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Direct conversion of biomass to nanoporous activated biocarbons for high CO2 adsorption and supercapacitor applications
Journal Pre-proof Direct conversion of biomass to nanoporous activated biocarbons for high CO2 adsorption and supercapacitor applications
Jarosław Serafin, Martyna Baca, Marcin Biegun, Ewa Mijowska, Ryszard J. Kaleńczuk, Joanna Sreńscek-Nazzal, Beata Michalkiewicz PII:
S0169-4332(19)32519-X
DOI:
https://doi.org/10.1016/j.apsusc.2019.143722
Reference:
APSUSC 143722
To appear in:
Applied Surface Science
Received date:
11 May 2019
Revised date:
8 August 2019
Accepted date:
16 August 2019
Please cite this article as: J. Serafin, M. Baca, M. Biegun, et al., Direct conversion of biomass to nanoporous activated biocarbons for high CO2 adsorption and supercapacitor applications, Applied Surface Science(2019), https://doi.org/10.1016/ j.apsusc.2019.143722
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© 2019 Published by Elsevier.
Journal Pre-proof Direct conversion of biomass to nanoporous activated biocarbons for high CO2 adsorption and supercapacitor applications Jarosław Serafina, Martyna Bacab, Marcin Biegunb, Ewa Mijowskab, Ryszard J. Kaleńczukb, Joanna Sreńscek-Nazzala, Beata Michalkiewicza* West Pomeranian University of Technology, Szczecin, Faculty of Chemical Technology and Engineering, a
Engineering, Institute of Inorganic Chemical Technology and Environment Engineering, Pulaskiego 10, 70-
322 Szczecin, Poland b
Nanomaterials Physicochemistry Department, Piastow Av. 45, Szczecin 70-311, Poland
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A series of nanoporous activated biocarbons have been successfully obtained from novel carbon
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precursor lumpy bracket. The textural properties of the biocarbons were tailored towards microporosity suitable for high CO2 adsorption. At the temperature of 298 K and at the pressure of 0.15 bar outstanding
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CO2 adsorption up to 1.67 mmol/g was achieved. The performance of biocarbons in the absorption/regeneration cycles towards CO2 adsorption remained stable. The selectivity of CO2 over N2
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calculated on the basis of Ideal Adsorbed Solution Theory was excellent (33.9 – 130.2). To the best to our
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knowledge, the prepared activated carbon labelled as 850_1 is one of the most promising sorbents tested in CO2 post-combustion capturing and pre-combustion capturing. Furthermore, the electrochemical properties
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of this sample were also evaluated. The specific capacitance reached over 220 F/g with excellent retention (nearly 92% of the original capacitance) after 5000 cycles. These results clearly indicate that activated carbons obtained from lumpy bracket can be used for the production of low cost and, more importantly,
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highly efficient electrode in supercapacitors as well. In this way, simple and efficient direct conversion of the lumpy bracket into the highly valuable multifunctional system is proposed.
*
Corresponding author. Tel. (+48)914494096. E-mail: [email protected] (B. Michalkiewicz)
Journal Pre-proof 1. Introduction
Development of new materials intended on a large scale requires certain industrial standards to be met such as: sustainability, easy or cheap production process and revealing improved or even new desirable properties [1]. Nanoporous activated carbons prepared from low cost precursors (especially biomass that can be valorised) via cost effective route meet all three above mentioned requirements. Such biocarbons are very attractive because of their tuneable porosity, pore size, and good chemical and thermal stability. They are considered now as key materials for carbon dioxide [2- 7], hydrogen [3, 5, 8], methane storage [3,9, 10] and electrode materials for supercapacitors[11, 12].
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The properties of activated carbons can be varied by changing carbon source (petroleum coke [13],
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biomass [2-6, 14,15], polymer [16], ion-exchange resins [17], coal [18]), carbon source pretreatment (addition of dopants [19],preoxidation [20]) process conditions activation temperature [21], activator to
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carbon source ratio [22], activating agent (KOH [23, 24], CO2 [25], K2CO3 [26, 27], H3PO4[28]) or by KOH and K2CO3 modification of commercial activated carbons [29]. Agricultural and food wastes have been
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widely studied in the production of activated carbons. Numerous examples have been described in excellent
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reviews [30 -32].
In recent times, there has been a great interest in the investigations of activated carbons production
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from biomass or waste and application for CO2 adsorption. Activated carbon prepared from waste tea, activated by NaOH and modified with ethylenediamine was used as CO2 sorbent [33]. The CO2 adsorption at 30oC and 1 bar was equal to 2.5 mmol/g. Porous carbon materials were obtained also by hydrothermal
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carbonization and KOH activation of camphor leaves and camellia leaves [34]. The maximum CO2 adsorption was 8.30 mmol/g at 25 oC at 40 bar (about 1 mmol/g at 10 bar) MPa.Our group also showed that biomass, namely: pomegranate peels, mistletoe branches, mistletoe leaves, carrot peels, kiwi fruit peels, fern leaves, sugar beet pulp [35] and also date [36] are very good CO2 sorbent. The highest CO2 capture capacity was observed at pomegranate peels (6.03 mmol/g at 0oC and 4.11 mmol/g at 25oC at 1bar [35]) and at date (4.36 mmol/g at 25oC and 6.4 mmol/g at 0oC at 1 bar [36]). Nitrogen-doped porous carbons were from hazardous waste oil sludge by mixing carbon precursor pyrolysis char with KOH and urea at 700oC [37]. CO2 adsorption capacity equal to 3 mmol/g at a temperature of 25oC was achieved. O-enriched porous carbonaceous adsorbents were derived from low cost, polyethylene terephthalate waste and activated by KOH [38]. The highest CO2 uptake of 1.31 mmol/g at 30°C was observed. Activated carbons were obtained from garlic peel using KOH as an activating agent [39]. The larges CO2 uptake was of 4.22 mmol/g at 25oC and 1 bar. Chars taken from five different commercial biomass gasifiers installed in South-Tyrol (Italy) we utilized as carbon precursor for activated carbons production for CO2 adsorption applications [40]. The CO2 adsorption tests were performed at 50oC and gas composition CO2:N2=1:1. The highest uptake (3.7%) was measured for char activated with KOH.
Journal Pre-proof Nitrogen-doped carbons were prepared by K2CO3 activation of urea-modified coconut shell [41]. The carbons demonstrate CO2 uptake at 1 bar, up to 3.71 mmol/g at 25oC and to 5.12 mmol/g at 0oC.Petroleum coke, which is the carbon residue remaining after heavy oil upgrading, was used to produce high-valueadded porous carbon for CO2 capture [42]. Activated carbons were prepared by KOH activation. The highest CO2 adsorption uptake of 3.68 mmol/g at 25oC and 1 bar was observed. N-doped activated carbons were obtained by using microalgae-sodium alginate (NaAlg) as renewable precursors [43]. The simple carbonization process with flowing N2 gas only was performed. Activated carbons exhibited CO2 adsorption capacity of 3.75 mmol/g at 1 bar and 25oC. The potential to use the activated carbons prepared from polypores such as birch polypore (Piptoporusbetulinus), turkey tail (Trametes versicolor (L.) Lloyd and, date for CO2 adsorption have been
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described by us [35].
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Conventionally, activated carbons from biomass are produced in a two-step process. In the first step, the conversion of biomass to carbon-rich matter takes place. In the second step, carbonaceous matter is
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activated by chemical or physical methods [14, 44, 45]. Searching for sustainable and cheap activated biocarbons, we decided to simplify the production route. In our procedure, there is no need to perform
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pyrolysis prior to the activation. The carbonization and chemical activation occurred in a single step
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simultaneously.
Another important worldwide issue is searching for new renewable and easily accessible energy
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storage technologies in response to constant and unavoidable fossil fuel depletion [46]. Wide group of energy storage devices are supercapacitors [47]. Various types of supercapacitors have been designed over the years, however nowadays two specific groups of supercapacitor sarouse researchers particular interest.
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In the first group, active electrodes are formed by individual components between which there is a synergistic effect. Chavan et. all synthesized electrochromic supercapacitor consisting of nanoflake NiMoO4 thin film with impressive specific capacity of 1853 Fg–1 at a current rate of 1 Ag–1 and greatly capacity retention over 2500 cycles [48]. Similarly, an oxygen-rich nanograin WO3 based smart supercapacitor was fabricated by Inamdar et. al via oblique-angle sputtering. The device displayed high specific capacitance of 228 F g−1 at 0.25 Ag−1 and maintained more than 75% of initial capacitance after 2000 cycles [49]. Li et ll. investigated the electrochemical performance of cobalt-Doped Nickel Phosphiteelectrode [50] and ultrathin Nickel–Cobalt Phosphate 2D Nanosheets [51]. Results revealed that both materials exhibited the maximum specific capacitance of 714.8 F g−1 and 1132.5 F g−1. The second group includes supercapacitors, in which a working electrode is activated carbon obtained from biomass. Therefore, in the present work, we report on the effective way to prepare activated microporous biocarbons from freely available biomass of saprophytic polypore; the lumpy bracket (Trametes gibbosa)- by carbonization combined with a chemical activation using KOH in one step. As prepared biocarbons have been tested as CO2 sorbent and as electrode material in an aqueous electrolyte Electrical Double Layer Capacitor (EDLC). Lumpy bracket derived nanoporous activated biocarbons were applied as CO2 sorbents at pressures up to 1 bar and up to 30 bar which are
Journal Pre-proof similar to the pressure of the post- and pre-combustion flue gas. Moreover, due to superior properties, such as well-developed porous structure and high surface area, the most promising activated carbon has been tested as electrode components in supercapacitor measured in two electrode system in KOH as an electrolyte. The properties of the activated carbons were controlled by varying temperature of carbonization combined with chemical activation step and by the mass ratio of the lumpy bracketto KOH. The effect of controlled textural parameters of the activated biocarbons on CO2 adsorption and electrochemical performance has been investigated in this study. To the best of our knowledge, there is no report on the synthesis of activated carbons from lumpy bracket. The lumpy bracket is a parasite and grows on most kinds of hardwood trees but most commonly on
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beech. It causes white rot. It is widespread and fairly common in southern Europe, North Africa and parts of
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the Middle East, extending to Southern Asia and China. Therefore, unlike most of biomass, the use of polypores does not interfere with food supplies. This work shows that highly efficient nanoporous activated
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biocarbons can be produced from a low-cost starting carbon source.
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2. Experimental
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2.1. Preparation of nanoporous activated biocarbons
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Lumpy bracket was dried and pulverized. The KOH and lumpy bracket in a ratio of 1:6, respectively, was mixed for 3 h. After drying, the mixture was carbonized for 1h in a tube furnace at the temperature range of 600 – 900oC under nitrogen flow. The obtained activated biocarbons were rinsed by distilled water to reach
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neutral pH, washed in dilute HCl (1 mol/dm3), rinsed by distilled water to neutral pH again, and finally dried. The resultingbiocarbons were denoted as X_Y. X refers to the temperature of carbonization and Y refers to mass ratios of KOH to dried lumpy bracket. Scheme 1 illustrates the preparation steps of nanoporous activated biocarbons derived from activated lumpy bracket.
Scheme 1. Schematic illustration of the preparation steps of nanoporous activated biocarbons derived fromactivated lumpy bracket
Journal Pre-proof 2.2. Measurement of textural properties
The textural properties such as surface area (SBET), total pore volume (Vtot), micropore volume (VmN2) and pore size distribution in the range of 1 – 30 nm were measured by N2 sorption at 77 K.The narrow micropore volume (VmCO2) and pore size distribution in the range of 0.3 – 1.5 nm were investigated via CO2 sorption at 273 K (Quadrasorbevo Gas Sorption Surface Area and Pore Size Analyzer). The surface area of the samples was calculated with the Brunauer–Emmett–Teller (BET) equations. The total pore volumes were calculated from the nitrogen amount adsorbed at a relative pressure, p/p0 of ~ 1. The micropore volumes and pore size distribution were calculated with a density functional theory (DTF). Prior the N2 and CO2
sorption isotherm measurements, all the samples were outgassed in the degassing station at 250°C for 12
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hours to a background pressure 22 · 10-3 mmHg to remove water and other impurities.
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2.3. X-ray diffraction (XRD)
Powdered activated biocarbons were pressed in the stainless steel sample holders for XRD analysis.
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The X-ray diffraction measurement was carried out on an Empyrean, PANalytical. The measurements were
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collected using copper radiation (K1 = 0.154056 nm) in the range of 10-60 in 2 scale. The scan rate was equal to 2o per minute. The patterns were analysed with X’PertHighScore software. The solid carbon materials are formed by monolayers of stacked graphenes. In the monolayers, each
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carbon atom is bonded to three other carbon atoms by covalent bonds-sp2hybridized type. The crystal structure of graphite is shown in Fig. 1. The crystal lattice of graphite is hexagonal, with A-B-A-B stacking
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of hexagonal monolayers i.e. half the carbon atoms have carbon atoms directly above and below them in the neighboring sheets (white circles in the Fig. 1), half of them has no direct neighbors in adjacent planes (black circles in the Fig. 1). This results in a hexagonal unit cell with dimensions of a = 0.2456 nm, c = 0.6708 nm (marked in red in Fig. 1). The interplanar distance (d002) between two consecutive layers of graphene by the c axis is ideally 0.3354 nm. The interactions between two consecutive layers of graphene are believed to be a van der Waals type but some authors suggested the mixture of Van der Waals and weak electrostatic interatomic interaction [52].
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Fig. 1. Crystal structure of graphite
The average crystallite thickness (Lc) and average graphene sheet diameter (Lc) can be approximately
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calculated using an empirical expression derived by Scherrer [53], which is:
where:
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(1)
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L is the mean crystallite dimension in nm along a line normal to the reflecting plane, λXDR is the X-ray wavelength (for copper K radiation λXDRis0.15409 nm),
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K is a constant depending on the reflection plane, is the full width at half-maximum of the (100/101) and (002) peaks in radians of 2after subtraction of the instrumental broadening,
is the scattering angle (in radians), The instrumental broadening effect on full width at half-maximum was subtracted out using Warren’s method assuming a Gaussian peak [53]: (2) For the dimension of turbostratic crystallites perpendicular to the graphene sheets (Lc ), (002) data is used and K equals 0.89. For the dimension in graphene sheet planes (La), (100/101) data is used and K equals 1.84 [54]. The La and Lc were calculated using equations:
(3)
Journal Pre-proof (4)
The average spacing between graphitic layers was calculated using the Bragg law (5)
where: n is an integer, the "order" of reflection is normally assumed to be equal to 1. The number of graphitic layers (N) was estimated using the equation:
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(6)
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2.4. Raman spectroscopy
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Raman spectra were collected by a Renishaw InVia equipped with a 514.5 nm line of an Ar+ laser.
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The relative intensity of the peaks was analysed with the assistance of the “Spectroscopy/Baseline and
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Peaks” function available at “Origin Pro 8” software.
2.5. FT_IR spectra
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Fourier transform infrared reflection (FTIR) spectra were carried out on Nicolet 6700 FT-IR
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spectrometer.
2.6. Electron microscopy
The morphology of the samples was observed via scanning electron microscopy with cold emission (UHR FE-SEM Hitachi SU8020) with Energy-Dispersive Spectroscopy (EDX) as its mode for analysis of the chemical composition. The sample preparation for SEM involved sprinkling of the powder sample on a double-sided carbon tape mounted on the SEM stub. Images were taken with a 20 kV accelerating voltage using a triple detector system. The morphology of the sample in greater details was examined using high resolution transmission electron microscopy (HR-TEM, Tecnai F20). The activated biocarbons were mounted on a copper grid.
2.7. The percentage of ash content and elemental analysis
The ash content (Ash%) of the activated biocarbons was established by standard methods (ASTM Designation D2866 11 [55]. Briefly dried and powdered activated biocarbons were burnt off for 3 h, at the temperature of 650 ± 25 °C in an electric muffle furnace.
Journal Pre-proof The ash content was calculated by: Ash% = weight of solids remaining [g] /original weight of biocarbon [g] · 100%
(7)
Elemental analysis of the ash was performed using energy dispersive X-ray fluorescence (EDXRF) (spectrometer Epsilon 3).
2.8. Measurement of equilibrium adsorption of CO2 Equilibrium adsorption isotherms of carbon dioxide at temperatures of 273 and 298 K for all the samples were measured volumetrically with Quadrasorbevo Gas Sorption Surface Area and Pore Size with pressure up to 1bar. The CO2 adsorption capacities at 1 bar and 273 and 298 K were denoted as qCO2(273, qCO2(298, 1), respectively.
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1)and
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Equilibrium adsorption isotherms of carbon dioxide in the most efficient CO2 sorbent in the temperature range of 273 - 353oC with pressure up to 30 bar were measured volumetrically with Hiden
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Isochema IMI – HTP Manometric Gas Sorption Analyzers. This method was described in details elsewhere [56]. Prior to all sorption isotherm measurements, the samples were outgassed.
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A good correlation of experimental data to theoretical models is useful for the mathematical
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modelling and simulation of CO2 separation process. Eight commonly used adsorption isotherms, two twoparameter equations: Langmuir, Freundlich and five three-parameter ones: Sips, Toth, Unilan, RadkePrausniz, Fritz−Schlunder isotherm were used to correlate the experimental data. The model parameters
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were estimatedusing Origin 8.0 using non-linear regression analysis. To find the best fitted model R2squared statistic was used. Moreover, standard errors (SE) of each parameter were calculated.
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The isosteric heat of adsorption was calculated using the Clausius – Clapeyron equation:
(8)
where: R is the universal gas constant, T is the temperature, p is the pressure, is the degree of surface coverage. After integration of equation (7) linear function is obtained:
(9)
The best fitted isotherm model was found. Then the values of pressure for each temperature for the same degree of surface coverage were calculated directly from the transformation of the isotherm model into function p=f(or by the interpolation. The ln(p) values vs. 1/T for the same degree of surface coverage
Journal Pre-proof were plotted. For each degree of surface coverage, one linear function was obtained. Isosteric heat of adsorption values was calculated from the slope of the linear function (equations (9). On the basis of these data isosteric heat of adsorption vs. degree of surface coverage plot was obtained.
2.9.Electrochemical Capacitor preparation
Three-electrode cell consists of Nickel foam, 15mm in diameter, working electrode (WE) with active material pressed at the surface containing active material, acetylene black and PTFE in mass ratio of 8:1:1, respectively, Saturated Calomel Reference Electrode (ALS-Japan) and Nickel Foam counter electrode with surface area of 20 cm2. WE contains ~6mg of the active material and cell works in 6M KOH as electrolyte.
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Electrode material for a two-electrode cell was prepared by mixing 100 mg of a solution containing
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active material, acetylene black and Kynar Flex PVDF with the mass ratio of 8:1:1, respectively and then anhydrous acetone was introduced to obtain a light slurry. Next, the active material was airbrushed onto a
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clean stainless-steel foil (10 μm 316L grade) or Al carbon covered foil (20um) followed by an IR radiation for drying. Steel or Al foil covered by an electrode material (mass density ~ 6 mg·cm-2) was cut to a circle
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shape electrode with a diameter of 13 mm by a mechanical cutter and dried for 24 h in a vacuum oven (15
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mbar, 70 C). The dried electrodes, after weighting, were transported to Ar (purity 5.0) filled glove-box (MBraun Germany). After 24h assembled together with a separator (TF4530 (NKK Japan)) and electrolyte
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(6M KOH Stainless Steel foil or 1-Methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide mpPYRTFSIIoLiTec Germany) in a type CR2032 button cell. Electrochemical measurements such as Cyclic Voltammetry, Galvanostatic Charging-Discharging,
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Electrochemical Impedance Spectroscopy, were conducted with the use of the multi-channel potentiostat VMP3 (BioLogic France).
3. Results and discussion
The N2 sorption isotherms measured for all activated biocarbons exhibited a sharp increase at very small p/p0. This indicates that activated biocarbons obtained from lumpy bracket are microporous. The hysteresis loops of the samples are very narrow and not clearly visible (see Fig.1). All the nitrogen adsorption−desorption isotherms are presented in details in Fig. S1. The appearance of a narrow hysteresis suggested that the activated biocarbons contain also mesopores.
Journal Pre-proof 800
600_1 750_1 900_1
700
650_1 800_1
800
a
700_1 850_1
500
3
3
800_1 800_3
400 300
500 400 300
200
200
100
100
0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 0.0
1.0
0.1
0.2
0.3
0.4
p/p0
850_1 850_3
0.7
0.8
0.9
1.0
c
850_1.5
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600 500 400
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3
0.6
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850_0.5 850_2
700
0.5 p/p0
800
N2 @ STP [cm /g]
b
800_1.5
600
N2 @ STP [cm /g]
600 N2 @ STP [cm /g]
800_0.5 800_2
700
300
100
0.1
0.2
0.3
0.4
0.5
0.6
p/p0
0.7
0.8
0.9
1.0
lP
0 0.0
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200
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Fig. 2. N2 adsorption−desorption isotherms on activated biocarbons prepared at different temperatures with KOH to biomass ratio equal to 1 (a), with different KOH to biomass ratio at the temperature of 800 (b), with
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different KOH to biomass ratios at the temperature of 850 oC (c)
The nitrogen adsorption isotherms of all the sample show the mixture of types I and IV isotherms according to IUPAC classification [57]. They are characteristic of an ordered microporous - mesoporous material. However, the limiting adsorption at high p/p0 was not observed. This indicates that the hysteresis loops of the activated samples showed type H3. The surface area of all activated biocarbons was calculated on the basis of the BET equation. The linear form of the BET equations is as follows: (10) where: n is amount adsorbed at the relative pressure p/p0 nm is monolayer capacity C is exponentially related to the energy of monolayer adsorption and it should be a positive value It is worth noting that the BET model is usually used to calculate the surface area of all materials but it actually does not apply to microporous materials. It is well known that in micropores the mechanism of
Journal Pre-proof adsorption is based on pore filling process rather than mono- or multilayer coverage [58]. According to this mechanism, the calculations of surface area on the basis of the standard BET method gives the overestimated values [59, 60]. The C value obtained from standard BET method calculations is usually negative, which has no physical sense. BET model can be applied to variousmaterials but extreme caution is needed for the materials containing micropores. The problem is how to find the linearrange of the BET plot for microporous materials in away that reduces any subjectivity in theassessment of the monolayer capacity. The solution of this problem was described by Rouquerol et al. [61]. Briefly, the following main consistency criteriamust be met: - the value of C parameter should be greater than zero (i.e. the y-intercept of the linear region should be positive
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- the pressure should be selected only in the range where the n(1 – p/p0) monotonically grows with p/p0
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- the p/p0 value corresponding to monolayer has to be within the selected linear range. It was found that for microporous materials BET method can be applied but the pressure range must
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be identified by criteria listed above [62, 63].
In our investigations “micropore BET assistant" available in Quantachrome software was used. This
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method is very useful to find the linear BET range for microporous materials taking into considerations
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above listed consistency criteria. However, it should be emphasized that the surface area calculated on the basis of BET equation for microporous materials can not be treated as a realistic value of the surface area.
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The calculated value is rather an apparent surface area. However, such surface area can be used for the comparison to the other materials. Rouquerol et al. [61] called surface area obtained in this way as “adsorbent fingerprint”.
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Table 1 lists the textural properties of the activated biocarbons. BET surface area, pore volume, and micropore volume were enhanced due to the increasing temperature of carbonization up to 850 oC. The high temperature of carbonisation favoured pore development but at 900 oC the oxidation of biomass was too intensive and porous structure was not well developed as at 850 oC. The similar phenomena were observed when the ratio of KOH to biomass increased. The BET surface area, pore volume, and micropore volume increased along with the increase in the ratio but there were the certain optimal values of the ratios (1:2 for 800 oC and 1:1 for 850 oC) to obtain the most porous structure. The highest BET surface area (SBET=1968 m2/g), pore volume (Vtot= 1.144cm3/g), micropore volume estimated on the basis of N2 adsorption (VmN2 = 0.552cm3/g) and narrow micropore volume estimated on the basis of CO2 adsorption (VmCO2= 0.401cm3/g) were obtained for lumpy bracket derived carbons activated at KOH: biomass ratio of 1 and temperature of 850 oC.
Journal Pre-proof Table 1. Textural properties of the lumpy bracket derived biocarbons and amount of CO2 uptake at a temperature of 273 and 298 K under a pressure 1 bar and CO2/N2selectivity calculated on the basis of ideal adsorbed solution theory (IAST) Activated
SBET
Vtot
VmN2
VmCO2
qCO2(273, 1)
qCO2(298, 1)
biocarbon
[m2/g]
[cm3/g]
[cm3/g]
[cm3/g]
[mmol/g]
[mmol/g]
SIAST
483
0.302
0.166
0.331
4.39
2.16
50.4
650_1
559
0.268
0.195
0.316
4.57
2.29
60.2
700_1
967
0.453
0.335
0.345
5.47
2.88
130.2
750_1
1482
0.756
0.485
0.386
6.53
3.96
49.1
800_1
1750
0.851
0.517
0.384
6.84
4.3
51
850_1
1968
1.144
0.552
0.401
4.62
46.4
900_1
1208
0.806
0.250
0.312
5.13
2.91
34.2
800_0.5
1321
0.602
0.418
0.352
6.10
3.59
51.5
800_1.5
1689
0.835
0.551
0.379
7.04
4.45
53.9
800_2
1833
0.960
0.457
0.357
6.54
4.13
45.2
800_3
874
0.546
0.255
0.321
5.01
2.70
40.5
850_0.5
1171
0.538
0.379
0.346
5.81
3.18
50.6
850_1.5
1604
0.931
0.485
0.371
6.59
4.06
45.4
850_2
1177
0.988
0.353
0.344
5.67
3.25
33.9
850_3
697
0.439
0.207
0.317
4.69
2.42
42.6
7.15
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na
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600_1
It was found that activated biocarbons contained micropores and small mesopores with a diameter below 6 nm (Fig. S2).
Due to kinetic restrictions, the estimation of the pore volume for pores lower than 1 nm using nitrogen at 77 K as sorbent is not advisable. CO2adsorption at a temperature of 273 K was proposed to complement N2 sorption for the characterization of the narrow microporosity [64]. CO2 sorption isotherms were used to calculate pore size distribution (Fig. 3) and pore volume for pores (0.3–1.1 nm) using the DFT method (Table 1). The pore volume of narrow micropores was ranging from 0.312 to0.401 nm. The trimodal pore size distribution with pore diameters of 0.35, 0.54 and 0.79 nm was observed.
Journal Pre-proof 1.6
a
1.6
600_1 650_1 700_1 750_1 800_1 850_1 900_1
1.0 0.8
1.2
0.6
1.0 0.8 0.6
0.4
0.4
0.2
0.2
0.0 0.3
0.4
0.5
0.6
0.7
0.8
800_0.5 800_1 800_1.5 800_2 800_3
3
1.2
b
1.4
dV/dD [cm /(g · nm)]
3
dV/dD [cm /(g · nm)]
1.4
0.9
1.0
0.0 0.3
1.1
0.4
0.5
0.6
D [nm] 1.6
0.8
0.9
1.0
1.1
c
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1.4
1.0
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0.8
ro
850_0.5 850_1 850_1.5 850_2 850_3
1.2
3
dV/dD [cm /(g · nm)
0.7 D[nm]
0.6
0.2
0.4
0.5
0.6
0.7 D [nm]
0.8
0.9
1.0
1.1
lP
0.0 0.3
re
0.4
Fig. 3. DFT pore size distribution of activated biocarbons calculated on the basis of CO2 sorption prepared at
na
different temperatures with KOH to biomass ratio of 1:1, respectively (a), with different KOH to biomass
Jo ur
ratios at the temperature of 800oC (b), with different KOH to biomass ratios at the temperature of 850 oC (c)
The main role of KOH was related to the oxidation of biomass. Potassium hydroxide reacts with part of carbon and stimulates the formation of the pores, especially micropores and small mesopores during the carbonization combined with the activation process. KOH removes also impurities such as SiO2 and dissolved the tar by-products. During the carbonization combined with chemical activation, the chemical reactions between biomass and KOH occurred. The oxidation of carbon leads to the formation of potassium carbonate, hydrogen, metallic potassium, water and carbon oxide (II). The main reactions of biomass and KOH were as follows: 4 KOH + C → K2CO3 + H2O + 2H2 6 KOH + 2 C → 2 K2CO3 + 2 K +3 H2 2 KOH + C + H2O → K2CO3 + 2H2 2 KOH + 2C → 2CO + 2 K + H2 The functional groups present in biomass decomposed also into volatiles such as carbon oxide, carbon dioxide, and water. These volatiles developed porosity escaped through the structure of the carbon. The volatiles also reacted with the biomass:
Journal Pre-proof C + H2O → H2 + CO C + H2O → H2 + CO2 C +CO2→ 2CO or with KOH: 2 KOH + CO2→ K2CO3 + H2O Potassium hydroxide was converted to potassium oxide: 2KOH → K2O + H2O Potassium carbonate and potassium oxide reacted with biomass as well: K2CO3+ 2 C → 2 K + 3 CO
of
K2CO3 + C → K2O + 2 CO K2O + C → 2 K + CO
ro
The activated carbons formation and development of high porosity can be ascribed to the above-
-p
mentioned reactions and evolution of volatiles in carbon that took place during the carbonization combined with the chemical activation. The potassium compounds and metallic potassium, which intercalated into the
re
carbon structure, were removed by washing with water and HCl. Highly porous activated biocarbons, with high surface area and pore volume, were obtained.
lP
The graphitic structure and the purity of activated biocarbons were analyzed by XRD method. Fig. 3 shows the XRD patterns of the samples prepared in the temperature range from 600 to 900oC with KOH to
na
biomass ratio equal 1:1 and at the temperatures of 800 and 850 oC with KOH to biomass ratios in the range of 1 :6. All the samples did not contain any of the sodium compounds residues. Two broad peaks centred at
Jo ur
about 2 =23 and 43o which correspond to the (002) plane and (100/101) of the graphitic structure, were observed. The broad peaks indicated a highly disordered carbon structure. It was found that the samples obtained at the lowest (600oC) and the highest (900oC) temperature and using the lowest (0.5) and the highest (0.3) impregnation ratios of KOH to lumpy bracket showed the peaks centred at about 2 =23 and 43 o with relatively narrower intensities. Similar results were obtained by Singh et al. [65]. The higher crystallinity in the samples can be ascribed to the insufficient and extensive reaction of the KOH with the biomass carbon occurred at the lowest and highest temperatures and KOH to biomass ratio. The optimization of the reaction conditions such as the amount of KOH and temperature required for the chemical reaction led to the formation of disordered nanoporous carbon. Therefore, their XRD peaks are broad. As an effect of applying the optimal combination of temperature and KOH, highly disordered and randomly oriented graphitic carbon layers (expansion of the graphitic carbon lattices, random distribution of layers, breakdown of aligned structural domains) were formed. What can be also related to the high surface area, total pore volume, and micropore volume. All these parameters play a key role in efficient CO2 adsorption.
Journal Pre-proof b
a
650_1 800_1
700_1 850_1
800_0.5 800_1.5
800_1 800_2
800_3
45
55
Intensity [a.u.]
Intensity [a.u.]
600_1 750_1 900_1
10
15
20
25
30
35
40
45
50
55
60
10
15
20
25
30
50
60
850_1 850_3
850_1.5
10
15
20
25
30
35
40
Angle 2 [degree]
45
50
lP
re
-p
Intensity [a.u.]
ro
850_0.5 850_2
40
of
c
35
A
Angle 2 [degree]
55
60
na
Fig.4. XRD patterns of activated biocarbons prepared at different temperatures with KOH to biomass ratio equal to 1 (a), with different KOH to biomass ratios at the temperature of 800oC (b), with different KOH to
Jo ur
biomass ratios at the temperature of 850 oC (c)
The values of average spacing between graphitic layers, average crystallite thickness, average graphene sheet diameter and an average number of graphitic layers are summarized in Table 2. The average interlayer spacing (distance between the turbostratic graphitic layers) in obtained materials was about 0.380 nm, which is approximately 13% increased in comparison to that of highly ordered pyrolytic graphite. The lowest interlayer spacing (0.373 nm) was observed for activated biocarbon obtained in the temperature of 850 oC and with KOH to the source of carbon ratio equal 1:2. The highest interlayer spacing (0.390 nm) was observed for activated biocarbons synthesized at the temperature of 750 o
C and with KOH : source of carbon ratio equal to 1:1. respectively. The higher inter-planer distance value for activated biocarbons may be caused by the sp3 defects
and/or by interlayer repulsion between the surfaces that are negatively charged because of surface functional groups[66]. The values of interlayer spacing and the width of the peaks corresponding to (002) and (101) reflection planes indicated the presence of disordered carbon structure [65]. It should be noticed that for
Journal Pre-proof biocarbons 800_1, 850_1 and 800_1.5 the values of d-spacing were not calculated because the peak corresponding to (002) reflection planes was not observed. The average crystallite thickness and average graphene sheet diameter values, listed in Table 2, are not exactly equal to Lc and La of the crystallites because equations (3) and (4) were derived for highly graphitized carbons. Therefore, they are not really suitable for turbostratic (i.e. fully disordered) carbons. Equations (3) and (4) can be applied as a convenient and relative estimation of crystallite thickness and average graphene sheet diameter [67]. In fact, the crystallite sizes are likely to be slightly higher than those presented in Table 2. The in-plane crystallite size values of the activated biocarbons were found to be small (2.443 – 3.716 nm). The average crystallite sizes along c axis values were within the range of 0.758 – 1.154 nm which gave the average number of graphitic layers in the crystallite from 2 to 3.
of
The values presented in Table 2 confirmed the conclusions derived from XRD patterns (Fig. 4) that
ro
activated biocarbons synthetized at the lowest and the highest temperatures and using the lowest and the highest impregnation ratios of KOH to lumpy bracket showed relatively higher crystallinity. These activated
-p
biocarbons exhibited the highest average crystallite thickness, average graphene sheet diameter and the
re
average number of graphitic layers.
lP
Table 2. The values of average spacing between graphitic layers d(002), average crystallite thickness La, average graphene sheet diameter Lc and average number of graphitic layers N calculated from XRD analysis d(002)
La
biocarbon
[nm]
[nm]
Lc
N
na
Activated
[nm]
0.381
3.117
1.154
650_1
0.387
2.970
1.018
2.6
700_1
0.377
2.755
1.020
2.7
750_1
0.391
2.890
1.017
2.6
Jo ur
600_1
800_1
2.564
850_1
2.443
3.0
900_1
0.382
3.331
1.147
3.0
800_0.5
0.382
3.679
0.824
2.2
0.758
2.0
800_1.5 800_2
0.386
800_3
0.382
2.890
0.932
2.4
850_0.5
0.373
3.716
0.913
2.5
850_1.5
0.373
3.274
0.874
2.3
850_2
0.375
3.186
0.887
2.4
850_3
0.379
3.155
0.955
2.5
Journal Pre-proof The activated biocarbons were characterized as well by Raman spectroscopy in order to supplement XRD results. The peak named G, centred around 1580 cm-1, refers to graphite.G-band arises from the stretching of the C-C bond in graphitic materials, and is common to all sp2 carbon systems. In the presence of graphitic disorder of the carbon structure the band around 1300 cm-1, named D is observed. Its intensity is proportional to the number of defects. So, the intensity ratio of the G and D band (IG/ID) is used for relative estimation of defects in the crystal structure. The higher IG/ID value the lower graphitic-disorder is observed. On the basis of Raman spectra normalized to the G band (see Fig. S3) IG/ID values were estimated. Fig. 5 shows the dependence of the IG/ID ratio on KOH: lumpy bracket mass ratio and the temperature. The values of IG/ID for activated biocarbons were in the range of 0.52 – 0.76. The higher IG/ID values were observed in the samples obtained in the lower and the higher temperatures and using the lowest and the
of
highest impregnation ratios of KOH to lumpy bracket. The values of IG/ID changed with a similar trend as Lc
ro
values (Fig. S4). FT-IR spectra of obtained activated carbon were also investigated and were attached to
o
600
650
850
900
0.8
lP
0.7 0.6 0.5 0.4
na
IG/ID
950
re
550
Temperature [ C] 700 750 800
-p
Supplementary (Fig. S5)
0.3
X_1 850_Y 800_Y
0.1 0.0 0.0
0.5
1.0
Jo ur
0.2
1.5
2.0
2.5
3.0
KOH : lumpy brackets mass ratio
Fig. 5. Dependence of the IG/ID ratio on KOH to lumpy bracket mass ratio and the temperature. SEM pictures showed the activated biocarbons have an irregular shape of thin flake-like particles with sharp corners ( Fig.6 and Fig. S6.) The similar shapes were observed for the activated carbons produced from Camellia japonica and Jujun grass [68].
Journal Pre-proof
of
Fig. 6. SEM (a) and TEM (b)image of activated biocarbon 850_1
image confirmed that 850_1 biocarbon is highly porous.
ro
TEM image of 850_1 sample is shown in Fig. 6. The pore channel geometry is clearly seen. TEM
-p
In order to determine the purity of the samples, the ash content in activated biocarbons was determined. The ash is the residue that remains after the combustion process (the detailed procedure was described in 2.6).
re
The ash content indicates the quality of activated biocarbons and affects the pore volume and surfaces area. The high ash content leads to the clogging of pores and reducing surface area and pore volume.
lP
The residual mass after activated biocarbon burning was assigned to the presence of mineral species. The presence of sodium was excluded by EDX and XRF method. High ash content is unfavourable because
na
obstructs the pore development, which leads to the low adsorption capacity. The activated biocarbons made from lumpy bracket exhibited low residual mass (see Fig. 7). The average ash content was equal to 3.0 wt%
Jo ur
and the highest value is 5.1 wt%. It means that biocarbons were nearly free of inorganic residues. Therefore, the activated biocarbons from lumpy bracket were composed of carbon. o
550
600
5
650
Temperature [ C] 700 750 800
850
900
950
Ash content [wt%]
4
3
2
1
X_1 850_Y 800_Y
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
KOH : lumpy brackets mass ratio
Fig. 7. Dependence of the ash content on KOH to lumpy bracket mass ratio and temperature
Journal Pre-proof The ash content depended on the carbon combustion and efficient removal of inorganic compounds due to reaction with KOH during carbonization and chemical activation. For example, SiO2 reacting with KOH formed water soluble K2SiO3, which is removed during purification step. The dependence of ash content on the temperature and KOH to lumpy bracket mass ratio is presented in Fig. 7. KOH to lumpy bracket mass ratio of 0.5 was too low to remove inorganic residue and the ash content was high. It can be assumed that an increase in ash content is related to the increase in the mass ratio of KOH to lumpy bracket from 1 to 3 and higher combustion temperature. The optimal KOH to lumpy bracket mass ratio was found to be 1. When KOH: lumpy bracket mass ratio was equal to 1 the increase in temperature caused the reduction of ash content. Probably, higher carbonization temperature combined with chemical activation led
of
to the faster reaction of inorganic compounds with KOH. The rate of inorganic species reaction grew faster
ro
along with the temperature increase than the rate of burning of the carbon. The ash content values were in good agreement with XRF results (Table S1). a
-p lP
3
re
600_1 650_1 700_1 750_1 800_1 850_1 900_1
1
0.4
0.6
0.8
Jo ur
0.2
na
2
0 0.0
CO2 adsorption [mmol/g]
4
CO2 adsorption [mmol/g]
b 4
1.0
Pressure [bar]
c
CO2 adsorption [mmol/g]
4
3
2 800_3 800_2 800_1.5 800_1 800_0.5
1
0 0.0
0.2
0.4
0.6
0.8
1.0
Pressure [bar]
3
2 850_3 850_2 850_1.5 850_1 850_0.5
1
0 0.0
0.2
0.4
0.6
0.8
1.0
Pressure [bar]
Fig. 8. Carbon dioxide adsorption isotherms at 298 K on activated biocarbons prepared at different temperatures with KOH to biomass ratio equal 1 (a), with different KOH to biomass ratios at the temperature of 800oC (b), with different KOH to biomass ratios at the temperature of 850 oC (c)
Journal Pre-proof Fig. 8 shows CO2 adsorption isotherms at 298 K under pressure up to 1 bar. The CO2 sorption up to 1 bar at the temperature of 273 K was investigated as well (Fig. S7). The data were obtained from theoretical calculations using the best fitted adsorption isotherm model. The CO2 adsorption isotherms at a temperature of 273 and 298 K had a steep rise at low pressure and slowly become less steeper at higher pressures. The highest CO2 adsorption under pressure of 1 bar was achieved in activated biocarbons obtained at 850oC and for KOH to lumpy bracket mass ratio equal to 1:1. The adsorption of carbon dioxide at 1 bar on 850_1 were 4.62 and 7.14 mmol/g at 298 and 273 K, respectively. Table 3 summarises the highest CO2 adsorption at 1 bar and at the temperature of 273K (qCO2(273, 1)) and 298 K (qCO2(298, 1)). The CO2 adsorption of the presented samples is the highest in the current state of the art (Table 3).
298 K
273 K
Ref.
activated templated carbons
3.4
5.4
[69]
sodium alginate
4.57
8.99
[6]
activated carbon spheres
4.55
7.48
[70]
lignin
4.6
7.4
eucalyptus sawdust
5.0
chitosan
3.86
5.90
[72]
N‐ doped carbons templated
4.4
6.9
[73]
from mesoporous silica polyaniline Camellia japonica Jujun grass data
4.5
4.3
6.35
[74]
[75]
5.0
[68]
4.1
[68]
4.4
-p
lP
6.4
Jo ur
N‐ doped carbons templated
[71] [4]
na
from zeolite
ro
Carbon source
re
of
Table 3. CO2adsorption of various carbons at 1 bar and273K and 298 K.
6.2
[36]
The typical composition of flue gas (post-combustion capture) is: nitrogen (70 – 75 vol%), carbon dioxide (15 – 16 vol%) and water (5 – 7 vol%) [76]. The pressure of it is 1 bar. Thus, it is desirable to find a good CO2 sorbent at low pressures, especially at 0.15 bar that is typical partial pressure in the flue gas. The activated biocarbons 850_1 exhibited very high CO2 adsorption (1.67 mmol/g) at the pressure of 0.15 bar and temperature of 298 K. This value is one of the highest described in the literature [77]. CO2 adsorption at 273 K was higher than at 298 K for all the activated biocarbons. The lowering adsorption with an increase in the temperature is widely described in the literature. This indicates that physisorption took place and the exothermic nature of CO2 sorption on biocarbons. The van der Waals forces played a fundamental role in the interaction between CO2 and biocarbons. These molecular forces are
Journal Pre-proof stronger at low temperature but increase in the temperature weaken them. This is also in good agreement with the Le Chatelier's principle. Fig. S8 shows a dependence of the carbonization temperature combined with chemical activation on the CO2 adsorption at fixed KOH: lumpy bracket mass ratio of 1:1. Lower CO2 adsorption of the activated biocarbons prepared at lower temperatures was observed because of the formation of a small amount of pores. The increase in carbonization temperature up to 850ºC caused an improvement of CO2 adsorption capability of biocarbons. It was related to the formation of the larger amounts of pores. CO2 adsorption occurred at 900°C decreased probably due to damage of porous structure. Thus, the optimal temperature of carbonization combined with chemical activation for production of the most efficient CO2 sorbents was 850oC.
of
The increase of KOH to lumpy bracket mass ratio led to an increase in CO2 adsorption to some point
ro
and then decrease. The optimal value of KOH to lumpy bracket mass ratio depended on the temperature. The recommended KOH: lumpy bracket mass ratio was equal to 1:1 for a temperature of 850oC and 3:2 for
-p
800oC. These behaviours was attributed to the textural properties of the samples.
re
To analyse how textural parameters influence CO2 sorption on activated biocarbons at 273 and 298
lP
K at 1 bar the relationships between them and CO2 sorption were investigated (see Fig 9).
7.5
6.5
273 K 298 K
5.5
2
R = 0.9938
5.0
Jo ur
CO2 adsorption [mmol/g]
6.0
na
7.0
4.5 4.0
2
R = 0.9656
3.5 3.0 2.5 2.0 0.1
0.2
0.3
0.4
0.5
0.6
3
VmN2 [cm /g]
Fig. 9. CO2 uptake at pressure 1 bar and temperature 273 and 298 K as a function of micropore pore volume (VmN2) It was supposed that CO2 adsorption capacity at 1 bar could depend on the surface area (Fig. S9), pore volume (Fig. S10), micropore volume determined by nitrogen (Fig. 9). It was found that there is no correlation between CO2 uptake and total pore volume. The correlation between CO2 uptake and surface area and micropore volume determined by nitrogen sorption was observed. The highest coefficient of determination at temperature 273 and 298 K was obtained for micropore volume. This suggested the best dependence of CO2 uptake on the micropore volume of the samples.
Journal Pre-proof Some authors [35, 78] stated that the most important for CO2 adsorption at 1 bar at 273 K are small micropores. The correlation between CO2 adsorption at 1 bar and 273 and 298 K, respectively, and the volume of specific pores smaller than a certain diameter was investigated (Fig. S11 and S12). On the basis of R2 values (Fig. S13) it was found that biocarbons contained mostly the pores below 0.899 nm favour the CO2 adsorption at 1 bar and 273 K, whereas at 298 K the samples with the pores below 0.548 nm were preferred. The detailed analysis of the influence of textural properties on CO2 adsorption at 1 bar showed that the volume of micropores obtained using nitrogen was the most important although a certain contribution from narrow micropores can not be excluded.
8 2
2
R =0.9133
4
CO2 adsorption [mmol/g]
5
3
-p
4 3
273 K, 1 bar
1
0.24
0.26
0.28
0.30
2
re
2
lP
CO2 adsorption [mmol/g]
6
0 0.22
of
R =0.9064
ro
7
5
0.32
0.34
3
Cumulative pore volume (0.300-0.899 nm) [cm /g]
1
0 0.08
298 K, 1 bar 0.10
0.12
0.14
0.16
0.18
Cumulative pore volume (0.300-0.548 nm) [cm3/g]
na
Fig. 10. CO2 uptake at 1 bar and 273 (left panel) and 298 K (right panel) as a function of the volume of pores below 0.899 and 0.548 nm respectively
Jo ur
The experimental data presented in Fig. 8 and S6 were fitted to Langmuir, Freundlich and five threeparameter ones – Sips, Toth, Unilan, Radke-Prausniz, Fritz−Schlunder equations. Using the squared statistics was found that Sips model fitted the best with the experimental data. Itis expressed as the following equation:
(11) where: q – is CO2equilibrium adsorption at p p - equilibrium pressure qm - the saturation capacity b - equilibrium constant n - exponential parameter indicating the heterogeneity of the sorbent Table S3 and S4 show the parameters of the Sips model, standard errors (SE), R-squared for 273 K and 298 K. The lines in Fig. 8 and S6 were obtained using Sips equation and the parameters are listed in Tables S3 and S4. All the values of saturation capacity at 273 K were higher than at 298 K. This confirmed that the adsorption of CO2 on activated biocarbons occurred via physisorption and it had the exothermic
Journal Pre-proof character. The value of n is a measure of the heterogeneity of the surface. For n =1 Langmuir equation is obtained and the surface is homogenous. The lower value of n indicates that the surface is more heterogenic. The values of n at 273 K were in the range of 0.637 – 0.841. This proves quite heterogeneity of the activated carbon surface. The activated biocarbons that adsorbed high amount of CO2 showed high heterogeneity. However, an obvious relationship between exponential parameter and CO2 sorption cannot be defined. The value of isosteric heat of adsorption (Qist) is important to characterise the interactions between adsorbent and adsorbate. It gives information about the strength of adsorption. The higher Qist value indicates stronger interaction between the carbon dioxide and activated biocarbons. High isosteric heat of adsorption causes high regeneration cost. The isosteric heat of adsorption was calculated using experimental
of
data obtained at 272 and 298 K and Sips equation parameters listed in Tables S3 and S4. The values of
ro
isosteric heat of CO2adsorption on activated biocarbons versus degree of surface coverage were presented at Fig. 10. The values of isosteric heats of adsorption varied from 26 to 32 kJ/mol for surface coverage in the
-p
range of 0.1 – 0.35. These values definitely confirm the physical character of CO2 sorption on activated biocarbons. The energies typical for physisorption are in the order of 20 – 40 kJ/mol, while the range 80 –
re
400 kJ/mol is characteristic for chemisorption. The isosteric heat of adsorption decreased with the surface
lP
coverage. The higher surface coverage, the weaker interaction between carbon dioxide and activated biocarbons. Carbon dioxide is bound to the surface of activated biocarbons by van der Waals forces and it
na
can be easily desorbed. Low values of isosteric heat of CO2 adsorption on activated biocarbons are very
Jo ur
desirable for post-combustion capture because of lower regeneration costs [79].
32
30 29
800_0.5 800_2 850_0.5 850_2
31
800_1.5 800_3 850_1.5 850_3
30
Eiso [Kj/mol]
31
Eiso [Kj/mol]
32
600_1 650_1 700_1 750_1 800_1 850_1 900_1
28
29 28
27
27
26
26 25
25 0.10
0.15
0.20
0.25
0.30
0.35
0.10
0.15
0.20
0.25
0.30
0.35
Fig. 11. Isosteric heat of adsorption CO2 on activated biocarbons calculated on the basis of adsorption at temperatures of 273 (left panel) and 298 K (right panel). However, the obtained values of isosteric heat of adsorption should be approached with a certain reserve because they were calculated on the basis of data measured at only two temperatures. For the best CO2 sorbent, namely activated biocarbon, CO2 adsorption up to 30 bar in the temperature range of 273 – 353 K was investigated. The results are presented in Fig. 12. The lines were
Journal Pre-proof calculated with the Sips model. The Sips isotherm equation was also recommended high pressure experimental data. The highest CO2 adsorption (14.31 mmol/g) was obtained at 30 bar and 273 K. CO2 adsorption at 20 – 30 bar is important for pre-combustion capture. The values obtained on 850_1 are the highest described in the literature [77]. 14
10 8 6 273 K 298 K 313 K 333 K 353 K
4 2
of
CO2 adsorption [mmol/g]
12
0
5
10
15
20
25
ro
0 30
-p
Pressure [bar]
Fig. 12. Carbon dioxide adsorption isotherms on activated biocarbon 850_1 up to 30 bar. The points
re
represent the experimental data, the lines were calculated with the Sips model.
Jo ur
na
lP
The parameters in Sips equation (11) are temperature dependent:
(12)
(13) (14)
In equations (12) – (14) qmo, Q, b0, n0, are the constants. R is ideal gas constant. T0 is the reference temperature. In this work, the lowest temperature of 273 K was chosen as the reference temperature. The parameter Q is the isosteric heat of adsorption at the surface coverage of 0.5. (15) In order to calculate qmo and ln(qm) vs temperature was plotted. The parameters Q and b0 were obtained from b vs. 1/T plot. On the basis of n vs. 1/T plot parameters n0 and were calculated. These plots are presented at Fig. S14. The parameters of the Sips equations (11) for activated biocarbons 850_1 at different adsorption temperatures and parameters of temperature dependent Sips equations (12) – (14) are presented in Tables S4 and S5.
Journal Pre-proof Logarithms of p values versus inverse temperature for various fixed surface coverage (adsorption isosteres) were plotted (Fig. S15). On the basis of slopes values of the obtained straight lines, the isosteric heat of adsorption was calculated. The isosteric heat of adsorption at various surface coverage are shown in Fig. 13
30
25
15
from high pressure adsorption from low pressure adsorption
10
of
Eiso [kJ/mol]
20
ro
5
0 0.2
0.3
0.4
0.5
-p
0.1
re
Fig. 13. Isosteric heat of adsorption on 800_1 calculated on the basis of low pressure adsorption and high pressure adsorption as a function of surface coverage
lP
The isosteric heat of adsorption for high pressure adsorption was calculated using adsorption experiments at five different temperatures. The values of isosteric heat of adsorption calculated on the basis
na
of low pressure adsorption at 273 K and 298K are presented inFig. 13. It was concluded that the values of isosteric heat of adsorption obtained on the basis of only two temperatures were very similar to the values
Jo ur
obtained on the basis of five temperatures. The isosteric heat of adsorption at the surface coverage of 0.5 obtained from Fig. 13 was consistent with the value obtained from equation (13) (25.6 kJ/mol). 14
CO2 adsorption [mmol/g]
12 10 8 6
Run 1 Run 5 Run 10 Run 15 Run 20
4 2 0 0
5
10
15
20
25
30
Pressure [bar]
Fig. 14. Multi-cycle CO2 adsorption isotherms for 850_1 at 289 K , 1st, 5th,10th, 15th , 20thadsorption isotherm The re-useability of 850_1 biocarbons as a CO2sorbent was examined as well. Twenty adsorption– desorption cycles at a temperature of 298 K were performed. Fig. 13 presents 1st, 5th,10th, 15th, 20th adsorption isotherms. The reproducibility and repeatability of CO2 adsorption isotherm are clear. No
Journal Pre-proof changes in the CO2 adsorption after twenty cycles were observed. The highest standard deviation was equal to 0.10. On the basis of these findings, it was found that 850_1 biocarbons was highly stable CO2 sorbent and it can be regenerated during post-combustion capture without loss of CO2 sorption capacity. It is very important to define the selectivity via evaluating of the sorbents in CO2removal from flue gas. Nitrogen adsorption isotherms were measured at a temperature of 298 K up to a pressure of 1 bar (Fig. S16). It was found that N2 adsorption depends on the total pore volume (Fig. S18). The higher total pore volume, the higher N2 adsorption capacity is observed. The selectivity ratio of CO2 over N2was calculated by dividing the CO2 adsorption capacity by the N2adsorption capacity at 298 K:
of
(16)
ro
qi(p) is the adsorption capacity [mmol/g] at the same partial pressure p and i is CO2 and N2.
800 600_1 650_1 700_1 (right axis) 750_1 800_1 850_1 900_1
700
800_0.5 800_1.5 800_2 800_3 850_0.5 850_1.5 850_2 850_3
re
80
500
40
400 300
0.2
Jo ur
20
0.4
0.6
0.8
CO2 selectivity
600
lP
60
na
CO2 selectivity
80
0 0.0
100
-p
100
200
60
40
20
100 0 1.0
0 0.0
Pressure [bar]
0.2
0.4
0.6
0.8
1.0
Pressure [bar]
Fig. 15. The CO2 over N2selectivity of activated biocarbons versus pressure, temperature 298 K Every CO2/N2selectivity ratio decreased very fast with the pressure (up to ~0.2 bar). For the pressure higher than 0.2, the slow increase in the CO2/N2selectivity ratio was observed. The CO2/N2 selectivity ratio at 0.01 bar for 700_1 was the highest (704) but at higher pressures, it was similar to the others biocarbons (about 20). The CO2/N2 selectivity ratio at 0.01 bar for the best CO2 adsorbent 850_1 was also very high (95). It was found that the CO2/N2 selectivity ratio depends on the total pore volume. The lower total pore volume the higher selectivity ratio was observed (Fig. S21). The CO2/N2 selectivity ratios values at 1 bar at 298 K were high (11.3 – 29.0). AchievedCO2/N2 selectivity ratio equal to 29. 0 is considerably higher than that of all described in the literature [77]. The ideal adsorbed solution theory (IAST) was also applied to predict CO2 and N2 mixture adsorption from single gas adsorption isotherms of CO2 and N2.
Journal Pre-proof (17) where: qi@pi – adsorption capacity of i at the pressure pi IAST method allows calculating the CO2 over N2 selectivity under flue gas conditions, namely pN2=0.85 and pCO2 =0.15. The selectivity calculated by IAST method is presented in Table 1. The SIAST values ranged from 33.9 to 130.2. These values were considerably higher than described by us earlier for activated carbons obtained from various biomass sources [35]. The selectivity calculated for the best CO2 adsorbent (850_1) was one of the highest and good enough for post-combustion capture.
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Well-developed porous structure and high surface area are two main characteristics responsible for the enhancement of CO2 sorption of the materials. However, these key features are also of great importance
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in the applications of energy storage, particularly in the supercapacitors. Therefore, the sample which
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indicated outstanding behaviour in CO2 sorption (850_1) will be investigated as active material in the electrodes in supercapacitor. Here, it is crucial that the material has the mesopores and micropores in its
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structure. The presence of micropores promotes the storage of electrolyte ion during the chargingdischarging process. Is also well-known that the micropores are responsible for a greater specific
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capacitance than mesopores at a low current density, whereas mesopores are more stable at high discharge current. According to the pore size distribution, calculated by DFT method (Fig. 16 and 17), this sample
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contains the mesopores in the range of 2.0 - 5.5 nm and the micropores larger than 0.5 nm [80,81]. Acquired active carbon was tested as electrode material in an energy storage device - supercapacitor.
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Electrochemical measurements include three-electrode cell test with 6M KOH as an electrolyte to get electrochemical characteristic of the carbon material. Next, symmetrical supercapacitor device, twoelectrode system, with 6M KOH and ionic liquid mpPYRTFSI electrolyte was procured to test material in a full working device.
Fig. 16 (a, b) present the cyclic voltammetry scans conducted at scan speed in range: (2,5,10,20,50,100) mV·s-1 in -0.1 V to -1.2 V potential window vs SCE. The CV plots show proper capacitor-like the behaviour of the active material with negligible faradays part at the ends of the voltage scan range. Charging/discharging curves, Fig. 16 (c), depict the symmetrical process of receiving and forwarding of the charge from mass of the electrode to the electrolyte bulk. Specific capacitance calculated according to equation 19 and 20 at different charge densities is presented in Fig. 16 (d) - the highest and lowest values are 351 and 187 F·g-1 . The numbers for the Specific Capacitance obtained from a threeelectrode test are mostly bigger than for a two-electrode device. However, only the latter value represents the real practical characteristic of the material. To further evaluate the electrochemical characterization of the active biocarbon material as EC electrode material the Electrochemical Impedance Spectroscopy was used and shown in Fig. 16 (e,f). The
Journal Pre-proof measurements were conducted in the frequency range: 100 kHz to 0.1 Hz, with 10mV amplitude at a steady point (Open Circuit Potential). The high-frequency range of the EIS spectra mainly provides information about the interaction between electrolyte ions and EC electrode material. To obtain the Equivalent Series Resistance ESR value a high-frequency range was fitted using Zfit software (BioLogic France) to an equivalent circuit presented on Fig. 16(g). This data analysis provide resistance: R1, R2 and Constance Phase Element Q described by equation 18: (18) where Zcpe is CPE impedance (Ω), Ycpe is CPE admittance (Ω), Qcpe is CPE value (F·s-α), ω is angular frequency (rad·s-1), where 0<α<1. When α =1 CPE element is a pure capacitor and for α=0 CPE is an ideal
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resistor. R1 element is mainly a resistance of the electrolyte and current collectors. R2 stands for resistance at electrolyte-electrode phase boundary with capacitance component as Q. Sum of the R1 and R2 is usually
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named Equivalent Series Resistance ESR.
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Lower frequency part of the EIS spectra represents the propagation characteristic of the ions in the porous structure of the electrode material. Cations and anions at 6M KOH electrolyte due to a small size and
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high diffusion rate could easily flow inside very developed porous carbon structure. On Nyquist plot, this corresponds to a line with ~45o slope which further tends into almost vertical line representing pure
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capacitance characteristic. Moreover, by fitting a vertical line by a linear equation, and calculation the cross point with real resistance axis (abscissa) at imaginary resistance equal 0, one could determine the segment
[78,80,84]
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from R2 which is called Equivalent Distributed Resistance EDR according to Transmission Line Theory.
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The values of the equivalent circuit obtained by fitting method R1, R2, Q and calculated ESR and EDR values for a three-electrode cell capacitor are presented at Fig. 16(f). Fig. 17 (a, b) and Fig.18 (a, b) shows the CV curves of two electrode capacitor device with active biocarbon at a different scan rate with 6M KOH and ionic liquid mpPYRTFSI electrolyte. The symmetric rectangular shape observed in 6M KOH electrolyte is associated with rapid ions diffusion and suitable charge propagation. This result confirmed that micropores size are comparable to the hydrated K+ ion size (0.331 nm), causing strong interaction of ions with the pores and thus enhancing the electroadsorption. Moreover, the shape is still maintained at a high scan rate. A large deviation from the rectangular toward triangle shape exhibited by the sample in an ionic liquid electrolyte is associated with deteriorate poor storage ability of ions charges. Cation and anion size of the mpPYRTFSI electrolyte is much bigger than in ions in KOH solution, i.e. approx. 0.7-0.8 nm. Thus, the microporous structure is too narrow to support free ion diffusion and cannot be fully used by ions. Therefore, the redox peaks were not observed in the CV curve. The specific capacitance (F·g-1) of the material was obtained from a charging-discharging test at a variable constant current density (0.1; 0.5; 1; 5; 10; 20 and 50)A·g-1. Numeric values were calculated according to equation 19 and 20 from a discharging slope of the charging-discharging test:
Journal Pre-proof (19) (20) where C is capacitance of the device (F), I is applied current (A) at time range dt (s), dQ is an electric charge (C) received from the EC during discharging action within voltage range dU (V), Csp is the specific capacitance of the material (F·g-1), Z is a 0.5 for a two-electrode device and 1 for a three-electrode cell, mel is a total mass of the electrode material in the cell. The specific capacitance in KOH electrolyte measured for a 0.1 A·g-1 was 223 F·g-1. The chargedischarge curves, Fig. 17(c), has an isosceles triangle shape and superb coulombic efficiency. Highly symmetrical form with a rectilinear characteristic of the charging and discharging step confirmed that none
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of the redox reactions occurred during the charging-discharging process [76- 82]. Furthermore, Fig. 17(d) showed that specific capacitance vs current density dependency of the
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850_1electrode material is greatly stable at values from 0.1-50 A·g-1. The specific capacitance retention is
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nearly 92 % resulting in a comparatively coulombic charge of charging and discharging stage [78] The specific capacitance with ionic liquid electrolyte was approximately 2-times lower, which is
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presented in Fig. 18(c). However, the shape of the charging-discharging slopes, Fig. 18(d), is only slightly
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deviated from a linear shape.
The values of the R1, R2, ESR and EDR for two-electrode device received from an analysis of the Electrochemical Impedance Spectroscopy test are presented in Fig. 17(f) for 6M KOH and 17(f)
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mpPYRTFSI electrolyte. A device with water-based electrolyte reveals much lower (355 mΩ) ESR value than for organic type electrolyte (2.74 Ω). Mainly it’s a consequence of the substantial differences between 25oC.
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ionic conductivity of these two electrolytes, 650 mS·cm-1 for 6M KOH vs. 4.92 mS·cm-1 for mpPYRTFSI at A tested capacitor with water-based electrolyte shows EDR at 185 mΩ level and with ionic liquid at 1.63 Ω. Lower EDR value indicates much faster charge redistribution inside electrode porous material. Furthermore, a slope of the liner segment of the 6M KOH capacitor is almost equal to 90o (88o) with comparison to a mpPYRTFSI device (78o), that indicates almost ideal capacitance characteristic. Fig. 17(g) and Fig. 18(g) displays Bode plot (phase shift φ/o vs frequency f/Hz) of tested material with two types of the electrolyte. Frequency characteristic is shifted towards higher values for a water-based device in respect to a mpPYRTFSI capacitor. Half of the power is dissipated as heat at internal resistance for a phase angle equal to 60o, which represents the point of lost factor d=0.5 [83]. Tested devices reached 1.9 Hz and 0.41 Hz at φ=60o for 6M KOH and mpPYRTFSI, respectively. The stability test was conducted under the constant current charge-discharge condition for 5000 cycles at 1 A·g-1, Fig. 17(i). The activated biocarbon exhibit excellent stability. The changes in specific capacitance are negligible, which makes this material promising for practical application.
Journal Pre-proof Specific energy (Wh·kg-1) of the material estimated for a real EC device was calculated from an equation 21, and specific power (W·kg-1) from an equation 22: (21) (22) where Esp is specific energy (Wh·kg-1), Csp is a specific capacitance (Fg-1), U stands for the working voltage range (V), t represents discharge time (h) and corr is a correction coefficient. Specific energy vs specific power data is shown in Fig. 19. For a water-based capacitor, specific energy mean value reached 1.6 Wh·kg-1 and for an ionic liquid 9.4 Wh·kg-1 . The value of the corr coefficient was equal to 12, which represent real capacitor device conditions [84].
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The electrochemical data reveal that 850_1 carbon material exhibits great power performance when
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6M KOH electrolyte is used. However, due to the higher working voltage range (3.5 vs 0.8 V) device with mpPYRTFSI reached superior specific energy value (9.4 vs1.6 Wh·kg-1). The specific capacitance of lumpy
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bracket based activated biocarbon is similar to the results obtained by other researchers. Comparison of the properties of activated carbons synthesized from different biomass sources using KOH as an activating agent
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for supercapacitor applications reported in the literature is presented in Table 4. [85-97]
Table 4 Specific capacitance of activated carbon reported in the literature.
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SBET
Carbon
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[m2g-1]
Partially graphitized ginkgo-based
Specific capacitance
Cell Electrolyte
[Fg-1]
configuratio Ref n
1775
178
6M KOH
3E
[82]
2841
330
2M KOH
3E
[86]
849
264
6M KOH
3E
[87]
Paulownia flower
1159
297
1M H2SO4
2E
[88]
Willow catkins
645
340
6M KOH
3E
[89]
Soybean nitrogen-doped
1749
243
6M KOH
3E
117
0.5MNa2SO4
2E
2959
260
6M KOH
2E
[91]
2869
287
6M KOH
3E
[92]
activated carbon Bi luochun
Human hair based heteroatom-doped activated hydrothermal carbon
Waste from a thermochemical process optimized for bio-oil production Fallen leaves mainly from Fraxinus Chinensis
[90]
Journal Pre-proof Coffee endocarp
361
69
1M H2SO4
3E
[93]
N- rich argan seed shells
2062
355
1M H2SO4
3E
[94]
O-rich argan seed shells
1654
259
1M H2SO4
3E
[94]
Corn grains activated carbon
3199
257
6M KOH
2E
[95]
Sunflower seed shell
1162
244
30% KOH
2E
[96]
Bamboo-based
3061
258
6 M KOH
3E
[97]
Lignin‐derived heteroatom‐doped porous carbons Nitrogen-doped porous carbon materials Nitrogen-doped lotus leaf porous
3E
99
2957
372
1 M KOH
3E
100
3401
346
6M KOH
3E
101
1087
6 M KOH
3E
102
327
6M KOH
2E
103
427
6M KOH
2E
104
2E
This
3E
work
2818
na
1968
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Lumpy brackets
[98]
6M KOH
1660
Garlic skin-derived 3D hierarchical
2E
240
lP
Corn straw porous carbons
3E
1270
266
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carbons
93
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Mangosteen peel waste porous carbon
6M KOH
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aerogel
porous carbon
179
1415
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Porous N-doped banana carbon
223 351
6 M KOH
na
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re
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Journal Pre-proof
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Fig. 16: (a) CV plots of three-electrode cell with 6M KOH electrolyte for 850_1 material at (2,5,10) mV·s-1 and (b) for (20,50,100) mV·s-1 scan speed, (c) charging-discharging curves for a three-electrode cell with 850_1 material 6M KOH electrolyte at (0.1, 0.5, 1, 5, 10, 20, 50) A·g-1 current densities, (d) specific capacitance - current densities dependence for a three-electrode cell 850_1 with 6M KOH electrolyte for 850_1 material, (e) EIS Nyquist plot for a 850_1 with 6M KOH electrolyte for 850_1 material, (f) magnification and analysis of the high-frequency region, (g) equivalent circuit electrical scheme.
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Fig. 17: (a) CV plots of two electrode cell with 6M KOH electrolyte for 850_1 material at (1,2,5,10) mV·s-1 and (b) for (50,100,200) mV·s-1 scan speed, (c) charging-discharging curves for a two-electrode capacitor
lP
device with 850_1material and 6M KOH electrolyte at (0.1, 0.5, 1, 5, 10, 20, 50) A·g-1 current densities,(d) specific capacitance - current densities dependence for a two-electrode 850_1and 6M KOH capacitor,(e) EIS
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Nyquist plot for a 850_1with 6M KOH capacitor, (f) magnification and analysis of the high-frequency region, (g) EIS Bode plot of the 850_1 with 6M KOH capacitor (h) equivalent circuit electrical scheme, (i)
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stability cycling test (above 4000cycles) for a 850_1with 6M KOH capacitor, specific capacitance, blue, and columbic efficiency, red, vs cycle number plot.
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Journal Pre-proof
Fig. 18: (a) CV plots of two electrode cell with mpPYRTFSI electrolyte for 850_1 material at (2,5,10) mV·sand (b) for (20,50,100) mV·s-1 scan speed, (c) charging-discharging curves for a two-electrode capacitor
re
1
lP
device with 850_1material and mpPYRTFSI electrolyte at (0.1, 0.5, 1, 5, 10, 20, 50) A·g-1 current densities, (d) specific capacitance - current densities dependence for a two-electrode 850_1 with mpPYRTFSI
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capacitor, (e) EIS Nyquist plot for a 850_1with mpPYRTFSI capacitor, (f) magnification and analysis of the high-frequency region, (g) EIS Bode plot of the 850_1 with mpPYRTFSI capacitor (h) equivalent circuit
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electrical scheme,
Fig. 19: Specific energy vs. specific power plot with 850_16M KOH and mpPYRTFSI data.
Journal Pre-proof 4. Conclusions The high-value-added nanoporous activated carbons produced from lumpy bracket was investigated and described for the first time. The influence of carbonization temperature in the range of 600 – 900 oC and mas ratio KOH to carbon precursor from 0.5:1 to 3:1 on various properties was investigated deeply. The resulted activated carbons exhibited a very high CO2 adsorption capacity and high specific capacitance with excellent retention (nearly 92% of the original capacitance) after 5000 cycles retention. It was proved that the properties of activated biocarbons produced from lumpy bracket can be tailored in order to obtain very good CO2 sorbents and efficient active material in the electrodes of supercapacitors. The activated biocarbons produced from lumpy bracket possess high surface area, high pore and
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micropore volume, narrow pore distribution and thermal stability, what makes them highly efficient
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candidates for CO2sorbent and active material for electrodes in supercapacitors. The porosity of the activated biocarbons carbons was tailored by the change of KOH: lumpy bracket
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mass ratio and the temperature of carbonization combined with chemical activation step. The changes in CO2 adsorption isotherms with the temperature and values of isosteric heats of
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adsorption indicates that CO2 sorption on activated biocarbons produced from lumpy bracket is typical for
lP
physisorption.
The highest CO2 adsorption was observed on activated carbon produced at 850oC using KOH to
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lumpy bracket mass ratio of 1:1. The adsorption of carbon dioxide of this sample at 1 bar were 4.62 and 7.14 mmol/g at 298 and 273 K, respectively, and 14.31 mmol/g at 30 bar and at 273 K. The high CO2 adsorption is due to the presence of micropores. Taking into consideration the typical composition (partial pressure of
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CO2=0.15) and conditions (1 bar, 298 K)of flue gas it was found that CO2 adsorption on 850_1 at 0.15 bar and 298 K was very high: 1.67 mmol/g. Moreover, this activated biocarbon was highly stabile and good enough selective CO2 sorbent (SIAST=46.4). The isosteric heat of CO2 adsorption on 850_1 was in the range of 25 – 27 kJ/mol. It means that 850_1 biocarbon is a potentially good candidate for both post-combustion and pre-combustion CO2 capture. Additionally, the potential application as a supercapacitor of 850_1 has been successfully confirmed. The specific capacitance retention of the material was approximately 92 after 5000 cycles, proving the great stability, which is particularly expected in energy storage devices. Presented here approach consisting of carbonization combined with chemical activation, is a facile method using cheap carbon sources which can be promising future alternatives for both CO2 adsorption and energy storage in practical applications.
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Graphical abstract
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[104] Q. Zhang, K. H. Han, S. J. Li, M. Li, J. X. Li and K. Ren, Nanoscale, 2018,10,2427–243
Highlights
nanoporous activated biocarbons from lumpy bracket
adsorption of CO2 14.31 mmol/g (30 bar, 273 K)
Journal Pre-proof CO2/N2 adsorption selectivity SIAST=46.4
specific capacitance retention 92% after 5000 cycles
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Jarosław Serafin, Martyna Baca, Marcin Biegun, Ewa Mijowska, Ryszard J. Kaleńczuk, Joanna Sreńscek-Nazzal, Beata Michalkiewicz PII:
S0169-4332(19)32519-X
DOI:
https://doi.org/10.1016/j.apsusc.2019.143722
Reference:
APSUSC 143722
To appear in:
Applied Surface Science
Received date:
11 May 2019
Revised date:
8 August 2019
Accepted date:
16 August 2019
Please cite this article as: J. Serafin, M. Baca, M. Biegun, et al., Direct conversion of biomass to nanoporous activated biocarbons for high CO2 adsorption and supercapacitor applications, Applied Surface Science(2019), https://doi.org/10.1016/ j.apsusc.2019.143722
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© 2019 Published by Elsevier.
Journal Pre-proof Direct conversion of biomass to nanoporous activated biocarbons for high CO2 adsorption and supercapacitor applications Jarosław Serafina, Martyna Bacab, Marcin Biegunb, Ewa Mijowskab, Ryszard J. Kaleńczukb, Joanna Sreńscek-Nazzala, Beata Michalkiewicza* West Pomeranian University of Technology, Szczecin, Faculty of Chemical Technology and Engineering, a
Engineering, Institute of Inorganic Chemical Technology and Environment Engineering, Pulaskiego 10, 70-
322 Szczecin, Poland b
Nanomaterials Physicochemistry Department, Piastow Av. 45, Szczecin 70-311, Poland
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A series of nanoporous activated biocarbons have been successfully obtained from novel carbon
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precursor lumpy bracket. The textural properties of the biocarbons were tailored towards microporosity suitable for high CO2 adsorption. At the temperature of 298 K and at the pressure of 0.15 bar outstanding
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CO2 adsorption up to 1.67 mmol/g was achieved. The performance of biocarbons in the absorption/regeneration cycles towards CO2 adsorption remained stable. The selectivity of CO2 over N2
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calculated on the basis of Ideal Adsorbed Solution Theory was excellent (33.9 – 130.2). To the best to our
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knowledge, the prepared activated carbon labelled as 850_1 is one of the most promising sorbents tested in CO2 post-combustion capturing and pre-combustion capturing. Furthermore, the electrochemical properties
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of this sample were also evaluated. The specific capacitance reached over 220 F/g with excellent retention (nearly 92% of the original capacitance) after 5000 cycles. These results clearly indicate that activated carbons obtained from lumpy bracket can be used for the production of low cost and, more importantly,
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highly efficient electrode in supercapacitors as well. In this way, simple and efficient direct conversion of the lumpy bracket into the highly valuable multifunctional system is proposed.
*
Corresponding author. Tel. (+48)914494096. E-mail: [email protected] (B. Michalkiewicz)
Journal Pre-proof 1. Introduction
Development of new materials intended on a large scale requires certain industrial standards to be met such as: sustainability, easy or cheap production process and revealing improved or even new desirable properties [1]. Nanoporous activated carbons prepared from low cost precursors (especially biomass that can be valorised) via cost effective route meet all three above mentioned requirements. Such biocarbons are very attractive because of their tuneable porosity, pore size, and good chemical and thermal stability. They are considered now as key materials for carbon dioxide [2- 7], hydrogen [3, 5, 8], methane storage [3,9, 10] and electrode materials for supercapacitors[11, 12].
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The properties of activated carbons can be varied by changing carbon source (petroleum coke [13],
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biomass [2-6, 14,15], polymer [16], ion-exchange resins [17], coal [18]), carbon source pretreatment (addition of dopants [19],preoxidation [20]) process conditions activation temperature [21], activator to
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carbon source ratio [22], activating agent (KOH [23, 24], CO2 [25], K2CO3 [26, 27], H3PO4[28]) or by KOH and K2CO3 modification of commercial activated carbons [29]. Agricultural and food wastes have been
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widely studied in the production of activated carbons. Numerous examples have been described in excellent
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reviews [30 -32].
In recent times, there has been a great interest in the investigations of activated carbons production
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from biomass or waste and application for CO2 adsorption. Activated carbon prepared from waste tea, activated by NaOH and modified with ethylenediamine was used as CO2 sorbent [33]. The CO2 adsorption at 30oC and 1 bar was equal to 2.5 mmol/g. Porous carbon materials were obtained also by hydrothermal
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carbonization and KOH activation of camphor leaves and camellia leaves [34]. The maximum CO2 adsorption was 8.30 mmol/g at 25 oC at 40 bar (about 1 mmol/g at 10 bar) MPa.Our group also showed that biomass, namely: pomegranate peels, mistletoe branches, mistletoe leaves, carrot peels, kiwi fruit peels, fern leaves, sugar beet pulp [35] and also date [36] are very good CO2 sorbent. The highest CO2 capture capacity was observed at pomegranate peels (6.03 mmol/g at 0oC and 4.11 mmol/g at 25oC at 1bar [35]) and at date (4.36 mmol/g at 25oC and 6.4 mmol/g at 0oC at 1 bar [36]). Nitrogen-doped porous carbons were from hazardous waste oil sludge by mixing carbon precursor pyrolysis char with KOH and urea at 700oC [37]. CO2 adsorption capacity equal to 3 mmol/g at a temperature of 25oC was achieved. O-enriched porous carbonaceous adsorbents were derived from low cost, polyethylene terephthalate waste and activated by KOH [38]. The highest CO2 uptake of 1.31 mmol/g at 30°C was observed. Activated carbons were obtained from garlic peel using KOH as an activating agent [39]. The larges CO2 uptake was of 4.22 mmol/g at 25oC and 1 bar. Chars taken from five different commercial biomass gasifiers installed in South-Tyrol (Italy) we utilized as carbon precursor for activated carbons production for CO2 adsorption applications [40]. The CO2 adsorption tests were performed at 50oC and gas composition CO2:N2=1:1. The highest uptake (3.7%) was measured for char activated with KOH.
Journal Pre-proof Nitrogen-doped carbons were prepared by K2CO3 activation of urea-modified coconut shell [41]. The carbons demonstrate CO2 uptake at 1 bar, up to 3.71 mmol/g at 25oC and to 5.12 mmol/g at 0oC.Petroleum coke, which is the carbon residue remaining after heavy oil upgrading, was used to produce high-valueadded porous carbon for CO2 capture [42]. Activated carbons were prepared by KOH activation. The highest CO2 adsorption uptake of 3.68 mmol/g at 25oC and 1 bar was observed. N-doped activated carbons were obtained by using microalgae-sodium alginate (NaAlg) as renewable precursors [43]. The simple carbonization process with flowing N2 gas only was performed. Activated carbons exhibited CO2 adsorption capacity of 3.75 mmol/g at 1 bar and 25oC. The potential to use the activated carbons prepared from polypores such as birch polypore (Piptoporusbetulinus), turkey tail (Trametes versicolor (L.) Lloyd and, date for CO2 adsorption have been
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described by us [35].
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Conventionally, activated carbons from biomass are produced in a two-step process. In the first step, the conversion of biomass to carbon-rich matter takes place. In the second step, carbonaceous matter is
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activated by chemical or physical methods [14, 44, 45]. Searching for sustainable and cheap activated biocarbons, we decided to simplify the production route. In our procedure, there is no need to perform
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pyrolysis prior to the activation. The carbonization and chemical activation occurred in a single step
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simultaneously.
Another important worldwide issue is searching for new renewable and easily accessible energy
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storage technologies in response to constant and unavoidable fossil fuel depletion [46]. Wide group of energy storage devices are supercapacitors [47]. Various types of supercapacitors have been designed over the years, however nowadays two specific groups of supercapacitor sarouse researchers particular interest.
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In the first group, active electrodes are formed by individual components between which there is a synergistic effect. Chavan et. all synthesized electrochromic supercapacitor consisting of nanoflake NiMoO4 thin film with impressive specific capacity of 1853 Fg–1 at a current rate of 1 Ag–1 and greatly capacity retention over 2500 cycles [48]. Similarly, an oxygen-rich nanograin WO3 based smart supercapacitor was fabricated by Inamdar et. al via oblique-angle sputtering. The device displayed high specific capacitance of 228 F g−1 at 0.25 Ag−1 and maintained more than 75% of initial capacitance after 2000 cycles [49]. Li et ll. investigated the electrochemical performance of cobalt-Doped Nickel Phosphiteelectrode [50] and ultrathin Nickel–Cobalt Phosphate 2D Nanosheets [51]. Results revealed that both materials exhibited the maximum specific capacitance of 714.8 F g−1 and 1132.5 F g−1. The second group includes supercapacitors, in which a working electrode is activated carbon obtained from biomass. Therefore, in the present work, we report on the effective way to prepare activated microporous biocarbons from freely available biomass of saprophytic polypore; the lumpy bracket (Trametes gibbosa)- by carbonization combined with a chemical activation using KOH in one step. As prepared biocarbons have been tested as CO2 sorbent and as electrode material in an aqueous electrolyte Electrical Double Layer Capacitor (EDLC). Lumpy bracket derived nanoporous activated biocarbons were applied as CO2 sorbents at pressures up to 1 bar and up to 30 bar which are
Journal Pre-proof similar to the pressure of the post- and pre-combustion flue gas. Moreover, due to superior properties, such as well-developed porous structure and high surface area, the most promising activated carbon has been tested as electrode components in supercapacitor measured in two electrode system in KOH as an electrolyte. The properties of the activated carbons were controlled by varying temperature of carbonization combined with chemical activation step and by the mass ratio of the lumpy bracketto KOH. The effect of controlled textural parameters of the activated biocarbons on CO2 adsorption and electrochemical performance has been investigated in this study. To the best of our knowledge, there is no report on the synthesis of activated carbons from lumpy bracket. The lumpy bracket is a parasite and grows on most kinds of hardwood trees but most commonly on
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beech. It causes white rot. It is widespread and fairly common in southern Europe, North Africa and parts of
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the Middle East, extending to Southern Asia and China. Therefore, unlike most of biomass, the use of polypores does not interfere with food supplies. This work shows that highly efficient nanoporous activated
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biocarbons can be produced from a low-cost starting carbon source.
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2. Experimental
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2.1. Preparation of nanoporous activated biocarbons
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Lumpy bracket was dried and pulverized. The KOH and lumpy bracket in a ratio of 1:6, respectively, was mixed for 3 h. After drying, the mixture was carbonized for 1h in a tube furnace at the temperature range of 600 – 900oC under nitrogen flow. The obtained activated biocarbons were rinsed by distilled water to reach
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neutral pH, washed in dilute HCl (1 mol/dm3), rinsed by distilled water to neutral pH again, and finally dried. The resultingbiocarbons were denoted as X_Y. X refers to the temperature of carbonization and Y refers to mass ratios of KOH to dried lumpy bracket. Scheme 1 illustrates the preparation steps of nanoporous activated biocarbons derived from activated lumpy bracket.
Scheme 1. Schematic illustration of the preparation steps of nanoporous activated biocarbons derived fromactivated lumpy bracket
Journal Pre-proof 2.2. Measurement of textural properties
The textural properties such as surface area (SBET), total pore volume (Vtot), micropore volume (VmN2) and pore size distribution in the range of 1 – 30 nm were measured by N2 sorption at 77 K.The narrow micropore volume (VmCO2) and pore size distribution in the range of 0.3 – 1.5 nm were investigated via CO2 sorption at 273 K (Quadrasorbevo Gas Sorption Surface Area and Pore Size Analyzer). The surface area of the samples was calculated with the Brunauer–Emmett–Teller (BET) equations. The total pore volumes were calculated from the nitrogen amount adsorbed at a relative pressure, p/p0 of ~ 1. The micropore volumes and pore size distribution were calculated with a density functional theory (DTF). Prior the N2 and CO2
sorption isotherm measurements, all the samples were outgassed in the degassing station at 250°C for 12
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hours to a background pressure 22 · 10-3 mmHg to remove water and other impurities.
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2.3. X-ray diffraction (XRD)
Powdered activated biocarbons were pressed in the stainless steel sample holders for XRD analysis.
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The X-ray diffraction measurement was carried out on an Empyrean, PANalytical. The measurements were
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collected using copper radiation (K1 = 0.154056 nm) in the range of 10-60 in 2 scale. The scan rate was equal to 2o per minute. The patterns were analysed with X’PertHighScore software. The solid carbon materials are formed by monolayers of stacked graphenes. In the monolayers, each
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carbon atom is bonded to three other carbon atoms by covalent bonds-sp2hybridized type. The crystal structure of graphite is shown in Fig. 1. The crystal lattice of graphite is hexagonal, with A-B-A-B stacking
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of hexagonal monolayers i.e. half the carbon atoms have carbon atoms directly above and below them in the neighboring sheets (white circles in the Fig. 1), half of them has no direct neighbors in adjacent planes (black circles in the Fig. 1). This results in a hexagonal unit cell with dimensions of a = 0.2456 nm, c = 0.6708 nm (marked in red in Fig. 1). The interplanar distance (d002) between two consecutive layers of graphene by the c axis is ideally 0.3354 nm. The interactions between two consecutive layers of graphene are believed to be a van der Waals type but some authors suggested the mixture of Van der Waals and weak electrostatic interatomic interaction [52].
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Fig. 1. Crystal structure of graphite
The average crystallite thickness (Lc) and average graphene sheet diameter (Lc) can be approximately
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calculated using an empirical expression derived by Scherrer [53], which is:
where:
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(1)
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L is the mean crystallite dimension in nm along a line normal to the reflecting plane, λXDR is the X-ray wavelength (for copper K radiation λXDRis0.15409 nm),
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K is a constant depending on the reflection plane, is the full width at half-maximum of the (100/101) and (002) peaks in radians of 2after subtraction of the instrumental broadening,
is the scattering angle (in radians), The instrumental broadening effect on full width at half-maximum was subtracted out using Warren’s method assuming a Gaussian peak [53]: (2) For the dimension of turbostratic crystallites perpendicular to the graphene sheets (Lc ), (002) data is used and K equals 0.89. For the dimension in graphene sheet planes (La), (100/101) data is used and K equals 1.84 [54]. The La and Lc were calculated using equations:
(3)
Journal Pre-proof (4)
The average spacing between graphitic layers was calculated using the Bragg law (5)
where: n is an integer, the "order" of reflection is normally assumed to be equal to 1. The number of graphitic layers (N) was estimated using the equation:
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(6)
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2.4. Raman spectroscopy
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Raman spectra were collected by a Renishaw InVia equipped with a 514.5 nm line of an Ar+ laser.
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The relative intensity of the peaks was analysed with the assistance of the “Spectroscopy/Baseline and
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Peaks” function available at “Origin Pro 8” software.
2.5. FT_IR spectra
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Fourier transform infrared reflection (FTIR) spectra were carried out on Nicolet 6700 FT-IR
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spectrometer.
2.6. Electron microscopy
The morphology of the samples was observed via scanning electron microscopy with cold emission (UHR FE-SEM Hitachi SU8020) with Energy-Dispersive Spectroscopy (EDX) as its mode for analysis of the chemical composition. The sample preparation for SEM involved sprinkling of the powder sample on a double-sided carbon tape mounted on the SEM stub. Images were taken with a 20 kV accelerating voltage using a triple detector system. The morphology of the sample in greater details was examined using high resolution transmission electron microscopy (HR-TEM, Tecnai F20). The activated biocarbons were mounted on a copper grid.
2.7. The percentage of ash content and elemental analysis
The ash content (Ash%) of the activated biocarbons was established by standard methods (ASTM Designation D2866 11 [55]. Briefly dried and powdered activated biocarbons were burnt off for 3 h, at the temperature of 650 ± 25 °C in an electric muffle furnace.
Journal Pre-proof The ash content was calculated by: Ash% = weight of solids remaining [g] /original weight of biocarbon [g] · 100%
(7)
Elemental analysis of the ash was performed using energy dispersive X-ray fluorescence (EDXRF) (spectrometer Epsilon 3).
2.8. Measurement of equilibrium adsorption of CO2 Equilibrium adsorption isotherms of carbon dioxide at temperatures of 273 and 298 K for all the samples were measured volumetrically with Quadrasorbevo Gas Sorption Surface Area and Pore Size with pressure up to 1bar. The CO2 adsorption capacities at 1 bar and 273 and 298 K were denoted as qCO2(273, qCO2(298, 1), respectively.
of
1)and
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Equilibrium adsorption isotherms of carbon dioxide in the most efficient CO2 sorbent in the temperature range of 273 - 353oC with pressure up to 30 bar were measured volumetrically with Hiden
-p
Isochema IMI – HTP Manometric Gas Sorption Analyzers. This method was described in details elsewhere [56]. Prior to all sorption isotherm measurements, the samples were outgassed.
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A good correlation of experimental data to theoretical models is useful for the mathematical
lP
modelling and simulation of CO2 separation process. Eight commonly used adsorption isotherms, two twoparameter equations: Langmuir, Freundlich and five three-parameter ones: Sips, Toth, Unilan, RadkePrausniz, Fritz−Schlunder isotherm were used to correlate the experimental data. The model parameters
na
were estimatedusing Origin 8.0 using non-linear regression analysis. To find the best fitted model R2squared statistic was used. Moreover, standard errors (SE) of each parameter were calculated.
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The isosteric heat of adsorption was calculated using the Clausius – Clapeyron equation:
(8)
where: R is the universal gas constant, T is the temperature, p is the pressure, is the degree of surface coverage. After integration of equation (7) linear function is obtained:
(9)
The best fitted isotherm model was found. Then the values of pressure for each temperature for the same degree of surface coverage were calculated directly from the transformation of the isotherm model into function p=f(or by the interpolation. The ln(p) values vs. 1/T for the same degree of surface coverage
Journal Pre-proof were plotted. For each degree of surface coverage, one linear function was obtained. Isosteric heat of adsorption values was calculated from the slope of the linear function (equations (9). On the basis of these data isosteric heat of adsorption vs. degree of surface coverage plot was obtained.
2.9.Electrochemical Capacitor preparation
Three-electrode cell consists of Nickel foam, 15mm in diameter, working electrode (WE) with active material pressed at the surface containing active material, acetylene black and PTFE in mass ratio of 8:1:1, respectively, Saturated Calomel Reference Electrode (ALS-Japan) and Nickel Foam counter electrode with surface area of 20 cm2. WE contains ~6mg of the active material and cell works in 6M KOH as electrolyte.
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Electrode material for a two-electrode cell was prepared by mixing 100 mg of a solution containing
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active material, acetylene black and Kynar Flex PVDF with the mass ratio of 8:1:1, respectively and then anhydrous acetone was introduced to obtain a light slurry. Next, the active material was airbrushed onto a
-p
clean stainless-steel foil (10 μm 316L grade) or Al carbon covered foil (20um) followed by an IR radiation for drying. Steel or Al foil covered by an electrode material (mass density ~ 6 mg·cm-2) was cut to a circle
re
shape electrode with a diameter of 13 mm by a mechanical cutter and dried for 24 h in a vacuum oven (15
lP
mbar, 70 C). The dried electrodes, after weighting, were transported to Ar (purity 5.0) filled glove-box (MBraun Germany). After 24h assembled together with a separator (TF4530 (NKK Japan)) and electrolyte
na
(6M KOH Stainless Steel foil or 1-Methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide mpPYRTFSIIoLiTec Germany) in a type CR2032 button cell. Electrochemical measurements such as Cyclic Voltammetry, Galvanostatic Charging-Discharging,
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Electrochemical Impedance Spectroscopy, were conducted with the use of the multi-channel potentiostat VMP3 (BioLogic France).
3. Results and discussion
The N2 sorption isotherms measured for all activated biocarbons exhibited a sharp increase at very small p/p0. This indicates that activated biocarbons obtained from lumpy bracket are microporous. The hysteresis loops of the samples are very narrow and not clearly visible (see Fig.1). All the nitrogen adsorption−desorption isotherms are presented in details in Fig. S1. The appearance of a narrow hysteresis suggested that the activated biocarbons contain also mesopores.
Journal Pre-proof 800
600_1 750_1 900_1
700
650_1 800_1
800
a
700_1 850_1
500
3
3
800_1 800_3
400 300
500 400 300
200
200
100
100
0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 0.0
1.0
0.1
0.2
0.3
0.4
p/p0
850_1 850_3
0.7
0.8
0.9
1.0
c
850_1.5
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600 500 400
-p
3
0.6
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850_0.5 850_2
700
0.5 p/p0
800
N2 @ STP [cm /g]
b
800_1.5
600
N2 @ STP [cm /g]
600 N2 @ STP [cm /g]
800_0.5 800_2
700
300
100
0.1
0.2
0.3
0.4
0.5
0.6
p/p0
0.7
0.8
0.9
1.0
lP
0 0.0
re
200
na
Fig. 2. N2 adsorption−desorption isotherms on activated biocarbons prepared at different temperatures with KOH to biomass ratio equal to 1 (a), with different KOH to biomass ratio at the temperature of 800 (b), with
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different KOH to biomass ratios at the temperature of 850 oC (c)
The nitrogen adsorption isotherms of all the sample show the mixture of types I and IV isotherms according to IUPAC classification [57]. They are characteristic of an ordered microporous - mesoporous material. However, the limiting adsorption at high p/p0 was not observed. This indicates that the hysteresis loops of the activated samples showed type H3. The surface area of all activated biocarbons was calculated on the basis of the BET equation. The linear form of the BET equations is as follows: (10) where: n is amount adsorbed at the relative pressure p/p0 nm is monolayer capacity C is exponentially related to the energy of monolayer adsorption and it should be a positive value It is worth noting that the BET model is usually used to calculate the surface area of all materials but it actually does not apply to microporous materials. It is well known that in micropores the mechanism of
Journal Pre-proof adsorption is based on pore filling process rather than mono- or multilayer coverage [58]. According to this mechanism, the calculations of surface area on the basis of the standard BET method gives the overestimated values [59, 60]. The C value obtained from standard BET method calculations is usually negative, which has no physical sense. BET model can be applied to variousmaterials but extreme caution is needed for the materials containing micropores. The problem is how to find the linearrange of the BET plot for microporous materials in away that reduces any subjectivity in theassessment of the monolayer capacity. The solution of this problem was described by Rouquerol et al. [61]. Briefly, the following main consistency criteriamust be met: - the value of C parameter should be greater than zero (i.e. the y-intercept of the linear region should be positive
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- the pressure should be selected only in the range where the n(1 – p/p0) monotonically grows with p/p0
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- the p/p0 value corresponding to monolayer has to be within the selected linear range. It was found that for microporous materials BET method can be applied but the pressure range must
-p
be identified by criteria listed above [62, 63].
In our investigations “micropore BET assistant" available in Quantachrome software was used. This
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method is very useful to find the linear BET range for microporous materials taking into considerations
lP
above listed consistency criteria. However, it should be emphasized that the surface area calculated on the basis of BET equation for microporous materials can not be treated as a realistic value of the surface area.
na
The calculated value is rather an apparent surface area. However, such surface area can be used for the comparison to the other materials. Rouquerol et al. [61] called surface area obtained in this way as “adsorbent fingerprint”.
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Table 1 lists the textural properties of the activated biocarbons. BET surface area, pore volume, and micropore volume were enhanced due to the increasing temperature of carbonization up to 850 oC. The high temperature of carbonisation favoured pore development but at 900 oC the oxidation of biomass was too intensive and porous structure was not well developed as at 850 oC. The similar phenomena were observed when the ratio of KOH to biomass increased. The BET surface area, pore volume, and micropore volume increased along with the increase in the ratio but there were the certain optimal values of the ratios (1:2 for 800 oC and 1:1 for 850 oC) to obtain the most porous structure. The highest BET surface area (SBET=1968 m2/g), pore volume (Vtot= 1.144cm3/g), micropore volume estimated on the basis of N2 adsorption (VmN2 = 0.552cm3/g) and narrow micropore volume estimated on the basis of CO2 adsorption (VmCO2= 0.401cm3/g) were obtained for lumpy bracket derived carbons activated at KOH: biomass ratio of 1 and temperature of 850 oC.
Journal Pre-proof Table 1. Textural properties of the lumpy bracket derived biocarbons and amount of CO2 uptake at a temperature of 273 and 298 K under a pressure 1 bar and CO2/N2selectivity calculated on the basis of ideal adsorbed solution theory (IAST) Activated
SBET
Vtot
VmN2
VmCO2
qCO2(273, 1)
qCO2(298, 1)
biocarbon
[m2/g]
[cm3/g]
[cm3/g]
[cm3/g]
[mmol/g]
[mmol/g]
SIAST
483
0.302
0.166
0.331
4.39
2.16
50.4
650_1
559
0.268
0.195
0.316
4.57
2.29
60.2
700_1
967
0.453
0.335
0.345
5.47
2.88
130.2
750_1
1482
0.756
0.485
0.386
6.53
3.96
49.1
800_1
1750
0.851
0.517
0.384
6.84
4.3
51
850_1
1968
1.144
0.552
0.401
4.62
46.4
900_1
1208
0.806
0.250
0.312
5.13
2.91
34.2
800_0.5
1321
0.602
0.418
0.352
6.10
3.59
51.5
800_1.5
1689
0.835
0.551
0.379
7.04
4.45
53.9
800_2
1833
0.960
0.457
0.357
6.54
4.13
45.2
800_3
874
0.546
0.255
0.321
5.01
2.70
40.5
850_0.5
1171
0.538
0.379
0.346
5.81
3.18
50.6
850_1.5
1604
0.931
0.485
0.371
6.59
4.06
45.4
850_2
1177
0.988
0.353
0.344
5.67
3.25
33.9
850_3
697
0.439
0.207
0.317
4.69
2.42
42.6
7.15
ro
-p
re
lP
na
Jo ur
of
600_1
It was found that activated biocarbons contained micropores and small mesopores with a diameter below 6 nm (Fig. S2).
Due to kinetic restrictions, the estimation of the pore volume for pores lower than 1 nm using nitrogen at 77 K as sorbent is not advisable. CO2adsorption at a temperature of 273 K was proposed to complement N2 sorption for the characterization of the narrow microporosity [64]. CO2 sorption isotherms were used to calculate pore size distribution (Fig. 3) and pore volume for pores (0.3–1.1 nm) using the DFT method (Table 1). The pore volume of narrow micropores was ranging from 0.312 to0.401 nm. The trimodal pore size distribution with pore diameters of 0.35, 0.54 and 0.79 nm was observed.
Journal Pre-proof 1.6
a
1.6
600_1 650_1 700_1 750_1 800_1 850_1 900_1
1.0 0.8
1.2
0.6
1.0 0.8 0.6
0.4
0.4
0.2
0.2
0.0 0.3
0.4
0.5
0.6
0.7
0.8
800_0.5 800_1 800_1.5 800_2 800_3
3
1.2
b
1.4
dV/dD [cm /(g · nm)]
3
dV/dD [cm /(g · nm)]
1.4
0.9
1.0
0.0 0.3
1.1
0.4
0.5
0.6
D [nm] 1.6
0.8
0.9
1.0
1.1
c
of
1.4
1.0
-p
0.8
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850_0.5 850_1 850_1.5 850_2 850_3
1.2
3
dV/dD [cm /(g · nm)
0.7 D[nm]
0.6
0.2
0.4
0.5
0.6
0.7 D [nm]
0.8
0.9
1.0
1.1
lP
0.0 0.3
re
0.4
Fig. 3. DFT pore size distribution of activated biocarbons calculated on the basis of CO2 sorption prepared at
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different temperatures with KOH to biomass ratio of 1:1, respectively (a), with different KOH to biomass
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ratios at the temperature of 800oC (b), with different KOH to biomass ratios at the temperature of 850 oC (c)
The main role of KOH was related to the oxidation of biomass. Potassium hydroxide reacts with part of carbon and stimulates the formation of the pores, especially micropores and small mesopores during the carbonization combined with the activation process. KOH removes also impurities such as SiO2 and dissolved the tar by-products. During the carbonization combined with chemical activation, the chemical reactions between biomass and KOH occurred. The oxidation of carbon leads to the formation of potassium carbonate, hydrogen, metallic potassium, water and carbon oxide (II). The main reactions of biomass and KOH were as follows: 4 KOH + C → K2CO3 + H2O + 2H2 6 KOH + 2 C → 2 K2CO3 + 2 K +3 H2 2 KOH + C + H2O → K2CO3 + 2H2 2 KOH + 2C → 2CO + 2 K + H2 The functional groups present in biomass decomposed also into volatiles such as carbon oxide, carbon dioxide, and water. These volatiles developed porosity escaped through the structure of the carbon. The volatiles also reacted with the biomass:
Journal Pre-proof C + H2O → H2 + CO C + H2O → H2 + CO2 C +CO2→ 2CO or with KOH: 2 KOH + CO2→ K2CO3 + H2O Potassium hydroxide was converted to potassium oxide: 2KOH → K2O + H2O Potassium carbonate and potassium oxide reacted with biomass as well: K2CO3+ 2 C → 2 K + 3 CO
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K2CO3 + C → K2O + 2 CO K2O + C → 2 K + CO
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The activated carbons formation and development of high porosity can be ascribed to the above-
-p
mentioned reactions and evolution of volatiles in carbon that took place during the carbonization combined with the chemical activation. The potassium compounds and metallic potassium, which intercalated into the
re
carbon structure, were removed by washing with water and HCl. Highly porous activated biocarbons, with high surface area and pore volume, were obtained.
lP
The graphitic structure and the purity of activated biocarbons were analyzed by XRD method. Fig. 3 shows the XRD patterns of the samples prepared in the temperature range from 600 to 900oC with KOH to
na
biomass ratio equal 1:1 and at the temperatures of 800 and 850 oC with KOH to biomass ratios in the range of 1 :6. All the samples did not contain any of the sodium compounds residues. Two broad peaks centred at
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about 2 =23 and 43o which correspond to the (002) plane and (100/101) of the graphitic structure, were observed. The broad peaks indicated a highly disordered carbon structure. It was found that the samples obtained at the lowest (600oC) and the highest (900oC) temperature and using the lowest (0.5) and the highest (0.3) impregnation ratios of KOH to lumpy bracket showed the peaks centred at about 2 =23 and 43 o with relatively narrower intensities. Similar results were obtained by Singh et al. [65]. The higher crystallinity in the samples can be ascribed to the insufficient and extensive reaction of the KOH with the biomass carbon occurred at the lowest and highest temperatures and KOH to biomass ratio. The optimization of the reaction conditions such as the amount of KOH and temperature required for the chemical reaction led to the formation of disordered nanoporous carbon. Therefore, their XRD peaks are broad. As an effect of applying the optimal combination of temperature and KOH, highly disordered and randomly oriented graphitic carbon layers (expansion of the graphitic carbon lattices, random distribution of layers, breakdown of aligned structural domains) were formed. What can be also related to the high surface area, total pore volume, and micropore volume. All these parameters play a key role in efficient CO2 adsorption.
Journal Pre-proof b
a
650_1 800_1
700_1 850_1
800_0.5 800_1.5
800_1 800_2
800_3
45
55
Intensity [a.u.]
Intensity [a.u.]
600_1 750_1 900_1
10
15
20
25
30
35
40
45
50
55
60
10
15
20
25
30
50
60
850_1 850_3
850_1.5
10
15
20
25
30
35
40
Angle 2 [degree]
45
50
lP
re
-p
Intensity [a.u.]
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850_0.5 850_2
40
of
c
35
A
Angle 2 [degree]
55
60
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Fig.4. XRD patterns of activated biocarbons prepared at different temperatures with KOH to biomass ratio equal to 1 (a), with different KOH to biomass ratios at the temperature of 800oC (b), with different KOH to
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biomass ratios at the temperature of 850 oC (c)
The values of average spacing between graphitic layers, average crystallite thickness, average graphene sheet diameter and an average number of graphitic layers are summarized in Table 2. The average interlayer spacing (distance between the turbostratic graphitic layers) in obtained materials was about 0.380 nm, which is approximately 13% increased in comparison to that of highly ordered pyrolytic graphite. The lowest interlayer spacing (0.373 nm) was observed for activated biocarbon obtained in the temperature of 850 oC and with KOH to the source of carbon ratio equal 1:2. The highest interlayer spacing (0.390 nm) was observed for activated biocarbons synthesized at the temperature of 750 o
C and with KOH : source of carbon ratio equal to 1:1. respectively. The higher inter-planer distance value for activated biocarbons may be caused by the sp3 defects
and/or by interlayer repulsion between the surfaces that are negatively charged because of surface functional groups[66]. The values of interlayer spacing and the width of the peaks corresponding to (002) and (101) reflection planes indicated the presence of disordered carbon structure [65]. It should be noticed that for
Journal Pre-proof biocarbons 800_1, 850_1 and 800_1.5 the values of d-spacing were not calculated because the peak corresponding to (002) reflection planes was not observed. The average crystallite thickness and average graphene sheet diameter values, listed in Table 2, are not exactly equal to Lc and La of the crystallites because equations (3) and (4) were derived for highly graphitized carbons. Therefore, they are not really suitable for turbostratic (i.e. fully disordered) carbons. Equations (3) and (4) can be applied as a convenient and relative estimation of crystallite thickness and average graphene sheet diameter [67]. In fact, the crystallite sizes are likely to be slightly higher than those presented in Table 2. The in-plane crystallite size values of the activated biocarbons were found to be small (2.443 – 3.716 nm). The average crystallite sizes along c axis values were within the range of 0.758 – 1.154 nm which gave the average number of graphitic layers in the crystallite from 2 to 3.
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The values presented in Table 2 confirmed the conclusions derived from XRD patterns (Fig. 4) that
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activated biocarbons synthetized at the lowest and the highest temperatures and using the lowest and the highest impregnation ratios of KOH to lumpy bracket showed relatively higher crystallinity. These activated
-p
biocarbons exhibited the highest average crystallite thickness, average graphene sheet diameter and the
re
average number of graphitic layers.
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Table 2. The values of average spacing between graphitic layers d(002), average crystallite thickness La, average graphene sheet diameter Lc and average number of graphitic layers N calculated from XRD analysis d(002)
La
biocarbon
[nm]
[nm]
Lc
N
na
Activated
[nm]
0.381
3.117
1.154
650_1
0.387
2.970
1.018
2.6
700_1
0.377
2.755
1.020
2.7
750_1
0.391
2.890
1.017
2.6
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600_1
800_1
2.564
850_1
2.443
3.0
900_1
0.382
3.331
1.147
3.0
800_0.5
0.382
3.679
0.824
2.2
0.758
2.0
800_1.5 800_2
0.386
800_3
0.382
2.890
0.932
2.4
850_0.5
0.373
3.716
0.913
2.5
850_1.5
0.373
3.274
0.874
2.3
850_2
0.375
3.186
0.887
2.4
850_3
0.379
3.155
0.955
2.5
Journal Pre-proof The activated biocarbons were characterized as well by Raman spectroscopy in order to supplement XRD results. The peak named G, centred around 1580 cm-1, refers to graphite.G-band arises from the stretching of the C-C bond in graphitic materials, and is common to all sp2 carbon systems. In the presence of graphitic disorder of the carbon structure the band around 1300 cm-1, named D is observed. Its intensity is proportional to the number of defects. So, the intensity ratio of the G and D band (IG/ID) is used for relative estimation of defects in the crystal structure. The higher IG/ID value the lower graphitic-disorder is observed. On the basis of Raman spectra normalized to the G band (see Fig. S3) IG/ID values were estimated. Fig. 5 shows the dependence of the IG/ID ratio on KOH: lumpy bracket mass ratio and the temperature. The values of IG/ID for activated biocarbons were in the range of 0.52 – 0.76. The higher IG/ID values were observed in the samples obtained in the lower and the higher temperatures and using the lowest and the
of
highest impregnation ratios of KOH to lumpy bracket. The values of IG/ID changed with a similar trend as Lc
ro
values (Fig. S4). FT-IR spectra of obtained activated carbon were also investigated and were attached to
o
600
650
850
900
0.8
lP
0.7 0.6 0.5 0.4
na
IG/ID
950
re
550
Temperature [ C] 700 750 800
-p
Supplementary (Fig. S5)
0.3
X_1 850_Y 800_Y
0.1 0.0 0.0
0.5
1.0
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0.2
1.5
2.0
2.5
3.0
KOH : lumpy brackets mass ratio
Fig. 5. Dependence of the IG/ID ratio on KOH to lumpy bracket mass ratio and the temperature. SEM pictures showed the activated biocarbons have an irregular shape of thin flake-like particles with sharp corners ( Fig.6 and Fig. S6.) The similar shapes were observed for the activated carbons produced from Camellia japonica and Jujun grass [68].
Journal Pre-proof
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Fig. 6. SEM (a) and TEM (b)image of activated biocarbon 850_1
image confirmed that 850_1 biocarbon is highly porous.
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TEM image of 850_1 sample is shown in Fig. 6. The pore channel geometry is clearly seen. TEM
-p
In order to determine the purity of the samples, the ash content in activated biocarbons was determined. The ash is the residue that remains after the combustion process (the detailed procedure was described in 2.6).
re
The ash content indicates the quality of activated biocarbons and affects the pore volume and surfaces area. The high ash content leads to the clogging of pores and reducing surface area and pore volume.
lP
The residual mass after activated biocarbon burning was assigned to the presence of mineral species. The presence of sodium was excluded by EDX and XRF method. High ash content is unfavourable because
na
obstructs the pore development, which leads to the low adsorption capacity. The activated biocarbons made from lumpy bracket exhibited low residual mass (see Fig. 7). The average ash content was equal to 3.0 wt%
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and the highest value is 5.1 wt%. It means that biocarbons were nearly free of inorganic residues. Therefore, the activated biocarbons from lumpy bracket were composed of carbon. o
550
600
5
650
Temperature [ C] 700 750 800
850
900
950
Ash content [wt%]
4
3
2
1
X_1 850_Y 800_Y
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
KOH : lumpy brackets mass ratio
Fig. 7. Dependence of the ash content on KOH to lumpy bracket mass ratio and temperature
Journal Pre-proof The ash content depended on the carbon combustion and efficient removal of inorganic compounds due to reaction with KOH during carbonization and chemical activation. For example, SiO2 reacting with KOH formed water soluble K2SiO3, which is removed during purification step. The dependence of ash content on the temperature and KOH to lumpy bracket mass ratio is presented in Fig. 7. KOH to lumpy bracket mass ratio of 0.5 was too low to remove inorganic residue and the ash content was high. It can be assumed that an increase in ash content is related to the increase in the mass ratio of KOH to lumpy bracket from 1 to 3 and higher combustion temperature. The optimal KOH to lumpy bracket mass ratio was found to be 1. When KOH: lumpy bracket mass ratio was equal to 1 the increase in temperature caused the reduction of ash content. Probably, higher carbonization temperature combined with chemical activation led
of
to the faster reaction of inorganic compounds with KOH. The rate of inorganic species reaction grew faster
ro
along with the temperature increase than the rate of burning of the carbon. The ash content values were in good agreement with XRF results (Table S1). a
-p lP
3
re
600_1 650_1 700_1 750_1 800_1 850_1 900_1
1
0.4
0.6
0.8
Jo ur
0.2
na
2
0 0.0
CO2 adsorption [mmol/g]
4
CO2 adsorption [mmol/g]
b 4
1.0
Pressure [bar]
c
CO2 adsorption [mmol/g]
4
3
2 800_3 800_2 800_1.5 800_1 800_0.5
1
0 0.0
0.2
0.4
0.6
0.8
1.0
Pressure [bar]
3
2 850_3 850_2 850_1.5 850_1 850_0.5
1
0 0.0
0.2
0.4
0.6
0.8
1.0
Pressure [bar]
Fig. 8. Carbon dioxide adsorption isotherms at 298 K on activated biocarbons prepared at different temperatures with KOH to biomass ratio equal 1 (a), with different KOH to biomass ratios at the temperature of 800oC (b), with different KOH to biomass ratios at the temperature of 850 oC (c)
Journal Pre-proof Fig. 8 shows CO2 adsorption isotherms at 298 K under pressure up to 1 bar. The CO2 sorption up to 1 bar at the temperature of 273 K was investigated as well (Fig. S7). The data were obtained from theoretical calculations using the best fitted adsorption isotherm model. The CO2 adsorption isotherms at a temperature of 273 and 298 K had a steep rise at low pressure and slowly become less steeper at higher pressures. The highest CO2 adsorption under pressure of 1 bar was achieved in activated biocarbons obtained at 850oC and for KOH to lumpy bracket mass ratio equal to 1:1. The adsorption of carbon dioxide at 1 bar on 850_1 were 4.62 and 7.14 mmol/g at 298 and 273 K, respectively. Table 3 summarises the highest CO2 adsorption at 1 bar and at the temperature of 273K (qCO2(273, 1)) and 298 K (qCO2(298, 1)). The CO2 adsorption of the presented samples is the highest in the current state of the art (Table 3).
298 K
273 K
Ref.
activated templated carbons
3.4
5.4
[69]
sodium alginate
4.57
8.99
[6]
activated carbon spheres
4.55
7.48
[70]
lignin
4.6
7.4
eucalyptus sawdust
5.0
chitosan
3.86
5.90
[72]
N‐ doped carbons templated
4.4
6.9
[73]
from mesoporous silica polyaniline Camellia japonica Jujun grass data
4.5
4.3
6.35
[74]
[75]
5.0
[68]
4.1
[68]
4.4
-p
lP
6.4
Jo ur
N‐ doped carbons templated
[71] [4]
na
from zeolite
ro
Carbon source
re
of
Table 3. CO2adsorption of various carbons at 1 bar and273K and 298 K.
6.2
[36]
The typical composition of flue gas (post-combustion capture) is: nitrogen (70 – 75 vol%), carbon dioxide (15 – 16 vol%) and water (5 – 7 vol%) [76]. The pressure of it is 1 bar. Thus, it is desirable to find a good CO2 sorbent at low pressures, especially at 0.15 bar that is typical partial pressure in the flue gas. The activated biocarbons 850_1 exhibited very high CO2 adsorption (1.67 mmol/g) at the pressure of 0.15 bar and temperature of 298 K. This value is one of the highest described in the literature [77]. CO2 adsorption at 273 K was higher than at 298 K for all the activated biocarbons. The lowering adsorption with an increase in the temperature is widely described in the literature. This indicates that physisorption took place and the exothermic nature of CO2 sorption on biocarbons. The van der Waals forces played a fundamental role in the interaction between CO2 and biocarbons. These molecular forces are
Journal Pre-proof stronger at low temperature but increase in the temperature weaken them. This is also in good agreement with the Le Chatelier's principle. Fig. S8 shows a dependence of the carbonization temperature combined with chemical activation on the CO2 adsorption at fixed KOH: lumpy bracket mass ratio of 1:1. Lower CO2 adsorption of the activated biocarbons prepared at lower temperatures was observed because of the formation of a small amount of pores. The increase in carbonization temperature up to 850ºC caused an improvement of CO2 adsorption capability of biocarbons. It was related to the formation of the larger amounts of pores. CO2 adsorption occurred at 900°C decreased probably due to damage of porous structure. Thus, the optimal temperature of carbonization combined with chemical activation for production of the most efficient CO2 sorbents was 850oC.
of
The increase of KOH to lumpy bracket mass ratio led to an increase in CO2 adsorption to some point
ro
and then decrease. The optimal value of KOH to lumpy bracket mass ratio depended on the temperature. The recommended KOH: lumpy bracket mass ratio was equal to 1:1 for a temperature of 850oC and 3:2 for
-p
800oC. These behaviours was attributed to the textural properties of the samples.
re
To analyse how textural parameters influence CO2 sorption on activated biocarbons at 273 and 298
lP
K at 1 bar the relationships between them and CO2 sorption were investigated (see Fig 9).
7.5
6.5
273 K 298 K
5.5
2
R = 0.9938
5.0
Jo ur
CO2 adsorption [mmol/g]
6.0
na
7.0
4.5 4.0
2
R = 0.9656
3.5 3.0 2.5 2.0 0.1
0.2
0.3
0.4
0.5
0.6
3
VmN2 [cm /g]
Fig. 9. CO2 uptake at pressure 1 bar and temperature 273 and 298 K as a function of micropore pore volume (VmN2) It was supposed that CO2 adsorption capacity at 1 bar could depend on the surface area (Fig. S9), pore volume (Fig. S10), micropore volume determined by nitrogen (Fig. 9). It was found that there is no correlation between CO2 uptake and total pore volume. The correlation between CO2 uptake and surface area and micropore volume determined by nitrogen sorption was observed. The highest coefficient of determination at temperature 273 and 298 K was obtained for micropore volume. This suggested the best dependence of CO2 uptake on the micropore volume of the samples.
Journal Pre-proof Some authors [35, 78] stated that the most important for CO2 adsorption at 1 bar at 273 K are small micropores. The correlation between CO2 adsorption at 1 bar and 273 and 298 K, respectively, and the volume of specific pores smaller than a certain diameter was investigated (Fig. S11 and S12). On the basis of R2 values (Fig. S13) it was found that biocarbons contained mostly the pores below 0.899 nm favour the CO2 adsorption at 1 bar and 273 K, whereas at 298 K the samples with the pores below 0.548 nm were preferred. The detailed analysis of the influence of textural properties on CO2 adsorption at 1 bar showed that the volume of micropores obtained using nitrogen was the most important although a certain contribution from narrow micropores can not be excluded.
8 2
2
R =0.9133
4
CO2 adsorption [mmol/g]
5
3
-p
4 3
273 K, 1 bar
1
0.24
0.26
0.28
0.30
2
re
2
lP
CO2 adsorption [mmol/g]
6
0 0.22
of
R =0.9064
ro
7
5
0.32
0.34
3
Cumulative pore volume (0.300-0.899 nm) [cm /g]
1
0 0.08
298 K, 1 bar 0.10
0.12
0.14
0.16
0.18
Cumulative pore volume (0.300-0.548 nm) [cm3/g]
na
Fig. 10. CO2 uptake at 1 bar and 273 (left panel) and 298 K (right panel) as a function of the volume of pores below 0.899 and 0.548 nm respectively
Jo ur
The experimental data presented in Fig. 8 and S6 were fitted to Langmuir, Freundlich and five threeparameter ones – Sips, Toth, Unilan, Radke-Prausniz, Fritz−Schlunder equations. Using the squared statistics was found that Sips model fitted the best with the experimental data. Itis expressed as the following equation:
(11) where: q – is CO2equilibrium adsorption at p p - equilibrium pressure qm - the saturation capacity b - equilibrium constant n - exponential parameter indicating the heterogeneity of the sorbent Table S3 and S4 show the parameters of the Sips model, standard errors (SE), R-squared for 273 K and 298 K. The lines in Fig. 8 and S6 were obtained using Sips equation and the parameters are listed in Tables S3 and S4. All the values of saturation capacity at 273 K were higher than at 298 K. This confirmed that the adsorption of CO2 on activated biocarbons occurred via physisorption and it had the exothermic
Journal Pre-proof character. The value of n is a measure of the heterogeneity of the surface. For n =1 Langmuir equation is obtained and the surface is homogenous. The lower value of n indicates that the surface is more heterogenic. The values of n at 273 K were in the range of 0.637 – 0.841. This proves quite heterogeneity of the activated carbon surface. The activated biocarbons that adsorbed high amount of CO2 showed high heterogeneity. However, an obvious relationship between exponential parameter and CO2 sorption cannot be defined. The value of isosteric heat of adsorption (Qist) is important to characterise the interactions between adsorbent and adsorbate. It gives information about the strength of adsorption. The higher Qist value indicates stronger interaction between the carbon dioxide and activated biocarbons. High isosteric heat of adsorption causes high regeneration cost. The isosteric heat of adsorption was calculated using experimental
of
data obtained at 272 and 298 K and Sips equation parameters listed in Tables S3 and S4. The values of
ro
isosteric heat of CO2adsorption on activated biocarbons versus degree of surface coverage were presented at Fig. 10. The values of isosteric heats of adsorption varied from 26 to 32 kJ/mol for surface coverage in the
-p
range of 0.1 – 0.35. These values definitely confirm the physical character of CO2 sorption on activated biocarbons. The energies typical for physisorption are in the order of 20 – 40 kJ/mol, while the range 80 –
re
400 kJ/mol is characteristic for chemisorption. The isosteric heat of adsorption decreased with the surface
lP
coverage. The higher surface coverage, the weaker interaction between carbon dioxide and activated biocarbons. Carbon dioxide is bound to the surface of activated biocarbons by van der Waals forces and it
na
can be easily desorbed. Low values of isosteric heat of CO2 adsorption on activated biocarbons are very
Jo ur
desirable for post-combustion capture because of lower regeneration costs [79].
32
30 29
800_0.5 800_2 850_0.5 850_2
31
800_1.5 800_3 850_1.5 850_3
30
Eiso [Kj/mol]
31
Eiso [Kj/mol]
32
600_1 650_1 700_1 750_1 800_1 850_1 900_1
28
29 28
27
27
26
26 25
25 0.10
0.15
0.20
0.25
0.30
0.35
0.10
0.15
0.20
0.25
0.30
0.35
Fig. 11. Isosteric heat of adsorption CO2 on activated biocarbons calculated on the basis of adsorption at temperatures of 273 (left panel) and 298 K (right panel). However, the obtained values of isosteric heat of adsorption should be approached with a certain reserve because they were calculated on the basis of data measured at only two temperatures. For the best CO2 sorbent, namely activated biocarbon, CO2 adsorption up to 30 bar in the temperature range of 273 – 353 K was investigated. The results are presented in Fig. 12. The lines were
Journal Pre-proof calculated with the Sips model. The Sips isotherm equation was also recommended high pressure experimental data. The highest CO2 adsorption (14.31 mmol/g) was obtained at 30 bar and 273 K. CO2 adsorption at 20 – 30 bar is important for pre-combustion capture. The values obtained on 850_1 are the highest described in the literature [77]. 14
10 8 6 273 K 298 K 313 K 333 K 353 K
4 2
of
CO2 adsorption [mmol/g]
12
0
5
10
15
20
25
ro
0 30
-p
Pressure [bar]
Fig. 12. Carbon dioxide adsorption isotherms on activated biocarbon 850_1 up to 30 bar. The points
re
represent the experimental data, the lines were calculated with the Sips model.
Jo ur
na
lP
The parameters in Sips equation (11) are temperature dependent:
(12)
(13) (14)
In equations (12) – (14) qmo, Q, b0, n0, are the constants. R is ideal gas constant. T0 is the reference temperature. In this work, the lowest temperature of 273 K was chosen as the reference temperature. The parameter Q is the isosteric heat of adsorption at the surface coverage of 0.5. (15) In order to calculate qmo and ln(qm) vs temperature was plotted. The parameters Q and b0 were obtained from b vs. 1/T plot. On the basis of n vs. 1/T plot parameters n0 and were calculated. These plots are presented at Fig. S14. The parameters of the Sips equations (11) for activated biocarbons 850_1 at different adsorption temperatures and parameters of temperature dependent Sips equations (12) – (14) are presented in Tables S4 and S5.
Journal Pre-proof Logarithms of p values versus inverse temperature for various fixed surface coverage (adsorption isosteres) were plotted (Fig. S15). On the basis of slopes values of the obtained straight lines, the isosteric heat of adsorption was calculated. The isosteric heat of adsorption at various surface coverage are shown in Fig. 13
30
25
15
from high pressure adsorption from low pressure adsorption
10
of
Eiso [kJ/mol]
20
ro
5
0 0.2
0.3
0.4
0.5
-p
0.1
re
Fig. 13. Isosteric heat of adsorption on 800_1 calculated on the basis of low pressure adsorption and high pressure adsorption as a function of surface coverage
lP
The isosteric heat of adsorption for high pressure adsorption was calculated using adsorption experiments at five different temperatures. The values of isosteric heat of adsorption calculated on the basis
na
of low pressure adsorption at 273 K and 298K are presented inFig. 13. It was concluded that the values of isosteric heat of adsorption obtained on the basis of only two temperatures were very similar to the values
Jo ur
obtained on the basis of five temperatures. The isosteric heat of adsorption at the surface coverage of 0.5 obtained from Fig. 13 was consistent with the value obtained from equation (13) (25.6 kJ/mol). 14
CO2 adsorption [mmol/g]
12 10 8 6
Run 1 Run 5 Run 10 Run 15 Run 20
4 2 0 0
5
10
15
20
25
30
Pressure [bar]
Fig. 14. Multi-cycle CO2 adsorption isotherms for 850_1 at 289 K , 1st, 5th,10th, 15th , 20thadsorption isotherm The re-useability of 850_1 biocarbons as a CO2sorbent was examined as well. Twenty adsorption– desorption cycles at a temperature of 298 K were performed. Fig. 13 presents 1st, 5th,10th, 15th, 20th adsorption isotherms. The reproducibility and repeatability of CO2 adsorption isotherm are clear. No
Journal Pre-proof changes in the CO2 adsorption after twenty cycles were observed. The highest standard deviation was equal to 0.10. On the basis of these findings, it was found that 850_1 biocarbons was highly stable CO2 sorbent and it can be regenerated during post-combustion capture without loss of CO2 sorption capacity. It is very important to define the selectivity via evaluating of the sorbents in CO2removal from flue gas. Nitrogen adsorption isotherms were measured at a temperature of 298 K up to a pressure of 1 bar (Fig. S16). It was found that N2 adsorption depends on the total pore volume (Fig. S18). The higher total pore volume, the higher N2 adsorption capacity is observed. The selectivity ratio of CO2 over N2was calculated by dividing the CO2 adsorption capacity by the N2adsorption capacity at 298 K:
of
(16)
ro
qi(p) is the adsorption capacity [mmol/g] at the same partial pressure p and i is CO2 and N2.
800 600_1 650_1 700_1 (right axis) 750_1 800_1 850_1 900_1
700
800_0.5 800_1.5 800_2 800_3 850_0.5 850_1.5 850_2 850_3
re
80
500
40
400 300
0.2
Jo ur
20
0.4
0.6
0.8
CO2 selectivity
600
lP
60
na
CO2 selectivity
80
0 0.0
100
-p
100
200
60
40
20
100 0 1.0
0 0.0
Pressure [bar]
0.2
0.4
0.6
0.8
1.0
Pressure [bar]
Fig. 15. The CO2 over N2selectivity of activated biocarbons versus pressure, temperature 298 K Every CO2/N2selectivity ratio decreased very fast with the pressure (up to ~0.2 bar). For the pressure higher than 0.2, the slow increase in the CO2/N2selectivity ratio was observed. The CO2/N2 selectivity ratio at 0.01 bar for 700_1 was the highest (704) but at higher pressures, it was similar to the others biocarbons (about 20). The CO2/N2 selectivity ratio at 0.01 bar for the best CO2 adsorbent 850_1 was also very high (95). It was found that the CO2/N2 selectivity ratio depends on the total pore volume. The lower total pore volume the higher selectivity ratio was observed (Fig. S21). The CO2/N2 selectivity ratios values at 1 bar at 298 K were high (11.3 – 29.0). AchievedCO2/N2 selectivity ratio equal to 29. 0 is considerably higher than that of all described in the literature [77]. The ideal adsorbed solution theory (IAST) was also applied to predict CO2 and N2 mixture adsorption from single gas adsorption isotherms of CO2 and N2.
Journal Pre-proof (17) where: qi@pi – adsorption capacity of i at the pressure pi IAST method allows calculating the CO2 over N2 selectivity under flue gas conditions, namely pN2=0.85 and pCO2 =0.15. The selectivity calculated by IAST method is presented in Table 1. The SIAST values ranged from 33.9 to 130.2. These values were considerably higher than described by us earlier for activated carbons obtained from various biomass sources [35]. The selectivity calculated for the best CO2 adsorbent (850_1) was one of the highest and good enough for post-combustion capture.
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Well-developed porous structure and high surface area are two main characteristics responsible for the enhancement of CO2 sorption of the materials. However, these key features are also of great importance
ro
in the applications of energy storage, particularly in the supercapacitors. Therefore, the sample which
-p
indicated outstanding behaviour in CO2 sorption (850_1) will be investigated as active material in the electrodes in supercapacitor. Here, it is crucial that the material has the mesopores and micropores in its
re
structure. The presence of micropores promotes the storage of electrolyte ion during the chargingdischarging process. Is also well-known that the micropores are responsible for a greater specific
lP
capacitance than mesopores at a low current density, whereas mesopores are more stable at high discharge current. According to the pore size distribution, calculated by DFT method (Fig. 16 and 17), this sample
na
contains the mesopores in the range of 2.0 - 5.5 nm and the micropores larger than 0.5 nm [80,81]. Acquired active carbon was tested as electrode material in an energy storage device - supercapacitor.
Jo ur
Electrochemical measurements include three-electrode cell test with 6M KOH as an electrolyte to get electrochemical characteristic of the carbon material. Next, symmetrical supercapacitor device, twoelectrode system, with 6M KOH and ionic liquid mpPYRTFSI electrolyte was procured to test material in a full working device.
Fig. 16 (a, b) present the cyclic voltammetry scans conducted at scan speed in range: (2,5,10,20,50,100) mV·s-1 in -0.1 V to -1.2 V potential window vs SCE. The CV plots show proper capacitor-like the behaviour of the active material with negligible faradays part at the ends of the voltage scan range. Charging/discharging curves, Fig. 16 (c), depict the symmetrical process of receiving and forwarding of the charge from mass of the electrode to the electrolyte bulk. Specific capacitance calculated according to equation 19 and 20 at different charge densities is presented in Fig. 16 (d) - the highest and lowest values are 351 and 187 F·g-1 . The numbers for the Specific Capacitance obtained from a threeelectrode test are mostly bigger than for a two-electrode device. However, only the latter value represents the real practical characteristic of the material. To further evaluate the electrochemical characterization of the active biocarbon material as EC electrode material the Electrochemical Impedance Spectroscopy was used and shown in Fig. 16 (e,f). The
Journal Pre-proof measurements were conducted in the frequency range: 100 kHz to 0.1 Hz, with 10mV amplitude at a steady point (Open Circuit Potential). The high-frequency range of the EIS spectra mainly provides information about the interaction between electrolyte ions and EC electrode material. To obtain the Equivalent Series Resistance ESR value a high-frequency range was fitted using Zfit software (BioLogic France) to an equivalent circuit presented on Fig. 16(g). This data analysis provide resistance: R1, R2 and Constance Phase Element Q described by equation 18: (18) where Zcpe is CPE impedance (Ω), Ycpe is CPE admittance (Ω), Qcpe is CPE value (F·s-α), ω is angular frequency (rad·s-1), where 0<α<1. When α =1 CPE element is a pure capacitor and for α=0 CPE is an ideal
of
resistor. R1 element is mainly a resistance of the electrolyte and current collectors. R2 stands for resistance at electrolyte-electrode phase boundary with capacitance component as Q. Sum of the R1 and R2 is usually
ro
named Equivalent Series Resistance ESR.
-p
Lower frequency part of the EIS spectra represents the propagation characteristic of the ions in the porous structure of the electrode material. Cations and anions at 6M KOH electrolyte due to a small size and
re
high diffusion rate could easily flow inside very developed porous carbon structure. On Nyquist plot, this corresponds to a line with ~45o slope which further tends into almost vertical line representing pure
lP
capacitance characteristic. Moreover, by fitting a vertical line by a linear equation, and calculation the cross point with real resistance axis (abscissa) at imaginary resistance equal 0, one could determine the segment
[78,80,84]
na
from R2 which is called Equivalent Distributed Resistance EDR according to Transmission Line Theory.
Jo ur
The values of the equivalent circuit obtained by fitting method R1, R2, Q and calculated ESR and EDR values for a three-electrode cell capacitor are presented at Fig. 16(f). Fig. 17 (a, b) and Fig.18 (a, b) shows the CV curves of two electrode capacitor device with active biocarbon at a different scan rate with 6M KOH and ionic liquid mpPYRTFSI electrolyte. The symmetric rectangular shape observed in 6M KOH electrolyte is associated with rapid ions diffusion and suitable charge propagation. This result confirmed that micropores size are comparable to the hydrated K+ ion size (0.331 nm), causing strong interaction of ions with the pores and thus enhancing the electroadsorption. Moreover, the shape is still maintained at a high scan rate. A large deviation from the rectangular toward triangle shape exhibited by the sample in an ionic liquid electrolyte is associated with deteriorate poor storage ability of ions charges. Cation and anion size of the mpPYRTFSI electrolyte is much bigger than in ions in KOH solution, i.e. approx. 0.7-0.8 nm. Thus, the microporous structure is too narrow to support free ion diffusion and cannot be fully used by ions. Therefore, the redox peaks were not observed in the CV curve. The specific capacitance (F·g-1) of the material was obtained from a charging-discharging test at a variable constant current density (0.1; 0.5; 1; 5; 10; 20 and 50)A·g-1. Numeric values were calculated according to equation 19 and 20 from a discharging slope of the charging-discharging test:
Journal Pre-proof (19) (20) where C is capacitance of the device (F), I is applied current (A) at time range dt (s), dQ is an electric charge (C) received from the EC during discharging action within voltage range dU (V), Csp is the specific capacitance of the material (F·g-1), Z is a 0.5 for a two-electrode device and 1 for a three-electrode cell, mel is a total mass of the electrode material in the cell. The specific capacitance in KOH electrolyte measured for a 0.1 A·g-1 was 223 F·g-1. The chargedischarge curves, Fig. 17(c), has an isosceles triangle shape and superb coulombic efficiency. Highly symmetrical form with a rectilinear characteristic of the charging and discharging step confirmed that none
of
of the redox reactions occurred during the charging-discharging process [76- 82]. Furthermore, Fig. 17(d) showed that specific capacitance vs current density dependency of the
ro
850_1electrode material is greatly stable at values from 0.1-50 A·g-1. The specific capacitance retention is
-p
nearly 92 % resulting in a comparatively coulombic charge of charging and discharging stage [78] The specific capacitance with ionic liquid electrolyte was approximately 2-times lower, which is
re
presented in Fig. 18(c). However, the shape of the charging-discharging slopes, Fig. 18(d), is only slightly
lP
deviated from a linear shape.
The values of the R1, R2, ESR and EDR for two-electrode device received from an analysis of the Electrochemical Impedance Spectroscopy test are presented in Fig. 17(f) for 6M KOH and 17(f)
na
mpPYRTFSI electrolyte. A device with water-based electrolyte reveals much lower (355 mΩ) ESR value than for organic type electrolyte (2.74 Ω). Mainly it’s a consequence of the substantial differences between 25oC.
Jo ur
ionic conductivity of these two electrolytes, 650 mS·cm-1 for 6M KOH vs. 4.92 mS·cm-1 for mpPYRTFSI at A tested capacitor with water-based electrolyte shows EDR at 185 mΩ level and with ionic liquid at 1.63 Ω. Lower EDR value indicates much faster charge redistribution inside electrode porous material. Furthermore, a slope of the liner segment of the 6M KOH capacitor is almost equal to 90o (88o) with comparison to a mpPYRTFSI device (78o), that indicates almost ideal capacitance characteristic. Fig. 17(g) and Fig. 18(g) displays Bode plot (phase shift φ/o vs frequency f/Hz) of tested material with two types of the electrolyte. Frequency characteristic is shifted towards higher values for a water-based device in respect to a mpPYRTFSI capacitor. Half of the power is dissipated as heat at internal resistance for a phase angle equal to 60o, which represents the point of lost factor d=0.5 [83]. Tested devices reached 1.9 Hz and 0.41 Hz at φ=60o for 6M KOH and mpPYRTFSI, respectively. The stability test was conducted under the constant current charge-discharge condition for 5000 cycles at 1 A·g-1, Fig. 17(i). The activated biocarbon exhibit excellent stability. The changes in specific capacitance are negligible, which makes this material promising for practical application.
Journal Pre-proof Specific energy (Wh·kg-1) of the material estimated for a real EC device was calculated from an equation 21, and specific power (W·kg-1) from an equation 22: (21) (22) where Esp is specific energy (Wh·kg-1), Csp is a specific capacitance (Fg-1), U stands for the working voltage range (V), t represents discharge time (h) and corr is a correction coefficient. Specific energy vs specific power data is shown in Fig. 19. For a water-based capacitor, specific energy mean value reached 1.6 Wh·kg-1 and for an ionic liquid 9.4 Wh·kg-1 . The value of the corr coefficient was equal to 12, which represent real capacitor device conditions [84].
of
The electrochemical data reveal that 850_1 carbon material exhibits great power performance when
ro
6M KOH electrolyte is used. However, due to the higher working voltage range (3.5 vs 0.8 V) device with mpPYRTFSI reached superior specific energy value (9.4 vs1.6 Wh·kg-1). The specific capacitance of lumpy
-p
bracket based activated biocarbon is similar to the results obtained by other researchers. Comparison of the properties of activated carbons synthesized from different biomass sources using KOH as an activating agent
lP
re
for supercapacitor applications reported in the literature is presented in Table 4. [85-97]
Table 4 Specific capacitance of activated carbon reported in the literature.
na
SBET
Carbon
Jo ur
[m2g-1]
Partially graphitized ginkgo-based
Specific capacitance
Cell Electrolyte
[Fg-1]
configuratio Ref n
1775
178
6M KOH
3E
[82]
2841
330
2M KOH
3E
[86]
849
264
6M KOH
3E
[87]
Paulownia flower
1159
297
1M H2SO4
2E
[88]
Willow catkins
645
340
6M KOH
3E
[89]
Soybean nitrogen-doped
1749
243
6M KOH
3E
117
0.5MNa2SO4
2E
2959
260
6M KOH
2E
[91]
2869
287
6M KOH
3E
[92]
activated carbon Bi luochun
Human hair based heteroatom-doped activated hydrothermal carbon
Waste from a thermochemical process optimized for bio-oil production Fallen leaves mainly from Fraxinus Chinensis
[90]
Journal Pre-proof Coffee endocarp
361
69
1M H2SO4
3E
[93]
N- rich argan seed shells
2062
355
1M H2SO4
3E
[94]
O-rich argan seed shells
1654
259
1M H2SO4
3E
[94]
Corn grains activated carbon
3199
257
6M KOH
2E
[95]
Sunflower seed shell
1162
244
30% KOH
2E
[96]
Bamboo-based
3061
258
6 M KOH
3E
[97]
Lignin‐derived heteroatom‐doped porous carbons Nitrogen-doped porous carbon materials Nitrogen-doped lotus leaf porous
3E
99
2957
372
1 M KOH
3E
100
3401
346
6M KOH
3E
101
1087
6 M KOH
3E
102
327
6M KOH
2E
103
427
6M KOH
2E
104
2E
This
3E
work
2818
na
1968
Jo ur
Lumpy brackets
[98]
6M KOH
1660
Garlic skin-derived 3D hierarchical
2E
240
lP
Corn straw porous carbons
3E
1270
266
re
carbons
93
of
Mangosteen peel waste porous carbon
6M KOH
-p
aerogel
porous carbon
179
1415
ro
Porous N-doped banana carbon
223 351
6 M KOH
na
lP
re
-p
ro
of
Journal Pre-proof
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Fig. 16: (a) CV plots of three-electrode cell with 6M KOH electrolyte for 850_1 material at (2,5,10) mV·s-1 and (b) for (20,50,100) mV·s-1 scan speed, (c) charging-discharging curves for a three-electrode cell with 850_1 material 6M KOH electrolyte at (0.1, 0.5, 1, 5, 10, 20, 50) A·g-1 current densities, (d) specific capacitance - current densities dependence for a three-electrode cell 850_1 with 6M KOH electrolyte for 850_1 material, (e) EIS Nyquist plot for a 850_1 with 6M KOH electrolyte for 850_1 material, (f) magnification and analysis of the high-frequency region, (g) equivalent circuit electrical scheme.
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Fig. 17: (a) CV plots of two electrode cell with 6M KOH electrolyte for 850_1 material at (1,2,5,10) mV·s-1 and (b) for (50,100,200) mV·s-1 scan speed, (c) charging-discharging curves for a two-electrode capacitor
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device with 850_1material and 6M KOH electrolyte at (0.1, 0.5, 1, 5, 10, 20, 50) A·g-1 current densities,(d) specific capacitance - current densities dependence for a two-electrode 850_1and 6M KOH capacitor,(e) EIS
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Nyquist plot for a 850_1with 6M KOH capacitor, (f) magnification and analysis of the high-frequency region, (g) EIS Bode plot of the 850_1 with 6M KOH capacitor (h) equivalent circuit electrical scheme, (i)
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stability cycling test (above 4000cycles) for a 850_1with 6M KOH capacitor, specific capacitance, blue, and columbic efficiency, red, vs cycle number plot.
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Fig. 18: (a) CV plots of two electrode cell with mpPYRTFSI electrolyte for 850_1 material at (2,5,10) mV·sand (b) for (20,50,100) mV·s-1 scan speed, (c) charging-discharging curves for a two-electrode capacitor
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1
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device with 850_1material and mpPYRTFSI electrolyte at (0.1, 0.5, 1, 5, 10, 20, 50) A·g-1 current densities, (d) specific capacitance - current densities dependence for a two-electrode 850_1 with mpPYRTFSI
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capacitor, (e) EIS Nyquist plot for a 850_1with mpPYRTFSI capacitor, (f) magnification and analysis of the high-frequency region, (g) EIS Bode plot of the 850_1 with mpPYRTFSI capacitor (h) equivalent circuit
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electrical scheme,
Fig. 19: Specific energy vs. specific power plot with 850_16M KOH and mpPYRTFSI data.
Journal Pre-proof 4. Conclusions The high-value-added nanoporous activated carbons produced from lumpy bracket was investigated and described for the first time. The influence of carbonization temperature in the range of 600 – 900 oC and mas ratio KOH to carbon precursor from 0.5:1 to 3:1 on various properties was investigated deeply. The resulted activated carbons exhibited a very high CO2 adsorption capacity and high specific capacitance with excellent retention (nearly 92% of the original capacitance) after 5000 cycles retention. It was proved that the properties of activated biocarbons produced from lumpy bracket can be tailored in order to obtain very good CO2 sorbents and efficient active material in the electrodes of supercapacitors. The activated biocarbons produced from lumpy bracket possess high surface area, high pore and
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micropore volume, narrow pore distribution and thermal stability, what makes them highly efficient
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candidates for CO2sorbent and active material for electrodes in supercapacitors. The porosity of the activated biocarbons carbons was tailored by the change of KOH: lumpy bracket
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mass ratio and the temperature of carbonization combined with chemical activation step. The changes in CO2 adsorption isotherms with the temperature and values of isosteric heats of
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adsorption indicates that CO2 sorption on activated biocarbons produced from lumpy bracket is typical for
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physisorption.
The highest CO2 adsorption was observed on activated carbon produced at 850oC using KOH to
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lumpy bracket mass ratio of 1:1. The adsorption of carbon dioxide of this sample at 1 bar were 4.62 and 7.14 mmol/g at 298 and 273 K, respectively, and 14.31 mmol/g at 30 bar and at 273 K. The high CO2 adsorption is due to the presence of micropores. Taking into consideration the typical composition (partial pressure of
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CO2=0.15) and conditions (1 bar, 298 K)of flue gas it was found that CO2 adsorption on 850_1 at 0.15 bar and 298 K was very high: 1.67 mmol/g. Moreover, this activated biocarbon was highly stabile and good enough selective CO2 sorbent (SIAST=46.4). The isosteric heat of CO2 adsorption on 850_1 was in the range of 25 – 27 kJ/mol. It means that 850_1 biocarbon is a potentially good candidate for both post-combustion and pre-combustion CO2 capture. Additionally, the potential application as a supercapacitor of 850_1 has been successfully confirmed. The specific capacitance retention of the material was approximately 92 after 5000 cycles, proving the great stability, which is particularly expected in energy storage devices. Presented here approach consisting of carbonization combined with chemical activation, is a facile method using cheap carbon sources which can be promising future alternatives for both CO2 adsorption and energy storage in practical applications.
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[104] Q. Zhang, K. H. Han, S. J. Li, M. Li, J. X. Li and K. Ren, Nanoscale, 2018,10,2427–243
Highlights
nanoporous activated biocarbons from lumpy bracket
adsorption of CO2 14.31 mmol/g (30 bar, 273 K)
Journal Pre-proof CO2/N2 adsorption selectivity SIAST=46.4
specific capacitance retention 92% after 5000 cycles
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