Design,Synthesis And Electrocatalytic Applications Of Cobalt-based Oxides Nanostructures And Their Composite Materials

Electrocatalytic water splitting can convert electric energy into clean hydrogen energy,which is a non-polluting and sustainable energy storage technology.Compared with the electrocatalytic hydrogen evolution reaction(HER),the oxygen evolution reaction(OER)process is complicated and the kinetics is slow,which is the decision step restricting the electrocatalytic water splitting.The performance of electrocatalytic materials is closely related to the number of surface active sites of the electrode,intrinsic catalytic activity and electrical conductivity.We designed and synthesized a series of cobalt-based hollow dodecahedral structures and ordered large mesoporous cobalt oxides and their composites with high specific surface area,adjustable skeleton size and skeleton chemical composition,which significantly enhanced the mass transport,conductivity and electrocatalytic activity of the electrocatalyst.Besides,the main factors that influence the OER performance was also investigated.1.ZIF-67,Mn/Co-ZIFs and Cu/Co-ZIFs with dodecahedrons structure and diameter of about 1?m were synthesized by chemical precipitation.Using these ZIFs as precursors,a series of Co3O4 and its composites were synthesized by controlling the selective exposure of the(111)crystal plane.The relationship between its orientation and the electrocatalytic OER was explored.It was found that the OER performance of hollow dodecahedron Co3O4 is related to the selective exposure of the(111)crystal plane,which is the active plane for OER.When the calcination atmosphere with an argon to oxygen is 9:1(VArgon/VOxygen),the obtained Co3O4-9:1 has the best catalytic performance.The overpotential is 307 mV to reach a current density of 10 mA·cm-2 and the Tafel slope is 55 mV·dec-1,which is far superior than Co3O4-4:1 and Co3O4-7:2.Further investigation confirmed that the optimal catalytic activity of Co3O4-9:1 is due to the high oxygen vacancy content(13.0%)and the increased electrochemically active surface area(ECSA),as well as the influence of Co2+content and charge transfer resistance.In addition,we also studied the influence of chemical composition and structure on the OER catalytic performance of cobalt-based composite oxide.To reach a current density of 10 mA·cm-2,the overpotential of Mn/Co-3 is 347 mV,which is lower than that of Mn/Co-2(359 mV)?Mn/Co-1(359 mV).We supposed that the high OER activity of Mn/Co-3 might be attributed to the high active surface area.In addition,the overpotentials is 385 mV to reach a current density of 10 mA·cm-2,implying that the CuCo2O4 also has good catalytic performance.In summary,we used different ZIFs as precursors to prepare(111)crystal plane selective exposure and controllable active sites of hollow Co3O4 dodecahedrons and their composites,which showed efficient OER catalytic performance.2.Using ordered mesoporous silica(KIT-6)as hard template,ordered mesoporous Co3O4-SnO2 composites with large mesoporous of 11 nm and high specific area of 161 m2.g-1 were synthesized.Besides,the influence of molar ratio of Co3O4 to SnO2 on the nanostructure of the composites was also studied.Electrochemical test results showed that the content of Co3O4 in ordered mesoporous Co3O4-SnO2 affected the conductivity.The higher the content of Co3O4,the better the conductivity and the better the catalytic performance.The over potential is 348 mV when the current density reaches 10 mA·cm-2,demonstrating the good OER catalytic performance.In addition,the electrocatalytic performance of Co3O4-SnO2 was also compared with ordered Co3O4 and hollow Co3O4 dodecahedrons.