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Nitrogen-doped hollow carbon spheres as highly effective multifunctional electrocatalysts for fuel cells, Zn–air batteries, and water-splitting electrolyzers
Journal of Power Sources 441 (2019) 227166
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Nitrogen-doped hollow carbon spheres as highly effective multifunctional electrocatalysts for fuel cells, Zn–air batteries, and water-splitting electrolyzers Jinhui Tong a, *, Wenmei Ma a, Lili Bo b, Tao Li a, Wenyan Li a, Yuliang Li a, Qi Zhang a a
Key Laboratory of Eco-environmental Polymer Materials of Gansu Province, Key Laboratory of Eco-functional Polymer Materials of Ministry of Education, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, 730070, China College of Science, Gansu Agricultural University, Lanzhou, 730070, China
b
H I G H L I G H T S
� An ionic liquid is used to prepare N-doped hollow carbon spheres (NHCS–W). � NHCS–W exhibits excellent electrocatalytic performance and durability. � As a cathode electrocatalyst in a fuel cell, NHCS–W shows good peak power density. � NHCS–W can be applied as an air electrode in Zn–air batteries. � NHCS–W can act as cathode and anode catalysts in water-splitting electrolyzers. A R T I C L E I N F O
A B S T R A C T
Keywords: Hollow carbon sphere Nitrogen dopant Fuel cell Zn–air battery Water splitting
Clean energy conversion technologies require low-cost multifunctional catalysts. In this work, nitrogen-doped hollow carbon spheres with nanoporous shells (NHCS–W) are prepared by using the ionic liquid 1-butyl-3-meth ylimidazolium chloride as a source of both carbon and nitrogen. A systematic investigation of the electrocatalytic performance and durability of NHCS–W reveals excellent performance and high stabilities for the oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction in alkaline electrolytes. This multifunctionality facilitates promising practical applicability for NHCS–W. An alkaline exchange membrane fuel cell (AEMFC) with NHCS–W as the cathode electrocatalyst has a peak power density of 109 mW cm 2, which is 7 mW cm 2 higher than that of an AEMFC with Pt/C (20%) as the cathode electrocatalyst. By using NHCS–W as an air electrode, two rechargeable Zn–air batteries in series can power a 5 mm red light-emitting diode (~2.2 V). Moreover, a cell voltage of only 1.61 V is needed to achieve a current density of 10 mA cm 2 in a water-splitting electrolyzer employing NHCS–W as both the cathode and anode catalysts. This work reveals the potential of ionic liquid precursors for constructing nitrogen-doped carbon-based materials for a wide variety of electrocatalytic applications.
1. Introduction The depletion of fossil fuels and pollution caused by their heavy use have encouraged the development of advanced clean energy conversion technologies, including fuel cells, metal–air batteries, and hydrogen evolution technologies [1,2]. Such technologies are primarily based on three electrochemical reactions, namely, oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution re action (HER) [3]. Commonly, clean energy conversion systems require
the integration of more than one reaction within a single device, e.g., HER–ORR–OER for regenerative fuel cells, HER–OER for water elec trolysis devices, and ORR–OER for rechargeable metal–air batteries [4]. Despite the efficient performance of noble-metal-based catalysts for these electrochemical reactions, large-scale applications have been limited by their low catalytic selectivity, prohibitive cost, and poor durability. Therefore, there is a necessity to develop low-cost multi functional catalysts that simultaneously exhibit high efficiencies, excellent catalytic activities, and long-term stabilities for the ORR, OER,
* Corresponding author. E-mail address: [email protected] (J. Tong). https://doi.org/10.1016/j.jpowsour.2019.227166 Received 7 May 2019; Received in revised form 13 September 2019; Accepted 15 September 2019 Available online 19 September 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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and HER [5]. In recent years, substantial efforts have been undertaken to develop non-precious-metal and metal-free multifunctional electrocatalysts. Transitional metal (Fe, Co, Mn, or Ni)-based dichalcogenides [6], phosphides [7], alloys [8], carbides [9], oxides [10], and particularly heteroatom (N, P, B, or S)-doped carbon materials have attracted much attention because of their remarkable electrocatalytic activities for the ORR, OER, or HER [11]. Notably, heteroatom dopants, particularly ni trogen, can significantly improve the electrocatalytic performance of carbon materials owing to charge polarization and differences in elec tronegativity and electron spin density between the heteroatom and carbon [12]. Furthermore, a porous structure has been revealed to be crucial to expose more active sites on the catalyst and facilitate electron and mass transport [1]. In particular, a hollow structure with abundant hierarchical pores is expected to enhance the catalytic performance by improving the effective surface area, exposing active sites, and estab lishing a triple-phase boundary to improve mass transport [13]. These advantages have been confirmed for heteroatom-doped hollow-car bon-sphere-based electrocatalysts developed for the ORR [9,14] and OER [15,16] as well as water electrolysis systems [17] and lithium-ion batteries [18]. The structures and morphologies of carbon materials, which are strongly dependent on the precursors used [19,20], significantly influ ence the physical properties and electrocatalytic performance of carbon catalysts. To realize the facile preparation of widely applicable carbon materials, various precursors have been investigated, including poly mers [19,21], N-containing heterocyclic compounds [9,22], supramo lecular aggregates [23], and metal–organic frameworks [24]. Compared with conventional polymers, ionic liquids (ILs) are versatile small-molecular-sized carbon precursors with negligible vapor pressures [2,25]. After appropriate molecular design, ILs can be used directly or indirectly to prepare various carbon materials with a wide range of applications [26]. Furthermore, because ILs usually contain hetero atoms, they can be used to prepare heteroatom self-doped carbon ma terials, which favor the formation of catalysts with uniform structures and good heteroatom distribution [26]. In this work, N-doped carbon spheres are prepared by pyrolyzing a mixture of 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), CoCl2⋅6H2O, and silica, followed by etching with HF. The selective synthesis of hollow or solid carbon spheres is achieved by using water or ethanol as a solvent. The as-prepared N-doped hollow carbon spheres with nanoporous shells (NHCS–W) exhibit excellent electrocatalytic activities and high stabilities as multifunctional electrocatalysts for the ORR, OER, and HER. The NHCS–W catalyst also exhibits excellent per formance in terms of energy conversion when applied in H2–O2 fuel cells, rechargeable Zn-air batteries, and water-splitting electrolyzers.
different hollow product (denoted as NHCS–W-MF) was obtained, and (2) when ethanol was used as the solvent instead of deionized water, Ndoped solid carbon spheres (denoted as NSCS-E) were formed. 3. Results 3.1. Catalyst characterization Fig. 1 presents the morphology of NHCS–W as well as the distribu tions of C and N in the sample. As shown by the SEM image of NHCS–W (Fig. 1a), our synthesis procedure led predominantly to the formation of spheres with diameters of 40–60 nm, although some broken and collapsed carbon fragments are also observed. The TEM image further reveals that the spheres are hollow with thicknesses of 10–15 nm (Fig. 1b). The element mapping images reveal homogenous distributions of C and N in the spheres (Fig. 1c and d). As the morphological features and elemental distributions of NHCS–W-MF are similar to those of NHCS–W, we do not show the corresponding images here. When the solvent is changed from water to ethanol, uniform solid carbon spheres with diameters of approximately 40 nm are obtained (Figs. S1a and b), which exhibit homogeneous distributions of C and N (Figs. S1c and d). The formation of hollow and solid carbon spheres can be ascribed to differences in the interaction mechanisms between the IL and silica in different solvents, as illustrated in Scheme 1. Generally, ILs form ag gregates of anions and cations linked by hydrogen bonds in both the solid and liquid states [27,28]. However, in aqueous solution, the IL aggregates can be effectively broken by water owing to its high dielectric constant and ability to form strong hydrogen bonds with the anion s/cations of the ILs [29]. In this work, dissociated [Bmim]Cl tends to assemble around the hydrophilic silica core owing to electrostatic in teractions between [Bmim]Cl. Thus, hollow carbon spheres are obtained following pyrolysis and removal of the silica core by acid etching. In contrast, organic solvents with low dielectric constants, e.g., ethanol in the present study, can enhance the ionic association of ILs [29]. As a result, [Bmim]Cl forms aggregates when ethanol is used as the solvent, which contributes to the formation of solid carbon spheres during the pyrolyzation process (Scheme 1). Fig. 2 shows the XRD patterns of the as-prepared samples. Two broad diffraction peaks at 2θ values of approximately 25� and 44� are observed for all samples. These peaks are associated with the (002) plane of graphite and the (100) plane of disordered amorphous carbon, respec tively [30]. Thus, the carbon spheres prepared in this work are partially graphitized, regardless of the solvent used. The nitrogen adsorption–desorption isotherms and pore size distri butions of the as-prepared carbon spheres are shown in Fig. 3. Isotherms with both type I and type II characteristic are observed for all three samples, indicating a broad pore size distribution from micro-to mac ropores [31]. This pore structure is also confirmed by the pore size distribution plots of the samples, in which the pore sizes range from 0.5 to 80 nm. Notably, the solvent used to prepare the carbon spheres exerts an obvious influence on the pore types. In the samples synthesized in water (NHCS–W and NHCS–W-MF), ink-bottle-type mesopores and some slip cracks are formed, as indicated by the combined type II and IV hysteresis loops in the isotherms (Fig. 3a, c), whereas the formation of pores with slip cracks can be inferred from the type IV hysteresis loop of NSCS-E, which was synthesized in ethanol (Fig. 3b). The BET specific surface areas and pore volumes of the samples are summarized in Table 1. Among the prepared samples, NHCS–W exhibits the largest BET surface area and pore volume (360 m2 g 1 and 0.39 cm3 g 1, respectively). Furthermore, as a solvent, water is appar ently beneficial for the formation of mesopores, as mesopores account for 80.3% of the BET surface area and 89.7% of the pore volume in NHCS–W. By contrast, NSCS-E is dominated by micropores, which ac count for 70.0% of the BET surface area and 67.7% of the pore volume. It is important to point out that micropores are not conducive to the transmission of electrolytes and reactants [32] and thus decrease the
2. Experimental The materials and reagents, characterization methods, and electro chemical measurements are described in the supplementary data. The N-doped hollow carbon spheres (denoted as NHCS–W) were prepared in deionized water as follows. First, 0.27 g of CoCl2⋅6H2O was dissolved in deionized water, and then 1.0 g [Bmim]Cl and 2.5 g silica were incorporated into the solution in sequence. Second, the mixture was ultrasonicated thoroughly to form a well-mixed suspension, which was then separated by centrifugation. The obtained solid was washed with deionized water several times and then dried at 80 � C under vac uum for 24 h. The dried solid was heated to 800 � C at a heating rate of 5 � C⋅min 1 and then pyrolyzed for 2 h in a nitrogen atmosphere to form a black powder. Finally, the product was etched using HF solution for 12 h to leach out the metal species and silica. Before being applied as a catalyst, the final product was washed thoroughly with deionized water and dried at 80 � C under vacuum overnight. For comparison, different types of N-doped carbon spheres were prepared by modifying the above procedures: (1) when CoCl2⋅6H2O was excluded from the first step, a 2
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Journal of Power Sources 441 (2019) 227166
Fig. 1. Typical (a) SEM and (b) TEM images of NHCS–W; elemental mapping of (c) carbon and (d) nitrogen in the selected area in (a).
Scheme 1. Illustration of carbon sphere formation processes in water and in ethanol. The sizes of the symbols do not correspond to the real sizes of the various species.
catalytic performance of solid carbon spheres, as discussed later. XPS analyses were performed to determine the atomic content and chemical states of C and N on the surface of the carbon spheres. The XPS survey spectrum and the high-resolution C1s and N1s spectra for NHCS–W are shown in Fig. 4, whereas those for NSCS-E and NHCS–W-
MF are shown in Fig. S2 and Fig. S3, respectively. The XPS survey spectra reveal that the as-prepared carbon spheres consist of C, N, and O. The high-resolution C1s spectra can be deconvoluted into three peaks at approximately 284.7, 285.6, and 287.7 eV, which can be attributed to C–C, C–N, and C¼O, respectively (Fig. 4b, Fig. S2b, and Fig. S3b) [33]. 3
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enhance the graphitization of carbon materials, as reported previously [35]. Furthermore, high graphitization is important for enhancing the conductivity of carbon catalysts, and pyridinic N is widely believed to play an essential role in nitrogen-doped carbon nanomaterials by providing the active sites for the ORR, OER, and HER [36,37]. 3.2. ORR activity tests The catalytic activities of the as-prepared catalysts and Pt/C (20%) for the ORR have been tested by strictly controlling the key factors such as glassware cleanliness, electrode film quality, catalyst loading, elec trolyte type, and electrolyte purity), as described in the literature [38]. The linear sweep voltammetry (LSV) curves and corresponding Tafel plots for the ORR in 0.1 M KOH are shown in Fig. 5, and the charac teristic data for evaluating the electrocatalytic activity are listed in Table 5. All the catalysts exhibit excellent electrocatalytic performance with large positive onset and half-wave potentials (Fig. 5a). In partic ular, NHCS–W has an onset potential of 0.949 V, which is only 11 mV more negative than that of commercial Pt/C (20%). Furthermore, the
Fig. 2. XRD patterns of the as-prepared samples.
The high-resolution N1s spectra reveal the existence of pyridinic N (~398.8 eV), graphitic N (~399.9 eV), and N-oxide (~404.2 eV) (Fig. 4c, Fig. S2c, and Fig. S3c) [34]. The atomic contents of the samples are listed in Table 2, and the amounts of various C and N surface species are shown in Tables 3 and 4. NHCS–W has the highest N content (3.8%) and predominately contains pyridinic N and graphitic N (26.5% and 68.1%, respectively; Table 4). NHCS–W also contains the highest content of graphitic C (77.4%), whereas the lowest content graphitic C is observed for NHCS–W-MF (65.7%), which is prepared without using CoCl2⋅6H2O (Table 3). This observation confirms that the cobalt species can act as a catalyst to
Table 1 Morphological parameters of the as-prepared samples. Sample 2
BET surface area (m ⋅g Pore volume (cm3⋅g
1
)
1
)
Total Microporous Mesoporous Total Microporous Mesoporous
NHCS–W
NSCS-E
NHCS–W-MF
360 71 289 0.39 0.04 0.35
230 161 69 0.31 0.21 0.10
339 102 237 0.34 0.05 0.29
Fig. 3. Nitrogen adsorption–desorption isotherms of (a) NHCS–W, (b) NSCS-E, and (c) NHCS–W-MF. The insets show the corresponding pore size distributions calculated using density functional theory. 4
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Fig. 4. (a) XPS survey spectrum, (b) high-resolution C1s spectrum, and (c) high-resolution N1s spectrum of NHCS–W.
The Tafel plots of the investigated catalysts all show two typical linear regions at low-current densities (from 1.5 to 1.0 on the loga rithmic scale) and high-current densities (from 0.5 to 0.3 on the log arithmic scale) (Fig. 5b; individual linear fits of the Tafel plots for each catalyst are shown in Fig. S4). These regions are characteristic of a current-density-dependent adsorption mechanism [39]. As shown in Table 5, NHCS–W has the lowest Tafel slope of 33/75 mV⋅dec 1 in both the low- and high-current density ranges, even when compared to Pt/C (20%) (38/92 mV⋅dec 1). These findings indicate that NHCS–W exhibits better kinetic properties. The excellent performance of NHCS–W for the ORR makes it superior to various types of carbon-based non- precious-metal catalysts, as listed in Table S1. To evaluate the active sites that may be involved in the electro chemical reaction, the electrochemically active surface area (ECSA) of each sample has been evaluated. The ECSA is proportional to the doublelayer capacitance (Cdl), which can be evaluated by CV in the nonfaradaic electrochemical potential range at different scan rates (Fig. S5) [40]. The measured Cdl values are 23.6, 19.8, and 10.6 F cm 2 for NHCS–W, NSCS-E, and NHCS–W-MF, respectively. Thus, NHCS–W has the largest ECSA and highest electrocatalytic activity for the ORR. Further kinetic investigations have been carried out to characterize the optimum catalyst NHCS–W. The LSV curves for NHCS–W at different rotating speeds are shown in Fig. S6a. The current density increases with increasing rotating speed because mass transport is improved at higher speeds. The corresponding Koutecky–Levich (K-L) plots exhibit good linearity at various potentials, indicating that the reaction is first order with respect to the dissolved O2 concentration under the investigated conditions (Fig. S6b). Based on the K-L plots, the transferred electron number (n) has been calculated as 3.71–3.83 in the potential range of
Table 2 Atomic contents (at%) of the samples determined by XPS analysis. Sample
Carbon
Nitrogen
Oxygen
NHCS–W NSCS-E NHCS–W-MF
92.7 90.6 88.4
3.8 3.7 3.2
3.5 5.7 8.4
Table 3 Relative integrated intensities (%) of carbon species determined by XPS analysis. Sample
Graphitic C
C–N
C¼O
NHCS–W NSCS-E NHCS–W-MF
77.4 74.2 65.7
18.4 16.8 15.2
4.2 9.0 19.1
Table 4 Relative integrated intensities (%) of nitrogen species determined by XPS analysis. Sample
Pyridinic N
Graphitic N
N-oxide
NHCS–W NSCS-E NHCS–W-MF
26.5 23.3 18.9
68.1 65.6 58.2
5.4 11.1 22.9
half-wave potential for NHCS–W is 0.860 V, which is 22 mV more pos itive than that of Pt/C (20%), and the limiting current densities achieved by NHCS–W and as Pt/C (20%) are similar. 5
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Journal of Power Sources 441 (2019) 227166
Fig. 5. (a) LSV curves of the catalysts in O2-saturated 0.1 M KOH at a scan rate of 10 mV s 1 and a rotation rate of 1600 rpm and (b) corresponding Tafel plots; (c) polarization and power density curves for NHCS–W and Pt/C (20%); and (d) 10 h durability test in a H2–O2 fuel cell with NHCS–W as the cathode catalyst at 60 � C.
under alkaline conditions. To investigate the structural stability of NHCS–W, SEM, TEM, and elemental mapping measurements have been obtained after the chronoamperometric test (Fig. S8). No evident changes in the morphology and microstructure of the sample are observed, confirming the high physical stability of this catalyst for the ORR in alkaline media.
Table 5 Electrocatalytic parameters of the catalysts for the ORR in 0.1 M KOH. Catalyst
Onset potential (V vs. RHE)
Half-wave potential (V vs. RHE)
Tafel slope (at low/high current densities) (mV⋅dec 1)
NHCS–W NSCS-E NHCS–WMF Pt/C (20%)
0.949 0.935 0.931
0.860 0.846 0.854
33/75 41/102 34/127
0.960
0.838
38/92
3.3. Application in fuel cells Considering the outstanding catalytic performance and stability of NHCS–W for the ORR in alkaline solution, the catalyst has been used to fabricate an MEA. An alkaline exchange membrane fuel cell (AEMFC) constructed using the MEA with carbon paper as the gas diffusion layer has been used to evaluate the single cell performance, as described in the literature [41]. The polarization and power density curves of AEMFCs employing NHCS–W and Pt/C (20%) as cathode catalysts are shown in Fig. 5c. An open-circuit voltage (OCV)/maximum power density of 0.86 V/109 mW cm 2 and 0.83 V/102 mW cm 2 are obtained with NHCS–W and Pt/C (20%), respectively. These results indicate that the catalytic activity of NHCS–W for the ORR is superior to that of Pt/C (20%) when employed as a cathode catalyst in AEMFCs. Furthermore, the NHCS–W catalyst shows high stability and little decrease in current density during a 10 h chronoamperometry test at 0.4 V (Fig. 5d).
0.40–0.60 V (Fig. S6c). This range of transferred electron numbers in dicates that the mass transfer resistance has not been completely over come. In this system, the 2e pathway for the ORR occurs together with the dominant 4e pathway, as the limiting current density does not reach the theoretical value of ~6 mA cm 2 that is expected when the reaction proceeds exclusively along the 4e pathway. The long-term stability of NHCS–W has been tested by accelerated continuous CV at a scan rate of 100 mV s 1. The results show that the potential window and catalytic activity of NHCS–W (with respect to the onset and half-wave potential) do not decrease appreciably after 10000 cycles (Figs. S7a and b). By comparison, a 45/12 mV decrease in the onset/half-wave potential is observed for Pt/C (20%) after 10000 cycles (Fig. S7c). The stabilities have also been tested by the chronoampero metric method and the current–time curves are shown in Fig. S7d. Although the current densities of both NHCS–W and Pt/C (20%) decrease with time, low attenuation is observed for NHCS–W. After 36000 s, NHCS–W retains 84.9% of its initial current density value, whereas the corresponding retention value for Pt/C (20%) is 57.3%. Thus, the NHCS–W catalyst exhibits higher stability than Pt/C (20%)
3.4. OER activity tests The rational design and synthesis of highly efficient ORR/OER bifunctional catalysts remain greatly challenging because of the large overpotential compelling the two reactions to proceed in opposite di rections away from the equilibrium state [10]. The abilities of the car bon spheres to act as electrocatalysts for the OER in 0.1 M KOH have also 6
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Journal of Power Sources 441 (2019) 227166
been evaluated, using commercial IrO2 as a control. The LSV curves and Tafel plots are shown in Fig. 6, and the overpotential and Tafel slope values are listed in Table 6. Among the as-prepared samples, NHCS–W exhibits the lowest overpotential of 362 mV, which is 24 mV lower than that of IrO2. The behavior of NHCS–W is superior to that of many highly efficient carbon-based noble-metal-free catalysts, as shown in Table S2. Electrochemical impedance spectroscopy (EIS) measurements (Fig. 6c) have been conducted and Cdl values (Fig. S9) have been measured to estimate the conductivity and ECSAs of our catalysts. The results show that NHCS–W has a charge-transfer resistance (Rct) of approximately 74 Ω, which is much lower than the values for NSCS-E (~191 Ω) and NHCS–W-MF (~299 Ω). The Cdl value for NHCS–W (14.9 mFcm 2) is also much higher than those for NSCS-E (10.7 mFcm 2) and NHCS–W-MF (5.3 mF cm 2). Again, NHCS–W shows excellent charge transformation and exhibits a high ECSA. Subsequently, the stability of the catalyst has been tested under OER conditions. The chronoamperometric test in Fig. 6d shows that after 36000 s, NHCS–W retains 72.2% of its initial current density, whereas this value is approximately 83.4% for IrO2. A chronopotentiometric test has also been performed to determine the long-term stability of the catalyst for the OER. The potential only shows a very minor change after 36000 s at 10 mA cm 2 (Fig. S11). The SEM and TEM images after the chronopotentiometric test also show no evident changes in the morphology and microstructure of NHCS–W (Fig. S12). These findings indicate that NHCS–W has high stability for the OER in alkaline electrolytes.
Table 6 Electrocatalytic parameters of the catalysts for the OER in 0.1 M KOH. Catalyst Overpotential (mV, at 10 mA cm Tafel slope (mV⋅dec 1)
2
)
NHCS–W
NSCSE
NHCS–WMF
IrO2
362 182
434 189
563 284
386 80
(j ¼ 10 mA cm 2) for the OER and E1/2 for the ORR. A small ΔE value indicates a more reversible oxygen electrode [42]. The overall activities of NHCS–W for the ORR and OER have been investigated between 0.2 and 1.8 V (vs. RHE) using a RDE in O2-saturated 0.1 M KOH at 1600 rpm, and the obtained LSV curve is shown in Fig. S13. The ΔE value is 0.732 V for NHCS–W, which is 38 mV lower than that for N-doped porous carbon (ΔE ¼ 0.770 V) [43] and 148 mV lower than that for N, S-codoped car bon nanosheets (ΔE ¼ 0.880 V) [4]. Thus, compared with most previ ously reported non-precious-metal catalysts, NHCS–W is a superior heteroatom-doped carbon bifunctional catalyst for the ORR and OER, as shown in Table S3. 3.6. Application in Zn–air batteries Encouraged by prominent bifunctional catalytic activities of NHCS–W for both the ORR and OER, we have evaluated the application of NHCS–W as a catalyst for the air electrode in a rechargeable Zn–air battery using a homemade cell (Fig. S14), as illustrated in Fig. S15. The open-circuit potential (OCP) of the Zn–air battery with NHCS–W is 1.41 V, which is similar to the value of 1.44 V for Pt/C (20%), and the OCP shows no appreciable decrease after 10 h (Fig. 7a). Fig. 7b shows the polarization and power density curves for the NHCS–W and Pt/C (20%) air electrodes. The NHCS–W catalyst exhibits a peak power
3.5. ORR/OER bifunctional activity tests Generally, the overall activity of a bifunctional oxygen catalyst can be evaluated using the potential difference (ΔE) between E
Fig. 6. (a) LSV curves for the OER at a scan rate of 10 mV s 1 in O2-saturated 0.1 M KOH and (b) corresponding Tafel plots; (c) Nyquist plots of the as-prepared samples; and (d) durability test for NHCS–W and IrO2 at 1.616 V (vs. RHE). The i-t curve for NHCS–W is the average result of three tests, and the curve for each test is shown in Fig. S10. 7
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Journal of Power Sources 441 (2019) 227166
density of 45.5 mW cm 2 (at 73.7 mA cm 2), which is similar to that of Pt/C (20%) (50.1 mW cm 2 at 81.9 mA cm 2), indicating the compara ble performance of these catalysts. The performance of the Zn–air bat tery based on NHCS–W is also superior to those of many others based on carbon or carbon-based noble-metal-free catalysts, as shown in Table S4. The charge and discharge polarization curves of the Zn–air batteries employing NHCS–W and Pt/C (20%) as air electrodes shown in Fig. 7c further confirm the comparable charge–discharge performance of these batteries. Using the commercial Pt/C (20%)-based air electrode as a reference, the robustness of the NHCS–W-based air electrode has been tested in a Zn–air battery at 10 mA cm 2 for 100 charge–discharge cycles (10 min for each cycle). The Zn–air battery employing the Pt/C (20%) air elec trode exhibits initial charge/discharge potentials of 2.05/0.91 V with a
voltage gap of 1.14 V and an initial round-trip efficiency of 44.4%. In the case of NHCS–W, the initial charge/discharge potentials are 1.96/1.14 V with a lower voltage gap of 0.82 V and a higher round-trip efficiency of 58.2%. After 100 cycles (~17 h), the Zn–air battery employing the NHCS–W air electrode shows little loss in performance, with an increase in the voltage gap of only 0.06 V and a decrease in the round-trip effi ciency of 2.4% (Fig. 7d). The robustness of the NHCS–W-based Zn–air battery has also been tested at higher current densities of 20 and 50 mA cm 2 over 100 charge–discharge cycles. Initial charge/discharge potentials of 2.04/0.98 V and 2.18/0.67 V with only 3.2% and 4.1% decreases in the round-trip efficiency are observed at 20 and 50 mA cm 2, respectively (Fig. 7e). These results confirm the high durability and excellent rechargeability of the Zn–air battery employing the NHCS–W air electrode. In the present study, we have also tested
Fig. 7. (a) Open-circuit potential test of Zn–air batteries for 10 h; (b) polarization and power density curves for NHCS–W and Pt/C (20%) air electrodes; (c) charge–discharge polarization curves for Zn–air batteries employing NHCS–W and Pt/C (20%) at a scan rate of 10 mV s 1; (d) galvanostatic charge–discharge cycling profiles for Zn–air batteries employing NHCS–W and Pt/C (20%) at 10 mA cm 2; (e) galvanostatic charge–discharge cycling profiles for the Zn–air battery employing NHCS–W at 20 and 50 mA cm 2; and (f) photograph of a red LED powered by two NHCS–W-based Zn–air batteries in series. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 8
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practical applications of the NHCS–W-based Zn–air battery. A 5 mm red light-emitting diode (~2.2 V) can be powered by two such Zn–air bat teries in series, which clearly reveals the high applicability of the NHCS–W catalyst for rechargeable Zn–air batteries (Fig. 7f and Movie S1). Supplementary data related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.227166.
Table 7 Electrocatalytic parameters of the catalysts for the HER in 1.0 M KOH. Catalyst Overpotential (mV) Tafel slope (mV⋅dec
1
)
NHCS–W
NSCS-E
NHCS–W-MF
Pt/C (20%)
196 101
276 123
305 167
64 60
recorded before and after cycling are shown in Fig. 8d. Clearly, only negligible attenuation of the current density is observed over 10000 cycles, indicating that the electrocatalytic activity does not decrease. The results of a 36000 s chronoamperometric test show that the reten tion of current density by NHCS–W (78.2%) is 38.6% higher than that for Pt/C (39.6%) (inset, Fig. 8d). This result demonstrates that NHCS–W displays superior stability in alkaline media for the HER.
3.7. HER activity tests The electrocatalytic activities of the as-prepared catalysts for the HER have been tested in 1.0 M KOH by the LSV method at a scan rate of 10 mV s 1. The HER activity of Pt/C (20%) has also been tested under the same conditions for comparison (Fig. 8a). The corresponding Tafel plots are shown in Fig. 8b, and the overpotential (at 10 mA cm 2) and Tafel slope values are listed in Table 7. Among the three as-prepared carbon sphere samples, NHCS–W exhibits the highest catalytic activity with the lowest overpotential (196 mV) and Tafel slope (101 mV dec 1). These results indicate that NHCS–W is among the most efficient heteroatom-doped carbon-based HER catalysts, as shown in Table S5. The ECSA values of the samples have been estimated by the CV method (Fig. S16). The Cdl values are 33.8, 23.7, and 5.4 mF cm 2 for NHCS–W, NSCS-E, and NHCS–W-MF, respectively. The much larger Cdl value for NHCS–W indicates a larger ECSA and thus superior electro catalytic activity for the HER. EIS has been used to investigate the electrode kinetics in the HER. The Nyquist plots of the samples are depicted in Fig. 8c. The Rct values for NHCS–W, NSCS-E, and NHCS–W-MF are approximately 73, 86, and 138 Ω, respectively. The low Rct value of NHCS–W allows an excellent charge-transfer rate to be achieved. The long-term stability of NHCS–W has been assessed by continuous CV over 10000 cycles at a scan rate of 100 mV s 1. The LSV curves
3.8. Application to overall water splitting To evaluate the practical application of NHCS–W for water elec trolysis, a water-splitting electrolyzer has been constructed, in which NHCS–W is employed as both the cathode and anode catalysts with loadings of 1.0 mg cm 2, in 1.0 M KOH at room temperature (Movie S2). The LSV curve reveals outstanding performance with a cell voltage of only 1.61 V required to reach a current density of 10 mA cm 2 with NHCS–W, which is much lower than the cell voltage of 1.78 V required with a pristine Ni foam electrode (Fig. S17a). The stability of the catalyst for water splitting has also been tested by the chronoamperometric method at 10 mA cm 2. As shown in Fig. S17b, the current density only decreases by 12.0% over 10 h, which confirms the high durability of the NHCS–W catalyst for overall water splitting under alkaline conditions. The results are comparable to the performance of state-of-the-art nonprecious-metal catalysts for overall water splitting, as shown in Table S6.
Fig. 8. (a) LSV curves for the HER in N2-saturated 1.0 M KOH and (b) corresponding Tafel plots; (c) Nyquist plots for the as-prepared samples at 0.2 V (vs. RHE); (d) LSV curves for NHCS–W before and after 10000 CV cycles, and chronoamperometric tests for NHCS–W and Pt/C (20%) at 0.196 V (vs. RHE). 9
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Supplementary data related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.227166.
multifunctional electrocatalysts for fuel cells, metal-air batteries, and water-splitting electrolyzers.
4. Discussion
Declarations of interest
Although it has been widely reported that the electrocatalytic ac tivity of carbon-based materials for the ORR, OER, and HER can be improved by doping N into the carbon structure, a greater understand ing of the effects of the type (graphitic N, pyridinic N, or pyrrolic N) and amount of nitrogen on the electrocatalytic activity of specific catalysts is required [44,45]. To determine the main active sites in NHCS–W, the pyridinic rings in the catalyst have been selectively grafted (approximate 60.8%, Fig. S18) with acetyl group at the pyridinic N atoms (denoted as N–Ac) (Scheme S1) [45]. The electrocatalytic activities for the ORR, OER and HER decrease sharply after modification (Fig. S19). This finding confirms that pyridinic N is the main active site in NHCS–W for the three reactions, which is consistent with previous reports [30,46]. However, the exis tence of active sites on other atoms, such as carbon atoms with Lewis basicity next to pyridinic N [11,47], graphitic N, topological defects and edge states [12,48], can not be eliminated. Based on our findings and previously reported results, the excellent catalytic activities of NHCS–W for the ORR, OER, and HER can be attributed to the combination of chemical functionalities (high C and N contents, particularly pyridinic N and graphitic C) and structural prop erties (high surface area and pore volume, hollow and mesoporous structure, and short diffusion paths). These factors create highly active and fully accessible catalytic sites and allow for high-flux mass trans portation [49]. In contrast, the low electrocatalytic performance of NHCS–W-MF is mainly caused by a low content of pyridinic N and graphitic C, whereas the inferior catalytic activity of NSCS-E is associ ated with impaired charge and mass transport owing to its solid structure. The high stability of NHCS–W may be attributable to its metal-free composition and the high chemical and mechanical stabilities of the catalyst active sites during electrocatalytic processes, as revealed by the microstructural characterizations after the stability tests (Fig. S8 and Fig. S12) [50]. It has been reported that carbon catalysts with nano porous and hollow structures exhibit greater resistance and oxidative stability than solid nanocarbons [13,51]. Furthermore, self-doping of nitrogen rather than introduction from an external source ensures a homogeneous distribution of the active sites in the catalyst. Thus, even if the top layer of the catalyst is destroyed during accelerated durability tests, equivalent active sites in the sublayers are exposed at the elec trochemical interface, which improves the electrocatalytic durability [49].
The authors declare no conflict of interest. Acknowledgment The authors are grateful to the National Natural Science Foundation of China (21363021, 51302222) for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227166. References [1] L.M. Dai, Y.H. Xue, L.T. Qu, H.J. Choi, J.B. Baek, Metal-free catalysts for oxygen reduction reaction, Chem. Rev. 115 (2015) 4823–4892. [2] M.H. Shao, Q.W. Chang, J.P. Dodelet, R. Chenitz, Recent advances in electrocatalysts for oxygen reduction reaction, Chem. Rev. 116 (2016) 3594–3657. [3] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q.X. Mi, E.A. Santori, N. S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473. [4] J. Zhang, L. Dai, Nitrogen, phosphorus, and fluorine tri-doped graphene as a multifunctional catalyst for self-powered electrochemical water splitting, Angew. Chem. Int. Ed. 55 (2016) 13296–13300. [5] Z.W. Seh, J. Kibsgaard, C.F. Dickens, I.B. Chorkendorff, J.K. Norskov, T. F. Jaramillo, Combining theory and experiment in electrocatalysis: insights into materials design, Science 355 (2017), eaad4998. [6] I.S. Amiinu, Z. Pu, X. Liu, K.A. Owusu, H.G.R. Monestel, F.O. Boakye, H. Zhang, S. Mu, Multifunctional Mo-N/C@MoS2 electrocatalysts for HER, OER, ORR, and Zn-air batteries, Adv. Funct. Mater. 27 (2017) 1702300. [7] P. Xiao, W. Chen, X. Wang, A review of phosphide-based materials for electrocatalytic hydrogen evolution, Adv. Energy Mater. 5 (2015) 1500985. [8] G.L. Chai, Z. Hou, D.J. Shu, T. Ikeda, K. Terakura, Active sites and mechanisms for oxygen reduction reaction on nitrogen-doped carbon alloy catalysts: stone-Wales defect and curvature effect, J. Am. Chem. Soc. 136 (2014) 13629–13640. [9] K.L. Ai, Z.L. Li, X.Q. Cui, Scalable preparation of sized-controlled Co-N-C electrocatalyst for efficient oxygen reduction reaction, J. Power Sources 368 (2017) 46–56. [10] Z.Q. Liu, H. Cheng, N. Li, T.Y. Ma, Y.Z. Su, ZnCo2O4 quantum dots anchored on nitrogen-doped carbon nanotubes as reversible oxygen reduction/evolution electrocatalysts, Adv. Mater. 28 (2016) 3777–3784. [11] D.H. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts, Science 351 (2016) 361–365. [12] M.H. Naveen, K. Shim, M.S.A. Hossain, J.H. Kim, Y.B. Shim, Template free preparation of heteroatoms doped carbon spheres with trace Fe for efficient oxygen reduction reaction and supercapacitor, Adv. Energy. Mater. 7 (2017) 1602002. [13] C. Zhang, R. Zhang, X. Li, W. Chen, PtNi nanocrystals supported on hollow carbon spheres: enhancing the electrocatalytic performance through high-temperature annealing and electrochemical CO stripping, ACS Appl. Mater. Interfaces 9 (2017) 29623–29632. [14] Y. Hu, J.O. Jensen, W. Zhang, L.N. Cleemann, W. Xing, N.J. Bjerrum, Q. Li, Hollow spheres of iron carbide nanoparticles encased in graphitic layers as oxygen reduction catalysts, Angew. Chem. Int. Ed. 53 (2014) 3675–3679. [15] Z. Jiang, Z.J. Jiang, T. Maiyalagan, A. Manthiram, Cobalt oxide-coated N- and Bdoped graphene hollow spheres as bifunctional electrocatalysts for oxygen reduction and oxygen evolution reactions, J. Mater. Chem. 4 (2016) 5877–5889. [16] M.Y. Song, D.S. Yang, K.P. Singh, J. Yuan, J.S. Yu, Nitrogen-doped hollow carbon spheres with highly graphitized mesoporous shell: role of Fe for oxygen evolution reaction, Appl. Catal. B Environ. 191 (2016) 202–208. [17] J. Chattopadhyay, T.S. Pathak, R. Srivastava, A.C. Singh, Ni nano-particle encapsulated in hollow carbon sphere electrocatalyst in polymer electrolyte membrane water electrolyzer, Electrochim. Acta 167 (2015) 429–438. [18] F. Zheng, M. He, Y. Yang, Q. Chen, Nano electrochemical reactors of Fe2O3 nanoparticles embedded in shells of nitrogen-doped hollow carbon spheres as highperformance anodes for lithium ion batteries, Nanoscale 7 (2015) 3410–3417. [19] G. Wu, K.L. More, C.M. Johnston, P. Zelenay, High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt, Science 332 (2011) 443–447. [20] R. Wang, H. Wang, T. Zhou, J. Key, Y. Ma, Z. Zhang, Q. Wang, S. Ji, The enhanced electrocatalytic activity of okara-derived N-doped mesoporous carbon for oxygen reduction reaction, J. Power Sources 274 (2015) 741–747. [21] S. Chen, Z. Chen, S. Siahrostami, D. Higgins, D. Nordlund, D. Sokaras, T.R. Kim, Y. Liu, X. Yan, E. Nilsson, R. Sinclair, J.K. Norskov, T.F. Jaramillo, Z. Bao, Designing boron nitride islands in carbon materials for efficient electrochemical synthesis of hydrogen peroxide, J. Am. Chem. Soc. 140 (2018) 7851–7859.
5. Conclusion In summary, novel N-doped hollow carbon spheres, prepared from ILs via a straightforward synthesis route, can be used as advanced electrocatalysts for the ORR, OER, and HER. The solvent, depending on the dielectric constant, affected the dis-/association of the IL precursor, which influenced the morphologies and structures of the as-prepared carbon spheres. Hollow carbon spheres were preferentially formed in water (high dielectric constant), whereas solid carbon spheres were formed in ethanol (low dielectric constant). NHCS–W exhibited excel lent multifunctional electrocatalytic activities and stabilities for the ORR, OER, and HER in alkaline media. The superior performance of NHCS–W was attributed to the hollow structure and high contents of graphitic carbon, pyridinic nitrogen, and graphitic nitrogen. Further more, excellent performance and stability were observed when NHCS–W was used as a cathode electrocatalyst in a H2–O2 fuel cell, an air elec trode in a rechargeable Zn–air battery, and both cathode and anode catalysts in a water-splitting electrolyzer. The present findings highlight the suitability of ILs for preparing nitrogen-doped carbon materials as 10
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Nitrogen-doped hollow carbon spheres as highly effective multifunctional electrocatalysts for fuel cells, Zn–air batteries, and water-splitting electrolyzers Jinhui Tong a, *, Wenmei Ma a, Lili Bo b, Tao Li a, Wenyan Li a, Yuliang Li a, Qi Zhang a a
Key Laboratory of Eco-environmental Polymer Materials of Gansu Province, Key Laboratory of Eco-functional Polymer Materials of Ministry of Education, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, 730070, China College of Science, Gansu Agricultural University, Lanzhou, 730070, China
b
H I G H L I G H T S
� An ionic liquid is used to prepare N-doped hollow carbon spheres (NHCS–W). � NHCS–W exhibits excellent electrocatalytic performance and durability. � As a cathode electrocatalyst in a fuel cell, NHCS–W shows good peak power density. � NHCS–W can be applied as an air electrode in Zn–air batteries. � NHCS–W can act as cathode and anode catalysts in water-splitting electrolyzers. A R T I C L E I N F O
A B S T R A C T
Keywords: Hollow carbon sphere Nitrogen dopant Fuel cell Zn–air battery Water splitting
Clean energy conversion technologies require low-cost multifunctional catalysts. In this work, nitrogen-doped hollow carbon spheres with nanoporous shells (NHCS–W) are prepared by using the ionic liquid 1-butyl-3-meth ylimidazolium chloride as a source of both carbon and nitrogen. A systematic investigation of the electrocatalytic performance and durability of NHCS–W reveals excellent performance and high stabilities for the oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction in alkaline electrolytes. This multifunctionality facilitates promising practical applicability for NHCS–W. An alkaline exchange membrane fuel cell (AEMFC) with NHCS–W as the cathode electrocatalyst has a peak power density of 109 mW cm 2, which is 7 mW cm 2 higher than that of an AEMFC with Pt/C (20%) as the cathode electrocatalyst. By using NHCS–W as an air electrode, two rechargeable Zn–air batteries in series can power a 5 mm red light-emitting diode (~2.2 V). Moreover, a cell voltage of only 1.61 V is needed to achieve a current density of 10 mA cm 2 in a water-splitting electrolyzer employing NHCS–W as both the cathode and anode catalysts. This work reveals the potential of ionic liquid precursors for constructing nitrogen-doped carbon-based materials for a wide variety of electrocatalytic applications.
1. Introduction The depletion of fossil fuels and pollution caused by their heavy use have encouraged the development of advanced clean energy conversion technologies, including fuel cells, metal–air batteries, and hydrogen evolution technologies [1,2]. Such technologies are primarily based on three electrochemical reactions, namely, oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution re action (HER) [3]. Commonly, clean energy conversion systems require
the integration of more than one reaction within a single device, e.g., HER–ORR–OER for regenerative fuel cells, HER–OER for water elec trolysis devices, and ORR–OER for rechargeable metal–air batteries [4]. Despite the efficient performance of noble-metal-based catalysts for these electrochemical reactions, large-scale applications have been limited by their low catalytic selectivity, prohibitive cost, and poor durability. Therefore, there is a necessity to develop low-cost multi functional catalysts that simultaneously exhibit high efficiencies, excellent catalytic activities, and long-term stabilities for the ORR, OER,
* Corresponding author. E-mail address: [email protected] (J. Tong). https://doi.org/10.1016/j.jpowsour.2019.227166 Received 7 May 2019; Received in revised form 13 September 2019; Accepted 15 September 2019 Available online 19 September 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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and HER [5]. In recent years, substantial efforts have been undertaken to develop non-precious-metal and metal-free multifunctional electrocatalysts. Transitional metal (Fe, Co, Mn, or Ni)-based dichalcogenides [6], phosphides [7], alloys [8], carbides [9], oxides [10], and particularly heteroatom (N, P, B, or S)-doped carbon materials have attracted much attention because of their remarkable electrocatalytic activities for the ORR, OER, or HER [11]. Notably, heteroatom dopants, particularly ni trogen, can significantly improve the electrocatalytic performance of carbon materials owing to charge polarization and differences in elec tronegativity and electron spin density between the heteroatom and carbon [12]. Furthermore, a porous structure has been revealed to be crucial to expose more active sites on the catalyst and facilitate electron and mass transport [1]. In particular, a hollow structure with abundant hierarchical pores is expected to enhance the catalytic performance by improving the effective surface area, exposing active sites, and estab lishing a triple-phase boundary to improve mass transport [13]. These advantages have been confirmed for heteroatom-doped hollow-car bon-sphere-based electrocatalysts developed for the ORR [9,14] and OER [15,16] as well as water electrolysis systems [17] and lithium-ion batteries [18]. The structures and morphologies of carbon materials, which are strongly dependent on the precursors used [19,20], significantly influ ence the physical properties and electrocatalytic performance of carbon catalysts. To realize the facile preparation of widely applicable carbon materials, various precursors have been investigated, including poly mers [19,21], N-containing heterocyclic compounds [9,22], supramo lecular aggregates [23], and metal–organic frameworks [24]. Compared with conventional polymers, ionic liquids (ILs) are versatile small-molecular-sized carbon precursors with negligible vapor pressures [2,25]. After appropriate molecular design, ILs can be used directly or indirectly to prepare various carbon materials with a wide range of applications [26]. Furthermore, because ILs usually contain hetero atoms, they can be used to prepare heteroatom self-doped carbon ma terials, which favor the formation of catalysts with uniform structures and good heteroatom distribution [26]. In this work, N-doped carbon spheres are prepared by pyrolyzing a mixture of 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), CoCl2⋅6H2O, and silica, followed by etching with HF. The selective synthesis of hollow or solid carbon spheres is achieved by using water or ethanol as a solvent. The as-prepared N-doped hollow carbon spheres with nanoporous shells (NHCS–W) exhibit excellent electrocatalytic activities and high stabilities as multifunctional electrocatalysts for the ORR, OER, and HER. The NHCS–W catalyst also exhibits excellent per formance in terms of energy conversion when applied in H2–O2 fuel cells, rechargeable Zn-air batteries, and water-splitting electrolyzers.
different hollow product (denoted as NHCS–W-MF) was obtained, and (2) when ethanol was used as the solvent instead of deionized water, Ndoped solid carbon spheres (denoted as NSCS-E) were formed. 3. Results 3.1. Catalyst characterization Fig. 1 presents the morphology of NHCS–W as well as the distribu tions of C and N in the sample. As shown by the SEM image of NHCS–W (Fig. 1a), our synthesis procedure led predominantly to the formation of spheres with diameters of 40–60 nm, although some broken and collapsed carbon fragments are also observed. The TEM image further reveals that the spheres are hollow with thicknesses of 10–15 nm (Fig. 1b). The element mapping images reveal homogenous distributions of C and N in the spheres (Fig. 1c and d). As the morphological features and elemental distributions of NHCS–W-MF are similar to those of NHCS–W, we do not show the corresponding images here. When the solvent is changed from water to ethanol, uniform solid carbon spheres with diameters of approximately 40 nm are obtained (Figs. S1a and b), which exhibit homogeneous distributions of C and N (Figs. S1c and d). The formation of hollow and solid carbon spheres can be ascribed to differences in the interaction mechanisms between the IL and silica in different solvents, as illustrated in Scheme 1. Generally, ILs form ag gregates of anions and cations linked by hydrogen bonds in both the solid and liquid states [27,28]. However, in aqueous solution, the IL aggregates can be effectively broken by water owing to its high dielectric constant and ability to form strong hydrogen bonds with the anion s/cations of the ILs [29]. In this work, dissociated [Bmim]Cl tends to assemble around the hydrophilic silica core owing to electrostatic in teractions between [Bmim]Cl. Thus, hollow carbon spheres are obtained following pyrolysis and removal of the silica core by acid etching. In contrast, organic solvents with low dielectric constants, e.g., ethanol in the present study, can enhance the ionic association of ILs [29]. As a result, [Bmim]Cl forms aggregates when ethanol is used as the solvent, which contributes to the formation of solid carbon spheres during the pyrolyzation process (Scheme 1). Fig. 2 shows the XRD patterns of the as-prepared samples. Two broad diffraction peaks at 2θ values of approximately 25� and 44� are observed for all samples. These peaks are associated with the (002) plane of graphite and the (100) plane of disordered amorphous carbon, respec tively [30]. Thus, the carbon spheres prepared in this work are partially graphitized, regardless of the solvent used. The nitrogen adsorption–desorption isotherms and pore size distri butions of the as-prepared carbon spheres are shown in Fig. 3. Isotherms with both type I and type II characteristic are observed for all three samples, indicating a broad pore size distribution from micro-to mac ropores [31]. This pore structure is also confirmed by the pore size distribution plots of the samples, in which the pore sizes range from 0.5 to 80 nm. Notably, the solvent used to prepare the carbon spheres exerts an obvious influence on the pore types. In the samples synthesized in water (NHCS–W and NHCS–W-MF), ink-bottle-type mesopores and some slip cracks are formed, as indicated by the combined type II and IV hysteresis loops in the isotherms (Fig. 3a, c), whereas the formation of pores with slip cracks can be inferred from the type IV hysteresis loop of NSCS-E, which was synthesized in ethanol (Fig. 3b). The BET specific surface areas and pore volumes of the samples are summarized in Table 1. Among the prepared samples, NHCS–W exhibits the largest BET surface area and pore volume (360 m2 g 1 and 0.39 cm3 g 1, respectively). Furthermore, as a solvent, water is appar ently beneficial for the formation of mesopores, as mesopores account for 80.3% of the BET surface area and 89.7% of the pore volume in NHCS–W. By contrast, NSCS-E is dominated by micropores, which ac count for 70.0% of the BET surface area and 67.7% of the pore volume. It is important to point out that micropores are not conducive to the transmission of electrolytes and reactants [32] and thus decrease the
2. Experimental The materials and reagents, characterization methods, and electro chemical measurements are described in the supplementary data. The N-doped hollow carbon spheres (denoted as NHCS–W) were prepared in deionized water as follows. First, 0.27 g of CoCl2⋅6H2O was dissolved in deionized water, and then 1.0 g [Bmim]Cl and 2.5 g silica were incorporated into the solution in sequence. Second, the mixture was ultrasonicated thoroughly to form a well-mixed suspension, which was then separated by centrifugation. The obtained solid was washed with deionized water several times and then dried at 80 � C under vac uum for 24 h. The dried solid was heated to 800 � C at a heating rate of 5 � C⋅min 1 and then pyrolyzed for 2 h in a nitrogen atmosphere to form a black powder. Finally, the product was etched using HF solution for 12 h to leach out the metal species and silica. Before being applied as a catalyst, the final product was washed thoroughly with deionized water and dried at 80 � C under vacuum overnight. For comparison, different types of N-doped carbon spheres were prepared by modifying the above procedures: (1) when CoCl2⋅6H2O was excluded from the first step, a 2
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Fig. 1. Typical (a) SEM and (b) TEM images of NHCS–W; elemental mapping of (c) carbon and (d) nitrogen in the selected area in (a).
Scheme 1. Illustration of carbon sphere formation processes in water and in ethanol. The sizes of the symbols do not correspond to the real sizes of the various species.
catalytic performance of solid carbon spheres, as discussed later. XPS analyses were performed to determine the atomic content and chemical states of C and N on the surface of the carbon spheres. The XPS survey spectrum and the high-resolution C1s and N1s spectra for NHCS–W are shown in Fig. 4, whereas those for NSCS-E and NHCS–W-
MF are shown in Fig. S2 and Fig. S3, respectively. The XPS survey spectra reveal that the as-prepared carbon spheres consist of C, N, and O. The high-resolution C1s spectra can be deconvoluted into three peaks at approximately 284.7, 285.6, and 287.7 eV, which can be attributed to C–C, C–N, and C¼O, respectively (Fig. 4b, Fig. S2b, and Fig. S3b) [33]. 3
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enhance the graphitization of carbon materials, as reported previously [35]. Furthermore, high graphitization is important for enhancing the conductivity of carbon catalysts, and pyridinic N is widely believed to play an essential role in nitrogen-doped carbon nanomaterials by providing the active sites for the ORR, OER, and HER [36,37]. 3.2. ORR activity tests The catalytic activities of the as-prepared catalysts and Pt/C (20%) for the ORR have been tested by strictly controlling the key factors such as glassware cleanliness, electrode film quality, catalyst loading, elec trolyte type, and electrolyte purity), as described in the literature [38]. The linear sweep voltammetry (LSV) curves and corresponding Tafel plots for the ORR in 0.1 M KOH are shown in Fig. 5, and the charac teristic data for evaluating the electrocatalytic activity are listed in Table 5. All the catalysts exhibit excellent electrocatalytic performance with large positive onset and half-wave potentials (Fig. 5a). In partic ular, NHCS–W has an onset potential of 0.949 V, which is only 11 mV more negative than that of commercial Pt/C (20%). Furthermore, the
Fig. 2. XRD patterns of the as-prepared samples.
The high-resolution N1s spectra reveal the existence of pyridinic N (~398.8 eV), graphitic N (~399.9 eV), and N-oxide (~404.2 eV) (Fig. 4c, Fig. S2c, and Fig. S3c) [34]. The atomic contents of the samples are listed in Table 2, and the amounts of various C and N surface species are shown in Tables 3 and 4. NHCS–W has the highest N content (3.8%) and predominately contains pyridinic N and graphitic N (26.5% and 68.1%, respectively; Table 4). NHCS–W also contains the highest content of graphitic C (77.4%), whereas the lowest content graphitic C is observed for NHCS–W-MF (65.7%), which is prepared without using CoCl2⋅6H2O (Table 3). This observation confirms that the cobalt species can act as a catalyst to
Table 1 Morphological parameters of the as-prepared samples. Sample 2
BET surface area (m ⋅g Pore volume (cm3⋅g
1
)
1
)
Total Microporous Mesoporous Total Microporous Mesoporous
NHCS–W
NSCS-E
NHCS–W-MF
360 71 289 0.39 0.04 0.35
230 161 69 0.31 0.21 0.10
339 102 237 0.34 0.05 0.29
Fig. 3. Nitrogen adsorption–desorption isotherms of (a) NHCS–W, (b) NSCS-E, and (c) NHCS–W-MF. The insets show the corresponding pore size distributions calculated using density functional theory. 4
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Fig. 4. (a) XPS survey spectrum, (b) high-resolution C1s spectrum, and (c) high-resolution N1s spectrum of NHCS–W.
The Tafel plots of the investigated catalysts all show two typical linear regions at low-current densities (from 1.5 to 1.0 on the loga rithmic scale) and high-current densities (from 0.5 to 0.3 on the log arithmic scale) (Fig. 5b; individual linear fits of the Tafel plots for each catalyst are shown in Fig. S4). These regions are characteristic of a current-density-dependent adsorption mechanism [39]. As shown in Table 5, NHCS–W has the lowest Tafel slope of 33/75 mV⋅dec 1 in both the low- and high-current density ranges, even when compared to Pt/C (20%) (38/92 mV⋅dec 1). These findings indicate that NHCS–W exhibits better kinetic properties. The excellent performance of NHCS–W for the ORR makes it superior to various types of carbon-based non- precious-metal catalysts, as listed in Table S1. To evaluate the active sites that may be involved in the electro chemical reaction, the electrochemically active surface area (ECSA) of each sample has been evaluated. The ECSA is proportional to the doublelayer capacitance (Cdl), which can be evaluated by CV in the nonfaradaic electrochemical potential range at different scan rates (Fig. S5) [40]. The measured Cdl values are 23.6, 19.8, and 10.6 F cm 2 for NHCS–W, NSCS-E, and NHCS–W-MF, respectively. Thus, NHCS–W has the largest ECSA and highest electrocatalytic activity for the ORR. Further kinetic investigations have been carried out to characterize the optimum catalyst NHCS–W. The LSV curves for NHCS–W at different rotating speeds are shown in Fig. S6a. The current density increases with increasing rotating speed because mass transport is improved at higher speeds. The corresponding Koutecky–Levich (K-L) plots exhibit good linearity at various potentials, indicating that the reaction is first order with respect to the dissolved O2 concentration under the investigated conditions (Fig. S6b). Based on the K-L plots, the transferred electron number (n) has been calculated as 3.71–3.83 in the potential range of
Table 2 Atomic contents (at%) of the samples determined by XPS analysis. Sample
Carbon
Nitrogen
Oxygen
NHCS–W NSCS-E NHCS–W-MF
92.7 90.6 88.4
3.8 3.7 3.2
3.5 5.7 8.4
Table 3 Relative integrated intensities (%) of carbon species determined by XPS analysis. Sample
Graphitic C
C–N
C¼O
NHCS–W NSCS-E NHCS–W-MF
77.4 74.2 65.7
18.4 16.8 15.2
4.2 9.0 19.1
Table 4 Relative integrated intensities (%) of nitrogen species determined by XPS analysis. Sample
Pyridinic N
Graphitic N
N-oxide
NHCS–W NSCS-E NHCS–W-MF
26.5 23.3 18.9
68.1 65.6 58.2
5.4 11.1 22.9
half-wave potential for NHCS–W is 0.860 V, which is 22 mV more pos itive than that of Pt/C (20%), and the limiting current densities achieved by NHCS–W and as Pt/C (20%) are similar. 5
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Fig. 5. (a) LSV curves of the catalysts in O2-saturated 0.1 M KOH at a scan rate of 10 mV s 1 and a rotation rate of 1600 rpm and (b) corresponding Tafel plots; (c) polarization and power density curves for NHCS–W and Pt/C (20%); and (d) 10 h durability test in a H2–O2 fuel cell with NHCS–W as the cathode catalyst at 60 � C.
under alkaline conditions. To investigate the structural stability of NHCS–W, SEM, TEM, and elemental mapping measurements have been obtained after the chronoamperometric test (Fig. S8). No evident changes in the morphology and microstructure of the sample are observed, confirming the high physical stability of this catalyst for the ORR in alkaline media.
Table 5 Electrocatalytic parameters of the catalysts for the ORR in 0.1 M KOH. Catalyst
Onset potential (V vs. RHE)
Half-wave potential (V vs. RHE)
Tafel slope (at low/high current densities) (mV⋅dec 1)
NHCS–W NSCS-E NHCS–WMF Pt/C (20%)
0.949 0.935 0.931
0.860 0.846 0.854
33/75 41/102 34/127
0.960
0.838
38/92
3.3. Application in fuel cells Considering the outstanding catalytic performance and stability of NHCS–W for the ORR in alkaline solution, the catalyst has been used to fabricate an MEA. An alkaline exchange membrane fuel cell (AEMFC) constructed using the MEA with carbon paper as the gas diffusion layer has been used to evaluate the single cell performance, as described in the literature [41]. The polarization and power density curves of AEMFCs employing NHCS–W and Pt/C (20%) as cathode catalysts are shown in Fig. 5c. An open-circuit voltage (OCV)/maximum power density of 0.86 V/109 mW cm 2 and 0.83 V/102 mW cm 2 are obtained with NHCS–W and Pt/C (20%), respectively. These results indicate that the catalytic activity of NHCS–W for the ORR is superior to that of Pt/C (20%) when employed as a cathode catalyst in AEMFCs. Furthermore, the NHCS–W catalyst shows high stability and little decrease in current density during a 10 h chronoamperometry test at 0.4 V (Fig. 5d).
0.40–0.60 V (Fig. S6c). This range of transferred electron numbers in dicates that the mass transfer resistance has not been completely over come. In this system, the 2e pathway for the ORR occurs together with the dominant 4e pathway, as the limiting current density does not reach the theoretical value of ~6 mA cm 2 that is expected when the reaction proceeds exclusively along the 4e pathway. The long-term stability of NHCS–W has been tested by accelerated continuous CV at a scan rate of 100 mV s 1. The results show that the potential window and catalytic activity of NHCS–W (with respect to the onset and half-wave potential) do not decrease appreciably after 10000 cycles (Figs. S7a and b). By comparison, a 45/12 mV decrease in the onset/half-wave potential is observed for Pt/C (20%) after 10000 cycles (Fig. S7c). The stabilities have also been tested by the chronoampero metric method and the current–time curves are shown in Fig. S7d. Although the current densities of both NHCS–W and Pt/C (20%) decrease with time, low attenuation is observed for NHCS–W. After 36000 s, NHCS–W retains 84.9% of its initial current density value, whereas the corresponding retention value for Pt/C (20%) is 57.3%. Thus, the NHCS–W catalyst exhibits higher stability than Pt/C (20%)
3.4. OER activity tests The rational design and synthesis of highly efficient ORR/OER bifunctional catalysts remain greatly challenging because of the large overpotential compelling the two reactions to proceed in opposite di rections away from the equilibrium state [10]. The abilities of the car bon spheres to act as electrocatalysts for the OER in 0.1 M KOH have also 6
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been evaluated, using commercial IrO2 as a control. The LSV curves and Tafel plots are shown in Fig. 6, and the overpotential and Tafel slope values are listed in Table 6. Among the as-prepared samples, NHCS–W exhibits the lowest overpotential of 362 mV, which is 24 mV lower than that of IrO2. The behavior of NHCS–W is superior to that of many highly efficient carbon-based noble-metal-free catalysts, as shown in Table S2. Electrochemical impedance spectroscopy (EIS) measurements (Fig. 6c) have been conducted and Cdl values (Fig. S9) have been measured to estimate the conductivity and ECSAs of our catalysts. The results show that NHCS–W has a charge-transfer resistance (Rct) of approximately 74 Ω, which is much lower than the values for NSCS-E (~191 Ω) and NHCS–W-MF (~299 Ω). The Cdl value for NHCS–W (14.9 mFcm 2) is also much higher than those for NSCS-E (10.7 mFcm 2) and NHCS–W-MF (5.3 mF cm 2). Again, NHCS–W shows excellent charge transformation and exhibits a high ECSA. Subsequently, the stability of the catalyst has been tested under OER conditions. The chronoamperometric test in Fig. 6d shows that after 36000 s, NHCS–W retains 72.2% of its initial current density, whereas this value is approximately 83.4% for IrO2. A chronopotentiometric test has also been performed to determine the long-term stability of the catalyst for the OER. The potential only shows a very minor change after 36000 s at 10 mA cm 2 (Fig. S11). The SEM and TEM images after the chronopotentiometric test also show no evident changes in the morphology and microstructure of NHCS–W (Fig. S12). These findings indicate that NHCS–W has high stability for the OER in alkaline electrolytes.
Table 6 Electrocatalytic parameters of the catalysts for the OER in 0.1 M KOH. Catalyst Overpotential (mV, at 10 mA cm Tafel slope (mV⋅dec 1)
2
)
NHCS–W
NSCSE
NHCS–WMF
IrO2
362 182
434 189
563 284
386 80
(j ¼ 10 mA cm 2) for the OER and E1/2 for the ORR. A small ΔE value indicates a more reversible oxygen electrode [42]. The overall activities of NHCS–W for the ORR and OER have been investigated between 0.2 and 1.8 V (vs. RHE) using a RDE in O2-saturated 0.1 M KOH at 1600 rpm, and the obtained LSV curve is shown in Fig. S13. The ΔE value is 0.732 V for NHCS–W, which is 38 mV lower than that for N-doped porous carbon (ΔE ¼ 0.770 V) [43] and 148 mV lower than that for N, S-codoped car bon nanosheets (ΔE ¼ 0.880 V) [4]. Thus, compared with most previ ously reported non-precious-metal catalysts, NHCS–W is a superior heteroatom-doped carbon bifunctional catalyst for the ORR and OER, as shown in Table S3. 3.6. Application in Zn–air batteries Encouraged by prominent bifunctional catalytic activities of NHCS–W for both the ORR and OER, we have evaluated the application of NHCS–W as a catalyst for the air electrode in a rechargeable Zn–air battery using a homemade cell (Fig. S14), as illustrated in Fig. S15. The open-circuit potential (OCP) of the Zn–air battery with NHCS–W is 1.41 V, which is similar to the value of 1.44 V for Pt/C (20%), and the OCP shows no appreciable decrease after 10 h (Fig. 7a). Fig. 7b shows the polarization and power density curves for the NHCS–W and Pt/C (20%) air electrodes. The NHCS–W catalyst exhibits a peak power
3.5. ORR/OER bifunctional activity tests Generally, the overall activity of a bifunctional oxygen catalyst can be evaluated using the potential difference (ΔE) between E
Fig. 6. (a) LSV curves for the OER at a scan rate of 10 mV s 1 in O2-saturated 0.1 M KOH and (b) corresponding Tafel plots; (c) Nyquist plots of the as-prepared samples; and (d) durability test for NHCS–W and IrO2 at 1.616 V (vs. RHE). The i-t curve for NHCS–W is the average result of three tests, and the curve for each test is shown in Fig. S10. 7
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density of 45.5 mW cm 2 (at 73.7 mA cm 2), which is similar to that of Pt/C (20%) (50.1 mW cm 2 at 81.9 mA cm 2), indicating the compara ble performance of these catalysts. The performance of the Zn–air bat tery based on NHCS–W is also superior to those of many others based on carbon or carbon-based noble-metal-free catalysts, as shown in Table S4. The charge and discharge polarization curves of the Zn–air batteries employing NHCS–W and Pt/C (20%) as air electrodes shown in Fig. 7c further confirm the comparable charge–discharge performance of these batteries. Using the commercial Pt/C (20%)-based air electrode as a reference, the robustness of the NHCS–W-based air electrode has been tested in a Zn–air battery at 10 mA cm 2 for 100 charge–discharge cycles (10 min for each cycle). The Zn–air battery employing the Pt/C (20%) air elec trode exhibits initial charge/discharge potentials of 2.05/0.91 V with a
voltage gap of 1.14 V and an initial round-trip efficiency of 44.4%. In the case of NHCS–W, the initial charge/discharge potentials are 1.96/1.14 V with a lower voltage gap of 0.82 V and a higher round-trip efficiency of 58.2%. After 100 cycles (~17 h), the Zn–air battery employing the NHCS–W air electrode shows little loss in performance, with an increase in the voltage gap of only 0.06 V and a decrease in the round-trip effi ciency of 2.4% (Fig. 7d). The robustness of the NHCS–W-based Zn–air battery has also been tested at higher current densities of 20 and 50 mA cm 2 over 100 charge–discharge cycles. Initial charge/discharge potentials of 2.04/0.98 V and 2.18/0.67 V with only 3.2% and 4.1% decreases in the round-trip efficiency are observed at 20 and 50 mA cm 2, respectively (Fig. 7e). These results confirm the high durability and excellent rechargeability of the Zn–air battery employing the NHCS–W air electrode. In the present study, we have also tested
Fig. 7. (a) Open-circuit potential test of Zn–air batteries for 10 h; (b) polarization and power density curves for NHCS–W and Pt/C (20%) air electrodes; (c) charge–discharge polarization curves for Zn–air batteries employing NHCS–W and Pt/C (20%) at a scan rate of 10 mV s 1; (d) galvanostatic charge–discharge cycling profiles for Zn–air batteries employing NHCS–W and Pt/C (20%) at 10 mA cm 2; (e) galvanostatic charge–discharge cycling profiles for the Zn–air battery employing NHCS–W at 20 and 50 mA cm 2; and (f) photograph of a red LED powered by two NHCS–W-based Zn–air batteries in series. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 8
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practical applications of the NHCS–W-based Zn–air battery. A 5 mm red light-emitting diode (~2.2 V) can be powered by two such Zn–air bat teries in series, which clearly reveals the high applicability of the NHCS–W catalyst for rechargeable Zn–air batteries (Fig. 7f and Movie S1). Supplementary data related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.227166.
Table 7 Electrocatalytic parameters of the catalysts for the HER in 1.0 M KOH. Catalyst Overpotential (mV) Tafel slope (mV⋅dec
1
)
NHCS–W
NSCS-E
NHCS–W-MF
Pt/C (20%)
196 101
276 123
305 167
64 60
recorded before and after cycling are shown in Fig. 8d. Clearly, only negligible attenuation of the current density is observed over 10000 cycles, indicating that the electrocatalytic activity does not decrease. The results of a 36000 s chronoamperometric test show that the reten tion of current density by NHCS–W (78.2%) is 38.6% higher than that for Pt/C (39.6%) (inset, Fig. 8d). This result demonstrates that NHCS–W displays superior stability in alkaline media for the HER.
3.7. HER activity tests The electrocatalytic activities of the as-prepared catalysts for the HER have been tested in 1.0 M KOH by the LSV method at a scan rate of 10 mV s 1. The HER activity of Pt/C (20%) has also been tested under the same conditions for comparison (Fig. 8a). The corresponding Tafel plots are shown in Fig. 8b, and the overpotential (at 10 mA cm 2) and Tafel slope values are listed in Table 7. Among the three as-prepared carbon sphere samples, NHCS–W exhibits the highest catalytic activity with the lowest overpotential (196 mV) and Tafel slope (101 mV dec 1). These results indicate that NHCS–W is among the most efficient heteroatom-doped carbon-based HER catalysts, as shown in Table S5. The ECSA values of the samples have been estimated by the CV method (Fig. S16). The Cdl values are 33.8, 23.7, and 5.4 mF cm 2 for NHCS–W, NSCS-E, and NHCS–W-MF, respectively. The much larger Cdl value for NHCS–W indicates a larger ECSA and thus superior electro catalytic activity for the HER. EIS has been used to investigate the electrode kinetics in the HER. The Nyquist plots of the samples are depicted in Fig. 8c. The Rct values for NHCS–W, NSCS-E, and NHCS–W-MF are approximately 73, 86, and 138 Ω, respectively. The low Rct value of NHCS–W allows an excellent charge-transfer rate to be achieved. The long-term stability of NHCS–W has been assessed by continuous CV over 10000 cycles at a scan rate of 100 mV s 1. The LSV curves
3.8. Application to overall water splitting To evaluate the practical application of NHCS–W for water elec trolysis, a water-splitting electrolyzer has been constructed, in which NHCS–W is employed as both the cathode and anode catalysts with loadings of 1.0 mg cm 2, in 1.0 M KOH at room temperature (Movie S2). The LSV curve reveals outstanding performance with a cell voltage of only 1.61 V required to reach a current density of 10 mA cm 2 with NHCS–W, which is much lower than the cell voltage of 1.78 V required with a pristine Ni foam electrode (Fig. S17a). The stability of the catalyst for water splitting has also been tested by the chronoamperometric method at 10 mA cm 2. As shown in Fig. S17b, the current density only decreases by 12.0% over 10 h, which confirms the high durability of the NHCS–W catalyst for overall water splitting under alkaline conditions. The results are comparable to the performance of state-of-the-art nonprecious-metal catalysts for overall water splitting, as shown in Table S6.
Fig. 8. (a) LSV curves for the HER in N2-saturated 1.0 M KOH and (b) corresponding Tafel plots; (c) Nyquist plots for the as-prepared samples at 0.2 V (vs. RHE); (d) LSV curves for NHCS–W before and after 10000 CV cycles, and chronoamperometric tests for NHCS–W and Pt/C (20%) at 0.196 V (vs. RHE). 9
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Journal of Power Sources 441 (2019) 227166
Supplementary data related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.227166.
multifunctional electrocatalysts for fuel cells, metal-air batteries, and water-splitting electrolyzers.
4. Discussion
Declarations of interest
Although it has been widely reported that the electrocatalytic ac tivity of carbon-based materials for the ORR, OER, and HER can be improved by doping N into the carbon structure, a greater understand ing of the effects of the type (graphitic N, pyridinic N, or pyrrolic N) and amount of nitrogen on the electrocatalytic activity of specific catalysts is required [44,45]. To determine the main active sites in NHCS–W, the pyridinic rings in the catalyst have been selectively grafted (approximate 60.8%, Fig. S18) with acetyl group at the pyridinic N atoms (denoted as N–Ac) (Scheme S1) [45]. The electrocatalytic activities for the ORR, OER and HER decrease sharply after modification (Fig. S19). This finding confirms that pyridinic N is the main active site in NHCS–W for the three reactions, which is consistent with previous reports [30,46]. However, the exis tence of active sites on other atoms, such as carbon atoms with Lewis basicity next to pyridinic N [11,47], graphitic N, topological defects and edge states [12,48], can not be eliminated. Based on our findings and previously reported results, the excellent catalytic activities of NHCS–W for the ORR, OER, and HER can be attributed to the combination of chemical functionalities (high C and N contents, particularly pyridinic N and graphitic C) and structural prop erties (high surface area and pore volume, hollow and mesoporous structure, and short diffusion paths). These factors create highly active and fully accessible catalytic sites and allow for high-flux mass trans portation [49]. In contrast, the low electrocatalytic performance of NHCS–W-MF is mainly caused by a low content of pyridinic N and graphitic C, whereas the inferior catalytic activity of NSCS-E is associ ated with impaired charge and mass transport owing to its solid structure. The high stability of NHCS–W may be attributable to its metal-free composition and the high chemical and mechanical stabilities of the catalyst active sites during electrocatalytic processes, as revealed by the microstructural characterizations after the stability tests (Fig. S8 and Fig. S12) [50]. It has been reported that carbon catalysts with nano porous and hollow structures exhibit greater resistance and oxidative stability than solid nanocarbons [13,51]. Furthermore, self-doping of nitrogen rather than introduction from an external source ensures a homogeneous distribution of the active sites in the catalyst. Thus, even if the top layer of the catalyst is destroyed during accelerated durability tests, equivalent active sites in the sublayers are exposed at the elec trochemical interface, which improves the electrocatalytic durability [49].
The authors declare no conflict of interest. Acknowledgment The authors are grateful to the National Natural Science Foundation of China (21363021, 51302222) for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227166. References [1] L.M. Dai, Y.H. Xue, L.T. Qu, H.J. Choi, J.B. Baek, Metal-free catalysts for oxygen reduction reaction, Chem. Rev. 115 (2015) 4823–4892. [2] M.H. Shao, Q.W. Chang, J.P. Dodelet, R. Chenitz, Recent advances in electrocatalysts for oxygen reduction reaction, Chem. Rev. 116 (2016) 3594–3657. [3] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q.X. Mi, E.A. Santori, N. S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473. [4] J. Zhang, L. Dai, Nitrogen, phosphorus, and fluorine tri-doped graphene as a multifunctional catalyst for self-powered electrochemical water splitting, Angew. Chem. Int. Ed. 55 (2016) 13296–13300. [5] Z.W. Seh, J. Kibsgaard, C.F. Dickens, I.B. Chorkendorff, J.K. Norskov, T. F. Jaramillo, Combining theory and experiment in electrocatalysis: insights into materials design, Science 355 (2017), eaad4998. [6] I.S. Amiinu, Z. Pu, X. Liu, K.A. Owusu, H.G.R. Monestel, F.O. Boakye, H. Zhang, S. Mu, Multifunctional Mo-N/C@MoS2 electrocatalysts for HER, OER, ORR, and Zn-air batteries, Adv. Funct. Mater. 27 (2017) 1702300. [7] P. Xiao, W. Chen, X. Wang, A review of phosphide-based materials for electrocatalytic hydrogen evolution, Adv. Energy Mater. 5 (2015) 1500985. [8] G.L. Chai, Z. Hou, D.J. Shu, T. Ikeda, K. Terakura, Active sites and mechanisms for oxygen reduction reaction on nitrogen-doped carbon alloy catalysts: stone-Wales defect and curvature effect, J. Am. Chem. Soc. 136 (2014) 13629–13640. [9] K.L. Ai, Z.L. Li, X.Q. Cui, Scalable preparation of sized-controlled Co-N-C electrocatalyst for efficient oxygen reduction reaction, J. Power Sources 368 (2017) 46–56. [10] Z.Q. Liu, H. Cheng, N. Li, T.Y. Ma, Y.Z. Su, ZnCo2O4 quantum dots anchored on nitrogen-doped carbon nanotubes as reversible oxygen reduction/evolution electrocatalysts, Adv. Mater. 28 (2016) 3777–3784. [11] D.H. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts, Science 351 (2016) 361–365. [12] M.H. Naveen, K. Shim, M.S.A. Hossain, J.H. Kim, Y.B. Shim, Template free preparation of heteroatoms doped carbon spheres with trace Fe for efficient oxygen reduction reaction and supercapacitor, Adv. Energy. Mater. 7 (2017) 1602002. [13] C. Zhang, R. Zhang, X. Li, W. Chen, PtNi nanocrystals supported on hollow carbon spheres: enhancing the electrocatalytic performance through high-temperature annealing and electrochemical CO stripping, ACS Appl. Mater. Interfaces 9 (2017) 29623–29632. [14] Y. Hu, J.O. Jensen, W. Zhang, L.N. Cleemann, W. Xing, N.J. Bjerrum, Q. Li, Hollow spheres of iron carbide nanoparticles encased in graphitic layers as oxygen reduction catalysts, Angew. Chem. Int. Ed. 53 (2014) 3675–3679. [15] Z. Jiang, Z.J. Jiang, T. Maiyalagan, A. Manthiram, Cobalt oxide-coated N- and Bdoped graphene hollow spheres as bifunctional electrocatalysts for oxygen reduction and oxygen evolution reactions, J. Mater. Chem. 4 (2016) 5877–5889. [16] M.Y. Song, D.S. Yang, K.P. Singh, J. Yuan, J.S. Yu, Nitrogen-doped hollow carbon spheres with highly graphitized mesoporous shell: role of Fe for oxygen evolution reaction, Appl. Catal. B Environ. 191 (2016) 202–208. [17] J. Chattopadhyay, T.S. Pathak, R. Srivastava, A.C. Singh, Ni nano-particle encapsulated in hollow carbon sphere electrocatalyst in polymer electrolyte membrane water electrolyzer, Electrochim. Acta 167 (2015) 429–438. [18] F. Zheng, M. He, Y. Yang, Q. Chen, Nano electrochemical reactors of Fe2O3 nanoparticles embedded in shells of nitrogen-doped hollow carbon spheres as highperformance anodes for lithium ion batteries, Nanoscale 7 (2015) 3410–3417. [19] G. Wu, K.L. More, C.M. Johnston, P. Zelenay, High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt, Science 332 (2011) 443–447. [20] R. Wang, H. Wang, T. Zhou, J. Key, Y. Ma, Z. Zhang, Q. Wang, S. Ji, The enhanced electrocatalytic activity of okara-derived N-doped mesoporous carbon for oxygen reduction reaction, J. Power Sources 274 (2015) 741–747. [21] S. Chen, Z. Chen, S. Siahrostami, D. Higgins, D. Nordlund, D. Sokaras, T.R. Kim, Y. Liu, X. Yan, E. Nilsson, R. Sinclair, J.K. Norskov, T.F. Jaramillo, Z. Bao, Designing boron nitride islands in carbon materials for efficient electrochemical synthesis of hydrogen peroxide, J. Am. Chem. Soc. 140 (2018) 7851–7859.
5. Conclusion In summary, novel N-doped hollow carbon spheres, prepared from ILs via a straightforward synthesis route, can be used as advanced electrocatalysts for the ORR, OER, and HER. The solvent, depending on the dielectric constant, affected the dis-/association of the IL precursor, which influenced the morphologies and structures of the as-prepared carbon spheres. Hollow carbon spheres were preferentially formed in water (high dielectric constant), whereas solid carbon spheres were formed in ethanol (low dielectric constant). NHCS–W exhibited excel lent multifunctional electrocatalytic activities and stabilities for the ORR, OER, and HER in alkaline media. The superior performance of NHCS–W was attributed to the hollow structure and high contents of graphitic carbon, pyridinic nitrogen, and graphitic nitrogen. Further more, excellent performance and stability were observed when NHCS–W was used as a cathode electrocatalyst in a H2–O2 fuel cell, an air elec trode in a rechargeable Zn–air battery, and both cathode and anode catalysts in a water-splitting electrolyzer. The present findings highlight the suitability of ILs for preparing nitrogen-doped carbon materials as 10
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