Study On The Cobalt-based(Hydro) Oxides To Promote Oxygen Evolution Reaction

Developing clean and effective energy technology to replace traditional fossil energy is of great significance to solve environment issues and energy crisis.Metal-air batteries and water-splitting devices have attracted researchers'much attention due to their high theoretical energy density.Oxygen evolution reaction(OER)as a core component in new energy techniques seriously hinders their commercialization.Due to the sluggishly thermodynamic reaction kinetic,it often needs effective electrocatalysts to facilitate OER.Noble metal electrocatalysts such as RuO₂and IrO₂process excellent electrochemical activities in OER.However,high cost,limited resources and poor stability make them hard to meet the need of market.Hence,many effects have been put in developing cheap effective and stable electrocatalysts.Cobalt-based(hydro)oxides play an important role in OER due to their rich resource,cheap cost and good activity.However,at present there are still many problems need to be solved such as unclear OER active sites in metal oxides and a vast promotion roomage for their electrocatalytic performance.This dissertation mainly focuses on the research of cobalt-based(hydro)oxides.The concrete research content is as follows:(1)Co₃O₄,a typical spinel oxide,is often explored as an efficient OER electrocatalyst but its activity origin is still not clear.To explore the OER origin of Co₃O₄,we have successfully synthesized Mg Co₂O₄,CoCr₂O₄and Co₂TiO₄,where only Co³⁺in the center of octahedron(Co³⁺(Oh)),Co²⁺in the center of tetrahedron(Co²⁺(Td))and Co²⁺in the center of Oh(Co²⁺(Oh)),respectively,can be active sites for OER.In Co₃O₄,Co³⁺(Oh)and Co²⁺(Td)are coexisted.The electrochemical results demonstrate that Co³⁺(Oh)sites are the best geometrical configuration for OER.Co²⁺(Oh)sites exhibit better activity than Co²⁺(Td).When Co³⁺(Oh)and Co²⁺(Td)are coexisted,the OER activity of Co₃O₄is the best.Density functional theory(DFT)calculations demonstrate that the rate-determining step(RDS)of both Co³⁺(Oh)and Co²⁺(Td)is the conversion from O*to OOH*.While the RDS of Co²⁺(Oh)is the desorption ofO₂.For Co₃O₄,the adsorption of O*is the RDS.When compared with the value of RDS,it can be seen that the free energy barrier needed to overcome for Co₃O₄is the lowest while CoCr₂O₄is the highest.Co₂TiO₄needs to overcome more energy barrier than that of Mg Co₂O₄.Moreover,DFT-calculated partial density of states results demonstrate that in Co³⁺(Oh)Co 3d and O 2p orbitals have the biggest hybridization percentage and strongest metal oxygen covalency,which can accelerate OER fastest.The post-characterizations of electrocatalysts after OER indicate that Co³⁺(Oh)can convert into CoOOH,which is well recognized as OER active site,most easily.This work here screens out the most optimal geometrical configuration of cobalt cations for OER and provides an important guidance for developing effective OER electrocatalysts.(2)Defects can effectively regulate the surface charge distribution of electrocatalysts to improve electrochemical activity.In this work,we successfully introduce Co²⁺vacancies(VCo(II))and Co³⁺vacancies(VCo(III))into Co₃O₄by etching Zn-doped Co₃O₄and Al-doped Co₃O₄using strong alkali solution.After the production of cobalt vacancies,the valance state of cobalt cations can be increased because of excess oxygen anions.Electrochemical results indicate that the OER activity of Co₃O₄can be enhanced greatly with the existence of cobalt vacancies.With the existence of VCo(II),the OER activity of Co₃O₄can be improved most greatly.DFT calculations demonstrate that the introduction of cobalt vacancies can endue Co₃O₄with metal-like conductivity,which is more beneficial for charge transfer.Moreover,the O p band center is the closest to Fermi level with the presence of VCo(II)leading to fast oxygen exchange kinetic at electrocatalyst surface,which can effectively accelerate OER.By calculating the reaction free energy of OER elementary reaction steps,it was found that the introduction of cobalt vacancies transformed the RDS from the formation of O*in original Co₃O₄to the conversion from O*to OOH*.When VCo(II)is introduced,the free energy barrier required to overcome is the lowest,which means that VCo(II)is more beneficial for the occurrence of OER than VCo(III).In this work,metal vacancies configurations that can maximize the catalytic performance of Co₃O₄have been successfully identified,which is of great significance for the design of defect-rich electrocatalysts.(3)Decoration of defects with heteroatoms is also an effective strategy to enhance the intrinsic activity of electrocatalysts.In this work,we used CHF₃-plasma technology to treat bulk Co₃Fe LDHs and successfully prepared defect-rich Co₃Fe LDHs ultrathin nanosheets.At the same time oxygen vacancies could be filled with F atoms.Exfoliation and doping can be achieved by plasma treatment but the crystalline structure of electrocatalysts would not be changed.Because of the strongest electronegativity,F^-ions could effectively absorb electron from neighbouring metal sites and change the electronic structure of electrocatalyst surface.Electrochemical results indicate that Co₃Fe LDHs that suffered from plasma treatment process outstanding OER activity and fast reaction kinetic rate.The enhanced electrochemical activity mainly is attributed to more exposed active sites and modulated electronic structure caused by F-filling.Unsaturated coordinated metal vacancies could also work as active sites to facilitate OER.This work provides a simple and safe method to achieve F-doping and extends the application of filling vacancies with heteroatoms in metal oxides to improve electrocatalytic activities.(4)The introduction of defects usually causes charge redistribution at electrocatalysts surface.Thus,in this work we created much oxygen vacancies,which can effectively decrease the valance state of cobalt cations at Co₃O₄surface,to induce the formation of Co₃O₄/CoO heterophase interface and make electrocatalyst stable and active.DFT calculations prove that with the presence of oxygen vacancies,the Co²⁺at Td center is easier to migrate to the adjacent interstitial Oh and act as interstitial atom.Electrochemical results show that after the formation of Co₃O₄/CoO heterophase interface,OER performance is greatly improved.Reaction kinetics rate is accelerated.Charge transfer resistance is smaller and Faraday efficiency is higher.At the same time,DFT calculations show that the RDS of Co₃O₄/CoO is the conversion from O*to OH*while the RDS of Co₃O₄is the remove of protons from OH*.The energy barrier of RDS needed to overcome in the Co₃O₄/CoO is lower.The change of RDS is mainly caused by the lower p-state energy of adsorbed-O on Co₃O₄/CoO heterophase interface.Post-characterizations of Co₃O₄/CoO after OER process indicate the increased valance state of coblat cations and the formation of CoOOH.This work successfully constructs heterophase interface by introducing defects and gives a deep demonstration about the relationship between heterophase interface and electrocatalytic ability.