Investigation On Fabrication And Electrocatalytic Properties Of Nanostructured Spinel/Perovskite-type Oxides

Oxygen reduction reaction(ORR)plays critical role in the development of fuel cells and metal-air batteries.However,the sluggish kinetics seriously hamper the further development of fuel cells and metal-air batteries.Up to now,Pt-based materials are known to be the preferred electrocatalysts with the highest catalytic activity towards ORR,however,the high cost and limited stability of precious metals grievously limit their commercial applications.One way to solve this problem is to develop active and stable electrocatalysts composed of inexpensive and earth abundant elements to replace precious catalysts.In recent years,spinel-type oxides and perovskite-type oxides have attracted great interests because of their special crystal structure,diversity of types and excellent performance in the oxygen electrode.In this paper,the Al-based precursor alloys were prepared by melting in an induction furnace,and then the precursor alloys were dealloyed in alkaline solutions.The as-dealloyed samples were annealed in air and then the nanostructured spinel/perovskite-type oxides with high BET surface area and excellent catalytic.performance were obtained.Firstly,the starting bi-component Al95Mn5 alloy was made by melting in an induction furnace,and then dealloyed in NaOH solution at room temperature.Finally,the as-dealloyed samples were annealed under a certain condition.The X-ray diffraction(XRD),electron microscope(SEM,TEM,HRTEM,STEM)and X-ray photoelectron spectra(XPS)results confirm that the Mn2Al04 spinel oxide with nanosheet structure was obtained.The N2 adsorption/desorption isotherms reveal that the samples have high BET surface areas because of the existence of the ultrathin nanosheets and the nanoporosities on the nanosheets.The electrochemical measurements show that the Mn2AlO4 oxide has superior ORR activities and methanol tolerance.Both the RDE and RRDE results show that the ORR on Mn2AlO4 is a 4e" process.Density functional theory(DFT)calculations verify the 4e" pathways of Mn2AlO4 in ORR and also unveil the reason why the Mn2AlO4 catalyst has much better methanol tolerance than the commercial Pt/C catalyst.Secondly,the ternary Al95.5Fe3M1.5(M = Co,Mn or Ni)alloys were fabricated by melting in an induction furnace,and then dealloyed and annealed under the defined conditions.Finally,the nanoporous MFe2O4 spinel oxides with high BET surface areas were obtained.The electrochemical measurements indicate that MFe2O4 oxides have superior ORR activities and show the M-dependent catalytic activities with CoFe2O4 being the most active electrocatalyst followed by MnFe2O4 and NiFe2O4.For the oxygen evolution reaction(OER),however,the activity increases in the order:CoFe2O4 ? NiFe2O4>MnFe2O4.Additionally,the MFe2O4 spinel oxides manifest outstanding methanol tolerance.The RDE and RRDE results show that the ORRs on MFe2O4 in 0.1 M KOH are 4e-processes.DFT calculations also verify the 4e-pathway of MFe2O4 in ORR,and unveil the reason why the MFe2O4 catalysts have excellent methanol tolerance.Thirdly,the ternary Al94.8Mn5Pt0.2 alloy was prepared by melting in an induction furnace,and then dealloyed and annealed under the defined conditions.Finally,Pt/(Mn,Al)3O4 metal/metal oxide composites with low Pt loading were obtained.The phase and characterization results reveal that the composites are composed of nanoporous Pt in intimate contact with lamellar(Mn,Al)3O4 nanosheets.Compared with commercial Pt/C,the np-Pt/(Mn,Al)3O4 NS catalyst shows a 11.5-fold increase in mass activity toward ORR and much better methanol tolerance,due to the SMSIs and electronic effect between Pt and(Mn,Al)3O4.Furthermore,both electrochemical measurements and DFT calculations verify that the ORR on the np-Pt/(Mn,Al)3O4 NS catalyst is a direct 4e-pathway in the alkaline solution.In addition,the electrocatalytic mechanisms have also been rationalized by DFT calculations.Finally,the ternary Al88La6M6(M = Co,Mn,Ni or Cr)alloys were fabricated by melting in an induction furnace,and then dealloyed and annealed in air under a certain temperature.Finally,perovskite-type oxides LaMO3 with high BET surface areas were obtained.The electrochemical measurements indicate that LaMO3 oxides have superior ORR and OER activities.In addition,the LaMO3 oxides reveal excellent methanol tolerance.The RDE,RRDE and DFT results verify the 4e-pathway of LaMO3 for ORR,and also unveil the reason why the LaMO3 catalysts have excellent methanol tolerance.