Preparation And Electrocatalytic Water Splitting Performance Of Cobalt-Based MOFs Derived Catalysts

Developing new green energy and reducing dependence on fossil fuels is the only way to achieve sustainable development.Nowadays,there is an interest in the preparation of cost-effective and efficient electrochemical catalysts for sustainable energy devices and technologies such as electrocatalytic water decomposition,metal-air batteries,and Li-batteries.Compared with noble metal catalysts,transition metal cobalt-based catalysts are considered to be promising electrochemical catalysts because of their abundant reserves,low cost and high activity.Cobalt-based Metal organic frameworks（MOFs）are three-dimensional（3D）network framework materials formed by metal ions and organic ligands through coordination bonds.MOFs materials have the characteristics of porosity,orderly pore structure,uniform pore size and large specific surface area,which is conducive to promoting the rapid transport and diffusion of reactants and has been applied in many research fields,especially in the field of electrochemistry.In this dissertation,cobalt-based catalysts with different morphologies and components were prepared by template method,and their performance for electrocatalytic water separation hydrogen reaction（HER）and oxygen evolution reaction（OER）were studied.The main results are summarized as follows:1.Rational construction of hierarchical Ni₂P-Co₂P-Fe₂P hybrid hollow nanoprism（abbreviated as NiCoFeP-HN）,which could efficiently work as the OER electrocatalysts.NiCoFeP-HN was synthesized via in situ ion exchange reaction of nickel-cobalt precursors with[Fe（CN）₆]^3-at room temperature and followed the subsequent phosphorization treatment at 300°C under N₂.The ion exchange procedure was found to be crucial to preserve the nanoprism morphology and enhance OER performance.Comparing to the conventional Ru O2+Pt/C Zn-air battery,NiCoFeP-HN+Pt/C based Zn-air battery has exhibited superior energy density and much better cycling stability.This work provided a facile and efficient avenue for the construction of hollow nanostructure and hybrid composition.2.Fe-doped NiCo P hybrid hollow nanocage（denoted as Fe-NiCo P）was synthesized through the phosphorization of the product from[Fe（CN）₆]^3-intercalated NiCo-LDH（layered double hydroxide）,where original ZIF-67-Co was used as the template.Owing to the hollow nanostructure and the synergistic effect between multiple components,the Fe-NiCo P catalyst exhibited promising HER and OER activities in 1.0 M KOH with the low overpotentials of 147 and 235 m V at 10 m A·cm^-2,respectively.Furthermore,Fe-NiCo P catalyst only achieve the current density of 10 m A·cm^-2 at the voltage of 1.60 V.The Zn-air battery constructed by Fe-NiCo P+Pt/C catalyst showed good cycling stability at the current density of 5 m A·cm^-2,and its performance was significantly better than that of Ru O₂+Pt/C catalyst.3.Fe-doped cobalt telluride nanoparticles encapsulated in nitrogen-doped carbon nanotube frameworks(Fe-Co_1.11Te₂@NCNTF)by tellurized of Fe-etched ZIF-67 under a mixed H₂/Ar atmosphere.Fe-doping was able to effectively modulate the electronic structure of Co_1.11Te₂,increase the reaction activity,and further improve the electrochemical performance.The optimized electrocatalyst exhibited superior HER and OER performances in an alkaline electrolyte with low overpotentials of 107 and 297 m V with a current density of 10 m A·cm^-2,in contrast to the undoped Co_1.11Te₂@NCNTF of 165 and 360 m V,respectively.The overall water splitting performance only required a voltage of 1.61 V to drive a current density of 10 m A·cm^-2.Density function theory（DFT）calculations indicated that the Fe-doping not only afforded rich exposed active sites but also decreased the hydrogen binding free energy.This work provided a feasible way to study non-precious-metal catalysts for an efficient overall water splitting.4.The nitrogen doped oxide/sulfide heterostructures（named N-NiCo₂S₄/Co O microsphere）microsphere was synthesized from annealing the NiCo-BTC MOF to multi-layered hollow structure of NiCo₂O₄microsphere,following the directly vulcanizing in the presence of NH₄HCO₃ to the resulted oxide/sulfide heterojunction.Benefiting from the nitrogen doping,the abundant multi-layered hollow heterostructure and the interfaces between multiple components,the N-NiCo₂S₄/Co O microsphere exhibited excellent OER activity with a low overpotential of 238 m V at 10 m A·cm^-2.The Zn-air battery based on the N-NiCo₂S₄/Co O+Pt/C catalyst displayed excellent cycling stability at the large current density of 5 m A·cm^-2,and its stability was superior to that of Ru O₂+Pt/C catalyst,suggesting the great potential application as the power source devices.5.Ru doped CoFeP nanocubes（Ru-CoFeP）were successfully synthesized by ion exchange reaction and phosphating process using CoFe-PBA as precursor.Benefitting from the synergistic effects of the Ru doping,porous structure and high density of active sites,the Ru-CoFeP nanocube exhibited excellent OER activity with a small overpotential of 185 m V at 10 m A·cm^-2 and superior long-term durability.Density functional theory（DFT）calculations revealed that the electrons would move toward to Ru after the Ru doping on the active Ru-CoFeP surface and the adsorption capacity to H₂O molecules was also strengthened to elongate and break O-H bond to enhance the OER performance.Furthermore,the cycle stability of Ru-CoFeP+Pt/C-based battery at the current density of 5 m A·cm^-2exhibited much durability,which was superior to that of the Ru O₂+Pt/C-based battery.