Interface Engineering of Biomass‐Derived Carbon used as Ultrahigh‐Energy‐Density and Practical Mass‐Loading Supercapacitor Electrodes

The development of flexible electrodes with high mass loading and efficient electron/ion transport is of great significance but still remains the challenge of innovating suitable electrode structures for high energy density application. Herein, for the first time, lignosulfonate‐derived N/S‐co‐doped graphene‐like carbon is in situ formed within an interface engineered cellulose textile through a sacrificial template method. Both experimental and theoretical calculations disclose that the formed pomegranate‐like structure with continuous conductive pathways and porous characteristics allows sufficient ion/electron transport throughout the entire structures. As a result, the obtained flexible electrode delivers a remarkable integrated capacitance of 6534 mF cm−2 (335.1 F g−1) and a superior stability at an industrially applicable mass loading of 19.5 mg cm−2. A pseudocapacitive cathode with ultrahigh capacitance of 7000 mF cm−2 can also be obtained based on the same electrode structure engineering. The as‐assembled asymmetric supercapacitor achieves a high areal capacitance of 3625 mF cm−2, and a maximum energy density of 1.06 mWh cm−2, outperforms most of other reported high‐loading supercapacitors. This synthesis method and structural engineering strategy can provide materials design concepts and a wide range of applications in the fields of energy storage beyond supercapacitors.


Introduction
Currently, long standby and fast charging wearable energy storage devices with high energy and power density are urgently needed driving by the expansion of portable electronics market. Flexible supercapacitors (FSCs) stand out by virtue of their high power density and long lifespan. However, the low energy density limits their wide-range applications. Aiming to develop high-performance FSCs, exploring novel active materials and electrode structures are two feasible strategies. [1] In view of active materials development, heteroatom-doped graphene-like carbon materials stand out from conductive polymers, metal oxides, and other carbon materials due to their 2D plane structures, high surface area, good electrical conductivity, and stability. Moreover, heteroatom-doping can greatly boost the electrochemical properties of graphene-like carbon materials owing to the change of electronic charge density and asymmetric spin density for additional redox reactions. [2] For example, nitrogen (N)-doping with the characteristics of electron donors can increase charge mobility and provide extra electrochemically active sites for carbon materials. [3] Sulfur (S) with electron-rich environment is also regarded as a promising doping element in the carbon matrix to improve capacitive performance and electrical conductivity due to the enhanced polarization and the overlap of p-orbitals between S atoms and sp 2 -hybridized carbon atoms. [4] By taking advantage of the synergistic effect between the 2D structure and heteroatom-doping, a large number of different heteroatom-doped graphene-like carbon materials have been developed, but they normally suffered from the inherent limitations of expensive precursors and complex synthesis procedures, such as hard-template methods, freeze-drying, as well as long time post-treatment, which severely limited their large-scale production and practical applications. [5][6][7][8][9][10][11] Therefore, from a perspective of economical and sustainable manufacture, reasonable design of heteroatomdoped graphene-like carbon materials via efficient synthesis processes together with low-cost renewable precursors will be more attractive for future large-scale energy storage applications.
In addition to developing high-performance active materials, realizing high-mass-loading electrodes is also a promising route to improve the total energy density through increasing active material loading ratios and offsetting the negative contribution of inactive components in devices. [12] Li et al. reported a surface-functionalized 3D-printed graphene aerogel electrode, which showed a remarkable energy density of 0.65 mWh cm −2 at a mass loading of 12.8 mg cm −2 . [13] Pan et al. demonstrated the alternate deposition of carbon nanocoils and poly(3,4-ethy lenedioxythiophene):poly(styrenesulfonate), the obtained electrode achieved a high energy density of 0.22 mWh cm −2 at a mass loading of 15 mg cm −2 . [1] Actually, the active mass loading is usually no less than 10 mg cm −2 for practical applications. [14] Nevertheless, increasing mass loading over the practical level generally results in severe degradation of the capacity and poor mechanical stability of electrodes because of sluggish electron/ ion transport kinetics caused by enlarged electrical resistance, reduced accessible surface area, and blocked ion transport pathways. Recent studies have demonstrated that aligning internal constructions through ice-template method and creating available pore channels by 3D-printing can effectively regulate the ion transport. [15][16][17] Although greatly improved performance has been achieved for high mass loading electrodes, these methods can only be valid for specific materials and special equipment, which are not suitable for flexible electrodes. Therefore, achieving high-mass-loading flexible electrodes with sufficient electron/ion transport is of great significance but still remains challenging, which highlights the necessities of electrode structure engineering strategies.
Herein, active materials exploitation coupled with electrode structure engineering were proposed to fabricate highperformance FSCs. First, we demonstrate the successful synthesis of N/S-co-doped graphene-like carbon with porous structures through a facile one-pot sacrificial template method using urea and lignosulfonate as mixture precursors. Lignosulfonate accounts for the majority of lignin derivatives with total annual worldwide production of 1.8 million tons. [18] Aromatic nature and sulfur-containing sulfonate group make lignosulfonate the most promising renewable precursor for nanocarbon materials. As a low-cost nitrogen source, urea can decompose and polymerize into g-C 3 N 4 under heating, in situ forming a 2D porous template. Importantly, g-C 3 N 4 decomposes completely at a higher temperature, thereby eliminating the requirement of further removal of the template. To achieve high-mass-loading flexible electrodes, an active nitrogen-doped carbon interface with high porosity is built on cellulose textile according to our previous works. [19] Then, the interface is served as a porous framework to accommodate N/S-co-doped graphene-like carbon, in which intertwined N/S-co-doped graphene-like carbon is anchored onto the nitrogen-doped carbon nanoparticles, forming a pomegranate-like structure for efficient electron and mass transfer. Moreover, the external hierarchical structure and interior void space of the interface provide sufficient space to achieve high mass loading while maintain electrode porosity for electrolyte migration. As a result, highloading flexible electrodes with ultrahigh capacitance and FSCs with excellent energy density can be achieved, which are much higher than all other carbonaceous FSCs reported to date.

Synthesis of N/S-Co-Doped Graphene-Like Carbon
N/S-co-doped graphene-like carbon was synthesized using the g-C 3 N 4 sacrificial template method by pyrolyzing urea and lignosulfonate mixed precursors (Figure 1a). To verify the feasibility of the sacrifice template method, the pyrolysis behavior of different precursors was tracked by thermogravimetric analysis (TGA). As a precursor for nitrogen, urea shows the first rapid weight loss at 227 °C originate from the decomposition of urea into cyanuric acid. Then the cyanuric acid was further transformed into melamine at ≈350 °C. The formation of g-C 3 N 4 was reached at 402 °C through the polycondensation of melamine (Figure 2a; Figure S1, Supporting Information). [20,21] The obtained product exhibits a 2D nanosheet structure with curled edges (Figures 2d and S2, Supporting Information). These sheets are thin and abundant in-plane nanopores are well-defined ( Figure 2e). As shown in X-ray diffraction patterns, two diffraction peaks at ≈13.2 o and 27.2 o are observed, which is consistent with the structure of g-C 3 N 4 composed of ordered tri-s-triazine sub-units connected through planar tertiary amino groups in a layer ( Figure S3, Supporting Information). [22] Importantly, it decomposes completely at the temperature above 600 °C, thereby eliminating the template removal process when used as a porous 2D-template ( Figure 2a).
The mixed urea and lignosulfonate show a similar pattern to that of urea but higher mass residue, indicating the formation and decomposition of g-C 3 N 4 within mixture. The in situ constructed g-C 3 N 4 provides a porous 2D-template, which subsequently guides the evolution of lignosulfonate during pyrolysis. The as-prepared materials possess an ultrathin 2D graphenelike structure with wrinkled surfaces as revealed by scanning electron microscopy (SEM) images (Figure 2f,g; Figure S4, Supporting Information). The morphology and nanostructures of the graphene-like materials were further characterized by transmission electron microscopy (TEM). As shown in Figure 2h,i, the graphene-like materials feature a transparent veil-like structure, while an abundance of in-plane nanopores are observed. On the contrary, heating the lignosulfonate displays a high mass residue and only produces a thick bulk structure, further indicating the critical role of the template ( Figure S5a, Supporting Information). The elemental composition in 2D graphene-like materials was investigated by X-ray photoelectron spectroscopy ( Figure S6a, Supporting Information). From the high-resolution N 1s spectra (11.49%), the peaks at 398.2, 399.3, 400.7, and 405.6 eV are corresponding to pyridinic N, pyrrolic N, graphitic N, and oxidized N, respectively ( Figure 2b). [23] Similarly, the highresolution S 2p spectra (1.73%) can deconvolute into CSC, C≐S, and oxidized sulfur groups (CSO x S) (Figure 2c). [24] The corresponding EDX mapping images also show a uniform distribution of C, N, and S elements, further indicating the existence of N and S elements ( Figure S6b-d, Supporting Information). Therefore, N/S-co-doped graphene-like carbon was successfully synthesized using this facile sacrificial template method. Interestingly, the morphology of the N/S-co-doped carbon can be controlled from 2D graphene-like structure to one-dimensional nanoribbons by varying the ratio between lignosulfonate and urea ( Figure S5b,c, Supporting Information).

Synthesis of Pomegranate-Like High-Mass-Loading Textile Electrode
With the aim of achieving high-mass-loading flexible electrodes with sufficient electron/ion transport, cellulose textile electrodes with N/S-co-doped graphene-like carbon intertwined N-doped carbon nanoparticles were designed. The design concept and electrode structures were examined by SEM and TEM. The cellulose textile (Cell) was chosen as the flexible substrate due to its excellent mechanical properties and wearability, which mainly consists of micron-sized carbon fibers with a smooth surface (Figure 3a,d). After in situ polydopamine modification, a hierarchical interface consisting of N-doped carbon nanoparticles with graphitized structure is coated on the fiber surface ((DA) n -Cell) (Figure 3b,e,g). Subsequently, this porous N-doped interface was used as a 3D framework to grow N/S-codoped graphene-like carbon.
To demonstrate the feasibility of the in situ growth of N/S-codoped graphene-like carbon within the interface, the pyrolysis behavior of different precursors was tracked by SEM images. Urea exhibits a thermally stimulated decomposition-polymerization process that gradually evolves toward a network structure within the interface (Figures S7a-e and S8a-e, Supporting Information). Lignosulfonate produces a large number of massive residues that block pores and destroy the porous character of the interface (Figures S7f-j and S8f-j, Supporting Information). Interestingly, mixed urea and lignosulfonate also gradually evolve toward a network structure within the interface due to the sacrificial template developed from urea ( Figures S7k-o and S8k-o, Supporting Information). As a result, N/S-co-doped graphene-like carbon was successfully grown within the interface, in which a graphene-like carbon network is anchored onto the nitrogen-doped carbon nanoparticles, forming a pomegranate-like structure (Figure 3f,i,h; Figures S9 and S10, Supporting Information). Among this unique pomegranate-like structure, N/S-co-doped graphene-like carbon network with high conductivity provides continuous conductive pathways for efficient electron transport. The porous N-doped interface provides sufficient room to achieve high mass loading of N/S-codoped graphene-like carbon while maintaining electrolyte ions migration. Benefitting from the synergistic effect of the novel graphene-like carbon and unique electrode structure, high electrochemical performance with excellent electron transfer and rapid ion diffusion are predictable for the high mass-loading flexible textile electrodes ( Figure S11, Supporting Information).

Electrochemical Characterizations and Theoretical Calculations
The electrochemical performance of textile anodes was evaluated in a three-electrode configuration using 1 m H 2 SO 4 as the electrolyte, a Hg/Hg 2 SO 4 , and a Pt wire as the reference and the counter electrodes, respectively. H 2 SO 4 (1 m) was chosen as the electrolyte due to it is favorable for fast electrochemical kinetics and robust redox reactions. After scrutinizing the effect of the ratio of urea to lignosulfonate on the electrochemical performances ( Figure S12, Supporting Information), we chose NS 2 -(DA) n -Cell electrode for further discussion. The NS-(DA) n -Cell electrode shows significantly increased area and discharge time, suggesting a substantially improved charge-storage capability (Figure 4a,b). In addition, the nearly rectangular cyclic voltammetry (CV) curve and symmetrical galvanostatic chargedischarge (GCD) curve of the NS-(DA) n -Cell electrode reflect the rapid electron and ion transport. According to electrochemical impedance spectroscopy (EIS), the NS-(DA) n -Cell electrode shows the smallest intercept with a horizontal axis and has no obvious semicircle in the high-frequency region (Figure 4c). To quantitatively analyze the value of the resistance, we fitted the EIS curves with an equivalent electric circuit. Compared with other electrodes, NS-(DA) n -Cell electrode delivers the smallest equivalent series resistance (R s , 2.06 Ω) and charge-transfer resistance (R ct , 0.08 Ω), indicating fast charge transport due to the pomegranate-like structure with continuous highly conductive pathways ( Figure S13, Supporting Information). [25] To gain insight into the ion diffusion process, the dependence of the impedance (Z') on the reciprocal of the square root of frequency (ω −0.5 ) was performed, where the values of σ were extracted from the slopes of the linear fitting lines representing ion diffusion resistance. [26] The hydrophilic N-doped interface with high porosity and porous N/S-co-doped graphene-like carbon enhances the electrolyte affinity and provide a large number of ion transport channels, which enable NS-(DA) n -Cell electrode the smallest ion diffusion resistance even at high mass loading (σ, Figure 4d).
The kinetic analysis was further performed to evaluate the electrochemical behavior. The current density of a supercapacitor, i, scales with the scan rate, ν, follows the relationship of i = kv b . The b is an important metric to evaluate the charge-storage kinetics, and b = 1 corresponds to an ideal supercapacitor with ultra-fast kinetics. [27] The b value of the NS-(DA) n -Cell electrode was calculated to be 0.92 in the scanrate range of 1-10 mV s −1 , approaching to that of an ideal capacitor and suggesting the ultra-fast kinetics (Figure 4e; Figure S14a,b, Supporting Information). We further decoupled the capacitance from surface-controlled contribution (fastkinetics) and diffusion-controlled contribution (slow-kinetics; Figure S14c,d, Supporting Information). [28] It is found that the capacitive contribution of the NS-(DA) n -Cell electrode dominates the capacitances at all scan rates (Figure 4f; Figure S14e, Supporting Information). By taking the advantage of the fast electrochemical kinetics and excellent electron/ion transport at high mass loading, the NS-(DA) n -Cell electrode displays an excellent integrated performance ( Figure S15, Supporting Information). Significantly, it exhibits a ultrahigh areal capacitance of 6 534 mF cm −2 (154.5 F cm −3 ), which is much higher than all other carbonaceous electrodes reported to date (Figure 4h; Figure S16c, Supporting Information). [1,13,[29][30][31][32][33][34][35][36][37][38] The corresponding gravimetric capacitance based on the total mass of the N/S-co-doped graphene-like carbon and N-doped carbon nanoparticles is calculated to be 335.1 F g −1 even at high areal mass loading (19.5 mg cm −2 ). Besides the remarkable capacitance, the electrode shows excellent cycling stability and high Coulombic efficiency after cycling for 10 000 charge-discharge cycles at a large current density of 15 mA cm −2 (Figure 4g; Figure S16a,b, Supporting Information). More importantly, a pseudocapacitive PANI-(DA) n -Cell cathode also achieves ultrahigh areal capacitance of 7000 mF cm −2 (386.7 F g −1 , 180.4 F cm −3 ) based on the same structure design, showing the versatility and applicability of this method (Figures S17-S23, Supporting Information).
To disclose the underlying mechanism of the remarkable electrochemical performance of the NS-(DA) n -Cell anode, theoretical calculations were conducted. As mentioned earlier, achieving sufficient electron/ion transport at high mass loading is of great significance but still challenging for high-performance FSCs. From the electrolyte ion diffusion aspect, the external hierarchical structure and interior void space of the interface deliver large surface area (386.5 m 2 g −1 ) and high porosity, which provide sufficient space to achieve high mass loading of N/S-co-doped graphene-like carbon while maintain electrode porosity for the electrolyte migration (344.3 m 2 g −1 ; Figure S24, Supporting Information). On the contrary, the carbonized Cell substrate without interface modification can only show a very small specific surface area of 47.5 m 2 g −1 (Figures S24 and S25, Supporting Information). In order investigate electrolyte ion transport within the electrodes, finite element analysis with COMSOL multiphysics simulation was performed. [39,40] For the NS-(DA) n -Cell electrode, the model composed of 300-400 nm nanoparticles with several to tens nanometers channels was constructed based on the characterizations from SEM and BET. As shown in Figure 5b, such porous structure provides abundant pathways for electrolyte ions to diffuse through the electrode, leading to a low concentration gradient along the electrode depth direction. On the contrary, the electrode without interface modification exhibits densely packed layer, which leads to a slow electrolyte ions diffusion rate and electrolyte ions only accumulate at the surface of the electrode (Figure 5c). According to the water contact angle measurement, electrolyte droplet is rapidly absorbed when in contact with NS-(DA) n -Cell electrode, indicating high electrolyte affinity and wettability by virtue of heteroatom-doped surface chemistry, which further accelerates electrolyte ions transport ( Figure S26, Supporting Information). [41] Therefore, this unique porous pomegranatelike structure with super hydrophilic surface can guarantee sufficient electrolyte ion transport at high mass loading.
Further, density functional theory calculations were conducted to understand electron transport process in detail. It is well known that doping carbon with heteroatoms of different electronegativities can tailor the electron donor-acceptor behavior and thus alter the electronic structure. [42] This can be directly observed from the electron density differences map, where the charge accumulates around the N, S doped sites   Figure 5d). The densities of states for the pristine graphic carbon and N/S-co-doped graphene-like carbon are shown in Figure 5e,f. Compared with pristine graphic carbon, the Fermi level in the N/S-co-doped graphene-like carbon shifts right up to the conduction band, which is favorable for enhancing the electronic conductivity. [43] Therefore, N/S-co-doped graphenelike carbon network within the NS-(DA) n -Cell electrode provides continuous highly conductive pathways for efficient electron transport. Moreover, the adsorption energy for the H + is 1.42 eV. After S, N co-doping, the adsorption energies of H + on the sulfur and nitrogen sites in the N/S-co-doped graphenelike carbon are 0.60 and −1.39 eV, respectively, indicating significantly enhanced H + adsorption capability (Figure 5g-i; Figure S27, Supporting Information). In conclusion, the sufficient ion transport ability guaranteed by unique porous pomegranate-like structure, the improved electronic conductivity and the enhanced H + adsorption capability facilitated by N, S co-doping contribute to the remarkable electrochemical performance of the NS-(DA) n -Cell anode.
An asymmetric FSC was assembled by pairing the NS-(DA) n -Cell anode with the PANI-(DA) n -Cell cathode in 1 m H 2 SO 4 electrolyte. The asymmetric FSC can combine different positive and negative electrodes operating in an opposite potential window to extend the device operating voltage to 1.6 V ( Figure S28, Supporting Information). All CV curves exhibit a nearly rectangular shape even at a high scan rate of 200 mV s −1 (Figure 6b). The GCD curves exhibit a nearly symmetrical shape, and the Nyquist plot shows a low R s and a small R ct , suggesting that the asymmetric device has an excellent reversibility and a fast charge transfer ability (Figures 6c,d). The asymmetric device exhibits an extraordinarily high areal capacitance of 3625 mF cm −2 (96.4 F g −1 , 37.5 mF cm −3 ) at 10 mV s −1 (Figure 6e). Even the scan rate is increased by 20 times (200 mV s −1 ), a high areal capacitance of 1 307 mF cm −2 can be still retained, indicating a moderate rate performance. The CV curves show almost the same profile at different bending states, confirming the good flexibility of the asymmetric device ( Figures S29 and S30, Supporting Information). Moreover, capacitance retention of 89.7% can be achieved after 3000 cycles, representing an excellent cycling stability ( Figure S31, Supporting Information). Significantly, the asymmetric device yields excellent energy density of 1.06 mWh cm −2 and power density of 21 mW cm −2 . As a result, our FSCs yield remarkably integrated areal capacitance, energy density, and power density by taking advantage of the strategy of active materials exploitation coupled with electrode structure engineering, surpassing most of the other reported supercapacitors (Figure 6f,g). [1,13,29,30,[44][45][46][47][48][49][50][51][52][53][54]

Conclusion
In summary, an effective strategy of active materials exploitation coupled with electrode structure engineering was proposed to fabricate high-loading FSCs. N/S-co-doped graphene-like carbon was first synthesized through a facile one-pot sacrificial template method using low-cost urea and renewable lignosulfonate as mixed precursors. For electrode structure engineering, an active nitrogen-doped carbon interface with high electrical conductivity and high porosity was built on cellulose textile to accommodate N/S-co-doped graphene-like carbon, forming a pomegranate-like structure. According to experimental and theoretical calculations, such a unique structure allows sufficient ion/electron transport and therefore enables the electrode fast electrochemical kinetics at high mass loading. By taking the advantage of the synergy of N/S-co-doped graphene-like carbon and pomegranate-like structure, the NS-(DA) n -Cell anode achieves remarkable integrated capacitance of 6534 mF cm −2 (335.1 F g −1 ) at a high mass loading of 19.5 mg cm −2 . Benefitting from this versatile electrode structure engineering, a pseudocapacitive cathode with ultrahigh capacitance of 7000 mF cm −2 can also be obtained. The assembled asymmetric supercapacitor delivers a high areal capacitance of 3625 mF cm −2 , and a maximum energy density of 1.19mWh cm −2 , substantially higher than other reported FSCs.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.