Nanoporous Activated Carbons Derived from Agro ...

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Nanoporous Activated Carbons Derived from Agro-waste Corncob for Enhanced Electrochemical and Sensing Performance Mandira Pradhananga Adhikari, Rina Adhikari, Rekha Goswami Shrestha, Raja Rajendran, Laxmi Adhikari, Partha Bairi, Raja Ram Pradhananga, Lok Kumar Shrestha,* and Katsuhiko Ariga

Advance Publication on the web April 27, 2015 by J-STAGE doi:10.1246/bcsj.20150092

© 2015 The Chemical Society of Japan

Nanoporous Activated Carbons Derived from Agro-waste Corncob for Enhanced Electrochemical and Sensing Performance Mandira Pradhananga Adhikari,a Rina Adhikari,b Rekha Goswami Shrestha,c Raja Rajendran,d Laxmi Adhikari,b Partha Bairi,e Raja Ram Pradhananga,b Lok Kumar Shrestha,*e and Katsuhiko Ariga e a

Department of Chemistry, Bhaktapur Multiple College, Tribhuvan University, Bhaktapur 44800, Kathmandu, Nepal. b Central Department of Chemistry, Tribhuvan University, Kirtipur 44613, Kathmandu, Nepal. c Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda Chiba 278-8510, Japan. d

Centre for Nanoscience and Technology, Anna University, Chennai-600025, India. e International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.

Contact: Dr. Lok Kumar Shrestha Email: [email protected] Tel: +81-29 860 4809 Fax: +81-29 860 4832

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Abstract Nanoporous activated carbons (AC) has been prepared from low-cost agro-waste corncob powder by phosphoric acid-activation and investigated their electrochemical supercapacitor and sensing properties. Surface areas and pore volumes are found in the range of 690 – 1288 m2 g–1 and 0.49 – 1.64 cm3 g–1, respectively and could be controlled by adjusting the weight ratio of corncob and phosphorous. The corncob-derived AC showed excellent electrochemical performance giving a maximum specific capacitance ca. 340.8 F g–1 at scan rate of 5 mV s–1. At relatively high scan rate of 100 mV s–1 the specific capacitance of 133.7 F g–1 was obtained. About 96% capacitance retention rate was achieved even after 1000 cycles demonstrating potential usages of the materials in high performance supercapacitor electrodes. Furthermore, our AC showed excellent solvent vapor sensing performance with high selectivity for ammonia molecule.

Keywords:

agro-waste,

corncob;

nanoporous

carbons;

cyclic

voltammetry;

supercapacitor; vapor sensing

2

Introduction Development of efficient supercapacitors (electrochemical capacitors, ECs) with high power density and long cycle life is essential in enormously growing demand of electronic devices, electronic systems, and hybrid electric vehicles.1-5 Therefore, intense research has been carried out focusing on increasing the energy and power densities of ECs. Major attention has also been paid to the rational design of materials to produce cost-effective and environmentally friendly nanomaterials.6 Several recent studies have demonstrated

that

nanoporous

activated

carbons

(AC),

mesoporous

carbon

nanostructures (fullerenes, carbon nanotubes, and graphene or graphite oxide), conducting polymers, and transition-metal oxides are potential candidates of EC electrode materials.7-12 Electric double layer capacitors (EDLCs) and pseudo-capacitors (PCs) are well-known ECs. They store energy by different mechanisms. EDLCs exhibit a non-faradic reaction involving electrostatic charge diffusion and accumulation of charges at the electrode and electrolyte interface,13 whereas PCs show a faradic process i.e., a fast surface redox reaction occurs.14 Among the several and various electrode materials tested, porous carbon materials, such as AC, carbon black, carbon nanotubes, and mesoporous carbons have been extensively used as efficient EC electrode material due to their stable physicochemical properties, good conductivity, low cost, and long cycle life.15-17 It has been described that capacitance of porous materials is directly proportional to their surface area that can be easily accessible by electrolyte ions. On the other hand, the power capability is restricted by the electrode kinetics such as ion transport within micropores, which severely diminishes the overall performances of EDLCs.18-21 Graphene nanosheets have also been proposed as the next generation electrode 3

materials for EDLCs22-25 due to their high surface area (2630 m2 g1), sufficient porosity, superior conductivity, a broad potential window, and rich surface chemistry.26-30 However, it has the inherent disadvantage of restacking of graphene sheets due to strong π-π interactions, i.e. graphene nanosheets undergo facile aggregation during preparation so that the actual performance is much lower than anticipated judging from the high surface area.31-33 AC on the other hand, have been extensively used in various practical applications including adsorption and separation, hydrogen storage, and supercapacitors due to high specific surface areas, large pore volumes, and very simple preparation methods.34-38 AC can be prepared from various natural lignocellulose precursors with high carbon contents, low levels of inorganic compounds and the natural precursors that are generally

abundant,

renewable,

inexpensive,

and

environmentally

friendly.

Conventionally, AC were produced from coal, wood, peach stone, olive stones, coconut.39-42 Recently, extensive efforts have been given to recycle waste materials from agro-industry to prepare efficient AC43-46 either by physical or chemical activation method. Recently, Zhang et al.47 demonstrated AC with ultrahigh surface area (3164 m2 g–1) and large pore volume (1.88 cm3 g–1) from waste litchi shells with channel-like macropore via KOH activation method. Okmam et al.48 also reported AC from grape seed via K2CO3 and KOH activation. Similarly, Joshi et al.49 has produced AC from Lapsi (Choerospondias axillaris) seed; a waste material, via ZnCl2 activation. In this contribution, we report electrochemical and sensing performances of nanoporous activated carbons (AC) prepared from low-cost agro-waste precursor; corncob via H3PO4 activation. The corncob-derived AC display the EDLC behavior giving specific capacitance ca. 340.8 F g–1 at 5 mV s–1 scan rate and good cyclic 4

stability with capacitance retention of about 96% even after 1000 cycles. The excellent electrochemical performance was attributed to the high surface area and large pore volumes. Additionally, the AC showed excellent sensing performance with high selectivity for ammonia molecule. Experimental Materials Corncobs were collected from a local market of Biratnagar, Nepal. After drying under the Sunlight for a few days, corncobs were crushed and grinded in a mill. The grinded particles were then sieved. Particles having sizes less than 425 µm were considered in this study. For chemical activation, an ortho-phosphoric acid (H3PO4) with purity greater than 88.29% was used. Ultra-pure nitrogen was supplied during the carbonation process. Thermogravimetry (TG) Thermogravimetric analysis with the temperature ramped from 25 to 800 C at a rate of 10 C/min in an Ar atmosphere was employed to study the course of pyrolysis of corncob powder. Measurement was performed on TG/DTA 6200 Seiko instrument. Preparation of activated carbon (AC) Corncob powder was chemically activated with H3PO4 at different impregnation ratios of corncob and phosphorous. Note that impregnation ratio of corncob and phosphorous indicates the weight ratio of corncob to phosphorous. It is not the weight ratio of corncob and H3PO4. The weight ratio of corncob and phosphorous were 1:0.1, 1:0.3, 1:0.6, 1:0.9, 1:1.2, 1:1.5 and 1:2.0, and the corresponding samples are represented

5

as AC0.1, AC0.3, AC0.6, AC0.9, AC1.2, AC1.5, and AC2.0, respectively. After treating corncob powder with H3PO4 for 24 h, they were dried at 110 C for 24 h and carbonized at 400 C under a continuous flow of nitrogen at the rate of 1000 mL/min for 2 h. The carbonized samples were washed with distilled water for several times, dried at 110 C for 2 h, and finally further grinded into powder and processed for advanced characterizations, electrochemical, and sensing studies. Characterizations Surface functional groups present in the prepared nanoporous AC were determined recording FTIR spectra on a Thermo Electron Corporation, Nicolet 4700 at 25 °C. The % of transmission of the samples was recorded over 400  4000 cm-1. Powder X-ray diffraction (XRD) patterns were recorded on Rigaku X-ray diffractometer, RINT, Japan operated at 40 kV and 40 mA with Cu  K radiation at 25 °C in the diffraction angles of 10 to 50 degrees. Raman scattering spectra were recorded on a Jobin-Yvon T64000 by exciting samples with green laser; 514.5 nm and 0.05 mW power. For Raman scattering measurements, samples were prepared on clean glass substrate. Chemical states of carbon in our AC were confirmed by X-ray photoelectron spectroscopy (XPS). The XPS measurements were performed on a Theta Probe spectrometer (Thermo Electron Co. Germany) using monochromated AlK radiation (photon energy 15 KeV, maximum energy resolution  0.47 eV, and maximum space resolution  15 m). High resolution spectra; core level C 1s, and O 1s were recorded in 0.05 eV steps. A built-in electronic charge neutralizing electron flood gun was used to prevent sample charging. Scanning electron microscopy (SEM) technique was used to study surface morphology and porous structure of the prepared AC. SEM samples were prepared on a carbon tape and SEM images were recorded using S-4800, Hitachi Co. Ltd. Japan at 10 kV. For microstructure 6

characterization, transmission electron microscopy (TEM) technique was used. TEM, high resolution –TEM (HR–TEM) images and selected area electron diffraction patterns were taken from the smallest components of the AC samples on JEOL, Model JEM– 2100F operating at 200 kV. TEM samples were prepared putting a drop of suspension of AC prepared in isopropanol on the carbon–coated copper grids. TEM samples were dried under reduced pressure for 24 h before TEM observations. Nitrogen adsorption/desorption isotherms of the prepared AC were recorded on an automatic adsorption instrument (Quantachrome Autosorb-iQ2 USA) and specific surface area, pore volume and average pore diameter were calculated. For each measurement about 20 mg of AC was taken and degassed for 24 h at 120 C prior to the measurement. The isotherms were recorded at liquid nitrogen temperature 77.35 K. Electrochemical performance Electrochemical performance of corncob-based nanoporous AC were studied recording cyclic voltammograms (CV) with a three-electrode system in 1 M aqueous H2SO4 solution at 25 C. A bare glassy carbon electrode (GCE) used as working electrode was mirror polished with Al2O3 slurry and cleaned with double-distilled water and sonicated in acetone for 5 min. 2 mg of AC sample was dispersed in 2 mL of ethanol (1 mg/mL) and the mixture was sonicated for 30 min in a sonication bath. 3 µL of this dispersion was added onto the GCE surface and dried at room temperature. After the solvent was evaporated, 5 µL Nafion solution (5%) was added as binder on the surface of the GCE and dried at 70 °C for 3 h. Platinum wire was used as a counter electrode and Ag/AgCl as the reference electrode. The cyclic voltammetry response and chronopotentiometry were performed on a CH instruments model: (CHI 850D Work station (USA)). For the calculation of specific capacitance (Cs) from CV curve, we have 7

used the following equation. 1

𝐶𝑠 = 𝑚𝑣(𝑉

𝑓

𝑉

𝑓 ∫ 𝐼(𝑉)𝑑𝑉 −𝑉 ) 𝑉 𝑖

𝑖

(1)

where ‘Cs’ is the specific capacitance, ‘m’ is the mass of the active electrode material, ‘v’ is scan rate, Vf and Vi are the integration limits of the voltammetry curve, and I(V) represents the current response, respectively. Specific capacitance was also calculated form charge-discharge curves using following equation. 𝐼𝑡

𝐶𝑠 = ∆𝑉×𝑚

(2)

Where, I, t, ΔV, and m are discharge current (A), the discharge time (s), potential window, and mass of the active material on the electrode, respectively. For specific energy (SE) and specific power (SP) following equations are used. 𝑆𝐸 =

0.5𝐶(∆𝑉)2 3.6

𝑆𝑃 = 𝑆𝐸 ×

3600 𝑡

(3) (4)

Quartz crystal microbalance (QCM) Using a resonance frequency of 9 MHz (AT-cut), the frequency of the quartz crystal microbalance (QCM) electrode was measured during adsorption and recorded when it became stable Corncob derived nanoporous carbon sample (4.0 mg) was dispersed in water (2 mg/mL). The mixtures were then ultrasonicated for 30 min in a bath sonicator and 3 L of this dispersion was drop casted on the QCM electrode. The electrode was dried at 80 C in vacuum for 24 h before measurements. The prepared QCM electrode was then applied to the QCM instrument and exposed to the solvent guest molecules in a sealed volume to prevent the escape of the vapors during the adsorption measurements. Between measurements, the electrode was exposed to air to

8

desorb the solvent vapor. The recovery of the initial frequency value was taken as an indication of complete desorption. Experiments were carried out at 25 °C. Since QCM is sensitive to the mass change, when the surface of a quartz crystal electrode is coated with nanoporous carbon sample mass changes. This change can be measured by the oscillating frequency of the electrode. The frequency change (Δf) corresponds to the sample amount loaded on the QCM electrode.50 Results and discussion The TG curve recorded with the temperature ramped from 25 – 800 C at a rate of 10 C/min in Ar atmosphere shows that about 5% mass loss occurred at about 100 C is due to evaporation of moisture (Figure S1 in Electronic Supporting Information). A major mass loss started at 300 C indicating the decomposition of cellulose and hemicellulose present in corncob powder. Pyrolysis of corncob powder seems to complete approximately at 400 C. Therefore, carbonization was carried out at this temperature. Figure 1 shows SEM, TEM, and HR-TEM images of phosphoric acid activated corncob-based nanoporous activated carbons prepared at different weight ratio of corncob and phosphorous. SEM images display a typical texture of porous materials containing non uniform carbon granules. Template free carbonization method generally leads to such morphology. Minute observation reveals that the granular sizes of AC prepared at lower phosphorous content are relatively bigger compared to those prepared at higher weight ratio of corncob and phosphorous. In high resolution SEM images of AC0.9 and AC1.5 (Figure 1a) disorderly arranged nanopores can be seen. Nanopores are formed due to release of moisture and volatile organic components from the precursor material during carbonization process. TEM images shown in Figure 1b were

9

taken from the smaller size grains of our samples and for HR–TEM images, thinner edge of spacemen was selected. TEM images display porous structure typical of disordered porous carbons. Broad and weak circular rings observed in SAED patterns (inset of panel b) indicate amorphous structure of carbon, which is further confirmed by HR–TEM image; carbon layers are randomly distributed without any fully developed graphitic microstructure. Surface functional groups present in the prepared AC were investigated by FTIR spectroscopy. FTIR spectrum of the precursor, corncob powder (Figure S2) displays peaks at 3410 cm-1 (stretching vibration of O–H bond in alcoholic or phenolic groups), 2927.0 cm-1 (stretching vibration of C–H bond in alkane and alkyl groups), 1465 cm-1 (carboxylic carbonates), 1636.5 cm-1 (stretching of C=C bond in aromatic compound), and broad peak in the range of 1166–1025 cm-1 (stretching of C–O in ether, phenol or lactones groups) demonstrating the presence of various surface functional groups such as OH, C=O, COOH and lactones. The oxygen containing surface functional groups remain present in the prepared AC and also remain unaffected by the weight ratio of corncob and phosphorous (Figure 2 and Figure S2). X-ray photoelectron spectroscopy (XPS) data presented in Figure 3 supports the FTIR results. The XPS survey spectra (Figure S3) clearly display core level peaks for carbon and oxygen. The XPS C 1s core level peak of AC0.1 could be deconvoluted into four peaks at 284.6 (C=C; sp2), 285.9 (C–C; sp3), 288.1 (O–C=O), and 290.9 eV (* shake up), respectively (Figure 3a).51 Similarly, XPS C 1s core level peak of AC0.9 can be deconvoluted into four peaks (Figure 3b) at 284.7 (C=C; sp2), 285.8 (C–C; sp3), 288.1 (O–C=O), and 290.9 eV (* shake up) demonstrating the hetero-carbon components with oxygenated functional groups, which is further confirmed by XPS O1s core spectra (Figure 3c,d). The XPS O 10

1s core level spectrum of AC0.1 (Figure 3c) and AC0.9 (Figure 3d), respectively could be deconvoluted into three peaks at 531.3 (C=O), 533.1 (O–C–O) and 536.2 eV (–OH), and 531.2 (C=O), 533.1 (O–C–O) and 536.4 eV (–OH). XRD patterns of AC mainly comprise of two broad peaks at diffraction angles of ~ 25 and 43 degrees (weak) (Figure 4a and Figure S4), which could be attributed to the (002) and (100) planes of graphitic clusters.20,52 The observed broad peaks may also indicate that the graphitic clusters in the AC are small and the ordered graphene layers are not fully developed. Absence of sharp diffraction peaks in the XRD patterns of our AC is a clear indication of amorphous structure equivalent to the commercially available carbons. Small peaks in XRD patterns of selected samples are coming from inorganic impurities. Raman spectra (Figure 4b and Figure S5) display two broad bands at 1350 (D) and 1597 cm-1 (G), typical of amorphous carbons.53,54 In order to investigate the effect of weight ratio of corncob and phosphorous on the graphitization degree of the prepared AC, we have calculated the relative intensity of G and D band (IG/ID)55 and plotted against the wt. ratio of corncob and phosphorous in H3PO4 (Figure S5b). The IG/ID increases from 0.95 to 1.19 upon increasing weight ratio of corncob and phosphorous from 0.1 to 0.9 and then remains apparently unchanged indicating that higher weight ratio of corncob and phosphorous causes higher graphitization in the prepared AC. Figure 5 shows the nitrogen adsorption/desorption isotherms and the corresponding pore size distributions of selected samples as typical example. The sample dependent adsorption isotherm indicates that the weight ratio of corncob to phosphorous in the chemical activation plays an important role on the surface textural properties (surface area and pore volume). Nitrogen adsorption/desorption isotherm for the AC0.1 exhibits 11

a type I, i.e. nitrogen adsorption drastically increased at low relative pressure region (P/P0 < 0.1) followed by saturation at higher relative pressure without any hysteresis loop. This corresponds to pure microporous material. The overall all nitrogen uptake of AC0.9 is much higher than AC0.1 and the isotherm displays mixture of Type-I and Type-IV behavior; nitrogen uptake increased significantly at lower relative pressure with a hysteresis loop at higher relative pressure indicating larger surface area of the AC0.9 sample. The large amount of N2 adsorption at low relative pressure attributes to the filling of the micropore and the hysteresis phenomenon attributes the capillary condensation occurring in the mesopore, corresponding to micro- and mesoporous material. The volume of nitrogen adsorption decrease in AC2.0 indicates a decrease in surface area caused due to pore coalescence at higher weight ratio of corncob and phosphorous. The pore size distributions obtained from Barrett-Joyner-Halenda (BJH) and density functional theory (DFT) are displayed in Figure 5b and Figure 5c, respectively. The nitrogen adsorption isotherms of rest of the samples are supplied in Figure S6. The textural properties of AC including Brunauer-Emmett-Teller (BET) surface areas, and pore volumes (obtained from BJH model) are summarized in Table 1. BET surface area and total pore volume (at P/P0 ~ 1) increases from 685 to 1288 m2 g–1 and 0.49 to 1.64 cm3 g–1, respectively upon increasing weight ratio of corncob to phosphorous from 0.1 to 0.9. Encouraged by the high surface area and large pore volume, we have studied the electrochemical properties of corncob-derived AC using cyclic voltammetry (CV). Figure 6a shows cyclic voltammograms of AC0.9 sample as typical example recorded in 1 M H2SO4 aqueous electrolyte in the potential range of 0 to 0.8 V against Ag/AgCl reference electrode at different scan rates ranging from 5 – 100 mV s–1. The CV curves 12

(Figure 6a and Figure S7) show a rapid current response to the voltage at each end potential with apparently rectangular shape, which is ideal for electrical double layer type capacitor.56,57 Although rectangular shape CV curve is seen in an ideal supercapacitor, in real systems the electrolyte ion diffusion may be prevented by the migration force and polarized resistance is expected to be produced, which make the CV curve different from the ideal rectangle. Almost rectangular shape CV curve obtained at higher scan rate (100 mV s–1) is an indication of faster electrolyte ion diffusion even at higher scan rates.57,58 Figure 6b shows calculated specific capacitance (Cs) at different scan rates (5 – 100 mV s–1) for AC0.1, AC0.6, AC0.9, and AC1.2. It is clear that Cs increases from AC0.1 to AC0.9 and then decreases slightly, which is good agreement with the BET results. Namely, larger the surface area and pore volume, higher the specific capacitance. The Cs calculated from CV curves for AC0.9 are 340.8, 312.5, 262.5, 184.2, 150.0, and 133.7 F g–1 at scan rates of 5, 10, 20, 50, 80, and 100 mV s–1, respectively showing the capacitance retention of about 40% at higher scan rate (100 mV s–1). Note that the obtained specific capacitance of 340.8 F g–1 (for AC0.9 at scan rate of 5 mV s–1) is much higher than capacitances of conventional AC having similar surface areas measured in aqueous electrolytes. The surface areas of commercial AC are found in the range of 1000 – 1200 m2 g–1 and specific capacitance in the range of 100 to 150 F g–1.58 Lota et al59 demonstrated that structural and chemical properties of commercial AC can be improved by KOH re-activation. For the re-activated carbons, they obtained specific capacitance of 200 F g–1 at low current density of 50 mA g–1 in aqueous electrolyte (1 M H2SO4 or 6 M KOH). Thus, it can be concluded that H3PO4-activated corncob derived nanoporous carbons find clear advantage over the conventional materials. 13

Further investigations on the electrochemical properties were also carried out recording galvanostatic charge-discharge curves at different current densities (1, 2, 3, 4, 5, and 10 A g–1) in the potential range of 0 to 0.8 V (Figure 7a). All the charge-discharge curves show typical linear discharge curve indicating well-balanced charge storage. From the discharge curve, specific capacitances were ca. 261.6, 199.5, 181.1, 161.0, 147.5, and 105.0 F g–1 at different current densities of 1, 2, 3, 4, 5, and 10 A g–1, respectively (inset of Figure 7a). Note that AC0.9 sample shows a high capacitance value of 105.0 F g–1 even at high current density of 10 A g–1 indicating good rate capabilities. Additionally, the supercapacitor electrode showed good cyclic stability (Figure S8a,b). Note that the specific capacitance retention of the supercapacitor electrode is very high. Almost 96% capacitance retention was achieved after 1000 cycles demonstrating stable performance of supercapacitor (Figure 7b). These results show that our material meets the criteria of high stability and good cyclic stability of supercapacitors. Figure S8c shows the Ragone plot. At current density of 1 A g–1, AC0.9 supercapacitor showed high specific energy ca. 23.2 Wh kg–1 at specific power of ca. 400 W kg–1. At high current density of 10 A g–1, the specific power increased to 4000 W kg–1, maintaining specific energy of 9.33 Wh kg–1. Thus it can be concluded that our material can be a suitable candidate for the design of high performance electrical energy storage supercapacitor devices. We have also investigated the sensing capacity of our material. Sensing of toxic substances using nanoporous carbons is receiving considerable attention in recent days.60-63 Investigations have shown that surface properties, particularly high surface area and large pore volume is required for effective sensing of organic substances. Considering high surface area and large pore volume, vapor sensing capacity of AC0.9 14

sample was investigated for different solvents using the quartz crystal microbalance (QCM) technique. Figure 8a shows the time dependencies of frequency shifts for a QCM electrode prepared using AC0.9 upon exposure to different solvent vapors. Frequency shifts are very rapid upon exposure of the sensor to solvent molecules and they largely depend on the nature of solvent molecules. Frequency shift caused due to adsorption of aliphatic hydrocarbon cyclohexane (62 Hz) is lower than the aromatic solvent vapors benzene (87 Hz) and toluene (152 Hz) showing higher selectivity of AC0.9 towards aromatic solvents. Adsorption of formaldehyde, acetic acid and ammonia lead to much larger frequency shifts. Frequency shift is estimated to 264 Hz for formaldehyde, 350 Hz for acetic acid and 768 Hz for ammonia, respectively. Among all the solvent molecules studied, AC0.9 showed a huge frequency shift for ammonia demonstrating its highest selectivity for sensing for ammonia, which could be due to presence of carboxylic acid surface functional group in the material. The selectivity of sensing decreases in the following order: ammonia > acetic acid > formaldehyde > toluene > benzene > cyclohexane. Figure 8b demonstrates that sensing of solvent vapors could be repetitively performed by alternate exposure and removal of acetic acid molecules. Conclusions In conclusions, we have prepared high surface area nanoporous activated carbons from corncob (an agro-waste material) via phosphoric acid activation and investigated the electrochemical and vapor sensing properties. Surface areas and pore volumes could be tuned from ca. 690 – 1288 m2 g–1 and 0.49 – 1.64 cm3 g–1, respectively by changing corncob and phosphoric acid impregnation ratio (weight ratio of corncob and phosphorous). The corncob-derived nanoporous activated carbons showed excellent 15

electrochemical performance giving maximum specific capacitance ca. 340.8 F g–1 at scan of 5 mV s–1. At relatively high scan rate of 100 mV s–1 specific capacitance of 133.7 F g–1 was obtained. Furthermore, about 96% capacitance retention rate was achieved after 1000 cycles demonstrating the potential use in high performance supercapacitors. Moreover, these materials showed excellent vapor sensing performance with a high selectivity for ammonia vapors. Acknowledgements This work is partially supported by the Grants-in-Aid for Young Scientists B (25790021) of the Japan Society for the Promotion of Science (JSPS). References 1 2

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Table Table 1: Textural properties of corncob derived nanoporous AC

Sample AC0.1 AC0.3 AC0.6 AC0.9 AC1.2 AC1.5 AC2.0

BET surface area (m2 g-1) 685.8 949.4 1235.4 1288 1196 1085 965.5

Total pore volume at P/P0 = 0.9966 (cm3 g-1) 0.49 0.91 1.31 1.64 1.36 1.32 1.04

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Figures and Figure captions

Figure 1. (a) SEM, and (b) TEM, HR–TEM images of AC0.1, AC0.3, AC0.9 and AC1.5 as typical examples. The inset of panel b shows selected area diffraction (SAED) patterns.

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Figure 2. FTIR spectra of AC0.1, AC0.9, and AC2.0 as typical examples.

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Figure 3. (a) XPS C 1s core level spectrum with deconvoluted peaks of AC0.1, (b) XPS C 1s core level spectrum with deconvoluted peaks of AC0.9, (c) O 1s deconvoluted XPS spectrum of AC0.1, and (d) O 1s deconvoluted XPS spectrum of AC0.9.

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Figure 4. (a) XRD patterns and (b) the corresponding Raman scattering spectra of AC0.1, AC0.9, and AC2.0 as typical example.

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Figure 5. (a) Nitrogen adsorption/desorption isotherms of AC0.1, AC0.9, and AC2.0 as typical examples, (b) pore size distributions obtained using BJH method, and (d) pore size distributions obtained using DFT method.

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Figure 6: (a) CV curves of AC0.9 measured against Ag/AgCl electrode in 1 M H2SO4 solution at different scan rates (5, 10, 20, 50, 80, and 100 mVs–1) as typical example, and (b) the calculated specific capacitance (Cs) for AC0.1, AC0.6, AC0.9, and AC1.2 samples at different scan rates.

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Figure 7 (a) Charge-discharge curves of AC0.9 at different current densities (1, 2, 3, 4, 5, and 10 Ag–1) in the potential range of 0 to 0.8 V, and (b) specific capacitances vs. cycle number up to 1000 cycles.

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Figure 8 (a) QCM frequency shifts of AC0.9 electrode upon exposure to different solvent vapors, and (b) repeatability test with acetic acid vapors.

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Graphical Abstract

Nanoporous activated carbons (AC) has been prepared from low-cost agro-waste corncob powder by phosphoric acid-activation and investigated their electrochemical supercapacitor and sensing properties. Surface areas and pore volumes are found in the range of 690 – 1288 m2 g–1 and 0.49 – 1.64 cm3 g–1, respectively and could be controlled by adjusting the weight ratio of corncob and phosphorous. The corncob-derived AC showed excellent electrochemical performance giving a maximum specific capacitance ca. 340.8 F g–1 at scan rate of 5 mV s–1. At relatively high scan rate of 100 mV s–1 the specific capacitance of 133.7 F g–1 was obtained. About 96% capacitance retention rate was achieved even after 1000 cycles demonstrating potential usages of the materials in high performance supercapacitor electrodes. Furthermore, our AC showed excellent solvent vapor sensing performance with high selectivity for ammonia molecule.

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