Study On Preparation And Electrochemical Performance Of Hierarchically Porous Carbon

In recent years,with the rapid development of the economy,human demand for energy has increased dramatically.However,with the large-scale exploitation and use of fossil energy,the energy crisis and environmental problems have become increasingly serious.Therefore,it is extremely urgent to transform the current energy structure and develop clean and pollution-free sustainable energy storage and conversion devices.Hierarchically porous carbon materials have great potential in electrochemical applications such as energy storage and conversion due to their rich and easily regulated pore structure,excellent electrical conductivity and stability,and diverse surface element doping characteristics.Nevertheless,the microstructure and surface properties of hierarchically porous carbon materials have a very complicated relationship with their electrochemical properties.The degree of graphitization,specific surface area,pore size distribution and surface functional group doping of the materials will affect the electrochemical performance of the materials.These performance indicators may be mutually restrictive.Therefore,only by precisely tuning the design of hierarchical porous carbon materials with different structures and properties can the most suitable electrode materials be obtained.Based on the above problems,a series of new hierarchically porous carbon electrode materials are designed and prepared in this work.The effects of porous structures and surface properties on electrochemical performance are further summarized and discussed.Theoretical and experimental support is provided for the preparation of hierarchically porous carbon with various properties and applications.?1?For the traditional carbon based capacitor material,The popular pursuit of high interior surface area,although leading to increasing capacitance records for carbon-based supercapacitor electrode materials,also produces low volumetric capacitances.In this work,nitrogen doped,hierarchically porous carbon material?NHC?with ultra-high gravimetric capacitance and volumetric capacitance is synthesized bycombining a foam frame with a soft and hard templatesroutes.The prepared NHC has a BET specific surface area as low as 403 m² g^-1 and total pore volume of only 0.29 cm³g^-1,which whereas exhibits an extremely high gravimetric capacitance of 400 F g^-1 and volumetric capacitance of 507 F cm^-3.Both values are indeed among the highest ones achieved so far.Particularly,the result means that the capacitance produced by a unit interior surface is especially high for which we systematically investigated the reasons by constructing three related control samples,and found that the critical structure features include:1)The evenly distributed macropores are necessary for rapid mass transfer;2)The micropores located inside the thin mesopore walls are more easily accessible for electrolyte ions;and 3)The pseudocapacitances resulted from near-surface redox reactions conducted by doped N also contribute much.Therefore,this work gives a nice and first illustration that a low-porosity material is possible to result in a high gravimetric and volumetric capacitance.And simultaneously there is still a big room for further performance improvement of carbon based supercapacitors,for which a synergistic combination of well-adjusted porous structure and surface doping may be required.?2?Although continuously increased capacitance records have been reported,the rate performance of carbon based supercapacitors is still short of expectation.We reported a general surface-engineering route to rapidly improve the rate performances for different types of carbon materials.Through physical characterizations,it is found that HNO₃-H₂O₂ mixed acid modificationcreate superhydrophilic carbon electrode surfaces by introducing copious N-,O-involved groups.Such a well-wettablesurface remarkably decreases the ion diffusion resistances,and improves the accessibility of the electrode interior surfaces at high current densities,thus enhancing both electrical double-layer and pseudo-capacitances to form the high rate capability.Combined electrochemical performance test,the achieved capacitance reaches 421 F g^-1 at 0.5 A g^-1and 304 F g^-1 at 20 A g^-1,and the rate performance is as high as 72.2%.Besides,a range of carbon-based control materials,no matter if heteroatoms are included or not and what porous structures are like,also demonstrate average 26%enhancement on the rate capability after applying the same treating method.This work demonstrates that the doping species and surface modification could largely change the electrode performances of the supercapacitors,which should be considered carefully when designing the electrode materials.?3?The FeNC catalyst obtained by introducing Fe and N into the carbon material is the most promising catalyst for replacing the noble metal for the oxygen reduction reaction.However,in the conventional carbonization process,the aggregation of Fe and the volatilization of N-containing small molecules often cause severe loss of doped Fe and N substances.And a large number of active sites exist in the bulk phase which are difficult to be exposed to the three-phase interface.In this work,a highly active FeNC catalyst was successfully synthesized using a ZnCl₂ assisted method.The effects of ZnCl₂ on the synthesis and the performance of the synthesized catalysts were discussed in depth through a series of physical characterization methods and electrochemical performance tests.It was found that in the molten state,excess ZnCl₂ forms a branched structure to prevent direct contact and aggregation of adjacent Fe species,by which highly dispersed FeN_x active sitescan be achieved.And the molten ZnCl₂ encapsulates the precursor,avoiding the rapid escape of volatile nitrogen-containing small molecules,thereby contributing to theincreased doping ratio of the N element species.The spheres-FeNC obtained by carbonizing FeP with the aid of ZnCl₂ have a total N content of up to 4.37%,and the Fe-N_x content is also as high as 0.71%,which is 3.2 and 13times of the control catalyst FeNC-none prepared without using ZnCl₂,respectively.Simultaneously,removal of ZnCl₂ produces a large number of micropores/mesopores which can expose more active sites.Finally,combined with the ORR performance test,the initial potential and half-wave potential of the sphere-FeNC in alkaline medium are1.080 V and 0.906 V?vs RHE?,respectively,showing excellent activity superior to commercial Pt/C catalyst.?4?In view of the difficulty in accurately controlling the particle size and the elemental composition for alloy nanoparticles when using a traditional chemical reductionroute,ultrafine alloy nanoparticles supported hierarchicalporous carbon catalystwith tunable elemental componentis prepared through a molecular self-assembly method in this work.The microstructure and alloy properties were investigated by TEM,BET,XRD and XPS methods.It is found that the PtRuOMC-2 catalyst hasordered mesopore structure,and the PtRu alloy particles are uniformly dispersed in the mesopore walls,particulalrly with an average particle size of 1.52 nm.The mass activity of PtRuOMC-2 for methanol electrooxidation is as high as 1379 mA mg_pt^-1,which is almost 4.9 times the mass activity of the commercial Pt/C catalyst and 3.4 times that of the commercial PtRu/C catalyst.After accelerated durability test,the PtRuOMC-2catalysthas much higher activity and intermediate tolerance than commercial Pt/C and PtRu/C catalysts.And the average particle size of PtRuOMC-2 catalyst alloy nanoparticles only increased by 0.07 nm to 1.59 nm after accelerated durability test,which is far superior to the previously reported catalysts and commercial catalysts.The synthesis mechanism is explored by applying amphiphilic block copolymer PEO-PS at differentPS lengths.Particularly,the amphiphilic block copolymer,resol,and metal precursors can be self-assembled into micelles,under strong interactions between hydrophobic chains of block copolymer and metal precursors,and those between hydrophilic chains of block copolymer and resol.During calcination,the resol framework could protect the embedded metal species from size growth,leading to monodisperse ultrasmall Pt alloy nanoparticles loaded on the mesoporous carbon.And a shorter PS block results in smaller alloy nanoparticles as fewer metallic precursor molecules could be included,and vice versa.More interestingly,this simple method is also suitable for the synthesis ofsingle-metal and a wide range ofPt-based bimetallic alloy loaded hierarchical porous carbon materials,which indeedprovides a new strategyfor the preparation of alloy nanoparticles loaded hierarchical porous carbon catalysts owing ultra-small sizes and tunable compositions.