty in identifying the nature of active sites is the diversity of carbon-based materials..Herein,a series of metal-free carbon-based catalysts is designed by processing of heteroatoms doping and removing.Therefore,oxygen concentration,nitrogen concentration and defect density are tuned.Correlation between defect density and ORR activity are observed in such metal-free carbon-based catalysts.While in the case of oxygen concentration or nitrogen concentration,none of dependence relationship with activity can be found.It is worth noted that defects derived from nitrogen-removed and oxygen-removed are both exibited a positive correlation with ORR activity.Furthermore,the effect of trace metal impurities on ORR activity are considered,since142 mg·kg^-1 of Fe are detected in NAr-1100 catalyst.SCN^-ion was used to stongly ligand with the metal center active sites to prevent ORR.Then,a real ORR activity of non-metal active sites was observed and found to correlate with defect density.Therefore,defect sites are one of the ORR active site in acid condition.Lewis base sites are proposed to directly absorb dioxygen molecular at the early stage of ORR.Therefore,it is suggested that basicity of carbon matrix will tune the ORR active.Hence,a series of metal-free carbon-based catalysts is prepared by nitrogen doping and removing.Lewis basicity and density of base sites of these catalysts are measured by Boehm titration.In this series of catalysts,the average Lewis basicity are all very close to 8.2 of pKa.And the density of base sites is changed from125.5 to 165.5?mol·g^-1,which is correlated to the change of nitrogen and oxygen concentration.Furthermore,a positive correlation between base density and ORR active are found.And turnover frequency,which is 0.219 e·sites^-1·s^-1 at 0.6 V vs.RHE,is estimated based on the density of base sites.