ZIF-derived Porous Carbon: Preparation And Their Electrocatalytic Performance Towards Oxygen Reduction Reaction

Fuel cell,as an energy conversion device,can directly convert chemical energy into electrical energy.Owing to its high efficiency and zero-pollution,fuel cell is expected to replace traditional electrochemical cells,ease or even solve global problems such as energy shortages and environmental pollution.However,many challenges block the wayto large-scale commercialization of fuel cells.One of the bottle neck problems is the slow kinetics of the oxygen reduction reaction?ORR?and high cost of catalysts at the cathode.Platinum-based catalysts are the most commonly used catalysts for oxygen reduction.The shortage of Pt resource and its expensive price are the main reasons for the high cost,which seriously hinder the commercialization of fuel cells.Therefore,developing non-precious metal catalysts with low-cost and high-efficiency to replace the current commercial Pt/C catalysts is of great significance for reducing the cost of fuel cell and accelerating the large-scale commercial application of fuel cell.Zeolitic Imidazolate Frameworks?ZIFs?,as a novel porous material,have excellent characteristics such as high specific surface area and porosity,adjustable pore size and easy modification of ligand functional groups.Thus,ZIFs become the ideal precursors for preparing porous carbon material catalysts with high catalytic performance.In this thesis,a series of ZIF-derived porous carbon materials were prepared by using ZIF-67 as the main precursor.Small-sized ZIF-67 crystals were synthesized using a conventional solution method.The obtained ZIF-67 with a typical rhombic dodecahedral sodalite?SOD?structure showed a high BET specific surface area of up to1880 m² g^-1 and a pore volume of 0.85 m³ g^-1,and excellent thermal stability.We prepared ZIF-derived porous carbon materials by carbonization at 700?,800?and900?,respectively.Among them,the ZIF-derived porous carbon material which was heat-treated at 900°C,exhibitedthe most excellent ORR catalytic activity,equivalent to the commercial Pt/C catalyst.A transfer electron number of about 4 demonstrates a dominant four-electron pathway for the ORR reaction.In order to further improve the catalytic activity of ZIF-derived porous carbon materials,we designed advanced catalysts according to the following two aspects:1)Increase the ion transport channels by optimizing the pore structure with high specific surface area and pore volume,which ensures the storage of the electrolyte and shorten the diffusion paths of reactants and intermediate products.2)Enhance the electron transfer during electrochemical process by the compersition of graphene.The experimental results show that,after KOH etching,the BET surface area of ZIF-derived porous carbon material increased from 260.52 m² g^-1?ZIF-C?to 342.37 m²g^-1?ZIF-C-KOH?,and the pore volume increased 2 times of the original pores.Accordingly,the opertimization of pore structure promoted the ORR performance of the ZIF-derived catalysts.On the other hand,the BET specific surface area of the ZIF-derived porous carbon material after a corperation of graphene?ZIF-C/graphene?did not change much,but the pore volume was increased by more than 60%,and the nitrogen content also increased from 3.81%to 5.36%.Graphene composites significantly improved the electrochemical performance of the ZIF-derived catalysts.Among these ZIF-derived porous carbon materials,ZIF-C/graphene exhibited the most excellent catalytic performance,especially in alkaline electrolyte,its onset potential?1.02 V vs.RHE?was comparable to the commercial Pt/C catalyst?1.04 V vs.RHE?,the half-wave potential reached to 0.865 V?vs.RHE?,higher than commercial Pt/C?0.850 V vs.RHE?,and the limiting current density(5.92 mA cm^-2 at 0.5 V)is also superior to the commercial Pt/C(5.83 mA cm^-2).Moreover,the stability of ZIF-C/graphene was better than commercial Pt/C catalysts.After a 10 h'chronoamperometry test,the limiting diffusion current of ZIF-C/grapheme could still maintain a current of 88%,compared with 70%of the commercial Pt/C.In addition,ZIF-C/graphene has good catalytic activity in acidic electrolytes and the stability is superior to the commercial Pt/C catalyst.