Synthesis Of Heteroatom-doped Carbon Nanomaterials And Their Electrocatalytic Performance For Oxidation And Reduction Of Oxygenated Small Molecules

The redox of oxygenated small molecules(such as oxygen and water)plays an important role in the areas of pollution control and clean energy.Electrocatalysis has shown potential for applications in the redox of oxygen and water,such as electro-Fenton,oxygen reduction and water splitting,due to its advantages of environmental compatibility,mild handling condition and high energy conversion efficiency.Electrocatalysts play a critical role in electrochemical process.It determines the feasibility and efficiency of the electrocatalytic process.However,the traditional electrocatalysts suffer from the scarcity and high cost of noble metals,the low activity of non-precious metal materials,which hinder the practical applications of electrocatalysis technology.To solve these issues,heteroatom doped sp2 hybrid carbon materials were designed as electrocatalysts for organic pollutants removal by electro-Fenton,oxygen reducion and oxygen evolution.The electrocatalytic performances of these materials were adjusted by heteroatom doping and the formation of nano structure.The mechanisms of these reactions on the electrocatalysts were also explored.The main research contents are shown as follows:(1)N-doped self-supporting carbon foam(NCF)was prepared by carbonizing the melamine foam and the electrochemical performance of NCF towards H2O2 production by oxygen reduction was evaluated.The conductivity and graphitization degree of NCF increased with the increasing of annealing temperature.NCF1000,the sample annealed at 1000?,shows the selectivity which is larger than 70%towards H2O2 production at pH=6-9 and the maximum selectivity of 82%.In the one-chamber reactor,NCF1000 presents the maximum equilibrium concentration of 0.87 mmol L-1 H2O2 at-0.6 V.This value is 1.8 times larger than that for graphite,2 times larger than that for carbon cloth and 2.6 times larger than that for carbon felt.For phenol degradation in neutral solution by electro-Fenton,the reaction kinetics of NCF1000 is 0.062 min-1,which is 4.1 times larger than that of commercial graphite.(2)Chitosan was converted into N-doped graphene-like carbon with high graphitization degree by a FeCl3 soft template induced method and the oxygen reduction reaction(ORR)performance was evaluated.The graphitization degree,surface area and the N species were adjusted by controlling the annealing temperature.Compared to DAC800(the sample prepared by directly annealing chitosan),the soft template induced sample annealed at 800?(STS800)presents higher graphtization degree,20.9 times larger surface area and higher contents of graphitic-N and pyridine-N.The value of onset potential for STS800 is just 25 mV negative than that for Pt/C(-0.08 V to-0.055 V)and the ORR current density at merely-0.3 V for STS800(-2.16 mA cm-2)is larger than that for Pt/C(-2.12 mA cm-2),demonstrating the superior ORR activity of STS800 even comparable to Pt/C.(3)N,S co-doped graphene/carbon nanotube(NS-GR/CNT)was prepared by hydrothermal method and its electrochemical performance towards oxygen evolution reaction(OER)was evaluated.NS-GR/CNT with varying S contents and species were employed to explore the structure-activity relationship between performance of NS-GR/CNT and S doping.The as-prepared NS-GR/CNT exhibits more negative onset potential and lower Tafel slope(560 mV,103 mV decade-1 vs.SCE)compared to single N doped graphene/carbon nanotube(650 mV,285 mV decade-1),which indicates S doping can significantly enhance the OER performance.The thiophene-like S(C-S-C)is the dominant S species in all the S doped samples and the catalytic kinetics are highly correlated with the content of C-S-C.(4)ZIF-67 was synthesized and converted into Co decorated N-doped porous carbons(Co/C)and its electron-Fenton performance for organic pollutants degradation under neutral conditions was evaluated.The doped N species,surface area,and nano-particle size of Co/C were controlled by adjusting carbonization temperatures.Co/C-1000,the sample annealed at 1000?,presents highest reaction kinetic of 0.044 min-1,which is 1.42 times higher than that of Co/C-800 and 1.69 times higher than that of Co/C-1100.The Co nanoprticles on Co/C surfaces can in-situ activate H2O2 converted into.OH via the Co2+/Co3+ transformation.Electro-Fenton with Co/C-1000 as cathode,which avoid from the pH adjustment process in traditional electro-Fenton and convenient in recycle or reuse,expanding the application of electro-Fenton technology.(5)Core-shell structured ZIF-67@ZIF-8 was synthesized and converted into core-shell Co decorated porous carbons(CS-Co/C)and its electrochemical performance towards OER was evaluated.The core derived from ZIF-67 is decorated by Co nanoprticles with diameter of 5-15 nm and the shell consists of metal-free porous carbon derived from ZIF-8 could lead to larger surface area and improve the wettability.Compared to individual ZIF-67 derived samples(Co/C-1000),CS-Co/C-1000 presents the 1.6 times larger surface area.The water contact angle of CS-Co/C-1000 is 14° smaller than that of Co/C-1000.The optimized CS-Co/C-1000 presents the low overpotential of 290 mV to deliver 10 mA cm-2 toward OER,which is lower than IrO2(330 mV)and among the best of the reported non-precious OER electrocatalysts.Besides,the core-shell structure also enhances the stability of CS-Co/C.In summary,the electrocatalytic performance of heteroatom doped carbon nanomaterials could be adjusted by heteroatom doping and the design of unique nano structure.The optimized heteroatom doped carbon nanomaterials exhibit good catalytic activity and stability as electrocatalysts for nitrate reduction,oxygen reduction and oxygen evolution.These results are valuable in developing non-precious metal electrocatalysts for environmental pollution control and clean renewable energy.