Research And Development Of Ferric-based Self-healing Hydrogel And Its Supercapacitor

Wearable devices are prone to the superposition of different external stresses,such as bending,twisting and even breaking during operation,which make the related energy storage devices with great safety and reliability to be highly demanded.Compared to batteries,supercapacitors can be great alternative energy storage devices due to their high power density and long cycle life.In addition,hydrogels have been introduced into supercapacitors to afford them multi-functions,especially the flexibility and self-healing.However,there are still considerable challenges in improving the energy density of supercapacitors and endowing them with multi-functions.In view of this,this work proposed novel ideas in the preparation and design of ferric-based hydrogel electrolytes for such functional supercapacitors.Hydrogel electrolyte with excellent electrochemical activity,mechanical properties and self-healing efficiency were prepared by combining the redox activity of the ferric ions with their simultaneous formation of dynamic metal-ligand coordination bonds.The main works were given as follows:（1）Redox active hydrogel electrolytes based on polyvinyl alcohol/sulfuric acid/polyvalent mixed iron ions(PVA/H₂SO₄/Fe^2+/3+)were prepared and used to enhance the electrochemical performance of the half-cell system of carbon-based flexible electrodes.In this section,unique carbon nanocoils/stainless steel flexible electrode（CNC/SS）was prepared by a flame deposition method recently developed in our laboratory.The growing mechanism of CNC without involving additional catalyst were proposed and their electrochemical behaviors were investigated in PVA/H₂SO₄/Fe^2+/3+electrolyte.The asymmetric supercapacitor assembled with the half-cell system exhibited an area specific capacitance of 1502 m F/cm² and an energy density of 0.252 m W h/cm² at the power density of 1.6 m W/cm²,which was better than most reported carbon-based supercapacitors.The experimental results confirmed the high redox activity of the Fe^2+/3+based hydrogel electrolyte and the flexibility of the devices assembled with them.（2）Design and fabrication of a dual-network self-healing gel electrolyte based on metal-ligand and hydrogen bonding interactions.To endow the redox active ferric ion-based hydrogel electrolyte with more functions,a PVA/Phytic acid（PA）/Fe³⁺hydrogel electrolyte（PPFe）was prepared by the"freezing-thawing"method.Research indicated that many properties of the PPFe hydrogel,including the mechanical properties,ionic conductivity,self-healing efficiency and the capacitance performance,were all dependent on the Fe³⁺concentration in the hydrogel.It is found that its self-healing efficiency was first increased and then decreased with increasing Fe³⁺concentration.Specifically,0.6 M Fe³⁺PPFe hydrogel showed the highest strain repair efficiency（72%）,fracture stress（0.618 MPa）,fracture strain（924%）and ionic conductivity（19.2 m S/cm）.This design utilized the redox active Fe³⁺as the coordination centers to build the metal-ligand coordination bonds,which provided a new strategy for the functionalization of PVA hydrogel electrolytes and the effective utilization of Fe³⁺simultaneously.（3）Inspired by the previous two sections,hydrogel electrodes with both self-healing and redox activities were developed and used to assemble PPFe hydrogel-based supercapacitors（PPFe HSCs）with both the promoted energy density and self-healing capability.By the combination of PPFe hydrogel electrolyte with polyaniline/carbon nanotube（PANI/CNT）composites,PPFe@PANI/CNT hydrogel electrodes were prepared.The hydrogel electrodes were presented in a silly putty state with excellent remodeling capability,which facilitated their recovery and re-utilization.They exhibited good mechanical properties,including a tensile stress of 0.267 MPa and fracture strain of 110%.In addition,the hydrogel electrode displayed a strain repair efficiency of 41%,which could be due to the retained dynamic cross-linking behavior originating from the metal-ligand and hydrogen bonding interactions in the composite hydrogel.More importantly,PPFe HSCs assembled with the optimized electrode displayed a specific capacitance of 1526 m F/cm²and energy density of 0.212 m Wh/cm²,which was much larger than that of the device without the incorporation of Fe³⁺.Also,92%of its original capacitance could be retained after 10000charging-discharging cycles.Moreover,the PPFe HSCs could recover 83%of its original specific capacitance after 5 healing cycles,demonstrating that the redox activity of Fe³⁺in the HSCs was exploited while ligating with other organic components in the hydrogel system at room temperature,thus realizing the self-healing capability and the high energy storage performance of the devices.