Preparation Of Iridium-based Nanocomposite Electrocatalysts For Oxygen Evolution Reaction In Acidic Media

Proton exchange membrane water electrolysis based on renewable energy is considered as the most promising hydrogen production technology without CO₂emission.However,as the half-reaction occurred at anode,oxygen evolution reaction（OER）involves multi-proton coupled electron transfer process and thus,the kinetics is very slow,which significantly limits the efficiency of hydrogen production.Therefore,it is urgent need to develop the high-performance and low-cost electrocatalysts for the OER in acidic media.At present,the Ir and its oxides have been considered as the state-of-the-art electrocatalysts for OER,but the high cost and scarcity of Ir has severely limited the commercialization of proton exchange membrane water electrolysis technology.Consequently,high-efficiency electrocatalysts are urgently to develop to enhance the intrinsic catalytic activity of iridium and simultaneously reduce the catalyst cost.In this dissertation,the preparation of nanocomposites composed of Ir and non-noble metal elements and their application in acidic OER are selected as research theme.Two goals are achieved by designing composite materials:1)The intrinstic catalytic activity of Ir is improved by the electronic interaction between Ir and non-noble metal;2)The cost of catalyst is reduced by reducing the Ir amount.The interplay between each component of nanocomposite is deeply analysed,and the evolution of surface species during the OER process will be deeply investigated.Finally,the relationship between structure and catalytic performance is established,which will provide new ideas for designing high-performance and low cost electrocatalysts for OER.The main achievements are summarized briefly as follows:（1）The Ir/Co₃O₄ nanocomposite catalyst was prepared by annealing the Co-MOF precursor modified with Ir³⁺at high temperature.The results of XPS and XANES analysis demonstrate the strong interaction between Ir and Co₃O₄ and the charge was transferred from Ir to Co atoms,which endows high activity and enhanced catalytic stability for Ir/Co₃O₄ toward OER.In 0.1 M HCl O₄ electrolyte,a low overpotential of292 m V is needed for Ir/Co₃O₄ to deliver the benchmark current density of 10 mA cm^-2and the corresponding Tafel slope is 52 m V dec^-1,superior to those of commercially available IrO₂.Moreover,no obvious potential change can be observed for Ir/Co₃O₄after continuous V-t measurement for more than 7 h,indicating the better stability than commercial IrO₂ and pure Co₃O₄.Besides,the Ir/Co₃O₄ composite owns faster kinetics than IrO₂ and Co₃O₄ evidenced by smaller Tafel slope and charge transfer impedance.This work demonstrates that it is an effective means to improve the catalytic performance of OER catalysts by carrier support effect.（2）LiLa₂IrO₆ electrocatalyst with a thin IrO₂ shell（IrO₂/LiLa₂IrO₆）was prepared by simple solid-state synthesis method,which achieves a win-win situation for both activity and stability under harsh acidic conditions.The synthesized IrO₂/LiLa₂IrO₆catalyst exhibits outstanding activity for the OER in 0.1 M HCl O₄ electrolyte,which only needs a potential of 1.508 V（vs RHE）to deliver the benchmark of 10 mA cm^-2,superior to commercially available IrO₂ and most recently reported catalysts.More significantly,IrO₂/LiLa₂IrO₆ shows enhanced electrochemical stability with only 160m V increase in potential after testing over 13 h,in contrast to commercial IrO₂ that almost lost its activity toward the OER in 7 h.The XPS and XANES analysis results show that a highly active IrO_x layer is formed on the catalyst surface during the initial OER process,which greatly contributs to the high activity.Moreover,the enhanced stability can be attributed to the strong interaction between IrO_x and bulk LiLa₂IrO₆that inhibited the excessive oxidation and dissolution of iridium to a certain extent.This work provides an idea for solving the problem of improving OER activity of non-precious metal-iridium composite oxides at the expense of stability.