Preparation Of Cobalt-based Micro-/nanostructured Composites By Biotemplating Method And Their Electrocatalytic Activity For Oxygen Reduction Reaction

Fuel cells mainly use noble metal Pt as the cathode's oxygen reduction reaction?ORR?catalyst.However,Pt has the disadvantages of high cost,low stability and poor methanol tolerance,which restricts its large-scale application.Therefore,developing non-precious metal electrocatalysts with low-cost,high-activity and high-durability properities is an urgent issue to be addressed in the market of fuel cells.Among them,transition metals have a variety of compound status and exhibit excellent oxygen reduction catalytic activity.The combination of transition metal compounds and micro-nanostructured carbon matrix can further increase the utilization of the active components,improve the electron transport efficiency and the material stability.Composite materials were prepared using biotemplating method and applied as ORR catalysts.Both biomass-derived carbon and micro/-nanostructures of biological tissue can be obtained,which makes the material exhibit the advantages of high specific surface area,large porosity and high electrical conductivity.This has great application potential as cathode catalysts for fuel cells.In this thesis,three kinds of biological materials,including butterfly wings,chloroplasts and Streptococcus thermophilus,were used as templates to prepare highly-efficient and cobalt-based micro-/nanostructured ORR composite electrocatalysts.The main research contents are as follows:?1?The black front wing of Troides aeacus butterfly is used as template and carbon source to obtain carbon with butterfly-wing pore array structure?CW?by high-temperature calcination;then Co₃O₄ is loaded onto the pore array structured carbon by hydrothermal method to obtian pore array structured Co₃O₄/carbon composite?Co₃O₄/CW?and used as ORR electrocatalyst.The as-synthesized catalyst had an onset potential of 0.89 V vs.RHE,a half-wave potential of 0.77 V vs.RHE,and a limiting current density of 4.59 mA cm^-2 in alkaline solution.The number of electron transfer?n?is⁴.1,indicating that four electron transfer processes occur.In addition,its durability and methanol tolerance are better than that of commercial Pt/C,which has great market application prospects.The excellent electrocatalytic performance of Co₃O₄/CW is due to its good electrical conductivity,unique porous structure,high surface area and synergistic effects of Co₃O₄ and CW.?2?Natural chloroplasts containing big amounts of chlorophyll?magnesium porphyrin,Mg-Chl?are employed both as template and porphyrin source.Co-substituted chlorophyll?Co-Chl?is obtained by substituting Co ions for chlorophyll center ions in the chloroplast.After calcining in nitrogen to 800?,Co-Chl is transferred to Co-N-C,while other parts of chloroplasts adsorbed with Co ions are transferred to CoO_x to synthesize the chloroplast submicron sphere Co-N-C/CoO_x composite,and used as an ORR electrocatalyst.The onset and half-wave potentials of Co-N-C/CoO_x are 0.89 and 0.82 V vs.RHE,respectively,and had better durability and methanol tolerance than commercial Pt/C.The abundant active Co-N-C sites in Co-N-C/CoO_x composites are protected by circumjacent biocarbon and CoO_x to avoid shedding and agglomeration;while CoO_x can directly transport electrons to the carbonaceous framework,overcoming the poor conductivity weakness of oxides.This unique synergistic effect,together with efficient bioarchitecture,leads to good electrocatalytical performance for the ORR.?3?Streptococcus thermophilus containing big amounts of teichoic acids are employed both as template and phosphorus source.The surface of bacteria is coated with cobalt acetate followed by adding graphene oxide.When calcining in nitrogen to 800?,cobalt acetate reacts with teichoic acids to form Co₂P;while graphene oxide is reduced to reduced graphene oxide and the remaining bacteria converts to biomass carbon.The bacteria structured Co₂P/CoO_x/C/rGO composite is finally obtained and used as electrocatalyst for ORR.The onset and half-wave potentials of Co₂P/CoO_x/C/rGO are 0.91 and 0.80 V vs.RHE,respectively,and the limiting current density is 4.96 mA cm^-2.In addition,the durability and methanol tolerance of Co₂P/CoO_x/C/rGO is superior to that of commercial Pt/C.The excellent electrocatalytic performance of ORR is attributed to the synergistic effect of components between Co₂P,CoO_x,C and rGO and the submicron sphere structure with high specific surface area.