Syntheses And Electrocatalytic Performance Of MOFs Derived Cobaltbased/Carbon Composites

In recent years,metal organic frameworks（MOFs）derived carbon materials obtained by high-temperature pyrolysis have great potential in developing high-performance electrocatalysts.By controlling the composition of metal/ligand in MOF precursors,and by introducing other precursors,the carbon-based composites with controllable compositions and adjustable morphologies can be obtained.This dissertation mainly focuses on several electrocatalytic reactions such as oxygen reduction（ORR）and oxygen evolution（OER）.By introducing other precursors such as phosphorus source and nitrogen source,adopting preparation methods such as all-in-one phosphorization-carbonization synthesis and micropore filling strategy,a series of highly active carbon-based composites with controllable composition and adjustable morphology were developed,effectively solving the problems of single active component and skeleton collapse of the MOFs-derived carbon catalysts.The synergy among multiple active components and the influence mechanism of structural stability on the performance of ORR/OER have been systematically studied.In addition,the zinc-air batteries based on the developed bifunctional catalyst in the dissertation exhibited decent cycling stability.The Co/P ratio has a great influence on the catalytic performance of cobalt phosphide materials.However,there are few studies on cobalt phosphides synthesis with controllable Co/P ratio towards ORR performance.The mechanism for the influence of Co/P ratio on ORR performance of the catalysts is still unclear.In this work,cobalt phosphide/N doped carbon nano-hybrid materials（Co P/NC and Co P₂/NC）with controllable Co/P ratio were successfully prepared by using ZIF-67 as precursor and Na H₂PO₂ as phosphorus source.The catalytic activity of cobalt phosphides can be adjusted by adjusting the Co/P ratio.Appropriate P content can adjust the electronic structure of the active sites,weaken the binding energy between*OH and Co atoms on the surface,thus promoting the ORR performance.The higher P content may weaken the oxygen adsorption on the surface of Co atom,which is not conducive to the O-O bond break,thus leading to 2 electrons reduction pathway.Co P/NC(E_1/2=0.824 V vs.RHE,j _L=-4.84 m A cm^-2)shows much better ORR performance than that of Co P₂/NC.In addition,this work also expands the application of Co P₂/NC in the electrocatalytic detection of chloramphenicol,which further confirms the significance of preparing cobalt phosphides with controllable Co/P ratio.In order to further regulate the bifunctional catalytic performance of cobalt phosphide materials,all-in-one phosphating carbonization method was employed to effectively regulate the composition and structure of the materials by using ZIF-67 with carbon nanotubes as the precursor.By combining the ORR active component（Co P）and OER active component（Co₂P）together,the core-shell type cobalt phosphides（Co₂P/Co P）decorated carbon based bifunctional catalyst（Co₂P-Co P/NC@CNT）was obtained.This bifunctional catalyst shows comparable ORR performance to commercial Pt/C and better OER performance than Ru O₂.In addition,Co₂P-Co P/NC@CNT besed liquid and solid-state zinc-air batteries showed good cycling performance.The flexible zinc-air batteries displayed good mechanical flexibility,which verifies their feasibility as a wearable electronic devices.High-temperature pyrolysis under the inert atmosphere is the most common method for the synthesis of MOFs derived carbon based materials.However,the inevitable collapse of the MOFs framework during pyrolysis will result in the sintering of the metal and the loss of active centers,which severely limits the electrocatalytic performance.In order to solve this problem,a new micropore filling strategy was proposed to strength the framework by using cyanamide as filler to fill up the cavities in MOFs.The cyanamide with high N content can not only increase the number of metal active sites but also modulate electronic structure of carbon matrix.Upon micropore filling,the as-prepared Fe,Co co-doped MOF derived carbon material（Fe Co@NC-N）delivers a significantly improved electrocatalytic performance for ORR than its unfilled counterpart,with the half-wave potential positively shifted by 30 m V（0.84 V vs.RHE）and enhanced power density when used as the cathode in zinc-air battery.This work demonstrates a reliable approach to mitigate framework collapse of MOFs,thus may open a new avenue to fabricate MOFs based catalysts with increased loading of active sites.At high OER potential region,electrochemical oxidation corrosion of the surface seriously affects the catalytic performance of cobalt based non oxide catalysts.However,for cobalt phosphide or phosphorus doped cobalt based catalysts,the reconstructed surface at high OER potential usually shows stronger catalytic activity than the original surface.Based on the Fe Co@NC-N material obtained by the above-mentioned micropore filling method,the mono functional catalyst was successfully transformed into bifunctional catalyst by phosphorus doping.The effects of P dopant on the structure and catalytic performance of the materials were studied.The results show that the Co OOH active sites are coupled with the oxidized Co（II）and phosphorus oxide species on the reconstructed surface of the catalyst,which promotes the electron transfer between Co atom and oxygen adsorbates,stabilizes the local structure of the catalyst,and makes the catalyst have high and stable OER performance and significantly improved reaction kinetics.Fe Co@NC-N-5P with P dopant shows excellent catalytic performance for both ORR and OER.The potential gap between E_1/2of ORR and E₁₀ of OER is 0.812 V.The liquid zinc-air battery based on Fe Co@NC-N-5P shows significantly higher power density and more stable cycle performance than that based on Pt+Ru O₂.