Preparation of activated carbon from alkali lignin using novel one-step process for high electrochemical performance application

2.1 Materials

The Na-lignin was collected from the delignification of coconut husk prepared by 6 wt% of sodium hydroxides at approximately 100 °C for 4 h (C: 47.87%; O: 46.29%; Na: 2.44%; S: 0.73%; atomic ratio). The coconut husk itself was obtained from the traditional market in Keputih Surabaya, Indonesia. The sodium hydroxide (NaOH) in analytical grade was purchased from Merck.

2.2 Experimental

2.2.1

2.2.1 Preparation of carbon

The aqueous liquor was first evaporated on a stirring hot plate at 85 °C to eliminate 80% water content and produce a thick Na-lignin. The thick liquor was then dried in the oven at 100 °C for 12 h. The dried Na-lignin size was reduced randomly using a crusher to make it fit in a ceramic boat. The following process was calcination in a flowing N₂ tubular furnace through a ceramic cylinder with a dimension of 25 mm inside diameter and 400 mm length. The first step of the temperature set was 100 °C, with the N₂ flowing rate was 150 mL/min for about 30 min to purge the oxygen content in the sample and cylinder itself. After that, the N₂ flowing rate was reduced to 50 mL/min, and the temperature was increased to 300, 500, 700, and 900 °C gradually. The ramp-up temperature–time was 30 min, then continued to the calcination for 1 h for each temperature observed, as shown in Fig. 1. After calcination, the samples were washed using the demineralized water until the conductivity obtained was the same as the solvent.

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Fig. 1

The calcination temperature scheme of Na-lignin using N₂ flow tubular furnace.

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3.1 Thermogravimetric analysis of Na-Lignin

The thermogravimetric analysis was performed to study the calcination temperature of Na-lignin further. The thermogravimetric analysis (TGA) curves obtained from 25 to 1000 °C at 10 °C/min heating rate are shown in Fig. 2.

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Fig. 2

TGA curves of Na-lignin sample under N_2.

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The thermal degradation profile of Na-lignin started from 85 °C and was completed at 1000 °C. The typical weight loss accompanying the structural change was 5 wt% at 85–110 °C, 11 wt% at 290–515 °C, 4 wt% slightly at 515–790 °C, and 15 wt% after the heating process. They correspond to the thermal stability of the compounds presents loss of moisture content (A), loss of other organic volatiles (B), loss due to the decomposition of lignin into carbon (C), and loss due to combustible matter after switching to an oxidizing atmosphere (D) (Bredin et al., 2011). Several references studied that there is no standardized definition of these components; however, it is generally agreed that they are defined as indicated in Fig. 2 and referred to as follows. The moisture content (section A) are classified as water, the other organic component (section B) are those responsible for the polymer degradation products that will be lost in the range of 300–500 °C (Rahmatika et al., 2018). The decomposition phase of lignin to be transformed as carbon (section C) and combustible matter (section D) refers to non-volatile but oxidizable material in its yet unoxidized form. Based on Fig. 2, the next calcination temperature applied, especially to the carbon properties, would be studied at 500, 700, and 900 °C.

3.2 The physical appearance of Na-lignin-based carbon at various temperature

The visual appearance of dried Na-lignin and carbon derived from Na-lignin via calcination at 300, 500, 700, and 900 °C are shown in Fig. 3. As seen, the dried Na-lignin color was brownish-red and bulky form and going darker with the increase of temperature. The presence of NaOH that hygroscopic makes the lignin was agglomerated. Carbon derived from Na-lignin via calcination at 500 to 900 °C shows black and shiner caused by the decomposition of some lignin's organic material into volatile gases and solid carbon, which carbon is physically not soluble in NaOH. It will make the separation process during washing with demineralized water become easier (Widiyastuti et al., 2020).

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Fig. 3

Visual appearance of (a) dried Na-lignin; and carbon derived from Na-lignin via calcination at (b) 300, (c) 500, (d) 700, and (e) 900 °C.

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3.3 Crystallography patterns of carbon derived from Na-lignin via calcination at various temperatures

Samples were analyzed by X-ray diffraction (XRD) to investigate further the carbon character at each temperature except the dried Na-lignin itself. All lignin and carbon fractions showed broad diffraction of amorphous carbon phase with a maximum at about 2θ = 26°. However, as displayed in Fig. 4, the diffraction of prominent peaks for Na-lignin calcined at 300 °C are at 28.53°, 26.46°, and 34.17°, which is identic to the prominent peak of sodium ICDD pattern (ICDD card No. 00–001-0850). It seems that sodium content remains in the carbon structure even though it has been washed with demineralized water. The physical properties of lignin are soluble in NaOH solution and carbon is not (Ponomarev and Sillanpää, 2019). Therefore, we can conclude that at the calcination temperature of 300 °C, lignin had not been entirely transformed into carbon structures yet.

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Fig. 4

X-ray diffraction (XRD) diffractogram of the carbon derived from Na-lignin via calcination at 300, 500, 700, and 900 °C.

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On the other hand, the X-ray patterns of Na-lignin calcined at 500, 700, and 900 °C did not exhibit a well-defined peak besides at range ∼ 26° as the prominent peak. The second peak is slightly shown at ∼ 43°, identical to the amorphous carbon phase (ICDD Card No.00–056-0159). Furthermore, the most important is the absence of sodium peak indicates that at calcination temperature of 500, 700, and 900 °C, the Na-lignin has already been entirely transformed into carbon.

3.4 Nitrogen adsorption–desorption analysis

The specific surface area of dried Na-lignin and carbon derived from Na-lignin were determined by measuring the Nitrogen adsorption–desorption isotherms and BET modeling. Their pore volume distributions were analyzed by the Barret-Joyner-Halenda (BJH) method using desorption isotherm data. The Nitrogen adsorption–desorption isotherms curves of dried lignin to the carbon at the various temperatures are shown in Fig. 5 and Table 1.

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Fig. 5

The adsorption–desorption curves of N₂ at − 195.79 °C for dried Na-lignin and carbon derived from Na-lignin via calcination at 300, 500, 700, and 900 °C.

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The lower portion of the loop was traced out on the adsorption, and the upper portion was traced out on the desorption. All samples revealed the mixed character of type IV of Brunauer's classification. That is a behavior that reflects the coexistence of mesopores (Widiyastuti et al., 2020). However, the particles of dried Na-lignin and calcined Na-lignin 300 °C represented a low N₂ adsorption volume. The lignin particle performed a flat adsorption curve, which indicates the low porosity content. Because the lignin content is soluble in NaOH, so the NaOH itself filled the pore of lignin. Like the dried Na-lignin, at a temperature of 300 °C, the lignin structure has not transformed into carbon completely. Therefore, the particle was still agglomerated, caused by the hygroscopic of NaOH and the less porous particle (Chatterjee and Saito, 2015).

The BET specific surface area (SSA) and pore properties of the rest treatment of carbon derived from Na-lignin via calcination at various temperatures are shown in Table 1. Calcined Na-lignin 700 °C has the highest BET SSA value (1573.99 m²/g). The increasing rate of N₂ volume adsorbed was slower at the lower relative pressure, indicating monolayer adsorption and a growing N₂ adsorption rate faster at the higher relative pressure due to the capillarity condensation (Tian et al., 2017). Calcined Na-lignin 900 °C shows a lower specific surface area compared to those which calcined at 700 °C. According to the pore volume and pore size diameter in Table 1, and after seeing the TGA pattern in Fig. 2, the phenomenon was possibly caused by the carbon structure's sintering effect from the sample of calcined Na-lignin 900 °C. The temperature also indicates the beginning of the carbon decomposition phase. Compared to the previous study (Widiyastuti et al., 2020), the performance of carbon derived from Na-lignin through conventional three-step synthesis shows that the specific surface area of lignin carbonization at 500, 700, and 900 °C were 157.621, 642.501, and 541.177 m²/g. It described a similar pattern that lignin at a calcination temperature of 900 °C showed a lower specific surface area compared to those which calcined at 700 °C. However, precipitation using HCl will generate NaCl salt as a byproduct trapped inside the pore, leading to decreased active pore. Even though the particles have already been washed using demineralized water, the salt content could not be entirely eliminated when they pass through the micropore.

3.5 Graphitic carbon type identified by Raman spectroscopic

Raman spectra (Fig. 6) of carbon derived from Na-lignin via calcination at various temperatures were deconvoluted into two pseudo-Voigt-shaped peaks centered at ∼ 1356 (D band) and ∼ 1594 (G band), respectively. Often the intensity ratio of D-band and G-band, which is respected as (I_D/I_G), is used as a calcination parameter, reflecting the amounts of the surface/edge defects of carbon sheets (Zhang et al., 2017). As the increasing calcination temperature from 300 to 700 °C, the D-band and G-band start to develop as the indication of the pyrolyzed material's aromatization process is begun (Paris et al., 2005). As reported before (Zhang et al., 2017), either the D-band or G-band indicated sp² bonded turbostratic carbon nanocrystallites and graphitic carbon crystallites, which exhibit a reduction of the amorphous phase for sp²-bonded carbon with the increasing of calcination temperature (Ishimaru et al., 2007).

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Fig. 6

Raman spectra for carbon derived from Na-lignin via calcination at various temperatures.

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The increase of I_D/I_G of carbon derived from Na-lignin via calcination from 300 °C (0.42), 500 °C (0.63) to 700 °C (1.03) confirms the grafting of oxygen-containing functional groups to the graphitic planes. After reaching 900 °C calcination temperature, the sintering effects make the surface reduced. Moreover, the high I_D/I_G has another advantage, such as a high electrocatalytic performance (Zhang et al., 2017; Zhang et al., 2017; Tian et al., 2017; Perumbilavil et al., 2015; Zhang et al., 2017). It would be discussed further in the following paragraph.

3.6 The morphology of carbon derived from Na-lignin via calcination

The SEM images of carbon derived from Na-lignin via calcination at 500–900 °C are demonstrated in Fig. 7. Carbon particles show an irregular shape and tend to agglomeration with a size of a few micrometers. The increase of calcination temperature led to the decomposition of more volatile components left behind the mesoporous carbon particles. The formation of pores for carbon derived from Na-lignin via calcination increases the specific surface area explained in Table 1.

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Fig. 7

SEM images of carbon derived from Na-lignin via calcination at (a) 500 °C (b) 700 °C, (c) 900 °C at a magnification of 3,000×.

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At higher magnification, superstructures at the single object level using Field Emission Scanning Electron Microscopy (FE-SEM, Fig. 8), the particles' structure can be observed as a rough porous. The pore number increase with the increase of calcination temperature from 500 to 700 °C. The increasing calcination temperature led to the decomposition of more volatile components left behind the microporous and mesoporous carbon particles. The formation of pores for carbon derived from Na-lignin via calcination increases the specific surface area, as explained in Table 1. However, the carbon derived from Na-lignin via calcination at 900 °C exhibits some denser carbon structures compared to the other lower temperature treatment. At a calcination temperature of 900 °C, a sintering effect and shrinkage of the carbon derived from Na-lignin reduced the pore areas. The carbon derived from Na-lignin via calcination at 900 °C also reduced the specific surface area because the sintering effect results in some pores' sealing. Overall, high calcination temperature had detrimental effects on micropore areas' development due to a shrinkage effect on the carbon microstructures (Guo and Chong Lua, 1998; Youseffi et al., 2000).

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Fig. 8

FE-SEM images of carbon derived from Na-lignin via calcination at (a) 500 °C, (b) 700 °C, and (c) 900 °C at a magnification of 40,000×.

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3.7 The electrocapacity performance of carbon derived from Na-lignin

As a highly porous and conductive material, carbon can commonly be used as a supercapacitor source and experimentally evaluate carbon's capacitive performance derived from Na-lignin. The electrochemical tests were conducted in 0.1 M Na₂S₂O₃ at a potential range of −1 to 0 V.

As shown in Fig. 9, all CV curves do not exhibit a precise rectangular curve as the double layer capacitive material commonly performs. However, the curves have no noticeable reaction peaks and have a countable area indicating that the carbon has a supercapacitor's behavior. Compared to the other two carbon samples, the carbon derived from Na-lignin via calcination at 700 °C presents a higher specific capacitance with 168.29F/g, determined by measuring the CV areas. This electrocapacity data is relevant to the highest specific surface area, the morphologic structure, and the Raman spectra data of carbon derived from Na-lignin via calcination at 700 °C, as explained previously. Moreover, as a comparison, the carbon derived from Na-lignin synthesized through traditional three-steps (Widiyastuti et al., 2020) exhibits lower specific capacitances of 1.96, 4.13, 28.84, and 17.90F/g for calcination temperature of 500, 700, and 900 °C, respectively. Therefore, it is clearly described that the effect of NaCl salt trapped inside the pore matrix can reduce the active side of carbon pore, impacting the reduction of specific capacitance as well.

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Fig. 9

Cyclic voltammograms of carbon derived from Na-lignin via calcination at 500–900 °C at 10 mV s⁻¹ on Ni-Foam surface in 0.1 M Na₂S₂O₃ solution.

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3.8 Electrocatalytic performance for ORR provided by cyclic and linear sweep voltammetry using rotating disk electrode

Referring to the previous study, alkali lignin is included in low-sulfur lignin that can be a sulfur co-doped carbon with good Oxygen Reduction Reaction (ORR) performance (Zhang et al., 2016; Shen et al., 2019; Demir et al., 2018). Therefore, to observe the electrocatalyst device's application, we need to identify the Oxygen Reduction Reaction (ORR) of the electrocatalyst agent. In this study, carbon derived from Na-lignin via calcination at 700 °C, as the best graphitic crystallites, was used for ORR performance. The cyclic voltammetry (CV) measurement was first evaluated using a comparison of a three-electrode system in 0.1 M KOH solution saturated with O₂– or N₂– using Ag/AgCl as a reference electrode with a scan rate of 10 mV s⁻¹ (Fig. 10).

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Fig. 10

ORR performance of (a) CV curves of carbon derived from Na-lignin via calcination at 700 °C in N₂-saturated and O₂-saturated solutions, and (b) CV curves of carbon derived from Na-lignin via calcination at 500, 700, and 900 °C in O₂-saturated solutions.

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The electrocatalyst's CV curves exhibited no cathodic peak in the N₂-saturated solution, as shown in Fig. 10(a). In contrast, a pronounced cathodic peak at −0.375 V was observed in O₂-saturated solutions at the current density of −0.67 mA/cm², attributed to the electrocatalytic oxygen reduction reaction on the electrode. Furthermore, to examine the best electrocatalytic performance among those three kinds of temperature treatment carbon, all CV measurements were performed using O₂-saturated 0.1 M KOH, as shown in Fig. 10(b). The carbon samples present distinct single oxygen reduction peaks, suggesting the conspicuous catalytic activity for the ORR. The shape of all samples' CV curves was similar and showed a reduction peak at −0.37 V for carbon derived from Na-lignin via calcination at 500 and 700 °C, and −0.41 V for carbon derived from Na-lignin via calcination at 900 °C with the approximate current density of −0.8 mA cm⁻². The phenomenon is indicating that the carbon structure from Na-lignin was sulfur-doped since the atomic ratio of sulfur to carbon content was 1.52% which prove the content of S co-doped porous carbon could further boost the catalytic performance due to a synergetic effect, so it has a potential usage to support the electrocatalyst (Ito et al., 2015; Wang et al., 2016; Wu et al., 2017; Su et al., 2016).

The carbon sample was further quantitatively evaluated to measure the electron transfer number using the rotating disk electrode (RDE) measurements. The carbon derived from Na-lignin via calcination at 700 °C has the best specific surface area and graphitic crystallites. The rotating rates from 400 to 2000 rpm were conducted, allowing further insight into the ORR kinetics and electrocatalytic processes on the catalysts.

As shown in Fig. 11(a), the J_L increased gradually with the increasing rotating speed due to the shorter diffusion distance of oxygen at higher speeds. Furthermore, the ORR kinetics of the electrocatalyst composite was further analyzed using the Koutecky-Levich plots of J⁻¹ versus ω^-1 calculated based on the potential linear range from the LSV curves. As shown in Fig. 11(b), the plots exhibit excellent linearity and parallelism at the potential range of −0.45 to −0.55 V, revealing first-order reaction kinetics (Shao et al., 2019). Moreover, the electron transfer number of carbon derived from Na-lignin towards ORR n = 2.23 is calculated based on the Koutecky-Levich equation (Xing et al., 2014). Those shows that the carbon derived from Na-lignin demonstrated the electrocatalytic activities for ORR via a two-step two-electron pathway in an alkaline solution as follows (Ma et al., 2019): $${\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{H}\mathrm{O}}_2^{-}+{\mathrm{O}\mathrm{H}}^{-}(-0.364\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{A}\mathrm{g}/\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l})$$ $${\mathrm{H}}_2\mathrm{O}+{\mathrm{H}\mathrm{O}}_2^{-}+2{\mathrm{e}}^{-}\to {3\mathrm{O}\mathrm{H}}^{-}(0.59\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{A}\mathrm{g}/\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l})$$

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Fig. 11

(a) LSV curves of the electrocatalyst at different rotating rates and (b) The Koutecky-Levich plots of electrocatalyst based on the RDE data.

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The defect-rich graphitic structure of carbon derived from Na-lignin produced by direct activation may increase the electrocapacity so it can potentially be developed in supercapacitor application and increase carbon conductivity. Therefore, it can support the Oxygen Reduction Reaction in a metal-air battery application. For further research, this inexpensive carbon product can be potentially composited with Pt material (Balgis et al., 2017) or some other non-noble metal to increase their function as support of metal-air battery electrocatalyst (Mahmudi et al., 2018; Nurlilasari et al., 2020).