Activated Carbon Modified with Carbon Nanodots as Novel Electrode

Jun 2, 2016 - The main goal of this work was to modify activated carbon (AC) with carbon nanodots (C-dots) and to explore the modified composites as ...
1 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

Activated Carbon Modified with Carbon Nanodots as Novel Electrode Material for Supercapacitors Vijay Bhooshan Kumar,†,‡ Arie Borenstein,†,‡ Boris Markovsky,† Doron Aurbach,*,† Aharon Gedanken,*,† Michael Talianker,§ and Zeev Porat∥,⊥

Downloaded via TU MUENCHEN on July 5, 2018 at 13:17:31 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Bar-Ilan Institute for Nanotechnology and Advanced Materials, Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel § Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel ∥ Division of Chemistry, Nuclear Research Center-Negev, P.O. Box 9001, Beer-Sheva 84190, Israel ⊥ Institute of Applied Research, Ben-Gurion University of the Negev, Beer-Sheva 841051, Israel S Supporting Information *

ABSTRACT: The main goal of this work was to modify activated carbon (AC) with carbon nanodots (C-dots) and to explore the modified composites as electrode materials for supercapacitors. C-dots were synthesized by sonication of polyethylene glycol followed by sonochemical modification of AC matrices with the preprepared C-dots. Sonication introduces the C-dots into the pores of the AC. The effect of the introduction of the C-dots into the AC and their incorporation into the pores was studied. The porosity of the AC/C-dots and the AC reference materials was explored, as well as the impact of the C-dot loading on the performance of the electrodes comprising these AC/C-dots. It was found that the AC/C-dot electrodes demonstrate a specific capacitance of 0.185 F/cm2 (per specific electrode area), three times higher than the capacitance of unmodified AC electrodes per specific electrode’s area. It was established that the new electrode’s material, namely, AC/C-dots, exhibits very stable electrochemical behavior. Many thousands of cycles could be demonstrated with stable capacity and a Coulombic efficiency of around 100%.

1. INTRODUCTION During the past few decades, energy consumption has increased due to high population growth and basic requirements of human life. That is why there are strong demands for new energy storage and conversion devices, which can improve our accessibility to energy resources and their judicious use.1−6 Batteries and electric double layer (EDL) capacitors are prime examples of traditional energy storage devices. EDL capacitors (known as supercapacitors) are based on electrostatic interactions of high surface area electrodes. Thereby, their energy density is low (2 orders of magnitude lower than that of rechargeable batteries), but their power density may be very high. Also, due to their energy storage mechanism, and based on electrostatic interactions, they can demonstrate a very long cycle life, with excellent capacity for retention and very rapid charge capability.5,7 In recent years, extensive work has been invested in further developing supercapacitors, especially toward higher energy density. Such a target can be obtained by increasing the specific capacity of the electrodes and the operating voltage.5 Innovative work is being done in developing new concepts of electrodes which combine capacitive8,9 and surface red−ox interactions.10 Thus, the development of efficient and cost-effective materials for supercapacitor electro© 2016 American Chemical Society

des has become an emerging challenge for researchers in the field of energy storage and conversion.11,12 Carbon is one of the most abundant elements on earth. Carbonaceous materials have a variety of unique formations and allotropes: diamond, graphite, nanotubes, fullerenes, graphene, glassy, hard, soft, disordered, and activated highly porous carbon. Most of these forms of carbon conduct electrons, and they are versatile in their mechanical, optical, chemical, and electronic properties.8,13 Consequently, carbonaceous materials have an increasingly wide range of uses in composite structures and electrodes in electrochemical devices.14,15 High surface area porous activated carbons (AC) are natural candidates for use as electrode materials in capacitive energy storage devices,14−16 as well as carbon electrode performance.17,18 They can be prepared from disordered carbons by a mild oxidation process with CO2 at high temperatures (as high as 900 °C), or with hot aqueous solutions of HNO3 and KOH (as high as 80 °C).19 Their porosity can be controlled and monitored, as has already been demonstrated.20,21 Porous carbons modified with nitrogen Received: April 22, 2016 Revised: May 30, 2016 Published: June 2, 2016 13406

DOI: 10.1021/acs.jpcc.6b04045 J. Phys. Chem. C 2016, 120, 13406−13413

Article

The Journal of Physical Chemistry C

Scheme 1. Scheme Representing the Synthesis Process for Producing Activated Carbon (AC) Modified with Carbon Dots (CDots) Shown as Yellow Circles in the Modified Material

2.2. Preparation Procedure. 15 mL of PEG-400 was transferred into a test tube which was dipped in a water bath at 75 °C, following Kumar et al.30,31 The ultrasonic transducer (Sonics and Materials Inc., USA, model VCX 750, frequency 20 kHz, voltage 230 V AC) was applied for 2.5 h with an amplitude of 70%. After the formation of the C-dots, the solution was cooled to room temperature (rt) and the activated carbon was added to the C-dot suspension of PEG-400. The sonication was continued for another 1−2 h at 40% amplitude in room temperature. This process was aimed at anchoring the C-dots on the AC. After sonication, the modified AC was washed with acetone to remove the PEG-400 and dried in vacuum. Dried AC/C-dot material was further characterized and used as electrodes for supercapacitors. The experimental procedure is illustrated in Scheme 1. 2.3. Electrochemical Testing. The electrodes used in the electrochemical cells were prepared by mixing the composite material (AC/C-dots) with poly vinylidene fluoride (PVDF) as a binder and carbon black as a conducting agent (with a ratio of 87:8:5 wt %). The slurry was spread on glass and dried at 100 °C overnight. Circular electrodes (0.8 mm in diameter) were prepared by loading with ca. 2 mg of the active material (AC or AC/C-dots). Two types of electrochemical cells were used: (a) A three-electrode cell contains the AC/C-dots as the working electrode, the counter electrode of the same material in mass excess (circles of 1.6 mm diameter), and a standard calomel electrode as reference. (b) A two-electrode cell containing pairs of symmetric AC/C-dot electrodes. The electrolyte solution in both cells was aqueous 6 M KOH. 2.4. Analytical Measurements. The fluorescence of Cdots was measured by a fluorescence spectrophotometer (Varian Cary Eclipse). High-resolution transmission electron microscopy (HRTEM) images are obtained and analyzed by a JEOL 2100 microscope, operated at accelerating voltage (200 kV). The samples for TEM analysis were prepared by dispersing the C-dots in isopropanol on a carbon coated copper grid, which is then dried under vacuum at 25 °C for 12 h. The size of the C-dots is calculated using the Image-J software. X-ray diffraction (XRD) is performed with the help of Bruker D8 Advance and with a Philips PW1050 X-ray diffractometer using Cu Kα radiation operating at 40 kV/40 mA with a 0.0019° step per 0.5 s. Gas adsorption measurements were carried out using the Autosorb-1 MP (Quantachrome, USA) system. Dynamic light scattering (DLS) measurements of the C-dots are performed on ZetaSizer Nano-ZS (Malvern Instruments Ltd., Worcestershire, U.K.). The specific surface area (SSA) is calculated using the Brunauer−Emmett−

functional groups or dopants have attracted much interest because nitrogen functional groups contribute to an increase in the charge density of the carbon materials. Hence, they contribute to certain higher capacities of electrodes in the adsorption process.18,22 Recently, the discovery of new generations of carbon dots (C-dots) with tunable properties has enabled the researchers to extend the range of applications in which carbon materials are used.11 C-dots are a new and intriguing class of luminescent nanoparticles with properties complementary to those of the traditional metal-based quantum dots.23 The combination of multicolor, tunable emission, controlled surface chemistry, and solvent dispensability in one simple platform has made C-dots attractive for a wide range of optical, sensing, and biomedical applications such as cell imaging, drug targeting, and drug delivery.24−26 In a recent paper27 by Lv et al., the authors demonstrated advantages for the use of C-dots and aerogel electrodes in supercapacitors. Liu et al. also prepared supercapacitor electrodes composed of graphene quantum dots, and attributed the increase in capacitance to the quantumsized graphene fragments.16 A few studies have reported on the improvement of capacitance in supercapacitors using electrodes that are based on C-dots in the form of modified graphene, graphene oxide, and carbon nanotubes, as the main active mass.9,28,29 The goal of our investigation is to develop new electrodes for supercapacitors, namely, activated carbon modified with C-dots, and to study their electrochemical behavior. Motivated by the unique electronic structure of C-dots, as reflected by their luminescent character, we presumed that their presence in activated carbon matrices might increase capacitance of activated carbon electrodes. To the best of our knowledge, there are no reports on supercapacitors containing activated carbon electrodes modified with C-dots. The novelty of this work lies in developing a new procedure for the deposition of C-dots on activated carbon materials, incorporating them into the pores by applying a simple sonochemical process without any catalyst. This study is a natural continuation of our recent work30,31 on the synthesis of C-dots and the elucidation of their physical and chemical properties.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Polyethylene glycol-400 (99.998%) was purchased from Sigma-Aldrich. The activated carbon (AC) powder was obtained from Energ2 (USA, ∼2200 m2/g). The carbon black used in this study as a conducting agent was super P carbon from Timcal, Switzerland. 13407

DOI: 10.1021/acs.jpcc.6b04045 J. Phys. Chem. C 2016, 120, 13406−13413

Article

The Journal of Physical Chemistry C

The crystalline nature of the carbon dots was detected on the basis of HRTEM observations. Figure 2a illustrates the

Teller (BET) model. Raman spectroscopic measurements were performed with a Jobin-Yvon Labram spectrometer. AC, Cdots, and AC/C-dots were excited using a 632.8 nm laser, with a spectral resolution of 2000 m2/g and mesopores of 10−15 nm. Also, further studies are focused on a full validation of the mechanism of capacity enhancement by adsorbed C-dots, suggested in this paper.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04045. TEM, Raman, and CV investigations of C-dots, AC, and AC/C-dots (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: 972-3-5318-315. Fax: 972-3-7384-053. *E-mail: [email protected]. Author Contributions ‡

V.B.K. and A.B. contributed equally to this paper.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS D.A. acknowledges the partial support for this work obtained from the ISF, Israel Science Foundation, in the framework of the INREP project. We also acknowledge support by the Israel Ministry of Science and Space.

■

REFERENCES

(1) Zhu, Y.; Murali, S.; Stoller, M. D.; Ganesh, K. J.; Cai, W.; Ferreira, P. J.; Pirkle, A.; Wallace, R. M.; Cychosz, K. A.; Thommes, M.; et al. Carbon-Based Supercapacitors Produced by Activation of Graphene. Science 2011, 332, 1537−1542. (2) Zhang, S.; Pan, N. Supercapacitors Performance Evaluation. Adv. Energy Mater. 2015, 5, 1−19. (3) Zhang, C.; Yin, H.; Han, M.; Dai, Z.; Pang, H.; Zheng, Y.; Lan, Y. Q.; Bao, J.; Zhu, J. Two-Dimensional Tin Selenide Nanostructures for Flexible All-Solid-State Supercapacitors. ACS Nano 2014, 8, 3761− 3770. (4) Simon, P.; Gogotsi, Y.; Dunn, B. Where Do Batteries End and Supercapacitors Begin? Science 2014, 343, 1210−1211. (5) Chen, L.-F.; Huang, Z.-H.; Liang, H.-W.; Yao, W.-T.; Yu, Z.-Y.; Yu, S.-H. Flexible All-Solid-State High-Power Supercapacitor Fabricated with Nitrogen-Doped Carbon Nanofiber Electrode Material Derived from Bacterial Cellulose. Energy Environ. Sci. 2013, 6, 3331. 13412

DOI: 10.1021/acs.jpcc.6b04045 J. Phys. Chem. C 2016, 120, 13406−13413

Article

The Journal of Physical Chemistry C Carbon Dots and Its Application as Fluorescent Imaging Agent. Talanta 2013, 117, 196−202. (27) Lv, L.; Fan, Y.; Chen, Q.; Zhao, Y.; Hu, Y.; Zhang, Z.; Chen, N.; Qu, L. Three-Dimensional Multichannel Aerogel of Carbon Quantum Dots for High-Performance Supercapacitors. Nanotechnology 2014, 25, 235401. (28) Yu, D.; Goh, K.; Wang, H.; Wei, L.; Jiang, W.; Zhang, Q.; Dai, L.; Chen, Y. Scalable Synthesis of Hierarchically Structured Carbon Nanotube-Graphene Fibres for Capacitive Energy Storage. Nat. Nanotechnol. 2014, 9, 555−562. (29) Iyyamperumal, E.; Wang, S.; Dai, L. Vertically Aligned BCN Nanotubes with High Capacitance. ACS Nano 2012, 6, 5259−5265. (30) Kumar, V. B.; Perelshtein, I.; Lipovsky, A.; Porat, Z.; Gedanken, A. The Sonochemical Synthesis of Ga@C-Dots Particles. RSC Adv. 2015, 5, 25533−25540. (31) Kumar, V. B.; Porat, Z.; Gedanken, A. Facile One-Step Sonochemical Synthesis of Ultrafine and Stable Fluorescent C-Dots. Ultrason. Sonochem. 2016, 28, 367−375. (32) Wang, Y.; Li, Y.; Yan, Y.; Xu, J.; Guan, B.; Wang, Q.; Li, J.; Yu, J. Luminescent Carbon Dots in a New Magnesium Aluminophosphate Zeolite. Chem. Commun. (Cambridge, U. K.) 2013, 49, 9006−9008. (33) Gao, S.; Chen, Y.; Fan, H.; Wei, X.; Hu, C.; Wang, L.; Qu, L. A Green One-Arrow-Two-Hawks Strategy for Nitrogen-Doped Carbon Dots as Fluorescent Ink and Oxygen Reduction Electrocatalysts. J. Mater. Chem. A 2014, 2, 6320. (34) Zheng, M.; Xie, Z.; Qu, D.; Li, D.; Du, P.; Jing, X.; Sun, Z. Onoff-on Fluorescent Carbon Dots Nanosensor for Recognition of Chromium (VI) and Ascorbic Acid Based on Inner Filter Effect. ACS Appl. Mater. Interfaces 2013, 5, 13242−13247. (35) Wang, J.; Cheng, C.; Huang, Y.; Zheng, B.; Yuan, H.; Bo, L.; Zheng, M.-W.; Yang, S. Y.; Guo, Y.; Xiao, D. A Facile Large-Scale Microwave Synthesis of Highly Fluorescent Carbon Dots from Benzenediol Isomers. J. Mater. Chem. C 2014, 2, 5028−5035. (36) Kumar, V. B.; Annamanedi, M.; Prashad, M. D.; Arunasree, K. M.; Mastai, Y.; Gedanken, A.; Paik, P. Synthesis of Mesoporous SiO2ZnO Nanocapsules: Encapsulation of Small Biomolecules for Drugs and “SiOZO-Plex” for Gene Delivery. J. Nanopart. Res. 2013, 15, 1904−1926. (37) Barman, M. K.; Jana, B.; Bhattacharyya, S.; Patra, A. Photophysical Properties of Doped Carbon Dots (N, P, and B) and Their In Fl Uence on Electron/Hole Transfer in Carbon Dots Nickel (II) Phthalocyanine Conjugates. J. Phys. Chem. C 2014, 118, 20034− 20041. (38) Song, Y.; Zhu, S.; Xiang, S.; Zhao, X.; Zhang, J.; Zhang, H.; Fu, Y.; Yang, B. Investigation into the Fluorescence Quenching Behaviors and Applications of Carbon Dots. Nanoscale 2014, 6, 4676−4682.

13413

DOI: 10.1021/acs.jpcc.6b04045 J. Phys. Chem. C 2016, 120, 13406−13413