Heteroatom-doped porous carbon derived from “all-in-one” precursor sewage sludge for electrochemical energy storage

Abstract

With the imposition of more stringent regulations governing the disposal and use of sewage sludge, the need to develop more cost-effective and environmentally benign re-uses of sewage sludge is of particular concern. Pyrolysis converting sewage sludge to porous carbons is an emerging technology for the disposal of huge amounts of sewage sludge. In this study, a unique heteroatom-doped porous carbon was prepared via the direct pyrolysis of sewage sludge with SiO₂, transition metals and organic matter, the special components of sewage sludge, acting as an in-built template, the graphitizing catalysts, and the ideal precursor and nature dopant, respectively. The as-prepared N, O-doped porous carbon had a high specific surface area, numerous heteroatoms, good electrical transport properties and a meso-/macroporous composite. It exhibited favorable charge storage capacity and good stability over 10 000 cycles. The supercapacitor performance results from its hierarchical porous structure and heteroatom doping effects, which combine electrical double-layer capacitors and Faradaic contributions. Our protocol demonstrates a new approach for the potentially eco-friendly benign re-use of sewage sludge and provides a proven technique for synthesizing electrode materials as promising candidates for electrochemical energy storage.

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1. Introduction

Sewage sludge, which is the residue generated from the activated sludge method, consists of organic material, mainly dead bacterial cells and organic pollutants, and inorganic components in the form of silica, iron salts, calcium oxide, alumina, magnesium oxide and a wide variety of transition metals (e.g., Cr, Co and Ni). The European Environmental Agency defines it as “a future waste problem”.^1,2 With the increasing generation of sewage sludge around the world, the disposal of sewage sludge has remained a thorny issue to date.³ Recently, this issue has been further aggravated by the imposition of more stringent regulations governing the disposal of sewage sludge. Traditional options for sludge disposal such as combustion, landfilling or ocean dumping are no longer acceptable.^4,5 In this situation, the initiation of more cost-effective, environmentally benign re-use of sewage sludge is certainly necessary from an environmental standpoint.^6,7

Sludge pyrolysis, different from sludge combustion, is a proven, innovative technology that can convert approximately half of the organic matter in sewage sludge into useful bioenergy (oil and gas). This process can also immobilize the rest of the organic and inorganic matter into a stabilized form of pyrolytic residue (biochar).^8,9 Furthermore, sludge pyrolysis is often carried out under inert atmospheric conditions, which precludes the production of highly toxic dioxin-like compounds and particulate matter (e.g., PM2.5). The gases and oils produced in the sludge pyrolysis process can be used as renewable liquid fuels and chemical feedstocks.^10,11 However, no value-added utilization has been found for biochar, which is a carbonaceous matrix byproduct that is environmentally resistant.

Supercapacitors are energy storage devices that accumulate energy in the form of electrical charge and bridge the gap between dielectric capacitors and batteries. These devices are attracting considerable attention due to their high specific power, short charging time and long cycle life.^12,13 Porous carbonaceous materials have been found to be promising candidates for electrical double-layer capacitors (EDLCs) because they possess a large specific area, a more favorable cycle durability and a high level of electrical conductivity.^14,15 Recently, the synthesis of porous carbonaceous materials derived from waste and biomass for making energy storage materials has attracted considerable attention. For instance, heteroatom-doped porous carbon flakes, porous carbon materials and hierarchically porous carbon nanosheets, which were prepared via carbonization of human hair fibers, broad beans and waste coffee grounds respectively,^16–18 were used for supercapacitor electrode materials, and exhibited high specific capacitance. Compared with conventional carbon precursors (e.g., wood, coal, pitch or nutshell), the cost-effectiveness and environmental friendliness of waste and biomass-derived porous carbon materials make them more suitable for large-scale production and for various practical applications.^16–18 Although biochar produced from sludge pyrolysis holds potential for carbon sequestration, the use of sewage sludge to prepare carbon nanomaterials for EDLCs has not yet been achieved.

In this study, for the first time, we demonstrate the synthesis of heteroatom-doped carbon materials for EDLCs via the direct pyrolysis of the “all-in-one” precursor sewage sludge. During this pyrolysis process, SiO₂, a special component of sewage sludge, acts as an in-built template that prevents agglomeration and results in the formation of a unique pore size distribution. The organic matter in sewage sludge exhibited as the structure-directing templates through carbonization and graphitizing under the catalytic action of the transition metals. These organic matter abundant in nitrogen and oxygen also serve as the source of heteroatom dopant, which leads to a significant enhancement in the charge storage capacity. The as-synthesized heteroatom-doped carbon material has a high specific surface area, numerous heteroatoms, good electrical transport properties and a meso-/macroporous composite. The electrochemical results demonstrate its favorable charge storage capacity and good stability. The results of our study provide a novel route for the synthesis of heteroatom-doped carbon materials for use in energy storage devices by using a simple, low-cost, green process. This study therefore demonstrates a new approach for the value-added re-use of sewage sludge.

2. Experimental section

2.1. Synthesis of the sewage sludge-derived carbon material

As the raw material for this process, a dewatered sewage sludge sample was obtained from the Anting wastewater treatment plant in Shanghai, China.⁷ The obtained sludge was stored at −20 °C before use. All other reagents were of analytical grade and were used as received unless otherwise stated. All of the water used was prepared with a purification system (Hitech Instrument Co., Shanghai, China).

The sewage sludge-derived heteroatom-doped carbon material was synthesized by a facile one-step pyrolysis process in a quartz tubular reactor (Fig. S1†). In brief, the lyophilized sewage sludge was first placed in the quartz tubular reactor under 100 ml min⁻¹ of N₂ flow for 30 min to remove air. Then the quartz tubular reactor was heated to 800 °C at a heating rate of 5 °C min⁻¹ under a 50 ml min⁻¹ of N₂ flow, and held at that temperature for 2 h. After being cooled to ambient temperature in the N₂ flow, a resultant black power was obtained and designated as SS-800. The SS-800 was immersed in 20 wt% HF to etch-out the SiO₂, then washed until the pH reached 7, and dried at 100 °C. The product of this process was the sewage sludge-derived carbon nanomaterial (SS-NC).

2.2. Characterization

Scanning electron microscopy (SEM; FEI Nova-Nano, The Netherlands) was used to image the morphology of the as-synthesized sewage sludge-derived nanomaterials. The analysis of the surface area, pore diameter and volume was carried out by using the Brunauer–Emmett–Teller (BET) method on an AUTOSORB-IQ instrument (Quantachrome Co., USA). The X-ray diffraction (XRD) patterns were measured with an X′ Pert PRO system (Philips Co., The Netherlands) to characterize the crystal structure of the nanomaterials. The functional groups of the as-prepared catalysts were determined by Fourier transform infrared (FTIR) spectra recorded with a VERTEX 70 FT-IR (Bruker Co., Germany) and by Raman analysis with a laser Raman spectrometer (ProTT-RZRaman-B2, Enwave Optronics Inc., USA). The electronic environment of the nanomaterials was investigated by using X-ray photoelectron spectroscopy (XPS, PHI-5000C, Perkin-Elmer Co., USA). The composition of the sewage sludge-derived carbon nanomaterials was measured via energy-dispersive X-ray spectroscopy (EDX) and inductively coupled plasma spectrometry (ICP, Agilent 720ES, USA) after total digestion in a microwave using a mixture of HNO₃ + HCl + HF. The C, O and N contents of the materials were also determined by a Vario EL III Elemental Analyzer (vario EL III, GmbH, Germany).

2.3. Electrochemical characterization

All of the electrochemical tests were carried out on a CHI760E electrochemical workstation (Shanghai Chenhua Instruments Co., China) at room temperature in a conventional three-electrode system. An aqueous solution of 0.5 M Na₂SO₄ was used as the electrolyte, a platinum wire was used as the counter electrode and an Ag/AgCl electrode served as the reference electrode. To prepare the work electrode, a slurry containing 80 wt% active material (e.g., SS-800, SS-NC), 10 wt% carbon black (Vulcan XC-72R, surface area 254 m² g⁻¹, Cabot Co., USA) and 10 wt% polytetrafluoroethylene was mixed and loaded on a nickel foam substrate. After being dried in an oven at 353 K for several hours, the as-prepared work electrode was pressed at 15 MPa to assure good electrical contact between the nickel foam substrate and the active material. The electrode was then further dried in an oven at 373 K for several hours. The total mass of the active materials on the nickel foam substrate was about 2–3 mg per electrode with a surface area of 1.0 cm².

Cyclic voltammetry (CV) curves were obtained in the potential range of 0–0.8 V vs. Ag/AgCl by varying the scan rate from 5 to 100 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was measured in a frequency range of 10⁵ Hz to 0.01 Hz at open circuit voltage with an alternate current amplitude of 5 mV. The EIS results were fitted by ZView software according to the corresponding equivalent circuits. Galvanostatic charge/discharge was carried out at 0.5–8 A g⁻¹ over a voltage range of 0–0.8 V vs. Ag/AgCl, and the specific capacitance was calculated from the galvanostatic discharge curves according to the following equation (eqn (1)):

C = IΔt/mΔV (1)

where C (F g⁻¹) is the discharge specific capacitance, I is the discharge current, Δt is the discharge time consumed in the potential range of ΔV and m (g) is the weight of the active materials loaded on the work electrode.

3. Results and discussion

3.1. Textural properties of the as-synthesized nanomaterials

The surface morphology and structure of the as-synthesized sewage sludge-derived nanomaterials were examined using SEM, and the typical images are shown in Fig. 1. Numerous particles with diameters ranging from about 20 nm to 2 µm were identified by EDX as being composed of SiO₂, a special component of sewage sludge. These particles, inlaid into the nanomaterials derived from the sewage sludge (Fig. 1A and B), acted as the in-built template during the synthesized process. This template prevented agglomeration and resulted in the formation of mesoporous structures combined with graphitization or evaporated of the organic matters in sewage sludge during the pyrolysis process (Fig. 1A and B). The highly porous structures around the SiO₂ particles (Fig. 1B) were formed by the partial graphitization of the organic matter as the carbon precursor.¹⁹ After being immersed in HF, all of these SiO₂ particles vanished. Also, the rough mesoporous structures displayed accumulations of numerous particles with nanometer-sized diameters that increased the surface area of the catalyst (Fig. 1C and D). This accumulation was made further evident by the increased BET surface area of the as-synthesized nanomaterial SS-NC compared with that of the SS-800 (Table 1).

Fig. 1 Textural properties of the as-synthesized catalysts. SEM images of the as-prepared SS-800 (A and B) and SS-NC (C and D).

Table 1 Properties of the as-synthesized sewage sludge-derived catalysts

	SS-800	SS-NC
a Calculated from the Barrett–Joyner–Halenda equation using the desorption isotherm. b The contents of C and N were obtained by elemental analysis. c The content of O was obtained by EDX. d The contents of Si and metals were obtained by ICP.
S BET (m^2 g^−1)	107.44	331.39
Average pore size^a (nm)	8.06	11.88
Conc. (wt%)
C^b	31.13	51.14
N^b	0.69	1.28
O^c	22.40	18.46
Fe^d	11.87	4.15
Si^d	6.67	—
Al^d	0.02	0.03
Mg^d	0.06	0.40
Ca^d	0.59	0.87
Cr^d	0.03	0.02
Mn^d	0.12	0.03
Ni^d	0.08	0.04
Cu^d	0.11	0.11

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The surface area and pore structure of the as-synthesized sewage sludge-derived nanomaterials were analyzed by the nitrogen adsorption–desorption method. The representative results for SS-800 and SS-NC (Fig. 2A) showed characteristic type IV curves (according to the IUPAC classification) with sharp capillary condensation steps that are typical features of mesoporous solids.²⁰ The isotherms of the SS-800 and SS-NC all had a clear upward trend at a low relative pressure (P/P₀ < 0.4), suggesting that the as-synthesized nanomaterials were rich in micropores. The appearance of a sharp upward-type H3 hysteresis loop at P/P₀ > 0.4 implied the presence of abundant mesopores in the nanomaterials.^16,21 At high relative pressures before 1.0, the curve exhibited a slight upward tendency, which could be ascribed to the internal macropores that represented the accumulation of particles present in the nanomaterials.

Fig. 2 N₂ adsorption–desorption isotherms (A), pore size distributions (B) (A represents absorption and D represents desorption) and (C) XRD spectrum (G represents graphene and P represents pentlandite) of the SS-800 and SS-NC.

The pore size distributions further suggested the intrinsic hierarchical porous structure of the as-synthesized nanomaterials (Fig. 2B). The peaks observed at 4 nm could be interpreted as mesopores generated by the graphitization or evaporation of organic matters in the sewage sludge. Compared with the SS-800, the increased broad peak observed between 5 and 100 nm for the SS-NC may have corresponded to the mesopores and macropores generated by the removal of SiO₂. The BET surface areas of the nanomaterials significantly increased from 107.44 to 331.39 m² g⁻¹ with the etch-out of the in-built SiO₂ template. This increase was accompanied with an expansion of the average pore size from 8.06 to 11.88 nm due to the generated mesopores and macropores. This higher surface area of the SS-NC is advantageous for charge storage.^16,21

3.2. Structural characterization of the as-synthesized nanomaterials

XRD was used to investigate the phase structure of the as-synthesized sewage sludge-derived nanomaterials (Fig. 2C). Obvious diffraction peaks at 2θ = 20.9° and 26.7° were observed for the SS-800. These peaks corresponded to the typical crystallite structures of SiO₂ (JCPDS, File no. 33-1161) that originate from sewage sludge. However, such peaks disappeared in the SS-NC, indicating the total removal of the hard template. This development was further confirmed by the disappearance of the characteristic asymmetric stretching vibrations of the Si–O–Si (1036 cm⁻¹) peak in the FTIR (Fig. S2A†) and the elemental composition of the nanomaterials (Table 1). The HF wishing process could remove most of the unwanted elements. No Si content was detected for the SS-NC, and this disappearance of Si was accompanied by an increase in the C content from 31.13% to 51.14%.

The amount of Fe content in the sewage sludge and in the as-synthesized nanomaterials was significant, while the content of other heavy metals was relatively low (Fig. S2† and Table 1). The presence of heavy metals such as Fe and Cr would tend to improve the degree of graphitization.¹⁹ A well-developed peak at 22.3° and a weak peak at 43.8°, which corresponded to the (002) and (100) spacing of the graphene stacks, respectively,¹⁶ could be clearly observed. The pattern of the SS-NC also showed several peaks at 15.4°, 29.5°, 30.9°, 39.2°, 47.1° and 51.5°, which could be attributed to the pentlandite (JCPDS, File no. 08-0090) (Fig. 2C). This pattern suggested the existence of iron sulphide, which is also a type of energy-storage material and may have contributed to the excellent electrochemical performance.²² It should be noted that there was still some N and O content detected (Table 1), which would suggest that the as-synthesized nanomaterials were N- and O-doped.

FTIR and Raman spectra were used to characterize the structures of the as-synthesized nanomaterial surfaces (Fig. S3†). Comparing the FTIR spectra of SS-800 and SS-NC, it could be clearly seen that the characteristic peaks of carbon nanomaterial ranging from 1250 to 1750 cm⁻¹ appeared on both curves. These peaks indicated that the framework structures and functional groups were analogous to each other (Fig. S2A†).²³ The two peaks in the Raman spectra at around 1335 and 1532 cm⁻¹ were the characteristic D and G bands of carbon, which reflected the degree of disorder and the level of graphitization in the carbon materials, respectively (Fig. S2B†).^16,18,23 These measurements suggested that the obvious G peak, which accorded with the XRD results, was formed by the partial graphitization of the organic materials as carbon precursors under the catalytic action of the critical toxic heavy metals (e.g., Fe, Cr, Co and Ni) that were uniformly distributed in the sewage sludge. The relatively higher D peak implied defects from the ideal graphitic lattice, which might have been caused by the heteroatom doping effects. The significantly decreased numbers of peaks in the SS-NC compared with those in the SS-800 indicated the removal of impurities, e.g., inorganic matter, accompanied with the etching-out of the in-built SiO₂ template. This process would tend to generate considerable quantities of porous structures, increase the surface areas of the as-synthesized nanomaterials and expose the functional groups in the carbon wall to the pore surfaces.

XPS analysis was used to further analyze the content and chemical state of the elements in the as-synthesized nanomaterials (Fig. 3A). The binding energies were calibrated with respect to the C 1s peak at 284.6 eV. Obvious C 1s and O 1s peaks and fine N 1s, Fe 2p and S 2p peaks were all observed as expected. Compared with SS-800, the intensity ratio of Si 2p vanished, demonstrating the etch-out of the in-built SiO₂ template in accordance with the above-described results. As the positions of the elemental peaks depended on the local chemical environment, high resolution scans of C, O and N were performed and deconvoluted to obtain the corresponding atom binding states by searching for the optimal combination of Gaussian bands (Fig. 3B–D). In the case of the C 1s XPS spectrum of SS-NC, the four peaks at 282.8, 284.4, 286.0 and 288.4 eV were attributable to the C–O, C=C, C–C/N and C=O species, respectively (Fig. 3B).²⁴ The three components of the O 1s XPS spectrum, with their peaks at binding energies of about 530.7, 532.1 and 534.6 eV, corresponded to the C=O, C–O and O=C–O species, respectively (Fig. 3C). These components indicated the existence of some oxygen-enriched functionalities in the carbon frameworks. Such oxygen-enriched functionalities have been reported to improve wettability and to produce pseudocapacitance, resulting in better supercapacitive performance.²⁴

Fig. 3 X-ray photoelectron spectra of the as-synthesized SS-800 and SS-NC (A). The high resolution C 1s, O 1s and N 1s XPS spectra of the SS-NC are shown in (B), (C) and (D), respectively.

Sewage sludge contains essential proteins that are abundant in nitrogen. The high-resolution N 1s peaks of SS-NC fit into four types of nitrogen-containing groups at about 398.0, 399.7, 400.8 and 402.5 eV (Fig. 3D). The two peaks located at about 398.0 and 399.7 eV were attributed to pyridinic N and pyrrolic N species, which were assumed to have contributed to the pseudocapacitance. The peak located at 400.8 eV was attributed to quaternary N, which could enhance the electrical conductivity of SS-NC.^25,26 These results further indicated that N atoms had been successfully doped into the carbonaceous nanomaterials. The introduction of nitrogen into the carbonaceous nanomaterials may have imposed nitrogen-related Faradaic reactions in the electrodes and changed the electron distribution of the materials, thus increasing their supercapacitor performance and enhancing the wettability between the electrode materials and electrolytes.

3.3. Electrochemical performance of the as-synthesized nanomaterials as supercapacitor electrodes

Electrochemical tests were performed to explore the potential applications of the as-synthesized nanomaterials as electrode materials for EDLCs. The CV curves of XC-72R, SS-800 and SS-NC in 0.5 M Na₂SO₄ aqueous solution at a scan rate of 50 mV s⁻¹ are shown in Fig. 4A. The control XC-72R exhibited regular rectangular-shaped CV curves, indicating an ideal EDLC nature of the charge–discharge process. The approximately rectangular-shaped and symmetric CV curves of the SS-800 and SS-NC also indicated a dominant contribution of the capacitance from the EDLC. The gravimetric capacitance of the control XC-72R (as calculated from its CV curve with a potential scan rate of 50 mV s⁻¹) was 21.29 F g⁻¹, which was very close to the values reported in the literature²⁷ and the 22.03 F g⁻¹ exhibit by the SS-800. The SS-NC exhibited a much higher specific capacitance of 120.98 F g⁻¹. The significant increase of capacitance was mainly attributed to the etch-out of the in-built SiO₂ template, which would not only increase the specific surface area of the nanomaterials but also increase the content of C and the heteroatoms, thereby promoting conductivity and wettability (Table 1).^25,26 A Faradaic hump was observed at ∼0.10 V in the CV curve at low scan rate of 5 mV s⁻¹ (Fig. 4B), which was probably due to redox reactions of the doped heteroatoms such as pyridinic N and pyrrolic N species, the oxygenated functional groups on the carbon frameworks, or the remaining Fe compounds.^25,26 As the scan rate increased, the plateau current increased accordingly. The specific capacitance of the SS-NC was up to 178.32 F g⁻¹ at a potential scan rate of 5 mV s⁻¹, which was comparable to the capacitance of many other EDLCs reported previously.^16–18 The quasi-rectangular shape of the CV curve could still be maintained with little distortion even when the scan rate rose to 100 mV s⁻¹ (Fig. 4B), which indicated that rapid ion transport and good rate capability could operate in the SS-NC. The distortion of the CV curve reflected the universal characteristic that ion diffusion and transport on the EDLC electrode surface are restricted at high scan rates.

Fig. 4 Electrochemical performance of the as-synthesized nanomaterials as supercapacitor electrodes. (A) CV measurements of XC-72R, SS-800 and SS-NC in 0.5 M Na₂SO₄ aqueous solution over a potential range from 0 to 0.8 V at a scan rate of 50 mV s⁻¹. (B) CV measurements of SS-NC at different scan rates. (C) Charge–discharge curves of SS-NC at different current densities. (D) Specific capacitances of SS-NC at different current densities.

Galvanostatic charge/discharge experiments were carried out at different current densities to further explore the capacitance performance and estimate the specific capacitance of the SS-NC. The appearance of shapes that were almost isosceles triangles demonstrated the ideal charge and discharge characteristics for EDLCs with low dynamic voltage drops and almost 100% Coulombic efficiency over a large voltage range.^18,26 The insignificant dynamic voltage drop indicated a low internal series resistance, which could also be demonstrated by the EIS (Fig. 5A). The SS-NC had a specific capacitance of 109.73 F g⁻¹ at a current density of 0.5 A g⁻¹ (Fig. 4D), which was marginally higher than the previous reports for 100% carbon nanotube (CNT) film (24 F g⁻¹) or pristine CNT paper (32 F g⁻¹).¹⁸ The 55.26% decrease of the capacitance with a current density increase from 0.5 to 8.0 A g⁻¹ was attributed to the insufficient electrolyte ion diffusion kinetics at higher operating current densities. These diffusion kinetics would reduce the amounts of electrolyte ions accumulated onto the electrode interfaces and result in the decrement of specific capacitance.^18,24

Fig. 5 (A) Partial enlargement of the Nyquist plot in the high-frequency region with its entire range shown in the inset. (B) Long-term cycling performance of the SS-NC during 10 000 charge/discharge cycles at a current density of 1 A g⁻¹.

Nyquist plots of the as-synthesized nanomaterials were measured over the frequency range from 10⁵ Hz to 0.01 Hz and simulated by ZView software (Fig. 5A). Unlike the SS-800 plot, the plot of SS-NC featured a vertical line in the low-frequency region, which indicated ideal capacitive behavior. This plot was composed of three distinct parts at different frequency ranges.^16,18,24 The real impedance (Z′), which was found at very high frequencies with the imaginary impedance near to zero, was the sum of the ohmic resistances (R_s) accounting for the electrolyte and the electrode. The R_s of the SS-800 (4.47 Ω) was much higher than that of the SS-NC (3.53 Ω), which confirmed that the promoted conductivity was due to the increased content of C and the heteroatoms. The semicircular loop observed at high frequencies represented the composite interfacial impedance and the expected pseudocapacitance impedance that were generated from the Faradaic redox reactions of the electrode material due to the content of N and O species in the carbon frameworks.^16,18 The inclined portion of the curve (of about 45°) at the middle frequency (which ranged from ∼175.8 to ∼3.8 Hz) was ascribed to the Warburg impedance, which was the level of encountered impedance to the diffusion of ions from the electrolyte to the electrode surface.^16,24 The short Warburg resistance section indicated the efficient access of the electrolyte ions to the porous carbon framework of the SS-NC electrode with its hierarchical pore structure. The almost vertical line of the Nyquist plot represented the dominance of ideal double-layer charge/discharge behavior by the as-synthesized SS-NC at low frequencies.^18,24 Furthermore, the as-synthesized SS-NC exhibited a superior capacitance retention performance. A greater than 99% retention of the initial capacitance of SS-NC was observed after 10 000 charge/discharge cycles at a current density of 1 A g⁻¹, which indicated its excellent cycling stability (Fig. 5B).

3.4. Significance of this work

We synthesized a unique heteroatom (N, O) doped porous carbon nanomaterial, SS-NC, via direct pyrolysis of the “all-in-one” precursor sewage sludge. This material exhibited favorable charge storage capacity with excellent stability and durability. The unique qualities of this carbon nanomaterial and its favorable electrochemical performance as a supercapacitor stem from the particular compositions of sewage sludge and the proper utilization of almost all of the content of the sludge in synthesizing this carbon material. The organic matter in sewage sludge, which is mainly composed of carbon, hydrogen, oxygen and nitrogen, worked as an ideal precursor and nature dopant for the synthesis of heteroatom (N, O) doped porous carbon nanomaterials. The doping of nonmetal heteroatoms in the sp² carbon framework not only improved the electrical conductivity and the wettability between the electrodes and electrolytes, but also induced additional pseudocapacitance via reversible redox reactions, consequently improving the capacitive performance of the SS-NC as the electrode.^16,18,24

The SiO₂ content in sludge was used as the in-built template. The critical toxic heavy metals (e.g., Fe, Cr, Co and Ni) that are uniformly distributed in sewage sludge with different phases were used as the “in-built catalysts” to catalyze the partial graphitization of the numerous special components of organic matter as carbon precursors during the pyrolyzation process.¹⁹ The intrinsic hierarchical pore structures of the SS-NC were formed due to the combined effects of the in-built SiO₂ template that prevented agglomeration and the uniformly in-built heavy metal catalysts that produced graphitized areas with a three-dimensional stacking order, resulting in transition-metal-containing carbon materials with essentially highly porous during the pyrolyzation process.^19,28 These hierarchical pore structures with their unique designs could offer abundant mesopore and macropore structures that could improve the migration and diffusion of the electrolyte ions, resulting in a decreased diffusion distance and high power performance.²⁹ The micropores within the walls of the mesopores and macropores could supply a highly effective specific surface area for double-layer capacitance to obtain a high specific capacitance and a favorable electrochemical energy storage performance.

The generation of sewage sludge around the world is continually increasing and now exceeds annual totals of 30 million tons in China, 10 million dry tons in the EU and 5.6–7 million dry tons in the US, respectively.^2,5,7 In this situation, the need to develop a more cost-effective and environmentally benign value-added re-use of sewage sludge is of particular concern. Our protocol uses the proven pyrolysis technique to convert sewage sludge into a unique porous carbon nanomaterial for the sustainable development of low-cost energy storage devices. This simple and easy-to-handle process is suitable for large-scale industrial production. We suggest that our approach deserves particular attention not only as an eco-friendly and value-added way of re-using sewage sludge, but also as a means of easy fabrication of a low-cost heteroatom-doped nanocarbon composite with a favorable charge storage capacity. The sludge source and property, especially the C content, do has important influence on this preparation. Thus dewatered sewage sludge from domestic wastewater treatment plant is more approach.

Although the specific capacitance of the as-synthesis SS-NC is intermediate in all of the reported carbon materials,^16–18,30 the excellent stability and durability of these materials makes them promising candidates for energy storage devices in many electronics products. Further studies are needed to investigate possible methods of chemical activation (KOH activation and carbonized in NH₃ flow for instance),¹⁶ which are expected to enable additional increases in the specific capacitance of the sewage sludge-derived carbon material by increase its specific surface areas and the N content.

4. Conclusions

In summary, a unique heteroatom (N, O) doped porous carbon nanomaterial, SS-NC, was synthesized via the direct pyrolysis of sewage sludge, with SiO₂ used as the in-built template, transition metals, the critical toxic components of sewage sludge, used as the graphitizing catalysts, and organic matter, which is mainly composed of carbon, hydrogen, oxygen and nitrogen, worked as an ideal precursor and nature dopant. The SS-NC exhibited favorable charge storage capacity, with a specific capacitance of 109.73 F g⁻¹ in 0.5 M Na₂SO₄ at a current density of 0.5 A g⁻¹, and excellent stability and durability over 10 000 charge/discharge cycles. This high supercapacitor performance can be attributed to the hierarchical porous structure, which provides a highly effective specific surface area for double-layer capacitance, and to the heteroatom doping effects that induce additional pseudocapacitance via a reversible redox reaction. Our protocol shows attractive prospects for establishing an eco-friendly, value-added re-use of sewage sludge. At the same time, our protocol demonstrates a proven technique for synthesizing alternative electrode materials for electrochemical energy storage.

Acknowledgements

The authors wish to thank the National Natural Science Foundation of China (51308401) and the National Key Technologies R&D Program of China (2014BAC29B01) for their partial support of this study.

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Footnote

† Electronic supplementary information (ESI) available: Energy dispersive X-ray, FTIR and Raman spectra. See DOI: 10.1039/c5ra07178j

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