Study On Preparation And Electrocatalytic Performance Of Nanosheet Array Structure LDH/Ni Foam Composite Electrode Materials For Water Splitting

Hydrogen evolution from the electrocatalytic water splitting,as a better method to produce hydrogen at present,has the advantages of high product purity and green environmental protection.In terms of the nature of the water electrolysis reaction,the slow kinetics of the anodic oxygen evolution reaction（OER）and the cathodic hydrogen evolution reaction（HER）,especially the OER process,directly leads to the high overpotential of the reaction system and the low efficiency of water splitting.Therefore,how to design and prepare efficient and stable catalysts to improve the efficiency of electrocatalysis process has become the key of hydrogen production from electrocatalytic water splitting.In addition,the H₂ and O₂ produced in the traditional process of hydrogen production from electrolytic water splitting are easily mixed in the reactor,thus leads to potential safety hazard.Meantime,considering the low efficiency of water electrolysis due to the sluggish anode reaction,introducting more easily oxidized urea molecules in the electrocatalytic reaction system to replace the traditional anodic OER reaction can be expected to effectively reduce the potential of the anodic reaction,improve the efficiency of the electrolytic hydrogen production.And the urea oxidation（UOR）assisted electrolytic water can be used to treat wastewater containing urea,and alleviate the pollution of industrial wastewater to the environment.However,how to reduce energy consumption and catalyst cost is a big challenge to promote the industrial application of hydrogen production from electrolytic water.Among the non-noble metal catalysts,layered double hydroxides（LDH）have attracted much attention in the field of electrocatalysis due to their unique two-dimensional ordered layered structure,atomically homogeneous dispersion of the elements in the layers,tunability of elemental composition and proportion,and structural diversity.Therefore,the present thesis is focused on typical alkaline electrocatalysts of LDH,aiming at the disadvantages of poor conductivity and limited inherent active sites,the synergistic enhancement and the modulation of active sites of LDH/nickel foam composite electrode materials were achieved by in situ incorporating LDH with r GO,metal doping and metal doping coupled H₂O₂ pre-oxidation to promote electrode reaction kinetics,thus significantly enhancing its electrocatalytic performance.The results of X-ray diffraction（XRD）,high-resolution transmission electron microscopy（HRTEM）,X-ray photoelectron spectroscopy（XPS）,X-ray absorption fine structure spectroscopy（XAFS）,in-situ Raman spectroscopy and in-situ electrochemical impedance spectroscopy（EIS）were combined with the DFT calculation to reveal the changes of the catalyst during the reaction process and the electrocatalytic reaction mechanism.The specific research content and results of the thesis are as follows:1.Synergistic enhanced overall water splitting by in situ incorporating Co Fe-LDH with grapheneNanosheet array Co Fe-LDH was grown in situ on reduced graphene oxide modified nickel foam（r GO/NF）were prepared by simple co-precipitation method,and the effects of different Co/Fe molar ratios for electrocatalytic performance were investigated.For the series of Co_xFe₁-LDH/r GO/NF（x=4,3,2）composites,the r GO layer is uniformly coated on the surface of the nickel foam skeleton,and the Co Fe-LDH nanosheets（～150-180 nm×15 nm）are grown vertically on the r GO/NF surface,and the Co₃Fe₁-LDH/r GO/NF displays the largest nanosheet size.The electrocatalytic results show that both the OER and HER properties of the series composites are better than those of Co₃Fe₁-LDH/r GO and Co₃Fe₁-LDH/NF in 1 M KOH solution.Among them,the Co₃Fe₁-LDH/r GO/NF has the lowest overpotential for both OER and HER reaction（250 m V and 110 m V）at the current density of 10 m A cm^-2.When Co₃Fe₁-LDH/r GO/NF is used as the cathode and anode simultaneously,the water decomposition voltages at 10 and 100 m A cm^-2 are 1.65 and 1.84 V,respectively.The activity at low current density is comparable to those of LDH-based catalysts at low current density,while superior to those at high current density.At the same time,the continuous reaction for 12 h still had good activity.These findings can be attributed to:（1）the more active sites upon nanosheet array structure increase the electrochemical specific surface area,promote the full contact between the active site and the electrolyte,and accelerate the electron transport;（2）Co-Fe electron interaction and Co Fe-LDH-r GO synergistic effect optimize the electronic structure,improve the conductivity,and enhance the catalytic activity;（3）in-situ growth of Co Fe-LDH on r GO/NF three-dimensional skeleton is conducive to bubble release and improves electrochemical stability.2.Enhanced HER performance of NiFe-LDH nanosheet array electrode materials by Mo-dopingA series of Mo-doped NiFe-LDH nanosheet array grown in situ on the surface of nickel foam were obtained by hydrothermal method.The LDH nanosheets of series Ni_xFe₁Mo₁-LDH/NF（x=4,6,8,10）composites were interlaced vertically on the surface of nickel foam,and the size of nanosheets（～300-700 nm×32-78 nm）gradually decreased with increasing Mo contents.In 1 M KOH solution,all the Mo-dopingcomposites exhibited superior HER activity than Ni₂Fe₁-LDH/NF.The smaller-sized Ni₆Fe₁Mo₁-LDH/NF has the best performance,requiring only 104 m V overpotential to achieve the current density of 10 m A cm^-2.And the current density remained 97%after 48 h test.The experimental and DFT theoretical calculations showed that after doping Mo⁶⁺ions in Ni²⁺Fe³⁺-LDH layer,electrons transfer from Fe and Nisites to Mo site,which provided a new active site Mo as the active site of H*adsorption instead of Fe site,optimizing the Gibbs free energy of H*adsorption（-0.224 e V）,and reducing the water dissociation energy barrier（2.10 e V）,and thus greatly promoting the entire Volmer-Heyrovsky process under alkaline conditions contributing to the HER performance.For further improving the activity of the composite,the optimized Ni₆Fe₁Mo₁-LDH/r GO/NF（～210 nm×20 nm）was prepared upon the optimal parameters,showing superior HER performance in alkaline solutions and requiring only 90 m V overpotential to achieve the current density of 10 m A cm^-2,which was superior to the majority LDH-based catalysts probably due to the synergistic effect of NiFe Mo-LDH and r GO.3.Synergistic promoted urea oxidation assisted hydrogen production of NiCo-LDH by V doping coupled H₂O₂ pre-oxidationA series of NiCo V_x-LDH/NF-y（x:V-doping ratio,y:volume of H₂O₂ added）nanosheet array composites were prepared by V doping coupled H₂O₂ pre-oxidation upon hydrothermal route.The obtained composites contained more high-valent nickel(Ni³⁺)and cobalt(Co³⁺)species.The NiCo V_1.0-LDH/NF-100 exhibits excellent electrocatalytic performance for UOR and HER requiring only a low operating voltage of 1.35 V and a low overpotential of80 m V,respectively,to reach the current density of 10 m A cm^-2.When it is used as both cathode and anode for urea-assisted water electrolysis,only 1.47 V voltage up to the current density of 10 m A cm^-2 is required,which is ca.310 m V lower than traditional water electrolysis（1.78 V）,and the sampe shows excellent stability within 36 h.The combinational characterization results of XPS,XAFS,in situ Raman and in situ EIS along with DFT calculations showed the electronic structure modulation of Niand Co by V-doping coupled with H₂O₂ pre-oxidation associated with 1)produced more highly valent active Ni³⁺species and auxiliary Co³⁺species;2)reduced the adsorption energy of urea at the Nisite in UOR（-1.757 e V）,simultaneously reduced the water adsorption energy（-0.846 e V）and water dissociation energy（1.49 e V）in HER,especially the active site of H*adsorption was shifted from the Co site to the V site,which optimized the Gibbs free energy change of H*adsorption（-0.124 e V）;and 3)lowered the bandgap energy and improved the electrical conductivity.These factors greatly promoted the urea adsorption under alkaline conditions and the whole Volmer-Heyrovsky process,speeding up the reaction kinetics of UOR and HER.To further improve catalytic performance,NiCo V-LDH-100/r GO/NF-100 nanosheet array composite（～135 nm×15 nm）was obtained using optimized synthesis parameters.The catalyst showed better UOR and HER performance,requiring only even a lower operating voltage of 1.33 V and overpotential of 70 m V,espectively,to reach the current density of 10 m A cm^-2.It is further used for urea-assisted water electrolysis displaying a voltage of only 1.45 V(10 m A cm^-2),which is better than most non-precious metal catalysts.This work demonstrates that the metal-doped coupled H₂O₂ pre-oxidation strategy can optimize the catalytic activity of the electrode materials for UOR and HER,and provides new ideas and methods for the research and development of urea-assisted electrolysis of water for hydrogen production technology.