| In the context of carbon peaking and carbon neutralization,fuel cells have received extensive attention,but their cathode-side oxygen reduction reaction（ORR）kinetics are slow and depend on Pt-based noble metal catalysts,which seriously hinders their wide application.Therefore,the development of low-cost,highly active oxygen reduction catalysts is a research focus in the fields of fuel cells and metal-air batteries.Fe Single-atom catalysts can catalyze the ORR and have excellent resistance to carbon monoxide,nitride,and sulfide poisoning,so they are considered as one of the most promising oxygen reduction catalysts to replace Pt/C.However,there is still a certain gap between Fe single-atom catalysts and Pt-based catalysts in terms of activity,which is mainly determined by the intrinsic activity and density of active sites.Therefore,this paper optimizes the activity of Fe single-atom catalysts by regulating the charge state of single-atom Fe sites and increasing the metal single-atom loading.The main research results are as follows:（1）The effects of metal with different outer electronic structures on the ORR performance of single-atom Fe catalysts,such as Ca in the s-block,Co in the d-block and Sn in the p-block,were investigated.Fe/Ca,Fe/Co and Fe/Sn bimetallic single-atom catalysts were prepared by a double isolation strategy,respectively.The combined characterization methods of XRD,TEM and XAS prove that the prepared Fe/Co and Fe/Sn are bimetallic single-atom catalysts.The electrochemical test results show that the Fe/Sn bimetallic single-atom catalyst has the highest activity,which may be due to the fact that Sn has no unsaturated d orbitals,it is difficult to provide active lone pair electrons,and the effect on the Fe active center is relatively mild,making it the most suitable state to improve the catalytic activity.By further tuning the Fe/Sn metal ratio（1:2,1:1 and 2:1,respectively）,and it was found that the prepared 2Fe Sn-1,4-PA/N-C catalyst had the best oxygen reduction activity when the Fe/Sn ratio was 1:1.The catalyst catalyzes ORR with an onset potential of 1.03 V,a half-wave potential of 0.857 V,a Tafel slope of 93.2 m V dec-1,and a kinetic current density and mass activity of 13.93 m A cm-2@0.8 V and 55.72 A g-1@0.8 V.（2）The effects of the addition amount of metal ions precursor（0.4,0.5 and0.6 mmol）on the structure,composition and performance of single-atom catalysts were investigated.XRD and TEM test results showed that the metals in the prepared materials remained dispersed at the single atomic level when the addition amount was≤0.5 mmol,and the metals began to agglomerate when the addition amount was 0.6 mmol.The as-prepared 5Fe Sn-1,4-PA/NC has the highest single-atom content with Fe and Sn loadings of 2.7 wt%and 1.3 wt%,respectively;the half-wave potential for catalytic oxygen reduction in alkaline solution reaches 0.871 V,and the current loss is 31.2%after 50,000 s i-t test at a constant potential of 0.75 V.The power density of Zn-air battery using it as cathode catalyst is as high as 224 m W cm-2,which is 29.5%higher than that of commercial Pt/C-based Zn-air battery(173 m W cm-2).At the same time,it has excellent stability,and the performance does not decay at a current density of10 m A cm-2 for 50 h,and maintains the same performance after replacing the electrolyte and Zn sheet. |
PDF Full Text Request