Fabrication Of Hexagonal Perovskite Oxides And Investigation Of Their Electrocatalytic Oxygen Evolution Reaction

The foremost efforts have been made to search for sustainable,clean and efficient energy production technologies to meet the ever-evolving needs of modern society.At present,oxygen evolution reaction(OER)plays an important role in advanced energy production technologies such as renewable fuel cells,metal-air batteries,and electrochemical water separation systems.However,the OER involves the transfer of 4 electrons and shows the characteristics of slow reaction kinetics,which limits the efficiency of gas evolution.The development of high-performance catalysts currently is one of the effective ways to promote the efficiency of oxygen evolution and solve the slow rate of energy production.Perovskite oxides have attracted widespread attention due to their easy-to-control element ratios,electronic structures and crystal structures.Moreover,perovskite oxides also have become a hot spot in the current electrocatalytic research fields due to the above-mentioned unique characteristics and high-efficiency OER catalytic performance.At present,the research of perovskite catalysts is mainly focused on oxides with cubic structure,but there are relatively few reports on perovskites with hexagonal or other crystal structures.However,a small number of reported hexagonal perovskites exhibit excellent OER performance,indicating that they may have great application potential in the field of electrocatalysis.Therefore,in this article,we synthesized 6H-type hexagonal perovskite oxides and investigated their catalytic activity.Then,10H-type hexagonal crystal system perovskite was synthesized and their OER catalytic performance was studied.Based on the researches on the catalytic mechanism of OER,it is believed that oxygen defects play an important role in the OER process.We introduce a series of A-site cation defects to increase the oxygen vacancies in the perovskites,thereby obtaining perovskite oxides with higher OER catalytic activity.The specific content is as follows:(1)The 6H-type hexagonal phase structure Ba0.5Sr0.5Co0.8Fe0.2O3-?(BSCF-H)was synthesized by simple sol-gel method.The crystal lattice constant and space group(P 6 m2)of the hexagonal phase perovskite was determined through structural refinement.High-resolution transmission electron microscopy(HRTEM)and selected area electron diffraction(SAED)patterns further proved the hexagonal structure of BSCF-H.Subsequently,the formation mechanism of perovskite shows that it is easier to form a hexagonal structure at low temperatures.Scanning electron microscopy(SEM)shows that the particle size of BSCF-H is approximately in the range of 200 nm-1 um.The specific surface area of BSCF-H sample is about 10 times that of BSCF-C by the Brunauer-Emmett-Teller(BET)method.BSCF-H showed extremely small overpotential,low Tafel slope and high mass activity density in 0.1 M KOH electrolyte,and its OER catalytic activity was far better than cubic phase Ba0.5Sr0.5Co0.8Fe0.2O3-?(BSCF-C).Besides,BSCF-H catalyst showed a good durability of electrolysis operation.The surface of hexagonal perovskite BSCF-H almost maintains the lattice morphology after CV cycles,which is completely different from the surface of the severely amorphized BSCF-C.The structure of well-known BSCF-C perovskite is classical cubic phase with a corner-sharing CoO6 octahedral unit,while BSCF-H adopts a 6H-type hexagonal crystal structure with face-sharing octahedron,which can maintain better structural stability.Compared with BSCF-C,BSCF-H has a stronger charge transfer ability,the higher electrochemical specific surface area(ECSA)and the larger amount of oxygen species(O22-/O-)associated with surface oxygen vacancies,which further explains the reason for the strong catalytic activity of BSCF-H.DFT results revealed that the higher Co 3d-band center and 0 2p-band center relative to Fermi level of BSCF-H correlates with the hybridization of metal and oxygen states,resulting in high OER activity of BSCF-H catalyst.The excellent catalytic activity and stability of BSCF-H indicate that it is a very potential OER electrocatalyst.The above research results also provide reference for the design and synthesis of the hexagonal phase perovskite.(2)Based on the excellent performance of the hexagonal phase BSCF-H,we have synthesized a series of hexagonal BaxSr1-xCo0.8Fe0.15Ru0.05O3-? and BaxSr1-xCo0.8Fe0.1 Ru0.1O3-?(x=1,0.8,0.6,0.5)perovskite oxides using BaxSr1-xCo0.8Fe0.2O3-? as the matrix by sol-gel method.Both the 0.05 Ru-doped and the 0.1 Ru-doped perovskite systems have a 10 H-type hexagonal perovskite structure,and the crystal space group is P63/mmc.As the amount of Sr element in the perovskite system increases,the unit cell parameters of the crystal slightly decrease.BaCo0.8Fe0.15Ru0.05O3-? and BaCoo.sFe0.1 Ru0.1O3-? perovskites without Sr doping presented the highest OER catalytic activity in their respective systems.The chemical kinetics and charge transfer rates of the catalysts decrease with the increase of Sr doping The long-term durability test of BaC0.8Fe0.15Ru0.05O3-? and BaCo0.8Fe0.1Ru0.1O3-?,which have the highest OER activity,proved their catalytic stability.Their excellent catalytic activity has also been attributed to the following reasons:faster charge transfer capability,larger ECSA,and higher content of O22-/O-related to oxygen vacancies.The catalytic activity of the 0.05 Ru doped perovskite system is higher than that of the 0.1 Ru doped system.The reason may be that the higher calcination temperature leads to a lower specific surface area of the catalyst,which exposes the less active sites on the surface of perovskite oxides.(3)A series of cubic perovskite(Ba0.5Sr0.5)1-xCo0.8Fe0.2O3-?(x=0,0.02,0.05,0.1 represents BSCF,BSCF-0.98,BSCF-0.95,BSCF-0.9 respectively)was synthesized using a simple and efficient sol-gel method,and their space group is Pm3m.X-ray diffraction(XRD)indicates that the higher the content of A-site cation defects in the perovskite,the smaller its lattice size.SEM shows that the series of perovskite oxides are lumps with a size of about 1-3 um.BET proves that the series of perovskite materials have similar specific surface area.In a series of OER catalysis studies,we found that the perovskites with A-site cation defects have higher catalytic activity.Among them,BSCF-0.95 exhibited the best OER catalytic property.Compared with the BSCF without A-site cation defects,the other three perovskites with A-site cation defects presented significantly lower overpotential,smaller Tafel slope and greater mass activity density.It found that the catalytic performance of these perovskites has been improved to a certain extent by the stability test.After CV tests,an amorphous layer of about 5 nm appeared on the surface of the BSCF-0.95,which is the main factor for the enhancement of catalytic activity.The reasons for the better OER catalytic performance of BSCF with A-site cation defects are stronger charge transfer ability,larger ECSA and higher oxygen vacancies concentrations.DFT results demonstrated that the introduction of A-site cation defects in perovskite will cause the O 2p-band to be closer to the Fermi level,leading to higher OER activity of the catalysts.