Microstructural Modulation And Electrocatalytic Performance Of Low-iridium Perovskite For Water Oxidation

Producing decarbonized"green hydrogen"by renewable energy power supply coupling with the water electrolysis,is not only the key path to achieve clean transformation of energy structure,but also the future trends of energy industry.Currently,proton exchange membrane（PEM）water electrolysis is the only hydrogen production technology that can be coupled to the fluctuant renewable energy generation system.Thus,some in the industry recognize it as the key technologies for energy transition.However,the anode catalyst in PEM water Electrolyzer（PEMWE）must maintain operation in the dual corrosive environment of strong acid media and high oxidation current.Hence,most of catalysts for oxygen evolution reaction（OER）are unqualified for it.At present,iridium oxide（Ir O₂）is still the only choice of anode catalyst in PEMWE.However,iridium is a precious element with the nearly lowest abundant in the Earth’s crust,resulting in the high cost of catalyst.Moreover,the catalytic activity of Ir O₂ is not good enough.The energy consumption required to overcome the anode overpotential usually accounts for30-50%of the whole energy consumption of PEMWE.Both of the above two major issues limit the wide application of PEMWE.Therefore,it is imperative to design substitutional catalysts of Ir O₂ with low-iridium mass,superior activity and robust stability.However,the restriction between low-iridium mass and high activity,as well as the inverse catalytic activity-structural stability relation is intractable for the design of substitutional catalysts.These bottlenecks make the development of PEMWE get into trouble.In this thesis,we broke the bottlenecks of designing substitutional catalysts of Ir O₂ by modulating the microstructures of iridium-based perovskite catalysts.The catalysts enabled the simultaneously improvement of cost,activity and stability.We verified the optimization mechanism for activity and stability of the low-iridium perovskite catalysts by combining experiments and theoretical calculations.More importantly,we provided the new ideas for the design and development of acidic OER electrocatalysts.This thesis includes the following parts:1.In order to solve the two major problems in designing substitutional catalysts of Ir O₂,we proposed the lattice anchoring strategy to select an ideal matrix and anchor the iridium atoms in it.On the one hand,the acid-stable matrix material can stabilize the iridium sites.On the other hand,the synergistic effect between iridium and the central metal atom can optimize the activity of catalyst.Taking into consideration that SrTiO₃ is a stable matrix in acid,we chose SrTiO₃ as the lattice anchor matrix and prepared a low-iridium substitutional catalyst—Ir-STO.Ir-STO reduced the mass of iridium by 57%compared with Ir O₂.Its geometric activity,effective electrochemical intrinsic activity and iridium mass activity were 10 times,26 times,and 34 times higher than those of Ir O₂,respectively.And it was currently one of the most active acidic water oxidation catalysts.Experimental and theoretical calculation results showed that there is a bimetallic synergistic effect between Ti and Ir.The anchored Ir can effectively regulate the electronic structure of Ti⁴⁺in SrTiO₃,significantly increasing the electron density around Ti⁴⁺.It also induced the Ir-O hybridization crossing Fermi level,and thus improved the electronic conductivity of Ir-STO.SrTiO₃ enabled excellent stability of Ir-STO as a robust anchoring material and further achieved the simultaneously improvement of cost,activity and stability.2.The insights of the inverse catalytic activity–structural stability relation of catalysts for acidic water oxidation is still not deep enough.Moreover,the optimization mechanisms of the activity and stability of iridium-based perovskite solid solutions are still not clear enough.In order to solve the above problems,we synthesized a series of solid solution materials（STO-SIO,SZO-SIO,SHO-SIO）of SrIrO₃ and the perovskite containing the metal of titanium subgroup（Ti,Zr,Hf）as B-site metal.The experimental results showed the geometric activity and intrinsic activity of solid solution materials all had been significantly improved comparing with SrIrO₃.And SZIO（with the molar ratio of Zr:Ir is 1:2）is ahead of most iridium-based catalysts for acidic water oxidation.In addition,SZIO maintained the superior catalytic activity after catalyzing OER and realized the reduced iridium-leaching mass compared with SrIrO₃.These indicate the SZIO possesses excellent catalytic stability and structural stability.Moreover,we found the microstructural modulation of Zr alleviated the excessively strong Ir-O covalent bond and effectively inhibited the leaching of cations on the surface of material.Finally,we take SZIO as an example to establish a theoretical model.The theoretical calculation results proved that Zr regulates the microstructure of perovskite and alleviates the excessive adsorption energy of oxygen intermediates on Ir sites.3.Applying the low-iridium perovskite catalysts from laboratory to hydrogen production industry to realize the transformation from research to industrialization is our goal of unremitting pursuit.However,the low-iridium perovskite catalysts are far from the application in PEMWE with high current density caused by the large operating resistance.Moreover,although the low-iridium perovskites we designed have achieved the breakthrough in both activity and stability,their iridium mass are still above 30%,which has a large space for reduction.1%of the reduction in iridium mass may economize hundreds of millions assets for our country.Therefore,further reducing the iridium content of the anode catalyst in the PEM cell is one of the most important catalyst design strategies.In order to further reduce the iridium levels,improve the electrical conductivity and regulate the microstructure of materials by multi-metal effect,we designed and synthesized the low-iridium perovskite catalyst with multiple abundant metals by introducing ingredients with high electrical conductivity and corrosion resistance,such as V,Nb and Ta.The experimental results showed that the conductivity of the materials is significantly enhanced after the high-conductivity metal doping into the perovskite.In addition,Nb-Ti-Ir multiple-metal perovskite can maintain its excellent activity and stability after the further dilution of iridium,which effectively promotes the industrialization of perovskite solid solution catalysts.