Compressible, Fatigue Resistant, and Pressure-Sensitive Carbon

Jul 14, 2019 - Eng.2019XXXXXXXXXX-XXX ... the stress–strain curves of TMCA; TGA and DTG curves of TOCN and MF; Pie chart of the proportion of C, N, ...
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Compressible, Fatigue Resistant, and Pressure-Sensitive Carbon Aerogels Developed with a Facile Method for Sensors and Electrodes Meng Wang,† Yanglei Chen,† Yanlin Qin,‡ Tiejun Wang,‡ Jun Yang,† and Feng Xu*,†

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†

Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, No. 35 East Qinghua Road, Haidian District, Beijing 100083, China ‡ School of Chemical Engineering and Light Industry, Guangdong University of Technology, No. 100, Xiaoguwei Street, Fanyu District, Guangzhou 510006, China S Supporting Information *

ABSTRACT: Carbon aerogels possess low density, high conductivity, and excellent electrochemical properties, which have potential applications in sensor and energy storage. However, the fabrication methods of carbon aerogels are very complicated, and the applications are usually restricted by their low compressibility, fragile structure, and poor electrical property. Herein, we report a very facile approach for the preparation of compressible, fatigue resistant, conductive, and pressure-sensitive carbon aerogels by pyrolysis of cellulose nanofibers aerogel using melamine foams as the skeleton. The wet aerogels are dried directly in ambient pressure without any volume shrinkage, which is remarkably contrasted to the complex and time-consuming drying process of traditional aerogels. The resulting carbon aerogels exhibit excellent performance, including a low density of 11.23 mg cm−3, high electrical conductivity of 0.378 S cm−1, high sensitivity of 1.841 kPa−1, and outstanding mechanical properties. The assembled carbon aerogel sensors can monitor human activities and pulse vibration, demonstrating the great potential application in wearable devices. Moreover, the high nitrogen content and hydrophilic property enable the carbon aerogels to be used as compressible electrodes with a specific and areal capacitance of 92.2 F g−1 and 461 mF cm−2, respectively, showing the promising prospect in flexible supercapacitors. KEYWORDS: Carbon aerogels, Cellulose nanofibers, Melamine foams, Sensors, Compressible electrodes

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structure collapse of the aerogels.19 In addition, the cellulose nanofibers-derived carbon aerogels were very fragile and easily destroyed due to the severe degradation of cellulose macromolecules during the pyrolysis.20 In order to improve the mechanical properties of cellulose nanofibers-derived carbon aerogels, various strategies were proposed. For example, the mechanical properties can be significantly improved by combining some robust materials such as graphene21 and carbon nanotube22 or designing the micromorphology of carbon aerogels.23 A nitrogen-doped nanocellulose carbon aerogel was fabricated by doping graphene and melamine followed by the pyrolysis process. The resulting carbon aerogels show outstanding resilience and mechanical strength.24 Moreover, the carbon aerogels derived from cellulose nanofibers and graphene oxide with high compressibility and fatigue resistance were fabricated by the bidirectional freezing technique.25 Although the as-prepared carbon aerogels exhibit outstanding mechanical properties, the

INTRODUCTION Carbon aerogels with the unique three-dimensional networks exhibit many outstanding properties including low density, large specific surface area, and high electrical conductivity, which have great application in widespread fields.1−5 The carbon aerogels are usually fabricated by pyrolysis of the various precursors, including cellulose nanofibers,6 resorcinol/ formaldehyde (RF),7 carbon nanotube (CNT),8 graphene oxide (GO),9 and their mixture aerogels.10−12 Among them, cellulose nanofibers with a high carbon content of 44.44% and high aspect ratio are remarkable precursors to prepare carbon aerogels, which can apply to energy storage, oil adsorption, catalysis, and so on.13−17 The carbon aerogels derived from cellulose nanofibers were generally prepared by drying wet cellulose nanofibers gel and followed with the pyrolysis of cellulose nanofibers aerogels.6,18 Although the reported cellulose nanofibers-derived carbon aerogels possess many marvelous properties, the preparation methods still have some limitation. For example, the drying process of wet cellulose nanofibers gel is inevitably used for the complex and time-consuming drying methods such as supercritical CO2 drying and freeze-drying to avoid the © XXXX American Chemical Society

Received: February 16, 2019 Revised: June 25, 2019 Published: July 15, 2019 A

DOI: 10.1021/acssuschemeng.9b00814 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

TOCN-coated-MF aerogels (TMA) were heated in a tubular furnace. The tubular furnace is full of N2 with a flow velocity of 30 mL/min. First, the TMA was heated to 400 °C at a heating rate of 2 °C/min and kept at 400 °C for 1 h. Then the sample was heated to 800 °C at a heating rate of 5 °C/min and kept at 800 °C for 2 h. The obtained TOCN-coated-MF carbon aerogels and MF carbon aerogels are abbreviated as TMCA and MFCA, respectively. Characterization. The morphologies of TOCN were observed under transmission electron microscopy (TEM) using JEM-1010 at an accelerator voltage of 80 kV. Scanning electron microscopy (SEM) imaging was performed in S-3400 N II. Thermogravimetric analysis (TGA) was performed using a thermal analyzer (NETZSCH STA 449) at a heating rate of 10 °C/min from 20 to 800 °C. The crystallinity of the samples was analyzed by X-ray diffraction (XRD) using a D8-Advance. Raman spectra analysis was conducted via Reflex Raman Spectrometer (Renishaw) with an operating wavelength of 532 nm. The surface chemical species and content were tested using an X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250). The mechanical property of TMCA was examined using a material testing machine (UTM6530) at a speed of 100 mm min−1. The piezoresistive tests of TMCA were tested by combining the material testing machine and electrochemical workstation (PGSTAT 302N). 2 V was applied to the two ends of carbon aerogels, and the current variation during compression was recorded by an electrochemical workstation. The cuboid carbon aerogels were nipped between two sheet copper, and the resistance of carbon aerogels was recorded. The conductivity of the sample is defined as

preparation methods are also complicated, and the precursors are expensive, which greatly limited their mass production and widespread application in sensors and energy storage. The carbon aerogels with excellent mechanical properties have potential application in many fields, such as sensors and flexible supercapacitors.11,26 The ideal flexible supercapacitors should be resilient and durable under various compression or bending conditions.27,28 Therefore, electrode materials are required to have high compressibility and fatigue resistance.29,30 The melamine foams (MF) with interconnected networks are composed of formaldehyde−melamine resin, which is a perfect precursor for the fabrication of carbon aerogels because of its high porosity (>99%), low density ( 40%), attributing to the densification of skeletons.30 In addition, the slopes of the hook region and the plateau region are very close so that they almost form a single quasi-linear slope region, indicating that the TMCA are very soft and without any collapse during compression.42 As revealed in Figure 3b, the stress−strain behaviors of TMCA were examined after 1, 100, and 500 cycles at 50% strain. After 100 cycles of compression at 50% strain, the TMCA showed a tiny plastic deformation of about 5%, and the maximum compressive stress at the 100th cycle only reduced by 11.5% of the first cycle. Even after 500 cycles at 50% strain, the TMCA could retain 85% of its original height and the maximum compressive stress at the 500th cycle was maintained at 82% of the first cycle, demonstrating the extraordinary mechanical stability and fatigue resistance property. Importantly, the 3D interconnected structures of TMCA were still maintained after 500 cycles of compression, and cellulose carbon still wrapped around MF carbon skeleton, showing excellent structural stability (Figure S6). Besides excellent compressibility and fatigue resistance, the TMCA also possesses an excellent electrical property. The conductivity of the TMCA was measured as 0.378 S cm−1, which is significantly higher than MF carbon aerogels (∼0.0031 S cm−1)43 and bacterial cellulose carbon aerogels (∼0.206 S cm−1).3 As shown in Table S1, with the increase of TOCN concentration, the conductivity of TMCA was gradually increased from 0.0132 to 1.81 S cm−1. Mostly reported carbon aerogels always possess hydrophobic and oleophilic properties, due to the fact that most of the hydrophilic groups are removed during the pyrolysis.13 Interestingly, the TMCA is both hydrophilic and lipophilic. As revealed in Figure S7, either in the water or in the oil pump, the TMCA absorbed liquids immediately and immersed into the liquids. The contact angles of TMCA are 0° because the high nitrogen content can improve the wetting ability of carbon aerogels.32 The strong wetting ability of TMCA can D

DOI: 10.1021/acssuschemeng.9b00814 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 4. (a) Pressure response curves for TMCA. (b) The current change in ten loading/unloading cycles. (c) Schematic of the strain sensors. The relative current response of the TMCA sensors corresponds to various motion signals including (d) Finger bending, (e) Eye blinking, (f) Facial expression, (g) Pulse vibration. (h) Schematic of the flexible 3 × 3 tactile sensor. (i) The distribution and scale of relative current change of the tactile sensor.

effectively facilitate the infiltration of electrolyte at the electrolyte/electrode interface, which is beneficial to electrochemical performance. Piezoresistive Property of TMCA and the Applications of the Pressure Sensors. As illustrated in Figure S8 and Movie S2, a LED lamp was connected with the TMCA in a closed circuit and the fluctuation of brightness depends on the compressive strain, suggesting strain-sensitive conductivity of the TMCA. When the pressure was gradually applied on TMCA, the contact areas between the adjacent carbon skeleton were increased and the electric resistance was decreased, thereby resulting in the increased brightness of the LED lamp. The electrical resistance variations were recorded using a material testing machine combined with an electrochemical workstation. As revealed in Figure 4a, the plot of the curve is divided into two stages with different sensitivity. The first stage exhibits a sharp slop with a higher sensitivity of 1.841 kPa−1, and the second stage has a lower sensitivity of 0.00131 kPa−1. The sensitivity of TMCA is higher than previously reported highly sensitivity graphene aerogels, such as 3D printing graphene aerogel (0.13 kPa−1),42 polymer-based graphene foams (0.18 kPa−1),12 and graphene/polyurethane sponge (0.26 kPa−1).44 The high sensitivity is ascribed to the soft characteristic of TMCA that exhibited large deformation at low pressure and the dramatically increased contact spots between carbon skeleton resulting in the high conductivity. As presented in Figure 4b, the cycling stability of resistance was investigated during the cyclic compression at 50% strain. During ten cyclic compressions, every cycle of the electric

current curve was very consistent, showing excellent resistance stability during cyclic compression. To demonstrate promising functionality of TMCA in the pressure sensor, the flexible TMCA was clipped between two conductive tape pieces with the aid of adhesive tape (Figure 4c). The TMCA sensor can be attached finger, temple, and wrist to monitor the human motion of finger bending, eye blinking, and facial expression, respectively. As revealed in Figure 4d, the relative current of the TMCA sensor was increased as the finger state from the straight to the bending. When the finger was maintained in the bending state, the relative current was unchanged until the finger was straight again. The TMCA sensor also could be stuck on the temple and cheek to detect the eye blinking movements (Figure 4e) and facial expression (Figure 4f), respectively. As eye blinking or pouting occurred, repeated square wave signals could be observed. In addition, the TMCA strain sensor can detect more weak responses of human motion such as the wrist pulse. As illustrated in Figure 4g, the TMCA sensor was stuck on the wrist to monitor the pulse in real-time. The change of relative current indicates the pulse rate was about 68 beat min−1, and every single pulse possesses the two main characteristic peaks, corresponding to the percussion wave (P-wave) and diastolic wave (D-wave), respectively.43 This further demonstrated that the TMCA sensors possess high sensitivity performance in monitoring subtle deformation of the human body. We expect this sensor would be used in preventing fatal traffic accidents caused by fatigue driving and in monitoring the vital signs of the patient in real-time. E

DOI: 10.1021/acssuschemeng.9b00814 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 5. (a) CV curves of the TMCA at the scan rates from 1 mV s−1 to 500 mV s−1. (b) GCD curves of the TMCA at the current densities from 1 to 10 mA cm−2. (c) Areal capacitance and specific capacitance calculated for the TMCA electrode. (d) CV curves of the TMCA at a scan rate of 1 mV s−1 under different strains. (e) GCD curves of TMCA at a current density of 1 mA cm−2 under different strains. (f) EIS spectra of TMCA under different strains.

In order to prove the potential application of TMCA in a compressible electrode, the electrochemical behavior under different strains was measured. Figure 5d shows the CV curves of TMCA under the different strains. All of the CV curves display approximately rectangular shapes, implying a typical EDLC behavior.45 In accordance with the discharge curves (Figure 5e), the capacitance of the TMCA electrode gradually increased as the strain increased, which is ascribed to the decrease of internal resistance at the larger strain. In contrast, the specific capacitance of MFCA electrode shows without any change during the compression process, indicating the significant contribution of capacitance from the cellulose carbon fibers.32 The cycling stability of the TMCA electrode at a different strain was also evaluated through GCD measurement. The initial capacitance retention of the TMCA electrode at a different strain (0%, 30%, and 60%) was about 98%, 97.8%, and 96%, and only a slight decay was observed after a respective 1000 cycles (Figure S9), showing excellent cycling stability under different strain. To further understand the electrochemical transfer mechanism of the TMCA, the EIS of TMCA from 0.1 to 100 kHz under various strains were tested (Figure 5f). All of the Nyquist plots consist of a typical semicircle and a straight line. The measured impedance spectra were analyzed by fitting the impedance data with the equivalent circuit. As the strain increased, the equivalent series resistance (ESR) was decreased from 10.5 to 6.2, suggesting the enhancement of charge-transfer capability at the electrode/ electrolyte interfaces and the decrease of ion diffusion resistance. This is because of the improved conductivity and the drastically shortened ion diffusion pathway at higher strains.30

Since the measurement of spatial pressure distribution is one of the important applications of the sensor, we designed a flexible 3 × 3 tactile sensor to detect the spatial pressure. As revealed in Figure 4h, there are three copper sheets in the horizontal and vertical direction, where column and rows are indicated as a, b, c and 1, 2, 3, respectively. The point of intersection of each two copper sheet was separated by TMCA (0.5 × 0.5 × 0.5 cm). The pressure-sensing mechanism of tactile sensors is related to the position and resistive variation of the TMCA. As shown in Figure 4h, a pen cap was put on the surface of the tactile sensor to detect the spatial pressure distribution. The current variation of each grid was recorded and plotted in the histogram (Figure 4i). The spatial pressure distribution was clearly displayed in the histogram by the height and position, which was highly consistent with the position of the pen cap (3a, 2b, and 1c). This precise and sensitive capability of the TMCA tactile sensor shows the great application in wearable skin electronics, human-machine interfacing, and touch sensors. TMCA Used as Compressible Electrodes for Supercapacitors. As results discussed above, TMCA possesses high N doping, hydrophilicity, and a coherent conductive network, making it applicable in supercapacitors. To evaluate the electrochemical performance of TMCA, CV, GCD, and EIS measurements were performed in a three-electrode configuration. As revealed in Figure 5a, all the CV curves were in nearly rectangular shapes, implying the typical electrical double-layer capacitive (EDLC) behavior.45 The slight distortion of the CV curves may be ascribed to the pseudocapacitance produced by the N heteroatoms. As revealed in Figure 5b, the GCD curves of TMCA maintained a roughly rectangular shape and an internal resistance (IR) drop was observed, which is attributed to the internal resistance of the TMCA.46 Through GCD curves, the specific and areal capacitance of TMCA were calculated as 92.2 F g−1 and 461 mF cm−2 at a current density of 1 mA cm−2 (Figure 5c). Although the capacitance of TMCA is still lower than the recently reported carbon materials such as graphene and CNT carbon aerogels (50−150 F g−1),47 the preparation method is very simple and TMCA electrode does not need any binder (PVDF) and pseudocapacitive active materials, revealing potentially commercializable application.

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CONCLUSIONS

In conclusion, we proposed a novel and facile strategy to fabricate the compressible, conductive, and pressure-sensitive carbon aerogels by pyrolysis of cellulose aerogel using melamine foams as the skeleton. The rigid skeleton of MF prevents the collapse of cellulose aerogel during the ambient drying process. The resulting carbon aerogels possess low density (11.23 mg cm−3), high nitrogen content (6.35%), high compressibility (60%), high sensitivity (1.541 kPa−1), and high electrical conductivity (0.378 S cm−1), exhibiting remarkable F

DOI: 10.1021/acssuschemeng.9b00814 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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(7) Li, W.; Reichenauer, G.; Fricke, J. Carbon aerogels derived from cresol−resorcinol−formaldehyde for supercapacitors. Carbon 2002, 40, 2955−2959. (8) Zhang, Z.; Wang, L.; Li, Y.; Wang, Y.; Zhang, J.; Guan, G.; Pan, Z.; Zheng, G.; Peng, H. Nitrogen-Doped Core-Sheath Carbon Nanotube Array for Highly Stretchable Supercapacitor. Adv. Energy Mater. 2017, 7, 1601814. (9) Hu, H.; Zhao, Z.; Wan, W.; Gogotsi, Y.; Qiu, J. Ultralight and highly compressible graphene aerogels. Adv. Mater. 2013, 25, 2219− 23. (10) Sun, H.; Xu, Z.; Gao, C. Multifunctional, ultra-flyweight, synergistically assembled carbon aerogels. Adv. Mater. 2013, 25, 2554−60. (11) Chen, C. J.; Song, J. W.; Zhu, S. Z.; Li, Y. J.; Kuang, Y. D.; Wan, J. Y.; Kirsch, D.; Xu, L. S.; Wang, Y. B.; Gao, T. T.; Wang, Y. L.; Huang, H.; Gan, W. T.; Gong, A.; Li, T.; Xie, J.; Hu, L. B. Scalable and Sustainable Approach toward Highly Compressible, Anisotropic, Lamellar Carbon Sponge. Chem-Us 2018, 4, 544−554. (12) Qin, Y.; Peng, Q.; Ding, Y.; Lin, Z.; Wang, C.; Li, Y.; Xu, F.; Li, J.; Yuan, Y.; He, X.; Li, Y. Lightweight, Superelastic, and Mechanically Flexible Graphene/Polyimide Nanocomposite Foam for Strain Sensor Application. ACS Nano 2015, 9, 8933−41. (13) Meng, Y.; Young, T. M.; Liu, P.; Contescu, C. I.; Huang, B.; Wang, S. Ultralight carbon aerogel from nanocellulose as a highly selective oil absorption material. Cellulose 2015, 22, 435−447. (14) Chen, W.; Li, Q.; Wang, Y.; Yi, X.; Zeng, J.; Yu, H.; Liu, Y.; Li, J. Comparative study of aerogels obtained from differently prepared nanocellulose fibers. ChemSusChem 2014, 7, 154−61. (15) Guilminot, E.; Fischer, F.; Chatenet, M.; Rigacci, A.; BerthonFabry, S.; Achard, P.; Chainet, E. Use of cellulose-based carbon aerogels as catalyst support for PEM fuel cell electrodes: Electrochemical characterization. J. Power Sources 2007, 166, 104−111. (16) Yang, X.; Shi, K.; Zhitomirsky, I.; Cranston, E. D. Cellulose Nanocrystal Aerogels as Universal 3D Lightweight Substrates for Supercapacitor Materials. Adv. Mater. 2015, 27, 6104−9. (17) Xu, X.; Zhou, J.; Nagaraju, D. H.; Jiang, L.; Marinov, V. R.; Lubineau, G.; Alshareef, H. N.; Oh, M. Flexible, Highly Graphitized Carbon Aerogels Based on Bacterial Cellulose/Lignin: Catalyst-Free Synthesis and its Application in Energy Storage Devices. Adv. Funct. Mater. 2015, 25, 3193−3202. (18) Meng, Y.; Young, T. M.; Liu, P.; Contescu, C. I.; Huang, B.; Wang, S. Ultralight carbon aerogel from nanocellulose as a highly selective oil absorption material. Cellulose 2015, 22, 435−447. (19) Zu, G.; Shen, J.; Zou, L.; Fang, W.; Wang, X.; Zhang, Y.; Yao, X. Nanocellulose-derived highly porous carbon aerogels for supercapacitors. Carbon 2016, 99, 203−211. (20) Allahbakhsh, A.; Bahramian, A. R. Self-assembled and pyrolyzed carbon aerogels: an overview of their preparation mechanisms, properties and applications. Nanoscale 2015, 7, 14139. (21) Zhuo, H.; Hu, Y.; Tong, X.; Chen, Z.; Zhong, L.; Lai, H.; Liu, L.; Jing, S.; Liu, Q.; Liu, C.; et al. A Supercompressible, Elastic, and Bendable Carbon Aerogel with Ultrasensitive Detection Limits for Compression Strain, Pressure, and Bending Angle. Advanced materials 2018, 30, 1706705. (22) Chen, Y.; Sheng, C.; Dang, B.; Qian, T.; Jin, C.; Sun, Q. High Mechanical Property of Laminated Electromechanical Sensors by Carbonized Nanolignocellulose/Graphene Composites. ACS Appl. Mater. Interfaces 2018, 10, 7344−7351. (23) Zhuo, H.; Hu, Y.; Tong, X.; Chen, Z.; Zhong, L.; Lai, H.; Liu, L.; Jing, S.; Liu, Q.; Liu, C.; Peng, X.; Sun, R. A Supercompressible, Elastic, and Bendable Carbon Aerogel with Ultrasensitive Detection Limits for Compression Strain, Pressure, and Bending Angle. Adv. Mater. 2018, 30, No. 1706705. (24) Zhang, X. F.; Zhao, J. Q.; He, X.; Li, Q. Y.; Ao, C. H.; Xia, T.; Zhang, W.; Lu, C. H.; Deng, Y. L. Mechanically robust and highly compressible electrochemical supercapacitors from nitrogen-doped carbon aerogels. Carbon 2018, 127, 236−244.

performance as pressure sensors and compressible supercapacitor electrodes. The assemble sensors can be used to monitor human activities including movement, expression, and pulse. Additionally, the carbon aerogels exhibit a specific and areal capacitance of 92.2 F g−1 and 461 mF cm−2, respectively, revealing excellent electrochemical properties. It is anticipated that the TMCA is very advantageous for large-scale production and can be widely used in sensors and supercapacitors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b00814. TEM image of TOCN; SEM images of MFCA and TMCA; tables of the density and conductivity of TMCA; the stress−strain curves of TMCA; TGA and DTG curves of TOCN and MF; Pie chart of the proportion of C, N, and O elements of TMCA; SEM image of TMCA after 500 cycles compression; the hydrophilic and oleophilic properties of TMCA; the pressure sensitive performance of TMCA; and the cycling performance of the TMCA electrode (PDF) (Movie S1) Compression process of TMCA (AVI) (Movie S2) Pressure sensitive performance of TMCA (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-10-62337993. ORCID

Meng Wang: 0000-0003-1691-7617 Yanlin Qin: 0000-0003-3043-0821 Feng Xu: 0000-0003-2184-1872 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by The National Key Research and Development Program of China (2017YFD0601004). REFERENCES

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DOI: 10.1021/acssuschemeng.9b00814 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX