A commercial sodium lignosulfonate (SLS) was provided by Shanghai Aladdin (Shanghai, China) and used as the raw material of porous carbons. Polyvinylpyrrolidone (PVP) was obtained from the same supplier and employed as a binder for particle forming. KOH and HCl were provided by Sinopharm Chemical Reagent (Shanghai, China). All the materials were used directly without any additional treatment.

A 20% aqueous solution of SLS was prepared, followed by the addition and dissolution of 1% PVP relative to SLS. Subsequently, lignin powders were obtained from the SLS solution using a centrifugal spray dryer with an inlet temperature of 150°C.

The lignin powders were converted into porous carbons through a simple curing and KOH activation method [27]. The lignin powders underwent curing at 300°C for 1 h, followed by immersion in KOH solution with three different sample-to-KOH ratios (1:1, 1:2, and 1:3). The KOH-treated powders were then dried and carbonized at 800°C for 2 h. They were then washed with a 1M HCl solution and deionized water multiple times. After drying at 80°C for 12 h, three distinct porous carbon samples were obtained and labeled as PC-1, PC-2 and PC-3, respectively.

A JSM 7500F scanning electron microscope was employed to characterize the lignin powders and porous carbons. The pyrolysis process of the KOH-modified powders was studied using a Netzsch STA 449F3 thermal analyzer-mass spectrometry system under an Ar atmosphere with a heating rate of 10°C/min. The porous carbons were characterized through X-ray diffraction (ADVANCE D8, Saarbrucken, Germany), Raman spectroscopy (LAbRAMHR Evolution, Paris, France), high-resolution transmission electron microscopy (JEM-2100, Akishima-shi, Tokyo, Japan), and nitrogen adsorption analyses (ASAP 2460, Micromeritics, Atlanta, GA, USA). The electrochemical characteristics of the porous carbons used as electrode materials in supercapacitors were evaluated using the same method as reported in our previous work [27].

Fig. 1 shows the SEM images of the lignin powder and porous carbons. As shown in Fig. 1a, the lignin powder exhibits a unique shrunken dimple shape. It is related to the inlet temperature during the spray drying process [27]. Porous carbons with varying morphologies have been prepared from the same lignin powder by adjusting the ratio of the sample to KOH. Specifically, when the ratio of sample to KOH is 1:1, the obtained porous carbon keeps the shrunken dimple shape of the lignin powder, but forms a porous structure on the surface (Fig. 1b). Upon increasing the ratio to 1:2, the original particle morphology is completely altered, transforming into a foam skeleton structure (Fig. 1c). Increasing the ratio to 1:3 results in the sample transformation from a foam structure to nanosheets (Fig. 1d). The sudden shift in the morphology of porous carbons is primarily due to the etching effect of KOH. KOH reacts with the carbon of lignin molecules, resulting in the original three-dimensional amorphous structure of lignin being destroyed and transformed into a layered carbon structure.

The pore structure of the porous carbons was analyzed through nitrogen adsorption and desorption isotherm measurements, and the results are presented in Fig. 2 and Table 1. As shown in Fig. 2a, the porous carbon samples, namely PC-1, PC-2 and PC-3, display a blend of type I and type IV isotherms. At extremely low relative pressure (P/P₀ < 0.01), the adsorbed amount rises sharply, and a hysteresis loop is observed between P/P0 = 0.45 and 0.9. The hysteresis loop is attributed to the capillary condensation of nitrogen molecules in mesoporous pores. The characteristics of these isotherms indicate that all the samples have micropores and mesopores. As shown in Fig. 2b, the pore size distribution of the three samples is primarily composed of micropores and mesopores. With the increase of the KOH ratio, the specific surface area gradually increases. The specific surface area of PC-1 is 1417 m² g⁻¹, while PC-2 has a value of 2060 m² g⁻¹ and PC-3 reaches 2475 m² g⁻¹. The total pore volume is also increasing, with values of 0.77 cm³ g⁻¹ for PC-1, 1.11 cm³ g⁻¹ for PC-2, and 1.33 cm³ g⁻¹ for PC-3. PC-1 is dominated by micropores (V_micro = 0.62 cm³/g), with a small number of mesopores (V_Meso = 0.02 cm³/g). The increase in KOH ratio leads to an increase in the volume of mesopores, while simultaneously reducing the volume of micropores. PC-3 is dominated by mesopores (V_Meso = 1.11 cm³/g), with a drastic reduction in its micropore volume (V_micro = 0.12 cm³/g). Generally speaking, KOH activation includes radial activation and lateral activation. The former will lead to the formation of micropores, while the latter will expand the space of micropores and form mesopores.

In Fig. 3a, the XRD patterns of the porous carbons activated with varying KOH ratios are presented. Both PC-1 and PC-2 exhibit a minor peak at 26.6°, suggesting the existence of a limited amount of ordered structure within the porous carbons. Upon increasing the ratio of sample to KOH to 1:3, the XRD pattern of the resulting PC-3 sample displays a distinct peak at 26.6°, indicating a highly ordered and crystalline structure. The results show that a higher concentration of KOH can promote the formation of more ordered carbon. In Fig. 3b, the Raman spectra of the porous carbons are illustrated. Two significant peaks at approximately 1351 and 1585 cm⁻¹, identified as D band and G band, respectively, are evident. The D band is associated with the defect and disorder of the carbon, while the G band corresponds to the in-plane bond stretching pattern of the C-C bond in the graphite structure. The D and G bands of PC-1 and PC-2 are broad and overlapping, indicating that they are predominantly amorphous. In comparison, PC-3 exhibits a weak D band, a sharp and strong G band and a distinct peak at around 2714 cm⁻¹, which corresponds to the 2D band. These features suggest that PC-3 has a graphite-like structure. The intensity ratio of the D band to the G band (I_D/I_G) is commonly employed to assess the level of order of carbon materials. A lower I_D/I_G ratio signifies a higher degree of order in the carbon materials. The I_D/I_G ratios of PC-1, PC-2 and PC-3 are 1.25, 1.19 and 0.21, respectively. The I_D/I_G value of PC-3 is the lowest, indicating the highest degree of order, which is consistent with the XRD results. Fig. 4 shows the TEM/HRTEM images and SAED pattern of PC-3. The layer structure of graphite can be clearly observed in the HRTEM image. It indicates that PC-3 indeed has a high degree of structural order. The appearance of graphite-like structure in PC-3 has further confirmed that KOH activation contributes to the improvement of the structural order of lignin-derived porous carbon. There are similar reports in the literature [28], but the mechanism that how KOH converts amorphous carbon into ordered carbon at a low temperature is still unclear.

Fig. 5 shows the TG/DTG curves of the cured lignin powder LP-300 and the KOH-modified LP-300 for the preparation of PC-3. LP-300 exhibits a slight weight loss below 200°C, because it has been previously cured at 300°C. LP-300 mainly undergoes two severe weight-loss stages, showing two peaks at about 413°C and 708°C in the DTG curve. A residual mass of about 60% is obtained at 800°C, which indicates that lignin has a relatively high carbonization yield. KOH-modified LP-300 starts weight loss at about 100°C. The thermal weight loss process is complex, as can be seen from the TG and DTG curves, which can be divided into many stages. KOH-modified LP-300 shows significant weight loss at about 750°C–800°C, which is probably due to the volatilization of potassium [29].

To elucidate the reaction mechanism of KOH activation, mass spectrometry (MS) was employed to detect the gases released during TG analysis. Four gaseous products with mass-to-charge (m/z) ratios of 2, 18, 28 and 44 were detected and assigned to H₂, H₂O, CO and CO₂, respectively.

As shown in Fig. 6a, LP-300 initiates hydrogen release at about 369°C, while the KOH-modified LP-300 starts to release hydrogen at around 317°C. KOH has significantly promoted the low-temperature dehydrogenation reaction, which is mainly due to the reaction of KOH with carbon (Eq. (1)) [30–32]. When the temperature exceeds 500°C, the hydrogen released by LP-300 gradually decreases, while the hydrogen released by the KOH-modified LP-300 remains stable and even increases after 750°C. The high-temperature dehydrogenation reaction may be attributed to the removal of hydrogen atoms at the edge of lignin-derived amorphous carbon [33]. This process is essential for the transformation of disordered amorphous carbon into ordered graphite-like carbon. Fig. 6b displays the MS curve of H₂O with an m/z of 18 for LP-300 and KOH-modified LP-300. It is evident that the KOH-modified LP-300 shows a peak at about 369°C, which occurs earlier than that of the unmodified LP-300. This suggests that the addition of KOH may promote the dehydration of lignin molecules. In addition, KOH undergoes thermal decomposition at about 400°C, resulting in the formation of H₂O and K₂O (Eq. (2)) [26]. Fig. 6c depicts the MS curve of CO for LP-300 and KOH-modified LP-300. There is a significant difference in these MS curves above 700°C. The release of CO from LP-300 begins to decrease significantly, and tends to stabilize at 800°C, suggesting that the carbonization process of LP-300 concludes at this temperature. In contrast, the release of CO from KOH-modified LP-300 continues to increase after 700°C, which is attributed to the reaction of K₂O and carbon (Eq. (6)) [34]. The MS curve of CO₂ for LP-300 and KOH-modified LP-300 is shown in Fig. 6d. LP-300 begins to release CO₂ at about 130°C, which may be attributed to the decarboxylation reaction and the desorption of CO₂ absorbed in lignin. In contrast, KOH-modified LP-300 does not exhibit a CO₂ signal before 330°C, which is probably due to the reaction between CO₂ and KOH (Eq. (3)). The released CO₂ was absorbed and reacted by KOH. KOH-modified LP-300 displays two new CO₂ peaks at 550°C and 790°C, which may be caused by the decomposition of KHCO₃ and K₂CO₃, respectively (Eqs. (4) and (5)).

(1) 4KOH+C→K2CO3+K2O+2H2

(2) 2KOH→K2O+H2O

(3) KOH+CO2→KHCO3

(4) 2KHCO3→K2CO3+H2O+CO2

(5) K2CO3→K2O+CO2

(6) K2O+2C→2K+CO

As can be seen from the above TG-MS results, KOH significantly promotes the removal of H₂ and CO from lignin-derived carbon at high temperatures. The release of a substantial quantity of gaseous molecules not only generates numerous micropores, but also facilitates the formation of ordered carbon structures. The dehydrogenation reaction enables the conversion of more sp³ hybridized carbon into sp² hybridized carbon. CO is a product of the reaction between K₂O and amorphous carbon. An increase in CO indicates that more amorphous carbon is etched and consumed, which in turn preserves more ordered carbon structures.

The porous carbon was used as a working electrode for a supercapacitor, and its electrochemical performance was evaluated in a three-electrode system using a 6M KOH electrolyte. In Fig. 7a, the cyclic voltammetry (CV) curves of PC-1, PC-2, and PC-3 at a scanning rate of 50 mV s⁻¹ are displayed. The CV curves of the three samples exhibit a quasi-rectangular shape with no obvious redox peak, behaving a typical electric double-layer capacitor (EDLC). Subsequently, the galvanostatic charge-discharge (GCD) curves were recorded to further analyze the capacitive performance of the samples. As depicted in Fig. 7b, all the samples exhibit nearly triangular GCD curves, highlighting substantial electrochemical reversibility. Notably, PC-2 exhibits the longest charge and discharge duration, suggesting the highest specific capacitance among the samples. In Fig. 7c, the specific capacitance of PC-1, PC-2, and PC-3 is depicted at various current densities. At current densities of 0.2, 0.5, 1, 2, 5, 10 and 20 A g⁻¹, PC-2 shows specific capacitance values of 256, 214, 199, 188.2, 177, 167 and 160 F g⁻¹, respectively, while PC-3 exhibits values of 172, 139, 130, 123, 114.5, 106 and 96 F g⁻¹, respectively. Both of them are significantly higher than that of PC-1. Even at a current density of 20 A g⁻¹, the specific capacitance of PC-2 and PC-3 retains 60.6% and 55.8%, respectively, relative to the values at 0.2 A g⁻¹. It indicates that the two samples have good rate performance.The excellent capacitance performance of PC-2 in the three-electrode system is mainly attributed to its exceptional pore structure. The synergistic effect of micropores and mesoporous pores makes it an excellent electrode material for supercapacitors.

Electrochemical impedance spectroscopy (EIS) was employed to further investigate the electrochemical properties of the porous carbons [35]. The Nyquist diagram and equivalent circuit diagram are shown in Fig. 7d. The experimental data from EIS were fitted using ZView software, and the results are presented in Table 2. As shown in the figure, the R_s values of PC-1, PC-2 and PC-3 are 0.89, 0.25 and 0.14 Ω respectively, indicating that the PC-3 sample has the best electrical conductivity in the system. Compared to PC-1, the Nyquist curves of PC-2 and PC-3 are nearly vertical in the low frequency region, indicating a faster charge transfer rate. The 45° diagonal line in the enlarged image is described as the Warburg impedance, which reflects the magnitude of the ion diffusion resistance. The relatively low Wo1-R value of PC-3 indicates its good ion diffusion ability. In the mid to high frequency region of the EIS, the charge transfer resistance (R_ct) magnitude is observed. PC-3 displays a small R_ct, indicating a minor interfacial migration resistance. The low resistance and high ionic mobility of PC-3 are attributed to its more ordered structure.