Conductive Materials Based On Natural Polysaccharide Polymers:Preparation And Applications In Flexible Supercapacitors

With the rapid development of science&technology,lots of portable,wearable and implantable electronic devices appear,and energy storage devices matching with them are gradually becoming the research hotspots of the scientific community.Nowadays,the most widely used lithium-ion battery possesses high energy density,but its low power density,poor safety and difficulty in flexibility limit its application in wearable electronics.On the other hand,flexible supercapacitors with many advantages,such as high power density,high charge/discharge rate,excellent cycle lifespan and mechanical flexibility,etc.,are considered to be one of the most ideal flexible energy storage devices in the future.Their development is crucial for flexible wearable devices.Among them,flexible supercapacitors with aqueous electrolytes are more suitable for the application requirements of wearable electronic devices due to their unique high security.However,the current development of aqueous flexible devices is mainly restricted by the low energy density,which is difficult to further improve,and the balance between flexibility and electrochemical performance.The key to solving the above problems lies in the development of high-performance flexible electrode materials and the preparation of gel electrolytes with excellent mechanical and electrochemical performance.Based on the above,this dissertation utilizes natural polysaccharide polymers(chitosan,cellulose,starch)that have the advantages of wide sources,low-cost,sustainable and renewable as raw materials.Electrode materials(Mn₃O₄@NPC,PC,Ti₃C₂T_x@Chitosan)with unique structures&performance and hydrogel electrolyte(HPMC/PDA/P(AAm-co-AA))with enhanced strength and self-adhesion properties are successively designed and constructed.By optimizing the design of each component of the device from different perspectives,several high-performance flexible aqueous supercapacitors have been assembled.The electrodes,electrolytes,and devices have been studied and discussed in detail.The details are as follows:1.In the first part,the goal is to overcome the shortcomings of aqueous supercapacitors such as narrow operating voltage and low energy density.Inspired by biomineralization technology,chitosan is used as a natural polymer skeleton to uniformly induce flower-like manganese phosphate crystals on its surface.Afterwards,the manganese tetroxide@nitrogen-phosphorus co-doped carbon conductive composite(Mn₃O₄@NPC)with a multi-intercalation structure is successfully prepared by annealing and hydrothermal methods.The composite is coated on carbon cloth to prepare a flexible electrode.Through electrochemical activation,the electrode deliveries a high potential window of 0-1.3 V(vs.Ag/Ag Cl)and a high specific capacitance of 370.8 F g^-1.Benefiting from the unique composite structure,Mn₃O₄@NPC also exhibits excellent rate performance and cycle stability.Studies have shown that the composite has a dual pseudocapacitance mechanism,which can effectively shift the oxygen evolution reaction(OER)to high potentials.On the other hand,electrochemical-reduced porous carbon(PC)that can work under the potential window of-1.3-0 V(vs.Ag/Ag Cl)is prepared based on high-gluten flour and KOH activator.Finally,the electrodes and PVA/Na₂SO₄ electrolyte are successfully assembled into a flexible aqueous asymmetric supercapacitor(FAAS)with a sandwich structure.The device has the characteristics of light weight and high flexibility,while also showing a high voltage window of 2.6 V and excellent rate performance and cycle stability.Its maximum energy density and max power density are up to 76.96 Wh kg^-1and 26.02 k W kg^-1,respectively,which are comparable to some of the organic-system supercapacitors and batteries.2.In the second part,to solve the problem of traditional electrode materials that are difficult to balance flexibility with good electrochemical performance,polymer substrate(cross-linked chitosan)is used to induce the assembly with highly conductive two-dimensional materials(MXene,Ti₃C₂T_x)through hydrogen bonds and electrostatic interaction to prepare free-standing film electrodes,Ti₃C₂T_x@Chitosan.The film exhibits well-defined ordered porous structure with three-dimensional ion transport channels.The structure,morphology,mechanical and electrochemical properties of Ti₃C₂T_x@Chitosan have been studied in detail.The introduction of chitosan network not only enhances the mechanical properties of the film,but also overcomes the self-accumulation of MXene nanosheets and the loss of electrochemical performance caused by it.At the same time,the high hydrophilicity of chitosan under acidic conditions ensures the full electrolyte infiltration of the Ti₃C₂T_x@Chitosan.The film exhibits a low mass loading dependence.Even when the mass loading reaches 4 mg cm^-2,the electrode can still contribute a high specific capacitance of 425.8 F g^-1 at 5m V s^-1.Owing to the rapid transmission of ions in the 3D ordered channels,even at a high scan rate of 2000 m V s^-1,Ti₃C₂T_x@Chitosan can still deliver a high specific capacitance of 243.2 F g^-1.Trasatti analysis method proves that the introduction of chitosan network skeleton and the 3D ordered porous structure can significantly improve the electrochemical active surface utilization rate of MXene nanosheets.Finally,a flexible symmetrical supercapacitor is assembled using a pair of Ti₃C₂T_x@Chitosan films as electrodes and PVA/H₂SO₄ hydrogel as electrolyte.The device achieves good flexibility and mechanical stability at low thickness(<400?m,capacitance loss is less than 5%after 180°bending 1000 times).It is worth noting that the device contributes a maximum energy density of 143.1?Wh cm^-2 and a maximum power density of 154.2 m W cm^-2,as well as excellent cycle stability.3.In the third part,we focus on the disadvantages of traditional PVA hydrogel electrolytes such as poor water retention and weak electrode/electrolyte interface,which makes it difficult to achieve three-dimensional flexibility(stretching,bending,and twisting).By introducing polydopamine(PDA)and hydroxypropyl methylcellulose(HPMC),which play the role of adhesion and mechanical enhancement,into the P(AAm-co-AA)hydrogel network skeleton,a new hydrogel electrolyte(HPMC/PDA/P(AAm-co-AA))with enhanced strength and self-adhesion properties is prepared.Multiple covalent and non-covalent crosslinks significantly improve the mechanical properties of the hydrogel and weaken its excessive swelling in electrolyte solution.The ionic conductivity of the hydrogel remains stable at a high value after repeated stretching.Moreover,the self-adhesive components significantly optimize the interface contact between hydrogel electrolyte and the flexible electrodes(free-standing film and carbon cloth).On the other hand,in order to further improve the potential window and energy density of MXene-based supercapacitors,a solvent-assisted self-assembly method is developed to prepare a PANI@Ti₃C₂T_x composite electrodes which achieves the uniform PANI distribution on the surface of Ti₃C₂T_x nanosheets.Furthermore,using PANI@Ti₃C₂T_x as the positive electrode,Ti₃C₂T_x@Chitosan as the negative electrode,and HPMC/PDA/P(AAm-co-AA)as the electrolyte,a FAAS was assembled.Due to the reasonable design of each component,the device exhibits excellent flexibility while achieving high energy storage performance(maximum energy density of 261.8?Wh cm^-2 and maximum power density of 89.5 m W cm^-2).The device can still retain 97%of the capacitance after 1000 times of 180°twisting without appearance damage.In summary,based on the cross-disciplinary research of polymer physics,polymer chemistry,physics and electrochemistry,this dissertation elaborates the advanced preparation technologies of high-performance flexible supercapacitors based on natural polysaccharide polymers.These technologies will pave the way for the development of flexible energy storage equipment,and have significant advantages and prospects.