Valorization of lignin waste: High electrochemical capacitance of lignin-derived carbons in aqueous and ionic liquid electrolytes

This report describes the utilization of waste lignin-derived activated carbons (LACs) as high-energy/high-power electrode materials for electric double layer capacitors (EDLCs). The influence of carbon-pore structure on the capacitance of the LACs in two aqueous (H 2 SO 4 and KCl) and two ionic liquid (1-ethyl-3-methylimidazolium ethylsulfate, [EMIm][EtSO 4 ], and 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIm][BF 4 ]) electrolytes is evaluated. In EDLCs containing aqueous H 2 SO 4 as electrolyte, the LACs exhibit specific capacitances of up to 223 F/g and good cycling stability, with energy density of 5.0 Wh/kg at a power density of 200 W/kg. EDLCs containing KCl achieved a specific capacitance of 203 F/g, and energy density of 7.1 Wh/kg at a power density of 510 W/kg. The specific capacitances of the LACs in [EMIm][EtSO 4 ] and [BMIm][BF 4 ] were up to 147 F/g and 175 F/g, respectively. The energy density in the IL electrolytes, is up to 25 Wh/kg at power density of 500 W/kg, and 16.4 Wh/kg at 15 kW/kg. We demonstrate that the electrochemical performance of the LACs depends not only the surface area and pore size, but also on the pore-wall thickness. Abstract Valorisation of waste lignin generates porous carbons with attractive properties as high-energy/high-power electrode materials for electric double layer capacitors (EDLCs); achieving energy density of 25 Wh/kg at power density of 500 W/kg in ionic liquid electrolytes.


Introduction
Electrical energy is stored in supercapacitors by the reversible adsorption of ions from electrolyte solutions onto high-surface-area electrodes. 1,2,3,4 Supercapacitors can be charged and discharged rapidly, 5 but suffer the drawback of only being able to store relatively low amount of energy compared to batteries as charge is only stored at the surface of the electrode materials. 1 Consequently, in recent decades, a number of studies have focused on the search for high-energy electrode materials for supercapacitors. 6,7 The energy density of supercapacitors is given by E = ½CV 2 , where C is capacitance, and V is cell voltage. 5,8 C can be increased by using high-surface-area electrode materials, such as nanostructured carbons and activated carbons (ACs), due to their large surface area, good electrical conductivity and low manufacturing costs. 9,10,11 A particularly-attractive property of ACs is the ease with which their porosity can be tailored to target specific applications, including gas 12 and energy 13 storage and the fact that they can be synthesized from low-value waste materials, thus achieving valorisation. [12][13][14] As well as increasing C, increasing the cell voltage, which depends on the electrochemical window of the electrolyte, can improve the energy and power density of supercapacitors. Aqueous electrolytes are highly conductive, have low viscosities, and can result in high specific capacitances, 15,16,17 but their operating voltage is limited to about 1 V, due to the decomposition of water at higher voltages. 16,17 The use of organic electrolytes can increase the cell voltage to about 3 V, 18,19,20 but their conductivity and achievable capacitance are relatively low compared to aqueous electrolytes. Recently, ionic liquids (ILs) have attracted interest as electrolytes for supercapacitors. 21 ILs have 4 interesting properties, including inherent conductivity and wide electrochemical windows (over 3 V), which could potentially result in the development of high-energy IL-based supercapacitors. 21,22 One of the most challenging tasks in supercapacitor electrode research is the search for the right material, and matching it to a suitable electrolyte to generate a combination of electrode material and electrolyte that can successfully maximize the energy and power of electrochemical devices. 3,[23][24][25] Herein, we describe the use of ACs derived from lignin waste as electrode materials. Lignin is an abundant natural and renewable raw material that is mainly generated as waste during the extraction of cellulose from woody precursors. Valorisation of lignin waste may be achieved by conversion to hydrochar via benign hydrothermal carbonisation processes, followed by activation to generate ligninderived activated carbons (LACs). 26 We herein evaluate the specific capacitance, energy density and power density, rate capability, and durability of LAC-based electrodes in contact with aqueous (H2SO4 and KCl) and IL (1-ethyl-3-methylimidazolium ethylsulfate, [EMIm][EtSO4], and 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIm][BF4]) electrolytes. The influence of carbon structure and porosity on the electrochemical properties of LACs was also evaluated, revealing insights into the role of the pore structure on the behavior of these novel carbons.

Materials synthesis and porosity characterisation
Hydrochar obtained from lignin waste via hydrothermal carbonization in high temperature water (300°C) was activated using established procedures. 11,12 In brief, a 5 mixture of hydrochar and KOH (at KOH/carbon ratio of 2 or 4) in an alumina boat was heated to between 600 and 900°C (at ramp rate of 3°C/min) under a flow of N2 in a horizontal tube furnace and the final temperature was held for 1 h. The resulting carbonaceous matter was allowed to cool under a flow of N2, washed repeatedly with 1.0 M HCl and distilled water at room temperature until the filtrate was at neutral pH, and dried in an oven at 120°C. The ACs were designated as LACT:x, where T is the activation temperature, and x is the KOH/hydrochar weight ratio used during activation.
Elemental compositions of the carbons were determined using an Exeter Analytical CE-440 Elemental Analyser. Powder XRD analysis was performed using a Bruker D8 Advance powder diffractometer. XRD analysis was performed using Cu Kα radiation (λ = 1.5406 Å) and the instrument was operated at 40 kV and 40 mA, with step size and time of 0.02 o and 2 s, respectively. A Horiba-Jobin-Yvon LabRAM Raman microscope (532 nm laser operating at ca. 4 mW (10%) and a 600 lines/mm grating was used to record Raman spectra. The spectra were averaged over 8 acquisitions of 60 s duration, while the Raman shift was calibrated using the Rayleigh peak and the 520.7 cm −1 Si line from a Si (100) reference sample. Nitrogen-sorption isotherms were recorded using a Micrometrics ASAP 2020 sorptometer, after outgassing the samples under vacuum at 200°C for 12 h. The BET equation was applied for surface area calculation using adsorption data in the relative pressure (P/Po) range 0.0-0.25. The amount of nitrogen adsorbed at P/P0~0.99 was used to calculate the total pore volume. Micropore surface area and micropore volume were estimated by t-plot analysis, while pore size was 6 obtained via a Non-Local Density Function Theory (NLDFT) method using nitrogen adsorption data.

Electrode preparation
Slurries containing 92 wt% of AC and 8 wt% of polytetrafluoroethylene, PTFE, binder were spread on polished glass-carbon current collectors to a loading of 4.3 mg/cm 2 .
Symmetric supercapacitors were assembled by separating the AC-coated plates by a Nippon Kodoshi separator that had been soaked in electrolyte for about 10 min. To prevent electrolyte loss during the course of the experiment, the sandwich electrode was wrapped with Paraffin film (Supporting Figure S1).

Electrochemical measurements
Electrochemical performances were determined using symmetric supercapacitors Galvanostatic charge/discharge was performed at current loads between 0.5 and 10 A/g. Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range 0.01Hz-100 kHz at AC amplitude of 5 mV. The cell was assembled and tested in ambient air at room temperature and allowed to equilibrate for 1 h before testing using a Model 760C Potentiostat (CH instruments Inc., Austin, TX).

Structural and textural properties of Lignin-derived carbons
The LACs prepared at a variety of temperature and KOH/Hydrochar ratio were rich in elemental C (Supporting Table S1), and depending of the extent of activation, the C content was in the range of 68-93 wt%, with the remainder consisting mainly of oxygen (7-31 wt%) and very small amounts of H (0.3-1.4 wt%). In general, the C content was greatest for samples prepared at high activation temperatures, while the converse was true for the H and O content. Thus, the H/C and O/C molar ratios were as low as 0.004 and 0.1, respectively, for LACs activated at 800 and 900°C (Table S1). Thermogravimetric analysis (TGA) indicated that almost no residual inorganic residues were present after thermal treatment in air (Supporting Figure S2). The chemical activation of the ligninderived hydrochar generated porous LACs (Supporting Figure S3 and S4) with a wide range of properties, 26 which are summarized in Table 1. Briefly, the LACs prepared at KOH/hydrochar ratio of 2 (LACT:2 series) were highly microporous ( Figure S3) with pore sizes in the range 7-13 Å, and surface areas and pore volumes of 1157-1924 m 2 /g and 0.59-0.95 cm 3 /g, respectively. Moreover, the average pore wall thickness for this highly microporous series of LACs was in the range 5-8 Å. The second series of LACs, prepared at KOH/Hydrochar of 4 (LACT:4 series) were, after activation at 600°C, mainly microporous but contained a small proportion of mesopores ( Figure S4). As the activation temperature rose up to 900°C, the pore size distribution (PSD) shifted to larger micropores and mesopores within the range 5-27 Å ( Table 1). The LACT:4 series LACs had higher surface areas and pore volumes of 1820-3235 m 2 /g and 0.91-1.77 cm 3 /g, 8 respectively. Their average pore wall thickness was in the range 3-5 Å and, therefore, lower than for the LACT:2 series LACs (5-8 Å). The differences in pore size and pore wall thickness were also evidenced by TEM analysis (Supporting Figure S5). The TEM images showed much thicker pore walls for sample LAC600:2 than for LAC800:4, while the pore size was smaller for the former. The level of pore order/disorder was similar, irrespective of level of activation. The values in the parenthesis refer to the following: a micropore surface area, b micropore volume, c mesopore surface area obtained from; Smeso = SBET-Smicro, d pore size distribution maxima obtained from NLDFT analysis, e pore wall thickness = 2/(SBET), where =2.2 g/cm 3 is density of carbon walls equated to the density of graphite. 27,28 Powder XRD patterns of the LACs (Supporting Figure S6)  domains. The peak at 2θ = 43°increased in magnitude upon activation at 800 and 900°C , presumably due to an increase in graphitization as the carbons were exposed to higher temperatures. However, the peaks remained very broad, which is an indication of the overall amorphous nature of the carbons. The amorphous nature of the carbons was also evidenced by Raman spectroscopy of the LACs (Supporting Figure S7), which revealed the D-peak (disordered carbon) at 1330-1350 cm −1 and G-peak (graphitic domains) at 1580-1595 cm −1 . Using the two-band fitting model, the D-peak to G-peak intensity ratio (ID/IG) was found to be in the range 0.87-0.98, with higher ratio for LACs activated at 800 and 900°C. Such an ID/IG ratio is consistent with the LACs being predominantly amorphous.  Table S2).

Electrochemical performance in aqueous electrolytes
For both electrolytes, the shape of the CVs in the voltage range 0.0-1.0 V is highly rectangular, suggesting that the LAC electrodes could be used for supercapacitor applications. As the voltage increased to 1.  Galvanostatic charge/discharge analysis performed at 0.5 A/g yielded triangular curves expected for predominantly capacitive response (Supporting Figure S8). [29][30][31] The specific capacitances, Csp, of the various LACs were determined from the slope of the discharge curve using equation 1, 32,33 and are summarized in Table 2.
where m is the mass of LAC on one electrode, V/t is the slope of upper half of the discharge current after subtracting the IR drop and the factor 2 accounts for the use of two electrodes in series.  it reached a plateau. We note that Csp of sample LAC800:4, which had the highest surface area of all LACs at 3235 m 2 /g, was comparable to that of sample LAC800:2 despite the fact that the latter had half the surface area of the former. Although previous studies have reported that the specific capacitance of electrode materials relies on surface area, 39,40 other factors such as microporosity, the nature of the electrolyte ions and mobility to active sites on the electrode may also play a role. [41][42][43][44][45] In order to evaluate the surface area of LAC samples accessible to the electrolyte, we determined the surface capacitance (Cs, µF/cm 2 ) values (Supporting Figure S10). The relationship between the specific capacitance and the surface capacitance is well preserved for surface area below 2000 m 2 /g, and at higher surface area, the surface capacitance decreases ( Figure S10). The highest surface capacitance achieved was 11.49 and 7.75 µF/cm 2 in 2 M H2SO4 and 2 M KCl, respectively. Moreover, it is observed that the most highly porous carbons i.e., samples LAC800:4 and LAC900:4, exhibited lower surface capacitance than expected.
This suggests that the specific capacitance of electrodes is not just dependent on surface area only; other factors that govern accessibility of electrolyte species and mobility within electrode pore channels should also be considered. 46,47 Barbieri et al. showed that Csp of ACs decreased as the pore-wall thickness decreased, 27 due to an increase in the screening length of electrical potential inside the pores. It is interesting to probe the link between the pore wall thickness of our LACs (especially those with surface areas above 2000 m 2 /g). Assuming a slit-shaped pore model, Equation 2 was used to estimate the pore wall thickness, w, of the LACs: 28 where SBET is total surface area and  = 2.2 g/cm 3 is the density of pore walls (equated to graphite density). The pore-wall thicknesses are given in Table 1  For all electrolytes, there was a reduction in Csp as the current increased, due to inaccessibility of the LACs to ions at high currents. The decrease in capacitance was especially significant at the highest current of 10 A/g, particularly for 2.0 M KCl. In general, though, the LAC samples exhibited good performance at high current rate in the electrolytes. In H2SO4, the highest specific capacitance of 223 F/g at current load of 0.5 A/g for sample LAC800:4 decreased to 174 F/g at 10 A/g, representing~78% retention of the initial capacitance. Lower rate stability was observed in KCl; for example, the specific capacitance of sample LAC800:4 in KCl was 203 F/g at 0.5 A/g and decreased to 102 F/g at 10 A/g, representing 50% capacitance retention. The lower capacitance retention in KCl may be ascribed to lower ionic conductivity of the electrolyte. It is noteworthy that sample LAC800:4 exhibited the highest specific capacitance retention at high current load in all electrolytes due to mesoporous nature of this sample. 56 Figure 3 (      and reversibility, and low ohmic resistance. As the current increased to 3 A/g, the IR drop (inset Figure S12B) became more visible, 47 Figure S14); Csp of LACs with surface area above 2000 m 2 /g was much higher than that of samples with surface area below 2000 m 2 /g. This can be related to the pore size distribution ( Figure S3 and S4). As shown in Table 1  . 23 The stability of LACs in ILs was investigated by CV cycling at scan rate of 100 mV/s. Figure 6 (C and D) shows the capacitance retention as a function of the number of cycles

Conclusion
High performance electrode materials for EDLCs were fabricated from lignin-derived activated carbons (LACs

Graphical Abstract
Valorisation of waste lignin generates porous carbons with attractive properties as highenergy/high-power electrode materials for electric double layer capacitors (EDLCs); achieving energy density of 25 Wh/kg at power density of 500 W/kg in ionic liquid electrolytes.