Research On Controlled Preparation And Applications Of Functionalized Graphene Oxide Based Porous Carbon Composites

Force sensors based on porous carbon composites have great potential for applications in wearable electronics,electronic skin,biomedicine,human motion monitoring,and soft robotics.In recent years,researchers have carried out a lot of research on porous carbon composite force sensors such as piezoresistive and capacitive sensors,which have high sensitivity and strong resistance signal in low stress conditions,but its resistive signal and functionality are poor in high stress conditions.In order to meet the testing needs of the increasing functional diversity and application range,how to prepare porous carbon composites with both versatility and working stability is an important direction for the future development of high-performance force sensing elements.The unique two-dimensional structure of graphene film makes it very sensitive to mechanical pressure,which is one of the preferred materials for force-sensitive sensors and will directly affect the mechanical and electrical properties of the force-sensitive sensor.Therefore,the outstanding properties of graphene oxide and the designability of its surface oxygen-containing functional groups were used to form porous organic framework materials,such as Graphene Oxide Frameworks（GOFs）and their spatially derived structures-Covalent Organic Frameworks（COFs）.Based on the functional modification of GO by GOFs and COFs,two kinds of porous carbide materials with tunable structure were obtained,that is reductive products with tunable structure-reduced graphene oxide/pillared carbon material（r GO/PC）and reduced graphene oxide/vertical oriented carbon framework material（r GO/C）was obtained,respectively.The theoretical model was built to guide the experimental testing,developing its applications in flexible piezoresistive sensors,capacitive sensing devices and enhancement of porous carbon materials,the main studies were as follows:（1）The atomic model and mechanical properties of the COFs-based porous carbon material r GO/PC were studied from the atomic and mesoscopic scales.At the atomic scale,a prediction model of r GO/PC was established through the inter-layer bonding between the"Bi-benzene rings"pillared carbon structure and graphene layers by density functional theory（DFT）.Equivalent Young’s modulus was calculated based on the adsorption energy and charge density during stretching and compression process.at the mesoscopic scale,the factors that caused the resistance change of the r GO/PC film material due to external loading were explored.The r GO/PC-PS film piezoresistive sensors were fabricated by encapsulating r GO/PC films in polydimethylsiloxane（PDMS）which exhibited high sensitivity(0.41 k Pa^-1)and wide operating range（2 k Pa-1200 k Pa）,with stable performance under 10000 cycles and different operating frequencies.Electrically and thermally conductive tungsten（W）atoms were deposited on the surface and cross section of the r GO/PC thin film,which improveed the sensitivity(6.03 k Pa^-1)and working range（up to 1300 k Pa）of the r GO/PC/W-PS sensor.Sensors can be used in human-computer interaction fields such as weight monitoring,temperature conversion,and speech recognition.（2）Layered structure-GOFs film grown on the surface of carbon foam（ACF）obtained by high-temperature carbonization and Na OH activation of sliced bread,and r GO/PC film with tiny particle morphology was formed on the surface of carbon foam after high-temperature reduction.With the help of the strong reducing agent Na BH₄to form oxygen vacancies in GOFs,the size of carbon particles on the surface of the membrane can be controlled,which revealed that large particles aggregated on the surface of the graphene membrane（ACF-r GO/LCP）.The test results show that the binder-free ACF-r GO/LCP electrode has a specific capacitance as high as 601 F g^-1 at a current density of 0.1 A g^-1,and the capacity retention rate retained 99.6%after 12000 charge-discharge cycles.The assembled symmetric supercapacitor ACF-r GO/LCP//ACF-r GO/LCP exhibited a high energy density of 23.063 W h kg^-1 and excellent cycling stability.The capacitive pressure sensing device composed of ACF-r GO/LCP electrode and PVA/KOH gel electrolyte could be applied in low-stress working condition,converting different mechanical pressures into capacitance variation.（3）The mechanical properties parameters of ACF,ACF-r GO and ACF-r GO/PC carbon foam materials were obtained through mechanical experimental tests,and the equivalent mechanical properties of the three materials were analyzed by numerical simulation extended finite element method（XFEM）,the stress distribution of the porous structure during the compression process was obtained,and the effect of the initial damage of the porous carbon material on its subsequent damage evolution and fracture behavior was qualitatively analyzed.The research showed that the addition of graphene/carbon coating contributed to improve the mechanical properties of the porous carbon structure,and the enhancement effect of r GO/PC was better than that of r GO.By testing the resistance variation of ACF-r GO/PC carbon foam material in the process of uniaxial compression,the stress sensing function of ACF-r GO/PC was discussed,and the effective sensing range of ACF-r GO/PC carbon foam was speculated from the perspective of force-electric effect.（4）Two-dimensional/one-dimensional（2D/1D）vertical carbon framework material r GO/C based on COFs structure was prepared,reacting with potassium permanganate to form three-dimensional（3D）r GO/C/Mn O₂ nanocomposites.The 2D/1D r GO/C framework not only provided abundant transport paths for electrons,but also formed good interfacial contacts with Mn O₂ nanosheets.r GO/C/Mn O₂ was used as the cathode material for capacitors,exhibiting high specific capacitance(215.2 F g^-1)at a current density of 0.5 m A cm^-2,and the capacitance retention rate retained 72%after 2500 cycles at a current density of 20 m A cm^-2.Activated carbon（AC）was used as the anode material,r GO/C/Mn O₂//AC asymmetric supercapacitors were constructed,exhibiting a high energy density of 21.2 Wh kg^-1(the power density was 190.4 Wh kg^-1).Supercapacitors in series could power a commercial LED light for more than 40s.