Graphene-wrapped nitrogen-doped hollow carbon spheres for high-activity oxygen electroreduction

Abstract

Graphene wrapping could be used to improve the electrochemical performance of electrodes via the formation of an effective “plane-to-point” conductive network to promote fast electron transfer to the active sites. In this paper, a series of N-doped hollow carbon spheres wrapped by flexible graphene nanosheets (NGCs) have been successfully prepared by the self-assembly of graphene oxide nanosheets onto the surface of hollow polymer spheres, followed by pyrolysis in flowing ammonia. The as-synthesized NGCs show unique hierarchical nanostructures, large hollow cores, large surface areas, and high relative contents of pyridinic and graphitic N groups, as well as uniform wrapping of graphene layers outside of the carbon spheres. The metal-free NGCs show much better activity for the electrocatalytic oxygen reduction reaction with a larger limiting current density and lower onset potential than those without graphene wrapping. They also showed superior long-term stability and fuel crossover effect to commercial Pt/C. These results indicate that graphene wrapping is an effective strategy for improving the activity of electrocatalytic materials via constructing a fast electron transfer network.

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Introduction

Polymer electrolyte membrane fuel cells (PEMFCs), directly converting chemical energy into electrical power, have the combined advantages of zero exhaust emission, high energy density and efficiency, quietness and close-to-room-temperature operation.¹ However, the cathodic oxygen reduction reaction (ORR) in the PEMFCs has a high reaction overpotential, which has become one of the major limitations for mass production of low-temperature fuel cells.² At present, carbon-loaded platinum (Pt) composites have been demonstrated as the most efficient and practical electrocatalyst for the ORR, but the scarcity and prohibitive cost of Pt make it difficult to commercialize the fuel cells on a large scale especially for the ORR requiring 40–70% of Pt loading due to its intrinsic sluggish reaction kinetics.^1a,3 Moreover, the Pt-based catalysts are also susceptible to carbon monoxide poisoning and methanol crossover effects, thus leading to decreased efficiency of Pt as the ORR electrocatalyst.⁴ Therefore, great efforts have been made to explore inexpensive and stable ORR electrocatalysts with high activity as the substitutes of Pt-based catalysts.

At present, two categories of nonprecious electrocatalysts have been developed. One is earth-abundant transition -based materials (e.g. Fe, Ni, Mn, Co, etc.), which still suffer from low activity and selectivity, and detrimental environmental effects derived from the catalyst waste.⁵ The other is heteroatom-doped metal-free carbon-based materials (e.g., N, B, S, P, etc.),⁶ which have been swiftly and intensively studied because of their many advantages of relative low cost, adjustable porosity, excellent chemical and thermal stability and good resistance to methanol crossover effects. Doping of heteroatoms into the carbon frameworks could adjust the electronic structures, surface basicity, surface hydrophilicity and electric conductivity, thus changing the chemisorption energy of O₂ and even the reaction mechanism (2e⁻/4e⁻) of the catalysts, resulting in enhanced performance for the ORR.⁷ The doped N atoms are beneficial for the ORR with a direct 4e⁻ pathway,⁸ while P, B, and S atoms incorporated into the carbon network could promote the formation of peroxide derived from a 2e⁻ process.⁹ Compared to other non-precious-metal ORR electrocatalysts, N-doped carbon materials have some unique merits of low-cost, environmental friendliness, and high activity. Recently, various kinds of N-doped carbon materials, including N-doped mesoporous carbons,¹⁰ N-doped carbon nanofibers,¹¹ N-doped graphene,¹² N-doped carbon aerogels,¹³ N-doped carbon spheres,¹⁴etc., have been reported toward the ORR. However, most of these materials demonstrate poor graphitization, decreasing the electron transfer and thus leading to both low electrocatalytic activity and reaction selectivity.

Graphene is a two-dimensional (2D) single-atom-thick sheet of sp²-hybridized carbon atoms.¹⁵ It possesses excellent properties including high carrier mobility (∼200 000 cm² V⁻¹ s⁻¹), superior thermal conductivity (∼5000 W m⁻¹ K⁻¹), large surface area (2630 m² g⁻¹), an ambipolar electric field effect, and the room-temperature quantum Hall effect.¹⁶ Therefore, flexible graphene nanosheets could act as the conductive pathway for electron transfer when used as the conductive wrapping layer for metal oxides.¹⁷ The resulting graphene/metal oxides have improved conductivity through forming an effective “plane-to-point” conductive network, showing increased performance as the electrode materials of lithium batteries. Also, flexible graphene nanosheets are prone to wrap on the surface of oxygen-containing solid surfaces via chemisorption and the force of electrostatic absorption.

Herein, we develop a facile strategy of synthesizing graphene nanosheet wrapped nitrogen-doped hollow carbon spheres (NGCs) as high-activity electrocatalysts for the ORR. This unique nanocomposite was prepared via graphene wrapping of hollow polymer spheres (HPSs) with large hollow cores, followed by direct pyrolysis in flowing ammonia to dope N atoms within the carbon framework. In the resulting composites of NGCs, hollow carbon spheres with diameters of 100–150 nm and a thickness of ∼20 nm are uniformly encapsulated by flexible graphene sheets. The thermal treatment in ammonia at high temperature not only introduces the doped N atoms as the active sites for the ORR, but also removes most of the oxygen-containing functional groups on the graphene oxide (GO) nanosheets, thus increasing the conductivity of the composites. The NGCs show not only hierarchical nanostructures, but also a large surface area of over 600 m² g⁻¹, pore volume of 0.68 cm³ g⁻¹ and variable N-doping level of 3.44–7.96 at% depending on the pyrolysis temperature. Graphene wrapping significantly enhances the ORR activity of hollow carbon spheres in terms of lower onset potential, larger limiting current density and higher reaction selectivity of water formed via a direct four-electron pathway. Moreover, the resulting NGCs also show a superior long-term durability and tolerance to MeOH crossover to commercial Pt/C electrocatalysts.

Experimental

Reagents

The Nafion solution (5 wt%) and triblock copolymers P123 (EO₂₀PO₇₀EO₂₀, M_v = 5800) were purchased from Aldrich. Sodium oleate (SO), hexamethylenetetramine (HMT), anhydrous ethanol (EtOH), 2,4-dihydroxybenzoic acid (DA), and potassium hydroxide (KOH) were bought from Sinopharm Chemical Reagent Co., Ltd. The commercial Pt/C catalyst (20 wt%) was bought from Johnson Matthey (United Kingdom). Graphene oxide (GO) was obtained from Nanjing Xianfeng Nanomaterials Company. All solvents and chemicals above were analytical grade and were utilized without further purification.

Material synthesis

Synthesis of N-doped hollow carbon spheres (NHCSs). The NHCSs were prepared by pyrolysis of hollow polymer spheres (HPSs) in flowing NH₃ at 650 °C for 3 h, followed by heating at 1100 °C for 2 h under Ar flow.¹⁸ HPSs were prepared using a hydrothermal self-assembly strategy, in which DA and HMT were used as the carbon source and double surfactants P123 and SO acted as the soft templates.¹⁹ In a typical process, 40 mL of aqueous solution containing SO (0.15 g) and P123 (0.09 g) was added into an aqueous solution (120 mL) containing DA (0.37 g) and HMT (0.14 g) under vigorous stirring. Then, this solution was hydrothermally treated at 160 °C for 8 h. The final reddish brown HPSs were collected via centrifugation, washed three times with deionized water, and dried at 80 °C.

Synthesis of graphene-wrapped nitrogen-doped hollow carbon spheres (NGCs). The commercial graphene oxide (GO, 200 mg) was dispersed into 100 mL of water by ultrasonication for 2 h to obtain a uniformly dispersed GO nanosheet aqueous solution. Then, this GO solution was mixed with a certain amount of HPSs by ultrasonication for 2 h, followed by heating at 80 °C to evaporate water to get a uniform black HPSs@GO composite. The N-doping was performed by pyrolysis of HPSs@GO in NH₃ at 650 °C for 3 h,²⁰ followed by heating at 800, 950 or 1100 °C for 2 h in Ar. The as-prepared sample was named as NGC-800, NGC-950 or NGC-1100 according to the carbonization temperature in Ar.

Structural characterization

Transmission electron microscopy (TEM) images were obtained using a JEOL 2010F electron microscope operating at 200 kV. Samples for analysis were deposited on a carbon micro-grid supported by a copper grid from ultrasonically processed ethanol solutions. Scanning electron microscopy (SEM) images were collected using a JEOL JSM-6700F field emission scanning electron microscope. X-ray photoelectron spectroscopy (XPS) was recorded with an ESCALAB 250 X-ray photoelectron spectrometer with Al Kα (hν = 1486.6 eV) radiation. The carbon monoliths were ground to a powder and then glued onto indium (In) metal particles by pressing for measurements. All spectra were calibrated using 284.5 eV as the line position of adventitious carbon. The source excitation energy was set as 15 keV with an emission current of 20 mA. An analyzer pass energy of 20 eV with a step size of 0.1 eV and dwell time of 100 ms were used. No charging effects were observed when studying the samples by XPS measurements. Nitrogen sorption isotherms were measured using a Micromeritics ASAP 2010 surface area and pore size analyzer at liquid nitrogen temperature (−196 °C). Prior to measurement, the samples were dehydrated under vacuum at 300 °C overnight. The specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) method. The total pore volume was calculated from the amount of nitrogen adsorbed at a relative pressure of 0.99. The pore size distribution curves were calculated from the analysis of the adsorption branch of the isotherm using the Barrett–Joyner–Halenda (BJH) model.

Electrode preparation and electrochemical measurements

After grinding, the active sample (5 mg) was dispersed in a mixture of deionized water (0.5 mL), ethanol (0.5 mL), and Nafion solution (5 wt%, 30 μL), followed by sonication until a homogeneous ink was formed.²¹ Then, the ink (20 μL) was dropped onto the glassy carbon (GC) electrode and then dried to form a catalyst film. A commercial 20 wt% Pt/C (Johnson Matthey) electrode was prepared following the same procedure for comparison. The electrocatalytic ORR activity of the working electrode was studied in a three-electrode system and a 0.1 M KOH solution was used as the electrolyte. Cyclic voltammetry (CV), rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques were performed in O₂-saturated solution using an electrochemical workstation (Pine Instrument Co.). A Pt flake and a saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. Before the ORR tests, the KOH electrode was saturated by bubbling high-purity O₂ for 15 min.

For the CV and RDE tests, a GC disk with 5 mm diameter was used for supporting the catalyst. The CV measurements were carried out at a scanning rate of 50 mV s⁻¹ and the working electrode was cycled at least 50 times before collecting the data in O₂-saturated KOH solution. The linear sweep voltammetry (LSV) tests were performed at an electrode rotating rate varying from 400 to 2025 rpm (scanning rate of 10 mV s⁻¹) in O₂-saturated KOH solution. The current densities were collected from the LSV data and then normalized by the geometric surface area (0.19625 cm²). The electron transfer number (n) is calculated by the Koutechy–Levich (K–L) equation (eqn (a) and (b)) as follows:

B = 0.2nFC_O2D_O2^2/3ν^−1/6 (b)

in which J is the measured current density, J_K is the kinetic limiting current density, ω is the electrode rotating rate, n is the electron transfer number, F is the Faraday constant (F = 96 485 C mol⁻¹), C_O2 is the bulk concentration of O₂ (C_O2 = 1.2 × 10⁻⁶ mol cm⁻³), D_O2 is the diffusion coefficient of O₂ in 0.1 M KOH (D_O2 = 1.9 × 10⁻⁵ cm² s⁻¹), and ν is the kinetic viscosity (ν = 0.01 cm² s⁻¹).^6e The constant 0.2 is adopted when the rotation rate is expressed in rpm.

For the RRDE tests, a GC disk electrode (0.2475 cm² for the geometric surface area) surrounded by a Pt ring (0.1866 cm² for the geometric surface area) was used as the working electrode. The ring current (I_r) and disk current (I_d) were collected from the Pt ring and GC electrode, respectively. The ring potential was held at 0.2 V (vs. SCE) at a scanning rate of 10 mV s⁻¹ and rotating speed of 1600 rpm. The peroxide (HO₂⁻) yields (%) and the transferred electron number (n) can be calculated via the following eqn (c) and (d):²²

in which I_d is the disk current, I_r is the ring current, and N is the current collection efficiency of the Pt ring (N = 0.37) provided by the company.

Results and discussion

Hollow polymer spheres (HPSs) were synthesized according to the previous literature.¹⁹ As shown in Fig. S1 (ESI†), the as-prepared HPSs are composed of well-monodispersed uniform spheres with hollow structures. The existence of uniform mesopores within the walls of the HPSs has also been proved in the previous research.¹⁹ With the evaporation of water from the aqueous mixture of HPSs and graphene oxide (GO) sheets, the hydrophilic GO sheets could gradually stick onto the surface of the carbon spheres due to the chemisorption on oxygen functional groups and the force of electrostatic adsorption and then encapsulate the spheres to form a HPS@GO composite (Fig. 1). The resulting composite was further calcined in NH₃ to carbonize the HPSs, dope the N atoms within the carbon framework and reduce the outer GO sheets via thermal removal of oxygen-containing functional groups at high temperature.²³ This high-temperature treatment not only affords the doping of the N-related species as the active centres,^22b,24 but also increases the carbonization and graphitization degree of the final graphene-wrapped N-doped hollow carbon spheres (NGCs). The outer graphene wrapping could act as a highway for electron transport to N-related catalytic sites within hollow carbon spheres, thus greatly improving the electrocatalytic reactivity for the oxygen reduction reaction (ORR).²⁵ This novel nanocomposite with a unique hierarchical nanostructure has been shown to possess enhanced performance for the ORR with a four-electron-dominant reaction process as discussed in the latter part of the paper.

Fig. 1 Schematic drawing of preparing graphene-wrapped N-doped hollow carbon spheres (NGCs) as highly active oxygen reduction electrocatalysts with a 4e⁻ reaction process.

As shown in Fig. 2a and b, the resulting NHCSs without graphene wrapping prepared by direct carbonization of HPSs still possess a good spherical morphology with a smooth surface, even though the HPSs suffered from a two-step heat-treatment at 650 °C in NH₃ and then at 1100 °C in Ar. This implies the good morphology maintenance of the robust hollow carbon spheres even after a high-temperature treatment. The diameters of the NHCSs are in the range of 100–150 nm as observed in the SEM images. After the encapsulation of HPSs by GO sheets, followed by pyrolysis, the resulting NGC-1100 shows a quite different morphology from NHCSs without graphene wrapping. As shown in Fig. 2c, many protruding particles indicated by yellow arrows are clearly observed due to the packing of the carbon spheres by graphene nanosheets. Also some macropores shown by red arrows are also formed and they are ascribed to the cavities that resulted from the removal of some carbon spheres. And these pores are more clearly seen in high-resolution SEM images (dashed red circles, Fig. 2d). Moreover, one can see that a spherical particle (dashed yellow ring) has a rough surface, indicating that this carbon sphere is completely wrapped by wrinkled graphene sheets (Fig. 2d). This comparison implies that the present wrapping strategy with graphene nanosheets is highly efficient.

Fig. 2 (a and c) SEM and (b and d) high resolution SEM images for (a and b) nitrogen-doped hollow carbon spheres (NHCSs) and (c and d) graphene-wrapped N-doped hollow carbon spheres (NGC-1100) prepared at 1100 °C. In panel c, red arrows indicate the macropores formed by the removal of carbon spheres, and yellow ones show the graphene-wrapped carbon spheres. In panel d, the yellow dashed circle shows a single carbon sphere covered by wrinkled graphene sheets, and the red dashed circles indicate the macropores.

The structure of the graphene-wrapped hollow carbon spheres could be further confirmed by TEM measurements. As presented in Fig. 3a, some rings, highlighted by yellow dashed circles, are clearly observed, indicating the unique hollow structures of the carbons spheres, well matching with the previous results. Moreover, the wrinkled graphene sheets can also be observed and well dispersed with these hollow-structured carbon spheres. As shown in Fig. 3b, graphene sheets well adhere onto the surface of a single hollow carbon sphere, which is well consistent with the observation via SEM. The uniform wrapping by graphene nanosheets could form the sheet-like conductive pathway on the surface of the carbon spheres, thus promoting fast electron transport to the active sites to efficiently improve the electrocatalytic activity and reaction kinetics.^17a,26

Fig. 3 (a) TEM image of NGC-1100 (red arrows indicate wrinkled graphene sheets and yellow dashed circles show some NHCSs embedded into graphene sheets). (b) TEM image of a single NHCS wrapped by wrinkled graphene sheets.

The N₂ adsorption–desorption isotherms, and pore size distribution curves of various samples are shown in Fig. 4a, c and d. And the textural properties are also shown in Table S1 (ESI†) and Fig. 4b. All curves showed a typical IV isotherm, indicating the dominant mesoporous characteristics of these samples.^18a,27 It is evident that NG has the smallest capacity for N₂ adsorption, while the NHCSs show the largest N₂ adsorption capacity. NG only has a low surface area of 119 m² g⁻¹, much smaller than 939 m² g⁻¹ for the NHCSs (Fig. 4b). The low surface area of NG could be ascribed to the re-stacking of graphene nanosheets after thermal treatment in ammonia.²⁸ Compositing graphene with NHCSs resulted in a little decrease in the surface area of the as-formed NGCs, but the resulting NGC-800 still has a large surface area of 819 m² g⁻¹. An increase in the pyrolysis temperature could gradually decrease the surface area to 722 m² g⁻¹ for NGC-950 and 618 m² g⁻¹ for NGC-1100, possibly due to the collapse of some micropores and narrow mesopores under high-temperature treatment.²⁹ The pore volume values for NGCs show the same changing trend as the specific surface area for all samples. The pore size distribution curves were calculated from the adsorption branches of the isotherms. As shown in Fig. 4c and d, NHCSs and NGCs show the existence of mesopores smaller than 10 nm. The wide mesopores at 40.9 and 42.9 nm are also found due to the aggregating pores of carbon spheres and graphene sheets.³⁰ So the packing of carbon spheres with graphene sheets led to the formation of unique hierarchical structures with high surface area and large pore volume as well as hierarchical pores, which is advantageous for fast mass transport of electrolyte and reactants to the active sites.³¹

Fig. 4 (a) N₂ sorption isotherms and (b) a diagram of the specific surface areas and pore volumes for NGCs, NHCSs, and NG. (c and d) Pore size distributions of NGCs, NHCSs and NG calculated from the adsorption branches.

The detailed elemental composition for the different samples was analyzed via X-ray photoelectron spectroscopy (XPS). As shown in Fig. 5a, three peaks at ∼531, 400, and 284 eV are found ascribed to the O 1s, N 1s, and C 1s peaks,³² respectively, implying the successful N-doping into the carbon framework. The N contents of NG and NHCSs are 2.65 and 3.88 at%, respectively (Fig. 5b and Table S2, ESI†). Increasing the treatment temperature from 800 to 1100 °C evidently led to a decrease in the nitrogen content from 7.96 at% for NGC-800 to 3.44 at% for NGC-1100. The higher temperature could cause the loss of doped heteroatoms from the carbon matrix.³³ All NGCs showed similar oxygen contents (3.30–3.66 at%) regardless of treatment temperature. The N/O ratio decreased from 2.41 for NGC-800 to 0.94 for NGC-1100. It is notable that the N/O ratio for NGC-1100 is smaller than 2.19 for NHCSs prepared at the same temperature, implying the prompting effect of the wrapped graphene conductive layer on the removal of doped oxygen atoms. The doped N atoms within the NGCs could modify the electronic structures and chemical activities due to the different electronegativity of the nitrogen and carbon atoms, advantageous for the activity improvement.⁸

Fig. 5 (a) XPS survey spectra, (b) N and O atom percentage (%), and N/O ratio of NGCs, NHCSs, and NG. (c) High-resolution N 1s XPS spectra of NGC-1100, and (d) the diagrams for the relative atomic percentage (%) of pyridine-N, pyrrolic-N, graphitic-N, and pyridine-N-O for NGCs, NHCSs, and NG.

The detailed chemical environments for the N-functional groups are analyzed by high-resolution N 1s XPS measurements. As shown in Fig. 5c and Fig. S2 (ESI†), the N 1s spectra can be divided into four peaks, corresponding to pyridinic N (∼398.4 eV), pyrrolic N (∼400.2 eV), graphitic N (∼401.1 eV) and oxidized N (∼402.2 eV).^{10b,22a,23a,34} The relative atomic percentages of these four groups are presented in Fig. 5d and Table S3 (ESI†). The thermal treatment has a great effect on the N species. By increasing the treatment temperature, the relative contents of both the pyridinic and pyrrolic N groups evidently decrease, while the graphitic N groups show an escalating trend. This complies with the previous reports that high-temperature treatment could transform pyridinic and pyrrolic N groups into graphitic N species.³⁵ Both pyridinic and graphitic N groups play a very important role in improving the electrocatalytic activity toward the ORR.^8,22b The former could lower the reaction barriers, while the latter could promote the fast electron transport and thus increase the limiting current density.

The rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) voltammograms were acquired in a conventional three-electrode system to investigate the electrocatalytic ORR activity of the NGCs and control samples.^6d,35 As displayed in Fig. 6a, there is an evident oxygen reduction peak in the CV curve of the GC working electrode coated with NGC-1100 in O₂-saturated 0.1 M KOH electrolyte. In sharp contrast, it completely disappeared when O₂ was replaced by N₂ to saturate the KOH solution. These results imply that O₂ could be efficiently reduced on the surface of the working electrode. The CV curves of all samples collected at a sweep rate of 50 mV s⁻¹ in O₂-saturated KOH solution are demonstrated in Fig. 6b. It is clearly observed that the NGCs show larger geometric current densities than the N-doped graphene (NG) and NHCSs, implying a higher catalytic activity of NGCs as compared to NG and NHCSs. Moreover, in contrast to those of NG and NHCSs, the CV curves of NGCs exhibit nearly rectangular shapes, showing their high conductivity with a superior capacitive current. It also means a good synergy effect of graphene and the hollow carbon spheres in NGCs on improving the conductivity and further promoting fast electron transfer to the active sites. Among these NGCs prepared at different temperatures, NGC-1100 shows the largest ORR current density at ∼0.29 V (vs. SCE), implying the best ORR activity of NGC-1100.^35,36

Fig. 6 (a) Cyclic voltammograms (CVs) of NGC-1100 in an O₂ or N₂-saturated 0.1 M KOH solution at a scan rate of 50 mV s⁻¹, (b) CVs of NGCs, NG, and NHCSs in an O₂-saturated 0.1 M KOH solution at a scan rate of 10 mV s⁻¹, (c) LSV curves of NGCs, NG, NHCSs, and commercial Pt/C in an O₂-saturated 0.1 M KOH solution at 1600 rpm (scan rate: 10 mV s⁻¹), (d) the comparison of onset potential and limiting current density (@−1.0 V vs. SCE) for various samples, (e) LSV curves for NGC-1100 collected at different rotation speeds of 400–2025 rpm, and (f) Koutecky–Levich (K–L) plots and the calculated electron-transfer number (n) of NGC-1100 at different potentials (vs. SCE). The lines are drawn by linearly fitting the corresponding dots.

The ORR activities for all samples were further studied by the linear sweep voltammetry measurements at 1600 rpm and 10 mV s⁻¹ in O₂-saturated 0.1 M KOH solution. As shown in Fig. 6c, the NG and NHCSs have moderate ORR activity with a similar onset potential of −0.17 to −0.18 V (vs. SCE) and the limiting current density of 3.3–3.6 mA cm⁻² (Fig. 6d). NG showed the lowest ORR activity possibly because its poor surface area restrained mass transfer to the active centers. It is unexpected that wrapping NHCSs by graphene sheets significantly improved the ORR activity in terms of onset potential, half-wave potential and current density. Among the three NGCs, NGC-1100 showed the best ORR activity reflected from its most positive onset potential (−0.12 V) and half-wave potential (−0.23 V), and largest limiting current density (5.7 mA cm⁻²) at −1.0 V (vs. SCE), although it is still inferior to those of commercial Pt/C (Fig. 6c). These results imply that graphene wrapping evidently improves the ORR activity of the NGCs. The outer wrapped graphene sheet layers could form conductive networks, accordingly lowering the barrier of electron transport to active sites. Moreover, the excellent ORR activity of NGC-1100 also results from the unique hierarchical structures with hollow cores, favorably promoting the fast diffusion of reactants, ions and electrons for the ORR. Notably, the lowest N-doping level of NGC-1100 among the NGCs obtained the best ORR activity among the NGCs, showing that the total N content is not directly determinative of high ORR activity.³⁷ This result also matches well with the previous studies. And the type of N-functional species is also pivotal to the electrocatalytic ORR activity. To study the detailed ORR process on the working electrode, the Koutechy–Levich (K–L) equation was used to calculate the electron transfer number (n) during the ORR process. With increasing rotation speed from 400 to 2025 rpm, the current densities increase which is ascribed to the improved mass transport on the electrode surface (Fig. 6e and Fig. S3, ESI†). The linearity of the K–L plots and near parallelism of the fitting lines imply first-order reaction kinetics toward the concentration of dissolved O₂ and approximate n values for the ORR in the potential range of −0.6 to −0.9 V (vs. SCE) (Fig. 6f). The n values for NGC-1100 ranging from 3.85 to 3.97 are very close to those for commercial Pt/C as the ORR electrocatalysts in alkaline solution, demonstrating a high ORR activity with a stable 4e⁻ transfer process for NGC-1100.

To further investigate the effects of graphene wrapping on the reaction mechanism and catalytic activity, the LSV curves of various samples including commercial Pt/C were collected via a RRDE technique in O₂-saturated 0.1 M KOH solution at 1600 rpm (ring potential: 0.2 V vs. SCE). And the polarization curves for the disk and ring electrodes are shown in Fig. 7a and b, respectively. The changing trends for the disk current from the RRDE results are similar to those from the RDE measurements. And NGC-1100 shows the best ORR activity with the most positive half-wave potential. The gap of the high-wave potential for Pt/C and NGC-1100 is only 72 mV, evidently superior to some of the previously-reported carbon-based ORR electrocatalysts including N, B-doped graphene,³⁷ nitrogen-doped hollow carbon hemispheres,³⁶ nitrogen-doped reduced-graphene oxide,³⁸etc. More comparison results are demonstrated in Table S4 (ESI†), showing the superior ORR activity of NGC-1000. The larger current density for the ring electrode suggests the formation of more amount of peroxide (HO₂⁻). It is evident that NGCs show much lower ring electrode current density than NG and NHCSs (Fig. 7b), implying that the binding of graphene and carbon spheres could suppress the HO₂⁻ formation. Indeed, the HO₂⁻ yields at −0.6 V (SCE) for NGC-800, NGC-950 and NGC-1100 are 11.4%, 16.7% and 9.5%, respectively, much smaller than 49.9% for NG and 34.0% for NHCSs without graphene wrapping. The outer graphene sheets could act as a highway for electron transfer to active sites, and thus are beneficial to impeding the formation of HO₂⁻. The measured yields of NGC-1100 are 6.9–12.1% in a wide potential range of −0.3 to −0.8 V (vs. SCE), very close to 4.8–8.9% for commercial Pt/C, suggesting that both NGC-1100 and Pt/C could catalyze an ORR process with less amount of HO₂⁻ as a side product. Accordingly, the calculated n values (3.76–3.86) for NGC-1100 are evidently larger than 3.30–3.52 for NHCSs and 2.96–3.26 for NG (Fig. 7d). And the n values for NGC-1100 are very similar to 3.81–3.90 for Pt/C, demonstrating a desirable four-electron transfer process for the ORR to obtain the maximum energy capacity. The four electron pathway is the most efficient pathway because oxygen could be directly reduced to OH⁻ without the intermediate product (HO₂⁻).^2b,39

Fig. 7 (a) Disk current and (b) ring current of the rotating ring-disk electrode (RRDE) voltammograms for NGCs, NHCSs, NG and commercial Pt/C (rotation speed, 1600 rpm; scan rate, 10 mV s⁻¹; ring potential, 0.2 V (vs. SCE); O₂-saturated 0.1 M KOH), and (c) the calculated peroxide (HO₂⁻) yields and (d) n values for NGCs, NHCSs, NG and commercial Pt/C in the potential range of −0.2 to −0.8 V (vs. SCE).

The long-term durability is one of the key challenges for the ORR electrocatalysts in fuel-cell technology.^39a The stability of NGC-1100 as well as commercial Pt/C was investigated via chronoamperometric responses at −0.6 V (vs. SCE) and 1600 rpm in O₂-saturated 0.1 M KOH solution for 5.5 h. As shown in Fig. 8a, 94.7% of the original current density was maintained for the NGC-1100 electrode, whereas the Pt/C electrode only retained 74.2% of the original one under the same testing conditions. This sharp comparison demonstrated the superior stability of NGC-1100 as the ORR electrocatalyst. NGC-1100 and Pt/C were further studied by measuring the methanol crossover via the chronoamperometric measurements in O₂-saturated 0.1 M KOH solution at 1600 rpm (Fig. 8d). When methanol was injected into the electrolyte at 200 s, there is a dramatic loss of the cathodic current for Pt/C and only 60.4% of the initial current density was retained at 1000 s. This implies a poor resistance to methanol crossover for commercial Pt/C. In sharp contrast, after injecting methanol into the cell, the NGC-1100 electrode could retain 83.6% of the cathodic current at 1000 s. Thus, NGC-1100 demonstrates an outstanding tolerance to the crossover effects compared to commercial Pt/C. These results show that metal-free NGC-1100 is a good electrocatalyst for the ORR with high stability.

Fig. 8 (a) Current–time (i–t) chronoamperometric response of NGC-1100 and commercial Pt/C on the RDE at −0.6 V (vs. SCE) in O₂-saturated 0.1 M KOH solution at 1600 rpm, and (b) the chronoamperometric responses of NGC-1100 and Pt/C at −0.6 V (vs. SCE) on the RDE at 1600 rpm in O₂-saturated 0.1 M KOH before (0–200 s) and after (200–1000 s) adding 3 M methanol.

Conclusions

In conclusion, graphene-wrapped N-doped hollow carbon spheres (NGCs) have been successfully prepared by the self-assembly of graphene oxide nanosheets on the surface of hollow polymer spheres (HPSs), followed by pyrolysis in flowing ammonia. The electron microscopy measurements confirmed the uniform wrapping of graphene sheets on N-doped hollow carbon spheres (NHCSs). Moreover, the carbon spheres could also impede the aggregation of the graphene sheets during the self-assembly and following high-temperature pyrolysis. The resulting nanocomposites show much better electrocatalytic performance for the ORR than NHCSs without graphene wrapping. The optimized NGC-1100 shows a larger limiting current density of 5.7 mA cm⁻² than N-doped graphene (NG) (3.3 mA cm⁻²) and pure NGCSs (3.6 mA cm⁻²), implying its superior ORR activity derived from the synergy of the graphene and carbon spheres. Moreover, the RRDE measurements indicate that the n values (3.85–3.97) of NGC-1100 are very close to 3.81–3.90 for commercial Pt/C, and are much higher than 2.96–3.26 for NG and 3.30–3.52 for NHCSs. In addition to the advantages of a unique hierarchical structure, large surface area, large hollow core, and high relative content of pyridinic and graphitic N doping in the NGCs, the wrapping graphene layer can play an important role as a conductive network to promote a fast electron transfer and thus improve the ORR activity and selectivity. Moreover, NGC-1100 demonstrates a very high stability during the chronoamperometric experiment compared to commercial Pt/C. The present strategy could be further extended to prepare other heteroatom-doped graphene-wrapped carbon materials for the ORR. Such a unique graphene-wrapped nanocomposite also has potential applications in supercapacitors, batteries, gas storage, etc.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful for the financial support from the National Key Research and Development Program of China (2016YFB0700204), NSFC (51602332 and 51502327), Science and Technology Commission of Shanghai Municipality (15520720400, 15YF1413800 and 16DZ2260603), and One Hundred Talent Plan of Chinese Academy of Sciences. M. Yang would like to thank the National Thousand Youth Talents program of China and Ningbo 3315 program.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S3 and Tables S1–S3 mentioned in the main text. See DOI: 10.1039/c8qm00160j

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