Bifunctional Oxygen Electrocatalysis of N, S-Codoped Porous Carbon

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Functional Inorganic Materials and Devices

The Bifunctional Oxygen Electrocatalysis of N, S Co-doped Porous Carbon with Interspersed Hollow CoO Nanoparticles for Rechargeable Zn-air Batteries Si Chen, Song Chen, Baohua Zhang, and Jintao Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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ACS Applied Materials & Interfaces

The Bifunctional Oxygen Electrocatalysis of N, S Co-doped Porous Carbon with Interspersed Hollow CoO

Nanoparticles

for

Rechargeable

Zn-air

Batteries Si Chen1, Song Chen1, Baohua Zhang1, and Jintao Zhang1,* 1Key

Laboratory for Colloid and Interface Chemistry of State Education Ministry, School

of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China. Email: [email protected].

ACS Paragon Plus Environment

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ABSTRACT: Exploring efficient bifunctional oxygen electrocatalysts is beneficial to promote the practical applications for rechargeable Zn-air battery. Herein, a highefficiency one-pot method is developed to synthesize porous carbon with N, S doping and

embedded

hollow

cobalt

oxide

nanoparticles.

The

coordination

of

polyethyleneimine (PEI) molecules with cobalt ions enables the formation of organicinorganic precursors via the co-precipitation with lignosulfonate due to the electrostatic interaction. Under thermal treatment, the hollow cobalt oxide nanoparticles can be welldispersed among the carbon matrix co-doped with N, S. The as-prepared composite catalysts exhibit efficient bifunctional activity for electrochemical reduction and evolution reactions of oxygen, thanks to the N, S co-doping nature and the hollow cobalt oxide with abundant oxygen vacancies. The bifunctional catalytic activity renders the assembly of high-performance Zn-air battery in an aqueous electrolyte with a specific capacities of 745 mA h gzn-1 and good cycling stability for over 100 h. More importantly, the all solid-state Zn-air battery is assembled with a polymer-based electrolyte, also exhibiting good cycling stability and flexibility under various bending status.


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KEYWORDS. Cobalt oxide ·N, S co-doping· Bifunctional electrocatalyst ·Zn-air battery·Porous carbon


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INTRODUCTION

To solve environmental deterioration and energy crisis problems, the development of sustainable energy is of importance.1 Batteries and supercapacitors have made an indelible contribution to the generation, storage and applications of clean energy.2-5 Among them, Zn-air batteries are considered as the particularly promising energy devices due to its good security, low price and good safety in comparison with Li-air batteries or Mg-air batteries.6-7 However, the sluggish kinetic reaction for air electrode involving the electrochemical reduction of oxygen and oxygen evolution reaction (OER) hinders the practical implementation of Zn-air batteries.8-10 Precious metal-based materials with intrinsic catalytic activity, such as Pt/C and RuO2 are able to reduce the overpotentials of oxygen reduction reaction (ORR) and OER. Nonetheless, the scarcity and exorbitant price of precious metal necessitate us to explore the high-efficiency, low cost and earth-abundant materials for the bifunctional electrocatalytic applications in Znair batteries.11-14


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Carbon materials have been extensively utilized as advanced electrocatalysts on account of their fascinating characteristics, such as high structure flexibility, striking thermal and electrical conductivity, good chemical and mechanical stability.10,

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Especially, the doping flexibility of graphitic carbons with various heteroatoms provides a universal approach to regulate the surface electron structure and status for electrocatalysis. Typically, the doping of heteroatoms including N, B et al. into the graphitic carbon matrix changes sp2 hybrid structure of carbon atoms, thus modifying the electron distribution and creating surface defects.16 Therefore, the enhanced electrocatalytic activity can be achieved via the heteroatom doping.17 Transition metal oxide nanostructures, such as cobalt oxides18, nickel oxides and iron oxides are identified as the efficient electrocatalytic catalysts for OER.19-22 However, the electrocatalytic performance of cobalt oxides is restricted because of the low electronic conductivity and poor structure stability. The incorporation of cobalt oxides with carbon materials is efficient to improve the conductivity and enhance the dispersion of cobalt oxides for exposing inner active sites23. Simultaneously, the stability of the catalysts would be enhanced. In order to obtain the desirable properties, the preparation of


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carbon materials as the support is also important. Especially, the incorporation of heteroatom doping and metal oxides on the graphitic carbons would improve the catalytic activity and stability due to the synergic effect.24 Therefore, the facile preparation of carbon-based electrocatalysts with good bifunctional activity is crucial to enhance the performance of Zn-air battery. Especially, it is still challenging to prepare the well-integrated carbon-hollow metal oxide bifunctional electrocatalysts for fabricating solid-state Zn-air batteries. Herein, we demonstrated a simple and efficient method to enhance the bifunctional oxygen electrocatalysis of porous carbon via the incorporation of N, S doping with the interspersed hollow cobalt oxide nanoparticles. Specifically, polyethyleneimine (PEI) molecule with the multiple amine groups and good coordination ability is efficient to bind cobalt ions on the polymer chain uniformly.25 The co-precipitation between PEI and sodium lignosulfonate (SLS) renders the formation of organic-inorganic precursor via the electrostatic interaction. The subsequent pyrolysis of such precursor results in the in-situ doping of N, S into porous carbon with interspersed hollow CoO nanoparticles among the graphitic carbon matrix. Such a composite electrocatalyst exhibited good


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bifunctional oxygen electrocatalysis. To demonstrate the practical applications, the Znair batteries assembled with aqueous and solid state electrolytes, respectively exhibit excellent charge-discharge cycling stability, superior to those of batteries with noble metal electrocatalysts. METHODS AND EXPERIMENTAL SECTION

Materials preparation Firstly, the mixed solution was obtained through adding 50 ml of polyethyleneimine (PEI) (0.2 g) solution into 20 ml Co2+ solution (0.2 mmol). The colourless solution was gradually changed brownish yellow, suggesting the successful coordination between PEI and Co2+ ions. Secondly, 0.5 g of sodium lignosulfonate (SLS) was dissolved into 50 ml deionized water. The above solutions were mixed together under stirring for 1 h. The yellow precipitate was obtained through centrifugal separation and washed with large amount of water. For comparison, the precipitate was also prepared without adding Co2+ ions. Finally, the obtained materials were thermally treated at an elevated temperature of 800-1000 ℃ , respectively in N2 atmosphere to prepare carbon-based composite. The obtained samples were denoted as NSC-900, CoO-NSC800, CoO-NSC-900 and CoO-NSC-1000, respectively. Materials characterization The detailed structure of as-prepared electrocatalysts were examined by using the field emission scanning electron microscopy (FESEM; Zeiss, Sigma 300) and high-resolution transmission


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electron microscopy (HRTEM; JEOL 2100 PLUS). The elemental mapping images were recorded by HRTEM with FEI TF20. X-ray diffractometer (Bruker D8) was used to reveal the crystalline structure of samples by using Cu Kα radiation (l=0.15418 nm). The surface composition was analyzed by using X-ray photoelectron spectroscopy (XPS; ESCALAB 250). Raman spectra were collected on LabRAM HR800 (HORLBA JY). The nitrogen sorption curves were recorded by using Kubo-X1000. The specific surface area and porosity were analyzed according to Barret-Joyner-Halenda method. Electrochemical methods The electrochemical workstation (CHI760E, Chenhua) with a three-electrode system was used for the electrochemical tests at room temperature. The platinum sheet and Ag/AgCl electrode were utilized as the counter and reference electrodes, respectively. The working electrode is the as-prepared catalysts coated glassy carbon electrode. The electrolyte is O2-saturated 0.1 M KOH. The applied potentials were normalized by using reversible hydrogen electrode (RHE) according to the equation: E(RHE) = E(Ag/AgCl)+0.059pH. Zinc-air Battery Assembly To assemble a liquid Zn-air battery, the anode and the air cathode electrode was the zinc plate (2 cm×2 cm) and a hydrophobic carbon cloth with gas diffusion layer covered with the electrocatalysts (1.0 mg cm-2). Homogeneous catalyst solution was obtained through mixing the electrocatalyst (10 mg), acetylene black (10 mg) and 12 mg of polytetrafluoroethylene (PTFE) emulsion (60%) in 2 ml of the mixed solution (deionized water: ethanol = 3:1). Then, the catalyst solution was ultrasonic dispersion for 30 min and dried at 80℃ for 40min. The alkali electrolyte


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was constituted of KOH (6 M) and Zn(CH3OO)2 (0.2 M). To prepare a gel polymer electrolyte, acrylic acid monomers were initiated by using K2S2O8 in the presence of cross-linking agent, N, N’-methylene-bisacrylamide (MBA), respectively. The battery testes were implemented on a LAND battery station (CT2001A). RESULTS AND DISCUSSION

As shown in Scheme 1, the nitrogen (N) and sulfur (S) co-doped mesoporous carbons with hollow CoO nanoparticles (CoO-NSC) catalysts were prepared via the pyrolysis of polyethyleneimine/Co2+/lignosulfonate nanocomposites in N2 atmosphere. When the cobalt ions were added into the polyethyleneimine solution, the gradual color change from colorless to brownish yellow suggests the formation of PEI/Co2+ complex via the coordination interaction (Figure S1). Abundant amino groups of polyethyleneimine provide efficient anchor sites for loading Co2+ ions uniformly. When sodium lignosulfonate (SLS) was added into the PEI/Co2+ solution, brownish yellow precipitate can be obtained by adding lignosulfonate with rich oxygenfunctional groups and sulfonic groups as an anionic surfactant. The electrostatic interaction results in the co-precipitation of lignosulfonate and polyethyleneimine with cobalt ions. The obtained organic-inorganic nanocomposite is named as PEI/Co2+/SLS, in which the intermolecular interactions and electrostatic interaction enable the uniform distribution of heteroatoms and cobalt ions. When used as the precursor, N, S co-doped carbon with hollow cobalt oxide nanoparticles can be prepared via the thermal treatment at an elevated temperature (named as CoO-NSC-T). The as-prepared samples were firstly examined by using XRD. The XRD pattern (Figure 1a) exhibits that the broaden peak at around 24.5 ° suggests the formation of carbon with small


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graphitic domains.26 In the presence of metal species, the XRD patterns of CoO-NSC-800 and CoO-NSC-900 samples exhibit that peaks at 36.5, 42.2 and 61.6° are ascribed to the (111), (200), and (220) plane diffraction of CoO (JCPDS 48-1719), respectively,27 suggesting the formation of CoO along with the carbonization of organic precursors. However, the new XRD peaks at 43.7 and 50.9 ° for the CoO-NSC-1000 sample can be indexed to the diffraction of (111) and (200) lattice plane of Co5.47N (JCPDS 41-0943), suggesting the formation of new crystalline cobalt nitride due to the large amount of nitrogen atoms in PEI. For the Raman spectra (Figure 1b), the two typical peaks are observed at 1350 and 1580 cm-1, respectively, which are ascribed to D- and G-bands.28 Generally, two bands would be attributed to the disorder sp3 and the sp2 graphitic carbons. The higher intensity ratio of D- over G-band (ID/IG) would be caused by the presence of abundant defects among the graphitic carbon matrix, which is beneficial to generate more efficient sites for improving electrocatalytic activity.29-30 The ID/IG ratios of samples were calculated to be 1.12, suggesting the presence of abundant defects. The typical N2 sorption curves (Figure 1c) would be ascribed to Type IV isotherms and the obvious hysteresis loops in the range of 0.4 ～ 1.0 suggest the existence of mesopores. Additionally, the abruptly increased volume at the low pressure would be contributed to the presence of micropores. Especially, the larger absorbed volume for CoO-NSC sample indicates the larger specific surface area than that of NSC. Indeed, the surface area of NSC-900, CoONSC-800, CoO-NSC-900 and CoO-NSC-1000 is 501, 567, 610 and 601 m2 g-1, respectively. The pore size distribution curves (Figure 1d) confirm the presence of abundant mesopores. In comparison with the nonporous feature of NSC-900, the obvious mesopores at around 3 and 5 nm are observed for CoO-NSC-800, CoO-NSC-900 and CoO-NSC-1000. The cobalt oxide nanoparticles are of importance for the generation of porous structure due to the etching effect of


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cobalt oxide because cobalt oxides are efficient catalysts for the decomposition of carbons at an elevated temperature. The rich mesoporous structure with large surface area would benefit to expose the inner active sites for enhancing electrocatalytic activities.31 For comparison, the FESEM images of NSC-900 sample prepared in the absence of metal species (Figure 2a & b) exhibit the bulk carbon particles. The CoO-NSC-800 sample exhibits the similar morphology with large interspaces (Figure 2c). As shown in the HRTEM image, the small cobalt oxide particles are embed in the carbon matrix (Figure 2d). However, with increasing the pyrolysis temperature, the porous structure composed of interconnected ligaments is observed for the CoO-NSC-900 sample (Figure 2e) and the ligament exhibits the smooth surface possibly due to the surface diffusion under thermal treatment. Notably, the TEM image (Figure 2f) reveals the presence of hollow nanoparticles among the carbon matrix. Further increasing the temperature to 1000 ℃ , CoO-NSC-1000 shows the surface smooth with few of holes. Furthermore, the white particles locate at the entrance of the holes. Obviously, the SEM images exhibit the enhanced surface cohesion process from the aggregated bulk particles, interconnected ligaments, to the smooth surface with holes (Figure 2g). The decomposition and carbonization of organic precursors (e.g., PEI and lignosulfonate) would contribute to the formation of carbon matrix. The thermal densification of these organic compounds would be driven by surface-energy reduction along with the carbonization process, leading to the gradual morphology changes. The TEM images reveal that the cobalt oxide particles embedded in the CoO-NSC-800 sample are observed (Figure 2d) whereas the aggregated particles with hollow structure are observed for the CoO-NSC-900 sample (Figure 2f). Furthermore, CoO-NSC-1000 exhibits the obvious nanoparticles with much larger size in comparison with CoO-NSC-800 (Figure 2h). The increasing particle size at the elevated temperature would be attributed to the


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Ostwald ripening, although the nanoparticles are trapped among the carbon matrix.32 It has been revealed that the presence of metal species is crucial to form porous structure because of the etching effect. Thus, the nanoparticles are thermal-driven moving and aggregated together to form hollow CoO structure for CoO-NSC-900. The thermal reduction in the presence of carbon would result in the formation of abundant oxygen vacancies for CoO nanoparticles. The TEM image of CoO-NSC-1000 obviously exhibits the channels with the moving trace of nanoparticles, resulting in the formation of porous structures. However, at such a high temperature, the sintering of small nanoparticles led to the aggregated particles with larger size. The HRTEM image in Figure 2i shows the enlarged hollow structure composed of aggregated nanoparticles. The nanoparticles are interconnected to improve the structure stability and expose much corners and edges as active sites. The HRTEM image verifies that the lattice spacing is around 0.25 nm, which is consistent with that of CoO (111).33 More interestingly, the larger lattice spacing is 0.34 nm, which is ascribed to the (002) spacing of graphitic carbon, suggesting the formation of carbon coating layer around the hollow particle, which also enhance the mechanical stability of nanostructured composite.28 The element mapping images (Figure 2j) exhibits that N, S, O and Co was uniformly distributed, which would be attributed to the N, S-codoped carbon with interspersed CoO nanoparticles. The chemical compositions of CoO-NSC samples are tested by using XPS. The N, S and Co, O elements are observed on the survey spectra (Figures 3a & S2), manifesting the formation of N, S co-doped carbon and cobalt oxide composite materials.34 The C 1s in Figure 3b can be divided into four noticeable bonds, which are indexed to C=C (284.6 eV)，C=N/C-S (285.7 eV), C-OC/C=N (287.4 eV) and O-C=O (290.1 eV), respectively.35 N 1s peak is deconvoluted into four peaks according to pyridinic (398.6 eV), pyrrollic (400.8 eV), graphitic (401.5 eV) and oxidized


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pyridinic N (404.0 eV). Thus, nitrogen doping is confirmed by the core-level XPS results (Figure 3c).36 The large ratio of pyridinic and pyrrollic N would provide more efficient sites to improve oxygen electrocatalysis.37 Additionally, a new peak at around 399.4 eV would be ascribed to the formation of Co-N binding when the pyrolysis temperature was increased to 1000 ℃ (Figure S3d) according to the XPS result. To demonstrate the configuration of sulfur, the S 2p peak is divided into three peaks, which are ascribed to C-S-C (164.3 eV for S 2p3/2, 167.9 eV for S 2p1/2) and CSOx-C (175 eV) (Figure 3d), respectively. It is believed that C-S bonding is considered to be the vital active sites and thus enhance the catalytic activity towards ORR.38 The Co 2p spectrum reveals that the binding energy at 780.8 eV and its adjacent peak are assigned to Co 2p3/2 while the peak at 796.8 eV with the satellites is ascribed to Co 2p1/2 of CoO (Figure 3e). Additionally, the lattice oxygen in CoO phase was also observed at the low binding energy of 530 eV (Figure 3f).33 More importantly, the oxygen coordination at 532 eV would be contributed to the formation of abundant oxygen vacancies for cobalt oxide.39 Especially, the Co 2p spectra exhibit the right shift to the large binding energy with increasing the annealing temperature to 1000 ℃ (Figure S3b & e). The results suggest the increasing oxygen deficiency due to the reducing ability of carbon materials.40 The presence of oxygen vacancies would modulate the electronic structure of cobalt oxide and provide rich active sites for the electrocatalytic reactions.41 To examine the electrochemical activity for ORR, the samples prepared at different temperatures were evaluated in aqueous electrolyte (0.1 M KOH). Cyclic voltammetry curves (Figure 4a and Figure S4a-b) exhibit an obvious cathodic peak in O2-satuated electrolyte in comparison with N2-satuated electrolyte, corresponding to the oxygen reduction reaction. The potential for ORR at the CoO-NSC-900 electrode (0.83 V) is higher than those at CoO-NSC-800 (0.81 V) and CoO-NSC-1000 (0.82 V) (Figure S4a & b), indicating the best catalytic activity.


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The liner sweep voltammetry (LSV) curves in Figure 4b were obtained by using a rotating disc electrode with a speed of 1600 rpm. The CoO-NSC-900 catalyst exhibited more positive halfwave than those of NSC-900 CoO-NSC-800 (0.74 V), CoO-NSC-1000 (0.65 V) and comparable with Pt/C (0.83 V) (Figure 4c). Furthermore, the diffusion-limiting current density of 5.5 mA cm2

for CoO-NSC-900 is larger than those of NSC-900 (3.5 mA cm-2), CoO-NSC-800 (5.0 mA cm-

2),

CoO-NSC-1000 (4.0 mA cm-2). Thus, CoO-NSC-900 exhibits the best electrocatalysis for

ORR, which is similar to that of Pt/C electrocatalyst. The porous structure with large surface area is beneficial to expose the active sites for oxygen reduction. However, the half-wave potential is negatively shifted and the current density is smaller when the CoO-NSC-900 electrocatalyst was washed with acid solution (Figure S5). It is believed that cobalt oxide is crucial to improve electrocatalytic activity. To examine the ORR mechanism, LSV curves of CoO-NSC-900 was tested at various rotation speeds in the range of 400 ~ 2500 rpm. By increasing rotation speed, the current density is enhanced (Figure 4d). The electron transferred number of CoO-NSC-900 (inset in Figure 4d) is about 4.0, which is calculated by using the Koutecky–Levich (K–L) equation. Thus, oxygen is able to be reduced via a four-electron process. In contrast, the calculated electron transfer numbers are only 3.5 for CoO-NSC-800 and CoO-NSC-1000 (Figure S6a-d). To further determine the ratio of intermediate for ORR, the disk and ring current densities were collected in Figure 4e by using a rotating ring disk electrode (RRDE). The current density at the ring electrode is quite lower than that at the disk electrode. The peroxide generated (%) is less than 1.5 % and the average electron transfer number is 4.0, which is consistent to the RDE results. The results further confirmed that oxygen can efficiently reduced at the CoO-NSC900 electrode (inset in Figure 4e). Additionally, negligible current decay is observed when additional methanol (1.5 M) was injected into the electrolyte for CoO-NSC-900 catalyst.


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However, the Pt/C electrode exhibits rapid current decay (Figure 4f). Thus, the results exhibit that the CoO-NSC-900 electrode has excellent resistance to methanol cross-over effect. To determine the bifunctional electrochemical catalytic activity, the LSV curves for OER were performed in alkaline electrolyte (Figure 5a). The onset potential at the CoO-NSC-900 electrode is around 1.55 V, which is lowest than these of NSC-900 (1.75 V), CoO-NSC-800 (1.68 V), CoO-NSC-1000 (1.69 V) and RuO2 (1.60 V). Moreover, a current density of 10 mA cm-2 at the CoO-NSC-900 electrode can be achieved at the small overpotential of 470 mV which is even smaller than that of RuO2 electrode (570 mV). Tafel plot (Figure 5b) is used to examine the reaction kinetics . The Tafel slope of OER at the CoO-NSC-900 electrode is 102 mV dec-1, smallest than these at NSC-900 (320 mV dec-1), CoO-NSC-800 (115 mV dec-1), CoO-NSC-1000 (133 mV dec-1) and RuO2 (112 mV dec-1), which suggests the best reaction kinetics process for the oxygen evolution. The electrochemical active surface area (EASA) of an electrocatalysts is generally calculated according to the electrochemical double-layer capacitance (Cdl). In Figure S7a-d, CV plots were implemented at various rates of 10 ~ 120 mV s-1. The largest Cdl for CoONSC-900 (3.5 mF cm-2) compared with these of NSC-900 (2.2 mF cm-2), CoO-NSC-800 (2.4 mF cm-2) and CoO-NSC-1000 (2.9 mF cm-2)indicates the largest electrochemical active area to enhance the catalytic activity (Figure S7e).42 According to the electrochemical impedance spectroscopy (EIS) in Figure S7f, the smallest charge transfer resistance of CoO-NSC-900 electrode (5 Ω) in comparison with NSC-900 (100 Ω), CoO-NSC-800 (19 Ω) and CoO-NSC1000 (10 Ω) further demonstrates the high electron transfer efficiency.43 Figure 5c shows the polarization curves for both ORR and OER. Generally, the low potential gap (E=Ej=10-E1/2) indicates the efficient bifunctional activity.44 Thus, the efficient oxygen reduction and evolution reactions can be achieved at the CoO-NSC-900 electrode due to the minimum potential gap of


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0.86 V (Figure 5d). In comparison with the reported results (Table Sl), it can be seen that the CoO-NSC-900 catalyst exhibits lower overpotentials for ORR and OER, suggesting the improved bifunctional electrochemical activity. The enhanced electrocatalytic performance of CoO-NSC-900 would be contributed to the synergistic effect of hollow cobalt oxide with abundant oxygen vacancies and the heteroatom-doping. Generally, the N, S doping is of importance for improving the electrocatalytic activity of graphitic carbon because the electron distribution could be modulated efficiently to adjust the adsorption of reaction intermediates for the ORR/OER process.45 The presence of oxygen vacancies and the hollow structure of cobalt oxides would also generate efficient sites for enhancing the electrocatalytic activity. The Zn-air battery with an aqueous electrolyte was fabricated by using CoO-NSC-900 as the catalyst to further explore its practical application. For comparison, the other samples were also assembled as the cathode electrodes, respectively.46 The open circuit potential of CoO-NSC-900 is maintained at 1.4 V and equivalent to that of the Pt/C catalyst (Figure S8a). The CoO-NSC900 electrode (Figure 6a) exhibits the largest power density, which would be contributed to large EASE with more efficient sites for enhancing the power density. Zn-air battery assembled with CoO-NSC-900 exhibit stable discharging curves with potentials of 1.29, 1.21 V at 2, 10 mA cm-2, respectively, suggesting the good high-rate performance. Furthermore, the specific capacities are 745 mA h gzn-1 and 714 mA h gzn-1, which are corresponding to the energy densities of 943 and 871 Wh kgzn-1(Figure 6b). The large specific capacity suggests that Zn can be fully consumed for energy generation by using CoO-NSC-900 electrocatalyst. The discharging potential of Zn-air battery fabricated with CoO-NSC-900 is around 1.13 V at a high current density of 15 mA cm-2 (Figure S8b). However, the discharge potential still comes up to 1.34 V by decreasing the current density to 0.1 mA cm-2. The negligible potential decay for the


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battery using CoO-NSC-900 catalyst suggests the good stability for 100 h, which would be contributed to the good catalytic stability of CoO-NSC-900 electrocatalyst for ORR (Figure S8c)47. The discharge-charge cycling stability is of importance for evaluating the battery performance.48 The smaller voltage gap of CoO-NSC-900 in comparison with Pt+RuO2 indicates the low polarization for the discharging/charging process in Figure S8d. As shown in Figure 6c, the discharging and charging potentials of the battery using CoO-NSC-900 electrocatalyst are maintained at around 1.2 and 2.0 V, respectively for 60 h at 2 mA cm-2. The battery performance is superior to those of batteries using NSC-900 (~0.8 and 2.5 V), CoO-NSC-800 (~0.9 and 2.5 V), CoO-NSC-1000 (~0.9 and 2.3 V) and Pt+RuO2 (~1.0 and 2.4 V). The potential efficiency could still maintain 56% at the 360th discharge-charge cycling, further suggesting the good cycling stability (inset in Figure 6c). Notably, the battery using CoO-NSC-900 as the bifunctional electrocatalyst exhibit very stable operation over 60 h at a higher current density of 10 mA cm-2 (Figure S8e). The low electron transfer resistance and the large surface area of CoONSC-900 would contribute to the rapid electron and mass transfer for oxygen electrocatalysis.31 The abundant mesopores could adjust the adsorption of oxygen and intermediate products to reduce the adverse impact of polarization.49 Thus, the good bifunctional electrocatalytic activity of CoO-NSC-900 contributes to the enhanced battery performance. Portable electronic devices are attracted increasing attention.50 Hence, the all-solid-state Zn-air batteries were fabricated with the polyacrylic acid (PAA) gel electrolyte.31 Figure S9a shows the open circuit potentials for the batteries using CoO-NSC-900 and Pt/C catalyst, respectively. The battery using CoO-NSC-900 electrocatalyst achieves the maximum power density of 65 mW cm2,

which outperforms the batteries using Pt/C (55 mW cm-2), NSC-900(40 mW cm-2), CoO-NSC-

800 (51 mW cm-2), and CoO-NSC-1000 (50 mW cm-2) (Figure 7a). When normalized to the Zn


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consumed during the discharging process, the specific capacities of CoO-NSC-900 (Figure 7b) are 677, 536 mA h gzn-1 at 1, 10 mA cm-2, respectively. The corresponding energy density is up to 858 and 661 Wh kgzn-1. Notably, the discharging voltage is as high as 1.17 V for the battery with CoO-NSC-900 electrocatalyst by increasing the current density to 15 mA cm-2 (Figure 7c), highlighting the good high-rate performance. Additionally, the stability test of the Zn-air battery (Figure S9b) exhibits negligible potential decay for 40 h, which would be contributed to the good catalytic stability. Figure 7d shows the polarization curves of Zn-air batteries using CoO-NSC-900 and Pt+RuO2 electrocatalyst, respectively. At the high current densities (> 25 mA cm-2), the low polarization for battery with CoO-NSC-900 electrocatalyst suggests the improved coulombic efficiency. It would stem from the large amount of the exposed active sites due to the synergic contribution of N, S co-doping and the hollow cobalt oxide nanoparticles for efficient bifunctional activities. Furthermore, the porous structure of conductive carbon skeleton enables the efficient mass and electron transfer ability, which also contributes the enhanced battery performance. As shown in Figure 7e, the discharging and charging potentials of CoO-NSC-900 are retained at 1.2 and 2.08 V, respectively, which is much better than these of NSC-900 (0.8 and 2.5 V), CoO-NSC-800 (1.1 and 2.08 V), CoO-NSC-1000 (1.0 and 2.5 V) and Pt+RuO2 (1.1 and 2.25 V). More importantly, in comparison with the obvious recession of battery using CoO-NSC-800, CoO-NSC-1000 and Pt+RuO2, the battery using CoO-NSC-900 deliver very stable potential plateau in the cycling stability test for 35 h. The polarization of the battery could be exacerbated by increasing to 10 mA cm-2. However, the rechargeable stability of CoO-NSC-900 still precedes the Pt+RuO2 catalyst (Figure S10). Comparing to the reported works, the battery fabricated with CoO-NSC-


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900 catalyst exhibit better cycling stability with lower overpotentials, showing the good battery performance (Table 2). CONCLUSION In summary, the N, S-codoped porous carbons with interspersed hollow CoO nanoparticles are successfully prepared through a facile one-pot method. The electrostatic interaction of PEI with metal ions and lignosulfonate leads to the formation of organic-inorganic precursors via the co-precipitation process. The subsequent pyrolysis of the homogenous nanocomposite resulted in the in-situ formation of N, Scodoped porous carbons with interspersed CoO nanoparticles. The resultant electrocatalyst exhibited good bifunctional electrocatalysis for ORR (half-wave voltage comparable with Pt/C) and OER (overpotential smaller than that of RuO2 at 10 mA cm-2). More importantly, the Zn-air battery assembled with a solid-state electrolyte exhibit a large power density of 65 mW cm-2, good cycling stability and flexibility. ASSOCIATED CONTENT Supporting Information


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TEM image, SEM image, XPS pattern, and electrochemical performance of the correlative samples. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected];

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was financially supported by the Natural Scientific Foundation of China (No. 21503116). Taishan Scholars Program of Shandong Province (No. tsqn20161004) and the Youth 1000 Talent Program of China are also acknowledged.


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Figures and captions

Scheme 1. Schematic illustration for preparing CoO-NSC.


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Figure 1. (a) XRD patterns of samples. (b) Rama spectrum. (c) Nitrogen sorption isotherm. (d) The distribution curves of pore sizes.


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Figure 2. SEM images for NSC-900 (a), CoO-NSC-800 (c), CoO-NSC-900 (e) and CoONSC-1000 (g). TEM images for NSC-900 (b), CoO-NSC-800 (d), CoO-NSC-900 (f) and CoO-NSC-1000 (h). (i) HRTEM image for CoO-NSC-900. (j) The element mapping images for CoO-NSC-900.


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Figure 3. (a) XPS survey spectra of CoO-NSC-900. The core-level XPS spectra of C 1s (b), N 1s (c), S 2p (d), Co 2p (e) and O 1s (f).


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Figure 4. (a) CV curves for CoO-NSC-900. (b) LSV curves and (c) half-wave potentials of NSC-900, CoO-NSC-800, CoO-NSC-900 and CoO-NSC-1000 at the rotation speed of 1600 rpm. (d) LSV curves and the K-L plots (inset) of CoO-NSC-900 at different rotation speeds. (e) RRDE test for CoO-NSC-900. The generation of peroxide (%) and the electron transfer number (inset). (f) Chronoamperometric response of CoO-NSC900 and Pt/C by adding 1.5 M methanol.


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Figure 5. (a) LSVs and (b) Tafel slopes of NSC-900, CoO-NSC-800, CoO-NSC-900 and CoO-NSC-100 and RuO2 for OER. (c) polarization curves for ORR and OER. (d) E values (E=Ej=10-E1/2).


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Figure 6. (a) Polarization and power density curves of Zn–air battery. (b) Long-term stability of Zn–air batteries. (c) Discharge–charge cycling curves at 2 mA cm−2. Inset is the enlarged curves with CoO-NSC-900 electrocatalyst.


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Figure 7. (a) Polarization and power density curves of the all-solid-state Zn–air battery. (b) The specific capacity curves of Zn–air battery normalized with the Zn consumed. (c) The discharging curves at various current densities. (d) Charge-discharge polarization curves. (e) The cycling stability tests at 1 mA cm−2.


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TOC Figure


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Bifunctional Oxygen Electrocatalysis of N, S-Codoped Porous Carbon

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