N-doped hard carbon nanotubes derived from conjugated microporous polymer for electrocatalytic oxygen reduction reaction

Renewable Energy 146 (2020) 2270e2280

Contents lists available at ScienceDirect

Renewable Energy journal homepage: www.elsevier.com/locate/renene

N-doped hard carbon nanotubes derived from conjugated microporous polymer for electrocatalytic oxygen reduction reaction Wanli Zhang, Hanxue Sun, Zhaoqi Zhu, Rui Jiao, Peng Mu, Weidong Liang, An Li* Department of Chemical Engineering, College of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou, 730050, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 January 2019 Received in revised form 21 June 2019 Accepted 12 August 2019 Available online 13 August 2019

The development of efficient non-metal catalysts (NMC) towards oxygen reduction reaction (ORR) in both acidic and alkaline electrolyte is of great importance for construction of new generation fuel cell. Herein, we demonstrate the fabrication of N-doped hard carbon nanotubes prepared by carbonization of melamine-incorporated nanotube-like conjugated microporous polymers (CMPs), as novel electrocatalysts for efficient ORR. As a kind of metal-free electrocatalyst, the as-prepared carbon nanotubes exhibit superior ORR activity not only in alkaline electrolyte but also in acidic condition. Interestingly, obvious enhancement on the diffusion-limited current density (increased by 1.8 mA cm
2) for the Ndoped products in acid solution is observed by comparison with that of carbonized CMPs nanotubes without N-doping. Furthermore the catalysts also show better methanol immunity than that of commercial 20 wt% Pt/C. And only a slight decrease (14 mV negative shift) in half-wave potential is detected after 5000 cycles in 0.1 M KOH, indicating an ideal electrochemical stability which makes the N-doped hard carbon nanotubes promising candidate as an efficient electrocatalyst for ORR, by combination with their desirable electrocatalytic ORR activity, methanol tolerance and stability. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Conjugated microporous polymer Oxygen reduction reaction Graphitic carbon nitride Nanotube Hard carbon

1. Introduction Fuel cells (FCs), as devices for direct conversion of chemical energy into electricity by electrochemical reactions, are among the key enabling technologies for the transition to a hydrogen-based economy [1,2]. One of the biggest obstacles hindering the commercialization of fuel cells is high cost and limited supply in nature of noble metals used as catalyst (e.g., Pt) at the cathode, in addition to their low stability and weak methanol tolerance [3,4]. To reduce the noble metal consumption meanwhile improve the comprehensive electrochemical properties of noble-metal-based catalysts, the construction of noble metal based alloy is therefore a priority. So far, Pt3M alloys (M ¼ Ni, Fe, Co, Ti) [5e9] have been created to possess better ORR performance than that of pure Pt catalysts. Particularly, Pt-based superlattices were reported to have high ORR activity such as Pt-Co-Fe [10]. And the enhancement of ORR activity of these superlattices lies in the fact high index facets and Pt rich surfaces, which derived from facets, size, and shape control [6]. Besides, the oxygen adsorption enthalpy of the top Pt

* Corresponding author. E-mail address: [email protected] (A. Li). https://doi.org/10.1016/j.renene.2019.08.071 0960-1481/© 2019 Elsevier Ltd. All rights reserved.

layer of Pt-M alloys could be dramatically decreased via alloying, thus resulting in an increase in ORR activity in another way. On the other hand, the development of non-noble metal or metal-free electrocatalysts for ORR is another one of most efficient approaches [11e13]. To date, a number of noble metal free electrocatalysts such as metal-nitrogen-carbon (M-N-C) catalysts [14e16], transition metal oxide [17,18], carbides [19], porous carbon materials [20,21], have been exploited. In this category, in particular, M-N-C catalysts derived from FeeN4 and CoeN4 macrocycles precursors [22] through carbonization treatment have been well investigated due to their superior ORR activity, better stability and, importantly, cost-efficient production. Comparatively, metal-free electrocatalyst such as N-doped carbon (e.g. N-doped graphene, CNTs, etc.) show great advantages owing to their low cost, superior stability and broader availability, making them promising candidates for practical applications [23]. These doped heteroatoms such as nitrogen, feature more electronegative, facilitating oxygen adsorption and subsequent reduction [24,25]. Though great progress has been made in this direction, there is still the challenge that remains in connection to the poor stability in acid solutions or sometimes complicated fabrications of those metal-free electrocatalysts. Therefore, further exploitation of novel electrocatalysts without using noble metal is of special interest.

W. Zhang et al. / Renewable Energy 146 (2020) 2270e2280

For non-noble metal or metal-free electrocatalysts for ORR, carbon-based materials play an important role as they can serve as conductive supporting materials for electrocatalysts or even be capable of providing active catalytic sites themselves. Except for the direct applying of carbon-based materials on this matter, a lot of natural products [26] or synthetic polymers [27] have been employed as precursors for preparation of electrocatalysts or conductive supporting materials for electrocatalysts via simple carbonization with or without dopants. In the most cases, however, the variance in chemical composition or structure of these mentioned precursors leads to unpredictable or unreproducible performance of resultant products (e.g. porosity, active sites, etc.) after carbonization, which limited their practical applications. Nitrogen atom has a relatively high electronegativity (c ¼ 3.04), and its introduction can increase the charge density of carbon atom with an effective enhancement of interaction between the guest molecules and carbon matrix [28]. Pyridinic N, N bonded to two carbon atoms, is planar sp2 hybridization. Excellent performance in oxygen adsorption of electrocatalysts with pyridinic N results from the strong electron adsorption capacity by the lone-pair electrons. Wei and his cooperators selectively prepared a kind of pyridinicand pyrrolic-nitrogen-doped graphene with virtually no quaternary N using layered montmorillonite as a quasi-closed flat nanoreactor. The results of electrochemical measurements showed that the synthesized catalyst exhibited excellent ORR activity, indicating quaternary N is not essential to catalyze the ORR [29]. Li and his cooperators [30] devised a series of experiments to locate active sites via detecting catalysts’ chemical composition variation before and after ORR. In their opinion, synchrotron-based X-ray photoelectron spectroscopy analyses illustrate ORR intermediate OH (ads) which is supposed to chemically attach to the active sites attached pyridinic N after ORR, revealing that pyridinic N plays a significant role in ORR. In addition, XPS analyses reveal that the content change of pyridinic N before and after ORR is much less important indicating no certain relation between pyridinic N and catalytic activity for ORR. And then highly oriented pyrolitic graphite model catalysts which possess well-defined p conjugation structures with well-controlled doping of N species were successfully synthesized to characterize the ORR active sites by Nakamura and his cooperators [31]. CO2 adsorption experiments demonstated that pyridinic N can create Lewis basic sites. Electrochemical measurements displayed that pyridinic N rather than graphitic N is beneficial to reduce the ORR overpotential and create the active site. The results suggested that the current densities is linear positive correlation with content of pyridinic N rather than the preparation method, revealing that the ORR performance is determined only by the concentration of pyridinic N. CMPs, as a class of porous organic polymers (POPs), consist of pconjugated skeletons and perpetual nanopores, featuring rich synthetic routes, tunable porous structures, large specific surface areas, easy functional modification and excellent physicochemical stability, which have generated enormous interest in many fields, e.g. separation and catalysis and so on [32e34]. Other than conventional porous materials, CMPs possess the characteristic of a high flexibility in construction of conjugated skeletons and nanopores at molecular level. In addition, it is worth noting that new functions have been extending because of the synergistic action of p conjugation and porosity [35]. Preceding work demonstrated that nanotube-like CMPs can be fabricated via appropriate monomers and solvents [36]. Significantly, the nanotube-like structure can afford to be well maintained on condition of high temperature even above 1000  C owing to the 3D interlinked carbon skeleton. As a result, nanotube-like CMPs are supposed to be ideal candidates as precursors in the field of electrocatalysis based on their unique cylindrical structure and excellent porosity [37]. However, few

2271

reports on the application of CMPs nanotubes in the field of catalytic ORR but also high-performance metal-free ORR catalysts applied to acid electrolyte. In this work, we report the creation of Ndoped hard carbon nanotubes (NHCNTs) derived from the carbonization of melamine-incorporated nanotube-like conjugated microporous polymers (CMPs) as novel electrocatalysts for efficient oxygen reduction reaction without doping metal, providing a novel idea for the design and synthesis of good property metal-free ORR catalysts. A significant amount of effort and expertise goes into figure out the touchy issue of low activity and poor stability of ORR electrocatalysts. As a kind of metal-free electrocatalyst, the resulting NHCNT exhibits superior ORR activity both in alkaline electrolyte and in acidic conditions, in addition to their ideal electrochemical stability and methanol tolerance, which makes the NHCNT promising candidate as efficient ORR electrocatalyst for construction of high performance fuel cells. 2. Experimental 2.1. Reagents 1,3,5-triethynylbenzene, 1,5-dibromonaphthalene, 2-amino3,5-dibromopyridine and tetrakis(triphenylphosphine)palladium(0) were purchased from TCI Development Co., Ltd. Commercial 20 wt% platinum-loaded carbon catalyst denoted as Pt/C for comparison and 5 wt% Nafion ionomer were purchased from Alfa Aesar. Perchloric acid, potassium hydroxide and melamine was purchased from Sinopharm Chemical Reagent Co., Ltd. Copper iodide was purchased from J&K Scientific. Toluene, triethylamine and other solvents were purchased from Tianjin Fuyu chemical reagent Co., Ltd. And each was analytically pure and used as received without further purification. 2.2. Preparation of materials CMP-based precursors were synthesized by SonogashiraHagihara cross-coupling reaction. 200 mg (1.3 mmol) 1,3,5triethynylbenzene and 572 mg (2.0 mmol) 1,5dibromonaphthalene were dissolved in the mixture of 15 ml toluene and 15 ml triethylamine in the presence of 100 mg tetrakis(triphenylphosphine)palladium(0) and 30 mg copper iodide. The reaction mixture solution was refluxed at a bath temperature of 80  C for 24 h under a nitrogen atmosphere. The precipitated polymer was purified with chloroform, ultrapure water, methanol, and acetone to get rid of catalyst or unreacted monomer residues, followed further purification by using Soxhlet extraction method for 72 h. The sample was dried at 70  C for 24 h in vacuum oven and denoted as CMP-1. The other CMP was obtained by the same way as mentioned above: 1,3,5-triethynylbenzene (750.9 mg, 5 mmol) and 2-amino-3,5-dibromopyridine (377.865 mg, 1.5 mmol) as monomers, denoted as CMP-2, which was discussed in supporting information. Turbostratic carbon nitride was prepared by two-step pyrolysis of melamine as followed. First, an appropriate amount of melamine was pyrolyzed at 300  C for 1 h in air. And then, the temperature was maintained at 600  C for 2 h. After milling, the powder was slowly heated to 300  C in Ar, and was calcined at a higher temperature of 650  C. The pyrolysis was performed at 300  C for 0.5 h, then at 650  C for 1 h. Finally, the brown powder of g-C3N4 was obtained. The calcination was carried out for the fabrication of HCNT and NHCNT. 0.03 g CMP-1 was fully ground with 0.08 g g-C3N4. Then the powder was transferred into a tube furnace for pyrolysis. The mixture was heated to 800  C and then maintained for 1 h under flowing argon gas. The resultant material was collected and ground,

2272

W. Zhang et al. / Renewable Energy 146 (2020) 2270e2280

denoted as NHCNT-1. In contrast, HCNT-1 and HCNT-2 was synthesized by heating the pure CMP-1, CMP-2 in argon atmosphere with the same method, respectively. The detailed fabrication procedures are schematically illustrated in Scheme 1.

materials was recorded by PhotronFastcamMini UX100 type high speed video camera. And water contact angle was measured by DSA100 of Drop Shape Analyzer. 2.4. Electrochemical measurement

2.3. Characterization The microstructures and morphologies of the CMP, HCNT and NHCNT were observed through scanning electron microscopy (SEM, JSM-6701F, JEOL, Ltd.) and transmission electron microscopy (TEM, JSM-2100F, JEOL, Ltd.). Phase identifications of samples were carried out by X-ray diffraction (XRD, D/max2550 V, Rigaku Japan) using a Cu Ka (l ¼ 1.5405 Å) radiation source, operated at 40 kV and 100 mA in the range of 2q from 2 to 80 . Raman spectra were recorded on a confocal microscopy Raman spectrometer (Renishaw, RM-1000) with 514.5 nm wavelength incident laser light. The specific surface areas and pore-size distribution of the resulting samples were computed from each corresponding nitrogen adsorption
desorption isotherm using the Brunauer-EmmettTeller (BET) equation and Barrett-Joyner-Halenda (BJH) method, which were recorded through a micromeritics ASAP 2020 apparatus with nitrogen adsorption at 77 K. And all these materials were degassed at 120  C for 12 h under vacuum condition before measurement. Elemental analysis was conducted by an Elementar (Vario EL) elemental analyzer in order to further investigate the relationship between the nitrogen content and ORR activity. Chemical composition and their existing valance state of catalysts were examined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) with a monochromatic Al Ka X-ray source (h ¼ 1486.6 eV). The droplet impregnation process on the surface of

The ORR activity, MeOH tolerance and durability tests of the asprepared electrocatalysts were evaluated by rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques. These electrochemical measurements were carried out in N2/O2-saturated 0.1 M KOH or 0.1 M HClO4 electrolyte using a Biopotentiostat (760E, CH Instruments) with a standard three-electrode cell at room temperature. A rotating disk electrode (RDE), 5 mm in diameter for glassy carbon (GC) substrate, or a rotating ring-disk electrode (RRDE, 5.61 mm in diameter for GC substrate) served as the working electrode, while a Pt wire or a graphite rod as counter electrode, a Hg/HgO (4.24 KOH) electrode or an Ag/AgCl (saturated KCl) electrode as reference electrode. And all the potentials were converted into a reversible hydrogen electrode (RHE), where ERHE ¼ EHg/HgO þ 0.098 V þ 0.059 pH, ERHE ¼ EAg/ AgCl þ 0.199 V þ 0.059 pH. As for the catalyst ink, 5 mg of prepared catalyst was ultrasonically dispersed in a water/isopropanol/nafionionomer solution (300 mL ultrapure water, 100 mL 5% nafion ionomer, 600 mL isopropanol) to form an ink. High quality catalyst films could be made by casting 10 mL of the catalyst inks onto a glassy carbon working electrode followed by drying in air. And commercial 20 wt% Pt/C catalyst was investigated under identical conditions for comparison. The electrochemical measurements were carried out following that N2/O2 was bubbled into the liquid subsurface and surface of the

Scheme 1. (a) Schematic representation of the synthesis routes for nanotube-like CMPs. (b) Simulation structure of the CMPs and their derivatives.

W. Zhang et al. / Renewable Energy 146 (2020) 2270e2280

electrolyte for 30 min in sequence to saturate it with N2/O2. Besides, a flow of N2/O2 was injected to the cell throughout the course of measurements. The activation of working electrode was performed by cyclic voltammetry (CV) technique (sweep segment:100e200, scan rate: 100 mV s
1) prior to the data were recorded. And CV measurements were carried out between 1.23 and 0 V vs. RHE in 0.1 M KOH or 0.1 M HClO4 at a scan rate of 50 mV s
1. Linear sweep voltammetry (LSV) measurements was performed at various rotating speeds of 400, 625, 900, 1225, 1600, 2025 and 2500 rpm in 0.1 M KOH or 0.1 M HClO4 with a scan rate of 10 mV s
1. And the potential window was from 1.23 to 0 V vs. RHE. The electron transferred number (n) for ORR can be extracted from the Koutecky-Levich (K-L) equation [38] given by:

1 1 1 1 1 ¼ þ ¼ þ J JL JK Bu1=2 JK

(1)

B ¼ 0:2nFðDo2 Þ2=3 y
1=6 Co2

(2)

where J is the measured current density (mA cm
2), JK , kinetic current density (mA cm
2), JL , the diffusion-limited current density (mA cm
2). In particular, u is the rotation rate expressed in rpm corresponding the constant 0.2 of Eq. (2), while constant 0.62 is adopted when u refered to angular velocity (rad/s) rather than 0.2 [39]. B is Levich slope, n, the electron transferred number, F, the Faraday constant (96485 C mol
1), Do2 , the diffusion coefficient of oxygen (1.9  10
5 cm2 s
1 in 0.1 M KOH, 1.67  10
5 cm2 s
1 in 0.1 M HClO4 [40]), y, the kinematic viscosity of the electrolyte (0.01 cm2 s
1), while Co2 is the bulk concentration of oxygen (1.2  10
6 mol cm
3 in 0.1 M KOH, 1.38  10
6 mol cm
3 in 0.1 M HClO4). The electron transferred number n can also be calculated as well as hydrogen peroxide yield (H2O2%) by RRDE tests in the light of the following equations [41]:

n¼

4 ID ID þ ðIR =NÞ

%H2 O2 ¼ 100

2 IR =N ID þ ðIR =NÞ

(3)

(4)

Here ID and IR are the Faradaic current (mA) corresponding to that is at the disk and ring, respectively. N ¼ 0:37 represents the H2O2 current collection coefficient of the Pt ring. The accelerated durability tests were carried out in the O2saturated electrolyte by CV technique between 0 and 1.23 V vs. RHE at a scan rate of 200 mV s
1 for 5000 cycles. The onset potential (Eonset) is designated as the potential at which the current density is
0.1 mA cm
2. Besides, the stability analysis of NHCNT-1 also can be evaluated by chronoamperometric experiments at a rotation speed of 1225 rpm with a constant potential (~0.7 V vs. RHE).

3. Results and discussion As illustrated in Scheme 1, the as-prepared CMP nanotubes were synthesized by 1,3,5-triethynylbenzene with two different monomers, namely 1,5-dibromonaphthalene (corresponding to CMP-1) and 2-amino-3,5-dibromopyridine (CMP-2), respectively. The morphology of CMPs is affected by many factors such as monomer, concentration of reactant, agitation rate and temperature. These resulting CMPs were constructed of 3D architectures composed of aromatic nodes arousing by the linkage of alkynyl groups, as displayed in its simulation structure (Scheme 1a). In addition, CMP-1 and CMP-2 possess stable tubular structures (Scheme 1b) which can also be proven by morphology analysis (see later, Fig. 1). After a

2273

high temperature carbonization treatment, the CMP-1 nanotube products which was loaded with g-C3N4 were then successfully transferred to the N-doped hard carbon nanotubes (denoted as NHCNT-1), as shown in Scheme 1b. For comparison, CMP-1 and CMP-2 were directly carbonized without N doped at the same temperature, denoted as HCNT-1 and HCNT-2, respectively. Just as that of shown in Scheme 1, both the CMP-1 (Fig. 1a, d) and CMP-2 (Figs. S1a and b) are formed in a nanotube-like morphology. Due to their rigid chemical structure composed by aromatic rings associating with stable p-conjugated network, the carbonization products of both two CMPs samples, i.e. HCNT-1 (Fig. 1a, d) and HCNT-2 (Figs. S1d and e), still remain nanotube-like morphology. For N-doped samples, in this case NHCNT-1, it also exhibits a carbon nanotubes morphology in presence of amorphous nitrogencontaining carbon particles loosely aggregated together (Fig. 1i). Compared with those bulk carbon-based materials, such cylindrical structure of NHCNTs well satisfy our primary design target, that is, such cylindrical structure would facilitate the exposure of active sites as well as a quick transportation of oxygen species [24], which in turn is beneficial to ORR performance. As regards the morphology of CMPs at high magnification, all of the CMP-1 (Fig. 1d), CMP-2 (Fig. S1b), HCNT-1 (Fig. 1e), HCNT-2 (Fig. S1d) and NHCNT-1 (Fig. 1f) exhibit a coarse surface with a tubular diameters of 200e400 nm. By comparison with the SEM images of the CMPs precursors, carbonization products and N-doped carbonization products, no distinct changes in structure can be observed after high-temperature processing according to the SEM images, as mentioned above. Fig. 1gej are the TEM images of these three samples. Clearly, the thickness of these CNTs wall was measured to be around 50 nm, while the sizes of inner diameter are in the range of 50e100 nm. Fig. 1k-l are the HRTEM images of outside and inside tubes of NHCNT-1, indicating an amorphous nature of carbonized HCNTs. The TEM elemental mapping results are in Fig. 1meo, revealing that nitrogen is uniformly distributed across the CNTs, where a certain amount of oxygen was also found. The typical XRD spectra of CMP-1, HCNT-1 and NHCNT-1 exhibits a broad peak with 2q ¼ 22.5 characteristic of amorphous in nature as shown in Fig. 2a, which would be ascribed to the periodicity perpendicular and parallel to the p-conjugated skeletons of the polymers. A distinct diffraction peak at 2q ¼ 27.4 which corresponds to typical layered stacking structure was observed, indicating that the synthesized g-C3N4 is of highly ordered graphite structure. As for NHCNT-1, an inconspicuous diffraction peak at 27.4 could be detected on account of decomposition of g-C3N4. The Raman characterization of HCNT-1 and NHCNT-1 are plotted in Fig. 2b, where two evident peaks are appeared at 1358 cm
1 and 1589 cm
1, respectively attributed to the D band (the disordered sp3) and G band (the graphitic sp2 stretching of graphene) [42]. The relative ratios of the D band to the G band demonstrate the crystallinity degree of carbon materials. The ID/IG of NHCNT-1 (1.23) is slightly higher than that of HCNT-1 (1.14), indicating more defects emerge after N doping, which facilitates the ORR [9]. The results of elemental analysis revealed that nitrogen contents of HCNT-1 and NHCNT-1 are 2.08 and 9.91 wt% (Table S1), respectively. The XPS survey spectra (Fig. S2a) show that the samples are dominated by carbon (C 1s, the strongest peak located at around 284 eV), along with some nitrogen (N 1s, 400 eV) and oxygen-containing moieties (O 1s, 531eV), while the HCNT-1 and NHCNT-1 exhibit the increase of nitrogen element (from 1.21 to 5.30 at%). The existence of oxygen might be due to the adsorbed small molecules such as hydrone and oxygen on nanotubes. However, no signals of Pd or Br were detected in the spectra (Table S2), indicating that residual monomers and catalysts had been removed. As can be observed from the Table S2, these as-prepared catalysts contained different amount of

2274

W. Zhang et al. / Renewable Energy 146 (2020) 2270e2280

Fig. 1. (a, d) SEM, (g, j) TEM images of CMP-1; (b, e) SEM, (h) TEM images of HCNT-1; (c, f) SEM, (i) TEM, (k, l) HRTEM images and (meo) corresponding the elements maps of NHCNT-1, (m) CeK, (n) NeK, (o) OeK.

nitrogen indicating the successful doping of N into CMPs, which is well in agreement with the TEM-mapping results (Fig. 1m, n) and elemental analysis (Table S1). In order to acquire more bonding information of the samples, high-resolution XPS spectra were collected. As depicted in Fig. S2b-c, C 1s peak was deconvoluted into several peaks. The bonding energy located at 284.7, 285.5, 286.4 and 287.2 eV [11] corresponded to C]C, C]N, CeN & CeO and C] O, confirming the incorporation of nitrogen into the CMPs basal

plane. And the deconvoluted high-resolution N 1s spectra of NHCNT-1 (Fig. 2d) demonstrates the concurrent existence of pyridinic N (398.3 eV, 36.09%), pyrrolic N (400.1 eV, 30.95%), graphitic N (401.2 eV, 11.84%) and oxide N (402.4 eV, 21.13%), respectively [11,43]. Attributing to one long pair of electrons, which arouse strong ability to attract electrons, the pyridinic nitrogen atoms regarded as efficient active sites contribute to the adsorption of oxygen, thus promoting the ORR [44]. Carbon atoms neighboring

W. Zhang et al. / Renewable Energy 146 (2020) 2270e2280

2275

Fig. 2. (a) XRD spectra. (b) Raman spectra. (c) and (d) High-resolution XPS spectra of N 1s for HCNT-1 and NHCNT-1.

Fig. 3. (a) N2 adsorption and desorption isotherms. (b) Pore size distribution curves. (c) Magnification of Fig. 3b from 1.75 to 6.0 nm pore diameter.

pyridinic N contribute to the spin density and atomic charges distributions in ORR process: the absorption and desorption of intermediates, the formation and cleavage of chemical bonds [45,46]. While quaternary N (also called graphitic N), whose N bonded to three carbon atoms, possesses 3D structures, giving rise to the interruption of p-p conjugation. And electrocatalysts with quaternary N are restricted in the ORR due to the poor electrical conductivity [29].

Nitrogen adsorption-desorption experiments were carried out at 77.3 K to investigate the porosities of the catalysts. As illustrated in Fig. 3a, the N2 adsorption/desorption isotherms of CMP-1, HCNT1 and NHCNT-1 are suitable for type IV with H3 hysteresis loops [47]. N2 adsorption isotherms close to 0 relative pressure (P/P0) exhibits strong N2 adsorption, indicating that the presence of a certain amount micropores in the polymers. Besides, the hysteresis loops of the three samples were attributed to capillary tube

2276

W. Zhang et al. / Renewable Energy 146 (2020) 2270e2280

coacervation. And the Brunauer-Emmett-Teller (BET) specific surface area of NHCNT-1 and HCNT-1 were calculated to be 520 m2 g
1 and 471 m2 g
1, respectively. The BET surface area of HCNT-1 and NHCNT-1 is much larger than that of CMP-1 un-carbonized nanotube (142 m2 g
1), indicating a significant enhancement in porosity of resultant during carbonization process. High BET surface area can contribute to the exposure of active sites. The N-doped sample possess rougher surface as well as higher porosity caused by the pyrolysis of g-C3N4. Pore size distribution curves originated from desorption branches of the isotherms by using BJH method are illustrated in the Fig. 3b and c. The single point adsorption total pore volume of pores at P/P0 ¼ 0.9936 was found to be 0.19 cm3 g
1 for CMP-1, 0.37 cm3 g
1 for HCNT-1 and 0.47 cm3 g
1 for NHCNT-1, respectively. As shown in Fig. 3b, the specimens pore size distributions of these three samples mainly focused on 1.93 nm, demonstrating the micropore feature of the resulting materials. In addition, the unique porous structure of p-conjugated skeletons can offer low-resistant diffusion channels facilitating the transmission of oxygen in electrolyte, which can enhance the ORR activity [11,48]. Droplet impregnation process on the surface of materials was recorded by the high-speed video camera in order to further evaluate the wettability of the as-prepared samples. As shown in Fig. 4a and b, the droplet bounced several times indicating CMP-1 and HCNT-1 possess excellent surface superhydrophobic properties, while NHCNT-1 treated by N-doped exhibit superhydrophilic

property and the droplet can be impregnated immediately, indicating good water diffusion property in Fig. 4c. And as shown in Fig. 4d, the water contact angle (CA) of CMP-1 is 152 and 149 for HCNT-1, which could be attributed to the rough surface of CMP-1 by random cross-coupling of nanoparticles. The treatment of N-doped can bring about a change in electronic structure of catalysts, as well as more defects of surface. On account of superhydrophilic property, NHCNT-1 has a promising application in the filed of electrocatalysis. The CV and LSV measurements were performed by using RRDE electrode to evaluate ORR performance of catalysts. As demonstrated in Fig. 5a, the CV measurement exhibits a pronounced cathodic ORR peak at 0.72 V vs. RHE in O2-saturated 0.1 M KOH electrolyte whereas only indistinctive double-layer charging current could be observed in N2-saturated electrolyte, indicating that N-doped CMP carbonized at 800  C, i.e. NHCNT-1, possess superior ORR activity in alkaline solution. It was found that NHCNT-1 exhibited the best performance, while HCNT-1 was less than satisfactory. The ORR activity of NHCNT-1 is also determined by its onset potential (~0.87 V vs. RHE, at which the current density is
0.1 mA cm
2) and half-wave potential (E1/2 ¼ 0.76 V vs. RHE) in the RRDE polarization curves. These values are higher than those of HCNT-1 measured under the identical conditions (Eonset ¼ 0.74 V and E1/2 ¼ 0.66 V). Furthermore, better performance for ORR of NHCNT-1 is also gleaned from its diffusion limiting current density (3.8 mA cm
2 at 0.2 V vs. RHE), whose value is much larger than

Fig. 4. Camera photos of droplet impregnation process on the surface of materials for (a) CMP-1, (b) HCNT-1 and (c) NHCNT-1. (d) Water contact angle of CMP-1 and HCNT-1.

W. Zhang et al. / Renewable Energy 146 (2020) 2270e2280

2277

Fig. 5. CV curves of (a) NHCNT-1, (b) HCNT-1 and (c) Pt/C in O2-saturated (red) and N2-saturated (black) 0.1 M KOH. (d) RRDE peroxide yield and electron transferred number of HCNT-1, NHCNT-1 and Pt/C in O2-saturated 0.1 M KOH. (e) RRDE voltammograms of HCNT-1, NHCNT-1 and Pt/C with a scan rate of 10 mVs
1 and rotation speed of 1600 rpm in 0.1 M KOH. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

that of HCNT-1. The oxygen adsorption mode of pure carbon nanotube surface is usually Pauling mode (single site), which against the strength of the OeO bond being suitable for 2-electron reaction, while it can change into Bridge mode afer N-doped, in

favour of 4-electron reaction [49,50]. And the nitrogen-induced may change the charge density and spin density of carbon, leading to the difference of chemisorption mode of O2 [51]. The electron transferred number (n) can be computed from the RRDE

Table 1 Summary of porous carbon electrocatalysts for the ORR in 0.1 M KOH unless stated. Sample NHCNT-1 1D CMP nanotubes C-POP-2-900 COPs

0.1 M KOH 0.1 M HClO4 1DPC-M3 1DPC-L3 COPePeSO3eCo-rGO CeCOP-4

S,N-codoped graphene MoS2QDs@Ti3C2TxQDs@MWCNTs-2 NDGNS (N-Doped Diamond/Graphite Hybrid Nanosheets)

Eonset (V)

E1/2 (V)

nK-L

nRRDE

H2O2%

0.87 1.15 0.96 0.97 0.95 0.88 1.01 0.88 0.87 0.94

0.76 0.45 0.73 0.75 0.78 0.72 0.78 0.72 0.75 0.76

3.95 3.81 / / 3.80 / 3.90 3.98 3.95 2.80

3.83 3.94 3.65 3.80 / 3.70 3.88 3.98 / 3.73e3.95

8.4 3.2 17.2 10.2 / / / 0.11e0.46 / 10e15

JL (mA cm
2) 3.8 4.4 4.2 4.4 5.0 4.0 5.2 4.3 4.0 4.5

Ref. Our work Our work [52] [53] [54] [55] [56] [57] [58]

Fig. 6. CV curves of (a) HCNT-1, (b) NHCNT-1 and (c) Pt/C in O2-saturated (red) and N2-saturated (black) 0.1 M HClO4. (d) RRDE peroxide yield and electron transferred number of HCNT-1, NHCNT-1 and Pt/C in O2-saturated 0.1 M HClO4. (e) RRDE voltammograms of HCNT-1, NHCNT-1 and Pt/C with a scan rate of 10 mVs
1 and rotation speed of 1600 rpm in 0.1 M HClO4. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

2278

W. Zhang et al. / Renewable Energy 146 (2020) 2270e2280

measurements, which is close to those values calculated from the LSV curves on the basis of the K-L equation i.e. Eq. (1). As shown in Fig. 5d, the average value of n (navg) at 0.2e0.8 V vs. RHE is 3.83 for NHCNT-1 in 0.1 M KOH, while navg ¼ 3.49 for HCNT-1 and 3.94 for Pt/C, implying a better 4-electron reduction of oxygen. Compared with the 1D CMP nanotubes- [52], POPs- [53] and COPs-based catalysts [54,55], S,N-codoped graphene [56], MoS2QDs@Ti3C2TxQDs@MWCNTs-2 [57], N-doped diamond/graphite hybrid nanosheets [58], the NHCNT-1 shows better onset potential, electron transferred number and comparative half-wave potential (as shown in Table 1). RRDE results suggest that the average value of H2O2 yield (H2O2 avg) for the NHCNT-1 is 8.4% at 0.2e0.8 V vs. RHE in 0.1 M KOH, while H2O2 avg ¼ 11.3% for HCNT-1 and 2.8% for Pt/C, indicating NHCNT-1 have superior ORR catalytic efficiency. And the good performance on half-wave potential and selectivity of 4-

electron reaction can be attributed to better mass transfer generated by more defects and large specific surface area. As shown in Fig. 6, CV and LSV measurements were also carried out in 0.1 M HClO4. For Fig. 6a and c, pronounced cathodic ORR peak was detected in O2-saturated 0.1 M HClO4 whereas only indistinctive double-layer charging current could be observed in N2-saturated electrolyte. As shown in Fig. 6d, the average value of n (navg) at 0.2e0.8 V vs. RHE is 3.94 for NHCNT-1 in 0.1 M HClO4, while navg ¼ 3.40 for HCNT-1 and 3.99 for Pt/C. And the average value of H2O2 yield (H2O2 avg) for the NHCNT-1 is 3.2% at 0.2e0.8 V vs. RHE in 0.1 M HClO4, while H2O2 avg ¼ 0.3% for Pt/C. As for HCNT-1, ORR was no longer 4-electron over 0.6 V vs. RHE. As shown in Fig. 6e, the values of half-wave potential for HCNT-1, NHCNT-1 and Pt/C in HClO4 are lower than those in KOH. And the detailed numerical results were summarized in Table S3.

Fig. 7. (a, b) LSV curves of NHCNT-1 in O2-saturated 0.1 M KOH and 0.1 M HClO4 at various rotation speeds. The inset is KouteckyeLevich (KeL) plots at different potentials. (c, d) CV, (e, f) LSV curves of NHCNT-1 and Pt/C in O2-saturated 0.1 M KOH and 0.1 M HClO4 with (dotted) and without (solid) 1 M MeOH. Electrode rotation speed: 1600 rpm; scan rate: 10 mV s
1.

W. Zhang et al. / Renewable Energy 146 (2020) 2270e2280

2279

Fig. 8. (a, b) Durability test of NHCNT-1 for 5000 cycles in O2-saturated 0.1 M KOH and 0.1 M HClO4. (c) Chronoamperometric experiments of Pt/C and NHCNT-1 in 0.1 KOH.

As shown in Fig. 7a and b, higher rotational speed, faster mass transfer and bigger diffusion current because of the exposure of active site. And the anti-poisoning on MeOH experiments of the NHCNT-1 and Pt/C catalysts were carried out by adding 1.0 M MeOH to the acidic and alkaline electrolyte, respectively. CV curves (Fig. 7c and d) for NHCNT-1 exhibit no visible variation in the presence of MeOH. For comparison, representative methanol oxidation behavior could be detected on Pt/C catalyst with the addition of methanol in 0.1 M KOH and 0.1 M HClO4. And the LSV curves (Fig. 7e and f) for NHCNT-1 exhibit tiny differences in the presence of 1 M MeOH, but a visible negative shift on E1/2 for commercial Pt/C catalyst is observed. Compared with Pt/C, to sum up, NHCNT-1 exhibits higher selectivity and stability during the ORR process. Furthermore, NHCNT-1 also exhibits better durability for ORR, as proved by the small shifts of half-wave potential after 5000 cycles (Fig. 8a and b). And the stability of NHCNT-1 was also determined by chronoamperometric experiments in 0.1 M KOH (Fig. 8c). The stability analysis of NHCNT-1 was evaluated by chronoamperometric experiments at a rotation speed of 1225 rpm with a constant potential (~0.7 V vs. RHE) in 0.1 M KOH. Normalized current plot suggests that the loss of current density on NHCNT-1 is only 5% after 15,000 s polarization, while the value of Pt/C is 19%, implying compared with Pt/C, it is more stable. 4. Conclusion In summary, we have demonstrated a new approach for fabrication of N-doped hard carbon nanotubes (NHCNTs) originated from conjugated microporous polymers as novel non-metal catalyst for efficient oxygen reduction reaction. The unique cylindrical structure accompanying with excellent porosity and large specific surface area of the resulting catalyst would facilitate the exposure of active sites as well as a quick transportation of oxygen and electrons. And the nitrogen-induced charge delocalization could weaken the OeO bonding to facilitate ORR. In addition, the high content of pyridinic N of NHCNTs promotes the ORR owing to the strong affinity of lone pair electron of pyridinic N to oxygen atom. Based on the inherent merits of NHCNTs mentioned above, the resulting catalysts exhibit good ORR activity not only in alkaline electrolyte but also in acidic condition with a high electron transferred number and better methanol immunity compared to commercial 20 wt% Pt/C. Due to the designable flexibility of CMPs which makes it possible to tune both their structure and chemical components by facile varying the building block of CMPs, further enhancement of the ORR activity as well as stability and methanol tolerance of CMPs-based catalysts can be anticipated, thus opening a new opportunity for rational design and fabrication of high performance catalysts by employing CMPs as precursors. Acknowledgements This work was supported by the National Natural Science

Foundation of China [grant numbers 51663012, 51462021]; the Natural Science Foundation of Gansu Province, China [grant number 1610RJYA001]; Project of Collaborative Innovation Team, Gansu Province, China [grant number 052005]; and Innovation and Entrepreneurship Talent Project of Lanzhou [2017-RC-33]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.renene.2019.08.071. References [1] A. Morozan, B. Jousselme, S. Palacin, Low-platinum and platinum-free catalysts for the oxygen reduction reaction at fuel cell cathodes, Energy Environ. Sci. 4 (2011) 1238e1254, https://doi.org/10.1039/c0ee00601g. [2] J. Zhu, M. Xiao, P. Song, J. Fu, Z. Jin, L. Ma, J. Ge, C. Liu, Z. Chen, W. Xing, Highly polarized carbon nano-architecture as robust metal-free catalyst for oxygen reduction in polymer electrolyte membrane fuel cells, Nano Energy 49 (2018) 23e30, https://doi.org/10.1016/j.nanoen.2018.04.021. [3] J. Yang, H. Sun, H. Liang, H. Ji, L. Song, C. Gao, H. Xu, A highly efficient metalfree oxygen reduction electrocatalyst assembled from carbon nanotubes and graphene, Adv. Mater. 28 (2016) 4606e4613, https://doi.org/10.1002/ adma.201505855. [4] M. Ruck, A. Bandarenka, F. Calle-Vallejo, A. Gagliardi, Oxygen reduction reaction: rapid prediction of mass activity of nanostructured platinum electrocatalysts, J. Phys. Chem. Lett. 9 (2018) 4463e4468, https://doi.org/10.1021/ acs.jpclett.8b01864. [5] M. Li, Z. Zhao, T. Cheng, A. Fortunelli, C.-Y. Chen, R. Yu, Q. Zhang, L. Gu, B. V Merinov, Z. Lin, E. Zhu, T. Yu, Q. Jia, J. Guo, L. Zhang, W.A. Goddard, Y. Huang, X. Duan, Ultrafine jagged platinum nanowires enable ultrahigh mass activity for the oxygen reduction reaction, Science 354 (2016) 1414e1419, https:// doi.org/10.1126/science.aaf9050. [6] Z.W. Seh, J. Kibsgaard, C.F. Dickens, I. Chorkendorff, J.K. Norskov, T.F. Jaramillo, Combining theory and experiment in electrocatalysis: insights into materials design, Science 355 (2017) 146e157, https://doi.org/10.1126/ science.aad4998. [7] K. Jiang, D. Zhao, S. Guo, X. Zhang, X. Zhu, J. Guo, G. Lu, X. Huang, Efficient oxygen reduction catalysis by subnanometer Pt alloy nanowires, Sci. Adv. 3 (2017), 1601705, https://doi.org/10.1126/sciadv.1601705. [8] S. Chen, Z. Niu, C. Xie, M. Gao, M. Lai, M. Li, P. Yang, Effects of catalyst processing on the activity and stability of Pt-Ni nanoframe electrocatalysts, ACS Nano 12 (2018) 8697e8705, https://doi.org/10.1021/acsnano.8b04674. [9] P. Zhang, F. Sun, Z. Xiang, Z. Shen, J. Yun, D. Cao, ZIF-derived in situ nitrogendoped porous carbons as efficient metal-free electrocatalysts for oxygen reduction reaction, Energy Environ. Sci. 7 (2014) 442e450, https://doi.org/ 10.1039/c3ee42799d. [10] M. Sial, H. Lin, M. Zulfiqar, S. Ullah, B. Ni, X. Wang, Trimetallic PtCoFe alloy monolayer superlattices as bifunctional oxygen-reduction and ethanoloxidation electrocatalysts, Small 13 (2017), 1700250, https://doi.org/ 10.1002/smll.201700250. [11] X. Cui, S. Yang, X. Yan, J. Leng, S. Shuang, P.M. Ajayan, Z. Zhang, Pyridinicnitrogen-dominated graphene aerogels with Fe-N-C coordination for highly efficient oxygen reduction reaction, Adv. Funct. Mater. 26 (2016) 5708e5717, https://doi.org/10.1002/adfm.201601492. [12] H. Zhang, W. Zhou, T. Chen, B.Y. Guan, Z. Li, X.W. Lou, A modular strategy for decorating isolated cobalt atoms into multichannel carbon matrix for electrocatalytic oxygen reduction, Energy Environ. Sci. (2018), https://doi.org/ 10.1039/c8ee00901e. [13] Y. Zhao, J. Wan, H. Yao, L. Zhang, K. Lin, L. Wang, N. Yang, D. Liu, L. Song, J. Zhu, L. Gu, L. Liu, H. Zhao, Y. Li, D. Wang, Few-layer graphdiyne doped with sphybridized nitrogen atoms at acetylenic sites for oxygen reduction electrocatalysis, Nat. Chem. (2018), https://doi.org/10.1038/s41557-018-0100-1. [14] T. Sun, Q. Wu, O. Zhuo, Y. Jiang, Y. Bu, L. Yang, X. Wang, Z. Hu, Manganese

2280

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33] [34]

[35]

[36]

W. Zhang et al. / Renewable Energy 146 (2020) 2270e2280 oxide-induced strategy to high-performance iron/nitrogen/carbon electrocatalysts with highly exposed active sites, Nanoscale 8 (2016) 8480e8485, https://doi.org/10.1039/C6NR00760K. Y.C. Wang, Y.J. Lai, L. Song, Z.Y. Zhou, J.G. Liu, Q. Wang, X.D. Yang, C. Chen, W. Shi, Y.P. Zheng, M. Rauf, S.G. Sun, S-doping of an Fe/N/C ORR catalyst for polymer electrolyte membrane fuel cells with high power density, Angew. Chem. Int. Ed. 54 (2015) 9907e9910, https://doi.org/10.1002/anie.201503159. J. Ding, P. Wang, S. Ji, H. Wang, V. Linkov, R. Wang, N-doped mesoporous FeNx/carbon as ORR and OER bifunctional electrocatalyst for rechargeable zinc-air batteries, Electrochim. Acta 296 (2018) 653e661, https://doi.org/ 10.1016/j.electacta.2018.11.105. S. Grewal, A. Macedo Andrade, A.J. Nelson, K. Thai, A. Karimaghaloo, E. Lee, M.H. Lee, Critical impact of graphene functionalization for transition metal oxide/graphene hybrids on oxygen reduction reaction, J. Phys. Chem. C 122 (2018) 10017e10026, https://doi.org/10.1021/acs.jpcc.8b01893. H. Osgood, S.V. Devaguptapu, H. Xu, J. Cho, G. Wu, Transition metal (Fe, Co, Ni, and Mn) oxides for oxygen reduction and evolution bifunctional catalysts in alkaline media, Nano Today 11 (2016) 601e625, https://doi.org/10.1016/ j.nantod.2016.09.001. Y. Zhong, X. Xia, F. Shi, J. Zhan, J. Tu, H.J. Fan, Transition metal carbides and nitrides in energy storage and conversion, Adv. Sci. 3 (2016), 1500286, https:// doi.org/10.1002/advs.201500286. H. Yu, L. Shang, T. Bian, R. Shi, G.I. Waterhouse, Y. Zhao, C. Zhou, L.Z. Wu, C.H. Tung, T. Zhang, Nitrogen-doped porous carbon nanosheets templated from g-C3N4 as metal-free electrocatalysts for efficient oxygen reduction reaction, Adv. Mater. 28 (2016) 5080e5086, https://doi.org/10.1002/ adma.201600398. X. Qin, Y. Huang, K. Wang, T. Xu, Y. Wang, P. Liu, Y. Kang, Y. Zhang, Novel hierarchically porous Ti-MOFs/nitrogen-doped graphene nanocomposite served as high efficient oxygen reduction reaction catalyst for fuel cells application, Electrochim. Acta 297 (2019) 805e813, https://doi.org/10.1016/ j.electacta.2018.12.045. A. Alsudairi, J. Li, N. Ramaswamy, S. Mukerjee, K.M. Abraham, Q. Jia, Resolving the iron phthalocyanine redox transitions for ORR catalysis in aqueous media, J. Phys. Chem. Lett. 8 (2017) 2881e2886, https://doi.org/10.1021/ acs.jpclett.7b01126. K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction, Science 323 (2009) 760e764, https://doi.org/10.1126/science.1168049. J. Shui, M. Wang, F. Du, L. Dai, N-doped carbon nanomaterials are durable catalysts for oxygen reduction reaction in acidic fuel cells, Sci. Adv. 1 (2015) 1e7, https://doi.org/10.1126/sciadv.1400129. H. Bin Yang, J. Miao, S.F. Hung, J. Chen, H.B. Tao, X. Wang, L. Zhang, R. Chen, J. Gao, H.M. Chen, L. Dai, B. Liu, Identification of catalytic sites for oxygen reduction and oxygen evolution in N-doped graphene materials: development of highly efficient metal-free bifunctional electrocatalyst, Sci. Adv. 2 (2016), https://doi.org/10.1126/sciadv.1501122 e1501122ee1501122. J. Deng, M. Li, Y. Wang, Biomass-derived carbon: synthesis and applications in energy storage and conversion, Green Chem. 18 (2016) 4824e4854, https:// doi.org/10.1039/c6gc01172a. 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) 443e447, https://doi.org/10.1126/science.1200832. Z.S. Wu, A. Winter, L. Chen, Y. Sun, A. Turchanin, X. Feng, K. Müllen, Threedimensional nitrogen and boron co-doped graphene for high-performance all-solid-state supercapacitors, Adv. Mater. 24 (2012) 5130e5135, https:// doi.org/10.1002/adma.201201948. W. Ding, Z. Wei, S. Chen, X. Qi, T. Yang, J. Hu, D. Wang, L.J. Wan, S.F. Alvi, L. Li, Space-confinement-induced synthesis of pyridinic- and pyrrolic-nitrogendoped graphene for the catalysis of oxygen reduction, Angew. Chem. Int. Ed. 52 (2013) 11755e11759, https://doi.org/10.1002/anie.201303924. T. Xing, Y. Zheng, L.H. Li, B.C.C. Cowie, D. Gunzelmann, S.Z. Qiao, S. Huang, Y. Chen, Observation of active sites for oxygen reduction reaction on nitrogendoped multilayer graphene, ACS Nano 8 (2014) 6856e6862, https://doi.org/ 10.1021/nn501506p. S. Saji, C. Akiba, D. Guo, R. Shibuya, J. Nakamura, T. Kondo, Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts, Science 351 (2016) 361e365, https://doi.org/10.1126/ science.aad0832. J.-X. Jiang, F. Su, A. Trewin, C.D. Wood, N.L. Campbell, H. Niu, C. Dickinson, A.Y. Ganin, M.J. Rosseinsky, Y.Z. Khimyak, A.I. Cooper, Conjugated microporous poly(aryleneethynylene) networks, Angew. Chem. Int. Ed. 119 (2007) 8728e8732, https://doi.org/10.1002/ange.200701595. A.I. Cooper, Conjugated microporous polymers, Adv. Mater. 21 (2009) 1291e1295, https://doi.org/10.1002/adma.200801971. Y. He, D. Gehrig, F. Zhang, C. Lu, C. Zhang, M. Cai, Y. Wang, F. Laquai, X. Zhuang, X. Feng, Highly efficient electrocatalysts for oxygen reduction reaction based on 1D ternary doped porous carbons derived from carbon nanotube directed conjugated microporous polymers, Adv. Funct. Mater. 26 (2016) 1e11, https://doi.org/10.1002/adfm.201603693. H. Wei, F. Wang, X. Qian, S. Li, Z. Hu, H. Sun, A. Li, Superhydrophobic fluorinerich conjugated microporous polymers monolithic nanofoam with excellent heat insulation property, Chem. Eng. J. 351 (2018) 856e866, https://doi.org/ 10.1016/j.cej.2018.06.162. P. Mu, Z. Zhang, W. Bai, J. He, H. Sun, Z. Zhu, W. Liang, A. Li, Superwetting

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

[57]

[58]

monolithic hollow-carbon-nanotubes aerogels with hierarchically nanoporous structure for efficient solar steam generation, Adv. Energy Mater. 9 (2019), 1802158, https://doi.org/10.1002/aenm.201802158. X. Feng, Y. Liang, L. Zhi, A. Thomas, D. Wu, I. Lieberwirth, U. Kolb, K. Müllen, Synthesis of microporous carbon nanofibers and nanotubes from conjugated polymer network and evaluation in electrochemical capacitor, Adv. Funct. Mater. 19 (2009) 2125e2129, https://doi.org/10.1002/adfm.200900264. L. Qu, Y. Liu, J.B. Baek, L. Dai, Nitrogen-doped graphene as efficient metal-free electrocatalyst for oxygen reduction in fuel cells, ACS Nano 4 (2010) 1321e1326, https://doi.org/10.1021/nn901850u. P. Yin, T. Yao, Y. Wu, L. Zheng, Y. Lin, W. Liu, H. Ju, J. Zhu, X. Hong, Z. Deng, G. Zhou, S. Wei, Y. Li, Single cobalt atoms with precise N-coordination as superior oxygen reduction reaction catalysts, Angew. Chem. Int. Ed. 55 (2016) 10800e10805, https://doi.org/10.1002/anie.201604802. N. Wakabayashi, M. Takeichi, M. Itagaki, H. Uchida, M. Watanabe, Temperature-dependence of oxygen reduction activity at a platinum electrode in an acidic electrolyte solution investigated with a channel flow double electrode, J. Electroanal. Chem. 574 (2005) 339e346, https://doi.org/10.1016/ j.jelechem.2004.08.013. s, R. Faure, R. Durand, Electrochemical reduction of oxygen on platL. Genie inum nanoparticles in alkaline media, Electrochim. Acta 44 (1998) 1317e1327, https://doi.org/10.1016/S0013-4686(98)00254-0. D. Yan, Y. Li, J. Huo, R. Chen, L. Dai, S. Wang, Defect chemistry of nonpreciousmetal electrocatalysts for oxygen reactions, Adv. Mater. 29 (2017) 1e20, https://doi.org/10.1002/adma.201606459. Z. Zhang, J. Sun, F. Wang, L. Dai, Efficient oxygen reduction reaction (ORR) catalysts based on single iron atoms dispersed on a hierarchically structured porous carbon framework, Angew. Chem. Int. Ed. 57 (2018) 9038es, https:// doi.org/10.1002/anie.201804958. L. Lin, Q. Zhu, A. Xu, Noble-metal-free Fe
N/C catalyst for highly E ffi cient oxygen reduction reaction under both alkaline and acidic conditions, J. Am. Chem. Soc. 136 (2014) 11027e11033, https://doi.org/10.1021/ja504696r. L. Zhang, Z. Xia, Mechanisms of oxygen reduction reaction on nitrogen-doped graphene for fuel cells, J. Phys. Chem. C 115 (2011) 11170e11176, https:// doi.org/10.1021/jp201991j. D.W. Boukhvalov, Y.W. Son, Oxygen reduction reactions on pure and nitrogen-doped graphene: a first-principles modeling, Nanoscale 4 (2012) 417e420, https://doi.org/10.1039/c1nr11307k. M. Thommes, K. Kaneko, A.V. Neimark, J.P. Olivier, F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing, Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report), Pure Appl. Chem. 87 (2015) 1051e1069, https://doi.org/10.1515/pac-20141117. J. Zhang, Y. Sun, J. Zhu, Z. Kou, P. Hu, L. Liu, S. Li, S. Mu, Y. Huang, Defect and pyridinic nitrogen engineering of carbon-based metal-free nanomaterial toward oxygen reduction, Nano Energy 52 (2018) 307e314, https://doi.org/ 10.1016/j.nanoen.2018.08.003. H. Li, G. Sun, N. Li, S. Sun, D. Su, Q. Xin, Design and preparation of highly active Pt-Pd/C catalyst for the oxygen reduction reaction, J. Phys. Chem. C 111 (2007) 5605e5617, https://doi.org/10.1021/jp067755y. C. Li, H. Tan, J. Lin, X. Luo, S. Wang, J. You, Y.M. Kang, Y. Bando, Y. Yamauchi, J. Kim, Emerging Pt-based electrocatalysts with highly open nanoarchitectures for boosting oxygen reduction reaction, Nano Today 21 (2018) 91e105, https://doi.org/10.1016/j.nantod.2018.06.005. G.A. Ferrero, A.B. Fuertes, M. Sevilla, M.-M. Titirici, Efficient metal-free Ndoped mesoporous carbon catalysts for ORR by a template-free approach, Carbon 106 (2016) 179e187, https://doi.org/10.1016/j.carbon.2016.04.080. Y. He, D. Gehrig, F. Zhang, C. Lu, C. Zhang, M. Cai, Y. Wang, F. Laquai, X. Zhuang, X. Feng, Highly efficient electrocatalysts for oxygen reduction reaction based on 1D ternary doped porous carbons derived from carbon nanotube directed conjugated microporous polymers, Adv. Funct. Mater. 26 (2016) 8255e8265, https://doi.org/10.1002/adfm.201603693. J. Yang, M. Xu, J. Wang, S. Jin, B. Tan, A facile approach to prepare multiple heteroatom-doped carbon materials from imine-linked porous organic polymers, Sci. Rep. 8 (2018) 4200, https://doi.org/10.1038/s41598-018-22507-2. J. Guo, C.Y. Lin, Z. Xia, Z. Xiang, A pyrolysis-free covalent organic polymer for oxygen reduction, Angew. Chem. Int. Ed. 57 (2018) 12567e12572, https:// doi.org/10.1002/anie.201808226. Z. Xiang, D. Cao, L. Huang, J. Shui, M. Wang, L. Dai, Nitrogen-doped holey graphitic carbon from 2D covalent organic polymers for oxygen reduction, Adv. Mater. 26 (2014) 3315e3320, https://doi.org/10.1002/adma.201306328. F. Liu, F. Niu, T. Chen, J. Han, Z. Liu, W. Yang, Y. Xu, J. Liu, One-step electrochemical strategy for in-situ synthesis of S,N-codoped graphene as metal-free catalyst for oxygen reduction reaction, Carbon 134 (2018) 316e325, https:// doi.org/10.1016/j.carbon.2018.04.007. X. Yang, Q. Jia, F. Duan, B. Hu, M. Wang, L. He, Y. Song, Z. Zhang, Multiwall carbon nanotubes loaded with MoS2quantum dots and MXene quantum dots: nonePt bifunctional catalyst for the methanol oxidation and oxygen reduction reactions in alkaline solution, Appl. Surf. Sci. 464 (2019) 78e87, https:// doi.org/10.1016/j.apsusc.2018.09.069. S. Wang, X. Ji, Y. Ao, J. Yu, Vertically aligned N-doped diamond/graphite hybrid nanosheets epitaxially grown on B-doped diamond films as electrocatalysts for oxygen reduction reaction in an alkaline medium, ACS Appl. Mater. Interfaces 10 (2018) 29866e29875, https://doi.org/10.1021/acsami.8b06101.