Zinc-salt assisted synthesis of three-dimensional oxygen and nitrogen co-doped hierarchical micro-meso porous carbon foam for supercapacitors

Nitrogen and oxygen co-doped hierarchical micro-mesoporous carbon foams has been synthesized by pyrolyzation treatment of a preliminary foam containing melamine and formaldehyde as nitrogen, carbon and oxygen precursors and Zn(NO3)2. 6H2O and pluronic F127 as micro-meso pores generators. Several characterizations including thermal gravimetric analysis (TGA), X-ray diffraction (XRD) and Raman spectroscopy, FTIR and X-ray photoelectron spectroscopy, N2 adsorption–desorption, field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) were performed on the prepared foams. X-ray diffraction patterns, Raman spectra and N2 adsorption–desorption results confirmed that ZnO has pronounced effect on the graphitization of the prepared carbon foam. From X-ray diffraction, thermal gravimetric and N2 adsorption–desorption analysis results it was confirmed that the carbothermal reaction and the elimination of ZnO and also the elimination of pluronic F127 are the main factors for the induction of porosities in the foam structure. The presence of Zn(NO3)2. 6H2O and pluronic F127 in the initial composition of the preliminary foam results in the specific surface area as high as 1176 m2.g−1 and pore volume of 0.68 cm3.g−1. X-ray photoelectron and FTIR spectroscopy analyses results approved the presence of nitrogen (about 1.9 at %) in the form of pyridinic, graphitic and nitrogen oxide and oxygen (about 7.5 at. %) functional groups on the surface of the synthesized carbon foam. Electrochemistry analysis results including cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) and also electrochemical impedance spectroscopy (EIS) analysis illustrated the formation of an electric double layer supercapacitor with the capacitance as high as 137 Fg−1.

www.nature.com/scientificreports/ as one of the nominated structure directing agent. The obtained solid foams were pyrolyzed for 3 h at the 1000 °C temperature under nitrogen inert atmosphere. The obtained carbon foams were nominated as CF-x-T which x denoted the amount of Zn(NO 3 ) 2 .6H 2 O (0, 2, 4, 6 g) and T denoted as the pyrolyzing temperature. For evaluation of the pluronic F127 contribution in the porosity formation, a sample with the same procedure and without the addition of pluronic F127 has been prepared, named as CF-2w-1000. In order to determine the synthesis mechanism of the porous carbon, a specimen with the mentioned synthesis method was pyrolyzed for 3 h at 450 °C, CF-2-450.

Characterization. Powder XRD analysis was performed on a BRUKER D8 ADVANCE diffractometer with
Cu Kα (λ = 1.54 Å). N 2 adsorption-desorption isotherms were performed on BELSORP mini II equipment at 77 K. Before beginning of any experiment, the specimens were degassed for 6 h at 150 °C. Teksan™ , Iran Raman spectrometer was applied for Raman Spectroscopy. FTIR spectra were obtained from Shimadzu 8400 s Spectrometer. X-ray photoelectron spectroscopy measurements were accomplished by an Al Ka source (XPS Spectrometer Kratos AXIS Supra). Field emission scanning and transmission electron microscopy (FE-SEM and TEM) analysis were conducted on 15 kV Mira3 TESCAN and 100 kV Philips TM 120, respectively. Thermal gravimetric analysis was performed under inert atmosphere using STA504 Bähr (Germany) up to 1000 °C.

Electrochemical measurements.
In order to carry out the electrochemical analysis, the working electrode was prepared as follows and coated on a graphite foam. About 4 mg of the synthesized porous carbon foam [CF-6-1000 (80 wt%)], 15 wt% carbon black and 5 wt% PTFE (polytetrafluoroethylene) as the binder were mixed in ethanol. The prepared slurry was then painted on the 1cm 2 graphite foam and after drying overnight at 80 °C used as the working electrode. The electrochemical analysis was performed on VERASTAT Potentiostat & Galvanostat instrument by three electrode system with platinum as the counter and Ag-AgCl as the reference electrodes, respectively. The kinetical performance of the prepared electrode was investigated using electrochemical impedance spectroscopy (EIS) analysis in the frequency range of 0.01 to 10 5 Hz. A solution of 6 M KOH was selected as the electrolyte for the galvanostatic charge-discharge and cyclic voltammetry analyses.

Results and discussion
Characteristics of the synthesized carbon foams. Synthesis mechanism of the prepared porous carbon foams can be inferred from the thermal gravimetric and powder X-ray diffraction analyses results of Fig. 1.
According to the JCPSD number 001-00702551, the X-ray diffraction peaks at about 32°, 34°, 37°, 47°, 57°, 63°, 67°, 68°, 69° of the as prepared solid, (CF-2 in Fig. 1a This reaction can take place due to the high oxidizing effect of Zn(NO 3 ) 2 32 . Due to the stability of the melamine-formaldehyde resin, this reaction is not the case in the present work. Therefore, based on the X-ray diffraction results and the brown color of the prepared foam, it can be concluded that Zn(NO 3 ) 2 .6H 2 O is not so strong to convert melamine-formaldehyde resin to carbon particles. During the pyrolysis of the as-prepared solid brown foam to 450 °C, (CF-2-450) some of the characteristic peaks of zinc oxide disappeared, the intensity of other peaks decreased and their characteristic peaks shifted to the lower angles. Thermal gravimetric analysis results of CF-2, CF-4 and CF-6 presented in Fig. 1b, shows three weight loss regions which are approximately the same in all three specimens. In the first region, from the ambient temperature up to about 250 °C, the remained water evaporates from the solid structure. The sharp weight loss from 250 to 450 °C can be attributed to the removing of pluronic F127 and to some extent evaporation of ZnO nanoparticles. Evaporation of ZnO particles results in the decrease of the ZnO content of the foam. This is in agreement with the reduction in the intensity of ZnO characteristic X-ray diffraction peaks in the Fig. 1a. Based on the thermodynamic and kinetic characteristics of the carbothermal reduction of zinc oxide to zinc vapor, i. e. ZnO + C = Zn (v) + CO , it seems no possibility for etching of carbon elements in this range of temperature [38][39][40] . Continues weight loss with less intensity than that of 250-450 °C takes place in the temperature range of 450-1000 °C. This weight loss can be attributed to ZnO evaporation and etching of some of the carbons of the foam structure. DTG plot of CF-2 in the inset of Fig. 1b shows a tiny valley in the range of 800-900 °C. This valley was marked with a blue circle. It seems that this change in the slope of the weight loss is related to the beginning of the thermal reduction of carbon with ZnO particles (etching process). At the end of the thermal process all of ZnO particles have been eliminated from the foam structure and only turbostratic carbons remained which can be inferred from the X-ray diffraction results of CF-2-1000, CF-4-1000 and CF-6-1000 in Fig. 1a. Sharp peak at around 45° in CF-4-1000 and CF-6-1000 can be assigned to the (101) planes of graphite structure 41 . The graphitization extent of the synthesized carbons and the effect of zinc nitrate content of the initial mixture on the graphite formation can be inferred from their Raman spectra of Fig. 1c. The distinct peak at nearly 1356 cm −1 can be related to the disordered carbons with sp 3 hybridization, named as D band and the peak at nearly 1634 cm −1 is attributed to the ordered graphitic carbons with sp 2 hybridization, named as G band. The intensity of D and G band named as I d and I g , respectively. The I d to I g ratios of the synthesized foams refers the graphitization extent of the synthesized carbons 24,[42][43][44] . This ratio differs from 0.92 in CF-2-1000 to 0.90 in CF-4-1000 and 0.91 in CF-6-1000, respectively which are approximately the same. Therefore, and from the Raman results, there is no any considerable effect of zinc oxide content on the graphitization of the synthesized carbon particles. The broad band around 2800 cm −1 can be assigned to the formation of local graphene like structures 45,46 .
Surface functionalities of the synthesized carbon foams can be inferred from the FTIR results of Fig. 2. The broad band from 476 to 726 cm −1 of CF-2 are related to Zn-O vibrational band 47 . From the FTIR result of the pyrolyzed carbon foam, CF-2-1000, it can be inferred that this band has been eliminated. The band ranging from 1000-1400 cm −1 is attributed to the C-O stretching 27,48 . For this range of wave number, pyrolysis treatment led to the broadening of the peak at 1100 cm −1 and elimination of the peak at 1350 cm −1 . This means that pyrolysis treatment, to some extent, resulted in the elimination of oxygen element from the carbon structure. The peak at about 1560 cm −1 can be related to C=O carboxylic and C=C groups 48 the intensity of which has been decreased during pyrolysis. The sharp peak for CF-2, located at about 1690 cm −1 which has been greatly decreased in www.nature.com/scientificreports/ CF-2-1000 is attributed to C-N and C-H groups 49 . Therefore, pyrolysis treatment results in the reduction of nitrogen element from the structure of the carbon foam, as well. The peak around 2900 cm −1 belongs to the C-H stretching 32,50 . The C-H functionality groups have also been eliminated after pyrolysis. The peak at around 3400 cm −1 which has been eliminated during the pyrolysis are attributed to the O-H hydroxyl group 50 . Therefore, and based on the FTIR results, it can be inferred that C-N, C-O and C=O functionality groups are present on the surface of the synthesized carbon foam. N 2 adsorption-desorption analysis results of the as-synthesized (CF-2) and the pyrolyzed foams (CF-2-450, CF-2-1000, CF-2w-1000, CF-4-1000 and CF-6-1000) in concomitant with the field emission scanning electron microscopy micrographs of CF-2 and CF-2-1000 and transmission electron microscopy micrographs of CF-6-1000 are presented in Fig. 3a-f. Surface characteristics of the synthesized foams which have been extracted from the adsorption-desorption analysis results were presented in Table 1. From these data, the dominant effects of pyrolysis temperature, the addition of pluronic F127 and also the amount of Zn(NO 3 ) 2 .6H 2 O on the specific surface area, pore volume and pore diameter is inferable. Before pyrolysis treatment, the synthesized foam, CF-2, has the surface area of about 0.75 m 2 g −1 . The surface area of this foam will be increased to 4.6 m 2 g −1 after pyrolysis at  Fig. 1c, with the big difference in their specific surface area and pore volume, declares that ZnO has positive effect in the graphitization process. The larger the surface area is equivalent with the larger defect level. Therefore, approximately the same I d to I g ratio means higher value of graphitization ratio. The sharp X-ray diffraction peaks at about 45° for CF-4-1000 and CF-6-1000 porous carbons which belong to (101) planes of graphite (Fig. 1a), also emphasize that ZnO can encourage the graphitization process. Regarding the average crystallite size of ZnO particles, extracted from the XRD results of Fig. 1, 13-15 nm, it can be concluded that etching of the surface carbon atoms during the carbothermal reaction is the main role of ZnO in pore formation. For both CF-4-1000 and CF-6-1000, about 21 percent of the porosities are mesopores which means that the foam structure is to some extent hierarchical. The hierarchical characteristics of the synthesized carbon foam can accelerate the ionic transportation and this is helpful for its supercapacitor performance 8 . Field emission electron microscopy micrographs of the synthesized foam before (CF-2) and after the pyrolysis treatment (CF-2-1000) are presented in Fig. 3d,e. These micrographs clearly demonstrate on the porosity formation during the pyrolysis treatment which are in complete agreement with the results of Fig. 3a,b. The presence of micro porosities in the synthesized carbon foam structure ca be inferred from the transmission electron microscopy micrograph of CF-6-1000 in Fig. 3f.
From the X-ray photoelectron spectroscopy analysis results of the pyrolyzed carbon foam with the highest surface area (CF-6-1000) which has been presented in Fig. 4, it is possible to further insight in to the surface functional groups. The survey XPS diagram in Fig. 4a shows the presence of carbon, nitrogen and oxygen with the atomic percent of 90.6, 1.9 and 7.5, respectively. Deconvoluted spectra of C1s in Fig. 4b shows three peaks at 284.6, 285.2 and 287.7 eV. The prominent peak at 284.6 eV belongs to the graphitic carbons with C-C and C=C bindings 5,51,52 . The peak at 285.2 eV is attributed to the C-N and C-O functional groups and finally the peak at 287.7 eV can be ascribed to C=O 53 . Deconvoluted N1s spectra of CF-6-1000 in Fig. 4c revealed that nitrogen atoms are present in three forms, pyridinic nitrogen at 398.7 eV, graphitic nitrogen at 401 eV and nitrogen oxide at 402.2 eV 3,25,42 . Finally, the O1s deconvoluted spectra of CF-6-1000 shows that can oxygen atoms are present in two forms, one at 531.6 eV which belongs to C=O functional groups and the other at 535.1 eV which is due to the carboxylic oxygen or adsorbed water 9,10,42,54 . Supercapacitor performance of the nitrogen and oxygen doped micro-meso porous carbon. Surface characteristics of the synthesized carbon foams including specific surface area, pore volume, pore size distribution and also surface functional groups can greatly affect their electrochemical performance. Based on these facts, CF-6-1000 with the highest specific surface area, pore volume, suitable ratio of micro-meso size porosities and surface nitrogen and oxygen functional groups has been selected for the electrochemical measurements. From the XPS results, the oxygen and nitrogen content of CF-6-1000 is about 7.5 and 1.9 at. %, respectively. Presence of these two heteroatoms on the surface can increase the hydrophilic characteristics of the synthesized carbon foam and improve the electrochemical performance 9,55 . The electrochemical performance of Table 1. Surface characteristics of the synthesized carbon foams which have been extracted from the adsorption-desorption results.
Total surface area (m 2 g −1 ) Micropore surface area (m 2 g −1 ) Total pore volume (cm 3 g −1 ) Micropore volume (cm 3 g −1 ) Mean pore diameter (nm) www.nature.com/scientificreports/ the nitrogen and oxygen doped hierarchical porous carbon including the cyclic voltammetry (CV), galvanostatic charge-discharge and gravimetric capacitance versus current density curves has been demonstrated in Fig. 5a-c. The quasi-rectangular shape of the cyclic voltammograms (CV) of CF-6-1000 almost in all the potential scan rates from 5 to 500 mVs −1 , Fig. 5a, confirms the creation of a double-layer capacitor. The rectangular shape of the cyclic voltammograms results from the ions adsorption on the interface of the electrolyte and the working porous carbon electrode 56 . In higher scanning rates, the shape of the cyclic voltammetry curves, to some extents, deviates from the rectangle which can be related to the poor contact of the pore surfaces and the electrolyte during the charge and discharge process 57 . There is no any obvious peaks for redox reaction in the CV curves, but the high oxygen and nitrogen content of the activated carbon has a pronounced effect on the capacitance of the prepared capacitor 58 . The galvanostatic charge-discharge (GCD) curves the capacitance of the manufactured capacitor has been shown in Fig. 5b. As can be seen in the Fig. 5b, with increasing the current density, the discharge process shifts to the lower time. This may be due to the fast ionic transport with the increasing of the current density and also due to the internal resistance which arises from the hierarchical porous structure of the carbon foam 56 . From the GCD curves the capacitance at 1 A.g −1 scanning rate is about 117.3 F.g −1 . This value for the capacitance of the prepared capacitor electrode is the result of relatively high surface area of the activated carbon (1176 m 2 g −1 ), high micropore surface area of about 893 m 2 g −1 and also the relatively high nitrogen and oxygen content of the porous activated carbon. In Table 2, the capacitance of some supercapacitors prepared from activated carbons has been compared. From this table it can be inferred that the ratio of the capacitance to the specific surface area of the activated carbon lies between 0.06 to 0.15, most of which are about 0.1. This emphasizes on the pronounced effect of the surface area, in comparison to the surface functionalities and the composition of the electrode materials, on the obtained capacitance. The capacitance to specific surface area ratio of the present work is about 0.1 which means that the obtained capacitance is in the expected value. With   Fig. 5c. This means that the rate capability of the prepared electrode is rather poor. High decreasing rates of the capacitance by increasing the current density may be attributed to the relatively high I d to I g ratio 56 of the synthesized carbon foam in Fig. 1c. High I d to I g ratio implies that the ratio of amorphous carbons in the prepared activated carbon is relatively high. This corresponds to the low electrical conductivity of the synthesized active carbons 13 . The poor retention of the capacitance with increasing the current density can also be attributed to, relatively, low ratio of the mesopore to micropore surface area of the synthesized active carbon (⁓ 0.21). Poor mesopore porosities in comparison to the micropores, results in the poor electrolyte transfer and this, in turn, results in the poor retention of the capacitance with increasing the current density.  www.nature.com/scientificreports/ In order to study the ionic transport mechanism of the prepared electrode, electrochemical impedance spectroscopy (EIS) was carried out on the supercapacitor electrode, the result of which has been shown in the Nyquist plot in Fig. 6. A small semicircle can be detected in the high frequency region (the inset in Fig. 6). This semicircle followed by a 45° slope line in the intermediate frequencies.
The series resistance which reflects the intrinsic resistance of the prepared electrode (Rp) and the electrolyte resistance in contact with the current collector (interface resistance) can be determined from the semicircle in the high frequency region. Rs can be determined from the left intercept of the Nyquist plot with the real x-axis which is about 1.7 Ω. Rp can be obtained from the right intercept of the Nyquist plot with the x-axis (about 2.1 Ω) and represents the internal or intrinsic resistance of the prepared electrode 59,60 . The obtained value for Rs and Rp is relatively high in comparison with some of the reported value 56,59,60 and this is in line with the low retained capacitance in Fig. 5c. The intermediate region of frequency represents the resistive behavior of the ions penetrating into the electrode pores (diffuse layer resistance) 61 . This value can imply the level of the mesopores available for the transportation of the ions into the micropores. For the prepared electrode the diffuse layer resistance is greater than 190 Ω which is to some extent high in comparison to some reported value 56,59,60 . This high value emphasizes that the ratio of meso to micropores in the prepared electrode is relatively low.

Conclusion
Nitrogen doped micro-mesoporous carbon foams has been synthesized using melamine and formaldehyde as nitrogen, oxygen and carbon precursors and Zn(NO 3 ) 2 .6H 2 O and F127 as the templates for pores generation. The surface characteristics of the carbon foams synthesized from different amounts of the Zn(NO 3 ) 2 .6H 2 O and pluronic F127 revealed the importance of these templates for the porosity formation. Pluronic F127 importance can be inferred from the specific surface area of carbon foams synthesized with (334.12 m 2 g −1 ) and without (1.02 m 2 g −1 ) using F127. The effectiveness of Zn(NO 3 ) 2 .6H 2 O is obvious from the specific surface area changes of the synthesized foams resulted from the different amounts of zinc nitrate hexahydrate. It has been confirmed that during the pyrolysis treatments zinc oxide particles have positive role in graphitization of carbons. The electrochemistry results confirm the formation of electric double layer capacitor with the capacitance as high as 117.3 Fg −1 . Figure 6. Nyquist plot of the supercapacitor electrode. The plot has been obtained from the electrochemical impedance spectroscopy (EIS) analysis results of electrode prepared from CF-6-1000, inset is the Nyquist plot at high frequency regions.