Rebirth of Coffee-Wiped Wastepaper: Flavor Activation of Cellulose for Eco-Friendly/Cost-Competitive Carbon Electrode Materials
Authors: Lee Dong-Gue , Kim Jung-Hwan , Kim Se-Hee , Kim Ju-Myung , Lee Sang-Young 
Published: February 2020

Forthcoming smart energy era, which will find widespread use of flexible/wearable electronics, electric vehicles, and stationary electricity storage systems, is in incessant pursuit of advanced rechargeable power sources with reliable/sustainable electrochemical performance. Among a wide variety of functional materials for use in energy storage systems, cellulose can be suggested as a promising candidate due to its natural abundance, recyclability, and low cost. Among numerous application fields of cellulose, cellulose-mediated activated carbons have been extensively investigated as an alternative electrode active material for energy storage systems including electric double-layer capacitors (EDLCs), A primary requirement of the activated carbons is the acquisition of well-defined microporous structure (pore size < 2 nm) with high specific surface area. To achieve this goal, chemical activation agents based on potassium ion (K+)-containing salts (e.g., potassium hydroxide (KOH)) are combined with cellulose. The salts play a viable role in catalyzing dehydration and decomposition reaction in initial pyrolysis stage, eventually facilitating pyrolysis kinetics. However, the chemical hazards and environmental pollution of the chemical activation agents remain formidable obstacles for their widespread use in the carbonization process.

In our daily lives, when we spill coffee on table, clothes or any substances, a paper towel is often used to wipe the spilled one. Here, inspired by such daily happenings, we demonstrate a new class of coffee-mediated flavor activation as a new eco-friendly strategy to address the aforementioned longstanding issue of chemical activation in the cellulose carbonization. Simply describing, wiping coffee with a paper towel is anticipated to open a new route for synthesis of cellulose-mediated activated carbon materials. Coffee, one of the most popular drinks in the world, is known to contain potassium ion (K+) as a core element. Espresso, which is known to have the highest content of K+ in a variety of coffee commercially available, is selected as an eco-friendly alternative chemical activation agent for cellulose carbonization. Kimwipes®, one of the most popular paper towels, is chosen as a model material of cellulose.

A piece of Kimwipes was soaked into the espresso solution and then subjected to heat treatment at 600 oC for 2 h, leading to the preparation of (espresso-assisted) carbonized Kimwipes (referred to as “c-EK”). As a control sample, simply carbonized Kimwipes without the aid of espresso (denoted as “c-K”) was synthesized. Structural uniqueness of the c-EK was investigated, with a particular focus on K+-induced micropore formation. The introduction of espresso led to the significant increase in the specific surface area of c-EK (= 255.78 m2 g-1) over the control sample (=198.89 m2 g-1). Moreover, the well-developed peaks (assigned to micropores) in the N2 adsorption-desorption isotherms were clearly observed at the c-EK. In addition to the progress in the microporous structure, the c-EK exhibited the process advancement in pyrolysis kinetics than the c-K, including the lower decomposition temperature and also higher yields of activated carbon. The aforementioned results demonstrate the advantageous effects of espresso as a new environmentally-benign chemical activation agent. Subsequently, the potential applicability of the c-EK as an alternative electrode active material for supercapacitors (SCs) was explored. The SC cell assembled with the c-EK showed the significant improvement in capacity (= 131 F g-1 at a scan rate of 1 mV s-1) and cycling performance (over 10,000 cycles) far beyond those achievable with a control cell containing the c-K.

Spurred by the structural/performance superiority of the c-EK as an exceptional activated carbon material, we developed all-paper flexible SCs that were composed of c-EK/CNT electrodes, Kimwipes separator, and PVA/KOH gel electrolyte. The physical compliance of Kimwipes, in combination with the highly-reticulated CNT networks in the electrodes, enabled the resultant paper SCs to maintain good electrochemical performance upon being subjected to mechanical deformation such as bending and folding. Notably, no appreciable loss in the capacity and cell resistance was observed even after repeated bending over 10,000 cycles.

We envision that the “wiping coffee with Kimwipes” strategy proposed herein, specifically, espresso-assisted flavor activation of cellulose for activated carbons and also all-paper flexible SCs, is highly facile and also effective, thus opening up a new opportunity for eco-friendly/cost-competitive materials/systems in urgent need for next-generation power sources.