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Coffee-Driven Green Activation of Cellulose and Its Use for All-Paper Flexible Supercapacitors Donggue Lee, Yoon-Gyo Cho, Hyun-Kon Song, Sang-Jin Chun, Sang-Bum Park, Don-Ha Choi, Sun-Young Lee, JongTae Yoo, and Sang-Young Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017

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ACS Applied Materials & Interfaces

Coffee-Driven Green Activation of Cellulose and Its Use for All-Paper Flexible Supercapacitors Donggue Lee,† Yoon-Gyo Cho,‡ Hyun-Kon Song,‡ Sang-Jin Chun,§ Sang-Bum Park,§ Don-Ha Choi,§ Sun-Young Lee,*§ JongTae Yoo,*† and Sang-Young Lee*† †Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea ‡Department of Chemical Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea §Department of Forest Products, Korea Forest Research Institute, Seoul 02455, Korea

Correspondence and requests for materials should be addressed to S.-Y. Lee§ (email: [email protected]), J. Yoo† (email: [email protected]), and S.-Y. Lee† (email: [email protected])

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ABSTRACT Cellulose, which is one of the most abundant and renewable natural resources, has been extensively explored as an alternative substance for electrode materials such as activated carbons. Here, we demonstrate a new class of coffee-mediated green activation of cellulose as a new environmentally benign chemical activation strategy and its potential use for allpaper flexible supercapacitors. A piece of paper towel is soaked in espresso coffee (acting as a natural activating agent) and then pyrolyzed to yield paper-derived activated carbons (denoted as “EK-ACs”). Potassium ions (K+), a core ingredient of espresso, play a viable role in facilitating pyrolysis kinetics and also achieving a well-developed microporous structure in the EK-ACs. As a result, the EK-ACs show significant improvement in specific capacitance (= 131 F g−1 at a scan rate of 1.0 mV s−1) over control ACs (= 64 F g−1) obtained from the carbonization of a pristine paper towel. All-paper flexible supercapacitors are fabricated by assembling EK-ACs/carbon nanotube mixture-embedded paper towels (as electrodes), polyvinyl

alcohol/KOH

mixture-impregnated

paper

towels

(as

electrolytes),

and

polydimethylsiloxane-infiltrated paper towels (as packaging substances). The introduction of the EK-ACs (as an electrode material) and the paper towel (as a deformable/compliant substrate) enables the resulting all-paper supercapacitor to provide reliable/sustainable cell performance and also exceptional mechanical flexibility. Notably, no appreciable loss in the cell capacitance is observed after repeated bending (over 5,000 cycles) or multiple folding. The coffee-mediated green activation of cellulose and the resultant all-paper flexible supercapacitors open new material and system opportunities for eco-friendly highperformance flexible power sources KEYWORDS: Wastepaper, Paper towels, Carbonization, Coffee, Activating agents, Flexible electrode, Supercapacitors


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1. Introduction Forthcoming flexible/wearable electronics era has spurred a relentless pursuit of advanced rechargeable power sources with reliable electrochemical performance, various form factors, and cost competitiveness1-3. Numerous approaches4-7, which focus mainly on the synthesis of new electrochemically-active8 materials and design of unique cell architecture9,10, have been undertaken to address the aforementioned challenging issues. Among the vast variety of energy materials reported to date, cellulose, which is one of the most abundant and renewable natural resources, has been extensively investigated as a low-cost and versatile building block for potential use in electrode materials, binders, separator membranes, and current collector substrates11-15. One interesting application field of cellulose is as a precursor source for activated carbons (ACs)16-18. The primary prerequisites for ACs to be used in supercapacitor electrode materials include a highly developed microporous (pore size < 2 nm) structure and large specific surface area19,20. However, the carbonization of pristine cellulose often fails to fulfill the aforementioned structural requirement. In an endeavor to resolve this problem, chemical activating agents based on potassium ion (K+)-containing salts (e.g., potassium hydroxide (KOH)) have been exploited21-22. KOH-based salts are known to facilitate a dehydration/decomposition reaction in the initial stage of activation, thus exerting a beneficial influence on the formation of the microporous structure23-25. However, the chemical toxicity and environmental hazards of the KOH-based salts, along with complex multi-step activation processes, pose formidable obstacles for the development of low-cost/high-performance ACs. In our daily lives, a paper towel is often used to wipe up the spilled coffee or drinks. Intriguingly, coffee is known to contain potassium ions (K+) as a core ingredient26. Here, intrigued by the facile acquisition and unique physicochemical features of paper towels and coffee, we demonstrate a new class of all-paper flexible supercapacitors based on coffee3 ACS Paragon Plus Environment


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mediated green activation of cellulose as a scalable and green material/electrochemical system strategy. Kimwipes® and espresso (possessing the highest amount of K+ ions among readily accessible coffees) are chosen as a paper towel and a natural activating agent, respectively. The K+ ions in espresso play viable roles in facilitating pyrolysis kinetics of coffee-soaked paper towels and also synthesizing ACs (hereinafter denoted as “EK-ACs”) with a highly developed microporous structure, eventually providing higher specific capacitance (= 131 F g−1 at a scan rate of 1.0 mV s−1) than that of control ACs (= 64 F g−1) obtained from the carbonization of a pristine paper towel (not soaked in espresso). The EK-ACs, together with single-walled carbon nanotubes (SWNTs), are incorporated into paper towels to produce paper electrodes. Subsequently, the paper electrodes are symmetrically assembled with polyvinyl alcohol (PVA)/KOH mixtureimpregnated paper towels (acting as electrolytes) and polydimethylsiloxane (PDMS)infiltrated paper towels (as packaging substances), leading to the fabrication of all-paper flexible electric double-layer supercapacitors. Benefiting from the EK-ACs as an electrode material and the paper towels as a deformable/compliant substrate, the resulting all-paper supercapacitor exhibits good cell performance and mechanical flexibility. Notably, the cell capacitance is not impaired after repeated bending (over 5,000 cycles) or after multiple folding. 2. Experimental 2.1. Espresso-assisted synthesis of activated carbons from wastepaper Espresso was brewed from an espresso pot using the ground coffee beans and deionized (DI) water. A piece of paper towel was soaked into the espresso coffee and then dried in a convection oven at 120 ˚C for 6 h. The espresso-soaked paper towel was carbonized at 600 ˚C for 10, 30, 120, and 180 min under an N2 atmosphere. The as4 ACS Paragon Plus Environment

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synthesized EK-ACs were then subjected to ball milling, yielding the EK-AC powders. The control sample (i.e., K-ACs obtained from the pristine Kimwipes® without the espresso treatment) was prepared using the same carbonization process mentioned above. 2.2. Fabrication of all-paper flexible supercapacitors The electrode suspension was prepared by mixing the EK-ACs with a 1 wt% SDBScontaining SWNT aqueous dispersion. Subsequently, a piece of paper towel was repeatedly dipped (= 10 times) into the above-prepared electrode suspension and followed by rinsing with DI water and isopropyl alcohol (IPA). To prepare the PVA/KOH polymer electrolyte, the PVA powders were added to the 6 M KOH aqueous solution under vigorous stirring at 85 ˚C, in which the composition ratio of PVA/KOH was 6/4 (w/w). A piece of paper towel was impregnated with the as-prepared PVA/KOH solution, resulting in the paper towel-reinforced PVA/KOH composite polymer electrolyte. The non-porous paper packaging substances were fabricated by infiltrating the PDMS solution (silicone/curing agent = 10/1 (w/w)) into a paper towel, followed by heat treatment at 60 ˚C for 3 h. The above-prepared paper electrodes (after being treated with the PVA/KOH polymer electrolyte acting as an ion conductor inside the electrodes) were assembled with the paper-reinforced composite polymer electrolyte. Subsequently, the paper electrode-paper electrolyte assembly was inserted inside the PDMSinfiltrated paper packaging substance and followed by high-temperature sealing at 160 ˚C, resulting in the fabrication of the all-paper flexible electric double-layer supercapacitor.

3. Results and discussion The overall synthetic procedure of the EK-ACs is conceptually illustrated in Figure 1A. The espresso was prepared using a typical steam pressure-driven coffee machine. The amount of K+ ions in the espresso was quantitatively analyzed and compared with those of 5 ACS Paragon Plus Environment


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other control samples (Figure 1B (left)). The as-prepared espresso exhibited the largest amount of K+ ions (= 2112 mg L−1), orders of magnitude higher than the amount of K+ ions in water (= 3 mg L−1) and caffè Americano (= 251 mg L−1). A piece of paper towel (i.e., Kimwipes®, Figure 1B (center and right)) was soaked in espresso and then subjected to carbonization at 600 ˚C for 2 h, eventually yielding the EK-ACs. The carbonization temperature chosen herein was the same as that used for the previously reported ACs derived from cellulose-based materials17,27. The scanning electron microscope (SEM) images (Figure 1C) of the EK-ACs show that the fibrous morphology of the paper towel was not disrupted after the carbonization. The presence of K+ ions in the EK-ACs was verified by the energy dispersive spectroscopy (EDS) mapping image (inset of Figure 1C). A large amount of elemental K (represented by red dots) was uniformly dispersed on the carbonized fibers. In contrast, elemental K was not detected in the control ACs (denoted as “K-ACs”, synthesized without the espresso treatment) (Supporting Information, Figure S1). The characteristic Raman spectra28 assigned to the Gband (1593 cm−1, graphite-like in-plane mode) and D-band (1344 cm−1, disorder-induced mode) were observed in the EK-ACs (Figure 1D (top)), verifying the formation of a graphitic carbon structure. Meanwhile, the archetypal X-ray diffraction (XRD) peaks at ~ 23 ˚ (corresponding to (002) plane) and ~ 44 ˚ ((100) plane) exhibited the disordered state of the graphitic carbons29-31 (Figure 1D (bottom)). Thermal decomposition (i.e., pyrolysis reaction) of cellulose, driven by its oxygencontaining functional groups (such as O-H, C-O, and C=O)32, is known to generate porous carbons, in which the porous structure becomes more developed when chemical activating agents, such as KOH salts, are added during the thermal decomposition23-25. In this study, K+ ions in the espresso are expected to play a vital role as a type of activating agent responsible for creating microporous carbons. The high-resolution transmission electron microscopy 6 ACS Paragon Plus Environment

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(HRTEM) image (Figure 1E) indicated that micropores are uniformly distributed in the EKACs33-34. The microporous structure was quantitatively elucidated by analyzing N2 adsorption-desorption isotherms. The EK-ACs exhibited a larger specific surface area (= 255.8 m2 g−1) than the K-ACs (=198.89 m2 g−1) (Figure 1F). Notably, a steep increase in N2 adsorption at a low relative pressure (P/P0 < 0.01) (inset of Figure 1F), which is known to arise from the filling of primary micropores35, demonstrates the evolution of the microporous (< 2 nm) structure. The pore size distribution, which was determined using the t-plot method36,37, confirmed the formation of micropores in the EK-ACs. These results underline the chemical functionality of espresso as a natural activating agent for the synthesis of cellulose-based microporous ACs. The comparison with the conventional KOH activation method (Supporting Information, Table S1) also exhibits the validity of the espresso coffee activation. In addition to the microstructure development described above, the espresso also contributed to facilitating the pyrolysis kinetics of cellulose. The thermal decomposition temperature was substantially lower for the espresso-soaked paper towel (Tdecomposition = 254 ˚C) compared to that (Tdecomposition = 322 ˚C) of the pristine paper towel (Supporting Information, Figure S2). Furthermore, a higher yield of ACs was obtained with the espressosoaked paper towel (= 24.3 % vs. 12.7 % for the pristine paper towel). The microstructural evolution during the carbonization of the espresso-soaked paper towel was investigated in detail, with a particular focus on the generation and removal of K2CO3 compounds. The K2CO3 compounds are

produced during KOH-assisted

carbonization38,39, in which the subsequent removal of K2CO3-occupied/K-bound sites via rinsing leads to the formation of the microporous structure in the resultant carbon substances. The X-ray photoelectron spectroscopy (XPS) spectra (Figure 2A) of the EK-ACs (before being rinsed with water) indicated the presence of K 2p peaks (at 293 and 296 eV40). In addition, the C 1s (at 287 and 289 eV41) peaks assigned to oxygenated carbon (i.e., C-O and 7 ACS Paragon Plus Environment


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C=O) groups were also observed in the EK-ACs (Figure 2B). These XPS results demonstrate that K2CO3 compounds were generated during the espresso-assisted carbonization. The presence of K2CO3 compounds in the EK-ACs was further verified by the FT-IR measurement, where the characteristic FT-IR peaks corresponding to carbonate groups42,43 were observed at 1452 and 1390 cm−1 (Figure 2C). Notably, the FT-IR peaks of the carbonate groups disappeared after water rinsing, indicating the removal of K2CO3 compounds from the resultant EK-ACs. This result was further verified by the XPS analysis (focusing on the K 2s and K 2p peaks) (Supporting Information, Figure S3). The effect of K2CO3 compounds on the microstructural evolution in the EK-ACs (after water rinsing) was quantitatively investigated as a function of carbonization time. The longer carbonization time (10 120 min) was favorable for the growth of characteristic XPS O 1s peaks ascribed to carbonate groups (Figure 2D). At the same time, the specific surface area and micropore volume of the EK-ACs were increased with the carbonization time (Figure 2E and F): 126.1 m2 g−1 (specific surface area)/0.0402 cm3 g−1 (micropore volume) after 10 min 193.7 m2 g−1/0.0557 cm3 g−1 after 30 min 255.8 m2 g−1 and 0.0772 cm3 g−1 after 120 min. On the other hand, excessively long carbonization time (e.g., 180 min) resulted in the decrease in the specific surface area (= 110.2 m2 g−1) and micropore volume (= 0.0323 cm3 g−1) (Supporting Information, Figure S4). These results demonstrate that the espressoassisted formation of K2CO3 compounds depends on the carbonization time and consequently exerts a significant influence on the micropore evolution in the EK-ACs, which is conceptually presented in Figure 2G. The potential applicability of the EK-ACs as an electrode material for supercapacitors was explored using an electric double-layer cell configuration. The EK-AC powders (Supporting Information, Figure S5), which were prepared through bead milling of the EKACs synthesized above, were mixed with multi-walled carbon nanotubes (MWNTs) (as a 8 ACS Paragon Plus Environment

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conductive additive) and polytetrafluoroethylene (PTFE) binder, leading to the formation of conventional supercapacitor electrodes (EK-AC/MWNT/PTFE = 85/10/5 (w/w/w)). The details on the electrode preparation and cell assembly are described in the experimental section. The cyclic voltammetry (CV) curves (measured at a scan rate of 1.0 mV s−1) of the EK-ACs appear almost rectangular in shape (Figure 3A), revealing a typical electric double layer (i.e., non-Faradaic) capacitive behavior39,44. A notable finding is that the EK-ACs showed substantial improvement in the capacitance (= 131 F g−1) over the K-ACs (= 64 F g−1). This capacitance value of the EK-ACs appeared nearly comparable to those of previously reported cellulose-derived ACs45-47 that were synthesized under the assistance of KOH salts. The higher capacitance of the EK-ACs relative to the K-ACs was also observed over a wide range of scan rates (1 – 500 mV s−1) (Supporting Information, Figure S6). These CV results demonstrate the effectiveness of espresso as a natural activating agent. Such superiority of the EK-ACs in the non-faradaic capacitive behavior was further verified by conducting galvanostatic charge-discharge (GCD) tests. The symmetrical triangular-shaped charge/discharge profiles were found at both the EK-ACs and K-ACs over a wide range of current densities (0.1 – 2.0 A g−1) (Figure 3B, Figure S7 (Supporting Information)). Notably, the EK-ACs exhibited a smaller IR drop (= 0.04 V at 0.5 A g−1) and longer charge/discharge time (= 44.2/42.9 s) than the K-ACs (= 0.08 V and 19.4/18.2 s). Meanwhile, no significant difference in the leakage current was observed between the cells with the K-ACs and EK-ACs (Supporting Information, Figure S8), indicating that the initial IR drop was predominantly affected by the internal resistance (R) of the cells. This result demonstrates the EK-ACs with the well-developed porous structure effectively reduce the ionic conduction resistance. Moreover, the higher capacitance of the EK-ACs was maintained over 10,000 charge/discharge cycles at a current density of 0.5 A g−1 (Figure 3C), manifesting the excellent long-term cycling stability. Comparison with the biomass-derived 9 ACS Paragon Plus Environment


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electrode materials (Supporting Information, Table S2) showed the advantageous effects of the coffee-mediated green activation of cellulose on the electrochemical performance. Meanwhile, open-circuit voltage (OCV) behavior of a cell containing the EK-AC was monitored as a function of elapsed time. No considerable drop in the OCV profile was observed at the EK-AC (Supporting Information, Figure S9), which appeared similar to those of previously reported works with conventional ACs48,49. The above-mentioned electrochemical performance of the EK-ACs is discussed in detail, with a particular focus on electrochemical activity of the microporous structure and consequential ionic conduction therein. A transmission line model with pore size distribution (referred to as “TLM-PSD”)50,51, which is a three-parameter model used to simulate the frequency response of entire electrodes in the complex nonlinear squares (CNLS) fitting52, provides information on total ionic conductance through pores (Yp) and a representative penetrability coefficient (α0) (for details, see TLM-PSD and Figure S10 (Supporting Information)). As shown in Figure 3D, the cell containing the EK-ACs presented dramatic enhancements in the Yp (= 1.4437 Ω−1) and α0 (= 0.6413 s−0.5) than the K-ACs (Yp = 0.3071 Ω−1 and α0 = 0.3731 s−0.5). This result demonstrates that the suppression of the IR drop in the EK-ACs (shown in Figure 3B) could be attributed to the increase in ionic conductance (enabled by the well-developed porous structure). Moreover, considering that the α0 reflects the effectiveness of utilizing porous surfaces at a given frequency or power density, the micropores (generated by the espresso-assisted activation) beneficially affect the electrochemically active porous sites, leading to the enhancement of electric double-layer capacitance in the EK-ACs. Paper has been widely used in a variety of application fields, ranging from traditional writing substrates/packaging materials/cleansing tissues to current state-of-the-art electronics and energy devices53-55, due to its exceptional deformability, compliance, and cost 10 ACS Paragon Plus Environment

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competitiveness12,14,56. Stimulated by such unique features of paper, we developed all-paper flexible supercapacitors using the EK-ACs and commercial paper towels. Figure 4A shows that the all-paper supercapacitors were composed of the EK-AC/SWNT-embedded paper towels (as electrodes), PVA/KOH-impregnated paper towels (as reinforced composite polymer electrolytes), and PDMS-infiltrated paper towels (as packaging substances). To fabricate the paper electrodes, the EK-AC powders were mixed with the SWNTs in water (herein, 1 wt% sodium dodecylbenzenesulfonate (SDBS)57 was added as a dispersing agent), yielding the electrode suspension (solid content = 1.08 %, EK-AC/SWNT = 10/1 (w/w)). The SWNTs were chosen to tightly hold the EK-AC powders as a type of binder and to secure sufficient electronic conductivity as a conductive additive even in the absence of metallic current collector foils. The SDBS additive contributed to the good dispersion state of the SWNTs (Supporting Information, Figure S11A) and also homogeneous complexation of the EK-AC and SWNTs (Supporting Information, Figure S11B). Specifically, the dispersion state of SWNTs was analyzed using visible-near infrared (visNIR) spectroscopy. The SWNT suspensions were subjected to centrifugation (10,000 × g) and then, the top layers (i.e., supernatants) of the centrifuged suspensions were exclusively collected for this measurement. The SDBS-containing SWNT suspension exhibited the stronger absorbance, whereas the control SWNT suspension without SDBS appeared transparent because most of the SWNTs had already been precipitated during the centrifugation. Subsequently, a piece of paper towel was repeatedly dipped into the aboveprepared electrode suspension until the resultant paper electrode sheet reached a saturated electronic resistance (= 20 Ω sq−1 after dipping ten times, Figure 4B). The overall fabrication procedure of the paper electrode, along with its photograph, is schematically illustrated in Figure S12 (Supporting Information).



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Figure 4C shows the morphological uniqueness of the paper electrode. The EK-AC powders, with being spatially besieged by the highly reticulated SWNT networks, were uniformly dispersed on the cellulose scaffold of the paper towel. Cellulose is known to have close affinity with CNTs58,59. This intimate interaction between the two components, in combination with the SWNT networks embracing the EK-AC powders and the mechanical compliance of cellulose, allowed the paper electrode to present good structural integrity and mechanical flexibility without the use of conventional polymer binders and metallic current collectors. No appreciable increase in the sheet resistance was observed after the repeated bending deformation (radius = 5 mm) over 10,000 cycles (Figure 4D). By contrast, control paper electrodes containing solely the EK-ACs without SWNTs exhibited electrically nonconductive behavior (Supporting Information, Figure S13) due to the absence of SWNTs that act as a current collector and conductive binder, underscoring the advantageous effects of the highly reticulated SWNT networks. PVA/KOH-based polymer electrolytes have been widely employed for solid-state supercapacitors60,61. Despite their well-balanced electrochemical properties, the presence of KOH liquid electrolyte often weakens the mechanical properties of the resultant polymer electrolytes. To address this concern, we used a porous substrate-reinforced composite electrolyte concept62,63. Compared to the conventional PVA/KOH polymer electrolyte chosen as a control sample (upper left image of Figure 4E), the paper towel-reinforced PVA/KOH composite polymer electrolyte showed significant improvement in the bending/twisting deformation (Figure 4E), underscoring the advantageous contribution of the paper towel substrate as a mechanical framework. The SEM image (Supporting Information, Figure S14) verified that the PVA/KOH polymer electrolyte (acting as an ion-conducting matrix) was impregnated into the paper towel substrate (as a structural skeleton). This paper-reinforced PVA/KOH composite polymer electrolyte, in combination with the aforementioned paper 12 ACS Paragon Plus Environment

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electrodes, is anticipated to play a critical role in enabling the highly all-paper flexible supercapacitors. The presence of the ionically-inert paper towel substrate may hinder the ionic transport of the reinforced PVA/KOH composite polymer electrolyte. However, the paper towel substrate has an intrinsically porous structure (porosity = 83 %), which allows for facile ion transport through the reinforced PVA/KOH composite polymer electrolyte. As a consequence, there was not a considerable difference in the ionic conductance between the reinforced PVA/KOH composite polymer electrolyte (= 4.7 S) and the control PVA/KOH polymer electrolyte (= 5.6 S) (Figure 4F). Here, the ionic conductance was analyzed instead of the ionic conductivity, because the electrochemical performance of the cells is directly affected by the ionic conductance (not the ionic conductivity) of the polymer electrolyte films56. The linear sweep voltammetry (LSV) profile indicated that the reinforced PVA/KOH composite polymer electrolyte was electrochemically stable up to 1.0 V (Supporting Information, Figure S15). In addition to the paper electrodes and paper-reinforced composite polymer electrolytes described above, the paper-mediated packaging substances were fabricated to realize all-paper supercapacitors. A paper towel was infiltrated with PDMS solution (silicone/curing agent = 10/1 (w/w)) and then subjected to heat treatment at 60 ˚C for 3 h (Supporting Information, Figure S16), resulting in a paper packaging substance with dense (i.e., nonporous) morphology (Figure 4G). The water contact angle of the paper packaging substances was found to be 96 ˚ (inset of Figure 4G), verifying the formation of a hydrophobic surface enabled by the PDMS impregnation. The all-paper flexible electric double-layer supercapacitor (inset of Figure 5A) was fabricated by symmetrically assembling the paper electrodes with the paper-reinforced composite polymer electrolyte and then followed by sealing with the paper packaging 13 ACS Paragon Plus Environment


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substance (Supporting Information, Figure S17). From the CV profiles (Figure 5A) of the paper supercapacitor, its specific and areal capacitance was respectively estimated to be 129 F g−1 (areal mass loading = 0.8 mg cm−2) and 103.5 mF cm−2 at 1.0 mV s−1. In addition, the GCD profiles (Figure 5B) show the symmetric triangular profiles over a wide range of charge/discharge current densities (= 0.1 – 2.0 mA cm−2), exhibiting the typical double-layer capacitive behavior. The cycling performance of the paper supercapacitor was examined by repeating the GCD test at a current density of 2.0 mA cm−2. No appreciable loss in the capacitance (> 89 %) was observed after 5,000 charge/discharge cycles (Figure 5C). The capacitance and cycling performance of the paper supercapacitor presented herein were compared with those of the previously reported studies (Supporting Information, Table S3). Meanwhile, the negligible level of capacitance was measured at the control paper electrode containing the SWNTs without the EK-ACs (Supporting Information, Figure S18), indicating that the double-layer capacitance of the paper electrode is predominantly attributed to the EK-ACs. The mechanical flexibility of the all-paper supercapacitor was quantitatively analyzed by conducting in situ measurement of CV profiles during mechanical deformation (Figure 5D). Upon repeated bending (radius = 5 mm, strain rate = 500 mm min−1) and twisting (twisting angle = 180 deg, twisting rate = 30 deg min−1) deformations, no significant distortion in the CV profiles (measured at a scan rate of 1.0 mV s−1) was observed after 5,000 bending and twisting cycles (Figure 5E (red and green lines)). Moreover, the paper supercapacitor was multiply folded (here, 5 times) and the change in its CV profiles was monitored. Notably, the double-layer capacitive behavior was not impaired after the multiple folding (Figure 5E (blue line)). The video clips showing the bending and folding deformation of the paper supercapacitor are provided in Movie S1. To highlight the flexibility of the paper supercapacitor, three cells were stacked in series and then subjected to an S-like shape 14 ACS Paragon Plus Environment

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deformation (Figure 5F). Even in the bent state, the series-stacked cells successfully lighted up a light-emitting diode (LED) lamp. These results underscore the exceptional flexibility and durability of the paper towel-based supercapacitor.

4. Conclusion In summary, intrigued by the facile acquisition/unique physicochemical features of paper towels and coffee that are commonly found in our daily lives, we presented the allpaper flexible supercapacitors incorporating the coffee/paper towel-derived activated carbons as a green material and electrochemical system strategy. The espresso coffee was used as a natural activating agent for the synthesis of paper towel (here, Kimwipes®)-based activated carbons (EK-ACs). The multifunctional roles of potassium ions (K+) in the espresso were investigated, with a particular focus on the microstructure evolution and carbonization kinetics of the EK-ACs. The EK-ACs showed significant improvement in the specific capacitance (= 131 F g−1 at a scan rate of 1.0 mV s−1) than the control ACs (= 64 F g−1) carbonized from a pristine paper towel. The all-paper flexible supercapacitors were composed of EK-AC/SWNT-embedded paper towels (as electrodes), PVA/KOH-impregnated paper towels (as electrolytes), and PDMS-infiltrated paper towels (as packaging substances). Driven by the EK-ACs as an electrode material and the paper towels as a compliant/deformable substrate, the all-paper flexible supercapacitors exhibited exceptional mechanical flexibility as well as sustainable cell performance, even in the absence of conventional polymeric binders and metallic current collectors. We anticipate that the coffeemediated green activation of cellulose and the resultant paper towel-based supercapacitors exhibit potential as an eco-friendly electrode material and an electrochemical system for



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high-performance flexible power sources, which are urgently needed for the upcoming smart ubiquitous electronics era.

ASSOCIATED CONTENT Supporting Information. Material and characterization details, experimental procedures, supporting characterization data, electrochemical analysis data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail:

(S.Y.L)

[email protected],

(J.Y)

[email protected],

(S.Y.L)

[email protected]

Author Contributions D.G.L. performed the experiments and analyzed the data. Y.G.C. performed the electrochemical analysis. H.K.S. participated in the electrochemical analysis. S.Y.L., J.Y., and S.Y.L. coordinated and supervised the overall project. D.G.L., S.Y.L., J.Y., and S.Y.L. wrote the manuscript and all authors discussed the results and participated in manuscript preparation.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This work was supported by the Korea Forest Research Institute (Grant No. FP 04002016-01), the energy efficiency and resources R&D program (20112010100150), the Industrial Technology Innovation Program 2015 (10050568), the Basic Science Research Program (2015R1A2A1A01003474 and 2015R1D1A1A01057004), and Wearable Platform Materials Technology Center (2016R1A5A1009926) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning.

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Figure 1. Espresso-assisted synthesis of activated carbons from wastepaper. (A) Schematic illustration depicting overall synthetic procedure of the EK-ACs (originating from 21 ACS Paragon Plus Environment


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the espresso-soaked wastepaper). (B) (left) Concentrations of the K+ ions (measured from the ICP-OES) in water, caffè Americano, and espresso. (center) Photograph of the coffee-soaked paper towel. (right) SEM image of the paper towel comprising the randomly piled cellulose fibers. (C) SEM image of the EK-ACs. An inset is the EDS mapping image (red dots represent K elements). (D) (top) Raman spectra and (bottom) XRD pattern of the EK-ACs. (E) HRTEM image of the EK-ACs. (F) Pore size distribution of the EK-ACs and K-ACs obtained from (inset) N2 adsorption-desorption isotherms.


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Figure 2. Microstructural evolution during carbonization of espresso-soaked paper towel. (A) XPS K 2p spectra of the EK-ACs and K-ACs. (B) XPS C 1s spectra of the EKACs and K-ACs, in which the blue- and green-colored regions indicate the presence of oxygenated carbon groups (i.e., C=O and C−O, respectively) in the EK-ACs. (C) FT-IR spectra of the EK-ACs before/after water rinsing. (D)-(G) Effect of K2CO3 compounds on the microstructural evolution in the EK-ACs (after water rinsing) as a function of carbonization (= 10, 30, and 120 min): (D) XPS O 1s spectra. Red-colored regions indicate the gradual growth of characteristic peaks assigned to carbonate groups (CO32−) with the carbonization time; (E) N2 adsorption-desorption isotherms; (F) Pore size distribution; (G) Conceptual illustration depicting the micropore development in the EK-ACs.



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Figure 3. Electrochemical properties of EK-ACs (or K-ACs) for use in supercapacitor electrodes. The composition ratio and areal mass loading of the electrodes were EK-AC (or K-AC)/MWNT/PTFE = 85/10/5 (w/w/w) and 43.6 ± 3 mg cm−2, respectively. (A) CV profiles at a scan rate of 1.0 mV s−1. (B) GCD profiles at a current density of 0.5 A g−1. (C) Capacitance retention with cycling (up to 10,000 cycles) at a current density of 0.5 A g−1. (D) AC impedance spectra (under a frequency range of 10−2 − 105 Hz) used for the TLM-PSD model. The total ionic conductance (Yp) through pores and penetrability coefficient (α0) of the K-AC and EK-AC are presented in the inset table.


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Figure 4. Preparation/characterization of major components for solid-state flexible allpaper supercapacitors. (A) Schematic illustration and photograph of the paper towel-based electrodes and composite polymer electrolyte for the solid-state flexible all-paper supercapacitor. (B) Variation in the electronic resistance of the paper electrode sheets as a function of dipping number. (C) SEM image of the EK-AC/SWNT-embedded paper electrode. (D) Variation in the relative electronic resistance of the EK-AC/SWNT-embedded paper electrode as a function of bending cycle (bending radius = 5 mm, deformation rate = 500 mm min−1). The inset images show the bending behavior of the paper electrode. (E) Photographs of the paper towel-reinforced PVA/KOH composite polymer electrolyte upon being subjected to bending/twisting deformation. The upper left image shows the mechanically broken state of the control PVA/KOH polymer electrolyte. (F) Nyquist plots of the paper towel-reinforced PVA/KOH composite polymer electrolyte and control PVA/KOH polymer electrolyte. (G) SEM image of the PDMS-infiltrated paper packaging substance. The inset shows the water contact angle of the paper packaging substance.



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Figure 5. Electrochemical performance and mechanical flexibility of solid-state allpaper supercapacitors. (A) CV profiles (at scan rates = 1 − 50 mV s−1). The inset is the photograph of the all-paper supercapacitor. (B) GCD profiles (at current densities = 0.1 – 2.0 mA cm−2). (C) Capacitance retention with cycling (up to 5,000 cycles) at a current density of 2.0 A cm−2. (D) Photographs depicting the mechanical (bending/folding) deformation of the all-paper supercapacitor. (E) CV profiles before/after mechanical deformation: (red line) bending after 5,000 cycles (radius = 5 mm, strain rate = 500 mm min−1), (green line) twisting after 5,000 cycles (twisting angle = 180 deg, twisting rate = 30 deg min−1), and (blue line) multiple folding after 5 times. (F) Photograph of the three paper cells stacked in series under an S-like shape deformation.


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Table of Contents Graphic Coffee-Driven Green Activation of Cellulose and Its Use for All-Paper Flexible Supercapacitors


Coffee-Driven Green Activation of Cellulose and Its ... - ACS Publications

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