nitrogen-doped carbon for robust oxygen reduction reaction

Carbon 155 (2019) 545e552

Contents lists available at ScienceDirect

Carbon journal homepage: www.elsevier.com/locate/carbon

Interfacial metal-nitrogen units of NiCo/nitrogen-doped carbon for robust oxygen reduction reaction Wangyan Gou a, 1, Jihong Bian a, 1, Mingkai Zhang a, Zhaoming Xia a, Yuxuan Liu a, Yaodong Yang a, Qingchen Dong b, **, Jiayuan Li a, ***, Yongquan Qu a, * a Center of Applied Chemical Research, Frontier Institute of Science and Technology and School of Chemical Engineering and Technology, Xi'an Jiaotong University, Xi'an, 710049, China b MOE Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, No. 79 Yingze West Street, Taiyuan, 030024, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 June 2019 Received in revised form 24 August 2019 Accepted 1 September 2019 Available online 2 September 2019

Corrosion-induced nitrogen (N) leaching limits the long-term applications of N-doped carbon (NC) catalysts for electrocatalytic oxygen reduction reaction (ORR). Anchored N for NC can suppress their corrosion and thus restrain the loss of catalytically active sites, resulting in the improved ORR durability. Herein, we report our strategies to suppress the leaching of N by embedding the NiCo alloy nanoparticles in NC nanowires and thus creating the interfacial metal-N units (M-N) for the robust ORR durability in alkali over 50 h. Also, the synergistic effects of Ni/CoeN, abundant active sites and favourable kinetics bring the high ORR activity over the Pt/C benchmark. Thus, our strategies with the improved durability and activity could create promising ORR catalysts with ~5 times of activity-durability factor over the NC catalysts. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Electrocatalysis Ni/CoeN units N leaching Oxygen reduction reaction Durability

1. Introduction Electrochemical oxygen reduction reaction (ORR) plays a key role in energy conversion and storage technologies, such as metalair batteries and fuel cells [1,2]. To date, most of state-of-the-art electrocatalysts for ORR are platinum (Pt)-based materials due to their low overpotentials and large catalytic current densities. Nevertheless, the further developments and applications of these catalysts are limited by their high-cost and declining activity [3,4]. Therefore, great efforts have been made in this field to explore various cost-effective alternatives [5e12]. Searching for economical and robust catalysts for ORR is essential. Among various substitutes, nitrogen (N)-doped carbon (NC) materials are attracting great interests in ORR electrocatalysis due to their great advantages in cost performance [13e17]. Thereinto, N

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (Q. Dong), [email protected] (J. Li), [email protected] (Y. Qu). 1 W. Gou and J. Bian contributed equally to this work. https://doi.org/10.1016/j.carbon.2019.09.001 0008-6223/© 2019 Elsevier Ltd. All rights reserved.

dopant (especially pyridinic-N) tailors the electron-donor properties of nearby carbon atoms, affording a favourable adsorption of OOH* intermediates and thus creating a series of active sites for ORR [18,19]. Thus, the superior ORR activity of NC can be ascribed to the existence of N. Except for activity, durability is equally a key factor in determining the performance of electrocatalysts for practical applications [20]. Unfortunately, the long-term use of NC is usually accompanied by the possible leaching of N, which leads to the loss of active sites and thereby the seriously declined activity [21,22]. As a result, NC catalysts suffer from the poor durability for ORR. Although significant advances have been achieved on the design and synthesis of various active NC catalysts, seldom efforts have been placed on the strategies to improve their durability. Up to now, the conception for improving their durability is still controversial due to the ambiguous cognition for the loss of the active sites and aforesaid N leaching [20e24]. The relatively convincible theory indicates that high carbonization temperature has the complicated effects, which improves the durability of ORR by enhancing the chemical resistance of catalysts against corrosion, but weakens the activity by suppressing the reservation of pyridinic-N in the meantime [25,26]. Therefore, to find a welldesigned strategy for pursuing both activity and durability of ORR

546

W. Gou et al. / Carbon 155 (2019) 545e552

catalyst is desirable. Recently, NC assisted by single-atom transition metals (M-N-C) have presented the most potential as the substitutes of noble-metal based electrocatalysts [27e33]. In M-N-C, pyridinic-N presents high affinity towards transition metal to form M-N units [34,35], which are widely considered to promote ORR effectively. In addition, for electrocatalysts, many efforts have demonstrated that bimetallic catalysts are superior to the monometallic counterparts owing to the synergistic effects between the two different metal components. For instance, among the various bimetallic alloys, NiCo alloy exhibits a great application potential because nickel and cobalt are earth-abundant and environmental friendly. Furthermore, it can provide rich valence state changes, which are important to exhibit high intrinsic activity for ORR [36,37]. Inspired by these, combining the bimetals and NC to form the interfacial M-N units could likely improve the overall ORR performance including both the catalytic activity and durability on account of anchoring pyridinic-N and thereby restraining N leaching. So far, a lot of studies concentrate on designing M-N-C with excellent activity towards ORR, while there are few investigations involving utilization of M-N units to improve durability in alkaline solution. In consideration of the difficulty of the structural construction on a large scale for single-atom metal catalysts and taking advantages of the superiority of synergistic effects for bimetal towards ORR, we proposed our strategy for improving both ORR durability and activity of NC catalysts by embedding the bimetallic alloy nanoparticles into the porous NC nanowires as well as creating the interfacial M-N units. Among various bimetallic systems, NiCo nanoparticles/N-doped carbon (NiCo/NC) system delivered the best ORR performance. Especially, the NiCo/NC catalysts with the Ni:Co molar ratio of 1:1 showed high ORR activity and robust durability over a period of 50 h with feeble N loss, presenting an activitydurability factor (ADF) of ~5 times over the NC catalysts. 2. Experimental section All chemicals are analytical-grade and commercially available, used without further purification. 2.1. Synthesis of NiCo/NC catalysts The precursor solutions were prepared by dissolving 93.6 mg of polyacrylonitrile (PAN, average molecular weight ¼ 150000, Aldrich) into 0.8 mL of N-dimethylformamide (DMF) with vigorous stirring at 80  C overnight. Afterwards, melamine (0.45 mmol) was added into the precursor solution under stirring overnight before adding metal salts. A total of 0.45 mmol of nickel nitrate (Ni(NO3)2$6H2O) and cobalt (II) acetate tetrahydrate (Co(Ac)2$4H2O) were dissolved into 0.3 mL of DMF under ultrasonication to form metal salts solutions, with molar ratios of (Ni(NO3)2$6H2O) : (Co(Ac)2$4H2O) ¼ 1:2, 1:1, 2:1, 0:1, respectively. As to the catalyst with molar ratio of 1:0, Ni(NO3)2$6H2O was replaced by nickel (II) acetate tetrahydrate (Ni(Ac)2$4H2O). The as-prepared precursors for electrospinning were prepared by pipetting the metal salts solution into the polymer precursor solution under stirring for 6 h and then reposing for 2 h, which were electrospun into fibers through a self-built electrospinning apparatus at 25 kV with aluminum foil as the collector. The distance between the needle and the collector was 17 cm, and the flow rate of the spinning solution was 0.5 mL h1. The electrospun-derived fibers were treated under 60  C for 6 h in a vacuum oven. The stabilized nanofibers were annealed at 300  C for 1 h and sequentially at 800  C for 2 h with a ramping rate of 5  C min1 in argon atmosphere to obtain the carbon nanofibers containing metal-based nanoparticles. The samples with Ni and Co molar ratios of 1:2, 1:1, 2:1, 0:1 and 1:0

were labeled as NiCo/NC-1, NiCo/NC-2, NiCo/NC-3, Co/NC and Ni/ NC, respectively. For the N-doped carbon fibers, the synthetic process was similar as aforementioned without adding metal salts solution and marked as NC. 2.2. Synthesis of FeCo/NC and NiFe/NC catalysts The FeCo/NC and NiFe/NC catalysts were synthesized by similar methods as aforementioned with different metal salts in precursors. A total of 0.45 mmol of Co(Ac)2$4H2O and iron (III) nitrate enneahydrate (Fe(NO3)3$9H2O), or Ni(NO3)2$6H2O and Fe(NO3)3$9H2O with 1:1 M ratio were dissolved into 0.3 mL of DMF under ultrasonication to form metal salt solutions, respectively. The obtained mixture solutions were electrospun and treated similarly as aforementioned. The samples with different metal salts were labeled as FeCo/NC and NiFe/NC, respectively. 2.3. Synthesis of NiCo/C catalysts The precursor sol-gel for NiCo/C was prepared by mixing Ni(NO3)2$6H2O and Co(Ac)2$4H2O solution with molar ratio of 1:1 (total 0.45 mmol in 300 mL of water) and a poly(vinyl alcohol) (PVA, average molecular weight ¼ 65000 g mol1, Aldrich) aqueous solution (300 mg in 2.5 mL of water). The mixture was vigorously stirred at 80  C for 5 h. The obtained solution was electrospun and treated similarly as aforementioned. 2.4. Material characterizations Raman spectrum analysis (INVIAREFLEX, with laser excitation at 633 nm sweep from 500 to 2000 cm1, 0.2 mW power) was adopted to distinguish the characteristic vibrational modes of the synthesized materials. X-Ray diffraction (XRD) measurements were performed for all catalysts on a Shimadzu X-ray diffractometer using Cu Ka radiation. Transmission electron microscopy (TEM) studies were conducted on a Hitachi HT-7700 transmission electron microscope with an accelerating voltage of 100 kV. High resolution TEM (HRTEM) images were performed on a FEI Tecnai G2 F20 S-Twin microscope with the accelerating voltage of 200 kV. The thermogravimetry analysis (TGA) was conducted at a ramping rate of 10  C min1 between 30 and 900  C in air on METTLER TOLEDO TGA1 STARe System. Nitrogen absorption/desorption isotherms were obtained on a Micromeritics ASAP 2020 HD88 analyzer at 77 K and the corresponding surface areas and pore distributions were determined by the Brunauere Emmette Teller (BET) equation. X-ray photoelectron spectra (XPS) data were acquired on a Thermo Electron Model K-Alpha with Al Ka as the excitation source. The valence band (VB) XPS spectra of the NiCo/NC and NiCo/C were obtained by subtracting carbon and nitrogen-doped carbon contributions from the as-obtained VB XPS spectra. The valence band maximum (VBM) was determined by extrapolating the excited leading edge to the energy axis to serve as a reference of the d-band center. The d-band center is located at the point which splits the VB XPS spectra into two parts of equal areas. Ion chromatography measurements were conducted on a DIONEX INTEGRION. 2.5. Electrochemical measurements The catalyst ink was prepared by dispersing 4 mg of catalysts into a mixture of 768 mL of H2O, 200 mL of ethanol and 32 mL of 5 wt% Nafion. Then, 5 mL of the catalyst ink was loaded onto a glass carbon (GC) electrode to reach a catalyst density of 0.28 mg cm2. Electrochemical measurements including linear sweep voltammograms (LSV), current voltammograms (CV) and chronoamperometry were carried out on a CHI 660D

W. Gou et al. / Carbon 155 (2019) 545e552

electrochemistry workstation (CH Instrument, Shanghai, China) with a typical three-electrode system, where the Pt wire and the Ag/AgCl electrode served as the counter electrode and the reference electrode, respectively. ORR electrocatalytic performances were evaluated on RDE (rotating disk electrodes) electrode system in O2saturated KOH (0.1 M). The kinetics parameters are calculated by using the KouteckyLevich equation (1):

1 1 1 1 1 ¼ þ ¼ þ jjj jjK j jjL j jjK j Bu1=2

(1)

where j and jL are the measured and diffusion limiting current densities, respectively; u is the electrode rotating rate. B is determined from the slope of the Koutechy-Levich (K-L) plots based on the Levich equation (2):

B ¼ 0:62nFC0 ðD0 Þ2=3 n1=6

(2)

where n represents the transferred electron number per oxygen molecule; F is the Faraday constant (96485 C mol1); C0 is the bulk concentration of O2 (1.2  106 mol cm3); D0 is the diffusion coefficient of O2 in 0.1 M KOH (1.9  105 cm2 s1); y is the kinetic viscosity (0.01 cm2 s1). The electrochemical impedance spectroscopy (EIS) measurements were conducted on the Autolab PGSTAT204 electrochemistry work station in a frequency range of 0.1 Hze100 kHz at a fixed voltage of 0.81 V vs. RHE. 3. Results and discussion Various NiCo/NC catalysts were synthesized through the electrospinning and subsequent calcination under the protection of Ar at high temperatures. Their Raman spectra displayed the graphitic features (1280 cm1 of D-band and 1520 cm1 of G-band) in these catalysts (Fig. S1) [36], indicating the formation of graphitic carbon in the catalysts. All NiCo/NC catalysts exhibited the similar structural and morphological features, as revealed from TEM and HRTEM images (Fig. 1a, S2 and S3). The uniformly distributed nanoparticles encapsulated within graphitic carbon layers embedded in the nitrogen-doped carbon nanofibers with a diameter of ~200 nm. The sizes of metal nanoparticles were around 5e20 nm. XRD patterns of various NiCo/NC catalysts presented the similar peaks at ~25.0 , which was ascribed to (002) plane of graphitic carbon, thus further ascertain the existence of graphitelike species in NiCo/NC catalysts. Moreover, XRD results delivered the peaks at 44.4 and 51.7 for NiCo/NC, which could be assigned to (111) and (200) planes of metallic Co (PDF-#15e0806) or Ni (PDF-#04e0850) (Fig. 1b). The zoom-in XRD patterns for NiCo/NC catalysts displayed the obvious peak-shift and located between the standard Ni and Co. Considering that metallic Ni and Co presented similar crystal structure (cubic phase) and different lattice parameters derived from different atomic radius, such XRD pattern strongly suggested an alloying behavior of Ni and Co in the NiCo/NC catalysts (Fig. S4) [36,38]. Elemental mapping further revealed the uniformly distributed N and the alloying nature for the embedded nanoparticles (Fig. 1c). The loadings of NiCo nanoparticles were examined by TGA, presenting the similar metal loading efficiency for various NiCo/NC catalysts (~23%, Fig. S5). BET results displayed that NiCo/NC-2 had the largest surface area and pore volume, which confirmed its hierarchical micro-/mesoporous structure (Fig. S6 and Table S1) and was beneficial to obtain the superior ORR activity [28]. XPS measurements were performed to exploit the surface properties of the NiCo/NC catalysts (Fig. 1d). Focusing on the Co 2p

547

and Ni 2p core level region, the sub-peaks at 778.3 and 852.9 eV further supported the presence of metallic NiCo alloys. Significantly, the sub-peaks at 779.1, 781.9, 854.8 and 857.4 eV reflected the presence of Co and Ni with high valence state, which was likely due to the formation of M-N units. In N 1s core level region, XPS peaks can be well decomposed into five sub-peaks at 397.9, 398.9, 399.8, 400.9 and 402.3 eV, matching well with pyridinic N, M-N units, pyrrolic N, graphitic N and oxidized N, respectively [36,38]. The O 1s XPS spectra can be fitted to three peaks centered at 531.0, 532.4 and 534.2 eV for NiCo/NC-2, representing CeOH, CeO and absorbed H2O respectively (Fig. 1d). The absence of lattice O manifested the scarce formation of metal oxide species on the surface of NiCo/NC-2 [39]. The high-resolution C 1s peak can be deconvoluted into several peaks corresponding to CeC bond at 284.6 eV, CeO and CeN bonds at 285.6 eV, C]O bond at 286.4 eV, COO bond at 287.5 eV, and p-p* at 288.9 eV, excluding the existence of metal carbides (Fig. S7) [40]. These findings depict a clear microstructure for NiCo/NC catalysts, which were composed of the NC skeletons and embedded NiCo nanoparticles with the interfacial Ni/CoeN units. The influences of various Ni/Co ratios on ORR activity were investigated. According to the CV measurements (Fig. S8), the catalysts exhibited no reduction peak in Ar-saturated 0.1 M KOH solution. In contrast, an obvious reduction peak appeared in O2saturated solution, which was attributed to the reduction of O2 [41]. NiCo/NC-2 showed the most positive peak centered at 0.82 V. Derived from LSV curves, NiCo/NC-2 demonstrated distinct advantages with much positive onset potential (Eonset, 0.94 V), half wave potential (Ehalf, 0.84 V) and larger limiting current density (IL, 6.7 mA cm2) compared to other catalysts with different metal ratios. In addition, the electrochemical surface area (ECSA) of the NiCo/NC-1, NiCo/NC-2 and NiCo/NC-3 were assessed via the regional CV investigations at different scan rates (Fig. S9). The calculated ECSA for NiCo/NC-1, NiCo/NC-2 and NiCo/NC-3 were 0.26, 0.54 and 0.35 mF, respectively. Above results demonstrated the significant superiority of NiCo/NC-2 towards ORR due to their superior activity and abundant active sites. Besides, the FeCo/NC, NiFe/NC bimetallic catalysts with the 1:1 M ratio of two metals, which were synthesized through the same procedure except using different metal sources, exhibited the similar structural features of the NiCo/NC catalysts (Fig. S10). Their ORR activity was also evaluated under the identical conditions. Among them, the NiCo/NC catalysts exhibited Eonset/Ehalf of 0.94 V/ 0.84 V as well as jL of 6.7 mA cm2, which were significantly superior to those of FeCo/NC (Eonset/Ehalf ¼ 0.88 V/0.74 V) and NiFe/NC (Eonset/Ehalf ¼ 0.87 V/0.72 V). Thus, the NiCo/NC catalysts with the a Ni:Co molar ratio of 1:1 delivered the best ORR activity among different bimetallic catalysts with the similar morphology structure. To get insight into the high ORR performance of the NiCo/NC catalysts, the bare NC (Figs. S11 and S12) and carbon embedded with NiCo alloy nanoparticles (NiCo/C) (Fig. S13) were also prepared by the similar approach except for the absence of metal or nitrogen source. As shown in Fig. S14, NC exhibited a much negative peak potential at 0.62 V in CV measurements compared with NiCo/ NC-2 (0.82 V), suggesting feeble ORR electrocatalytic activity of NC. Fig. 2a showed the LSV curves of NiCo/NC-2, NiCo/C, NC and Pt/C catalysts. Introduction of M-N units increased the Eonset from 0.83 V for NC or 0.88 V for NiCo/C to 0.94 V for NiCo/NC-2, Ehalf from 0.62 V for NC or 0.72 V for NiCo/C to 0.84 V for NiCo/NC-2 and jL from 3.7 mA cm2 for NC or 4.7 mA cm2 for NiCo/C to 6.7 mA cm2 for NiCo/NC-2. The derived Tafel slope of NiCo/ NC-2 (45 mV dec1) was significantly smaller than those of the NC (71 mV dec1) and NiCo/C (70 mV dec1), and even lower than that of commercial Pt/C (64 mV dec1) (Fig. 2b) [42]. The superior

548

W. Gou et al. / Carbon 155 (2019) 545e552

Fig. 1. Characterizations of the catalysts. (a) TEM and HRTEM (inset) images of NiCo/NC-2. (b) XRD patterns of NiCo/NC-1, NiCo/NC-2, NiCo/NC-3, Co/NC and Ni/NC. (c) STEM image and elemental mapping images of N (green), Co (violet) and Ni (blue) in NiCo/NC-2. (d) XPS spectra of Co2p, Ni 2p, N 1s and O1s for NiCo/NC-2. (A colour version of this figure can be viewed online.)

Fig. 2. (a) ORR polarization curves of the NiCo/NC-2, NiCo/C, NC and Pt/C catalysts at a rotation rate of 1600 rpm in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s1. (b) Tafel plots of NiCo/NC-2, NiCo/C, NC and Pt/C catalysts. (c) High resolution valence-band spectra of NiCo/NC-2 and NiCo/C after background subtraction; bars indicate the d-band centers. (d) Koutecky-Levich plots of NiCo/NC-2 and Pt/C catalysts at 0.4 V. (A colour version of this figure can be viewed online.)

W. Gou et al. / Carbon 155 (2019) 545e552

ORR activity of NiCo/NC-2 is comparable to or even better than those of the Pt/C benchmark and state-of-the-art earth-abundant ORR catalysts in alkali (Table S2). The results demonstrated the importance of the synergistic effects of Ni/CoeN units on the superior ORR activity for NiCo/NC-2. The different electronegativities (1.88 for Co and 1.91 for Ni) and lattice parameters of Ni (a ¼ b ¼ c ¼ 3.524 Å) and Co (a ¼ b ¼ c ¼ 3.5447 Å) derived from different atomic radius in the nitrogen-doped carbon systems can establish intrinsic polarity when forming the bond between Ni and Co. It led to the weakened metallic bond (CoeCo and NieNi) by each other and thus the relatively stronger electropositive feature of metal when incorporating Ni into Co, which promote the formation of M-N active species [38]. To support this cognition, high-resolution XPS spectra in N 1s region was further investigated (Fig. S15). As shown in Table S3 and Fig. S16, the monometallic Co/NC catalysts showed an M-N unit yield of 1.2%. With the introduction of Ni, the M-N unit yields were raised to 2.4% for NiCo/NC-2. Correspondingly, the NiCo/NC-2 catalysts presented a significant improvement in their ORR performance, compared to the Co/NC catalysts. Clearly, the synergistic effects for Ni and Co could allow the easier formation of the active Ni/CoeN units and facilitate the ORR performance. To further clarify the synergistic effects on ORR activity, the valence band spectra of NiCo/NC-2 and NiCo/C were measured, which corresponded to the density of states (DOS, Fig. 2c). The filling degree of the antibonding states can be described by the position of the d-band center. The d-band shift of 0.7 eV for NiCo alloy in the NiCo/NC-2 can be ascribed to there being fewer valence electrons available for filling the antibonding DOS states due to the coordination with N. The formation of M-N units leads to a significant shift of the d-band center of NiCo/NC-2, resulting in an upward shift of the antibonding DOS states, lower occupation of them, and thus stronger interaction with guest molecules (oxygen herein) [43,44]. It would facilitate the mass transport in ORR and thus the overall catalytic kinetics. To further understand the ORR activity of NiCo/NC-2, their ORR kinetics were investigated. Derived from the LSV curves of NiCo/ NC-2 and Pt/C (Fig. S17), the onset potentials remained constant under various rotating speeds, while the current densities increased with the rotation speed owing to the enhanced mass transport, suggesting the kinetics-controlled process of ORR [28]. According to the Koutechy-Levich (K-L) equation, the plots of NiCo/ NC-2 manifested good parallel linearity at the potential of 0.4 V, as shown in Fig. 2d. It is noted that NiCo/NC-2 exhibits the same slope with that of Pt/C electrodes, whose electron transfer number is well-known as four [45]. EIS technique was also utilized to understand the charge-transfer kinetics for the above two catalysts (Fig. S18). The results clearly suggested the decreased chargetransfer resistance (Rct) for NiCo/NC-2, compared to that for NC, which was characterized by the significantly damped semicircle in the medium frequency region. Such decreased Rct for NiCo/NC-2 indicates the accelerated charge-transfer kinetics [46]. Overall, the high ORR activity of NiCo/NC-2 can be attributed to the intrinsically superior catalytic activity of M-N units assisted by the synergistic effects of bimetal NieCo, the abundant catalytic active sites and accelerated kinetics. To prove our conceptions that M-N units can improve ORR durability, the current-time (i-t) chronoamperometric responses for NiCo/NC-2, NiCo/C and NC were investigated in Fig. 3a. As expected, the NC catalysts presented the severe degradation of catalytic current density of 46% and NiCo/C also showed a serious degradation of 38% for 10 h. In contrast, the NiCo/NC-2 catalysts afforded the stabilized catalytic current density with 9% degradation during 10 h and 14% during 50 h, which were distinctly better than the performance of Pt/C benchmark (degradation of 55% for

549

10 h). As shown in Fig. S19, such spent catalysts showed the slightly decreased ORR catalytic current density after durability test, further supporting their catalytic robustness. The durability of NiCo/NC-2 and NC at much negative potential (0.4 V vs RHE) was also tested. As shown in Fig. S20, NC presented a severe active loss of 67% within only 15 h. In contrast, the degradation of the NiCo/NC-2 catalysts was only 6% after 15 h. Clearly, such superior ORR durability can be ascribed to the presence of M-N units. Moreover, the crossover effects caused by small organic molecules (e.g., methanol) were also investigated by chronoamperometric measurement at 0.4 V. In comparison with Pt/C, the NiCo/NC-2 exhibited higher resistance to the methanol (Fig. S21), supporting their potentials for practical applications [29]. The structural stability of NiCo/NC-2 was a key factor for their catalytic durability. As shown in Fig. S22, the morphology of the NiCo/NC-2 after durability test was well preserved with the wrapped alloy particles by graphitic carbon shells, demonstrating the robustness of the constructed interface between the alloy particles and N-doped carbon. To understand the mechanism of M-N units for improving durability, the leached N in electrolyte was directly examined by ion chromatography [47]. As shown in Fig. 3b, there were 0.99 ppm (z 0.99 mg/L) of nitrate (NO 3 ) appearing in the electrolyte for NC after long-term catalysis, which was calculated to the 48 at% of total N in the freshly prepared catalysts. In the case of NiCo/NC-2, the detected concentration of NO 3 in electrolyte was lower than the limitation of detection (LOD). These findings suggest that NC indeed suffers from the severe N leaching and thereby results in their poor ORR catalytic durability. Fortunately, the presence of MN units in NiCo/NC-2 could effectively protect the N species from leaching and thus contribute to the prolonged catalytic durability. To examine the protection of N, XPS investigations were further conducted on the fresh and spent NiCo/NC-2, which indeed presented the well-preserved nitride species at 399.5 eV after longterm catalysis (Fig. 3c). Considering the final products of NO 3 in electrolyte and the rapid activity loss of NC, such leaching process is likely to be a galvanic corrosion involving cathodic and anodic reactions at the catalyst surface. The anode involves a complex process of oxidation reactions, in which N reacts with hydroxyl ions to produce nitrates, water and electrons. The cathodic reaction is an O2 reduction reaction, in which O2 is employed to accept the electrons from anodic reaction. To prove such spontaneous corrosion process, NC catalysts was immersed in electrolyte (0.1 M KOH) with saturated oxygen for 20 h. Significantly, the NC catalysts suffered from a severe leaching without the applied external voltage. Ion chromatography investigations presented 0.99 ppm of leached NO 3 in electrolyte, which was almost equal to the values of the NC catalysts under ORR operation and thus reflected the nature that such N leaching was actually a spontaneous process in presence of electrolyte (water and conductive medium) and O2. By contrast, the detected concentration of NO 3 for NiCo/NC-2 case was lower than LOD (Fig. S23), which further confirmed that NiCo/NC-2 was not haunted by such galvanic corrosion due to the anchored N by M-N units. Such suppressed corrosion process was further examined by the corresponding corrosion polarization curves for NC and NiCo/NC-2 catalysts [48]. As shown in Fig. 3d, the NC catalysts presented an obvious corrosion phenomenon with a low corrosion potential at 0.54 V and large self-corrosive current density (jCorr ¼ 0.051 mA cm2). In the case of NiCo/NC-2, its corrosion potential was significantly raised to 0.73 V, revealing the unfavorable corrosion process. Also, the corrosion current density for NiCo/NC-2 (0.018 mA cm2) was diminished and thus revealed the nearly stagnant corrosion kinetics for NiCo/NC-2. Such a corrosion was considered to be suppressed, effectively restraining the N from leaching. Thus, the mechanism of M-N units for improving

550

W. Gou et al. / Carbon 155 (2019) 545e552

Fig. 3. (a) Current-time (iet) chronoamperometric responses for NiCo/NC-2, NiCo/C, NC and Pt/C during ORR process at their half-wave potentials. (b) Ion chromatography results of NO 3 in the electrolytes for NiCo/NC-2 and NC after ORR durability. (c) N 1s XPS spectra of NiCo/NC-2 before and after ORR durability measurements. (d) Potentiodynamic steady state scan for NiCo/NC-2 and NC in O2 purged 0.1 M KOH. Inset image indicate the ratio between activity and durability as quantified by ASF of NiCo/NC-2 (red) and NC (black). (A colour version of this figure can be viewed online.)

Fig. 4. Schematic of durability towards ORR in alkaline solution for NC and NiCo/NC-2. (A colour version of this figure can be viewed online.)

durability could be illustrated in Fig. 4. In NiCo/NC-2, the pyridinic N could easily interact with metal to form M-N units and strengthen the anchored N, which thus suppresses the N corrosion

as well as their leaching during ORR. Considering the indispensable role of N for creating ORR active sites [49], the restrained N leaching is responsible for the improved ORR durability of NiCo/NC-2.

W. Gou et al. / Carbon 155 (2019) 545e552

To comprehensively evaluate the ORR performance for above catalysts, an activity-durability factor (ADF), a ratio between ORR rate (expressed as catalytic limiting current density, jL) and rate of active site loss (equivalent jCorr), was utilized to evaluate their ORR performance, and the details of calculations could be found in Supporting Information. As expected, the ADF of NiCo/NC-2 was 5 times higher than that of NC counterpart due to the contributions of M-N units to activity and durability (inset of Fig. 3d). Such NiCo/NC2 catalyst provides a promising case to obtain robust ORR electrocatalysts based on carbon materials. 4. Conclusions In summary, we report the design conception of suppressing the leaching of N into electrolyte by forming the interfacial M-N units for the sake of improving both activity and durability. NiCo alloy nanoparticles with interfacial M-N units embedded in carbon nanofibers perform as highly active and durable oxygen reduction electrocatalysts. Due to their M-N units, abundant active sites and enhanced kinetic process, NiCo/NC-2 exhibits excellent ORR performance and robust durability of 50 h that are better than the state-of-the-art Pt/C catalysts. Moreover, NiCo/NC-2 presents an activity-durability factor (ADF) of ~5 times over the NC catalysts. This work provides a novel strategy for prolonging the catalytic durability and contributes to the design of ORR catalysts with high performance. Notes The authors declare no competing financial interest. Acknowledgements We acknowledge the financial support from the National Natural Science Foundation of China (21872109 and 61774109). Y. Qu is also supported by the Cyrus Tang Foundation through Tang Scholar program. Q. Dong thanks the support from the Youth “Sanjin” Scholar Program and the Key R&D Project of Shanxi Province (International cooperation program, No. 201603D421032). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.09.001. References [1] M.K. Debe, Electrocatalyst approaches and challenges for automotive fuel cells, Nature 486 (2012) 43e51. [2] D.U. Lee, J.-Y. Choi, K. Feng, H.W. Park, Z. Chen, Advanced extremely durable 3D bifunctional air electrodes for rechargeable zinc-air batteries, Adv. Energy Mater. 4 (2014) 1301389. [3] 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. [4] X. Han, X. Wu, C. Zhong, Y. Deng, N. Zhao, W. Hu, NiCo2S4 nanocrystals anchored on nitrogen-doped carbon nanotubes as a highly efficient bifunctional electrocatalyst for rechargeable zinc-air batteries, Nano Energy 31 (2017) 541e550. [5] H.T. Chung, D.A. Cullen, D. Higgins, B.T. Sneed, E.F. Holby, K.L. More, et al., Direct atomic-level insight into the active sites of a high-performance PGMfree ORR catalyst, Science 357 (2017) 479e484. [6] H. Fei, J. Dong, Y. Feng, C.S. Allen, C. Wan, B. Volosskiy, et al., General synthesis and definitive structural identification of MN4C4 single-atom catalysts with tunable electrocatalytic activities, Nat. Catal. 1 (2018) 63e72. [7] Y. Yang, K. Mao, S. Gao, H. Huang, G. Xia, Z. Lin, et al., O-, N-atoms-coordinated Mn cofactors within a graphene framework as bioinspired oxygen reduction reaction electrocatalysts, Adv. Mater. 30 (2018) 1801732. [8] L. Zhang, J.M.T.A. Fischer, Y. Jia, X. Yan, W. Xu, X. Wang, et al., Coordination of atomic Co-Pt coupling species at carbon defects as active sites for oxygen

551

reduction reaction, J. Am. Chem. Soc. 140 (2018) 10757e10763. [9] R. Jiang, L. Li, T. Sheng, G. Hu, Y. Chen, L. Wang, Edge-site engineering of atomically dispersed Fe-N4 by selective C-N bond cleavage for enhanced oxygen reduction reaction activities, J. Am. Chem. Soc. 140 (2018) 11594e11598. [10] Z. Liang, X. Fan, H. Lei, J. Qi, Y. Li, J. Gao, et al., Cobalt-nitrogen-doped helical carbonaceous nanotubes as a class of efficient electrocatalysts for the oxygen reduction reaction, Angew. Chem. Int. Ed. 57 (2018) 1e6. [11] B. Ni, C. Ouyang, X. Xu, J. Zhuang, X. Wang, Modifying commercial carbon with trace amounts of ZIF to prepare derivatives with superior ORR activities, Adv. Mater. 29 (2017) 1701354. [12] Q. Wei, X. Yang, G. Zhang, D. Wang, L. Zuin, D. Banham, et al., Bifunctionally active and durable hierarchically porous transition metal based hybrid electrocatalyst for rechargeable metal-air batteries, Appl. Catal. B Environ. 237 (2018) 85e93. [13] L. Yang, X. Zeng, W. Wang, D. Cao, Recent progress in MOF-derived, heteroatom-doped porous carbons as highly efficient electrocatalysts for oxygen reduction reaction in fuel cells, Adv. Funct. Mater. 28 (2018) 1704537. [14] J. Han, G. Huang, Z. Wang, Z. Lu, J. Du, H. Kashani, et al., Low-temperature carbide-mediated growth of bicontinuous nitrogen-doped mesoporous graphene as an efficient oxygen reduction electrocatalyst, Adv. Mater. 30 (2018) 1803588. [15] X. Fu, F.M. Hassan, P. Zamani, G. Jiang, D.C. Higgins, J.-Y. Choi, et al., Engineered architecture of nitrogenous graphene encapsulating porous carbon with nano-channel reactors enhancing the PEM fuel cell performance, Nano Energy 42 (2017) 249e256. [16] J. Zhang, Y. Sun, J. Zhu, Z. Kou, P. Hua, L. Liu, et al., Defect and pyridinic nitrogen engineering of carbon-based metal-free nanomaterial toward oxygen reduction, Nano Energy 52 (2018) 307e314. [17] W. Chen, L. Xu, Y. Tian, H. Li, K. Wang, Boron and nitrogen co-doped graphene aerogels: facile preparation, tunable doping contents and bifunctional oxygen electrocatalysis, Carbon 137 (2018) 458e466. [18] L. Zhang, Z. Xia, Mechanisms of oxygen reduction reaction on nitrogen-doped graphene for fuel cells, J. Phys. Chem. C115 (2011) 11170e11176. [19] G. Wu, A. Santandreu, W. Kellogg, S. Gupta, O. Ogokea, H. Zhang, et al., Carbon nanocomposite catalysts for oxygen reduction and evolution reactions: from nitrogen doping to transition-metal addition, Nano Energy 29 (2016) 83e110. [20] D. Banham, S. Ye, K. Pei, J.-i. Ozaki, T. Kishimoto, Y. Imashiro, A review of the stability and durability of non-precious metal catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells, J. Power Sources 285 (2015) 334e348. [21] M. Shao, Q. Chang, J.-P. Dodelet, R. Chenitz, Recent advances in electrocatalysts for oxygen reduction reaction, Chem. Rev. 116 (2016) 3594e3657. [22] V. Goellner, C. Baldizzone, A. Schuppert, M.T. Sougrati, K. Mayrhofer, F. Jaouen, Degradation of Fe/N/C catalysts upon high polarization in acid medium, Phys. Chem. Chem. Phys. 16 (2014) 18454e18462. [23] A.A. Gewirth, J.A. Varnell, A.M. Di Ascro, Nonprecious metal catalysts for oxygen reduction in heterogeneous aqueous systems, Chem. Rev. 118 (2018) 2313e2339.
vre, S. Sun, J.-P. Dodelet, Is iron involved in the [24] G. Zhang, R. Chenitz, M. Lefe lack of stability of Fe/N/C electrocatalysts used to reduce oxygen at the cathode of PEM fuel cells? Nano Energy 29 (2016) 111e125.
vre, J.-P. Dodelet, Activity, [25] L. Yang, N. Larouche, R. Chenitz, G. Zhang, M. Lefe performance, and durability for the reduction of oxygen in PEM fuel cells, of Fe/N/C electrocatalysts obtained from the pyrolysis of metal-organicframework and iron porphyrin precursors, Electrochim. Acta 159 (2015) 184e197. [26] J. Xu, L. Shi, J. Wang, S. Lu, Y. Wang, G. Gao, et al., Hierarchical micro/mesoporous nitrogen-doped carbons derived from hypercrosslinked polymers for highly efficient oxygen reduction reaction, Carbon 138 (2018) 348e356. [27] Y. Tong, X. Yu, H. Wang, B. Yao, C. Li, G. Shi, Trace level Co-N doped graphite foams as high-performance self-standing electrocatalytic electrodes for hydrogen and oxygen evolution, ACS Catal. 8 (2018) 4637e4644. [28] M. Kuang, Q. Wang, P. Han, G. Zheng, Cu, Co-embedded N-enriched mesoporous carbon for efficient oxygen reduction and hydrogen evolution reactions, Adv. Energy Mater. 7 (2017) 1700193. [29] X. Cui, S. Yang, X. Yan, J. Leng, S. Shuang, P.M. Ajayan, et al., Pyridinic-nitrogen-dominated graphene aerogels with Fe-N-C coordination for highly efficient oxygen reduction reaction, Adv. Funct. Mater. 26 (2016) 5708e5717. [30] J. Wang, Z. Huang, W. Liu, C. Chang, H. Tang, Z. Li, et al., Design of N-coordinated dual-metal sites: a stable and active Pt-free catalyst for acidic oxygen reduction reaction, J. Am. Chem. Soc. 139 (2017) 17281e17284. [31] Y. Tian, L. Xu, J. Qian, J. Bao, C. Yan, H. Li, et al., Fe3C/Fe2O3 heterostructure embedded in N-doped graphene as a bifunctional catalyst for quasi-solidstate zinc- air batteries, Carbon 146 (2019) 763e771. [32] Y. Tian, L. Xu, J. Bao, J. Qian, H. Su, H. Li, et al., Hollow cobalt oxide nanoparticles embedded in nitrogen-doped carbon nanosheets as an efficient bifunctional catalyst for Zneair battery, J. Energy Chem. 33 (2019) 59e66. [33] D. Deng, Y. Tian, H. Li, L. Xu, J. Qian, J. Pang, NiCo alloy nanoparticles encapsulated in multi-dimensional N-doped carbon architecture as efficient bifunctional catalyst for rechargeable zinc-air batteries, J. Alloy. Comp. 797 (2019) 1041e1049. [34] X.-R. Wang, J.-Y. Liu, Z.-W. Liu, W.-C. Wang, J. Luo, X.-P. Han, et al., Identifying the key role of pyridinic-N-Co bonding in synergistic electrocatalysis for reversible ORR/OER, Adv. Mater. 30 (2018) 1800005. [35] R. Arrigo, M.E. Schuster, Z. Xie, Y. Yi, G. Wowsnick, L.L. Sun, et al., Nature of the

552

[36]

[37]

[38]

[39]

[40]

[41] [42]

W. Gou et al. / Carbon 155 (2019) 545e552 N-Pd interaction in nitrogen-doped carbon nanotube catalysts, ACS Catal. 5 (2015) 2740e2753. Y. Fu, H.-Y. Yu, C. Jiang, T.-H. Zhang, R. Zhan, X. Li, et al., NiCo alloy nanoparticles decorated on N-doped carbon nanofibers as highly active and durable oxygen electrocatalyst, Adv. Funct. Mater. 28 (2018) 1705094. X. Liu, M. Park, M.G. Kim, S. Gupta, G. Wu, J. Cho, Integrating NiCo alloys with their oxides as efficient bifunctional cathode catalysts for rechargeable zincair batteries, Angew. Chem. Int. Ed. 54 (2015) 9654e9658. L. Zeng, X. Cui, L. Chen, T. Ye, W. Huang, R. Ma, et al., Non-noble bimetallic alloy encased in nitrogen-doped nanotubes as a highly active and durable electrocatalyst for oxygen reduction reaction, Carbon 114 (2017) 347e355. J. Bao, X. Zhang, B. Fan, J. Zhang, M. Zhou, W. Yang, et al., Ultrathin spinelstructured nanosheets rich in oxygen deficiencies for enhanced electrocatalytic water oxidation, Angew. Chem. Int. Ed. 54 (2015) 7399e7404. ~ es, P.J. Ramírez, J.A. Rodriguez, F. Illas, Combining theory and C. Kunkel, F. Vin experiment for multi technique characterization of activated CO2 on transition metal carbide (001) surfaces, J. Phys. Chem. C 123 (2019) 7567e7576. J. Wang, F. Ciucci, Boosting bifunctional oxygen electrolysis for N-doped carbon via bimetal addition, Small 13 (2017) 1604103. W. Zhao, K. Huang, Q. Zhang, H. Wu, L. Gu, K. Yao, et al., In-situ synthesis, operation and regeneration of nanoporous silver with high performance toward oxygen reduction reaction, Nano Energy 58 (2019) 69e77.

[43] T. Hofmann, T.H. Yu, M. Folse, L. Weinhardt, M. Bar, Y. Zhang, J. Phys. Chem. C 116 (2012) 24016e24026. [44] L. Bai, X. Wang, Q. Chen, Y. Ye, H. Zheng, J. Guo, et al., Explaining the size dependence in platinum-nanoparticle-catalyzed hydrogenation reactions, Angew. Chem. Int. Ed. 55 (2016) 15656e15661. [45] W. Wang, J.-Q. Chen, Y.-R. Tao, S.-N. Zhu, Y.-X. Zhang, X.-C. Wu, Flower like Ag-supported Ce-doped Mn3O4 nanosheet heterostructure for a highly efficient oxygen reduction reaction: roles of metal oxides in Ag surface states, ACS Catal. 9 (2019) 3498e3510. [46] P. Joshi, T. Okada, K. Miyabayashi, M. Miyake, Evaluation of alkylamine modified Pt nanoparticles as oxygen reduction reaction electrocatalyst for fuel cells via electrochemical impedance spectroscopy, Anal. Chem. 90 (2018) 6116e6123. [47] MdE.E. Alahi, S.C. Mukhopadhyay, Detection methods of nitrate in water: a review, Sens. Actuators A Phys. 280 (2018) 210e221. [48] A.R.J. Kucernak, V.N.N. Sundaram, Nickel phosphide: the effect of phosphorus content on hydrogen evolution activity and corrosion resistance in acidic medium, J. Mater. Chem. 2 (2014) 17435e17445. [49] S.K. Singh, K. Takeyasu, J. Nakamura, Active sites and mechanism of oxygen reduction reaction electrocatalysis on nitrogen-doped carbon materials, Adv. Mater. 31 (2019) 1804297.