t enables production of nitrogen-doped porous carbons in an air oven †

The production of nitrogen-doped porous carbons in an air oven at 750 Cwith high surface area ($500m g ), carbonization yield up to 35wt% and tuneable nitrogen content from ionic liquids was reported. It utilized inorganic halide salts as the reaction medium, the activation agents, and the physical barrier. This method was successfully expanded to recyclable sea salt and natural sources, such as nucleobase (adenine) and biomass (oak leaves).


Introduction
Nitrogen-doped porous carbons have attracted considerable interest in the past decade due to their unique physical and chemical properties and a wide application scope in the elds of catalysis, environment techniques and energy generation and storage. 1,2The doping of nitrogen atoms in the carbon framework is well known to adjust the physical properties of carbon materials by increasing the overall electron density at the Fermi level, which improves the electric conductivity and oxidation stability. 3,4][7][8][9][10][11][12][13][14] Meanwhile, the unusual benet received via nitrogen doping in carbons can be further amplied in a porous conguration that bears a high surface area to accelerate the mass/energy transport and boost their materials performance in electrochemical devices, such as lithium-ion batteries, electrochemical double layer capacitors and fuel cells. 2,14,15][18][19][20][21] The former deposits nitrogen atoms preferentially on the carbon surface, while the latter provides incorporation of structural nitrogen atoms into the bulk.Independent of the choice of these two methods, production of nitrogen-doped porous carbon under an oxygen-poor atmosphere is considered as a prerequisite, since the presence of oxygen at the carbonization temperature (typically 600 C or above) will heavily attack the carbon product and turn it into gaseous compounds, usually CO or CO 2 .This oxidation effect is much strengthened for porous carbons, where enormous surface area is available to undergo and accelerate the oxidation reaction.A literature survey indicates that the production of porous carbons with high specic surface area, S BET ($500 m 2 g À1 ) directly in air is a rare case, not to mention the possible doping of nitrogen.Stucky et al. reported recently that an ultrasonic spray pyrolysis method with consecutive thermal treatment of conned coreshell microparticles could create porous carbon microspheres (nitrogen-free).In this case the porous carbon core of micrometer size was conned and protected by a thin, dense TiO 2 shell for the pyrolysis in air. 22However, the necessity of particular equipment, complex design of the core-shell morphology, and the post-synthesis treatment to etch the TiO 2 scaffold limits the advantage of this process over the common preparation methods under inert atmosphere. 23In fact not only the noble gas as protecting atmosphere but also the engineering setups increase the nal production cost and complexity.From this point of view, production of nitrogen-doped porous carbons bearing a high S BET without noble gas protection is not only scientically challenging but also of practical values.
In this work a sustainable and easy-to-operate method to produce porous nitrogen-doped carbon materials in air via a salt-assisted approach is demonstrated.Specically speaking, common inorganic halide salts, NaCl and ZnCl 2 were employed as the high-temperature ionic solvent, the activation agents to introduce pores into carbons, and the protecting barrier to conne a local oxygen-poor environment.Very interestingly this concept can be straightforward extended to natural sources, such as sea salt as reaction medium, and nucleobase or oak leaves as carbon precursor.The general synthetic route is illustrated in Fig. 1.

Materials
All chemicals were used as supplied.Reagents for the syntheses of IL 4 and IL 5 were purchased from IoLiTec company, and the rest were synthesized by ourselves according to our previous route. 24The salts NaCl (99%) and ZnCl 2 (99%) were purchased from Sigma-Aldrich.All solvents used were of analytic grade.Ultrapure Milli-Q water was used in all experiments.Oak leaves were collected from a thicket located in Potsdam, Germany.

Synthesis of nitrogen-doped porous carbon from ILs
In a general procedure 1.5 g of carbon precursors was mixed with salt combination of NaCl and ZnCl 2 or Red Sea salt in a dened weight ratio listed in Table 1.The starting mixture was dissolved in 6 ml of water in a porcelain crucible, heating at 60 C for 15 minutes under stirring.The sample was heated up to 250 C (heating rate 4 C min À1 ) in air oven and the temperature was maintained stable for 2 hours.Aerwards the sample was directly covered with NaCl or sea salt (roughly 60 g). 3 ml of water were dropped on the surface in order to melt a thin layer of salt and then let it evaporate at room temperature for 30 minutes.The sample was placed in an air oven previously heated at 750 C for the desired time and aer that the sample was pulled out of the oven and cooled down to room temperature.The salt was then simply dissolved in water and the resulting carbon was treated with 1 N HCl aqueous solution for two days in order to remove nal impurities from the pores.The product was separated by centrifugation and washed several times with water until neutral pH and then with mixtures of water-ethanol.The as-prepared carbon material was nally dried under vacuum at 80 C overnight.

Synthesis of nitrogen-doped porous carbon from IL 1 under N 2 ow, with and without ceiling
The mixture of IL 1 : ZnCl 2 : NaCl (weight ratio of 1 : 0.3 : 1) in water was dried in air oven at 250 C for 2 hours, and the resulting carbonaceous matrix was then covered with ned grinded NaCl.The starting material was placed in the oven at room temperature and ushed with nitrogen ow for one hour before starting the heating in order to avoid eventually oxidation reactions during thermal treatment.The temperature was then increased with a heating rate of 10 C min À1 up to 750 C and the sample was held at this temperature for 30 minutes.Aer the treatment the sample was le inside the chamber until the temperature was cooled down naturally.The same purication procedure as the carbon produced in air oven was performed (see Section 2.2).When carbonization under nitrogen without NaCl ceiling on the top was performed, the procedure is exactly the same, except the sample was directly carbonized aer the drying process (heating rate 10 C min À1 , holding time 30 minutes).

Synthesis of nitrogen-doped porous carbon from adenine
We used the same procedure reported in Section 2.2 with adenine instead of IL.The weight ratio is adenine : ZnCl 2 : NaCl ¼ 1 : 0.3 : 1.

Synthesis of porous carbon from oak leaves and red sea salt
Oak leaves were rstly washed with water and then dried at 80 C overnight.Aer grinded 1.5 g of leaves, a 1 : 1 weight ratio of sea salt was added and mixed carefully.The solid mixture was transferred into a crucible and fresh sea salt (roughly 60 g) was used to generate the connement.3 ml of water were dropped on the surface to melt a thin layer of salt and then it was dried at room temperature for 30 minutes.The sample was then placed directly at 750 C in air oven for 30 minutes.The work-up of the product purication was the same as reported for ILs (see Section 2.2).

Characterization methods
Nitrogen sorption measurements were performed on Quantachrome Autosorb-1 at 77 K (liquid nitrogen temperature).Samples were degassed prior to the measurements at 150 C for 20 hours.The specic surface area is calculated according to the Brunauer-Emmett-Teller (BET) equation, while quenched solid density functional theory (QSDFT) for cylindrical pores is applied for the evaluation of pore size distribution.This latter method takes in consideration the surface roughness and heterogeneity of the material, representing a more precise and realistic model for the surface area interpretation here.Combustion elemental analysis (C/H/N content) was carried out for all samples using Vario Micro setup (version 1.4.1).Thermal gravimetric analyses (TGA) were performed on a Netzsch TG209-F1 apparatus at a heating rate of 10 C min À1 .High resolution transmission electron microscopy (HR-TEM) and elemental mapping investigation were carried out on a JEOL JEM-2200FS instrument (JEOL GmbH, Eching, Germany).For sample preparation, one drop of the sample dispersion in ethanol was placed on a 200 mesh carbon coated copper grid and dried in air.TEM investigations were carried out at temperature around 90 K, operating at an acceleration voltage of 200 kV.All images were recorded digitally by a bottom-mounted 4k*4k CMOS camera system using low dose conditions (TemCam-F416, TVIPS, Gauting, Germany) and processed with a digital imaging processing system (EM-Menu 4.0, TVIPS, Gauting, Germany).Xray diffraction experiments were done with a Bruker D8 diffractometer using Cu K a radiation (l ¼ 0.154 nm) and a scintillation counter.XPS data were obtained on Thermo ESCALAB250 instrument with a monochromatized Alkaline source (200 W).A Controller P320 professional furnace (Nabertherm Company) was used to perform all the carbonization processes.Porcelain crucibles of 40 mm in diameter, purchased from VWR Company, were used for all the syntheses.FT-IR spectra were recorded on a Varian1000 FT-IR spectrometer.SEM image was obtained on a LEO 1550-Gemini instrument with platinum sputtering.

Results and discussion
Imidazolium ionic liquids (ILs) are employed as model carbon precursors in this preparation process because of their negligible vapor pressure, high thermostability and the signicant presence of nitrogen in the nal carbon. 6Some recent reports have proven that a carbonization yield of 10-30 wt% at 800-1000 C and a tunable nitrogen content in terms of the chemical design can be reached via an IL route. 6,9Particularly from a structure point of view, the network-forming groups in ILs are known to build up crosslinked intermediates to improve the structural stability and correspondingly the carbonization yield.In this study ILs with different functional groups, shown in Fig. 1, are tested.In a typical run a mixture of IL, NaCl and ZnCl 2 in a dened weight ratio is homogeneously dissolved in water in a porcelain crucible, and then thermally treated at 250 C in air for 2 h.This step dried the mixture and produced a solid black solution.In the presence of cyano and vinyl groups in ILs, trimerisation and the thermal polymerization occurred in parallel, which formed a crosslinked network (Fig. S1 †) and favorably stabilized the carbon intermediate.Aerwards, nely grinded NaCl powder was packed directly above the black composite in the crucible.It is important to spread a few water drops to partially wet the upper NaCl surface, which becomes highly dense aer thermal evaporation of water to reach a maximum space connement.The NaCl ceiling acts here as a solid physical barrier to minimize the oxygen entrance.In addition, this connement can retard the diffusion of the gasied decomposition products, mostly the hydrocarbons, out of the reactor, thus raising the overall carbonization yield.
The carbonization took place via inserting the crucible reaction in an air oven at 750 C, i.e. below the melting point of NaCl (801 C) for 30 min.At this stage, several events occur parallel.ZnCl 2 (m.p. $292 C) melts and dissolves NaCl at the bottom of the crucible, creating an ionic, homogeneous, liquid environment for the carbon formation, in which ZnCl 2 initiates the carbon activation reaction.It should be claried that although the NaCl top layer remains solid at the carbonization temperature to create a closed environment, the bottom part may join the molten salt due to the direct contact with the ZnCl 2 /NaCl liquid mixture.Finally, the mixture was pulled out of the oven for cooling down to room temperature.The dark mixture was treated by acidic water before completely dried.Table 1 collects characterization data of the as-synthesized carbon products.To reach a comprehensive view of the scope and applicability of this method, we systematically varied the experimental parameters, such as the amount/ratio of ZnCl 2 and NaCl, the carbonization time and the IL chemical structure.The S BET , pore volume, carbonization yield and elemental analysis results of the nal carbon product were measured and recorded.For the sake of conciseness and clearness, the carbon products derived from ILs were denoted as IL w Z x S y T z , in which w, x, y and z stand for the IL type, the wt% of ZnCl 2 with regard to IL, the wt% of NaCl, and the carbonization time at 750 C in min, respectively.As shown in Table 1, porous carbons with large S BET and high nitrogen doping were indeed received via this route.As a representative product, IL 1 Z 0.30 S 1 T 30 (Entry 3 in Table 1), which used IL 1 as the carbon precursor and a salt mixture of 30 wt% of ZnCl 2 and 100 wt% of NaCl, was chosen to demonstrate the structural properties of the as-synthesized porous carbons.Fig. 2A displayed the nitrogen sorption isotherm measured at 77 K.A type-I isotherm is clearly identi-ed and proves the dominant existence of micropores and a low fraction (<10%) of small mesopores in this carbon product.The pore volume is 0.453 cm 3 g À1 , and a S BET of 950 m 2 g À1 is reached.Quenched solid density functional theory (QSDFT) is used to determine the average pore size, which is in the range of 1-5 nm (Fig. S5 †, red curve).It should be mentioned that in some carbon products prepared at different conditions listed in Table 1, their sorption isotherms (Fig. S5-S8 †) have a broad hysteresis at high values of relative pressure.This indicates interstitial pore formation that is related to the morphology of the carbon particle aggregation discussed later.The crystalline status is characterized via X-ray diffraction (XRD) measurements.As shown in Fig. 2B, characteristic bands at 26 , 43 and 81 were observed, which are assigned to the (002), (100) and (110) diffraction bands of graphitic phase, respectively.The weak intensity at 81 implies that the obtained carbon lacks of a long range order, i.e. turbostratic.
To visualize the local atomic order, high resolution transmission electron microscopy (HR-TEM) was employed.Irregularly bent planes of various lengths are clearly visible in Fig. 2C.Scanning and transmission electron microscopy shows that IL 1 Z 0.30 S 1 T 30 is composed of aggregated primary carbon nanoparticles of 20-50 nm in size (Fig. S2 †).This kind of morphology of carbon particles was also observed in many other carbons in Table 1.The packing of the carbon nanoparticles densely together creates interstitial voids, responsible for the broad hysteresis at high values of relative pressure of the nitrogen sorption isotherms of some carbon products.Combustion elemental analysis reveals a high nitrogen content of 15 wt%, which is homogeneously distributed, as conrmed by elemental mapping of the C and N atoms based on energy dispersive X-ray spectroscopy (EDX, Fig. 2D and E).The elemental composition of the sample surface was further checked by X-ray photoelectron spectroscopy (XPS) which depicts the presence of carbon, nitrogen and oxygen in 77.4,14.1 and 8.5 wt%, respectively (Fig. S3 †).The C and N values are close to that obtained by combustion elemental analysis method, which agrees with the EDX analysis on a homogenous distribution of the elements across the sample.A close view of the chemical states of N1s is reported in Fig. 2F.Typical binding energies of pyridinic (30.6%, 398.2 eV), pyrrolic (28.1%, 399.8 eV), quaternarygraphitic (34.0%, 400.8 eV) and oxidic-N species (7.3%, 405.1 eV) are found, proving the presence of structural nitrogen atoms intercalated in the material matrix.The existence of an oxidic-N species is a nature outcome of the carbonization process, where a small fraction of oxygen is incorporated (Fig. S4 †).For the sake of completeness, control experiments under nitrogen ow were also performed (named "N 2 carbon").In detail, the production of the sample IL 1 Z 0.30 S 1 T 30 was tested under inert gas ow both with and without NaCl ceiling.The resultant N 2 carbons showed lower surface areas and pore volumes compared with the carbon obtained in air oven.Specically 766 m 2 g À1 and 0.371 cm 3 g À1 for the salt-conned N 2 carbon and 763 m 2 g À1 and 0.328 cm 3 g À1 for the N 2 carbon without NaCl ceiling were obtained (Fig. S11 and S12, † respectively).The nitrogen content of N 2 carbons was similar to that produced in air oven (15.7 wt% for the salt-conned N 2 carbon and 14.7 wt% for the N 2 carbon without NaCl ceiling), while a slightly higher yield was observed (34.5 wt% for the salt-conned N 2 carbon and 36.4 wt% for the N 2 carbon without NaCl ceiling).
As a commonly used activation agent, the effect of ZnCl 2 is rst investigated (Entry 1-6 in Table 1, Fig. 3A and S5 †).While xing the weight ratio of IL 1 : NaCl to 1 : 1 and varying that of ZnCl 2 : IL 1 gradually from 0 to 100 wt%, the S BET increased dramatically upon the initial addition of ZnCl 2 until 30 wt%.Beyond this value, it remains constant around 900 m 2 g À1 .In comparison, the ZnCl 2 -effect on the carbonization yield is more complex.Addition of a small portion (#10 wt%) leads to a high carbonization yield up to 31 wt%.Above 10 wt% of ZnCl 2 , the carbonization yield drops.At the maximum ZnCl 2 addition (100 wt%) in the tests (Entry 6 in Table 1), an unfavorable low yield of only 8.4 wt% was received.The complex effect of ZnCl 2 is caused by the double role of ZnCl 2 in the carbonization process.It serves as the activation agent for the pore formation, which creates surface area and suppresses the yield.Meanwhile, it catalyzes the trimerisation of the cyano groups, which construct stable intermediates to reduce the structural fragmentation at the carbonization temperature, thus increasing the yield.Our data supports that a low fraction of ZnCl 2 promotes the formation of crosslinked networks, and overcompensates the weight loss caused by the activation.Too much addition of ZnCl 2 however makes the etching process more dominant.It is not surprising that the highest yield is found at a balanced content of ZnCl 2 , here 10 wt% (Entry 2 in Table 1).
It is curious to nd that even in the absence of ZnCl 2 (Entry 1 in Table 1), a porous carbon with a medium S BET of 200 m 2 g À1 was obtained.This nding tells that in the current process, NaCl seems to be not only a salt merely to dilute the ZnCl 2 concentration, but also a co-activation agent, though weak.The role of NaCl (not the top NaCl layer) was thus thoroughly investigated (Entry 7-10 in Table 1, Fig. S6 †).As illustrated in Fig. 3B, in the presence of 30 wt% of ZnCl 2 , NaCl in a low fraction (30 wt%) increases the S BET from 419 up to 809 m 2 g À1 , indicating its participation in creation of porosity.Unlike ZnCl 2 , at even higher concentrations of NaCl above 30 wt%, the pore features and the yield of the nal carbon products are unaffected.
As it is true for all carbonization processes, the dwelling time is crucial, necessary means to control the physical properties of the carbons.To understand the pore development, experiments with several duration times were performed.We xed the IL 1 : ZnCl 2 : NaCl ratio to 1 : 0.3 : 1 (Entry 11-13 in Table 1, Fig. 3C and S7 †).It was found that the S BET increases from 813 (15 min) to 1130 m 2 g À1 (2 h), indicating the activation continuously introduces more porosity.In contrast, the carbonization yield decreases from 28.3 wt% to 8.4 wt% gradually due to the prolonged etching time and a possible entrance of oxygen into the reactor upon prolonged dwelling times.It is particularly worth noting that only aer 15 min the S BET jumps already to 812 m 2 g À1 , implying a fast conversion of the precursor to porous carbons.
The chemical structure of ILs is important to modulate the physical property of the carbon product.Besides IL 1 , four other imidazolium-based ILs (Fig. 1) with different functional groups are tested in order to reveal the relationship between the IL chemical structure and the physical feature of the as-synthesized carbons.A carbonization time of 30 min, and a starting mixture of IL : NaCl : ZnCl 2 in ratio of 1 : 1 : 0.3 were xed for all runs (Entries 14-17 in Table 1, Fig. 3D and S8 †).
When the cyano group in IL 1 is replaced by a methyl group in IL 2 , a yield of 11.5 wt%, and a surface area of 502 m 2 g À1 was observed (Entry 14 in Table 1).Both values are signicantly lower than that of IL 1 .It proves the necessity to bring the cyano group into the IL chemical structure.Keeping the cyano functionality unchanged but substituting the vinyl with a methyl group, i.e.IL 3 , a comparable surface area of 920 m 2 g À1 is obtained (Entry 15).Yet the carbonization yield drops by 9% to 20.5 wt%.As discussed above, the absence of the vinyl group fails to generate polymer chains that undergo crosslinking by the cyano groups, thus amplifying the structural fracture effect.A benecial of IL 3 is the high nitrogen content, 22 wt%.When both cyano and vinyl groups are absent, i.e.IL 4 , it is not astonishing to observe a carbonization yield below 1 wt% (Entry 16), a useless precursor for carbon production.Finally IL 5 , bearing a cyano-containing anion was tested.A signicant high nitrogen content, 32 wt% (C/N ratio of 1.4) was observed.Unfortunately, the yield is as low as 4.9 wt%.Therefore, both the vinyl and cyano groups on the cation are of key importance.
The central spirit of carrying out carbonization process without noble gas protection is to create a local oxygen-poor milieu to run oxygen-sensitive high temperature reaction.We should note that an absolute oxygen-free circumstance throughout the entire carbonization process is only idealistic.Though the top NaCl oor is dense, microchannels do exist.The reaction mixtures aer 10 and 30 min were illustrated in Fig. 4. The 10 min sample displays, besides a black IL/ZnCl 2 /NaCl bottom body, a grey top NaCl zone.This is an indication that following the initial violent decomposition of the ILs at 750 C, the gaseous fracture compounds, majorly the carbohydrates, build up inner pressure to force the gas diffusion through the interstice in the top NaCl ceiling.The grey color thus stems from the carbon deposition of the gaseous compounds.This overpressure blocks the intrusion of the oxygen into the reactor and guarantees a true inert atmosphere at this stage.The white color of the top NaCl zone aer 30 min however implies that aer the initial conversion of the ILs quickly into carbons, the major reaction switches to the chemical activation by ZnCl 2 , a relatively mild process.The high pressure built up at the initial IL thermal decomposition stage vanished, which enables the (partial) entrance of oxygen, though slow, through the microchannels in the NaCl zone into the reactor and oxidizes the formed carbons into CO or CO 2 .This is in part responsible for the low yield upon elongated carbonization (Fig. 3C).From this point of view, 30 min is an ideal reaction period to minimize the oxygen disturbance.It should be noted that due to the oxygen penetration, it may very possibly create a gradient oxidative carbon product along the sample from the top to the bottom.
The implementation of the carbonization in air without noble gas protection relies on the isolation of the reactor from the surrounding by a salt shielding below its melting point.We were motivated to modify this concept into a general and more sustainable methodology.Two key factors attract our attention, the salt mixture and the carbon precursor.In the former case, we could use sea salt to replace ZnCl 2 /NaCl.The sea salt we tested is provided from RedSea© and it is usually used to replicate reef water system.Since the dominant fraction is NaCl, it serves favorably as the ceiling layer.Meanwhile, due to the coexistence of a small fraction of MgCl 2 (3.3 wt%), CaCl 2 (1.1 wt%) and KCl (1.0 wt%) species, it can conduct activation like ZnCl 2 .In the test run using IL 1 as a carbon precursor, a yield of 10 wt% in 30 min and 11 wt% of nitrogen doping were obtained (Entry 18 in Table 1).The S BET is 750 m 2 g À1 , with pore volume of 0.406 cm 3 g À1 .These exciting results prove that the natural sea salt can replace the NaCl/ZnCl 2 binary system.An additional merit of sea salt is the easy recyclability due to the identical salt composition all over the reactor.In a 3-recycle test, the sea salt preserves well its shielding and activation function (Entry 19 and 20 in Table 1 and Fig. S9 †).
In the latter case to replace ILs as carbon precursor, we were motivated to employ natural raw materials.Nucleobases such as adenine, possess a high atomic N/C ratio, high oxidation stability, and suitable atomic N-C-N bonding motifs.They can be isolated from nature in larger quantities, and have been explored for functional carbon synthesis very recently. 1Here adenine was tested in the ZnCl 2 /NaCl system.The formed nitrogen-doped porous carbons showed a high carbonization yield of 32 wt%, a high nitrogen content of 31 wt% and a satisfactory S BET of 412 m 2 g À1 (Entry 21 in Table 1, Fig. S10 †).In addition to nucleobase, biomass is also applied.For example, the oak leaves have been successfully converted to porous carbon using sea salt (Entry 22 in Table 1, and Fig. S13 †).As shown in Fig. 5, the oak leaves were collected and grinded with the sea salt into ne powders to reach a homogeneous mixture.Upon the carbonization procedure mentioned above (see Section 2.5), ne carbon powders with S BET of 600 m 2 g À1 were achieved.The nitrogen sorption isotherm is shown in Fig. 5D.Beside a signicant amount of micropores, the sample contains a considerable fraction of mesopores.The pore volume is 0.45 cm 3 g À1 , which contains a mesopore volume of 0.33 cm 3 g À1 .Its nitrogen content is relatively low, only 1.6 wt%, which comes from the proteins existing in the oak leaves (elemental analysis of fresh oak leaves showed nitrogen content of 1.1 wt%).

Conclusions
In summary, we introduced a general synthetic route to produce nitrogen-doped porous carbons with high surface area and  tunable nitrogen content in an air oven.This conceptually straightforward production procedure is easy to implement.Not only the salt can be replaced by recyclable sea salt, but also the carbon precursor is accessible from natural sources, thus pointing out a more sustainable way for the nitrogen-doped carbon production.Furthermore, the method is expected to be able to conduct many other high temperature chemical processes in air instead of under inert gas environment, for instance, the preparation of metal nanoparticles, and the synthesis of metal carbide or nitride.

Fig. 1
Fig. 1 Schematic representation of the carbonization process in an air oven.The chemical structures of the used ILs are illustrated beneath.DCA represents the dicyanamide anion.

Fig. 2
Fig. 2 Characterization of the carbon product IL 1 Z 0.30 S 1 T 30 : (A) nitrogen adsorption/desorption isotherm curve; (B) XRD pattern; (C) HRTEM image; (D and E) elemental mapping of carbon and nitrogen elements spectroscopy, respectively (obtained via SEM-EDX spectroscopy); (F) XPS spectrum of the nitrogen element.

Fig. 3
Fig.3The variation of the specific surface area and the carbonization yield of the nitrogen-doped porous carbons prepared at different concentrations of ZnCl 2 (A), NaCl (B) and different period (C).Yields from different IL precursors are also reported (D).

Fig. 4
Fig. 4 Illustration of the reaction mixture after 10 and 30 min.The two photographs on the right side show the reaction mixture at 10 and 30 min.

Fig. 5 (
Fig.5 (A-C) Photographs of pristine oak leaves, grinded powder of oak leaves and sea salt, and the carbonization mixture after 30 min at 750 C. (D) N 2 sorption isotherm of the carbon prepared from the oak leaves.

Table 1
Characterization of nitrogen-doped porous carbons prepared via carbonization in an air oven under different reaction conditions.The EA results were obtained from the combustion EA analysis a SS: sea salt.These are the 3 recycling tests.IL 1 : SS weight ratio is 1 : 1. b Using adenine and oak leaves (OL) as the carbon precursor.