The Role of Morphology in Optimized Electrochemical Performance of KOH-Activated Cellulose for Supercapacitor Applications
Authors: Ibrahim Abouelamaiem Dina , Belen Jorge Ana , Titirici Maria-Magdalena , Shearing Paul R. , Brett Daniel J.L. 
Published: February 2020

Supercapacitors, also known as ultracapacitors or electrochemical capacitors, have recently attracted significant attention, mainly due to their long cycle life and high power density, in turn bridging the energy/power gap between dielectric capacitors (high power input) and fuel cells/batteries (high energy storage) (1). One of the functions of a supercapacitor is to compliment the power dispatchibility of fuel cells and batteries in hybrid systems, by providing the necessary power for acceleration and recuperating brake energy (2). Further developments in this device have shown that it can play an important role in coupling batteries and fuel cells in energy storage functions by offering back-up power supplies to protect systems against power disruption (3). However, the disadvantages of a supercapacitor including high production cost and low energy density have still to be overcome. Development of new materials for supercapacitor electrodes has been investigated to resolve the obstacle of low energy density in which the most widely used ones are carbon-based materials. Activated carbons, in various forms, are believed to lead the next generation of supercapacitors due to their versatile properties including low cost, high electronic conductivities, controllable porosity and high surface area values (4).

In this study, we focus on the activation of bio-based cellulosic materials with potassium hydroxide (KOH) and the optimization of the KOH/cellulose ratio to yield the best electrochemical performance. The synthesis of electrodes with high specific capacitances and power densities has been achieved upon optimizing the porous morphology of the carbons via activation with different KOH loadings. Various characterization techniques of the carbons were carried including transmission electron microscopy, x-ray tomography, mercury intrusion porosimetry and nitrogen adsorption-desorption isotherms. The effect of each of the specific surface areas, pore size distribution, porosity and tortuosity on the electrochemical performance was then investigated in three-electrode systems and coin cells. The samples with KOH/cellulose ratios of 0.5:1 and 1:1 proved to acquire the best performance with specific capacitances as high as 187 F g-1 at current density of 1 A g-1 and a retention rate of 72%. This was attributed to the hierarchical porous network structure, high surface areas and low cell resistances. It was established that a well-balanced porous network structure of micro-, meso- and macro-pores is crucial for achieving high electrochemical performance, specifically at low current densities.

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Figure 1