Researches On The Preperations And Electrocatalytic Performance Of Transition Metal-based Catalysts

With the rapid development of industry and population growth,traditional fossil energy resources can no longer meet social needs.Hydrogen has the advantages of the highest energy density,small molecular weight,environmental friendliness and no carbon pollution,which is an ideal renewable clean energy for future development.The production of hydrogen and oxygen by water electrolysis is a green and sustainable energy conversion technology.Efficient and robust electrocatalysts are required to greatly reduce the over-potential of the cathode hydrogen evolution reaction（HER）and anode oxygen evolution reaction（OER）,which ensure the effective overall water splitting.At present,precious metal-based materials（Pt,RuO₂,IrO₂）are the most efficient catalysts,but they are expensive and scarce,which greatly hinders the further development of water electrolysis technology.Transition metals with the characteristics of low cost,unique electronic configuration and high activity are efficient and strong electrocatalysts which show good stability and excellent electrocatalytic activity under long-term catalytic conditions.In this thesis,we design and synthesize a series of transition metal-based compounds（such as oxides,hydroxides,phosphides）as electrocatalysts.The composition,structure,and morphology of the materials are adjusted and ameliorated to improve the conversion capacity of HER and OER in the electrohydrolysis process.The specific research work is as follows:1.Transition metal cobalt hydroxide nanosheet array deposited on a carbon cloth decorated with a trace amount of precious metal ruthenium is synthesiezd through two-step electrochemical deposition strategy（Ru-Co（OH）₂/CC）.The doping of a trace amount of precious metal ruthenium in the catalyst activates the bifunctional electrocatalytic water electrolysis of Co（OH）₂ in alkaline medium.Ru-Co（OH）₂/CC drives a current density of 10 mA cm^-2 at low overpotentials and Tafel slopes of 55 m V(61 mV dec^-1)and 203 mV(44 mV dec^-1)for HER and OER in alkaline solution.Furthermore,a cell voltage of 1.54 V is obtained to deliver 10 mA cm^-2 for overall water splitting in a two-electrode electrolyzer by using Ru-Co（OH）₂/CC as bifunctional catalysts.Stable current output is maintained at 10 mA cm^-2 for at least 60h,and the attenuation of the current density can be neglected.Meanwhile,a single AAA battery can successfully drive the continuous overall water splitting in the same two-electrode device.2.A heterogeneous composite of transition metal cobalt Co@CoO-PNC/CC with core-shell structure is prepared by using the strategy of combining electrochemical deposition and high-temperature calcination.The heterogeneous Co@CoO encapsulated in P,N co-doped carbon nanofibers supported on carbon cloth（Co@CoO-PNC/CC）has been prepared to pursue attractive synergism for the application of pH-tolerant catalyst for OER in both alkaline and neutral solutions.The prepared Co@CoO-PNC/CC exhibits advanced OER performances,generating a current density of 10 mA cm^-2 at overpotential of 289 mV in alkaline media and 265mV in neutral media.The high OER activity can be assigned to the synergistic effect of abundant active sites and enhanced charge transfer between Co and CoO,as well as the specific P,N co-doped carbon nanofiber architecture,superior to the OER catalytic performance of many catalysts in 1.0 M PBS.This experimental system provides a new design idea for the development of OER catalyst materials in neutral solution.3.Double-layered heterogeneous structured composite material which deposited on the P,N co-doped carbon nanofibers（Fe₃（PO₄）₂-NiFe₂O₄@PNC/CC）is composed by the combination of phosphate and iron ore.This catalyst can provide more active sites and accelerate the charge transfer,greatly improves the intrinsic electrocatalytic OER performance and the reaction kinetics of the NiFe₂O₄ spinel catalyst.This phosphate compound combined with spinel special double layer heterogeneous structure can produce a current density of 10 mA cm^-2 at an overpotential of 290 mV in1.0 M KOH electrolyte.Moreover,the electrocatalyst presents outstanding OER performance in 1.0 M PBS which delivers 10 mA cm^-2 at the low overpotential of only240 mV,superior to most of the transition metal oxide and heterostructured transition metal compounds.This type of catalysts provides a new development idea for the study of electrocatalytic oxygen evolution in wide pH range.