Effect of Charge Density on the Mechanical and Electrochemical Properties of Poly (acrylic acid) Hydrogel Electrolytes Based Flexible Supercapacitors

Abstract

The conventional hydrogels have lack of structural sites which dissipate energy while applying load, this energy dissipation provides strength to hydrogel structure but due to shortage of energy dissipation mechanism, conventional hydrogels suffer from low stretchability and weak mechanical strength. To tackle this problem, much research has been done to improve the properties of hydrogel. Here, we report double network (DN) hydrogel electrolytes of poly (acrylic acid) synthesized through free radical mechanism. Ammonium persulfate (APS) and N, N-methylene bisacrylamide (MBA) were used as an initiator and crosslinking agent, respectively. Hydrogel electrolytes were prepared by soaking the as-prepared hydrogels into 1 M KCl, 1 M CaCl2, and 1 M FeCl3 salt solutions. Cations (K+, Ca2+, and Fe3+) ionically interacted with carboxylate ions of poly (acrylic acid) network to form double network AA (KCl), AA (CaCl2), and AA (FeCl3) hydrogel electrolytes. The synthesized hydrogel and hydrogel electrolytes were structurally characterized via Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) analysis. The surface morphology was observed using field emission scanning electron microscopy. The pore size of the hydrogel (AA) and hydrogel electrolytes was measured by Brunauer- Emmet-Teller (BET). Ionic conductivity study was conducted at room temperature and found that AA (KCl), AA (CaCl2), and AA (FeCl3) had ionic conductivity of 7.8 × 10-3, 10.4 × 10-3, 1.3 × 10-3 S/cm at room temperature, respectively. Furthermore, transport analysis was examined to observe the movement of ions in the hydrogel electrolytes. This analysis indicated the transference number of 0.995, 0.997, and 0.902, respectively. The hydrogel electrolytes were used to fabricate symmetric electric double capacitors (EDLCs) (AC/AA (KCl)/AC, AC/AA (CaCl2)/AC, AC/AA (FeCl3)/AC) and electrochemical performance such as cyclic voltammetry (CV) and galvanic charge discharge (GCD) was performed. AC/AA (CaCl2)/AC achieved highest performance with a specific capacitance of 158 F/g at 3 mV/s and 144.65 F/g at 0.50 A/g. This cell obtained energy density of 19.54 W h/kg at a power density of 493.29 W/kg. The life cycle of the AC/AA (CaCl2)/AC was accomplished for 5000 reversible charge/discharge cycles at 5 A/g and the cell retained 99.93 % capacitance. In addition, two symmetric cells were connected in series to power up the 2 V light emitting diode (LED). Our results offer a new design strategy to improve the strength of DN hydrogel electrolytes by controlling the structure and interactions in the second network. We hope that this work provides an alternative view for the design of tough hydrogels with desirable properties and is useful for smart flexible electronic devices.