Synthesis Of Nanoscale Multinuclear Iron-oxo Clusters And Their Electrochemical Properties

Electrochemical technology has attracted much attention due to its clean and efficient advantages among modern energy storage and conversion technologies.The design and construction of stable and efficient electrocatalysts are of great importance in improving the overall conversion rate.Polyoxometalates（POM）are considered to be a class of potential electrocatalytic materials with the well-defined molecular structures,nano-size effects and multi-electron redox activities.Among them,the development of high-yield element iron-oxo clusters is considered to have the possibility of replacing noble metal catalysts.However,in the actual electrocatalytic process,metal-oxo clusters are facing the inherent disadvantages of being easily soluble in electrolyte solution and poor electrical conductivity,resulting in the instability and deactivation of catalysts,and further the specific catalytic mechanism cannot be understood.In this dissertation,in considering of nano-size effect,unique Fe-O bonds and multi-electron redox activity of iron-oxo clusters,four types of highly efficient and stable iron-oxo clusters have been designed and synthesized.Furthermore,they were used from the aspects of electrolyte or electrode materials for electrochemistry research.The specific research contents are as follows:1.In the first chapter,the oxygen reduction reaction（ORR）catalytic activity of commercial Pt/C electrocatalysts in the solution containing polyoxoanion[Fe₂₈（μ₃-O）₈（L-（-）-tart）₁₆（CH₃COO）₂₄]^20-has been greatly improved.Density functional theory（DFT）calculation suggests that the unique structure of iron-oxo clusters can assist in the transport of oxygen during the ORR reaction and play an important role in the oxygen reduction process.When this discovery was applied into a direct formic acid microfluidic fuel cell（DFMFC）,the effect of Fe₂₈ acting as the electrolyte was proved to significantly improve the performance of the battery compared to the phosphate buffered solution,the control electrolyte,with a 9.5-fold increase in maximum power density.This study provides the first example of Fe oxo-cluster acting as electrolyte in electrochemical energy conversion and storage.2.In the second chapter,{Bi₆Fe₁₃}bimetallic iron-oxo cluster is selected as the acidic electrolyte to explore the improvement of the oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）catalytic activities of commercial Pt/C catalysts.By comparison,it is found that{Bi₆Fe₁₃}acts as an electrolyte in the ORR reaction system,its own redox potential window is in the same interval as the electrochemical oxygen reduction potential occurrence window of the working electrode.The catalytic activity curve of the actual electrochemical ORR was obtained by the background deduction.The ORR half-wave potential and OER overpotential of Pt/C catalyst were greatly improved in the{Bi₆Fe₁₃}electrolyte solution when compared with the electrolyte Na₂SO₄/H₂SO₄ mixed solution,which the{Bi₆Fe₁₃}is expected to be bifunctional oxygen-activated electrolyte.As the“electron accommodator”,{Bi₆Fe₁₃}undertook the function of electron transport in the system,that is,the electrons generated during the self-redox process participate in the electron transport turnover of the system,which greatly improves the overall reaction rate.3.In the third chapter,three tetranuclear iron-oxo clusters were synthesized by low temperature solvothermal method.The different coordination environments,functional groups,three-dimensional packing and stability of iron-oxo clusters were analyzed by single crystal X-ray diffraction,infrared,thermogravimetric and other testing methods.Different coordination modes around metal atoms lead to different redox abilities,this unique structure and redox activity are beneficial to the intercalation and deintercalation of lithium ions,which is very beneficial for increasing the capacity as anode materials for new lithium-ion batteries.The transfer of electrons and charges during lithiation and delithiation was further analyzed by XPS,XANES and other electrochemistry methods.Meanwhile,its discharge specific capacity,cycle stability and structural stability were investigated,which further proved that the Fe oxo-cluster was a good choice for high-performance new lithium-ion battery anode materials.4.In the fourth chapter,trinuclear iron-oxo clusters composed of formic acid,acetic acid and propionic acid,were synthesized by evaporating the solvent at room temperature,respectively.It is found that the series of clusters have good purity and nano-size effect through XRD and SEM tests,et al.In the process of exploring the electrocatalytic activity of the unique iron-oxo clusters,it was found that the conductivity of the acetic acid-coordinated clusters was greatly improved after complexing with the ZIF-8（Zn）.Further pyrolysis treatment improves the stability of the composite catalyst.At 1050℃,the composite system completes the phase transformation into an electrocatalyst Fe/Fe_n-NC₁₀₅₀ with dual catalytic active species of iron atoms and iron nanoparticles.In 0.5 M H₂SO₄ solution,the dual-active species Fe/Fe_n-NC₁₀₅₀catalyst exhibited excellent stability and resistance to methanol.This provides a new idea for designing dual-active species electrocatalysts with coexistence of atoms and nanoparticles by iron-oxo clusters as precursors.