Doping carbon nanotubes with N, S, and B for electrocatalytic oxygen reduction: a systematic investigation on single, double, and triple doped modes

Sen Liu a, Guozhu Li *ab, Yuying Gao a, Zhourong Xiao a, Junfeng Zhang ab, Qingfa Wang ab, Xiangwen Zhang ab and Li Wang ab
aKey Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. E-mail: gzli@tju.edu.cn; Fax: +86 22 27892340; Tel: +86 22 27892340
bCollaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China

Received 15th March 2017 , Accepted 17th June 2017

First published on 19th June 2017


In this work, polydopamine (PDA)-coated multi-walled carbon nanotubes (CNTs) have been employed as reactive platforms for the anchoring of multiple heteroatom dopants. Single-, double-, and triple-doped CNTs with the elements N, S and B have been successfully prepared and systematically investigated as electrocatalysts for the oxygen reduction reaction (ORR). The obtained catalysts have been fully characterized by the nitrogen-adsorption technique, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Combining with the physicochemical properties of different doped CNTs and their catalytic performance, various doping modes are compared and the mechanism of these processes has been investigated. The synergetic effects on double and triple doped CNTs with N, S, and B are found. The new non-metal catalyst N–S–B-CNTs exhibits high electrocatalytic activity and good stability for ORR.


Introduction

The oxygen reduction reaction (ORR) as a cathodic half-reaction draws particular attention for fuel cells and metal–air batteries with high efficiency, high power density, and low/zero emissions.1 Traditionally, platinum (Pt) and its alloys have been regarded as the best electrocatalysts for ORR. Meanwhile, Pt-based electrodes suffer from multiple drawbacks, such as prohibitive cost, scarce resources, poor durability and susceptibility to methanol and CO poisoning effects.2,3 Exploring efficient, low-cost, and durable electrocatalysts is of great importance. Metal-free catalysts, particularly heteroatom-doped carbon materials, are some of the most promising alternatives. They are non-electron-neutral and consequently favor the molecular adsorption of oxygen and its reduction.4–6

With the progress in tuning intrinsic properties of carbon materials with a series of heteroatoms, such as B,7–17 N,4,18–34 S,35–42 P,17,30,43–48 and F,49–52 plenty of doped carbon materials used as ORR electrocatalysts have been reported. Among them, the N-doped ones are the most studied. In 2009, N-doped vertically aligned carbon nanotubes (VA-CNTs) were discovered to be superior to Pt for ORR without CO deactivation and fuel crossover effects in alkaline media.20 Subsequently, N-doped graphene was also found to be an efficient metal-free catalyst. Many groups reported that N-doped graphene25,31–34 exhibited much higher durability and selectivity than Pt/C for ORR in alkaline/acid medium. N-doped active carbon was also reported to have excellent ORR properties.27 Recently, Fu et al. reported a N-doped-dual-carbon-material, 3D N-doped reduced graphene oxide-PPV calcined-carbon nanotubes (N-RGO-PPV-CNTs), which shows good ORR performance.19 Other heteroatoms, like B and S, also exhibit a promising research prospect. Experiments have shown that B-doped carbon nanotubes (BCNTs),13 B-doped graphene (BG)11 and S-doped graphene (SG)40 all possess quite good performance for ORR.

Carbon materials doped with binary heteroatoms were consequently focused on. Experiments have proved that introducing two kinds of heteroatoms in carbon materials can increase the activity sites and spin density to improve electrocatalytic performance. Qiao et al. observed that the ORR activities of N, S-co-doped mesoporous carbon nanosheets (N, S-CN)35 and N, B-graphene8 were more active than single-doped (S-, B- or N-) carbon materials. Wang et al. prepared a series of vertically aligned boron carbonitrides (VA-BCNs),14 which were far better than vertically aligned N-doped carbon nanotubes (VA-NCNTs) and vertically aligned B-doped carbon nanotubes (VA-BCNTs). Besides, catalysts of S–N-co-doped carbon foams prepared by She et al.39 and S–N-co-doped 3D graphene frameworks prepared by Su et al.38 show excellent catalytic performance.

Unfortunately, most works involve single-doped or double-doped carbon materials. What is the situation of multidoped carbon materials with more than two kinds of heteroatoms? Are there any differences among single-doped, double-doped and triple-doped catalysts? What are the action mechanisms and exact interactions among various dopants in carbon materials? Generally, the preparation of doped carbon materials for ORR applications remains at an early stage. Herein, we conduct systematic research on a series of doped CNTs ranging from single-doped and double-doped CNTs to triple-doped CNTs based on the use of dopamine. The self-polymerization of dopamine can form adhesive substrate-independent coatings of polydopamine (PDA). PDA serves not only as a N-containing precursor, but also a versatile platform for subsequent modification.53 It possesses excellent structural tunability, strong chelation capability to metal ions and capability of facile post-modification. It can react with amine or thiol groups via Schiff base or Michael addition reactions, which provides opportunities to introduce other heteroatoms. And these reactions proceed efficiently at room temperature without any harsh reaction conditions. Those distinguished natures make it a good candidate for a N-containing precursor.

In this work, three well-studied elements N, S and B have been selected. We used multi-walled CNTs as carriers and PDA as both a N source and reactive platform for the anchoring of S and B elements. The obtained catalysts have been fully characterized and used as ORR electrocatalysts. Moreover, their behaviors together with the one based on reduced graphene oxide (rGO) have been fully compared for a better understanding of the doping process.

Experimental section

Materials

Multi-walled carbon nanotubes (MWCNTs) (10–20 nm in diameter and 20–100 μm in length) and reduced graphene oxide (rGO) were purchased from Chengdu Organic Chemicals Company. Dopamine hydrochloride (DA), 2-mercaptoethanol (2-ME), 4-mercaptophenylboronic acid (4-MPBA) and 3-aminobenzeneboronic acid (3-ABBA) were purchased from Aladdin. Tris(hydroxymethyl)aminomethane (Tris, AR) was purchased from Guangfu Chemical Company in Tianjin. All chemicals were used directly without further purification. Milli-Q water (18.2 MΩ) was used throughout all experiments.

Material characterization

The morphology of the catalysts were characterized using a transmission electron microscope (TEM) (Tecnai G2 F20, USA) working at an accelerating voltage of 200 kV. Electron energy loss spectroscopy (EELS) mapping analysis was carried out on a Titan G2 60-300 Probe Cs Corrector UHRSTEM. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a Thermo ESCALAB 250XI using a monochromatic Al Kα X-ray source (1486.6 eV photons). Nitrogen adsorption–desorption isotherms were measured on an ASAP 2460 (Micromeritics Instrument Corp., USA) at 77 K. Before measurements, the samples were degassed in a vacuum at 300 °C overnight. Raman characterization was carried out using a DXR Raman microscope (Thermo Fisher Scientific) with a 532 nm-wavelength laser.

Preparation of the catalysts

For the preparation of doped-CNT catalysts, 50 mg of CNTs was added to 50 ml Tris buffer (0.01 M, pH = 8.5). The mixture was sonicated for 10 min. Then, 26.5 mg of DA was added. The mixture was continuously stirred at room temperature for 24 h after sonicated for 30 min. Subsequently, 132.5 mg of various binding compounds (2-ME, 3-ABBA, or 4-MPBA) was added and stirred continuously for another 12 h. For the preparation of N-CNTs, no binding compound was added. The obtained hybrid was collected by centrifugation and washed with water and ethanol. At last, the heteroatom(s)-doped catalysts were prepared through the carbonization of the obtained hybrids in a temperature-programmable tube furnace under a N2 atmosphere at 700 °C for 2 h with a heating rate of 5 °C min−1. A similar method was applied to prepare N–S–B-rGO, in which the CNT support was replaced by rGO.

Electrochemical analysis

A glassy carbon rotating disk electrode (RDE, 5 mm in diameter, Tianjin AidaHengsheng Science-Technology Development Company, China) was polished mechanically prior to use with 1 μm, 0.5 μm and 0.05 μm alumina slurry respectively to obtain a mirror-like surface, followed by washing with Milli-Q water and ethanol alternately and then drying in air.

For the electrochemical tests, 3 mg of the fabricated catalysts was dispersed in 1 ml of 75 vol% isopropanol. The mixture was ultrasonicated to obtain homogenous catalyst ink. Then, 25 μl of 0.5 wt% Nafion (Sigma-Aldrich) aqueous solution was added, and the mixture was continuously sonicated for 2 h.

To prepare the working electrode for electrochemical measurements, 15 μl of the ink was dripped on a mirror polished glass carbon electrode. Then, it was dried at room temperature overnight. Before the test, the electrode was further dried in a vacuum drying oven under 60 °C for 30 min. After that, the working electrode was inserted into a three-electrode cell setup, which also included a platinum counter electrode and a Ag/AgCl/KCl (saturated) reference electrode in a glass cell containing 250 ml of 0.1 M KOH as an electrolyte.

All cyclic voltammetry curve (CV) measurements were performed at a scan rate of 100 mV s−1 in the potential range of −0.9–0.2 V. Linear sweep voltammetry (LSV) measurements, which were obtained in O2-saturated 0.1 M KOH solution, were scanned cathodically from −0.9 V to 0.2 V at a scan rate of 10 mV s−1. Before each test, the electrolyte was purged with O2 for at least 1 h to achieve O2 saturated solution. RDE measurements were conducted at different rotation speeds from 400 to 2800 rpm by using an electrochemical analysis station (Ivium Technologies Co., Netherlands) combined with a rotation speed controller (Pine Instrument Co., USA).

Calculation for electron transfer number

Koutecky–Levich plots (J−1vs. ω−1/2) were analyzed at potentials of −0.8 to −0.5 V and were fitted into the linear curves, where the slopes and intercepts can be used to calculate the electron transfer number (n) and kinetic current density (jk) number according to the Koutecky–Levich equation,
image file: c7cy00491e-t1.tif

B = 0.62nF(D0)2/3ϑ−1/6C0

jk = nFkC0
where j is the measured current density (mA cm−2), jl is the diffusion-limiting current density (mA cm−2), jk is the kinetic current (mA cm−2), and ω is the electrode rotation rate (rpm). B could be determined from the slope of the Koutecky–Levich plots, n represents the electron transfer number, F is the Faraday constant (F = 96[thin space (1/6-em)]485 C mol−1), D0 is the diffusion coefficient of O2 in 0.1 M KOH (1.9 × 10−5 cm2 s−1), ϑ is the kinetic viscosity (0.01 cm2 s−1), C0 is the bulk concentration of O2 (1.2 × 10−6 mol cm−3), and k is the electron-transfer rate constant. A constant of 0.62 is adopted when the rotation speed is expressed in rad s−1.

Results and discussion

As illustrated in Scheme 1, N-CNT hybrids were firstly synthesized by mixing a given amount of dopamine (DA) with CNTs in Tris buffer (pH = 8.5). DA polymerized to form a PDA thin film directly onto the surface of CNTs.35 After pyrolysis under a N2 atmosphere, N-doped multi-walled CNTs were obtained.
image file: c7cy00491e-s1.tif
Scheme 1 Fabrication of single-doped, double-doped and triple-doped CNTs (or rGO) with the elements N, S, and B based on a PDA platform.

Double-doped and triple-doped CNTs can be synthesized by the anchoring of different organic species on the reactive PDA ad-layer. This process can be realized by simple immersion of PDA-coated CNTs in a solution of a thiol- or amine-containing compound with target element(s). PDA can react with thiols or amines via Michael addition or Schiff base reactions. For the catalyst co-doped with N and S (N–S–CNTs), 2-mercaptoethanol (2-ME) was used as the precursor of doped heteroatoms, while 3-aminobenzeneboronic acid (3-ABBA) was employed as the source of the B element for the preparation of the N–B-co-doped catalyst (N–B-CNTs). 4-Mercaptophenylboronic acid (4-MPBA) containing both S and B elements was selected for the preparation of the N-, S-, and B-tridoped catalyst (N–S–B-CNTs) using the same synthesis strategy.

Typical as-synthesized catalysts including all the types of single, double and triple doped ones have been characterized systematically. The morphology of the three samples of N-CNTs, N–S-CNTs, and N–S–B-CNTs was firstly investigated by transmission electron microscopy (TEM). The typical TEM images in Fig. 1 show that all the three catalysts (N-CNTs, N–S-CNTs, and N–S–B-CNTs) keep a good tube structure with a smooth surface after surface modification using PDA and the dopant followed by pyrolysis in N2. The small bumps on the tube surface increase surface roughness, which may generate more active sites. The distribution of various elements on the catalyst was also investigated on the tri-doped N–S–B-CNTs for instance. Typical elemental mapping images of N–S–B-CNTs illustrate the presence of N, S, and B elements which are homogeneously distributed on CNTs (Fig. 1g).


image file: c7cy00491e-f1.tif
Fig. 1 TEM images of N-CNTs (a and b), N–S-CNTs (c and d), and N–S–B-CNTs (e and f). Elemental mapping of C, B, N, and S on N–S–B-CNTs (g).

Furthermore, the porosity of the catalysts (N-CNTs, N–S-CNTs, N–B-CNTs and N–S–B-CNTs) was characterized by the nitrogen-adsorption technique. The obtained nitrogen-adsorption isotherms are of type IV with a distinct hysteresis loop in the high-pressure regions (P/P0 = 0.8–1). As summarized in Table 1, the surface areas of N-CNTs, N–S-CNTs and N–B-CNTs are 126 m2 g−1, 130 m2 g−1 and, 122 m2 g−1, respectively, which are comparable to that of pristine CNTs (130 m2 g−1). This indicates that the structure of the nanotubes is mostly preserved after the anchoring and doping of N, N–S and N–B. In comparison, the surface area of N–S–B-CNTs is 82 m2 g−1, which decreased by 37% in comparison to that of CNTs. This indicates the change of the packing structure due to the triple doping of N, S and B, which is different from those of the single doping and double doping.

Table 1 BET surface area, pore volume, and pore size of CNTs, N-CNTs, N–S-CNTs, N–B-CNTs and N–S–B-CNTs
Sample Surface area (m2 g−1) Pore volume (cm3 g−1) Pore size (nm)
CNTs 130 0.51 15.66
N-CNTs 126 0.44 19.67
N–S-CNTs 130 0.6 22.53
N–B-CNTs 122 0.51 17.04
N–S–B-CNTs 82 0.41 23.66
N–S–B-rGO 95 0.34 11.32


The chemical states of the key elements were accurately evaluated by X-ray photoelectron spectroscopy (XPS), and the results are displayed in Fig. 2. The XPS survey scans of all the four samples (N-CNTs, N–B-CNTs, N–S-CNTs and N–S–B-CNTs) indicate the presence of carbon together with the target doping element(s), which is consistent with the results of TEM mapping. No obvious signal of any other element can be observed in the full scan range, indicating that the catalysts are free of metal. The relative amounts of different elements on the surfaces of the samples have been summarized in Table 2. The compositions of various element states are shown in Table 3. Only the elements C and N are detected in N-CNTs with a N content of 1.25%. In comparison, both N and S are recorded in N–S-CNTs. 0.3% S and 0.56% N had been successfully doped after pyrolysis. The fraction of doped N in N–S-CNTs decreased by 55.2% compared with that in N-CNTs. This can be explained by the structure of 2-ME added during catalyst preparation. The elements of S and O with the same mole ratio in 2-ME were also introduced simultaneously in the ad-layer. Therefore, more N will be cost via gasification caused by O during pyrolysis. Moreover, the high resolution peaks of S 2p are deconvoluted into three peaks associated with C–S–C (163.2 eV for S 2p3/2, 164.4 eV for S 2p1/2) and C–SOx–C (167.8 eV) species. 38.2% of the doped S is in the oxide state of C–SOx–C. The C–S–C structure (61.8%) referring to S substitution in the carbon lattice suggests successful incorporation of S into the carbon lattice network. In comparison, the fraction of doped N in N–B-CNTs is as high as 5.95%. And 1.04% of doped B was also detected in the catalyst N–B-CNTs. The B 1s peak ranging from 189.0 eV to 193.2 eV indicates the formation of various oxides of boron (BC2O, BCO2, or B2O3) on the surface.15 The increased N content can be explained by the introduction of 3-ABBA with the amino group as an extra N source and the boric group as an oxygen consumer. The formation of boron oxides can fix oxygen to prevent gasification of nitrogen during pyrolysis. Then more N is reserved in the final sample than that of N–S-CNTs. Moreover, no oxidized N (NOx) was detected in N–B-CNTs, which further indicates the effect of oxygen capture by B during catalyst preparation.


image file: c7cy00491e-f2.tif
Fig. 2 XPS spectra of typical catalysts. (a) N 1s, B 1s and survey of N–B-CNTs. (b) N 1s, S 2p and survey of N–S-CNTs. (c) N 1s, S 2p, B 1s and survey of N–S–B-CNTs. (d) N 1s and survey of N-CNTs.
Table 2 Heteroatomic doping level for N-CNTs, N–S-CNTs, N–B-CNTs, and N–S–B-CNTs
Sample (at%) N-CNTs N–S-CNTs N–B-CNTs N–S–B-CNTs
C 98.75 99.14 93.01 91.50
N 1.25 0.56 5.95 2.29
S 0.3 0.32
B 1.04 5.89


Table 3 Peak assignments for N 1s, S 2p, and B 1s for N-CNTs, N–S-CNTs, N–B-CNTs and N–S–B-CNTs
Sample (at%) N-CNTs N–S-CNTs N–B-CNTs N–S–B-CNTs
Notes: a Represents relative amount. b Represents absolute amount.
N Pyridinic-N 28.0a 0.35b 27.6a 0.16b 5.3a 0.31b 18.2a 0.42b
Pyrrolic-N 24.2 0.30 27.5 0.15 17.8 1.06 30.5 0.70
Graphitic-N 27.6 0.35 24.3 0.14 76.9 4.58 37.1 0.85
Oxidized-N 20.2 0.25 20.6 0.12 0 0 14.3 0.33
S S 2p3/2 34.6 0.10 40.2 0.13
S 2p1/2 27.2 0.08 59.8 0.19
C–SOx–C 38.2 0.12 0 0
B Boron oxides 100 1.04 100 5.89


For the case of tri-doped N–S–B-CNTs, 0.32% S, 5.89% B and 2.29% N have been detected on the surface. The fractions of N, S, B and C elements substantially changed compared with those of N-CNTs, N–S-CNTs and N–B-CNTs. The N content of N–S–B-CNTs is 1.8 times and 4.1 times that of N-CNTs and N–S-CNTs, respectively. The doped B element with a high proportion (5.89%) was also detected in N–S–B-CNTs in the states of various oxides (BC2O, BCO2, or B2O3), which is similar to that of N–B-CNTs. The formation of boron oxides can cost more oxygen (with an amount of a multiple of 1–2 times) during pyrolysis. Considering the structures of 4-MPBA and 2-ME, the mole ratios of S/O are 1/2 and 1/1, respectively. Though 4-MPBA can introduce twice as much oxygen compared to 2-ME, it also has boron with a 1/1 S/B mole ratio. Based on the final state of boron oxides (BC2O, BCO2, or B2O3), it can be deduced that 1 mole of boron will fix 1–2 moles of oxygen. Therefore, more than 50% oxygen in 4-MPBA will be fixed by boron during the synthesis of N–S–B-CNTs. Then the gasification of N and S elements via reaction with oxygen is restricted due to the limited amount of free oxygen. This conclusion is further verified by the different compositions of the two states of S element (C–S–C and C–SOx–C) in the catalysts prepared from 2-ME and 4-MPBA. When 2-ME is used, 38.2% of the S element is in the oxide state of C–SOx–C. In comparison, no C–SOx–C was detected and the entire S element is in the state of C–S–C for N–S–B-CNTs synthesized with 4-MPBA. The absence of C–SOx–C can also be explained by the consumption of more oxygen by doped boron oxides.

The N contents of the catalysts (0.56%–5.95%) reported in our work are much lower than that reported in the work of the Qiao group35 (8.6%). In both works, similar precursors have been adopted, but the procedures of carbonization are different. A slow pre-calcination step with a heating rate of 1 °C min−1 had been employed in Qiao's experiments, which causes the preservation of a large fraction of N in the sample. In our experiments, the pre-calcination step has been omitted and the heating rate was always kept at 5 °C min−1. A higher heating rate during carbonization leads to less N content in our samples. A similar N content had been reported by Cui et al.54 (∼1.5 at%) and Borghei et al.55 (∼0.5 at%). In both works, good electrocatalytic activity for ORR had been achieved. Additionally, the high-resolution N 1s peak is deconvoluted into four peaks located at 403 eV, 401.2 eV, 399.8 eV, and 398.5 eV, which can be assigned to nitrogen oxide, graphitic nitrogen, pyrrolic nitrogen and pyridinic nitrogen (the data are shown in Fig. 2 as insets). It should be noticed that the XPS signal of N 1s of N–S-CNTs is too weak to be reliably deconvoluted. Therefore, no definite conclusion can be made based on the comparison of the N compositions. It is still disputed about which type of N plays a major role in ORR. Some groups showed that pyridinic nitrogen34,56,57 is catalytically active, while some others thought that graphitic nitrogen33 is more effective. In the work of Jung et al., both pyridinic nitrogen and graphitic nitrogen were claimed to contribute high catalytic activity by using density functional theory (DFT) calculations.58 However, a study by Lin et al. indicated that pyridinic nitrogen is not an effective promoter for ORR activity of carbon materials.59 The detailed roles of different N are still not clear. More in-depth work on the catalytic mechanism is to be further investigated.

The Raman spectra of CNTs, N-CNTs, N–S-CNTs, N–B-CNTs and N–S–B-CNTs (Fig. 3) display the typical D band and G band at roughly 1350 cm−1 and 1580 cm−1. The estimation of the amount of defect and disordered structures can be conducted by the measurement of the intensity ratio of the D band to G band (ID/IG). The ID/IG ratio of pristine CNTs is 0.621. After heteroatom doping, the ID/IG ratio increased to 0.687, 0.865, 0.871 and 0.770, for N-CNTs, N–S-CNTs, N–B-CNTs and N–S–B-CNTs, respectively, because the introduction of extra heteroatoms (S, N and B) results in the decomposition of the graphitic structure and produces more defects and disordered moieties on CNTs.


image file: c7cy00491e-f3.tif
Fig. 3 Raman spectra of N-CNTs, N–B-CNTs, N–S-CNTs, N–S–B-CNTs, and pristine CNTs.

Previous literature studies had performed DFT calculations to investigate the mechanism and influence of the heteroatom-doped CNTs.7 When CNTs are doped with N, carbon atoms near nitrogen atoms are positively charged.60 The Fermi level is shifted towards the positive energy, so that new states appear above the Fermi energy level.61 N and S co-doped nanotubes have a narrow energy gap with metal conductivity.62 The synergistic effect of N–S co-doping improves the positive charge distribution at active centers and the number of active sites. The co-doping of N and S induces the significant spin density and has strong chemical bonding with oxygen molecules which is not observed in the mono-doped cases.63 In comparison, doping of B in CNTs leads to electron transfer from B atoms to the nearby carbon atoms.64 Boron impurities cause deficiency of electrons which leads to the shift of the Fermi level towards the valence band. This shift introduces new states below the Fermi level.61 B doped nanotubes have overlapping energy gaps with semi-metal conductivity.62 For the B–N co-doping of CNTs, the energy gaps increase with increasing doping concentration both along the tube axis and around the tube, because the mirror symmetry of MCNTs is broken by doping.65 It can be deduced that a new electron density distribution and Fermi level will be achieved based on complex interactions from the triple doping of N, S, and B in MCNTs.66

All the catalysts of single-, double-, and triple-doped carbon materials (N-CNTs, N–S-CNTs, N–B-CNTs, N–S–B-CNTs and N–S–B-rGO) have been used to catalyze ORR comparatively. Their electrocatalytic performances were evaluated by carrying out three-electrode cyclic voltammetry (CV) experiments in Ar-saturated and O2-saturated 0.1 M KOH solution at a scan rate of 100 mV s−1. As shown in Fig. 4a, when the electrolyte was saturated with Ar, a quasi-rectangular voltammogram without an obvious redox peak was obtained as a result of the typical super capacitance effect on porous carbon materials. In contrast, a well-defined characteristic ORR peak, centered at around −0.20 V, was observed for all the samples when O2 was introduced. All the multi-doped CNTs show a high reaction current of about 4 mA cm−2, which is much higher than that of single-doped N-CNTs (1.6 mA cm−2).


image file: c7cy00491e-f4.tif
Fig. 4 (a) Cyclic voltammograms of N-CNTs (■), N–S-CNTs (image file: c7cy00491e-u1.tif), N–S–B-CNTs (image file: c7cy00491e-u2.tif) N–S–B-rGO (image file: c7cy00491e-u3.tif), N–B-CNTs (image file: c7cy00491e-u4.tif) and 20% Pt/C (image file: c7cy00491e-u5.tif) in Ar-saturated (gray) and O2-saturated (colored) conditions. (b) Linear sweep voltammograms recorded in O2-saturated 0.1 M KOH at a scan rate of 10 mV s−1 and a rotation speed of 1600 rpm. (c) Electron transfer number at various potentials of different samples. (d) Koutecky–Levich plots at −0.6 V. (e) Chronoamperometric responses of N–S–B-CNTs and 20% Pt/C at −0.3 V vs. Ag/AgCl in 0.1 M KOH (O2-saturated). The arrows represent the addition of O2 and methanol. (f) Durability of N–S–B-CNTs and 20% Pt/C in 0.1 M KOH (O2-saturated). (g) Linear sweep voltammograms recorded at different rotation speeds from 400 to 2800 rpm for N–S–B-CNTs.

To gain quantitative understanding and additional insight into the ORR process catalyzed by the doped CNTs, the linear sweep voltammograms (LSVs) of all the catalysts were collected on a rotating disk electrode (RDE) at 1600 rpm and compared in Fig. 4b. N–S–B-CNTs exhibit the highest ORR onset potential of −0.05 V (vs. Ag/AgCl, all potential will be relative to Ag/AgCl unless specifically illustrated hereinafter), which is slightly positive than those of N-CNTs (−0.08 V), N–S-CNTs (−0.06 V), N–B-CNTs (−0.06 V) and N–S–B-rGO (−0.06 V). The half-wave potentials (E1/2, viz. a potential at which the current is half of the limiting current in LSV curve, which is commonly used to assess ORR catalysts) of N-CNTs, N–S-CNTs, N–B-CNTs, and N–S–B-CNTs are all around −0.20 V, which is about 50 mV more negative than that of Pt/C (−0.15 V). Moreover, the wide current plateau from −0.3 V to −0.8 V observed on N-CNTs, N–S-CNTs, N–B-CNTs, and N–S–B-CNTs represents a stable diffusion-controlled process. Significantly, N–S–B-CNTs shows the highest reaction current density of 4.8 mA cm−2 at −0.8 V and 1600 rpm, as compared to N-CNTs (4.0 mA cm−2). In comparison, the current densities of both N–S-CNTs and N–B-CNTs at −0.8 V are 4.2 mA cm−2.

Based on the PDA platform for heteroatom doping, rGO was also employed as a support for the preparation of doped rGO. As shown in Fig. 4b, when rGO is used as the carrier of the dopants with N, S and B elements, the obtained catalyst behaves differently from that of CNTs. The ORR performance of N–S–B-rGO is relatively bad according to its unstable diffusion-limiting current density. Therefore, the steric configuration of carbon materials on the nanoscale also plays a key role in catalytic performance. CNTs with high specific surface areas, large pore volumes, and well-developed mesoporosity are suitable supports in this method. The resulting doped CNTs not only assure facile access of reactants to the active sites, but also facilitate electron delivery through the continuous framework structure.

Furthermore, electron transfer numbers (n) under various potentials (−0.5–0.8 V) are calculated from the Koutecky–Levich (K–L) plots on the basis of the K–L equation. The data are displayed in Fig. 4c and d. The K–L plots (J−1vs. ω−1/2) for all the samples were obtained from the reaction current at different potentials (−0.5 V, −0.6 V, −0.7 V, −0.8 V) on the LSVs at various rotation speeds (400–2800 rpm). The plots of all samples exhibit good linearity. As shown in Fig. 4c, triple-doped N–S–B-CNTs show larger n values of 3.98–4.05 than single-doped and double-doped CNT catalysts. This suggests the perfect selectivity for N–S–B-CNTs for the efficient four-electron-dominated ORR pathway. Obviously, the stable and high values of electron transfer numbers and limited current density of N–S–B-CNTs (n = 3.98–4.05; J = 4.8 mA cm−2 at −0.8 V) are extremely close to those of 20 wt% Pt/C (n = 4.00–4.10; J = 5.2 mA cm−2 at −0.8 V).

Therefore, superior electrocatalytic ORR performance of N–S–B-CNTs can be confirmed. When the results of both the electron transfer number (n) and the limited current density (J) are considered, an apparent principle of the doping modes for our system can be concluded. The catalysts of double-doped CNTs (N–S-CNTs and N–B-CNTs) exhibit better performance than single-doped CNTs with nitrogen (N-CNTs). Moreover, the triple-doped catalyst (N–S–B-CNTs) is superior to the double-doped ones. The synergy gained from the co-doping of S and N can effectively increase the catalytic activity. This conclusion can be deduced from the fact that N–S-CNTs is superior to N-CNTs in ORR even though the doping amount of N, S, and their sum in N–S-CNTs are lower than the N amount of single-doped N-CNTs. It should be noticed that only 0.3% S has been doped in N–S-CNTs, which is much lower than that reported previously.35,41,42 Similarly, the synergy among N, S and B via triple doping also plays a key role in the improvement of catalytic performance. The addition of B changes the catalyst in two ways. First, during the preparation of the catalyst, the doping amount of N is increased and the oxide state of S element (C–SOx–C) is eliminated due to the oxygen consumption of doped boron oxides. Second, in the catalytic reaction, the doped boron oxides themselves together with N and S can elevate the catalytic activity, because N–S–B-CNTs have higher activity than the N-, S-co-doped catalyst developed in a previous work by Liang et al., even though the efficiencies of both S doping (0.3%) and N doping (2.29%) are lower than Liang's results (2.0% S and 4.5% N).41 The N and S dual-doped graphene (N–S-G) developed by Liang et al. showed a reaction current density of −3.3 mA cm−2 at −0.24 V and the n values of 3.3–3.6 over most of the potential range. Positively charged B helps facilitate chemisorption of O2 on the catalyst, meanwhile the π* electrons accumulating on B are beneficial to transfer the chemisorbed O2.13 Therefore, the outstanding ORR catalytic performance of triple-doped N–S–B-CNTs can be explained as compared with single-doped and double-doped CNTs. In our triple-doped system, the three kinds of heteroatoms have different electronegativities (λ) (λB = 2.04, λN = 3.04 and λS = 2.19) compared with C (λC = 2.55). The difference in the four types of elements can induce asymmetric spin density of atoms, and then improve the catalytic performance. Therefore, N–S–B-CNTs probably possess a unique electronic structure for facilitating ORR.

High selectivity towards ORR is important for potential fuel-cell catalysts to prevent possible fuel-crossover effects. In this study, the tolerance of N–S–B-CNTs to methanol crossover was also assessed in comparison with that of the commercial Pt/C catalyst (with a resulting methanol concentration of 3 M under −0.3 V). When methanol was introduced into the testing cell, the current response of N–S–B-CNTs remained unchanged, indicating their high catalytic selectivity for ORR against methanol oxidation. In contrast, the catalyst of 20 wt% Pt/C showed an instantaneous current jump upon methanol addition, reflecting its sensitivity to fuel crossover (Fig. 4e).

The durability of N–S–B-CNTs was also evaluated by the measurement of chronoamperometric response under a constant voltage of −0.3 V (Fig. 4f). N–S–B-CNTs show a reliable stability. 93.1% of the initial current has been retained after 65[thin space (1/6-em)]000 s. Meanwhile, the Pt/C catalyst lost 23% of its initial current after the same period of time. The higher stability of the metal-free catalyst can be attributed to the strongly bonded heteroatoms in N–S–B-CNTs. Its chemical and mechanical stabilities are improved via pyrolysis of the coated CNTs compared with the carbon-black supported Pt. Then the loss of active sites can be effectively prevented. In addition, a series of linear sweep voltammograms (LSVs) were collected on a rotating disk electrode (RDE) at different rotation speeds (400–2800 rpm) for better evaluation of N–S–B-CNTs (Fig. 4g).

Conclusions

In summary, various non-metal catalysts of heteroatom(s)-doped CNTs have been prepared and studied for ORR. Based on the reactive platform of PDA coating, single, double, and triple doping modes of heteroatom(s) have been realized on CNTs using the elements N, S and B. The obtained catalysts have been fully characterized and systematically investigated as electrocatalysts for ORR. Results show that double-doped CNTs with N and S (N–S-CNTs) are superior to single-doped CNTs with N (N-CNTs), even though the doping amounts of both N and S in N–S-CNTs are lower than that of N in N-CNTs. The synergy between N and S can efficiently improve the catalytic performance of doped CNTs. The addition of B together with N and S plays dual roles. First, the compositions of N and S can be effectively adjusted by the presence of B in catalyst preparation. The amount of doped N is increased and the oxide state of S (C–SOx–C) is eliminated due to the oxygen consumption of doped boron oxide. Second, the doped boron oxides themselves together with N and S can increase their catalytic activity in the reaction. The catalyst N–S–B-CNTs has comparable or even superior activity compared with the N-, S-co-doped catalysts reported in previous works, in the case of lower S doping (0.32%) and N doping (2.29%) in N–S–B-CNTs. The onset potential is −0.05 V and the reaction current density is 4.8 mA cm−2 at −0.8 V with an explicit platform zone of diffusion-limiting current density in a wide potential range for N–S–B-CNTs in ORR. Results also demonstrate that the electron transfer over N–S–B-CNTs is a 4e transfer process, indicating a highly selective surface electrocatalytic reaction. Moreover, N–S–B-CNTs also exhibit good stability and immunity towards methanol. The new non-metal catalyst N-, S-, B-tri-doped CNTs developed in this work brings forward bright developing prospects for non-metal catalysts in ORR applications.

Acknowledgements

This work was supported by the research fund of the National Natural Science Foundation of China (No. 21306132).

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