In recent years, significant research has aimed at developing environmentally friendly supercapacitors by introducing sustainable natural polymerse. This review summarizes the advances on electrically active cellulose-based devices and the challenges of this exciting field of study.

Biosynthetic poly-γ-glutamic acid (-PGA) has been used to produce hydrogels using cystamine as cross-linker and 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide methiodide (EDC methiodide) as condensing agent. Eight different hydrogels with different properties were formulated by varying both the molecular weight of -PGA and the -PGA/EDC/cystamine ratio, and subsequently characterized. The most appropriate γ-PGA hydrogel was selected to perform as solid electrolytic medium in organic electrochemical supercapacitors (OESCs) using poly(3,4ethylenedioxythiophene) (PEDOT) electrodes based on their mechanical behaviour (consistency and robustness to hold the PEDOT electrodes), morphology, and influence on the electrochemical response of the organic electrode (i.e. specific capacitance, and both maximum energy and power densities values). Hence, PEDOT/γ-PGA energy storage devices fabricated using the most adequate hydrogel formulation displayed a supercapacitor response of 168 F/g and a capacitance retention of 81%. Moreover, after evaluating the maximum energy and power densities (Ragone plot), cyclability, longterm stability, leakage-current, and self-discharging response of PEDOT/γ-PGA OESC devices, results allow us to highlight the merits and great potential of γ-PGA hydrogels as sustainable ion-conductive electrolytes for environmentally friendly energy storage technologies. Our daily activities have become dependent on the available energy supply. Indeed, portable energy storage devices have become essential, not only for operating laptops and smart cell phones, but also for other electronic and electrical systems used in motor vehicles, satellites, sensors, or medical and military equipment. Among other features, these applications require energy storage devices to be cost-affordable, light-weight, durable (long-term stability) and, if possible, sustainable by using eco-friendly materials. Although electrolytic/ceramic batteries and capacitors are both used for energy storage applications, these devices differ in the power and energy densities they can provide. Thus, batteries are characterized by high energy density values (10-100 Wh/kg), while capacitors display high power density values (i.e. they can release the stored energy much faster). 1 In low-energy applications (e.g. motor starting, signal processing and sensing) or when fast pulsed power supply is needed (e.g. electromagnetic forming, Marx generators, pulsed lasers, and particle accelerators, among others), capacitors offer a series of advantages, such as cost-effectiveness, faster charging-discharging times, and long-lasting cycling stability. Electrochemical supercapacitors (ESCs) have emerged as promising energy storage devices displaying higher energy values (i.e. 1-10 Wh/kg for carbon ESCs) than conventional capacitors (i.e. < 0.1 Wh/kg), evidencing a potential future use in largescale high-energy applications. 2 In ESCs, the electric double-layers formed at the electrode-electrolyte interface (i.e. Helmholtz and diffuse layers) contribute to ...

Different carboxymethyl cellulose sodium salt (NaCMC)-based pastes and hydrogels, both containing a salt as supporting electrolyte, have been prepared and characterized as potential solid state electrolyte (SSE) for solid electrochemical supercapacitors (ESCs).The characteristics of the NaCMC-based SSEs have been optimized by examining the influence of five different factors in the capacitive response of poly(3,4-ethylenedioxythiophene) (PEDOT) electrodes: i) the chemical nature of the salt used as supporting electrolyte; ii) the concentration of such salt; iii) the concentration of cellulose used to prepare the paste; iv) the concentration of citric acid employed during NaCMC cross-linking; and v) the treatment applied to recover the supporting electrolyte after washing the hydrogel. The specific capacitance of the device prepared using the optimized hydrogel as SSE is 81.5 and 76.8 F/g by means of cyclic voltammetry and galvanostatic charge/discharge, respectively, these values decreasing to 60.7 and 75.5 F/g when the SSE is the paste.

Review: 143 refs.