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Macropores and micropores dominated carbon derived from polyvinyl alcohol and polyvinylpyrrolidone for supercapacitor and capacitive deionization Kexin Tang, Junjun Chang, Hongbin Cao, Chunlei Su, Yuping Li, Zisheng Zhang, and Yi Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02307 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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ACS Sustainable Chemistry & Engineering

Macropores and micropores dominated carbon derived from polyvinyl alcohol and polyvinylpyrrolidone for supercapacitor and capacitive deionization Kexin Tang, a, b, c Junjun Chang, a Hongbin Cao, a, b, c Chunlei Su, a Yuping Li, a,* Zisheng Zhang, b, d Yi Zhang a, b, c

a

Chinese Academy of Sciences, Institute of Process Engineering, Division of Environment Technology and

Engineering, Beijing Engineering Research Center of Process Pollution Control, Beijing 100190, PR China b

Tianjin University, School of Chemical Engineering and Technology, National Engineering Research

Center for Distillation Technology, Tianjin 300072, PR China c

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, PR

China d

University of Ottawa, Department of Chemical & Biological Engineering, Canada

* Corresponding author. Tel.: +86-10-82544844-810. Fax: +86-10-82544844-816. E-mail address: [email protected] (Yuping Li). Mailing Address for all authors Kexin Tang, No.1 North Second Road, Zhongguancun Area, Haidian District, Beijing, China 10109 Junjun Chang, No.1 North Second Road, Zhongguancun Area, Haidian District, Beijing, China 10109 Hongbin Cao, No.1 North Second Road, Zhongguancun Area, Haidian District, Beijing, China 10109 Chunlei Su, No.1 North Second Road, Zhongguancun Area, Haidian District, Beijing, China 10109 Yuping Li, No.1 North Second Road, Zhongguancun Area, Haidian District, Beijing, China 10109 Zisheng Zhang, 161 Louis Pasteur Street, Ottawa, Ontario, K1N 6N5, Canada Yi Zhang, No.1 North Second Road, Zhongguancun Area, Haidian District, Beijing, China 10109

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ACS Paragon Plus Environment


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Abstract

This work developed a kind of macropores and micropores dominated carbon (HPAC) derived from polyvinyl alcohol and polyvinylpyrrolidone for electric double-layered capacitive (EDLC) applications, e.g. supercapacitor and capacitive deionization (CDI). By comparing the EDLC performance of HPAC with ordered mesoporous carbon (OMC) and commercial activated carbon (AC), the pore size effects were evaluated. Cyclic voltammetry (CV) was employed for static and flowable CDI process to discuss the disparities between supercapacitor and CDI. HPAC demonstrates specific capacitance of 309 F g−1 at specific current of 0.5 A g−1 (6 M KOH) in a three-electrode half-cell, and has salt removal capacity of 16.3 mg g−1 (1.2 V, 500 mg L−1 NaCl), which is better than AC and OMC. Cycling tests of HPAC in supercapacitor and CDI show excellent stability. The properties of HPAC, fine, hydrophilic, macroporous and microporous, endow HPAC with promising performance in supercapacitor and capacitive deionization. The disparities of supercapacitor and CDI include ions species and concentration and solution hydromechanics. The CV analysis of static and flowable CDI equipped with HPAC electrodes suggests that increasing salt concentration in CDI is beneficial for carbon electrode to show high capacitance and to reduce pumping energy during CDI process. Keywords: macroporous; microporous; carbon; supercapacitor; capacitive deionization; desalination; salt concentration; pumping energy

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INTRODUCTION Energy shortage and water crisis are the two of the major issues in 21st. 1-2 One of the solutions for energy shortage is to store clean energy generated from solar, wind and tidal energy through battery, solar cell and capacitor, etc.

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To relieve fresh water demand, desalination technologies

have been intensively developed, like heat-driven process of distillation, multistage flash evaporation and pressure-driven process of reverse osmosis. 4 Supercapacitor, a method for fast and reversible energy storage, 5 and capacitive deionization (CDI), an environmental desalination technology to directly remove salt from saline water,

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both employ electric double-layered

capacitance (EDLC) of electrodes to migrate ions among a certain electrical field. Generally, the performance of supercapacitor and CDI is primarily determined by the properties of electrode materials.

Carbon materials are popularly employed for electrochemical applications, mostly due to their easily tuned morphology, surface property, pore structure and electrical conductivity for specific application.

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The pore structure and electrical conductivity of carbon materials are two of the

major factors to enable electrodes express high performance in supercapacitor and CDI process. 9-10

Regarding to the pore structure effects, Chmiola et al. 11 reported that micropores less than 1

nm in carbon materials contribute most to the capacitance of supercapacitor. However, there was a controversy about the influence of pore structure on salt removal capacity in CDI: a. micropore (< 2 nm) dominates in ions adsorption, removal ability,

13-14

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b. mesopore (2 ~ 50 nm) is beneficial for high salt

c. hierarchical pore determines the fast ions removal. 3


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Different from


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micropore sand mesopores, the transport of small molecules in macropores (> 50 nm) of nanoporous materials can approach to the diffusion rates in the bulk solution, because the ionbuffering reservoirs formed inside macropores can minimize the ion diffusion distance from bulk solution to the interior surfaces. 16-17 To be noted, the above pore structure is classified according to IUPAC. In transfer theory, micropores and macropores respectively refer to the pores in media (e.g. carbon) and the space between particles.18-19 Nevertheless, ions buffer effect could be formed in either those pore width bigger than 50 nm or the space between particles (much bigger than 50 nm). Carbon electrodes with abundant macropores, like graphene with 3D frameworks 20

and macroporous carbon with 3D interconnected architectures,

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can simultaneously achieve

high energy and power density in supercapacitor, and high sorption capacity and efficiency in CDI.

Supercapacitor and CDI share the similar cell design and mechanism (EDLC) to store energy or remove salt. 22 Nevertheless, when applying electrode materials in these two systems, electrodes usually behave better in supercapacitor than in CDI. The disparities of supercapacitor and CDI include cell size, potential range, electrolyte type and concentration, distance between two electrodes and solution hydromechanics. Supercapacitor cell is usually compressed with high pressure to decrease the distance between two electrodes (higher electric field intensity),

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in

comparison, conventional CDI cell must leave a space between two electrodes for flowable solution.

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Despite the cell assembly differences, high concentration electrolyte is adopted in

supercapacitor to minimize the ion diffusion effects on electrodes performance, 4


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but CDI

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normally deal with low salty solution.

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Another difference is that electrolyte is static and

adsorbed by porous separator in supercapacitor, whereas electrolyte is flowing during CDI process.

The present work primarily aims to develop a macropores and micropores dominated carbon (HPAC) material to use as electrode for supercapacitor and CDI. Secondly, by systematically comparing its performance in supercapacitor and CDI with commercial activated carbon and ordered mesoporous carbon, we try to determine materials structure influences in both applications. By using static and flowable state of CDI process to represent supercapacitor and CDI cell, we finally try to analyze the disparity of capacitive performance in these two systems.

EXPERIMENTAL Carbon materials preparation Macropores and micropores dominated carbon (HPAC) was synthesized through one-pot method followed with freeze-drying, carbonization and ultrasonic treatment. The detailed synthesis route was presented in the supporting information (SI). To address the importance of molecular sieve on the structure and performance of sample, a simple without MCM-41 was prepared through the same route of HPAC, which is termed porous activated carbon (PAC). Ordered mesoporous carbon (OMC) was prepared from the route reported by Liu et al. 25 A brief introduction for the synthesis of OMC was described in SI. Commercial carbon powder (AC, 99+%, 325 mesh) was purchased from STERM Chemicals, Inc. (USA) and used as received.

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Application in supercapacitor The application in supercapacitor was conducted in an Autolab electrochemical workstation (PGSTAT302N, Metrohm). Working electrode was prepared by mixing carbon materials, carbon black and polytetrafluoroethylene with mass ratio of 8:1:1 in dimethylacetamide and pasting on nickel foams, resulting in mass loading of ~ 3 mg and thickness of 210 µm. In three-electrode cell, platinum plate (1.0 cm × 1.0 cm) and mercuric oxide electrode (Hg/HgO, 1 M KOH) were employed as counter and reference electrode, respectively. 6 M KOH and 1 M TEA-BF4 in acetonitrile were used as electrolyte. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were applied to measure the electrochemical performance of electrodes. CV scanned from 20 mV s−1 to 2 V s−1 with potential window ranging from −1.0 to 0.0 V to avoid water splitting and oxidation of nickel foam. Specific current was applied from 0.5 to 16 A g−1 in GCD. The impedance frequency decreased from 105 to 0.01 Hz with signal amplitude of 10 mV. Electrode stability of HPAC was evaluated by voltage hold test and GCD method in a full cell. For voltage hold test, the full cell was hold at – 1.0 V (6 M KOH) or 2.7 V (1 M TEA-BF4) for 100 h, and occasionally tested using GCD method (1 A g–1, refers to the total mass of two electrodes) every 1 h for three times.26 For cycling test, a specific current of 1 A g–1 was applied.

Capacitive deionization experiments The electrochemical performance of CDI cell (two-electrode) was measured using CV method. CV scanned at 5 mV s−1 with potential window of 1.4 V. To consider the influence of flow 6


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disturbance, CV measurements were applied for both static CDI cell (supercapacitor) and flowing CDI process. The salt concentration ranged from 50 mg L−1 to 10 g L−1.

The CDI module was symmetrically assembled by four parts: support endplate, current collector of 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 described as follows: initial salt concentration from 50, 100, 200 to 500 mg L−1, total solution volume of 60 ml, flow rate of 40 ml min−1, charging voltage of 1.2 V (1 h), discharging by short-circuiting (1 h), water temperature of 25 ºC. 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, no further treatment was applied. To evaluate salt removal ability of HPAC, CDI cell was cycling 100 times assisted with ion exchange membranes (1.2 V, 500 mg L– 1

, 20 ml min−1).

RESULTS AND DISCUSSION Morphology and structure The SEM images of HPAC with low-resolution (Figure 1a) show fine particles. Finer particles are suggested to provide more external surface area when contact with ions and to reduce the particle contact resistance between aggregates due to their greater number.

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Meanwhile, well-

distributed macropores are found for HAPC (Figure 1b) and PAC (Figure S2a), which may be developed by freeze-drying and molecular sieve, whereas AC shows layer structure and OMC has relatively smooth surface (Figure S3). In addition, HPAC holds smaller macropore size and 7



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denser macropore distribution than PAC. The macropore structure of HPAC was further determined by mercury intrusion porosimetry (Figure S6), where the average pore diameter (4V/A) is 353.2 nm with total pore volume of 2.307 cm3 g–1. In contrast, AC only shows macroscopic average pore diameter of 33.3 nm. Macropores can provide ion-buffering reservoirs,

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which could enhance ions transport from bulk solution to the internal surface of

electrode for electrochemical application. The element mapping of HPAC shows uniformly distributed carbon, oxygen and nitrogen on the surface, indicating the homogeneous synthesis of samples.

Figure 1 (a-c) SEM images and (d) the elements mapping of highly porous activated carbon.

Nitrogen isothermal sorption analysis was conducted to obtain pore structure of samples (Figure 2a). PAC and HPAC demonstrate typical type-I isothermal sorption curves due to the intense adsorption at relatively low pressure, 27 which represents high content of micropores. HPAC has nearly twice sorption volume than PAC, which implies more micropores developed inside carbon 8


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structure by MCM-41. AC and OMC show micropores (intense sorption at low pressure) and mesopores characteristics (hysteresis loop at high pressure), and has similar pore volume, except that OMC has more centralized mesopores than AC (Figure 2b). The pore volume of PAC is only about half of OMC, but PAC shows higher SBET (Table 1). Comparably, the pore volume (1.103 cm3 g−1) and SBET (2254 m2 g−1) of HPAC were significantly developed by MCM-41. The average pore width of HPAC is 2 nm ranging from 1 ~ 3 nm, where 2 nm is exactly the boundary of micropores and mesopores.

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As shown in Table 1, the proportion of micropores volume

(Vmic) to the total pore volume (Vt) increases from OMC (0.23), AC (0.40), PAC (0.55) to HPAC (0.80). These four carbon materials demonstrate distinguishable pore structure, therefore, analyzing their electrochemical and desalination performance can thoroughly understand the pore structure effects.

Table 1 Pore structure and elemental composition of carbon materials Samples SBET 2

Vt −1

Vmic 3

−1

APW1 VPW

Macropore- and Micropore-Dominated Carbon ... - ACS Publications

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