Multi-component Low-iridium Oxide Electrocatalysts: Component Synergistic Mechanism And Oxygen Evolution Electrocatalytic Performance

Carbon-based fuels have been the main energy source in recent decades.The large amount of usage on fossil fuels has brought about severe environmental problems,including air pollution and global warming.The above problems have prompted attempts to replace fossil fuels with renewable energy,such as solar energy,wind energy and hydropower,but the intermittency and unpredictability of renewable energy limit its practical application.Proton exchange membrane water electrolysis(PEMWE)is a strategic technology for large-scale production of hydrogen,which is considered as a carrier of clean energy and effective response to carbon emissions.However,the selection of anode materials for this technology is strongly limited by the strong acidic and oxidizing operating environment,iridium dioxide(Ir O₂)is the best choice for both electrocatalytic activity and stability.However,iridium is expensive due to its low crustal abundance and low annual production.How to effectively reduce the amount of precious metal iridium without sacrificing catalytic performance is the key to achieve large-scale application of PEMWE technology.So far,the most common method is to synthesize multi-component iridium oxide and reduce the precious metal iridium by introducing other non-precious metal components.However,the relationship between the component synergistic mechanism and catalytic performance of multi-component low-iridium electrocatalysts is still unclear,and the structural evolution during the catalytic process is not well understood.In this paper,a variety of multi-component oxides with low-iridium content are designed and synthesized,and the synergistic effect of different components on catalytic performance is explored by combining experimental and theoretical calculation methods,deepen the understanding of the structural evolution in the catalytic process.This paper will be discussed from the following three aspects:1.Iridium-based double perovskite oxides are promising electrocatalysts to replace commercial Ir O₂ due to its inclusive crystal structure and diverse element composition,which can greatly reduce the amount of precious metal iridium.Six representative iridium double perovskite oxide were used for the systematic research and we first observed a phenomenon beyond our expectation:In the non-catalytic condition,most of iridium-based double perovskite oxides once contact with acidic electrolyte,the instant iridium element leaching occurs,and this process are not persistent,also not determined by thermodynamics.The degree of iridium leaching was strongly correlated with the amount of non-iridium elements in the double perovskite oxides.In addition,we found that titanium metal can effectively stabilize the structural framework of double perovskite oxides,achieving the excellent structural stability of Sr₂Ti Ir O₆ sample in acid corrosion test.Therefore,we propose a more comprehensive screening protocol for multi-component low-iridium oxygen evolution electrocatalysts and emphasize that instant acid corrosion test is an indispensable evaluation criterion for the evaluation of structural stability on the potential electrocatalysts.2.Multi-component iridium oxides often have complex crystal structure,and it is a great challenge to identify the key structural subunits and establish the correlations between different structural subunits and catalytic performance.The triple-perovskite oxide Ba₃M'M"₂O₉ materials have special crystal structure,containing two different structural subunits,namely the isolated M'O₆ subunit and the Ir₂O₉ dimer subunit in a face-shared connection mode.In a series of Ba₃M'M"₂O₉ materials,the overpotential of Ba₃Ti Ir₂O₉ is only 270 m V,when the current density reaches 10 m A/cm² during the oxygen evolution reaction under acidic conditions.The mass fraction of iridium in Ba₃Ti Ir₂O₉ is 47%lower than that of the benchmark catalyst Ir O₂,while its mass activity is 28 times higher than that.In addition,it is found that the Ir₂O₉ dimers structural subunit is a necessary condition for achieving reasonable electrocatalytic stability,and the synergistic effect between the two subunits can effectively regulate the electronic structure and the metal-oxygen bond covalency,which contribute to achieve efficient and stable oxygen evolution reaction.Experimental and theoretical results show that Ba₃Ti Ir₂O₉ sample is an excellent electrocatalyst for acidic oxygen evolution reaction.3.It is an important topic in catalytic reaction to regulate the catalytic performance by crystal-facet effect.In acidic oxygen evolution reactions,the understanding of the relationship between crystal-facet effect and catalytic performance is scarce.The crystal-facet effect on catalytic performance of multi-component iridium oxide Ca₂Ir O₄ was investigated.The crystal growth of Ca₂Ir O₄ material along the specific(001)crystal plane was controlled by precise control of experimental synthetic method.The experimental and theoretical results show that in the(001)crystal plane of Ca₂Ir O₄,the Ca ion is difficult for dissolution,while in the(110)crystal plane,the Ca ion is prone to dissolve from the structure,resulting in different electrocatalytic performances of Ca₂Ir O₄ materials with different crystal sizes in the oxygen evolution reaction under acidic conditions.In addition,the overpotential of Ca₂Ir O₄-1 material is 301 m V when the current density reaches 10 m A/cm².Therefore,Ca₂Ir O₄ can be regarded as a potential electrocatalyst for oxygen evolution reaction in acidic conditions.