Preparation Of Lanthanide-iridium Mixed Metal Oxide And Their Electrocatalytic Performance For Oxygen Evolution In Acidic Medium

Proton exchange membrane（PEM）water electrolysis is considered as the most promising green hydrogen production technology.However,the overpotential of the anodic oxygen evolution reaction（OER）is high,resulting in significant efficiency loss and material degradation of the catalyst in industrial applications.Therefore,it has become an urgent matter to develop the catalyst for catalyzing the oxygen evolution reaction in acidic media to reduce the anodic reaction barrier.IrO₂ is still the state-of-the-art catalysts for OER in acidic electrolytes due to its higher current density and better corrosion resistance.However,the high cost,scarcity and insufficient durability of precious metal IrO₂ greatly limits the large-scale application.It is necessary to reduce the precious metal content in the catalyst and improve the durability without at expense of the inherent activity.In this dissertation,the preparation of lanthanide-iridium mixed metal oxide and their application in acidic OER are selected as the research theme.The catalysts with special crystal structure constructed by lanthanide metals and Ir are firstly developed for catalyzing the OER in acidic media,and the catalytic mechasim is preliminary studied by combining experimental results with density functional theory calculations,which provides new ideas for designing novel electrocatalysts with high OER performance.The main achievements are summarized briefly as follows:（1）The preparation of lanthanum iridium oxides with surface deficient-lanthanum structures（La₃IrO₇-SLD）and their application in acidic oxygen evolution reaction.The La₃IrO₇-SLD catalyst was prepared by annealing a mixture of lanthanum acetate and iridium chloride and subsequent electrochemical activation.The results of elemental mappings and inductively coupled plasma atom emission spectrometry（ICP-AES）indicate that the severe leaching of lanthanum from the surface lattices during the acidic OER process,which induces the surface atomic reconstruction,forming a La-poored and Ir-riched surface.X-ray photoelectron spectroscopy（XPS）and X-ray absorption fine structure spectroscopy（XAFS）analysis show the increase of electronic density and decrease of valence state of Ir ions after forming the SLD.The OER performance of the prepared La₃IrO₇-SLD was evaluated in 0.1 MHClO₄ solution.As a result,the catalyst exhibited a low overpotential of 296 mV at a current density of 10 mA cm^-2,which is smaller than that of the commercial IrO₂ catalysts and most catalysts.Moreover,the potential increased less than 50 mV after continuous v-t measurement for more than 60000 s,demonstrating the excellent stability of the catalyst.The catalyst also shows high mass activity when normalized to the mass of noble metal Ir.The mass activity(A g _Ir^-1)of La₃IrO₇-SLD is 5times higher than that of commercial IrO₂ at potential of 1.6 V.Theoretical calculations revealed the prepared catalyst follows the lattice oxygen participating mechanism with central Ir atoms serving as active sites.In the SLD,the energy level of 5d orbital of the active site Ir is lower than that of the coordinated O 2p orbital,making the oxygen atoms easier to escape from the surface lattices and thus,improving the OER activity.（2）The preparation of IrO_x-terminated praseodymium iridium oxides（IrO_x/Pr₃IrO₇）and their application in acidic oxygen evolution reaction.A cubic fluorite-type catalyst was prepared by annealing of a mixture of praseodymium acetate hydrate and iridium chloride hydrate.The large pore structure between the blocks of catalysts is beneficial to the storage of electrolyte and gas diffusion,while the defects generated by the jagged edge provides more exposed active center.As the acidic OER processing,a large amount of Pr was leached out the catalyst surface,which induces the restruction of surface atoms and thus,forming highly ative IrO_x layer.Theoretical calculations reveal that the catalyst follows the lattice oxygen participating mechanism.The energy level of active Ir 5d orbital of IrO_x is lower than that of the O 2p orbital,which facilitates the escape of oxygen atoms from the lattice to participate in the OER reaction.IrO_x/Pr₃IrO₇ shows high activity and stability in a 0.1 MHClO₄ solution.The overpotential of 305 mV is required for the catalyst to achieve a current density of 10 mA cm^-2,and the Tafel slope is 37 mV dec^-1,demonstrating the faster reaction kinetics and competitive activity than those of commercial IrO₂.After continuous test by chronopotentiometry method for 60,000 s,the initial potential of IrO_x/Pr₃IrO₇ only increased by 0.07 V,while the IrO₂ was almost lost its activity within 20000 s.