Integrated Electrodes Of Transition Metal Based Oxides For Water Electrolysis

With depletion of fossil fuels,deterioration of ecological environment and frequent occurrence of extreme weather,hydrogen energy is considered as the most promising clean energy.Coupled with renewable energy generation,hydrogen production via water electrolysis can make efficient use of wasted renewable energy.However,the unsatisfactory activity and stability of water splitting electrocatalysts are becoming the major bottlenecks in the industrialization of water splitting technology for hydrogen production,which is mainly attributed to the inferior intrinsic activity,the insufficient exposure of active sites,and the poor transfer of electron,electrolyte and bubbles.Herein,strategies such as heteroatomic doping and heterointerface constructing have been proposed to design and develop a series of integrated electrode with high intrinsic activity toward HER and OER.The high-performance HER electrodes are achieved by means of lowering the energy barrier for water decomposition and balancing the ratio of hydrogen adsorption sites to water decomposition centers.As for OER electrode,optimizing the adsorption energies of intermediates and reducing their difference,breaking the scaling relation,activating the lattice oxygen and triggering more favorable oxygen evolution route are efficient for lowering the theoretical OER overpotential.Using metal-organic frameworks as the sacrifice template,integrated electrodes with in-site grown framework of ultrathin Molybdenum bronze type oxide nanosheets were constructed for efficient HER/OER.Evolution of structure and chemical states of the molybdenum bronze type oxides was systematically investigated during HER and OER and the philosophy of performance tuning was unraveled through DFT calculation.Being characteristic of open and interconnected passage network,the nanocages forming from nanosheets can significantly facilitate the transfer of electron,electrolyte and bubbles.1.In-situ electrochemical production of Ni-Based microprisms as high-performance HER/OER electrocatalystsA two-step hydrothermal method was developed to construct Ni-based Bimetal-Organic Framework（Ni M-MOF NSs/Ni）with open and interconnected passage network by selective etching its precursor,Ni M-MOF MPs/Ni.After partial phosphorization and in-site activation,Ni M（OH）2/Ni MP NSs/Ni and Ni Fe P（OH）x NSs/Ni with controllable morphology,components and heterointerface were acquired as high-performance HER electrode and OER electrode,respectively.Compared with the traditional electrode,the obtained electrodes have the following advantages:1)Being characteristic of open and interconnected passage network,the active sites are readily accessible.2)The interaction between in-situ formed Ni Co（OH）₂ and Ni Co P is strong,which can boost water dissociation and simultaneously optimize hydrogen adsorption energy.3)The in-situ formed Ni Fe P（OH）x NSs/Ni decorated with amorphous phosphate on the surface is capable of activating the lattice oxygen.Electrochemical tests demonstrate that the Ni Co（OH）₂/Ni Co P NSs/Ni have the best HER activity,which can deliver current densities of-10 and-500 m A cm^-2 at relatively low overpotentials of 22 and 167 m V,respectively.The amorphous Ni Fe P（OH）x NSs/Ni requires overpotentials of 190 and 280 m V to drive current densities of 10 and 500 m A cm^-2,respectively.2.Rational design of molybdenum bronze oxides as high-performance HER electrocatalysts in alkaline solutionCombining density functional theory calculation and experiments,the origins of high activity of molybdenum bronze oxides（A₂Mo₃O₈,A=Co、Fe、Mg、Zn et al.）were explored.Two different functional active sites were found in A₂Mo₃O₈,Mo O₆ with optimum hydrogen adsorption energy and AO₆ being highly active for water dissociation.Furthermore,the hydrogen adsorption energy changes(?G_H*)is determined by the Mo-Mo bond length.Increasing the Mo-Mo bond length can lead to increased the overlap of electron cloud and downshift of d-band center（?_d）and thus decrease the hydrogen adsorption energy.The favorable Mo-Mo bond lengths in Mg₂Mo₃O₈and Zn₂Mo₃O₈ensure their optimum hydrogen adsorption energy changes(?G_H*),which were confirmed by their excellent HER activity in 0.5 M H₂SO₄.DFT calculation discovered that the water dissociation kinetics is determined by the AO₆ octahedral,mainly the metal at A site.Benefiting from the relatively low water dissociation barriers and optimized?G_H*,Co₂Mo₃O₈ shows excellent HER activity in 1 M KOH.Furthermore,DFT calculation and experiment also discovered that doping Zn and constructing heterointerface can further optimize the?G_H*and water dissociation kinetics.Through this project,the tuning mechanisms of adsorption free energies of reactive intermediates have been revealed and strategies for rationally designing molybdenum bronze-like oxides established.3.Rational design of cation vacancy and heterointerface for constructing high-performance OER electrocatalystsOwing to the strong electron transfer between metal ions and oxygen,the electron conductivity of traditional metal oxide is poor.Through ex situ XRD and XPS,the changes of crystal structure and surface chemical statement in A₂Mo₃O₈ under OER conditions were probed.It was found that Mo⁴⁺will be gradually oxidized to Mo⁶⁺,which then reacts with Na OH to form soluble Mo O₄^2-ion.The dissolution of Mo in A₂Mo₃O₈can promote the formation of amorphous and cation vacancy rich AMo OOH.DFT calculation discovered that the rate limiting step of AOOH in OER is the formation of O from OH.The in-situ formed AMo OOH can enhance the adsorption of OH and therefore reduce the theoretical overpotentials.Furthermore,DFT calculation discovered that the cation vacancy can enhance the covalency of M-O bond and therefore promote the lattice oxygen process.4.Metal-organic framework derived Co/（Co M）₂Mo₃O₈ nanocages with orderly ultrathin nanosheets as high-performance HER and OER electrodeRegarding the difficulty in morphology and interface control,metal-organic framework was exploited to construct the molybdenum bronze-like oxide nanocages.The Co-based bimetal-organic framework precursors have been prepared with 1,4-dicarboxybenzene as organic ligands and Co Cl₂,Mn Cl₂,Fe Cl₂ and Zn Cl₂ as metal sources.Then,the Co-based bimetal molybdate（Co MMo O₄HNPs/Ni）were prepared by a second hydrothermal reaction in ammonium molybdate（di）solution.The molybdenum bronze oxide with controllable heterointerface and components were finally prepared by annealing the Co MMo O₄HNPs/Ni in H₂/N₂ atmosphere.Modulating the metal ions and the dosages,molybdenum bronze oxide with heteroatom homogeneously distributed were achieved.Meanwhile,the heterointerface in molybdenum bronze were modulated by varying the p H of ammonium molybdate（di）.Thanks to the synergy effect,the Co/（Co Zn）₂Mo₃O₈ HNPs/Ni and Co/（Co Fe）₂Mo₃O₈ HNPs/Ni present excellent HER and OER activity,requiring overpotentials of 12 m V and 160 m V to achieve 10 m A cm^-2 for HER and OER,respectively.The overpotentials are even lower than the benchmarks of commercial Pt/C and Ru O₂/C.Furthermore,owing to the optimized morphology,the Co/（Co Zn）₂Mo₃O₈ HNPs/Ni and Co/（Co Fe）₂Mo₃O₈ HNPs/Ni can attain the industrial current density of 500 m A cm^-2 at relatively low overpotentials of 157 m V and 240 m V,respectively.