Study On The Preparation And Performance Of Plant-based Hydrogel Supercapacitors

Supercapacitor is an important electrochemical energy storage device,which has attracted much attention because of its fast charging and discharging capability,long cycle life and high power density.Conventional supercapacitors mainly use liquid electrolytes,and supercapacitors prepared from them are prone to deformation or damage under external forces.There is a risk of toxic and flammable electrolyte leakage.Compared with liquid electrolytes,solid electrolytes,especially gel-type polymer electrolytes,have high ionic conductivity,high energy density,high dielectric constant and high safety,and have become a hot research topic in the field of capacitors.In this thesis,a gel-type polymer electrolyte containing three conductive metal ions,Ca²⁺,Fe³⁺and Zn²⁺,is prepared by protonation of plant-based materials sodium carboxymethyl cellulose（CMC）and Lignin using citric acid（CA）.Protonation reaction induces hydrogen bonding cross-linking of carboxyl groups by freezing process,and then high-valent metal ions further crosslink the polymers.The microscopic morphology,interfacial properties,electrochemical properties,mechanical properties and degradation properties of the hydrogel electrolytes are characterized by SEM,FT-IR,EIS,CV,mechanical tests,soil burial and other tests.The effects of CMC concentration content,cross-linking conditions and metal ion content on the performance of polymer electrolytes are investigated and the mechanism is studied.Experimental results of the CMC/Ca²⁺gel polymer electrolyte show that the compressive strength of the hydrogel is improved by the addition of Ca²⁺,and the maximum compressive fracture stress of the prepared hydrogel is 4.32 MPa when the Ca²⁺concentration is 25 wt%and the citric acid concentration is 10 wt%,and the hydrogel can be completely degraded within 8 days at room temperature.In addition,the maximum energy density of the assembled supercapacitor is 63.3 Wh·kg^-1at a power density of 1 080.0 W·kg^-1,the specific capacitance is 87.08 F·g^-1at a current density of 0.5 A·g^-1.The capacitance retention rate reachs 93.9%after 5 cuttings and92.0%after 1000 charge/discharge cycles.Experimental results of the Lignin/CMC/Ca²⁺/Fe³⁺hydrogel polymer electrolyte show that with two kinds of ions,the compressive stress,ionic conductivity and specific capacitance reaches the highest level,which is 2.35 MPa,50.13 m S·cm^-1and166.04 F·g^-1,respectively.When the Lignin content was 0.8 wt%and the Fe³⁺concentration is 1.0 M the maximum compressive strength of the prepared hydrogels is 2.35 MPa and they can be completely degraded within 21 days at room temperature.At this time,the assembled supercapacitor has a maximum energy density of 119.0Wh·kg^-1at a power density of 1 173.6 W·kg^-1and a specific capacitance of 166.04F·g^-1,at a current density of 0.3 A·g^-1.The capacitance retention rate after 5 cuttings reaches 92.5%,and also has excellent cycling performance with a capacitance retention rate of 94.1%after 1,000 consecutive charge/discharge cycles.Experimental results of the CMC/Zn²⁺gel polymer electrolyte showed that the maximum compressive fracture stress of the hydrogel prepared at Zn²⁺concentration of 1.0 M is 2.33 MPa and can be completely degraded within 13 days at room temperature.The maximum energy density of the assembled supercapacitor is 785.7Wh·kg^-1at a power density of 1 621.2 W·kg^-1,and the specific capacitance is 485.38F·g^-1at a current density of 0.5 A·g^-1,showing excellent energy storage performance.The capacitance retention rate is 80.0%after 10,000 charge/discharge cycles,and the capacitance retention rate reached 82.9%after 5 cuttings.