The Designed Synthesis And Electrochemical Performances Of Crystalline Nanocarbon-based Composites

With the improvement of the required energy and the emergence of the crisis of energyshortage, the applications of energy storage and conversion devices have been attracted widelyattention in the world. Supercapacitors and fuel cells are considered as promising energystorage and conversion devices, and their elecrodes are mainly composed of carbon materials.Supercapacitors have the advantages of high specific capacitance, high energy and powerdensity, and long cycle life. The electrode materials play important role for the performancesof supercapacitors. The carbon materials must have plentiful porous structures, which couldfacilitate to electrolyte-accessibility, rapid diffusion/transport of ions and high ion storage.Besides, the excellent conductivity of the materials could facilitate the fast transport ofelectrons during the electrochemical processes. However, conventional carbon materials, suchas activated carbon, carbon nanotubes, could not meet these needs. Consequently, it isnecessary to develop new methods to synthesize the novel carbon materials with large specificsurface area （SBET）, excellent electronic conductivity and outstanding capacitive performance.Fuel cell is a kind of energy conversion devices. In various fuel cells, direct methanol fuel cell（DMFC） have been attracted considerable attention owing to their high-energy conversionefficiency, low operating temperature, low pollutant emission, and ease of handling andprocessing of the liquid fuels. Nevertheless, the low activity, low stability and vast usage of Ptcatalysts have been hindered the commercialization of fuel cells. Therefore, on one hand, it isvery important to improve the activity and stability of Pt catalyst, as well as reducing the usageof precious metal Pt. On the other hand, it is necessary to develop a novel non-precious metalcatalyst with enhanced activity and stability, which could push farword the applications ofDMFC. In this paper, the main research contents in this paper have been shown as follows:1. A novel soft template method is developed to synthesize a mesoporouscarbon/graphene （MCG） composite. The resulting MCG composite exhibits a outstandingcapacitance as high as242F/g in6M KOH electrolyte at the current density of0.5A/g,which is much higher than mesoporous carbon, graphene and a sample made bymechanical mixing of mesoporous carbon with graphene. A series of experimental resultsshow that the thickness, SBETand carbonized temperatures seriously affect the structure andenergy storage performance of the as-prepared MCG composite. Remarkably, thesynthesized MCG composite displays good cyclic stability, and the final capacitance wasup to105%compared to the initial capacitance after2000cycles of the composite. The mesoporous carbon in the MCG composite is beneficial to the accessibility and rapiddiffusion of the electrolyte, and the graphene in MCG can facilitate the transport ofelectrons during the processes of charging and discharging owing to its high conductivity,which leads to an excellent energy storage performance.2. Mesoporous-carbon-coated graphite nanosheet （GNS@MC） composites have beensynthesized by the intercalation of resol prepolymer into the interlayers of expandablegraphite （EG） under vacuum-assisted conditions, followed by the exfoliation of EGthrough in situ polymerization, the growth of resol under hydrothermal conditions, andcarbonization under Ar. The GNS@MC composites exhibit enhanced capacitiveperformance compared to mesoporous carbon （MC）, microwaved EG after thermaltreatment （T-EG）, and the physical mixture of MC and T-EG （MC+T-EG）. The highestcapacitance of the synthesized GNS@MC composite is up to203F/g at1A/g in6M KOHelectrolyte. Furthermore, the GNS@MC composite exhibits a good cyclic stability with95%capacitance retention and a high columbic efficiency of99%after5000cycles. Theenergy density of the symmetric supercapacitor GNS@MC/GNS@MC achieved was ashigh as11.5Wh/kg at a high power density of10kW/kg. This good performance isattributable to the GNSs in the GNS@MC composite facilitating electron transport owingto its excellent conductivity; moreover, the MC in GNS@MC favors the rapid diffusion ofions by providing lowresistance pathways.3. Porous graphitic carbon nanosheets （PGCS） have been synthesized by an in-situself-generating template strategy based on the carburized effect of iron from cornstalks. Inthis synthesis, cornstalks firstly coordinate with [Fe（CN）₆]⁴ ions to form thecornstalks-[Fe（CN）₆]⁴ precursor. After carbonization and removal of catalyst, the PGCS isobtained. Series experiments indicate that PGCS could be formed only in the case of usingthe iron-based catalyst which can generate carburized phase during the pyrolytic process.The unique structures of PGCS would endow excellent capacitive performance. Especially,the PGCS-1-1100sample (synthesized from0.1M [Fe（CN）₆]⁴ with the carbonizedtemperature of1100oC), shows the excellent electrochemical capacitance （up to213F/g at1A/g）, cycle stability and rate performance in6KOH electrolyte. In the two-electrodesymmetric supercapacitors, the high energy densities of8.3and40.6Wh/kg are achievedat the high power density of10.5kW/kg in aqueous and organic electrolytes, respectively.4. Platinum nanocrystals/graphene nanosheets （Pt-NCs/GNS） composite has beensynthesized by a chemical reduction of PtCl26with formic acid （HCOOH） at roomtemperature. TEM analyses indicate that most of the Pt-NCs with a length of10nm and width of5nm has uniformly growth along the graphene basal plane, making the effectivecontact of Pt and graphene, which would be favorable for enhancing the utility of Ptcatalyst. Electrochemical tests show that the current density of the resulting Pt/GNScatalyst for methanol electro-oxidation is up to412.6A/g Pt, which is about2.3times asthat of the commercial Pt/C（JM） catalyst. Moreover, the Pt/GNS catalyst exhibits betterdurability toward methanol electro-oxidation and outstanding CO electro-oxidation activitythan the commercial Pt/C（JM） catalyst.5. Graphitic carbon nanocapsules （GCNs） with high crystallinity and a large SBEThavebeen prepared in high yield by a solid-state pyrolysis route using a polyacrylic weak-baseanion-exchange resin （PAWBA） as the carbon source. The synthesized GCNs were used asan excellent support of platinumnanoparticles （3–5nm） for methanol electro-oxidation.The Pt/GCN electrocatalyst exhibited a higher catalytic activity and better durabilitytowards methanol electro-oxidation than Pt/C （Vulcan XC-72R support） and commercialPt/C（JM） catalysts. The current density of the Pt/GCN catalyst for methanol electro-oxidation is as high as275A/g Pt, whereas for the Pt/C and Pt/C（JM） catalysts the valuesare only190and184A/g Pt, respectively.6. We have successfully synthesized Fe₂N–NGC composite derived from PWAR resin toform Fe²⁺-PWAR precursor via ion-exchanged process firstly, followed by a carbonizedand a subsequent nitridation process. Because of the synergistic effect between Fe₂N andNGC, the resulting Fe₂N/NGC composite as efficient non-precious metal electrocatalystshows excellent electrocatalytic activity for ORR in alkaline electrolytes. Furthermore,Fe₂N/NGC exhibits superior methanol tolerance and excellent cyclic stability compared toa commercial Pt/C catalyst. Moreover, the ORR proceeds of the synthesized Fe₂N/NGCcatalyst through the preferred four-electron reaction pathway.