Single Co Atoms Anchored in Porous N-Doped Carbon for Efficient

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Single Co Atoms Anchored in Porous N-doped Carbon for Efficient Cathode in Zinc-air Battery Wenjie Zang, Afriyanti Sumboja, Yuanyuan Ma, Hong Zhang, Yue Wu, Sisi Wu, Haijun Wu, Zhaolin Liu, Cao Guan, John Wang, and Stephen J. Pennycook ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02556 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018

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Single Co Atoms Anchored in Porous N-doped Carbon for Efficient Cathode in Zinc-air Battery Wenjie Zang1, Afriyanti Sumboja2, Yuanyuan Ma1,2, Hong Zhang1, Yue Wu1, Sisi Wu1, Haijun Wu1, Zhaolin Liu2*, Cao Guan1*, John Wang1*, Stephen J. Pennycook1* 1 Department of Materials Science and Engineering, Faculty of Engineering, National University of Singapore, 117574, Singapore 2 Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis #8-13, 138634, Singapore

ABSTRACT: Exploration of cheap, efficient and highly durable transition metal-based electrocatalysts is critically important for the renewable energy chain, including both energy storage and energy conversion. Herein, we have developed cobalt (Co) single atoms anchored in porous nitrogen-doped carbon nanoflake arrays, synthesized from Co-MOF precursor and followed by removal of the unwanted Co clusters. The well dispersed Co single atoms are attached in the carbon network through the N-Co bonding, where there is extra porosity and active surface area created by the removal of the Co metal clusters. Interestingly, compared with those electrocatalysts containing excess Co nanoparticle, Co single atoms alone demonstrates a lower oxygen evolution reaction (OER) overpotential and much higher oxygen reduction reaction (ORR) saturation current, showing that the Co metal clusters are redundant in driving both OER and ORR. Given the bifunctional electrocatalytic activity and mechanical flexibility, the electrocatalyst assembled on carbon cloth is employed as the air

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cathode in solid-state Zn-air battery, which presents good cycling stabilities (2500 minutes, 125 cycles) as well as a high open circuit potential (1.411V).

KEYWORDS: Single Co atoms in carbon, Oxygen reduction reaction, Oxygen evolution reaction, Zinc-air battery, electrocatalyst.


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1 Introduction For the past several years, there has been constant search for better energy storage and conversion systems, such as development of improved metal-air batteries, supercapacitors and electrocatalysts, which are the crucial parts of the entire energy chain1–6. For metal-air batteries, such as Zn-air battery, ORR and OER are two significant processes that make a determining effect on the overall efficiency of the charge and discharge processes7–10. Considering the low onset and half-wave potential of ORR and high overpotential of OER in several of the known electrocatalyts, highly efficient cathodes are required in order to reduce the energy barrier and improve the overall efficiency. Noble metal-based nanomaterials with high catalytic activity are known as among the best electrocatalysts in terms of performance. However, their high cost, scarcity and poor long-term stability (in some parameters) greatly restrict their widespread and large scale utilization11–15.Thus, there is urgent need in development of cheap, efficient and durable non-precious metal-based electrocatalysts. For the above purpose, cobalt-based nanomaterials, such as Co3O416–19, CoP20–22, CoSe223,24 and cobalt single atom catalysts (Co-SAC)25–29, have attracted extensive attention recently, owing to their low cost and generally high catalytic activities, depending on both the type of the compounds nanostructural features. Among these cobalt-based catalysts, Co single atoms anchored in an appropriate matrix (Co-SAC) exhibit several advantages, including high atom utilization, high active site exposure, low-coordination and unique catalytic performance compared to those cobalt nanoparticle and nanocluster counterparts30–35. Co single atoms can be bonded to the supporting matrix through strong electronic and covalent interactions, subsequently forming rather stable catalysts32,36,37. In a couple of recent studies, nitrogendoped carbon nanomaterials have been selected as the supporting matrix for Co-SAC to develop optimal Co-N4 sites, which can be active sites for oxygen adsorption and subsequent


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O=O bond breaking during the ORR38,39. In addition, as has been documented in previous studies, Co-SAC have been studied for energy conversion, such as hydrogen evolution40, oxygen reduction25,26,41,42 and hydrogenenration38. By comparison, there is little that has been done with Co single atoms (or other metal) for energy storage. Zn is the fourth most abundant metal on earth. Zn–air batteries, exhibiting high theoretical energy density (1086 Wh kg−1, including oxygen) and of low cost, have been suggested to be one of the most promising energy storage systems. One of the challenges is the general lack of stable and efficient bifunctional air electrodes, which has been a major obstacle to their practical application43–48. Although several powder-based materials, such as transition metal oxides17,49,50 and phosphides51,52, have been studied for rechargeable Zn-air batteries, use of polymeric binders and conductive additives often adds extra weight/volume to the cathode materials53,54. These binders can also significantly reduce the catalytic activity, enlarge the interfacial impedance, as well as affect the long-term stability55. Although Co SAC have been tested for OER and ORR, to the best of our knowledge, there is no report on the successful application of Co SAC to flexible Zn-air batteries. Therefore, to improve the durability and flexibility of Zn-air batteries, it would be of considerable interest to grow Co SAC on conductive matrix directly and use it as a binder-free air cathode in Zn-air batteries. Herein, we report a Co SA electrocatalyst assembled in nitrogen-doped porous carbon nanoflake arrays (NC-Co SA), which was fabricated by a facile carbonization-acidification process using Metal Organic Frameworks (MOF) as the precursor material. Compared with the Co nanoparticle counterpart (NC-Co), the NC-Co SA shows significantly enhanced ORR and OER performance. Furthermore, for the first time, the NC-Co SA electrocatalyst is studied as an air cathode without any binder and additives, and applied to both aqueous and


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solid-state Zn-air batteries. In addition to the energy storage behavior, the device shows excellent cycling stability and flexibility as well as a high open-circuit potential. 2 Experimental section 2.1 Materials synthesis 2.1.1 Synthesis of Co based MOF nanoflake arrays (Co-MOF) on carbon cloth 40 ml of 2-methylimidazole solution (0.4 M) was first mixed with 40 ml of Co(NO3)2•6H2O solution (50 mM) at room temperature. One piece of acidized carbon cloth (3555 mm2) was then put in the solution and retained for 4 hours. Then the carbon cloth was washed with deionized water and dried overnight at 55 oC. 2.1.2 Synthesis of Co nanoparticles in nitrogen-doped carbon nanoflake arrays (NC-Co) A piece of the carbon cloth with Co-MOF arrays was put in a ceramic boat and transferred into a tube furnace. The sample was then annealed in nitrogen atmosphere at 800 oC for 4h with a ramping rate of 1 oC min−1. When the furnace was cooled to the room temperature, the NC-Co is recovered. 2.1.3 Synthesis of NC-Co SA A piece of carbon cloth with NC-Co (1555 mm2) being grown upon was immersed in 40ml of concentrated hydrochloric acid (HCl, 37 wt%), and the suspension was then stirred for 6 hours by a magnetic bar to dissolve Co metal clusters. After that, the carbon cloth with NCCo was washed with deionized water several times until the PH of the solution reached 7, and then dried overnight at 55 oC. 2.2 Characterization


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Scanning Electron Microscopy (SEM, SUPRA 40 Zeiss) was used to study the particle morphologies in all samples. X-ray diffraction (XRD) patterns and X-ray photoelectron spectroscopy (XPS) data were acquired by a Bruker D8 diffractometer and a AXIS Ultra XPS, respectively. Scanning transmission electron microscopy (STEM), energy-dispersive Xray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) were conducted using a JEOL ARM200F equipped with cold field emission gun, Gatan Quantum ER spectrometer and Oxford X-max 100TLE SDD EDS under acceleration voltage of 80 and 200 kV. The images at 200 kV had improved resolutions, and no damage was observed. All results reported here were obtained at 200 kV. Raman spectroscopy studies were made using a LabRAM HR Evolution Raman microscope. Inductive coupled plasma mass spectrometry (ICP-MS) was conducted using a Perkin Elmer Optima 5300DV. Nitrogen adsorption– desorption isotherms were measured using a Micromeritics 3Flex at 77 K. The speciﬁc surface areas and pore size distribution were calculated by the Brunauer–Emmett–Teller (BET) and density functional theory (DFT) method, respectively. 2.3 Electrochemical measurements OER and ORR activities of NC-Co and NC-Co SA catalysts were evaluated by linear sweep voltammetry (LSV) and rotating disk electrode (RDE) studies in O2-saturated potassium hydroxide electrolyte (KOH, 1 M, pH=14). RDE measurements were recorded by an Autolab PGSTAT302N working station with the standard three electrode system. The samples were fixed on the glassy carbon RDE by acrylic tape, which formed the working electrode. A platinum plate was used as the counter electrode, and Ag/AgCl electrode acted as the reference electrode. The disk electrode was scanned at a rate of 5 mV s−1 and all potentials were calibrated against a reversible hydrogen electrode (RHE), according to ERHE=EAg/AgCl (3M


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KCl)

+0.059pH+0.209 V. The number of electrons transferred (n) is calculated using the

Koutecky-Levich equation: 1 1 1 1 1 = + = + 1/2 𝑖 𝑖𝑑𝑙 𝑖𝑘 𝐵 𝑖𝑘 2/3

𝐵 = 0.62𝑛𝐹𝐴𝐶𝑂 𝐷𝑂 −1/6 where i, idl, and ik represent the measured current, diffusion-limiting current and kinetic current, respectively. F is the Faraday’s constant (98485 C mol-1), A is the electrode’s geometric area (0.35 cm2), CO is the bulk concentration of oxygen, DO is the diffusion coefficient of oxygen,  is the kinematic viscosity of the electrolyte,  is the electrode rotation speed. DO, CO, and v in 1 M KOH are 1.65 × 10−5 cm2 s−1, 8.3 × 10−7 mol cm−3, and 9.5 × 10−3 cm2 s−1, respectively. The mass activity (A g−1 of catalyst) was determined from the measured current density (mA cm−2) and the amount of catalysts grown on carbon cloth (mA cm−2). The turnover frequency (TOF) is calculated using the following equation: 𝑇𝑂𝐹 =

𝑗∗𝐴 4 ∗ 𝐹 ∗ 𝑛′

where j is the measured geometrical current density, n’ is the number of moles of Co atoms on the working electrode, which was obtained from ICP-MS. 2.4 Assembly and electrochemical test of Zn-air batteries For aqueous Zn-air battery, a Zn plate was used as the anode and 6 M KOH consisting of 0.1 M ZnAc served as the electrolyte. The air electrode was composed of one layer of NC-Co SA and a layer of carbon paper (38 BC SGL Carbon). Cycling tests were performed by a Maccor 4300 battery test system, using recurrent galvanostatic pulses for 10 minutes of discharge followed by 10 minutes of charge at 10 mA cm-2. For comparison purpose, commercial Pt/C catalyst was used as a reference.


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To assemble the solid-state sandwich-like layered Zn-air battery, the solid-state electrolyte was prepared by the following process. Firstly, 5 ml of 11.25 M KOH and 0.25 M ZnO were mixed with 0.5 g of acrylic acid and 0.075 g N,N’-methylene-bisacrylamide. The white precipitate was filtered out after 5 min of stirring. 75 μl of 0.3 M K2S2O8 (potassium persulfate) was then added into the solution, where a small portion of the solution was poured into the acrylic well (thickness: 1.0 mm) once the solution started to polymerize56. Zn foil (0.5 × 3 cm2, with 0.5 × 1.5 cm2 left blank as current collector), solid-state electrolyte and flexible air cathode (0.5 × 1.5 cm2, adhered to 0.5 × 3 cm2 Ti mesh current collector) were assembled first, and then two pieces of acrylic tape with 0.5 mm thickness were used to bond the device into a rectangular shape with a thickness of 2.0 mm. Cycling tests were performed using Maccor 4300 battery test system, as before. Mechanical flexibility and stability were tested by flattening and folding of the battery alternately for five cycles. Commercial Pt/C catalyst and NC-Co sample were used as references. 3 Results and discussion 3.1 NC-Co SA electrocatalyst The schematic process for fabrication of NC-Co SA is illustrated in Figure 1a. Initially, two dimensional Co MOF nanoflake arrays were synthesized on the carbon cloth by mixing an appropriate amount of Co(NO3)2•6H2O and 2-methylimidazole for 4 hours. From the Scanning Electron Microscopy image in Figure 1b and Figure S1 (Supporting Information), these Co MOF precursors grew uniformly on the carbon cloth, typically showing a triangular shape with a smooth surface. Subsequently, the Co MOF was carbonized in a nitrogen atmosphere at 800 oC for 4 hours. During this process, the nanoflake morphology was well retained on carbon cloth with a mass loading of 1.56 mg cm−2, and some Co nanoparticles had become encapsulated in carbonaceous shells to form the NC-Co sample, as shown in


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Figure 1c and Figure S2. After acid leaching treatment for 6 hours, the morphologies of NCCo SA were well retained. Most of the large Co particles were removed (Figure 1d and Figure S3), resulting in the decline of mass loading to 1.35 mg cm−2. The N2 adsorptiondesorption isotherm of NC-Co SA shows a typical type-IV curve with a hysteresis loop, suggesting the existence of mesopores (Figure S4). The specific surface area and total pore volume of NC-Co SA are measured to be 319.35 m2 g−1 and 0.20 cm3 g−1, respectively, which are higher than those corresponding values of NC-Co (268.02 m2 g−1, 0.10 cm3 g−1). Such increase of specific surface area can accommodate more active sites for electrocatalytic reactions. Meanwhile, as shown in Figure S5, the NC-Co SA is rich in Co single atom sites, which are dispersed well both in the flakes and the ripple-like nitrogen-doped carbon shells. The EDS results in Figure S6 also confirm that the Co and N are dispersed evenly in the catalyst. According to the results of ICP-MS, the loading content of Co in NC-Co SA is 1.84 wt%, which is much smaller than that in NC-Co (74.9 wt%), suggesting that most Co particles were removed by the acid leaching treatment. To better reveal the chemical nature of bonding Co single atoms in the NC-Co SA, we have employed the atomic level HAADF-STEM coupled with EELS (Figure 1e-g). When we placed the electron beam on the bright dot (red circle in Figure 1e), both Co and N signals were detected in the EELS, confirming the coexistence of Co and N in the form of Co-Nx. By comparison, when the electron beam was moved to the dark matrix (blue circle in Figure 1e), no Co and N signals were found. The above EELS results were also confirmed in many areas throughout the sample, as provided in Figure S7. This atomic level spectroscopic analysis testifies that the single atom sites composed of Co are coordinated with N, and there are sufficiently stable Co-Nx sites anchored in the porous flakes, which survived under the high voltage electron beam. The Co-Nx coordination and Co aggregates (Co-Co coordination, small Co dots) were formed from the Co MOF precursor during the high temperature


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carbonization process. Some of the Co-Co bonds were broken, when large Co clusters were removed by the washing of strong acid, and those Co single atoms coordinated with N were remained in the N-doped carbon flakes. The nanocrystalline nature of the NC-Co and NC-Co SA were further confirmed by X-Ray diffraction (Figure 2a). NC-Co exhibited the cubic phase of Co metal (PDF #15-0806) and graphitic carbon its (002) plane. After the acid leaching, no strong Co-containing peaks could be detected in NC-Co SA, due to the insensitivity of XRD towards single atoms and those Co dots. The chemical composition of NC-Co SA was further investigated by using XPS. The Co 2p spectra in Figure 2b exhibit two main peaks at 778.8 eV and 780.34 eV with one satellite peak at 783.1 eV, and one main peak at 795.5 eV with one satellite peak at 799.19 eV. The N 1s spectra in Figure 2c can well be deconvoluted into four peaks, which are assigned to the pyridinic and N-Co (398.54 eV), pyrrolic (399.56 eV), graphitic (401.06 eV), and N-oxide (402.71 eV)40. As suggested by previous work, the graphitic N would affect the geometric and electronic structures of carbonaceous matrix, whereas the pyridinic N are most likely to bond with atomically dispersed Co atoms in the form of CoN438. The binding energies of the N-Co and pyridinic N is too small to differentiate, thus the peak at the binding energy of 398.54 eV includes contribution both from N-Co and the pyridinic N. This result is also in accordance with the above EELS results, which have testified the coexistence of Co and N. Furthermore, compared with the N spectra in NC-Co (Figure S8), the ratio of graphitic N to pyrrolic N in NC-Co SA remains nearly unchanged, but the ratio of N-Co to graphitic N increases significantly. This reveals that more Co-Nx sites are exposed on the surface of NCCo SA than NC-Co. In addition to Co and N, elements of C and O were also detected by XPS, as shown in Figure S9. Raman spectra in Figure 2d show the characteristic G and D bands of the carbon which are related to the mesoporous carbonaceous flakes. The ID/IG ratio of NC-Co SA is larger than that of NC-Co, which is further verified to be the high contents of


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pores and defects (such as vacancies and edge sites) in NC-Co SA. Such abundant defects are believed to enhance the ORR and OER catalytic activities. The peak centered at 675 cm-1 in NC-Co is ascribed to the metallic Co43,57, whereas it is absent in the sample of NC-Co SA. This result indicates the removal of Co large aggregates in NC-Co SA after acidification, which is consistent with the above XRD and STEM data. 3.2 Electrochemical evaluation of NC-Co SA for ORR and OER activities N-coordinated Co sites, which exhibit a similar chemical environment of CoN4 in the Co porphyrin structure, are believed to be the active sites for ORR activities58. The carbon flakes with high level of porosity and high density of defects are also crucial for accommodation of these active sites. To elucidate the activity of Co-Nx coordination, linear sweep voltammetry (LSV) measurements of NC-Co SA, NC-Co and commercial Pt/C were performed at different rotation speeds in 1M KOH solution (Figure 3a-b). As shown in Figure 3a, although the onset potential (Eonset) and half wave potential (E1/2) of NC-Co SA (Eonset= 1.00 V, E1/2= 0.87 V, vs. RHE) are quite close to those of NC-Co (Eonset= 1.02 V, E1/2= 0.86 V, vs. RHE), its saturation current at 0.60 V (vs. RHE) can reach 10.38 mA cm−2, which is dramatically larger than that of NC-Co (6.93 mA cm−2) and Pt/C (8.52 mA cm−2). Its mass activity at 0.9 V (vs. RHE) can reach 1.25 A g−1, which is also higher than that of NC-Co (1.09 A g−1). This suggests that highly porous carbon flakes and high density of Co-Nx active sites in NC-Co SA facilitate oxygen adsorption and subsequent O=O bond breaking during the ORR reaction. By contrast, the large Co particles existed in NC-Co are inactive and play insignificant role in boosting ORR activity. The ORR LSV curves of NC-Co SA under different rotation speeds and corresponding Koutechky-Levich (K-L) plots are illustrated in Figure 3b. The K-L plots show a good linear relationship between i-1 and -0.5 under three different voltages. According to the K-L equation, the electron transfer number per oxygen molecule (n) is calculated to be


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4.07, suggesting a preferable four electron pathway for the ORR. However, for the NC-Co sample, n is only 2.75 (Figure S10). Such a big difference is attributed to the agglomeration of Co particles and the low content of Co-Nx active sites, reducing the capacity for oxygen adsorption and mass transport of ionic species. After acid leaching, most Co particles are removed, and a large number of Co-Nx sites are exposed on the surface of NC-Co SA to absorb oxygen molecules and promote the electron transfer. As for the OER catalytic activities shown in Figure 3c, when the current density reaches 10 mA cm−2, the NC-Co SA presents a lower overpotential (360 mV) than that of NC-Co (401 mV), showing its promising OER activities. The double layer capacitance in Figure S11 further testifies that the NC-Co SA has a larger active surface area than that of NC-Co. Furthermore, we have calculated the difference between the OER voltage at 10 mA/cm2 and ORR half-wave potential (E=Ei=10-E1/2). The smaller the E value, the better the catalyst performance as a reversible oxygen electrode43. We found that E of NC-Co SA (0.72 V) is significantly smaller than that of NC-Co (0.77 V). The larger TOF of NC-Co SA compared to NC-Co also suggests a faster ORR and OER kinetic rate (Figure S12). Based on the above experimental results, the superior bifunctional activity of NC-Co SA can be ascribed to the following features. Firstly, a large number of Co-Nx active sites can promote oxygen adsorption and mass transport of ionic species. Secondly, the porous carbonaceous flakes with large surface area accommodate a large number of active site exposures and facilitate the reactions. Thirdly, the existence of N in the mesoporous carbon support can induce uneven charge distribution, so the positively charged carbon atoms are favorable to adsorb oxygen species. By contrast, for the sample of NC-Co, although its Co content is higher than the NC-Co SA, most Co-Co sites exist in the interior Co aggregates and cannot be exposed to the electrolyte, thus they will be useless for promoting the ORR activities.


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Having proven the superior ORR and OER catalytic properties of NC-Co SA, we assembled two aqueous Zn-air batteries using NC-Co SA and Pt/C as the air cathodes, and then compared their catalytic stability by alternating 10 minutes of discharge and 10 minutes of charge, as shown in Figure 3d. The battery using NC-Co SA as the cathode did not show any obvious voltage change under 570 charge/discharge cycles (equal to 180 hours), which was more stable than the battery using Pt/C with 325 cycles (equal to 108 hours), presenting the excellent cycling stability of the former. The morphologies of NC-Co SA after cycling (Figure S13) reveal that the overall structure of the nitrogen-doped carbon nanoflake arrays is well retained on the carbon cloth after the long term charging-discharging process, suggesting that the Co single atoms remain stable in the nanoflakes. 3.3 NC-Co SA as air cathode for the flexible solid-state Zn-air batteries Bifunctional electrocatalysts for ORR and OER are highly desirable and important for metalair batteries. Considering the excellent ORR and OER performance of flexible NC-Co SA, we have used our sample as a binder-free air cathode and assembled a flexible solid-state Znair battery with Zn foil anode, to test the flexibility and stability performance (see the experimental section for details). To the best of our knowledge, no literature has reported the application of Co single atom electrocatalysts in metal-air batteries until now. The mechanical flexibility and stability of NC-Co SA, NC-Co and Pt/C were tested by flattening and folding the batteries alternately per five cycles (see details in Figure S14), as shown in Figure 4a. Compared with the Pt/C and NC-Co, the NC-Co SA can withstand larger mechanical change and exhibit less voltage change as well as better cycling stability (up to 600 minutes) under continuous charging and discharging process. In addition to the excellent mechanical stability, the battery with the NC-Co SA cathode can generate a high open-circuit potential, up to 1.41 V (as shown in Figure 4b, 4.232 V for three NC-Co SA batteries in


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series), which was much better than that of Pt/C cathode (1.27 V), previously reported Fe single atom cathode (1.35 V) and Co particle based cathode43,59. Such high open circuit potential also remained stable under 90o and 180o bending angles (Figure S15). The device with two batteries connected in series was capable of lighting up six light emitting diodes (LEDs) in both flat and bent states, showing a promising practical usage for flexible electronics (Figure 4c). The charging-discharging performance is shown by the polarization curves in Figure 4d. When the current density reaches 10 mA cm-2, the voltage gaps of NCCo SA (0.45 V in flat states and 0.51 V in bent states) are much smaller than those of NC-Co (1.140 V in flat states and 0.52 V in bent states), demonstrating better charging-discharging abilities. Figure 4e shows the power density-current density curves for the solid state Zn-air batteries. The NC-Co SA in its flat state shows a maximal current density of 31.0 mA cm−2 and a peak power density of 20.9 mW cm−3, which are also better than those of NC-Co (24.7 mA cm−2, 16.9 mW cm−3). In addition, we compared the long-term cycling stabilities of batteries using NC-Co SA, NC-Co and Pt/C as air electrodes in both flat and bent states, as shown in Figure 4f. The battery with NC-Co SA cathode showed very stable chargingdischarging potentials for 2500 minutes (125 cycles, one cycle for 20 minutes) in its flat state and 2200 minutes (110 cycles) in its bent state, outperforming the batteries with NC-Co cathode (48 cycles in bent and flat states) and Pt/C cathode (56 cycles in its flat state and 13 cycles in its bent state). These results indicate that the NC-Co SA electrocatalyst developed in the present work has great potential in the rechargeable Zn-air battery fields. 4 Conclusions In summary, a mechanically flexible electrocatalyst NC-Co SA comprising Co single atoms anchored in porous nitrogen-doped carbon flakes was synthesized by a carbonizationacidification process from Co-MOF precursor. The well-dispersed Co single atoms were


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stably attached to the N-doped carbon flakes through chemical bonding with N during the carbonization process. Compared with Co nanoparticles grown on NC, the NC-Co SA electrocatalyst with high density of Co-Nx active sites demonstrates a lower OER overpotential and much higher ORR saturation current, showing that the Co metal clusters are redundant in driving both OER and ORR. Benefiting the excellent bifunctional activity, the flexible NC-Co SA electrocatalyst was studied as binder-free air cathodes for the Zn-air batteries, which delivered a high open circuit potential of 1.411 V with excellent cycling stabilities in both flat and bent states. The single atom catalysts show great promise for the development of non-noble metal based electrocatalysts for flexible energy conversion and storage devices. AUTHOR INFORMATION Corresponding Author *Zhaolin Liu, *Cao Guan, *John Wang, *Stephen J. Pennycook *Email: [email protected], [email protected], [email protected], [email protected] Author contributions The manuscript was written through contributions of all authors. ASSOCIATED CONTENT Supporting Information The morphology of Co-MOF, NC-Co and NC-Co SA studied using SEM, HAADF-STEM, EDS and EELS analysis of NC-Co SA, N2 adsorption-desorption isotherms and XPS results of NC-Co and NC-Co SA, electrochemical test and battery test of NC-Co SA. ACKNOWLEDGEMENT


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This project of supported by the Ministry of Education (Singapore), grant number: MOE 2015-T2-2-094, conducted at the National University of Singapore. Z.L. acknowledges the financial support from Advanced Energy Storage Research Programme (IMRE/12-2P0503 and IMRE/12-2P0504), Institute of Materials Research and Engineering of the A*STAR, Singapore. REFERENCES (1)

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Figure 1. Illustration of the formation of NC-Co SAC nanoflake arrays on carbon cloth, and STEM-EELS analysis. (a) Schematic illustration of fabrication process, local magnification of obtained materials. (b) SEM image of Co MOF. (c) STEM image of NC-Co. (d) STEM image of NC-Co SA. (e) HAADF STEM image of Co atoms distributed across the nitrogendoped carbonaceous support. (f) EEL spectra taken at the bright atom in the red circle in (e)


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showing Co and N edges. (g) EEL spectra taken at the dark support area in the blue circle in (e) showing neither.


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Figure 2. Phase identification and surface chemical state of NC-Co SA and NC-Co. (a) XRD spectra. (b) XPS spectra of Co 2p in the sample of NC-Co SA. (c) XPS spectra of N 1s in the sample of NC-Co SA. (d) Raman spectra.


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Figure 3. Electrochemical characterization of the catalysts. (a) Oxygen reduction polarization curves at the rotation speed of 1600 rpm. (b) Oxygen reduction polarization curves of NC-Co SA at different rotation speeds and corresponding Koutecky-Levich plots. (c) Oxygen evolution curves. (d) Stability test of aqueous rechargeable Zn-air batteries using NC-Co SA and Pt/C as the air cathodes.


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Figure 4. Performance of flexible solid-state rechargeable Zn-air batteries. (a) Mechanical flexibility and stability tests with continuous mechanical alternation between flat and bent states. (b) Photograph of the three Zn–air batteries assembled with a minimum open circuit voltage of ≈4.232 V measured by a voltammeter. (c) Photograph of 6 LEDs powered by two assembled Zn–air batteries with different bending angles (0o, 90o, 180o). (d) Discharge and charge polarization curves. (e) Power-current density curves. (f) Comparison of the cycling stabilities in both bent and flat states.


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TOC


Single Co Atoms Anchored in Porous N-Doped Carbon for Efficient

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