Cobalt nitride embedded holey N-doped graphene as advanced bifunctional electrocatalysts for Zn-Air batteries and overall water splitting

Journal Pre-proof Cobalt nitride embedded holey N-doped graphene as advanced bifunctional electrocatalysts for Zn-Air batteries and overall water splitting Xinxin Shu, Song Chen, Si Chen, Wei Pan, Jintao Zhang PII:

S0008-6223(19)31032-2

DOI:

https://doi.org/10.1016/j.carbon.2019.10.023

Reference:

CARBON 14685

To appear in:

Carbon

Received Date: 18 August 2019 Revised Date:

29 September 2019

Accepted Date: 12 October 2019

Please cite this article as: X. Shu, S. Chen, S. Chen, W. Pan, J. Zhang, Cobalt nitride embedded holey N-doped graphene as advanced bifunctional electrocatalysts for Zn-Air batteries and overall water splitting, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.10.023. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Graphical abstract Tannic acid (TA) as a dispersing agent for chelating Co2+ ions on the surface of GO renders the in-situ formation of nitrogen doped holey graphene sheets with embedded cobalt nitride nanoparticles via the thermal treatment for bifunctional electrocatalysis in Zn-air battery and water splitting.

Cobalt Nitride Embedded Holey N-doped Graphene as Advanced Bifunctional Electrocatalysts for Zn-Air Batteries and Overall Water Splitting Xinxin Shu,a Song Chen,a Si Chen,a Wei Pan,b Jintao Zhang a,*

a

Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China b

College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, China.

*Corresponding author. E-mail: [email protected] (Jintao Zhang)

ABSTRACT Exploration of cost-effective and highly durable carbon-based multifunctional electrocatalysts for energy conversion and storage devices (e.g., metal-air batteries) is of critical significance. Herein, we present a unique worm-like structure of hierarchically porous nitrogen-doped graphene (N-rGO) embedded with cobalt nitride (Co5.47N) nanoparticles (named as Co5.47N@N-rGO) by tannic acid assisted nitridation method for Zn-air batteries and overall water splitting. Benefiting from the unique worm-like structure to expose more active sites and the synergy advantages of a close contact between Co5.47N nanoparticles and the N-rGO sheets, the Co5.47N@N-rGO exhibits efficient bifunctional catalytic activities toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Notably, Zn-air batteries assembled with Co5.47N@N-rGO-750 show a power density of 120.7 mWcm-2 and excellent cycling stabilities for 330 h in an aqueous electrolyte. More interestingly, when assembled into a flexible solid-state rechargeable Zn-air battery, the Co5.47N@N-rGO-750 displays a specific capacity of 610 mAh gzn-1 and good cycling stability over 40 h. Moreover, the integrated device for water splitting powered by Zn-air batteries is also fabricated by using the Co5.47N@N-rGO-750 electrocatalyst, exhibiting a good gas generation rate. This work offers a new strategy to design and synthesize efficient multifunctional carbon-based electrocatalysts applied in electrochemical devices.

1. Introduction The massive consumption of traditional fossil fuels results in serious climate and environment issues, which has drawn much attention to develop clean and efficient electrochemical energy conversion and storage technologies, such as fuel cells, metal-air batteries and water splitting devices [1, 2] [3]. The performance of these energy storage and conversion devices is determined by a couple of fundamental electrochemical reactions. For example, rechargeable zinc-air batteries attract increasing attention in light of their advantages such a slow cost, environmental benignity and high theoretical energy density [4]. Oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are two key half-reactions, dominating the final performances of zinc-air batteries. Additionally, water splitting couples the hydrogen evolution reaction (HER) with OER, which is the highly promising approach to obtain clean hydrogen energy. Due to multi-electron transfer process and the sluggish kinetics of ORR, OER and HER, advanced electrocatalysts are highly desirable to decrease the reaction overpotentials and improve the conversion efficiency [5-8]. To date, important progress has been made to develop electrocatalysts for OER or ORR separately [8-12]. For instance, platinum (Pt) and its alloys are proved to be the efficient ORR/HER catalyst whereas ruthenium (Ru) / iridium (Ir) and their oxides represent the state-of-the-art materials for OER. However, such noble metal-based catalysts generally are incapable to serve as multifunctional electrocatalysts for ORR, HER and OER. Therefore, various alternative materials including transition metal base (Fe, Co, Ni) materials and non-metallic base materials (N, S, P heteroatom doping) have been widely studied by researchers [13-15]. Especially, cobalt-based nanomaterials have attracted tremendous attention as

alternative electrocatalysts for ORR and OER (or HER and OER) due to their multifunctional catalytic activity, low cost, and easy availability [16-18]. For example, Co3O4 nanosheets have been used as electrocatalyts to farbricate a Zn-air battery with a peak power density of 106.6 mW cm-2 [16]. For ORR, the half wave potential at CNTs-Co/NC composite electrocatalyst was up to 0.84 V in an alkaline electrolyte [19]. However, most of cobalt-based oxides still suffer from poor electronic conductivity, while cobalt nanoparticles have the disadvantage of easy agglomeration. In contrast, cobalt nitrides exhibit good catalytic activities and much better stabilities as well as good conductivities due to their typical metallic properties [20] [21]. The N2 radio frequency plasma treatment of Co3O4 nanowire arrays enabled the preparation of cobalt nitride nanowires, achieving a current density of 10 mA cm-2 for OER at a low overpotential of 290 mV [20]. The depostion of brush-like Co4N nanorods on N-doped carbon nanofiber rendered the long-term cycling stability for over 177 cycles in Li-O2 battery [22]. Although the big advances on the development of cobalt nitride electrocatalyts have been achieved [23], it is still highly desirable to develop efficient strategies for fabricating the cobalt nitride-based electrocatalysts for multifunctional applications, such as rechargable Zn-air batteries and water splitting. Additionally, substantial research efforts have demonstrated that graphene provides efficient platform to enhance the dispersion of additional species for various electrochemical applications [21, 24-29]. However, the chemically deriven graphene are easily aggreagated due to the π-π re-stacking [21]. Herein, we present a green and simple method to prepare multifunctional electrocatalysts of nitrogen-doped graphene embedded with Co5.47N nanoparticles (Co5.47N@N-rGO). Tannic acid (TA), as a

plant-derived polyphenol with strong chelating ability to various metal ions [30, 31], provides a facile and efficient approach to anchoring cobalt ions uniformly on the surface of graphene oxide sheets without obvious aggregation. The subsequent thermal treatment in the presence of NH3 leads to the in-situ formation of Co5.47N nanoparticles embedded on the nitrogen-doped reduced graphene oxide (N-rGO). Interestingly, the thermal etching effect of cobalt nitride nanoparticles results in the formation of worm-like pores on rGO sheets during the nitridation process. It can be expected that such well-designed holey N-rGO with uniformly dispersed and highly conductive Co5.47N nanoparticles could not only facilitate fast interfacial electron transfer and ion diffusion for the corresponding reactions but also effectively suppress the aggregation and pulverization of cobalt nitride nanoparticles. Thus, the Co5.47N@N-rGO exhibits efficient multifunctional catalytic activity towards ORR, HER and OER in the alkaline solution. On the basis of the bifunctional electrocatalytic activity, the Co5.47N@N-rGO-750 electrocatalyst are used as the electrocatalysts for rechargeable Zn-air batteries and overall water splitting to realize self-powered generation of oxygen and hydrogen. The high conductive Co5.47N nanoparticles incorporated with the hierarchically porous N-doped rGO sheets contributes to the multifunctional electrocatalytic activity 2. Experimental 2.1. Materials Cobalt (II) acetate tetrahydrate (Co(OAc)2·4H2O, 99.5%) and N,N’-Methylenebis (acrylamide) (MBA) were purchased from Shanghai Macklin biochemical technology Co., Ltd. Tannic acid (TA) was purchased from Shanghai Aladdin biochemical technology Co., Ltd. Zinc acetate dihydrate (Zn(OAc)2·2H2O, 99%), Znic oxide (ZnO),

Potassium peroxydisulfate (K2S2O8) and KOH were purchased from Sinopharm Chemical Reagent Co., Ltd. Ultrapure ammonia and nitrogen were purchased from Jinan Deyang gas Co., Ltd. Carbon paper (HCP-135) and Carbon cloth (HCP-330N) were purchased from Shanghai hesen Co., Ltd. All the regents were used without further purification. 2.2. Sample preparation Synthesis of Co5.47N@N-rGOs: Graphene oxide (GO) was prepared from natural graphite flake via the modified Hummers' method [32]. 40 mL of GO dispersion (1.5mg mL-1) was sonicated for 20 min and then Co(OAc)2·4H2O (0.4982 g) was added into the above solution under strong stirring. Tannic acid (0.75 g) was dissolved in 10 ml deionized (DI) water under magnetic stirring for 10 minutes and added into the above mixed solution to abtain an grey pink precipitate. The precipitates were collected by centrifugation and washed repeatedly with DI water to remove the excess tannic acid and Co2+ ions. The sample obtained by freeze-drying for 12 h was loaded into a tube furnace and heated to the desired temperatures (650~800 °C) with a heating rate of 10 °C/min in the gas flow with a N2:NH3 ratio of 3:1. The obtained samples were defined as Co5.47N@N-rGO-650, Co5.47N@N-rGO-700, Co5.47N@N-rGO-750 and Co5.47N@N-rGO-800, respectively. The samples were also prepared by changing the pyrolysis time (2, 3h) and named as Co5.47N@N-rGO-750-2h, Co5.47N@N-rGO-750-3h, respectively. Synthesis of Co@N-rGO-750: For comparison, in the absence of TA, the sample was prepared by using the same procedure at 750 °C and named as Co@N-rGO-750. 2.3 Material characterizations Scanning electron microscopy (SEM) was performed by a Gemini-SEM-300, Carl

Zeiss Microscopy GmbH and transmission electron microscopy (TEM) were carried out on JEOL 2100 PLUS. X-ray diffraction (XRD) was conducted by using a X’Pert3 Powder X-ray diffractometer. High-resolution transmission electron microscopy (HRTEM) was carried out on a FEI-TF20 at Shiyanjia Lab. X-ray photoelectron spectroscopy (XPS) was performed with a photoelectron spectrometer (ESCALAB 250). Raman spectra were carried out on a LabRAM HR800 with an excitation laser of 532 nm. N2 sorption isotherms were measured at 77 K using BJ Builder Kubo-X1000 instruments. 2.4 Electrochemical characterization The electrochemical tests were performed by using a CHI 760E electrochemical workstation (CH Instrument, Shanghai) with a three-electrode system. A glassy carbon rotating ring-disk electrode (RRDE) coated with the as-prepared catalyst was served as the working electrode, while Ag/AgCl electrode was considered as the reference electrode. A graphite rod was used as the counter electrode for hydrogen evolution reaction (HER) tests, and a Pt plate was served as the counter electrode for oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) tests. All the potentials were measured versus Ag/AgCl electrode and calibrated to reversible hydrogen electrode according to the following equation: ERHE = EAg/AgCl + 0.197 + 0.0591*pH. When the electrolyte was 0.1M KOH, the PH value was 13. To prepare the catalyst ink, 5 mg of as-prepared samples were blending with 50 µL Nafion (5 wt%) in 950 µL ethanol solution under sonication to obtain a homogeneous catalyst ink. Then, 12 µL catalyst ink was dropped onto the glassy carbon electrode surface and dried at room temperature. For comparison, the Pt/C (20 wt%, ETEK) electrode was prepared using the same procedure.

Cyclic voltammetry measurements were performed in N2 or O2 saturated 0.1 M KOH electrolyte from -1.0 to 0.2V (vs. Ag/AgCl) at a scan rate of 10 mV s-1. LSV curves for ORR were performed by using RDE in O2 saturated 0.1 M KOH at a scan rate of 5 mV s-1 and the catalyst loading is 0.24 mg cm-2 (RRDE, 5.61 mm), while the mass loading of the commercial Pt/C catalyst was 0.20 mg cm-2. OER and HER measurements were carried out in O2 or N2 saturated 1.0 M KOH (pH = ~14) at a scan rate of 5 mV s-1, respectively, and the catalyst loading is 0.5 mg cm-2 on the surface of carbon paper (1×1 cm). The electron number (n) transferred per oxygen molecule for ORR procedure can be determined by Koutechy-Levich equation.[33] 1

=

1

+

1

=

1

+

1

1

where j is measured current, jd is the diffusion-limiting current, jk is the kinetic current and ω is the electrode rotating rate. B is calculated from the slope of Koutechy-Levich (K-L) plots according to the Levich equation as given below: = 0.2

2

where F is the Faraday constant (96485 C mol-1), oxygen-saturated 0.1 M KOH (1.2 × 10-6 mol cm-3),

is the bulk concentration of O2 in is the diffusion coefficient of

O2 in 0.1 M KOH (1.9 × 10-5 cm2 s-1), ν is the kinematic viscosity (0.01 cm2 s-1), The constant 0.2 is adopted when the rotation speed is expressed in rpm. The HO2- yield and n (the number of electrons transferred during ORR) were estimated by the followed equations [34]:

!

%

= 200 ×

= 4 × +

!

" +

!

3

"

4

"

where Ir is ring current, Id is disk current, N is current collection efficiency of the Pt ring. N was determined to be 0.40 [33]. 2.5 Zn-air battery measurements The liquid Zn-Air battery measurements were performed in a home-made electrochemical cells. The air cathodes were prepared by coating the prepared catalyst ink (5 mg ml-1) onto carbon paper with polytetrafluoroethylene (PTFE) binder, and dried at 80 °C for 2 h. The mass loading was 0.75 mg cm-2 unless otherwise noted. A Zn plate (0.3 mm thick) acted as the anode and a solution containing 6 M KOH and 0.2 M zinc acetate was utilized as electrolyte to ensure reversible Zn electrochemical reactions in the rechargeable Zn-air batteries. The as-fabricated Zn-Air batteries were characterized under ambient conditions using a LAND CT2001A multi-channel battery testing system. To assemble the solid-state zinc-air battery, polyacrylic acid (PAA) polymer gel electrolyte was prepared. In particularly, 6.3 g KOH was blended with 0.20 g ZnO in 9 ml H2O, and the obtained solution was named as solution A. Solution B was prepared by adding 0.15 g N, N′-methylene-bisacrylamide (MBA) into 0.95 mL acrylic acid. Then, two solutions were mixed together slowly and kept stirring for 5min. Then, 120 µL K2S2O8 was added to irritate the polymerization process. The as-prepared air electrode and the zinc foil (0.1 mm thick) were assembled with a piece of polymer electrolyte and sealed by using acrylic tape (0.5 mm thick).

2.6 Water splitting tests 5 mg of electrocatalyst was ultrasonically dispersed in mixed solution of 950 µL ethanol solution and 50 µL Nafion (5 wt%) to obtain the electrocatalyst ink. The anode and cathode electrodes were prepared by homogeneously coating the catalyst ink onto the carbon paper (1×1 cm2) to achieve a mass loading of 0.5 mg cm-2. The water splitting tests were performed in a home-made device with two zinc-air batteries. 3. Results and discussion 3.1. Microstructure and composition characterization The scheme diagram of the synthesis process is demonstrated in Fig. 1. Firstly, cobalt ions would be adsorbed on the GO surface via the electrostatic interaction due to the ionization of oxygenate functional groups (such as carboxyl, hydroxyl, and epoxyl groups) [35]. TA, a typical polyphenol with a number of pyrogallol hydroxyls, is able to chelate with Co2+ ions, because two adjacent phenolic oxygen molecules donates a lone pair of electrons to the empty d-orbitals of Co2+ ions, resulting in the formation of stable five-membered chelating rings [36]. The reaction is extremely fast, accompanied with an immediate color change from brownish black to grey pink (Fig. S1). Then, the thermal annealing enables the synchronous reduction of GO and the in-situ formation of the Co5.47N nanoparticles in the presence of NH3. Notably, TA molecules is of importance to prevent the GO sheets form stacking in the presence of cobalt ions as well as disperse the cobalt ions on the surface of GO due to the π-π stacking interactions between the aromatic rings of TA and GO sheets as well as the electrostatic interaction.

Figure. 1. The schematic illustration for the preparation of Co5.47N@N-rGO.

The morphology and structure of all samples were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 2a, the TA-Co/GO precursor exhibits the layered structure with lots of wrinkles. After thermal treatment, the sample obtained in the absence of TA (Co@N-rGO-750) exhibits the obvious aggregation of reduced graphene oxide (rGO) sheets to form graphite structure with lots of nanoparticles. (Fig. 2b and Fig. S2). The electrostatic interaction between cobalt ions and the negatively charged GO sheets would result in the serious aggregation of GO sheets [37]. In contrast, the highly porous structure composed of two-dimensional rGO sheets (Fig. 2c) is observed in the presence of TA， on which Co5.47N nanoparticles are uniformly dispersed. Interestingly, the enlarged SEM image (Fig. 2d) reveals that the nanoparticles are uniformly dispersed on the surface of rGO and many worm-shaped holes can be seen on the surface.

Figure. 2. The SEM images of TA-Co/GO (a), Co@N-rGO-750 (b), the SEM (c and d) and TEM images (e and f) with different magnifications of Co5.47N@N-rGO-750. HRTEM (g) and HAADF-STEM images and the corresponding EDS elemental mapping images (h) of Co5.47N@N-rGO-750. Especially, the enlarged TEM images reveal that the worm-like traces on the surface of rGO sheets with nanoparticles at the end of the traces (Fig. 2e) and even holes were formed (Fig. 2f). The thermal etching motion of Co5.47N nanoparticles on the rGO surface would lead to the formation of worm-like pores during the nitridation process, which would create more surface defects for enhancing electrocatalytic activity. At the same pyrolysis temperature, the particle size of Co5.47N@N-rGO-750 is much smaller

than that of Co@N-rGO- 750 prepared in the absence of TA (Fig. S2). Furthermore, the X-ray diffraction (XRD) patterns (Fig. S3) exhibit that the sharp diffraction peaks at about 2θ = 44.3, 51.5, 75.8°would be identified as the (111), (200), (220) reflection of metallic cobalt, respectively. In the absence of tannic acid, the cobalt ions adsorbed would be trapped among the stacked GO sheets. During the thermal treatment, the inner space could not be reached easily by the NH3 gas. Thus, cobalt nanoparticles were formed gradually due to the trapping effect of the stacked graphene sheets along with the generation of large pores around the nanoparticles owing to the thermal etching of carbon[37, 38]. For Co5.47N@N-rGO-750, tannic acid is helpful to prevent from stacking of GO sheets in the presence of cobalt ions[39]. During the thermal treatment, NH3 gas is able to diffuse into the highly porous structure of non-stacked sheets to form cobalt nitride on the surface of rGO sheets [40], and only few nanoparticles are formed along with the thermal etching of rGO sheets, leading to the formation of worm-like structure. The high resolution transmission electron microscopy (HRTEM) image (Fig. 2g) reveals that the lattice fringe with a d-spacing of 0.21 nm is in good agreement with that of the (111) plane of Co5.47N. The presence of graphite layers outside of the Co5.47N nanoparticles indicates the compact interaction between Co5.47N and the N-doped graphitic carbons matrix also improves the electrocatalytic activity of catalysts. Furthermore, Fig. 2h shows the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and the energy dispersive X-ray spectroscopy (EDS) mapping image of theCo5.47N@N-rGO-750. The images demonstrate the uniform distribution of C, N and Co components. It is obvious that the relatively strong signals of Co and N confirm the formation of Co5.47N nanoparticles on

the N-doped graphene framework. Comparing the samples obtained at different pyrolysis temperatures, many wrinkles and few pores are observed on the rGO surface at 650 °C (Fig. S4a-c), which is similar to that of bare rGO. With increasing the pyrolysis temperature to 700 °C, the particle size on the rGO surface becomes larger and some worm-like channels are observed due to the thermal etching of the nanoparticles (Fig. S4d-f). At the temperature of 750 °C, it is obvious that the structure of worm-like channels is observed clearly (Fig. 2d and 2e). However, when the temperature reached to 800°C, the rGO sheets with large holes and sintered particles are formed (Fig. S4g-i). The results suggest that the nanoparticles are moving to agglomerate together at a high temperature and at the same time the rGO sheet would be etched due to the thermal decomposition of carbons in the presence of metal catalysts. It is further evidenced that the rGO sheets are gradually etched and disappeared with increasing the annealing time from 2 to 3 h (Fig. S5 and S6). At longer calcination duration, the Co5.47N nanoparticles with larger size are formed due to the thermal aggregation (Fig. S7). N2 sorption isotherms were used to investigate the surface areas and pore structure of the as-synthesized materials. The isotherms exhibit a typical adsorption curve of type Ⅳ (Fig. S8a), suggesting the mesoporous structures (Fig. S8b). Co5.47N@N-rGO-750 possesses a relatively large surface area of 143.63 m2 g-1, which would facilitate to expose active sites for electrochemical reactions. The X-ray diffraction (XRD) analysis was performed to examine the crystal structure of the as-prepared Co5.47N@N-rGO. As shown in Fig.3a, the diffraction peaks located at 43.7, 50.8, and 74.9° are corresponding to the (111), (200), and (220) planes of Co5.47N, confirming the formation of Co5.47N [41]. A broad diffraction peak at 26° is considered to

be the (002) graphitic plane of N-rGO [42]. As previously reported [21], the crystal structure of the Co5.47N phase is that some N atoms are absent in the octahedral interstitials of the Co metal lattice and formed corresponding vacancies, which not only ensures the metallic property of Co5.47N but also enhances more active sites via the generation of vacancies. When the calcination temperature was increased from 650 to 800 °C, the diffraction peak intensity of graphite carbon gradually decreased because GO sheets were etched gradually with increasing the temperature.

Figure. 3. (a) XRD patterns and (b) Raman spectrum of Co5.47N@N-rGO-650, Co5.47N@N-rGO-700, Co5.47N@N-rGO-750 and Co5.47N@N-rGO-800. (c) XPS survey spectrum of Co5.47N@N-rGO-750. (d-f) High-resolution XPS spectra of the C1s, N1s and Co2p of Co5.47N@N-rGO-750. For the Raman spectra, the D-band at 1336 cm-1 is ascribed to the introduced defects in the graphene framework, and the G-band at 1591 cm-1 represents the sp2 graphite carbon [12, 43]. The Co5.47N@N-rGO-750 sample exhibits that the highest ID/IG ratio of

1.46 (Fig. 3b) suggests the presence of more defect sites for enhanced catalytic activity. The surface chemical composition of different samples can be confirmed by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 3c, the corresponding peaks of C, N and Co elements are clearly observed in the survey spectra of the Co5.47N@N-rGO-750. The high-resolution C1s spectrum displays the presence of C=C (284.5 eV), C-N (285.6 eV), C-OH (286.3 eV) and C-O (289.2 eV) species in the Co5.47N@N-rGO-750 sample, indicating the successful doping of N into the graphene matrix (Fig. 3d) [18, 44, 45]. The core-level N1s region (Fig. 3e) of Co5.47N@N-rGO-750 is fitted with four different peaks at binding energy of 397.3, 400.5, 404.0 eV and 399.3 eV, corresponding to the presence of pyridinic N, pyrrolic N, graphitic N and Co−N bond in Co5.47N. Such a high ratio of pyridinic N (50.6%) would enhance the surface wettability and electrical conductivity for improving the electrocatalytic activity towards ORR [17, 43, 46]. For Co2p spectra (Fig. 3f), the peaks located at 781.6 and 800.2 eV further reveal the presence of Co-N bond [41]. Two prominent peaks at 779.1 and 795 eV for Co 2p3/2 and Co 2p1/2, respectively indicate the metallic state of cobalt in Co5.47N@N-rGO-750 sample. However, the presence of satellite peaks at 785.4 and 802.2 eV would be attributed to the surface oxidation of nanoparticles. These results confirmed the formation of hybrid materials of Co5.47N nanoparticles and the N doped rGO. 3.2.ORR and OER electrocatalytic performance

To investigate the ORR catalytic activities of Co5.47N@N-rGO samples, cyclic voltammetry (CV) experiments were performed in N2-and O2-saturated 0.1 M KOH solution. As shown in Fig. S9, no obvious redox peak was observed for Co5.47N@N-rGOs in N2-saturated electrolyte. In contrast, when the electrolyte was

saturated with O2, a well-defined reduction peak clearly appeared, which is ascribed to the reduction of oxygen. The observed oxygen reduction peak shifted to more positive potentials with increasing pyrolysis temperature from 650 to 750 °C, but slightly reversed on further increasing the temperature to 800 °C. The reduction potential at the Co5.47N@N-rGO-750 electrode is superior to that of the commercial Pt/C catalyst (Pt/XC-72 20wt%), suggesting better electrocatalytic activity. Considering the 2D structure of GO sheets combined with Co5.47N, the samples with porous structure would provide large electrochemical active surface area for ORR. CV measurementsat different scan rates were used to evaluate the electrochemical double-layer capacitance (Cdl) (Fig. S10). As shown in Fig. S11, the value of Cdl for Co5.47N@N-rGO-750 is 13.5 mF cm-2, highest than those of Co5.47N@N-rGO-650 (4.8 mF cm-2), Co5.47N@N-rGO-700 (4.9 mF cm-2) and Co5.47N@N-rGO-800 (2.3 mF cm-2), suggesting the largest active surface area. To evaluate the electrocatalytic activity of the Co5.47N@N-rGO for the ORR, linear sweep voltammetry (LSV) was carried out at a rotating disk electrode (RDE) in alkaline solution. The Co5.47N@N-rGO-750 catalyst (Fig. 4a) exhibited a comparable onset potential to commercial Pt/C catalyst. Notably, the high half-wave potential (E1/2) of 0.94 V is around 21 mV higher that of the commercial Pt/C, which is superior to most of the reported

carbon-based

Co5.47N@N-rGO-700

catalysts and

[47-49].

Co5.47N@N-rGO-750

The

Co5.47N@N-rGO-650,

catalysts

showed

a

temperature-dependent activity with increasing the annealing temperature from 650 to 750 °C, which would be contributed to the improved electronic conductivity and the formation of highly porous structure. However, the electrocatalytic activity of ORR

decreased when the annealing temperature increased to 800 °C. As revealed previously, the higher temperature resulted in the serious aggregation of cobalt nitride nanoparticles and the etching of rGO sheets, leading to the poorer activity. Thus, Co5.47N@N-rGO-750 exhibited the best catalytic activity with more positive half-wave potential and larger limiting current density (5.6 mA cm-2).

Figure. 4. (a) Linear scan voltammogram (LSV) curves at a RDE (1,600 r.p.m.) with a scan rate of 5 mV s-1. (b) The polarization curves of the ORR at the Co5.47N@N-rGO-750 electrode and the corresponding Koutecky-Levich plots (inset). (c) Electron transfer numbers and HO2– yields based on the corresponding RRDE data. (d) LSV curves for OER. (e) The corresponding Tafel plots. (f) The overall LSV curves for bifunctional activities to ORR and OER in 0.1 M KOH. The tafel slope of the Co5.47N@N-rGO-750 (79 mV dec-1) catalyst is close to that of that of Pt/C (91 mV dec-1). Thus, the first electron transfer would be the rate determining step of ORR (Fig. S12). To reveal the electrocatalytic mechanism for ORR, the electron

number transferred per oxygen molecule (n) in a typical ORR was determined from the LSV

curves

(Fig.

4b)

according

to

the

Koutechy-Levich

(K-L) equation.

Co5.47N@N-rGO-750 showed an electron transfer of ~4.0 at the potential range of 0.4-0.7V, suggesting a direct four-electron reduction of oxygen into hydroxide [50]. To further analyze peroxide (H2O2) and the transferred electron number (n), a rotating ring disk electrode (RRDE) test was performed. As shown in Fig. 4c, the value of n for Co5.47N@N-rGOs are in the range of 3.99 and 4.02, the yield of H2O2 is less than 1% within the potential range of 0.2 to 0.8V, which are basically consistent with the results

calculated

by

K-L

equation.

Particularly,

the

yield

of

H2O2

at

Co5.47N@N-rGO-750 is less than that at Pt/C electrode, indicating the better selectivity for the four-electron reduction of oxygen. The synergistic effect of metallic Co5.47N nanoparticles in combination with the unique worm-like structure of N-doped graphene enable the higher oxygen reduction efficiency. The catalytic performance of Co5.47N@N-rGOs for OER was further evaluated at a scan rate of 5 mV s-1. As shown in Fig. 4d, Co5.47N@N-rGO-750 electrode exhibits the better OER electrocatalytic activity with the comparable onset potential (1.50 V vs. RHE) in comparison with RuO2 (1.49 V), Co5.47N@N-rGO-650 (1.54 V), Co5.47N@N-rGO-700 (1.53 V), Co5.47N@N-rGO-800 (1.51 V) and Pt/C (1.56 V). Additionally, the overpotential to achieve the current density of 10 mA cm-2 at the Co5.47N@N-rGO-750 electrode is 0.35 V vs. RHE (Fig. S14), which is lower than those of Co5.47N@N-rGO-650

(0.41

V),

Co5.47N@N-rGO-700

(0.39

V)

and

Co5.47N@N-rGO-800(0.38 V). As shown in Fig. 4e, the corresponding Tafel slope is 80 mV dec-1 for Co5.47N@N-rGO-750, which is slightly larger than that of RuO2 (72 mV

dec-1), but much lower than those of Co5.47N@N-rGO-650 (143 mV dec-1), Co5.47N@N-rGO-700 (89 mV dec-1), Co5.47N@N-rGO-800 (100 mV dec-1) and Pt/C (147 mV dec-1). The smaller Tafel slope suggests that Co5.47N@N-rGO-750 electrode is capable of achieving larger anodic current for OER at a smaller overpotential, leading to the favorable kinetic process for the OER. The overall polarization curves for ORR and OER were shown in Fig. 4f. Co5.47N@N-rGO-750 catalyst exhibits the smallest ∆E = 0.77 V (∆E = EOER − EORR, in which EOER is the potential to obtain the current density of 10 mA cm-2 and EORR is the half-wave potential for ORR), suggesting Co5.47N@N-rGO-750 processing excellent bifunctional electrocatalytic activity and good application prospect of zinc air batteries. In contrast, ∆E of Co5.47N@N-rGO-650, Co5.47N@N-rGO-700, Co5.47N@N-rGO-800 and Pt/C are 0.88, 0.83, 0.91 and 0.97 V, respectively. Thus, the porous structure of Co5.47N@N-rGO-750 would benefit to the enhanced electrocatalytic activity [51]. For practical applications, the large resistance to methanol cross-over effect of the catalyst is very important. To evaluate the fuel cross-over effect of Co5.47N@N-rGO-750 and Pt/C, current-time chronoamperometric curves were successfully tested in O2-saturated electrolyte. As shown in Fig. S13a, after the injection of methanol solution at 1000 s, the current density for ORR at Pt/C electrode was significantly decreased due to the oxidation of methanol. In contrast, no obvious current density changing at the Co5.47N@N-rGO-750 electrode is observed, indicating high tolerance to the methanol crossover

effect.

It

is

also

evidenced

by

the

cyclic

voltammograms

of

Co5.47N@N-rGO-750 and Pt/C in O2-saturated electrolyte with 3 M CH3OH (Fig. S13b). Methanol

oxidation

peaks

was

observed

at

the

Pt/C

electrode

whereas

Co5.47N@N-rGO-750 only exhibited the cathodic peak for oxygen reduction in the same methanol solution. 3.3. Zinc-air battery performance

On the basis of the bifunctional activity for ORR and OER, the liquid electrolyte rechargeable zinc-air battery was assembled with Co5.47N@N-rGO-750 as the air electrode catalyst using 6 M KOH with 0.2 M zinc acetate as the electrolyte (Fig. 5a). In the

liquid

electrolyte

zinc-air

battery,

the

open-circuit

voltage

(OCV)

of

Co5.47N@N-rGO-750 and Pt/C were 1.45 and 1.42 V, respectively (Fig. S18a).

Figure. 5. (a) Schematic illustration of the rechargeable Zn-air battery. (b) Power density plot. (c) Discharge and charge polarization curves. (d) Galvanostatic discharge curves at different current densities. (e) Cycling stability of the Zn-air battery. The discharge polarization and power density plots were shown in Fig. 5b. Co5.47N@N-rGO-750 electrode displayed the maximum peak power density (120.7 mW cm-2) at 0.67 V, higher than that of commercial Pt/C-RuO2 catalyst (85.8 mW cm-2). Fig. 5c shows the discharging and charging polarization curves of Zn-air batteries. The

potential gap of Co5.47N@N-rGO-750 is 1.03 V at the charging and discharging current density of 20 mA cm-2, which is smaller than that of Pt/C-RuO2 electrode (1.25 V). The smaller potential gap of Co5.47N@N-rGO-750 would be contributed to the good catalytic activity of ORR and OER in zinc-air battery. At the different current densities from 1 to 20 mA cm-2, the discharging potentials of Zn-air batteries are above 1.1 V at 20 mA cm-2, higher

than that of Pt/C (1.0 V), suggesting good

rate performance of

Co5.47N@N-rGO-750 catalyst（Fig. 5d). Additionally, according to the mass of Zn consumed, the specific capacity of the zinc-air battery using Co5.47N@N-rGO-750 electrocatalyst is up to 788.5 mAh gzn-1 at a current density of 10 mA cm-2, superior to Pt/C-RuO2 catalyst (713.4 mAh gzn-1), indicating the enhanced zinc utilization (Fig. S21a). The corresponding energy densities are 997.3 and 882.0 Wh kgzn-1 at the current density of 10 and 20 mA cm-2, respectively (Fig. S21b-c). Fig. 5e shows the cycling performance of the Zn-air battery. It can be found that Co5.47N@N-rGO-750 electrode can be continuously cycled for 2000 cycles in 330 h and only slight potential changes were observed. However, the commercial Pt/C-RuO2 catalyst showed obvious increase on the charging potential. These results indicate the good cycling stability of Co5.47N@N-rGO-750 electrode. To meet the development trend of portable flexible devices, solid zinc air batteries were assembled with polyacrylic acid (PAA) gel electrolytes. As displayed in Fig. S22, the open

circuit

voltage

of

the

solid-state

Zn-air

battery

assembled

with

Co5.47N@N-rGO-750 is 1.40 V, which is relatively close to that of the liquid electrolyte zinc-air battery. As shown in Fig. 6a, the charge and discharge voltage difference of Co5.47N@N-rGO-750 electrode is smaller than that of Pt/C-RuO2 electrode, indicating

better efficiency of Co5.47N@N-rGO-750 electrocatalyst. Furthermore, Fig.6b exhibits a higher power density of 54.6 mW cm-2 for Co5.47N@N-rGO-750 in comparison with Pt/C-RuO2 (24.7 mW cm-2). It's worth noting that the specific capacity at a discharging current density of 10 mA cm−2 is around518 mAh gzn-1 (normalized with the mass of zinc consumed) which is also slightly larger than that of Pt/C-RuO2 (506 mAh gzn-1) suggesting its excellent discharge multiplier performance (Fig. 6c). The flexibility and stability tests Co5.47N@N-rGO-750 is also evaluated at 1 mA cm−2, as shown in Fig. 6d. It can be seen that the charging and discharging voltage of Co5.47N@N-rGO-750 electrode has almost no attenuation, which indicates that the solid zinc air battery has good flexible application potential. Fig. 6e shows the long-term cycling stability and reversibility for Co5.47N@N- rGO-750 air cathode. No obvious potential decay is observed forover 40 h, suggesting the good reversibility. Moreover, the potential efficiency

of

Co5.47N@N-rGO-750

could

still

maintain

56%

at

the 240th

discharge-charge cycling, it further indicates the good stability (Fig. 6f). Meanwhile, red LED (2V) and yellow LED (3V) could be also lighted up by using two series-connected Co5.47N@N-rGO-750-based solid-state Zn-air batteries (Fig. S19). In short, the Co5.47N@N-rGO-750 catalyst showed good catalytic activity for both liquid zinc-air battery and solid flexible zinc-air battery, proving its potential application under practical conditions.

Figure. 6. (a) Discharge and charge polarization curves of solid-state Zn-air batterie. (b) Power density plot and discharge polarization. (c) Specific capacity curves at 1 mA cm−2 and 10 mA cm−2, respectively. (d) The flexibility and stability tests of the solid-state rechargeable Zn-air batteries under flat and folded status. (e) Cycling stability of the solid-state Zn-air battery at 1 mA cm−2. (f) Corresponding the voltage efficient of Co5.47N@N-rGO-750. 3.4. HER electrocatalytic performance

Figure. 7. (a) LSV curves for HER. (b) The overpotentials at a current density of 10 mAcm-2. (c) The H2 and O2 gas evolution rates with the Co5.47N@N-rGO-750 electrocatalyst. The HER catalytic activities of Co5.47N@N-rGO and the commercial Pt/C were shown in Fig. 7a. The Co5.47N@N-rGO-750 electrode exhibits reasonably good HER electrocatalytic activity with a positive onset potential (-0.1 V vs. RHE). The corresponding overpotential is only 0.19 V (Fig. 7b) to reach a current density of 10 mA cm-2, outperforming to those of Co5.47N@N-rGO-650 (0.26 V), Co5.47N@N-rGO-700 (0.22 V), Co5.47N@N-rGO-800(0.23 V). A smaller Tafel slope (123 mV dec-1) is observed for Co5.47N@N-rGO-750 in comparison with Co5.47N@N-rGO-650(127 mV dec-1), Co5.47N@N-rGO-700(128 mV dec-1), Co5.47N@N-rGO-800(158 mV dec-1), which suggests a favorable kinetic process for HER (Fig. S15). These results indicate the superior electrocatalytic activity of the Co5.47N@N-rGO-750 electrode for HER. To explore the multifunctional applications, two identical Co5.47N@N-rGO-750 electrodes are used as cathode and anode for water splitting, which is derived by Zn-air batteries in series (Fig. S16 and S17). The volumes of produced gas displayed a good linear relationship with the electrolysis time for both O2 and H2 (Fig. 7c). The slope revealed that the gas ratio generated between the anode and cathode was 1:2, which suggested the

complete decomposition of water into oxygen and hydrogen. The gas evolution rates of O2 and H2 were calculated to be about 0.98, 1.95 mL s-1, respectively. 4. Conclusions In summary, we have developed a green and facile process to in situ synthesize cobalt nitride nanoparticles on hierarchically porous rGO sheets. Interestingly, the in-situ formation and thermal motion etching of Co5.47N nanoparticles led to the formation of worm-like channels and holes on the rGO surface. Benefiting from the heteroatom doping and intrinsic high conductivity of Co5.47N, the Co5.47N@N-rGO-750 exhibits excellent electrocatalytic activities towards ORR, HER and OER, which enable the fabrication of rechargeable Zn-air batteries and overall water splitting devices. Typically, the rechargeable Zn-air using Co5.47N@N-rGO-750 as air cathode shows a high open circuit potential (1.45 V), large power density (120.7 mWcm-2) at 0.67V and excellent cycling stability for over 330 h. The solid-state Zn-air batteries also showed good rechargeable performance (about 40 h). Furthermore, the water splitting driven by Zn-air batteries exhibited good gas generation rates by using the as-prepared electrocatalysts. This study provides a new strategy for exploring bifunctional electrocatalysts for metal-air batteries and water splitting devices. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grants No. 21503116). The Taishan Scholars Program of Shandong Province (No. tsqn20161004) and the Youth 1000 Talent Program of China are also acknowledged. References [1] H.F. Wang, C. Tang, Q. Zhang, A review of precious-metal-free bifunctional oxygen electrocatalysts: rational design and applications in Zn-air batteries, Adv. Funct. Mater. 28(46) (2018) 1803329.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: