Currently, carbon materials are the promising electrode materials in supercapacitors due to its outstanding electrical conductivity, high surface area, extraordinary electrochemical stability and low cost^[1,2,3]. Nevertheless, the specific capacitance of commercial activated carbon is about 200 F·g^-1, falling short of the practical applications^[4]. Heteroatom doping (e.g., N, P and S) is an efficient approach to improve the capacitance^[5,6]. Among different N doping types, pyridinic nitrogen (N-6) and pyrrolic nitrogen (N-5) are effectively active sites, which can provide much pseudocapacitance. Hence, how to achieve high active nitrogen content in carbon materials is the key issue for next generation supercapacitors.

Two methods are proposed in recent works. One common way is to lower the pyrolysis temperature at the cost of conductivity loss^[7,8]. Generally, low-temperature pyrolysis leads to high nitrogen content in the carbon materials, because the increasing sintering temperature enhances the graphitization degree, reduces the defects and edge sites. The other approach is to dope phosphorus to adjust the active N proportion but not total nitrogen content^[9,10]. Previous work suggests that P-doping could result in high distortion of carbon structures, consequently producing defects and open edge sites in the N-doped carbon materials^[10]. The defects and edge sites in the carbon materials play important roles in achieving a high proportion of pyridinic-N. Therefore, the construction of N-rich carbon materials with high electrical conductivity is extremely challenging.

Herein, we designed the N-rich carbon material by introducing both Si-O-Si network and aluminum oxide at 1000 ℃ for supercapacitors. The interaction between Si-O-Si network and aluminum oxide can promote the activity of oxygen atoms in oxides, making O in oxides bond with N, main active N in carbon, which is beneficial for achieving high nitrogen content. Additionally, the Si-O-Si network and aluminum oxide as templates also empower the carbon material with large pore volume, 3D interconnected and hierarchical porous structure. Thus, these features lead to the excellent performance for supercapacitors with this N-rich carbon material.

1.1.1 Preparation of precursors

All reagents were of analytical grade and used without any purification. In a typical synthesis, 2 g of triblock copolymer polyethylene oxide-polypropylene oxide- polyethylene oxide (PEO-PPO-PEO) (P-123) as the surfactant, 1 mL of pyrrole as the nitrogen and carbon source, 3 g of Al(NO₃)₃·9H₂O or 4 mL of tetraethyl orthosilicate (TEOS) was added, respectively, or both Al(NO₃)₃·9H₂O and TEOS were added. Then, 0.4 mL of 1 mol∙L^-1 HCl and 50 μL of FeCl₃ (50 mg·mL^-1) were continuously dissolved in 10.0 mL of ethanol. Finally, the solution was kept at 60 ℃ in oven overnight. For comparison, the gel without TEOS and Al(NO₃)₃·9H₂O, was prepared by the above procedure.

1.1.2 Preparation of variable-structure carbon materials

The precursors were ground into powders, and heated under H₂ (20 sccm) and Ar (300 sccm) flows at 1000 ℃ (heating rate: 10 ℃·min^-1) for 2 h. The furnace was cooled to 400 ℃ at 10 ℃·min^-1 before natural cooling to the room temperature. The residual oxides were removed in a mixed solution of 6 mol·L^-1 H₃PO₄ and 8 mol·L^-1 HF to obtain the carbon materials. To improve electrochemical performance, N doping was promoted by providing NH₃ flow (100 sccm) in the heating process. The prepared carbon materials were denoted as NC (without the addition of Al(NO₃)₃·9H₂O and TEOS), AlO-NC (with the addition of Al(NO₃)₃·9H₂O), SiO-NC (with the addition of TEOS), and SiAlO-NC (with the addition of both Al(NO₃)₃·9H₂O and TEOS), respectively.

The microstructures were observed by an emission scanning electron microscope (SEM, Hitachi S-4800) and a transmission electron microscope (TEM, JEOL 2100F). Raman spectra were collected on a thermal dispersive spectrometer (λ=532 nm, 10 mW). X-ray diffraction (XRD) was performed on a Bruker D8 Advanced diffractometer with Cu-Kα irradiation (λ=0.15406 nm, 2θ= 10°-80°). Specific surface areas and pore size distributions were obtained by nitrogen adsorption measurements, performed with ASAP 2020M (Micromeritics). X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific) was carried out under vacuum using Al-Ka irradiation (1486.6 eV) generated at 400 W and 15 kV.

Variable-structure carbon materials were prepared by tuning the compositions of precursors, as shown in Fig. 1. Firstly, self-assembly process of Pluronic P123 takes place^[11]. Then, TEOS gradually hydrolyzes to form Si-O-Si network. In the sintering process, Al(NO₃)₃·9H₂O is dehydrated into aluminum oxide, and Al-N bonding forms in the NH₃ atmosphere. Meanwhile, Si-O-Si network interacts with Al-N bonding. On one hand, the interaction between Si-O-Si network and Al-N bonding introduces more defects and promotes O in oxides bond with N in carbon materials during the reaction, which makes it difficult to escape, achieving high nitrogen content. On the other hand, this empowers carbon materials with high pore volume and broad pore size distribution. The detailed discussion is provided below.

The addition of Al(NO₃)₃·9H₂O and TEOS affects the morphology and structures of as-prepared carbon materials. First, the morphology of these carbon materials varies by adjusting the precursor composition, which is coral-like for NC, sphere-like for AlO-NC and 3D for SiO-NC and SiAlO-NC in Fig. 2(a-d). The coral-like morphology for NC is attributed to the co-assembling between pyrrole and the surfactant template^[11]. NC shows microporous structure in Fig. 2(e). After adding Al(NO₃)₃·9H₂O and lowering solution pH, self-assembling process of P123 is influenced. Therefore, pyrrole monomer polymerizes to sphere-like polypyrrole precursor, independently^[12]. Then, after chemical vapor deposition (CVD), the sphere-like carbon material, AlO-NC, with abundant mesopores is prepared in Fig. 2(f). The 3D skeletons of SiO-NC and SiAlO-NC are formed by Si-O-Si network and P123, separated by many disordered mesopores^[13]. These structures are in accordance with what are observed in high-resolution TEM (HRTEM) (Fig. 2(g-h)). The corresponding elemental mappings of SiAlO-NC confirm homogeneous distributions of C, N and O elements in SiAlO-NC, as shown in Fig. 2(i-l).

Moreover, these carbon materials have diverse pore size distributions. With the interaction of oxides, the pore structure becomes richer. This phenomenon is demonstrated by the nitrogen adsorption-desorption isotherms (Fig. 3(a)) and pore size distributions (Fig. 3(b)). SiAlO-NC possesses the richest pore structure that distributes from 0.5 to 60 nm. As listed in Table 1, SiAlO-NC has large surface area (703.20 m²·g^-1) and total pore volume (1.78 cm³·g^-1) resulting from the interaction between the Si-O-Si network and Al(NO₃)₃·9H₂O. Wide pore size distribution is beneficial for the rate capability of the material. However, all of the other materials display narrow distribution. In addition, Raman spectra reflect the difference (Fig. S1(a)). I_D/I_G is 1.00 for NC, 1.01 for SiO-NC, 1.03 for AlO-NC and 1.02 for SiAlO-NC. The D band at ~1340 cm^-1 is commonly ascribed to the defects and edges of graphene-like carbon materials, and the intensity ratio of D band to G band (~1570 cm^-1) typically suggests the defect degree of carbon materials^[14].

Furthermore, the presence of oxides also increases the N contents of carbon materials even at high temperature (1000 ℃) (Table 1). From XPS spectra, nitrogen content of SiAlO-NC is 5.29at%, which is higher than those of NC (1.42at%), SiO-NC (1.66at%), and AlO-NC (3.52at%).

The phenomenon is rarely reported for N-doped carbon materials, tuning nitrogen content particularly at temperatures as high as 1000 ℃. For demonstrating the phenomenon, C1s and N1s XPS spectra of the carbon materials (Fig. 3(c-d) and Fig. S2) are deconvoluted. The C1s spectrum of SiAlO-NC is deconvoluted at 284.7, 285.3, 286.5, and 288.5 eV, corresponding to sp² C, N-bonded sp² C, N-bonded sp³ C and C-O, respectively^[15]. Its N1s band is deconvoluted at 398.5, 400.0, and 401.3 eV, corresponding to pyridinic nitrogen, pyrrolic nitrogen, and graphitic nitrogen, separately^[16]. After etching, the atomic percentage of active nitrogen is 0.24% for NC, 0.58% for SiO-NC, 2.53% for AlO-NC and 3.70% for SiAlO-NC. SiAlO-NC owns the highest nitrogen and active nitrogen contents, which are attributed to the interaction between Si-O-Si network and aluminum oxide. This hypothesis can be demonstrated by the following analysis.

Firstly, previous studies suggest that the high surface area with more defects and edges in the carbon materials facilitates the formation of the high nitrogen and active nitrogen contents^[17]. Therefore, considering that AlO-NC has more defects than SiO-NC and NC, the nitrogen content of AlO-NC should be higher than those of SiO-NC and NC, in accordance with the result of XPS spectra.

Although the specific surface area and defects of SiAlO-NC are less than those of AlO-NC, the nitrogen content of SiAlO-NC (5.29at%) is higher than that of AlO-NC (3.52at%). Thus, to further explore the mechanism, we deconvolute N1s band of SiO-NC, AlO-NC and SiAlO-NC without removing templates (Fig. 3(d)). The Al-N bonding forms in AlO-NC and SiAlO-NC. The peak position (binding energy) of the Al-N bonding in SiAlO-NC is higher than that in AlO-NC, suggesting the interaction between Si-O-Si network and Al-N. This interaction could promote the activity of oxygen atoms in oxides because SiAlO-NC owns relatively strong N-O bonding in contrast with SiO-NC and AlO-NC. After etching, nearly no N-O bonding exists in SiAlO-NC, SiO-NC and AlO-NC, so that oxygen atoms in oxides bond with nitrogen atoms in carbon materials for SiAlO-NC in the sintering process, making the nitrogen atoms difficult to escape.

Besides, the N1s bands of the carbon materials, which are prepared at 900 and 1100 ℃ for SiAlO-NC, are also deconvoluted, as shown in Fig. S3 and Table S1. Clearly, the interaction between Si-O-Si network and Al-N bonding is independent of the sintering temperature for carbon materials, which suggests that this method is promising for tuning nitrogen species and content in carbon materials, achieving high active N content for electrochemical fields, such as supercapacitors and electrocatalysis.

To explore the importance of tuning N species and content in these carbon materials, three-electrode electrochemical cells are assembled in aqueous solution. Considering that the prepared carbon materials have the best electrochemical performances in 1.0 mol·L^-1 H₂SO₄, it is chosen as aqueous solution electrolyte. Cyclic voltammetry (CV) curves at 10 mV·s^-1 (Fig. 4(a)) are approximately rectangular, even at 100 mV·s^-1 (Fig. S4(a)). These results show that the prepared carbon materials own good capacitive behavior. For SiAlO-NC, two Faradaic peaks (one during charging, another during discharging) are almost symmetric and obvious in Fig. 4(a), owing to the high active N content. With the introduction of oxides, the capacitance is gradually improved due to higher specific surface area and active N content. Specifically, SiAlO-NC has the best performance at all current densities and achieves 302 F·g^-1 at 1 A·g^-1, which is higher than those of NC (160 F·g^-1), SiO-NC (180 F·g^-1) and AlO-NC (240 F·g^-1) (Fig. 4(b) and Fig. S5). To find the optimal nitrogen-rich carbon material, we tried different heating temperatures, and SiAlO-NC at 1000 ℃ still shows the best performance (Fig. S6). SiAlO-NC achieves 218 F·g^-1 at the current density of 20 A·g^-1 and 177 F·g^-1 at 120 A·g^-1 (Fig. S4(b)). The cycling stability of SiAlO-NC is extraordinary at 20 mV·s^-1. After 20000 cycles, the capacitive retention is about 92% in 1 mol·L^-1 H₂SO₄ (Fig. S7). The excellent capacitive performance origins from the high-temperature sintering process, leading to the interaction between Si-O-Si network and aluminum oxide, thereby empowering high electronic conductivity, active nitrogen content, hierarchical pore structure and 3D interconnected structure in SiAlO-NC.

We assembled two-electrode electrochemical cells, using symmetric SiAlO-NC electrodes, and tested at operating voltage (1.2 V) in aqueous solution (1.0 mol·L^-1 H₂SO₄) for validating electrochemical performance. CV curves at 10 mV·s^-1 are nearly rectangular (Fig. 4(c)), even at 100 mV·s^-1 (Fig. S8). GCD curves (Fig. S9(a)), even at 40 A·g^-1 (Fig. S9(b)), are almost linear and symmetrical. All these results reflect the purely capacitive behavior of SiAlO-NC. Ragone plots for SiAlO-NC and other reported nitrogen-carbon materials are shown in Fig. 4(d). At 300 W·kg^-1, the energy density of SiAlO-NC is calculated to be 13.41 Wh·kg^-1, which is higher than those of most reported nitrogen-carbon materials^{[18,19,20,21,22,23]}, indicating that it is a comparable electrode material for supercapacitors (Table S2).

In summary, we successfully tuned nitrogen species and content by the interaction between Si-O-Si network and aluminum oxide. Different morphology designs also affect the defect concentration, specific surface area, pore size distribution and pore volume. This interaction between Si-O-Si network and Al-N affects the activities of oxygen atoms in oxides, which provides oxygen- induced nitrogen fixation. As-prepared N-rich carbon material is endowed with high nitrogen doping level of 5.29at%, high pore volume of 1.78 cm³·g^-1, hierarchical pore structure of 0.5-60 nm, and 3D interconnected structure. Therefore, these synergetic effects give rise to high capacitance of 302 F·g^-1, excellent rate capability of 177 F·g^-1 at 120 A·g^-1 and good durability (92% after 20 000 cycles). This strategy to tune nitrogen species and content in carbon materials at high temperature is a promising method to prepare high performance N-rich carbon materials.

Supporting materials related to this article can be found at https://doi.org/10.15541/jim20200498

Supporting materials:

Electrochemical characterization

To fabricate supercapacitor electrodes, 20 mg of the active material and 10 mg of poly(vinylidene fluoride) PVDF were mixed in 2 mL of N-methyl-2-pyrrolidone. Then 100 μL slurry was coated onto a graphene foam, which was used as the collector. The electrode had exposed surface area of 1 cm×1 cm and was dried at 120 ℃ under vacuum. Electrochemical tests were performed on a CHI 760E electrochemical workstation (Shanghai Chenhua).

Results and discussion

There are some similarities among as-prepared carbon materials. In Raman spectra (Fig. S1(a)), there is a weak signal of 2D in all carbon materials, which suggests that they are composed of crystalline carbon, identical with the observation in XRD (Fig. S1(b)). Two broad peaks in all samples at about 24.0° and 43.5° correspond to the (002) and (101) crystal planes of the graphite structure. However, the broad peaks at about 24.0° are slightly shifted to the left compared to graphite, which suggests that the interlayer distance, d-spacing, increases through this method.

Resistances of devices can be evaluated by electrochemical impedance spectroscopy (EIS) (Fig. S10), which is presented in Nyquist plots, fitting with the equivalent Randles circuit. According to the Nyquist plots, R_b (Ohmic resistance) is 1.62 Ω for NC, 1.56 Ω for SiO-NC, 1.59 Ω for AlO-NC and 1.44 Ω for SiAlO-NC, suggesting the resistance for SiAlO-NC is the lowest. This low resistance probably results from the wetting of materials improved by nitrogen doping, which lowers the interface resistance. R_ct (charge-transfer resistance) is 0.44 Ω for NC and 0.40 Ω for SiAlO-NC, which suggests that the charge-transfer kinetic improves with the interaction between Si-O-Si network and aluminum oxide. The absence of a semicircle at high frequencies for all four kinds of materials likely signifies fast ion diffusion in the electrodes. The impedance plots for NC, SiO-NC, AlO-NC and SiAlO-NC are all parallel to vertical lines at low frequencies, indicating the pure capacitive behavior.

Electrochemical analysis

To further explore components of the capacitance in SiAlO-NC, the total capacitance is divided into two parts, which are rate-independent (such as EDLC) and diffusion- limited component (such as intercalation), according to the equation^[1]:

Here, on the right-hand side, k₂ν^-1/2 is the diffusion- limited component. The potential scan rate, v, is inversely proportional to the half-cycle time (T) in CV and CC tests. In CV test, T is the time required to sweep over the voltage window. In the CC test, it is the time required to discharge. The rate-independent part, k₁, dominates in SiAlO-NC at about 235 F·g^-1, from Fig. S11(a). The high k₁ suggests that SiAlO-NC owns good rate capability and high double-layer capacitance, owing to the high- temperature sintering process and the interaction between Si-O-Si network and aluminum oxide. On one hand, SiAlO-NC is prepared at high temperature (1000 ℃), which results in high electronic conductivity in the carbon material. On the other hand, the interaction between Si-O-Si network and aluminum oxide leads to high pore volume (1.78 cm³·g^-1), the hierarchical pore structure (0.5-60 nm) and 3D interconnected structure in SiAlO-NC, which is convenient for ions in the electrolyte to approach the active sites in the carbon material. The other part of the capacitance, k₂ν^-1/2, originating from the pseudocapacitance is ~70 F·g^-1. Thus, this material shows the combination of pseudocapacitance and electric double-layer capacitance.

The currents at each potential are deconvoluted into currents proportional to v and v^1/2 in the CV curves. The former is a capacitive CV loop, as shown in Fig. S11(b-c)^[1]. Here, the shaded loop represents the capacitor-like charge storage. The fraction of capacitive capacitance C_c in the total capacitance C_t is calculated according to the shaded loop (Fig. S11(d)).

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