- Email: [email protected]
Shaddock peel derived nitrogen and phosphorus dual-doped hierarchical porous carbons as high-performance catalysts for oxygen reduction reaction
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Shaddock peel derived nitrogen and phosphorus dual-doped hierarchical porous carbons as highperformance catalysts for oxygen reduction reaction Lei Lu a,1, Jia Yu a,1, Zongdeng Wu a, Jiawei Fan a, Wu Lei a, Yu Ouyang a, Xifeng Xia a, Guangyu He b, Qingli Hao a,* a Key Laboratory for Soft Chemistry and Functional Materials, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China b Key Laboratory of Advanced Catalytic Materials and Technology, Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou, Jiangsu 213164, China
highlights
graphical abstract
One-step pyrolysis can accomplish carbonization, activation and N,Pdual doping. NPSPs own hierarchical porous structures with high BET surface areas. Higher temperature can enhance ORR
activities
with
improved
doping N/P species. NPSP-900 owns a close ORR activity compared with Pt/C, following 4e transfer way. NPSP-900
shows
a
remarkable
methanol tolerance and superior stability.
article info
abstract
Article history:
Oxygen reduction reaction (ORR) plays a key role in the application of fuel cells. Here, we
Received 13 November 2018
used shaddock peel to fabricate a set of N, P dual-doped hierarchical porous carbons,
Received in revised form
abbreviated as NPSPs, where the carbons were carbonized, activated and dual-doped via a
24 July 2019
simple one-step pyrolyzation. Contrast results indicate the contents of pyridinic-N,
Accepted 16 August 2019
graphitic-N and PeC species increase with the rising of temperatures, and the tempera-
Available online 12 September 2019
ture also affects the degree of graphitization, surface area, morphology, thus influences the ORR performance. More importantly, the NPSP-900 demonstrates an outstanding ORR activity with a comparable half-wave potential (0.83 V vs. RHE) and higher current density
* Corresponding author. E-mail addresses: [email protected], [email protected] (Q. Hao). 1 The authors contribute equally to this work. https://doi.org/10.1016/j.ijhydene.2019.08.133 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
Keywords: Shaddock peel Nitrogen phosphorus dual-doping Hierarchical porous carbon
26983
with respect to commercial Pt/C, following 4e transfer pathway. Our work illustrates that NPSP-900 is a promising cathode for fuel cells because of its simple preparation, waste utilization, excellent ORR performance, good methanol tolerance and superior stability. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Temperature Oxygen reduction reaction
Introduction Up to now, oxygen reduction reaction (ORR) has always been a bottleneck problem in the application of fuel cells due to its slow kinetics. Although Pt-based materials are the wellknown best ORR catalysts, their high costs and rare source limit their mass commercialization. Therefore, it is highly desirable to develop alternative low-cost, efficient Pt-free ORR catalysts with high stability [1e3]. Among these ORR candidates, heteroatoms (eg: N [4], P [5], B [6], S [7], I [8], etc.) doped hierarchical porous carbons, have aroused extensive attention owing to their excellent activity, stable skeleton, large surface area and porous structure. Especially for N and P, many groups have developed N, P dual doped hierarchical porous carbon (NPHPC) materials towards ORR attributed to the following reasons [9e11]. Firstly, according to the reported theoretical calculation and experimental results, when nitrogen atoms are doped into carbon framework, the difference in electronegativity (N: 3.04, C: 2.55) can induce charge polarization and change spin density towards the adjacent C atoms and then such asymmetrical charge distribution can in turn improve the oxygen adsorption mode on the catalyst surface to accelerate the OeO bond breakage [9e14]. Secondly, the lower electronegativity of P (2.19) than that of C could cause a break of electro neutralities to produce new active sites in alkaline solutions [15,16]. Thirdly, a lot of defects can be created in the nearby sites of the doping N and P atoms owing to the differences of bond length and atomic size between heteroatoms and C atoms, in addition, these defects could not only serve as catalytic active sites, but also improve the wettability of an electrode surface by the electrolyte [15,17]. Finally, hierarchical porous nanostructure can afford higher specific surface area and vast pores, which can act as additional reaction sites and ion-transport paths to further enhance their ORR performance [18,19]. To develop porous carbon frameworks, a lot of carbon sources have been studied as precursors such as polymers [20], metal-organic frameworks [21,22], and natural biomasses [23]. Among these frequently-reported carbon sources, sustainable biomass is highly desirable to derive porous carbons due to its rich resource, low cost and environment-friendly properties, such as, pistachio shells [24], enteromorpha [25], banana peels [26], bagasse [27], fermented rice [28], willow [29] and so on. Shaddock peel (SP), as a kind of waste biomass, is widely utilized to synthesize nitrogen-doped activated carbons by various methods as ORR catalysts due to its nitrogencontaining and porous structure nature, rarely for N, P dual-doped carbons. For example, Yuan, W., et al. [30] have
demonstrated a pomelo peel derived nitrogen-doped nanoporous carbon (N-PC-1000) as a metal-free electro-catalyst towards ORR using a hydrothermal treatment and subsequent 1000 C annealing under NH3 atmosphere; Wang, Nan, et al. [31] have reported a nitrogen-doped porous carbon derived from pomelo peel as an ORR catalyst by 400 C precarbonization, 550 C KOH-activation and finally 900 C annealing under NH3 atmosphere. Nevertheless, some serious problems still exist in the preparation process. Firstly, doping nitrogen by NH3 annealing method needs long-time tail gas treatment. Secondly, multistep activation is a timeconsuming and complicated technology, and it will cause a great loss of carbon source. Thirdly, phosphor doping is ignored. Finally, there is no research about effect of temperature on the doped N, P modes and morphology changes. Herein, shaddock peel (SP) waste was initially soaked in 4 M NaOH þ 0.7 M (NH4)3PO4 þ 0.7 M urea solution under vacuum condition, to achieve thoroughly impregnation in a short time in a fume hood. Then the dried SP precursor was carbonized, activated and N,P dual-doped in one-step pyrolysis treatment under N2 atmosphere for 2 h with different temperatures. The samples were marked as NPSP-temperature (600, 700, 800 and 900). According to the contrast results, it was found that the contents of nitrogen and phosphorus species exhibited a disciplinary change, indicating the contents of pyridinic-N, graphitic-N and PeC increase with the rising of temperatures. The mentioned N, P species are well-known ORR active sites [9,32e34], especially for pyridinic-N. In addition, temperature can play a positive role in tuning the degree of graphitization, surface area and morphology, so the NPSP-900 exceeds the other three in ORR catalytic activity. What’s more, when compared with the commercial (20%) Pt/C, the NPSP-900 shows higher current density and a comparable 0.83 V (vs. RHE) half-wave potential in 0.1 M KOH solutions. The calculated electron transfer numbers (n) suggest that all the NPSPs follow 4e transfer pathway during the oxygen reduction process. Last but not least, the unchanged polarization curve after 2500 cycles and the 91.2% current retention for 16 h rotating of NPSP-900 demonstrate its outstanding long term stability, which together with its excellent methanol tolerance could further support its potential application in fuel cells.
Experimental Synthesis of NPSPs The shaddock peel (SP) was collected from the shaddocks bought from the market. The SP was firstly washed with the
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
ethanol-water mixture for five times and then dried in 60 C oven. Then the clean and dry SP was cut into small pieces (V < 1 cm3). 2 g SP pieces were soaked in a conical flask with 100 mL 4M NaOH solution, then a certain amount of (NH4)3PO4 and urea (CH4N2O) were added and stirred to achieve a 4M NaOH þ 0.7 M (NH4)3PO4 þ 0.7 M urea mixture. The whole process was performed in a fume hood due to the generated NH3. After the solid dopants were totally dissolved, the conical flask was placed in a vacuum doughnut. After repeating vacuumizing and air inflation operation until the SP was completely soaked in the solution and then stayed at vacuum state over night. After that, the soaked brown SP was taken out and dried in 100 C oven. Then the prepared SP was heated to 600 Ce900 C and kept for 2 h with a heating rate of 5 C/ min in N2 atmosphere. After cooling down, the N,P-doped SP carbon was successively washed with dilute hydrochloric acid solution and water until the pH z 7. After the final freezedrying, the samples at different pyrolysis temperatures were marked as NPSP-600, NPSP-700, NPSP-800 and NPSP-900, respectively.
1 j ¼ 1
=
26984
jk
þ1 ¼1 þ1 jd jk ðBu1=2 Þ
1=6 CO2 B ¼ 0:62nFD2=3 O2 y
Results and discussion
X-ray photoelectron spectroscopy (XPS) tests were executed on Thermo ESCALAB 250 using a monochromic Al X-ray source. X-ray diffraction (XRD) characterization was obtained on a Bruker D8 Advance diffractometer using a Cu-Ka radiation (l ¼ 0.15406 nm) with a scanning range of 10 ~80 . The Brunauer-Emmett-Teller (BET) data were performed on an Accelerated Surface Area and Porosimetry (ASAP) 2020 system at 77 K. Raman spectra were characterized on a LabRam Aramis (HORIBA JobinYvon S.A.S) using a 514 nm laser, Morphology results of NPSPs were obtained on a field emission gun scanning electron microscope (SEM, Quant 250FEG) and a transmission electron microscope (TEM, JEOL JEM-2100).
Materials characterization
The electrochemical measurements for ORR activities of NPSPs were performed on a rotating ring disk electrode (ALSRRDE-3A) by a three-electrode cell configuration in 0.1 M KOH aqueous solutions at room temperature. A graphite rod, an Ag/AgCl (saturated KCl) and a rotating disk electrode (RDE, 3 mm) were employed as counter, reference and working electrodes, respectively. 1 mg sample powder was equably dispersed in 500 mL aqueous solution (25 mL 5 wt% Nafion þ 475 mL DI water) by ultrasonication. Then 10 mL of asprepared inky catalyst dispersion was carefully dropped onto the RDE with a mass loading of 283 mg cm2. Commercial 20% Pt/C electrode was modified by the same method with the same mass loading. All the potentials in this work vs. RHE are calculated using the Nernst equation: ERHE ¼ EAg/AgCl þ 0.0591*pH þ0.197 V. Cyclic voltammetry (CV) was performed in the N2-saturated and O2-saturated 0.1 M KOH with a scan rate of 10 mV s1. Linear sweep voltammetry (LSV) was tested in O2-saturated 0.1 M KOH solution applying different rotating speeds, including 400, 625, 900, 1225, 1600, 2025 rpm, with a scan rate of 10 mV s1. The electron transferred numbers (n) of NPSP-900 was calculated by the following equations [35]:
(2)
where j, jk and jd are the disk current density, kinetic current density, and the diffusion-limiting current density, respectively; B is the Levich constant and u is the rotating rate. F (96485 C mol1) is Faradic constant; DO2 (1.86 105 cm2 s1) is the diffusion coefficient of O2 in the electrolyte; y (0.01 cm2 s1) is kinematic viscosity of the electrolyte; CO2 (1.21 106 mol cm3) is the O2 concentration in the electrolyte. Methanol tolerance property was measured by currenttime (i-t) chronoamperometry in O2-saturated 0.1 M KOH with a rotating speed of 1600 rpm at the potential of 0.3 V (vs. Ag/AgCl), where 2M CH3OH was added into the electrolyte at about 700s. The long-term durability was characterized by the polarization curves initially and after 2500 cycles by continuous CV scanning for 2500 cycles in O2-saturated 0.1 M KOH.
Characterization
Electrochemical measurements
(1)
The chemical composition of all NPSPs was investigated by X-ray photoelectron spectroscopy (XPS). The XPS survey spectra (Fig. S1) shows the coexistence of C, O, N, P and the calculation comparison of elemental content (Table 1) for each sample, which confirms the successful doping of both nitrogen and phosphorus for all the NPSPs. It is noteworthy that the carbon content increases while the oxygen content decreases with rising temperature due to the poor thermal stability of some oxygen species, therefore the NPSP-900 owns the highest carbon content (81.3 at%) and the lowest oxygen content (11.2 at%). Moreover, the N/C and P/C atom ratios of these samples were also displayed in Table 1, which reflect that the high temperature may be more beneficial to the total doping content of nitrogen instead of phosphorus. The high-resolution N1s and P2p spectra of each NPSP were analyzed and shown in Fig. 1 and Table 1 for comparison. All the N1s spectra of four NPSP materials can be typically deconvoluted into four types: 398.5 ± 0.2 eV, 399.8 ± 0.2 eV, 401.2 ± 0.2 eV and 403.0 ± 0.2 eV, which are assigned to pyridinic-N (N1), pyrrolic-N (N2), graphitic-N (N3), and pyridine-N-oxide groups (N4), respectively. Evident N4 peaks are shown in the N1s spectra of the NPSP-600 and NPSP-700, while there are no corresponding signals found in those of the NPSP-800 or NPSP-900. According to the previous theoretical and experimental results, pyridinic-N (N1) is proved to be the most active N specie for oxygen reduction and the graphitic-N (N3) can also play an important role to enhance ORR performance [9,32e34]. In this case, the N1 content and N1/N ratio are both increased by the order of NPSP-600 < NPSP700 < NPSP-800 < NPSP-900. Furthermore, the graphitic-N (N3) content and N3/N ratios also follow this trend. These results indicate that the higher temperature can not only increase of N1, but also improve the graphitization degree of carbon materials. For the phosphorus doping, the P2p spectra of NPSPs can be divided into two types: 132.4 ± 0.2 eV, 133.5 ± 0.2 eV,
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
26985
Table 1 e Elemental content of NPSPs calculated by XPS results. Sample
NPSP-600 NPSP-700 NPSP-800 NPSP-900
At% C1s
O1s
73.1 74.9 75.8 81.3
21.1 18.2 16.7 11.2
corresponding to the features of PeC (P1) and PeO (P2) [5], respectively. It is obvious that the higher temperature favors more P1 and less P2. The PeO phase means that P is in oxidation state, referring to surface oxidized P species. While the lower value of 132.4 eV is attributed PeC direct bonding, revealing the P atom has been incorporated into the carbon framework [16,36,37]. As known, PeC can change the charge and spin density resulted from the lower electronegativity of P compared with C, and structural defects could be introduced by the bigger size of P atom to offer as catalytic active sites [15,16]. Although the phosphorus content decreases when temperature goes up, the PeC content increases with the rising of temperature. The increased N1, N3 and P1 can all contribute to the ORR activities of higher-temperature catalysts. XRD patterns (Fig. 2) of all as-prepared NPSPs exhibit a broad peak located at about 25 , which is accord with the (002) diffraction, indicating the amorphous structure of the NPSPs [38]. In addition, the intensity of the (002) diffraction peak increases to some extent with the rising of temperature, revealing the positive effect of high temperature on the graphitization of NPSPs. Interestingly, there is a weak peak at 43.7 in the XRD pattern of NPSP-900, which originates from a graphite-like nature, indicating an improvement of graphitization degree [39]. The Raman spectra of NPSP are displayed in the Fig. 2b, where the characteristic D and G bands of carbon materials locate at about 1350 cm1 and 1590 cm1,
N1s (N1, N2, N3, N4/N) 1.1 (22.2, 4.1 (27.0, 5.2 (29.3, 4.9 (34.3,
13.0, 34.0, 30.8) 18.7, 45.0, 9.23) 21.3, 49.4, 0) 8.1, 57.6, 0)
P2p (P1, P2/P) 4.7 (22.7, 2.9 (36.6, 2.3 (55.2, 2.6 (59.3,
77.3) 63.4) 44.8) 40.7)
respectively. The D band represents defects and disordered areas and the G band is identified as the graphitized region. So ID/IG is always used to evaluate the integrity degree of graphite structure. As the calculated ID/IG ratio of each sample shown in Fig. 2, the higher temperature affords higher degree of graphitization. Thus, NPSP-900 exhibits the highest graphitization, which is accord with the XPS and XRD results. Fig. 2c shows the adsorption-desorption BET isotherms of NPSPs, in which all the isotherms give a dominant type Ⅳ plots. The BET surface area increased by: NPSP-600 (301 m2 g1) < NPSP-700 (361 m2 g1) < NPSP-800 (503 m2 g1) < NPSP-900 (548 m2 g1). The BJH pore size distributions of the NPSPs in the Fig. 2d suggest that all the NPSPs mainly reveal ~4 nm mesoporous structure. The NPSP-800 owns increased capillary condensation, indicating improved mesoporous porosity, while NPSP900 shows higher micropore distribution. The SEM images are applied to characterize the morphologies of the NPSPs materials with relatively larger scale, as shown in Fig. 3. It can be seen that with the rising of annealing temperature, the smaller size carbon sheets are derived from original SP. When temperature goes up, the SP and dopants can be decomposed more sufficiently and more gases generate from the SP inner part, which causes more macropore morphologies and rougher surfaces. NPSP-800 and NPSP900 both exhibit better macropore dispersion than the NPSP-600 or NPSP-700 do.
Fig. 1 e N1s high-resolution XPS spectra comparison of NPSP-600 (a), NPSP-700 (b), NPSP-800 (c), NPSP-900 (d) with the P2p high-resolution XPS spectra of NPSP-600 (e), NPSP-700(f), NPSP-800 (g), NPSP-900 (h), respectively. N1: pyridinic-N, N2: pyrrolic-N, N3: graphitic-N, N4: pyridine-N-oxide groups; P1: PeC, P2: PeO.
26986
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
Fig. 2 e XRD patterns (a), Raman spectra (b) and BET isotherms (c) of NPSP samples with their corresponding pore size distributions (d).
The pore structure of each NPSP sample was further investigated by TEM technology, as shown in Fig. 4. Rich micropores and mesopores can be easily observed in the TEM images of all the NPSPs, and the pore sizes (<5 nm) are in accord with the BJH pore size distributions. Obviously, NPSP900 and NPSP-800 present thinner carbon sheets than
NPSP-700 or NPSP-600 do, which indicates more sufficient carbonization under higher annealing temperature and agrees with the SEM results. SEM and TEM images together illustrate the hierarchical pores construction of the NPSPs, including micropores, mesopores and macropores, which are very advantageous to improve ORR activities.
Fig. 3 e SEM images of NPSP-600 (a), NPSP-700 (b), NPSP-800 (c) and NPSP-900 (d).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
26987
Fig. 4 e TEM images of NPSP-600 (a), NPSP-700 (b), NPSP-800 (c) and NPSP-900 (d).
ORR performance The ORR performance of each NPSP catalyst were measured and compared by the CV and LSV polarization curves in Fig. 5. A typical quasi-rectangular CV curve of carbon material was obtained for each NPSP under N2-saturated electrolyte without
any redox peak. While in O2-saturated electrolyte, well-defined oxygen reduction peaks were observed at different potentials for all NPSP catalysts, suggesting that all the samples have ORR catalytic activities. Furthermore, it is found that NPSP-900 shows the most positive reduction peak potential and highest current than those of other samples as shown in Fig. 5a.
Fig. 5 e (a) CV curves of NPSP samples in N2-saturated (black) and O2-saturated (red) 0.1 M KOH with a scanning rate of 10 mV s¡1. (b) LSV curves of each NPSP catalyst and commercial Pt/C in O2-saturated 0.1 M KOH at a scan rate of 10 mV s¡1 with 1600 rpm rotating. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
26988
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
Fig. 6 e (aed) LSV curves of NPSP-600, NPSP-700, NPSP-800 and NPSP-900 at a scan rate of 10 mV s¡1 under different rotating speed in O2-saturated 0.1 KOH solutions, with the K-L plots inset of the former three, respectively. (e) The K-L plots at varied potentials of NPSP-900, (f) the transferred electron number (n) of the NPSPs.
Therefore, the ORR performance in CV curves increases as follows: NPSP-600 < NPSP-700 < NPSP-800 < NPSP-900. The LSV polarization curves on a rotating-disk electrode for the NPSPs and commercial Pt/C (20%) were also performed in O2-saturated 0.1 M KOH with a rotation speed of 1600 rpm for comparison. Similar trend was also observed in LSV case, the NPSP900 demonstrates a half-potential of 0.83 V (vs. RHE), which is evidently more positive than those of NPSP-800 (0.81 V vs. RHE), NPSP-700 (0.78 V vs. RHE) and NPSP-600 (0.76 V vs. RHE). In addition, current densities are also improved as this order: NPSP-600 < NPSP-700 < NPSP-800 < NPSP-900, corresponding to the CV results. More importantly, NPSP-900, as a metal-free catalyst, manifests very close ORR half-wave potential and higher current density when compared with commercial Pt/C
(0.85 V vs. RHE) in alkaline medium. To further explore the intrinsic activities of the NPSPs, the current densities normalized by electrochemical active surface areas and mass loading are calculated and displayed in Figs. S2e3, respectively. The results further demonstrate the NPSP-900 owns the best ORR activity of the NPSPs, and detail description is shown in supporting materials. The ORR polarization curves of NPSPs were obtained with different rotating speeds from 400 to 2025 rpm, as shown in Fig. 6aed with the corresponding Kouteckye-Levich (K-L) plots inset. All the KouteckyeLevich (K-L) plots at varied potentials display good linearity and parallelism for all the NPSP catalysts, revealing first-order reaction kinetics toward the dissolved O2 concentration and adjacent electron transfer
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
numbers (n). As for NPSP-900 in Fig. 6e, it can be seen that K-L plots at 0.3e0.7 V (vs. RHE) are almost parallel with the standard plot for 4e transferred way. The electrons transferred per O2 for all the NPSP catalysts were calculated based on the corresponding K-L plots according to the Equations (1) and (2), as shown in Fig. 6f. It demonstrates that all the NPSP materials follow a dominant four-electron-transfer pathway and the NPSP-900 gives an average n of 4.01, higher than those of NPSP-800 (3.95), NPSP-700 (3.83) and NPSP-600 (3.60), further suggesting the superior ORR activity of NPSP-900. All the results suggest that NPSP-900 owns better catalytic activity with respect to the other three. The reasons for its superior ORR activity can be explained as below: Firstly, the highest doping N, pyridinic-N (N1) and PeC content affords more active sites for oxygen reduction than the other samples do, Secondly, the highest graphitic-N (N3) content and lowest ID/IG both indicate that NPSP-900 owns highest degree of graphitization, and deeper graphitization is beneficial for the electrical conductivity of materials, Thirdly, the BET proves the largest surface area for NPSP-900 among these NPSPs, and ORR is an interfacial/surface reaction, the O2 adsorption or dissociation mainly occurs on the interface/surface of catalyst, therefore, large surface area and hierarchical pores can make remarkable contribution for ORR performance. The electrocatalytic selectivity of ORR catalysts is very important in the cathode application of fuel cells. Therefore, the methanol crossover effect of NPSP-900 was also examined
26989
using chronoamperometry in the presence of methanol with comparison of commercial Pt/C as shown in Fig. 7a, where the reduction currents in O2-saturated solutions before adding CH3OH are set as 1. After the addition of 2 M CH3OH to electrolyte at about 700 s, an avalanche current decrease for Pt/C appears resulted from the inhibition of methanol oxidation [40], while hardly any current change was observed for the NPSP-900 electrode, suggesting that methanol could hardly affect the ORR activity of NPSP-900. On the other hand, stability is a quite critical index to evaluate an ORR catalyst in the potential application of fuel cells. We examined the stability performance by the comparison between initial polarization curves (1600 rpm) and those after 2500 cycles at NPSP-900 and Pt/C electrodes in the O2-saturated 0.1 M KOH. Very encouraging as shown in Fig. 7b, indistinguishable change of ORR current or potential between the 1st and 2500th polarization curves illustrates a stable catalytic activity sustained by NPSP900. While for commercial Pt/C, it suffers an obvious current decrease and potential shift after cycling 2500 times, revealing a serious degeneration of ORR activity. In addition, as shown in Fig. 7d, after rotated for 16 h under 1600 rpm in O2-saturated 0.1 M KOH, NPSP-900 still remains about 91.2% of initial current, much better than some recently reported doped biomass derived ORR catalysts [41e43]. With the excellent catalytic activity, methanol tolerance and stability, NPSP-900 could serve as a promising catalyst for ORR to replace commercial Pt/C in alkaline fuel cells.
Fig. 7 e (a) Current-time responses of NPSP-900 and Pt/C RDEs at ¡0.3 V (Ag/AgCl) with a rotating speed of 1600 rpm, where 2.0 M methanol was added into the electrolyte at about 700 s. Polarization curves at 1600 rpm for NPSP-900 (b) and Pt/C (c) initially and after 2500 cycles. Current-time response of NPSP-900 for 16 h in O2-saturated 0.1 M KOH at 0.7 V (vs. RHE) with a rotating speed of 1600 rpm. All the tests were obtained in O2-saturated 0.1 M KOH solutions.
26990
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
Conclusion In conclusion, we have successfully proposed a simple onestep pyrolysis approach to synthesize NPSPs with N, P dualdoped hierarchical porous structures, which can set carbonization, activation and doping into the same heating process. Temperature plays an important role to influence the degree of graphitization, surface area and morphologies. With higher annealing temperature, NPSP-900 gives higher contents of pyridinic-N, graphitic-N and PeC, all of which are catalytic active species. Beyond the other three NPSPs, the NPSP-900 suggests a more positive half-wave potential (0.83 V vs. RHE) and a higher current density, which is close to the commercial Pt/C. What’s more, the NPSP-900 holds a superior methanol tolerance and much better stability with respect to commercial Pt/C. Therefore, the advantages of our work, simple preparation, waste utilization, excellent ORR performance, good methanol tolerance and superior stability of NPSP-900 could support its potential application in fuel cells.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21576138, 51572127), China-Israel Cooperative Program (2016YFE0129900), Program for NCET-12-0629, Natural Science Foundation of Jiangsu Province (BK20160828), Post-Doctoral Foundation (1501016B), PAPD of Jiangsu Province, and the program for Science and Technology Innovative Research Team in Universities of Jiangsu Province, China.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.133.
references
[1] Qu L, Liu Y, Baek J-B, Dai L. Nitrogen-doped graphene as efficient metal-free electrocatalyst for oxygen reduction in fuel cells. ACS Nano 2010;4:1321e6. [2] Ben Liew K, Daud WRW, Ghasemi M, Leong JX, Su Lim S, Ismail M. Non-Pt catalyst as oxygen reduction reaction in microbial fuel cells: a review. Int J Hydrogen Energy 2014;39:4870e83. [3] He Q, Cairns EJ. Review-recent progress in electrocatalysts for oxygen reduction suitable for alkaline anion exchange membrane fuel cells. J Electrochem Soc 2015;162:F1504e39. [4] Sui ZY, Li X, Sun ZY, Tao HC, Zhang PY, Zhao L, Han B-H. Nitrogen-doped and nanostructured carbons with high surface area for enhanced oxygen reduction reaction. Carbon 2018;126:111e8. [5] Lei W, Deng YP, Li G, Cano ZP, Wang X, Luo D, Liu Y, Wang D, Chen Z. Two-dimensional phosphorus-doped carbon nanosheets with tunable porosity for oxygen reactions in zinc-air batteries. ACS Catal 2018;8:2464e72.
[6] Ishii T, Maie T, Kimura N, Kobori Y, Imashiro Y, Ozaki J-i. Enhanced catalytic activity of nanoshell carbon co-doped with boron and nitrogen in the oxygen reduction reaction. Int J Hydrogen Energy 2017;42:15489e96. [7] Liang J, Jiao Y, Jaroniec M, Qiao SZ. Sulfur and nitrogen dualdoped mesoporous graphene electrocatalyst for oxygen reduction with synergistically enhanced performance. Angew Chem 2012;124:11664e8. [8] Yao Z, Nie H, Yang Z, Zhou X, Liu Z, Huang S. Catalyst-free synthesis of iodine-doped graphenevia a facile thermal annealing process and its use for electrocatalytic oxygen reduction in an alkaline medium. Chem Commun 2012;48:1027e9. [9] Chai G-L, Qiu K, Qiao M, Titirici M-M, Shang C, Guo Z. Active sites engineering leads to exceptional ORR and OER bifunctionality in P, N co-doped graphene frameworks. Energy Environ Sci 2017;10:1186e95. ~ ldsepp E, Johansson L-S, [10] Borghei M, Laocharoen N, Kibena-Po Campbell J, Kauppinen E, Tammeveski K, Rojas OJ, Porous N. P-doped carbon from coconut shells with high electrocatalytic activity for oxygen reduction: alternative to Pt-C for alkaline fuel cells. Appl Catal B Environ 2017;204:394e402. [11] Bai J, Zhu Q, Lv Z, Dong H, Yu J, Dong L. Nitrogen-doped graphene as catalysts and catalyst supports for oxygen reduction in both acidic and alkaline solutions. Int J Hydrogen Energy 2013;38:1413e8. [12] Li H, Liu H, Jong Z, Qu W, Geng D, Sun X, Wang H. Nitrogendoped carbon nanotubes with high activity for oxygen reduction in alkaline media. Int J Hydrogen Energy 2011;36:2258e65. [13] Oh H-S, Oh J-G, Lee WH, Kim H-J, Kim H. The influence of the structural properties of carbon on the oxygen reduction reaction of nitrogen modified carbon based catalysts. Int J Hydrogen Energy 2011;36:8181e6. [14] Wang L, Yin F, Yao C. N-doped graphene as a bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions in an alkaline electrolyte. Int J Hydrogen Energy 2014;39:15913e9. [15] Choi CH, Park SH, Woo SI. Binary and ternary doping of nitrogen, boron, and phosphorus into carbon for enhancing electrochemical oxygen reduction activity. ACS Nano 2012;6:7084e91. [16] Liu Z-W, Peng F, Wang H-J, Yu H, Zheng W-X, Yang J. Phosphorus-doped graphite layers with high electrocatalytic activity for the O2 reduction in an alkaline medium. Angew Chem Int Ed 2011;50:3257e61. [17] Zhao Y-Q, Lu M, Tao P-Y, Zhang Y-J, Gong X-T, Yang Z, Zhang G-Q, Li H-L. Hierarchically porous and heteroatom doped carbon derived from tobacco rods for supercapacitors. J Power Sources 2016;307:391e400. [18] Liang H-W, Zhuang X, Bru¨ller S, Feng X, Mu¨llen K. Hierarchically porous carbons with optimized nitrogen doping as highly active electrocatalysts for oxygen reduction. Nat Commun 2014;5:4973. [19] Liang J, Du X, Gibson C, Du XW, Qiao SZ. N-doped graphene natively grown on hierarchical ordered porous carbon for enhanced oxygen reduction. Adv Mater 2013;25:6226e31. [20] Zhang J, Zhao Z, Xia Z, Dai L. A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions. Nat Nanotechnol 2015;10:444. [21] Xia BY, Yan Y, Li N, Wu HB, Lou XW, Wang X. A metaleorganic framework-derived bifunctional oxygen electrocatalyst. Nat Energy 2016;1:15006. [22] Zhou H, He D, Saana AI, Yang J, Wang Z, Zhang J, Liang Q, Yuan S, Zhu J, Mu S. Mesoporous-silica induced doped
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
carbon nanotube growth from metaleorganic frameworks. Nanoscale 2018;10:6147e54. Chen P, Wang L-K, Wang G, Gao M-R, Ge J, Yuan W-J, Shen YH, Xie A-J, Yu S-H. Nitrogen-doped nanoporous carbon nanosheets derived from plant biomass: an efficient catalyst for oxygen reduction reaction. Energy Environ Sci 2014;7:4095e103. Wu F-C, Tseng R-L, Hu C-C. Comparisons of pore properties and adsorption performance of KOH-activated and steam-activated carbons. Micropor Mesopor Mat 2005;80:95e106. Lu L, Jiao X, Fan J, Lei W, Ouyang Y, Xia X, Xue Z, Hao Q. Cobalt ferrite on honeycomb-like algae-derived nitrogendoped carbon for electrocatalytic oxygen reduction and ultra-cycle-stable lithium storage. Electrochim Acta 2019;295:461e71. Liu B, Zhang L, Qi P, Zhu M, Wang G, Ma Y, Guo X, Chen H, Zhang B, Zhao Z, Dai B, Yu F. Nitrogen-doped banana peelederived porous carbon foam as binder-free electrode for supercapacitors. Nanomaterials 2016;6:18. Hao P, Zhao Z, Tian J, Li H, Sang Y, Yu G, Cai H, Liu H, Wong CP, Umar A. Hierarchical porous carbon aerogel derived from bagasse for high performance supercapacitor electrode. Nanoscale 2014;6:12120e9. Gao S, Chen Y, Fan H, Wei X, Hu C, Luo H, Qu L. Large scale production of biomass-derived N-doped porous carbon spheres for oxygen reduction and supercapacitors. J Mater Chem A 2014;2:3317e24. Ma Y, Zhao J, Zhang L, Zhao Y, Fan Q, Li X a, Hu Z, Huang W. The production of carbon microtubes by the carbonization of catkins and their use in the oxygen reduction reaction. Carbon 2011;49:5292e7. Yuan W, Feng Y, Xie A, Zhang X, Huang F, Li S, Zhang X, Shen Y. Nitrogen-doped nanoporous carbon derived from waste pomelo peel as a metal-free electrocatalyst for the oxygen reduction reaction. Nanoscale 2016;8:8704e11. Wang N, Li T, Song Y, Liu J, Wang F. Metal-free nitrogendoped porous carbons derived from pomelo peel treated by hypersaline environments for oxygen reduction reaction. Carbon 2018;130:692e700. Guo D, Shibuya R, Akiba C, Saji S, Kondo T, Nakamura J. Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science 2016;351:361e5.
26991
[33] Lu L, Fan J, Lei W, Ouyang Y, Yao D, Xia X, Hao Q. Multiple metal (Cu, Mn, Fe) centered species simultaneously combined nitrogen-doped graphene as an electrocatalyst for oxygen reduction in alkaline and neutral solutions. ChemCatChem 2018;10:2471e80. [34] Lu L, Hao Q, Lei W, Xia X, Liu P, Sun D, Wang X, Yang X. WellCombined magnetically separable hybrid cobalt ferrite/ nitrogen-doped graphene as efficient catalyst with superior performance for oxygen reduction reaction. Small 2015;11:5833e43. [35] Bard AJ, Faulkner LR. Electrochemical methods: fundamentals and aplications. Wiley; 2001. [36] Yang D-S, Bhattacharjya D, Inamdar S, Park J, Yu J-S. Phosphorus-doped ordered mesoporous carbons with different lengths as efficient metal-free electrocatalysts for oxygen reduction reaction in alkaline media. J Am Chem Soc 2012;134:16127e30. [37] Wu J, Yang Z, Li X, Sun Q, Jin C, Strasser P, Yang R. Phosphorusdoped porous carbons as efficient electrocatalysts for oxygen reduction. J Mater Chem A 2013;1:9889e96. [38] Su F, Poh CK, Chen JS, Xu G, Wang D, Li Q, Lin J, Lou XW. Nitrogen-containing microporous carbon nanospheres with improved capacitive properties. Energy Environ Sci 2011;4:717e24. [39] Ma W, Xie L, Dai L, Sun G, Chen J, Su F, Cao Y, Lei H, Kong Q, Chen C-M. Influence of phosphorus doping on surface chemistry and capacitive behaviors of porous carbon electrode. Electrochim Acta 2018;266:420e30. [40] Schmidt TJ, Gasteiger HA, Behm RJ. Methanol electrooxidation on a colloidal PtRu-alloy fuel-cell catalyst. Electrochem Commun 1999;1:1e4. [41] Liu Z, Li Z, Tian S, Wang M, Sun H, Liang S, Chang Z, Lu G. Conversion of peanut biomass into electrocatalysts with vitamin B12 for oxygen reduction reaction in Zn-air battery. Int J Hydrogen Energy 2019;44:11788e96. [42] Wang Y, Zuo S, Miao M, Liu Y, Gu Z, Jin Y. Cost-effective preparation of metal-free electrocatalysts by phosphoric acid activation of lignocellulosic materials for oxygen reduction reaction. Int J Hydrogen Energy 2019;44:2811e22. [43] Mao X, Cao Z, Chen S, Jia J, Li X, Yin Y, Yang S. Facile synthesis of N, P-doped hierarchical porous carbon framework catalysts based on gelatin/phytic acid supermolecules for electrocatalytic oxygen reduction. Int J Hydrogen Energy 2019;44(12):5890e8.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Shaddock peel derived nitrogen and phosphorus dual-doped hierarchical porous carbons as highperformance catalysts for oxygen reduction reaction Lei Lu a,1, Jia Yu a,1, Zongdeng Wu a, Jiawei Fan a, Wu Lei a, Yu Ouyang a, Xifeng Xia a, Guangyu He b, Qingli Hao a,* a Key Laboratory for Soft Chemistry and Functional Materials, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China b Key Laboratory of Advanced Catalytic Materials and Technology, Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou, Jiangsu 213164, China
highlights
graphical abstract
One-step pyrolysis can accomplish carbonization, activation and N,Pdual doping. NPSPs own hierarchical porous structures with high BET surface areas. Higher temperature can enhance ORR
activities
with
improved
doping N/P species. NPSP-900 owns a close ORR activity compared with Pt/C, following 4e transfer way. NPSP-900
shows
a
remarkable
methanol tolerance and superior stability.
article info
abstract
Article history:
Oxygen reduction reaction (ORR) plays a key role in the application of fuel cells. Here, we
Received 13 November 2018
used shaddock peel to fabricate a set of N, P dual-doped hierarchical porous carbons,
Received in revised form
abbreviated as NPSPs, where the carbons were carbonized, activated and dual-doped via a
24 July 2019
simple one-step pyrolyzation. Contrast results indicate the contents of pyridinic-N,
Accepted 16 August 2019
graphitic-N and PeC species increase with the rising of temperatures, and the tempera-
Available online 12 September 2019
ture also affects the degree of graphitization, surface area, morphology, thus influences the ORR performance. More importantly, the NPSP-900 demonstrates an outstanding ORR activity with a comparable half-wave potential (0.83 V vs. RHE) and higher current density
* Corresponding author. E-mail addresses: [email protected], [email protected] (Q. Hao). 1 The authors contribute equally to this work. https://doi.org/10.1016/j.ijhydene.2019.08.133 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
Keywords: Shaddock peel Nitrogen phosphorus dual-doping Hierarchical porous carbon
26983
with respect to commercial Pt/C, following 4e transfer pathway. Our work illustrates that NPSP-900 is a promising cathode for fuel cells because of its simple preparation, waste utilization, excellent ORR performance, good methanol tolerance and superior stability. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Temperature Oxygen reduction reaction
Introduction Up to now, oxygen reduction reaction (ORR) has always been a bottleneck problem in the application of fuel cells due to its slow kinetics. Although Pt-based materials are the wellknown best ORR catalysts, their high costs and rare source limit their mass commercialization. Therefore, it is highly desirable to develop alternative low-cost, efficient Pt-free ORR catalysts with high stability [1e3]. Among these ORR candidates, heteroatoms (eg: N [4], P [5], B [6], S [7], I [8], etc.) doped hierarchical porous carbons, have aroused extensive attention owing to their excellent activity, stable skeleton, large surface area and porous structure. Especially for N and P, many groups have developed N, P dual doped hierarchical porous carbon (NPHPC) materials towards ORR attributed to the following reasons [9e11]. Firstly, according to the reported theoretical calculation and experimental results, when nitrogen atoms are doped into carbon framework, the difference in electronegativity (N: 3.04, C: 2.55) can induce charge polarization and change spin density towards the adjacent C atoms and then such asymmetrical charge distribution can in turn improve the oxygen adsorption mode on the catalyst surface to accelerate the OeO bond breakage [9e14]. Secondly, the lower electronegativity of P (2.19) than that of C could cause a break of electro neutralities to produce new active sites in alkaline solutions [15,16]. Thirdly, a lot of defects can be created in the nearby sites of the doping N and P atoms owing to the differences of bond length and atomic size between heteroatoms and C atoms, in addition, these defects could not only serve as catalytic active sites, but also improve the wettability of an electrode surface by the electrolyte [15,17]. Finally, hierarchical porous nanostructure can afford higher specific surface area and vast pores, which can act as additional reaction sites and ion-transport paths to further enhance their ORR performance [18,19]. To develop porous carbon frameworks, a lot of carbon sources have been studied as precursors such as polymers [20], metal-organic frameworks [21,22], and natural biomasses [23]. Among these frequently-reported carbon sources, sustainable biomass is highly desirable to derive porous carbons due to its rich resource, low cost and environment-friendly properties, such as, pistachio shells [24], enteromorpha [25], banana peels [26], bagasse [27], fermented rice [28], willow [29] and so on. Shaddock peel (SP), as a kind of waste biomass, is widely utilized to synthesize nitrogen-doped activated carbons by various methods as ORR catalysts due to its nitrogencontaining and porous structure nature, rarely for N, P dual-doped carbons. For example, Yuan, W., et al. [30] have
demonstrated a pomelo peel derived nitrogen-doped nanoporous carbon (N-PC-1000) as a metal-free electro-catalyst towards ORR using a hydrothermal treatment and subsequent 1000 C annealing under NH3 atmosphere; Wang, Nan, et al. [31] have reported a nitrogen-doped porous carbon derived from pomelo peel as an ORR catalyst by 400 C precarbonization, 550 C KOH-activation and finally 900 C annealing under NH3 atmosphere. Nevertheless, some serious problems still exist in the preparation process. Firstly, doping nitrogen by NH3 annealing method needs long-time tail gas treatment. Secondly, multistep activation is a timeconsuming and complicated technology, and it will cause a great loss of carbon source. Thirdly, phosphor doping is ignored. Finally, there is no research about effect of temperature on the doped N, P modes and morphology changes. Herein, shaddock peel (SP) waste was initially soaked in 4 M NaOH þ 0.7 M (NH4)3PO4 þ 0.7 M urea solution under vacuum condition, to achieve thoroughly impregnation in a short time in a fume hood. Then the dried SP precursor was carbonized, activated and N,P dual-doped in one-step pyrolysis treatment under N2 atmosphere for 2 h with different temperatures. The samples were marked as NPSP-temperature (600, 700, 800 and 900). According to the contrast results, it was found that the contents of nitrogen and phosphorus species exhibited a disciplinary change, indicating the contents of pyridinic-N, graphitic-N and PeC increase with the rising of temperatures. The mentioned N, P species are well-known ORR active sites [9,32e34], especially for pyridinic-N. In addition, temperature can play a positive role in tuning the degree of graphitization, surface area and morphology, so the NPSP-900 exceeds the other three in ORR catalytic activity. What’s more, when compared with the commercial (20%) Pt/C, the NPSP-900 shows higher current density and a comparable 0.83 V (vs. RHE) half-wave potential in 0.1 M KOH solutions. The calculated electron transfer numbers (n) suggest that all the NPSPs follow 4e transfer pathway during the oxygen reduction process. Last but not least, the unchanged polarization curve after 2500 cycles and the 91.2% current retention for 16 h rotating of NPSP-900 demonstrate its outstanding long term stability, which together with its excellent methanol tolerance could further support its potential application in fuel cells.
Experimental Synthesis of NPSPs The shaddock peel (SP) was collected from the shaddocks bought from the market. The SP was firstly washed with the
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
ethanol-water mixture for five times and then dried in 60 C oven. Then the clean and dry SP was cut into small pieces (V < 1 cm3). 2 g SP pieces were soaked in a conical flask with 100 mL 4M NaOH solution, then a certain amount of (NH4)3PO4 and urea (CH4N2O) were added and stirred to achieve a 4M NaOH þ 0.7 M (NH4)3PO4 þ 0.7 M urea mixture. The whole process was performed in a fume hood due to the generated NH3. After the solid dopants were totally dissolved, the conical flask was placed in a vacuum doughnut. After repeating vacuumizing and air inflation operation until the SP was completely soaked in the solution and then stayed at vacuum state over night. After that, the soaked brown SP was taken out and dried in 100 C oven. Then the prepared SP was heated to 600 Ce900 C and kept for 2 h with a heating rate of 5 C/ min in N2 atmosphere. After cooling down, the N,P-doped SP carbon was successively washed with dilute hydrochloric acid solution and water until the pH z 7. After the final freezedrying, the samples at different pyrolysis temperatures were marked as NPSP-600, NPSP-700, NPSP-800 and NPSP-900, respectively.
1 j ¼ 1
=
26984
jk
þ1 ¼1 þ1 jd jk ðBu1=2 Þ
1=6 CO2 B ¼ 0:62nFD2=3 O2 y
Results and discussion
X-ray photoelectron spectroscopy (XPS) tests were executed on Thermo ESCALAB 250 using a monochromic Al X-ray source. X-ray diffraction (XRD) characterization was obtained on a Bruker D8 Advance diffractometer using a Cu-Ka radiation (l ¼ 0.15406 nm) with a scanning range of 10 ~80 . The Brunauer-Emmett-Teller (BET) data were performed on an Accelerated Surface Area and Porosimetry (ASAP) 2020 system at 77 K. Raman spectra were characterized on a LabRam Aramis (HORIBA JobinYvon S.A.S) using a 514 nm laser, Morphology results of NPSPs were obtained on a field emission gun scanning electron microscope (SEM, Quant 250FEG) and a transmission electron microscope (TEM, JEOL JEM-2100).
Materials characterization
The electrochemical measurements for ORR activities of NPSPs were performed on a rotating ring disk electrode (ALSRRDE-3A) by a three-electrode cell configuration in 0.1 M KOH aqueous solutions at room temperature. A graphite rod, an Ag/AgCl (saturated KCl) and a rotating disk electrode (RDE, 3 mm) were employed as counter, reference and working electrodes, respectively. 1 mg sample powder was equably dispersed in 500 mL aqueous solution (25 mL 5 wt% Nafion þ 475 mL DI water) by ultrasonication. Then 10 mL of asprepared inky catalyst dispersion was carefully dropped onto the RDE with a mass loading of 283 mg cm2. Commercial 20% Pt/C electrode was modified by the same method with the same mass loading. All the potentials in this work vs. RHE are calculated using the Nernst equation: ERHE ¼ EAg/AgCl þ 0.0591*pH þ0.197 V. Cyclic voltammetry (CV) was performed in the N2-saturated and O2-saturated 0.1 M KOH with a scan rate of 10 mV s1. Linear sweep voltammetry (LSV) was tested in O2-saturated 0.1 M KOH solution applying different rotating speeds, including 400, 625, 900, 1225, 1600, 2025 rpm, with a scan rate of 10 mV s1. The electron transferred numbers (n) of NPSP-900 was calculated by the following equations [35]:
(2)
where j, jk and jd are the disk current density, kinetic current density, and the diffusion-limiting current density, respectively; B is the Levich constant and u is the rotating rate. F (96485 C mol1) is Faradic constant; DO2 (1.86 105 cm2 s1) is the diffusion coefficient of O2 in the electrolyte; y (0.01 cm2 s1) is kinematic viscosity of the electrolyte; CO2 (1.21 106 mol cm3) is the O2 concentration in the electrolyte. Methanol tolerance property was measured by currenttime (i-t) chronoamperometry in O2-saturated 0.1 M KOH with a rotating speed of 1600 rpm at the potential of 0.3 V (vs. Ag/AgCl), where 2M CH3OH was added into the electrolyte at about 700s. The long-term durability was characterized by the polarization curves initially and after 2500 cycles by continuous CV scanning for 2500 cycles in O2-saturated 0.1 M KOH.
Characterization
Electrochemical measurements
(1)
The chemical composition of all NPSPs was investigated by X-ray photoelectron spectroscopy (XPS). The XPS survey spectra (Fig. S1) shows the coexistence of C, O, N, P and the calculation comparison of elemental content (Table 1) for each sample, which confirms the successful doping of both nitrogen and phosphorus for all the NPSPs. It is noteworthy that the carbon content increases while the oxygen content decreases with rising temperature due to the poor thermal stability of some oxygen species, therefore the NPSP-900 owns the highest carbon content (81.3 at%) and the lowest oxygen content (11.2 at%). Moreover, the N/C and P/C atom ratios of these samples were also displayed in Table 1, which reflect that the high temperature may be more beneficial to the total doping content of nitrogen instead of phosphorus. The high-resolution N1s and P2p spectra of each NPSP were analyzed and shown in Fig. 1 and Table 1 for comparison. All the N1s spectra of four NPSP materials can be typically deconvoluted into four types: 398.5 ± 0.2 eV, 399.8 ± 0.2 eV, 401.2 ± 0.2 eV and 403.0 ± 0.2 eV, which are assigned to pyridinic-N (N1), pyrrolic-N (N2), graphitic-N (N3), and pyridine-N-oxide groups (N4), respectively. Evident N4 peaks are shown in the N1s spectra of the NPSP-600 and NPSP-700, while there are no corresponding signals found in those of the NPSP-800 or NPSP-900. According to the previous theoretical and experimental results, pyridinic-N (N1) is proved to be the most active N specie for oxygen reduction and the graphitic-N (N3) can also play an important role to enhance ORR performance [9,32e34]. In this case, the N1 content and N1/N ratio are both increased by the order of NPSP-600 < NPSP700 < NPSP-800 < NPSP-900. Furthermore, the graphitic-N (N3) content and N3/N ratios also follow this trend. These results indicate that the higher temperature can not only increase of N1, but also improve the graphitization degree of carbon materials. For the phosphorus doping, the P2p spectra of NPSPs can be divided into two types: 132.4 ± 0.2 eV, 133.5 ± 0.2 eV,
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
26985
Table 1 e Elemental content of NPSPs calculated by XPS results. Sample
NPSP-600 NPSP-700 NPSP-800 NPSP-900
At% C1s
O1s
73.1 74.9 75.8 81.3
21.1 18.2 16.7 11.2
corresponding to the features of PeC (P1) and PeO (P2) [5], respectively. It is obvious that the higher temperature favors more P1 and less P2. The PeO phase means that P is in oxidation state, referring to surface oxidized P species. While the lower value of 132.4 eV is attributed PeC direct bonding, revealing the P atom has been incorporated into the carbon framework [16,36,37]. As known, PeC can change the charge and spin density resulted from the lower electronegativity of P compared with C, and structural defects could be introduced by the bigger size of P atom to offer as catalytic active sites [15,16]. Although the phosphorus content decreases when temperature goes up, the PeC content increases with the rising of temperature. The increased N1, N3 and P1 can all contribute to the ORR activities of higher-temperature catalysts. XRD patterns (Fig. 2) of all as-prepared NPSPs exhibit a broad peak located at about 25 , which is accord with the (002) diffraction, indicating the amorphous structure of the NPSPs [38]. In addition, the intensity of the (002) diffraction peak increases to some extent with the rising of temperature, revealing the positive effect of high temperature on the graphitization of NPSPs. Interestingly, there is a weak peak at 43.7 in the XRD pattern of NPSP-900, which originates from a graphite-like nature, indicating an improvement of graphitization degree [39]. The Raman spectra of NPSP are displayed in the Fig. 2b, where the characteristic D and G bands of carbon materials locate at about 1350 cm1 and 1590 cm1,
N1s (N1, N2, N3, N4/N) 1.1 (22.2, 4.1 (27.0, 5.2 (29.3, 4.9 (34.3,
13.0, 34.0, 30.8) 18.7, 45.0, 9.23) 21.3, 49.4, 0) 8.1, 57.6, 0)
P2p (P1, P2/P) 4.7 (22.7, 2.9 (36.6, 2.3 (55.2, 2.6 (59.3,
77.3) 63.4) 44.8) 40.7)
respectively. The D band represents defects and disordered areas and the G band is identified as the graphitized region. So ID/IG is always used to evaluate the integrity degree of graphite structure. As the calculated ID/IG ratio of each sample shown in Fig. 2, the higher temperature affords higher degree of graphitization. Thus, NPSP-900 exhibits the highest graphitization, which is accord with the XPS and XRD results. Fig. 2c shows the adsorption-desorption BET isotherms of NPSPs, in which all the isotherms give a dominant type Ⅳ plots. The BET surface area increased by: NPSP-600 (301 m2 g1) < NPSP-700 (361 m2 g1) < NPSP-800 (503 m2 g1) < NPSP-900 (548 m2 g1). The BJH pore size distributions of the NPSPs in the Fig. 2d suggest that all the NPSPs mainly reveal ~4 nm mesoporous structure. The NPSP-800 owns increased capillary condensation, indicating improved mesoporous porosity, while NPSP900 shows higher micropore distribution. The SEM images are applied to characterize the morphologies of the NPSPs materials with relatively larger scale, as shown in Fig. 3. It can be seen that with the rising of annealing temperature, the smaller size carbon sheets are derived from original SP. When temperature goes up, the SP and dopants can be decomposed more sufficiently and more gases generate from the SP inner part, which causes more macropore morphologies and rougher surfaces. NPSP-800 and NPSP900 both exhibit better macropore dispersion than the NPSP-600 or NPSP-700 do.
Fig. 1 e N1s high-resolution XPS spectra comparison of NPSP-600 (a), NPSP-700 (b), NPSP-800 (c), NPSP-900 (d) with the P2p high-resolution XPS spectra of NPSP-600 (e), NPSP-700(f), NPSP-800 (g), NPSP-900 (h), respectively. N1: pyridinic-N, N2: pyrrolic-N, N3: graphitic-N, N4: pyridine-N-oxide groups; P1: PeC, P2: PeO.
26986
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
Fig. 2 e XRD patterns (a), Raman spectra (b) and BET isotherms (c) of NPSP samples with their corresponding pore size distributions (d).
The pore structure of each NPSP sample was further investigated by TEM technology, as shown in Fig. 4. Rich micropores and mesopores can be easily observed in the TEM images of all the NPSPs, and the pore sizes (<5 nm) are in accord with the BJH pore size distributions. Obviously, NPSP900 and NPSP-800 present thinner carbon sheets than
NPSP-700 or NPSP-600 do, which indicates more sufficient carbonization under higher annealing temperature and agrees with the SEM results. SEM and TEM images together illustrate the hierarchical pores construction of the NPSPs, including micropores, mesopores and macropores, which are very advantageous to improve ORR activities.
Fig. 3 e SEM images of NPSP-600 (a), NPSP-700 (b), NPSP-800 (c) and NPSP-900 (d).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
26987
Fig. 4 e TEM images of NPSP-600 (a), NPSP-700 (b), NPSP-800 (c) and NPSP-900 (d).
ORR performance The ORR performance of each NPSP catalyst were measured and compared by the CV and LSV polarization curves in Fig. 5. A typical quasi-rectangular CV curve of carbon material was obtained for each NPSP under N2-saturated electrolyte without
any redox peak. While in O2-saturated electrolyte, well-defined oxygen reduction peaks were observed at different potentials for all NPSP catalysts, suggesting that all the samples have ORR catalytic activities. Furthermore, it is found that NPSP-900 shows the most positive reduction peak potential and highest current than those of other samples as shown in Fig. 5a.
Fig. 5 e (a) CV curves of NPSP samples in N2-saturated (black) and O2-saturated (red) 0.1 M KOH with a scanning rate of 10 mV s¡1. (b) LSV curves of each NPSP catalyst and commercial Pt/C in O2-saturated 0.1 M KOH at a scan rate of 10 mV s¡1 with 1600 rpm rotating. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
26988
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
Fig. 6 e (aed) LSV curves of NPSP-600, NPSP-700, NPSP-800 and NPSP-900 at a scan rate of 10 mV s¡1 under different rotating speed in O2-saturated 0.1 KOH solutions, with the K-L plots inset of the former three, respectively. (e) The K-L plots at varied potentials of NPSP-900, (f) the transferred electron number (n) of the NPSPs.
Therefore, the ORR performance in CV curves increases as follows: NPSP-600 < NPSP-700 < NPSP-800 < NPSP-900. The LSV polarization curves on a rotating-disk electrode for the NPSPs and commercial Pt/C (20%) were also performed in O2-saturated 0.1 M KOH with a rotation speed of 1600 rpm for comparison. Similar trend was also observed in LSV case, the NPSP900 demonstrates a half-potential of 0.83 V (vs. RHE), which is evidently more positive than those of NPSP-800 (0.81 V vs. RHE), NPSP-700 (0.78 V vs. RHE) and NPSP-600 (0.76 V vs. RHE). In addition, current densities are also improved as this order: NPSP-600 < NPSP-700 < NPSP-800 < NPSP-900, corresponding to the CV results. More importantly, NPSP-900, as a metal-free catalyst, manifests very close ORR half-wave potential and higher current density when compared with commercial Pt/C
(0.85 V vs. RHE) in alkaline medium. To further explore the intrinsic activities of the NPSPs, the current densities normalized by electrochemical active surface areas and mass loading are calculated and displayed in Figs. S2e3, respectively. The results further demonstrate the NPSP-900 owns the best ORR activity of the NPSPs, and detail description is shown in supporting materials. The ORR polarization curves of NPSPs were obtained with different rotating speeds from 400 to 2025 rpm, as shown in Fig. 6aed with the corresponding Kouteckye-Levich (K-L) plots inset. All the KouteckyeLevich (K-L) plots at varied potentials display good linearity and parallelism for all the NPSP catalysts, revealing first-order reaction kinetics toward the dissolved O2 concentration and adjacent electron transfer
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
numbers (n). As for NPSP-900 in Fig. 6e, it can be seen that K-L plots at 0.3e0.7 V (vs. RHE) are almost parallel with the standard plot for 4e transferred way. The electrons transferred per O2 for all the NPSP catalysts were calculated based on the corresponding K-L plots according to the Equations (1) and (2), as shown in Fig. 6f. It demonstrates that all the NPSP materials follow a dominant four-electron-transfer pathway and the NPSP-900 gives an average n of 4.01, higher than those of NPSP-800 (3.95), NPSP-700 (3.83) and NPSP-600 (3.60), further suggesting the superior ORR activity of NPSP-900. All the results suggest that NPSP-900 owns better catalytic activity with respect to the other three. The reasons for its superior ORR activity can be explained as below: Firstly, the highest doping N, pyridinic-N (N1) and PeC content affords more active sites for oxygen reduction than the other samples do, Secondly, the highest graphitic-N (N3) content and lowest ID/IG both indicate that NPSP-900 owns highest degree of graphitization, and deeper graphitization is beneficial for the electrical conductivity of materials, Thirdly, the BET proves the largest surface area for NPSP-900 among these NPSPs, and ORR is an interfacial/surface reaction, the O2 adsorption or dissociation mainly occurs on the interface/surface of catalyst, therefore, large surface area and hierarchical pores can make remarkable contribution for ORR performance. The electrocatalytic selectivity of ORR catalysts is very important in the cathode application of fuel cells. Therefore, the methanol crossover effect of NPSP-900 was also examined
26989
using chronoamperometry in the presence of methanol with comparison of commercial Pt/C as shown in Fig. 7a, where the reduction currents in O2-saturated solutions before adding CH3OH are set as 1. After the addition of 2 M CH3OH to electrolyte at about 700 s, an avalanche current decrease for Pt/C appears resulted from the inhibition of methanol oxidation [40], while hardly any current change was observed for the NPSP-900 electrode, suggesting that methanol could hardly affect the ORR activity of NPSP-900. On the other hand, stability is a quite critical index to evaluate an ORR catalyst in the potential application of fuel cells. We examined the stability performance by the comparison between initial polarization curves (1600 rpm) and those after 2500 cycles at NPSP-900 and Pt/C electrodes in the O2-saturated 0.1 M KOH. Very encouraging as shown in Fig. 7b, indistinguishable change of ORR current or potential between the 1st and 2500th polarization curves illustrates a stable catalytic activity sustained by NPSP900. While for commercial Pt/C, it suffers an obvious current decrease and potential shift after cycling 2500 times, revealing a serious degeneration of ORR activity. In addition, as shown in Fig. 7d, after rotated for 16 h under 1600 rpm in O2-saturated 0.1 M KOH, NPSP-900 still remains about 91.2% of initial current, much better than some recently reported doped biomass derived ORR catalysts [41e43]. With the excellent catalytic activity, methanol tolerance and stability, NPSP-900 could serve as a promising catalyst for ORR to replace commercial Pt/C in alkaline fuel cells.
Fig. 7 e (a) Current-time responses of NPSP-900 and Pt/C RDEs at ¡0.3 V (Ag/AgCl) with a rotating speed of 1600 rpm, where 2.0 M methanol was added into the electrolyte at about 700 s. Polarization curves at 1600 rpm for NPSP-900 (b) and Pt/C (c) initially and after 2500 cycles. Current-time response of NPSP-900 for 16 h in O2-saturated 0.1 M KOH at 0.7 V (vs. RHE) with a rotating speed of 1600 rpm. All the tests were obtained in O2-saturated 0.1 M KOH solutions.
26990
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
Conclusion In conclusion, we have successfully proposed a simple onestep pyrolysis approach to synthesize NPSPs with N, P dualdoped hierarchical porous structures, which can set carbonization, activation and doping into the same heating process. Temperature plays an important role to influence the degree of graphitization, surface area and morphologies. With higher annealing temperature, NPSP-900 gives higher contents of pyridinic-N, graphitic-N and PeC, all of which are catalytic active species. Beyond the other three NPSPs, the NPSP-900 suggests a more positive half-wave potential (0.83 V vs. RHE) and a higher current density, which is close to the commercial Pt/C. What’s more, the NPSP-900 holds a superior methanol tolerance and much better stability with respect to commercial Pt/C. Therefore, the advantages of our work, simple preparation, waste utilization, excellent ORR performance, good methanol tolerance and superior stability of NPSP-900 could support its potential application in fuel cells.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21576138, 51572127), China-Israel Cooperative Program (2016YFE0129900), Program for NCET-12-0629, Natural Science Foundation of Jiangsu Province (BK20160828), Post-Doctoral Foundation (1501016B), PAPD of Jiangsu Province, and the program for Science and Technology Innovative Research Team in Universities of Jiangsu Province, China.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.133.
references
[1] Qu L, Liu Y, Baek J-B, Dai L. Nitrogen-doped graphene as efficient metal-free electrocatalyst for oxygen reduction in fuel cells. ACS Nano 2010;4:1321e6. [2] Ben Liew K, Daud WRW, Ghasemi M, Leong JX, Su Lim S, Ismail M. Non-Pt catalyst as oxygen reduction reaction in microbial fuel cells: a review. Int J Hydrogen Energy 2014;39:4870e83. [3] He Q, Cairns EJ. Review-recent progress in electrocatalysts for oxygen reduction suitable for alkaline anion exchange membrane fuel cells. J Electrochem Soc 2015;162:F1504e39. [4] Sui ZY, Li X, Sun ZY, Tao HC, Zhang PY, Zhao L, Han B-H. Nitrogen-doped and nanostructured carbons with high surface area for enhanced oxygen reduction reaction. Carbon 2018;126:111e8. [5] Lei W, Deng YP, Li G, Cano ZP, Wang X, Luo D, Liu Y, Wang D, Chen Z. Two-dimensional phosphorus-doped carbon nanosheets with tunable porosity for oxygen reactions in zinc-air batteries. ACS Catal 2018;8:2464e72.
[6] Ishii T, Maie T, Kimura N, Kobori Y, Imashiro Y, Ozaki J-i. Enhanced catalytic activity of nanoshell carbon co-doped with boron and nitrogen in the oxygen reduction reaction. Int J Hydrogen Energy 2017;42:15489e96. [7] Liang J, Jiao Y, Jaroniec M, Qiao SZ. Sulfur and nitrogen dualdoped mesoporous graphene electrocatalyst for oxygen reduction with synergistically enhanced performance. Angew Chem 2012;124:11664e8. [8] Yao Z, Nie H, Yang Z, Zhou X, Liu Z, Huang S. Catalyst-free synthesis of iodine-doped graphenevia a facile thermal annealing process and its use for electrocatalytic oxygen reduction in an alkaline medium. Chem Commun 2012;48:1027e9. [9] Chai G-L, Qiu K, Qiao M, Titirici M-M, Shang C, Guo Z. Active sites engineering leads to exceptional ORR and OER bifunctionality in P, N co-doped graphene frameworks. Energy Environ Sci 2017;10:1186e95. ~ ldsepp E, Johansson L-S, [10] Borghei M, Laocharoen N, Kibena-Po Campbell J, Kauppinen E, Tammeveski K, Rojas OJ, Porous N. P-doped carbon from coconut shells with high electrocatalytic activity for oxygen reduction: alternative to Pt-C for alkaline fuel cells. Appl Catal B Environ 2017;204:394e402. [11] Bai J, Zhu Q, Lv Z, Dong H, Yu J, Dong L. Nitrogen-doped graphene as catalysts and catalyst supports for oxygen reduction in both acidic and alkaline solutions. Int J Hydrogen Energy 2013;38:1413e8. [12] Li H, Liu H, Jong Z, Qu W, Geng D, Sun X, Wang H. Nitrogendoped carbon nanotubes with high activity for oxygen reduction in alkaline media. Int J Hydrogen Energy 2011;36:2258e65. [13] Oh H-S, Oh J-G, Lee WH, Kim H-J, Kim H. The influence of the structural properties of carbon on the oxygen reduction reaction of nitrogen modified carbon based catalysts. Int J Hydrogen Energy 2011;36:8181e6. [14] Wang L, Yin F, Yao C. N-doped graphene as a bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions in an alkaline electrolyte. Int J Hydrogen Energy 2014;39:15913e9. [15] Choi CH, Park SH, Woo SI. Binary and ternary doping of nitrogen, boron, and phosphorus into carbon for enhancing electrochemical oxygen reduction activity. ACS Nano 2012;6:7084e91. [16] Liu Z-W, Peng F, Wang H-J, Yu H, Zheng W-X, Yang J. Phosphorus-doped graphite layers with high electrocatalytic activity for the O2 reduction in an alkaline medium. Angew Chem Int Ed 2011;50:3257e61. [17] Zhao Y-Q, Lu M, Tao P-Y, Zhang Y-J, Gong X-T, Yang Z, Zhang G-Q, Li H-L. Hierarchically porous and heteroatom doped carbon derived from tobacco rods for supercapacitors. J Power Sources 2016;307:391e400. [18] Liang H-W, Zhuang X, Bru¨ller S, Feng X, Mu¨llen K. Hierarchically porous carbons with optimized nitrogen doping as highly active electrocatalysts for oxygen reduction. Nat Commun 2014;5:4973. [19] Liang J, Du X, Gibson C, Du XW, Qiao SZ. N-doped graphene natively grown on hierarchical ordered porous carbon for enhanced oxygen reduction. Adv Mater 2013;25:6226e31. [20] Zhang J, Zhao Z, Xia Z, Dai L. A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions. Nat Nanotechnol 2015;10:444. [21] Xia BY, Yan Y, Li N, Wu HB, Lou XW, Wang X. A metaleorganic framework-derived bifunctional oxygen electrocatalyst. Nat Energy 2016;1:15006. [22] Zhou H, He D, Saana AI, Yang J, Wang Z, Zhang J, Liang Q, Yuan S, Zhu J, Mu S. Mesoporous-silica induced doped
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 9 8 2 e2 6 9 9 1
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
carbon nanotube growth from metaleorganic frameworks. Nanoscale 2018;10:6147e54. Chen P, Wang L-K, Wang G, Gao M-R, Ge J, Yuan W-J, Shen YH, Xie A-J, Yu S-H. Nitrogen-doped nanoporous carbon nanosheets derived from plant biomass: an efficient catalyst for oxygen reduction reaction. Energy Environ Sci 2014;7:4095e103. Wu F-C, Tseng R-L, Hu C-C. Comparisons of pore properties and adsorption performance of KOH-activated and steam-activated carbons. Micropor Mesopor Mat 2005;80:95e106. Lu L, Jiao X, Fan J, Lei W, Ouyang Y, Xia X, Xue Z, Hao Q. Cobalt ferrite on honeycomb-like algae-derived nitrogendoped carbon for electrocatalytic oxygen reduction and ultra-cycle-stable lithium storage. Electrochim Acta 2019;295:461e71. Liu B, Zhang L, Qi P, Zhu M, Wang G, Ma Y, Guo X, Chen H, Zhang B, Zhao Z, Dai B, Yu F. Nitrogen-doped banana peelederived porous carbon foam as binder-free electrode for supercapacitors. Nanomaterials 2016;6:18. Hao P, Zhao Z, Tian J, Li H, Sang Y, Yu G, Cai H, Liu H, Wong CP, Umar A. Hierarchical porous carbon aerogel derived from bagasse for high performance supercapacitor electrode. Nanoscale 2014;6:12120e9. Gao S, Chen Y, Fan H, Wei X, Hu C, Luo H, Qu L. Large scale production of biomass-derived N-doped porous carbon spheres for oxygen reduction and supercapacitors. J Mater Chem A 2014;2:3317e24. Ma Y, Zhao J, Zhang L, Zhao Y, Fan Q, Li X a, Hu Z, Huang W. The production of carbon microtubes by the carbonization of catkins and their use in the oxygen reduction reaction. Carbon 2011;49:5292e7. Yuan W, Feng Y, Xie A, Zhang X, Huang F, Li S, Zhang X, Shen Y. Nitrogen-doped nanoporous carbon derived from waste pomelo peel as a metal-free electrocatalyst for the oxygen reduction reaction. Nanoscale 2016;8:8704e11. Wang N, Li T, Song Y, Liu J, Wang F. Metal-free nitrogendoped porous carbons derived from pomelo peel treated by hypersaline environments for oxygen reduction reaction. Carbon 2018;130:692e700. Guo D, Shibuya R, Akiba C, Saji S, Kondo T, Nakamura J. Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science 2016;351:361e5.
26991
[33] Lu L, Fan J, Lei W, Ouyang Y, Yao D, Xia X, Hao Q. Multiple metal (Cu, Mn, Fe) centered species simultaneously combined nitrogen-doped graphene as an electrocatalyst for oxygen reduction in alkaline and neutral solutions. ChemCatChem 2018;10:2471e80. [34] Lu L, Hao Q, Lei W, Xia X, Liu P, Sun D, Wang X, Yang X. WellCombined magnetically separable hybrid cobalt ferrite/ nitrogen-doped graphene as efficient catalyst with superior performance for oxygen reduction reaction. Small 2015;11:5833e43. [35] Bard AJ, Faulkner LR. Electrochemical methods: fundamentals and aplications. Wiley; 2001. [36] Yang D-S, Bhattacharjya D, Inamdar S, Park J, Yu J-S. Phosphorus-doped ordered mesoporous carbons with different lengths as efficient metal-free electrocatalysts for oxygen reduction reaction in alkaline media. J Am Chem Soc 2012;134:16127e30. [37] Wu J, Yang Z, Li X, Sun Q, Jin C, Strasser P, Yang R. Phosphorusdoped porous carbons as efficient electrocatalysts for oxygen reduction. J Mater Chem A 2013;1:9889e96. [38] Su F, Poh CK, Chen JS, Xu G, Wang D, Li Q, Lin J, Lou XW. Nitrogen-containing microporous carbon nanospheres with improved capacitive properties. Energy Environ Sci 2011;4:717e24. [39] Ma W, Xie L, Dai L, Sun G, Chen J, Su F, Cao Y, Lei H, Kong Q, Chen C-M. Influence of phosphorus doping on surface chemistry and capacitive behaviors of porous carbon electrode. Electrochim Acta 2018;266:420e30. [40] Schmidt TJ, Gasteiger HA, Behm RJ. Methanol electrooxidation on a colloidal PtRu-alloy fuel-cell catalyst. Electrochem Commun 1999;1:1e4. [41] Liu Z, Li Z, Tian S, Wang M, Sun H, Liang S, Chang Z, Lu G. Conversion of peanut biomass into electrocatalysts with vitamin B12 for oxygen reduction reaction in Zn-air battery. Int J Hydrogen Energy 2019;44:11788e96. [42] Wang Y, Zuo S, Miao M, Liu Y, Gu Z, Jin Y. Cost-effective preparation of metal-free electrocatalysts by phosphoric acid activation of lignocellulosic materials for oxygen reduction reaction. Int J Hydrogen Energy 2019;44:2811e22. [43] Mao X, Cao Z, Chen S, Jia J, Li X, Yin Y, Yang S. Facile synthesis of N, P-doped hierarchical porous carbon framework catalysts based on gelatin/phytic acid supermolecules for electrocatalytic oxygen reduction. Int J Hydrogen Energy 2019;44(12):5890e8.