Starch Derived Porous Carbon Nanosheets for High-Performance

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Starch Derived Porous Carbon Nanosheets for Highperformance Photovoltaic Capacitive Deionization Tingting Wu, Gang Wang, Qiang Dong, Fei Zhan, Xu Zhang, Shaofeng Li, Huiying Qiao, and Jieshan Qiu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01629 • Publication Date (Web): 12 Jul 2017 Downloaded from http://pubs.acs.org on July 13, 2017

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Environmental Science & Technology

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Starch Derived Porous Carbon Nanosheets for High-performance Photovoltaic Capacitive Deionization

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Tingting Wu†, Gang Wang†, *, Qiang Dong†, Fei Zhan†, Xu Zhang‡, Shaofeng Li†,

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Huiying Qiao†, Jieshan Qiu†, *

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†

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Lab for Energy Materials and Chemical Engineering, Dalian University of

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Technology, Dalian 116024, Liaoning, China.

State Key Lab of Fine Chemicals, School of Chemical Engineering, Liaoning Key

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‡

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Panjin Campus, Panjin 124221, China.

School of Petroleum & Chemical Engineering, Dalian University of Technolgy,

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Corresponding Author:

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Gang Wang, *E-mail: [email protected]

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Jieshan Qiu, *E-mail: [email protected], Tel/Fax: +86-411-84986080

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Address: Dalian University of Technology, High Technology Zone, No. 2 Ling Gong

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Road, Dalian 116024, China

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(~4090 words, 1 small figures and 4 large multipart figures)

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1 ACS Paragon Plus Environment


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ABSTRACT

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Capacitive deionization (CDI) is an emerging technology that uniquely integrates

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energy storage and desalination. In this work, porous carbon nanosheets (PCNSs)

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with an ultrahigh specific surface area of 2853 m2/g were fabricated by the simple

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carbonization of starch followed by KOH activation for the electrode material of

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photovoltaic CDI. The CDI cell consisting of PCNSs electrodes exhibited a high salt

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adsorption capacity (SAC) of 15.6 mg/g at ~1.1 V in 500 mg/L NaCl as well as high

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charge efficiency and low energy consumption. KOH activation played a key role in

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the excellent CDI performance as it not only created abundant pores on the surface of

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PCNSs but also made it fluffy and improved its graphitization degree, which are

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beneficial to the transport of ions and electrons. PCNSs are supposed to be a

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promising candidate for CDI electrode materials. The combination of solar cells and

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CDI may provide a new approach to reduce the energy cost of CDI and boost its

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commercial competitiveness.

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KEYWORDS: porous carbon nanosheets; KOH activation; solar cells; capacitive

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deionization

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1. INTRODUCTION Surging demands on fresh water have attracted intense interests in developing

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new

desalination

technologies.

Capacitive

deionization

(CDI)

based

on

42

electrosorption is being increasingly investigated to complement or replace

43

conventional desalination technologies in treating low-concentration salty water. A

44

typical CDI cell consists of two electrodes and a spacer, and salty water flows in the

45

compartment between the electrodes. Ions in the water are adsorbed and stored in the

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electrical double layers (EDLs) of the electrodes when the CDI cell is charged and are

47

desorbed when the electrodes are short-circuited. The EDL based electrosorption

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mechanism endows CDI with many advantages such as low pressure, low cell voltage

49

and ambient operation, ecofriendly and highly energy efficient1-3. Moreover, as CDI is

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direct current powered, it possesses the potential to be combined with solar cells4-8 or

51

other renewable energy technologies9-11 to reduce fossil-fuel consumption and

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promote the application in remote areas.

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Properties of electrode materials play a key role in CDI performance. To obtain

54

excellent CDI performance, the electrode materials for CDI should possess large

55

specific surface area available for ion adsorption, well-distributed pore structure to

56

realize rapid ion adsorption/desorption and good conductivity for fast electron

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transport12, 13. Up to now, porous carbon is the most promising candidate for CDI

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electrode materials and activated carbon based materials14, 15, carbon aerogels16, 17,

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mesoporous carbon18, graphene based materials19-21, metal-organic framework (MOF)

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derived carbon22-25, etc. have been explored to acquire good CDI performance. 4 ACS Paragon Plus Environment

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Among various carbon materials, two dimensional (2D) porous carbon, especially

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carbon nanosheets, has attracted considerable attention due to its broad application

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prospects. Compared with particulate carbon materials, the ion diffusion path into

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carbon nanosheets is much shorter due to their thin structure26, which is beneficial to

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the rapid adsorption/desorption of ions. Thus graphene based materials have emerged

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as a class of promising CDI electrode candidates and an ultrahigh SAC value of 21.0

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mg/g (500 mg/L NaCl, 1.2 V) was reported for nitrogen-doped graphene sponge21.

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Nevertheless, the tedious preparation processes, high-cost and low yield of graphene

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based materials may greatly limited their application in CDI. Worse still, the

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preparation processes usually involve toxic reagents, such as concentrated H2SO4 and

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KMnO4, which may cause serious environmental issues. Recently, biomass including

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glucosamine hydrochloride27, bacterial cellulose28, Leucaena leucocephala wood29,

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cotton30, etc. have been used as the carbon source of CDI electrode materials. As

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biomass materials is abundant, low-cost and clean, developing carbon materials from

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biomass is advantageous from both environmental and economic perspectives.

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Therefore, biomass-derived carbon is considered as a promising candidate for CDI

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electrode materials.

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As a typical kind of biomass, starch is inexpensive and widely distributed in

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plants. Starch has been used as the carbon source to prepare carbon materials with

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different properties and morphologies31-33. In this work, porous carbon nanosheets

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(PCNSs) were fabricated by the self-blowing of starch without any additional agents

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followed by KOH activation for the electrode material of CDI. The CDI cells were 5 ACS Paragon Plus Environment


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integrated with commercial solar cells constituting a hybrid desalination system. The

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CDI performance of the electrode materials and energy consumption/recovery of the

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system were thoroughly investigated. The CDI system exhibited a high SAC of 15.6

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mg/g in 500 mg/L NaCl at ~1.08 V, charge efficiency above 65% and low energy

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consumption in various NaCl concentrations.

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2. MATERIALS AND METHODS

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2.1 Preparation of PCNS

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PCNSs were prepared by the direct carbonization of starch and subsequent

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activation. Typically, 10 g soluble starch (AR, Shantou Xilong Chemical Co. Ltd,

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China) was heated to 500 ºC under nitrogen flow at a ramping rate of 10 ºC/min and

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maintained at this temperature for 2 h. The resulting carbonaceous solid, denoted as

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biochar, was then chemically activated using KOH. The biochar was thoroughly

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mixed with KOH in water with the mass ratio of 1:3 and the mixture was dried at 110

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ºC overnight. After that, the mixture was heated at 800 ºC for 1 h under nitrogen flow.

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Finally, the activated sample was washed with 1 M HCl and distilled water

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respectively, and dried at 80 ºC for 6 h. Carbon nanosheets (CNSs) were prepared in

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the same way as PCNSs without the addition of KOH.

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2.2 Characterization

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The morphologies of PCNSs and CNSs were visualized by scanning electron

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microscopy (SEM, FEI Quanta 450), field-emission scanning electron microscopy

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(FESEM, FEI NOVA NanoSEM 450), transmission electron microscopy (TEM, JEOL

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JEM-2100 and FEI Tecnai G20) and high-resolution TEM (HRTEM, FEI, Tecnai F30). 6 ACS Paragon Plus Environment

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The microstructure was analyzed by a Rigaku D/Max 2400 X-ray diffractometer

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(XRD, Cu Kα radiation, λ=1.5406 Å) and a Thermo Scientific DXR Raman

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spectrometer with an excitation wavenumber of 532 nm. The pore structure and

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surface properties of the samples were measured on a Micromeritics 3-Flex surface

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characterization analyzer and a Thermo ESCALAB 250 X-ray photoelectron

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spectrometer, respectively. The electrical conductivity of CNSs and PCNSs was

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examined by a four-point probe conductivity meter (Four Probes Tech, Guangzhou,

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China, RTS-9).

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2.3 CDI tests

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To prepare CDI electrodes, a slurry of PCNSs or CNSs, acetylene black,

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polyvinyl butyral (PVB) and polyvinylpyrrolidone (PVP)34 with the mass ratio of

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82.5:10:6:1.5 in ethanol was coated onto a piece of graphite paper (57 cm2) and then

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dried at 80 ºC overnight. The thickness of the dry films was 130  20 m and the

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mass was about 0.1 g. A symmetric CDI cell was assembled for deionization tests

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using two pieces of the as-prepared electrodes, a piece of non-woven fabric as the

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separator, a silicon gasket (~1.3 mm thick), two titanium strips as the current

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collectors and two polymethyl methacrylate plates as the support. A schematic of the

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CDI setup used in this work is illustrated in Figure 1. CDI tests were performed in

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single-pass mode during which NaCl solution was continuously pumped from a tank

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(2.5 L) into the CDI cell by a peristaltic pump at a flow rate of 9.2 mL/min and then

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flowed into another tank. The changes of effluent conductivity and pH were recorded

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by an ion conductivity meter and a pH meter, respectively. The concentration of NaCl 7 ACS Paragon Plus Environment


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solution was calculated by a calibration curve according to the conductivity profiles.

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The CDI cells were powered by commercial solar cells with an output of 1.1 V/700

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mA and a photoelectric conversion efficiency of 19.6%. Pictures of the solar cells and

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a CDI cell are depicted in Figure S1. A household bulb was utilized to simulate

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sunlight. During the charging process, the CDI cells were connected with the solar

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cells while discharging is done by connecting the CDI cells with an external resistor

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absorbing the recovered energy. The cell voltage and current were recorded

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simultaneously by a multimeter (Model 2700, Keithley Instruments Inc., Cleveland,

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OH, USA) linked to a differential multiplexer (Model 7701, Keithley Instruments

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Inc.). The SAC (, mg/g), charge consumed (Σ, C/g) and charge efficiency () were

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calculated according to Eq. 1, Eq. 2 and Eq. 3, respectively:

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 

    C0  Ct 

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

 idt

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

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where Ф (mL/min) is the flow rate, C0 and Ct (mg/L) are the influent and the effluent

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NaCl concentration respectively, m refers to the total mass of the two electrodes (g), i

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refers to the current during the adsorption process (A), F is the Faraday constant

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(96485 C/mol) and M is the molar mass of NaCl (58.5 g/mol).

(1)

m

(2)

m

 F  

(3)


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Figure 1. CDI setup composed of a CDI cell, pH and conductivity meters,

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commercial solar cells and a multimeter.

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3. RESULTS AND DISCUSSION

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3.1 Characterization of PCNSs

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The thermal processing of starch is very complex involving dehydration and

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thermal decomposition. The TG curve of the soluble starch exhibits two major weight

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loss steps (Figure S2). The first step commencing at about 70 ºC corresponds to

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physical dehydration. The second one starting at about 240 ºC represents the chemical

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dehydration and thermal decomposition of starch during which water and other

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volatile molecular species are generated35. It is supposed that the released gases

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induced the foaming of starch and the formation of carbon nanosheets. From Figure

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S3a and b, it can be seen that the biochar obtained by carbonizing starch at 500 ºC for

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2 h at the ramping rate of 10 ºC/min is carbon frameworks composed of thin

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nanosheets. The formation of carbon nanosheets is closely related to the ramping rate.

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As shown in Figure S4, bulk structure instead of carbon nanosheets was obtained

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when the ramping rate is lower than 5 ºC/min possibly because the release of gases is 9 ACS Paragon Plus Environment


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too slow to induce the foaming process.

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CNSs were prepared by further carbonizing the biochar at 800 ºC for 1 h. A side

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view of CNSs is shown in Figure 2a. The thickness of CNSs is about 80 nm. Some

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bubbles possibly caused by the gases not released timely during the foaming process

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can be observed on the surface of CNSs. By contrast, PCNSs is a little thinner (~ 70

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nm) and the surface is very rough as a result of KOH etching (Figure 2d). TEM

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images at low magnification indicate that CNSs and PCNSs both are large carbon

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nanosheets (Figure S5). From Figure 2b and e, it can be observed that CNSs are very

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dense, while PCNSs are fluffy. The fluffy structure of PCNSs is supposed to be

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beneficial to the transport and accumulation of ions. Moreover, there is a layer of

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ultrathin carbon nanosheets and some small particles on the surface of PCNSs (Figure

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2e, as marked in the red rectangle). HRTEM was performed to further characterize the

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structure of the samples. The atoms on the surface of CNSs are randomly oriented,

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indicative of its low crystallinity (Figure 2c). Interestingly, the HRTEM image of

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PCNSs shows obvious lattice fringes and the corresponding fast Fourier transform

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(FFT) pattern exhibits six-fold symmetry feature (Figure 2f), indicating that some

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well-crystallized carbon domains in favor of fast electron transfer exists on the surface

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of PCNSs.


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Figure 2. Morphologies of the samples. FESEM images of CNSs (a) and PCNSs (d),

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TEM images of CNSs (b) and PCNSs (e), and HRTEM images of CNSs (c) and

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PCNSs (f). Inset in (f) is a FFT pattern of PCNSs.

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The microstructure of the samples was characterized by XRD, XPS and Raman

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spectroscopy. From Figure 3a, it can be seen that two broad diffraction peaks

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representing the (002) and (100) reflections of graphite emerge in the XRD patterns of

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CNSs and PCNSs, which reveals the amorphous structure of the samples. Broad peaks

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representing the (002) reflection can be observed at 22.3ºand 26.1ºfor CNSs and

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PCNSs, respectively. Compared with CNSs, the (002) reflection peak of PCNSs shifts

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to higher angles, indicating a decrease of d002 and an increase in the crystallinity.

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Figure S6 depicts the XPS survey spectra and the atomic ratio on the surface of CNSs

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and PCNSs. The ratio of carbon atoms on the surface of PCNSs is 94.6% while that

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on the surface of CNSs is 92.3%, indicating that KOH activation removed a portion of

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heteroatoms from the carbon structure. Figure 3b shows the Raman spectra of CNSs 11 ACS Paragon Plus Environment


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and PCNSs. G bands at around 1584 cm-1 correspond to in-plane vibration of sp2

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bonded carbon structure and D bands at about 1345 cm-1 mainly reflect defects and

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disorders in the graphitic structure. The intensity ratio of D bands and G bands is

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usually used to evaluate the graphitization degree of carbon. The ID/IG of CNSs was

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calculated to be 1.03, implying the domination of disordered structure. In comparison,

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the ID/IG of PCNSs decreases to 0.87, indicating increased graphitization degree of

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PCNSs. Moreover, a sharp 2D band at about 2689 cm-1 which is the second order of

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zone-boundary phonons and related to the number of layers of graphene emerges in

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the Raman spectrum of PCNSs. Generally, improved graphitization degree of carbon

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leads to higher electrical conductivity. The electrical conductivity of the samples was

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measured using the four-point probe method and the results revealed that the

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conductivity of PCNSs is an order of magnitude higher than that of CNSs (30.3 S m-1

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vs. 2.5 S m-1).

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Figure 3c shows the N2 adsorption-desorption isotherms of CNSs and PCNSs.

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CNSs exhibit very limited nitrogen uptake and the Brunauer–Emmett–Teller surface

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area (SBET) is only 90 m2/g (Figure 3c, inset), indicating that few pores exist in CNSs.

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The N2 adsorption-desorption isotherm of PCNSs is type I according to the IUPAC

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classification. The steep increase of the isotherm at very low relative pressure is

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caused by the capillary filling of micropores while the wide isotherm knee in the

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pressure range of P/P0  0.05 indicates the existence of narrow mesopores, which can

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be demonstrated by the pore size distribution. As shown in Figure 3d, PCNSs exhibit

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a broad pore size distribution in the range of 0.5-4 nm and the average pore diameter 12 ACS Paragon Plus Environment

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is 2.5 nm. The SBET of PCNSs is calculated to be 2853 m2/g which is much higher

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than that of CNSs.

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The morphology transition, improved graphitization degree and significantly

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increased specific surface area of PCNSs are ascribed to the reactions between KOH

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and carbon. KOH can react with carbon at high temperature and the reactions produce

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various potassium containing species (K2CO3, K2O, K) and gases (H2O, CO2) which

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can further etch carbon through redox reactions, generating rough surface and creating

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abundant pores in the carbon structure36, 37. In addition, as amorphous carbon is more

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reactive than the crystalline part, it is supposed that KOH first etches amorphous

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carbon away while graphitic structure is attained, leading to the formation of

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well-crystallized carbon structure and increased graphitization degree of PCNSs38.

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Figure 3. XRD patterns (a), Raman spectra (b), nitrogen adsorption-desorption

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isotherms (c) and pore size distribution (d) of CNSs and PCNSs. 13 ACS Paragon Plus Environment


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3.2 CDI performance

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Before CDI tests, capacitive performance of the electrodes was first investigated

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in 1 M NaCl electrolyte. The PCNSs electrode exhibited a specific capacitance of 196

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F/g at the current density of 1 A/g while that of the CNSs electrode is only 1 F/g

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(Figure S7c). Moreover, the specific capacitance of the PCNSs electrode remains at

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154 F/g with a capacitive retention rate of 78% at a high current density of 20 A/g,

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indicative of fast ion adsorption/desorption and excellent rate capability. CDI

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performance of the CNS and PCNSs electrodes was tested in a single-pass mode

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utilizing commercial solar cells as the power source. Figure 4a depicts the effluent

240

NaCl concentration profiles during the CDI tests. When the CDI cells are charged by

241

the solar cells, the effluent NaCl concentration decreases rapidly to a minimum value

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as a result of the fast electro-adsorption of ions into the electrodes. Then the NaCl

243

concentration increases slowly towards the influent level because of the gradual

244

saturation of the electrodes. After 10 min of adsorption, the CDI cells were connected

245

to an external resistor of 20 Ω. The ions previously adsorbed into the electrodes were

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desorbed and released back to the electrolyte, leading to an increase in the effluent

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NaCl concentration. The NaCl concentration variation range for the PCNSs electrodes

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is much larger than that for the CNSs electrodes. Correspondingly, the SAC of the

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PCNSs electrodes is 15.6 mg/g which is much higher than that of the CNSs electrodes

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(1.0 mg/g, Figure 4b). The big difference between the SAC of the CNSs and PCNSs

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electrodes is attributed to their structural discrepancies. Firstly, as the specific surface

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area of PCNSs is much higher than that of CNSs, the PCNSs electrode can provide 14 ACS Paragon Plus Environment

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more sites for ion adsorption. Secondly, the electrical conductivity of PCNSs is much

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better than that of CNSs, which is beneficial to fast electron transport. Thirdly, ion

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diffusion in the PCNSs electrode is much easier than that in the CNSs electrode. As

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shown in Figure S7d-f, for the CNSs electrode, the restacked and large nanosheets

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without pores form a very tortuous way for ion transport from the electrolyte into the

258

electrode, resulting in a high diffusion resistance39. By contrast, the abundantly porous

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and fluffy structure of PCNSs can ensure good ion transport, leading to excellent CDI

260

performance.

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Figure 4c exhibits the SAC profiles of the PCNSs electrodes at different influent

262

NaCl concentrations. It can be seen that the adsorption process reaches equilibrium

263

faster at higher NaCl concentrations. Moreover, SAC of the electrodes is strongly

264

dependent on the influent NaCl concentration. When the influent NaCl concentration

265

increases from 100 to 300, 500 and 1000 mg/L, SAC of the PCNSs electrodes

266

increases from 8.3 to 13.1, 15.6 and 20.8 mg/g, respectively. The increase of SAC

267

with the influent NaCl concentration is due to that the NaCl concentration is relatively

268

low under this condition and improving influent NaCl concentration can facilitate the

269

building-up of EDLs in the micropores of the PCNSs electrodes. When the influent

270

NaCl concentration further increases to 3 g/L and 5 g/L, SAC of the PCNSs electrodes

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decreases slightly to 18.8 and 16.8 mg/g, respectively, which may be caused by

272

aggravated co-ion repulsion and side reactions. Salt adsorption rate (SAR) was

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calculated based on the total time of the adsorption and desorption processes (Figure

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S8). In consistent with the SAC results, a maximum SAR of 1.0 mg/g/min or 66.9 15 ACS Paragon Plus Environment


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mg/m2/min was obtained in 1000 mg/L NaCl for PCNSs electrodes.

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A comparison between the SAC of PCNSs and previously reported carbon

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materials is shown in Table S1. The SAC of PCNSs is relatively high while the

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preparation process is much simpler and the carbon source is much more inexpensive,

279

which make them promising and favorable as the electrode material of CDI.

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The regeneration performance of the PCNSs electrodes was investigated in 100

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mg/L NaCl. As shown in Figure 4d, ions are adsorbed into the electrodes when the

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cell voltage increases to ~1.1 V, and are desorbed when the cell voltage gradually

283

decreases to 0 V, leading to a consecutive concentration change in the effluent NaCl

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solution. During the test, SAC of the PCNSs electrodes exhibited little recession (7.9

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mg/g for the 1st cycle and 7.6 mg/g for the 10th cycle), indicative of their acceptable

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regeneration capability. Cyclic stability of carbon electrodes for CDI is usually limited

287

by anode oxidation caused by irreversible Faradiac reactions40,

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improved by various methods, including adding membranes42, surface treatment43, 44

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and optimization of operational processes45.

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41

, but it can be

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Figure 4. CDI performance of the CNSs and PCNSs electrodes. Effluent

293

concentration profiles (a) and SAC (b) of the electrodes in 500 mg/L NaCl; SAC of

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the PCNSs electrodes at different influent NaCl concentrations (c) and cyclic

295

performance of the PCNSs electrodes in 100 mg/L NaCl (d).

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3.3 Energy consumption and recovery

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Cell voltage and electric current during the CDI tests of the PCNSs electrodes at

298

different influent NaCl concentrations were recorded by a multimeter. As shown in

299

Figure S9a, the cell voltage gradually increases to about 1.08 V after the CDI cell was

300

connected to the solar cells and then stays constant during the adsorption process.

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Correspondingly, the electric current reaches the maximum value instantly and then

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decreases as the CDI cell gradually became fully charged (Figure S9b). During the

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desorption process, the CDI cell acted as a power source and charged the external

304

resistor. The cell voltage and electric current gradually decreased to zero as the

305

external resistor absorbed the released energy. Figure S9c shows the power input and 17 ACS Paragon Plus Environment


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output of the CDI cell obtained by multiplying the cell voltage by the electric current.

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The peak power density output by the CDI cell can reach 10 W/m2 in 500 mg/L NaCl.

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Figure 5a shows the charge efficiency during the adsorption process. Charge

309

efficiency of the CDI cell stays above 65% at various influent NaCl concentrations,

310

indicating that most of the charge was consumed by ion adsorption. High charge

311

efficiency usually implies low energy consumption. Energy consumed by the CDI cell

312

was calculated from the integral of the power density during the adsorption process

313

(Figure S9c) and then normalized by the amount of salt removed (Figure 5b). As the

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influent NaCl solution increases from 100 mg/L to 300, 500, 1000, 3000 and 5000

315

mg/L, energy consumption changes from 141 kJ/mol to 121, 103, 96, 102, and 105

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kJ/mol, respectively. A minimum energy consumption of about 96 kJ/mol was

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obtained when the influent NaCl concentration is 1000 mg/L. Table S1 shows several

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previously reported energy consumption results of CDI under constant voltage

319

operation. It can be seen that energy consumption varies greatly between different

320

CDI systems. The values reported in this work are relatively low compared with that

321

reported by previous works.

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Energy recovery was calculated as the ratio of the energy recovered during the

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desorption process to the energy consumed by the CDI cell during the adsorption

324

process. As shown in Figure 5b, energy recovery of the CDI cell increases from

325

21.3% to 55.6% when the influent NaCl concentration increases from 100 mg/L to

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5000 mg/L. The gradual increase of energy recovery is mainly attributed to the

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change of equivalent series resistance (ESR) with influent NaCl concentration. ESR 18 ACS Paragon Plus Environment

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refers to the resistance existing in a capacitor, mainly including the electrical

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resistance of the electrodes, electrolyte, separator and various contact resistance. ESR

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can be determined from the ohmic drop (Figure S9a) and the instant current at the

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beginning of the discharge process (Figure S9b)46. As shown in Figure. 5c, ESR of the

332

CDI system decreases gradually from 17.2 Ω to 0.7 Ω when the NaCl concentration

333

increases from 100 mg/L to 5000 mg/L. As the energy consumed to overcome the

334

ESR cannot be recovered, the decrease of ESR greatly enhanced the energy recovery.

335 336

Figure 5. Charge efficiency (a), energy consumption/ recovery (b) and equivalent

337

series resistance (c) of the CDI cell consisting of PCNSs electrodes at different

338

influent NaCl concentrations; Energy output and recovery of the CDI cell consisting

339

of PCNSs electrodes at different external resistances in 500 mg/L NaCl (d).

340

The effects of external resistance on the energy output, energy recovery was

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studied in 500 mg/L NaCl. As shown in Figure 5d, energy output and recovery of the 19 ACS Paragon Plus Environment


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CDI cell both increases rapidly as the external resistance increases from 0 Ω to 20 Ω

343

and then changes slightly when the resistance further increases to 100 Ω. On the other

344

hand, as the external resistance increases, desorption of ions becomes more and more

345

slow (Figure S10) which is undesirable for application. For these reasons, the

346

optimum external load under this condition was determined to be about 20 Ω.

347

The results of CDI test and energy consumption/recovery indicated that on the

348

one hand CDI cells can be powered by the commercial solar cells and on the other

349

hand they can store the energy generated by the solar cells and serve as energy storage

350

devices. Nevertheless, as solar energy is intermittent and the intensity changes

351

throughout a day, introduction of power supply management and energy storage

352

devices is still needed to meet the steady energy demand of CDI and realize the

353

practical application of the self-sustainable desalination system.

354

3.4 Environmental implications

355

In this work, porous carbon nanosheets (PCNSs) were prepared by the direct

356

carbonization of starch followed by KOH activation as the electrode material of CDI

357

coupled with solar cells. The system exhibited excellent CDI performance while the

358

preparation of PCNSs is simple, template-free, scalable and environmental-friendly.

359

The combination of CDI with solar cells can effectively reduce fossil energy

360

consumption and carbon emission. The hybrid system also can promote the

361

application of CDI in remote areas or other occasions without electric power supply.

362

Moreover, in view of the excellent desalination performance of the CDI system, it is

363

expected that it can also exhibit good performance in the removal of heavy metal ions 20 ACS Paragon Plus Environment

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364

or other charged hazardous species.

365

ACKNOWLEDGEMENTS

366

This work was supported by the National Natural Science Foundation of China

367

(NSFC, No. 21336001), the Qaidam Salt Lake Chemical Joint Research Fund Project

368

of NSFC and Qinghai Province State People's Government (No. U1507103), the Star

369

of the Youth Science and Technology of Dalian (No. 2015R053) and the Fundamental

370

Research Funds for the Central Universities (No. DUT16TD14).

371

SUPPORTING INFORMATION

372

Optical images of the commercial solar cells and a CDI cell; the TGA curve of

373

soluble starch; SEM images of the biochar; SEM images of samples obtained by

374

carbonizing soluble starch at 800 ºC with different ramping rates; TEM images of the

375

samples at low magnification; XPS survey spectra of samples; Electrochemical

376

performance of the electrodes and possible ion transport ways in the electrodes; Salt

377

adsorption rate of the PCNSs electrodes; Cell voltage, current density and power

378

density during the CDI tests in different NaCl concentrations; Comparison between

379

the SAC of PCNSs and various carbon electrode materials; Variations in effluent

380

NaCl concentration, current density, cell voltage and power density of the CDI cell

381

consisting of PCNSs electrodes with different external resistances. This information is

382

available free of charge via the Internet at http://pubs.acs.org.

383

NOTES

384

The authors declare no competing financial interest.

385

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Starch Derived Porous Carbon Nanosheets for High-Performance

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