P/N/O co-doped carbonaceous material based supercapacitor with voltage up to 1.9 V in aqueous electrolyte

Abstract

Phosphorus, nitrogen and oxygen co-doped carbonaceous materials (PNODC) were facilely prepared by pyrolyzing phosphorus, nitrogen and oxygen-rich phytic acid doped polyanilines. ICP analysis and elemental analysis demonstrate that the maximum phosphorus content of the as-prepared PNODC reaches 6.02 wt% with the total heteroatom (phosphorus, nitrogen and oxygen) content up to 32.76 wt%. The symmetric supercapacitor built from PNODC could be operated at a very high working voltage of 1.9 V. A maximum energy density of 21.8 W h kg⁻¹ is achieved at a power density of 238 W kg⁻¹ and a voltage load of 1.9 V. The excellent electrochemical performance can be attributed to the high heteroatom content, the synergetic effect of phosphorus and nitrogen/oxygen co-doping and the protection of abundant heteroatom functionalities on the surface of the carbon electrodes.

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1. Introduction

Supercapacitors have drawn much attention as novel energy devices with high power density and long cycling life.^1–6 Carbonaceous materials are the most frequently used electrode materials for supercapacitors due to their excellent electric conductivity, remarkable physicochemical stability and relatively cheap price.^2–8 Various nanostructured carbons have been reported for fabricating high performance supercapacitors, such as activated carbon,^8,9 carbon fiber,^10–12 carbon nanotube,^13,14 mesoporous carbon,^15,16 carbon aerogel,^17,18 and graphene.^19,20

Carbonaceous supercapacitors, commonly based on the electric double-layer capacitance (EDLC) mechanism, show dependent capacitance on the porous structure and the specific surface area of carbonaceous electrodes.² Optimized pore size distribution and high specific surface area are essential to obtain good capacitive properties.^21–24 However, the excessive increase of specific surface area may damage the mesoporosity and limit the volumetric capacitance inevitably, which is unfavorable for the electrochemical performance of supercapacitors.⁸ Recently, researchers found that surface functionalization is also an effective way to improve the specific capacitance of carbon materials. The heteroatoms such as nitrogen, oxygen, phosphorus, boron, sulfur, etc. on the surface of carbonaceous materials could help to not only provide substantial Faradaic pseudocapacitance from the redox reactions of these electrochemically active groups,^25,26 but also introduce surface defects onto the surface of the carbon materials to improve the wettability of carbonaceous electrodes to electrolyte solution, and thus promote the overall electrochemical properties.^27,28

Nitrogen and/or oxygen-doped carbons have been extensively studied for their attractive pseudocapacitance and ready availability of plentiful sources from natural biomaterials to common synthetic polymers.^29–31 Meanwhile, phosphorus-doped carbons are also appealing because they could widen the electrochemical widow of supercapacitors in the aqueous electrolyte.^32,33 Based on the well-known formula E = ½CU², the energy density (E) is in proportional to the specific capacitance (C) and the square of the electrochemical window (U). Therefore, in consideration of the synergetic effects of phosphorus and nitrogen/oxygen heteroatoms,³⁴ the phosphorus (P) and nitrogen (N)/oxygen (O) co-doped carbonaceous materials should exhibit the enhanced energy density and the superior pseudocapacitance. Previous work on the preparation of the P-doped carbon materials by using inorganic phosphorus sources, such as phosphoric acid, phosphate sates, have been reported.^34–42 However, it has rarely been mentioned to prepare the P-doped carbon materials from organic phosphorus sources, in which phosphorus groups are well bonded to their carbon skeletons.^43–45

Phytic acid (PA) is a biogenic organic phosphorus source with theoretic phosphorus content up to 28%.⁴⁶ It is especially suitable for doping polyaniline because of abundant phosphate groups and the highly acidic environment offered. In addition, its stereoscopic configuration (Fig. 1) also benefits the construction of porous structure.⁴⁷ In this work, PA doped polyanilines were prepared by the oxidation polymerization of aniline monomers in the presence of PA, and were then pyrolyzed under argon atmosphere to give phosphorus, nitrogen and oxygen co-doped carbonaceous materials (PNODC). Abundant heteroatom functionalities on the surface of the carbon materials endow PNODC with high specific capacitance, wide electrochemical window and large energy density.

Fig. 1 Chemical structure of phytic acid.

2. Experimental section

2.1 Preparation of PA or HCl doped polyaniline

For preparation of PA doped polyaniline (PA-PANI), 75 μL aniline was added into 8 mL 0.1 M PA solution in vial I, which was then shaked vigorously to achieve homogenous dispersion. In another vial II, 185 mg ammonium persulfate (APS) was dissolved in 8 mL DI water. Afterwards, the solutions in both vials were respectively precooled to 0–5 °C in ice water, and then mixed together to react for another 6 h at the same low temperature to give a green floccule. The floccule solution was filtered and washed with water and alcohol for several times to obtain PA doped polyaniline, labeled as PA-PANI1. When increased the concentration of PA solution to 0.2 M and repeated the above reaction, we got the product PA-PANI2.

For comparison purpose, conventional HCl doped polyaniline (HCl-PANI) was prepared as described above except that the concentrations of both aniline and APS are 1 M and 0.1 M PA solution was replaced with 1 M HCl solution. All samples were dried in vacuum desiccator at 40 °C before pyrolysis.

2.2 Preparation of multi-heteroatom co-doped carbon materials

All PANI samples were placed in a quartz tube furnace and were pyrolyzed at 800 °C for 4 hours under argon atmosphere to get black powder. The corresponding pyrolyzed products of HCl-PANI, PA-PANI1 and PA-PANI2 are labeled as NODC, PNODC1 and PNODC2, respectively.

2.3 Characterizations

The morphologies of all samples were observed with a Hitachi S-4800 field-emission scanning electron microscope under an acceleration voltage of 5.0 kV and a JEOL JEM-2100 transmission electron microscope of 200 kV. The phase analysis was carried out on a Shimadzu XRD-6000 X-ray diffractometer with Cu-K_α radiation at 40 kV and 40 mA, and the data were collected in the 2θ range of 10°–60°. Nitrogen sorption isotherms were measured by a Micromeritics ASAP2020 analyzer, and all samples were degassed in vacuum at 150 °C for 6 hours before measurements. The specific surface area and the average pore width were acquired based on the Brunauer–Emmett–Teller (BET) model. The pore size distribution was obtained using the non-local density functional theory (NLDFT) approach. The pore volume was gained from adsorbed nitrogen at a relative pressure of about 0.98. Elemental analysis was taken out on a Heraeus CHN-O-Rapid analyzer, and inductive coupled plasma (ICP) analysis was dealt with a PE Optima 5300DV spectrometer. X-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI 5000 Versaprobe system, using monochromatic Al K_α radiation (1486.6 eV) operating at 25 W.

2.4 Electrochemical test

Electrochemical measurements were separately carried out in a three electrode system and in a two electrode system on a CHI600E electrochemical workstation at room temperature. In the three electrode system, a platinum sheet and an Hg/HgO electrode (or a saturated calomel electrode, SCE) were employed as the counter and the reference electrodes, respectively. 6 M KOH or 1 M H₂SO₄ aqueous solution was used as the electrolyte solution. The working electrode with a mass loading of 1.6 mg cm⁻² was prepared by casting a Nafion-impregnated sample onto a glassy carbon electrode (GCE) with a diameter of 4 mm. Typically, 4 mg sample was added to 4 mL DI water containing 0.5 mg carbon black, and sonicated for at least 30 min. 200 μL of this sample solution and 50 μL 0.05 wt% Nafion solution was sequentially dropped onto the GCE and left to dry before the electrochemical test. In the two electrode system, the working electrode with a mass load of 3–5 mg was prepared by casting mixtures of samples, carbon black and Nafion solution (mass ratio of 8 : 1 : 1) on a platinum sheet. A pair of working electrodes were separated by a filter paper soaked with 6 M KOH electrolyte and were packed tightly. Afterwards, the package was dipped in the 6 M KOH electrolyte under vacuum before tests.

3. Results and discussions

X-ray diffraction (XRD) patterns of the as-prepared carbon samples are shown in Fig. 2. Two broad peaks located at around 25° and 44° respectively indicate the carbon samples are amorphous. The intensity of those two peaks decreases gradually with the increase of PA contents in the carbon precursors, proving that the bulky PA molecule could prevent the π-stacking of nanographene sheets effectively. In addition, the release of possibly volatile gas during pyrolyzation of the PA doped polyanilines is also one of the reasons for the formation of the carbon materials with amorphous structure.

Fig. 2 XRD patterns of NODC, PNODC1 and PNODC2.

Elemental compositions of the samples were analysed and listed in Table 1. It can be seen that, except NODC only with nitrogen and oxygen, PNODC1 and PNODC2 have high nitrogen, oxygen and phosphorus content, indicating that nitrogen, oxygen and phosphorus were successfully incorporated into the carbon materials. Highly volatile sulphur/chlorine-containing groups have been eliminated under the high-temperature condition of pyrolyzation. For NODC, partial amino groups in the polyaniline chains were decomposed for that its N/C ratio is far below that of the carbon precursor polyaniline. For PNODC, since phosphate groups are not volatile, both the content of phosphorus and oxygen increase steadily with the accretion of PA, accompanying simultaneously by the decrease of the nitrogen content. In such case, the decomposition of amino groups and the introduction of foreign atoms from PA together contributed to the loss of the nitrogen content. Noticeably, the decrease of the nitrogen content doesn't offset the overall growth of the total heteroatom content. In PNODC2, the heteroatom doping reaches a balance of phosphorus content of 6.02 wt%, oxygen content of 21.89 wt% and nitrogen content of 4.85 wt%, with total heteroatom content up to 32.76 wt%. The phosphorus content is much greater than those of the earlier reports, in which the material was made using organic triphenylphosphine as the phosphorus precursor.^48,49 These results suggest that PA is an effective heteroatom, especially phosphorus, dopant for carbonaceous materials.

Table 1 Elemental compositions of all carbon samples (wt%)^a

Sample	C	N	H	P	O	N + P + O
a C, H and N contents were elemental analysis results, P contents were measured from ICP, and O contents were the remainder excluded C, N, H and P contents.b After being subjected to galvanostatic charge–discharge at 10 A g^−1 for 10 000 cycles.
NODC	77.74	8.58	2.40	0	11.28	19.86
PNODC1	66.77	6.17	2.18	5.32	19.56	31.05
PNODC2	64.53	4.85	2.71	6.02	21.89	32.76
PNODC2^b	63.48	4.66	2.89	5.91	23.06	33.63

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X-ray photoelectron spectroscopy (XPS) was also utilized to investigate the surface concentrations and chemical bonding of the elements of the samples. The results shown in Fig. S1 and Table S1† also demonstrate that the surface compositions have the same growth trend as the overall materials. Fine XPS spectrum enables us to examine the chemical bonding of the elements (Fig. 3). The P2p spectrum (Fig. 3a) of NODC and PNODC samples contains a major peak at about 132.5 eV due to C–PO₃ or C₂–PO₂ groups,^42,50 and two minor peaks at around 131.1 eV and 134.2 eV, which are characteristics of C₃–P groups and C–O–PO_x (x = 1, 2, 3) groups respectively.⁴² Since the C–O–PO_x groups could damage the electrochemical performance,³⁴ the prepared carbon materials with a low content of C–O–PO_x groups are favorable for high capacitance. The N1s spectrum (Fig. 3b) of NODC and PNODC samples can be fitted into four individual subpeaks at 398.2 eV, 400.2 eV, 401.2 eV and 405.2 eV, which are ascribed to pyridinic nitrogen (N-6) and/or P=N bond, pyridonic/pyrrolic nitrogen (N-X), quaternary nitrogen (N-Q) and pyridine-N oxide (N-O) and/or P-N bond, respectively.⁴⁵ It can be seen that the percentage of 405.2 eV peak in the N1s spectrum has sequence of NODC < PNODC1 < PNODC2 (Table 2), implying the presence of the interaction between amino groups in polyaniline and phosphate groups in PA for PNODC, and the increase of the N–P bond amount with the augment of PA doping amount in polyaniline. Meanwhile, the N-6 group decreases from 24.1% of NODC to 12.2% of PNODC2 along with N-X (Table 2) also due to the generation of P–N bonds. The O1s spectrum (Fig. 3c) of NODC and PNODC samples can be divided into four different components such as the quinone-type C=O and non-bridged oxygen of P=O at 531.2 eV (O-I), the single bonded oxygen of C–O and C–O–P at 532.6 eV (O-II), and the single bonded oxygen of O–H at 533.6 eV (O-III). Table 2 shows that with the introduction of phosphate groups, the O-I and O-II content increase gradually, while the O-III content decrease simultaneously, indicating that the phosphate groups could stabilize the unstable surface oxygen groups by forming C–P bond, and thus improve the long-term electrochemical stability.⁴²

Fig. 3 The subsets of XPS spectra of all carbon samples: (a) P2p, (b) N1s and (c) O1s.

Table 2 The components of N1s and O1s XPS spectra (%)

Sample	N-6	N-X	N-Q	N-O	O-I	O-II	O-III
NODC	24.1	55.5	6.5	13.9	34.2	25.0	40.7
PNODC1	18.0	42.7	10.4	28.9	34.3	28.6	37.0
PNODC2	12.2	38.9	11.6	37.2	34.5	29.8	35.6

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Fig. 4 displays the morphologies of pyrolyzed products of PA or HCl doped polyanilines. NODC preserves well the original nanofiber network of HCl-PANI (Fig. S2a and S2b†), while PNODC shows no comparative hierarchical morphologies like PA-PANI (Fig. S2c to S2f†) but rather smoother grains with several hundred nanometers in size. Nevertheless, abundant pores could be observed in both nanofibers and grains, which are helpful to accommodate electrolytes and facilitate ion accessibility in the electrochemical process.

Fig. 4 SEM, TEM and magnified TEM (indicated in the red box) images of (a–c) NODC, (d–f) PNODC1 and (g–i) PNODC2, respectively.

To further check texture properties, nitrogen sorption isotherms and pore size distribution (PSD) of all carbon samples were investigated (Fig. S3†), and the derived results are summarized in Table 3. It can be seen that NODC with the nanofiber network structure possesses both the greatest SSA and the greatest pore volume with the pore width mainly in the micropore zone. In contrast, PNODC1 and PNODC2 have a relatively smaller SSA and pore volume but with larger pores of 60 nm, which makes the average pore size increased from 4.4 nm of NODC to about 6.0 nm. The larger pores is believed to benefit accessibility of the electrolyte ions to the porous electrode material.^8,51

Table 3 The textual parameters, specific capacitance and interfacial capacitance of all carbon samples^a

Sample	SSA (m^2 g^−1)	SSAm (m^2 g^−1)	PV (cm^3 g^−1)	PVm (cm^3 g^−1)	PW (nm)	C (F g^−1)	Ci (F m^−2)
a SSA—specific surface area; PV—pore volume; PW—average pore width; C—specific capacitance measured at a current load of 0.5 A g^−1 in 6 M KOH; Ci—interfacial capacitance per unit specific surface area; The subscript letter m—micropore.
NODC	136	68	0.149	0.031	4.4	169	1.24
PNODC1	24.4	3.7	0.040	0.002	6.6	202	8.28
PNODC2	11.7	3.8	0.017	0.002	6.0	236	20.2

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The electrochemical performances for the as prepared carbon materials were investigated by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) in the three electrode system in 6 M KOH aqueous solution. It can been seen that, from Fig. 5a, the CV curves are getting more and more rectangular as the total heteroatom content grows, suggesting that the multi-heteroatom doping including P, N and O atoms could improve the wettability of electrodes to the electrolyte, and thus promotes the formation of electric double layers.^27,28,34,52 Practically, the CV curve of PNODC2 keeps a nearly rectangular shape even at a scan rate of 200 mV s⁻¹ (Fig. 6a), implying ideal capacitor behavior and fast arrangement of electric double layers. GCD tests (Fig. 5b) were performed at a current density of 0.5 A g⁻¹, and the calculated specific capacitances of all samples were summarized in Table 3. The longer the discharge time, the better the capacitive property. PNODC2 gives the largest specific capacitance of 236 F g⁻¹ compared to 169 F g⁻¹ of NODC and 202 F g⁻¹ of PNODC1 in spite of a smaller SSA and pore volume. The interfacial capacitance (C_i) reveals the more obvious growth trend in the sequence of NODC < PNODC1 < PNODC2 as C_i depends only on the heteroatom doping level. The fairly higher specific capacitance of PNODC2, even larger than those of reported P and N co-doped carbons,^36–38,42 can be ascribed to the pseudocapacitance contributed from high-content heteroatoms doping and the improved wettability to the electrolyte.^25–28,34 The high total content of O-I groups, N-6 groups and N-X groups together contributed to the overall specific capacitance through Faradic redox reactions.⁴² Among them, the pseudocapacitive effect of O-I groups play a major role in relation to the relatively lower-content N-6 groups and N-X groups. In addition, the abundant P–N bonds with high affinity to electrolyte ions also favor the pseudocapacitive Faradic reactions.^33,52 Fig. 5c displays the rate capability of all three samples in the GCD processes. The capacitance retention grows in the sequence of NODC < PNODC1 < PNODC2. For PNODC2, the specific capacitance remains 189 F g⁻¹ even at a current density of 10 A g⁻¹ with a retention ratio of 80% (Fig. 5c and 6b), demonstrating a good rate capability. The results could be explained by the higher phosphorus content in the carbon materials since phosphorus shows higher electron donating capacity relative to nitrogen, and enhanced charge density of the carbon surface.^53–56 GCD tests were further performed in the electrolyte of 1 M H₂SO₄ aqueous solution (Fig. S4 and S5†), and the obtained data are summarized in Table 4. It should be mentioned that the specific capacitance in the acidic electrolyte is slightly smaller than that in alkaline one, possibly for that P–N bonds show superior affinity to donor-type electrolyte.⁵² EIS could provide detailed information on the ion transport and charge transfer and be displayed as Nyquist plots of imaginary impedance versus real impedance. Fig. 5d shows the Nyquist plots of all carbon samples. The increase of the slop value of the line at the lower frequency in the sequence of NODC < PNODC1 < PNODC2, suggests that the ion diffusion into the electrode materials faster as the total heteroatom content grows. The semicycles in the high-frequency region reveals the charge transfer behavior of the porous carbons. The equivalent series resistances (ESR) could be determined as the intersection of Nyquist curve with the real axis. The greater semicycle and intersection (Fig. 5d insert) of PNODC1 and PNODC2 demonstrate that the charge transfer resistances and ESR are both greater than those of NODC. This can be attributed to the lower N content.⁵⁷

Fig. 5 (a) CV curves at 100 mV s⁻¹; (b) galvanostatic charge and discharge curves; (c) specific capacitance vs. current density plots from 0.5 A g⁻¹ to 10 A g⁻¹; (d) Nyquist plots, the inset is the magnified view of the high-frequency zone, of all carbon samples in the 6 M KOH aqueous solution.

Fig. 6 (a) CV from 5 mV s⁻¹ to 200 mV s⁻¹, (b) galvanostatic charge and discharge curve from 0.5 A g⁻¹ to 10 A g⁻¹ and (c) cycling stability at a current load of 10 A g⁻¹ of PNODC2 sample in the 6 M KOH aqueous solution.

Table 4 The summary of electrochemical performance in 6 M KOH and 1 M H₂SO₄

Electrolyte	Sample	Capacitance/F g^−1
		0.5 A g^−1	1 A g^−1	2 A g^−1	5 A g^−1	10 A g^−1
6 M KOH	NODC	169	133	112	94	85
	PNODC1	202	165	148	133	120
	PNODC2	236	214	203	194	189
1 M H2SO4	NODC	132	110	94	78	68
	PNODC1	171	148	135	122	114
	PNODC2	207	185	171	158	151

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GCD cycling was performed to investigate the long-term stability of PNODC2. Fig. 6c shows the galvanostatic cycle curve of PNODC2 at a voltage load from −1.2 V to −0.2 V (vs. Hg/HgO) with an applied current density of 10 A g⁻¹. As can be seen from the figure, PNODC2 remains a high capacitance retention of 93% with a slightly capacitance decay of 7% after 10 000 times charge and discharge cycles, demonstrating that the surface functionalities on the well-doped PNODC could improve and keep effectively the high stability of the carbon electrodes.

To expand the applications of electrochemical supercapacitors, the energy density (E), in proportion to the capacitance and square of the cell voltage based on the well-known equation E = ½CU², should be improved. Generally, the cell voltage depends largely on the electrochemical window of the electrolyte used. For aqueous electrolytes, the electrochemical window is narrow with a theoretical voltage of 1.23 V and a practical one around 1 V for symmetric capacitors with carbon electrodes,⁵⁸ which limits inevitably the improvement of both energy and power densities. Phosphorus and oxygen doped carbon has reportedly widened the electrochemical window of carbonaceous supercapacitors to 1.4 V or more.^37,59 In our work, the CV curve shows that the electrochemical windows of PNODC2, measured in a three electrode system in the aqueous electrolyte of 1 M H₂SO₄ or 6 M KOH (Fig. 7), could be read as 1.6 V (from −0.35 V to 1.25 V) and 2.2 V (from −1.4 V to 0.8 V), respectively, which can be ascribed to the blockage of the phosphorus group to oxidation and/or reduction of the electrochemical active oxidation sites.^36,37,60,61 The further test result obtained in the two electrode system in 6 M KOH aqueous electrolyte demonstrates that the electrochemical window of PNODC2 could reach to 1.9 V (Fig. 8), far beyond that of phosphorus and nitrogen co-doped carbons reported earlier (1.4 V).^37,59 The supercapacitor cell made from PNODC2 could be operated stably at the high potential window of 1.9 V for 10 000 GCD cycles with no significant loss of the capacitance (Fig. 9). For carbon electrodes after cycling, elemental analysis showed no apparent degradation of carbon (Table 1), and XPS analysis of O spectrum gave no peaks at around 535 eV (Fig. S6†), which is the characteristic of the oxidization product of carbon. The two results prove that carbon electrodes withstood the prolonged cycling. Rather high coulombic efficiency (96.2–100.1%) also demonstrated the decomposition of the electrolyte was negligible. The results could be explained by the fact that the dopant phytic acid has six phosphate groups attached to its cyclohexane skeleton and the phosphate groups could also react respectively with surface oxygen groups or amino groups of the polyaniline chain to form C–P bond or P–N bond, and thus protect effectively the carbon materials from oxidation, as proved by the XPS characterization.

Fig. 7 CV (100 mV s⁻¹) in the three electrode system showing the electrochemical stability windows of PNODC2 in 1 M H₂SO₄ or 6 M KOH aqueous solution.

Fig. 8 (a) CV (100 mV s⁻¹) and (b) GCD (1 A g⁻¹) showing the electrochemical window PNODC2 in two electrode system in 6 M KOH aqueous solution.

Fig. 9 Cycling tests and the corresponding coulombic coefficiencies of PNODC2 in two electrode system operated at 10 A g⁻¹ and 1.9 V.

CV and GCD in the two electrode system were further employed to investigate the electrochemical performance of PNODC2. CV measurements were conducted within the voltage window of 0–1.9 V at different scan rates from 5 mV s⁻¹ to 200 mV s⁻¹ and were shown in Fig. 10. An approximate rectangular CV curve can be observed at a scan rate of 5 mV s⁻¹ and the shape is well maintained at the scan rate of 200 mV s⁻¹, indicating the ideal capacitive property.

Fig. 10 CV curves of PNODC2 in the two electrode system in 6 M KOH electrolyte.

The GCD measurements with a potential range from 0–1.9 V were carried out at a current load from 0.5 A g⁻¹ to 50 A g⁻¹ and the results are shown in Fig. 11 and Table 5. PNODC2 also demonstrates a large capacitance of 174 F g⁻¹ at the current density of 0.5 A g⁻¹ in the two electrode system and a good rate capability with a retention ratio of about 70% (from 0.5 A g⁻¹ to 10 A g⁻¹). The results confirmed the excellent electrochemical properties of PNODC2. Energy density and power density were also calculated from GCD tests and summarized in Table 5. The Ragone plots showing relationship between power output and energy density are displayed in Fig. 11c. It reveals that PNODC2 based supercapacitor exhibits a high energy density of 21.8 W h kg⁻¹, which is 4–5 folds of that of commercial carbonaceous materials (4–5 W h kg⁻¹).⁶² The high energy density originates from the wide electrochemical window and rather large specific capacitance.

Fig. 11 (a and b) GCD curves and (c) the corresponding Ragone plots of PNODC2 in 6 M KOH electrolyte in the two electrode system.

Table 5 Specific capacitance, energy density and power density of PNODC2^a

a I—current density; Cs—specific capacitance of a single electrode; E—energy density; P—power density.
I (A g^−1)	0.5	1	2	5	10	20	30	40	50
Cs (F g^−1)	174	158	144	130	119	110	104	96.8	94.8
E (W h Kg^−1)	21.8	19.8	18.1	16.3	14.9	13.7	13.1	12.1	11.9
P (W kg^−1)	238	475	950	2375	4750	9500	14 250	19 000	23 750

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4. Conclusions

In summary, phosphorus, nitrogen and oxygen co-doped carbon materials (PNODC) with high heteroatom content were successfully prepared from phytic acid doped polyanilines. Organic phosphorus source phytic acid was exploited for preparing the P/N/O co-doped carbon materials for the first time. All reactants are cheap and commercially available, and the amplified preparation could be achieved. The as-prepared PNODC has a maximum phosphorus content of 6.02 wt% and total heteroatom content of 32.76 wt%, respectively. The maximum specific capacitance of 236 F g⁻¹ is achieved at a current density of 0.5 A g⁻¹ in 6 M KOH aqueous solution. And the specific capacitance could still remain 189 F g⁻¹ at 10 A g⁻¹ with a high capacitance retention ratio of 80%. Besides, 93% of the capacitance was retained after PNODC was subjected to 10 000 times galvanostatic charge and discharge cycles at a current density of 10 A g⁻¹. Noticeably, a symmetric carbon/carbon supercapacitor based on PNODC shows a very high voltage of 1.9 V and a maximum energy density of 21.8 W h kg⁻¹. The excellent electrochemical performance can be attributed to the synergetic effect of multi-heteroatoms co-doping and their well protection of against the oxidation for carbon electrodes.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 21174059, 21374046), Program for Changjiang Scholars and Innovative Research Team in University, Open Project of State Key Laboratory of Superamolecular Structure and Materials (SKLSSM201416), the Testing Foundation of Nanjing University, and the Scientific Research Foundation of Graduate School of Nanjing University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06336h

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