In situ activation graphitization to fabricate hierarchical porous graphitic carbon for supercapacitor

In situ activation–graphitization method based on the atomically dispersed K and Fe in organic salts is developed to synthesize hierarchical porous graphitic carbon by directly pyrolysis potassium citrate and iron citrate. Moreover, (NH4)2C2O4 is also employed as both N dopant and porogen to open up internal structure and regulate pore structure. The inside-out activation leads to the homogeneous reaction and interconnected hierarchical porous structure with few dead pores. Accompanied by high specific surface area, appropriate pore distribution, good conductivity, and N/O functional groups, the sample exhibits high capacitance of 322.6 F g−1 at 0.5 A g−1, good rate capability, and excellent cycling stability with 101.5% capacitance retention after 15,000 cycles. The supercapacitor shows an energy density of 21.3 W h kg−1 at 456.7 W kg−1 in 1 M Na2SO4. Easy synthesis, cost-effective, and environmentally benign, the work provides a promising strategy to produce hierarchical porous graphitic carbon applied in energy storage.


Results
Morphology and structural properties. During the high temperature treatment of potassium citrate, a certain amount of K compounds can be generated, such as K 2 CO 3 and K 2 O. K 2 CO 3 acts as a crucial role in the development of micropores, which can play as an activating agent and corrode carbon skeleton (K 2 CO 3 + 2C → 2 K + 3CO). And the K vapors can intercalate into carbon materials, making swelling and disruption of carbon microstructure and creating additional porosity 18 . Besides, Fe 3 C can be formed during the thermal decomposition process of iron citrate, and it can transform amorphous carbon into graphitized carbon at high temperature. The morphology and pore geometry of the prepared carbon materials were observed by SEM images. Both PC-750 (Fig. 1a) and PGC-750 (Fig. 1b) exhibit a bulky monolithic morphology, and large pores are hard to be observed. The solid block of carbon with few macropores or mesopores is disadvantageous for ions transport and energy storage. Surprisingly, when (NH 4 ) 2 C 2 O 4 is incorporated during the carbonization process, the bulk carbon particles are converted into unique, well-defined 3D honeycomb framework constructed by hierarchical porous carbon skeletons (Fig. 1d), showing good pore connectivity. The macropores are evenly distributed in HPGC-750, which is attributed to physical activation effect of the chemically released gases (NH 3 , CO, CO 2 ) from (NH 4 ) 2 C 2 O 4 during thermal pyrolysis. The different structures of PGC-750 and HPGC-750 indicate that the employment of (NH 4 ) 2 C 2 O 4 is contributed to the formation of interconnected porous structure. Figure 1c-e manifest that carbonization temperature is the key element influence the morphologies of carbon materials. With the increase of temperature, the honeycomb structure of HPGC-T step-by-step becomes irregu- www.nature.com/scientificreports/ lar, which caused by the collapse of the carbon skeleton at high temperature 21 . TEM images also confirm that HPGC-750 has a honeycomb-like porous structure with macropores, mesopores (Fig. 2a) and dense micropores (Fig. 2b). The combination of macropores, mesopores, and micropores can buffer electrolyte to shorten ion diffusion distance and facilitate fast ion transport to the interior surface, providing numerous effective electrochemical sites. In Fig. 2b, some ordered lattic fringes with the interlayers spacing of ~ 0.34 nm, originated from the graphitization of carbon. More ordered lattic fringes can be observed in HPGC-800 (Fig. 2c), indicating a higher content of graphitic carbon.   www.nature.com/scientificreports/ The N 2 adsorption-desorption isotherm of PC-750 in Fig. 3a displays a type-I isotherm, indicating the existence of a large amount of micropores, which originate from the in-suit activation effect of potassium citrate. PGC-750 shows a type-II isotherm with an increased adsorption at high relative pressure region (P/P 0 > 0.9), revealing that PGC-750 consists of micropores and macropores. Obviously, when using (NH 4 ) 2 C 2 O 4 as pore-forming agent, the adsorbed quantity of HPGC-750 dramatically increases and the isotherm can be classified as type-IV isotherm, which is attributed to a large number of mesopores in the sample as verified by pore size distribution curves in Fig. 3c. Mesopore can effectively minimize ion transport resistance and improve the wettability of the material 22 . Detailed texture properties of the samples are summarized in Table 1. When only potassium citrate used, PC-750 has the lowest S BET of 1194.0 m 2 g −1 and the smallest total pore volume (V total , 0.60 cm 3 g −1 ). With the presence of ferric citrate, S BET and V total of PGC-750 increase to 1445.0 cm 3 g −1 and 1.25 cm 3 g −1 , respectively. The distinct differences in porosity properties suggest that the presence of Fe species offer additional templates to create pores and increase S BET . Significant increase in both S BET and V total have been observed in HPGC-750 because the released gases (NH 3 , CO, CO 2 , and H 2 O) from (NH 4 ) 2 C 2 O 4 , which may act as pore-forming agents to create more pores 23 . Carbonization temperature also has a great influence on porous structure (i.e. S BET , pore size distribution, and total pore volume). As seen in Fig. 3b, HPGC-700 and HPGC-750 have the same features about isotherms, but the isotherm of HPGC-800 has obvious change. The S BET of HPGC-T follows the trend of HPGC-700 (3118.3 m 2 g −1 ) > HPGC-750 (2973.3 m 2 g −1 ) > HPGC-800 (2951.1 m 2 g −1 ). The pore size distribution curves in Fig. 3d indicate an enlargement of pore size, especially mesoporous, as the temperature increases. The average pore size enlarges from 2.03 nm (HPGC-700) to 2.99 nm (HPGC-800). These phenomenons are caused by the collapse of carbon skeleton at higher carbonization temperature.
The graphitic structures of the prepared carbon materials were investigated by XRD and Raman. As shown in Fig. 4a, both PC-750 and PGC-750 show a broad diffraction peak (2θ ≈ 20-30°), which is the characteristic of amorphous carbon framework. While, PGC-750 has an additional peak at 2θ = 26.26° assigned to the (002) diffraction of graphitic carbon, indicating its partly graphitic structure. Interestingly, with the presence of (NH 4 ) 2 C 2 O 4 , the intensity of (002) diffraction peak of HPGC-750 becomes sharper and stronger, suggesting a higher graphitization degree. Moreover, a higher intensity of peak at the low-angle scattering region indicates a high density of micropores, thus providing more active sites for charge storage. Carbonization temperature is one of the key factors affecting the graphitic structure of carbon materials. As shown in Fig. 4b, high temperature results in an increased intensity of (002) peak, which is related to the development of amorphous structure towards the graphitic structure.
Raman spectra further prove the characteristic of disorder carbon and graphitic carbon (Fig. 4c,d). All Raman spectra of samples show the well-known D-band (stemming from the disordered carbon) at around 1375 cm −1 and typical G-bond (originating from the sp 2 -hybridized graphitic carbon) at around 1620 cm −1 . The I G /I D value of carbon material can be taken as an index to reflect the graphitic degree. The I G /I D values of PC-750, PGC-750, and HPGC-750 are 0.99, 1.00, and 1.02, respectively, which are significantly higher than that of commercial Norit activated carbon (I G /I D = 0.52) 24 . In addition, the I G /I D value increases from 1.00 (HPGC-700) to 1.02 (HPGC-800) with carbonization temperature increases from 700 to 800 °C, which is in accordance with the XRD patterns. The appropriate coexistence of graphitic and amorphous carbon could enhance electrical conductivity, improve wettability, and increase active surface area, benefitting for capacitance improvement 22 .
The surface functional groups of carbon materials were investigated by FTIR spectroscopy ( Figure S1). All PC-750, PGC-750, and HPGC-750 have strong and broad bands at ~ 3435 cm -1 corresponding to the O-H stretching. The bands at ~ 2916 and ~ 1639 cm -1 are ascribed to C-H and C=O functionalities, respectively. The band at 1389 cm -1 is attributed to either C-H bending vibration or O-H deformation. The bands at 1123 cm -1 can be ascribed to the C-O-C stretching vibration 25 . The XPS survey spectra of HPGC-750 are shown in Fig. 5. In Fig. 5a, two pronounced peaks locate at ~ 284.8 and ~ 533.3 eV, attributed to carbon (C 1 s, 85.10 at%) and oxygen (O 1 s, 11.65 at%), respectively. And a weak peak appeared at ~ 401.7 eV should be attributed to N 1 s, and the N content is 3.24 at%, indicating that NH 3 gas generated by pyrolysis process of (NH 4 ) 2 C 2 O 4 subsequently doped into HPGC-750 to bring N into carbon matrix. The high-resolution of C 1 s spectra can be deconvoluted into four peaks (Fig. 5b) Table 1. Porosity properties of as-prepared carbon samples. a Micropore surface area. b Mesopore surface area. c Micropore volume. d Total pore volume. e Average pore size.

Samples
S BET (m 2 g −1 ) S mic a (m 2 g −1 ) S mes b (m 2 g −1 ) V mic c (cm 3 g −1 ) V total d (cm 3 g −1 ) D ap e (nm) Electrochemical measurements. Due to honeycomb-like hierarchical porous structure, large accessible surface area, and sufficient heteroatoms, HPGC-750 is expected to be an ideal electrode for supercapacitors. Figure 6a compares the CV curves of PC-750, PGC-750, and HPGC-750 at a current density of 10 mV s −1 . In the case of HPGC-750, the CV curve displays the largest CV curve area, indicating the highest specific capacitance, evidencing the advantages of 3D honeycomb-like hierarchical porous graphitic carbon. A slight hump is also observed, which is corresponding to the redox reactions of N and O functional groups 21,27 . A pair of sharp peaks at the edge of − 1 and 0 V could be ascribed to limited ion transport and adsorption in some irregular micropores with narrow bottlenecks 28 . HPGC-750 also has the maximum CV curve area among HPGC-T samples (Fig. 6b).
Although HPGC-700 possesses the largest S BET , the relatively poor graphitization degree limits electron transfer and charge storage. Figure 6c shows the CV curves of HPGC-750 at various scan rates from 10 to 100 mV s −1 .
With increasing scan rate, a slight deviation from the rectangular nature of CV plots is associated with limited ions diffusion at higher scan rate. The GCD curves of HPGC-750 are shown in Fig. 6d. The approximately symmetric triangle shapes of curves at all current densities indicate a predominantly double-layer charge storage mechanism. Figure 6e displays the specific capacitances of PC-750, GC-750, and HPGC-T at different current densities. HPGC-750 has the highest capacitance of 322.6 F g −1 at 0.5 A g −1 . Besides, it maintains a high capacitance of 258.4 F g −1 at an ultrahigh current density of 30 A g −1 . The high capacitance of HPGC-750 could be attributed to the multiple synergistic effects: (1) hierarchical porous structure integrates the advantages of different pore structures, simultaneously providing ion-buffering reservoirs, rapid ion transfer channels, and abundant locations for ions accumulation; (2) owing to the effects of chemical activation and physical activation, HPGC-750 has a ultrahigh S BET , providing enough active sides for energy storage; (3) the coexistence of graphitic and amorphous carbon structure could enhance electrical conductivity and improve wettability, benefitting for capacitance improvement. The rate performance is a key parameter to evaluate the electrochemical performance of electrode. In hierarchical porous graphitic texture, micropores provide active sides for charge storage, macropores serve as ion-buffering reservoirs, mesopores act as ion-highways to facilitate fast ion transmission, and graphitic structure provides conductive network for rapid charge transfer. When the current density increased by 30 times, HPGC-750 and HPGC-800 achieve excellent rate capabilities of 80.1% and 79.2%, respectively. The outstanding rate capability www.nature.com/scientificreports/ can be attributed to large mesopore contribution with bigger pore size and higher graphitization degree those facilitate ion and electron transport especially at high current density. HPGC-700 possesses relatively smaller pore size of mesopores and lower graphitization degree, and exhibits a poor rate capability of 66.7%. Figure 6f shows the cycling stability of HPGC-750 at a scan rate of 100 mV s −1 for 15,000 cycles. In the initial 1750 cycles, the capacitance maintains at 99.2% of the initial capacitance, and then increases to 101.3% after 2500 cycles. The cycling-induced increase of capacitance is also reported in many references, which attributed to the penetration of electrolyte ions and in-situ activation of electrode so as to provide additional active surface area for charge storage [29][30][31] . After 2500 cycles, the electrochemical stability remains somewhat steady, and it maintains 101.5% of initial capacitance after 15,000 cycles. In comparison with the first CV curve at a scan rate of 10 mV s −1 , CV curves after 10,000 cycles and after 15,000 cycles still maintain similar shape without obvious distortion, which demonstrates that the electrode has an excellent cycling performance.
Based on the above analysis, 750 °C is an optimum temperature from the view of the morphologies, structures, and electrochemical performances of the carbon materials. On the basis of these, we further investigated the effects of the content of (NH 4 ) 2 C 2 O 4 and the mass ratio of citrate/iron citrate on the structures and capacitive properties of the samples, denoted as HPGC x-y-z (where x, y, z refer to the mass of citrate, iron citrate, and  Figure S2 displays the morphologies and structures of HPGC x-y-z . HPGC 3-1.5-6 shows little macropores ( Figure S2a), and HPGC 3-1.5-10 ( Figure S2b) maintains 3D honeycomb framework. As for HPGC 2-2.5-8 ( Figure S2c) and HPGC 3.5-1-8 ( Figure S2d), the honeycomb structure is disappeared. With different mass ratio of potassium citrate/iron citrate/(NH 4 ) 2 C 2 O 4 , HPGC x-y-z exhibits tunable S BET and pore size distribution ( Figure S3, Table S1). The pore sizes of all HPGC x-y-z are mainly around 1.27 and 2.51 nm. The S BET of HPGC 3-1.5-z decreases with increased content of (NH 4 ) 2 C 2 O 4 , while excessive (NH 4 ) 2 C 2 O 4 results in an increased S BET . HPGC 2-2.5-8 has a lower S BET (1911 m 2 g −1 ) than that of HPGC 3-1.5-8 due to insufficient chemical activation. But when the mass ratio of potassium citrate/iron citrate increased to 3.5:1, the S BET of HPGC 3.5-1-8 (2125.7 m 2 g −1 ) decreases, probably caused by the excessive activation effect of potassium citrate. Figure S4 displays the electrochemical performances of all HPGC x-y-z electrodes. When the mass ratio of potassium citrate/ iron citrate/(NH 4 ) 2 C 2 O 4 is 3:1.5:8, the carbon electrode possesses optimal capacitive performances, although its S BET is not largest, indicating that the S BET is not completely directly proportional to capacitance. With 3D hierarchical porous texture and localized graphitic structure, CV curves of symmetric supercapacitor HPGC-750//HPGC-750 performed in 6 M KOH show rectangular shapes at various scan rates (Fig. 7a). The rectangular nature even at a higher scan rate of 200 mV s −1 is related to the presence of mesopores. All the GCD curves in Fig. 7b show regular symmetric triangle, which again confirms the double-layer formation at the electrode-electrolyte interface. The maximum specific capacitance for a single electrode calculated from GCD curves is 245.1 F g −1 at 0.5 A g −1 and it retains 201.6 F g −1 at 10 A g −1 , corresponding to 82.2% capacitance retention.
The symmetric supercapacitor HPGC-750//HPGC-750 was also tested in 1 M Na 2 SO 4 , owing to its lower H + and OH − , which allows it providing a larger operating voltage. Figure 7c shows the CV curves of HPGC-750//HPGC-750 at 40 mV s −1 within different potential windows, and they present well reversible cycle even at a potential window of 0−1.8 V. The rectangle-like CV curves (Fig. 7d) and symmetrical GCD curves (Fig. 7e) within the voltage windows 0−1.8 V further demonstrate excellent rate capability. In 1 M Et 4 NBF 4 /AN, the rectangular nature of CV curves ( Figure S5a) and the linear and symmetrical characteristics of GCD curves at various current densities ( Figure S5b Electrochemical impedance spectroscopy was also measured to evaluate the ion and electron transport kinetics in different electrolytes. Figure 8a shows the Nyquist plots of HPGC-750 supercapacitors in 6 M KOH and 1 M Na 2 SO 4 and Fig. 8b shows the Nyquist plots in 1 M Et 4 NBF 4 /AN. The vertical nature of curves in the low frequency region indicates nearly ideal capacitive performance. The semicircle in high frequency region is related to charge transfer resistance (R ct ). It is observed that the diameters of the semicircles for aqueous electrolytes are much smaller than that for organic electrolyte. The value of equivalent series resistance (R s ) for KOH electrolyte is calculated to be 0.60 Ω, and it is 2.07 Ω for 1 M Na 2 SO 4 , 2.14 Ω for 1 M Et 4 NBF 4 /AN. The smaller R s and R ct for aqueous electrolytes than that for organic electrolyte could be ascribed to high conductivity, small ion size, and quick ionic mobility of aqueous electrolyte 38 . Figure 8c shows the Bode phase angle plots. The large phase

Preparation of materials.
In a typical synthesis, 3 g potassium citrate tribasic monohydrate, 1.5 g iron citrate, and 8 g diammonium oxalate monohydrate were grinded homogeneously to achieve a sufficient contact. Subsequently, the mixture was carbonized in a N 2 atmosphere at high temperatures (700, 750, and 800 °C) for 2 h at a heating rate of 5 °C min −1 . The calcined products were treated with 3 M HCl solution and then washed with deionized water several times. After dried at 80 °C, the final product HPGC-T was obtained (where T represents carbonization temperature).
For comparison, the sample synthesized by solely pyrolysis potassium citrate at 750 °C was denoted as PC-750. To evaluate the effect of (NH 4 ) 2 C 2 O 4 on the structure of the resultant carbon materials, synthesis without (NH 4 ) 2 C 2 O 4 was also performed, and the resultant sample was labeled as PGC-750.

Materials characterization.
Scanning electron microscopy (SEM, JSM-6510F) was conducted to investigate and morphologies and structures of the samples. Transmission electron microscopy (TEM) was taken using FEI-G2F20. X-ray diffraction (XRD) patterns were obtained by Miniflex 600 diffractometer. Raman spectra were measured by using a Jobin-Yvon, HR 800 spectrometer. The N 2 adsorption-desorption isotherms were obtained by using Micromeritics ASAP 2460 instrument 39 . Surface chemical composition of the sample was investigated by X-ray photoelectron spectroscopy (XPS, ESCALAB 250) and fourier transformation infrared spectroscopy (FTIR, Bruker Tensor 27).
Electrochemical measurement. The electrochemical performances of samples were evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) through a CHI760e electrochemical workstation. Active carbon material (2.4 mg, 80 wt%), acetylene black (10 wt%), and polytetrafluoroethylene (10 wt%) were mixed and coated on a nickel foam to prepare a working electrode. In a three-electrode system, Hg/HgO and Pt foil electrodes were used as the reference electrode and the counter electrode, respectively. The electrolyte was 6 M KOH solution. The specific capacitance C (F g -1 ) in the three-electrode system can be obtained by following equation: where I, Δt, m, and ΔV are the discharge current (A), the discharge time (s), the mass of active material (g), and the potential window (V), respectively.
In two-electrode system, the cell consists of two symmetric electrodes with equal mass were separated by a polypropylene membrane using 6 M KOH, 1 M Na 2 SO 4 , or 1 M Et 4 NBF 4 /AN electrolyte. The specific capacitance C sp (F g -1 ) for a single electrode, energy density E (W h Kg -1 ), and power density P (W Kg -1 ) of supercapacitor were calculated according to the following Eqs. where m is the total mass of active materials for two electrodes (g), V is the maximum discharging potential (V) 39 .