Facile route to achieve N, S-codoped carbon as bifunctional electrocatalyst for oxygen reduction and evolution reactions

Journal Pre-proof Facile route to achieve N, S-codoped carbon as bifunctional electrocatalyst for oxygen reduction and evolution reactions Jinrui Guo, Yue Yu, Jicheng Ma, Tingting Zhang, Shuangxi Xing PII:

S0925-8388(19)34730-9

DOI:

https://doi.org/10.1016/j.jallcom.2019.153484

Reference:

JALCOM 153484

To appear in:

Journal of Alloys and Compounds

Received Date: 14 October 2019 Revised Date:

18 December 2019

Accepted Date: 19 December 2019

Please cite this article as: J. Guo, Y. Yu, J. Ma, T. Zhang, S. Xing, Facile route to achieve N, S-codoped carbon as bifunctional electrocatalyst for oxygen reduction and evolution reactions, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153484. 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 B.V.

Credit Author Statement Jinrui Guo: Conceptualization, Methodology, Experiment, Writing-Original draft preparation. Yue Yu: Data organization, Draft editing. Jicheng Ma: Physical characterization. Tingting Zhang: Investigation. Shuangxi Xing: Supervision, Writing-reviewing and editing.

Facile route to achieve N, S-codoped carbon as bifunctional electrocatalyst for oxygen reduction and evolution reactions Jinrui Guo, Yue Yu, Jicheng Ma, Tingting Zhang, Shuangxi Xing* Faculty of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun, Jilin, P. R. China Email address: [email protected]

Abstract Zn-air batteries with high theoretical energy density and low cost are potential devices for future energy storage technology. Whereas, the performance of the battery is dependent on the catalyst on the air cathode. Efficient catalysts can advance the kinetics of oxygen reduction and oxygen evolution (ORR/OER) reactions. Polydopamine is a commonly used carbon source with a simple and non-toxic synthetic route. Here, the N, S-codoped carbon composite is prepared by carbonizing the mixture of polydopamine/sodium dodecyl sulfate precursors. The obtained sample displays an onset potential of 0.876 V vs. RHE, a half wave potential of 0.805 V, and a current retention of 94% after 20000s chronoamperometric response. Meanwhile, it shows a good OER catalytic property with the overpotential of 0.545 V. Owing to the favorable dual function catalytic ability of the sample, the sample can be used as an air–cathode catalyst to construct rechargeable Zn–air batteries. The battery exhibits an open circuit voltage of 1.526 V and a 54.1 % round trip efficiency. Moreover, it also presents satisfactory durability and benign charge and discharge stability over long cycles. Keywords: Polydopamine; Sodium dodecyl sulfate; Carbon; Oxygen reduction 1 / 22

reaction; Oxygen evolution reaction; Rechargeable Zn–air batteries 1. Introduction Rechargeable Zn-air batteries are considered to be a kind of efficient and green device for energy conversion. However, their sluggish kinetics in air electrode requires effective catalysts, generally including Pt, Rh, and Ir-based noble metals. [1-4] Therefore, variety of inexpensive materials have been designed and synthesized for replacement, such as transition-metal oxides and heteroatom-doped carbons.[5-9] Polydopamine (PDA) is a substance that can be self-polymerized by dopamine at indoor temperature and pH around 8.5. PDA contains a large number of phenol hydroxy groups, which can form hydrogen bonds with water molecules, making PDA perform outstanding hydrophilicity and adhesion. Therefore, PDA can be used in many

fields,

such

as

biomedicine

for

drug

carrier,[10]

electrocatalytic

nanobiomaterials for bioreactors[11] and electrochemistry for supercapacitor and electrocatalysis.[12, 13] For its application in catalyzing oxygen reduction reaction (ORR) or evolution (OER), the following strategies have been reported: 1) PDA can act as supporting substrate for depositing or growing active materials. The carbonized PDA provides high specific surface area and effectively avoids the aggregation of the electrocatalytically active nanoparticles. Besides, the N doping in the carbonized PDA can provide abundant active sites for the catalyst growth;[14, 15] 2) Owing to the functional groups for surface modification, PDA is often utilized as a linker to lock the catalyst strongly anchored onto the substrate, for example, graphene oxide. This can reinforce the interaction between the support and the active materials and the 2 / 22

stability of the catalysts can be greatly improved;[16] 3) Actually, PDA can also be a carbon source with nitrogen doping.[17] Various routes have been adopted to enhance the electrocatalytic property of the PDA-derived carbons. On one hand, kinds of sacrificed templates, such as ZnO and SiO2, were used to obtain hollow nanostructured carbon materials.[18] On the other hand, some structure-guidance reagents have been adopted during the polymerization of dopamine and the mesoporous structure was generated.[19] In both cases, a high specific surface area was achieved and the mass transport was greatly enhanced. Apart from that, some transition metals could be co-doped with the PDA-derived N-doped carbon. For example, Fe, N-codoped carbon has been synthesized using Fe3O4@PDA as starting material, and the formation of the Fe-N centers led to the outstanding ORR activity. [20, 21] In the case of the carbon-based ORR or OER catalysts, kinds of non-metal atoms have been doped into the carbon framework except N element, including B, P, F and S. [22-24] The S doping induces the change of the electronic spin density, imparts high conductivity and produces extra electrocatalytic active sites to the catalyst, which plays a significant role to promote the ORR/OER activity. [25, 26] In order to achieve S-doped or S, N-codoped carbons, many S sources have been selected, such as S8, CS2, H2S, benzyl disulfide and sulfur trioxide, via using ball-milling followed by pyrolysis or direct annealing process in corresponding gas atmosphere. [27-29] In this work, we reported the preparation of N, S-codoped carbon via direct mixing PDA and sodium dodecyl sulfate (SDS) in solid state followed by a pyrolyzing 3 / 22

process. Aim to achieve S doping, grind PDA and SDA sufficiently. Grinding is a way of mechanochemistry processes, which include several energies.[30, 31] These energies can cause chemical bonds breaking and generate free radicals in different ways with occurring in solution. Apart from its role to introduce S element, SDS was beneficial to tuning the porous structure of the carbonized PDA. Therefore, the obtained materials illustrated excellent electrocatalytic activity for ORR and OER. Furthermore, the assembled rechargeable zinc-air battery using the obtained N, S-codoped carbon as air electrode displayed satisfactory power density and cycle life. 2. Experimental Section 2.1 Materials Dopamine hydrochloride (DA, 98%), sodium dodecyl sulfate (SDS, 99%), tris (hydroxymethyl) aminomethane (Tris, 99.8%), hydrochloric acid, Pt/C (20 wt% Pt on Vulcan XC72R carbon), RuO2 (99.9%, Aldrich), Nafion (5wt%, Aldrich) and isopropyl alcohol (AR) were used as received. Zinc and copper sheet were used for zinc–air batteries. The carbon fiber paper was cut to fit the size of the Zinc-air battery. All the solutions in the experiment were prepared with deionized water (18.25 MΩ cm). 2.2 Synthesis of PDA nanoparticles 0.189 g of DA was dispersed in 20 mL of Tris buffer solution (pH=8.5) via ultrasonic treatment. After ultrasonication for 30 min, the system was put into a water bath followed by reaction at 30 oC for 24 h to allow the self-polymerization of DA into PDA. The product was centrifuged and washed with deionized water for two 4 / 22

times, and the obtained black precipitate was dried in an oven at 50 oC for 12 h. 2.3 Synthesis of C-PDA/S microspheres The above obtained PDA was mixed with SDS with molar ratio of 1:1 (based on DA monomer) in an agate mortar and thoroughly ground for 30 min. After that, the mixture was annealed in N2 atmosphere at 900 oC for about 2 h with a heating rate of 5 oC min−1. After cooled to ambient temperature, the carbonized product was prepared and labeled as C-PDA/S. 2.4 Physical characterization The morphology of the samples was investigated via transmission electron microscopy (TEM, JEM-2100F) and scanning electron microscopy (SEM, JSM-IT300). TEM elemental mapping images were acquired by the EDAX detector equipped on the JEM-2100. X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was performed with a mono-chromatic Al Kα X-ray source (1486.6 eV). FT-IR spectrum measurement was carried out on an FT-IR-8400s (Shimadzu) spectrometer in the range of 4000-600 cm− 1. Raman spectrum was collected on a LabRAM XploRA laser Raman spectrometer (HORIBA Jobin Yvon Co. Ltd) ranging from 500 to 2000 cm− 1. The pore size distribution of obtained sample was performed by the Barret-Joyner-Halenda

(BJH)

method

(Micromeritics,

ASAP

2020).

The

thermogravimetric analysis (TGA) was conducted on a NETZSCH STA 449C thermogravimetric analyzer with a heating rate of 10 °C min-1 under nitrogen atmosphere. 2.5 Electrochemical measurements 5 / 22

All electrochemical measurements were performed on an electrochemical workstation (CHI660E, Shanghai, China) equipped with a ring-disk electrode (RDE, glassy carbon, d = 5 mm). Ag/AgCl electrode and platinum sheets and were used as the reference and counter electrodes, respectively. 4 mg of sample powder was dispersed in the mixture of 760 µL of deionized water, 190 µL of isopropanol, and 50 µL of nafion under ultrasonication for 30 min. Later, 14.4 µL of the obtained suspension was dropped on a glassy carbon electrode and dried at room temperature. Before each experiment, high-purity O2 was pre-purged to an aqueous KOH solution (0.1 M) as an electrolyte solution for at least 30 min until the solution was saturated. The CV measurements were performed from 0.2 to −0.8 V after purging with O2 or N2 for 30 min at a scan rate of 10 mV s−1. LSV measurements were made by varying the speed from 400 to 1600 rpm (scan rate: 5 mV s−1). In this work, the overpotential was defined as the difference value between 1.23 V and the potential at 10 mA cm-2, when the LSV curves at 1600 rpm. The transferred electron number (n) for the ORR was estimated by the Koutecký– Levich (K–L) equation (1) and (2) = +

=

/

+

B=0.2nFCo2(Do2)2/3v-1/6

(1) (2)

where J represents current density, JK and JL represent the kinetic current density and the diffusion limiting current density, ω is the rotating rate of RDE, F is the Faraday constant (96485 C mol-1), and Co is bulk concentration of O2 (1.2×10-6 mol cm-3 for 0.1 M KOH solution), Do is the diffusion coefficient of O2 (1.9×10-5 cm2 s-1 for 0.1 M 6 / 22

KOH solution), ν is the kinetic viscosity of the electrolyte (0.01 cm2 s−1 for 0.1 M KOH solution). 2.6 Fabrication of Rechargeable Zn–Air Batteries The zinc-air battery was prepared by using zinc plate, carbon fiber paper and 6.0 M KOH as anode, cathode and electrolyte, respectively. The ink was manufactured by dispersing 4 mg of C-PDA/S or 2 mg of Pt/C and 2 mg of RuO2 in 1 mL mixture solution consisting 760 µL deionized water, 190 µL of isopropanol and 50 µL nafion. The air cathode was prepared by casting the above catalyst ink on carbon fiber paper and dried at indoor temperature, resulting in a mass loading of 0.8 mg cm−2. The battery cycling reversibility, open circuit voltage and discharge/charge performance were test on LAND-CT2001A devices. The cycle performance was measured at a current density of 4 mA cm−2 with each cycle of 20 min. 3. Results and Discussion The FT-IR spectrum shown in Figure 1 confirms the polymerization of DA with the characteristic peaks located at 3434, 1638 and 1434 cm-1. The peak of 3434 cm-1 is attributed to the O-H and N-H stretching vibrations of PDA. As for the peak of 1638 cm-1, it could be assigned to aromatic rings (νC=C). After carbonization, the intensity of the two peaks greatly increases, indicating the existence of more νC=C and O-H or N-H groups.[32] In the case of C-PDA/S, the weak peak at 685 cm-1 corresponds to the vibration of the C-S bond, implying successful conjugation of the carbonized PDA and S.[13] The two peaks in the region of 2700-2900 cm-1 are assigned to saturated C-H bond stretching vibrations, and it was broken to form C-S bond after 7 / 22

carbonization. Therefore, these two peaks disappear upon carbonization.

Figure 1. FTIR spectra of C-PDA/S before and after carbonization. C-PDA presents uniform spheres with the size of 200±100 nm (Figure S1), however, upon addition of SDS during the carbonization process, the product gives irregular morphology with mixing bulk, spherical and lamellar particles, as shown from Figure 2a, SEM image. This reveals the strong interaction between SDS and PDA under high temperature. The TEM image (Figure 2b) confirms the connected particles with irregular shape. Apart from that, as shown in Figure 3 the EDS mapping images verify the homogeneous distribution of S, promising the effective doping of S into the product.

Figure 2. SEM (a) and TEM (b) images of C-PDA/S.

8 / 22

Figure 3. The element mapping for C, O, S and N in C-PDA/S. Figure 4a shows a N2 adsorption-desorption isotherms of C-PDA/S with an obvious hysteresis loops at the medium and high pressure ranges (P/P0 = 0.4–1.0), assigned to the type IV, combined with the pore size distribution curve of C-PDA/S, together indicating the mesoporous-dominant structure of the synthesized C-PDA/S sample. [33] The Barrett-Joyner-Halenda (BJH) pore size distribution of samples is displayed in Figure 4b, and C-PDA illustrates considerable microporous structure, endowing it with a relatively large specific surface area (488.01 m2 g-1). However, in the case of C-PDA/S, a great deal of mesoporous structure can be constructed. Although a higher surface area can generate much more active sites, in electrocatalysis, the existence of mesopores is beneficial to facilitate the storage and transport of electrolyte ions and enhance the transport of O2 in the electrolyte, resulting in improvement of ORR catalytic ability. [34, 35] On the other hand, mesoporous structure can also reduce the overpotential of OER process. [36] In the Raman spectrum of C-PDA/S (Figure 4c), the D band at 1350 cm-1 can put down to defects or sp3 C, while the G band at 1588 cm-1 is the characteristic peak of 9 / 22

graphitic carbon. The intensity ratio of the two bands (ID/IG) is 1.54; however, it is only 1.03 for the product carbonized in the absence of SDS, revealing more active sites exist in C-PDA/S than C-PDA (Figure 4d). [37] Moreover, the peak at around 657 cm−1 is caused by the C-S vibration.

Figure 4. a) N2 sorption of C-PDA/S; b) Pore size distribution of C-PDA/S and C-PDA; Raman spectra of C-PDA/S (c) and C-PDA (d). XPS analysis was further performed to obtain insights into the chemical states and compositions of the C-PDA/S and the surface element content for N and S is estimated to be 2.3 % and 2.25 %, respectively. The spectrum of C 1s in Figure 5a can be decomposed into two peaks, corresponding to C=C (284.3 eV) and C-N/C-S (284.9 eV). In the case of N 1s spectrum (Figure 5b), the deconvoluted three peaks locating at 398.4, 400.1 and 401.2 eV, confirm the existence of pyridinic N, pyrrolic N, and graphitic N, respectively. [38] The graphite and pyridine N are considered as effective components to enhance the catalytic ability for ORR and OER, and the relative 10 / 22

content for graphite and pyridinic N is estimated to be 56.7% and 15.4%, respectively, higher than those of C-PDA with the corresponding values of 43.5% and 12.8% (Figure S2b). S 2p spectrum in Figure 5c can be resolved into three S species that locate at 163.9, 164.9, and 168.5 eV, assigning to S 2p2/3, S 2p1/2, and S-O, respectively. Among them, S 2p3/2 and S 2p1/2 are derived from the splitting of S 2p spin orbits (C-S-C). Previous studies have shown that C-S-C can effectively render high spins in surrounding C atoms, and facilitate the ORR reaction process thereby. [27, 39] On the other hand, C-S-C is reported to be ORR active site, which can boost ORR performance. Meanwhile, pyridinic N and C-S-C could also synergistically promote the OER activity, leading the lower overpotential in OER. As for the spectrum of O (Figure 5d), it can be separated into three peaks locating at 530.8, 531.9, and 532.6 eV, corresponding to Oα, Oβ, and Oγ, respectively. Among them, the Oβ peak is considered to provide more oxygen vacancies, and hence improve the ORR and OER catalytic performance. [40] The calculated Oβ content is 52.7% in C-PDA/S, much larger than that in C-PDA with the corresponding value of 35.4% (Figure S2c).

11 / 22

Figure 5. XPS spectra of C 1s (a), N 1s (b), S 2p (c) and O 1s (d) for C-PDA/S. TGA was used to study the thermal stabilities of C-PDA/S sample and the TGA curve of C-PDA/S is shown in Figure S3. The initial weight loss from room temperature to 180 oC is due to the removal of moisture. The second weight loss from 300 to 400 oC is due to the presence of S in the sample. The loss from 600 to 750 oC should be related to the incompletely pyrolized PDA. Moreover, the weight retention percentage between room temperature and 900 °C is 46.6 %. Previous work has shown that the thermal decomposition temperature of pure S is about 200 oC,[41] therefore, the sample exhibits higher thermal stability compared to pure sulfur, which is attributed to the presence of C-S-C bonds.

12 / 22

Figure 6. a) CV curves of C-PDA/S in N2 and O2 saturated 0.1 M KOH; b) LSV curves for C-PDA/S, C-PDA and Pt/C in O2 saturated 0.1 M KOH in rotating rate of 1600 rpm; c) ORR LSV curves for C-PDA/S at different rotating rates (inset) and corresponding

K-L

plots

of

C-PDA/S

in

various

potential

range;

d)

Chronoamperometric curves of different samples. In order to investigate the electrocatalytic ORR performance, CV curves of the C-PDA/S (Figure 6a) was firstly collected under alkaline conditions. No apparent redox peak is observed in a saturated N2 electrolyte; on the contrary, an obvious reduction peak can be detected in a saturated O2 electrolyte, indicating the occurrence of redox reaction. The LSV was measured for evaluating the ORR performance of the C-PDA/S. The LSV curve displays a half-wave potential of 0.805 V (Figure 6b), close to that of Pt/C (0.852 V). It can be seen from Figure 6b, an onset potential of 0.876 V is demonstrated, meanwhile, the kinetic limiting current density reaches 4.22 mA cm-2 at the potential of 0.4 V. As for the C-PDA, the corresponding values are 0.643 V, 0.739 V and 2.92 mA cm-2, much worse than those the C-PDA/S, revealing the 13 / 22

advantage of introducing SDS or doping S into the sample. The mixing of SDS during carbonization leads to the S doping into the N-doped carbon, which has been assumed to change the spin density and hence increase the ORR activity of the product. [42-44] Furthermore, in the presence of SDS during pyrolysis, the content of pyridine and graphite N with electrocatalytic activity are greatly increased and more defect is detected to provide much more active sites (vide ante). [45, 46] As the RDE speed increases, the onset potential of C-PDA/S is unchanged, while the current density increases sharply due to the improvement of mass transport. As shown in Figure 6c, a corresponding K-L curve with favorable linearity at different potentials (0.35-0.55 V) is plotted, verifying the reaction belongs to first-order reaction kinetics. The almost same slope of all five lines indicates that the electron transfer number n of each O2 molecule is similar at different potentials. [47] The n of C-PDA/S is calculated to be 3.83, demonstrating the ORR process of C-PDA/S follows a four-electron approach. However, it is only 2.73 for C-PDA (Figure S4), revealing it is a mixture of 2 and 4 electron transfer paths. The ORR durability of C-PDA/S in O2 saturated 0.1 M KOH solution was evaluated by a current-time (i-t) test at 1600 rpm (Figure 6d). After 20000 s, the current retention keeps 94%, while Pt/C and C-PDA give a corresponding value of 81% and 73%, respectively, illustrating the benign stability of C-PDA/S. The OER catalytic performance of C-PDA/S was measured on RDE in 0.1 M KOH electrolyte at 1600 rpm. As can be seen in Figure 7, the potential of the C-PDA/S is 1.78 V at 10 mA cm-2, a slightly larger than that of RuO2 (1.64 V). Considering the 14 / 22

counterpart of C-PDA only displays a tiny current (3.58 mA cm-2), the OER activity of C-PDA/S is well improved. The improvement of OER activity can be attributed to the dual-doping of N and S [48, 49], which produces more active sites. These active sites can increase the electron transfer rate and reduce the overpotential of C-PDA/S. In addition, co-doping of N and S triggers asymmetric spin and charge density, which can increase the number of active C atoms and effectively improve OER catalytic activity.

Figure 7. OER LSV curves for C-PDA/S, C-PDA and RuO2 in 0.1 M KOH.

Figure 8. ORR (a) and OER (b) LSV curves for different samples obtained with the molar ratio of DA and SDS in O2 saturated 0.1 M KOH. The ORR LSV curve was obtained at a rotating rate of 1600 rpm. 15 / 22

As shown in Figure 8, the ORR and OER properties of the samples differ when the molar ratio of PDA and SDS is changed. When the ratio is increased (based on the corresponding ratio of DA and SDS), the sample exhibits a half-wave and onset potential of 0.768 and 0.832V, respectively, meanwhile, it shows an overpotential of 0.652 V in OER process. When the amount of PDA is decreased, the sample exhibits a half-wave and onset potential of 0.742 and 0.812 V, respectively, and an overpotential of 0.683 V in OER process. All these results reveal the worse electrocatalytic performance of these samples. The XPS spectra of the two samples demonstrate the content of electrocatalytic active sites, including the graphite N, C-S-C and Oβ, is lower than the sample with the ratio of DA:SDS of 1:1 (Figure S5-7 and Table S1).

Figure 9. a) Schematic of the configuration of a rechargeable zinc–air battery; b) Charge and discharge polarization curves of zinc–air batteries using C-PDA/S catalyst 16 / 22

for air cathode; c) Discharge/charge cycling curves of the znic–air battery using C-PDA/S catalyst for air cathode; d) An LED powered by two zinc-air batteries using C-PDA/S catalyst for air cathode. RDE measurements are currently insufficient to predict catalyst performance because 0.1 M KOH is well below the concentration typically used in battery systems (6 M KOH). Considering the bifunctional performance for ORR/OER of C-PDA/S, a rechargeable zinc–air battery was assembled using zinc plate as anode, and C-PDA/S as air electrode in 6 M KOH electrolyte (Figure 9a). The battery shows an open circuit voltage of 1.526 V (Figure 9b), revealing the significant catalytic activity of C-PDA/S. The discharge/charge cycling curves (Figure 9c) of the rechargeable zinc–air battery are collected using Pt/C-RuO2 or C-PDA/S for the air cathode at a constant current density of 4 mA cm−2. During the discharge/charge cycling, the constant current gives a charge voltage of 1.11 V and a discharge voltage of 2.05 V. The rechargeable Zinc-air battery using the C-PDA/S catalyst has a small voltage change during 100 discharge/charge cycles (1000 min) (Figure 9c). Though the 54.1% round trip efficiency is slightly worse than Pt-RuO2 (Figure S8), it demonstrates an impressive performance in zinc-air batteries.[50, 51] Owing to the satisfactory durability of the C-PDA/S catalyzed Zinc-air battery, a LED (light-emitting diode) was illuminated as a demonstrated in Figure 8d. Two Zinc-air batteries can lighten up one LED (3.0 V) with no significant performance degradation in 48 h. 4. Conclusions Nitrogen/sulfur co-doped carbon materials have been successfully prepared via 17 / 22

high temperature annealing on PDA/SDS mixture precursor (C-PDA/S). The introduction of SDS during the carbonization on PDA led to the formation of S2p2/3 and S2p1/2 that could change the electron spin. At the same time, a mesoporous-dominant structure was formed, enhancing the transport of ions and oxygen in electrolyte solutions. Furthermore, the intensity ratio of D band to G band of the sample was increased from 1.03 to 1.54, augmenting the number of active sites. Apart from that, the content of graphite and pyridinic N, which were considered to be beneficial for improving the electrocatalytic process, was higher than that obtained in the absence of SDS. Therefore, C-PDA/S exhibited a half-wave potential of 0.805 with a 4-electron transfer pathway, and a current retention of 94%. Besides, is also showed a low overpotential of 1.78 V for OER process. When the air cathode of Zinc-air batteries was constructed by the as-synthesized catalyst, its performance was close to the most advanced Pt/C-RuO2 with a high open circuit voltage of 1.526 V. Therefore, C-PDA/S is a prominent high-performance metal-free dual functional catalyst in Zinc-air batteries, looking forward to be applied to electric vehicles and emerging flexible electronics. Acknowledgements The authors thank the Fundamental Research Funds for the Central University (2412017FZ016), Jilin Provincial Science and Technology Development Foundation and Jilin Provincial Key Laboratory of Advanced Energy Materials (Northeast Normal University) for financial support. References 18 / 22

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Highlights ★ N, S-codoped carbons are achieved via carbonizing the mixture of polydopamine and sodium dodecyl sulfate. ★ The effective N and S doping and formation of mesoporous structures ensure the good ORR and OER catalytic activity. ★ The rechargeable zinc-air battery using the sample as air electrode exhibits excellent performance.

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: