Design Of Hirerachical Transition Metal-based Catalysts And Study On Their Electrocatalytic Water Splitting

With the rapid consumption of fossil fuels,the exploration of alternative clean,sustainable,and environment-friendly energy sources has become imperative.Hydrogen is a potential energy carrier because of its superior properties.An effective way to produce high-quality hydrogen is via water splitting,in which an electrolytic cell composed of two electrodes is typically employed.On the anode side,oxygen evolution reaction（OER）will take place,while hydrogen evolution reaction（HER）is involved on the cathode side.Due to the high thermodynamic barrier and the slow reaction kinetics,a low over-potential is usually required for an effective electrocatalyst.The conventional catalysts based on noble metals（Pt,Ir,Ru,etc.）can effectively catalyze the electrolysis reactions with satisfactory yields of products obtained.However,the high price and the limited resources significantly hinder their wide applications in water splitting.Therefore,the development of non-precious transition metal electrode materials is of great significance.In this thesis,a series of research work has been carried out towards the development and applications of advanced transition metal-based electrocatalysts.On this basis,the thesis is aimed to clarify the relationship between the structures and electrocatalysis performances of the catalysts and to explore the underlying reaction mechanisms.Collectively,the content of the thesis can be summarized as below:（1）The search for cost-effective and highly active transition metal-based electrocatalysts is of great importance for overall water splitting to generate clean energy hydrogen.In this work,we present a controllable structural transformation engineering strategy to construct 3D hierarchical CoP porous microscale prism-like superstructure（assembled with nanoflakes）arrays grown on surface-phosphatized Ni foam（CoP/SPNF）.Specifically,Zn/Co-based composite arrays with nanowires@prism hierarchical structure were prepared on Ni foam first.Then,porous Co-based compound arrays with nanoflakes@prism hierarchical structure were obtained through removing the Zn-based compound by alkaline etching.Finally,CoP arrays were produced through phosphatization of the prepared Co-based arrays precursor,using Na H₂PO₂·H₂O as P source.The fabricated CoP/SPNF electrocatalyst exhibits impressive bifunctional performance for hydrogen evolution reaction(HER,overpotential of 45 m V at 10 m A cm^-2)and oxygen evolution reaction(OER,overpotential of 215 m V at 80 m A cm^-2),and consequently enables efficient electrolytic water splitting with a low cell voltage of1.547 V at 30 m A cm^-2and a prominent durability.Versatile CoP with its porous superstructure arrays on surface-phosphatized Ni foam can promote fast electron transport,increase the exposure of electrochemically active sites and render easy contact with the electrolyte,thus facilitating effective electrolyte diffusion and quick release of the product gas bubbles during the electrocatalytic process.This work provides an effective strategy for the design and preparation of non-noble metal bifunctional electrocatalysts for overall water splitting electrolysis.（2）3D superhydrophilic/superaerophobic hierarchical“rod-sheet”structure of NiFe-based layered double hydroxides coupled with cobalt carbonate hydroxide（Co-CH@NiFe-LDH/NF）has been synthesized on Ni foam by aFe³⁺ion hydrolysis-induced etching-growth approach under hydrothermal condition.The Co-CH@NiFe-LDH/NF electrode demonstrates excellent oxygen evolution reaction（OER）performances,which mainly results from the synergic effect betweenCo-CH and NiFe-LDH.The unique stepped hierarchical“rod-sheet”structure of Co-CH@NiFe-LDH/NF not only benefits the inherent catalytic activity,but also enhances the electron transfer and increases the exposed active sites.Moreover,the superhydrophilic/superaerophobic joint properties of Co-CH@NiFe-LDH/NF promote the diffusion of aqueous electrolyte and ensure the rapid release of bubbles.Furthermore,a water splitting device has been assembled with commercial Pt/C（coated on Ni foam）and Co-CH@NiFe-LDH/NF as cathode and anode,respectively,which can work stably for 200 h without obvious degradation.Hence,the strategy of engineering stepped hierarchical structures with synergistic effect may offer a new pathway to synthesizing highly efficient OER electrocatalysts.（3）Oxygen evolution reaction（OER）is a key reaction in many renewable energy technologies,such as metal-air batteries,water splitting and regenerative fuel cells.However,this reaction is known to be kinetically sluggish and proceeds at rather high overpotential,which are linearly correlated and cannot be optimized simultaneously.The factor determining OER kinetics is the Tafel slope.Herein,a novel hierarchical particles-sheets NiFeP electrocatalyst is developed.Experimentally,the OER overpotential is reduced to 204 m V,225 m V and 231 m V at current density of 20 m A cm^-2,100 m A cm^-2,300 m A cm^-2,respectively,with an outstanding Tafel slope value(25 m V dec^-1).To study the overall water-splitting performance using NiFeP/NF as anode and Pt/C/NF as cathode,the required cell voltage toward overall water splitting is only as low as 1.446 V for driving the current density of 10 m A cm^-2.The NiFeP/NF demonstrates more advanced OER catalyst performance.