Studies On Controllable Preparation Of Co-based Electrocatalysts And Catalytic Performance For Water Splitting

Transition metal electrocatalysts?such as cobalt,nickel,tungsten,etc.?are widely used in catalytic materials,magnetic materials,etc.due to their low cost,abundant reserves and easy regulation of d-electron structure.which is considered to be promising.However,most of the traditional transition metal catalysts are located at the half-peak position of the Volcano plot,which is not conducive to the hydrogen evolution reaction because the adsorption of active H^*is too strong or too weak.Based on this,a core-shell structured Co@WO₂@Ni₁₇W₃/Ni hydrogen evolution electrode was prepared.To accelerate the slow kinetic oxygen evolution reaction,a high-efficiency P-CoFeP NPs/Ni bifunctional electrocatalyst with interconnected macropores were further prepared.This thesis conducted experiments by considering the following two aspects:?Reducing the hydrolysis energy barrier under alkaline conditions and optimizing the hydrogen adsorption free energy(?G_H*)to increase the intrinsic activity of the catalysts;?Optimizing the morphological structure of the catalysts to increase the specific surface areas,the number of catalytically active sites,and provide more accessible transport channels for gases and ions.The main works of the thesis is as follows:?1?Studies of core-shell structured Co@WO₂@Ni₁₇W₃/Ni as Hydrogen Evolution Electrocatalyst Prepared by Template MethodThe Co₃O₄ precursor nanowire arrays were grown in situ on the nickel substrate by hydrothermal synthesis method,and then NiWO₄ was grown on the surface of the precursor to form the core-shell structured Co₃O₄@NiWO₄/Ni.Finally,the high-performance Co@WO₂@Ni₁₇W₃/Ni hydrogen evolution electrode with core-shell structure was prepared by hydrogen reduction at a set temperature.The morphological characterization of Co@WO₂@Ni₁₇W₃/Ni showed that“-finger-like”nanomaterials covered with a layer of uniform nanoparticles grow on the nickel substrate.XRD characterization showed that the crystal phases of the final catalyst included Ni₁₇W₃?JCPDS:65-4828?,WO₂?JCPDS:71-0614?and Co?JCPDS:89-7373?.XPS characterization showed the presence of Co,Ni,W,O in the Co@WO₂@Ni₁₇W₃/Ni sample.The as-prepared Co@WO₂@Ni₁₇W₃/Ni electrode after 8 hours of second-step hydrothermal method and then after hydrogenation at 600°C required respectively only22,124 and 200 mV to drive HER 10,300 and 800 mA cm^-2current densities in 1.0M KOH,showing satisfactory HER catalytic activity and stability.The superiority of the catalyst stems from:?using Co₃O₄core precursor as a template for NiWO₄ growth,providing a larger specific surface area;?the metal Co in the core-shell structure has good electrical conductivity,reducing electron transfer resistance;?The interface between WO₂ and Ni₁₇W₃ in situ by hydrogen reduction method is beneficial to the adsorption and desorption of active H^*,and the Ni₁₇W₃ alloy formed in situ can effectively reduce the hydrolysis energy barrier,and at the same time producing synergistic effect on HER,improving the bonding ability of the surface of the electrode to active H^*,WO₂ also optimized the hydrogen adsorption free energy(?G_H*).?2?MOF-derived porous CoFeP/Ni nanosheet array electrocatalysts for high-efficiency Water SplittingHigh performance,stable and abundant bifunctional electrocatalysts are extremely important for producing sustainable hydrogen by water splitting.However,it is difficult for catalysts in previous studies to precisely regulate the electronic structure and fully expose the active sites.Therefore,in this study,using CoFe bimetallic-organic framework nanosheets as precursors,we have developed a new strategy to prepare P-CoFeP NPs/Ni nanosheet arrays with interconnected macroporous structures.MOFs materials are widely used in electrocatalytic reactions,gas adsorption and separation due to their charateristics such as ultra-high specific surface area,adjustable pore structure and monodisperse unsaturated metal sites.However,due to the intrinsic non-conductivity and the small pore size?generally less than 5 nm?of MOFs materials,which also limits the application of MOFs materials in the field of electrocatalysis.Therefore,this work started from the weak binding of organic ligands to metal ions in MOFs materials,using(Co_xFe_1-x)₂?OH?₂?BDC?as a sacrificial template,and after two hydrothermal reactions and phosphating treatments,the prepared nanosheet array P-CoFeP NPs/Ni electrode with interconnected macroporous structure was used as highly efficient water spitting.Morphological characterization showed that a layer of rough-surfaced nanosheet arrays?a thickness of several hundred nanometers?with interconnected macroporous structures is uniformly grown in situ on the nickel substrate,thus providing more specific surface area,more active sites and more gas transport channels for full water splitting.XRD analysis showed that FeP?JCPDS:89-2746?was present in the composite phosphide,and the incorporation of Co did not change the crystal phase structure of the catalyst.XPS and EDS characterization showed the presence of Co,Fe,P in the sample and the molar ratio of Co,Fe and P was 1:2:1.The TEM characterization showed that the catalyst was in the form of nanosheet,and the interconnected macroporous morphology randomly distributed on the surface of the nanosheet can be clearly seen.Electrochemical characterization showed that the P-CoFeP NPs/Ni electrode requires only 49,98,149 and 250 mV overpotentials when driving HER at 10,100,300 and 900mA cm^-2current densities in 1.0M KOH,respectively;Providing 10,100,300 and 700mA cm^-22 current densities for OER requires only 198,250,290 and 335 mV overpotentials respectively;exhibits excellent HER and OER catalytic activity.When driving 10mA cm^-2current density,P-CoFeP NPs/Ni||P-FeCoP NPs/Ni requires a tank pressure as low as 1.47V to perform full water splitting.And the activity and stability of P-CoFeP NPs/Ni in 1.0M KOH is also better than the existing best transition metal phosphide electrodes?TMPs?.The excellent performance of this catalyst stems from:?The incorporation of Co optimizes the adsorption and desorption of hydrogen on the surface of FeP,making?G_H*closer to 0 eV,which improves the intrinsic activity of the catalyst;?The porous structure with interconnected macropores prepared by using MOFs as a sacrificial template increases the catalytic specific surface area,exposes more active sites,and improves the gas and ion transport efficiency.