Selenium-Doped Carbon Nanosheets with Strong Electron Cloud

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Energy, Environmental, and Catalysis Applications

Selenium Doped Carbon Nanosheets with Strong Electron Cloud Delocalization for Non-Deposition of Metal Oxides on Air Cathode of Zinc-Air Battery Sisi Liu, Mengfan Wang, Tao Qian, Jie Liu, and Chenglin Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04870 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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ACS Applied Materials & Interfaces

Selenium Doped Carbon Nanosheets with Strong Electron Cloud Delocalization for Non-Deposition of Metal Oxides on Air Cathode of Zinc-Air Battery Sisi Liu, Mengfan Wang, Tao Qian,* Jie Liu, and Chenglin Yan* College of Energy, Key Laboratory of Advanced Carbon Materials and Wearable Energy Technologies of Jiangsu Province, Soochow University, Suzhou 215006, China. KEYWORDS: selenium-doped carbon, oxygen reduction reaction, zinc-air battery, metal oxides deposition, electron cloud delocalization

ABSTRACT: The deposition of metal oxides on the air cathode is a well-known problem for metal-air batteries since it can cover the active surface and block the oxygen gas diffusing pathway, resulting in poor battery performance and serious cell degeneration. Herein, through deliberately introducing selenium doping in nitrogen-doped carbon, a strong electron cloud delocalization among the carbon matrix is realized, which can prevent the air cathode from zinc oxide poisoning during zinc-air battery operation, as confirmed by experimental results and density functional theory simulations. In-situ X-ray powder diffraction observation confirms that the increased electron cloud density of the surrounding carbon caused by electron delocalization from the selenium atom could repulse the access of zincate ions, effectively prohibiting the oxide deposition

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on the air cathode. An amazingly long zinc-air battery cycle life reaching 780 cycles is thus obtained.

INTRODUCTION Metal-air batteries, especially zinc-air batteries, have been attached great importance due to their impressively high theoretical energy density, cost effectiveness, and environmental friendliness, so that they hold great potential in sustainable energy storage systems.1-5 Constituting of a specifically unique half-closed structure, the power output of metal-air batteries is generated from the redox reactions between the metal anode and oxygen in the cathodic surrounding atmosphere.6,7 However, this process logically results in the formation of a discharge by-product, namely insoluble metal oxides, which continuously accumulates on the surface of air cathode, leading to a short cell service life with an undesirable performance far below the theoretical level. Specifically, upon the discharging process of zinc-air battery, zinc is oxidized at the anode and oxygen is reduced at the cathode, leading to rise of the zincate ions (i.e. Zn(OH)42-) concentration in the alkaline solution.8 After the zincate ions gradually supersaturate in the electrolyte, they decompose into insoluble zinc oxide. Once zinc oxide deposits on the cathode, it will clog the active surface of the air electrode, covering the active sites and preventing the oxygen reactant diffusion, thus causing a premature battery death.9 Therefore, an effective strategy to overcome this obstacle is to exclude the zincate ions from attaching the surface of the cathode, which could enhance the durability of the air electrode and the total energy system. In addition, the overall performance of zinc-air batteries is largely depended on the oxygen electrochemical reactions. But the lack of efficient catalysts significantly impedes the practical


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application of zinc-air batteries, as the sluggish kinetics of the oxygen reactions causes large overpotential and low round-trip efficiency.10-12 Since the most common noble metal-based catalysts on boosting the oxygen reactions are unfortunately hindered by their scarcity, prohibitive cost, as well as the undesirable long-term stability,13 carbon materials have been widely investigated for zinc-air battery’s cathode as they have great potential on catalytic activity, superiority on electron conductivity, and low price.14-16 It has been proven that substitutional doping with heteroatoms into the carbon skeleton can generate unique electron distribution that leads to novel catalytic activity of the original carbon material.17,18 The doped heteroatom could introduce electronic states near the carbon Fermi energy level in carbon framework, which is confirmed to induce local high spin density on the basal plane to facilitate the oxygen chemisorption that benefits the oxygen reduction reaction (ORR) process.19,20 Besides, co-doping is a further strategy that intensively studied to modulate the regional electronic structure of the original carbon.21 Different electronegativity of the doped elements brings about synergistic effect, as the charge redistribution mostly happens around the dopant cluster that further enhances the catalytic activity.22 Taking these into consideration, if suitable heteroatoms are doped among the carbon framework, it can not only generate superior catalytic activity, but also redistribute the electronic structure that greatly inhibit the access of zincate ions to the air cathode. Herein, we unveil that, for the first time, deliberate introduction of selenium atom in nitrogen-doped carbon nanosheet (denoted as NCNS) unexpectedly causes a strong electron cloud delocalization that prevent the air cathode from zinc oxide poisoning, which is urgently demanded in long life-targeted zinc-air batteries. Distinct catalytic activity towards ORR is also realized (Figure 1a), which even exceeds traditional noble-metal materials. The fantastic nature of selenium was deeply dissected by density


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functional theory (DFT) simulations, and we found that selenium, featured by strong electron withdrawing capability among the carbon skeleton, can act as highly efficient ORR active site. Insitu X-ray powder diffraction (XRD) characterization demonstrate much enhanced chemisorption of oxygen molecules as well as oxygen-containing intermediates in the selenium-doped material through zinc-air battery discharging, showing great potential for practical application. More importantly, no evident peak of zinc oxide can be captured in the cathode of zinc-air battery assembled with selenium-doped catalyst through discharge operation, while several peaks representing zinc oxides were detected in the counterpart batteries along with the discharging. As further confirmed by DFT calculation, the insert of selenium causes strong electron cloud delocalization around the doped area. The electron cloud density of the adjacent carbon atoms evidently increases, which effectively restrains the access of zincate ions, thus protecting the air cathode from zinc oxide poisoning. Given the above excellent properties, an amazingly long cycle life reaching 780 cycles with extremely stable discharging/charging potential plateaus of zinc-air battery was obtained. The breakthrough of our work offers up a powerful guidance in the design principle of carbon materials for non-deposition of metal oxides on air cathode as well as high catalytic activity of zinc-air batteries, and holds brilliant prospect in a broad spectrum of future commercial application.

RESULTS AND DISCUSSION The selenium-doped NCNS (denoted as Se-NCNS) was fabricated by annealing the mixture of benzyl diselenide and NCNS under argon atmosphere. As sulfur is the most common chalcogen element that shows several similar chemical properties to selenium,23 it was chosen as the


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comparison guest dopant to explore the regularity of doping, and the corresponding product was named as S-NCNS. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) (Figures 1b, c, and S1a) are combined to illustrate the graphene-like morphology of Se-NCNS. This structure largely benefits the exposure of catalytic active sites for the oxygen reactions, as illustrated by the Brunauer-Emmett-Teller (BET) surface area (Figure S2). Energy dispersive X-ray spectroscopy (EDS) analysis (Figure S1b) and the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images in Figure 1d display the homogeneous dispersion of selenium throughout the whole carbon framework, indicating the successful doping of selenium atoms in the NCNS. To reveal the chemical composition and element valence state of different samples, X-ray photoelectron spectroscopy (XPS) was conducted (Figure S3 and Table S1). The high-resolution Se 3d XPS spectrum of Se-NCNS (Figure 1e) illustrates two fitted peaks, including Se 3d5/2 (55.6 eV) and Se 3d3/2 (56.4 eV) that result from the spin-orbit coupling. As for pure Se, the Se 3d5/2 and 3d3/2 peaks are located at about 55.2 and 56.1 eV, respectively, with a spin-orbit splitting of ~0.86 eV. However, the Se 3d5/2 and 3d3/2 peaks here in Se-NCNS are located at higher binding energy, indicating that the Se atoms have successfully been doped in carbon frameworks to form C-Se bonds.24,25 Also, a fitted peak representing the formation of C-N/C-Se bonds is obtained in the C 1s spectrum (Figure 1f).26,27 The S 2p spectrum of S-NCNS can be deconvoluted into S 2p3/2 at 163.9 eV, and S 2p1/2 at 165.1 eV, representing the characteristic of thiophene-S (Figure S4a). The peak representing C-N/C-S can be divided from the C 1s spectrum of both S-NCNS and NCNS (Figures S4b and S5a). The curve fitted N 1s spectra of the three samples (Figures 1g, S4c and 5b) were all deconvoluted into four peaks: pyridinic-N at 398.5 eV, graphitic-N at 401.1 eV, pyrrolic-


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N at 399.4 eV, and N-O at 404.1 eV, confirming that the import of chalcogen atoms do not means transformation of the original nitrogen state. Bearing these in mind, electrochemical experiments were conducted to test the catalytic performances of different samples in the actual oxygen reaction process. Linear sweep voltammetry (LSV) measurements were first carried out in O2-saturated 0.1 M KOH using a conventional three-electrode system. As shown in Figures 2a and S6, NCNS exhibits the most negative ORR performance. After the sulfur doping, an evident increase of the onset potential (Eonset) from 0.88 V to 0.93 V compared with NCNS was obtained, and its half-wave potential (E1/2) also rivals the commercial Pt/C. Expectedly, Se-NCNS possesses the optimal ORR activity with a Eonset of 0.98 V, a high E1/2 of 0.87 V and the largest reduction current, which even outperform the noble-metal benchmark. Its superior reaction kinetics can be further verified by its smallest Tafel slope derived from the corresponding LSV curve (Figure 2b). Rotating ring-disk electrode (RRDE) tests were executed to capture ORR reaction pathway of different samples (Figures 2c and S7). At the voltage range of 0.50-0.80 V, the electron transfer number of Se-NCNS per O2 is over 3.9, and the peroxide yield was determined to be below 5%, suggesting a highly efficient ORR progress. Chronoamperometry tests were conducted in alkaline media to evaluate the durability of different samples. The responses show no obvious difference of the performance loss for Se-NCNS, SNCNS, and NCNS after a continuous operation period of 25 000 s (Figure 2d), illustrating superior stability of the materials. On the contrary, the normalized current of Pt/C exhibits an unsatisfactory drop to 79.3% compared with its initial value. Simultaneously, the OER performance of different samples was also evaluated (Figure S8a). Clearly, the OER activity increases in the order of NCNS < S-NCNS < Se-NCNS, indicating that the introduction of selenium is also helpful on boosting


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the adsorption of OER intermediates, thus enhancing the OER activity. The more favorable OER kinetics is further confirmed by its relatively smaller OER Tafel slope (Figure S8b). To the best of our knownledge, though heteroatom doping in carbon material was confirmed to generate catalytic activity, the promotion is rather restricted, limiting its further application.28 Here, as an ultrahigh performance of Se-NCNS towards ORR was discovered, it is surely necessary to further investigate the activity origin. Thus, density functional theory (DFT) calculations grounded on the ORR four-electron pathway were conducted (Figure S9). Since graphitic-N is the major nitrogen existential form in all materials, which is well-known to benefit the catalytic process,29,30 only it was considered for simplicity. Based on the chemical composition of the chalcogen dopant, five possible active site models are proposed including nitrogen-activated carbon in NCNS (C@NCNS), sulfur and nitrogen-activated carbon in S-NCNS (C@S-NCNS), sulfur in S-NCNS (S@S-NCNS), selenium and nitrogen-activated carbon in Se-NCNS (C@SeNCNS), and selenium in Se-NCNS (Se@Se-NCNS), as illustrated in Figure 3a. The models of different active sites showing the ORR reaction pathway for DFT calculation and the corresponding free energy diagrams are set out in Figures S10-S19. The calculated free energy diagrams of the ORR substeps on different active sites at 0 V and equilibrium potential (1.23 V) are compared in Figures 3b and S20, respectively. According to the free energy diagrams at 0 V, step by step downhill energy variations are observed in all models except for the S@S-NCNS, declaring a nonspontaneous reaction, and thus S@S-NCNS is not the active site for ORR. Among the carbon active sites in all three models, C@S-NCNS illustrates the highest catalytic activity. Though C@Se-NCNS performs slightly weaker, it still obviously ourperforms C@NCNS, confirming that the further introduction of chalcogens can surely induce a higher spin density on the surrounding carbon, which benefits the ORR process.31 Interestingly, we find that, for the first


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time, the introduced selenium atom in nitrogen-doped carbon material can serve as the active site during ORR catalyzing, whereas the function of other dopants in this report is only to boost the activity of the adjacent carbon atom. In a spontaneous reaction, the least exergonic of the elementary steps in the given pathway represents the most sluggish reaction step, and thus is the rate-determining step in the overall ORR process.32,33 For C@NCNS, C@S-NCNS, and C@SeNCNS, the smallest Gibbs free energy change ΔG1 reflects that the first electron transfer reaction that the hydrogenation of adsorbed O2 to OOH* is the rate-limiting step, which has been identified in other reported heteroatom-doped carbon materials.34 Differently, the rate-determining step of Se@Se-NCNS is revealed to be the fourth step, in which the OH* is removed to form OH−. The same rate-limiting step was commonly confirmed in transition metal-based materials,35 illustrating the unique role played by selenium as the ORR active site. Furthermore, the largest calculated limiting potential, and the highest calculated working potential (0.70 V) of Se@Se-NCNS (Figure 3c) also indicate it the most efficient active site towards ORR among all models. Also, the theoretical OER working potentials were calculated, and both of the active sites considered in SeNCNS perform well (Figure S21). Therefore, the DFT calculations matches well with the electrochemical experimental responses, thus highlighting the unprecedented function of the selenium doping in NCNS that to promote the oxygen reaction performance through enhancing the activity of the neighboring carbon as well as creating new active sites. As demonstrated by the electrochemical results and DFT calculations, selenium doping in NCNS could generate a more evident increase of the charge population and density than sulfur doping can do that benefits the ORR process. To further investigate the actual oxygen adsorption capacity for different catalysts as air cathode in zinc-air battery directly, in-situ XRD was conducted through the discharging process (Figures S22-26).36,37 The discharge curves of different


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cathodes operated under in-situ XRD test is provided in Figure S23. As shown in Figure 4a, there are two major peaks located at approximately 44 ° and 54 ° relating to carbon diffractions in all three cases. Along the discharging process, all the carbon peaks gradually weaken due to the continuous chemisorption of oxygen and oxygen-containing intermediates that covering the material surface. The transformation of the usual end-on adsorption model to the side-on adsorption model of oxygen also further blocks the carbon diffraction.38 Remarkably, the decline extent of the carbon peaks for different samples is dramatically disparate, as the value increases in the order of NCNS < S-NCNS < Se-NCNS. Though the carbon peak decrease in S-NCNS is more obvious than that in NCNS, the most evident intensity decrease is observed in Se-NCNS air cathode, indicating the strongest chemisorption of oxygen at the catalyst surface, and thus the most efficient activity towards ORR. In-situ XRD results are in great accordance with the electrochemical responds and DFT calculations, reconfirming that Se-NCNS performs the best in the ORR process. Interestingly, apart from the above observations, several ZnO peaks (JPCDS card no. 36-1451) were detected in the later period of discharging for zinc-air battery with NCNS and S-NCNS cathode, especially in the former, whereas no evident ZnO signals appeared in the case of SeNCNS. As reported before, ZnO is originated from the anodic discharge product, namely zincate ions (i.e. Zn(OH)42-), which gradually deposits on the air electrode when reaching supersaturation. This phenomenon is not expected in zinc-air batteries, as the insoluble oxides will clog the active surface and gas diffusion pathway of the catalyst, resulting in poor battery service life. Considering that the ZnO peaks in zinc-air battery with S-NCNS are much weaker than that with NCNS, and almost no signals were captured in that with Se-NCNS cathode through the discharging process, theoretical analyses were thus carried out via DFT simulations. Figure 3b implies the calculated


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charge density of the three catalysts. In the S-NCNS and Se-NCNS models, the electron cloud is shifted away from the dopant, resulting in an increased electron cloud density of the neighboring carbon. With a more evident electron cloud delocalization happens around selenium, Se-NCNS generates a stronger repulsion of zincate ions compared with S-NCNS, thus effectively protect the air cathode from ZnO poisoning. The results were further confirmed by zeta potentials tested in alkaline media (Figure 5a). Specifically, the negative potential of different catalysts increases in the order of Pt/C+IrO2 (-3.7 mV) < NCNS (-7.5 mV) < S-NCNS (-9.9 mV) < Se-NCNS (-20.4 mV). The larger negative value indicates a higher electron cloud density in the carbon matrix. Considering that there are two dopants in the Se-NCNS, one may doubt whether the inhibition effect of zincate ions to air-cathodes is attributed to the Se doped in carbon or the synergistic effects of Se and N. Thus, we carried out controlled experiments to address this problem. Single Se-doped sample (denoted as Se-CNS) was fabricated using the same method as Se-NCNS except replacing the NCNS precursor by graphene oxide (Figure S27). The unsatisfactory electrocatalytic performance of Se-CNS (Figure S28) leads to an almost unchanged carbon peak intensity over the discharging process of in-situ XRD measurement (Figures S29-S31). However, no ZnO signal was detected, declaring that single Se-doping can prevent the air cathode from ZnO poisoning. The zeta potential of Se-CNS was measured to be as negative as -21.2 mV (Figure S32), which indicates an even higher electron cloud density than Se-NCNS in the carbon matrix. As a result, it can be concluded that the inhibition effect on the access of zincate ions to air-cathodes is due to the strong electron cloud delocalization caused by introduction of Se in carbon framework. The zinc-air battery performances were then tested in a home-built electrochemical cell (Figure S33). To verify whether the excluding of oxides really benefits the battery performance under practical operating condition, galvanostatic discharging/charging stability were tested at a


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cyclic current density of 25 mA cm–2 for 100 cycles with 10 min for each cycle (Figure 5b). As clearly shown in the enlarged cycle potential profiles (Figure S34), an obvious degradation of the cyclic performance is observed in NCNS owing to the lack of protection for the air cathode, whereas the cyclic performance drop is minished to some extent in the case of S-NCNS. Notably, Se-NCNS exhibits ultrahigh stability with no notable change of the charging-discharging potential gap after 100 cycles. As for the Se-CNS counterpart, though the charging-discharging platform is unsatisfactory, the battery demonstrates very constant operation (Figure S35). Therefore, all the results correspond well to the XRD conclusions. On account of the similar stability of Se-NCNS, S-NCNS and NCNS in the ORR chronoamperometry test, the different cyclic performance drop of NCNS and S-NCNS can be ascribed to the varied degrees of ZnO deposition, thus highlighting the vital role of selenium for enhancing the durability of zinc-air batteries. On the other hand, the zinc-air battery assembled with Se-NCNS cathode exhibits the highest open circuit potential of 1.49 V, as well as the optimal power density of 154.45 mW cm-2 compared with the counterparts (Figure S36). With all the above satisfactory features, a long cycle life of over 780 discharging/charging cycles of Se-NCNS air cathode was obtained (Figures 5c and S37), which is top-ranking among the previously reported devices (Table S3). The enlarged cycle potential profiles demonstrate that the zinc-air battery with Se-NCNS electrode could maintain a round-trip efficiency of 60.9% at the 780th cycle, with only a negligible drop compared to the 1st cycle (61.8%). In great contrast, the zinc-air battery with noble-metal-based catalyst shows dramatic recession in the cycling test (Figure S38). The highly efficient active sites, accompanied with the unique feature of cathodic protection, pushes this discovery forward to future contribution for highly stable and efficient zinc-air batteries.


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CONCLUSION In summary, we have dissected, for the first time, the introduction of selenium atom in nitrogen-doped carbon can not only further enhance the catalytic activity, but is also able to generate a strong electron cloud delocalization that protects the air cathode from zinc oxide poisioning, which is urgently demanded for the long battery service life. The resultant material exhibits superior catalytic activity, which even outperforms the commercial noble metal-based benchmark. By DFT calculations, we confirmed that the inserted selenium active site is the most crucial source of the catalytic activity of the material, since it not only enhances the activity of the surrounding carbon, but also acts as a highly efficient active site itself. In-situ XRD characterization clearly demonstrates an indeed enhanced chemisorption of the ORR reactants in the selenium-doped material, and, more importantly, the non-deposition of zinc oxide on the air electrode through zinc-air battery operation. The strong electron cloud delocalization from the dopant to the adjacent carbon results in a higher electron cloud density of the latter, and is the origin of the non-deposition of zinc oxide, as substantiated by DFT simulations. A desirably long cycle life of over 780 cycles with a steady discharging/charging potential gap is obtained in zincair battery assembled with this catalyst. Our work provides an in-depth exploration of heteroatomdoped carbon materials for non-deposition of metal oxides on air cathode and depicts a grand blueprint towards future highly stable metal-air batteries with great cost-effectiveness.

EXPERIMENTAL SECTION Fabrication of NCNS: Sodium chloride was disolved in deionized water, to which 2 ml pyrrole was added and treated by sonication to form a uniform suspension. The system was cooled down to 0 °C in ice/water bath. Followed by the addtion of 1ml 1 M HCl, (NH4)2S2O8 solution


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was dropwise added into the system under vigorous stirring to start the pyrrole polymerization. The reaction was kept at 0 °C for 24 h, and the obtained mixture was freeze-dried. The obtained material was triturated and annealed at 900 °C for 2 h with argon protection. After cooling down to room temperature naturally, the production was washed thoroughly with deionized water to remove the salt and dried under vacuum to obtain black powders. The material was then treated with hydrochloric acid to remove the possible impurities. Finally, the product was successively washed with deionized water and ethanol, and dried under vacuum at 60 C overnight to obtain NCNS. Fabrication of Se-NCNS, S-NCNS and Se-CNS: Se-NCNS was synthesized as follows: NCNS and benzyl diselenide were mixed and ultrasonically dispersed in ethanol for 1 h. Then, the suspension was heated at 60 °C under stirring to obtain a uniform dry mixture. The mixture was placed in a quartz tube and annealed at 900 °C with argon protection. After cooling down to room temperature naturally, the sample was successively washed with deionized water and ethonal, and dried under vacuum at 60 C overnight. S-NCNS was fabricated through the same strategy as SeNCNS, except changing benzyl diselenide with benzyl disulfide. Se-CNS was fabricated through the same strategy as Se-NCNS, except changing the NCNS precursor with graphene oxide. Electrochemical measurements: All electrochemical measurements were carried out in a conventional three-electrode system using WaveDriver 20 bipotentiostat (Pine Instrument Company, USA). A Rotating Disk Electrode (RDE) with a glassy carbon disk served as the substrate was used as the working electrode. Ag/AgCl (4 M KCl) was used as the reference electrode while graphite rod was used as the counter electrode. Se-NCNS catalyst ink was prepared by dispersing 2 mg catalysts and 0.5 mg Acetylene Black in 380 μL ethanol and 20 μL Nafion solution (5 wt%) in an ultrasonic bath for at least 30 min. 8 μL of catalyst ink was then coated on


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glassy carbon disk to achieve a catalyst loading of 0.2 mg cm-2. For comparison, Commercial Pt/C (20 wt%, Johnson Matthey) and IrO2 (99 wt%, Alfa Aesar) catalyst ink was prepared by dispersing 1 mg catalysts in 250 μL ethanol and 10 μL Nafion solution (5 wt%). 10 μL of catalyst ink was then coated on glassy carbon disk to achieve a catalyst loading of 0.2 mg cm-2. The electrolyte was 0.1 M KOH. RDE tests were conducted at 400, 625, 900, 1,225, 1,600, 2,025, 2,500 rpm with a scan rate of 10 mV s-1. Zinc-air battery test: The catalyst ink was prepared by blending 1 mg catalysts with 6 μL Nafion solution (5 wt %) and 250 μL ethanol in an ultrasonic bath for 60 min. The mass ratio of commercial Pt/C and IrO2 mixed catalyst was 1:1. The cathode was prepared by uniformly loading the catalyst ink onto carbon paper (TGP-H-060, Toray Carbon, Japan) with mass loading of 1 mg cm-2. Commercial zinc plate was used as the anode, and was polished and then sonicated in ethanol for an hour to remove the oxidation layer and impurities before use. Zinc-air batteries were assembled using home-built electrochemical cells with 6 M KOH and 0.2 M Zn(CH3COO)2 as electrolyte. The open circuit potential measurement and voltage-current polarization curve were tested in the aqueous alkaline electrolyte pre-saturated with high-purity oxygen with the CHI760D (CH Instruments, Inc., Shanghai, China) electrochemical workstation. The galvanostatic discharge and charge tests were conducted using the CT2001A cell test instrument (Wuhan LAND Electronic Co., Ltd) at a constant current density, and the cell was maintained with continuous O2 bubble through the electrolyte at room temperature. In-situ XRD characterization: The in-situ XRD measurements were recorded at the X-ray powder diffractometer (D8 ADVANCE, Bruker AXS GmbH Co., Ltd). The special battery was assembled using a tailor-made mould with a window for X-ray penetration, in which the Se-NCNS, S-NCNS, NCNS and Se-CNS coated carbon paper as air cathodes, Whatman glass microfiber filter


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as separator, a polished Zn plate as anode, and 6 M KOH with 0.2 M zinc acetate as the electrolyte. The galvanostatic discharge measurements (2 h discharge) were carried out by CHI660E (Shanghai Chenhua instrument Co., Ltd) electrochemical workstation at a current density of 10 mA cm-2. Other experiment and characterization details are provided in the Supporting Information.


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Figure 1. (a) Schematic illustration of the reaction procedure derived from the selenium active site in selenium-doped NCNS. (b) SEM image and (c) TEM image of Se-NCNS. (d) HAADF-STEM images showing the presence of C, O, and Se of Se-NCNS. High resolution XPS spectrum of the (e) Se 3d, (f) C 1s and (g) N 1s of Se-NCNS.


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Figure 2. (a) ORR polarization curves tested in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s−1, (b) ORR Tafel plots derived from the corresponding LSV curves, (c) peroxide yield and electron transfer number based on the RRDE data, and (d) ORR current-time chronoamperometric responses of different samples.


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Figure 3. (a) The models used for theoretical calculations for NCNS, S-NCNS and Se-NCNS, with the dotted circle lines marking out the active sites. The calculated free-energy diagrams of the *OOH, *O, *OH intermediates adsorbed on the active sites in different samples in the standard 4e- reaction-pathway for ORR under (b) 0 V, and (c) corresponding working potentials.


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Figure 4. (a) In-situ XRD intensity map of zinc-air battery with NCNS, S-NCNS and Se-NCNS air electrodes during discharging. (b) The corresponding models for DFT calculations, showing the charge-density wave of the NCNS, S-NCNS and Se-NCNS. The brown, blue, red, and purple spheres refer to carbon, nitrogen, sulfur, and selenium atoms, respectively.


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Figure 5. (a) The zeta potential of each sample in alkaline media. (b) Galvanostatic cyclic performance of zinc-air batteries with different air cathodes at a steady current density of 25 mA cm–2 for 100 cycles with 10 min for each cycle. (c) Long-term cyclic performance of zinc-air battery using Se-NCNS cathode with inset showing the enlarged cycle voltage profiles of the 1st and 780th cycle.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental methods, SEM, AFM and EDX of Se-NCNS, BET surface areas, XPS survey spectra and elemental ratios of different samples, high-resolution XPS spectra of S-NCNS, high-resolution XPS spectra of NCNS, LSV curves at different rotation speeds, RRDE curves of different samples, OER LSV curves and Tafel plots of different samples, computational models for DFT calculation, calculated free energy diagrams of different models, calculated theoretical OER working potentials, schematic diagram of the device for in-situ XRD test, discharge curves of zinc-air batteries with different cathodes operated under in-situ XRD test, in-situ XRD patterns of zinc-air batteries with different cathodes, TEM image, XPS spectra and elemental ratios of Se-CNS, electrocatalytic performance of Se-CNS, in-situ XRD responses of Se-CNS, zeta potential of different samples in alkaline media, photograph of the home-built electrochemical cell for zinc-air battery tests, enlarged cycle voltage profiles of different samples, galvanostatic discharge and charge performance of zinc-air batteries with Se-NCNS and Se-CNS cathodes, open-circuit plots and peak power densities of zinc-air batteries with different cathodes, long-term cyclic performance of zinc-air battery using SeNCNS cathode, comparison of Se-NCNS to reported electrocatalysts for rechargeable zincair batteries, comparison of galvanostatic discharge and charge performance of zinc-air batteries with Se-NCNS, and Pt/C+IrO2 cathodes.

AUTHOR INFORMATION Corresponding Authors


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*E-mail for T.Q.: [email protected] *E-mail for C.Y.: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge the support from the National Natural Science Foundation of China (No. 51622208 and No. 21703149).

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Table of Contents


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Selenium-Doped Carbon Nanosheets with Strong Electron Cloud

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