Softwood Lignin as a Sustainable Feedstock for Porous Carbons as

Research Article pubs.acs.org/journal/ascecg

Softwood Lignin as a Sustainable Feedstock for Porous Carbons as Active Material for Supercapacitors Using an Ionic Liquid Electrolyte Markus Klose,† Romy Reinhold,†,‡ Florian Logsch,§ Florian Wolke,† Julia Linnemann,†,‡ Ulrich Stoeck,† Steffen Oswald,† Martin Uhlemann,† Juan Balach,†,∥ Jens Markowski,§ Peter Ay,§ and Lars Giebeler*,† †

Leibniz Institute for Solid State and Materials Research (IFW) Dresden e.V., Institute for Complex Materials, Helmholtzstraße 20, D-01069 Dresden, Germany ‡ Department of Chemistry and Food Chemistry, Technische Universität Dresden, Bergstraße 66, D-01069 Dresden, Germany § Lehrstuhl Aufbereitungstechnik, Brandenburgische Technische Universität Cottbus-Senftenberg, Siemens-Halske-Ring 8, D-03046 Cottbus, Germany ∥ Department of Chemistry, Universidad Nacional de Río Cuarto-CONICET, Route 36 Km 601, AR-X5804ZAB Río Cuarto, Argentina S Supporting Information *

ABSTRACT: We report on the facile synthesis of porous carbons based on a biopolymer lignin employing a two-step process which includes the activation by KOH in various amounts under an inert gas atmosphere. The resulting carbons are characterized with regard to their structural properties and their electrochemical performance as an active material in double-layer capacitors using for the first time an ionic liquid (EMIBF4) as the electrolyte for this type of carbon material to enhance storage ability. A capacitance of more than 200 F g−1 at 10 A g−1 is achieved for a carbon with a specific surface area of more than 1800 m2 g−1. One of the most crucial factors determining the electrochemical response of the active materials was found to be the strong surface functionalization by oxygen-containing groups. Furthermore, the sulfur content of the carbon precursor lignin does not result in a significant amount of sulfur-containing surface functionalities which might interact with the electrolyte. KEYWORDS: Carbon, Precursor, Microporous, Electrolyte, Double-layer capacitor, Surface functionalization, Electrochemistry

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INTRODUCTION The worldwide growth of population and the mounting demand for energy are considered two of the most pressing issues of our time. As a consequence, more research is devoted toward new and more sustainable ways of processing materials and the exploitation of byproducts as feedstocks for novel raw and functional materials. Even though lignin is one of the most abundant and renewable natural biopolymers, it is mostly regarded as a low-value residue and mainly used as an energy source in the pulping industry.1 For the most part, this circumstance also arises from the fact that the molecular constitution and structure of lignin strongly depends on the type of plant and specific process it is generated from, which in turn complicates attempts to establish it as an industrial feedstock for platform chemicals.2 Another approach consists of the utilization of the carbon content of lignin for the generation of activated carbons.3−5 Typically, this process involves a high-temperature treatment of lignin under an inert gas atmosphere and the subsequent activation of the resulting carbon material via a variety of © 2017 American Chemical Society

different activation agents such as KOH, NaOH, H3PO4, or ZnCl2, for example.4,6,7 Unsurprisingly, since activated carbons play a major role in many energy applications, lignin-based carbons have already been investigated as active materials or additives for the said field.1,4,8 Hence, the overall idea of using a waste compound such as lignin for the generation of materials for energy storage is very appealing in terms of merging two concepts of global importance for the exploitation of their synergies for increased sustainability.9 In this context, electrochemical energy storage systems seem to be one of the most promising fields of application for these materials. For example, lignin has been tested as a possible precursor for carbonaceous anodes for lithium or sodium batteries. However, in contrast to the commercially used graphite, these electrode materials suffer from a number of significant drawbacks, such as a relatively high potential of Received: January 9, 2017 Revised: March 10, 2017 Published: March 22, 2017 4094

DOI: 10.1021/acssuschemeng.7b00058 ACS Sustainable Chem. Eng. 2017, 5, 4094−4102

Research Article

ACS Sustainable Chemistry & Engineering

and stirred until a homogeneous suspension was obtained. The suspensions were dried in alumina boats at 100 °C for 24 h, after which they were transferred into a horizontal quartz glass tube furnace, where the activation was performed under an argon atmosphere at 800 °C for 1 h with a heating rate of 300 K h−1. After cooling naturally, samples were washed with a mixture of HCl (Merck KGaA, 37%, p.a.) and ethanol (Berkel AHK, 99%) (1:3 v/v) and then with a mixture of ethanol and water (1:3 v/v). Samples are denoted with the prefix “SC” for “skunk-carbon” due to their characteristic odor after the activation process, followed by the mass ratio of lignin to KOH which was used for synthesis of the respective sample (e.g., for SC-1:0.5, 1 g of KOH was mixed with 2 g of lignin). The sample denoted “pure lignin” was obtained employing the same thermal treatment as described above, omitting the addition of KOH. Characterization. For X-ray powder diffraction (XRD) experiments, powder samples were glued as a thin layer on an acetate film. Data were collected in flat sample transmission mode on a STOE STADI P powder diffractometer equipped with a 6°-position sensitive detector and a curved Ge(111) monochromator. The samples were measured with Co Kα1 radiation with a step size of Δ2θ = 0.02°. Nitrogen sorption experiments were carried out using a Quantachrome Quadrasorb SI. Prior to the measurement, the samples were degassed under vacuum at 423 K for 24 h. The pore size distributions were obtained employing the Quenched Solid Density Functional Theory (QSDFT) for slit-shaped pores. Values for surface area and pore volume were also obtained from the QSDFT method according to the BET method. Raman spectra were recorded using a Thermo Scientific DXR Smart Raman spectrometer with an excitation laser wavelength of 532 nm and a spot size of 2.1 μm. Here, ID/IG ratios were obtained by fitting and integrating the individual peaks with Lorentzian functions via common data analysis software. The XPS measurements were carried out on a PHI 5600 CI (Physical Electronics) spectrometer equipped with a hemispherical analyzer operated at a typical pass energy of 29 eV and with an analysis area of 800 μm in diameter. Mg Kα excitation (350 W) was used without applying an additional energy electron charge neutralizer. To avoid any contact of the samples with air and moisture, a transfer chamber (Physical Electronics) was used for the sample transport from the argon-filled glovebox to the XP spectrometer. Charge correction was not necessary. The C 1s peak of sp2-hybridized carbon was always observed at around 284.5 eV. For sputtering, Ar+ ions with an acceleration voltage of 3.5 eV were used. The sputtering depth itself was referenced using a SiO2 standard, with a rate of approximately 3.5 nm/min. Concentration quantification was done using standard single element sensitivity factors. For elemental analysis, a varioMACRO cube from Elementar with a predrying temperature of 135 °C and an ashing temperature of 815 °C was used. For electrode preparation, a mixture of 20 mg of carbon material, 600 μL of acetone (Sigma-Aldrich, > 99%)m and 50 μL of binder solution of PVDF-21216 (Solvay) in acetone were homogenized using an ultrasonic bath and then drop coated onto platinum current collectors (diameter: 12 mm; mass loading: 3.4−3.9 mg cm−2) before being dried for 16 h at 80 °C and then 24 h under vacuum. Platinum current collectors were used to ensure that the electrochemical properties of the synthesized active carbon materials are characterized by reducing potential influences of the current collector material. Figure S1 demonstrates that similar performance is achieved when using aluminum current collectors which are of more practical relevance. The resulting binder content was 5 wt % based on the active mass. Electrochemical tests were conducted at a constant temperature of 25 °C within a climate chamber, using a symmetrical two-electrode arrangement and a glass fiber Whatman Separator. The determination of capacitance employing an ionic liquid as the electrolyte was conducted using 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) from Ionic Liquids Technologies (io-li-tec) GmbH, Germany. Furthermore, a multichannel VMP3 potentiostat from Bio-Logic, France, was used. The capacitance was calculated using the discharge cycles of the respective experiments without employing iR

lithiation/sodiation, a pronounced hysteresis between the potentials of cell charge and discharge, and low Coulombic efficiencies.10,11 Other approaches focus on the use of ligninbased activated carbons as the active material in supercapacitors. Since the capacitance of supercapacitor electrodes scales with the amount of accessible surface area of the active material, a high degree of porosity is usually required for this application.12 In order to increase the surface area of ligninbased carbons or to introduce pseudocapacitive entities, a number of different synthesis strategies have been investigated, such as electrospinning,13,14 addition of surfactants such as Pluronic F127 or P123,15,16 and the self-assembly of NiO nanoparticles.17 Yet, the use of activation agents, in particular, KOH, still appears to be the most facile and economically viable method for this purpose.3 Here, we report on the successful synthesis of activated porous lignin-based carbons with specific high surface areas of more than 1800 m2 g−1 by employing a simple two-step process, which consists of a high-temperature thermal treatment of a lignin/KOH composite under an inert gas atmosphere without any addition of templating agents, followed by a washing step in order to remove byproducts of the activation procedure. The resulting carbons were characterized with regard to their microstructural properties and their performance as active material in electrodes for supercapacitors for energy storage applications. Since the type of lignin which was used for this studylignin from softwoodis exposed to a certain amount of sulfur-containing additives during preprocessing, X-ray photoelectron spectroscopy (XPS) combined with Ar+-sputtering was employed as a means of investigating the content of sulfur in various depths of the resulting carbons.18,19 This investigation was performed in order to assess the possible influence of sulfur on the electrochemical properties of this material system since heteroatoms in carbon are known to facilitate electrocatalytic reactions and might destabilize the electrolyte.20 This aspect is especially relevant as it is not uncommon for activated carbons from lignin to contain a significant portion of oxygen or other heteroatom surface functionalities which can strongly impact the electrochemical performance. A number of research groups in the field of ligninbased carbons for supercapacitors employ beaker-type threeelectrode test cells for electrochemical measurements, which are very well suited to investigate the fundamental properties of the respective materials.15−17 However, for the present work, we chose to employ a symmetrical two-electrode setup which resembles the conditions of a real application to a much larger extent21 and three electrode Swagelok-type cells to explain the surface group redox behavior. Furthermore, in this work, an ionic liquid (IL) was used as the electrolyte, which is considered more advantageous compared to aqueous electrolytes due to the increased potential window under which ILs can operate, as well as their nonvolatility.22,23 Scant attention has been paid to lignin-derived carbons as the active material for supercapacitors. To the best of our knowledge, investigations of supercapacitors in combination with an ionic liquid as the electrolyte have still to be realized, and first results are therefore reported herein.

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EXPERIMENTAL SECTION

Material Preparation. Softwood lignin obtained via a kraft pulping process was used as the starting material for all carbon samples. For a typical experiment, 2 g of lignin was added to an aqueous solution of the respective mass of KOH (Merck KGaA, p.a.) 4095


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ACS Sustainable Chemistry & Engineering compensation. For measurements in a three-electrode configuration in Swagelok-type cells, a platinum pseudoreference electrode was used.

long-range order, i.e., no lattice planes, are visible, as would be characteristic for graphitic carbon with a long-range order. Therefore, the carbon material is amorphous and highly disordered, which results from the KOH activation as well as the rather moderate carbonization temperature of 800 °C. The effects of the disordered structure of our lignin-derived carbons are also reflected in the corresponding Raman spectra presented in Figure S3, as well as in Table 1 which contains the

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RESULTS AND DISCUSSION Structural Characterization. Figure S2 (Supporting Information) shows the X-ray diffraction pattern of the activated carbons obtained from lignin via KOH activation. For comparison, one sample, denoted as “pure lignin” which was carbonized under the same conditions but without the addition of KOH, is also included. The carbon materials presented here exhibit no distinct reflections but one broad feature with a maximum at 2 V vs. Pt. The current−potential curve exhibits a maximum (A5) indicating that the electrochemical decomposition process happens at a finite number of reaction sites. Therefore, the decomposition reaction involves functional groups on the surface at lower potentials, and for higher potentials, a further decomposition process proceeds at the carbon material itself. This finding is in line with studies of Dyatkin et al.45 who observed the reduction of breakdown reactions for defunctionalized carbon surfaces. The first CVs of a freshly prepared electrode measured with 2 mV s−1 show four distinct anodic current signals (A1−A4) and six cathodic current signals, as well as the onset of the beginning anodic electrolyte decomposition. These signals may correspond to interactions between electrolyte ions and functional surface groups involving charge transfer and the 4098


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decomposition proceeding similarly to the initial process A5. As the 26th CV shows, the onset potential is shifted back to more positive potentials with cycle number, and again, the stability limitation of reactive species emerges. Finally, reactive surface groups influence the capacitor properties but seem not to stay unchanged during reaction compared to the situation before and after starting electrochemical cycling. Similar observations were analyzed and proposed earlier for surface-modified model carbons.39 In order to evaluate the electrochemical performance of our material under conditions that resemble the conditions of a real application more closely than cyclic voltammetry experiments, galvanostatic cycling with potential limitation (GCPL) experiments were conducted. In contrast to cyclic voltammetry in GCPL, the current is controlled as would also be the case in a real device.12,21 Figure 6 shows charging−discharging curves at different specific currents and the corresponding capacitance derived from these measurements. At a specific current of 2 A g−1, all carbons display somewhat steady slopes of the cell voltage during the charging and discharging process (Figure 6a). The small sudden drop (or increase) in voltage at these points, where the reversal of the current occurs, is referred to as “iR drop”, which results from the electrical serial resistance (ESR) of the cell.46 This effect becomes more pronounced at higher currents as the ions of the electrolyte at the surface of the active material have to be reorganized very rapidly (Figure 6b). While for this process the viscosity of the electrolyte is considered to be rate-determining, other factors influencing the ESR are the electrical conductivity of the carbon as well as, in a broader context, also its ionic conductivity.21 The overall time an individual system requires to reach the given voltage limit corresponds to the extent of which electrosorption at the surface of this active material can occur. Thus, a longer time for charging or discharging also corresponds to a higher achievable capacitance. As shown in Figure 6c, the samples pure lignin, SC-1:0.25,and SC-1:0.5 allow for a capacitance of less than 150 F g−1 reached at a comparatively low current of 1 A g−1 and less than 100 F g−1 at 8 A g−1. In contrast, SC-1:1 yields 231 F g−1 at 1 A g−1, which only slightly decreases to still 203 F g−1 once the specific current is increased 1 order of magnitude to 10 A g−1. This exceptional performance is mainly attributed to its high specific surface area in combination with the slightly higher electric conductivity, compared to the other carbons presented here. For assessing the long-term electrochemical performance of our material, SC-1:1 was chosen as an example. Figure 7 displays the capacitance of SC-1:1 during the course of 10,000

Figure 5. Cyclic voltammograms of the SC-1:1 carbon-based electrode measured with 2 mV s−1 vs. a platinum pseudoreference electrode, a CV measured over a wide potential range (inset a) and CVs measured at different scan rates displaying always the third recorded CV (inset b).

formation of chemisorptive bonding. During the initial three cycles, the peak currents increase. This behavior may be related to increased reactivity of functional groups on the surface through changes due to previous interactions with the electrolyte ions. However, through further cycling, some current signals disappear, and the peak current of A3 and the corresponding cathodic signal slightly decrease. This observation suggests that the stability of the formed species is limited. The peak potential of A3 is shifted to more negative potentials, while the peak separation to the corresponding cathodic signal is decreased indicating higher reversibility of the related processes. At higher scan rates (Figure 5, inset b), peak separation increases which could be explained by kinetical hindrance of the processes due to further involved reaction steps as deprotonation, charge transfer reactions, or transport limitations to the active surface sites. The current for potentials close to 2 V vs. Pt does not increase for higher scan rates which proves that signal A5 corresponds to an electrolyte decomposition process. From the first to the second cycle recorded with 2 mV s−1, the onset potential of the electrolyte decomposition (A5) is significantly shifted to more negative potentials. This shift indicates the formation of species which are more reactive toward electrolyte

Figure 6. Charging−discharging curves of lignin-derived carbons using GCPL: (a) at a current rate of 2 A g−1, (b) at a current rate of 8 A g−1, and (c) the corresponding capacitance values. 4099


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the boundaries of performance of the said energy storage systems.

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CONCLUSIONS In this report, we have shown that a facile two-step activation process using KOH as the activation agent and lignin from softwood mainly yields microporous carbon materials with a specific surface area of up to 1886 m2 g−1. These carbons are highly disordered and amorphous as evidenced by the results of XRD, TEM, and Raman experiments. Furthermore, due to the fact that the addition of KOH leads to an oxidative environment during the activation step, a considerable portion of oxygen-containing surface groups are present along with sulfur from the carbon precursor. The latter one however is predominantly located in the inner volume of the carbons, as shown by the results of depth profile XP spectroscopy investigations. Electrochemical experiments using a symmetrical two-electrode arrangement and the ionic liquid EMIBF4 as the electrolyte yielded the highest specific capacitance for sample SC-1:1 with 231 F g−1 at 1 A g−1 and still 203 F g−1 if the current is increased 10-fold to 10 A g−1. However, in the course of 10,000 charging−discharging cycles, a decay in capacitance of about 50% is observed, which seems to be the result of the large voltage window and the surface functionalization.

Figure 7. (a) Capacitance of SC-1:1 during 10,000 charging− discharging cycles and (b) corresponding voltage profiles.

charging−discharging curves together with the respective voltage profiles. Starting at 204 F g−1 (Figure 7a), the capacitance drops asymptotically to a value of 100 F g−1 at the end of the experiment. This observation is accompanied by a decrease in the time needed for the completion of individual cycles (Figure 7b). There are multiple possible reasons for this fatigue of the system. The two most important are (1) the high voltage window used and (2) the influence of the oxygencontaining surface groups.39 It is well known that even very minor decomposition reactions at high cell voltages can significantly contribute to a decay in capacitance over time. This behavior is especially found for mainly microporous materials since the clogging of the pore entrance by products of such reactions already renders the entire pore inaccessible.46 Using a smaller potential window of 0−2.5 V results in a smaller drop of capacitance after 5000 cycles (Figure S4). One possible reason might be the beginning anodic decomposition of the electrolyte for high voltages greater than 3 V (Figure 4a and b) as further investigated using a reference electrode (Figure 5) and discussed above. The lowering of the capacitance with cycle number also observed for the smaller potential window of 0−2.5 V is related to the limited stability of functional groups on the surface involved in electrochemical processes contributing to the current. Comparing the electrochemical performance of the materials presented in this study with other lignin-derived carbons that have been reported by other groups is a somewhat intricate task due to the fact that conditions under which the respective experiments have been conducted vary by a large degree. Table 4 summarizes the results of reports where lignin-based carbons have been employed as active material in supercapacitors. The material system reported in the present study outperforms the other approaches. This comparison further confirms our assessment that the use of an ionic liquid as an electrolyte for lignin-derived carbons in supercapacitors can significantly push

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00058. Results on element analysis of softwood lignin, electrochemical influence of platinum vs aluminum current collector, X-ray diffraction pattern of the nonpyrolized lignin and the carbon-derivatives depending on the amount of added KOH, respective Raman spectra with the D- and G-mode fits, and comparison of long-term cycling with different potential windows. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +49 351 4659-652. Fax: + 49 351 4659-452. E-mail: [email protected]. ORCID

Julia Linnemann: 0000-0001-6883-5424 Juan Balach: 0000-0002-7396-1449

Table 4. Comparison of Properties, Test Conditions, and Performance of Lignin-Derived Carbons as Active Materials in Supercapacitors material

electrolyte

test conditions

specific surface area (m2 g−1)

capacitance (F g−1)

ref

SC-1:1 KOH-activated mesoporous carbon from lignin lignin-activated carbon (LAC4) KOH-activated ACFs ECNFs KOH-activated carbon from natural lignin

EMIBF4 6 M aq. KOH

GCPL, 1 A g−1 CV, 2 mV s−1

1886 1184

231 91.7

this work 15

6 M aq. KOH 6 M aq. KOH 6 M aq. KOH 1,5 M NEt4BF4/ acetonitrile 1 M aq. H2SO4 1 M aq. H2SO4

GCPL, 8 A g−1 CV, 50 mV s−1 GCPL, 0.4 A g−1 GCPL, 0.1 A g−1

3775 − 583 1406

207.1 196 64 87

47 13 14 48

GCPL, 10 A g−1 GCPL, 0.5 A g−1; CV, 5 mV s−1

769 1092

123.5 91,114

49 50

LHPC carbonized pre-extracted lignin

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Lars Giebeler: 0000-0002-6703-8447 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Robert R. Rottenkügler and Mönke M. Mautzbach for valuable scientific discussion, as well as to Dr. Alexey Popov and Marco Naumann for providing the Raman spectrometer. Further gratitude is expressed to Romy Keller for help with the SEM measurements. The European Union (European Regional Development Fund − ERDF) and the Free State of Saxony are gratefully acknowledged for financial support in the NaSBattSy project (SAB Grant No.100234960) for partially funding this work.

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Softwood Lignin as a Sustainable Feedstock for Porous Carbons as

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