Rational Design And Synchrotron Radiation Investigation Of Electrocatalyst For Water Splitting Based On Binary Metal Nanoarchitectures

Energy crisis and climate warming are two major challenges to the sustainable development of human society at present.It is urgent to rebuild a Modern New Energy Resources System with renewable energy as the main source.As an important medium of the energy revolution for the third time,hydrogen energy plays a prominent role in the Modern New Energy Resources System,and is even called the "ultimate energy" in the 21st century.Electrocatalytic water splitting is a key process to convert renewable energy into hydrogen energy without producing other carbon products.The design of stable and efficient new electrocatalysts for water splitting and the research of its energy conversion mechanism are the hot spots and front directions in Modern New Energy Resources System.The introduction of the second metal into the single metal electrocatalyst will lead to a series of interesting phenomena,such as morphology and size change,electronic structure reforming,phase structure conversion of crystal,etc.The removal of a metal element from the binary metal electrocatalyst may also lead to defects,change the coordination environment or expose the active center,which provides infinite possibilities for the design of stable and efficient electrocatalysts for water splitting.Therefore,the exploration on the design strategy of electrocatalysts for water splitting based on binary metal has become a hot field for the scientific community.In this dissertation,based on binary metals,the morphology,size,electronic structure and coordination environment of the electrocatalysts for water splitting were tailored by the strategies of design of unique morphology,induced defect sites and in situ construction of flexible-redox-sites,which effectively solve the key scientific issues of insufficient kinetics,unstable structure and low conversion efficiency of electrocatalysts for water splitting in acidic and alkaline electrolytes.Ultimately,the electrocatalytic properties of water splitting was improved remarkably.The changes of electronic structure and coordination environment of the designed electrocatalysts for water splitting were ascertained through X-ray photoelectron spectroscopy(XPS),synchrotron radiation absorption spectroscopy(XAS)and other advanced characterization techniques.We tracked the dynamic evolution of key reaction intermediates using the independently established operando synchrotron radiation Fourier transform infrared spectroscopy(SR-FTIR)detection technology.We revealed the kinetic process and microscopic reaction mechanism of electrocatalytic water splitting by combining experimental results with theoretical calculations,aiming to establish the structure-activity relationship between the microstructure and catalytic performance of electrocatalytic water splitting reaction.This work provides reference and guidance for the design and synthesis of stable and efficient electrocatalyst for water splitting.The specific research contents of this dissertation are as follows:1.Design and acid OER study of donut-like RuCu bimetallic nanocrystalsRationally architectural design and accessible construction of an efficient electrocatalyst featuring with high activity and stability in acid solution are fundamentally important to advance the renewable energy conversion technologies nowadays.We deliberately conceive and successfully synthesize a donut-like architecture of RuCu bimetallic nanocrystals with well-defined nanoscale shell,by a facile "in-situ galvanic replacement" strategy,to boost Ru-based electrocatalysts with prominent water oxidation performance in acid condition.The as-prepared donut-like RuCu nanostructures with ultrathin shell of?1 nm thickness could catalyze the oxygen evolution reaction(OER)under a small overpotential of 270 mV at 10 mA·cm-2 with excellent long-term stability and ultrahigh mass activity of?1000 A·gRu-1,two-orders of magnitude larger than Ru and commercial RuO2 nanoparticles.Experimental and theoretical analyses reveal that the well-dispersed Cu element plays a key role in the architectural engineering and catalytic activity improvement of donut-like RuCu nanoalloy catalysts via dual regulation of coordination environment and electron structure of Ru active sites.Especially,by the merits of ultrathin shell structure,the strong surface electron transfer via robust Ru-Cu bonds could effectively promote the appearance of active and stable Ru2+throughout donut-like RuCu nanoalloy,resulting in much more thermodynamically favor for H2O adsorption and*OOH formation during OER process compared with metallic Ru0 nanoparticle.Undoubtedly,this approach may open a new avenue for strategically designing highly active and performance-oriented electrocatalytic materials for tremendous energy applications.2.Defect induction and electrocatalytic HER study of Ru based nanocrystalsA reasonable design strategy with single variable to improve the utilization of noble metal electrocatalysts for hydrogen evolution reaction(HER)is crucial to simplify the process flow and accelerate the future energy cycling economy.A single variable.the abundant defects.has been intentionally created on Ru nanoparticles of 2.4 nm towards unprecedently high mass-specific reactivity in harsh acid and alkaline electrolytes.The obtained defects-enriched Ru(DR-Ru)exhibits ultra-high alkaline HER turnover frequencies of 16.4 s-1 at 100 mV overpotential,and it also remains an excellent value of 20.6 s-1 in acidic media,superior to any other Ru catalysts reported.Accordingly,a record low loading amount of 2.5?g·cm-2 for the DR-Ru catalysts,and low overpotentials of 28.2 and 25.1 mV at 10 mA.cm-2 in alkaline and acid media,respectively,can be realized.Furthermore,the less-coordinated Ru surface sites and partial lattice oxygen weaken the bonding between H and DR-Ru catalysts to facilitate the H-H bond formation and help dissociation of the water molecule to overcome the major challenge of HER in alkaline electrolyte,leading to the comparable activity with that under acidic conditions.This result provides a guideline for the defect engineering on noble metal nano-catalysts to effectively improve the utilization of catalyst and optimize the reactivity.3.In situ construction and investigation of alkaline OER electrocatalysis of NiV-MOF nanosheet arraysAtomic-level design and construction of synergistic active centers are central to develop advanced oxygen electrocatalysts toward efficient energy conversion.Herein,we skillfully develop an in-situ construction strategy to introduce flexible-redox-sites of V-Ni centers onto Ni-based metal-organic framework(MOF)nanosheet arrays(NiV-MOF NAs)as a promising oxygen electrocatalyst.The abundant redox V-Ni centers with flexible metal valence states of V+3/+4/+5 and Ni+2/+3 enable NiV-MOF NAs excellent oxygen evolution(OER)activity and a long-term stability under high current densities,achieving the current density of 10 and 100 mA·cm-2 at recorded overpotentials of 189 and 290 mV,respectively,and showing ignorable decay of initial activity at 100 mA·cm-2 after 100-hour OER operation.Operando SR-FTIR spectra combined with quasi in-situ XAFS spectroscopies reveal at the atomic level that the flexible V sites can continuously accept electrons from the adjacent active Ni sites to accelerate OER kinetics for NiV-MOF NAs catalysts,accompanied by a self-optimized structural distortion of VO6 octahedron for promoting electrochemical stability during OER.