Flexible and Self-Healing Aqueous Supercapacitors for Low Temperature Applications: Polyampholyte Gel Electrolytes with Biochar Electrodes

Abstract

A flexible and self-healing supercapacitor with high energy density in low temperature operation was fabricated using a combination of biochar-based composite electrodes and a polyampholyte hydrogel electrolyte. Polyampholytes, a novel class of tough hydrogel, provide self-healing ability and mechanical flexibility, as well as low temperature operation for the aqueous electrolyte. Biochar is a carbon material produced from the low-temperature pyrolysis of biological wastes; the incorporation of reduced graphene oxide conferred mechanical integrity and electrical conductivity and hence the electrodes are called biochar-reduced-graphene-oxide (BC-RGO) electrodes. The fabricated supercapacitor showed high energy density of 30 Wh/kg with ~90% capacitance retention after 5000 charge–discharge cycles at room temperature at a power density of 50 W/kg. At −30 °C, the supercapacitor exhibited an energy density of 10.5 Wh/kg at a power density of 500 W/kg. The mechanism of the low-temperature performance excellence is likely to be associated with the concept of non-freezable water near the hydrophilic polymer chains, which can motivate future researches on the phase behaviour of water near polyampholyte chains. We conclude that the combination of the BC-RGO electrode and the polyampholyte hydrogel electrolyte is promising for supercapacitors for flexible electronics and for low temperature environments.

Direct visualization of nano and microscale polymer morphologies in as-prepared and dialyzed polyampholyte hydrogels by electron microscopy techniques

“…Here, microscopic cellular structures were observed in the DIW-dialyzed sample, whereas there is no visible macroscopic structure in the KOH-dialyzed sample. This morphologic observation is consistent with high mechanical strength [21,22] and high low-temperature ionic conductivity, [16,20] respectively, reported in previous literatures.…”

“…[14] Moreover, the upper critical solution temperature of polyampholyte hydrogel can be manipulated by tuning the strength of ionic interactions. [15] Based on these unique properties of ionic cross-linking, polyampholyte hydrogels have been used as electrolyte for flexible and selfhealing supercapacitors, [16] and as an optically modulating substance for thermosensitive smart windows. [17] Such diverse applications of polyampholyte hydrogels are possible by controlling morphology.…”

“…[21,22] Another example is dialysis in aqueous solution of potassium hydroxide (KOH) to introduce mobile ions in the hydrogel as the charge carriers for the ionic conduction to achieve gel electrolyte for electrochemical energy storage devices. [16] While it is well known that polymer structures govern the physical and chemical properties of hydrogels, the morphologic impact on the dialysis processes in different solutions have not been studied to date.…”

The synthesis and performance analysis of various biomass‐based carbon materials for electric double‐layer capacitors: A review

“…In the process of low‐rate charge and discharge, ionic electrolyte is easier to transfer in the pores with large sizes, making full use of the surface area of carbon . A certain amount of mesopores and macropores can be necessary for the rapid diffusion of electrolyte ions, which not only can effectively improve the accessibility of micropores, also serve as a channel for rapid diffusion of ions, promoting the rapid transport of electrolyte ions, thereby greatly improving electrochemical performance of SCs . Mi et al observed the equivalent series resistance decreased with the increase of V meso / V total of the porous carbons, which led to a high capacitance.…”

“…87 A certain amount of mesopores and macropores can be necessary for the rapid diffusion of electrolyte ions, which not only can effectively improve the accessibility of micropores, also serve as a channel for rapid diffusion of ions, promoting the rapid transport of electrolyte ions, thereby greatly improving electrochemical performance of SCs. 159,160 Mi et al 34 observed the equivalent series resistance decreased with the increase of V meso /V total of the porous carbons, which led to a high capacitance. HPCs have a higher specific capacitance and capacity retention ratio at a large current density than the porous carbon having a single microporous structure.…”