Research Article pubs.acs.org/journal/ascecg
Macropore- and Micropore-Dominated Carbon Derived from Poly(vinyl alcohol) and Polyvinylpyrrolidone for Supercapacitor and Capacitive Deionization Kexin Tang,†,‡,§ Junjun Chang,† Hongbin Cao,†,‡,§ Chunlei Su,† Yuping Li,*,† Zisheng Zhang,‡,∥ and Yi Zhang†,‡,§ †
Institute of Process Engineering, Division of Environment Technology and Engineering, Beijing Engineering Research Center of Process Pollution Control, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ School of Chemical Engineering and Technology, National Engineering Research Center for Distillation Technology, Tianjin University, Tianjin 300072, P. R. China § Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China ∥ Department of Chemical & Biological Engineering, University of Ottawa, Ottawa, Ontario, Canada S Supporting Information *
ABSTRACT: We developed a kind of macropore- and micropore-dominated carbon (HPAC) derived from poly(vinyl alcohol) and polyvinylpyrrolidone for electric double-layer capacitive (EDLC) applications, e.g., supercapacitors and capacitive deionization (CDI). By comparing the EDLC performance of HPAC with those of ordered mesoporous carbon (OMC) and commercial activated carbon (AC), we evaluated the pore size effects. Cyclic voltammetry (CV) was employed for static and flowing CDI processes to identify the disparities between supercapacitors and CDI. HPAC exhibits a specific capacitance of 309 F g−1 at a specific current of 0.5 A g−1 (6 M KOH) in a three-electrode half-cell and has a salt removal capacity of 16.3 mg g−1 (1.2 V, 500 mg L−1 NaCl), which is better than those of AC and OMC. Cycling tests of HPAC in supercapacitors and CDI show excellent stability. The properties of HPAC, fine, hydrophilic, macroporous, and microporous, endow HPAC with the promising possibility of use in supercapacitors and capacitive deionization. The disparities of supercapacitors and CDI include ionic species and concentrations and solution hydromechanics. CV analysis of static and flowing CDI equipped with HPAC electrodes suggests that increasing the salt concentration in CDI is beneficial for the carbon electrode to show high capacitance and to reduce the pumping energy during the CDI process. KEYWORDS: Macroporous, Microporous, Carbon, Supercapacitor, Capacitive deionization, Desalination, Salt concentration, Pumping energy
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INTRODUCTION
and CDI is primarily determined by the properties of electrode materials. Carbon materials are popularly employed for electrochemical applications, mostly because of their easily tuned morphology, surface properties, pore structure, and electrical conductivity for specific applications.7,8 The pore structure and electrical conductivity of carbon materials are two of the major factors that allow electrodes to perform well in supercapacitors and CDI.9,10 With regard to the pore structure effects, Chmiola et al.11 reported that micropores that are 50 nm or the space between particles (≫50 nm). Carbon electrodes with abundant macropores, like graphene with three-dimensional (3D) frameworks20 and macroporous carbon with 3D interconnected architectures,21 can simultaneously achieve high energy and power density in supercapacitors and high sorption capacity and efficiency in CDI. Supercapacitors and CDI share a similar cell design and mechanism (EDLC) for storing energy or removing salt.22 Nevertheless, when electrode materials are applied in these two systems, electrodes usually behave better in supercapacitors than in CDI. The disparities of supercapacitors and CDI include the cell size, potential range, electrolyte type and concentration, distance between two electrodes, and solution hydromechanics. A supercapacitor cell is usually compressed with high pressure to decrease the distance between two electrodes (higher electric field intensity);9 in comparison, a conventional CDI cell must leave a space between two electrodes for a flowing solution.22 Despite the cell assembly differences, a high-concentration electrolyte is adopted in supercapacitors to minimize the ion diffusion effects on electrode performance,23 but CDI normally employs a weakly salty solution.24 Another difference is that the electrolyte is static and adsorbed by a porous separator in supercapacitors, whereas the electrolyte is flowing during CDI.
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EXPERIMENTAL SECTION
Preparation of the Carbon Materials. Macropore- and micropore-dominated carbon (HPAC) was synthesized through a one-pot method followed by freeze-drying, carbonization, and ultrasonic treatment. The detailed synthesis route is presented in the Supporting Information. To address the importance of the molecular sieve for the structure and performance of a sample, a sample without MCM-41 was prepared through the same route of HPAC, which is termed porous activated carbon (PAC). Ordered mesoporous carbon (OMC) was prepared via the route reported by Liu et al.25 A brief introduction for the synthesis of OMC is given in the Supporting Information. Commercial carbon powder (AC, >99%, 325 mesh) was purchased from Strem Chemicals, Inc., and used as received. Application in Supercapacitors. The application in supercapacitors was conducted in an Autolab electrochemical workstation (PGSTAT302N, Metrohm). A working electrode was prepared by mixing carbon materials, carbon black, and polytetrafluoroethylene in a mass ratio of 8:1:1 in dimethylacetamide and pasting on nickel foams, resulting in a mass loading of ∼3 mg and a thickness of 210 μm. In the three-electrode cell, a platinum plate (1.0 cm × 1.0 cm) and a mercuric oxide electrode (Hg/HgO, 1 M KOH) were employed as the counter and reference electrodes, respectively. Aqueous KOH (6 M) and TEABF4 (1 M) in acetonitrile were used as the electrolytes. Cyclic voltammetry (CV), galvanostatic charge−discharge (GCD), and electrochemical impedance spectroscopy (EIS) were used to measure the electrochemical performance of the electrodes. CV scanned from 20 mV s−1 to 2 V s−1 with a potential window ranging from −1.0 to 0.0 V to avoid water splitting and oxidation of nickel foam. A specific current was applied from 0.5 to 16 A g−1 during GCD. The impedance frequency decreased from 105 to 0.01 Hz with a signal amplitude of 10 mV. The electrode stability of HPAC was evaluated by a voltage hold test and the GCD method in a full cell. For the voltage hold test, the full cell was held at −1.0 V (6 M KOH) or 2.7 V (1 M TEA-BF4) for 11325
DOI: 10.1021/acssuschemeng.7b02307 ACS Sustainable Chem. Eng. 2017, 5, 11324−11333
Research Article
ACS Sustainable Chemistry & Engineering
Figure 2. (A) Nitrogen isothermal sorption curves. (B) QSDFT pore size distribution. (C) X-ray diffraction patterns of activated carbon (AC), ordered mesoporous carbon (OMC), porous activated carbon (PAC), and highly porous activated carbon (HPAC). (D) Raman spectra of AC and HPAC. (E) Fourier transform infrared spectrum and X-ray photoelectron spectroscopy survey of HPAC.
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100 h and occasionally tested using GCD (1 A g−1, which refers to the total mass of two electrodes) every 1 h three times.26 For the cycling test, a specific current of 1 A g−1 was applied. Capacitive Deionization Experiments. The electrochemical performance of the CDI cell (two-electrode) was measured using the CV method. CV scanned at 5 mV s−1 with a potential window of 1.4 V. To consider the influence of flow disturbance, CV measurements were applied for both the static CDI cell (supercapacitor) and the flowing CDI process. The salt concentration ranged from 50 mg L−1 to 10 g L−1. The CDI module was symmetrically assembled from four parts: support end plate, current collector of the graphite plate, carbon electrodes (∼0.15 g for each electrode), and insulated separator (Figure S1). The running parameters for the batch-mode CDI process were as follows: initial salt concentrations, 50, 100, 200, and 500 mg L−1; total solution volume, 60 mL; flow rate, 40 mL min−1; charging voltage, 1.2 V (1 h); discharging by short-circuiting (1 h); water temperature, 25 °C. The electric conductivity was recorded by a conductivity meter (S470-K, Mettler-Toledo) every minute. Saline water was prepared by dissolving sodium chloride crystals (AR) in deionized water, and no further treatment was applied. To evaluate the salt removal ability of HPAC, the CDI cell was cycled 100 times with the assistance of ion exchange membranes (1.2 V, 500 mg L−1, 20 mL min−1).
RESULTS AND DISCUSSION
Morphology and Structure. The scanning electron microscopy (SEM) images of HPAC with low resolution (Figure 1A) show fine particles. Finer particles are thought to provide more external surface area when they make contact with ions and to reduce the particle contact resistance between aggregates because of their greater number.9 Meanwhile, welldistributed macropores are found for HAPC (Figure 1B) and PAC (Figure S2a), which may be developed by freeze-drying and molecular sieve, whereas AC shows a layer structure and OMC a relatively smooth surface (Figure S3). In addition, the macropore size and macropore distribution are smaller than and denser than, respectively, those of PAC. The macropore structure of HPAC was further determined by mercury intrusion porosimetry (Figure S6), where the average pore diameter (4 V/A) is 353.2 nm with a total pore volume of 2.307 cm3 g−1. In contrast, AC shows a macroscopic average pore diameter of only 33.3 nm. Macropores can provide ionbuffering reservoirs,16 which could enhance the transport of ions from the bulk solution to the internal surface of the electrode for electrochemical application. Element mapping of HPAC shows uniformly distributed carbon, oxygen, and 11326
DOI: 10.1021/acssuschemeng.7b02307 ACS Sustainable Chem. Eng. 2017, 5, 11324−11333
Research Article
ACS Sustainable Chemistry & Engineering Table 1. Pore Structure and Elemental Composition of Carbon Materials
elemental composition (wt %)
a
sample
SBET (m2 g−1)
Vt (cm3 g−1)
Vmic (cm3 g−1)
Vmeso (cm3 g−1)
APWa (nm)
VPW
