**3. Results**

Nitrogen self-doped 3D honeycomb-like porous carbon was prepared through one-step activation from waste cottonseed husk (CSH). Cotton seeds were generally collected for the preparation of cottonseed oil; large amounts of cotton seed husks were not effectively used and were abandoned. Therefore, we recycled them to prepare high-performance biomass-derived electrode materials (Figure 1a,b). The material preparation process is shown in Figure 1b–d. Pretreated CSH powder (Figure 1c) was directly stirred with aqueous KOH solution and dried for carbonization and activation. The temperature for activation was adjusted from 600 ◦C to 800 ◦C, and the carbon that was obtained was washed and dried. The entire preparation process was cost-efficient, simple, and easily achieved the industrialized requirements.

**Figure 1.** Schematic diagrams for the fabrication of a-CSH-700 and the corresponding scanning electron microscopy (SEM) image. (**a**) Images of cotton. (**b**–**d**) Schematic of the synthesis of a-CSH-700 derived from cottonseed husk, and (**e**) the corresponding SEM images of a-CSH-700.

Scanning electron microscopy (SEM) images of a-CSH-600, 700, and 800 are shown in Figure 2a–f. The micromorphologies of the obtained samples showed typical 3D inter-connected honeycomb-like microstructures at different pyrolysis temperatures. The chemical composition of the waste cottonseed husk had a certain degree of degradation after stirring and evaporation with an aqueous KOH solution. Subsequently, the following chemical reaction of CSH and KOH during carbonization and activation processes occurred: 6KOH + C → 2K + 3H2 + 2K2CO3, followed by the decomposition of K2CO3, and the simultaneous generation of 3D pore structures and graphite sheet-like layer structures [8,37,38]. The lateral size of the 3D porous carbon varied in the range 400 nm to 4 μm (Figure 1d–f). With the increasing pyrolysis temperature, the characteristics of the 3D structure were slightly damaged, which was mainly because the higher temperature was bad for obtaining the 3D structure. The 3D linked carbon skeleton caused the obtained carbon material to exhibit a higher specific surface area. Moreover, this unique 3D structure generated abundant interconnected pore structure that allowed the electrolyte to be stored therein, reducing the distance that the electrolyte travelled on the surface of the electrode material. At the same time, abundant micropores and mesoporous structures existed on the surface of the carbon material (Figure 3a,b). With the help of these multi-level pore structures, it was easy to obtain a high energy storage performance.

**Figure 2.** SEM images of (**<sup>a</sup>**,**d**) a-CSH-600, (**b**,**<sup>e</sup>**) a-CSH-700, and (**<sup>c</sup>**,**f**) a-CSH-800.

**Figure 3.** Transmission electron microscopy (TEM) image (**a**) and high-resolution TEM (HRTEM) image (**b**) of a-CSH-700.

The powder X-ray diffraction (XRD) patterns of as-prepared a-CSHs samples are shown in Figure 4a. There were two distinct peaks at around 2θ = 22.1◦ and 43.5◦, which were ascribed to the (002) and (100) reflections of the amorphous graphitic carbon structure. The high intensity values at the low angles indicated high specific surface areas of carbon materials. The obvious peak at 43.5◦ revealed a higher degree of interlayer condensation in a-CSHs, which also significantly increased the electrical conductivity. Raman spectroscopy was further used to characterize the a-CSHs samples. As shown in Figure 4b, there were three distinct peaks at 1343 cm<sup>−</sup><sup>1</sup> (D band), 1590 cm<sup>−</sup><sup>1</sup> (G band), and 2800 cm<sup>−</sup><sup>1</sup> (2D band). The D band represented the degree of the defects and disordered sp<sup>3</sup> carbon atoms in the sample, and the G band was in line with graphite sp<sup>2</sup> hybridized carbon atoms in the sample. The presence of a 2D peak indicated that there existed an ordered graphite-like structure in the a-CSHs. The intensity ratio of D band to G band (ID/IG) represented the disorder degree of the samples [39,40]. The ID/IG ratios of a-CSH-600, a-CSH-700, and a-CSH-800 were 0.83, 0.84, and 0.87, respectively. We confirmed that the chemical reaction became deeper with rise in activation temperature, which promoted the defects and disordered structures in a-CSH-700 and a-CSH-800.

**Figure 4.** Powder X-ray diffraction (XRD) patterns (**a**) and Raman spectra (**b**) of a-CSHs samples. (**c**) Nitrogen adsorption-desorption isotherms and (**d**) pore size distributions of a-CSHs.

The nitrogen adsorption/desorption measurements were further used to examine the pore properties of a-CSHs. The nitrogen adsorption-desorption isotherm and pore size distribution curve of a-CSHs are shown in Figure 4c,d, respectively. It could be seen that all of the a-CSHs samples displayed type I isotherms [41]. With the increase in pyrolysis temperature, the corresponding quantity adsorbed value also increased [42]. This suggested that the specific surface area increased with an increase in the activation temperature. Figure 4d displays the pore size distribution isotherms. The dominant pore size distribution was located in the micropores (0.5–2 nm), and a part in mesopores (2–4 nm). a-CSH-600, a-CSH-700, and a-CSH-800 exhibited hierarchical porous structures, abundant micropores, and profuse mesopores, respectively, which were consistent with the results of the SEM images. Table 1 also summarizes the information on the specific BET surface areas and pore sizes of all of the a-CSHs samples. The specific surface areas of a-CSH-600, a-CSH-700, and a-CSH-800 were determined to be 1257.8, 1694.1, and 2063.0 m2/g, respectively, while the pore volumes were 0.64, 0.87, and 1.07 cm3/g, respectively. This showed that the temperature of activation was a dominating factor for the development of pore structure.

**Table 1.** Pore characteristics of the a-CSHs samples.


a Total surface area calculated using the Brunauer-Emmett-Teller (BET) method; b Micropore surface area and volume calculated from the t-plot method; c Total pore volume calculated at P/Po = 0.99; d Average pore diameter calculated from the (Barrett-Joyner-Halenda) BJH desorption.

The chemical compositions and surface functional groups of as-prepared a-CSH-600, a-CSH-700, and a-CSH-800 were further characterized by XPS measurements. As shown in the survey spectra

(Figure 5a), there existed three distinct peak signals corresponding to the C 1s peak near 286 eV, the N 1s peak near 400 eV, and the O 1s peak near 534 eV. Table 2 lists the statistical results of the corresponding elemental contents of the as-prepared carbon materials. The carbon content decreased and the associated oxygen content increased with rise in activation temperature. The nitrogen content of a-CSHs ranged from 1.51 to 2.56 atom %, the main reason being the complex chemical processes and structure features of these groups at a higher temperature. Figure 6 shows the element mapping images of as-obtained a-CSH-700. The nitrogen was uniformly distributed on the surface of the carbon, and the nitrogen content was 2.62 atom %.

Figure 5b–d displays high resolution C 1s, O 1s, and N 1s spectra of the as-prepared carbon materials. The high resolution C 1s spectrum (Figure 5b) of a-CSHs was resolved into four individual peaks at 284.6, 285.7, 286.7, and 289.2 eV, corresponding to C=C, C–C, C–O, and C=O, respectively [43,44]. The high-resolution O 1s spectrum at 530.8, 531.9, 533, and 534.1 eV were associated with COOH, O–C=O/N–C=O, C–O=C, and C–OH/N–O–C, respectively [45,46]. The high-resolution N 1s spectrum was composed of pyridinic-N (398.4 eV), pyrolic-N (400.1 eV), quaternary-N (401 eV), and oxidized-N (402.7 eV) [47–49]. These nitrogen and oxygen functional groups enhanced the surface wettabilities of the a-CSHs, and thereby promoting the specific capacitances of the as-prepared carbon materials [8,50,51].

**Table 2.** Elemental contents of a-CSHs samples from X-ray photoelectron spectroscope (XPS) and EDS.


**Figure 5.** XPS images of the a-CSH-600, a-CSH-700, and a-CSH-800 (**a**). High-resolution C 1s (**b**), O 1s (**c**), and N 1s (**d**) of the a-CSH-600, a-CSH-700, and a-CSH-800.

**Figure 6.** (**a**) SEM image of a-CSH-700, and elemental mapping images of (**b**) C, (**c**) O, and (**d**) N.

The general standards for designing high performance supercapacitor electrodes were associated with high specific capacitance, good rate capability, and long cycle stability. According to the above conclusions, the a-CSHs derived from one-step synthesis possessed many advantages for supercapacitor electrode materials. The a-CSHs had superior specific surface area for forming ideal electrochemical double layers. The interconnected micropores and mesopores could increase ion transport and the abundant surface nitrogen functional groups improved the wettability and promoted electrical conductivity. The 3D structure provided space for electrolyte storage. These characteristics endowed a-CSHs with a high supercapacitor performance.

The electrochemical performances of the obtained a-CSHs were tested by a three-electrode system in a 6 M KOH aqueous electrolyte and the CV curves are shown in Figure 7. The CV curves of a-CSH-600 (Figure 7a), a-CSH-700 (Figure 7b), and a-CSH-800 (Figure 7c) showed typical quasi-rectangular shapes at scan rates from 5 mV/s to 50 mV/s. This suggested that all of the samples displayed ideal electrochemical double layer capacitances. The CV curves of a-CSH-700 had the largest current responses and areas, suggesting the highest capacitance. Furthermore, galvanostatic charge/discharge (GCD) was further used to assess the electrochemical performance, which is shown in Figure 8. The GCD curves of a-CSH-600 (Figure 8a), a-CSH-700 (Figure 8b), and a-CSH-800 (Figure 8c) showed typical quasi-linear shapes at current densities from 0.5 A/g to 20 A/g. It also suggested that the as-obtained carbon samples possessed good electrochemical performances. Figure 8d summarizes the gravimetric specific capacitances of a-CSHs, calculated from galvanostatic charge/discharge curves at a current density ranging from 0.5 to 20 A/g. The specific capacitance value of the a-CSH-700 sample was as high as 238 F/g at a current density of 0.5 A/g, which was higher than those of a-CSH-800 (217 F/g) and a-CSH-600 (204 F/g). The capacitance was also as high as 200 F/g for a-CSH-700 with an excellent capacitance retention of 92%, indicating its outstanding rate capability. For a better comparison, Table 3 lists the specific capacitances of other biomass-derived porous carbons that were reported in the recent literature. Although the electrode of a-CSH-700 exhibited the best electrochemical performance, its specific area and surface functional groups were not the highest among all of the samples, meaning that there were synergistic effects.

**Figure 7.** Electrochemical performance characteristics of a-CSHs measured in a three-electrode system in a 6 M KOH electrolyte: the cyclic voltammetry (CV) curves of (**a**) a-CSH-600, (**b**) a-CSH-700, and (**c**) a-CSH-800 at different scan rates.

**Figure 8.** Electrochemical performance characteristics of a-CSHs measured in a three-electrode system in a 6 M KOH electrolyte: galvanostatic charge/discharge (GCD) curves of (**a**) a-CSH-600, (**b**) a-CSH-700, and (**c**) a-CSH-800 at different scan rates. (**d**) Specific capacitances at different current densities.

**Table 3.** Electrochemical performance of biomass derived porous carbons.


The capacitive performance of a-CSH-700 was further evaluated by assembling it in a symmetric two-electrode cell in 6 M KOH. Figure 9a shows the CV curves that were tested in different potential windows. It could be seen that there no obvious promotion of anodic current when the operating voltage was 1.2 V. The GCD curves (Figure 9b) had a symmetric triangular shape as the current density increased from 0.5 A/g to 10 A/g, and there were no evident IR drops. The testing results also showed

that it had a good capacitive performance. The specific capacitance for the entire electrochemical supercapacitor was estimated to be 52 F/g at 0.5 A/g with an energy density of 10.4 Wh/kg and power density 300 W/kg (Figure 9c,e). Figure 9d shows the cycling stability of the device at a higher current density of 10 A/g. It could be seen that the charge/discharge curves of the last two cycles were the same as the first two cycles (Figure 9f). Capacitance retention was also as high as 91% after 5000 cycles. These results revealed that the synthesized a-CSH-700 sample was a superior electrode material for high power and cost-effective supercapacitors.

**Figure 9.** Electrochemical measurements of an as-assembled a-CSH-700//a-CSH-700 symmetric supercapacitor in a 6 M KOH electrolyte: (**a**) CV curves of the cell operated in different voltage windows at scan rate 50 mV/s; (**b**) galvanostatic charge/discharge curves of the cell at various current densities; and (**c**) specific capacitances for the supercapacitor at different current densities. (**d**) Cycling stabilities of the devices at a current density of 10 A/g. (**e**) Ragone plot. (**f**) First two cycle charge-discharge and last two charge-dischage plots.
