Preparation Of Graphene-based Functional Materials And Their Application In Energy Storage

With the exhaustion of fossil energy,the development and utilization of novel energy(especially renewable energy)will become an inevitable trend in the future.However,renewable energy has the characteristics of intermittency and instability,and it is urgent to use matched energy storage equipment(especially electrochemical energy storage equipment with high power density/energy density and excellent cycle stability)to improve the controllability and utilization rate of renewable energy.The current solution is to develop new alternative energy storage devices(such as sodium batteries,lithium sulfur batteries,potassium batteries,etc.)and upgrade existing technologies(such as supercapacitors,lithium batteries,lead-acid batteries,etc.)in order to meet the practical requirements.As the core technology of electrochemical energy storage equipment,the research and development of new generation high-performance energy storage materials has become a research hotspot in recent years.Due to its good electrical conductivity,stable structure,high theoretical specific surface area and strong structural controllability,graphene has been widely and deeply studied as an energy storage material However,although diferent electrochemical energy storage devices have similarities,their energy storage mechanisms are not the same,and the design strategy of their electrode materials structure is also completely different.Therefore,on the basis of deep understanding of energy storage mechanisms in various devices,the rational control of the fine microstructure of graphene and its composite electrode materials in different energy storage device is the key to realize the practical application of graphene in the field of energy storage,and the related work will also provide certain theoretical guidance for the research of other high-performance electrode materials.In the thesis,on one hand,in view of the large ionic radius,small adsorption energy and poor ion transfer performance of sodium ion batteries,the nitrogen doped carbon quantum dots and/or metal Sn quantum dots pillared graphene dense blocks have been designed and synthesized,respectively.During the construction of the conductive network,in order to promote the rapid transmission of sodium ions,more active sites are introduced in the structure and the graphene interlayer spacing is also obviously enhanced.Finally,the electrode materials with high specific capacity,high volume/weight rate performance and ultra-long cycle stability was prepared.On the other hand,graphene can provide low electric double layer capacity for supercapacitors.In view of this problem,the reasonable regulation and control of the sp2 and sp3 hybrid carbon on graphene surface are conducted.On the premise of guarantee the high electric conductivity,introduce more sp3 hybrid carbon and oxygen containing functional groups can provide additional pseudocapacitance,and realize the balance of conductivity and pseudocapacitance of electrode materials.Finally,the "paddy land" structural electrode materials of graphene with high specific capacity was prepared.The main contents and conclusions are as follows:(1)Multilayered graphene oxide blocks(GOB)was synthesized through the modified Hummers' method.Due to the high oxidizability of functional groups on GOB surface and high reducibility of aniline monomer,polyaniline nanodots are easify formed in-between GO sheets through self-limited polymerization.And then,the edge-nitrogen-rich carbon dots pillared graphene blocks(N-CDGB)through subsequent carbonization is developed.Due to high bulk density(1.5 g cm-3)and integrated lamellar structure with large edge-interlayer spacing(4.2 A)pillared by nitrogen-doped carbon dots(95%edge-nitrogen content),the dense N-CDGB shows robust structural stability,fast ion/electron transfer pathways,and more active sites for sodium storage.As a result,the N-CDGB electrode exhibits ultrahigh reversible volumetric and gravimetric capacities(780 mAh cm-3/520 mAh g-1 at 0.02 A g-1),excellent rate capability(118 mAh g-1/177 mAh cm-3 at 10 A g-1),and superior cycling stability up to 10 000 cycles with almost no capacity loss at 10A g-1.DFT calculations suggest that the edge-nitrogen-rich carbon dots embedding into graphene sheets can largely enhance the Na+adsorption energy by introducing negative electron densities,leading to energetically favorable sodiation/desod iatio n.(2)Graphene quantum dots(GQD)was prepared through acid oxidation method from the precursor of coal At the edges of GQD,there is a lot of carboxyl groups(COOH),like an octopus.DFT calculations indicate that the COOH groups around GQD shows strong electronegativity which could adsorb Sn2+ based on the electrostatic adsorption or chemical complexation,forming the stable GQD/Sn2+ complex.And then the nitrogen doped graphene quantum dots/Sn nanodots(dimension of?2 nm)pillared nitrogen doped graphene dense blocks was synthesized through self-assembly with graphene nanosheets and subsequent reduction under NH3.Because of the both chemical(improved Sn-O and Sn-N covalent bonding)and physical(multilayered graphene block structure)confinement effects,the GQD/Sn-NG electrode exhibits excellent rate performance(555.4 mAh g-1 at 0.02 A g-1 and 197.8 mAh g-1 at 10 A g-1)and ultra-long cycling stability(184.1 mAh g-1 remaining even after 2000 cycles).DFT calculations confirm that the strong binding energy of COOH groups,pyrrolic N and pyridine N to Sn could effectively prevent the aggregation and decrepitation of Sn nanoparticles during long sodiation/desodiation process.(3)The kind,content and distribution of oxygen functional groups can be easily adjusted by the long-time and low-temperature thermal annealing process.In this part,we find that the oxygen functional groups distributed on the graphene nanosheet could be selectively migrate.Eventually,after annealing at 160 ? for 8 days,the graphene with "paddy-land" structure(GO-160-8D)was synthesized,in which the pseudocapacitive oxygen functional groups(COOH and C-OH)clusters distributed at the edges and defects of graphene nanosheet,greatly enhancing the conductivity and rate capacity.As a result,the GO-160-8D exhibits ultra-high specific capacitance of 436F g-1 at 0.5 Ag-1,exceeding the values ofpreviously reported RGO materials,as well as excellent rate performance(261 F g-1 at 50 A g-1)and cycling stability(94%of capacitance retention after 10 000 cycles).DFT calculations demonstrate that "paddy land"structure exhibits conjugated carbon network,ultralow HOMO-LUMO gap,and long span of atomic charge values,which are beneficial for the enhanced pseudocapacitance and rate performance.