Cellulose nanofibrils-reduced graphene oxide xerogels and cryogels for dielectric and electrochemical storage applications

Highlights

• CNF-RGO composites of different density and porosity (size and orientation) are prepared.

• Dielectric properties are investigated at highest frequency of 1 MHz.

• Denser and layer-structured film gave temperature dependent dielectric properties.

• Aligned micro-porous structure gave high specific capacitance and lower electrochemical resistance.

Abstract

The effect of structure (density and porosity) and composition (ratio) of composites prepared from cellulose nanofibrils (CNF) and reduced graphene oxide (RGO) as film-like xerogels and porous cryogels, respectively, were investigated related to their dielectric performance and electrochemical storage abilities. A very small content (5 wt%) of RGO incorporated xerogelic film of a denser and layered structure shows relatively high dielectric constant of ∼85 and ∼164 and an ac electrical conductivity of 0.0028 S/m² and 0.0072 S/m² at 270 K and 370 K, respectively, at a high frequency of 1 MHz showing their suitability for high temperature dielectric storage applications. On the other hand, a high specific capacitance of ∼460 F/g and electrochemical resistance of 12–15 Ω cm² was observed for well-aligned and longitudinal-oriented micro-porous cryogel, indicate on its suitability as electrodes for electrochemical storage devices.

Introduction

Flexible and light weight energy storage devices are crucial to meet the increasing demand of energy and to compete with the current issues like global warming and natural energy depletion. Polymeric composites with high dielectric constant have, thus, been demanded increasingly compared to ceramic based composites [1] for energy storage devices being used in microelectronics [2], electric actuators [3], electromagnetic interference shielding [4], embedded capacitors [5], flexible electronics [6], as well as gas sensing and supercapacitor applications [7].

Much attention has been paid recently to the polymeric composites with incorporated highly conducting nanofillers, especially graphene or carbon nanotubes, because of their good electrical properties and high aspect ratio [[8], [9], [10], [11]]. The unique honeycomb structures, as well as cheap and easy processing, makes graphene sheets even more favourite. Great intention has thus been made to further enhance the dielectric properties of graphene filled composites [6,[12], [13], [14], [15]]. Recent reports have lightened up also with graphene filled cellulose-based composites [[16], [17], [18], [19], [20], [21], [22], [23]]. However, the effect of cellulose based dielectric composites regarding temperature and frequency has not been much explored so far [21]. Furthermore, research was focused mainly on the low frequency region of the dielectric properties, while studies in the higher frequency region to explore the real material properties have been presented recently only by our study [19].

Since the energy storage properties of polymer composites is dependent mainly on the homogeneous dispersion of the additive, interfacial interaction, surface area and pore structure, several kinds of strategies were used to alter these parameters. Cellulose composites incorporated with graphene derivatives were reported as both paper [24,25] and three-dimensional porous structures [26,27] for supercapacitor electrode applications. However, to achieve an outstanding performance of material for such an application, the transfer of ions through the electrolyte must be stable and continue to ensure the effective diffusion or mass transfer through it [28]. The large specific surface area (∼2630 m² g⁻¹) and electronic transport of graphene sheets [29] with the advantage of long and entangled Cellulose Nanofibrils (CNFs) to form a differently dense and porous network with a fibrillar morphology and strong molecular interactions [30,31], may be a promising combination to attain the tremendous electrochemical storage properties. The renewable and lightweight nature of CNFs with huge mechanical strength (2–6 GPa) and high Young's modulus (ca. 138 GPa), as well as low thermal expansion coefficient (0.1 ppm/K) [[32], [33], [34]] is an additional argument.

In this study, CNFs and Reduced Graphene Oxide (RGO) sheets in various ratios were thus used to prepare film-based xerogels (by vacuum filtration and room drying) and porosity-structured cryogel composites (by one-directional freezing and lyophilisation), respectively. At the same time, this study aims the potential integration of energy storage technologies as dielectric papers and electrochemical electrode materials. There are still no reports on high temperature dielectric properties of cellulose and graphene based composites for potential applications in high temperature dielectric storage technologies as dielectric papers, which is very important for their usage in temperature related applications [35]. In addition, the comparison of electrochemical properties of xerogels and cryogel composites having same composition but prepared at different freezing temperatures and having different bulk structure (density and porosity) is original compared with the existing literature. The composites were characterised by X-Ray Diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopies, and Scanning Electron Microscope (SEM) imaging. The temperature dependence of dielectric properties using different frequencies (100 Hz, 1 KHz, 10 KHz, 100 KHz and 1 MHz) were studied systematically, and the samples were compared related to their electrochemical charge storage properties evaluated by Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS).