Study On Preparation And Electrochemical Performance Of Ca₂CoFeO_5+δ Catalytic Materials

The traditional fossil energy belongs to nonrenewable energy,since human overuse has caused serious consequences such as energy shortage and environmental pollution,it is urgent to develop new and sustainable energy sources.In many electrochemical energy storage devices,metal-air battery has attracted more and more attention because of its high energy conversion rate,cleanliness and environmental protection.Oxygen reduction reaction（ORR）and oxygen precipitation reaction（OER）are very important reactions on its electrode.However,due to the slow kinetic rate on its electrode,a large number of electrocatalysts must be used to reduce overpotential and reduce energy loss.Precious metal materials are recognized as electrocatalysts with obvious effect,but they are expensive and unstable,so they can’t be used in large scale.Therefore,the development of low-cost and high stability electrocatalyst with dual function of ORR and OER is the direction of scientific research.Perovskite type metal oxides have been used as cathode materials of fuel cells due to their high energy density and wide sources,and can show high power density.Recently,some researchers have studied them as cathode of zinc-air battery,and found that they can obtain high catalytic activity and stability,which has set off a wave of upsurge since then.This article first synthesized Brownmillerite（BM）-type metal oxide Ca₂CoFeO_5+δ（C₂CF）by EDTA-CA combined with complexed wet chemical method.Then,in order to improve its electrochemical performance further,we conducted the following research on this basis:（1）Several F-doped catalysts Ca₂CoFeO_4.95+δF_0.05(C₂CFF_0.05),Ca₂CoFeO_4.9+δF_0.1(C₂CFF_0.1)and Ca₂CoFeO_4.85+δF_0.15(C₂CFF_0.15)were prepared with NH₄F as F source.Firstly,it is proved that they are orthorhombic crystal phase by XRD and TEM analysis,and fluorine has been successfully incorporated into C₂CF crystal lattice by mapping analysis.Electrochemical tests revealed that F-doping enhanced the ORR and OER activity of the catalyst.The corresponding voltages of C₂CFF_0.05,C₂CFF_0.1and C₂CFF_0.15 were 1.60V,1.58 V and 1.60 V at the current density of 10 m A/cm^-2,respectively,which were greatly improved compared with C₂CF（1.66 V）,and exceeded commercial Ru₂O（1.64 V）.The results of i-t test also show that the stability and methanol resistance of C₂CFF_x are better than C₂CF.This is due to the reduction of the valence electron density of oxygen ions after F ions replacing O ions,weakening the chemical bond between oxygen ions and metal ions,after F ions replacing O ions,and more conducive to the replacement of lattice oxygen(O^2-)by active oxygen(O₂^2-/O^-),Which increases the oxygen vacancies.（2）Using diammonium hydrogen phosphate as the P source,three P-doped catalyst samples Ca₂Co_0.9Fe P_0.1O_5+δ（C₂CPF）,Ca₂CoFe_0.9P_0.1O_5+δ（C₂CFP）,and Ca₂（CoFe）_0.9P_0.1O_5+δ（C₂（CF）P）were synthesized by EDTA-CA complex wet chemical method.XPS and EDS were used to determine the successful incorporation of P,and XRD and TEM showed that the incorporation of P did not change the orthogonal structure of the catalyst.OER electrochemical test showed that the potential of C₂CPF,C₂CFP and C₂（CF）P was 1.63 V,1.65 V and 1.62 V respectively at the current density of 10 m A/cm^-2.We found that the substitution of P,especially the double substitution（C₂（CF）P）,showed the most outstanding OER activity.ORR test also proves that P doping has a positive effect on the improvement of its performance.Which may be due to the incorporation of P⁵⁺high valence state ions into the lattice,according to the charge conservation,reducing the average valence state of B metal.It is more conducive to electronic conduction,accelerating the electrode reaction rate,and making its electrocatalytic performance significantly improved.（3）We synthesized three catalysts,Ca₂Co_0.95Feo_5+δ(C₂C_0.95F),Ca₂Co_0.9Feo_5+δ(C₂C_0.9F)and Ca₂Co_0.85FeO_5+δ(C₂C_0.85F),by making defects at the Co site of the catalyst Ca₂CoFeO_5+δ.The structural characterization proved that they all maintained the crystal structure of the parent catalyst C₂CF.Electrochemical test revealed that the ORR curve half wave potential of C₂C_0.9F at 1600 rpm was positively shifted by 30 m V compared to C₂CF,Moreover,the OER potential was also increased to 1.61V at the current density of 10 m A/cm^-2,which was30 m V higher than that of commercial Ru₂O.i-t test showed better methanol interference resistance than that of commercial Pt/C.Further study found that its excellent performance mainly depends on the presence of B-site metal defects,which increases the active oxygen content on the catalyst surface,increases the active sites,and is more conducive to electron conduction,which works together to improve catalytic activity.