China rose-derived tri-heteroatom co-doped porous carbon as an efficient electrocatalysts for oxygen reduction reaction

Abstract

Developing Pt replacement catalysts for the cathode oxygen reduction reaction (ORR) is crucial for the application of energy conversion technology. Here, the N, O and S tri-heteroatom-doped, hierarchically porous carbon material was obtained by pyrolysis of natural China rose petals as a precursor, followed by chemical activation and post-treatment with melamine. The results demonstrated that the micro/nano-structures of rose petals provide a hard template to form hierarchically porous structures, favorable for the mass transportation and electrolyte, while the saccharides in the rose petals were used as precursors for carbon. Moreover, the amino acids of rose petals and the additive melamine acted as the source of heteroatoms, thus creating enormous active sites for ORR. The as-prepared catalyst therefore displays an excellent electrocatalytic activity similar to the performance of Pt/C in both alkaline and acid media, long term durability and better tolerance for methanol crossover/CO poisoning effects than commercial Pt/C, making it a promising alternative for platinum and other expensive metal-based catalysts toward ORR.

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Introduction

With rapid progress in the global economy and industry, the impending energy crisis has stimulated intense research on energy conversion and storage technology. Due to their high energy efficiency and low pollutant emission, fuel cells are considered to be a promising technology for clean and efficient power generation.¹ However, the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode is a crucial bottleneck in the development of fuel cells.^2–6 So far, precious platinum and its alloys have been considered to be the most reliable ORR electrocatalysts because of their relatively high activity. In addition to the high cost, however, they suffer from the poor durability, methanol crossover/CO poisoning effect.⁷ Tremendous attention has, therefore, been paid to developing non-noble metal,⁸ or even metal-free electrocatalysts for ORR,^13,15 including titanium dioxide supported electrocatalysts,¹⁶ iron-based catalyst,¹⁷ graphene-based carbon nitride,¹⁸ and heteroatom-doped carbon nanomaterials.^19–23 In particular, heteroatom-doped carbon nanomaterials with hierarchically porous structures have been extensively studied as efficient ORR electrocatalysts to replace Pt-based catalysts, because carbon is more abundant and durable, as well as less expensive than Pt.⁹ The heteroatoms doping into carbon frameworks, including nitrogen (N), phosphor (P), sulphur (S) and boron (B) have been certified to improve their electrical conductivity, electrocatalytic activities and long-term stability.¹³ For example, L. Dai et al. reported that N-doped carbon nanotube arrays had surprisingly high electrocatalytic activity for oxygen reduction, which was almost comparable with that of the commercial Pt/C.¹⁰ After 10 h of operation, our group reported that S, trace N and Fe co-doped carbon foams had excellent catalytic durability for ORR, whose catalytic current density only decreased by 4% with the initial value, but Pt/C catalysts had lost 16% of its original current density.³² Up to now, the synthesis of heteroatom-doped carbon nanomaterials has been achieved by a variety of methods, including post treatment with ammonia,²⁵ amines or urea,²⁶ or direct pyrolysis using acetonitrile,²⁷ polypyrrole,²⁸ or polyaniline²⁹ as precursor. Although tremendous effort has been made, it is still desirable to exploit low-cost precursors for the large-scale fabrication of heteroatom-doped carbon nanomaterials as promising electrocatalysts for ORR.

Flowers are a constant delight to both the nose and the eyes of the visitors, regardless of seasonal changes. It is highly desirable to know if flowers themselves can act as precursor for the ORR electrocatalysts. The recent report presented that plant Typha orientalis was used as a precursor to produce nitrogen-doped nanoporous carbon aerogel by a hydrothermal process and post-treatment in ammonia, which showed a promising catalytic activity for ORR.^24,30 Moreover, it was also found that heteroatom-doped carbon materials can be fabricated from a wide variety of biomass, such as microorganism,⁴¹ seaweed,⁴⁴ cellulose,⁴⁵ and so on.^42,43 All these results demonstrated that biomass can be considered as the best option for providing low-cost, efficient, durable electrocatalysts for ORR on the larger scale.

Herein, we presented an effective ORR electrocatalysts, heteroatoms doped-carbon materials derived from China rose petals by combining chemical activation and pyrolysis, which were denoted as RPC-X (RP and C represent rose petal and carbon, respectively, while X represents process procedure). The reasons for choosing China rose petals as precursor are as follows: (1) China rose is perennial evergreen shrub that can grow anywhere and bloom almost year-round, and is also cheaper and more available than other flowers, thus providing abundant and affordable source for ORR catalyst. (2) The China rose petal was used as the template for the petal-like carbon nanomaterials due to its hierarchical porous structures. (3) China rose also contains large quantities of inorganic salt components containing potassium and magnesium salts, etc., which will form numerous pore structures after treatment with hydrochloric acid.^13–16 (4) The saccharides in the petals, for instance, cellulose from cytoderm, serves as suitable precursor for carbon, while the amino acids of rose petal act as the source of heteroatoms. As a result, the heteroatoms doped-carbon materials displayed unique hierarchical porous structure, thus facilitating the transport of oxygen and electrolyte. Moreover, N, O and S elements were doped into carbon framework, thus providing enormous catalytic sites for ORR. As expected, a high Brunauer–Emmett–Teller (BET) surface area of 1478.6 m² g⁻¹ was obtained for RPC-M. Thus, RPC-M demonstrated excellent ORR activity in both alkaline and acid media, it displayed a relatively positive onset potential, half-wave potential and high current density of 0.07 V, −0.15 V and −4.58 mA cm⁻² in alkaline medium, and 0.87 V, 0.30 V and −4.81 mA cm⁻² in acid medium, respectively. In addition, RPC-M showed a long term durability after 5.5 h with the retained 3.2% of its initial current density, and better tolerance against both CO and methanol than that of Pt/C catalyst in alkaline solution. Taken together, the strategy presented here may provide a simple way for cost-effective production of ORR electrocatalysts on a large scale for practical applications, which could be a promising substitute for commercial Pt/C catalyst.

Experimental section

Preparation of the catalysts

China roses were purchased from Beijing Dasenlin Flower Market. Fresh rose petals were washed with distilled water to remove dirt and freeze-dried under reduced pressure for 48 hours to completely remove water in the rose petal. 1.8 g of the freeze-dried petals were directly placed in tube furnace and heated from room temperature to 200 °C at a rate of 5 °C min⁻¹ and then kept at this temperature for 3 h under nitrogen atmosphere. After cooling down to room temperature, the pre-carbonized petal showed black grey in color. The pre-carbonized petal was then immersed in 20 mL of KOH aqueous solution (5 mol L⁻¹) and kept in a 50 °C water bath for 3.5 h for chemical activation. After activation, the products were washed with ethanol several times and dried in oven at 80 °C for 120 min. The as-activated product was heated from room temperature to 900 °C at a rate of 5 °C min⁻¹ and kept for 1 h under nitrogen atmosphere. The product was treated with 0.5 mol L⁻¹ hydrochloric acid for 15 min to remove inorganic salts, and then rinsed with ultrapure water for several times until reaching a neutral pH, and finally dried in oven at 80 °C for 120 min. Then it was grinded into fine powder and mixed with melamine with a mass ratio of 1 : 2. The mixture was heated from room temperature to 900 °C at a rate of 5 °C min⁻¹ and kept for another 1 h under nitrogen atmosphere, which was donated as RPC-M (M for post-treatment of melamine). The schematic diagram for the formation of porous carbon was shown in Fig. 1. For comparison, RPC-A was fabricated by the same procedure as above, except for the melamine treatment (A means chemical activation). RPC-D was also synthesized by directly carbonizing the freeze-dried rose petal at 900 °C for 3 h (D represents direct pyrolysis). Both RPC-A and RPC-D should be treated with 0.5 mol L⁻¹ hydrochloric acid to remove inorganic salts and rinsed with ultrapure water.

Fig. 1 Schematic diagram for the formation of porous carbon derived from the natural China rose. Images of the natural rose (a), dried rose petal (b), as-activated product (c), RPC-A (d) and RPC-M (e).

Characterizations

The morphologies of the freeze-dried rose petal, RPC-M, RPC-A and RPC-D were observed by field emission scanning electron microscopy (FESEM, JEOL7500). Molecular structures of all samples were characterized by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. XPS spectra were collected on the Thermo Scientific ESCALab 250Xi using 200 W monochromated Al Kα radiation. Raman spectra were recorded on a JobinYvon (Laboratory RAM HR1800) confocal micro-Raman spectrometer backscattered geometry through a 10× (NA = 0.25) microscope objective. An Ar⁺ laser emitting at a wavelength of 514.5 nm was used as a source of excitation. Thermo-gravimetric analysis (TGA) analysis of dried rose petals was carried out on Thermo-LABSYS evo TGA sensor in nitrogen atmosphere. Nitrogen absorption/desorption isotherm was performed with ASAP 2020M (Micromeritics, USA) to obtain Brunauer–Emmett–Teller (BET) specific surface area. The pore size distribution was obtained from the adsorption branch of the isotherms derived from the Barrett–Joyner–Halenda (BJH) analysis. TZY-XRD (D/MAX-TTRIII, Cu Kα) was used to get the X-ray diffraction (XRD) pattern.

Electrochemical measurements

The electrochemical characterization for ORR was conducted in a three-electrode glass cell (Pine Research Instrumentation, USA) using a bipotentiostat (CHI 760D, Shanghai). The catalyst-coated a glass carbon (GC) disk with 5 mm in diameter was employed as the working electrode, a platinum wire as counter electrode reference and an Ag/AgCl electrode filled with saturated KCl aqueous solution as reference electrode. 0.1 M KOH aqueous solution and 0.1 M HClO₄ were the electrolytes, which were saturated with pure nitrogen or oxygen gas for 30 min prior to the electrochemical test. To prepare the catalyst-loaded working electrode, 2 mg catalyst powder was ultrasonically dispersed in absolute ethanol for 3 min to obtain a homogeneous catalytic ink (2 mg mL⁻¹). Then, 15 μL of the catalytic ink was pipetted onto the surface of GC electrode by using a micropipettor, followed by dropping 7.5 μL of Nafion solution in isopropanol (0.05 wt%). The catalyst loading was 0.15 mg cm⁻¹. For comparison, Pt/C (20 wt% Pt) on GC with the same concentration of the catalyst was prepared in the same way. All the electrodes were prepared by depositing the same loading mass of active materials on GC electrode using the same method. The ORR activity was evaluated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) techniques on a rotating disk electrode (RDE) in O₂-saturated 0.1 M KOH or 0.1 M HClO₄ aqueous solutions. The tolerance to the methanol crossover and CO poisoning effect, and durability tests were carried out by a chronoamperometric technique at the bias potential of −0.3 V (vs. Ag/AgCl) in O₂-saturated 0.1 M KOH solutions with a rotation rate of 900 rpm.

Results and discussion

The microstructures of the freeze-dried rose petal and RPC-Xs were characterized by field emission scanning electron microscope (FE-SEM). Fig. 2a showed the SEM image of the fresh petal. It can be seen that the surface of petal displayed many bud-like protrusions of 16.8 μm in average length. The magnified SEM image (Fig. 2b) showed that the protrusions contained a large number of irregular folds with a diameter of about 600 nm, and the surface of the folds on the protrusions was relatively smooth (Fig. 2c). Fig. 2d showed the cross section SEM image of the petal, from which the micrometer-sized pore structures, approximately 15.3 μm in diameter, were observed. Fig. 3 presented the SEM images of the as-prepared RPC-M. As seen from the Fig. 3a and d, after activation and carbonization, RPC-M still maintained the hierarchical micro/nanostructures of rose petal, but the diameter of protrusions and porous structure decreased to about 13.8 and 9.5 μm, respectively, due to thermal decomposition of organic composites. From the enlarged SEM image in Fig. 3b and c, it can be seen that the surface protrusions have macropores ranging from 100 to 500 nm in diameter as well as myriad mesopores ranging from 3 to 20 nm in diameter, resulting from the decomposition of organic residues by reaction with KOH during the activation process.¹¹ Moreover, the element mapping images of RPC-M (Fig. 3e–h) showed the uniform distribution of carbon, oxygen, nitrogen and sulfur on RPC-M, which derived from the nitrogen and sulfur-rich organic compounds of rose petal or melamine. Thermo-gravimetric analysis (TGA) curve of dried rose petals was presented in Fig. S1a (see ESI†). It can be seen that the first step loss of dried rose petal occurs in the temperature range from 150 to 450 °C, which is due to release of either water or small organic molecules. The weight loss of the rose petal occurs in the temperature range from 450 to 900 °C with a mass loss of 85%, which could be associated to the transformation organic materials into carbon framework.⁴⁵ Therefore, the decomposition and release of organic components may result in the formation of numerous pores on the residue carbon framework. Nitrogen adsorption–desorption isotherms were employed to further study the porous nature of the RPCs, as shown in Fig. S1b.† The BET surface areas were calculated to be 1174, 1523 and 1478 m² g⁻¹ for RPC-D, RPC-A and RPC-M, respectively, while the dried rose petal showed a negligible surface area of less than 10 m² g⁻¹. The high surface area of RPC-D can be ascribed to the porous precursor and high temperature carbonization, and the even higher surface area of RPC-A and RPC-M should be the chemical activation with KOH, in accordance with the FE-SEM result. Pore size distribution curves of the RPCs, calculated from the adsorption branches of the nitrogen isotherms by the Barrett–Joyner–Halenda (BJH) model, was shown in Fig. S1c.† The RPC-D had mainly pore size distribution with around 11 nm. Apart from this pore size, RPC-A and RPC-M also exhibited a new wide pore size distribution ranging from 18 to 30 nm that may be attributed to KOH activation. The high surface area and the presence of nano-pores observed in RPCs are anticipated to provide more surface active sites, thus leading to enhanced electrocatalytic activity toward ORR. The high surface area and the uniform pore size distribution of RPCs are anticipated to provide more surface active sites, thus leading to enhanced electrocatalytic activity toward ORR.

Fig. 2 FE-SEM images of the freeze-dried rose petal surface (a–c) and lateral structure (d).

Fig. 3 FE-SEM images of RPC-M surface (a–c) and lateral structure (d); (e–h): element mapping for Fig. 2c of carbon, oxygen, nitrogen and sulfur.

The chemical compositions of RPC-D, RPC-A and RPC-M were investigated by Raman spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The Raman spectra (Fig. 4a) of RPC-D, RPC-A and RPC-M showed two feature peaks of graphite: the G peak centered at 1580 cm⁻¹ is due to the bond stretching of all pairs of sp² atoms in both rings and chains, whereas the D peak centered at 1360 cm⁻¹ is attributable to the breathing modes of sp³ atoms in rings.³¹ For RPC-M, however, its D peak was shifted to 1325 cm⁻¹ that could be ascribed to the introduction of more heteroatoms into carbon frameworks.^46,51 Moreover, the integrated intensity ratio of the D and G band (I_D/I_G) is widely employed to assess defects of the graphite materials, where a higher ratio suggests more defects on these materials.⁹ The I_D/I_G values were calculated to be 1.19 for RPC-D, 1.20 for RPC-A and 1.24 for RPC-M, respectively, indicating the fact that RPC-M has a high defect density as a result of high heteroatom-doped content. The presence of defect sites was further verified by XRD analysis. As shown in Fig. S2,† the XRD patterns of RPC-D, RPC-A and RPC-M displayed a broad peak at around 26.5° (002), which could be attributed to the graphitic framework.³⁷ It is noticed that, compared to RPC-A and RPC-D, the peak of RPC-M had a negative shift to 25.8°, which could be a result of carbon material doped with N and O atoms that can introduce more defect in the graphitic structure due to the difference of atomic radius between nitrogen, oxygen and carbon.⁵⁰ Thus, the result of XRD pattern was consistent with that of the Ramen spectra. XPS survey spectra of RPC-D, RPC-A and RPC-M revealed the presence of C, O, N, and S elements, which also verified the N, O and S elements were doped into carbonaceous frameworks via carbonization or post-treatment of melamine, as shown in Fig. 4b. After acid treatment, moreover, metal species were not detected in samples due to the removal of inorganic components from samples,^13–16 which may be responsible for mesopores channel formation. Quantitative analysis of the XPS survey spectrum elucidated the corresponding atom contents of RPC-M, including 88.64% for C, 7.40% for O, 3.80% for N and 0.06% for S (Table S1†), in which the relatively higher nitrogen content of RPC-M than that of RPC-D and RPC-A was ascribed to the post-treatment of melamine. Compared to the fresh rose petal with a relative content for N atoms of 2.2%, however, those of RPC-D and RPC-A reduced gradually to 1.19% and 0.49%, due to the N and S-containing volatile fraction removed during high temperature pyrolysis and KOH activation. As presented in Fig. 4c, the high resolution C1s XPS spectrum in RPC-M showed a dominant peak at 284.8 eV and two weak peaks at 286.4 and 289.6 eV, corresponding to the sp²-hybridized graphitic C,^37,52 C–O configuration³³ and C–N configurations,³⁴ respectively. The high-resolution N1s XPS spectrum of RPC-M shown in Fig. 4d can be deconvoluted into three typical peaks with binding energies of 398.5, 400.6 and 402.7 eV, which corresponded to pyridinic N, graphitic N, and oxidized nitrogen, respectively. As shown in Fig. S3c and d,† however, the high resolution N1s spectra of RPC-D and RPC-A revealed the presence of pyridinic N, pyrrolic N and graphitic N species. It should be noted that the configuration of doping nitrogen could be adjusted by varying the treatment methods.^25–29 As was reported, pyridinic and graphitic N species could effectively improve the onset potential and increase the limiting current density, while pyrrolic and oxidized N species have little effect on the electrochemical performance of N-doped carbon material.^12,34,35 The according contents of different types of nitrogen in RPC-D, RPC-A and RPC-M were shown in Table S2,† from which it was found that RPC-M had the highest content of pyridinic and graphitic N species, which was consistent with literature that treatment with melamine resulted in higher relative contribution of pyridinic nitrogen.⁴⁸ It can be therefore deduced that RPC-M may provide the better ORR electrocatalytic performance than RPC-D and RPC-A. The high-resolution O1s XPS spectrum of RPC-M (Fig. 4e) showed two peaks at 531.6 and 532.9 eV, which can be attributed to carbonyl groups (C=O/O–C=O) and other oxygen-containing groups (–N–O–).⁴⁷ As demonstrated by previous report,⁴⁸ the oxidized nitrogen was associated with oxygen from hydroxyl or carbonyl groups located in the close vicinity, which lead to a positive charge in nitrogen, thus enhancing electron transport. Fig. 4f showed five peaks in the high resolution S2p spectrum of RPC-M, out of which peaks at 168.4, 167.2 and 163.5 eV were assigned to S–S bonds between two aromatic rings,³⁸ while the broad peaks at 175.9 and 171.7 eV were attributed to the sulfonated aromatic groups in the form of –C–SO₂–C–.⁴⁰ The catalytic mechanism of sulfur doping was suggested that the spin density arisen from the mismatch of the outermost orbitals of S and C may participate in the oxygen reduction process and electrons transfer with nitrogen atoms.^39,49 In addition, the mentioned literature sufficiently revealed that electrocatalysts for ORR was introduced by multiple heteroatoms into carbon framework, thus enhancing the chemical stability by the strong strength of covalent heteroatom-carbon bonds.³⁶ Taken together, these results, therefore, made RPC-M a promising candidate as feasible metal-free electrocatalysts for ORR even in harsh environment.

Fig. 4 (a) The Raman and (b) XPS survey spectra of RPC-D, RPC-A and RPC-M. (c–f) The high resolution XPS spectra of C1s, O1s, N1s and S2p for RPC-M.

To evaluate the ORR electrocatalytic activities of the as-prepared catalysts, CV measurements were performed on RPC-D, RPC-A and RPC-M catalysts as well as commercial Pt/C with a scan rate of 50 mV s⁻¹ in N₂ or O₂-saturated 0.1 M KOH solutions. CV curves for all samples in Fig. 5a showed a single cathodic reduction peak ranging from −0.36 V to −0.10 V (vs. Ag/AgCl), which were attributed to the electrocatalytic reduction of oxygen on the electrode. In contrast, the negligible electrochemical response was observed in the N₂-saturated KOH aqueous solution. It can be seen from Fig. 5a that the peak potential at −0.15 V vs. Ag/AgCl electrode afforded by the RPC-M catalyst was comparable to that of commercial Pt/C (−0.12 V), but the peak current (−2.1 mA cm⁻²) of RPC-M was much higher than that of commercial Pt/C (−1.5 mA cm⁻²). The other comparative RPC-D due to increasing the pore structures. Compared to both RPC-D and RPC-A, RPC-M catalyst displayed much higher onset potential of 0.07 V and half-wave potential of −0.15 V, which were even better than those of Pt/C, whose onset and half-wave potential was at 0.03 V and −0.14 V. Moreover, the limiting current density of RPC-M (−4.7 mA cm⁻²) was higher than that of commercial Pt/C (−4.6 mA cm⁻²). Compared to the NCS-800,³⁰ Scup-1000,⁴¹ BZ-800 (ref. 44) derived from the other biomasses reported previously (Fig. S4a†), not only did RPC-M display most positive onset potential, but also had highest current density per loading mass, as listed in Table S3,† thus proving a promising alternative to commercial Pt/C catalyst. Moreover, ORR catalytic performance in acidic medium is also very important for practical applications. ORR measurements were also conducted in O₂-saturated 0.1 M HClO₄, as shown in Fig. 5c, and other conditions remain the same. It is apparent that both RPC-D and RPC-A showed no obvious reduction peak, showing poor ORR activity in acid medium. RPC-M showed a single peak at 0.44 V with, which was only 10 mV inferior to that of commercial Pt/C (0.54 V). As given in Fig. 5d, the onset and half-wave potentials, as well as the limiting current density of RPC-M were 0.87 V, 0.30 V and 4.8 mA cm⁻², respectively, which were much better than those of RPC-D and RPC-A, and slightly inferior to those of commercial Pt/C (0.84 V, 0.41 V and 4.1 mA cm⁻²). The results above demonstrated that RPC-M show excellent ORR activity in both alkaline and acid media. The excellent ORR performance of RPC-M can be attributed to a synergistic effect of porous structures which have abundant micropores and mesopores that are beneficial to oxygen and electrolyte transport, and the co-doped heteroatoms created the active sites for promoting ORR.

Fig. 5 Cyclic voltagrammetry (CV) curves of RPC-D, RPC-A, RPC-M and commercial Pt/C in N₂ (dotted lines) or O₂ (solid lines) saturated 0.1 M KOH solution (a), and 0.1 M HClO₄ solution (c). Linear sweep voltammetry (LSV) curves of RPC-D, RPC-A, RPC-M and commercial Pt/C catalysts on a glassy carbon rotating disk electrode (RDE) in O₂ saturated 0.1 M KOH solution (b) and 0.1 M HClO₄ solution (d) at the sweep rate of 10 mV s⁻¹ with the rotation speed of 1600 rpm.

For further insight into the kinetic processes, LSV measurements on rotational disk electrode were carried out at rotation rates from 400 to 1600 rpm, the results shown in Fig. 6a. Typically, the current density at RPC-M electrode increased obviously with increasing rotating speed, proving the diffusion controlled oxygen reduction. The transferred electron number (n) during ORR on RPC-M was calculated by the Koutecky–Levich (K–L) equations:

where, j_k is kinetic current and ω is rotating rate. B can be determined by the slope of K–L plots based on K–L equation where n, F, D_O2, ν, C_O2 represent the transferred electron number per oxygen molecule, Faraday constant (96 485 C mol⁻¹), diffusion coefficient of O₂ (1.9 × 10⁻⁵ cm² s⁻¹), the kinetic viscosity (1.1 × 10⁻² cm² s⁻¹), and the bulk concentration of O₂ (1.2 × 10⁻⁶ mol cm⁻³) in 0.1 M KOH solutions, respectively. The constant 0.2 that is used to correct the equation is applied when the rotation speed is expressed in rpm. As shown in the set of Fig. 5b, the corresponding K–L plots showed fairly good linearity and near coincidence, indicating first-order reaction kinetics toward the concentration of dissolved oxygen and similar electron transfer number for ORR at different potentials. The transferred electron number (n) for RPC-M was calculated to be 3.6, suggesting that ORR of RPC-M was dominated by an almost four-electron pathway, similar to the commercial Pt/C catalyst. In contrast, the comparative samples showed a relatively lower electron transfer number of 3.5 for RPC-D (Fig. S5b), and 3.4 for RPC-A (Fig. S5d†), indicating poorer electrocatalytic selectivity for these catalysts. Moreover, the rotating ring-disk electrode (RRDE) was carried out to further confirm the ORR reaction pathway and the electron transfer number. The results of RPC-D, RPC-A, RPC-M and commercial Pt/C were shown in Fig. S4(b).† It can be seen that the ring and disk currents of RPC-M were almost comparable to that of commercial Pt/C catalyst. The percentage of HO₂⁻ and the number of transferred electrons (n) can be calculated in the following equations:where, j_D is the disk current, j_R is the ring current and N is current collection efficiency of the Pt ring, which equals 0.24. The results derived from RRDE were presented in Fig. S4c and d.† The percentage of HO₂⁻ formation for RPC-M catalyst was below 20% over a potential range of −0.3 to −0.9 V, which was close to that of commercial Pt/C, whereas RPC-D and RPC-A produced a large amount of peroxide over the same potential range. The electron transfer number was calculated to be 3.6 for RPC-M, 3.2 for RPC-D and 3.4 for RPC-A, which is in good agreement with the results obtained from RDE measurements. Those results clearly illustrated that the RPC-M catalyst can efficiently reduce oxygen via a 4-electron reduction pathway in alkaline medium.

Fig. 6 (a) LSV curves of RPC-M at different rotation speed in the O₂ saturated 0.1 M KOH solution at the sweep rate of 10 mV s⁻¹, (b) the K–L plots of RPC-M catalyst. Inset is the electron transfer number at different electrode potentials.

For the consideration of practical application on fuel cells, the stability and resistance to methanol crossover and CO poisoning effects are also important indexes to measure the performance of the catalysts. At a constant potential of −0.30 V and in O₂-saturated 0.1 M KOH solution, the durability of RPC-M and Pt/C were tested, as given in Fig. 7a. After 5.5 hours of continuous operation, about 3.2% loss of current density occurred at the RPC-M electrode, whereas the corresponding current loss at the commercial Pt/C electrode under the same condition was as high as 16%. As previously mentioned, the strong strength of covalent heteroatom-carbon bonds could improve the chemical stability of the doped carbon materials,³⁶ the superior stability of RPC-M was therefore mainly due to the co-doped N, O and S atoms. The resistances to methanol crossover and CO poisoning of RPC-M and Pt/C through a chronoamperometry technique were shown in Fig. 7b and c. When 3 M methanol was added, almost no response was observed for RPC-M at 2100 s, but the catalytic activity of commercial Pt/C dropped severely. Moreover, RPC-M also showed a good tolerance to CO poisoning effect with about 20% current decrease upon aerating CO, compared to about 83% decrease for commercial Pt/C catalyst under the same condition. Taken together, RPC-M catalysts showed excellent stability and tolerance to CO poisoning and methanol crossover effects, which may make it a promising candidate for ORR in the direct methanol fuel cells.

Fig. 7 (a) Durability evaluation of RPC-M and Pt/C for 5.5 hours in O₂-saturated 0.1 M KOH solutions at −0.3 V with a rotation rate of 900 rpm. (b and c) Chronoamperometric responses of Pt/C and RPC-M in O₂-saturated 0.1 M KOH solution at −0.3 V. The arrows indicate the introduction of 3 M methanol or CO.

Conclusions

In summary, N, S and O co-doped hierarchically porous carbon materials (RPC-M) were obtained through pyrolysis of China rose petal, followed by KOH activation and post-treatment with melamine. The as-prepared RPC-M catalyst showed hierarchically porous structures that were composed of microporous structure derived from fresh rose petal and nanoporous from high temperature pyrolysis and KOH activation, thus leading to high specific areas. As expected, the RPC-M catalyst displayed the improved ORR electrocatalytic performance that showed positive onset and peak potential, and higher current density in both alkaline and acid media, which were similar to or even better than those of commercial Pt/C catalyst. We proposed that the hierarchically porous structures provided abundant channels for efficient oxygen transfer, and multi-heteroatom doping created more catalyst sites, thereby resulting in the enhanced synergistically ORR performance. Moreover, RPC-M showed better tolerance against CO and methanol, as well as longer term stability than that of commercial Pt/C. The outstanding electrocatalytic performance, easy fabrication, and low-cost therefore make it a very promising non-precious metal catalyst substitute to commercial Pt/C catalyst in fuel cell systems.

Acknowledgements

The authors thank the financial support by the National Natural Science Foundation of China (51273008, 51473008), the National Basic Research Program (2012CB933200) and 863 Program (2012AA030305). The authors also thank the financial support of the 26th National College Students Innovation and Entrepreneurship Training Program (201510006151).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14619h

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