Studies On The Photoelectrochemical Water Splitting Of ?-Fe₂O₃ Photoanode Modified By LDHs

The rapid development of social economy has cased energy and the environment to be two majorissues that humans have to face.Therefore,it is particularly important to develop clean,safe and renewable energy sources.Compared with traditional fossil fuels,solar energy is rich,clean and environment friendly.Hydrogen energy has the advantages of high energy density,easy storage and transportation,environmental friendliness,zero carbon emission,and it is the most ideal energy carrier.Photoelectrochemical?PEC?splitting water is considered as one of the most promising ways to convert solar energy into hydrogen energy,and which has attracted widespread attention from researchers home and abroad.Among many photoanode materials,?-Fe₂O₃ is considered as one of the most promisingphotoanode due to its suitable band gap?1.9-2.2 eV?,high theoretical solar to hydrogen efficiency,chemical stability and rich storage in earth.However,the poor conductivity,short lifetime of photo-generated charge carrier and high on-set potential have limited the performance of?-Fe₂O₃ severely.The surface modification with co-catalyst is an effective means to increase the photoelectric conversion efficiency of?-Fe₂O₃.Layered double hydroxides?LDHs?have the advantages of unique lamellar structure,low cost and environmental protection,and has become the most promising promoter for PEC water oxidation.This thesis mainly focused on the photoelectric conversion efficiency of surface modified?-Fe₂O₃ by layered double hydroxides.The main content are as follows:?1?The study on the photoelectrochemical water oxidation performance of?-Fe₂O₃ photoanode modified by self-assembled CoAl-LDH nanosheets:CoAl-LDHs were synthesised by hydrothermal method.The CoAl-LDHs nanosheet colloidal solution was prepared by NO₃^-ion exchange and formamide delaminating.PO₄^3--CoAl-LDH/?-Fe₂O₃,SO₄^2--CoAl-LDH/?-Fe₂O₃,CO₃^2--CoAl-LDH/?-Fe₂O₃ and NO₃^--CoAl-LDH/?-Fe₂O₃ were synthesized by layer-by-layer self-assembly method.Photoelectrochemical performance confirmed that the photoelectrocatalytic activity of PO₄^3--CoAl-LDH/?-Fe₂O₃,SO₄^2--CoAl-LDH/?-Fe₂O₃,CO₃^2--CoAl-LDH/?-Fe₂O₃ and NO₃^--CoAl-LDH/?-Fe₂O₃ composite electrodes are much higher than that of?-Fe₂O₃.X-ray powder diffraction?XRD?,ultraviolet-visible diffuse reflectance spectroscopy?UV-Vis?,scanning electron microscopy?SEM?,and high-resolution transmission electron microscopy?TEM?were used to characterize the structure,optical properties,and morphology.Through the electrochemical impedance,the mechanisms discovered that Co²⁺in CoAl-LDH in the reassembled composite structure provides active sites for the oxygen evolution,thus reducing the recombination of photogenerated charges and improving the photoelectric conversion efficiency.In addition,PO₄^3-,SO₄^2-,CO₃^2-and NO₃^-anions have two effects on the composite composite electrode materials.On the one hand,four anions can increase the LDH layer spacing,providing a channel for the diffusion of water molecules.On the other hand,the anion formed by the negative layer is conducive to the transmission of electrons,thereby increasing the photoelectric conversion efficiency.The results prove that the designed composite electrode could provide experimental basis and theoretical basis for the modification of?-Fe₂O₃.?2?The study on the photoelectrochemical activity of NiCoAl-LDH modified?-Fe₂O₃ photoanode materials:NiCoAl-LDH was modified on the surface of?-Fe₂O₃ by hydrothermal method.Through XRD,SEM and TEM,the structure,morphology and valence of the elements were studied.Through photoelectrochemical performance tests,the resulting NiCoAl-LDH/?-Fe₂O₃ photoanode exhibits an onset potential as low as 0.55 V vs.reversible hydrogen electrode?RHE?and a significantly enhanced photocurrent density of 2.56 mA cm^-2 at 1.23 V vs.RHE for PEC water oxidation in neutral electrolyte,which is ca.13,2 and 1.4 times as high as that of?-Fe₂O₃,NiAl-LDH/?-Fe₂O₃ and CoAl-LDH/?-Fe₂O₃.Comprehensive electrochemical and structural analysis reveal the synergistic effect of Ni and Co should be responsible for the enhanced PEC performance.Ni and Co could cause local structure distortion of MO₆?M=Ni or Co?in NiCoAl-LDH,which improve the conductivity of NiCoAl-LDH,thus enhance the separation and the transfer of photogenerated charges and slightly change the chemical environment of Co and Ni and facilitate the formation of low-valent oxidation states,thus lower the energy barrier of Co²⁺or Ni²⁺to high oxidation states and accelerate the kinetics of water oxidation.The excellent stability of NiCoAl-LDH is attributed to the layered framework allowing the rapid transfer of H⁺and counter anions as well as the thicker layer structure of NiCoAl-LDH to prevent?-Fe₂O₃ corrosion.The results demonstrate that NiCoAl-LDH is one of the most promising candidates for semiconductor-based electrode protection materials.?3?The preliminary research on photoelectrochemical activity of?-Fe₂O₃ photoanode modified by MgCoAl-LDH:MgCoAl-LDH was modified on the surface of?-Fe₂O₃ by hydrothermal method.Through XRD,SEM and other physical charaterizations,the structure,morphology and other information were studied.Under neutral conditions,MgCoAl-LDH/?-Fe₂O₃ exhibits an onset potential as low as 0.60 V vs.RHE and a significantly enhanced photocurrent density of 2.78 mA cm^-2 at 1.23 V vs.RHE for PEC water oxidation,which is ca.9.3,1.4,and 1.6 times as high as that of?-Fe₂O₃,CoAl-LDH/?-Fe₂O₃ and MgAl-LDH/?-Fe₂O₃.Under alkaline conditions,MgCoAl-LDH/?-Fe₂O₃ exhibits an onset potential as low as 0.56 V vs.RHE and a significantly enhanced photocurrent density of 4.12 mA cm^-2 at 1.23 V vs.RHE for PEC water oxidation,which is ca.4.5,2.0,and 2.2 times as high as that of?-Fe₂O₃,CoAl-LDH/?-Fe₂O₃ and MgAl-LDH/?-Fe₂O₃.Comprehensive electrochemical and structural analyses indicate that the synergistic effect of Mg and Co can enhance the performance of?-Fe₂O₃ PEC.The Mg²⁺radius is similar to the Co²⁺radius but slightly larger than the Co²⁺radius.Mg doping causes the octahedral structure formed by Co²⁺to be distorted,thereby increasing the Co²⁺action and accelerating the water oxidation kinetics.The results show that MgCoAl-LDH broadens the application range of?-Fe₂O₃and provides new ideas for the design of photoanode materials.