Improved oxygen reduction reaction activity of three-dimensional porous N-doped graphene from a soft-template synthesis strategy in microbial fuel cells

Abstract

Electrocatalysts with high oxygen reduction reaction (ORR) activity are still of great significance to improve the performance of microbial fuel cells (MFCs). We report herein an approach to synthesize three-dimensional porous nitrogen-doped graphene (PNG) using cetyltrimethyl ammonium bromide (CTAB) micelles as soft templates. Based on this strategy, one type of PNG series, denoted PNG-15, is obtained by optimizing the mass ratios of CTAB and GO. This material exhibits a well-developed porous structure with a high surface area of 615.2 m² g⁻¹. The results of XRD, Raman spectroscopy, and XPS analyses verify that the superior porous structure of PNG-15 is beneficial to the formation of active centers. Therefore, this material shows an ORR electron transfer number of 3.88 ± 0.10 in a neutral medium and comparable activity to commercial Pt/C. Such comparable performance of PNG-15 is preserved as a cathode electrocatalyst in MFCs in terms of voltage output, coulombic efficiency, and power density output. Furthermore, the PNG-15 demonstrates superior long-time stability to Pt/C during over two-months of operation. The findings confirm that the new material can serve as a promising cathode electrocatalyst in MFCs.

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Introduction

Microbial fuel cells (MFCs) are a promising platform to convert chemical energy in wastewater into electrical energy,^1,2 and consist of bio-oxidation processes at an anode and reduction at a cathode. For the sake of easy operation and large-scale application, the oxygen reduction reaction (ORR) has become a dominant cathode reaction during the last decade. In particular, the four-electron transfer pathway of ORR with H₂O as a final product can help MFCs to achieve optimal cell performance.³ Thus, searching for appropriate ORR electrocatalysts with high performance and low cost becomes significant for the large-scale application of MFCs in the future.

Nitrogen-doped graphene (NG) has been verified as an efficient ORR electrocatalyst via the four-electron transfer pathway.^4–8 Meanwhile, the price of NG is much lower than that of state-of-the-art platinum. NG is theoretically expected to possess a high surface area because of the one atom-thick and two-dimensional structure of the graphene sheets, such as in pristine graphene. However, this unique property deteriorates because of the stacking of graphene sheets via strong van der Waals forces during chemical fabrication processes, which reduce the surface area and porosity.^9–11 The deterioration of the physical properties of NG further adversely affects its chemical properties. For example, the shrunken surface area reduces the formation of active sites on NG and the sites for reactant adsorption. The inferior mass transport of reactants or products would cause severe concentration polarization, which results in the loss of electromotive force. Therefore, the incompletely exerted ORR activity of NG inhibits the performance of energy-conversion cells.

Assembling NG into three-dimensional (3-D) porous architectures is one of the promising approaches to prevent activity deterioration, and many efforts have been carried out in this respect using template-assisted approaches.^12–15 Template-directed chemical vapor deposition can manipulate the layer number of 3-D NG, thus affecting the electrocatalytic activity for the ORR, in which a nickel foam with a porous structure was used as a hard template.^16,17 Whether silica spheres or a nickel foam are used, these hard templates must be removed by HCl or NaOH etching, which complicates the fabrication procedures. Thus, it is essential to develop a simple and efficient template-induced approach to synthesize 3-D NG.

Aside from the hard-template approach, the soft-template method has the advantages of easy processing, high efficiency, and low cost.^18–20 Soft-template approaches are widely employed to efficiently synthesize porous carbon materials. Compared with other 3-D porous carbon materials (e.g. mesoporous carbon), 3-D porous graphene materials prepared through the soft template method are rarely reported.^21,22 Huang et al. reported a general emulsion soft-template method to synthesize porous graphene foams with a surface area of 451 m² g⁻¹.²⁰ They also showed that micelle-template synthesis can efficiently produce mesoporous graphene materials with pore sizes of 25.1 nm on average.²³ Mesoporous NG was obtained by sintering a mixture of the as-prepared mesoporous graphene and cyanamide. Wang et al. used crimson phenol-formaldehyde prepolymer as the soft template to synthesize a mesoporous carbon/graphene composite.²⁴

Cetyltrimethyl ammonium bromide (CTAB) is a cationic surfactant that is also widely used to synthesize metal nanoparticles and mesoporous silica nanoparticles.²⁵ As a typical surfactant, it can form micelles in aqueous solution. Through the interaction principle, graphene oxide (GO) with negative charge can interact with the cation CTA⁺ via electrostatic forces. Therefore, it is expected to form a composite consisting of a porous GO shell and a CTAB micelle core. Graphene with a 3-D porous structure can be obtained by pyrolyzing the composite at a high temperature in an inert atmosphere. The CTAB-based soft-template approach, which does not employ expensive chemicals or equipment, is potentially simpler and more time-saving than other previously developed methods. However, this approach to synthesize 3-D porous NG (PNG) has not yet been reported.

In this work, CTAB micelles in aqueous solution were selected as the soft template. Fig. 1a shows that composites with a porous GO shell and a CTAB micelle core were formed via electrostatic forces, followed by the adsorption of cyanamide onto this composite. Finally, PNG was obtained by pyrolyzing the CTAB–GO–cyanamide composites in an inert atmosphere. The ORR activity of the as-prepared PNG in a neutral medium was evaluated. The PNG was also tested as a cathode electrocatalyst in MFCs to generate electricity.

Fig. 1 Synthesis route of PNG (a) and SEM images (b and c) and TEM images (d–g) of PNG-15.

Results and discussion

Characterization of PNG

As depicted in Fig. 1a, the synthesis of PNG was based on the electrostatic interaction between CTA⁺ cations and negatively charged GO. It is assumed that charge-neutralization after the interaction may result in a superior product. Thus, the charge property of the supernatant was monitored to obtain the optimal ratio of GO to CTAB. The zeta potentials of the supernatants of different GO–CTAB mixed solutions are depicted in Fig. S1.† One can see from this figure that as the CTAB dosage was increased, the zeta potential of the GO–CTAB mixed solutions positively shifted. This phenomenon indicated that the negatively charged GO interacted with the positively charged CTAB micelles and reached charge-neutralization at 15 wt% CTAB.

Fig. 1(b)–(g) and S2† show the SEM and TEM images of PNG-X, respectively. The images demonstrate that all the PNG-X samples were composed of typical thin graphene nanolayers with porous and wrinkled structures, which were possibly formed because nitrogen atoms were incorporated into the graphene lattice.²⁶ Thus, the samples exhibited structural defects and distortion because the carbon atoms were substituted with heteroatoms in the graphene sheets.²⁷ Although all the samples demonstrated a similar porous and wrinkled structure, a subtle difference was observed for PNG-15. A comparison of the SEM images (Fig. 1b and c) with the other images (Fig. S2†) indicates that clear edges of the graphene nanolayer could be observed in the case of PNG-15. Its TEM images also exhibited the same phenomenon. Thus, PNG-15 was composed of fewer graphene layers and was expected to possess a superior porous structure.

Nitrogen adsorption–desorption measurements were carried out to investigate the porous features of these materials. The N₂ adsorption–desorption isotherms of PNG-X, as shown in Fig. 2, exhibited a IV type isotherm with an obvious hysteresis loop at a medium relative pressure, indicating the existence of mesopores. Furthermore, the rapid increase at low relative pressure indicated the presence of micropores.¹² The BET surface areas of these materials were calculated based on the isotherms and the results are presented in Table 1. The pore size distribution curves of PNG-X depicted in the insets of Fig. 2 were obtained according to the Barrett–Joyer–Halenda model. The corresponding pore diameters are also summarized in Table 1. The results showed that the NG porosity was enhanced with CTAB, unlike PNG-0, in terms of BET surface area and pore diameter. PNG-15 possessed the largest BET surface area of 615.2 m² g⁻¹ and smallest pore diameter of 11.87 nm. Through the soft-template strategy, the surface area of PNG-15 was enlarged over 1.7 times in comparison with PNG-0. Moreover, the surface area of PNG-15 was comparable or even higher than other porous graphene derivatives, which were obtained using either hard templates or other kinds of soft templates.^{12,20,24,28,29}

Fig. 2 N₂ adsorption and desorption isotherms of PNG-0 (a), PNG-5 (b), PNG-10 (c), PNG-15 (d) and PNG-20 (e). The insets are the corresponding pore size distribution curves plotted from the Barrett–Joyer–Halenda models.

Table 1 Results of BET surface area, pore diameter and ratio of I_D/I_G for the PNG-X electrocatalysts

Type of catalyst	BET surface area/m^2 g^−1	Pore diameter/nm	ID/IG
PNG-0	353.3	21.10	1.23
PNG-5	395.6	17.51	1.16
PNG-10	477.7	13.35	1.20
PNG-15	615.2	11.87	1.14
PNG-20	485.8	15.47	1.20

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By contrast, the materials synthesized using an insufficient or excessive CTAB dosage had decreased BET surface areas and pore diameters. This phenomenon may be ascribed to the fact that the CTAB micelle failed to form under those circumstances. With a moderate ratio of CTAB and GO, a fluffy and stable structure of the CTAB–GO composite was formed, resulting in NG with fine porosity. The insufficient CTAB dosage for PNG-5 could not facilitate the formation of a fluffy CTAB–GO composite, which led to a small BET surface area and large pore diameter. Meanwhile, excessive CTAB caused aggregation of the CTAB–GO composite, resulting in the poor porosity of PNG-20.

XRD and Raman tests were carried out to further reveal the structural information of 3-D NG. The XRD patterns in Fig. 3a showed broad peaks of C(002) at around 25° and C(100) at around 43° for all PNG materials,¹² indicating the amorphous structure of carbon in the NG lattice. This result is consistent with the SEM images. The amorphous structure was generated because of the nitrogen atom incorporation and reduction of GO at a high temperature. The Raman spectra (Fig. 3b) demonstrated typical D and G bands at around 1350 cm⁻¹ and 1585 cm⁻¹ for PNG-X. The G band was associated with the E_2g mode of graphitic carbon, and the D band corresponded with the defect-induced mode, which resulted from the possible amorphous carbon with doped heteroatoms.^30,31 The values of I_D/I_G for PNG-X are indicated in Table 1. Unlike that of pristine graphene, the high ratio of I_D/I_G demonstrated the amorphous carbon structure of PNG-X, which further confirmed that nitrogen atoms were incorporated into the graphene sheets.³² The I_D/I_G of PNG-15 was slightly lower because of the enhanced graphitization of carbon, indicating the superior electrochemical property of this material.

Fig. 3 XRD (a) and Raman (b) spectra of PNG-X electrocatalysts.

XPS measurements were carried out to probe the chemical composition of PNG-X. The survey spectra in Fig. 4a indicate the presence of C 1s, N 1s, and O 1s for all PNG-X samples. The corresponding atomic weight of each element is listed in Table 2. The atomic percentage of O in PNG-15 was higher than that in the other materials. A previous study hypothesized that O species behaved as the active sites for O₂ adsorption and electron transfer, which was presumed to decrease the active energy carrier of ORR in a neutral pH medium.³³ This finding indicated that PNG-15 exhibited high electrocatalytic activity.

Fig. 4 XPS survey of PNG-X electrocatalysts (a) and high resolution of N 1s of PNG-15 (b); N1: pyridinic N, N2: pyrrolic N, and N3: graphitic N. The inset shows the nitrogen species in the graphene layers.

Table 2 Elemental analysis of CTAB-NG-X electrocatalysts^a

Type of catalyst	C 1s/at%	O 1s/at%	N 1s/at%
			N1	N2	N3
a N1: pyridinic N, N2: pyrrolic N, and N3: graphitic N.
PNG-0	88.80	4.63	1.22	1.44	3.52
PNG-5	88.45	5.21	1.29	1.49	3.65
PNG-10	88.48	5.34	1.48	1.64	3.17
PNG-15	87.62	5.95	1.73	2.00	2.84
PNG-20	89.14	4.57	1.39	1.34	3.60

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N-containing functional groups have been widely accepted as the active centers for the ORR of N-doped carbon materials.^5,7,8 Fig. 4b and S3† display the N 1s peak with a high resolution of PNG-X. The deconvoluted high-resolution N 1s spectrum revealed the presence of three N species, including pyridinic N (398.5 eV), pyrrolic N (399.8 eV), and graphitic N (401.5 eV).^34,35 The content of each type of N species was calculated, and the results are presented in Table 2. The N contents of PNG-X were almost identical to each other; thus, the soft-templating approach hardly influenced the incorporation of nitrogen atoms into the graphene lattice.

However, the specific contents of the individual N species showed significant differences. The sum of pyridinic N (1.73 at%) and pyrrolic N (2.00 at%) for PNG-15 was higher than that of the other materials. This finding was probably attributed to the fact that the fluffy structure of the CTAB–GO composite of this material resulted in more exposed edges, which facilitated the incorporation of nitrogen atoms into the nearby carbon atoms, thus forming pyridinic N and pyrrolic N.³⁶ Previous reports indicated that different types of N species played different roles in the process of the ORR. Pyridinic and pyrrolic N can reduce the adsorption energy of O₂, resulting in the ORR occurring more readily on N-doped carbon materials. Graphitic N can reduce O₂ to H₂O₂ via adsorbed OOH intermediates through a two-electron pathway, whereas pyridinic and pyrrolic N species may convert the ORR mechanism from a two-electron dominating process to a four-electron dominating process.^34,37 Based on DFT calculations, Fan et al. pointed out that heteroatom-doped graphene possessed strong adsorption of OOH and could carry out the four-electron pathway.³⁸ Therefore, the higher content of pyridinic and pyrrolic N of PNG-15 may indicate enhanced electrocatalytic activity.

Electrocatalytic activity for ORR in neutral medium

Both CV and RRDE tests were performed to evaluate the electrocatalytic activity of PNG-X. Fig. 5a and S4† demonstrate the results of CV performed with and without saturated O₂ in 0.1 M PBS solution. Obvious ORR peaks were observed in the potential range of −0.1 V to −0.03 V (vs. Ag/AgCl) in the presence of O₂ for all the PNG-X samples. No significant discrepancy in the ORR onset potential between PNG-15 and its counterparts was observed from these figures. However, the ORR current density of this electrocatalyst was significantly higher than the others. This was indicative that the pyridinic and pyrrolic N played an important role in enhancing the ORR activity of the N-doped carbon materials instead of the total amount of nitrogen species. The figures showed that PNG-15 may exhibit enhanced electrocatalytic activity for ORR among the PNG-X series. In comparison with Pt/C, the ORR peak of PNG-15 negatively shifted, but possessed higher current density. Furthermore, the CV areas of PNG-X were observed to be higher than those of Pt/C, indicating that PNG-X possessed a larger electroactive area. Besides serving as an ORR catalyst, the N-doped graphene has already served as an efficient capacitive material for energy storage due to its high surface area.^39,40 This result was in good agreement with the BET data listed in Table 1.

Fig. 5 Cyclic voltammograms (a) and polarization curves of ORR at different rotary rates (b) of PNG-15, and comparison of polarization curves of PNG-X and Pt/C at a rotating rate of 1600 rpm (c) in O₂-saturated 0.1 M PBS. Insets in (b) and (c) are the Koutecky–Levich plots and a comparison of n values of different electrocatalysts, respectively. Current–potential profiles (d) and corresponding H₂O₂ yield and electron transfer number results (e) at a rotation rate of 1600 rpm from RRDE tests.

Fig. 5b and S5† present the results of linear polarization sweeping of the electrocatalysts at different rotating rates. The electron-transfer number is an essential indicator in evaluating the ORR electrocatalysts. The K–L plots were profiled as insets in the figures based on eqn (3) and found to exhibit good linearity with parallelism. Meanwhile, the electron-transfer number of PNG-15 was 3.88 ± 0.10, followed by PNG-10, PNG-5, PNG-20, and PNG-0, with electron-transfer numbers of 3.67 ± 0.18, 3.42 ± 0.10, 3.32 ± 0.05, and 3.21 ± 0.09, respectively. The electron-transfer number of PNG-15 was close to that of state-of-the-art Pt/C, which was 3.93 ± 0.03 as shown in Fig. S4e.† These results indicated that the ORR of PNG-15 proceeded through a main four-electron reaction pathway, which could provide a more positive electrode potential. For the other electrocatalysts, hybrid two-electron and four-electron reaction pathways with partial H₂O₂ generation for ORR were present.

Fig. 5c compares the linear polarization curves of the electrocatalysts at the same rotating rate. The figure clearly shows that the current density of PNG-15 was significantly higher than that of the other PNG-X samples, possibly because the higher surface area and better porosity of PNG-15 provided more sites for O₂ adsorption and faster mass transport, which resulted in more occurrences of ORR, thus a higher current density. Pt/C also exhibited excellent activity in terms of current density and onset potential. Although the onset potential of PNG-15 was more negative than that of Pt/C, its current density was close to that of Pt/C.

Fig. 5(e)–(d) and S6† illustrate the results of RRDE tests and the corresponding H₂O₂ yield and electron-transfer number of different electrocatalysts. The results indicated identical values of electron-transfer number to those obtained from the K–L plots. The low H₂O₂ yield of PNG-15 testified that this electrocatalyst facilitated a quasi-four-electron ORR pathway in neutral medium.

The electrocatalytic activity of PNG-X was determined based on its physicochemical properties. Generally, a high surface area could provide more reaction sites for ORR, and the mesoporous structure facilitated the mass transport of reactants and products. The enhanced graphitization of the carbon of electrocatalysts was beneficial to electron transfer. The large amounts of pyridinic- and pyrrolic-type N reduced the adsorption energy of O₂ and facilitated ORR via a four-electron pathway. On the other hand, PNG-0, obtained from conventional synthesis without a soft template, displayed lower ORR activity than other PNG-X samples. It was probably due to its poorer porosity and lower amount of pyridinic- and pyrrolic-type N. Therefore, this material possessed a smaller electron transfer number for ORR. In particular, the PNG-15 obtained from an appropriate mass ratio of CTAB to GO exhibited superior electrocatalytic activity of ORR in a neutral medium.

Application as cathode electrocatalysts in MFCs

Air-cathode MFCs equipped with different electrocatalysts were prepared to investigate the feasibility of PNG as an alternative to Pt/C. The voltage output and power generation of the different MFCs were compared. Fig. 6a shows the voltage output of the MFCs equipped with these electrocatalysts in the initial 5 cycles. A similar trend in cell-voltage variation for the MFCs equipped with PNG-15 and Pt/C was observed in terms of maximum cell voltage and duration. The COD removal and coulombic efficiency of the MFCs were also calculated for each cycle. Table 3 shows the average values for each type of MFC. The values of maximum cell voltage for PNG-15-MFC and Pt/C-MFC were 546 ± 14 mV and 541 ± 10 mV, respectively. However, the maximum cell voltage of the counterpart of PNG-0-MFC was slightly lower at 520 ± 8 mV but with a longer duration. Identical COD removal and CE for PNG-15-MFC and Pt/C-MFC were observed. By contrast, PNG-0-MFC exhibited inferior cell performance based on the two aspects.

Fig. 6 The voltage–time profiles (a), power density curves (b) and electrode potentials (c) of MFCs equipped with different cathode electrocatalysts.

Table 3 Performance comparison of MFCs equipped with different cathode electrocatalysts

Type of MFC	Maximum voltage/mV	COD removal/%	CE/%	MPD/mW m^−2
Pt-MFC	541 ± 10	93.44 ± 1.54	87.52 ± 2.19	938.74 ± 16.38
PNG-0-MFC	520 ± 8	90.43 ± 1.36	80.19 ± 2.57	800.48 ± 29.52
PNG-15-MFC	546 ± 14	92.79 ± 2.15	87.49 ± 1.54	930.86 ± 33.26

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Power generation is one of the most significant evaluation parameters of MFC performance. Fig. 6b shows the power density curves of MFCs. PNG-15-MFC possessed a similar maximum power density (MPD) to Pt/C-MFC, which was substantially higher than that of PNG-0-MFC. Table 3 demonstrates that the value of MPD of PNG-15-MFC was 930.86 ± 33.26 mW m⁻², followed by Pt/C-MFC with 938.74 ± 16.38 mW m⁻² and PNG-0-MFC with 800.48 ± 29.52 mW m⁻². It should be noted that the power density was normalized to the project area of the cathode because of the cubic shape anode. The electrode potentials shown in Fig. 6c suggested the difference in activity for each electrocatalyst. The anode potentials of each MFC were nearly identical because the same electrode materials and inoculum were used in the anode chamber. This result suggests that the cathode performance determines the power generation of the MFCs. The similar cathode potential profiles of PNG-15 and Pt/C illustrated that the PNG possessed similar ORR properties to Pt/C. It indicated that PNG-15 exhibited comparable activity to the state-of-the-art commercial Pt/C. The lower power generation of PNG-0-MFC was attributed to the lower ORR activity and smaller electron transfer number of PNG-0, which consequently caused a lower cathode potential of the MFC.

Long-time stability is another important index to assess electrocatalysts. The results of maximum voltage output and power density during over 70 day of continuous operation are summarized and depicted in Fig. S7.† As shown in this figure, the MFCs equipped with PNG-15 even PNG-0 displayed superior stability. Whereas the performance of Pt/C-MFC declined gradually as operation proceeded. More than 27% decay of performance was observed for Pt/C-MFC after 70 days of operation. Meanwhile, the performance decays for PNG-15-MFC and PNG-0-MFC were 11.51% and 17.37%, respectively. Biofouling has been widely observed in membrane-less single-chamber MFCs, which severely hinders the ORR activity of Pt/C.^41,42 Biofilm on the cathode was also observed in dual-chamber MFCs due to the migration of ammonia from the anode chamber to the cathode chamber, resulting in formation of a denitrifier on the surface of the cathode catalysts.⁴³ Therefore, the decay of the MFC performance was probably ascribed to biofouling on the cathode in this work. As shown in Fig. S8,† all the cathodes were covered by biofilm after 70 days of operation. It seemed that the biofilm on Pt/C was somewhat thicker than on the others. As for the PNG series, the generated active oxygen species including H₂O₂ during the ORR process were assumed to be the key reason for the antimicrobial activity. However, the higher performance decay of PNG-0-MFC than PNG-15-MFC was probably due to the transport of active oxygen species from the cathode chamber to the anode chamber, causing inactivation of anodic microorganisms. As a consequence, excessive generation of active oxygen species like in PNG-0 would weaken the power generation of the MFC. On the other hand, it was presumed that the mechanism of the adverse effect of biofilm formation on the catalyst activity was different in the Pt/C compared to the PNG. The active centers on Pt/C might be more sensitive to being inactivated by biofilm, e.g. extracellular polymeric substances.⁴² The results indicated that PNG-15 could have antimicrobial characteristics as well as demonstrate the quasi-four-electron ORR pathway.Exploration of the antimicrobial properties of heteroatom doped carbon materials with the quasi-four-electron ORR pathway is under way.

The performance of these MFCs was determined by the activity of the corresponding electrocatalysts. The results above were consistent with those of electrochemical measurements. The electrocatalyst that underwent the four-electron pathway of the ORR could provide a more positive electrode potential, which could enhance and accelerate the electron transfer from the anode to the cathode. Consequently, the CE of PNG-15-MFC was higher than that of PNG-0-MFC. Furthermore, the electricity generation of PNG-15-MFC could be achieved in a shorter duration with a higher cell voltage compared with PNG-0-MFC. This result is also meaningful for engineering applications in that a smaller water-treatment apparatus could be constructed. Meanwhile, a low amount of electrocatalyst with high activity was required, which could reduce the thickness of the catalyst-layer and further improve the mass transfer of O₂ and H₂O. Therefore, the resistance of the entire cell and the cost could be decreased.

In conclusion, the results confirmed that the soft-template method using CTAB could efficiently synthesize PNG, which acted as an ORR electrocatalyst. This method was also cost effective because expensive chemicals and instruments were not used. PNG-15 exhibited high performance, thus is a promising cathodic catalyst for MFCs that could be used to replace the expensive Pt/C in large-scale application.

Experimental

Electrocatalysts synthesis

GO was synthesized using a modified Hummers’ method, as described in a previous study.⁴⁴ In a typical synthesis of PNG, 100 mL of 1.0 mg mL⁻¹ GO solution was added dropwise to a certain volume of CTAB aqueous solution with a concentration of 1000 mg L⁻¹, followed by continuous stirring of the solution for 2 h. During this procedure, a GO–CTAB composite was formed due to the electrostatic interaction between CTA⁺ cations and the negatively charged GO. 4.0 mL of cyanamide solution (50 wt% in water) was then added to the solution, followed by an additional 30 min of stirring. The cyanamide was adsorbed onto the GO–CTAB composite and further used as a nitrogen source. The water in the mixture was thoroughly removed in an oven at 90 °C to get a dark brown solid. The obtained solid was subsequently pyrolyzed as described in a previous work; the solid was initially pyrolyzed at 550 °C under an Ar atmosphere for 4 h at a heating rate of 2 °C min⁻¹, followed by further heating to 900 °C for 1 h at a heating rate of 5 °C min⁻¹.^33,45 The sample was washed using deionized water to remove residues and the water was removed via lyophilization. NG without using CTAB as a soft template was also prepared and used as the control. The as-prepared electrocatalysts were denoted as PNG-X (X is the mass percentage of CTAB/GO; X = 0, 5, 10, 15, and 20).

Ink preparation for electrochemical measurements

For electrochemical measurements, 2.0 mg mL⁻¹ catalyst ink was prepared by ultrasonically dispersing 5.0 mg of catalyst in 2.5 mL of ethanol. Approximately 50 μL of 5 wt% Nafion (DuPont Company) was added to the system, and the resultant mixture was ultrasonicated for 10 min. The above ink (9.8 μL) was dispersed on a glassy carbon electrode (GC; 5 mm in diameter; Pine Instrument Co.), which was initially polished using a 0.05 μm alumina slurry. For comparison, a commercial Pt/C catalyst containing 20 wt% Pt supported on carbon black (Johnson Matthey Corp., HISPEC 3000) was used, and the ink was prepared using the same procedure described above. The loading of all catalysts on the GC was 100 μg cm⁻².

Electrode preparation and MFC setup

An air-cathode containing poly(dimethylsiloxane) as a diffusion layer was fabricated as described in a previous study.⁴⁶ Approximately 2.5 mg cm⁻² PNG and Pt/C catalyst was sprayed on a waterproof carbon cloth.

The MFCs were composed of dual-chamber cubic-shaped reactors. Each chamber was 4.0 cm long and 3.0 cm in diameter. The anode and cathode chambers were separated by a proton-exchange membrane (Nafion® 117, DuPont Company) after pretreatment.⁴⁷ The carbon felt (Alfa Aesar) (20.0 mm × 20.0 mm × 2.0 mm) was pre-treated using ammonia gas before being used as the anode. The anode chamber was inoculated with a solution obtained from an MFC that had been operating for over a year in the laboratory. The anode chamber was filled with a mixture of 0.1 M phosphate-buffered saline (PBS, pH 7.0) containing 1.0 g L⁻¹ NaAc, 0.13 g L⁻¹ KCl, 0.31 g L⁻¹ NH₄Cl, 12.5 mL L⁻¹ mineral solution, and 5 mL L⁻¹ vitamin solution.⁴⁸ The anode feed solution was purged with N₂ before inoculation to maintain an anaerobic condition. The cathode chamber was filled with the same mixture media but without NaAc, minerals, and vitamin solutions. The anode and cathode were connected with a 1000 Ω external resistor using a Ti wire, and all exposed metal surfaces were sealed with nonconductive epoxy. All the reactors were operated in fed-batch mode at 30 ± 1 °C. The feed solution was replaced once the cell voltage decreased to below 10 mV, which was noted as a complete cycle of electricity generation.

Measurement and analysis

The cell voltage across the external resistor was measured using a multimeter equipped with a data-acquisition system (Advantech, PCI-1747U). The maximum power densities of the cells were obtained from the polarization curves by varying the value of the external resistor from 10 Ω to an open-circuit voltage as the performance of the MFCs approached steady-state conditions. Data were recorded as <1 mV variation in voltage. All the experiments were conducted twice at room temperature (30 ± 1 °C). Power density was calculated based on the anode area as follows:where E_cell is the cell voltage, R_ext is the external resistance, and A_cat is the cathode project area. Coulombic efficiency (CE) over time was calculated using eqn (2):where V_An is the volume of liquid in the anode chamber, and Δ_COD is the change in COD over time t.

Electrochemical measurements were conducted on a 760E workstation (CH Instrument, USA) equipped with an MSR electrode rotator (Pine Research Instrumentation, USA). A conventional three-electrode cell was used. Here, a Ag/AgCl electrode (saturated KCl aqueous solution) and a Pt mesh were used as the reference and counter electrodes, respectively. Cyclic voltammetry (CV) experiments were performed in 0.1 M PBS (pH 7.0) after purging with N₂ and O₂ for 15 min at a potential sweep speed of 50 mV s⁻¹. Linear polarization experiments on a rotary ring-disk electrode (RRDE) were performed in the same PBS with saturated O₂. The potentials were varied from −0.6 V to +0.6 V (vs. Ag/AgCl) at a potential sweep rate of 5 mV s⁻¹, and the rotating speed was changed from 400 rpm to 2400 rpm. All electrochemical measurements were carried out at 25 °C using a water bath as the temperature controller.

The electron-transfer number of the ORR was calculated from the slopes of the K–L plots using eqn (3):

where j_K is the kinetic current, ω is the electrode rotating speed in rad per s, and b is the reciprocal of the slope determined from the slope of the K–L plots and calculated based on the Levich equation:

b = 0.62nFAν^−1/6C_O2D_O2^2/3 (4)

where n is the number of transferred electrons per O₂ molecule, F is the Faraday constant (96 485 C mol⁻¹), D_O2 is the diffusion coefficient of O₂ in 0.1 M PBS (2.7 × 10⁻⁵ cm² s⁻¹), ν is the kinetic viscosity (0.01 cm² s⁻¹), and C_O2 is the bulk concentration of O₂ (1.26 × 10⁻³ mol L⁻¹).⁴⁹ All kinetic parameters were obtained in 0.1 M PBS (pH 7.0) at 25 °C.

The yield percentage of hydrogen peroxide (H₂O₂) and the electron transfer number (n) were also determined using the following equations:

where I_d is the disk current, I_r is the ring current, and N is the current collection efficiency of the Pt ring. N was determined to be 0.16 ± 0.01 from the reduction of K₃Fe[CN]₆.

The zeta potentials of GO, CTAB and the supernatant of the mixed GO–CTAB solutions were measured using a Zetaplus analyzer (Malvern Zetasizer Nano ZS, UK). The morphology of the NG electrocatalysts was observed using a scanning electron microscopy (SEM) system (FEI Nova 400) and a transmission electron microscopy (TEM) system (TECNAI FEI G2). The TEM instrument was combined with an energy-dispersive X-ray spectrometer to analyze the elements present in the sample. The X-ray diffraction (XRD) patterns of the NG catalysts were obtained using an X’Pert3 powder diffractometer with Cu-Kα radiation (1.54056 Å). The Brunauer–Emmett–Teller (BET) surface area was obtained from 77 K N₂ sorption isotherms using a Belsorp-max instrument. The Raman spectra were obtained on a laser confocal Raman spectrometer (Renishaw inVia Reflex) using a 532 nm laser. The X-ray photoelectron spectroscopy (XPS) studies were conducted using an ESCALAB 250XI spectrometer (Thermo Electron, UK) with an Al-Kα X-ray source, and the C 1s peak at 284.8 eV was used as an internal standard.

Conclusions

In summary, a simple and efficient soft-template strategy for the synthesis of 3-D porous NG materials was proposed. The results verified that this strategy was suitable for fabricating NG with a high surface area. The mass ratio of CTAB to GO was controlled at 15 : 100. Thus, the as-prepared PNG-15 outperformed its counterparts concerning the BET surface area and ORR activity. PNG-15 exhibited comparable ORR activity in a neutral medium to commercial Pt/C in terms of electron-transfer number and current density. The MFC with PNG-15 exhibited a similar cell performance to that with Pt/C regarding maximum power density, cell voltage, and coulombic efficiency. In addition, the PNG-15 equipped MFC showed improved long-time stability in comparison with the Pt/C equipped one. These results indicated that the proposed soft-template strategy is a promising approach for PNG synthesis. Furthermore, the as-prepared 3-D porous NG can be used as a cathode electrocatalyst in MFCs.

Acknowledgements

This work was supported by Natural Science Foundation of China (No. 51525805, 51378494, 51578526), and partially by the Western Action Research Program granted by Chinese Academy of Science (No. KZCX2-XB3-14), Youth Innovation Promotion Association of Chinese Academy of Sciences (No. 2016341), and Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2015jcyjBX0063).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23971d

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