Preparation Of Several Cobalt-based Hydroxides,Phosiphides And Research Of Their Electrocatalytic Performance Toward Water Splitting

With the global burning of fossil fuels and increasing energy demand,the increasingly environmental and energy crisis forces us to find new energy sources to replace the fossil fuels.Hydrogen energy,as an efficient and nonsecondary pollution“green energy,”has many advantages over most other energy sources,such as high energy density,clean,and renewable traits,which is also considered as a valuable source of renewable energy.Generally,electrochemical water splitting,including oxygen evolution reaction（OER）in the anode and hydrogen evolution reaction（HER）in the cathode,is deemed as an efficient approach to produce hydrogen（H₂）.To date,the Pt/C and RuO₂ or IrO₂ are regarded as the most advanced HER and OER electrocatalysts,respectively.However,the scarcity,high-cost and poor sability of these catalysts hinders the widespread commercial application in the water electrolysis technique.Therefore,the design and synthesis of cheap,high efficiency and good stability of electrochemical catalysts is the key to the industrial application of hydrogen production from water electrolysis.Based on this consideration,the experiments about several cobalt hydroxides and phosphides were carried out in this paper.The efficient and stable catalysts were designed by those strategies including adjusting the morphology and structure,optimizing the chemical composition,modifying the electronic structure.The main contents include the following three parts:（1）The controlled synthesis of cobalt layered-double-hydroxide（CoLDH）nanocages was realized via morphology enginnering with metal-organic frameworks（MOFs）as precursors for oxygen evolution reaction（OER）.The etching process of MOFs into hollow CoLDH nanocages was investigated in detail,and the results demonstrate that the structure and morphology of the electrocatalysts could be regulated by the concentration of Co²⁺cation.The obtained hollow CoLDH nanocages have obvious mesoporous characteristics and high electrochemical active surface areas by precise regulation.Benefiting from their unique hollow nano-architecture,the CoLDH electrocatalysts exhibit excellent electrocatalytic activity for OER in alkaline electrolyte,with a overpotential of 290 mV(10 mA cm^-2),a Tafel slope(60 mv dec^-1).It also has good stability and its current density maintains the same after 20 h test.（2）On the basis of the former work,the layered-double-hydroxide CrCoLDH nanosheets were synthesized by a mild etching co-deposition method using the MOF ZIF-67 as the precursor.The experimental results show that the incorporation of Cr can not only increase the BET surface area of the catalyst,but also change the etching process to form CrCoLDH nanosheets,and further improve the OER activity.The prepared CrCoLDH nanosheets exhibit more excellent OER activity due to higher electrical conductivity,faster charge transfer process and more catalytic active sites,which can drive the 10 mA cm^-2 at the overpotential of 268 mV,with a small Tafel slope(79 mV dec^-1)and a good stability（25 h）.（3）3D self-supported Cr-doped CoP nanoarrays（Cr-CoP/CP）were constructed by a two-step method,and took as a prototype to uncover the activation process of active sites on Cr-CoP/CP.Due to the electronic modulation effect,Cr-CoP/CP exhibits preeminent activity toward pH-universal hydrogen evolution reaction（HER）,with low overpotentials of 47,131 and 67 mV at 10 mA·cm^-2 in acidic,neutral,and alkaline medium,respectively.Cr-CoP/CP also shows the outstanding performance for oxygen evolution reaction（OER）with an overpotential of 251 mV at 10 mA·cm^-2 in alkaline electrolyte.And the voltages of 1.59 and 1.73 V are required to achieve 10 and 100mA·cm^-2 for the Cr-CoP/CP（+,-）cell.More importantly,both the X-ray photoelectron spectroscopy and theoretical calculations confirmed the activation process of active sites for transition metal phosphides（TMPs）.In the early stage of electrocatalysis,M-P bonds of transition metal phosphides undergo the transformation from covalent to ionic bonds,and finally form metal cations as active sites.This work is significanct to understand the process of electrochemical water splitting catalyzed by TMPs.