Hierarchically Structured Platinum-Iron Alloy Catalysts For Efficient Oxygen Reduction Reaction

The oxygen reduction reaction（ORR）plays an important role in sustainable energy conversion and storage devices,such as fuel cells and metal-air batteries.It is a four-electron process with a complex mechanism and slow kinetics,which severely restricts the commercialization of such devices.The precious metal Pt can reduce the activation energy of ORR and promote the kinetics.The 20 wt.%Pt/C is currently the most widely used ORR catalyst.However,the catalytic activity and atomic utilization of the Pt/C catalyst is unsatisfactory.Furthermore,the nano-scale Pt particles dispersed on porous activated carbon are prone to migration,aggregation,and shedding during long-term operation,resulting in poor stability.Therefore,the development of platinum-based catalysts with lower platinum loading,higher activity and stability has been noticeably considered.In this study,the atomic utilization and activity of Pt-based catalysts through component regulation,morphology design,and surface engineering strategies have been improved,achieving the purpose of reducing the platinum loading.Besides,the well-designed Pt-based catalyst has a strong interaction with the carbon support,which can inhibit the migration,aggregation,and shedding of the catalyst,improving the stability.The main research contents are as follows:1.The transition metal Fe is introduced into Pt crystals to prepare Pt Fe binary metal catalysts through component regulation.The Fe atom can downshift the d-band center energy of the Pt atom,increasing the ORR activity of Pt-based catalysts.In this section,the Pt Fe binary metal catalysts were successfully prepared by an impregnation-annealing method,and the effects of annealing temperature and Fe-precursor on the structure and ORR activity of the Pt Fe binary metal catalyst were studied.The results show that the transformation from disordered Pt Fe alloy to ordered intermetallic compounds Pt₃Fe and Fe Pt can be obtained by changing the annealing temperature;The main component of the intermetallic compounds in the catalysts can be regulated by changing the Fe-precursor.The intermetallic compound Fe Pt displayed the best ORR catalytic activity with a mass activity of 0.4790 A mg _Pt^-1 at 0.9 V versus reversible hydrogen electrode（vs.RHE）,which was about 5 times higher than that of the commercial Pt/C catalyst.The excellent ORR performance could be ascribed to the order and size effect of the intermetallic compound Fe Pt.2.The Pt Fe alloy is devised into a three-dimensional interconnected nanowire network structure via morphology design.The three-dimensional structure could expose a larger specific surface area,boosting the ORR activity.Moreover,the anisotropic three-dimensional structure can enhance the interaction between the catalyst and the carbon support,improving the stability.The three-dimensional nanowire network structure of Pt Fe alloy catalyst was successfully prepared by annealing Fe₃O₄nanoparticles with massive Pt nucleation sites under reducing atmosphere.The formation of the nanowire network structure was attributed to the magnetic dipole interaction of the Fe₃O₄.The structural properties of the alloy catalysts can be regulated by changing the annealing condition,thus affecting the ORR catalytic activity.The optimized structure of the Pt Fe alloy catalyst annealed at 900°C under H₂ atmosphere for 2 h displayed a mass activity of 1.69 A mg _Pt^-1 at 0.9 V vs.RHE,which was roughly9 times higher than that of the commercial Pt/C catalyst.3.Defects are introduced on the surface of the Pt Fe alloy with an interconnected nanowire network structure through surface engineering technology.Surface defects with unique physical and chemical properties can improve the ORR activity of the catalyst.Surface vacancies were prepared on Pt Fe alloy by electrochemically dealloying which could dissolve the most of Fe atoms on the topmost and near-surface.The interconnected nanowire network structured Pt Fe alloy catalyst with abundant Fe vacancies exhibited excellent ORR activity and durability.The mass activity at 0.9 V vs.RHE were 3.65 and 1.10 A mg _Pt^-1 in alkaline and acidic electrolyte,which were approximately 20 and 10 times than that of the commercial Pt/C catalyst,respectively.And the catalytic activity exhibited no significant decrease after 10,000 accelerated durability tests.In addition,density functional theory calculation was used to explore the mechanism of vacancy in the ORR process.The results show that vacancy can weaken the bonding strength of Pt and the intermediate oxygen species,thus increasing the ORR activity.