ABSTRACT: Due to their high surface areas and large pore volumes, porous carbons (PCs) are valuable materials for use as electrodes in energy storage and conversion devices. Biomass is an ideal precursor for the preparation of PCs in part because it is sustainable and eco-friendly. Herein, new methodology for converting agarose, a naturally occurring type of biomass that forms robust hydrogels, into PCs with tunable pore structures and high electrochemical performance is described.
The synthetic process is straightforward and entails heating a gel that is composed of agarose and potassium oxalate (K2C2O4). Since the salt transforms into gaseous byproducts at elevated temperatures, the decomposition process was harnessed to create activated, open pores as the hydrogel underwent carbonization. For example, a PC with a surface area of 1754.9 m2g−1and a pore volume of 2.643 cm3g−1was obtained by heating a mixture of agarose and K2C2O4in a 1:3 weight ratio at 700
°C. The material was subsequently used as the electrode material in a
supercapacitor and found to display a specific capacitance of 166.0 F g−1at 0.125 A g−1. Varying the quantity of added K2C2O4 resulted in predictable changes in porosity and thus offered a means to tune the textural properties and the electrochemical performance of the PCs. For example, changing the feed ratio of agarose to K2C2O4 to 1:6 afforded a PC that exhibited a high persistent specific capacitance (64.1 F g−1at 5 A g−1after 10,000 cycles) and a high-power density (20 kW kg−1at 10 A g−1).
1. INTRODUCTION
Due to their extremely large specific surface areas, large pore volumes, high electrical conductivities, excellent physical properties, and high chemical stabilities, porous carbons (PCs) hold promise for use in a wide range of technological applications,1 including toxic gas adsorption,2 photonic materials,3 and energy conversion and storage devices.4−10 PCs are most commonly synthesized using a template, which can be a hard template11−14or a soft template,15−18although other methods are also known.19,20The templates are used to orient pores and direct them to form in a hierarchical manner via processes that includes self-assembly21or other advanced forms of controlled physical arrangement.22 Despite these advances, templated methods have several shortcomings. First, the templates must be prefabricated from well-defined porous organic or inorganic materials, which can be time consuming and costly to obtain.23 Second, the template removal step is often beleaguered by calcination24,25 or by tedious washing with copious amounts of acid or base.26 Collectively, these factors can be detrimental and prolong the production cycle, increase the cost of the PC, and/or limit mass production.
The use of biomass such as lychee shells,27tamarind,28lotus seed pods,29 glucose,30,31 or starch32 as carbon precursors provides an alternative approach for the preparation and development of PCs. The structures found in such materials
are intricate and, as such, obviate the need for templates when they are carbonized.33Moreover, PCs that are prepared from biomass typically exhibit outstanding characteristics, such as good mechanical strength, broad chemical resistance, and high electrical conductivity, and are often comparable to the properties exhibited by PCs that are prepared using templates.34,35 Biomass is also environmentally friendly, inexpensive, highly processable, and frequently showcased within the context of the growingfield of green chemistry.36
The preparation of PCs from biomass typically requires two steps, (1) carbonization and (2) activation, although both steps can be performed simultaneously. Biomass carbonization usually occurs at a temperature between 400 and 900°C and is often performed under an inert gas. The purpose of the activation step is to introduce functionality into the biochar,37 and reagents such as KOH, ZnCl2, NaOH, or FeCl3 are typically employed.38These reagents modify or“activate” the surfaces of the PCs through chemical reaction and do so
Received: August 24, 2021 Revised: October 17, 2021 Published: November 11, 2021
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without the release of gases. Since chemical reactions are governed by diffusion, the level of activation is intrinsically limited and can even reduce the porosity of resulting PCs.
Moreover, the use of strongly corrosive or toxic activators may restrict utility on large scales. Beyond activator selection, another challenge of using biomass to prepare PCs is the difficulty associated with controlling or regulating pore size and structure.
Since biomass often consists of ultralarge molecular weight polymers, which can become insoluble and complicate prefabrication of the desired structures prior to carbonization, balancing the textural properties and the specific surface areas exhibited by PCs that are prepared from biomass remains an outstanding opportunity. A particular biomass that was envisioned to meet such requirements is agarose, which is a naturally occurring polysaccharide that exhibits myriad attractive properties, including good mechanical strength, broad chemical resistance, low cost, and high processability.
Moreover, mixtures of the material adopt sol states when heated and form gels upon cooling; thus, phases and constituent pores can be tuned by modulating temperature and concentration.39−41
Herein, we describe a convenient and tunable synthetic methodology for preparing PCs using agarose and demonstrate that supercapacitors prepared from this material exhibit high electrochemical performance characteristics. While carbonized agarose typically features mesopores and macropores, we show that the addition of K2C2O4 as an activator during the carbonization step effectively introduces micropores into the resulting material as well. Indeed, as will be described below, hierarchical PCs with high surface areas (up to 1754.9 m2g−1) and high pore volumes (up to 2.643 cm3g−1) were obtained at an agarose to K2C2O4weight ratio of 1:3 and, when used as an electrode material in supercapacitors, the material displayed high specific capacitance values (i.e., 166.0 F g−1 at 0.125 A g−1). A unique advantage of the methodology described is its synthetic simplicity in conjunction with its high modularity.
For example, the performance and properties of the carbons can befinely tuned by changing readily accessible parameters, such as the quantity of the added activator and/or the carbonization temperature, and thus, the resulting materials can be tailored for use in a broad range of contemporary energy storage and conversion applications.
2. RESULTS AND DISCUSSION
Agarose was selected as a carbon precursor in part due to its renewability, environmental friendliness, relatively low cost, and high carbon processability. Figure 1 shows the synthetic approach that was used to convert agarose-based hydrogels to PCs. In brief, a predetermined ratio of agarose and an activator were dissolved in hot water, and a robust hydrogel was readily obtained after the solution was cooled to room temperature.
The hydrogel was converted to an aerogel by freeze drying and then calcined at high temperature to generate a three- dimensional porous structure. A key feature of the method- ology is that carbonization and activation occur simultaneously during the calcination step and thus obviates separate hydrothermal or thermal steps prior to activation. The synthetic simplicity is attributed to the use of agarose as the carbon precursor and to the soluble activators used to introduce pores in a manner that effectively circumvents templates. A key aspect of the method is the use of K2C2O4as an activator. The salt converts to CO2, CO, and other gaseous byproducts at elevated temperatures and thus functions as a pore-generating agent.
To optimize the quantity of K2C2O4added to the agarose, a series of materials were prepared, wherein the weight ratio of agarose to K2C2O4was systematically varied from 1:1 to 1:6.
Gel formation was not observed when the weight of K2C2O4 exceeded the aforementioned upper limit. As part of a series of control experiments, potassium hydroxide (KOH), potassium carbonate (K2CO3), and potassium chloride (KCl) were also used as activators. However, the former two activating agents hindered hydrogel formation, presumably because the basic characteristics of KOH and K2CO3 disrupted the requisite networks of hydrogen bonds.42The carbonization temperature was also varied from 600 to 800°C. The PCs produced using this methodology were labeled as“PC-x-y”, wherexrefers to the weight ratio of added K2C2O4to agarose and y refers to the temperature (expressed in °C) used for carbonization and activation. The PCs that were produced using other activating agents were labeled as “PC-z-y”, where z indicates the activating agent that was used.
The morphologies of the resulting carbonized products were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Open pores with diameters of several hundred nanometers were observed in a SEM image recorded for PC-6-700, and the porous morphology was subsequently observed by TEM (Figure 2a Figure 1.Schematic illustration of the synthetic process used to prepare agarose-based, hierarchical porous carbons.
and b). The walls of the pores appeared to be relatively thin, and a network formed via the interconnection of the pores was evident. Such unique structural features can be ascribed to the K2C2O4 activator, which may synergistically act as a
“surfactant” that establishes the network while generating gas upon degradation in a manner that facilitates pore interconnection. This hypothesis was supported by the observation of a significantly reduced number of pores for PC-0-700 (Figure 2c and d). The pore walls in PC-0-700 also appeared to be thicker than those observed in the images recorded for PC-6-700. Likewise, closed pores were observed in carbons prepared using KCl as an activator (Figure 2e and f), presumably because gaseous byproducts were not generated during carbonization, although the walls of the pores were found to be thin. Since an interconnected porous network is needed to facilitate charge transfer,43,44it was concluded that PC-6-700 was the most ideal candidate for use as electrode material in supercapacitors as it featured a large number of three-dimensional, cross-linked pores, particularly when compared to PC-0-700 which exhibited almost no pores or PC-KCl-700 which featured a closed porous structure.
The impact of the K2C2O4content on the porous structures was investigated by changing the ratio of agarose to K2C2O4. When a 1:1 weight ratio of agarose to K2C2O4was used, the
surface of the resulting material exhibited relatively shallow pores, thick pore walls and nonporous surface features that were partially flat (Figure 3a and b). This is because the proportion of the activator used was too low to facilitate sufficient surface activation of the carbon material. As the agarose to K2C2O4ratio was increased to 1:3 and then to 1:6 (Figure 3c−f), the walls of the pores found in the corresponding PCs became thinner and the distribution of pore size became more uniform. Notably, the pores found in PC-6-700 appeared to be relatively dense and more uniform when compared to the other samples analyzed (Figure 3e and f), a result that was attributed to not only the quantity of activator used but also to its unique characteristics. As noted above, K2C2O4can efficiently activate the surface of the carbon material while generating a gas that can further manipulate the structures of the pores produced. Since further increasing the quantity of K2C2O4 prevented hydrogel formation, a 1:6 weight ratio of agarose to K2C2O4 was deemed to be the optimal ratio.
To gain additional insight in the effect of carbonization temperature on the textural properties of the resulting PCs, a series of samples were prepared using a 1:6 weight ratio of agarose to K2C2O4and then separately heated at 600, 700, or 800 °C to effect carbonization. As summarized in Figure 4, Figure 2.Morphological characterization data recorded for various carbon materials. (a) SEM and (b) TEM images of PC-6-700. (c) SEM and (d) TEM images of a material that was prepared without an activating agent and carbonized at 700°C (i.e., PC-0-700). (e) SEM and (f) TEM images of a material that was prepared using KCl as an activating agent and carbonized at 700°C (i.e., PC-KCl-700).
Figure 3.Morphological characterization data recorded for carbon materials obtained at different ratios of agarose to K2C2O4. (a) SEM and (b) TEM images of PC-1-700. (c) SEM and (d) TEM images of PC-3-700. (e) SEM and (f) TEM images of PC-6-700.
fragmented pores with fine particles were identified on the pore walls found in PC-6-600 (Figure 4a and b) and attributed to amorphous carbon. By contrast, PC-6-700 exhibited an unobstructed and intact pore structure (Figure 4c and d).
Further increasing the carbonization temperature to 800 °C led to formation of cracks on the walls of the porous carbon (i.e., PC-6-800) (Figure 4e and f). The differences in the pore structures can be attributed to the decomposition character- istics of K2C2O4under different temperatures. To produce PC- 6-600, a temperature of 600 °C was used. At 500−600 °C, K2C2O4 decomposes to K2CO3 and carbon monoxide (K2C2O4→ K2CO3 + CO) after losing adventitious water at
∼100 °C. Since the decomposition process is incomplete, activation is insufficient which ultimately results in the formation of fragmented pores. To produce PC-6-700, a temperature of 700 °C was used. At this temperature and in the presence of carbon, it has been reported that liquid metallic potassium can form via a redox reaction (K2CO3+ 2C→2K + 3CO) and accelerate etching by intercalating the carbon network.45,46 Regardless, the carbon material obtained at 700
°C (PC-6-700) appeared to be fully activated and featured an interconnected, porous structure that was uniformly distrib- uted. To produce PC-6-800, a temperature of 800 °C was used. When the calcination temperature exceeds the boiling point of potassium metal (∼762 °C), gaseous potassium may be produced which can readily diffuse between the carbon layers. Such a diffusion process can be expected to result in the formation of micropores via continuous etching and then eventually mesopores via pore fusion or collapse (Scheme S1).47,48 Indeed, the irregular pores as well as the reduced micropore content and low specific surface observed in PC-6- 800 can be attributed to such a process.
The textural properties of the PCs were evaluated by a series of N2 adsorption−desorption measurements. As summarized inFigure S1, the adsorption−desorption data measured for the series of PCs exhibited Type-I and Type-IV isotherms, according to the IUPAC classification scheme, including
steep uptakes at low relative pressures (P/P0< 0.1) and high relative pressure (P/P0close to 1) as well as a hysteresis loop at intermediate relative pressures (0.4 <P/P0<∼0.8). The steep increases at low and high relative pressures were attributed to the presence of abundant micropores and macropores, respectively, and the hysteresis loop indicated that mesopores were present. Hysteresis loops may arise from capillary condensation and have been previously reported to be present in materials with mesoporous structures.49,50 By gleaning the data obtained from N2adsorption−desorption measurements in conjunction with the SEM and TEM observations, it can be concluded that the materials described herein feature hierarchical pore structures and include micro-, meso-, and macropores. Such hierarchical porous structures are critically important when using the PCs as electrodes in supercapacitor applications since ions first enter the macropores along with the electrolyte, then pass through the diffusion channel (mesopores), and finally reach the surface of the micropores to build electrochemical double layers. The hierarchical structure effectively shortens the diffusion path of the ions and reduces the diffusion resistance.
Certain textual properties, such as the specific surface area and pore volume, play decisive roles in determining the electrochemical performance characteristics displayed by PCs.
As summarized inTable 1, the specific surface area and pore volume of the carbon materials were found to strongly depend on the quantity and type of activator used. For example, the specific surface area of PC-0-700 (50 m2g−1), which was prepared without the use of an activator, was measured to be 1 to 2 orders of magnitude lower than that of the PCs the were prepared with an activator. Although the sample prepared with KCl exhibited a relatively high pore volume (3.907 cm3g−1), it was composed entirely of meso- and macropores, and thus the specific surface area of the material was relatively low (530.2 m2g−1). When K2C2O4was used, the specific surface area of PC-6-700 was measured to be as high as 1196.9 m2g−1which, as stated above, can be attributed to ability of the activator to Figure 4.Morphological characterization data recorded for carbon materials obtained at different temperatures. (a) SEM and (b) TEM images of PC-6-600. (c) SEM and (d) TEM images of PC-6-700. (e) SEM and (f) TEM images of PC-6-800.
convert to gaseous byproducts during the carbonization/
activation process.
Inspection of the textural property data revealed that the specific surface area and the pore volumes of PC-1-700, PC-3- 700, and PC-6-700 tended tofirst increase and then decrease with the ratio of activator used; optimal results were obtained for PC-3-700. The material exhibited a total surface area of 1754.9 m2g−1, a microporous surface area of 1444.3 m2g−1, a total pore volume of 2.643 cm3g−1, and a micropore volume of 0.573 cm3 g−1. The specific surface area and pore volume values were relatively large when compared to the other PCs described herein. While pore volume increased with the quantity of activator used (i.e., the pore volume measured for PC-3-700 was higher than that measured for PC-1-700), the use of excessive K2C2O4may result in the formation of caustic byproducts (e.g., metallic potassium) that ultimately destroy the pores. Such correlation may explain why the pore volume measured for PC-6-700 was less than the value measured for PC-3-700.
The carbonization temperature also has a significant influence on the textural properties of carbon materials due to the different decomposition characteristics of K2C2O4 at different temperatures. As seen inTable 1, PC-6-700 featured a high quantity of micropores (39% of micropore volume proportion vs 21% for PC-6-600 and PC-6-800). PC-6-700 also exhibited the largest surface area (1196.9 m2g−1) among
using X-ray diffraction (XRD) and Raman spectroscopy. The broad and low-intensity (002) and (100) reflections found in the XRD pattern, which were centered at diffraction angles of 24.7°(interplanar spacingd= 3.60 Å) and 43.4°(d= 2.08 Å), respectively, indicated that the material features graphitic-like carbon (Figure 5a).51,52 Although slight differences in the position of the (002) diffraction were observed, the XRD patterns recorded for the other PC materials were similar to that of PC-6-700 and also reflected graphitic features (Figure S2). The Raman spectrum recorded for PC-6-700 displayed signals centered at 1331 and 1588 cm−1, which were assigned to the D and G bands, respectively (Figure 5b). The former reflects defects and disorder in the carbon lattice, whereas the latter stems from the vibration modes of the sp2-hybridized carbons.53,54 Since the bands were not well resolved, the carbon was assumed to contain defects and may derive from the activating agent which introduces pores and oxygen functional groups.55 A broad and weak 2D band centered at 2767 cm−1was also identified and assigned to the graphitic-like structure.56,57In addition, PC-6-700 was also found to contain an intermediate oxygen content among the series of prepared materials, as determined by combustion elemental analysis (Table S1), which may be beneficial to the electrochemical properties by balancing carbon wettability with ion trans- port.58−62
To explore the electrochemical performance exhibited by the PCs, symmetric supercapacitors were fabricated and tested;
key data are shown inFigure 6. Cyclic voltammetry (CV) data were first recorded at different scan rates for a device containing PC-6-700 as an electrode (Figure 6a). The device exhibited CV curves with rectangular shapes, consistent with ideal electrochemical double-layer capacitance. The rectangular shape was retained at elevated scan rates which indicated that area as calculated using the t-plot method. cPercentage of micro-
porous surface area within the total surface area.dVtotalrefers to the total pore volume calculated at a relative pressure of 0.99. eVmicro refers to micropore volume as calculated using the t-plot method.
fPercentage of micropores within the total pore volume.gValue was obtained at a relative pressure of 0.35.
Figure 5.Representative (a) XRD pattern and (b) Raman spectrum recorded for PC-6-700.
charge transfer in the electrodes is relatively fast.63Figure 6b shows the discharge data as recorded at different current densities for supercapacitors prepared from PC-6-700. The linear characteristics of the discharge curves are consistent with the CV data and indicated that an electrochemical double-layer energy storage mechanism is operative. The specific capacitances for the electrode materials were calculated to be 135.9, 117.6, 110.5, 103.3, 93.7, 73.3, and 48.8 F g−1at current densities of 0.125, 0.25, 0.5, 1, 2, 5, and 10 A g−1, respectively (Figure 6c).
Similar measurements were conducted on supercapacitors that were prepared from the other materials described herein (seeFigures S3 and S4for the CV and charge/discharge data, respectively), and the specific capacitance data are summarized in Figure 6c. The following conclusions were derived upon gleaning the data: (1) All of the materials that were prepared with an activator exhibited higher specific capacitance values than those that were prepared in the absence of the additive, and thus, the formation of porous structures is salient to electrochemical performance. (2) Large specific surface areas benefit specific capacitance at low current densities, and PC-3- 700 (Stotal = 1754.9 m2 g−1) exhibited the highest specific capacitance (166.0 F g−1) measured at 0.125 A g−1. (3) A higher content of mesopores is beneficial to promote specific capacitance at high current rates presumably because the mesopores generally facilitate ion transport and ensure rapid diffusion of ions at a high rate. For example, PC-6-800 (which was found to contain a relatively large number of mesopores;
see Figure S1f) exhibited high specific capacitances at high current densities (e.g., 70.9 F g−1 at 10 A g−1). PC-6-700 possesses balanced amounts of micropores and mesopores such that this material featured a relatively high specific capacitance value at a high current density as well as at a low current density (e.g., 135.9 and 48.8 F g−1at 0.125 and 10 A g−1, respectively).
Electrochemical impedance spectroscopy (EIS) is often used to evaluate the serial resistance (Rs) and charge-transfer resistance (Rct) of electrodes as well as charge transfer rates.
EIS data that were recorded for various materials before cycling are shown inFigure 6d. Among the materials tested, the sample labeled as PC-6-700 exhibited relatively optimal results. For example, the data recorded in the low frequency region for the material were linear and nearly vertical, consistent with a high charge transfer rate. The Rs and Rct values were measured to be 0.49 and 9.12Ω, respectively. EIS curves were also collected after 100 charge/discharge cycles. As shown inFigure S5, the semicircles that correspond to theRct values were found to be smaller when compared with those recorded before cycling. Moreover, theRctvalue recorded for the PC-6-700 electrode was the smallest (2.20 Ω) when compared to the values recorded for the other samples. From the corresponding discharge curves, the energy and power density values were calculated and are summarized inFigure 6e. The highest power density was measured to be 20 kW kg−1 at a current density of 10 A g−1, and the highest energy density obtained was 18.9 Wh kg−1at a current density of 0.125 A g−1. Under balanced conditions, an energy density of 14.4 Wh kg−1 was measured at a power density of 2000 W kg−1. The cycling stability of the PC-6-700 electrode material was evaluated at a current density of 5 A g−1(Figure 6f). Over thefirst 500 cycles, the specific capacitance was found to increase from 63.1 to 84.7 F g−1, potentially due to surface activation of the porous material. After 10,000 cycles, the specific capacitance measured (64.1 F g−1) was still comparable to the initial value (63.1 F g−1). These performance metrics (i.e., specific capacitance, and energy density, and power density) are comparable to state-of- the-art values reported in the literature.64−66
Figure 6.Electrochemical characterization of the supercapacitors that were fabricated using different PC materials. (a) CV data as recorded at different scan rates for an electrode containing PC-6-700. (b) Charge/discharge curves collected at different current densities (indicated) for an electrode containing PC-6-700. (c) Rate performance of different electrodes measured at various current densities (indicated). (d) EIS profiles of different electrodes before cycling (indicated). (e) Ragone plot generated for an electrode containing PC-6-700. (f) Cycling stability of a supercapacitor prepared from PC-6-700 at a current density of 5 A g−1. The inset shows the discharge curves of thefirst, 500th, 5000th, and 10,000th cycle.
Changing the ratio to 1:6 afforded a PC that exhibited excellent electrochemical performance at both low and high current densities as well as outstanding capacitance retention characteristics. For example, the specific capacitance measured for PC-6-700 after 10,000 cycles at 5 A g−1(64.1 F g−1) was comparable to its initial value. Furthermore, increasing the carbonization temperature to 800°C led to a PC that showed the best rate performance (70.9 F g−1 at 10 A g−1).
Collectively, these results compare favorably with other materials used in high performance supercapacitors recently reported in the literature. The methodology described herein can be expected to afford scalable and broadly accessible PCs thatfind utility in contemporary supercapacitors as well as in other energy storage and conversion applications.
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ASSOCIATED CONTENT*sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.energyfuels.1c02875.
Experimental section, illustration of a process that explains how micropores fuse to form mesopores and macropores, textural properties of different PCs, summary of elemental composition data, CV data, and discharge curves collected at different scan rates/current densities for different electrodes (PDF)
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AUTHOR INFORMATION Corresponding AuthorsChristopher W. Bielawski−Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan National Institute of Science and Technology (UNST), Ulsan 44919, Republic of Korea; Department of Chemistry and Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea; orcid.org/0000-0002- 0520-1982; Email:[email protected]
Jianxin Geng−Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China; State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China;
Present Address: School of Material Science and Engineering, Tiangong University, No. 399 BinShuiXi Road, XiQing District, Tianjin 300387, China;
orcid.org/0000-0003-0428-4621; Email:jianxingeng@
tiangong.edu.cn
Republic of Korea
Karel Goossens−Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan National Institute of Science and Technology (UNST), Ulsan 44919, Republic of Korea;Present Address: Johnson &
Johnson, Janssen R&D, Turnhoutseweg 30, B-2340 Beerse, Belgium; orcid.org/0000-0003-0134-9565 Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.energyfuels.1c02875 Author Contributions
#S.H. and J.Z. contributed equally to this work.
Notes
The authors declare no competingfinancial interest.
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ACKNOWLEDGMENTSThe Institute for Basic Science (IBS-R019-D1) and the National Natural Science Foundation of China (51773211, 21961160700) are acknowledged for support.
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