Synthesis And Electrocatalytic Properties Of Metal-Organic Frameworks(Co,Fe) And Their Derivatives

To solve the serious issues of energy crisis and environmental pollution caused by excessive consumption of fossil fuels,it is necessary to develop the new efficient and environmentally-friendly energy conversion and storage technologies,such as fuel cells and supercapacitors.The performances of these electrochemical related energy technologies are highly dependent on the activities of electrode materials.Currently,precious metal materials such as Pt,Ru,Ir,etc.are the most active electrocatalysts for fuel cells,however,their expensive cost and scarce resource have limited their large-scale production applications.Consequently,it is of great significance to develop electrode materials with availability,high efficiency,low price and abundance for electrochemical related energy technologies.In this thesis,we aim to develop high-efficient and abundant non-precious metal based electrode materials for energy conversion and storage applications,using non-noble metal based organic framework materials?MOFs?as the precursors to obtain metal nanoparticles anchored on porous graphite carbon under different heat treatment conditions.The obtained electrocatalysts materials have uniform metal particle size and stable nitrogen doping content.It enhances the active sites on the inner electrode materials per unit volume,reduces the thickness of the catalyst layer and reduces the mass transfer resistance.The main contents and achievements of this thesis are as follows:?1?Synthesis and electrocatalytic properties of cobalt nanoparticles anchored on N-doped porous carbonIn this chapter,metallic Co nanoparticles?10?30 nm?anchored on N-doped porous carbon layers?denoted as Co@NPC?was obtained by using the macroscale Co-MOFs crystals as the precursor through pyrolysis method in N₂ atmosphere.The characterization results demonstrate that the sample was obtained at 900 ??Co@NPC-900?with a porous structure??1.9 nm and-20 nm?and surface area of 110.8 m2 g-1.The synthesized Co@NPC-900 used as the electrocatalyst exhibited bifunctional electrocatalytic activities toward the oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?in alkaline media.The results indicated that the Co@NPC-900 can not only afford an onset potential of 0.88 V?vs RHE?and limited current density of 4.5 mA cm2 in ORR,close to the commercial Pt/C catalyst,but also give an onset potential of 1.50 V?vs.RHE?and an overpotential of 380 mV at a current density of 10 mA cm-2 for OER with high applicable stability,respectively.The bifunctional oxygen catalytic activity of Co@NPC-900 can be ascribed to the N doping and metallic Co nanoparticles in carbon structure,providing rich catalytic active sites,and the hierarchical porous structure to facilitate the electrocatalysis-related mass transport.?2?Synthesis and electrocatalytic properties of Co/CoO nanoparticles anchored on N-doped porous carbonIn this chapter,Co-MOFs micron-rods were firstly fabricated using Co2+ source,triethylamine and trimesic acid?H3BTC?as reaction precursors by a simple solvothermal method,followed by pyrolysis treatment at 900 ? in N₂ atmosphere to obtain metallic Co encapsulated into graphitic carbon structure?Co@C?.Interestingly,ultrafine Co/CoO nanoparticles with an average size of?2 nm anchored on graphitic carbon surface?Co/CoO-C?can be obtained through further acid/alkali rinsing treatment and then thermal process of Co@C precursor at 900? in N₂ atmosphere.As the electrocatalyst,Co/CoO-C demonstrated superior bifunctional electrocatalytic activities of ORR and OER in alkaline media,obviously better than that of Co@C catalyst.The results indicated that the Co/CoO-C can afford an onset potential of 0.90 V?vs.RHE?and half-wave potential of 0.82 V?vs.RHE?in ORR,and give an onset potential of 1.46 V?vs.RHE?and an overpotential of 340 mV at a current density of 10 mA cm-2 for OER,close to commercial Pt/C and RuO2/C catalyst,respectively.Furthermore,the Co/CoO-C with superior bifunctional ORR and OER activities as air cathode material was assembled into a rechargeable zinc-air battery,exhibiting high power density and long-term stability.Our experimental results revealed that the formed Co-Nx and N doping in graphitic carbon structure of Co/CoO-C should be responsible for its ORR activity,while its high OER activity can be ascribed to the Co-related catalytic active species resulted from highly exposed Co/CoO nanoparticles on graphitic carbon in alkaline media.?3?Synthesis and electrochemical properties of Fe/Fe₃C nanoparticles anchored on graphitic carbon nanosheetsIn this chapter,ultrafine Fe/Fe₃C nanoparticles anchored on ultrathin graphitic carbon nanosheets?GCN?were fabricated from Fe-based metal organic frameworks?hexagonal rod-like structure?through a sectionalized pyrolysis process.The ultrafine Fe₃C nanoparticles on graphitic carbon nanosheets?Fe₃C/GCN?can be first formed from the Fe-MOFs at 500 ? for 2 h in N₂ atmosphere.Further increasing the temperature to 900 ? for 2 h,the ultrafine Fe/Fe₃C nanoparticles with the sizes of?2 nm were achieved on graphitic carbon nanosheets?Fe/Fe₃C-GCN?.The Fe/Fe₃C-GCN as the electrode material for supercapacitor exhibitsed high specific capacitance of 1186 F g-1 at 1 A g-1 and long-term cycling stability in 6.0 M KOH electrolyte.Moreover,a hybrid capacitor assembled with Fe/Fe₃C-GCN and activated carbon presented an energy density of 164 Wh kg-1 at a power density of 1.03 kW kg-1,and the energy density can still reach 148 Wh kg-1 even at higher power density of 7.25 kW kg-1,indicating its great potential as high-performance electrode material for supercapacitor.The ultrafine Fe and Fe₃C nanoparticles in the Fe/Fe₃C-GCN not only provide more exposed active sites,but also readily transform into Fe2+ and Fe₃+ active species respectively in alkaline media to establish Fe2+/Fe₃+ electrochemical redox cycles,combined with its high surface area and good conductivity,thus contributing high supercapacitor performance.?4?Synthesis and electrocatalytic properties of CoFe alloy nanoparticles anchored on N-doped porous carbonsIn this chapter,bimetallic CoFe-MOFs nanocubes were first synthesized by hydrothermal method,and then the CoFe alloy nanoparticles loaded on N-doped porous carbon?CoFe-NC?were obtained by pyrolysis treatment in H₂/Ar atmosphere.Through a series of characterization and electrochemical tests,the obtained CoFe-NC-700 can catalyze both ORR and OER under alkaline conditions,showing excellent bifunctional catalytic activity and stability.The results indicated that the CoFe-NC-700 can afford an onset potential 0.92 V?vs.RHE?and a half-wave potential of 0.83 V?vs.RHE?in ORR,and shows an onset potential of 1.42 V?vs.RHE?and an overpotential of 260 mV at a current density of 10 mA cm-2 for OER,comparable to commercial Pt/C and RuO2/C,respectively.Furthermore,the fabricated CoFe-NC-700 with superior bifunctional ORR and OER activities as the electrode material was assembled into a rechargeable zinc-air battery,exhibiting high power density and long-term stability.