Fabrication of flexible nitrogen and phosphorus-codoped porous carbon membrane and investigation of capacitance performance

Porous carbon materials have been used extensively in new energy storage materials. They have the advantages of high stability, large specific surface area, and good electrical conductivity. However, carbon material capacitance still needs to be increased. In this paper, nitrogen and phosphorus-codoped porous carbon membrane (NPCM) was prepared by electrostatic spinning using polyacrylonitrile (PAN) as a carbon and nitrogen source, triphenylphosphine (TPP) as a phosphorus source and SBA-15 as a templating agent. When PAN/TPP was set to 2:1, the resulting NPCM has good flexibility, with N and P doping content of 13.8% and 0.5%, respectively, and a specific surface area of 872 m2/g. The NPCM was used as the working electrode with 1.0 M H2SO4 electrolyte and a specific capacitance of 215 F/g was achieved at a current density of 0.5 A/g by a three-electrode test, The capacitance retention rate could reach 103% after 1000 charge/discharge cycles at a current density of 2.0 A/g. Therefore, these results indicate that NPCM has great potential for application in high-performance flexible supercapacitors.


Introduction
Compared to traditional supercapacitors, flexible supercapacitors are resistant to bending and are not limited by mass, volume, or mechanical properties [1] .They are resistant to bending and can be used in applications like clothing, implantable medical devices, and health monitoring [2] .For flexible supercapacitor electrodes, carbon-based materials are preferred due to their high specific surface area, superior thermal conductivity, high mechanical strength, and Young's modulus.But its specific capacitance needs to be enhanced [3] .In this paper, high-specific capacity flexible nitrogen-phosphorus co-doped porous carbon membranes were prepared by introducing nitrogen and phosphorus atoms codoped in carbon materials by in-situ method and obtaining three-dimensional flexible continuous membranes by electrostatic spinning and programmed heating carbonization.

Fabrication of nitrogen and phosphorus-codoped porous carbon membrane
1.2 g of polyacrylonitrile (PAN) and a certain quality of triphenylphosphine (TPP) were both dissolved in 8 g of DMF solvents, and then 0.6 g of hard templating agent SBA-15 was dispersed in 8 g of DMF.
The above two solutions were stirred at 70 °C for 8 h to obtain the composite solution.The composite fiber membrane is obtained by electrospinning from the complex solution.The carbon membranes were dried under vacuum at 80 °C for 3 h, followed by pre-oxidation, carbonization, and CO 2 activation in sequence.The carbon membrane was soaked in 10% hydrofluoric acid for 24 h and etched to remove SBA-15.The porous carbon membrane NPCM was obtained by washing and drying at 110 °C for 6 h.The carbon membrane without triphenylphosphine was named NCM.The preparation process is shown in Figure 1.

Characterization
FSEM, Hitachi S-4800, Japan; XPS, Karats Axis Ultra DLD; N 2 adsorption-desorption test, Quantachrome SI-21, USA; Raman, Renishaw InVia Reflex; thermal performance test (TGA, DSC), Netzsch STA 449 F3, Germany; A titanium mesh of 2 cm in diameter and treated with HF acid, washed and dried was used as the current collector; a carbon membrane was used as the active material and placed directly between the two pieces of titanium mesh without binder, with a titanium strip as the lead, pressed at 10 MPa and held for 5 minutes on a YP-24T tablet press, and the pressed piece was used as the working electrode.In a 1.0 mol/L H 2 SO 4 electrolyte, using a platinum sheet as the counter electrode and a saturated glycolic as the reference electrode, three electrodes (CHI660E, Shanghai Chenhua Instruments) were used to perform (CV, GCD, EIS).

Morphological structure of NPCM
Figure 2(a) depicts a SEM image of NPCM, revealing the coarse shape of the NPCM fiber, and (b) depicts a magnified cross-section with a loose porous structure.The SEM pictures and associated EDS spectra of NCM and NPCM are shown in Figures 2(c) and (d), respectively.The SEM photos demonstrate that the fiber shape is rougher after phosphorus doping.The EDS spectra show that there are some phosphorus atoms in the carbon layer following TPP doping, while the oxygen content has also increased.1 shows the atomic percentages of each bonded atom's content in the two specimens after splitting the N1s, and O1s peaks.Among these, pyridine nitrogen (N-6) can improve the material's capacitive properties; pyrrole nitrogen (N-X) can improve the specific capacity of the carbon material, and graphite nitrogen (N-Q) can improve the carbon material's electrical conductivity.The addition of phosphorus is advantageous not only for increasing the pseudocapacitance of the carbon material, but also for improving its electrical conductivity.The pyrrole nitrogen content of the carbon membrane NPCM obtained by P-doping is significantly higher than that of the carbon membrane obtained without P-doping, indicating a mutually stable coexistence of the heteroatoms N and P.     4(c) shows the Raman spectrum of NPCM.The I D /I G of this carbon membrane is 0.74, indicating that the N and P doping process of the carbon membrane did not affect its graphitization process and the resulting carbon membrane NPCM has good order, which is conducive to electron transport.Figure 4(d) shows the TEM image of a single microfibre, which can be seen to have a diameter of about 140 nm.The finer the fiber is, the greater the specific surface area of the carbon membrane is.

Conclusion
In this paper, nitrogen and phosphorus co-doped porous carbon membrane NPCM was prepared using PAN as carbon and nitrogen source and TPP as phosphorus source by the electrostatic method and programmed heating carbonization and CO 2 activation.The following conclusions were obtained: TPP was successfully used to dope the porous carbon membrane with P element in situ, and P and N can stably exist in the carbon membrane with each other, increasing the specific capacitance of the carbon membrane and laying a good structural foundation for it to be an excellent supercapacitor electrode material.When PAN/TPP=2:1, the specific surface area of NPCM after doping increased by about 27% and mesoporous content by about 12% compared to NCM without phosphorus doping.This indicates that TPP also has a good porogenic effect.The specific capacitance of NPCM is as high as 215 F/g (0.5 A/g), with a capacitance retention rate of 76% at a high current of 10 A/g and 103% through 1000 charge/discharge cycles at a current density of 2.0 A/g.The as-made NPCM exhibits more desirable capacitive characteristics and reversibility, and has greater potential for development in the application of electrode materials for supercapacitors.

Figure 1 .
Figure 1.Flow chart of nitrogen and phosphorus co-doped porous carbon membrane preparation.

Figure 3
Figure 3 shows the XPS analysis spectra of NCM and NPCM.The analytical figure (a) shows the full spectrum of the specimen, and the results of peak splitting of P2p, N1s, and O1s are shown in Figure 3(b), (c), and (d).Table1shows the atomic percentages of each bonded atom's content in the two specimens after splitting the N1s, and O1s peaks.Among these, pyridine nitrogen (N-6) can improve the material's capacitive properties; pyrrole nitrogen (N-X) can improve the specific capacity of the carbon material, and graphite nitrogen (N-Q) can improve the carbon material's electrical conductivity.The addition of phosphorus is advantageous not only for increasing the pseudocapacitance of the carbon material, but also for improving its electrical conductivity.The pyrrole nitrogen content of the carbon membrane NPCM obtained by P-doping is significantly higher than that of the carbon membrane obtained without P-doping, indicating a mutually stable coexistence of the heteroatoms N and P.

Figure 4 (
Figure 4(a) depicts the specimen's N 2 adsorption-desorption curves, while Figure 4(b) depicts the pore size distribution curves.Figure (a) shows that the phosphorus-doped carbon membrane NPCM has a more developed pore structure.The specific surface area of the phosphorus NPCM obtained by triphenylphosphine doping increased by about 27% compared to that of the NCM, and the proportion of mesopores also increased by about 12%, according to the BET pore structure and specific surface area characteristics of the two specimens (shown in Figure 4(b)).Figure4(c) shows the Raman spectrum of NPCM.The I D /I G of this carbon membrane is 0.74, indicating that the N and P doping process of the carbon membrane did not affect its graphitization process and the resulting carbon membrane NPCM has good order, which is conducive to electron transport.Figure4(d)shows the TEM image of a single microfibre, which can be seen to have a diameter of about 140 nm.The finer the fiber is, the greater the specific surface area of the carbon membrane is.

Figure
Figure 4(a) depicts the specimen's N 2 adsorption-desorption curves, while Figure 4(b) depicts the pore size distribution curves.Figure (a) shows that the phosphorus-doped carbon membrane NPCM has a more developed pore structure.The specific surface area of the phosphorus NPCM obtained by triphenylphosphine doping increased by about 27% compared to that of the NCM, and the proportion of mesopores also increased by about 12%, according to the BET pore structure and specific surface area characteristics of the two specimens (shown in Figure 4(b)).Figure4(c) shows the Raman spectrum of NPCM.The I D /I G of this carbon membrane is 0.74, indicating that the N and P doping process of the carbon membrane did not affect its graphitization process and the resulting carbon membrane NPCM has good order, which is conducive to electron transport.Figure4(d)shows the TEM image of a single microfibre, which can be seen to have a diameter of about 140 nm.The finer the fiber is, the greater the specific surface area of the carbon membrane is.

Figure 5
Figure 5(f) shows the cycle curve of NPCM charged and discharged 1000 times at 2.0 A/g.The high capacitance retention of 103% is mainly due to the large specific surface area of NPCM (873 m 2 •g -1 , Figure 4(a)), abundant mesopores (38.1%, Figure 4(b)), very small impedance (0.54 Ω), activation of carbon membrane after about 400 cycles and increase in active sites which results in more capacitance being stored.

Figure 5 .
Figure 5. (a) Histogram of specific capacitance of carbon membrane with different PAN/TPP ratios of NPCM; (b) CV curves of the NPCM at different scan rates; (c) GCD curves of NPCM at different current densities; (d) AC impedance diagram; (e) Specific capacitance at different current densities; (f) Cyclic performance graph.