Biomass-Derived N/O/P Tri-Doped Hierarchically Porous Carbon with a Wider Potential Window for Flexible Energy Storage Devices

Synthesis of APCs

Characterization

The synthetic process of APCs was schematically illustrated in Figs. 1a and 1b. Firstly, dissolved chitosan and phytic acid were combined together to form a chitosan-phytic acid hydrogel through electrostatic interaction and hydrogen bonding. Then, carbonize and activate of the chitosan-phytic acid gel precursors to obtain the APCs. In which, the amino group (–NH₂) in chitosan took place protonated reaction to form –NH³⁺ under the action of hydrogen ion (H⁺) in phytic acid, and solvation occurred to break the original hydrogen bond of chitosan. The ratio of chitosan to phytic acid can affect the structure of gel and ultimately affect the morphology and properties of the obtained carbon materials. Next, the gel precursor was pyrolyzed at 400 °C for 2 h, and carbonized at 850 °C for 2 h in N₂ atmosphere. In this process, chitosan was the main source of C, N and O, and phytic acid was not only the main source of P, but also catalyzed the conversion of carbon to low molecular level due to its rich phosphate group. Lastly, KOH activation further enriched the pore structure, and obtained N/O/P tri-doped hierarchal porous carbon APCs. ^12,43 The activated process involved K reduction and C oxidation. ²⁶

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Firstly, KOH was reduced to K₂CO₃ and metallic K at 400 °C–600 °C. Next, above 700 °C, K₂CO₃ was pyrolyzed into K₂O and gassy CO₂. Lastly, K₂O further reacted to form metallic K. In the course of the reaction, carbon materials realized high microporosity. ^16,26,44

The morphology and microstructure of the samples were characterized via SEM, as shown in Figs. 2a–2f. In Fig. 2a, the PC-1 added 0.005 mol phytic acid showed a block structure. Interestingly, the SEM image of PC-2, PC-3 (Figs. 2b–2c) also showed the block structure, but the block of carbon materials became smaller, and the thickness became thinner. This phenomenon was attributed to the difference in amount of phytic acid, thus affecting the degree of loose of the carbon aerogel. Figure 2d–2f showed the morphology of PCs by KOH activation. With the addition of a small amount of KOH, the edge of bulk carbon materials began to become loose (APC-1) (Fig. 2d). As the amount of KOH increases, the morphology of carbon materials gradually changed from block structure to honeycomb structure (Figs. 2e–2f). This was because in the pyrolysis process, KOH can react with the surrounding C, and then leave porous structure in carbon materials after washing with HCl. In addition, Figs. 2g–2j demonstrated the N/O/P elements were homogeneously doped in the carbon skeleton.

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XRD and Raman spectroscopy were used to further analyze the crystallographic structure of the carbon materials, as shown in Figs. 3a and 3b. The XRD measurement revealed that two characteristic peaks located at 23.4° and 43°, which are consistent with the (002) and (100) planes respectively, indicating the amorphous nature of all as-prepared samples (Fig. 3a). ⁴³ According to Bragg equation (2dsin θ = λ), the (002) peak of APC-2 and APC-3 shifted to a lower angle than that of PCs, demonstrating that the layer spacing was expanded due to more defects, which was consistent with the more loose morphology shown by SEM images. Raman spectroscopy was shown in Fig. 3b to evaluate the degree of graphitization of carbon materials. It can be seen that all samples reveal two obvious peaks at about 1340 cm⁻¹ and 1590 cm⁻¹, ^45,46 which could be assigned to the D-band (sp3-type disordered carbon) and the G-band (sp²-type graphitized carbon), respectively. ^14,24,47,48 The I_D/I_G ratios of PC-1, PC-2, PC-3, APC-1, APC-2, APC-3 were 0.987, 1.001, 1.027, 1.049, 1.055, 1.098, respectively. Distinctly, APCs exhibited a relatively high I_D/I_G ratio in comparison to PCs, indicating with the increase of the phytic acid and activation dose, the disorder of the as-prepared carbon materials was gradually enhanced, which might be caused by the deeper etching degree of the carbon skeleton by the phosphate radical group and KOH.

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The pore structure and specific surface area of carbon materials have conferred electrode outstanding functional properties in energy storage. Nitrogen adsorption and desorption curves were applied to analyze the pore size distribution of carbon materials, as shown in Fig. 4a. The resulting isotherms indicated that APCs displayed a combined isotherm of type I and IV, revealing the presence of micropores and mesopores. ¹¹ It can be seen that adsorption-desorption isotherm showed a dramatic increase at low pressure of 0.1 < P/P₀ < 0.45 with an abrupt inflection point for P/P₀ ≈ 0.1, which indicated the abundance of micropores in samples. Furthermore, desorption branch of carbon materials with P/P₀ > 0.45 demonstrated the presence of a slight hysteresis loop (type-IV isotherm), displaying that a mass of mesopores existed in samples. ^7,14,49 In addition, the pore size distributions of carbon materials were calculated and shown in Fig. 4b. It can be observed that the pore diameters of the samples were basically distributed between 0.5–10 nm, with both micropores (<2 nm) for abundant active sites and mesopores (2–50 nm) for rapid ion transport. ⁴² The calculated total specific surface areas, total/micropores pore volumes were summarized in Table I. Based on the Brunauer–Emmett–Teller (BET) formula, the APC-2 exhibited a specific surface area of 1650 m² g⁻¹ and a pore volume of 1.37 cm³ g⁻¹, which was much higher than PC-2 (1140 m² g⁻¹ and 0.788 cm³ g⁻¹). This phenomenon can put down to KOH activation resulted in the increase of micropores, which increased the surface area of carbon materials and was conducive to the increase of active sites. Moreover, the S_BET and pore volume of APC-1 and APC-3 can be respectively up to 1900 m² g⁻¹, 2020 m² g⁻¹, 1.51 cm³ g⁻¹ and 1.24 cm³ g⁻¹. It can be found that although the APC-2 owned lower surface area and pore volume in APC-x samples, it exhibited the best capacitive performance. The reason may be that the chemically accessible specific surface area was affected by the unique pore size distribution of porous materials. Large specific surface area can provide more active sites for ion adsorption, but if ions were adsorbed through pores, additional pores (such as mesopores and macropores) were needed. The optimal pore size distribution can help increase the effective surface area and achieve the best capacitance performance. Therefore, the excellent porosity structure of APC-2 was more favorable for ion adsorption and ion diffusion, thus being beneficial to improve the energy storage capacity.

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XPS was applied to characterize chemical states of specified elements in carbon materials. In Fig. 5a, the full spectrum exhibited four peaks, which were consistent with C 1 s (at 284.6 eV), N 1 s (at 400 eV), O 1 s (at 530 eV), and P 2p (at 134 eV), indicating the N, O and P atoms were successfully embedded into carbon skeleton. High-resolution spectra of samples further research the chemical nature of N, O and P elements, as shown in Fig. 5b–5d and Fig. S1a–I (available online at stacks.iop.org/JES/168/100534/mmedia). The O 1 s XPS spectra can be fitted into three different signals with binding energy of 531.5 eV, 532.4 eV and 533.7 eV, assigning to C–O, C=O, and O=C–O bonds, respectively. ^6,50 Additionally, in the high resolution N 1 s spectra, four peaks at 398.7 eV, 399.6 eV, 401 eV and 404.2 eV were assigned to pyrrolic-N, pyridinic-N, quaternary-N and pyridine-N-oxide, respectively. ^1,19 It has been reported that pyrrolic-N can contribute extra free or delocalized electrons to the carbon materials to improve the pseudocapacitance of electrodes. Pyridinic-N had excellent electron donor characteristics, which improved the catalytic activity in the electron transfer process and effectively improved the electrochemical performance. ⁵¹ Moreover, quaternary-N and pyridine-N-oxide can provide higher electron conductivity and faster electron transfer rate, which was conducive to enhance the storage capacity of devices. ^29,32 The P 2p spectra displayed three chemical environments of phosphorus, corresponding to the P-C (132.6 eV), P-O (133.5 eV) and P-N (134.5 eV) bonds in Fig. 5d, revealing the successful doping of phosphorus in samples. ⁵² Additionally, it can be seen that all the samples above contained N, O and P elements, but their contents varied due to the different preparation processes in Table I. Compared with PCs, the APCs samples revealed less N, O, and P contents, which might be ascribed to KOH activation leading to the increase of more unstable and easy loss elements. In conclusion, APC-2 had the most considerable specific surface area, pore volume and element contents, therefore these advantages made it a promising electrode material for electrochemical energy storage.

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The electrochemical performances of as-prepared PCs and APCs electrodes were studied in a three-electrode system with an potential range of −1 to 0 V using 6 M KOH solution as the electrolyte. ^53–55 In Figs. 6a and S2a, the cyclic voltammetry (CV) curves of all samples showed a rectangular shape at a scan rate 100 mV s⁻¹, suggesting outstanding capacitive performance for electrodes. ^56,57 However, the PC-2 notably displayed a smallest CV integral area, indicating the relatively poor energy storage ability. Inversely, the CV curve of APC-2 possessed the larger integral area, revealing the existence of ideal electric capacity performance due to the outstanding specific surface area and porous structure. More importantly, by adding the biomass derivative phytic acid, there was a weak redox reaction in CV curve of APCs, which was caused by extra pseudocapacitance from P-doping. Additionally, the galvanostatic charge/discharge (GCD) curves of carbon materials displayed a linear and symmetrical isosceles triangle shape in Figs. 6b and S2b. The results also proved the samples took on the distinguished electrochemical performance of supercapacitor, consisted with the CV test. ^58,59 The APC-2 presented the longer charging/discharging times, showing the great electrochemical reversibility and electrochemical performance. According to GCD curves further calculation, the specific capacitance of APC-2 was significantly higher than other carbon materials at similar current densities. For example, the APC-2 showed a high capacitance of up to 320 F g⁻¹ at a current density of 1.0 A g⁻¹, which was higher than APC-1 (212.8 F g⁻¹), APC-3 (317.2 F g⁻¹), individually. Using the CV and GCD further researched the capacitance property of APC-2 at various scan speeds between 2 mV s⁻¹ and 100 mV s⁻¹ and current densities from 1 A g⁻¹ to 50 A g⁻¹, respectively. In Fig. 6c, the CV profiles of APC-2 almost remained the same rectangle shapes with the scan rate increased from 2 to 100 mV s⁻¹, showing fast kinetics for electric double-layer formation and pseudocapacitive behavior. The GCD profiles of carbon materials at different current densities from 1 A g⁻¹ to 50 A g⁻¹ were shown in Fig. 6d. These profiles kept the hardly similar isosceles triangle shape, which proved that the samples had a good rate capability and can adapt to charge/discharge process even at the large current density. Additionally, these results indicated that more P-doping may enhance the wettability of the carbon surface and accelerated ion transport. However, we can see that when the current densities were gradually increased to 50 A g⁻¹, the isosceles triangle slowly distorted, which might be ascribed to the reduction of effective ion concentration at electrode/electrolyte interface with uninterruptedly increase of current density. Compared APC-2 with other carbon electrode materials, it was found that APC-2 had a great advantage in the performance of specific capacitance (Fig. S3).

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Furthermore, the capacitive performance of the carbon materials can be analyzed by electrochemical impedance spectroscopy (EIS) measurements in the frequency range of 0.01 Hz-100 kHz at open circuit potential. In Fig. 6e, the Nyquist plot of all samples revealed the enhanced kinetics for ion diffusion and charge transfer. ^60,61 The value of equivalent series resistance (R_s) can be expressed by x-intercept in the high-frequency part, related to the resistance of the electrode material and the electrolyte. The value of charge transfer resistance (R_ct) can be represented by diameter of semicircle, which was matched with the electrical double layer and faradaic redox reaction. Moreover, the nearly vertical line in low frequency region of the Nyquist plot exhibited low ion diffusion resistance. ^26,33,35,43 To be noted, the equivalent series resistance of PC-1, PC-2, PC-3, APC-1, APC-2 and APC-3 were 2.23, 2.01, 2.31, 0.7, 0.5 and 1.01 Ω, respectively. The APC-2 with the almost vertical line in low frequency and smallest semicircle diameter in high frequency exhibited the lowest interface resistance than other electrodes attributed to the N/O/P doping and hierarchically porous structure. In addition, the specific capacitance retention rate of sample APC-2 was 97% after 10000 cycles, with an average attenuation of 0.03% per 100 cycles (Fig. 6f). At the same time, the coulomb efficiency of APC-2 sample maintained 99%-100% after 10000 charge-discharge cycles, demonstrating the excellent structural stability during the charge/discharge process.

Considering the practical application of supercapacitors, the APC-2 was assembled as electrode active material into a symmetric supercapacitor APC-2//APC-2 (KOH) in a two-electrode system using the 6 M KOH electrolyte. ^62,63 The Fig. 7a showed CV curves of the APC-2//APC-2（KOH）at 100 mV s⁻¹ scan rate under different voltage windows. The device exhibited typical rectangular shape, indicating an ideal capacitive behavior, ^64–67 and breaking the traditional 0–1.0 V operating voltage window limit, which can be attributed to the effective P-doping in the electrode active material. We studied the CV curves at different scanning rates (Fig. 7b) and the GCD curves at different current densities (Fig. 7c) to further investigate the electrochemical performances of APC-2//APC-2 (KOH). The results showed that the CV curves can maintain a similar rectangular shape at different scanning rates (from 2 to 100 mV s⁻¹), and there was no obvious distortion even at higher scanning rates, proving the good electric double-layer capacitance behavior. ⁶⁸ It can be ascribed the porous structure provided the ions diffusion channel to rapidly transfer to the electrolyte surface. In Fig. 7c, the all curves of GCD at different current densities exhibited the symmetrical triangle shapes, indicating the good rate capability and the highly reversible charge-discharge process. ^53,69–73 The symmetric device delivered a specific capacitance of 211.23 F g⁻¹ at the current density of 0.5 A g⁻¹. In addition, the Ragone plot revealed the energy density and power density of the device at different current densities in Fig. 7d. The energy density can achieve 11.15 W h kg⁻¹ at the power density of 162.5 W kg⁻¹, and which still remained 8.85 W h kg⁻¹ at a high power density of 1625 W kg⁻¹. In order to meet the practical requirements of high output voltage and current of the APC-2//APC-2 (KOH) device in practical applications, two APC-2//APC-2 (KOH) devices were measured in series and in parallel, respectively. As can be seen from the GCD curve, the operating voltage can extend from 0 to 2.6 V when connected in series, while still maintained the similar symmetrical triangular shape. Additionally, when the two devices were measured in parallel, the discharge time can be extended to twice the single capacitor in the same voltage range, indicating the great potential for high energy output. At the same time, two supercapacitors in series were used to supply power to other equipment. It can be observed that two APC-2//APC-2 (KOH) devices can provide power for red LED bulbs for about 2 min after series connection. To evaluate the applications of the prepared supercapacitors in flexible devices, the CV curves of an all-solid symmetric supercapacitor APC-2//APC-2 (KOH) was measured at different bending angles. As shown in Fig. 7f, the CV curves showed negligible variation after bending at various angles from 0°, 45°, 90°, 135° and 180°, demonstrating the good flexibility and feasibility in wearable devices.

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The voltage window in KOH electrolyte can be limited by the decomposition voltage of water. In contrast, the neutral electrolyte containing the same amount of H⁺ and OH^- will not react with the electrode to generate H₂ or O₂. Therefore, it was expected to provide a wider voltage window than alkaline electrolyte. The strong solvation of Na⁺ and SO₄ ²⁻ was conducive to maintain the stability of the electrolyte even under high voltage, ²⁹ so the symmetrical supercapacitor was assembled with a voltage range of 0–1.7 V using 1 M Na₂SO₄ electrolyte. The CV and GCD tests at diverse scan speeds from 10 mV s⁻¹ to 1000 mV s⁻¹ and current densities between 1 A g⁻¹ and 50 A g⁻¹ were applied to manifest electrochemical performance of device, individually. In Figs. 8a and 8b, we can observe that the GCD curves and CV curves showed the symmetrical triangle and rectangle shapes, revealing the excellent rate performance and ideal reversibility. The CV curves of the APC-2//APC-2 (Na₂SO₄) at 100 mV s⁻¹ scan rate under different voltage windows was shown in Fig. 8c. The device exhibited typical rectangular shape, indicating an ideal capacitive behavior. The cyclic stability of APC-2//APC-2 (Na₂SO₄) under 5000 cycles at 5 A g⁻¹ was shown in Fig. S4. It shows that even after 5000 cycles, APC-2//APC-2 (Na₂SO₄) retained 84% of its initial specific capacitance, indicating excellent cycling stability. The performance of power density and energy density has also been effectively improved (Figs. 8d). The device had an energy density of 17.3 W h kg⁻¹ at a power density of 212.5 W kg⁻¹. And when the power density was increased to 21.25 kW kg⁻¹, the energy density can still maintain 11.81 W h kg⁻¹.

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In summary, we have successfully synthesized N/O/P tri-doping hierarchical porous carbon materials through using chitosan as biomass precursor and N/O source, phytic acid as solvent, porogen, and P/O source. The hierarchical pore structure ensured effective electron and ion transport routes and provided more adsorption sites. The N/O/P heteroatoms doping increased the electric conductivity and provided additional pseudocapacitance. As electrode materials, APC-2 displayed high specific capacitance and good cycling stability. Additionally, the as-prepared symmetrical supercapacitor exhibited a wide voltage window and excellent flexibility.

This work was supported by the National Natural Science Foundation of China (21776061, 21978069); the program for Science & Technology Innovation Team in Universities of Henan Province (19IRTSTHN029).