Preparation Of CoP₃ Nanostructured Electrodes With Different Morphologies And Study For Electrocatalytic Hydrogen Evolution From Water

Environment-friendly clean energy supply has been urgent and received extensive research in the past few decades due to the continuous growth of environment pollution and energy demand.Hydrogen,an abundant and sustainable fuel with high energy density and zero environmental impact,is a promising candidate to replace fossil fuels in the future.Among the technologies to produce hydrogen,electrochemical water splitting by using the efficient catalysts,which can achieve large cathodic current densities at low overpotentials,can offer a simple and promising route for large scale production of highly pure hydrogen.Up to now,noble metal Pt and Pt-based compounds have been known as the most active electrocatalysts toward hydrogen evolution reaction（HER）,however,their high expensity and low earth-abundance dramatically hinder their widespread usage.Therefore,it motivates intensive research efforts to explore and develop high-efficiency,stable and non-noble metal HER electrocatalysts.In the past few years,Cobalt phosphides（CoP_x）have been reported as excellent electrochemical catalysts toward HER.Therefore,this doctoral thesis focuses on the fabrication of the CoP₃ nanostructures with different morphologies and study of their electrochemical performance.The main contents in this thesis are as follows:（1）Self-supported cobalt phosphide（CoP_x,x=0.5,1,3）nanoneedle arrays on carbon cloth was successfully synthesized by using different phosphorus sources and were evaluated as HER electrocatalysts.As the rich-phosphorus phosphides have more phosphorus active sites,longer M-P bonds and lower hydrogen adsorption energy barrier,it shows more excellent electrocatalytic activity.By comparison,we found rich-phosphorus CoP₃ exhibit better electrocatalytic activity than Co₂P and CoP with a lower overpotential for achieving cathodic current density of 10 mA cm^-2 and a smaller Tafel slope.（2）Metal-organic framework（MOF）derived porous cobalt poly-phosphide（CoP₃）concave polyhedron was prepared by phase-controlled phosphidation route via vacuum capsulation and explored as a superior electrocatalyst for HER.The prepared MOF derived CoP₃ concave polyhedrons,due to unique polyhedral structure and carbon modification by calcining Co-MOF,show excellent electrocatalytic activity and stability towards HER in acidic media,with Tafel slopes of 53 mV dec^-1 and a current density of10 mA cm^-2 at overpotentials of-78 mV for HER,which are remarkably superior to those of transition metal phosphides（TMPs）and comparable to those of commercial precious-metal catalysts.（3）Three dimensional hierarchical porous cobalt poly-phosphide hollow spheres（CoP₃ HSs）were prepared by topotactic phosphidation of the cobalt-based precursor via vacuum encapsulation technique.As a porous HER cathode,the special layered hollow nanosphere structure of CoP₃ have large specific surface area and expose more catalytic active sites,thus delivers remarkable electrocatalytic performance over the wide pH range.Both experimental measurements and density functional theory（DFT）calculations are performed to explore the mechanism behind the excellent HER performance.（4）Self-supported three-dimensional CoP₃ nanospheres on carbon cloth（CoP₃NSs/CC）were fabricated via topotactic phosphidatio n of Co₃O₄ NSs/CC.Such binder-free flexible HER cathode with integrated three-dimensional nanostructures can not only provide a large surface area to expose luxuriant active sites,but also facilitate the electrolyte penetration for electrons and electrolyte ions.The self-supported CoP₃NSs/CC,as a novel non-precious-metal electrocatalyst,exhibits efficient bifunctional electrocatalytic performance in alkaline medium,with Tafel slopes of 66 mV dec^-1 and72 mV dec^-1,and a current density of 10 mA cm^-2 at overpotentials of-121 and 291 mV for HER and OER,respectively.In addition,it just needs a cell voltage of 1.54 V to acquire a current density of 10 mA cm^-2 for overall water splitting.（5）Self-supported three-dimensional porous CoP₃ nanoplate arrays in situ grown on carbon cloth（CoP₃ NPAs/CC）,which was synthesized by a facile topotactic phosphidation method under hydrothermal condition.Experimental measurements and theoretical calculation results indicate that self-supported porous CoP₃ nanoplate arrays have good conductivity property,great intrinsic catalytic activity,synergistic effect between Co and P and unique rich-phosphorous CoP₃ nanoplate arrays structure promote the adsorption and adsorption of hydrogen in the electrocatalytic process of HER,thus exhibit great catalytic activity that is comparable to Pt,over a wide pH range of 0-14.