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Synthesis of Silicon Carbide-derived Carbon as an Electrode of a Microbial Fuel Cell and an Adsorbent of Aqueous Cr(VI) Shally Gupta, Ashish Yadav, Shiv Singh, and Nishith Verma Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03832 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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Industrial & Engineering Chemistry Research

Synthesis of Silicon Carbide-derived Carbon as an Electrode of a Microbial Fuel Cell and an Adsorbent of Aqueous Cr(VI) Shally Gupta1, Ashish Yadav1, Shiv Singh1, Nishith Verma1,2* 1

Department of Chemical Engineering, Indian Institute of Technology Kanpur, India 208016

2

Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, India 208016

* Corresponding author: Prof. Nishith Verma Telephone: (91)-512-2596352/7704 Fax: (91) (512) 2590104 E-mail address: [email protected]; [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT: Micron-sized non-porous silicon carbide (SiC) powder of the spent heating elements of a graphite furnace were used as the common precursor of two different forms of carbide-derived carbon (CDC) synthesized by chlorination at different temperatures: (1) graphitic, (2) amorphous Si-CDCs. Whereas the former material having high electroconductivity was used as an efficient electrode of a microbial fuel cell (MFC), the latter material having high specific surface area was used as an efficient adsorbent for aqueous hexavalent chromium (Cr(VI)). The MFCs generated a significantly high maximum power density of ~1570 ± 30 mW/m2 and open circuit potential of ~460 ± 5 mV. The adsorbents exhibited a significantly large adsorption capacity of ~95 ± 5 mg/g. This study has developed for the first time two types of SiCDCs having different physico-chemical characteristics, from the common SiC precursor via the facile route of different temperature conditions, for bioelectricity generation and environmental remediation applications. Keywords: carbide-derived carbon; silicon carbide; graphitic carbon; amorphous carbon; microbial fuel cells; adsorbents.

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1. INTRODUCTION Carbide-derived carbons (CDCs) have received much attention in the past decade because of the high specific surface area, and more importantly, tunable and narrow pore size distribution (PSD) of the materials.1,2 Considering that these materials have significantly large surface area and microporosity, they have been mostly applied as supercapacitor electrodes3-6 and an energystorage material for the high calorific value-gases, such as hydrogen, methane and n-butane7-10. Carbides of various metals have been used as the precursor for CDCs. Common examples are titanium9, molybdenum11, tungsten12, vanadium13 and silicon14. A low-cost material, silicon carbide (SiC) is the preferred choice for the precursor of CDCs. The selective etching of silicon by chlorination is the most common method used to synthesize SiC-derived carbons (Si-CDC).3,14-16 Difference in these methods lies in the posttreatment steps and the source (raw material) of SiC. Tee et al. treated Si-CDC by physical activation using CO2 and showed that the activation significantly (approximately two times) increased the BET surface area and microporosity in the material, resulting in the enhanced performance of the prepared Si-CDC supercapacitors.3 Duan et al. synthesized CDC from the commercial polymethyl(phenyl)siloxane resin.14 In the later study the authors treated the synthesized Si-CDC with NH3 and showed that the introduction of nitrogen-containing surface functional groups in Si-CDC increased the CO2 adsorption capacity of the material.15 Contrary to these studies, Kaarik et al. modified a commercial SiC, using the mixed metal (Co/Ni/Fe) catalysts and showed that the presence of metals in SiC decreased the specific surface area.16 However, the graphitic characteristics of the material increased.

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There are a few recent studies which have focused on the synthesis of the mesoporous- or hierarchical micro-mesoporous Si-CDC, with the hypothesis that relatively larger pores (> 2 nm) in the material mitigated hindrance to the ionic transport in the pores, thereby increasing the capacitance of the material. Along this direction, Korenblit et al. synthesized mesoporous CDCs from SiC derived from the high quality commercial SBA-15 mesoporous SiO2 template and polycarbosilane.17 Tsai et al. used an ordered mesoporous SiC that was derived from the magnesio-thermal reduction of the templated carbon-silica to synthesize a mesoporous Si-CDC. 18

Yan et al. chemically activated Si-CDC, using KOH, and showed a seven times-increase of

mesoporosity in the material.19 The enhanced capacitance of the prepared Si-CDC was attributed to increase in the mesoporosity content of the material. From the overview of the aforementioned studies it is clear that Si-CDC has been prepared as supercapacitor and/or for gas storage. To the best of our knowledge no study has been performed to prepare Si-CDC as an electrocatalytic electrode and as an adsorbent. In the present study, Si-CDC was derived from the waste SiC heating elements of a high temperature graphite furnace and used for the first time as an efficient electrode of microbial fuel cells (MFCs) for bioelectricity generation and as an efficient adsorbent for aqueous Cr(VI). Notably, two forms of carbon having characteristically different properties were prepared from a common source (SiC) via the facile route of different temperature conditions. One form of the prepared Si-CDC, used as the electrocatalytic electrode, contained relatively higher graphitic content, whereas the other form, used as adsorbents, contained higher specific surface area. The following section describes the synthesis steps for the Si-CDC-based electrode/adsorbent materials. The physico-chemical characteristics of the synthesized materials, 4


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such as BET surface area, surface morphology, surface functional groups, crystallographic planes and the graphitic content are discussed next. The electrochemical performance parameters (maximum power density/open circuit potential (OCP)) and Cr(VI)-adsorption capacity of both types (electrodes and adsorbents) of Si-CDC, each type subjected to the electrochemical and adsorption tests, are presented and compared with the materials discussed in literature. 2. MATERIAL AND METHODS 2.1. Materials. The phenolic resin precursor-based activated carbon fibers (ACFs), used as the substrate for electrodes, were provided by Gun Ei Chemical Industry Co. Ltd. (Japan). Potassium dichromate (K2Cr2O7), potassium hexacynoferrate (K3Fe(CN)6), potassium hydroxide (KOH), binder polyvinylidene fluoride (PVDF) and solvent N-methyl-2-pyrrolidone (NMP) were purchased from Merck (Germany). All chemicals used in the preparation of phosphate buffer solution (PBS) were of analytical grade with high purity and purchased from Merck (Germany). The waste/spent SiC-heating rod elements of a high temperature (~1200 oC) graphite furnace were used as the precursor for synthesizing Si-CDC. Such heating elements had a service life of 5-6 months before they were replaced in the furnace. The high purity chlorine (Cl2) and nitrogen (N2) gases were purchased from Sigma Gases (India). The Nafion 117 proton exchange membrane (PEM) was purchased from Sinsil International (India). The E. coli (K 12) culture was indigenously procured. Milli-Q water was used to prepare all solutions. 2.2. Synthesis of Si-CDC. The as-received SiC rod samples (15 mm-diameter and 200 mm-length) were brushed to remove dust or any materials deposited on the surface of the rods. The samples were crushed into small pieces (~1 mm in size), using a 400 g-iron hammer. The samples were milled for 6 h to produce the SiC powder (~1 µm), using the ball mill (Retsch, 5



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Germany). Approximately 2.5 g of the SiC powder were placed in the quartz crucible mounted inside a vertical quartz tubular reactor. A high-temperature furnace was used to heat the reactor. The furnace could be moved up or down, using a pulley-chain system (supplementary Figure S1). The samples were heated 15 oC per min to the set temperature. Four different temperatures (850, 900, 1050, 1100 °C) were considered. The Cl2 gas was passed over the SiC powder sample, contained in the crucible, at a flow rate of 150 standard cubic meter per min (sccm), using a 2.5 cm diameter and 100 cm long vertical quartz tube. The spent gas from the reactor was absorbed in a 1 M-KOH solution. Chlorination was performed for 3 h. The furnace was moved down and the reactor was allowed to cool to room temperature (~30 oC). The sample was simultaneously purged with N2 at the 50 sccm-flowrate to remove excess Cl2 from the sample. The prepared samples (Si-CDC) were directly used for the adsorption study. The electrodes were prepared by dip coating of the ACFs in the Si-CDC ink. Approximately 0.54 g of PVDF was dissolved in 10 ml-NMP and the binder solution was continuously stirred at 80 rpm for 1 h, using a magnetic stirrer (REMI, 2 MLH). The temperature of the mixture was held constant at 50 oC. Approximately 0.4 g of the Si-CDC sample was dispersed in the prepared binder solution and ground for 15 min, using a mortar-pestle. The ACF samples, cut into 2 cm x 1 cm pieces, were dipped three times in the prepared Si-CDC ink. A uniform layer of the ink was formed on the ACF substrate. The Si-CDC-coated ACF samples were dried at 120 °C in a vacuum oven for 12 h to remove the solvent. After drying, the prepared samples were used as the electrodes of the MFC. Figure 1 describes the schematics of the preparation steps for the Si-CDC-based adsorbent and electrode samples. The different Si-CDC samples prepared at 850, 900, 1050, and 1100 °C were termed as Si- CDC_A, Si-CDC_B, Si-

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CDC_C and Si-CDC_D, respectively, for the references purposes in this study. The prepared samples were directly used as electrodes at anode and cathode in MFC.

Post filtration

Post adsorption

Cr(VI) solution(50-400ppm)

Etching with Cl2 for 3 h 150 sccm

Ball milling for 6h

(850, 1100 °C) Si-CDC powder (~1 µm)

SiC powder (~1 µm)

SiC rods

Dispersed in binder solution

ACF After dip coating and heating

Si-CDC/ACF

Si-CDC ink

Binder solution (NMP + PVDF)

Figure 1. Schematic illustration of the preparation and applications of Si-CDC. 2.3. MFC Set-up and Electrochemical Measurements. A double chambered MFC was fabricated for the electrochemical study of the Si-CDC-based electrodes. The cell was fabricated from five acrylic 7 cm x 7 cm sheets of 0.6 cm-thickness. The sheets were vertically stacked in series. Three inner sheets contained an opening of 40 mm-diameter at the center and were 7



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stacked between the outermost sheets (without opening), using threaded screws (Figure S2). The electrode (anode/cathode) samples were held in the opening of the sheets, using a vertical Cuwire. The PEM was sandwiched between the two electrodes. Nylon O-rings were placed in the groove of the openings to seal the two (anode/cathode) chambers of the MFC. The anode and the cathode (projected area = 4 cm2) were separated through the PEM (exposed area = 0.0755 m2). Provisions were made for purging the anode and cathode chambers, using atmospheric air and N2, respectively. The protocols used to prepare the anolyte and catholyte solutions are described in the previous study.19 The anolyte solution (PBS) was purged with N2 for 30 min after inoculation with E. coli, whereas the catholyte solution (K3Fe(CN)6) was continuously purged with atmospheric air, using a high precision peristaltic pump (Miclims, India). Electrochemical measurements were performed on the complete cell, using a potentiostat (AUTOLAB-PGSTAT302N,

Netherlands).

Linear

sweep

voltammetry

(LSV)

and

electrochemical impedance spectroscopy (EIS) analyses were performed to determine the electrochemical parameters of the MFC. The protocols followed for performing LSV and EIS analysis have been previously described.19,20 Briefly, power density and OCP were measured using LSV. The measurements were taken at the 1 mVs-1-scan rate over the potential range of 1 to 0 V. The EIS analysis was performed at the OCP of the cell to measure the different internal resistances of the MFC, namely, solution resistance (Rs), charge transfer resistance (Rct) and Warburg mass transfer diffusion resistance (W). The measurements were performed over a frequency range of 100,000 to 0.01 Hz at the potential amplitude of 0.01 V. The frequency response analyzer software was used to determine the resistances. All electrochemical measurements were performed using two-electrode assembly in which the reference and counter electrodes of potentiostat were shorted and connected to the cathode, and the working electrode 8


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was connected to the anode of the MFC. The electrodes were connected via an external 5000 Ωload. The cell was left idle for stabilization (~3 days) and the measurements were taken after the stabilization of the cell voltage. Electrode potentials were measured using the prepared electrodes as working electrode and Ag/AgCl as reference electrode. The cyclic voltammetry (CV) measurements were performed for different anode materials in a three-electrode assembly cell. Tests were performed in anolyte solution at a scan rate of 10 mV/s over the potential range of -1.0 to +1.0 V. All electrochemical measurements were performed in the batch mode at room temperature and repeated in triplicate to check the reproducibility. 2.4. Batch Adsorption Study. A stock solution of 1000 ppm-Cr(VI) in water was prepared using K2Cr2O7. Adsorption study was performed over the Cr(VI) concentration range of 50-400 ppm, using different doses (0.01-0.05 g) of the prepared Si-CDCs. A fixed amount of the adsorbents was mixed in the 10 ml-test solutions contained in different vials. The vials were kept on a mechanical shaker (150 rpm-speed). Adsorption tests were performed at room temperature for 48 h, without adjusting solution pH, which was measured to be ~4.7. The spent adsorbents were separated from the solution using a 0.25 µm-Millipore filter paper. The concentration of Cr(VI) in the test solutions was determined using the UV-vis spectrophotometer (Carry 100, Varian, USA) and the standard 1,5-diphenylcarbazide (DPC) method.21 The wavelength for the maximum absorbance of Cr(VI) was determined to be 541 nm. The amount of Cr(VI) adsorbed by the Si-CDC samples was calculated from the species balance equation: q = V(Ci − Ce)/W, where q is the loading (mg/g) of Cr(VI); Ci and Ce are the initial and final (equilibrium) concentrations (mg/L) of Cr(VI) in the solution, respectively; V is the volume (L) of the solution, and W is the weight (g) of the adsorbents.

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3. MATERIAL CHARACTERIZATION The physico-chemical characteristics of the Si-CDC samples were determined using the different analytical and spectroscopic techniques, namely, Brunauer, Emmett and Teller (BET) surface area (SBET) and pore-size distribution (PSD) measurement analyzer, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Raman spectroscopy, broad angle X-Ray diffraction (XRD) spectroscopy and Fourier transform infrared spectroscopy (FTIR). The SBET, PSD, and total pore volumes (Vt) of the prepared samples were determined using the Autosorb-1C (Quantachrome, United States) instrument and N2 as the probe molecule at 77 K. Before starting the analysis, the samples were degassed at 150 °C in vacuum for 12 h. The SBET was calculated using the BET isotherm for P/P0 < 0.1 and Vt was calculated at a relative pressure near unity (P/Po = 0.9994). Micropore volume (Vmicro) was calculated from t-plot and meso-macropore volume (Vmeso-macro) was calculated by subtracting Vmicro from Vt. The surface morphology of the samples was examined using FE-SEM (Zeiss, Supra 40 VP, Germany). The microstructure was examined using TEM (LaB6 FEI Tecnai, 120 kV TWIN). The samples for the TEM analysis were prepared by the ultrasonication of the SiC/Si-CDC samples in methanol solution for 15 min and then depositing the sample-dispersed methanol solution on the copper grids (300 mesh). Raman analysis (Alpha, Germany) was performed over the wavenumber range of 500-4000 cm-1 at 25 °C, using a charge couple device detector and an Ar-ion laser as the excitation source. The relative graphitic characteristics (ID/IG) of the prepared samples were determined from the Raman spectra. The XRD analysis was performed over the 2θ range of 5 to 120° at the scan rate of 3° per min, using the Cu Kα radiation. The crystal size and the crystallographic planes of the samples were determined using XRD spectra and the JCPDS 10


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software (No: 29-1129 and 41-1487). A Tensor 27 (Bruker, Germany) FTIR was used to determine the transformation of SiC into carbon powder. The measurements were performed over the wavelength range of 500−4000 cm−1 with a resolution of 4 cm-1 in the attenuated total reflectance mode using the germanium crystal. Prior to the analysis, the background spectrum was recorded using air. The sample chamber was continuously purged with N2 to minimize the interference from atmospheric moisture. The zeta potential of the prepared material was measured in Milli Q water using the Delsa Nano (Beckman Coulter, Inc. USA) instrument 4. RESULTS AND DISCUSSION 4.1. Surface Morphology. Figures 2(A-A’) and 2(B-B’) show the low and high magnification-SEM images of the SiC and Si-CDC_D samples, respectively. The low magnification images did not show any distinct difference between the morphologies of the two materials. However, the high magnification images clearly showed that the SiC precursor was non-porous (Figure 2A’), whereas Si-CDC_D was porous (Figure 2B’). Pores were developed in the Si-CDCs during chlorination. The SEM images of the Si-CDC samples that were chlorinated at low temperatures showed relatively less porous characteristics in the materials and are not included here for brevity. Figure 2(C-C') shows the SEM images of the produced biofilm at the surface of the Si-CDC_A electrode sample used in the MFC for 3 days. The growth of the film was uniform and dense. Figure 2(D-D’) shows the EDX spectra of the SiC and Si-CDC_D samples. The spectra confirmed the presence of silicon (Si) and carbon (C) in the pre- and post-chlorinated samples. The decrease in the intensity of the peak corresponding to Si showed that roughly all SiCs were converted into C and only a small amount of SiC remained unetched. Elemental compositions 11



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A

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A’

25 µm

250 nm

B

B’

25 µm

250 nm

C

C’

5 µm

D

500 nm Element C O Si

%(w/w) 29.1 8.82 62.07

D'

Element C Si Cl

%(w/w) 89.8 4.17 6.03

Figure 2. SEM images of (A-A') SiC, (B-B') Si-CDC_D, (C-C') uniform and dense bio-film at the surface of Si-CDC/ACF electrode, and (D-D') the EDX spectra of SiC and Si-CDC_D. 12


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A

A’ 0.36 nm

100 nm

B

nm 55 nm

a

B’

100 nm

C

100 nm

Graphitic ribbons

nm 5 nm

C’

5 nm

Figure 3. TEM images of (A-A') SiC, (B-B') Si-CDC_A and (C-C') Si-CDC_D.

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(w/w %) of the samples confirmed decrease and increase of Si and C in the Si-CDC samples, respectively. The TEM images showed the detailed microstructure of the SiC and Si-CDC samples. The as-received SiC particles were black and crystalline with the interplanar spacing determined to be ~0.36 nm (Figure 3(A-A’)). The color transformation from dark to light grey confirmed the formation of carbon during chlorination. Figures 3(B-B’) and 3(C-C’) show two different phases of carbon, namely, graphitic and amorphous, produced from the common SiC precursor. Graphitic ribbons were induced in the Si-CDC_A sample, i.e., in the SiC chlorinated at 850 oC, confirming the graphitic characteristics of the material. Disordered structure was developed in the high-temperature (1100 °C)-chlorinated samples, with the absence of ribbons in Si-CDC_D (Figure 3C’). These findings were consistent with the Raman analysis discussed in the subsequent section. 4.2. BET analysis and pore volume measurement. Figure 4A shows the N2 adsorption isotherms of the prepared materials. The amount of adsorbed N2 in Si-CDC gradually increased with increasing relative pressures, before leveling off, which is the characteristics of the Type-I isotherms. The SiC precursor was nonporous and the Si-CDCs were predominantly microporous. Table 1 shows the SBET and pore volumes of the prepared samples. It is evident that the SBET increased in the samples with increasing chlorination temperatures, as relatively greater amounts of Si were etched from SiC, creating a greater number of pores in the material. The Si-CDC_Ds and Si-CDC_As showed the largest and smallest SBET and micropore volume, respectively. Figure 4B shows t-plots of the prepared materials. The micropore volumes were determined from the intercept of the dashed black lines shown in the figure. 14


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Table 1. SBET, Pore Volume and ID/IG Ratio of the Si-CDC Samples Pore volumes (cc/g) SBET (m2/g)

Vt (cc/g)

Si-CDC_A

259

Si-CDC_B

Sample

ID/IG Vmicro

Vmeso-macro

0.104

0.103

0.001

1.01

883

0.369

0.335

0.034

1.19

Si-CDC_C

1255

0.513

0.482

0.031

1.27

Si-CDC_D

1751

0.736

0.680

0.056

1.39

4.3. Raman analysis. Figure 5 shows the Raman spectra of the Si-CDC samples prepared at different chlorination temperatures. Two distinct peaks were observed in the spectra. The peak located at 1598 cm-1 (G-band) corresponded to the graphitic phase and the second peak located at ~1338 cm-1 (D-band) corresponded to the disordered phase in the material. The ID/IG ratios were calculated to determine the degree of disorderedness in the material, or conversely, their graphitic characteristics. The intensity of D-band increased in the materials that were chlorinated at relatively higher temperatures. The ID/IG ratio increased from 1.01 in Si-CDC_A to 1.39 in SiCDC_D.

15



500

A

Si-CDC_D

Volume adsorbed (cc/g)

400

300

Si-CDC_C

200

Si-CDC_B

100 Si-CDC_A

0 0.0

0.2

0.4

0.6

0.8

1.0

P/Po

Volume adsorbed (cc/g)

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

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500 450 400 350 300 350 300 250 200 250 200 150 100 150 100 50 0

Si-CDC_D

B

Si-CDC_C

Si-CDC_B

Si-CDC_A

0

1

2

3

4

5

6

7

8

9

10

t-plot (Å) Figure 4. (A) BET isotherms and (B) t-plots of the Si-CDC samples. Actual micropore volume was evaluated using dashed black line; red and blue lines show micropore volumes calculated using conventional approaches, which yield an approximately 40% underestimated value.22 16


G band

Si-CDC_D Si-CDC_C Si-CDC_B Si-CDC_A

Intensity (a.u)

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


D band

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600

900

1200

1500

1800

2100

2400

Wavenumber (cm-1) Figure 5. Raman spectra of the Si-CDC samples. 4.3. FTIR and XRD analysis. Figure 6A shows the FTIR spectra of the SiC (black), SiCDC_A (red) and Si-CDC_D (blue) samples. The peaks at 730-840 and 1100 cm-1 in the SiC sample corresponded to the characteristic vibration peaks of Si-C and Si-O-Si bonds, respectively.23 Diminution of these peaks in the spectra of Si-CDC_A and Si-CDC_D confirmed the etching of Si from SiC. The etched samples showed two peaks at ~1520 and 1640 cm-1 corresponding to the absorption peak of C-C and the C=C stretch of the aromatic ring of graphitic carbon, respectively.24 The spectra also show the presence of the -CO, -C=O and -CH stretching vibrations at ~1050, 1750 and 2924 cm-1, respectively.15 The significance of these stretches in the adsorption of Cr(VI) has been discussed later.

17



Figure 6B shows the XRD spectra of the SiC (black), Si-CDC_A (red) and Si-CDC_D (blue) samples. The spectra of SiC showed the diffraction peaks at the 2θ-angles of ~35.6, 41.4, S i- C D C _ D S i- C D C _ A S iC

Intensity (a.u.)

A

C=C C -H

S i-O -S i

C -C C=O

3000

2400

S i-C

C -O

1800

1200

600

W a v e n u m b e r (c m -1 ) SiC graphite

B

SiC Si-CDC_D Si-CDC_A

Intensity (a.u.)

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

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15

30

45

60

75

Wavenumber (cm-1) Figure 6. (A) FTIR spectra, (B) XRD patterns of the SiC and Si-CDC samples. 18


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60, 71.8 and 75.5°, corresponding to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) crystal planes, respectively. The presence of these peaks confirmed the crystalline characteristics of the SiC sample. After chlorination, the intensity of these peaks decreased. At higher temperature no such peaks were observed, which confirmed the etching of Si from the SiC samples. The spectra of the Si-CDC_A and Si-CDC_D sample exhibited two peaks at the 2θ angles of ~26 and 44°, which corresponded to the (0 0 2) and (1 0 1) crystallographic planes of the graphite, respectively. A narrow peak was observed at ~26° in the spectra of Si-CDC_A, attributed to the presence of graphitic ribbons, and the broadening of this peak in Si-CDC_D confirmed the formation of amorphous carbon.9,25 The crystallite size of the SiC and Si-CDC samples was calculated to be in the range of 19 to 26 nm, using Scherrer's equation (r = kλ/β cos θ ). 4.5. Adsorption study. The percentage removal of Cr(VI) increased while equilibrium loading decreased with increasing adsorbent dosages (Figure 7A). Figure 7B shows the equilibrium loading at different aqueous phase concentrations, using 0.024 g of the adsorbents. The data are shown for Si-CDC_A and Si-CDC_D, the material having the smallest and largest SBET or micropososity, respectively. Equilibrium loading asymptotically reached a constant value in both materials with increasing Cr(VI) concentrations and the maximum adsorption capacity of Si-CDC_A and Si-CDC_D were determined to be ~51 ± 3 and 95 ± 5 mg/g, respectively. The relatively higher adsorption capacity of Si-CDC_D is attributed to the high SBET of the material, which was earlier discussed. The inset in Figure 7B shows the linear form of the Langmuir adsorption isotherms fitted to the adsorption data for both samples: (1)

19



90

Cr(VI) removal (%)

75 80 60

60 45

(0.024)

40 0.01

0.02

Cr(VI) loading (mg/g)

A

100

30

0.03

0.04

0.05

Amount of Si-CDC (g)

B

100

6.0 4.5 C e /q e

Si-CDC_D 75

qe (mg/g)

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

Page 20 of 36

3.0 1.5 0.0

0

100 200 Ce (ppm)

300

50

Si-CDC_A

25

0

75

150

225

300

Ce (ppm) Figure 7. (A) effects of the adsorbent-dosages on the adsorption of Cr(VI), (B) comparative equilibrium adsorption data of Si-CDC_A and Si-CDC_D.

20


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, where qe is the solute loading (mg/g); qmax is the maximum adsorption capacity (mg/g); Ce is the aqueous phase concentration (mg/L) and K is the Langmuir constant. A reasonable good fit (R2 = 0.99) indicated the monolayer coverage of Cr(VI) at the adsorbent surface over the aqueous phase concentration range of 50-400 ppm. Notably, the least performing adsorbent (Si-CDC_A) prepared in this study also showed a significant Cr(VI)-loading. The pH at which the adsorbent surface was electrically neutral, i.e. pH of zero charge (pHpzc), played a significant role in the adsorption of Cr(VI) on the surface of Si-CDC. Figure S3(A) shows the pHpzc to be ~6.5 for Si-CDC_D. The adsorption experiments were performed at the solution pH of ~4.7. Considering that the solution pH < pHpzc, the Si-CDC surface was positively charged. Therefore, the negatively charged ions in the solution were attracted towards the surface. The aqueous Cr(VI) existed in the form of hydrogen chromate (HCrO4-), chromate (CrO42-) and dichromate (Cr2O72-). The relative amounts of these ions in the solution were determined by the solution pH. At low (acidic) pH, HCrO4- and Cr2O72- ions were predominant, whereas CrO42- was stable under alkaline pH.26 The surface functional groups of Si-CDC, namely, -CO, -C=O and -CH (refer FTIR spectra in Figure 6A) were protonated under acidic pH, rendering the surface of the adsorbent positively charged. Therefore, adsorption was facilitated by the electrostatic attraction between the protonated surface of the adsorbents (SiCDC) and the negatively charged ions (HCrO4- and Cr2O72-) present in the solution. Figure S3(B) shows the zeta potentials of Si-CDC_D in Milli Q water whose pH was adjusted to ~4.7, and in Cr(VI) solution also at the same pH. Post-adsorption, the zeta potential decreased from ~20 to 16 mV, corroborating the pHpzc data. Adsorption tests were also performed on the Si-CDC_B and Si-CDC_C samples (Figure S4). The adsorption capacities of these samples were expectedly found to be in between that of 21



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Si-CDC_A and Si-CDC_D, the least and best performing adsorbents prepared in this study, respectively. The results are consistent with the SBET of the materials, discussed earlier. The adsorption capacity of the best performing Si-CDC_D adsorbent was compared, wherever possible, with that of the carbon-based adsorbents discussed in literature for the Cr(VI) removal. Table 2 shows the comparative data of the adsorption studies.27-38 Although a direct comparison may not be appropriate because of the dependency of the Cr(VI)-adsorption on the initial solution concentration and pH of the solution, it is reasonably accurate to mention that the adsorption capacity of the prepared adsorbent in this study is larger than most of the adsorbents discussed in literature. Recent studies have shown the emergence of graphene oxides as an efficient adsorbent for Cr(VI), showing a significantly high adsorption capacity in the range of 80-200 mg/g. Such materials were, however, functionalized with different reagents to enhance the number of active adsorption sites. Therefore, it can be mentioned that the adsorption capacity of Si-CDC_D can also be potentially enhanced by functionalizing it with a suitable reagent.

Table 2. Comparative Cr(VI)-Adsorption Capacities of the Various Carbon-based Materials

Reference

This study 26 26 27 28

29 30

Material

Aqueous phase equilibrium concentration (ppm)

~Loading (mg/g)

~pH

Si-CDC_D

125

~95 ± 5

4

powdered activated carbon SWCNTs GAC and CSAC triethanol amine-grafted hollow carbon nanofibers carbonaceous waste biomass graphene oxide functionalized with cyclodextrin chitosan

45 48 18 and 19

47 20 7 and 0.3

4 4 7

98

51

4

37

53

2

16

68

3 22


Page 23 of 36

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31 32 33 34 35 36

37

38

crushed coal functionalized CNTs PANI/MWCNTs oxidized MWCNTs activated carbon graphene oxide/chitosan composite triethylene tetramine-graphene oxide ternary nanocomposite amino functionalized graphene oxide with Fe3O4 nanoparticles

10 61 66 9 38

3 2 56 1 29

9 5 2 2 2

32

86

2

74

184

2

28

123

2

4.5. Performance of MFCs using the Si-CDC electrodes. Figure 8A shows the polarization and power density curves for the MFCs based on the Si-CDC_A- and Si-CDC_D electrodes having the maximum and minimum graphitic contents, respectively. The data are also shown for the ACF alone, used as the substrate for the Si-CDC electrodes. The ACF substrate showed negligible power density, OCP and limiting current density. The Si-CDC_A-based MFC showed high performance, with the corresponding electrochemical parameter values determined to be 1570 ± 30 mW/m2, 460 ± 5 mV and 11900 ± 60 mA/m2, respectively. These values were ~4, 1.5 and 2 times greater than that of the Si-CDC_D-based MFC, respectively. The superior performance of Si-CDC_A than Si-CDC_D is attributed to the higher graphitic content in the former electrode, which increased the electrical conductivity of the material. The coating of the Si-CDC ink on the ACFs roughened the substrate surface, enhancing the growth of biofilm (E. coli) at the anode, as was corroborated from the SEM images shown earlier. A high growth of the biofilm facilitated the electron transfer between bacteria and the anode, thereby improving the performance of the Si-CDC-based MFCs.39

23



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The OCP was relatively lower during initial stages of the cell operation. This is attributed to discontinuity in the formation of biofilm at the anode surface, resulting in small electron transfer rate at the anode. The discontinuity in the biofilm is possibly because of a relatively lesser number of E. coli produced during initial stages of the MFC operation. As the metabolic activity progressed, the biofilm formed on the electrode surface became stable.19 E. coli can transfer electrons to the anode surface in the absence of mediator(s) via the pili. They can also self-mediate the electron transfer through the electrochemically activated excretion of electron shuttling redox molecules.19,40,41 The graphitic ribbons present in the SiCDC_A sample, as was corroborated from the TEM images, facilitated the electron transfer from the c-type cytochrome of E. coli to anode.42 Therefore, the charge transfer resistance (shown later by the EIS analysis) was small in the case of the Si-CDC_A-based MFC, resulting in the enhanced generation of current density (11900 ± 60 mA/m2). EIS analysis was also performed to get an insight of voltage losses in the MFCs. The losses were attributed to the ohmic (solution) resistance (Rs), charge transfer resistance (Rct) and the Warburg mass transfer diffusion resistance (W). The Rs depends on the conductivity of electrolytes, PEM, and electrical interconnections between electrodes, whereas Rct depends on the electro-conductivity of the cathode material. The internal resistances of the materials were calculated from the Nyquist plots (Figure 8B). The plots were constructed from the EIS data. The supplementary Table S1 presents the calculated resistances. The resistances of the materials expectedly decreased in the following order: ACF > Si-CDC_D > Si-CDC_C > Si-CDC_B > SiCDC_A. The presence of the graphitic Si-CDC_A at the ACF surface decreased the interfacial

24


Page 25 of 36

contact resistance between the anode surface and electrolyte, thus enhancing the electron transfer.19,43,44

A

Power density Si-CDC_A Si-CDC_D ACF

2000

2

600

Potential Si-CDC_A Si-CDC_D ACF

1500

OCP (mV)

Power density (mW/m )

800

400

1000

200

500

0

0 0

3000

6000

9000

12000

Current density (mA/m2)

Si-CDC_A Si-CDC_D ACF

200

B Rs 150

Rct

4

100

Z"(Ω )

Z" (Ω)

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


2

0

28

Z'(Ω )

32

50

0 100

200

300

400

500

Z' (Ω) Figure 8. (A) polarization and power density curves and (B) Nyquist plots for the Si-CDC samples. 25



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The significant decrease of Rct in Si-CDC_A is attributed to the enhanced biofilm growth at the anode surface and graphitic content of the material. The graphitic Si-CDC_A facilitated charge transfer via the planar structure of the graphitic sp2 hybridized carbon.45 The electrochemical performance of the MFCs using Si-CDC_B and Si-CDC_C as electrodes was also evaluated, and the polarization and power density curves are presented in Figure S5. Expectedly, the electrochemical performance of these electrodes lie in between that of Si-CDC_A and Si-CDC_D, the best and the least performing electrode material prepared in this study, respectively. The previously discussed Raman spectra had clearly shown increasing graphitic contents of the materials in the following order: Si-CDC_D < Si-CDC_C < Si-CDC_B < Si-CDC_A. The bioelectrochemical activity of the anode was investigated by performing CV test. The ACF and Si-CDC_A were used as the working electrodes in a three-electrode assembly cell. In each case, the KCl-saturated Ag/AgCl and Pt were used as the reference and the counter electrode, respectively. The test was performed after 3 days of the colonization of biofilm at the anode surface. The capacitive current was measured to be higher in the CDC-based electrode than in the ACF electrode (Figure 9). A significant redox peak was observed in Si-CDC_A. The redox peak current was, however, absent in ACF. The relative higher capacitive current in SiCDC_A is attributed to the graphitic characteristics of the material, which enhanced the electron transfer rate in the electrode. The rough surface of the CDC-based material also favored the biofilm formation and improved the interactions between microbe and electrode by facilitating the electron transfer. The biofilm formation was corroborated from the SEM images (Figure 2(CC')).

26


Page 27 of 36

0.008 ACF Si-CDC_A 0.004

Current (A)

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


0.000

-0.004

-0.008 -1.0

-0.5

0.0

0.5

1.0

Potential (V) vs Ag/AgCl

Figure 9. CV data for ACF and Si-CDC_A as anode in MFCs.

The long-term stability of the best performing Si-CDC_A electrode in this study was investigated by operating the MFC over a long period (580 h or approximately 25 days). An external load of 5000 Ω was applied and generated voltage was measured (Figure 10). A steady state value of ~0.32 V was measured after ~60 h, which remained constant for another ~100 h. The potential gradually decreased to ~0.05 V in the next ~24 h. The electrolytes were replaced and the voltage again increased to ~0.32 V. No significant change occurred in the voltage generation for 580 h. The results showed approximately three reproducible cycles, demonstrating the long-term stability of the prepared electrode without any degradation.

27



0.4

0.3 Potential (V)

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

Page 28 of 36

0.2

0.1

0.0 0

100

200

300

400

500

600

Time (h)

Figure 10. Cyclic stability test using the Si-CDC_A electrode.

A comparison of the performance of the Si-CDC_A-based MFC was made, wherever possible, with the performance of the MFCs based on different carbon electrode materials discussed in literature (Table 3).39,46-58 Similar to the previous comparative study (adsorption), although a direct comparison may not be possible between different MFCs in this case because of the dependency of the MFC performance on both anode and cathode materials, the comparative data show that the electrochemical performance of the MFC using Si-CDC_A electrodes is higher than or comparable to most of the MFCs mentioned in the table. It is evident from the data of Sharma et al.55 and Lv et al.58 that the noble metal-based electrodes showed superior performance than non-noble metal-based electrodes or the electrodes without metal. The enhanced performance in the former study was also attributed to the use of a mediator and a nano-fluid in electrolytes. Similarly, the high electrochemical performance reported in the latter study was attributed to the use of an electrochemically active bacteria. A relatively high 28


Page 29 of 36

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


Table 3. Comparative performance of the carbon electrode-based MFCs.

Si-CDC_A

power density (mW/m2) 1570 ± 30

current density (mA/m2) 11900 ± 60

OCP (mV) 460 ± 5

carbon paper

460

3250

400

reticulated vitreous carbon electrodes

11

140

500

bare carbon felt

166–1326

2350–7530

700–800

carbon cloth with gas diffusion layers carbon cloth porous carbon fiber

4250

5670

749

1098 686–1487

7200 ∼4000

∼720

plain carbon cloth

53

260

700

bare carbon felt

166-1326

2350-7530

700-800

Pt-coated carbon cloth

300-830

1150-1900

390-570

granular activated carbon Ru-Pt/MWNT and SnPt/MWNT

17

140

450

2470

7000

940

Pt-coated carbon cloth

120

450

1070

carbon paper bare carbon felt carbon fiber brush

228 3080 789

1278 12940 ~3200

180 800 ~800

reference

anode material

cathode material

present study

Si-CDC_A polyaniline modified carbon felt reticulated vitreous carbon electrodes PPy/GO-modified graphite felt

39

46

47

48

carbon brush

49

CNT coated fiber graphite rods graphene deposited carbon cloth PPy/GO-modified graphite felt Fe3O4/CNT dispersed carbon paper granular activated carbon

50 51

52

53 54 55

graphite, Sn-Pt/MWCNT

56

CNT/MnO2 dispersed carbon paper PPy-coated CNTs RuO2-carbon felt MWCNTs-GO hybrid

57 58 59

29



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Page 30 of 36

performance was also reported in the study of Zou et al.59 because of the use of the threedimensional hierarchically porous multi-walled CNTs and the reduced graphene oxide hybrid as anode materials. Therefore, it is noteworthy to mention that even when compared with the noble metal containing materials discussed in literature, Si-CDC_A showed a relatively high performance, attributed to the synergistic effects of the presence of graphitic ribbons, high surface area and biocompatibility of the electrode material. 5. CONCLUSIONS The waste SiC-heating rods were successfully used as the common precursor of two characteristically different CDC materials synthesized by chlorination at different temperatures. The chlorination at 850 °C produced a graphitic carbon, whereas the same at 1100 oC produced an amorphous carbon. Raman analysis showed a low ID/IG ratio (~1.01) in the former material. The TEM images confirmed the presence of graphitic ribbons in the material. Such CDCs were used as an efficient electrode of the MFCs producing a significantly high limiting current density of 11900 ± 60 mA/m2 and maximum power density of ~1570 ± 30 mW/m2. The high temperature-synthesized CDCs were highly microporous with the SBET measured to be ~1750 m2/g. Such CDCs were used as an efficient adsorbent having ~95 ± 5 mg/g-adsorption capacity for aqueous Cr(VI), which was considerably greater than that of the carbon-based adsorbents discussed in literature. The method of producing the different types of Si-CDCs, namely graphitic and amorphous, is simple, cost-effective and scalable, and such materials can be efficiently used as an adsorbent for environmental remediation and/or as an electrode for electrochemical applications.

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■ SUPPORTING INFORMATION Figure S1 showing a schematic illustration of the experimental setup used for the chlorination of SiC; Figure S2 showing a schematic illustration of the double-chambered MFC fabricated in this study; Figure S3(A) showing point of zero-charge on Si-CDC_D; Figure S3(B) showing zeta potentials of Si-CDC_D in Milli Q water and Cr(VI) solution, both at pH ~4.7; Figure S4 showing comparative equilibrium adsorption data of Si-CDC_B and Si-CDC_C; Figure S5 showing (A) polarization and power density curves and (B) Nyquist plots for the Si-CDC_B and Si-CDC_C samples; Figure S6 showing polarization curves of anode and cathode; Table S1 listing internal resistances of the MFCs using the Si-CDC electrodes, determined from Nyquist plots.

■ AUTHOR INFORMATION Corresponding Author Tel.: +91 512 259 7704; fax: +91 512 259 0104. *E-mail address: [email protected], [email protected] (N. Verma). Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS The authors gratefully acknowledge the Gun Ei Chemical Industry Co. Ltd., Japan for supplying ACFs. The authors are also thankful to the Center for Environmental Science and Engineering at IIT Kanpur for carrying out the research.

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