Amorphous Cobalt-based Metal Oxide As Electrocatalysts For Oxygen Evolution Reaction And Oxygen Reduction Reaction

The inherent intermittent and discontinuous feature of clean renewable energy（such as wind energy,solar energy,etc.）makes it necessary to develop large-scale energy storage and conversion systems.The use of excess electricity to electrolyze water to produce H₂ is regarded as a very promising hydrogen production technology in the 21st century.Fuel cells and metal-air batteries are very promising energy storage devices due to their high energy density.There are oxygen evolution reactions（OER）or/and oxygen reduction reactions（ORR）in these systems,but these processes are relatively slow in kinetics,which seriously restricts the improvement of device performance.Therefore,it is very necessary to develop cheap,high-performanceOER/ORR catalysts.This thesis focuses on Co-based amorphous metal oxide catalysts,and systematically studies theirOER/ORR catalytic behavior by adjusting the doping elements,elemental ratio,heat treatment temperature and other strategies.The main research content and conclusions are as follows:Co-Ce binary metal oxide(Co_1-yCe_yO_x)thin films were prepared on FTO and nickel foam（NF）substrates using photochemical metal organic deposition（PMOD）method.Three-electrode linear scanning voltammetry（LSV）was used to study theOER activity,and the effect was systematically studied from the perspective of geometric and electronic factors.The results show that the amorphous film with controlled composition,uniform element distribution,and thickness of～200 nm can be obtained by the PMOD method.When the Ce addition amount y<60%,its catalytic activity is significantly better than CoO_x.Co_0.9Ce_0.1O_x/FTO has the highest activity:the overpotentials are 158（1）and 320（2）m V,respectively,at the current density of 1 and 10 m A cm^-2（in 1 M KOH solution）.By loading Co_1-yCe_yO_x film on NF,theOER activity is further increased.The overpotentials of Co_0.9Ce_0.1O_x/NF at the current density of 10 and 100 m A cm^-2（in 1 M KOH solution）are reduced to 175（0.1）and 230（1）m V,respectively.Scanning electron microscope and electric double layer capacitance measurement show that a small amount of Ce can increase the specific surface area of Co_1-yCe_yO_x thin films.Cyclic voltammetry（CV）,X-ray photoelectron spectroscopy（XPS）,and electrochemical impedance spectroscopy（EIS）results show that the addition of Ce can increase the content of Co³⁺and reduce the charge transfer resistance.Simultaneously,the addition of Ce can increase the strength of the metal-OH bond,so that the rate control step is changed from the generation of the adsorbedOH radical to the step of generating the physically adsorbed H₂O₂.Based on the synergistic effect of specific surface area,Co³⁺content and M-OH bond strength,Ce doping can significantly improve theOER activity of CoO_x when the Ce content is less than 60%,.In addition,the catalyst shows good long-term stability.With KB as electrical conduction substrates,the amorphous Co-Mn binary oxide hybrid catalysts were prepared by a one-step ultraviolet photolysis method.The effect of elemental ratio,KB content and heat treatment temperature onORR activity were studied using rotating disk electrode（RDE）and rotating ring disk electrode（RRDE）.The results show that when the KB content is less than 33.3%,it is helpful for complete photolysis.The proportion of Co-Mn element has a great influence on the catalytic activity.The onset potential(E_onset),half-wave potential(E_1/2)and electron transfer number（n）of the amorphous Co_yMn_3-yO_x/KB（0<y≤1.5）catalyst are significantly better than MnO_x/KB and Co₂MnO_x/KB.Co_1.5Mn_1.5O_x/KB showed best E_1/2（0.73 V vs.RHE）and electron transfer number 3.84（at 0.3 V vs.RHE potential）.The limiting current density（J_L）and n of crystalline Co Mn₂O_x/KB catalyst obtained by high temperature treatment are higher than those of their amorphous counterparts,but the E_onset and E_1/2 remain almost unchanged.Adjusting the ratio of metal elements in the amorphous cobalt-manganese catalyst system can increase theORR activity in terms of E_onset,E_1/2 and n whereas the crystalline metal oxide can increase theORR activity in terms of such as J_L and n.It provides a new strategy of tailoring theORR performance.