Hierarchical sulfur and nitrogen co-doped carbon nanocages as efficient bifunctional oxygen electrocatalysts for rechargeable Zn-air battery

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Hierarchical sulfur and nitrogen co-doped carbon nanocages as efficient bifunctional oxygen electrocatalysts for rechargeable Zn-air battery Hao Fan a, Yu Wang a, Fujie Gao a, Longqi Yang b,c, Meng Liu a, Xiao Du a, Peng Wang b,c,∗∗, Lijun Yang a, Qiang Wu a,∗, Xizhang Wang a,∗, Zheng Hu a

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Key Laboratory of Mesoscopic Chemistry of MOE and Collaborative Innovation Center of Chemistry for Life Sciences, Jiangsu Provincial Laboratory for Nanotechnology, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, Jiangsu, China b National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210023, Jiangsu, China c College of Engineering and Applied Sciences and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, Jiangsu, China

a r t i c l e

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Article history: Received 2 August 2018 Revised 11 September 2018 Accepted 12 September 2018 Available online xxx Keywords: 3D hierarchical Carbon nanocages S, N co-doping Bifunctional electrocatalysis Zn-air battery

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Exploring inexpensive and efficient bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is critical for rechargeable metal-air batteries. Herein, we report a new 3D hierarchical sulfur and nitrogen co-doped carbon nanocages (hSNCNC) as a promising bifunctional oxygen electrocatalyst by an in-situ MgO template method with pyridine and thiophene as the mixed precursor. The so-prepared hSNCNC exhibits a positive half-wave potential of 0.792 V (vs. reversible hydrogen electrode, RHE) for ORR, and a low operating potential of 1.640 V at a 10 mA cm−2 current density for OER. The reversible oxygen electrode index is 0.847 V, far superior to commercial Pt/C and IrO2 , which reaches the top level of the reported bifunctional catalysts. Consequently, the hSNCNC as air cathodes in an assembled Zn-air battery features low charge/discharge overpotential and long lifetime. The remarkable properties arises from the introduced multiple heteroatom dopants and stable 3D hierarchical structure with multi-scale pores, which provides the abundant uniform high-active S and N species and efficient charge transfer as well as mass transportation. These results demonstrate the potential strategy in developing suitable carbon-based bi-/multi-functional catalysts to enable the next generation of the rechargeable metal-air batteries. © 2018 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences

1. Introduction Rechargeable metal-air batteries have been targeted as a promising technology to meet the energy requirements for electric vehicles and other electric devices [1–4]. The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are two key electrochemical processes in the battery but have to suffer from sluggish kinetics, which needs high-performance electrocatalysts to reduce the overpotentials and enhance the energy conversion efficiency [5–8]. Noble metal Pt, Ir and Ru-based electrocatalysts are conventionally high-active either ORR- or OER-related ∗

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Corresponding authors. Corresponding author at: College of Engineering and Applied Sciences and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, Jiangsu, China. E-mail addresses: [email protected] (P. Wang), [email protected] (Q. Wu), [email protected] (X. Wang). ∗∗

catalysts [9,10]. However, such single noble metal-based catalyst is not able to effectively catalyze both ORR and OER, which involved in the different electron transfer mechanisms and intermediates [3,11]. Therefore, it is highly desirable to develop cheap and efficient bifunctional electrocatalysts for both ORR and OER. Carbon-based nanomaterials with the features of low-cost, good conductivity, abundant morphology and tunable surface chemistry have gained paramount interest in recent decades. Since Dai et al. reported that N-doped carbon nanotube arrays (NCNTs) could demonstrate excellent ORR activity comparable to Pt in alkaline medium [12], it has opened a new avenue to developing advanced carbon-based metal-free catalysts [11,13,14]. Subsequently, the other heteroatom-doped (B [15,16], S [17,18], P [19], edge-halogenated [20,21], etc.) and even dopant-free defectsenriched [22–27] carbons demonstrated considerable activities for ORR and/or OER. In fact, the dopants- and defects-induced charge redistribution could tune the electronic structure and surface

https://doi.org/10.1016/j.jechem.2018.09.003 2095-4956/© 2018 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences

Please cite this article as: H. Fan et al., Hierarchical sulfur and nitrogen co-doped carbon nanocages as efficient bifunctional oxygen electrocatalysts for rechargeable Zn-air battery, Journal of Energy Chemistry (2018), https://doi.org/10.1016/j.jechem.2018.09.003

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properties, thereby balance the chemisorption energies of reaction intermediates on suitable moieties and thus boost the catalytic activity [12,15,18,22,24,28]. Comparatively, the co-doped carbons based on two or more selected heteroatoms (B/N [29,30], N/P [31,32], N/S [33,34], N/O [35], P/S [36], N/O/S [37], B/N/P [38] and N/P/S [39,40]) enable more tailorable active moieties and efficient synergistic effect for different electrocatalytic reactions, showing more attractive promise in electrocatalysis [11,13,41]. For instance, the separated B, N co-doping turned inert CNTs into ORR-active catalysts while the bonded B, N co-doping didn’t [30]. The S and N co-doped configurations in carbon tubes accelerated electrocatalytic ORR and hydrogen evolution activities [42]. The synergism of N/S and N/P dopants in mesoporous carbon [31], graphene microwires [43], graphene/carbon nanosheets [32] and graphene/carbon nanotubes [44] can greatly promote the bifunctional performance for ORR and/or OER. However, most of the studies have been mainly focused on developing co-doping methods/materials while few reports correlate the electrocatalytic performance with the heteroatom amount/ratio. Therefore, it is very significant to explore the controlling of the heteroatom amount/ratio and investigate their effects on electrocatalytic performance. It is well known that the electrocatalytic performance not only relies on the active moieties but also on their densities [45–47]. Among the widely used sp2 carbon nanomaterials, carbon-based nanotubes suffer from the low specific surface area (SSA) [48,49], and graphene is restricted by the difficulty in doping and the π π restacking to loss SSA [50,51], usually leading to less exposed high-active sites. Therefore, it is of great importance to obtain new carbon materials with abundant high-active sites and stable structure, used as cheap and efficient bifunctional electrocatalysts towards ORR and OER. We have recently developed an in-situ MgO template approach to construct three-dimension (3D) hierarchical carbonbased nanocages featuring high SSA, high conductivity, and tunable doping contents and status, which demonstrated high ORR activities as metal-free catalysts [22,52]. Herein, by similar template method, we have constructed 3D hierarchical S and N co-doped carbon nanocages (hSNCNC) using pyridine and thiophene mixture as precursor. The optimized hSNCNC demonstrates a positive halfwave potential of 0.792 V (vs. reversible hydrogen electrode, RHE) for ORR and a low operating potential of 1.640 V at 10 mA cm−2 for OER, and thus leads a low potential difference of 0.847 V between ORR and OER, far superior to commercial Pt/C and IrO2 , which locates at the top-level of ORR/OER bifunctional catalysts reported to date. Consequently, the obtained hSNCNC catalyst presents a low charge/discharge overpotential and long lifetime when employed as the air cathode in an assembled Zn-air battery. The introduced uniform S- and N-rich active species and stable 3D hierarchical structure with multi-scale pores are responsible for the excellent bifunctional electrochemical performances. The results suggest the great potential of hierarchical co-doped carbon nanocages as multifunctional electrode materials for advanced energy applications.

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2. Experimental

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2.1. Preparation of hSNCNC The hSNCNC was prepared by in-situ MgO template method with mixed pyridine and thiophene precursors, similar to our recent reports [53–56]. In a modified procedure, 4.0 g basic magnesium carbonate (4MgCO3 ·Mg(OH)2 ·5H2 O) with a 3D hierarchical structure was decomposed at 800 °C into a quartz tube in a vertical furnace. The deposition temperature was adjusted to the designated temperatures (70 0, 80 0 and 90 0 °C), then the mixture of thiophene and pyridine (1:1, V/V) was introduced into the tube

at the feeding rate of 60 μL min−1 by a syringe pump for 25 min. The reactor was thus naturally cooled down to ambient temperature. The black powder was first stirred in 6.0 mol L−1 HCl (aq.) for 48 h to remove the MgO template, followed by filtration, repeated washing with de-ionized water and absolute ethanol, and vacuum drying at 80 °C overnight. Three control samples, i.e., hierarchical un-doped and S or N mono-doped carbon nanocages, were also obtained at 800 °C with benzene, pyridine, and thiophene as precursors, respectively, which are labeled as hCNC, hNCNC and hSCNC in turn.

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2.2. Characterizations

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The structures and compositions of samples were characterized by scanning electron microscopy (SEM, Hitachi S4800 at 5.0 kV), transmission electron microscope (TEM, JEM-2100F operating at 200 kV), scanning TEM (STEM, FEI, TF20 operating at 200 kV, equipped with the energy dispersive X-ray spectrometry (EDS)), X-ray photoelectron spectroscopy (XPS, ULVAC-PHI INC, PHI 50 0 0 VersaProbe, Al Kα ), X-ray diffraction (XRD, Bruker D8 Advance A25, Co Kα 1 radiation of 1.78897 A˚ with Fe filter of 0.02 mm thickness), and Raman spectroscopy (LabRAM Aramis, laser excitation at 532 nm). STEM was also used to collect high angle annular dark field (HAADF) and elemental mappings. The binding energy (Eb ) of XPS spectrum was in reference to C 1s at 284.6 eV. N2 adsorption/desorption isotherm was recorded by Thermo Fisher Scientific Surfer Gas Adsorption Porosimeter at 77 K after degassed at 300 °C for 6 h. The specific surface area and pore size distributions were calculated by the Brunauer–Emmett–Teller (BET), Horvath–Kawazoe (HK, for < 2 nm pores) and Barrett–Joyner–Halenda (BJH, for > 2 nm pores) methods from the adsorption isotherm, respectively.

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2.3. Electrochemical measurements

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All electrochemical tests were conducted at 25 °C with a CHI 760E workstation (CH Instruments) coupled with a MSR electrode rotator (Pine Instrument Co.) in a three-electrode electrochemical cell using Ag/AgCl (3.0 mol L−1 KCl) as reference electrode, a platinum wire as counter electrode, the sample-modified glassy carbon (GC) electrode as working electrode, and 0.1 mol L−1 KOH as electrolyte. All potentials in this study are converted to ones versus reversible hydrogen electrode (RHE) at pH = 13 (E = Evs. Ag/ AgCl + 0.978 V). To prepare the electrocatalyst ink, 2.0 mg sample and 50 μL Nafion solution (5 wt%) were ultrasonically dispersed in 1.0 mL water/ethanol (4:1, V/V) mixed solvent for 30 min to form a homogeneous ink. Then 15.0 μL catalyst ink was dropped onto a GC electrode of 5.5 mm in diameter. Cyclic voltammetry (CV) and linear sweep voltammogram (LSV) were performed at a scan rate of 10 mV s−1 in the N2 - or O2 -saturated electrolytes. For the ORR tests, the rotating ring-disk electrode (RRDE) voltammetries were measured in O2 -saturated electrolyte at 10 mV s−1 with typical rotating rates of 400, 625, 900, 1225, 1600, 2025 and 2500 revolutions per minute (rpm). The stability of catalyst was examined with LSV measurements in the electrolyte at 0 cycle (initial) and after 50 0 0 CV cycles. The onset potential Eonset is defined as the separating point of LSV curves measured in the O2 - and N2 -saturated electrolytes, and the half-wave potential E1/2 as the critical potential where the reduction current density reaches the half of the limiting current density (jL ) at 0.5 V. Commercial Pt/C (20 wt%, Johnson Matthey) electrode was measured in exactly the same way for comparison. For the OER measurements, the LSV curves were recorded in 0.1 mol L−1 KOH at a scan rate of 10 mV s−1 with iR drop compensation (80%). In order to obtain a stable current, the LSV data

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Please cite this article as: H. Fan et al., Hierarchical sulfur and nitrogen co-doped carbon nanocages as efficient bifunctional oxygen electrocatalysts for rechargeable Zn-air battery, Journal of Energy Chemistry (2018), https://doi.org/10.1016/j.jechem.2018.09.003

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Fig. 1. The morphology and elemental mapping of hSNCNC: (a) SEM image, (b and c) TEM and HRTEM images. The arrowed locations in (c) represent three typical defects, i.e., broken fringes, holes and corners. (d and e) HAADF-STEM images. (f–h) Elemental mapping of C, N and S, respectively. The images of (e–h) are corresponding to the orange square area in (d).

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were collected at the second sweep. The overpotential (η) of OER was calculated according to the Nernst equation as follows: η = Ej =10 − 1.23 (Ej =10 : the operating potential at 10 mA cm−2 in OER). The stability of catalyst was examined by the LSV measurements in the electrolyte at 0 cycle (initial) and after 20 0 0 CV cycles. IrO2 (Aladdin Industrial Corporation) and Pt/C electrodes were carried out in the same way for comparison. The bifunctional ORR and OER activity can be assessed by the variance of Ej =10 in OER and E1/2 in ORR metrics, i.e., ࢞E = Ej =10 − E1/2 . The rechargeable Zn-air battery was assembled with air electrode, Zn foil (99.9%, 16 mm in diameter and 0.5 mm in thickness) and 6.0 mol L−1 KOH. The air electrode with a hSNCNC loading of 0.5 mg cm−2 was prepared by uniformly coating the electrocatalyst ink onto hydrophobic carbon cloth (Fuel Cell Earth) followed by drying at 80 °C for 2 h. The Zn-air batteries were tested in a multichannel potentiostat (LANHE CT2001A, Wuhan) at room temperature, and the rechargeability tested by a galvanostatic recurrent pulse method at a current density of 5 mA cm−2 , with a pulse period of 3 min. The Zn-air battery with the mixed catalyst of 20 wt% Pt/C and IrO2 (1:1, m/m), denoted as Pt/C + IrO2 , was tested for comparison. 3. Results and discussion The morphology, structure and elemental distribution of hSNCNC are shown in Figs. 1 and 2. The SEM image presents a 3D hierarchical micron-sized sphere-like architecture composed of micron-sized carbon nanosheets with submicron-sized interspace (Fig. 1a), similar to our recent studies [55,56]. The nanosheets consist of interconnected cuboidal hollow nanocages of ca. 10–50 nm in size and 3–5 well-graphitized layers in thickness (Fig. 1b and c), endowing the hSNCNC with high conductivity as measured by four-probe method [55] (Table S1). The nanocages possess abundant broken fringes, holes and corners (Fig. 1c), but the clear C (002) peak in the XRD pattern indicates the good graphitic degree (Fig. S1), in agreement with the HRTEM observations (Fig. 1c). Such defects-enriched graphitic structures could facilitate electrocatalysis [22,46]. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) with corresponding elemental mapping images reveal the uniform distribution of C, N and S elements in hSNCNC (Fig. 1d–h), confirming the formation of S and N co-doped carbon nanocages. The N2 adsorption/desorption isotherms of hSNCNC, hCNC, hSCNC and hNCNC show typical type-IV characteristics with large

SSA (ca. 1400 m2 g−1 ) and coexistence of micro-meso-macro-pores (Fig. 2a and Table S1), which favors the mass transport during electrochemical processes [55–57]. XPS survey spectrum confirms that S and N are co-doped into the hSNCNC (Fig. S3), in consistence with elemental mapping analysis. The S and N contents in hSNCNC are 1.86% and 5.87 at%, respectively, while the S content in mono-doped hSCNC is 2.12 at% and the N content in hNCNC is 7.50 at% (Table S1). The high-resolution S 2p spectra for hSNCNC and hSCNC can be divided into three peaks at ca. 163.8, 165.1, and 168.2 eV, which are ascribed to 2p3/2 , 2p1/2 , splitting of the S 2p spin orbital (C-S-C, S1) and oxidized sulfur (C-SOx -C, S2), respectively (Fig. 2b) [18,33,42]. The high-resolution N 1s for hNCNC is deconvolved to three peaks at ca. 398.7 eV (pyridinic-N, N1), 400.1 eV (pyrrolic-N, N2) and 401.1 eV (graphitic-N, N3), while the case for hSNCNC is mainly at ca. 398.1, 399.7 and 401.1 eV, respectively (Fig. 2c) [57]. The slight negative shifts of 0.6 and 0.4 eV for pyridinic- and pyrrolic-N for the latter indicate the charge transfers from S to N species after S doping, reflecting the varying degrees of interaction between them with the order of pyridinic-N > pyrrolicN. The absence of N–S bonds (ca. 397.0 eV) further implies that S and N are mostly isolated in hSNCNC [42,58]. These compositions and status of nitrogen species lead to different chemical/electronic environments for neighboring carbon atoms and hence different electrocatalytic activities. The S and N contents of the hSNCNC samples decrease with increasing the growth temperature from 700 to 900 °C (Table S1), whilst higher temperature favors the formation of graphitic-N due to its stability better than those of the pyridinic- and pyrrolic-N species [59]. It is noticed that hSNCNC prepared at 800 °C generates the higher absolute amount of pyrrolic- and graphitic-N (N2 + N3, 4.61 at%) and thiophene-S (S1, 1.67 at%) bonding configurations (Fig. S4 and Table S1), which are deemed the main factor to enhance the electrocatalytic performances as discussed below. The area ratio of the D band to G band (ID /IG ) in Raman spectra for hSNCNC is 2.83, higher than 1.95 for hCNC, 2.13 for hSCNC, and 2.51 for hNCNC (Fig. 2d). The ID /IG ratios for the hSNCNC samples decrease with increasing growth temperatures (Fig. S5). This indicates that the doping of heteroatoms (S and/or N) into the graphitic structure introduces more defects and disordered moieties on hCNC, which could contribute to the electrocatalytic reactions [22,24]. Based on the preceding results, the 3D hSNCNC possesses the features of (i) adjustable S and N contents with electronic interaction and rich defects for providing high active catalytic moieties, (ii) large specific surface area for supporting high-density active sites, and (iii) multi-scale pore structure and high conductivity for

Please cite this article as: H. Fan et al., Hierarchical sulfur and nitrogen co-doped carbon nanocages as efficient bifunctional oxygen electrocatalysts for rechargeable Zn-air battery, Journal of Energy Chemistry (2018), https://doi.org/10.1016/j.jechem.2018.09.003

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Fig. 2. Structures and compositions of hSNCNC. (a) N2 adsorption/desorption isotherms and corresponding pore size distribution curves (inset). (b and c) High-resolution S 2p (b) and N 1s (c) XPS. (d) Raman spectra. The corresponding results of hCNC, hSCNC and hNCNC are presented for comparison.

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facilitating charge transfer, therefore the hSNCNC has high possibility for exploring bifunctional oxygen electrocatalysts. Electrocatalytic performances of ORR and OER were evaluated in 0.1 mol L−1 KOH solution, as shown in Fig. 3. LSV curve for hSNCNC shows a positive Eonset and E1/2 of 0.898 and 0.793 V (vs. RHE), respectively, which are higher than 0.861 and 0.768 V for hNCNC, 0.835 and 0.736 V for hSCNC, and 0.811 and 0.692 V for hCNC, but still lower than 0.993 and 0.820 V for commercial Pt/C (Fig. 3a). The high Eonset and E1/2 for hSNCNC are in accordance with the characteristic ORR peak in the O2 -saturated CV (Fig. S6), which reflect the remarkable ORR activity. The electron transfer number n and HO− yield for hSNCNC are 3.81–3.94 and 2.35–8.56% 2 in the wide potential range from 0 to 0.80 V, close to 3.90–3.97 and 1.40–3.60% for Pt/C (Fig. 3b), indicating a 4e-dominated pathway, in accordance with the Koutecky–Levich results (Fig. S7). The n and HO− yield for hCNC are 3.15–3.69 and 15.3–32.3%, suggest2 ing a mixed 2e and 4e pathway. With S or N mono-doping, the corresponding n and HO− values are changed to 3.55–3.83 and 2 8.15–21.75% for hSCNC, and 3.73–3.81 and 10.51–13.33% for hNCNC (Fig. 3b). This indicates the hSNCNC has the high 4e selectivity in comparison with the mono-doped and undoped carbon nanocages. After continuous CV tests for 50 0 0 cycles, the E1/2 for hSNCNC only negatively shift by 5.0 mV, much lower than 32 mV for Pt/C (Fig. S8), indicating the excellent ORR stability. The co-doped hSNCNC also presents the high OER activity with Ej =10 of 1.640 V, lower than 1.718 V for hSCNC, 1.761 V for hNCNC, 1.827 V for hCNC, and 1.779 V for Pt/C (Fig. 3c), but higher than 1.588 V for IrO2 . The corresponding Tafel slope for hSNCNC is 288 mV decade−1 (Fig. 3d), comparable to the carbon-based metalfree OER catalysts [31,59,60], suggesting a good OER kinetic process. The difference in Tafel slopes for carbon-based nanocages in

Fig. 3(d) could be ascribed to the complexity of the microstructures such as N/S functionality distribution, defects, and surface functional groups [11,61]. These results imply that the co-doping of S and N can synergistically catalyze the OER activity. Furthermore, hSNCNC shows the outstanding stability after 20 0 0 continuous CV testing with negligible decay (Ej = 10 < 5 mV), superior to IrO2 (Ej = 10 = 48 mV) (Fig. S8). The superb ORR and OER stability of hSNCNC is ascribed to the robust structure composed of interconnected crystalline carbon nanocages, which retain its original structure after the cycling tests in the harsh operating environments (Fig. S8). In addition, the ORR and OER activities for the series of hSNCNC samples are higher than those for the unand mono-doped hCNC, and present the tendency of first increasing then decreasing in the temperature range from 700 to 900 °C (Fig. S9). Hence, hSNCNC obtained at 800 °C is the optimal catalyst, which may result from the high-density S and N active species (Table S1). The bifunctional activity can be assessed by the variance of OER and ORR metrics, i.e., ࢞E = Ej =10 − E1/2 . The optimal hSNCNC exhibits superior bifunctional activity with a low E value of 0.847 V, which locates at the top level for metal-free carbons (e.g., SHG [46], CNS [59], NSOG [62], NGM [26], NCHCs [63], PCN–CFP [64], and NG/CNT [65]), and even outperformed some transitional metal-based bifunctional catalysts (e.g., Co, Ni and Mn [66–70]) (Fig. 3e and Table S2). These results indicate that the hSNCNC with abundant S and N active species and stable 3D hierarchical structure is a promising low-cost, efficient bifunctional catalyst for both ORR and OER simultaneously. The influence of the N and S dopants was further explored. Fig. 4 shows the relationship between ORR-/OER-related indexes (i.e., potential, n and HO− %) and the amount of N and S sites 2 for the catalysts. The doped N, with its larger electronegativity,

Please cite this article as: H. Fan et al., Hierarchical sulfur and nitrogen co-doped carbon nanocages as efficient bifunctional oxygen electrocatalysts for rechargeable Zn-air battery, Journal of Energy Chemistry (2018), https://doi.org/10.1016/j.jechem.2018.09.003

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Fig. 3. Bifunctional electrocatalytic performances for hSNCNC. (a) RRDE curves. (b) Plots of n and HO− yield. (c) LSV curves. (d) Tafel slopes derived from (c). (e) Comparison 2 of E values for our catalyst with some reported state-of-the-art bifunctional catalysts.

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could result in the redistribution of the charge densities in the carbon matrix which is usually ORR-active [13,52,59,71]. Specifically, the electron-donating pyrrolic-N (N2) and graphitic-N (N3) species (Fig. 4a) could increase the nucleophile strength for the adjacent C rings to enhance the O2 adsorption, thereby accelerate the formation of the OOH species and boost the ORR [37,59,71,72]. The S1 (C-S-C) moiety in S dopant is effective in rendering high spin densities within the surrounding C atoms and thus also promotes the ORR activity [13,33,59]. Control experiments (Fig. 4a and Fig. S10) show that the non-doping (hCNC) and S2-rich (C-SOx -C moiety in hSCNC and hSNCNC prepared at 700 °C) catalysts can easily go through an inefficient 4e pathway, while the high N2 and N3 doping catalysts with/without little S2 species feature better Eonset and E1/2 for ORR (Fig. 2b and c and Table S1). The N2-, N3- and S1-enriched catalyst (hSNCNC obtained at 800 °C), most promisingly, gives the best ORR activity, suggesting the synergic role of the co-doped high-active N and S sites in ORR. On the other hand, recent progresses suggested that the high spin-density S1

and electron-withdrawing pyridinic-N (N1) sites within carbon matrix can make the neighboring C atoms positively charged, which could facilitate the adsorption of OH− and OOH− intermediates in OER and then improve the catalytic kinetics process [18,35,44,71]. The incorporated S1 and N1 sites could synergistically promote the OER activity with the former contributed more than the latter, leading the lower overpotential in OER (Fig. 4b). Additionally, due to the differences in the ORR- and OER-active species contents, the combined synergistic effect by N and S in the catalysts may vary them different work functions [44,71,73]. This makes them regulate the surface electron structure of catalysts and then discriminatively adsorb the O2 and ∗ OH− or ∗ OOH intermediates, leading to the different activity and selectivity in ORR and OER (Fig. S11). Therefore, we consider that the proper high-active pyrrolic-N, graphitic-N and thiophene-S species in the carbon matrix account for the excellent ORR activity of the catalysts, whilst the OER activity originates primarily from the thiophene-S and pyridinic-N species.

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yield plots in Fig. 4. ORR- and OER-related indexes versus amount of high-active N and S sites in hCNC, hSCNC, hNCNC and hSNCNC, respectively. (a) Eonset , E1/2 , n and HO− 2 ORR. (b) Overpotential histogram at 10 mA cm−2 in OER. The type and amount of N and S sites were determined by XPS. N1: pyridinic-N. N2: pyrrolic-N. N3: graphitic-N. S1: C-S-C. S2: C-SOx -C.

(a)

(b)

KOH

(c)

0

2.0

50

2.5

Voltage (V)

Carbon Cloth

e

Catalyst Layer

e

Charge

2.0 hSNCNC Pt/C+IrO2

1.5 1.0 Discharge

0.5 0.0

0

Cycle number (n) 100

20 40 60 80 100 Current density (mA cm-2) 190

195

hSNCNC

E ’=0.84 V

Pt/C+IrO2

E ’=1.07 V

200

Voltage (V)

1.5 1.0 2.0 1.5 1.0

5 mA cm 0

-2

5

10 Time (h)

19.0

19.5

20.0

Fig. 5. Schematic diagram and performances of the two-electrode rechargeable Zn-air battery. (a) Schematic diagram of the basic battery configuration. (b) Discharge-charge polarization curves of the battery using hSNCNC as catalyst (mass loading: 0.5 mg cm−2 ). (c) Stability tests of the battery at 5 mA cm−2 for 200 cycles (20 h). The Pt/C + IrO2 mixture is presented for comparison.

Please cite this article as: H. Fan et al., Hierarchical sulfur and nitrogen co-doped carbon nanocages as efficient bifunctional oxygen electrocatalysts for rechargeable Zn-air battery, Journal of Energy Chemistry (2018), https://doi.org/10.1016/j.jechem.2018.09.003

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To evaluate the practical application of bifunctional hSNCNC catalyst, a rechargeable Zn-air battery was assembled with hSNCNC catalyst on carbon cloth as air cathode, Zn foil as anode and 6 mol L−1 KOH as electrolyte (Fig. 5a). The Zn-air battery exhibits an ORR potential of 1.18 V (vs. Zn) and an OER potential of 1.47 V at a current density of 2 mA cm−2 . The difference of charge and discharge potentials increases with increasing the current density (Fig. 5b). Specifically, the charging overpotential for hSNCNC was overwhelmingly low compared with the benchmark Pt/C + IrO2 , which verifies its superior OER activity. For discharge process, the overpotential of hSNCNC was slightly higher than that of Pt/C + IrO2 , which reflects its excellent ORR activity. The much lower charge potential and comparable discharge potential for hSNCNC indicate the superior bifunctional electrocatalytic activity to Pt/C + IrO2 , in agreement with the LSV characterizations (Fig. 3). The rechargeable Zn-air battery shows remarkable cycling stability during a continuous test at 5 mA cm−2 for 200 cycles (i.e., 20 h). And the potential difference (࢞E’) stabilizes at 0.84 V, which is much lower than 1.07 V for Pt/C + IrO2 (Fig. 5c), and among the top level in the reported Zn-air batteries (Table S3) [59,74–76]. The lower potential difference implies the lower polarization, demonstrating the great potential of bifunctional hSNCNC in the rechargeable Zn-air batteries and beyond.

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4. Conclusions

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In summary, we have successfully prepared a new 3D hierarchical sulfur and nitrogen co-doped carbon nanocages by co-doping strategy using in-situ MgO as template and pyridine and thiophene mixture as precursor. The so-made hSNCNC presents excellent bifunctional ORR and OER activities with a positive half-wave potential of 0.792 V for ORR and a low operating potential of 1.640 V at 10 mA cm−2 for OER, and thus lead a low potential difference of 0.847 V, far superior to commercial Pt/C and IrO2 . Consequently, the obtained hSNCNC catalyst features low charge/discharge overpotential and long lifetime when employed as the air cathode in an assembled rechargeable Zn-air battery. The remarkable properties arises from the introduced multiple heteroatom dopants and the stable 3D hierarchical structure with multi-scale pores, which provides the abundant uniform S- and N-abundant high-active sites and efficient charge transfer as well as mass transportation. These results demonstrate the potential of this strategy in developing low cost, high activity, and long-term stability bi-/multi-functional catalysts to enable the next generation of metal-air batteries.

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Acknowledgments

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We appreciate the financial support from the National Natural Science Foundation of China (21773111, 21473089, 21573107 and 51571110), the National Key Research and Development Program of China (2017YFA0206503, 2018YFA0209103), Priority Academic Program Development of Jiangsu Higher Education Institutions, Fundamental Research Funds for the Central Universities, and the program B for outstanding PhD candidate of Nanjing University (201702B049).

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Supplementary materials

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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2018.09.003.

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Please cite this article as: H. Fan et al., Hierarchical sulfur and nitrogen co-doped carbon nanocages as efficient bifunctional oxygen electrocatalysts for rechargeable Zn-air battery, Journal of Energy Chemistry (2018), https://doi.org/10.1016/j.jechem.2018.09.003

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