Metal-Organic Frameworks-Derived Bimetallic Cobalt Based Materials For Electrocatalytic Water Splitting

Hydrogen is considered as a principal energy carrier for the future due to its high energy density,huge reserves and zero-emission.In recent years,electrocatalytic water splitting has been widely regarded as a promising approach of producing clean hydrogen fuel.However,due to the existence of inherent energy barriers in the process of water splitting,so it is necessary to design of efficient catalysts to reduce the energy barrier.Cobalt-based materials are considered as one of the most promising candidate for water splitting owing to their environmental friendliness,abundant reserves,thermal stability,and low cost.The introduction of other transition metal elements into cobalt-based materials to form bimetallic cobalt-based materials could be an effective way to increase the diversity of active sites and charge transfer efficiency of materials toward higher electrocatalytic performance.Metal-organic frameworks?MOFs?are a novel class of porous materials consisting of metal ions coordinated with organic ligands.MOFs have been demonstrated as very promising templates/precursors to obtain high surface area,appropriate porosity and good chemical stability derivatives.In this dissertation,a series of bimetallic cobalt-based materials with controllable morphology and composition were prepared by using the zeolite imidazole skeleton?ZIFs?in the MOFs branch as templates/precursors.The synthesis processes,formation mechanism and electrocatalytic properties of the prepared materials were investigated.It mainly includes the following aspects.Firstly,bimetallic cobalt based oxides with yolk-shell polyhedron(M_xCo_3-xO₄ YSP,M=Zn,Ni,and Cu)structure were fabricated by facile pyrolysis of bimetallic zeolitic imidazolate frameworks?MCo-ZIFs?.The unique mesoporous yolk-shell structure combined with appropriate metal ions selection could not only increase exposed active sites,but also improve their reaction kinetics and catalytic stability.Particularly,Zn_xCo_3-xO₄ YSP exhibits a low overpotential of 337 mV at 10 mA cm^-2,which is smaller than that of Co₃O₄ YSP?422 mV?and comparable to the commercial RuO₂catalyst?332 mV?.Zn_xCo_3-xO₄ YSP electrode also exhibits stable OER current density and retains over 90%of its original current density after testing for 8h in alkaline solution.Metal sulfides have lower band gap and higher conductivity than the corresponding metal oxides.Hence,we report a novel strategy to prepare of porous hollow nickel-cobalt sulfide?NiCoS?by employing ZIF-67 as template underwent hydrolysis and ion exchange reaction in this chapter.The mass ratio of Ni?NO₃?₂·6H₂O/ZIF-67 in precursor has an important influence to the final sulfide products composition,morphology and OER activity.When the mass ratio of Ni?NO₃?₂·6H₂O/ZIF-67 is 3:1 in precursor,the resulted NiCoS-3 through the hydrothermal sulfidation with the optimized composition shows superior OER activity,affording small OER overpotentials of 320mV at the current density of 10 mA cm^-2.NiCoS-3 can achieve enhanced OER activity compared to Co₃S₄,which can be assigned to the incorporation of Ni ions into the Co₃S₄could effectively increase electrochemical surface area and improve the electron transfer.Considering the practical application in industrial production,it is highly desirable to design of bifuncational electrocatalysts with both HER and OER activities for overall water splitting.Bimetal cobalt based carbides have been widely concerned as a bifuncational electrocatalysts owing to their high electron conductivity,good thermal conductivity and low cost.However,more progress is still needed to improve the activity and stability of transition metal carbides in alkaline solution.Hence,we report the synthesis of Co₃ZnC/Co embedded in N-doped carbon nanotubes nanopolyhedra composite?denote as Co₃ZnC/Co-NCNT/NP?by carbonization of ZnCo-ZIFs in Ar/H₂atmosphere.Benefting from their structural and compositional merits,the as synthesized Co₃ZnC/Co-NCNT/NP composite exhibited excellent OER and HER performance,affording a current density of 10 mA cm^-2 at overpotentials of 295 mV for the OER and 188 mV for the HER.Moreover,after 10 h consistent operation,there was no obvious decline in its performance.Furthermore,Co₃ZnC/Co-NCNT/NP exhibits overall water splitting at 10 mA cm^-2 with only 1.65 V,which is close to 1.62 V of Pt/C-RuO₂.Through the comprehensive investigation of the activity and stability of the catalysts mentioned above.The Co₃ZnC/Co-NCNT/NP is designed as the precursor to obtain the Zn_0.975Co_0.025S/CoS₂ embedded in N,S-codoped carbon nanotubes nanopolyhedra composite?denoted as ZnCoS-NSCNT/NP?in a high temperature vulcanization process.Compared with the Co₃ZnC/Co-NCNT/NP,the as-prepared ZnCoS-NSCNT/NP composite exhibits a remarkably enhanced OER and HER performance,affording a current density of 10 mA cm^-2 at overpotentials of 270 mV for the OER and 152 mV for the HER,which is lower than those of many other previously reported non noble metal catalysts.Moreover,an electrolyzer constructed by ZnCoS-NSCNT/NP also exhibited excellent catalytic activity in alkaline solution.Meanwhile,the ZnCoS-NSCNT/NP can still retain⁸0%of its original current density after continuous chronoamperometric testing for 40 h,indicating high stability of ZnCoS-NSCNT/NP catalysts.