Innovative approaches of porous carbon materials derived from energy waste and their electrochemical properties

INTRODUCTION

Global warming is a result of climate change driven by human energy consumption and the release of significant carbon dioxide emissions, adversely affecting lives and leading to mounting issues. In 2020, China introduced its goals of carbon peaking and neutrality at the UN General Assembly. Subsequently, the nation outlined five key strategies, which include enhancing energy efficiency and establishing a green, low-carbon circular economy. Many fossil fuels are not fully converted during use, leaving byproducts rich in unburned carbon that can harm the environment. Thus, the path to carbon neutrality involves repurposing energy waste into advanced energy storage components for sustainable energy cycles^[1-3].

Supercapacitors (SCs) are energy storage devices that operate on the principle of electrochemical conversion, offering quick charge-discharge capabilities, high power density, and long cycle life^[4-10]. They are categorized into electric double-layer capacitors (EDLCs) and pseudocapacitors based on their storage mechanisms^[11-13]. In EDLCs, the charging and discharging processes involve the complete adsorption and desorption of charges. The electrode materials used in EDLCs are primarily carbon materials with large surface areas, such as activated carbon (AC), hierarchically porous carbon (HPC), carbon nanotubes (CNT), graphene (GH), etc.^[14-18]. The pore structures of these materials can be classified as micropores (less than 2 nm), mesopores (2-50 nm), and macropores (greater than 50 nm). Each pore structure offers unique advantages, but also comes with specific performance limitations. The microporous structure provides more active sites, but is not conducive to the transport of electrolytes and active substances. Macropores form the foundation and skeleton of the construction of conductive network and pore structure. They have large pore volume and transport channels, but low specific surface area and insufficient reactive sites. Enhancing EDLC performance relies on designing and creating multi-pore structures to increase the surface area of carbon materials^[19-22].

Biomass materials, with advantages such as being environment-friendly, sustainable, rich in heteroatoms, and cost-effective, have become the preferred choice for preparing EDLC electrode materials in recent years^[23-26]. So far, a large amount of plant- or animal-derived biomass materials have been used to produce electrode materials for SCs with excellent electrochemical properties^[26-28]. Ghosh et al. prepared AC from banana stem, corncob (CC) and potato starch by KOH activation or simple carbonization, in which the specific capacitance of KHC was about 225 F g^-1 at 5 A g^-1. In addition, they assembled KHC as a SC; after 6,000 cycles, the capacitance retention is 72.88%^[29]. Lin et al. used nitrogen-rich soybean as the precursor to prepare nitrogen-doped graded mesoporous micro carbon (ANPC) by coupling template method and KOH activation. They also studied its SC performance and REDOX ability^[30]. Finally, the material prepared by the mass ratio of KOH to carbon of 3:1 exhibits the largest specific surface area and the best performance, with a specific capacitance of 243.2 F g^-1 at 0.5 A g^-1. Furthermore, Jung et al. also proposed a method for preparing graded porous biomass-derived GH-based carbons by glucose-based polymers^[31]. They first transformed the glucose-based polymer into a three-dimensional (3D) foam-like structure, then carbonized it to form the corresponding structure, and finally activated the carbon by physical and chemical ways to produce micro and mesopores, resulting in layered porous biomass-derived GH-based carbon. The specific capacitance of this material is 175 F g^-1 at 1 A g^-1, and the assembled SC has a maximum energy density of 74 Wh kg^-1, corresponding to a maximum power density of 408 kW kg^-1. Despite the advancements in biomass materials for SCs, several challenges remain: (1) Limited specific surface area. For example, plant-derived biomass has an abundance of macroporous and mesoporous structures due to the presence of lignin and cellulose, but it rarely contains microporous structures, making it difficult for the material to provide more active sites; (2) Complex preparation process. Biomass, as a natural carbon source, contains a deficiency of heteroatoms in its original composition, which is typically used as a hard template to form hierarchical porous carbon structures through a complex heteroatom doping process; and (3) Uneven pore size distribution. Biomass, when used as a carbon-based material obtained through traditional carbonization, hydrothermal, or activation procedures, presents challenges in precisely regulating the pore size distribution on its surface. To further address these issues, we are using biomass and industrial wastes containing inorganics of various sizes as raw materials, providing a new perspective for the preparation of electrode materials for SCs with high surface area, reasonable porosity, and high performance.

CC, a lignocellulosic byproduct of corn threshing and refinement, is rich in cellulose, hemicellulose, and lignin, endowing it with desirable mechanical properties such as superior tensile strength, enhanced hygroscopicity, and exceptional abrasion resistance. As a biomass residue, CC has found applications across various industries, including abrasive polishing, desiccant materials, and nutritional feed additives, which positions it as a promising precursor for the large-scale synthesis of carbonaceous substrates^[32]. Moreover, the intrinsic carbohydrate and crude protein content within CC offers the potential for in situ heteroatom doping, specifically nitrogen and oxygen, within the carbon matrix. Fly ash (FA), a particulate byproduct of fossil fuel combustion, is composed primarily of inorganic oxides such as SiO₂, Al₂O₃, Fe₂O₃, and CaO, along with a residual carbon content^[33,34]. The fine particle size and smooth surface texture of FA necessitate its responsible management to prevent environmental contamination. Although FA has been repurposed in various applications, ranging from cementitious materials to agronomic fertilizers, as well as in wastewater treatment and industrial recycling, its potential in the synthesis of electrode materials for energy storage devices such as SCs is a rapidly emerging field of interest^[35].

In light of the above considerations and with a commitment to eco-friendly materials science, we have crafted an advanced strategy for the two-step hybrid conversion of materials. This process involves hydrothermal carbonization followed by activation to fabricate N/O co-doped, porous biomass-derived carbon. The hydrothermal carbonization phase promotes the selective decomposition of lignin and hemicellulose, thereby enhancing the mesoporosity and oxygenation of the carbon framework, which are crucial for improving electrochemical performance, particularly specific capacitance. This innovative strategy not only leverages the synergistic integration of these two waste streams but also introduces a groundbreaking method for fabricating advanced electrode materials for energy storage applications, highlighting the potential for valorizing industrial byproducts. The porous carbons derived from CC and FA, designated as CFCs, boast a 3D macroporous framework combined with microporous and mesoporous structures. They exhibit a high specific surface area of 1,768 m² g^-1, which marks an enhancement factor exceeding 70, thereby significantly amplifying the void fraction of the material. The specific capacitance of the sample in 6 M KOH electrolyte, measured using a three-electrode system, was as high as 240 F g^-1 (at 1 A g^-1). In addition, we found that the material has surprising lithium storage properties, with an initial energy density of 160 mAh g^-1 (at 0.1 A g^-1). After 100 cycles, the specific capacity remains at 50 mAh g^-1, with a coulomb efficiency of nearly 100%. The successful preparation of this material provides a new idea for energy recycling.

EXPERIMENTAL

RESULTS AND DISCUSSION

Figure 1 shows a schematic diagram depicting the preparation of CFC. Low-cost CC and industrial waste FA are used as a self-template and template, respectively. The precursors are obtained by hydrothermal reaction to remove the existing metal oxides and retain the microporous structure. Then, the microporous and mesoporous structures are realized by high temperature carbonization and activation of KOH under N₂ atmosphere.

Figure 1. A schematic illustration of CFC preparation by CC and FA.

Figure 2 shows the surface morphology of CC and FA before and after treatment respectively. As depicted in Figure 2A, untreated CC exhibits a substantial volume characterized by numerous large pores on its surface, approximately 25 μm in diameter. As can be further observed from the illustration, its surface is smooth with almost no pore structure. After being treated with hydrothermal and KOH activation [Figure 2B], the original pores on the CC surface disappear and are replaced by large pores. Through careful observation, it can be found that the size of these pores is similar to that of the original pores. It acts as a support and frame for the material. In addition, it can also be observed from the illustration that the surface of the CC at this time is still smooth, and there is only a large hole structure. Figure 2C shows FA without any treatment where it is clearly observed to consist of many balls and lumps of varying sizes. In order to explore its main components, the material content of FA is analyzed with the help of XRF technology [Supplementary Tables 1 and 2]. It can be inferred that in addition to unburned carbon, it also mainly contains silicon oxide, calcium oxide, iron oxide, alumina and some trace metal oxides. During the reaction process, these metal oxides become embedded in the CC surface and are subsequently removed through treatment to facilitate initial pore formation. As illustrated in Figure 2D, a limited number of mesopores and larger pores emerge on the surface of FA following hydrothermal and KOH activation treatments, with pore sizes ranging from 10 to 500 nm. This observation suggests that unburned carbon within FA can also interact with metal oxides to generate pore structures. However, given the restricted availability of carbon sources in FA, the energy conversion efficiency remains minimal.

Figure 2. SEM images of (A and B) Before and after processing of CC; (C and D) Before and after processing of FA.

Figure 3A and B illustrates the morphology of carbon foam composites (CFC) synthesized from the two materials which reveals two distinct morphologies: block and spherical forms within the CFC structure. Furthermore, they demonstrate that both morphological contain macroporous and mesoporous structures with pore sizes ranging from 10 to 500 nm, indicating a significant increase in pore quantity compared to treatments involving FA alone. The emergence and proliferation of these pores suggest that porous carbon is successfully synthesized through the synergistic interaction between CC and FA, resulting in a novel morphology characterized by porous carbon microspheres.

Figure 3. (A and B) SEM images of CFC at different magnifications; (C and D) TEM images of CFC.

To investigate the co-pore formation mechanism between CC and FA and the origin of porous carbon microspheres, we delve into an extensive examination of their preparation process. Supplementary Figure 1 presents the micro-morphology of CC, FA, and CFC precursors following hydrothermal treatment. As observed in Supplementary Figure 1A, numerous carbon microspheres are generated on the surface of CC after water heating. These microspheres exhibit smooth surfaces but display uneven diameters; some appear to be dissolving. Consequently, it can be inferred that these carbon microspheres may gradually detach from larger blocks or spheres over time. Subsequently, controlling hydrothermal duration allows for regulation of the size of these carbon microspheres. Supplementary Figure 1B and C presents scanning electron microscopy images of FA following hydrothermal treatment at varying magnifications. It is evident that the aforementioned phenomenon does not manifest in FA post-hydrothermal reaction; however, under high magnification, irregularly sized pores are observable on the material's surface. This observation supports the hypothesis that metal oxides within FA interact with unburned carbon during hydrothermal processing to create these voids. In contrast, the morphology of the CFC precursor exhibits slight differences compared to the two previously discussed materials [Supplementary Figure 1D and E]. Notably, this material displays both block structures and carbon spheres derived from those blocks simultaneously. As illustrated in Supplementary Figure 1D, the surface of carbon microspheres in this sample is not smooth but rather features numerous fine particles. Based on prior analyses regarding CC hydrothermal behavior, it can be inferred that these carbon microspheres primarily originate from direct fragmentation of larger blocks or by division of individual microspheres into smaller ones; thus, it is reasonable to conclude that these minute particles result from contributions made by FA. Furthermore, high-magnification images [Supplementary Figure 1E] reveal a limited presence of carbon microspheres on the surface of corn pellets while indicating significant preliminary pore formation effects as well. Overall, the porous structure development within CFC predominantly arises from metal oxide intercalation found in FA; concurrently, some remain adhered to the carbon surfaces as fine particles. Following activation and high-temperature carbonization processes, these particles may either be removed or etched away to yield porous characteristics on the underlying carbon substrate. Additionally, CC - serving as biomass-derived carbon - produces a substantial quantity of carbon microspheres during hydrothermal treatment which enriches material architecture and provides critical value for subsequent strategies aimed at preparing porous carbon microsphere structures.

Figure 3C and D shows TEM images of CFC. It can be clearly seen that the material is amorphous carbon and has mesoporous structure with a diameter of about 10-50 nm. From the marked area and the inset part, it can be observed that there are some bright spots of light transmission on the material, which we speculate may be microporous structures. The existence of these microporous structures may be attributed to two factors: on the one hand, the rich element composition inside the CC will react into gas and escape at high temperatures, and on the other hand, CC, as a plant, originally has many micropores. In summary, we successfully prepared CFC porous biomass carbon with abundant macroporous, mesoporous and microporous structures through the synergistic effect of CC and FA. The existence of these pores plays a positive role in electrolyte transport and the active site of electrochemical reaction, which can effectively improve the electrochemical performance of electrode materials.

The elemental composition and chemical bonds of the CFC are further analyzed through the XPS spectra presented in Figure 4. First, the C 1s peak of CFC can be divided into five main components, C=C (284.3 eV), C-C (284.9 eV), C-N (286.6 eV) and C-O/C=N (288.2 eV), and the peak at 294.6 eV is characterized by π-π oscillation^[42-44]. As can be seen from Figure 4B, the N 1s XPS spectrum of CFC consists of four peaks - quinoid amine (=N-, 398.6 eV), pyrrole -N (399.6 eV), benzene (-NH-, 400.5 eV), and nitrogen cation radical (N⁺,401.8 eV) - with azane type (402.3 eV) at the center^[23,45,46]. The presence of the azane type indicates a cross-linking structure. The combination of nitrogen functionalities such as pyrrole, pyridine, and graphitic nitrogen boosts the pseudocapacitance characteristics of the electrode, thus enhancing its energy storage capability. Moreover, nitrogen doping improves the wettability of the electrode material, facilitating a more efficient interaction with electrolyte ions. This outstanding interface lowers the equivalent series resistance (ESR), promotes charge transfer kinetics, and eventually raises the power density and accelerates the charge and discharge kinetics. Additionally, nitrogen doping is capable of enhancing the conductivity of the carbon substrate. The insertion of nitrogen atoms disrupts the sp² carbon matrix, generating additional defects that make electron transfer more effortless. This increase in conductivity leads to an enhanced rate capability and overall superior performance of SCs^[36,37]. Finally, the O 1s peak, as seen in Figure 4C, can be deconvoluted into three characteristic peaks: C=O (531.4 eV), C-O (532.4 eV) and -COOH (533.2 eV), respectively^[47-49]. The existence of oxygen-containing groups can enhance the hydrophilicity, allowing the ions in the water system electrolyte to permeate the material more easily. The Raman spectra for CC, FA, and CFC are depicted in the Supplementary Figure 2. Their I(D)/I(G) ratios are 1.03, 1.01, and 0.99, respectively, indicating that CFC exhibits a greater degree of graphitization. A sharp peak at 2,949 cm^-1 is observed in all three materials, signifying the stretching vibration of the O-H bond. Notably, FA possesses a distinctive 2D peak characteristic of GH at 2,549 cm^-1.

Figure 4. XPS deconvoluted spectrum of (A) C 1s, (B) N 1s and (C) O 1s of CFC.

In order to investigate the specific surface area and pore size distribution of the material, nitrogen adsorption/desorption test was carried out. First of all, it can be seen from the above SEM photos that the surfaces of CC and FA before treatment are smooth, and the existence of holes is almost invisible. Since their nitrogen absorption and desorption curves are irregular and do not close, and their specific surface areas are less than 1 m² g^-1, the test results will not be shown or detailed. Supplementary Figure 3 shows the nitrogen absorption and desorption of the three materials after hydrothermal treatment. It can be seen that the absorption and desorption curves of the three materials are all type IV isotherms^[50,51]. The failure of the absorption and desorption curves of the CC in Supplementary Figure 3A may be caused by the small specific surface area of the material itself and the insufficient amount of test samples. There is no obvious rapid rise of adsorption lines in the low-pressure region, indicating almost no microporous structure in the three curves. Platforms and hysteresis loops appear in the middle pressure region and the high-pressure region, indicating that they have mesoporous and macroporous structures. In addition, their specific surface area is slightly increased compared with that before treatment, while the specific surface area of FA and mixture is larger, indicating that the addition of FA plays a positive role in the formation of pores, and the preliminary pore-making begins to take effect.

The sample adsorption and desorption curves after activation and carbonization are shown in Figure 5. It can be seen from Figure 5A that the adsorption-desorption isotherm of CC after complete porosity is a type I isotherm^[52,53], and there is a steep long platform in this curve, reflecting the reversible adsorption process of a single layer, indicating that the material has a microporous structure. Therefore, it can be concluded that the simple CC porosity can only have a microporous structure, but cannot achieve multi-layer porous carbon structure.

Figure 5. (A-C) Nitrogen adsorption/desorption isotherms and pore size distribution diagram of CC, FAC, and CFC.

Figure 5C shows that the adsorption-desorption isotherm of CFC exhibits a type IV mesoporous characteristic curve; when P/P₀ < 0.01, the adsorption line obviously rises rapidly, indicating the existence of micropores in CFC. Further adsorption of N₂ reveals a plateau in the adsorption curve where P/P₀ is between 0.2-0.5. With the increase of P/P₀ from 0.5 to 0.9, the adsorption line gradually rises, forming a H2 hysteresis loop related to capillary condensation, which manifests the characteristics of mesoporous surface. In the range of P/P₀ = 0.9-1.0, the adsorption line continues to rise, and does not show an obvious adsorption plateau as it approaches 1.0, exhibiting an H3 hysteresis loop, which indicates the existence of macropore distribution. In addition, the pore size distribution diagram in the illustration shows that the pore size of CFC is mostly concentrated in the ranges of 1.5-1.3, 3.5-4 and 10-15 nm, indicating a combination of microporous, mesoporous and mesoporous structures^[54,55].

The same phenomenon can be found in the nitrogen adsorption/desorption curve of FAC [Figure 5B], but the sample has a small distribution of large pores, with a specific surface area of only one-third that of CFC, possibly due to insufficient carbon sources in FA. The above results show that the core-making of FA and CC can greatly increase the specific surface area of carbon materials. On the one hand, CC, as a biomass material, provides a large number of microporous and macroporous channels and a stable carbon source. On the other hand, the macromolecules and unburned carbon in FA itself can be used as templates and self-templates to generate mesoporous structures.

In Figure 6A, the CV plot of the CFC measured by 6 M KOH electrolyte in the three-electrode system is plotted. At negative potential (-0.6-0 V), the curve appears a plateau, indicating that the electrochemical reaction of the CFC at low potential is dominated by charge absorption and desorption, which shows a double-layer capacitance behavior. It can complete the transmission and diffusion of electrons and ions within a short time, manifesting that it has good capacitance characteristics. As the potential increases, the curve reveals the emergence of REDOX peaks within the range of 0.6-1.3 V, indicative of pseudocapacitive behavior. These peaks are attributed to the reversible REDOX reactions of the functional groups doped with heteroatoms during the charge and discharge processes. The XPS spectrum reveals the presence of various nitrogen-containing functional groups within the CFC, including quinoid amine, pyrrole-N, benzene-NH-, and nitrogen cation radicals. The integration of these functional groups contributes to the enhancement of the electrode’s pseudocapacitive characteristics, thereby augmenting its energy storage capacity. At different scanning speeds (1-8 A g^-1), the shape of each curve is similar, the response current increases with the rise of scanning speed, and the REDOX peak position rarely has displacement, which indicates that the electrode material has good cycle reversible stability and fast REDOX current response. This phenomenon can also be easily seen from the CV curves of CC and FAC materials in Supplementary Figure 4A and B. It can be found from Figure 6B that the CV area and response current of CFC prepared by CC and FA are larger than those of the other two materials prepared by monomers; meanwhile, a pair of REDOX peaks possessed by CC (at 0.6 and 1.1 V approximately) and FAC (at 0.8 and 0.9 V approximately) are reflected in CFC respectively. It is worth noting that we can also observe that the potential range of CFC is -0.6-1.3 V, and the potential difference reaches 1.9 V, which is relatively scarce in aqueous electrolytes. Figure 6C, Supplementary Figure 4C and D illustrate that the charge-discharge profiles of the various samples at distinct current densities (1-8 A g^-1) exhibit favorable rate capabilities. Notably, CFC demonstrates the longest discharge duration at a current density of 1 A g^-1. Figure 6D visually depicts that the specific capacitance of CFC is markedly superior to that of CC and FAC across different current densities, with CFC achieving a specific capacitance of 240 F g^-1 at 1 A g^-1. This suggests that the CFC, prepared through the collaborative efforts of CC and FA, possesses a greater specific surface area and a higher number of functional groups, enabling a greater number of active sites to be utilized on the surface of the CFC. Nevertheless, the specific capacitance of the CFC diminishes progressively as the current density increases, which we attribute to the possibility that the REDOX reactions and charge transfer rates of the active material may not suffice to accommodate the abrupt rise in current. The collective results of these tests underscore the outstanding performance of CFC, which also compares favorably with several previously reported materials in the literature [Table 1].

Figure 6. (A) CV curves at different scanning speeds of CFC, (B) CV curve of CFC, CC and FAC with a scanning speed of 0.1 V s^-1, (C) the constant current charge and discharge curve at different current densities of CFC, and (D) the specific capacitance of CFC, CC and FAC.

Except for the research on the electrical properties for SC of CFC, we also assemble a half-cell using the lithium sheet as the counter electrode to test its lithium storage performance in a lithium electrolyte. Figure 7A is the CV map of CFC measured at different sweep speeds under the voltage window of 1.5-4 V. All curves are roughly symmetrical and rectangular, which is a typical double-layer capacitance characteristic, indicating that the main occurrence of the material is the adsorption and desorption of lithium ions on the material surface. The rate characteristics of CFC are recorded in Figure 7B. When the current density increases to 2 A g^-1, the capacity is maintained at 53.2%. After switching back to 0.1 A g^-1, the reversible capacity is maintained at 67.7%, showing good rate performance. Figure 7C shows the cycle performance of the half-cell at 0.1 A g^-1. The initial capacity is about 160 mAh g^-1. Although there is significant attenuation in the first 10 cycles, the capacity gradually rises and basically remains stable thereafter. After 100 charging and discharging cycles, the specific capacity stabilizes at 50 mAh g^-1, demonstrating excellent cycle stability. The aforementioned results suggest that the materials also possess lithium storage properties, but the lithium storage mechanism and performance enhancement still require further investigation.

Figure 7. (A) CV curves at different scanning speeds of CFC half-cell, (B) magnification performance of CFC half-cell, (C) cycle performance of CFC half-cell.

CONCLUSIONS

In this study, biomass carbon (CC) and industrial waste (FA) are successfully employed together for the preparation of CFC through the hydrothermal method and KOH activation, thereby achieving waste recycling. The results showed that the participation of FA effectively increased the mesoporous ratio of the material and thus enriched the specific surface area of the material. Simultaneously, a considerable number of carbon microspheres generated in the hydrothermal process of biomass CC enriched the microstructure of the electrode material and increased the heteroatom doping site. In the three-electrode test, CFC shows a maximum specific capacitance of 240 F g^-1 (at 1 A g^-1), and even at 8 A g^-1, the specific capacitance can reach over 100 F g^-1, with good reversibility and magnification performance. In addition, CFC also displays excellent lithium storage capacity. When the current density is 0.1 A g^-1, the first discharge specific capacity of the assembled Li|CFC capacitor is about 160 mAh g^-1, and the specific capacity can be maintained at 50 mAh g^-1 after 100 cycles. This strategy leverages the synergistic action between different materials to prepare porous carbon, offering a new idea for increasing the specific surface area of materials and comprehensively improving electrochemical properties.