The Design,Preparation And Application Of New High-Efficiency Electrochemical Oxygen Reduction Catalysts

Fuel cells(FCs)represent promising energy conversion technology,but commercializing this technolgy on the industrial scale remians a challenging task.One of the main problems with FCs is its sluggish cathodic oxygen reduction reaction(ORR).It always needs catalysts to lower its electrochemical overpotential for high-voltage output,and so far,platinum(Pt)has been the universal choice.However,there is not enough of Pt on Earth for large-scale application.So it is of great priority to develop new efficient catalysts for ORR.Currently,two efficent strategies have been identified.One is to boost the ORR activity with high Pt ultilization efficency by means of forming alloys,manipulating architectures,tuning morphologies and so on.The other one is to explore non-Pt catalysts for ORR.Our early work has discovered that the sturctural ordering degree of AuCu alloys would affect the arrangement of their outer atoms.On this basis,the first part of our experiments selectively replaced the outer Cu atoms on the surface of AuCu alloys with Pt atoms and synthesized core-shell structured AuCu@Pt/C catalysts.The local environment of outer Pt atoms had been adjusted by controling the structural ordering degree of AuCu core.It was found that the obtained O-AuCu@Pt/C catalysts,whose core material is ordered AuCu,could display high mass activity(0.75 A·mgPt-1)and specific activity(0.491 mA·cm-2),or nearly 7.5 times and 3.6 times higher as compared with commmercial Pt/C.The catalysts exhibited high stability with negligible activity decay after 10,000 cycles.X-ray absorption spectroscopy proved that changes in the local environment of Pt would change the electronic state of the outer Pt atoms.Theoretical calculations revealed that changes in the local environment of Pt could change the value of Gibbs free energy when the intermediate product OOH*is generated,thereby affecting the catalytic activity of the obtained catalysts.Among all the explored non-Pt catalysts,Fe-N-C electrocatalysts are considered to be the most promising substitute of commercial Pt/C.In our previous work,we used FePc as the precursor and synthesized a three-diamensional Fe-Nx/CNTs/rGO catalysts for ORR.However,the expensive macrocycle compound precursor,low catalytic stability and limited effective iron content limited its further application in practice.On this basis,the second part of our experiment used the low-cost melamine and FeCl2·4H2O as precursors and synthesized another Fe-Nx-CNTs/rGO catalysts for ORR.It could exhibited oxygen reduction activity comparable to Pt/C with a half-wave potential(E1/2)at 0.88 V(vs.RHE)in alkaline solution.In addition,it could also showed a better stability than Pt/C with a lower decay of E1/2(10 mV)after 2000 cycles.The CN-poisioning experiment of Fe-Nx-CNTs/rGO demonstrated the effective Fe speceis in Fe-Nx-CNTs/rGO for ORR was Fe-N4 specie.However,its 57Fe Mossbauer spectroscopy and X-ray absorption spectroscopy revealed that the content of Fe-N4 in Fe-Nx-CNTs/rGO was still low.To further improve its effective iron content,the third part of our experiment still used the low-cost iron salt and nitrogen-containing compound as precursors.We firstly polymerized an iron-containing polyphthalocyanine compound.It is a two-dimensional covalent organic framework(COF)compound with only Fe-N4 species.After ball-milling,its suspension was mixed with graphene oxide.After freeze-drying,a COF derived three-dimensional Fe-Nx-C catalysts were synthesized.Fe-Nx-C could displayed a higher ORR activity than Fe-Nx-CNTs/rGO with a E1/2 at 0.90 V(vs.RHE)in alkaline solution.Besides,It could also exhibited excellent stability with negligible activity decay after 2,000 cycles.Metal clusters often appear in the preparation of single-atom site catalysts by pyrolysis.And they are usually removed by acid leaching.However,the introduction of acid leaching not only increases the preparation cost of the catalyst but also causes significant pollution to the environment.Drawing on the technology of preparing core-shell catalysts in the first part of our paper,the metal clusters generated in the pyrolysis of ZIF-67 were skillfully converted into PtCox/C catalysts.The prepared catalysts could exhibit extremely high ORR activity in the acidic electrolyte.Its mass activity could reach 1.03 A·mgPt-1 and specific activity could reach 1.202 mA·cm-2 which were 10-and 8.8-fold enhancements compared to commercial Pt/C.In addition,The catalysts exhibited high stability with negligible activity decay after 10,000 cycles.All the catalysts prepared above have showed excellent oxygen reduction activity and stability,which provides new ideas for the development of new low-platinum and non-platinum oxygen reduction catalysts.