Hierarchical Porous Carbon Materials Derived from Kelp for Superior

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Hierarchical Porous Carbon Materials Derived from Kelp for Superior Capacitive Applications Na Sun, Zeyang Li, Xian Zhang, Wenxiu Qin, Cuijiao Zhao, Haimin Zhang, Dickon H.L. Ng, Shenghong Kang, Huijun Zhao, and Guozhong Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00635 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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ACS Sustainable Chemistry & Engineering

Hierarchical Porous Carbon Materials Derived from Kelp for Superior Capacitive Applications Na Sun,a,b Zeyang Li,a,b Xian Zhang,a Wenxiu Qin,a Cuijiao Zhao,a,b Haimin Zhang*a,b, Dickon H. L. Ng,c Shenghong Kang,a Huijun Zhaoa,d and Guozhong Wang*a,b a.

Anhui Key Laboratory of Nanomaterials and Nanotechnology,

Centre for Environmental and Energy Nanomaterials, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, (P. R. China). Tel: (+86) 551 65595616; E-mail: [email protected], [email protected]. b.

Science Island Branch of Graduate School,

University of Science and Technology of China, Hefei, Anhui 230026, China. c.

Department of Physics,

The Chinese University of Hong Kong, New Territory, Hong Kong d.

Centre for Clean Environment and Energy,

Gold Coast Campus, Griffith University, Queensland 4222, Australia.

KEYWORDS: kelp, hierarchical pore, supercapacitor, capacitive deionization

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ABSTRACT: The lack of high-performance electrode materials is the main factor restricting the breakthrough of capacitive applications. Recently, the design of hierarchical porous carbon (HPC) becomes a hot spot for the reason of integrating the advantages of different pore structures to optimize the material’s performance. The renewable, economical and widely available kelp was selected as the carbon source to obtain mesoporous structure using its naturally contained salts (Ca, Na and so on) as template in this work. Subsequent chemical activation was proceeded to get multimodal pores, and enrich the micropore structure. The micropores increase the specific surface area and provided abundant available adsorption sites, while mesopores improve not only the ionic conductivity but also the wettability of the material which are crucial in electrochemically related applications. When assembled in pouch cell (capacitor), HPC showed high specific capacitance of 190 F g-1 (1 A g-1, 1 mol L TEABF4/AN) with a broad operation voltage range (0-2.7 V). Further applied to electric double-layer (EDL) based capacitive deionization (CDI), it exhibited excellent salt removing capacity (27.2 mg g-1) with rapid response and efficient circularity. These excellent properties are mainly resulted from its high surface area (2613.7 m2 g-1) and unique multimodal porous structure. We also verified the superior performance of HPC material assembled CDI device driven by pouch cell for energy-integrated applications.

INTRODUCTION ACS Paragon Plus Environment

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EDL based CDI has been considered as a promising technology with the superiorities of high efficiency, low cost, easy regeneration and non-secondary pollution.1-2 However, the lack of efficient electrode material is a key limiting its rapid development of EDL based technology. In addition to this, the commonly used CDI power source is still the direct-current (DC) power mode, severely limiting the portable CDI applications, especially in state of emergency with insufficient electrical power. Combination a new power approach such as electrochemical capacitors (ECs) with CDI will be a promising option to solve the above problem.3-4 ECs is a high-performance storage system with high power, energy density, fast charge-discharge rates and extraordinary long cycle life. It is necessary for portable electronics, hybrid electric vehicles, backup power systems and pulse techniques.5-6 By far, the most common types of capacitor are EDLCs and pseudo capacitors. The pseudo capacitors possess superior specific capacitance or energy density,7-8 but the EDLCs tend to have a prominent rate capability, especially cycle stability, which is critical to its practical application.9 Noteworthy, the capacitive performance of EDL based CDI technology and EDLCs are all generally depend on the ions transportation and adsorption on the electrodes surface, hence, high surface areas and appropriate pores, high electrical conductivity, good wettability and recyclability are required.10-12 In this case, multimodal porous carbon material emerges as it can integrate the advantages of different pore structures to bring the performance to the extrem: micropores (< 2 nm) can greatly increase the material’s specific surface area (SSA) and provide abundant of available adsorption sites, thus enhancing the adsorption capacity of the double electric layer. Mesoporous (2-50 nm) can effectively increase the ionic conductivity of the material, improves the wettability of the material and reduces the resistance during molecular diffusion. Macropores ( ＞ 50 nm) facilitate the storage and transmission of electrolyte.13-16 In recent years, multimodal porous carbon materials become the research hotspot in both CDI and ECs research.17 Many technical approaches have been explored to synthesize HPC. Most often, majority of them relied on the dual template strategy incorporate with hard/soft templates or



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mixture of hard and soft templates.18-19 Other techniques such as sol-gel method20 or the breath figures approach21 are also utilized. Although they have been confirmed to be useful, there are still concerns with the red tape multi-step fabrication processes including the variation in templates and the concomitant tedious cleaning process of removing the template. Sometimes the course of template removal may introduce extra highly corrosive substance (for instance hydrofluoric acid).22 In addition to the synthetic method, the use of chemicals as raw materials is also a limiting factor to the application of materials in industries. Thus, more straightforward and cost-effective strategies are urgently needed. Biomass-derived porous carbons (from natural carbohydrates and plants) arise at the right time benefit by their characteristics of low cost, natural sustainability and environmental friendliness.23-25 At the same time, the activation of biomass carbon material by KOH is a common method which has the advantages of low activation temperature, short activation duration, high yield, high SSA and well-developed pores.26-27 Up to now, sustainable biomass derived HPCs have been used in a variety of applications such as supercapacitors,28 water purification29-31 and others.32-33 In this work, an easily available and low-cost marine alga kelp is acted as ingredient. The mesoporous framework is formed by a direct thermal treatment approach with the salt (such as Na, Ca and so on) and organic components contained in the kelp in N2 atmosphere.34 To enrich the pore structure and generate higher SSA, a chemical activation process is indispensable. Through the further carbonization of mesoporous framework precursor by KOH, the HPC materials are obtained, in which ample mesopores and micropores are constructive to the formation of active centres.35 When the mass ratio of KOH/PC is 4, the optimal product (HPC800-4) has the highest SSA of 2613.7 m2 g-1, large pore volume (PV) of 1.4 cm3 g-1 and pore size distribution (PSD) range from 1.4 to 3.8 nm. When HPC800-4 is used as supercapacitor’s electrode, in 1 mol L-1 NaCl solution and at the current density of 1 A g-1, the maximum specific capacitance is 202 F g-1 with viable stability after 10000 cycles. As the prominent capacitive


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property is a good predictor for desalination performance, when HPC800-4 is used as CDI electrode, it exhibits out-bound desalination capacity (27.2 mg g-1) and excellent regenerability. When HPC800-4 was used as the pouch cell electrode, it’s specific capacitance was measured up to 190 F g-1 at 1 A g-1 in the 0-2.7 V working voltage. When charged to 1.2 V, the pouch cell can drive the CDI device as a power supply. Thus, its performance is equivalent to that of a DC mains voltage power one in the sense of salt adsorption capacity (26.7 mg g-1) and rate. To the best of our knowledge, such HPC assembled pouch cell driven capacitive deionization has not been reported before. This work offers an easy, green and batch manufactured way to obtain HPCs material by using a plentiful inexpensive and eco-friendly biologic material as precursor. The advantages of the prepared sample are solidly demonstrated by its application in CDI and ECs. Beyond that, it may also hold the potential application in many related fields such as catalyse or water treatment.36-37 EXPERIMENTAL SECTION Materials Kelp was collected in market (Hefei, Anhui Province, China) and used directly after washing with water before freeze drying. NaCl, HCl and KOH were purchased from the Sinopharm Chemical Reagent Co., Ltd. Acetylene black and 5.0 wt% Nafion were acquired from Aladdin Chemistry Co., Ltd. (Shanghai, China). Titanium belt and Nickel foams were obtained from Oudifu Flagship store and the Cyber Chemical Material Network Co., respectively. Preparation of hierarchical pore carbon (HPC) and porous carbon (PC) In a typical synthesis, raw kelp was used after fully washing and freeze drying, subsequent carbonized at 700 oC for 1 h. After washing with HCl (2 mol L-1) and deionized water, the porous carbon (PC) was collected and dried at 60 oC. Then the PC product was further activated in various mass ratios of KOH to PC (1:1, 2:1, 4:1, 6:1) at different temperatures (700, 800 and 900 oC

with heating rate of 5 oC min-1 and holding time of 2 h). The above products were further

washed to neutral and finally dried at 60 oC to obtain the HPC samples. The resultants are denoted



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as HPCT-X standing for HPC obtained from chemical activation at a temperature of T with a KOH/PC mass ratio of X. Characterizations The morphologies and structural features of samples were analyzed by scanning electron microscopy (SEM, SU 8020) with an energy dispersive X-ray spectrometer (EDS Oxford, Link ISIS) using accelerating voltage of 5 kV, transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, JEOL-2010) with 200 kV accelerating voltage. The phase structural features were evidenced by X-ray diffraction (XRD, X’Pert Pro Super, Philips Co.) with Cu Kα radiation (1.5478 Å), X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250) with Al Kα X-ray source and calibrated with the C 1s peak (284.8 eV). Renishaw Micro-Raman Spectrometer (Renishaw in Via Reflex) using 532 nm laser excitation. The surface structure analysis of samples were carried by Surface Area and Porosity Analyzer (Autosorb iQ, Quantachrome Instruments), and the pore size analysis was performed using the instrument's after N2 adsorption and desorption tests, via the inherent software. The electrochemical workstation (CHI 760D, Chenhua Instruments Co.,) was used to measure the electrochemical performance. Electrode preparation and electrochemical measurements of ECs A mixture of active component (with an active material loading amount of about 2.0 mg cm-3), acetylene black and Nafion (8:1:1) were daubed on the Ni foam substrate to prepare electrode. After being dried overnight at 80 oC, the material was pressed at 15 MPa to become an electrode slice before being used for the following tests. A three-electrode system was used to evaluate the performance of the supercapacitor with 1.0 mol L-1 NaCl as electrolyte, using the Ni foam substrate loaded with active materials as the working electrodes, Hg/HgO as the reference electrode and platinum mesh as the counter electrode. Cyclic voltammetry curves (CV) were made at different scanning rates from 5 to 100 mV s-1 in the potential range of -1.0 to 0 V vs. Hg/HgO. Under the open circuit voltage of 10 mV, the electrochemical impedance spectroscopy (EIS) was measured in a frequency range of 0.01 Hz ACS Paragon Plus Environment

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to 10 mHz. In the voltage range of -0.9 - -0.2 V vs. Hg/HgO, galvanostatic charge-discharge (GCD) was measured at different current densities (0.5-10 A g-1). The cycle performance was evaluated by GCD cycling on an RST5200F work station (Suzhou Risetest Instrument Co., Ltd.) at a current density of 2.0 A g-1. Based on the discharge curves, the specific capacitance can be calculated by the following equations:38 C=I∆t/m ∆V

(1)

E=0.5C∆V2

(2)

P=E/∆t

(3)

where C, I, ∆t, m, ∆V, E and P denote the specific capacitance (F g-1), the response current (A), the discharge time (s), the mass of the active material (g), the change of voltage during discharge (V), specific energy (Wh kg-1) and specific power (kW kg-1), respectively. Meanwhile, using 1 mol L-1 TEABF4 /AN as electrolyte, cellulosic paper (NKK TF45) as a separator and Al foil as current collector, the target product of the pouch cell of capacitor was obtained. In this system, the specific capacitance of the single electrode can be calculated by the GCD values according to the following equation:39 C=2I∆t/m∆V

(4)

Capacitive deionization (CDI) performance measurements CDI device can evaluate the CDI performance of electrode materials. The adsorption capacity (SAC, mg g-1) and the corresponding salt adsorption rate (SAR, mg g-1 min-1) of salt can be obtained by the following equations:40 SAC=(C0-Ct) V/m

(5)

SAR=SAC/t

(6)

where C0 and V are the initial concentration (mg L-1) and the volume (L) of NaCl solution, respectively, m (g) represents the total active components mass of both electrodes in CDI device (the mass of carbon materials is about 60 mg), and Ct (mg L-1) is the NaCl concentration at t min.



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Our CDI device consists of an organic glass and rubber gaskets with an area of 10×10 cm2, a current collector with a Titanium belt area of 8.0×8.0 cm2 and a carbon electrode with an active area of 4.0×4.0 cm2. They were assembled symmetrically, A 2.0 mm thick hollow rubber gasket was mounted between the two electrodes to prevent short circuit. A DC power supply with 0 - 3 A and 0 - 30 V or a pouch cell was used to provide 1.2 V voltage. A peristaltic pump (BT-200SD, Shanghai, China) was used to keep the flow rate of NaCl stable at 20 mL min-1 into the CDI cell. The relationship between the conductivity of the sample and the concentration of NaCl solution could be measured by a conductivity meter (DDSJ-308, Shanghai, China) and the concentration of NaCl solution was obtained according to a calibration table before the experiment. RESULTS AND DISCUSSION Kelp is an important raw material widely used in industrial production.41 A serial of pore carbon materials had been successfully synthesized using kelp as the precursor as shown in Figure 1a. The PC was obtained by a simple heat-treating process. Then a series of HPCs was further acquired with further KOH activated process. SEM and TEM were used to examine the morphology. In a low-magnification image, the surface of raw kelp was wrinkled as shown in Figure 1b and Figure S1a, and the surface could be seen to be smooth without obvious pore structure under high-magnification image (Figure S1b). The kelp then showed rugged surface with a small amount of pore structure after a heat treatment in N2 atmosphere (Figure S1c). The formation of the pore structure was due to the reaction of the salts (K, Ca, Mg


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Figure 1. (a) The presentation of the building-up process of HPC and PC; Morphological and structural characterizations of SEM images: (b) original kelp, (c)-(e) HPC-800-4; TEM images of (f)-(g) HPC-800-4. and Na were detected in the burnt sample’s EDS mapping (Figure S1d)) and bio-polymers in kelp. In order to obtain the samples with rich pore structure, the above obtained precursors were further activated by KOH. The product of HPC800-4 was obtained after the activated process with KOH at 4:1 mass ratio at 800 °C for 2 h. As shown in Figure 1c, HPC800-4 remained the sheet-like morphology as the original kelp (Figure 1b), and the slice is about 0.5-1 μm thick filled with numerous pores (Figure 1d and Figure 1e). Notably, HPC800-4 product possessed mesh-like structure with more pore after KOH activation. TEM also confirmed that HPC800-4 had the pore structures with the mesopores (Figure 1f) and micropores (Figure 1g). From the SEM images of all samples, it is noted that the activation temperature and the dosage of KOH were the two key factors affecting the morphology of the final products. Elevating the treatment temperature from 700 °C (Figure S2a) to 800 °C (Figure 1f), there appeared more pores



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Figure 2. (a) N2 adsorption-desorption isotherms, (b) Corresponding PSD curves. (c) and (d) corresponding enlarged regions of (b). in the sample when the temperature was increased to 900 °C, the interconnected carbon walls were found partially destroyed. They appeared decomposed resulting in bigger holes (Figure S2b). In addition, as the dose of KOH increased (i.e. KOH/C mass ratio from 0 to 4), the density of the pores in the sample increased gradually (Figure S3a-c and Figure 1f), but when the ratio increased to 6, some of the pores fused together, probably due to the overraction of the KOH with the carbon skeleton (Figure S3d).42 To further understand the changes in pore structure with the variation of temperature and dosage, N2 adsorption/desorption measurements were conducted. All the samples indicated a typical IV isotherm with hysteresis behaviour (seen from Figure. 2a), which are characteristics of micropores and mesoporous.26 The total PSD is shown in Figure 2b. The peaks located in the region with pore size between 0 and 40 nm, which could be further grouped into two parts as shown in Figure 2c (0-5 nm) and Figure 2d (5-40 nm). All the samples treated by KOH activation exhibited similar pore size distribution pattern with two extra peaks located at about1.8 nm and 2.4 nm compared to the pattern of PC (Figure 2c). Without the KOH treatment, the PC sample exhibited abundant mesoporous structure, which ACS Paragon Plus Environment

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was due to the reaction of inherent salts (K, Ca, Mg and Na) and biological compounds in kelp. At the same time, the pores widen as there was increase of the temperature or the KOH ratio (Figure 2c and Figure 2d). The values of SSA and the total PV of the kelp-derived carbons were listed in Table S1. As the KOH ratio changing from 0 to 4, SSA and PV were increased from 328.6 m2 g-1 and 0.22 cm3 g-1 in the PC to 2613.7 m2 g-1 and 1.4 cm3 g-1 in the HPC800-4, respectively. This was expected that the increase of KOH ratio enhanced the formation of the micropore structure. Nevertheless, as the KOH ratio further scaled up, the corresponding SSA and PV of HPC800-6 would decrease to 1379.1 m2 g-1 and 0.7 cm3 g-1, which was caused by the fracture of carbon groups. The above results were consistent with the pore structure produced by KOH activation as in some previous report.43 As for the activation temperature of the reaction, it was also a crucial factor which affected the porous structure. Raising the activation temperature from 700 °C to 800 °C, the SSA increased from 1551.1 to 2613.7 m2 g-1 and meanwhile the PV expanded from 0.8 cm3 g-1 to 1.4 cm3 g-1, respectively, but decreased at 900 °C. This is because high activation temperature was conducive to the activation process, while too high temperature caused the violent gasification reactions, resulting in pore collapse and reducing the SSA.44 These phenomena appeared correspond to the SEM and TEM results. In short, the HPC800-4 sample had the largest SSA, wide PSD and maximal PV. The porosity of the materials can be optimized by adjusting the reaction temperature and the amount of KOH. The influence of the state of the carbon-based materials on the electrochemical performance in water was further discussed. The results of XRD show that PC and HPCs samples possessed the characteristic peaks (002) at 2θ∼25°, representing partially graphitic stacking structures. A broad hump of (100) at 2θ ∼45° indicated the amorphous carbon with disorderly stack up (Figure S4a). The intensity of HPC800-4 at the low-angle scattering peak was higher than that of PC, which suggested that high density of micropores and unordered texture had been formed, thus providing more active sites.45 Raman spectra further gave the evidence of characteristic disorder carbon (D band: 1360 cm-1) and graphitic carbon (G band: 1586 cm-1) respectively (Figure S4b). The weaker and broader peek accounted for more disorder carbon. The specific values of ID/ IG (ID:D band, IG: G band) indicated the structural order information. ACS Paragon Plus Environment


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All the results were summarized in table S2, the IG/ID values of HPC700-4, HPC800-4 and HPC900-4 were 0.97, 1.0 and 1.02, respectively, that was because higher carbonization temperature led to a higher structure alignment. Nevertheless, the graphitization degree decreased as amount of KOH increased, suggesting that the reaction between carbon and KOH had resulted in the formation of pores and disordered carbon. The appropriate coexistence of graphitic and amorphous carbon would greatly improve the electrical conductivity, wettability and ion storage.35 Surface chemical compositions analysis of PC and HPC800-4 were conducted by XPS measurements. These materials were confirmed to consist of C, N and O. A significant signal reduction of N and O was observed in HPC800-4 comparing to that of PC (Figure. S5a). More detailed analysis was shown in Table S3, HPC800-4 possessed higher C contents while lower-level N and O doping than that of PC, in that the non-carbon elements are more inclined to decompose in the presence of KOH.1, 46 The N1s XPS results were fitting for four peaks represent pyridinic-N (398.3 eV), pyrrolic-N (399.57 eV), graphitic-N (400.8 eV) and quaternary-N+-O- (402.1 eV) (Figure. S5b), respectively. More accurate as analyzed in Table S3, even the pyridinic-N in favor of improving pseudo capacitance decreased, HPC800-4 had higher pyrrolic-N, graphite-N, quaternary-N+-O- that will respectively increase the pseudo capacitance, enhance conductivity and promote wettability.47-48 In brief, suitable combination of distinct nitrogen form will jointly contribute to the capacitive performance. In order to evaluate the capacitive performance of the samples, CV tests were performed. CV curves of all the samples were almost rectangular


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Figure 3. (a) CV curves of all samples measured at a scan rate of 50 mV s-1; (b) CV curves of HPC800-4; (c) Charge-discharge curves; (d) Specific capacitance obtained at different current densities of PC, HPC800-1, HPC800-2, HPC800-4, HPC800-6, HPC700-4 and HPC900-4. (Figure 3a and Figure S6), indicating they are ideal EDL electrode materials. The integral area of CV curve indicated the electrode’s ion adsorption capacity. It was evident that the HPC800-4 sample had the largest integral area, showing the maximal specific capacitance among samples benefited by its highest SSA and presence of applicable pores (Figure. 3b). In addition, the capacitive performance could also be explored by GCD measurement (Figure S7 and Figure S8). All the GCD results appeared regular symmetric triangles shape. The HPC800-4 sample exhibited the longest discharge time (Figure 3c), which demonstrated that it had the optimal capacitive performance. Therefore, the results of CV and GCD were consistent, HPC800-4 possessed the most outstanding capacitance owing to the hierarchical porous frame equipped with larger surface area and accessible pores, allowing larger ion storage, faster ions adsorption and diffusion capacity.49 At different current densities, the specific capacitance values were calculated from the symmetric GCD curves according to equation (1). Figure 3d presented all the samples’ specific capacitance values decreasing with the current density increasing from 0.5 to 10 A g-1 due to the insufficient surface reaction ACS Paragon Plus Environment


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and ion transportation. Within the range of current density studied, the specific capacitance of HPC800-4 was the highest one and retained at 107 F g-1 even at 10 A g-1. This excellent performance was attributed to its efficient charge storage characteristics, rapid charge, ion transfer and buffering effect of the hierarchical pores. The ion-transport features and resistance of the electrode were measured by Nyquist plot tests (NaCl solution: 1 mol L-1, frequency range: 104 ～10-2 Hz). Obviously, the obtained lines nearly perpendicular to the real axis indicated the dominant EDL charge-storage characteristic, and the small semicircle diameter suggested the low-resistance charge transformation, which is beneficial to give fully play to the electrochemical properties.24 As shown in Figure S9, the results from the HPC800-4 sample exhibited the smallest semicircle and the steepest slope, manifesting the best ion-transport performance and electroconductibility due to the suitable surface conditions such pore structure and state of carbon. The Ragone plots (Figure 4a) are calculated from the GCD results. The specific energy density of HPC800-4 was 13.7 Wh kg-1 at a current density of 1 A g-1, highly outdistancing the commercial equipment (< 3 Wh kg-1). Moreover, the HPC800-4 assembled capacitor also showed a long-term applicable stability with the ~95% capacitance retention after 10000 cycles at a current density of 2.0 A g-1 in the 1.0 mol L-1 NaCl solution (Figure. 4b), suggesting the promising for supercapacitors electrode materials. The inset of Figure 4b revealed the typical charge discharge curves for the 10th, 2000th, 5000th and 10000th cycles, the curves have nearly the same shapes without obvious deviation showing the excellent capacitance properties. In order to show a practical application of the HPC materials, a pouch cell was prepared with multilayers electrodes (6 + 6 = 12 layer) using 1 mol L-1 TEABF4 /AN as electrolyte with a total mass of


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Figure 4. (a) Ragone plots of symmetrical of all samples; (b) Cyclic stability test of HPC800-4 electrode at a GCD current density of 2 A g-1 for 10000 cycles in 1.0 mol L-1 NaCl solution; The inset is GCD curves of the 10th, 2000th, 5000th and 10000th cycles. (c) GCD curves of HPC800-4 at different current density in 1 mol L-1 TEABF4/AN; (d) Lightened “CDI” LED pattern. the active material was 10 g. In Figure 4c, at a current density of 1 A g-1 and a maximum charging voltage of 2.7 V, the HPC800-4 assembled symmetrical capacitor exhibited the specific capacitance up to 190 F g-1. Furthermore, when charged to 2.0 V, the pouch cell supercapacitor was used to light-up parallel LEDs in shape of CDI as shown in Figure 4d. These indicated that the practical supercapacitor properties of our HPC800-4 materials. In 1.0 mol L-1 NaCl solution, the superior capacitance performance of HPC800-4 indicated that this material could be used in CDI. A signal mode CDI device was composed of a power supply, a pump, a conductivity meter, a water container and a homemade CDI unit. The desalination performance was studied in room temperature, a voltage of 1.2 V (maintain the maximum electrostatic force while avoiding the hydrolysis of H2O molecules split into H2 and O2), a NaCl solution of 500 mg L-1 and a water supply speed of 20 mL min-1. In addition, HPC800-4 assembled pouch cell can also be used as power source for driving CDI device



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Figure 5. (a) Current response profiles of HPC800-4 powered by pouch cell, inset of (a) is the diagram of pouch cell powered CDI course; (b) Part of the discharge curve at 8 mA g-1 of HPC800-4 in 1 mol L-1 TEABF4 /AN and inset of (a) is the original charge-discharge curve; (c) home-made CDI device with bisymmetric section and ilka contains 1: plexiglas covers and rubber gaskets; 2: a current collector, 3: HPC800-4; (d) Schematic illustration single of a pouch cell composed by 3: HPC800-4; 4: Al foil, 5: cellulosic paper (NKK TF45). with a charged of 1.2 V (as shown in inset of Figure. 5a). During the deionization process driven by the pouch cell, the maximum current could reach up to 8 mA Figure 5a). When discharging at the maximum current, the pouch cell maintained enough voltage output constant of about 1.2 V for long enough time with slight voltage attenuation. (Figure 5b), which further verified the applicability of pouch cell powered CDI. More specifically, Figure. 5c demonstrated the complete capacitive deionization reactor make up of two half cells each contains a plexiglas covers and rubber gaskets, a current collector and HPC800-4 as electrode material. At the same time, one of the multilayers electrodes in pouch cell contains a current collector (Al foil), a separator (cellulosic paper (NKK TF45)) and HPC800-4 as electrode material as shown in Figure 5d.


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24

24 -1

-1

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(a) 30 18 12 6

18 12 6 0

0 1000

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0

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(d)

24

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10

1500

Time (s)

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-1

-1

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0

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


18

DC Powered (HPC800-4) Pouch Cell Powered DC Powered (PC)

12 6 0 0

200

Time (s)

400

600

1

0.1

1

-1

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SAC (mg g )

Figure 6. Electrosorption and regeneration cycles of HPC800-4 actuate by (a) DC Power; (b) Pouch cell in 500 mg L-1 NaCl; (c) SAC curves; (d) Ragone plots of (SAC vs. SAR) of DC powered CDI with PC and HPC800-4 as electrode and pouch cell powered CDI with HPC800-4 as electrode. As CDI is driven by the principle of an electric double-layer capacitor (EDLC), it absorbs cations and anions to the opposite charged side of the electrodes, stores them in the electric double layer by applying voltage, and releases ions when removing or reversing the voltage. Typically, four steps are included in the CDI course: (i) mass transfer of salt ion in the treating water; (ii) salt ions transfer into the interior of porosint; (iii) electrosorption and storage of ions in the EDL exists between the electrode interface and solution; (iv) ions diffusion between the treating water and the porosint.50 When the voltage was applied by a DC power supply or pouch cell, the SAC of each electrode increased rapidly and attained equilibrium within 500 s, much shorter than the time of other materials,51 then the SAC declined as the voltage removed as shown in Figure 6a and Figure 6b. Good regeneration is necessary in practical application. In this work, we demonstrated that the electrodes could be nearly entirely rebirthed for 4 cycles by removing voltage. The HPC800-4 sample exhibited higher salt adsorption capacity of 27.2 mg g-1 than PC (6.8 mg g-1) (Figure 6c) and was comparable with the current reported values as shown in Table S4. It also showed faster ACS Paragon Plus Environment


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salt adsorption rate, which was crucial to practical application of CDI (Figure 6d). The high SSA, proper aperture and excellent conductivity of HPC800-4 sample can promote the salt ions transfer and ion adsorption and these were the main reasons for the excellent CDI performances as high SAC, rapid SAR and good reversibility.52 The electrosorption isotherms of the electrodes were studied by changing the concentrations of the initial salt solution. When the NaCl concentration was adjusted from 40 to 6000 mg L-1, the desalination capacities changed from 6.8 to 60 mg g-1, respectively. This attributed to the fact that higher concentration is more likely to form a tight EDL, resulting in high SAC (Figure S10). The power was supplied by the 1.2 V pouch cell, its performance was comparable to that of DC powered supply in both salt adsorption capacity (26.7 mg g-1) and adsorption rate (Figure 6d). The slight difference in adsorption capacity and adsorption rate might be caused by the small voltage drop produced by the current in a short-time during the desalt course. All the experimental results showed that the HPC800-4 sample possessed good desalination behaviour with rapid responsiveness and superior renewability. It also revealed the feasibility of utilizing the HPC800-4 sample as electrode for the application of pouch cell powered CDI. CONCLUSIONS The electrode material with high SSA of 2613.7 m2 g-1, multimodal structure and excellent conductivity had been successfully fabricated from kelp via a KOH activation step. The superior material HPC800-4 demonstrated outstanding capacitance storage feature (202 F g-1) at a current density of 1.0 A g-1 and long-time stability. A pouch cell was assembled to illustrate its applicability. The HPC800-4 symmetrical pouch cell exhibited the specific capacitance (190 F g -1,

1 A g-1) with a broad operation voltage range (0-2.7 V) using 1 mol L-1 TEABF4/AN as

electrolyte. When charged to 1.2 V, it exhibited potential application in driving CDI process with HPC800-4 as electrode compared with DC power supply mode. Its salt adsorption capacity (26.7 mg g-1) was comparable to that of DC (27.2 mg g-1) powered CDI process. Most importantly, we demonstrated the possibility of developing an integrated technology for energy-saving and


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portable CDI with a new power supply mode. It is also expected that target porous material of HPC800-4 can be used in other electrochemical field. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Tables for the summary Texture Properties and Peak intensities calculated from Raman results; Performance comparison of HPC800-4 and the currently reported CDI materials. SEM images of the original kelp; SEM image of kelp after thermal treatment; corresponding EDS results; SEM images of HPC700-4, HPC900-4, PC, HPC800-1, HPC800-2, HPC800-6; XRD patterns and Raman spectra; XPS results; CV curves; GCD curves; and Nyquist plots of PC, HPC800-1, HPC800-2, HPC800-4, HPC800-6, HPC700-4 and HPC 900-4; Desalination capacities of HPC800-4 at different concentrations of NaCl solution. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected], [email protected]. Tel: (+86) 551 65595616; Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the National Key R&D Program of China (2017YFA0207203), the Natural Science Foundation of China (Grant No. 51672277, 51432009), the CAS Pioneer Hundred Talents Program, China.



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