N-Doped Carbon Nanosheet Networks with Favorable Active Sites

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Letter

N-doped Carbon Nanosheet Networks with Favorable Active Sites Triggered by Metal Nanoparticles as Bifunctional Oxygen Electro-catalysts Xiaoxiao Huang, Yelong Zhang, Haoming Shen, Wei Li, Tong Shen, Zeeshan Ali, Tianyu Tang, Shaojun Guo, Qiang Sun, and Yanglong Hou ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01717 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 6, 2018

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N-doped Carbon Nanosheet Networks with Favorable Active Sites Triggered by Metal Nanoparticles as Bifunctional Oxygen Electro-catalysts Xiaoxiao Huang†,§,⊥, Yelong Zhang⊥, Haoming Shen⊥, Wei Li†,§,⊥, Tong Shen†, §,⊥,

Zeeshan Ali†,§,⊥, Tianyu Tang†,§,⊥, Shaojun Guo⊥, Qiang Sun⊥,*, Yanglong

Hou†,§,⊥,* † Beijing

Innovation Center for Engineering Science and Advanced Technology

(BIC-ESAT), § Beijing

Key Laboratory for Magnetoelectric Materials and Devices (BKLMMD),

⊥Department

of Materials Science and Engineering, College of Engineering,

Peking University, Beijing 100871, China Corresponding Author * [email protected]; [email protected].

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ABSTRACT

Developing noble metal free bifunctional oxygen electro-catalysts is vital for metal-air batteries. Herein, we present a facile approach to fabricating N-doped carbon nanosheet networks with metal nanoparticles (M/N-CNSNs) readily converted from metal-organic frameworks. The resultant Co/N-CNSNs show superior bifunctional oxygen catalytic activity attributed to the efficient active sites and fast mass diffusion enabled by the nanosheet structure. It is worth noting that the first principle studies proved the Co/N-C sites to be the oxygen reduction reaction active sites, where the most favorable ones are the carbon atoms next to Co coordinated pyridinic N. Interestingly, the cobalt content plays an important role in Co/N-C sites but was not directly involved in the catalytic process. In a Zn-air battery, small voltage gap without obvious voltage loss is found for the Co/N-CNSNs. This facile approach enables scalable synthesis, representing an essential step towards the popularization of metal-air batteries.

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Developing cost-effective and earth abundant bifunctional electro-catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is vital for the

commercialization

of

Zn-air

batteries.1-2

Carbonized

metal-organic

frameworks (MOFs) with abundant transition metal-nitrogen/carbon (M-N/C) sites have been explored with impressive oxygen electro-catalytic performance.3-4 However, an activity gap still exists between the traditional MOF-derived catalysts and noble-metal catalysts.5 The inferior catalytic performance is partially due to their reduced exposure of active sites and increased mass diffusion resistance, which is caused by the destruction of MOF-intrinsic pore structure during the pyrolysis process.6-7 To solve this problem, MOF-derived nanosheet networks with high surface area and abundant mesopores are expected to offer a favorable mass transport platform and efficient exposure of active sites. Herein, we present an approach for fabricating N-doped carbon nanosheet networks with metal (M/N-CNSNs, M=Co, Ni). It is worth noting that the dodecahedral ZIF-8 can be readly converted to 2D nanosheets by a room-temperature hydrolysis strategy, followed by a pyrolysis process to produce M/N-CNSNs. The Co/N-CNSNs present superior ORR and OER performance with excellent stability and tolerance to methanol. Density functional theory (DFT) calculations reveal that the most favorable ORR active sites are the carbon atoms next to Co coordinated pyridinic N (PD-C), where the cobalt content in 4 ACS Paragon Plus Environment

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Co/N-Cs is part of the active sites. Furthermore, the Zn-air battery with Co/N-CNSNs electrode exhibits larger open circuit voltage and slighter performance loss than that of the Pt/C+IrO2.

Figure 1. (a) Schematic illustration of the fabrication of M/N-CNSNs (M=Co, Ni). (b) -(e) TEM images of ZIF-8, intermediate product in hydrolysis, 2-MI-LDHs and Co/N-CNSNs, respectively. The synthetic route of M/N-CNSNs from ZIF-8 includes a hydrolysis and pyrolysis processes as illustrated in Figure 1a. To synthesize Co/N-CNSNs, the polyhedral ZIF-8 (Figure 1b, S1)

8

was dispersed in aqueous cobalt ions solution

during the hydrolysis process.9 Subsequently, wrinkled nanosheets were observed around ZIF-8 (Figure 1c). The nanosheets extended progressively accompanied by the gradual disappearance of ZIF-8, producing only nanosheets 5 ACS Paragon Plus Environment


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eventually (Figure 1d). The hydrolysis derived nanosheets present flexible, wrinkled lamellar morphology (Figure S2 and 3), which is denoted as 2-MI-LDHs. Subsequently, 2-MI-LDHs can be pyrolyzed into Co/N-CNSNs (Figure 1e), in which Zn2+ ions will evaporate due to its low boiling temperature.10 Furthermore, ten times of the precursor weights were employed in a single batch to demonstrate the scalability of this approach. The resulting product weighs as high as ~1 g without phase change or morphology damages (Figure S4). Notably, the strategy can also be generalized for Ni species (Figure S5). The hydrolysis-derived nanosheets present wrinkled lamellar morphology, which can be pyrolyzed into Ni/N-CNSNs.


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Figure 2. (a-b) SEM images, (c-d) HRTEM images and (e-h) HAADF and corresponding EDS mapping images of Co/N-CNSNs. (i) XRD patterns and (j) N2 sorption isotherms of Co/N-CNSNs, Co/N-CNFs, and N-CNFs, where * corresponds to the (002) plane of graphitic carbon (PDF#41-1487). (k) Raman spectra of Co/N-CNSNs, Ni/N-CNSNs, Co/N-CNFs and N-CNFs. In Figure 2a and b, scanning electron microscopy (SEM) images show that the resultant Co/N-CNSNs display wrinkled lamellar morphology with uniform distribution of nanoparticles (NPs).11 In Figure 2c and d, the high resolution



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transmission electron microscopy (HRTEM) images illustrate that the Co NPs are surrounded by few layers of graphitic carbon shells. The lattice fringes with

d-spacing of 2.05 Å and 3.37 Å correspond to Co (111) and C (002), respectively. High-angle annular dark-field (HAADF) and energy dispersive spectrometer (EDS) mapping analysis also confirm the uniform distribution of N dopant on the carbon nanosheets (Figure 2e-h). In Figure 2i, X-ray diffraction (XRD) patterns of Co/N-CNSNs present (111), (200), and (220) peaks of metallic Co and the (002) plane of graphitic carbon. Carbon nano-frameworks with Co nanoparticles (Co/N-CNFs) were also synthesized by annealing the Co2+ salt and ZIF-8, presenting the same peaks as Co/N-CNSNs. The metallic Co is produced by reducing Co2+ ions and the graphitic carbon is generated by Co-assistant graphitization in the pyrolysis process.12-13 XRD pattern of N-doped carbon nano-frameworks (N-CNFs, synthesized by annealing ZIF-8) presents no cobalt peaks and broader carbon peak for the absence of Co. The Brunauer-Emmett-Teller (BET) surface area and pore distribution of Co/N-CNSNs investigations (Figure 2j) show that the surface area of Co/N-CNSNs reaches 483.96 m2/g with mesopores extending to ~40 nm (Figure S6).14 In comparison, Co/N-CNFs show much smaller surface area of 242.75 m2/g and less mesopores (Table S1 and Figure S6), resulting from the destruction of pore structure during pyrolysis. The N-CNFs exhibit much larger 8 ACS Paragon Plus Environment

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surface area of 1856.30 m2/g due to the absence of Co. In Figure 2k, the intensity value ratios of D and G bands (ID/IG) are both 1.01 for Co/N-CNSNs and Co/N-CNFs, respectively, revealing the formation of highly ordered graphitic structure.15 The ID/IG of Ni/N-CNSNs (0.88) is lower due to nickel’s better catalytic property for carbon’s graphitization.16 Whereas, the N-CNFs present higher ID/IG of 1.08 for less graphitization of carbon, which is good agreement with the XRD results. The ORR activity of M/N-CNSNs catalysts was investigated in an O2-saturated alkaline solution. The M/N-CNSNs and Co/N-CNFs present an ORR peak in the cyclic voltammetry (CV) curves respectively in Figure S7a, implying their ORR catalytic property. The Ni/N-CNSNs show inferior onset potential (Eonset) of 0.88V than Co/N-CNSNs due to the intrinsic properties of metals.

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The

high-performance Co/N-CNSNs present an Eonset of 0.98 V and a half-wave potential (E1/2) of 0.84 V, comparable to those of the commercialized Pt/C (Eonset =0.98 V and E1/2 =0.83 V, Figure 3a and b). To reveal the role of the nanosheets structure, ORR performance of Co/N-CNFs was also evaluated. In Figure 3a and b, the Co/N-CNFs present almost similar Eonset to Co/N-CNSNs with much lower

E1/2 and current density, with similar electrochemical impedance (Figure S7b). Co/N-CNSNs possess a higher double-layer capacitance (Cdl) of 18.47 mF/cm2 than that of Co/N-CNFs (11.00 mF/cm2) as shown in Figure 3c and S7c, d and e. 9 ACS Paragon Plus Environment


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In Figure S7f, the Co/N-CNSNs present a smaller Tafel slope (70 mV/dec) than those of Co/N-CNFs (99 mV/dec) and Pt/C (85 mV/dec), indicating the faster reaction kinetics of Co/N-CNSNs in ORR.These results evidence the superior catalytic property of Co/N-CNSNs is credited to sufficient exposure of active sites and efficient mass diffusion, benefiting from the large surface area and abundant mesopores of the nanosheet networks. To further investigate the reaction kinetics during ORR, LSV curves at various rotating speeds with corresponding Koutechy-Levich (K-L) plots of Co/N-CNSNs were collected in Figure 3d. The electron transfer number (n) of Co/N-CNSNs is calculated to be around 4 from 0.3 V to 0.6 V, suggesting a dominant 4 e- ORR catalytic process. In Figure 3e, the current-time (i-t) curve shows that the addition of 3 M methanol causes a sharp decrease in the current of Pt/C, while Co/N-CNSNs present no obvious change. In Figure 3f, the Co/N-CNSNs deliver a slight current loss with retention of more than 90% after 35,000 s, exceeding that of Pt/C (70 %). TEM images show the catalyst remain undamaged after the stability test (Figure S8a and b), benefiting from the protection of graphitic carbon layers.3, 18


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Figure 3. (a) LSV curves and (b) corresponding E1/2 and Eonset of the Co/N-CNSNs, Ni/N-CNSNs, Co/N-CNFs and Pt/C in O2-saturated 0.1 M KOH at 1600 rpm. (c) Plots of the extraction of the Cdl from CV curves at various scan rates. (d) RDE voltammograms of Co/N-CNSNs at various rotation speeds and corresponding K-L plots. (e) Chronoamperometric responses of Co/N-CNSNs 11 ACS Paragon Plus Environment


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and Pt/C at 0.60 V in O2-sarurated 0.1 M KOH followed by addition of 3 M methanol. (f) i-t responses of Co/N-CNSNs and Pt/C at 0.60 V in O2-saturated 0.1 M KOH at 400 rpm. To identify the ORR active sites of Co/N-CNSNs, we first characterized the elemental chemical states by X-ray photoelectron spectroscopy (XPS). In Figure 4a, the regional Co 2p spectrum shows a higher energy band of Co 2p1/2 and a lower energy band of Co 2p3/2, including Co(0), Co(II) and Co-N.19-20 In Figure 4b, the high resolution N 1s spectrum shows the presence of Co coordinated pyridine-N (Co-N, slight upshift from pristine pyridine-N (PD-N)), pyrrolic N (PR-N), graphitic N (G-N).21-22 In Figure 4c, the N-CNFs (Figure S9a) show much inferior ORR catalytic performance to Co/N-CNSNs even with larger surface area and more N dopant. These results illustrate the vital role of Co/N-Cs in ORR catalysis.


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Figure 4. (a) Co 2p and (b) N1s high resolution XPS spectra of Co/N-CNSNs. (c) LSV curves of the pristine, SCN- poisoned and acid-treated Co/N-CNSNs, N-CNFs in O2 saturated 0.1 M KOH. (d) Schematic illustration for DFT simulation of Co/N-CNSNs (blue balls for Co, gray balls for C, yellow balls for N). (e) ∆G of reaction coordinates on various C sites, inset: adsorption structure of PD-C with red balls for O and white balls for H. (f) ∆G1 and ∆G2 of various active sites. Whether the metal is part of the active sites is one of the hot debates.23 In order to explore the role of metal, a harsh acid leaching process was adopted to remove the encapsulated Co NPs (Figure S9b and c), which resulted in severe performance decay of ORR activity (Figure 4c).24 Furthermore, DFT simulations were also carried out to explore the role of Co with a slab model to represent the



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Co/N-CNSNs surface (Figure 4d). In Figure S9d, N-doped carbon shell is adopted to represent Co/N-CNSNs after acid-etching. Gibbs free energetics (∆G) of the adjacent carbon of PD-C were investigated for its potential as active sites,25 among which the 1st and 2nd steps are generally considered with higher

∆G among the five elementary steps of ORR.24 In Figure 4e and Table S2, the corresponding ∆G1 and ∆G2 (0.24 eV and 0.41 eV) of N-doped carbon shells illustrate a higher energy barrier than the one with cobalt (0.19 eV and -0.36 eV), implying the key role of cobalt in active sites. Meanwhile, little activity deterioration was caused after SCN− ions selectively attacking the metal centres (Figure 4c), indicating that the outer surface of the networks are free of Co species.26-27 We can draw the conclusion that despite the encapsulated cobalt nanoparticles are not in direct contact with the electrolyte and reactant, they still play a key role in the catalysis. The encapsulated cobalt nanoparticles can activate the surrounding N-doped graphitic layers, making the outer surface of the carbon layer active towards the ORR. 27 DFT simulations were also carried out to explore the most favourable active sites within Co/N-Cs. ∆G1 and ∆G2 of PR-N, PD-N, G-N and their adjacent C (PR-C, PD-C, and G-C) as well as the graphene C (GE-C) were investigated. Consequently, The PD-C atoms coordinated to underlying Co with strong bonding to OOH* exhibit much lower free energy of -0.67 eV (∆G1 + ∆G2) than 14 ACS Paragon Plus Environment

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that of the others (-0.33 eV to -0.10 eV) (Figure 4f and Table S2). On the PD-C sites, O2 can be converted into H2O following an associative mechanism via a four-electron transfer pathway (Figure 4e). The most favorable active sites of Co/N-CNSNs are the cobalt coordinated PD-Cs.

Figure 5. (a) OER polarization curves and (b) corresponding Tafel plots of Co/N-CNSNs, Co/N-CNFs and IrO2 in O2-saturated 0.1 M KOH solution. (c) i-t responses of Co/N-CNSNs and IrO2 at 1.50 V vs RHE in O2-saturated 0.1 M KOH at 400 rpm. We further investigated the electro-catalytical OER activity of these Co/N-CNSNs, Co/N-CNFs and IrO2. As shown in Figure 5a, the Co/N-CNSNs deliver a current density of 10 mA/cm2 at a potential of 1.57 V, whereas higher potentials of 1.66 V and 1.59 V are required for Co/N-CNFs and IrO2. The OER kinetics of these two electrodes have been further estimated by Tafel plots. In Figure 5b, the Tafel slope of the Co/N-CNSNs is 88 mV/dec, much lower than that of IrO2 (97 mV/dec) and Co/N-CNFs (135 mV/dec). The lowest Tafel slope of 15 ACS Paragon Plus Environment


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the Co/N-CNSNs implies the favorable OER kinetics. The outstanding OER performance of Co/N-CNSNs can be attributed to the positively charged carbon atoms active sites for OER. 28 Furthermore, the high surface area and abundant mesopores of the Co/N-CNSNs could boost reaction kinetics and mass transport in OER.28 Catalyst stability of bifunctional catalysts are crucial and challenging, especially for carbon based ones. Nano-carbon based electro-chemical catalysts are sensible to high potential since carbon is thermo-dynamically unstable when the potential is high.29 Furthermore, the current of the Co/N-CNSNs at a fixed potential of 1.50 V has been monitored for 30,000 s (Figure 5c) with remaining of 79%, larger than that of IrO2. The outstanding stability of the OER catalyst is believed from the protection of highly graphitic and thick outer carbon shells.29-30 Based on the excellent bifunctional catalytic performance for OER and ORR, the home-made Zn−air battery was fabricated to assess the practical applicability of Co/N-CNSNs (Figure 6a). The Co/N-CNSNs-based Zn-air battery possesses an open circuit voltage of 1.471 V (Figure 6b), larger than that of the Pt/C+IrO2 battery under the same conditions (1.389 V, Figure S10). Note that the Co/N-CNSNs-based Zn−air battery reaches a specific capacity of 638.4 mAh g−1 and gravimetric energy density of 703.2 Wh kgZn−1, which is larger than that of the Pt/C+IrO2 battery (specific capacity of 532.7 mAh g−1; energy density of 562.9 Wh kgZn−1, Figure S11). Moreover, a commercial LED can be illuminated by two 16 ACS Paragon Plus Environment

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Co/N-CNSNs-based Zn–air batteries in series, with no obvious brightness decay over 2.5 h (Figure S12 and 6c). Notably, the Co/N-CNSNs battery can achieve a maximum power density of 81.7 mW/cm2 at 140.0 mA/cm2, far exceeding that of Pt/C+IrO2 (73.1mW cm−2 at 110.0 mA/cm2) in Figure S13, demonstrating outstanding applicability for both high-energy densities and high-power capabilities needs.

Figure 6. (a) Scheme of a rechargeable Zn–air battery. (b) and (c) Photographs of the Zn–air battery employing the Co/N-CNSNs cathode and commercial LED driven by two Co/N-CNSNs-based Zn−air batteries. Charge/discharge profiles of the rechargeable Zn–air battery with the (d, e) Co/N-CNSNs and (f, g) 17 ACS Paragon Plus Environment


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commercial Pt/C + IrO2 catalysts at a current density of 10 mA/cm2 (20 min per cycle). The stability and rechargeability of the batteries were also explored in Figure 6d-g. Obviously, a smaller voltage gap without any obvious voltage loss is found in 100 continuous cycles (over 33 h) for the Co/N-CNSNs air cathode compared to that of Pt/C+IrO231 cathode in Figure 6d and f. In the initial cycle processes at 10 mA/cm2, the potential of the battery with Co/N-CNSNs (1.223 V) cathode is also larger than batteries using Pt/C+IrO2 (1.114 V). The Co/N-CNSNs battery delivers an initial voltage gap of 0.793 V and a high voltaic efficiency of 60.6%. Moreover, slighter performance loss has been found on the Co/N-CNSNs cathode after 100 cycles than that of the commercial cathode (Figure 6e and g). Conspicuously, the rechargeable performance of the Zn–air battery with Co/N-CNSNs is also comparable to the recently reported batteries with various catalysts (Table S3). The superb performances of Co/N-CNSNs air cathode can be mainly ascribed to their nanosheets structure as well as abundant reactive sites. In conclusion, a synthetic route of N-doped carbon nanosheet networks with metal NPs has been developed by a combination processes of hydrolysis and pyrolysis. The resultant Co/N-CNSNs show superior ORR and OER activity,


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benefiting from the sufficient exposure of active sites and efficient mass diffusion for its nanosheet network structure. Note that the Co/N-Cs consisted of the Co NPs, graphitic carbon, and N dopants are proved to be ORR active sites, where the most favorable ones are the cobalt coordinated PD-Cs. The cobalt content in Co/N-Cs is part of the active sites, but not directly involved in the catalytic process. In a practical demonstration, the Zn–air battery with Co/N-CNSNs presents a small voltage gap and slight performance loss. Moreover, the stated approach is also scalable, representing an essential step-forward for the developing electrochemical catalysts. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXXXXXXXXXX. A description of the material included experimental details; electrochemical measurements; DFT Models and Methods; SEM, TEM, BET and XRD results of ZIF-8; SEM, TEM, XRD, FT-IR and EDS analysis of 2-MI-LDHs; Pictures, TEM image and XRD patterns of Co/N-CNSNs after enlargement. SEM, TEM, XRD and BET results of Ni/N-CNSNs; CV curves of Co/N-CNSNs, Co/N-CNFs and Ni/N-CNSNs; EIS curves of the Co/N-CNFs and Co/N-CNSNs;; CV curves at various scan rates and Tafel plots of Co/N-CNSNs and Co/N-CNFs; TEM images of Co/N-CNSNs after current-time test, 19 ACS Paragon Plus Environment


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Co/N-CNFs, N-CNFs, and acid treated Co/N-CNSNs. Photograph, discharge curves and polarization curves of Zn-air batteries assembled from the CoN-CNSNs and Pt/C+IrO2 catalysts; BET surface area, Raman analysis (ID/IG) and chemical composition from XPS results of Co/N-CNSNs, Co/N-CNFs, N-CNFs and Ni/N-CNSNs; Gibbs free energy of various sites after the five elementary steps; The performance of rechargeable Zn−air batteries with various electrocatalysts. Acknowledgements We acknowledge the financial support from the National Natural Science Foundation of China (51631001, 51590882, 51672010, 81421004) and the National Key R&D Program of China (2017YFA0206301, 2016YFA0200102). References: (1) Han, S.; Hu, X.; Wang, J.; Fang, X.; Zhu, Y. Novel route to Fe-based cathode as an efficient bifunctional catalysts for rechargeable Zn-air battery. Adv. Energy

Mater. 2018, 1800955. (2) Lambert, T. N.; Vigil, J. A.; White, S. E.; Davis, D. J.; Limmer, S. J.; Burton, P. D.; Coker, E. N.; Beechem, T. E.; Brumbach, M. T. Electrodeposited Ni(x)Co(3-x)O4 nanostructured films as bifunctional oxygen electrocatalysts. Chem. Commun. 2015, 51, 9511-9514.


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