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Article

Hierarchical Porous Carbon Aerogel Derived from Sodium Alginate for High Performance Electrochemical Capacitor Electrode

by
Xuequan Li
*,
Liting Lai
,
Fangdi Wu
,
Wenju Xie
and
Junshao Liu
*
Key Laboratory of Green Chemical Technology of Fujian Province University, Fujian Provincial Key Laboratory of Eco-Industrial Green Technology, Wuyi University, Wuyishan 354300, China
*
Authors to whom correspondence should be addressed.
Processes 2023, 11(12), 3355; https://doi.org/10.3390/pr11123355
Submission received: 7 October 2023 / Revised: 5 November 2023 / Accepted: 27 November 2023 / Published: 2 December 2023
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Versions Notes

Abstract

:
To improve the performance of electrochemical capacitors, there is a notable focus on carbon materials characterized by a large surface area, reasonable pore size, pore size distribution, appropriate electronic conductivity, and excellent chemical durability. Herein, the hierarchical porous carbon aerogel originating from sodium alginate (SA) with well-defined porosity are proposed. The resultant hierarchical porous carbon aerogel shows a substantial specific surface area of 2050.6 m2 g−1 with macropores, mesopores and micropores confirmed by techniques such as TEM, SEM, BET, etc., resulting from a sequence of aerogel formation-carbonization-activation. By electrochemical measurement, the hierarchical porous carbon aerogel exhibits a specific capacitance of 204 F g−1 at an operating current density of 0.2 A g−1 employing 6 M KOH aqueous solution. The hierarchical carbon aerogel displays outstanding cycling stability with a 96.2% capacity maintenance for 10,000 cycles at an operating current density of 1 A g−1. This study presents a viable method for for preparing hierarchical porous carbon aerogel derived from biopolymer for electrochemical capacitors.

1. Introduction

Electrochemical capacitors (ECs) have garnered significant interest owing to the need for energy storage solutions across various applications, including emergency power systems, electric cars, and wind energy. This can be attributed to their impressive characteristics, such as high power density, substantial specific capacitance, and exceptional cycle life stability. Up to now, electric double layer capacitors (EDLC) employing porous activated carbon are still the main commercial electrochemical capacitors. To further improve the energy density of electrochemical capacitors, metallic compounds [1,2] and conducting polymers [3,4] are introduced, because the electrical energy storages result from the redox reactions giving rise to the high energy density and specific capacitance. However, poor cycle stability and rate capability contribute to the failure of commercialization so far. Therefore, a number of recent research efforts still focus on various carbon materials [5,6]. Moreover, the porous carbon with hierarchical pores (i.e., macropores, mesopores, and micropores) is ideal candidate for electrochemical capacitor. Macropores act as ion-buffering reservoirs and can minimize the diffusion distances of ions to the interior surfaces. Mesoporous channels play an important role in low-resistant pathways for the ions diffusing through the porous network, and micropores improve charge storage. In order to improve the porosity and specific surface area, KOH [7], NaOH [8], ZnCl2 [9], or other chemicals [10,11] are usually applied as chemical activators to activate the carbon materials at high temperature to develop more effective structures and optimize the porosity. Recently, porous carbon material developed by green sustainable biomass or their derivative [12,13,14,15] becomes a significant subset owing to the increasing green development and environmental protection. Therefore, porous carbons derived from various biomass resources, such as corncob [16], corn straw [17], pomelo peel [18], celery [19] and rice straw [20] have been exploited for high-performance EDLC. Among them, carbon aerogel is one of carbon materials showing good electrochemical energy storage performance due to 3D hierarchical pores with tunable porous structure, high specific surface area and good conductivity, facilitating ions efficient diffusion in the carbon network [21,22,23].
Sodium alginate is one of these reproducible plants with similar structural units to cellulose. And it is extracted from seaweed mainly applied to food [24] and health care [25,26] and can be produced at low cost using environmentally friendly processes. Its large-scale production has been very mature. SA can be easily dissolved in aqueous solution, and then form a gel. Subsequent freeze-drying and carbonization result in a 3D hierarchical porous aerogel and carbon aerogel, respectively. The dissolving, gelation and de-gelation process destroys the compact structure of SA and reconstructs a hierarchical porous architecture. The carbon aerogel can inherit the hierarchical porous structure of the SA aerogel. Therefore, transforming sodium alginate (SA) aerogel into an electrode material is anticipated to be a cost-effective and eco-friendly approach, presenting another viable biomass alternative.
Herein, hierarchical porous carbon aerogel derived from SA aerogel was fabricated. A number of macro- and meso-pores were developed by the direct carbonization of SA aerogel. KOH extensively applied to manufacture porous carbon was employed to further activate the mesoporous carbon aerogel to create micropores. Therefore, a general method, exhibiting effectiveness, easy operation, cost-effectiveness, and environmentally friendly, is proposed to prepare hierarchical porous carbon applied to EDLC with reproducible biomass. Furthermore, the as-prepared hierarchical carbon aerogel exhibits not only reasonable pore size distributions but also high specific surface area and hence shows outstanding energy storage characteristic and cycle stability. Additionally, the impact of various activation temperatures on the distribution of pore sizes was also explored and the comparison of their respective electrochemical properties was carried out.

2. Experimental

2.1. Chemicals

All of the chemicals were purchased from Aladdin Co., Ltd. (Shanghai, China). and used as received, including the sodium alginate (SA), potassium hydroxide, and poly(vinylidene fluoride).

2.2. Preparation of Sodium Alginate Aerogel

SA aerogels were prepared by lyophilization. Here, 2 g of SA powder was dispersed 100 mL of deionized water with vigorous stirring at 300 rotations per minute until the transparent SA was obtained, and then the transparent SA solution was moved to the refrigerator at −80 °C for pre-freezing for 1 h, and then all the water was removed to obtain SA aerogel by a lyophilizer.

2.3. Carbonization of Sodium Alginate Aerogel

The carbonization of the SA aerogel was carried out in a tubular furnace. The initially prepared SA aerogels underwent pyrolysis at various temperatures (e.g., 500 °C, 600 °C, 700 °C, 800 °C) for 3 h in a flowing nitrogen (N2) atmosphere with a heating rate of 10 °C per minute to yield carbonized aerogels after washing and drying.

2.4. Carbon Aerogel Activation

The KOH activation procedure follows the methodology outlined in prior research with slight modification [27]. Briefly, The carbon aerogels, as prepared, were blended with a 6 M KOH solution, maintaining a KOH/carbon aerogel weight ratio of 3:1. The mixture was placed in an air dry oven at 100 °C for 24 h to remove water. The samples were thermally treated at activation temperature for 2 h under argon gas flow with a heating rate of 10 °C min−1 at 700 °C, 800 °C and 900 °C. Following activation, the sample underwent extensive rinsing with deionized water until neutral. Ultimately, the hierarchical porous carbon was acquired through the drying of the sample at 100 °C for a duration of 24 h.

2.5. Electrochemical Measurements

The electrochemical performances were tested in two-electrode cells with the electrolyte of 6 M KOH and a glass fiber paper separator. The working electrode materials were prepared through mingling poly(vinylidene fluoride), carbon black and the as-prepared samples with a weight ratio of 1:1:8. The cyclic voltammetry (CV) and the galvanostatic charge–discharge (GCD) tests were attested with a potentiostat (CHI 660 Instruments, Shanghai, China). An Autolab potentiostat (PGSTAT100N, Switzerland) was applied to carry out the electrochemical impedance spectroscopy (EIS) tests with 0 V as the applied potential, 5 mV as the amplitude and the frequency from 0.1 Hz to 100 KHz.
For CV curves, the capacitance was calculated with the equation C = 4ʃIdt/mV in the two electrode system, where I is the current, m is the total active carbon mass for the two electrodes, and V is the applied voltage window. The specific capacitance of the specimen by GCD was calculated by the formula C = 4It/mV, where I is the used constant current, m is the gross mass of both active material electrodes, V is the used potential window, and t is the discharging time. The energy density and power density were calculated with the equations of E = (CV2)/2 and P = E/t, respectively, where C is the specific capacitance through charge–discharge by galvanostatic measurement and t is the discharging time.

2.6. Materials Characterization

The cold-field emission scanning electron microscope (SEM, HITACHI SU8200, Tokyo, Japan) was applied to investigate the microstructures of the specimen. The transmission electron microscopy (TEM) was surveyed employing a FEI Tecnai G2 F20 instrument (Columbia, SC, USA) operated at an accelerating voltage of 200 kV. The total surface area was analyzed with the micromeritics surface area and porosity analyzer (ASAP 2020 HD88, Atlanta, GA, USA) through nitrogen sorption. The degas process was conducted at 250 °C for 4 h under a vacuum of 500 mm Hg (~0.67 mbar). Liquid nitrogen was applied to keep the temperature throughout adsorption–desorption process. The Barrett–Joyner–Halenda (BJH) method was used to analyze the surface area and pore diameter. The pore volume of the nanopores and the specific surface area were calculated by the t-method.

3. Results and Discussion

The primary procedure for fabricating hierarchical porous carbon aerogels derived from sodium alginate (SA) is depicted in Scheme 1. Initially, sodium alginate was dissolved, and the solution was subjected to freezing at −80 °C for 1 h to induce gel formation. Carbon aerogels were subsequently obtained through the processes of freeze-drying and carbonization at different temperatures. To further enhance the distribution of pore sizes and create hierarchical porous structures, the as-prepared carbon aerogels were mixed with a KOH solution. Following drying at 100 °C, the mixture underwent activation at various temperatures under an argon flow for 2 h. The resultant mixture was then washed and dried, resulting in the formation of hierarchical nanoporous structures.
For an overview of the overall microstructures, SEM analysis was conducted on all carbon aerogel samples prepared at different temperatures. The results are presented in Figure 1, illustrating the general morphologies of the carbon aerogels before any post-activation treatment. The SEM images clearly demonstrate porous structures, which exhibit pitting and channels with average diameters of about several micrometers. Furthermore, with the increase in the pyrolysis temperature, the number of nanopores on the surface of the samples increases. The morphology is different from that of wood-based carbon and chitosan- derived carbon aerogel [28,29]. In detail, at 500 °C, fewer pores was produced with a small diameter on the relatively smooth surface (Figure 1a), and the number of pores increased slightly at 600 °C (Figure 1b). As the pyrolysis temperature increased to 700 °C (Figure 1c), in addition to a mass of pores on the surface, a great many dark zones can also be observed under the unbroken carbon film, which are the pore structures existing under the unbroken film. While the pyrolysis temperature increased to 800 °C, the surface pore size became larger, and at the same time, lots of white dots could be observed (Figure 1d), which were supposedly the bulging carbon film that has not been completely ruptured to form the pores. This phenomenon results from the development of CO, CO2, H2O, etc., due to SA aerogel continuous pyrolysis with the carbonization temperature increasing. On the one hand, the consumption of carbon elements makes carbon gasification, resulting in the rupture of the original intact carbon plane, on the other hand, the resulting gas will break through the weak carbon film, giving rise to some pores. This process can be better understood from the change in specific surface area and pore volume of porous carbon. As displayed in Table 1, at 500 °C, the total specific surface area and pore volume are only 64 m2 g−1 and 0.088 m3 g−1, respectively. As pyrolysis temperature increases, both the specific surface area and pore volume are increasing. When the pyrolysis temperature increased to 800 °C, the total specific surface area and pore volume of the porous carbon is up to 718 m2 g−1 and 0.488 m3 g−1, respectively. This confirms with pyrolysis increasing, more carbon is consumed by the gasification contributing to more pore structures of various sizes.
To assess the electrochemical capabilities of the porous carbon aerogels, cyclic voltammetry (CV) experiments were conducted using a two-electrode cell at various scanning rates within the potential range of 0 to 1 V. The outcomes are illustrated in Figure 2a–d. As shown in Figure 2a, for the 500 °C carbonized sample, the cyclic voltammetry curves with a small, enclosed area deviate severely from the rectangle, implying a smaller capacitance and poorer capacitive performance, while all the deviations of cyclic voltammetry curves of the 600 °C carbonized samples are improved (Figure 2b). When the temperature was increased to 700 °C, the electrochemical performance is further improved and the cyclic voltammetry curves are closer to the rectangle (Figure 3c), and the cyclic voltammetry curves already show a good rectangle when the temperature was increased to 800 °C (Figure 2d). Figure 2e,f shows the CV curves for the four carbonization temperatures at 2 mV s−1 and 200 mV s−1 scan rates, respectively. The cyclic voltammetry curves of the 800 °C carbonized sample, at both 2 mV s−1 and 200 mV s−1 scan rates, show an approximate rectangle which is much better than that of previous reports [27,30] with the area enclosed by the curves being significantly larger than that of the other three samples. At scan rates of 2 mV s−1 and 200 mV s−1, the specific capacitances are 91 F g−1 and 57 F g−1, respectively. In comparison, the specific capacitances of the samples carbonized at 500 °C are only 32 F g−1 and 7 F g−1 at the corresponding scan rates. The sample carbonized at 800 °C exhibits the most favorable electrochemical performance due to more thorough carbonization, improved conductivity, a more fully developed porous structure, and a higher specific surface area.
To further improve its electrochemical performance, the carbonized sample at 800 °C was chemically activated using potassium hydroxide activator. The scanning electron micrographs of the samples activated at 700 °C, 800 °C and 900 °C are shown in Figure 3a–c. All of them show three-dimensional continuous pores, including a continuous carbon skeleton portion and a continuous porous structure. As mentioned above, the carbon materials chemically activated with potassium hydroxide will undergo a series of reactions with the carbon during the activation. The activation process involved subjecting the as-prepared carbon aerogels to high temperature treatment, allowing KOH to permeate the aerogel pores. At elevated temperatures, the reaction between the carbon on the surface of the aerogel channel walls and KOH can take place as described by Formula (1).
4KOH + C → K2CO3 + K2O + 2H2
This will further consume the carbon, causing some intact carbon films to rupture and form a porous structure, and the size of some already existing pores may further increase. Moreover, when the temperature is higher than 700 °C, the resulting potassium metal can penetrate between graphitic layers [31,32], expanding the space between the atomic layers of the graphite layered structure and giving rise to increased pore capacity. The detailed morphology and structure of the carbon aerogels were further observed employing transmission electron microscope. The typical transmission electron micrograph (the sample activated at 800 °C) is shown in Figure 3d. Some parts of the sample display approximate ordered lattice stripes, indicating a certain degree of graphitization, and slit-like voids between the stripes, which is conducive to charge transfer. While some parts are disordered carbon, which are carbon particles with a diameter of several nanometers. A combination of scanning electron microscopy and TEM reveals that the samples contain porous structures and particles of different sizes, which is very favorable for double layer electrochemical capacitors.
Electron microscopy is difficult to fully observe pore structures of various sizes, and the observed porous structures are not statistically significant. Gas adsorption/desorption isotherm can statistically analyze and illustrate the pore distribution. Therefore, it shows the porous structure information more truly and fully. Figure 4a displays the typical N2 isothermal adsorption/desorption curves of the samples prepared under different conditions, and the four samples show typical type-IV nitrogen sorption isotherms, indicating various pore size distributions from micropores to mesopores according to the IUPAC classification. The probe gas adsorptions of all samples increase rapidly at relative gas pressure P/P0 < 0.1, demonstrating the high micro-porosity in the samples. On the one hand, the plot of the non-activated sample displays almost horizontal plateau at higher relative pressure without a continuous increase in adsorption, indicating the sample contains a high proportion of micropores, which is consistent with the statistical data shown in Table 1. The specific surface areas of micropores and mesopores are 492 m2 g−1 and 226 m2 g−1, respectively. On the other hand, the observed hysteresis between the adsorption and desorption branches (at 0.5–1.0 P/P0) for the three activated samples clearly indicates the obvious existence of mesopores. In addition, as displayed in Table 1, the specific surface area of mesopores of the three activated samples is higher than that of micropores, which indicates not only a large number of micropores, but also a lot of mesopores are developed during the activation. Furthermore, the mesopores of the four samples are concentrated in the range of 2 nm–5 nm (Figure 4b). Furthermore, the sample activated at 800 °C exhibits the highest specific surface area (2050.6 m2 g−1) and pore volume (2.26 m3 g−1). In conclusion, the as-prepared porous carbon shows hierarchical structure containing micropores (<2 nm), mesopores (2–50 nm), macropores(>50 nm).
In addition to the specific surface area, the electrochemical capacitor’s performance is influenced by the effective distribution of pore sizes and electrical conductivity (to enhance power density). Raman spectra were conducted to assess the degree of graphitization of the four samples, as depicted in Figure 5a. All samples display the D band, arising from disordered carbon at 1350 cm−1, and the G band, indicating the vibration of ordered carbon at 1580 cm−1. Furthermore, the relatively wide width of both the D and G bands suggests the presence of amorphous carbon as well. These findings align with the observations made through TEM. Furthermore, the presence of G band suggests that the as-prepared SA carbon aerogels show a certain extent of graphitization. The intensity ratio (ID/IG) of the D and G bands serves as an indicator of the proportion of disordered carbon to ordered carbon [33,34], This is due to the fact that the degree of graphitization exhibits an inverse correlation with the intensity ratio (ID/IG). Thus, the ID/IG ratio is applied to evaluate the amount of the amorphous carbon in hierarchical carbon aerogels. As shown in (Figure 5b), the ID/IG ratios of the samples (carbonized at 800 °C, activated at 700 °C, 800 °C and 900 °C) are 0.96, 0.95, 0.94, and 0.91, respectively. It indicates an inverse proportionality between the ratio (ID/IG) and the activation temperature. Therefore, elevating the activation temperature enhances the graphitization level of the hierarchical carbon aerogels. This phenomenon results from the activation reaction process. Normally, the KOH tends to corrode the less graphitized portion first, leaving the more highly graphitized nanoparticles intact. It is well-established that the degree of graphitization is a factor influencing the conductivity of carbon aerogel. Consequently, the conductivity of the activated samples should be superior to that of the non-activated counterpart.
The performance of porous carbon electrode materials for double layer electrochemical capacitors ultimately needs to be examined by electrochemical tests. To measure the electrochemical properties of the four hierarchical carbon aerogels (carbonized at 800 °C, activated at 700 °C, 800 °C and 900 °C), all CV tests were conducted employing a two-electrode system with the 0–1 V voltage window. The cyclic voltammetry plots of the four samples in Figure 6a–d display nearly rectangular shapes. Especially for the activated samples, all the cyclic voltammetry curves at a scan rate of 200 mV s−1 are still very close to the standard ideal rectangles, demonstrating very desirable double layer electrochemical behaviors. In detail, the specific capacitances of the four samples at a scan rate of 200 mV s−1 are 57 F g−1, 138 F g−1, 155 F g−1 and 150 F g−1, respectively. Therefore, the sample activated at 800 °C shows the highest specific capacitance at high scan rate (200 mV s−1). Furthermore, in Figure 6e, the cyclic voltammetry (CV) curves for the four samples at a scan rate of 2 mV s−1 are depicted. It is evident that the sample activated at 800 °C displays the most substantial enclosed area, while the non-activated sample displays the most modest enclosed area. In detail, the specific capacitances of the four samples are 91 F g−1, 175 F g−1, 210 F g−1 and 191 F g−1, respectively, indicating the sample activated at 800 °C has the highest that specific capacitance as well. To further estimate the electrochemical performance of the samples, the Nyquist plots and corresponding fitted equivalent circuit model are summarized as shown in Figure 6f.
At high frequency regions, all the samples display a semicircle behavior, which is associated with the charge transfer resistance, and the samples activated with smaller radii demonstrate a decreased charge transfer resistance (Rct, 0.6 Ω (800 °C activation), 0.7 Ω (900 °C activation), 1.4 (700 °C activation) Ω and 3.2 Ω (800 °C carbonization), respectively), implying a enhanced electrical conductivity in the electrodes, aligning with the findings from the cyclic voltammetry results. And the intercepts (RΩ) are less than 1 ohm, indicating that the equivalent series resistance of all four samples is relatively small, which is consistent with the characteristics of the aqueous double layer electrochemical capacitor. Subsequently, straight lines are observed at intermediate frequencies, and these linear behaviors refer to the diffusion of electrolyte. It can be found that the plots exhibit almost perpendicular lines for all samples. The activated samples exhibit more pronounced vertical lines, which reveals the mass transfer in the electrolyte is promoted. Furthermore, the 800 °C activated sample showing the most vertical line approaches the characteristics of an ideal capacitor. Comprehensive analysis of the above data displays that the sample activated at 800 °C has the best electrochemical performance, therefore the detailed electrochemical properties of at 800 °C were investigated further.
To further evaluate the performance of the 800 °C activated sample, more electrochemical measurements were carried out with a two-electrode symmetrical system. The cyclic voltammetry curves at various scan rates are demonstrated in Figure 7a, and the corresponding specific capacitances are calculated and plotted in Figure 7b. The highest specific capacitance is up to 210 F g−1 at 2 mV s−1, and the specific capacitance is 155 F g−1 at the scan rate of 200 mV s−1, indicating good rate capability. Figure 7c exhibits the GCD curves at various current densities, displaying almost isosceles triangles with good symmetry and sensitive current-voltage response indicating the high Coulombic efficiency, suggesting the typical double-layer capacitor behaviors. The IR drop is relatively low at various current densities, illustrating the low internal resistance of the two-electrode system. This phenomenon results from the high speed ions diffusion and the fast charge transfer. By GCD curve calculation, the specific capacitances at various current densities are displayed in Figure 7d. The specific capacitance is up to 204 F g−1 at 0.2 A g−1, and remains 132 F g−1, as the current density increases to 10 A g−1. The capacitances are lower than that of the carbon composites [35,36,37] and doped porous carbons [38,39], but they are higher than the ones demonstrated by previous reports, such as graphene (117 F g−1) [40], carbon black/graphene composites (175 F g−1) [41], carbon/manganese dioxide composite [29], self-doped hierarchical porous carbon aerogels (197 F g−1) [34], glucose-based porous carbon (158 F g−1) [42] and commercial activated carbon (153 F g−1) [43]. To investigate the cycling stability of the as-fabricated hierarchical.
Carbon aerogel (activated at 800 °C), the GCD test was carried out at a constant current density (1 A g−1) with 10,000 charge–discharge cycles. In Figure 7f, a 96.2% capacity retention is illustrated, with a minor decrease from 182 F g−1 to 175 F g−1, indicating excellent cycling stability. Additionally, the Ragone plot in Figure 7e depicts both energy density and power density. At an energy density of 28 W h kg−1, the corresponding power density is 400 W kg−1, and with an energy density of 18 W h kg−1, the corresponding power density reaches up to 20 kW kg−1.

4. Conclusions

In summary, hierarchical porous carbon aerogels derived from SA with good performance have been prepared by carbonization followed by activation at various temperatures. The resulting carbon aerogels exhibit hierarchical porous structures (e.g., macropores, mesopores, and micropores) and display a high specific surface area of 2050.6 m2 g−1. The specific capacitance, determined through GCD in a two-electrode symmetrical supercapacitor cell, reaches 204 F g−1 at 0.2 A g−1, with an impressive 96.2% maintenance of capacitance over 10,000 cycles at 1 A g−1. Furthermore, the energy density of 28 W h kg−1 at a power density of 400 W kg−1 and 18 W h kg−1 at a power density of 20 kW kg−1 are revealed, respectively. The excellent electrochemical performance can be ascribed to the distinctive characteristics of hierarchical carbon aerogels, with a high degree of graphitization and well-defined porosity. This work exhibits the promise for massive-scale preparation of hierarchical porous carbon aerogels employing a cost-effective biopolymer, i.e., sodium alginate.

Author Contributions

Conceptualization, X.L. and J.L.; Methodology, X.L.; Investigation, X.L., L.L., F.W. and W.X.; Data curation, X.L. and L.L.; Funding acquisition, X.L. and J.L.; Writing—original draft, X.L.; Writing—review and editing, X.L. and J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work is financially supported by the Natural Science Foundation of Fujian Province (2023J011053), the Science and technology plan of Fujian Province, China (No. 2022G02020, 2022H6002) and College student innovation and entrepreneurship project, Wuyi University (S202110397050).

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author, X.L., upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Processes 11 03355 sch001
Scheme 1. Schematic of the production process for sodium alginate-derived carbon aerogels featuring a highly porous structure and hierarchical pore structure.
Scheme 1. Schematic of the production process for sodium alginate-derived carbon aerogels featuring a highly porous structure and hierarchical pore structure.
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Figure 1. SEM of SA carbonized by various temperature: (a) 500 °C carbonization; (b) 600 °C carbonization; (c) 700 °C carbonization; (d) 800 °C carbonization.
Figure 1. SEM of SA carbonized by various temperature: (a) 500 °C carbonization; (b) 600 °C carbonization; (c) 700 °C carbonization; (d) 800 °C carbonization.
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Figure 2. CV curves of the four samples at various scan rates: (a) 500 °C carbonization; (b) 600 °C carbonization; (c) 700 °C carbonization; (d) 800 °C carbonization; (e) CV curves of the four samples at scan rate 2 mV s−1; (f) CV curves of the four samples at scan rate 200 mV s−1.
Figure 2. CV curves of the four samples at various scan rates: (a) 500 °C carbonization; (b) 600 °C carbonization; (c) 700 °C carbonization; (d) 800 °C carbonization; (e) CV curves of the four samples at scan rate 2 mV s−1; (f) CV curves of the four samples at scan rate 200 mV s−1.
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Figure 3. Electron microscope of nanoporous carbon activated by various temperature: (a) SEM of 700 °C activation; (b) SEM of 800 °C activation; (c) SEM of 900 °C activation; (d) TEM of 800 °C activation.
Figure 3. Electron microscope of nanoporous carbon activated by various temperature: (a) SEM of 700 °C activation; (b) SEM of 800 °C activation; (c) SEM of 900 °C activation; (d) TEM of 800 °C activation.
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Figure 4. (a) Nitrogen adsorption/desorption isotherms of porous carbon; (b) pore size distribution.
Figure 4. (a) Nitrogen adsorption/desorption isotherms of porous carbon; (b) pore size distribution.
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Figure 5. (a) Raman spectra of samples; (b) the ratio of D band and G band.
Figure 5. (a) Raman spectra of samples; (b) the ratio of D band and G band.
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Figure 6. Electrochemical tests for samples: (a) CV curves of the sample carbonized at 800 °C without activation; (b) CV curves of 700 °C activation; (c) CV curves of 800 °C activation; (d) CV curves of 900 °C activation; (e) CV curves of the four samples at scan rate 2 mV s−1; (f) The electrochemical EIS of the four samples.
Figure 6. Electrochemical tests for samples: (a) CV curves of the sample carbonized at 800 °C without activation; (b) CV curves of 700 °C activation; (c) CV curves of 800 °C activation; (d) CV curves of 900 °C activation; (e) CV curves of the four samples at scan rate 2 mV s−1; (f) The electrochemical EIS of the four samples.
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Figure 7. The electrochemical tests results of the sample activated by 800 °C. (a) CV curves with various scan rates; (b) specific capacitance obtained from (a); (c) GCD profiles at different current densities; (d) specific capacitance obtained from (c); (e) The capacitance across different cycles at current density 1 A g−1; (f) Ragone plots of the sample.
Figure 7. The electrochemical tests results of the sample activated by 800 °C. (a) CV curves with various scan rates; (b) specific capacitance obtained from (a); (c) GCD profiles at different current densities; (d) specific capacitance obtained from (c); (e) The capacitance across different cycles at current density 1 A g−1; (f) Ragone plots of the sample.
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Table 1. Porous structure information of samples.
Table 1. Porous structure information of samples.
Sample IDSmicro (m2 g−1)Smeso (m2 g−1)Vmicro (m3 g−1)Vmeso (m3 g−1)
500 °C carbonization25.538.50.0140.074
600 °C carbonization107.534.50.0570.051
700 °C carbonization145.331.60.0770.048
800 °C carbonization491.6226.30.2630.225
700 °C activation524.11167.20.2831.701
800 °C activation735.41315.20.4131.847
900 °C activation534.21309.40.3161.803
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Li, X.; Lai, L.; Wu, F.; Xie, W.; Liu, J. Hierarchical Porous Carbon Aerogel Derived from Sodium Alginate for High Performance Electrochemical Capacitor Electrode. Processes 2023, 11, 3355. https://doi.org/10.3390/pr11123355

AMA Style

Li X, Lai L, Wu F, Xie W, Liu J. Hierarchical Porous Carbon Aerogel Derived from Sodium Alginate for High Performance Electrochemical Capacitor Electrode. Processes. 2023; 11(12):3355. https://doi.org/10.3390/pr11123355

Chicago/Turabian Style

Li, Xuequan, Liting Lai, Fangdi Wu, Wenju Xie, and Junshao Liu. 2023. "Hierarchical Porous Carbon Aerogel Derived from Sodium Alginate for High Performance Electrochemical Capacitor Electrode" Processes 11, no. 12: 3355. https://doi.org/10.3390/pr11123355

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