Efficient Capacitive Deionization Using Natural Basswood Derived

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Energy, Environmental, and Catalysis Applications

Efficient Capacitive Deionization Using Natural Basswood Derived, Free Standing, Hierarchically Porous Carbon Electrodes Mingquan Liu, Min Xu, Yifei Xue, Wei Ni, Silu Huo, Linlin Wu, Zhiyu Yang, and Yiming Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08232 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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ACS Applied Materials & Interfaces

Efficient Capacitive Deionization Using Natural Basswood Derived, Free Standing, Hierarchically Porous Carbon Electrodes Mingquan Liua, Min Xua, Yifei Xuea, Wei Nia, Silu Huoa, Linlin Wua, Zhiyu Yangb, Yi-Ming Yan*b a: School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, 100081, People’s Republic of China b: State Key Lab of Organic-Inorganic Composites, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China *Corresponding Author. Email: [email protected] Tel (Fax): +86-10-64451521

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Abstract Carbon electrodes are of great importance in constructing high performance capacitive deionization (CDI) devices. However, the conventional carbon electrode for capacitive deionization (CDI) is limited by its poor mechanical stability and low mass loading. Herein, we report a binder-free, free-standing, robust and ultrathick carbon electrode derived from wood carbon framework (WCF) for CDI applications. The WCF inherits the unique structures of natural basswood, containing straightly aligned channels interconnected with highly ordered, open and hierarchical pores. A CDI device based on thick WCF electrodes (1200 µm, equals to a mass loading of 50 mg cm-2) exhibits a remarkable areal salt adsorption capacity of 0.3 mg cm-2, a high volumetric salt adsorption capacity of 2.4 mg cm-3 and a competitive gravimetric salt adsorption capacity of 5.7 mg g-1. Also, the good mechanical strength and water tolerance of WCF electrodes improve the cycling stability of the CDI device. Finite element simulations of ion transport behavior indicate that the unique structure of WCF substantially facilitates the ion transport within the ultrathick CDI electrodes. This work provides a viable route to the rational design of free-standing and ultrathick electrodes

for

CDI

applications,

as

well

as

offers

insights

into

the

structure-performance relationship of CDI electrodes. Keywords: basswood, free-standing, high mass loading, carbon electrode, capacitive deionization

1. Introduction Capacitive deionization (CDI) has been drawn extensive attention as an emerging technique for water desalination, due to its minimal energy consumption and environmental friendliness.1,2 The water desalination performance of the CDI devices is largely dependent on the electrodes. Commonly, the conventional CDI electrodes


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are prepared through mechanically mixing the active materials (such as porous activated carbon, graphene and carbon nanotube, etc.), conductive additives and polymer binders, followed by coating onto a current collector.3-5 Unfortunately, two drawbacks of such a CDI electrode have to be pointed out: First, the addition of binders increases the internal resistance of the electrodes and blocks micropores of the active materials, leading to poor water desalination performance.6,7 Second, the usage of current collectors brings high contact resistance and extra production cost.8 To tackle these issues, researchers have recently devoted to developing novel CDI electrodes. For instance, the activated carbon nanofiber,9-13 3D graphene aerogel,14 carbon aerogel,15-17 carbon nanotube sponge18 and their composites7,19-22 have been utilized as binder-free electrodes for CDI applications, while the activated carbon cloth23,24 also has been employed as free-standing electrodes to assemble CDI devices. Although these binder-free and free-standing electrodes show competent desalination efficiency, they usually suffer from high cost and unsatisfied mechanical properties.19,25,26 Therefore, it is imperative to develop a cost-effective strategy to fabricate binder-free, free-standing and robust CDI electrodes. On the other hand, the salt adsorption capacity (SAC) of CDI devices relies on the electrical double layer capacitance (EDLC) of the electrodes.27 Specifically, the charge transport and ion diffusion inside the electrodes have known to significantly affect the charge/discharge efficiency of the electrodes.28 To achieve this goal, a conventional CDI electrode based on porous carbon (especially for the activated carbon) is usually fabricated by randomly coating an ultrathin carbon layer with a thickness of ca. 100-300 µm,29-31 resulting in a disordered structure onto the current collector. Unfortunately, such an ultrathin carbon layer leads to several serious problems for CDI applications, such as low mass loading (usually less than 20 mg



cm-2), poor salt removal capacity and undesired volumetric utilization efficiency of the whole CDI devices (assuming that the extraordinary usage of inactive additives, binders and current collectors for salt adsorption).32-34 To address these problems, a CDI electrode with high mass loading (or namely an ultrathick CDI electrode) is highly desired because it owns high packing volumetric density and low production cost.28 In view of practical applications, the ultrathick electrode ensures high areal salt adsorption capacity (SACA) and volumetric salt adsorption capacity (SACV), which are meaningful and scientific parameters to evaluate water desalination performance of the CDI devices.35,36 However, to our best knowledge, very few studies have focused on exploring an ultrathick electrode for CDI applications, due to its unsatisfied desalination performance. Several factors should be considered for limiting the performance of a conventional carbon based ultrathick CDI electrode: (1) The higher active materials loading means more binders would be used, resulting in decreased conductivity of the electrodes.6,7 (2) The carbon powders usually exfoliate from the current collector due their poor adhesive ability and unsatisfied water tolerance, especially at high mass loading.34 (3) The ion diffusion rate inside the ultrathick electrodes would be sluggish because of their blocked micropores and disordered macropores structure.28 Moreover, the development of ultrathick CDI electrodes based on coating carbon materials on current collectors is severely hindered by the lacking of available fabrication processes and preparation techniques. Of note, developing an ultrathick electrode also has been ignored by the reported works on binder-free and free-standing CDI electrodes. Therefore, it is very desirable but challenging to fabricate an ultrathick electrode for high performance CDI applications. More recently, natural wood has been employed as effective templates or starting


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materials for constructing well-ordered, open and hierarchical porous structure with straight align channels for versatile applications in energy storage

and

conversion.28,37-39 We expect that such a low-tortuosity and well-connected structure might be beneficial for electrolyte permeation through the ultrathick electrodes, allowing for the efficient transfer of electrons and ions with low resistance. Therefore, the unique properties of natural wood derived carbon materials should offer special opportunities of constructing a high performance electrode for CDI applications, but no trials have been devoted in this field. Herein, for the first time, we report the fabrication of an ultrathick CDI electrode derived from natural basswood, which is prepared by a simple carbonization and activation process. We demonstrate that the naturally porous structure of basswood is successfully inherited by the as-prepared wood carbon framework (WCF). When being used as a binder-free, free-standing electrode, the WCF exhibits excellent physical-chemical properties including high mass density, large specific capacitance, good mechanical strength and sufficient electrical conductivity. The CDI device based on ultrathick WCF electrodes (1200 µm, equals to an unprecedented mass loading of 50

mg

cm-2)

exhibits

superior

CDI

performance

regarding

SAC,

ion

adsorption/desorption rate and cycling stability. Remarkably, the WCF electrode delivers the highest SACA of 0.3 mg cm-2 among the reported binder-free and free-standing CDI electrodes, an impressive SACV of 2.4 mg cm-3 and a competitive gravimetric salt adsorption capacity (SACG) of 5.7 mg g-1. Finite-element simulations based on COMSOL Multiphysics reveal that the ion transport rate is greatly enhanced in the well-ordered and hierarchical pores distributed along with the straight aligned channels of WCF. This work not only provides a viable route to fabricate a free-standing and ultrathick CDI electrode, but also offers insights into the effect of



mass transport on the water desalination performance. 2. Experimental Section 2.1 Synthesis of the wood carbon framework (WCF) The WCF was prepared according to a modified procedure reported in literature,28 as shown in Figure 1a. Briefly, the natural basswood blocks were cut into cubical pieces with a certain size of 50 mm × 50 mm × 5 mm and dried at 60 ℃ for 72 h in air to evaporate all the moisture. The dried cubical wood pieces were carbonized in Ar at 1000 ℃ for 6 h followed by activated in CO2 at 750 ℃ for 10 h. The carbonized wood pieces were polished with 1000 grit sandpaper to obtain the desired thickness of 1200 µm with a size of 20 mm × 35 mm. Finally, the as-prepared wood pieces were cleaned with DI water several times and dried. The mass density of a WCF was ca. 50 mg cm-2 (equals to the mass loading). 2.2 Preparation of the activated carbon based electrode (AC) The AC electrode with a mass loading of 50 mg cm-2 was fabricated by coating a mixture of activated carbon, carbon black and PTFE onto a graphite plate (the thickness was ca. 0.5 mm) with the mass ratio of 8: 1: 1. The slurry was pressed into a cubical piece with a thickness of 1200 µm with a size of 20 mm × 35 mm, and then the AC electrode was dried at 130 ℃ for 12 h.34 2.3 Characterization of carbon materials Scanning electron microscopy (SEM, JEOL, Japan) and Transmission Electron Microscopy (TEM, JEM-2010, Japan) were used to characterize the morphologies and microstructures of the carbon materials. The specific surface area (SSA) was calculated according to the nitrogen adsorption-desorption isotherms tested using a


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Porosimetry System (ASAP 2010, Micro-metitics, Norcross, Ga). The volume of micropores and the pore size distribution (PSD) were measured by using the density functional theory (DFT) method. The volume of mesopores was determined by subtracting the micropore volume from the total pore volume of N2 absorbed at P/P0 equals to 0.95.25 The surface wettability of the electrodes was tested using goniometric equipment (HARKE-SPCA, China). Renishaw RM 2000 with 633 nm laser was used to characterize the Raman spectra. The X-ray powder diffraction (XRD) analysis was performed by using a PW-1710 (Philips, Netherlands) diffractometer with Cu Kα radiation. The chemical compositions of the carbon electrodes were characterized by X-ray photoelectron spectroscopy (XPS, Physical Electronics 5400 ESCA). Fourier transform infrared (FTIR) was carried out to test the surface functional groups on a Bruker Vector-22 FTIR spectrometer at 4000-40 cm-1. The mechanical compression experiments were conducted using the Discovery HR-1 hybrid rheometer. The pH changes during the CDI cycling tests were recorded with a pH meter (F-51, HORIBA, Japan). 2.4 Electrochemical measurement To investigate the electrochemical performance, cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy were carried out in a three-electrode system by using a CHI 760E electrochemical workstation. The three-electrode setup contained a platinum plate as the counter electrode, a piece of WCF or AC electrode as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and 1.0 M NaCl solution as the electrolyte. The gravimetric capacitances (F g-1) were measured from CV profiles according



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to the following equation (1): 34 = ∫ /2 ∆

(1)

where I (A) is the corresponding current density, v (V s-1) is the scan rate, m (g) is the mass of the active material, ∆ (V) is the potential window. 2.5 Desalination performance test The desalination experiments were performed by using a laboratory-scale CDI reactor with a size of 20 mm × 35 mm CDI unit. The CDI device was composed of two working electrodes with the same mass loading separated by a piece of insulating grid spacer, as shown in Figure 1b. The circulation flow of NaCl solution (the volume was 50 mL) was driven by a peristaltic pump with a flow rate of 20 mL min-1. The initial concentration of NaCl solution was 100 mg L-1. The conductivity value measurement of NaCl solution in real time was tested by using a conductivity meter connected with the CDI device. The schematic diagram of the CDI experiment setup is illustrated in Figure 1c, and the WCF based CDI device is shown in Figure S1. The operating potential was 1.2 V. The gravimetric salt adsorption capacity (SACG, mg g-1), areal salt adsorption capacity (SACA) and volumetric salt adsorption capacity (SACV) were calculated according to formula 2, 3 and 4, respectively: 34,40 SAC = V( − )/m

(2)

SAC = V( − )/

(3)

SAC = V( − )/

(4)

where V (L) is the solution total volume, m (g) is the total mass of the active materials of two working electrodes, C0 (mg L-1) is the original concentration of NaCl solution and Ce (mg L-1) is the final NaCl concentration, Sm is the total surface area of two


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electrodes, Vm is the total volume of two electrodes except for current collectors. The charge efficiency (φ) was obtained from formula 5:34 φ = ( × F)/C

(5)

in which SACG (mol g-1) is the adsorption capacity, F is Faraday constant (96485 C mol-1), and C (charge, C g-1) is obtained from the integrating the current. The energy consumption (W) of one salt adsorption process can be calculated based on the equation (6): 41,42 W = V ∫ "

(6)

The Kim-Yoon plot can evaluate the relationship between adsorption capacity and the adsorption rate for the CDI performance. The adsorption capacity at a certain time (SACt, mg g-1) and the corresponding adsorption rate (vt, mg g-1 min-1) of the electrodes were calculated according to the formula 7 and the formula 8: 34 = (( − ))/

(7)

= /"

(8)

where Ct (mg L-1) is the concentration of NaCl solution at t min, t (min) is the corresponding time of the adsorption process. 3. Results and Discussion We first characterized the structure of the WCF electrode by using SEM measurements. The top-view SEM results of the WCF electrode, as shown in Figure 2a-2c, exhibit a highly ordered and interconnected porous structure, including numerous big channels surrounded by relatively small channels. Moreover, there exists a large number of macropores with the pore width of ca. 1 µm inside the walls



of channels, confirmed by the image at a high magnification (Figure 2c). The cross-section view SEM images of the WCF electrode in Figure 2d-2e show that abundant open channels are straightly aligned along the growth direction. Figure 2f gives a direct observation of channels from the cross-sectional view, further confirming numerous macropores uniformly distributed on the walls of channels. It is well known that natural wood has abundant ordered channels for water and minerals transport from the root to branches and leaves of the tree.37-39 Therefore, these SEM results strongly demonstrate that the well-aligned elongated channels of natural basswood are perfectly inherited by WCF after the carbonization process. We expect that the well-ordered pore structure with low-tortuosity channels should significantly facilitate the electrolyte infiltration into the ultrathick electrode. Furthermore, the macropores inside the walls of the straight channels can also act as bridging pathways for accelerating the electrolyte transport rate between the parallel channels. For comparison, we also investigated the structure of the conventional AC electrode. As shown in Figure S2a-S2c, the top-view SEM images of the AC electrode demonstrate that the AC electrode exhibits a disordered, randomly connected and highly dense surface morphology. The cross-section view SEM images as shown in Figure S2d-S2f, display a thick AC layer attached to the graphite paper with observable cracks. The thick AC layer with disordered structure may lead to low transfer efficiency of ions, thus limiting the utilization efficiency of active materials. In general, SEM results demonstrate significantly different structures between the WCF electrode and the AC electrode. The effect of the electrode structure on the performance of water desalination will be discussed later in detail. Hydrophilicity is an essential parameter that evaluate the performance of the CDI electrode.27 The contact angles measurements were conducted to assess the


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hydrophilic properties of the electrodes. As shown in Figure 2g, the contact angle of the WCF electrode decreases rapidly from 15° to 0° in 20 seconds. In contrast, the AC electrode shows a nearly unchanged contact angle of 60° as time increased (Figure 2h). It suggests that the WCF electrode has a better water affinity, which endows easily access of salt solution into the WCF electrode and consequently improves desalination performances. The different hydrophilicity of WCF and AC electrodes may be due to following reasons: First, the WCF electrode possesses numerous ordered channels for water droplet penetration, by contrast, both the disordered structure and dense surface of the AC electrode lead to unfavourable water penetration.39,43 Second, the addition of hydrophobic binders might also reduce the water affinity of the AC electrode, which would not happen in the WCF electrode due to its binder-free feature.43 Third, the surface functional groups of the carbon materials also play an essential role in the hydrophilicity of the electrodes.44 Figure S3 shows the EDS mappings of C, O and N elements in the WCF, indicating the homogeneous distribution of elements. Figure S4 compares the XPS survey results of the WCF and AC. Obviously, the WCF (as shown in Figure S4a) displays a much stronger signal (at 532 eV) of oxygen-containing group than AC (as shown in Figure S4d), suggesting that the WCF owns more abundant oxygen functional groups at the surface, which may be contributed from the activation process of the WCF and its natural abundant hydroxyl groups on cellulose, hemicellulose and lignin.37-39 The amount of oxygen in the WCF is 10.4%, which is higher than that in the AC (6%). The high resolution patterns of C 1s and O 1s of the samples (Figure S4b-S4c for WCF, Figure S4e-S4f for AC) confirm the occurrence of C-O (at 285.2 eV) and C=O (at 289.0 eV). This result can be further proved from the FTIR spectra of the WCF and AC (as shown in Figure S5), where the peaks at 1080 cm-1 and 1580 cm-1



represent the C-O and carboxylate anion stretch mode, respectively.45 More information about the intensities of the peaks of C-related components are summarized in Table S1. The high amount of oxygen-containing functionalities of the WCF provide strong evidence for its promising hydrophilicity.44,45 On the other hand, the hydrophilicity of the electrodes would be improved during CDI experiments due to more hydrophobic carbon materials would become wetted along with the electrochemical cycling.46 Figure S6 displays the contact angles of the AC electrode after CDI cycling tests. It is clearly that the contact angle of the AC electrode decreases to smaller than 15°, suggesting an improvement of the water affinity. There are two reasons for such an improvement of hydrophilicity for the AC electrode. First, the ions stored on the surface of the electrodes by the electrostatic forces can drag along the solvent molecules.47,48 Second, the occurred electrochemical oxidation of carbon materials contributes to increasing the oxygen groups of the carbon electrodes, thus leading to an enhancement of hydrophilicity.46,49 N2 adsorption/desorption isotherms measurements were conducted to further investigate the porous structure of the WCF. As shown in Figure 2i, the WCF shows a steep increase in the amount of nitrogen absorbed at relatively low pressure (P/P0 < 0.1), indicating the existence of a large number of micropores.50 Moreover, the isotherm has a relatively unobtrusive hysteresis loop at higher pressures (P/P0 > 0.5), suggesting the mesoporous structure in the WCF.50 The unobtrusive hysteresis loop also indicates that the WCF contains small amount of mesopores. The corresponding SSA of the WCF is 839 m2 g-1 calculated by using BET method. The volume of micropores of the WCF is calculated to be 0.43 cm3 g-1 with 74% of the total pore volume (0.58 cm3 g-1), and the average pore size of the WCF is 2.17 nm. The cumulative pore volume distribution derived from DFT model (Figure 2j) and the


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calculated differential pore size distribution (PSD) pattern of the WCF (Figure 2k) further confirm the predominantly micropores structure for the WCF.25,46 Of note, there exists a weak peak between 2 and 4 nm of the PSD pattern for the WCF (Figure 2k),

again

reflecting

its

mesopores

structure.

Figure

S7

shows

the

macropore/mesopore size distribution calculated by BJH method, revealing the WCF contains peaks at 25, 45, 70 and 80 nm, suggesting its hierarchically porous structure. As mentioned above, there exist macropores with a pore width of ca. 1 µm in the WCF. Figure S8-S9 provide the TEM images of the WCF, where some mesopores can be observed (marked by red tags). Therefore, SEM and TEM results further confirm the hierarchically porous structure of the WCF. It has been proved that micropores are employed as active sites for ions adsorption, mesopores would act as the pathways for ions transport, and macropores can function as ion-buffering reservoirs to reduce the ions diffusion distance.10 As such, the hierarchical pore structure of WCF offers unique advantages in CDI applications. For comparison, the N2 adsorption/desorption isotherms measurements of the AC electrode were also performed. Figure 2i shows that the AC electrode exhibit a Type-I isotherm without an evident hysteresis loop, suggesting its typical micropores structure. The SSA of the AC electrode is measured to be 688 m2 g-1, which is slightly smaller than that of WCF. The PSD (Figure 2k and Figure S7) of the AC electrode shows that it only contains peaks smaller than 2 nm, again confirming its micropores structure. The obvious absence of macropores and mesopores in the AC electrode should limit the ions transportation. We noted that the AC electrode was fabricated using binders and conductive additives. Therefore, to understand

the

effects

of

binders

on

pore

structure

of

AC,

the

N2

adsorption/desorption isotherm of pristine AC (P-AC) was measured, as shown in Figure 2i. Also, a Type-I isotherm is observed and the SSA of the P-AC is calculated



to be 1370 m2 g-1, which is nearly twice as high as that of the AC electrode. The micropores volume of P-AC is 0.62 cm3 g-1(85% of the total pore volume), which decreases to 0.32 cm3 g-1 (82% of the total pore volume) after the addition of binders. The average pore size of P-AC is 1.88 nm, which is slightly smaller than that of the AC electrode (1.90 nm). Notably, the strong micropores peaks intensity is observed for P-AC, as shown in Figure 2k, suggesting that the high SSA of P-AC is mainly contributed from micropores. By contrast, the sharply decreased SSA of AC electrode should be caused by the loss of micropores, arising from blocking effect by the binders.6,7,34,43 For clear comparisons, the structural parameters of WCF, AC electrodes and P-AC are listed in Table S2. Electrochemical performances of the WCF and AC electrodes were first evaluated by using CV tests (Figure 3a, S12a, S12b). No redox peaks are observed from the CV curves of the WCF and AC electrodes, suggesting that they have a typical EDLC behaviour.34 However, Figure 3a displays that the WCF electrode possesses a rectangular like shape of the CV curve, while the AC electrode shows a distorted shape of the CV curve. Also, the capacitive current of the WCF electrode is higher than that of AC electrode. Figure 3b presents the plot of gravimetric capacitance (CG) versus scan rates, revealing that the CG at the scan rate of 1 mV s-1 is 87.1 F g-1 and 65.9 F g-1 for the WCF electrode and the AC electrode, respectively. These results confirm that the WCF electrode possesses a better EDLC behaviour and a higher specific capacitance than the AC electrode. GCD profiles are shown in Figure 3c. The WCF electrode shows a longer charging-discharging time than AC electrode at the same current density, indicating a better ion storage behaviour for the WCF electrode.37 In addition, GCD profiles of the WCF electrode remain well triangular shape with a low voltage (IR) drop at the current density of 5 mA cm-2, indicating its


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good conductivity and excellent rate performance. However, GCD profiles of the AC electrode show an unsymmetrical shape with a higher IR drop, suggesting it has a high inherent resistance and a poor ionic motion performance.36 EIS was further carried out to understand electron/ion transport properties of WCF and AC electrodes, as shown in Figure 3d. To fit the Nyquist plots, an equivalent electric circuit was applied (inset of Figure 3d), and the corresponding resistor elements derived from Nyquist plots are summarized in Table S3. Both electrodes show nearly vertical line at low frequencies, suggesting a pure capacitive behaviour.36 The intrinsic ohmic resistances (Rs, the intersection of the curve at the real part in the high frequency region) of the WCF electrode is 0.81 Ω, while the Rs value of the AC electrode is 3.66 Ω, reflecting that the WCF electrode has less internal loss and faster charge/discharge rates.50 The high Rs value of the AC electrode should be ascribed to its high intrinsic resistances and contact resistance between the current collector and electrode materials.34,36 Furthermore, we measured the resistances of electrodes by using a multimeter, as shown in Figure S13, where 8.5 Ω for the WCF electrode and 122.3 Ω for the AC electrode were recorded, respectively. Again, it suggests the WCF electrode has a high electrical conductivity. The diameter of a semicircle refers to the interfacial charge transfer resistance (Rct). The WCF electrode shows a smaller Rct of 0.52 Ω than the AC electrode (0.84 Ω), indicating a much faster ion transfer rate at the WCF electrode/electrolyte interface.36,50 The Warburg diffusion resistance (Rw, the projected length of Warburg region on the real axis) of the WCF electrode is 0.08 Ω, which is much lower than that of the AC electrode (0.62 Ω), further indicating a faster ion diffusion rate from electrolyte into the WCF electrode.50 In addition, Bode plots (Figure S12c) derived from EIS spectroscopy can provide parameters about frequency response of the impedance or



phase angle. It is clearly that the phase angles are -63° and -45° at a low frequency of 10-3 Hz for the WCF electrode and the AC electrode, respectively, confirming an ideal capacitive behaviour for the WCF electrode.34 The water CDI performances of the electrodes were tested by using batch-mode experiments. Figure 4a describes the conductivity variation of the salt solution in one adsorption/desorption process. Particularly, the WCF based CDI cell (WCF-CDI cell) reaches a conductivity of 40 µS cm-1, whereas AC based CDI cell (AC-CDI cell) gives a conductivity of 140 µS cm-1, indicating a higher SAC of the WCF-CDI cell. As a result, the SACG of WCF-CDI cell is measured to be 5.7 mg g-1, which is nearly 2.6 times as high as that of AC-CDI cell (2.2 mg g-1). Notably, although the WCF is an ultrathick CDI electrode, its SACG is still an average value among reported carbon based CDI electrodes.51 Salt adsorption/desorption time was calculated to assess desalination rate of electrodes. As displayed in Figure 4a, to reach a conductivity decrease of 60 µS cm-1, the adsorption times are 32 mins and 180 mins for the WCF electrodes and the AC electrodes, respectively. Similarly, the desorption time of WCF electrodes is 30 mins, which is shorter than that of AC electrodes (68 mins). The results demonstrate that the WCF electrodes not only display a high SAC, but also possess a fast ion adsorption/desorption rate. The regeneration performances of the WCF and AC electrodes were evaluated by performing several cycles of charge-discharge experiments. As shown in Figure 4b, it can be observed that the salt adsorption/desorption curves reach a stable state after several cycles of CDI processes. Under a given operation time (1600 mins), the WCF electrodes can achieve a larger desalination-regeneration cycles (7 cycles) than AC electrodes (6 cycles), indicating that the WCF electrodes have a higher working efficiency. The Kim-Yoon plots were also tested to investigate the desalination


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performance of the CDI electrodes.1 As shown in Figure 4c, the Kim-Yoon plot of WCF electrodes is located more in the upper and right region in comparison with that of the AC electrodes, again indicating that the WCF electrodes combine the key performances of both higher SAC and faster desalination rate. The charge efficiency is very essential for assessing the performance of CDI electrodes. Figure 4d depicts the corresponding current response of the WCF and AC electrodes during a desalination process. The charge efficiency is 30.25 % and 13.86 % for the WCF electrodes and the AC electrodes, respectively. Notably, the charge efficiency of WCF electrodes is also higher than that of CDI electrodes based on 3D graphene sheet-sphere composites with an ultrathin active layer coating (17% for 100 mg L-1 salt concentration).52 These results indicate the WCF electrode has low energy consumption, which is favourable for water desalination applications. The cycling stability of a CDI electrode is of practical importance. Figure 4e displays

the

cycling

stability

of

the

WCF

and

AC

electrodes

after

desalination/regeneration process for 50 cycles. The SACG of WCF electrodes could keep nearly unchanged during the lifetime test (5.7 mg g-1 for the last cycle with nearly 99.8 % retention of the highest SACG). However, the SACG of AC electrodes decreased to 2.0 mg g-1 for the last cycle with 91% SACG retention, reflecting a poor cycling stability. This result can be further confirmed by corresponding current responses of the first and last desalination cycles, as recorded in Figure S15a and Figure S15b. Obviously, the WCF electrodes have a smaller current loss than AC electrodes after the cycling tests. The excellent cycling stability of WCF electrodes may be related to their promising mechanical properties and water tolerance. As shown in Figure 4f, the engineering compression test shows that the WCF electrode can bear a high loading stress of 25.67 MPa, clearly suggesting that the WCF has



substantially mechanical strength as a free-standing CDI electrode. As shown in inset of Figure 4f, WCF electrode can be connected to a crocodile clip directly without showing cracking and deformations. Such a high mechanical strength of the WCF is consistent with that of ever-reported works.37,53,54 The robust structure of WCF electrodes helps to maintain the CDI performance for long-term tests. For the AC electrode obtained by the conventional slurry coating method, the carbon layer is easily cracked and detached from the current collector when the thicknesses exceeding 500 µm, which has been commonly observed in literature.53-56 The fragile structure of the AC electrode is possibly responsible for its poor cycling stability. In addition, after soaking the WCF electrode into salty solution for one month, as shown in Figure S16a, the solution was clean without any exfoliations were observed. However, the carbon powder was observed to easily exfoliate from the AC electrode (Figure S16b), suggesting its poor water tolerance.43 These results indeed prove that the WCF is robust enough as a binder-free and free-standing electrode for long-time CDI applications. The pH changes during a certain long-time CDI cycling tests were also performed, as shown in Figure S17. The pH values fluctuate during CDI experiments with a slight acidification of the salt solution during adsorption (charging) processes, which may be ascribed to the faradaic reactions along with the reduction of dissolved oxygen.57 To better understand the desalination performance of WCF electrodes, we conduct a comprehensive comparison of SAC values for various carbon based CDI electrodes. Table S4 summarizes the comparison of SACA and SACV for reported binder-free and free-standing CDI electrodes. The SACA and SACV of WCF electrodes are measured to be 0.3 mg cm-2 and 2.4 mg cm-3, respectively. Significantly,


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the SACA of WCF electrodes is the highest value among reported binder-free and free-standing CDI electrodes, while SACV of WCF electrodes is just slightly inferior to that of activated carbon cloth based electrodes with an ultrathin active layer (3.7 mg cm-3). In addition, we further compared the SACG of our WCF electrodes with reported binder-free and free-standing CDI electrodes, recorded in Table S5. The SACG of WCF electrodes is slightly smaller than that of reported binder-free and free-standing CDI electrodes. However, considering our ultrahigh thickness and relatively low initial salt concentration used in this work, the CDI performance of WCF electrodes is still impressive. Table S6 shows SACG values of various modified carbon based CDI electrodes. Of note, WCF electrodes exhibit a compatible SACG value, which is higher than that of the modified carbon electrodes with an extremely low mass loading. Alencherry et. al. developed an ultrathick (1350 µm) modified AC electrode (namely Ag/AC/CNT) with the SACG of 5.3 mg g-1 tested in an initial salt concentration of 500 mg L-1.58 Considering that a high salt concentration is beneficial for achieving high SACG, therefore, the 5.7 mg g-1 of WCF electrodes obtained with an initial salt concentration of 100 mg L-1 is very competitive and pronounced. These comparisons strongly prove that the WCF is a promising candidate for developing ultrathick CDI electrodes without sacrifice their desalination performances. Several reasons can be summarized to understand the high CDI performance of the WCF ultrathick electrode: (1) the good wettability of WCF electrodes favours the contacting between electrodes and salt solution; (2) the binder-free feature contributes to enhancing the electrical conductivity; (3) the unique highly ordered and well-developed hierarchical porous structure with straight aligned channels is propitious to solution penetration into the inner active sites for salt adsorption. (4) The good mechanical strength and free-standing properties help to improve the water



tolerance and cycling stability. To further shed light on structure-performance relationship of the CDI electrodes, we performed finite element simulations to unveil the ions transport behaviour insight the WCF and AC electrodes, the details are summarized in Supporting Information. Figure 5a and 5b present the cartoon depictions of the electrode structures of WCF and AC, as well as the possible ions migration pathways. In first case, we set a constant concentration gradient between the inlet and outlet of electrodes, and calculate the ion transport rate in two types of electrode configurations. Figure S19a reveals that the flux density in WCF electrode configuration is apparently higher than that of the AC electrode during all the diffusion process, indicating that the ions can transport much faster in WCF electrode than that in AC electrode. Moreover, the steady apparent diffusion coefficient of the ions in WCF electrode is calculated to be 1.16 × 10-9 m2 s-1, which is two orders of magnitude larger than that in AC electrode (6.62 × 10-11 m2 s-1). In second case, we hypothesize that the flux density in the outlet of electrodes is zero, and the flux density-time profiles are depicted in Figure S19b. When diffusion time is less than 2 seconds, the flux density of the WCF is obviously larger than that of the AC. Specifically, the flux density of the WCF model reaches to zero after 3 seconds diffusion, revealing a complete concentration polarization inside the electrode. For compassion, it needs 6 seconds to achieve complete concentration polarization for the AC electrode. To give an intuitive illustration, Figure 5c and Figure 5d show the variations of ions concentration along with the diffusion time inside two electrode configurations, revealing a faster change tendency of ions concentration in the WCF electrode than that in the AC electrode. All these results strongly demonstrate that there is a fast transport kinetic and less diffusion resistance for WCF electrode configuration, which is in well-agreement with the results obtained


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with electrochemical tests, such as EIS measurements. Finally, we assessed the environmental and economic benefits of WCF electrode based CDI devices. First, the cost should be significantly decreased due to the usage of WCF as the binder-free and free-standing electrode, thus avoiding the utilization of conductive additives, binders and current collectors. Noteworthy, wood is one of the most abundant biomass on Earth, which can be used as inexpensive starting materials to essentially reduce the production cost of CDI devices. Second, the raised concept of ultrathick electrodes is of practically important for CDI applications, as it can rationally deliver high SACA and SACV for CDI devices. Third, the energy consumption of the WCF electrodes is calculated to be significantly as low as 210.4 kJ mol-1 for one single adsorption process, which is economically prior to that of the AC electrodes (456.5 kJ mol-1 for our work, 377 kJ mol-1 for ever-reported work 42). Fourth, the simple carbonization and activation strategy is template-free, scalable, toxic reagents free, cost effective and environmentally friendly. Therefore, in comparison with other carbon materials, such as active carbon,58 graphene14, carbon nanotubes18 and ordered carbon59, etc., the basswood derived WCF is a competitive candidate of binder-free, free-standing and the ultrathick electrode for CDI applications. 4. Conclusion In summary, we have successfully fabricated a binder-free, free-standing, robust and ultrathick CDI electrode by a simple carbonization and activation of natural basswood. Our results demonstrate that the WCF electrodes inherited structures from natural wood, such as abundant low-tortuosity channels with highly ordered hierarchical pores, significantly contribute to boosting the CDI performances of the



ultralthick electrodes. An as-constructed CDI device exhibits a remarkable SACA of 0.3 mg cm-2, a high SACV of 2.4 mg cm-3, a competitive SACG of 5.7 mg g-1, as well as a fast salt absorption/desorption rate and excellent desalination cycling stability. The high CDI performance can be attributed to the outstanding electrical conductivity, high mechanical strength, and good water tolerance of the WCF electrode. Moreover, finite element simulations confirm that the ions transport rate in the WCF electrode is greatly accelerated. The present work provides a facile and low-cost strategy for designing and fabricating ultrathick electrodes for CDI applications, which can be certainly extended to other fields such as sensors, catalysis, energy storage and conversion. Conflicts of interest There are no conflicts to declare. Associated Content Supporting Information. Details on surface morphology and structure of the electrodes, additional electrochemical characterization, additional CDI performance metrics, and finite element simulations of ions transport behaviour in two electrode models are provided in Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org. Author Information *Corresponding Author: Yi-Ming Yan, [email protected] Tel (Fax): +86-10-64451521 ORCID Mingquan Liu: 0000-0002-3023-3900


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Funding sources National Natural Science Foundation of China (grant nos. 21575016). Acknowledgment Financial support from the National Natural Science Foundation of China (grant nos. 21575016) and from the National Program for Support of Top-notch Young Professionals is gratefully acknowledged. References (1) Matthew, S.; Slawomir, P.; Xiaowei, S.; Maarten, B.; Jeyong, Y.; Volker, P. Water Desalination via Capacitive Deionization: What Is It and What Can We Expect from It? Energy & Environmental Science 2015, DOI: 10.1039/C5EE00519A. (2) Younghyun, C.; Ki Sook, L.; SeungCheol, Y.; Jiyeon, C.; Hong-ran, P.; Dong Kook, K. A Novel Three-Dimensional Desalination System Utilizing Honeycomb-Shaped Lattice Structures for Flow-Electrode Capacitive Deionization. Energy & Environmental Science 2017, DOI: 10.1039/C7EE00698E. (3) Zou, L.; Morris, G.; Qi, D. Using Activated Carbon Electrode in Electrosorptive Deionisation of Brackish Water. Desalination 2008, 225 (1-3), 329-340, DOI: 10.1016/j.desal.2007.07.014. (4) Zhang, D.; Shi, L.; Fang, J.; Dai, K.; Li, X. Preparation and Desalination Performance of Multiwall Carbon Nanotubes. Materials Chemistry and Physics 2006, 97 (2), 415-419. (5) Ruiz, V.; Blanco, C.; Granda, M.; Menéndez, R.; Santamaría, R. Influence of Electrode Preparation on the Electrochemical Behaviour of Carbon-Based Supercapacitors. Journal of Applied Electrochemistry 2007, 37 (6), 717-721. (6) Wang, M.; Huang, Z.-H.; Wang, L.; Wang, M.-X.; Kang, F.; Hou, H. Electrospun Ultrafine Carbon Fiber Webs for Electrochemical Capacitive Desalination. New Journal of Chemistry 2010, 34 (9), 1843-1845. (7) Dong, Q.; Wang, G.; Qian, B.; Hu, C.; Wang, Y.; Qiu, J. Electrospun Composites Made of Reduced Graphene Oxide and Activated Carbon Nanofibers for Capacitive Deionization. Electrochimica Acta 2014, 137, 388-394. (8) Cong, H.-P.; Ren, X.-C.; Wang, P.; Yu, S.-H. Flexible Graphene-Polyaniline Composite Paper for High-Performance Supercapacitor. Energy & Environmental Science 2013, 6 (4), 1185-1191. (9) Wang, G.; Pan, C.; Wang, L.; Dong, Q.; Yu, C.; Zhao, Z.; Qiu, J. Activated Carbon Nanofiber Webs Made by Electrospinning for Capacitive Deionization. Electrochimica Acta 2012, 69, 65-70. (10) Wang, G.; Dong, Q.; Ling, Z.; Pan, C.; Yu, C.; Qiu, J. Hierarchical Activated Carbon Nanofiber Webs with Tuned Structure Fabricated by Electrospinning for Capacitive Deionization. Journal of Materials Chemistry 2012, 22 (41), 21819-21823. (11) Pan, H.; Yang, J.; Wang, S.; Xiong, Z.; Cai, W.; Liu, J. Facile Fabrication of Porous Carbon Nanofibers by Electrospun PAN/Dimethyl Sulfone for Capacitive Deionization. Journal of Materials Chemistry A 2015, 3 (26), 13827-13834. (12) Liu, Y.; Lu, T.; Sun, Z.; Chua, D. H. C.; Pan, L. Ultra-Thin Carbon Nanofiber Networks



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Figure. 1 (a) Illustration of the preparation procedure of the WCF. (b) Digital photograph and graphical illustration of the two-electrode system. (c) Schematic diagram of the wood based capacitive deionization experiment setup.


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Figure 2. SEM images of the wood carbon framework (WCF): (a-c) top view; (d-f) cross-sectional view. Contact angles measurements of the WCF electrode (g) and the AC electrode (h). (i) Nitrogen absorption/desorption isotherms for AC electrode, P-AC and WCF. (j) The cumulative pore volume distribution derived from DFT model. (k) The calculated differential pore size distribution pattern of the samples.



Figure 3. Electrochemical performance of the WCF and the AC electrodes. (a) CV curves at a scan rate of 1 mV s-1 in a 1.0 M NaCl aqueous solution. (b) The specific capacitance at different scan rates. (c) GCD curves at a current density of 5 mA cm-2. (d) Nyquist plots of the electrodes.


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Figure 4 (a) Variation of solution conductivity in one desalination-regeneration process. (b) Desalination-regeneration curves of the WCF electrode and the AC electrode. (c) The Kim-Yoon plots. (d) Current response of the WCF electrode and AC electrode and corresponding charge efficiency. (e) Cycling stability of the WCF and AC electrodes. (f) Mechanical compression stress-strain curves of the WCF electrode and the digital photographs showing the structural integrity of the freestanding WCF electrode connected to a crocodile clip directly.



Figure 5. The schematic diagrams of two possible ion migration pathways, (a) the WCF electrode, (b) the AC electrode. (c, d) The simulation results considering that the flux density of outlet of the electrodes was zero, (c) the AC electrode, (d) the WCF electrode.


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Efficient Capacitive Deionization Using Natural Basswood Derived

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