(177b) Bio-Based Polymer Reinforced Thermo-Responsive Polyelectrolyte Hydrogels for Self-Protective Supercapacitors

Supercapacitors have attracted considerable attention in recent years due to their ability to store and deliver energy efficiently over a wide range of applications, from portable electronics to electric vehicles and renewable energy systems. In addition, supercapacitors have advantages over battery energy storage as they have higher power density and cycle stability. One of the key challenges in advancing supercapacitor technology is enhancing their mechanical strength and safety while maintaining excellent electrochemical performance, particularly in applications where they may be subjected to mechanical stress or temperature fluctuations. Electrolyte leakage and thermal runaway are also contributing challenges in supercapacitor development because these issues can compromise their performance and safety, especially in demanding environments. Thus, there is a growing study for mechanically robust solid electrolytes for supercapacitors, as they can potentially reduce leakage by maintaining their structural integrity, preventing electrolyte evaporation or seepage, and ensuring stable ionic conductivity.

Poly(N-isopropyl acrylamide) (PNIPAm) hydrogels have been used as solid polymer matrices for aqueous salt electrolytes due to its unique temperature-responsive behavior and swelling properties. These characteristics allow for non-restricted movement of salt ions, facilitate efficient ion transport, and thus improve ionic conductivity, which is essential for supercapacitors. Although PNIPAm hydrogels can be beneficial in many applications, the gels are typically fragile and lack the mechanical strength required for practical uses, including as polyelectrolytes for energy storage. Currently, PNIPAm hydrogels suffer from cracking or deformation under repeated mechanical stress due to poor mechanical strength. By incorporating mechanically strong plant-based polymers, such as lignin, as sustainable reinforcing agents, a semi-interpenetrating polymer network (sIPN) PNIPAm hydrogel with high compressive modulus and fatigue resistance properties can be developed. The PNIPAm-based hydrogel composite will also help prevent any aqueous electrolyte leakage that can hinder the long-term performance of the supercapacitor. This leakage prevention is due to the highly porous 3D hydrogel polymer structure and the hydrogen bonding interaction between unreacted hydroxyl groups of lignin and amide groups of PNIPAm with water molecules, which leads to water retention in the polymer network. Additionally, the phase transition ability of PNIPAm-based hydrogel can also regulate the ionic movement of the composite polyelectrolyte at different temperatures. Above the lower critical solution temperature (LCST), the polyelectrolyte becomes hydrophobic, creating phase-separated domains within the hydrogel that hinder the ion migration. While below the LCST, the gel is in a hydrophilic state, allowing efficient ions to flow in polyelectrolyte without restriction. This thermal-responsive phase transition behavior of the gel allows for controlling the ion conductivity by modulating the temperature below (high conductivity) or above (low conductivity) the LCST. As a result, the PNIPAm-based polyelectrolyte can potentially prevent overheating (thermal runaway) issues and catastrophic failure in energy storage devices—currently a key concern for batteries and supercapacitors—through temperature-conductivity control.

In this study, phenolated lignin-co-poly(N-isopropyl acrylamide) (PL-co-PNIPAm) copolymers are utilized as a reinforcing agent in PNIPAm-based hydrogel at different sodium chloride (NaCl) concentrations ranging from 0.10 – 0.5M. The temperature-responsive hydrogel polyelectrolyte is used together with activated carbon (AC) electrodes and stainless steel (SS) mesh current collectors to develop a self-protective supercapacitor. The physicochemical properties of the mechanically enhanced PNIPAm-based hydrogel electrolyte, such as chemical structure, morphological change, thermo-responsive behavior, and compressive modulus, are assessed using different characterization methods such as Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy with energy-dispersive x-ray (SEM-EDX) spectroscopy, transmittance measurements, and mechanical compressive testing. The electrochemical properties of assembled supercapacitors, including ionic conductivity, specific capacitance, energy density, and power density are evaluated via different techniques such as electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge-discharge (GCD), and stability after 1000 cycles are evaluated at a temperature range of 15 – 65 °C. The LCST and compressive modulus of PL-co-PNIPAm/PNIPAm/Am/NaCl-0.5 are ~ 17 °C and 36.08 kPa. The ionic conductivity and specific capacitance achieved for PL-co-PNIPAm/PNIPAm/Am/NaCl-0.5 are 42.3x10^-3 ± 7.6x10^-4 S/cm and 126.11 F/g at 10 mV/s, respectively. The research project holds exciting possibilities for its ability to develop self-regulated and mechanically enhanced PNIPAm-based hydrogel polyelectrolytes for supercapacitors. The knowledge gained in this study will allow for broader and more effective use of PNIPAm-based hydrogels in industries seeking innovative and eco-friendly supercapacitor applications without any mechanical and thermal runaway limitations.