Nitrogen- and sulfur-doped carbon nanoplatelets via thermal annealing of alkaline lignin with urea as efficient electrocatalysts for oxygen reduction reaction

Abstract

Nowadays, the development of metal-free oxygen reduction reaction (ORR) catalysts based on heteroatom-doped carbon materials has become one of the most attractive topics in fuel cells. Here, we describe a green one-step pyrolysis method for the synthesis of N and S dual-doped carbon nanoplatelets by using alkaline lignin (AL) as the C and S sources combining with urea as a nitrogen dopant. After carbonization at 900 °C, such a hybrid material (N–S–C 900) possesses an excellent electrocatalytic activity towards ORR in both alkaline and acidic media, which is superior to the benchmark Pt/C catalyst in terms of the half-wave potential and diffusion-limiting current density in an alkaline medium. Meanwhile, the obtained hybrid also shows better stability and excellent methanol tolerance than the commercial Pt/C catalyst for ORR in both alkaline and acidic media. In particular, the N–S–C 900 has prominent operational stability in alkaline media, retaining 93.1% of the initial current density after 10 000 s. In this way, using natural biological resources provides a promising alternative to noble-metal catalysts.

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1. Introduction

Currently, the polymer-electrolyte membrane fuel cell is regarded as a promising power source, which converts chemical energy directly into electrical energy with low emission and high energy efficiency.¹ The oxygen reduction reaction (ORR) in the cathode is kinetically sluggish and therefore needs to use highly efficient electrocatalysts. Pt or Pt-based alloys are considered as the high-efficiency ORR catalysts. The traditional state-of-the-art Pt-based catalysts, however, are very expensive and scarce in catalyzing oxygen reduction.² What is more, the Pt-based electrode materials still suffer from their susceptibility to CO deactivation, poor tolerance to fuel crossover, and time-dependent drift, which hamper their large-scale commercial applications. In this case, exploring low cost, highly active and endurable alternative non-noble-metal or metal-free ORR catalysts has been zealously pursued over the past few years.^3,4

In recent years, heteroatom-doped (e.g. N, B, S, F, P doped) carbon nanomaterials (graphene, carbon nanotubes, nanofibers, and ordered mesoporous graphitic arrays) have been widely investigated because of their competitive ORR activity as compared to traditional state-of-the-art Pt-based catalysts.^5–11 Particularly, nitrogen-doped carbon materials have attracted intensive interest because of their prominently high ORR activity in alkaline and acidic media.¹² It is generally believed that the intrinsic reactive sites for ORR in N-doped carbon materials may include pyridinic-N, pyrrolic-N, and graphitic-N species.¹³ Many theoretical and experimental results have proven that the carbon atoms adjacent to N have more positive spin density and atomic charge density and thus become the active sites for ORR.¹⁴ Furthermore, it has been demonstrated that the carbon nanomaterials codoped with two heteroatoms, such as N and S, can produce more active sites than mono-doped counterparts.^15–17 A variety of extreme methods including chemical-vapor deposition, laser ablation, electrical arcs, and the carbonization of polymer aerogels, have been employed for their synthesis.^18–21 However, most of the synthesis methods are complicated, dangerous and time-consuming. Under these circumstances, the development of facile, green and time-saving synthesis strategies is fascinating but challenging.

Lignin is an amorphous, polyphenolic, highly cross-linked polymer consisting of three primary precursors, as shown in Fig. S1.† Lignin is the third most abundant natural polymer, next to cellulose and chitin, and ranks as one of the most abundant phenolic natural polymers. Its main function is to cement the cellulose fibres in plants. Lignin is generally obtained from black liquor, a waste discharged from paper mills in large quantities. Currently, much of the lignin produced by the paper industry is consumed as a fuel. Moreover, lignin also has some other marginal applications in the fields of the adhesives, tanning agents,²² adsorbents²³ and supercapacitor electrodes.^24–26 Alkaline lignin can be extracted from lignin when it is dissolved in alkaline environments. Fig. S2† shows the molecular structure of AL. The availability of a handful of sulfhydryl and low cost make it a preferred precursor for the synthesis of sustainable catalyst. In this work, we utilize a one-step pyrolysis method to thermally synthesize metal-free N- and S-doped porous graphene-like nanoplatelets. In the high-temperature annealing process, alkaline lignin is used as carbon and sulfur sources, and urea serves as the N dopant. The catalyst pyrolysed at 900 °C (N–S–C 900) possesses the superb ORR performance in an alkaline medium, superior the commercial Pt/C catalyst. In addition, the composite can regulate the ORR process in an acidic medium, making it a promising alternative to the commercial Pt/C catalyst for applications in both acidic and alkaline fuel cells.

2. Experimental section

2.1. Materials

Alkaline lignin and Nafion® perfluorinated resin solution (5 wt%) were bought from Sigma-Aldrich. Pt/C (20 wt%, Pt on Vulcan XC-72) was obtained from Alfa Aesar. Urea was purchased from the Beijing Chemical Company (China). All other reagents were of analytical grade and used without further purification, and all the solutions used in electrochemical experiments were prepared with ultrapure water (18.2 MΩ cm).

2.2. Preparation of N–S–C electrocatalysts

Fig. 1 illustrates the synthesis steps of N–S–C and S–C. The N–S–C hybrids were prepared via a simple one-step pyrolysis procedure. Simply, alkaline lignin (40 mg) was dispersed into 5 mL distilled water by ultrasonic vibration for 15 min. Then, 160 mg urea was added to the above aqueous dispersion of alkaline lignin with ultrasonic vibration for 30 min at room temperature. Afterwards, the mixture was dried overnight at 60 °C in the oven. The as-prepared mixture was transferred into a crucible, and then annealed at the set temperatures (800, 900, and 1000 °C) for 1 h under an Ar atmosphere. The heating ramp rate was 5 °C min⁻¹. After cooling to room temperature, the obtained black powder was grinded in an agate mortar. The N–S–C pyrolyzed at 800, 900, and 1000 °C was denoted as N–S–C 800, N–S–C 900, and N–S–C 1000, respectively.

Fig. 1 Schematic illustration for easy preparation of N–S–C 900 from alkaline lignin and urea at controlled temperatures under Ar protection.

The S–C 900 was synthesized by a similar procedure as N–S–C 900, except that urea was not added to the synthesis mixture.

2.3. Physical and chemical characterizations

The morphologies and structures of the as-prepared samples were characterized under transmission electron microscopy (TEM, JEOL-2010 transmission electron microscope operating at 200 kV) and X-ray diffractometer (RIGAK, D/MAX2550 VB/PC, Japan). Raman spectroscopy measurements were operated on a TriVista™555CRS Raman spectrometer at 532 nm. X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCLAB 250 spectrometer with a monochromatized Al Kα X-ray source (1486.6 eV photons) to identify surface chemical compositions and the bonding states. Nitrogen sorption isotherms were measured at 77 K with a Quantachrome adsorption instrument. The amount of carbon, nitrogen, and sulfur in samples were accurately determined by elemental analyzer (C, H, N, S analysis).

2.4. Preparation of modified electrode

The working electrode was firstly polished with 1.0, 0.3 and 0.05 mm aluminum oxide slurry in turn, then thoroughly rinsed with distilled water, absolute ethanol and distilled water for 5 min. Afterwards, the cleaned glassy carbon (GC) electrode was blow-dried with N₂ at ambient temperature. In order to modify GC electrode, 1.0 mg N–S–C and 1.0 mL ethanol were mixed ultrasonically to obtain a homogeneous catalyst ink. A certain amount of the catalyst ink at a concentration of 1.0 mg mL⁻¹ was dropped onto the freshly polished electrode surface to prepare a catalyst film, resulting in the catalyst loading per area of 0.42 mg cm⁻² and 0.28 mg cm⁻² in an alkaline medium and an acidic medium, respectively. After drying at room temperature, 1 μL of a diluted Nafion solution (v_Nafion : v_ethanol = 1 : 9) was covered on the surface of the catalyst layer to form a thin protective film.

The electrochemical properties of the as-prepared materials were measured on a CHI660E (Chenhua, Shanghai) with a three-electrode cell. A catalyst modified GC electrode was employed as the working electrode, a saturated calomel electrode (SCE) as the auxiliary electrode and a Pt wire as the counter electrode. Rotating disc electrode (RDE, d = 5 mm) tests were conducted by recording linear sweep voltammetry (LSV) curves in oxygen-saturated 0.1 M KOH or 0.5 M H₂SO₄ solutions. The LSV curves for ORR were recorded between 1.2 and −0.2 V in an alkaline medium or 1.1 and 0.1 V in an acidic medium with a scan rate of 10 mV s⁻¹. The rotating ring disk electrode (RRDE) tests were operated on a CHI660E electrochemical workstation. For RRDE experiments, the working electrode was a glassy carbon disk (d = 5.61 mm) and a platinum ring. The collection efficiency of the Pt ring electrode, N was 37%. The potentials for all electrochemical measurements were reported in terms of reversible hydrogen electrode (RHE). All potentials in the RHE scale were converted from the SCE scale using E(RHE) = E(SCE) + 0.0591pH + 0.244.²⁷ In order to avert any incidental errors, each electrochemical measurement was carried out at least 3 times.

3. Results and discussion

Fig. 2a gives the X-ray diffraction (XRD) patterns of alkaline lignin (AL), S–C 900 and N–S–C 900. As revealed, the diffraction peaks of N–S–C 900 exhibits a wide diffraction peak at 2θ of around 24.0°, which can be assigned to a turbostratic carbon.^12,28 It is worth noting that the carbon (002) peak shifts from 24.5° of S–C 900 to 24.0° of N–S–C 900, indicating the increase of interlayer distances with the doping of nitrogen. In contrast, the wide peak centered at 23.4° for alkaline lignin is ascribed to the amorphous-carbon. Additionally, the weak peak located at approximately 44.3° for S–C 900 and N–S–C 900 is a characteristic feature of carbon (101), which suggests the formation of the graphitic structure after pyrolysis.

Fig. 2 (a) The XRD patterns of alkaline lignin, S–C 900, and N–S–C 900. (b) The Raman spectra of S–C 900, N–S–C 800, N–S–C 900, and N–S–C 1000. (c and d) SEM images of S–C 900 (c) and N–S–C 900 (d).

Raman spectroscopy is a useful tool to confirm significant structural changes for carbon materials.²⁹ Fig. 2b presents the Raman spectra of S–C 900, N–S–C 800, N–S–C 900 and N–S–C 1000. Generally, the intensity ratio of the D band to the G band (I_D/I_G) is applied to estimate the defect density and edge plane exposure resulted from the heteroatom doping into carbon materials. The D band is related to the structural defects, and the G band is attributed to the E_2g mode of vibration in the sp² carbon domain.^30,31 The I_D/I_G ratio for N–S–C 900 and S–C 900 is 1.07 and 1.01, respectively, which reflects that N–S–C 900 has more structural defects due to the doping of nitrogen into the carbon layers. The ratios of I_D/I_G for N–S–C 800, N–S–C 900, and N–S–C 1000 are found to be 1.06, 1.07, and 1.08, respectively. These values suggest that the N–S–C 800 sample exhibits more ordered graphitic carbon while N–S–C 1000 exhibits the most disordered graphitic carbon. This phenomenon is probably related to the in-plane C=C crack due to decomposition of most of the epoxy and hydroxyl groups at higher temperatures.¹³ In addition, a wide and weak 2D bands showed at about 2800 cm⁻¹ for the as-prepared catalysts are characteristic features of few-layered graphene.^32,33

Scanning electron microscopy (SEM) was used to gain an insight into the morphologies of as-obtained samples. The SEM results reveal that the S–C 900 shows the irregular carbon nanoplatelet morphology with the smooth and flat surface (Fig. 2c), while the N–S–C 900 presents rough carbon nanoplatelet appearance covered by destroyed carbon layers (Fig. 2d). Moreover, it can be seen from the enlarged SEM images in Fig. S3,† the N–S–C 900 reveals crumpled morphology due to the nitrogen doping in the carbon matrix. The folded and wrinkled surface can produce lots of open edge sites, which may be conducive for catalyzing the ORR.^34,35

Nitrogen adsorption desorption isotherms were obtained to analyze the materials porosity (Fig. S4†). The S–C 900 presents a type of I/IV sorption isotherm with a Brunauer–Emmett–Teller (BET) surface area of 656 m² g⁻¹. The N–S–C 900, however, exhibits a typical I sorption isotherm with a BET surface area of 1209 m² g⁻¹, which is the feature of a microporous structure (d ≤ 2 nm). Besides, the microporous sorption shows the typical H4 hysteresis loop at low relative pressure, which demonstrates the presence of mesopores in S–C 900 and N–S–C 900. Furthermore, inset in Fig. S4† gives the pore size distribution curves originated from the adsorption branch of the isotherms utilizing the nonlocal density functional theory (NLDFT) method. The S–C 900 sample possesses a low pore volume (0.47 cm³ g⁻¹), while the N–S–C 900 exhibits the microporous structure as well as a large pore volume (0.74 cm³ g⁻¹). The microporous surface area (S_mic, 1035 m² g⁻¹) of N–S–C 900 is much larger than that (602 m² g⁻¹) of S–C 900 (see Table S1†). It is well known that urea is a bio-molecule rich in nitrogen and undergoes decomposing into CO₂ and NH₃ in the pyrolysis process, and therefore the larger S_mic of N–S–C 900 can be related to the releasing of gas.³⁶ Moreover, the high BET surface area of N–S–C 900 correlates well with the released CO₂. The CO₂, which can act as the active agent,³⁷ significantly increases the adsorption amount, with the N–S–C 900 sample showing 500 cm³ g⁻¹ adsorption at the highest relative pressure (see Fig. S4b†). In addition, the BET surface area of our N–S–C 900 material is also compared with those of recently reported microporous nanomaterials in literature and results are listed in Table S2.† In general, our N–S–C 900 catalyst is comparable to or even better than previously reported ones in terms of the BET surface area, microporous surface area and pore volume. High microporosity is required to obtain high catalytic activity, which can result in the increase of the active site density of such catalyst.^38,39 Whereas small amount of mesoporosity is beneficial for efficient transports of dioxygen and electrolytes in the catalyst layer.^40,41 These structural characteristics indicate that doping of nitrogen into alkaline lignin effectively increases the pore volume and BET surface area, which may facilitate the electron and electrolyte transports on N–S–C 900 and help to improve its electrocatalytic activity towards ORR.⁴²

Fig. 3 shows the high-resolution TEM (HRTEM) images of S–C 900 and N–S–C 900. The structure of the samples is significantly affected by addition of urea. TEM images reveal that the N–S–C 900 (Fig. 3b) shows more crumpled structure when compared with the S–C (Fig. 3a). HRTEM images of S–C (Fig. 3c) and N–S–C (Fig. 3d) demonstrate the amorphous nature of carbon structure. Additionally, numerous dislocation defects can be observed in the graphite layers of N–S–C 900 owing to the structural distortions stemmed from the incorporation of nitrogen and sulfur atoms into the graphite lattice, in agreement with XRD analysis. In the inset of Fig. 3c and d, the selected area electron diffraction (SAED) patterns disclose the presence of the diffraction rings, in accordance with the typical hexagonal pattern of graphene-like carbon with poor crystallinity.⁴³ The energy dispersive spectroscopy (EDS) mapping analysis displays the presence of C, N, O and S (see Fig. S3†) in the N–S–C 900, and the N and S elements are distributed evenly in the carbon structure, suggesting that nitrogen and sulfur were all doped into carbon matrix successfully. This result was further confirmed by X-ray photoelectron spectroscopy (XPS) analysis.

Fig. 3 TEM and HRTEM images of (a and c) S–C 900, (b and d) N–S–C 900.

To confirm the chemical compositions and bonding states, XPS measurements were conducted. In the broad scan spectra (Fig. 4a), the peaks at around 400.0 and 165.0 eV correspond to the nitrogen and sulfur atoms, respectively, demonstrating the successful incorporation of N and S into the carbon structure during the thermal annealing process. The invisibility of corresponding peaks of S is due to the less total S content, as shown in Table S2.† Moreover, the weaker peak of S becomes inconspicuous compared with the stronger peak of C. The C1s and O1s peaks are centered at about 283.2 and 531.7 eV, respectively. To characterize the chemical states of nitrogen in co-doped carbon nanoplatelets. The high-resolution N1s of N–S–C (800, 900, 1000) were analyzed (Fig. 4). As seen from Fig. 4, along with introduction of N in alkaline lignin, a decreasing trend in the N doping degree is observed with increasing the annealing temperature (3.97% for N–S–C 800, 2.98% for N–S–C 900 and 0.61% for N–S–C 1000). The complex N1s peaks deconvolution further indicates that there are four components in N–S–C located at 398–398.5, 399.1–399.6, 400.5–400.8 and 403 eV, which correspond to pyridinic-N,⁴⁴ pyrrolic-N,⁴⁵ graphitic-N²⁷ and oxidized-N,⁴⁶ respectively. The pyridinic-N (N1) and pyrrolic-N (N2) species are converted to graphitic-N (N3) and oxidized-N (N4) at higher treatment temperatures, as shown in Table S2.† Previous reports have found that graphitic N is two p-electron donors and the pyridinic N has a lone-electron pair to provide more electrons in the delocalized π-orbitals of the carbon framework,⁴⁷ which can facilitate electron transfer from the carbon electronic bands to the antibonding orbitals of O₂,^47,48 and therefore is highly efficient for the ORR. It is seen from Table S2,† the graphitic-N becomes dominant with the increase of pyrolysis temperature from 800 to 1000 °C, and the N–S–C 900 possesses the higher content of the sum of pyridinic-N and graphitic-N (71.0%), which may contribute a lot to promotion of the ORR performance. Moreover, the quantitative analyses of N–S–C samples displayed that the ratio of the total N to C content (N/C ratio) of N–S–C 800, N–S–C 900 and N–S–C 1000 are estimated to be around 0.046%, 0.031% and 0.006%, respectively. For the presented N–S–C, the N/C ratios derived from element analysis significantly exceed the XPS-derived ones (Table S4†). Such differences are very typical and are correlated with the fact that element analysis detects all nitrogen atoms, while XPS is a surface sensitive technique and therefore reflects surface species of functional groups.

Fig. 4 (a) XPS broad scan spectra of S–C 900, N–S–C 800, N–S–C 900, and N–S–C 1000. High resolution N1s spectrum of N–S–C 800 (b), N–S–C 900 (c), N–S–C 1000 (d).

Fig. S6† presents the high-resolution S2p XPS spectrum of N–S–C. The S2p spectrum can be deconvoluted into three peaks located at binding energies of 163.4 eV, 164.8 eV and 169.2 eV, which correspond to the S2p_3/2 and S2p_1/2 energy positions of thiophene-S and oxidized S,⁴⁹ respectively. As exhibited in Fig. S6c,† with introduction of N in alkaline lignin, the S doping level gradually decreases from S–C 900 to N–S–C 1000, following the order of 0.67% for S–C 900, 0.51% for N–S–C 800, 0.31% for N–S–C 900, and 0.19% for N–S–C 1000 (Fig. S6c†). In Fig. S6b,† the total content of S2p_3/2 and S2p_1/2 becomes dominant for N–S–C 900 and N–S–C 1000. Previous studies demonstrated that the thiophene-S was a catalytic active site for ORR.¹³ Indeed, the significantly enhanced electrocatalytic activity of N–S–C 900 and N–S–C 1000, when compared with that of S–C 900, can be proved by the observations of linear sweep voltammograms (LSVs) (Fig. 5a and 6a). The above-mentioned results illustrate that high pyrolysis temperature is conducive for both nitrogen and sulfur to generate highly-effective active sites for ORR.

Fig. 5 Electrochemical activity studied using RRDE technique in an O₂-saturated 0.1 M KOH solution at 1600 rpm. (a) Effect of carbonization temperature on LSV curves, (b) the electron transfer number per oxygen molecule, (c) % HO₂⁻ production, and (d) current density on Pt ring electrode during ORR reaction for S–C 900, N–S–C 800, N–S–C 900, N–S–C 1000, and 20% Pt/C.

Fig. 6 Electrochemical activity studied using RRDE technique at 1600 rpm in O₂-saturated 0.5 M H₂SO₄ solution. (a) Effect of carbonization temperature on LSV curves, (b) electron number transferred per oxygen molecule, (c) % HO₂⁻ production, and (d) current density on ring electrode during ORR reaction for S–C 900, N–S–C–T (800, 900 and 1000 °C), and 20% Pt/C.

The electrocatalytic activity of as-prepared samples was studied employing rotating ring disk electrode (RRDE) technique in 0.1 M KOH/0.5 M H₂SO₄ at a rotation speed of 1600 rpm with a scan rate of 10 mV s⁻¹ (Fig. 5 and 6). Fig. 5a shows LSVs of all the catalysts in an oxygen-saturated 0.1 M KOH electrolyte. Interestingly, an almost inactive S-doped carbon (S–C 900) with the onset potential of 0.86 V becomes highly active with incorporation of N into carbon layers. The N–S–C 900 shows a more positive onset potential (0.97 V) than N–S–C 800 (0.92 V) and N–S–C 1000 (0.95 V), and the same with the commercial Pt/C catalyst (0.97 V). Moreover, the N–S–C 900 catalyst even possesses a higher peak current density (7.06 mA cm⁻²) than the commercial Pt/C catalyst (6.09 mA cm⁻²). These results indicate that N–S–C 900 can be applied as an highly active ORR catalyst in an alkaline medium. In order to assess the catalytic efficiency comprehensively, some ORR kinetics parameters including the onset potential, half-wave potential and diffusion limiting current density, are compared according to the polarization curves, as shown in Table S5.† The ORR kinetics was further analyzed using the Koutecky–Levich (K–L) eqn (1) and (2) (see ESI†). The kinetic current density obtained at the potential of 0.5 V on N–S–C 900 is higher than those on other N–S–C materials (see Table S5†), indicating that the N–S–C 900 is more kinetically facile toward ORR than other as-obtained samples. The less negative onset and half-wave potentials, and the higher limiting current density on N–S–C 900 may be ascribed to the large BET surface area and hierarchical porous structure, providing sufficient reaction space and facilitating the electron and electrolyte transports. In addition, the introduction of highly active sites (pyridinic N, graphitic N and thiophene S) and the synergistic effect of N and S contribute a lot to the enhancement of the ORR activity.

In order to further understand the reaction kinetics, the n value and the percentage of peroxide (% HO₂⁻) generated during ORR on different catalysts were obtained from RRDE profile using eqn (3) and (4) (see the ESI†). Fig. 5b shows the number of electron transferred for each catalyst including Pt/C. Following the excellent kinetics of Pt (4-electron process) in an alkaline medium for ORR, the N–S–C 900 displays the electron transfer number of 3.8 very close to Pt/C, whereas the N-free counterpart S–C 900 shows the value of 3.4, which is much lower than that of N–S–C 900. Fig. 5c and d presents the percentage of H₂O₂ yield and current density obtained at the ring electrode during oxygen reduction, respectively. As can be seen, the N–S–C 900 outperforms all the other prepared catalysts. The % H₂O₂ formation of N–S–C 900 is 9.85%, which is a bit higher than 3.78% for Pt/C. However, the value is much better than 27.9% for S–C 900, 15.2% for N–S–C 800, and comparable to 8.8% for N–S–C 1000. Because of N doping, a reduction in ring current for N–S–C 900 (0.16 mA cm⁻²) is observed in Fig. 5d as compared to that of S–C 900 (0.30 mA cm⁻²). Furthermore, the ring currents of N–S–C 800 (0.21 mA cm⁻²) and N–S–C 1000 (0.14 mA cm⁻²) are also less than that of S–C 900. These results clearly display the superiority of N–S–C 900 over N-free S–C 900.

We also measured the ORR in an acidic medium vs. RHE using RRDE as shown in Fig. 6a. The onset potential and limiting current density of N–S–C 900 (0.78 V and 5.82 mA cm⁻²) are apparently better than those of S–C 900 (0.57 V and 3.4 mA cm⁻²), and comparable to those of Pt/C (0.81 V and 5.73 mA cm⁻²). The variation in ORR activity, similar with that in basic conditions, is observed in the acidic conditions as well. N–S–C 900 presents the onset potential of 0.78 V and limiting current density of 5.82 mA cm⁻², superior to other as-prepared samples and close to the benchmark Pt/C catalyst (Table S6†). Fig. 6b gives the electron transfer number in all the prepared catalysts during catalyzing ORR. The S–C 900, due to the absence of N, is found to display very weak kinetics with electron transfer number of ∼3.2. However, the N–S–C 900 shows the n value of 3.92, a little less than that (3.95) of the state-of-the-art Pt/C catalyst, whereas for N–S–C 800 and N–S–C 1000, the n values are found to be 3.74 and 3.89, respectively. Similarly, as can be seen from Fig. 6c, the H₂O₂ production for N–S–C 900 catalyst (4.1%) is comparable to that of Pt/C (2.7%) in the applied potential range, while the S–C 900, N–S–C 800, and N–S–C 1000 catalysts lead to high H₂O₂ production with 40.6, 13.1, and 5.2%, respectively. Above results indicate that our proposed N–S–C 900 also possesses high ORR activity in an acidic medium.

Cyclic voltammogram (CV) at a scan rate of 100 mV s⁻¹ were conducted in 0.1 M KOH and 0.5 M H₂SO₄ (Fig. S7a and b†). As shown in Fig. S7a,† in N₂-saturated 0.1 M KOH solution, no redox peak is found. In contrast, a cathodic peak appears in the O₂-saturated 0.1 M KOH solution (Fig. S7a†), demonstrating the electrocatalytic activity of the catalysts towards oxygen reduction. The N–S–C 900 (0.91 V) reveals the ORR peak at more positive potential than the S–C 900 (0.84 V). In both cases, the area of the voltammograms are large in O₂- and N₂-saturated solution due to faradaic currents (i.e., current produced by charge transfer between reacting species). Because the N–S–C 900 are less active in acidic than in alkaline media, the significant cathodic peaks found in 0.1 M KOH are not obvious in 0.5 M H₂SO₄. In 0.5 M H₂SO₄, the wide and small redox peaks observed for S–C 900 and N–S–C 900 in O₂- and N₂-saturated solution may be attributed to heteroatom protonation on the carbon surface.⁵⁰

Furthermore, for a robust catalyst, the stability and tolerance to fuel crossover are also vital for practical applications in fuel cells. Consequently, the stability of the N–S–C 900 catalyst was first assessed with the current–time (i–t) chronoamperometric responses, and commercial 20% Pt/C catalyst was used as the reference. It reveals that the relative current of the ORR on N–S–C 900 is only declined by 6.9% during a 10 000 s test in oxygen-saturated 0.1 M KOH (Fig. 7a), and in contrast, the relative current of the commercial Pt/C is reduced by 15.2% under the same conditions. The catalyst was then exposed to methanol to test its tolerance to methanol. In an alkaline medium, when 3 M methanol is added, almost no response is observed for N–S–C 900, but the catalytic activity of 20% Pt/C severely drops (Fig. 7b). As seen in Fig. 7c, the current density for the N–S–C 900 retains 56.4% of the initial value in a 0.5 M H₂SO₄ solution after 10 000 s, which is also superior to 42.2% current retention for the Pt/C. As shown in Fig. 7d, after the addition of 3 M methanol, the current density for the N–S–C 900 retains 81.8%, which is significantly higher than that on the Pt/C (49.3% current retention). These results indicate that the N–S–C 900 catalyst has better operational stability and better methanol tolerance for ORR than the commercial Pt/C catalyst in both alkaline and acidic media.

Fig. 7 (a) The stability and (b) methanol-tolerance evaluation of N–S–C 900 tested by the current–time chronoamperometric responses at 0.70 V versus RHE in O₂-saturated 0.1 M KOH solution (commercial 20% Pt/C is used for comparison). The arrow in (b) represents the addition of 3 M methanol into the electrolyte. (c) The stability and (d) methanol-tolerance evaluation of N–S–C 900 tested by the current–time chronoamperometric responses at 0.49 V versus RHE in O₂-saturated 0.5 M H₂SO₄ solution (commercial 20% Pt/C is used for comparison). The arrow in (d) represents the addition of 3 M methanol into the electrolyte.

The activity enhancement of N–S–C 900 for ORR suggests that our fabricated catalyst is conducive for energy generation. Firstly, the nitrogen adsorption/desorption isotherm of N–S–C 900 indicates that the N–S–C 900 catalyst possesses a high BET surface area and plenty of microporous structure, thus providing abundant reaction sites explored.^38,39 Meanwhile, a handful of mesoporous structure helps to facilitate electron and mass transports.⁴⁰ Secondly, our XPS measurements show that the N–S–C 900 not only has a relatively high nitrogen and sulfur content, but also possesses dominant active species like pyridinic N (18.7%), graphitic N (52.3%) and thiophene-S (65.9%),¹³ which are beneficial for ORR. Thirdly, the synergistic effect between N and S enhances the overall electrocatalytic performance.

4. Conclusions

In conclusion, we synthesized, for the first time, novel N, S co-doped carbon catalysts by a facile one-step pyrolysis method and measured their electrocatalytic activity toward ORR in both alkaline and acidic media. Our fabricated N–S–C 900 was superior to the commercial Pt/C in terms of half-wave potential and diffusion limiting current density in alkaline media. What is more, the N–S–C 900 also displayed excellent ORR activity, comparable to the benchmark Pt/C catalyst. The unusual high catalytic activity should be ascribed to the combination of the high surface area and micropore-rich structure, more highly active sites (pridinic-N, graphitic-N and thiophene-S) and the synergetic effect between nitrogen and sulfur. The excellent ORR performance and reliable stability of N–S–C 900 in an alkaline medium indicate that this new catalyst is a promising candidate for the next generation of highly efficient ORR electrocatalysts particularly in alkaline methanol fuel cells.

Acknowledgements

This research has been financed by the National Natural Science Foundation of China (no. 21273024) and the Natural Science Foundation of Jilin Province, China (no. 20160101298JC).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21958f

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