Pyridinic-N-Dominated Doped Defective Graphene as

Pyridinic-N-Dominated Doped Defective Graphene as a Superior Oxygen Electrocatalyst for Ultrahigh-Energy-Density Zn−Air Batteries Qichen Wang,†,‡,⊥,○ Yujin Ji,∥,○ Yongpeng Lei,*,†,‡ Yaobing Wang,∇ Yingde Wang,§ Youyong Li,*,∥ and Shuangyin Wang*,# †

School of Aeronautics and Astronautics & Science and Technology on High Strength Structural Materials Laboratory, Central South University, Changsha 410083, China ‡ College of Basic Education, National University of Defense Technology, Changsha 410073, China ⊥ College of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha 410004, China ∥ Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Jiangsu 215123, China ∇ Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China § Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, National University of Defense Technology, Changsha 410073, China # State Key Laboratory of Chem/Bio-Sensing and Chemometrics, Provincial Hunan Key Laboratory for Graphene Materials and Devices, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China S Supporting Information *

ABSTRACT: Identification of catalytic sites for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in carbon materials remains a great challenge. Here, we construct a pyridinic-N-dominated doped graphene with abundant vacancy defects. The optimized sample with an ultrahigh pore volume (3.43 cm3 g−1) exhibits unprecedented ORR activity with a halfwave potential of 0.85 V in alkaline. For the first time, density functional theory results indicate that the quadri-pyridinic Ndoped carbon site synergized with a vacancy defect is the active site, which presents the lowest overpotential of 0.28 V for ORR and 0.28 V for OER. The primary Zn−air batteries display a maximum power density of 115.2 mW cm−2 and an energy density as high as 872.3 Wh kg−1. The rechargeable Zn−air batteries illustrate a low discharge−charge overpotential and high stability (>78 h). This work provides new insight into the correlation between the N configuration synergized with a vacancy defect in electrocatalysis.

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been experimentally and theoretically proved to efficiently catalyze reversible ORR/OER in alkaline electrolyte.21−25 Currently, it is regretful that there is no consensus on the actual catalytic mechanism of metal-free N-doped nanocarbon systems.26−28 It is reported that N doping induces uneven charge distribution of adjacent C atoms, facilitating ORR catalysis.29 Conversely, other reports believe that doped pyridinic-N creates the active sites (e.g., pyridinic-N-based

lectrochemical oxygen electrode catalysis, such as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), is especially crucial for next-generation renewable energy conversion applications.1−5 However, the sluggish kinetics of ORR/OER restrict the overall energy conversion efficiency. At present, the commonly used platinum (Pt) and ruthenium (Ru)/iridium (Ir) oxides cannot simultaneously catalyze ORR/OER or present unsatisfactory performance.6−12 To date, enormous efforts have been therefore spent on the development of bifunctional ORR/ OER catalysts based on earth-abundant elements.13−20 Metalfree catalysts (e.g., heteroatom-doped carbon materials) have © 2018 American Chemical Society

Received: February 21, 2018 Accepted: April 5, 2018 Published: April 5, 2018 1183

DOI: 10.1021/acsenergylett.8b00303 ACS Energy Lett. 2018, 3, 1183−1191

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Cite This: ACS Energy Lett. 2018, 3, 1183−1191

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Figure 1. (a) Schematic illustration of the fabrication process series of samples. (b) SEM, (c) TEM, (d) high-resolution TEM, (e) AFM image, (f) elemental mapping images, and (g) N2 adsorption/desorption isotherms of NDGs-800.

mechanism for ORR).30 Recently, the defect−activity relationship for ORR has been carefully discussed and summarized.31−34 For example, Yao et al. proposed that catalytic activity was dependent on the carbon defects (e.g., edge pentagon, 5−8−5 defect, etc.) within the structure instead of heteroatom doping.35−37 Also, Zhang et al. concluded that a curved configuration with a five-carbon ring adjacent to a seven-carbon ring (C5 + 7) exhibited a small overpotential. In general, it is a fact that defects often induced upon heteroatom doping cannot be ignored, which poses a huge challenge for the confirmation of active sites. Therefore, a clear understanding of the active sites (heteroatom doping, defect effect, or the synergistic effect of doping and defects) is essential for the synthesis of efficient ORR/OER catalysts. Besides, insufficient mass transport undoubtedly degrades the catalytic performance at high overpotential, during which the transport and consumption of O2 are vast. It is known that rotating disk electrode (RDE) measurements significantly improve O2 diffusion and minimize the mass transfer resistance owing to the forced convection at high rotating speeds (such as 1600 rpm).38,39 However, full-cell measurement is generally performed under static conditions, where reactant diffusion resistance is an extremely critical factor to deliver energy output. To solve this problem, constructing a hierarchically macro/mesoporous nanostructure with an open framework is necessary to promote fast mass exchanges and improve

transport kinetics.40,41 Recently, Duan et al. reported a threedimensional (3D) Nb2O5/HGF composite for ultrahigh-rate energy storage at practical levels of mass loading, giving emphasis to the critical role of porosity in mass transport.42 Thus, rational design of highly efficient metal-free carbon-based catalysts featured with a hierarchical porous structure and prominent activity is preferentially desirable but still a fundamental challenge. As known, the reduced graphene oxides have plenty of structural defects (such as vacancies etc.).43−45 It is anticipated to introduce more targeted pyridinic-N to defective graphene. Herein, we report a novel 3D defective graphene enriched with pyridinic-N, exhibiting superior bifunctional ORR/OER performance. For the first time, density functional theory (DFT) calculations reveal that the quadri-pyridinic N-doped carbon site synergized with a vacancy defect is the reactive site with the lowest overpotential for ORR (0.28 V) and OER (0.28 V). As a proof-of-concept, the constructed primary/rechargeable Zn−air batteries display better performance than those of noble-metal-based Zn−air batteries. The fabrication process is illustrated in Figures 1a and S1. Field-emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM) images in Figure 1b,c reveal the open porous structure and typical graphene character of the as-prepared NDGs-800. Further high-resolution TEM (HRTEM) characterization in Figure 1d indicates the few-layer 1184


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Figure 2. (a) Raman spectra of different samples. (b) XPS survey spectra. (c) High-resolution N 1s spectrum of the NDGs-800. (d) Distribution of pyridinic-N, pyrrolic-N, graphitic-N, and pyridinic N+−O− obtained from the N 1s spectra of different samples.

ratio values of ID/IG were calculated to be 1.23−1.09, indicating the highly defective structure including vacancies etc. In the Xray photoelectron spectroscopy (XPS) spectra (Figure S5), no signal of metal species (Mn) was detected because the GO was prepared by an improved Hummers method. In Figure 2b, the C 1s peak of NDGs-800 can be fitted to C−C (284.7 eV), C− N (285.9 eV), and CO (288.6 eV).52 Meanwhile, the highresolution N 1s spectrum (Figure 2c) shows four fitted peaks at around 398.4, 399.7, 400.9, and 403.0 eV, associated with pyridinic-N, pyrrolic-N, graphitic-N, and pyridinic N+−O−, respectively.53 The overall N content decreases from 11.04 to 1.48 atom %. Specifically, NDGs-800 possesses the highest proportion of pyridinic-N (47.9%) among all of the NDGs-x samples (Figure 2d). We speculate that g-C3N4 prefers to react with defects of GO to promote the formation of pyridinic-N during thermal treatment.54,55 Such unique pyridinic-Ndominated doping and rich defects are expected to make a great contribution for ORR/OER. The catalytic ORR activity of NDGs-x and Pt/C catalysts was measured. The linear sweep voltammetry (LSV) result demonstrates that NDGs-800 has unprecedented ORR activity with an onset potential (Eonset) of 0.98 V (vs reversible hydrogen electrode) and a half-wave potential (E1/2) of 0.85 V (Figure 3a). Additionally, the NDGs-800 also reaches a higher current density of 5.6 mA cm−2 at 0 V. The Jk of NDGs-800 (13.91 mA cm−2) at 0.8 V is higher than that of Pt/C (13.32 mA cm−2), NDGs-900 (6.03 mA cm−2), NDGs-600 (5.55 mA cm−2), and NDGs-700 (2.80 mA cm−2) (Figure 3b). Such superior performance makes NDGs-800 one of the best ORR

feature. The atomic force microscopy (AFM) analysis in Figure 1e shows a nanosheet structure with a thickness of 3.0 nm, corresponding to about nine layers of graphene sheets. Also, the elemental mapping images of NDGs-800 (Figure 1f) confirm the homogeneous distribution of C, N, and O species on a graphene scaffold. A local graphene-like structure with N doping is believed to alter its properties. For instance, in comparison to the electrical conductivity (1.81 S cm−1) of DGs-800 prepared via heating GO at 800 °C in N2, the electrical conductivity of NDGs-800 was much higher (3.07 S cm−1). Furthermore, the porosity of NDGs-x was studied in Figure S2. All samples show a typical type-IV isotherm curve with an obvious hysteresis, implying the presence of a mesoporous structure.46,47 Figure 1g displays a BET surface area of 443.2 m2 g−1 and an ultrahigh pore volume of 3.43 cm3 g−1 for NDGs-800. Also, the rapid N2 rise in the P/P0 > 0.9 region indicates the existence of much larger pores, arising from the 3D self-assembled DGs sheets. In contrast, two comparison samples (NDGs-800-1# and NDGs-800-2#) show decreased BET surface area and pore volume (Figure S3). We believe that the highly interconnected 3D graphene framework with excellent charge (electron and ion) transport capability facilitates effective exposure of more accessible active sites. A broad peak at 25.5° in the X-ray diffraction (XRD) pattern was assigned to the (002) of graphitic carbon for NDGs-800 (Figure S4).48,49 Figure 2a presents Raman spectra of NDGs-x under the 532 nm laser, with a typical D-band (∼1350 cm−1) and G-band (∼1580 cm−1) corresponding to the disorder and the vibration of sp2-bonded carbon atoms, respectively.50,51 The 1185


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Figure 3. (a) LSV curves of NDGs-x and Pt/C catalysts for ORR in 0.1 M KOH. Scan rate: 5 mV s−1. (b) Comparison of the kinetic current density (Jk) and E1/2 of NDGs-x and Pt/C catalysts. (c) LSV curves at different rotation speeds from 400 to 1600 rpm for NDGs-800. (d) Corresponding K−L plots of NDGs-800. (e) LSV curves of DGs-800, NDGs-800, RuO2/C, and Pt/C catalysts for OER in 1 M KOH. Scan rate: 2 mV s−1. (f) Corresponding Tafel plots for OER catalysis.

S11−S15). Furthermore, the mechanism of ORR and OER is investigated based on first-principles calculations in different pyridinic-N-contained configurations synergized with common vacancy defects. As shown in Figure 4a, seven types of pyridinic-N configurations (i.e., 1N, 2N, 3N-1, 3N-2, 4N, 5N, and 6N) in the graphene model were constructed at the edge of the vacancy defect. According to analysis of the molecular orbital, the C and N at the edge mainly contribute to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) shown in Figure 4e, while the carbon atom bonded with pyridinic-N will become the potential active site due to electron transfer from the N to C atom. Then, the optimal free energy reaction pathway was calculated, and the overpotential of each site is given to reflect the practical performance of ORR and OER. Figure 4b demonstrates that the 4N (quadri-pyridinic N) configuration exhibited the best OER/ORR performance due to the lowest overpotential of 0.28/0.28 V, while the 2N site is followed because the overpotential (0.33 V) of OER is relatively higher than that (0.28 V) of ORR, indicating that the 2N site is propitious for ORR. However, according to the formation energy shown in Table S2, 3N-2 configurations are the most common, with the overpotential of OER/ORR corresponding to 0.52/0.52 V, which might be the reason why our NDG catalysts are better than other traditional N-doped nanocarbon systems, and more exposure of the 2N/4N site will be responsible for the distinguished performance. Further, the ORR/OER reaction pathway is given in Figures 4c and S16. The formation of intermediate *OOH adsorption and *OH desorption tend to be the rate-limiting step during ORR and OER (Figure 4d). Previous research shows that the edge effect of pyridinic-N (1N-R) would enhance the catalytic activities of

metal-free catalysts, even surpassing the most nonprecious metal-based electrocatalysts reported.17,37,56−60 The increased current plateau ranging from 0.6 to 0 V with an increment of rotating speed represents a surface-controlled kinetics process (Figure 3c). Accordingly, the value of the electron transfer number (n) based on the Koutecky−Levich (K−L) equation for NDGs-800 was estimated to be close to 4.0 in Figure 3d, implying an ideal four-electron ORR pathway with a high catalytic efficiency. In addition, the superior catalytic performance is also confirmed by a lower Tafel slope of 81 mV dec−1 (Figure S6). The chronoamperometric response (i−t) and accelerated degradation test demonstrate the excellent stability of NDGs-800 (Figure S7). The acid ORR activity of NDGs-800 was also measured in 0.5 M H2SO4 (Figure S8). We then investigated the electrocatalytic OER performance. The required overpotential to reach a current density of 10 mA cm−2 is 450 mV (Figure 3e), which is slightly larger than that of RuO2/C (375 mV). The Tafel slope of 132 mV dec−1 (Figure 3f) for NDGs-800 demonstrates a good kinetic process. The NDGs-800 also maintains good catalytic stability to at least 40000 s (Figure S9). The OER activity of NDGs-800 still cannot catch up with RuO2/C, which is a crucial problem needed to be tackled in future study. The excellent ORR/OER was comparable to those of known bifunctional electrocatalysts reported previously (Table S1).17,34,59−67 To identify the N doping states (p-type or n-type), the slopes of Mott−Schottky plots (Figure S10) in the p-type region are much lower than those of the n-type region, indicating more positive charge carrier density due to the electron-withdrawing capability of pyridinic-N.60 In order to unfold the distinguished performance of NDGs-800, we performed a series of experiments described in the Supporting Information (Figures 1186


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Figure 4. (a) Seven types of pyridinic-N-contained sites (1N, 2N, 3N-1, 3N-2, 4N, 5N, and 6N) in the graphene model and (b) corresponding overpotential versus adsorption energy of *OH along the ORR and OER pathways without considering the effect of pH. (c) Calculated Gibbs free energy diagrams of ORR and OER in the 4N (quadri-pyridinic N) site and (d) optimized adsorption configurations of ORR/OER intermediates (*OOH, *O, and *OH). (e) HOMO and LUMO distribution in the 4N site. Gray, blue, red, and white balls represent the C, N, O, and H atoms, respectively.

graphene.34,37 In order to reveal the distinct chemical activity of quadri-pyridinic N, the difference of the HOMO and LUMO was calculated because the HOMO−LUMO gap plays a vital role in the catalytic performance of ORR/OER.68,69 A smaller HOMO−LUMO gap will lead to stronger adsorption of *OOH and *OH with a lower overpotential due to more filling of the bonding orbital. In Figure S17, it is found that the HOMO−LUMO gap has a linear relationship with the performance of ORR/OER in N-doped configurations, and the distinct activity of quadri-pyridinic N is attributed to its lower HOMO−LUMO gap compared other pyridinic-N and graphitic-N configurations. The seven pyridinic-N configurations in our work could further improve OER/ORR performance and thus demonstrate excellent performance. Besides, it is widely accepted that the electrochemical active surface area (ECSA), estimated from the double-layer capacitance (Cdl), makes a great contribution to enhanced electrochemical activity for nanostructured catalysts.70 As a result, NDGs-800 displays the highest Cdl of 18.2 mF cm−2 in Figure S18, larger than that of NDGs-600 (13.0 mF cm−2), NDGs-700 (8.3 mF cm−2), and NDGs-900 (17.0 mF cm−2), confirming the better exposure and enhanced utilization of active sites of NDGs-800. As a proof-of-concept, we assembled primary Zn−air batteries (Figure S19) with an NDGs-800-loaded carbon cloth/gas diffusion layer as the air cathode. The open-circuit voltage (OCV) and maximum power density are 1.45 V and 115.2 mW cm−2, respectively, superior to those of Pt/C (1.43 V; 110.3 mW cm−2) (Figure 5a). In Figure 5b, we also compared the maximum power density vs discharge current

density with other advanced ORR catalysts.34,56,60,67,71−77 Moreover, the discharge voltage platforms at different current densities are more stable than those of Pt/C-based Zn−air batteries (Figure S20). The specific capacity normalized to the weight of the consumed Zn electrode was calculated to be 750.8 mAh g−1 at a constant current density of 10 mA cm−2, corresponding to a much higher energy density of 872.3 Wh kg−1, which is one of the highest values among carbon-based materials. Meanwhile, we also constructed rechargeable Zn−air batteries owing to the excellent ORR/OER activity of NDGs800, and a rechargeable Zn−air battery based on the mixture of Pt/C + Ir/C (1:1 by weight) catalysts was also tested as a reference. As shown in Figure 5c, the discharge−charge overpotential of NDGs-800 is 0.76 V at a current density of 10 mA cm−2, slightly larger than that of the Pt/C + Ir/C counterpart (0.68 V), suggesting efficient reversibility of the rechargeable Zn−air batteries. In Figure 5d, when cycled at a constant current density of 10 mA cm−2 at 20 min per cycle, the voltage difference remains stable after more than 78 h (up to 234 cycles). Oppositely, the Pt/C + Ir/C shows fast activity decay at the same condition. The batteries with a Pt/C + Ir/C electrode exhibited a voltage gap increase of 0.24 V, almost three times that of the NDGs-800 electrode (0.08 V). These results sufficiently prove the robust stability of NDGs-800. In general, such a distinguished result highlights that the 3D hierarchical porous architecture in the NDGs-800 electrode leads to a much lower internal resistance, thus showing much less discharge−charge voltage degradation.32 As an illustration, red light-emitting diodes (LED, 3.0 V) can be powered by two 1187


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Figure 5. (a) Galvanostatic discharge voltage and power density curves of the single Zn−air battery with NDGs-800 and Pt/C as air cathodes. (b) Maximum power density and corresponding discharge current density as well as comparison with previous works. (c) Charge and discharge polarization curves of rechargeable Zn−air batteries. (d) Charge−discharge cycling performance of rechargeable Zn−air batteries at a constant charge−discharge current density of 10 mA cm−2. (e) Photograph of a two-series liquid Zn−air red LED light (≈3.0 V).

series-connected Zn−air batteries using NDGs-800 as the aircathode (Figure 5e). In summary, we have prepared a novel pyridinic-Ndominated doped defective graphene toward efficient oxygen electrocatalysis. The quadri-pyridinic N-doped carbon site synergized with a vacancy defect acts as the active site, displaying the lowest overpotential for ORR (0.28 V) and OER (0.28 V). Furthermore, the assembled Zn−air batteries could deliver a maximum power density of 115.2 mW cm−2 and an energy density as high as 872.3 Wh kg−1. The corresponding rechargeable Zn−air batteries display a low discharge−charge overpotential value and excellent stability (more than 78 h). In addition, the significance of this work is new insight into the correlation between a quadri-pyridinic N-doped carbon site synergized with a vacancy defect and ORR/OER catalysis, which also provides a platform to support various electrochemically active species (e.g., oxides, carbides, sulfides, etc.) to fabricate more efficient and robust electrocatalysts applied in energy conversion devices.

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formation energy of seven types of pyridinic-N-doped active sites (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y. Lei). *E-mail: [email protected] (Y. Li). *E-mail: [email protected] (S. Y. Wang). ORCID

Yongpeng Lei: 0000-0002-8061-4808 Yaobing Wang: 0000-0001-6354-058X Youyong Li: 0000-0002-5248-2756 Author Contributions ○

Q.C. Wang and Y.J. Ji contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT

ACKNOWLEDGMENTS Yongpeng Lei thanks the Research Project of NUDT (ZK1603-32). Yingde Wang acknowledges support from the National Natural Science Foundation of China (51773226). The authors also acknowledge support from the Provincial Natural Science Foundation of Hunan (Grant no. 2016TP1009).

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b00303. Experimental section, computation details, photographs, SEM, BET, XRD, XPS, and Tafel plots, stability data for ORR and OER, Mott−Schottky plots, Raman spectra, LSV curves, schematic and photograph of the rechargeable Zn−air battery, galvanostatic discharge curves, table of the electrocatalytic activities of the recently reported bifunctional catalysts for ORR/OER, and table of the

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