Letter Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Amorphous MOF Introduced N‑Doped Graphene: An Efficient and Versatile Electrocatalyst for Zinc−Air Battery and Water Splitting Wenhan Niu† and Yang Yang*,†,‡ †
NanoScience Technology Center, University of Central Florida, Orlando, Florida 32826, United States Department of Materials Science and Engineering, University of Central Florida, Orlando, Florida 32826, United States
‡
S Supporting Information *
ABSTRACT: Recently, developing metal−organic framework (MOF) derived carbon-based electrocatalysts has become more and more popular for large-scale application of renewable energy devices. However, the rational conversion of MOFs into a versatile platform for high-efficiency catalyst is still very challenging. Moreover, the relationship between the crystallinity of MOF precursor and the catalytic activity of resultant carbon-based catalyst is still not well-understood. In this work, a strategy for the synthesis of sheet-like mesoporous nitrogendoped graphene (MNG) derived from amorphous MOFs is demonstrated. The amorphous MOF derived MNG showed much higher catalytic activity than that of nitrogen-doped carbon (MNC) derived from highly crystallized MOFs. This rationally designed MNG catalyst served as a multifunctional electrode in a zinc−air battery and a water splitting device, both of which showed electrocatalytic performance superior to those of platinum group metal (PGM) catalysts. The characterization analysis confirmed that the enhanced activity of amorphous MOF derived MNG was primarily attributed to the optimal properties of electronic conductivity, graphitization degree, and high specific surface area. KEYWORDS: metal−organic frameworks, nitrogen-doped graphene, electrocatalyst, zinc−air battery, water splitting
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purification, and recrystallization, which undoubtedly lead to the low yield of MOFs.13−15 In addition to the drawbacks mentioned above, there have been reports that the highly crystallized MOF derivatives (e.g., nitrogen-doped porous carbons) generally suffer from a close-ended polyhedron structure, hydrophobic microporous shell, and low graphitization.16 These shortcomings greatly restrict their catalytic performances to some extent, which, especially, lead to the isolation of active sites within the core of these MOF-derived carbons.8,16 To this end, it is imperative to design a strategy for rationalizing the synthesis of MOF precursors with an openended structure, and efficiently converting MOFs into highly active carbon-based catalysts with an optimal conductivity, mass transfer, and active site density for diverse electrocatalytic reactions. Herein, we developed a strategy for the synthesis of mesoporous nitrogen-doped graphene (MNG) compounds using amorphous zeolitic imidazolate framework-67 (ZIF-67) fibers as a precursor. More specifically, the amorphous ZIF-67 fibers (Supporting Information Figures S1 and S2) were initially prepared on NaCl substrates through grinding of the metal salt with 2-methylimidazole-coated NaCl crystals (Figure 1). Then, the amorphous ZIF-67 fiber-coated NaCl crystals
xygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER) reactions are the most significant electrochemical processes in energy conversion applications such as metal−air batteries, proton exchange membrane fuel cells (PEMFCs), and water electrolyzers.1−5 Typically, platinum group metals (PGMs) are widely reported as commercial catalysts for ORR, OER, and HER. However, PGMs present with many disadvantages including scarcity, poor stability, and low poison resistance, which hamper the scalable commercialization of energy devices by using PGM electrocatalysts.6,7 Thus, massive efforts have been invested in the development of cost-effective, PGM-free, and highly efficient electrocatalysts. Nitrogen-doped porous carbon catalysts derived from metal−organic frameworks (MOFs), which are considered to be promising alternatives to PGMs, have been intensively studied recently due to their controllable structure and porosity.8−10 Although many types of MOFs and their derived carbon catalysts have been developed, the relationship of the electrochemical performance of MOF-derived carbon catalysts to the structure, composition, and crystallization degree of the MOF precursor is still ambiguous. In addition, the traditional methods for the synthesis of MOF precursors such as solvothermal synthesis and crystal breeding in liquid remain the challenge for the scalable and cost-effective production of carbon-based catalysts.11,12 For example, these traditional methods involve several complicated processes, e.g., aging, © XXXX American Chemical Society
Received: April 13, 2018 Accepted: May 23, 2018 Published: May 23, 2018 A
DOI: 10.1021/acsaem.8b00594 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
Figure 1. Schematic illustration of the preparation process of MNG.
were transferred into a tube furnace for further carbonization in N2 gas flow. Finally, the MNG was obtained after dissolving NaCl crystals in acid solution and subsequent washing with deionized water. The resultant MNG compounds showed a mesoporous structure with a highly electrochemically active surface area (ECSA), which was much larger than that of porous carbon derived from typical polyhedral ZIF-67. In addition, with many advantages in structure, composition, and graphitization degree, the as-prepared MNG-CoFe with graphene-encapsulated CoFe alloy nanoparticles (NPs) showed enhanced ORR, OER, and HER performances, which were comparable and even superior to the benchmark PGM electrocatalysts. As expected, the MNG-CoFe electrocatalyst was assembled as a multifunctional electrode in the rechargeable zinc−air battery (ZAB) and water splitting device, respectively, which showed a high power density and cycling durability in ZAB and a low overpotential for water splitting, outperforming those of PGM and recently reported carbonbased electrocatalysts. To further improve the catalytic performance of MNG, we first prepared the MNG encapsulating CoFe NPs (denoted as MNG-CoFe) using a trace amount of Fe-doped amorphous ZIF-67 as MOF precursor (see the Experimental Section in Supporting Information). Scanning electron microscopy (SEM) images in Figure S3 reveal the sheet-like morphology of MNG-CoFe. Transmission electron microscopy (TEM) images, as shown in Figure 2a, indicate that MNG-CoFe has a mesoporous structure containing a large number of open-ended pores that could effectively expose interior active sites to electrolyte. Note that this structure is considered to be more favorable for electrocatalytic reactions when compared with that of the close-ended structure of porous nitrogen-doped carbon (MNC-Co) prepared with typically polyhedral ZIF-67 (Figures S4 and S5). Brunauer−Emmett−Teller (BET) reveals the higher specific surface area of MNG-CoFe with high porosity within the mesoporous range, as compared to that of MNC-Co (Figure S6). TEM observation (Figure 2b) showed that the mesoporous structure of MNG-CoFe comprises densely interconnected graphene layers (indicated by yellow arrows). The formation of high specific surface area and mesoporosity could be attributed to the pyrolysis at elevated
Figure 2. (a, b) TEM images with different magnification. (c) HRTEM image, (d) linear scan EDS, and (e−i) EDS elemental mapping of MNG-CoFe.
temperature leading to the volatilization of a large number of organic groups within amorphous Fe-ZIF-67 compounds, which results in part of the organic groups acting as a carbon source facilitating the growth of graphene layers on the crystalline metal (Co or Fe) nanoparticle surface. Highresolution transmission electron microscopy (HRTEM) observation further confirmed this assumption. As shown in Figure 2c, the well-organized graphene layers are only observed on the metal surface, implying the assistance of metal NPs in the growth of graphene layers during pyrolysis. Because of the postetching process during synthesis, the mesoporous structure with graphene walls was finally formed. As depicted in Figure 2c, the crystalline space of the graphene-encapsulated metal is calculated to be 0.10 nm, which matches well with the (220) face of the CoFe alloy structure. The linear scanning of energy dispersive spectroscopy (EDS) confirms again that metal NPs encapsulated within graphene layers are formed from CoFe alloy (Figure 2d). The selected EDS mapping shows that abundant C and N distribute throughout the entire sample, with only scattered Co and Fe signals existing within graphene layers (Figure 2e−i). X-ray diffraction (XRD, Figure S7) suggests the existence of graphite in all samples, besides the detection of the CoFe alloy phase in MNG-CoFe and the Co phase in MNG-Co and MNC-Co, respectively. The full width at half-maximum (fwhm) values of the graphite (002) peak in both MNG-CoFe and MNG-Co are smaller than that of MNCCo, implying a high graphitization degree in both MNG-CoFe and MNG-Co samples. The D, G, and 2D bands are, respectively, identified at 1349, 1583, and 2696 cm−1 for MNG-CoFe and MNG-Co (Figure S8), indicating the formation N-doped graphene.17 In general, the higher ID/IG ratio implies a more disordered carbon structure within the nanocarbon system and a lower crystallization quality.18,19 The ID/IG ratios are estimated to be 0.85, 0.86, and 1.09 for MNGCoFe, MNG-Co, and MNC-Co, respectively, indicating the highly crystallization degree of graphene in MNG-CoFe and B
DOI: 10.1021/acsaem.8b00594 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Figure 3. (a) LSVs and (b) kinetic current densities (jk) normalized to ECSA in all samples for ORR (electrode rotating speed: 1600 rpm). (c) LSVs for OER with iR compensation. (d) LSVs for both ORR and OER. (e) LSV for HER with iR compensation. (f) Overpotential at the different current density and onset potential for HER (all tests were conducted in 0.1 M KOH with a scan rate of 5 mV s−1).
MNG-Co. Furthermore, the in-plane crystallite size of sp2 hybridization in the nanocarbon can also be estimated using La (nm) = (2.4 × 10−10) λ4(ID/IG)−1 (λ represents the Raman excitation wavelength).20 The calculated La values are 22.6, 22.4, and 17.6 for MNG-CoFe, MNG-Co, and MNC-Co, respectively, indicating larger average sizes of crystalline graphite domains in MNG-CoFe and MNG-Co than that in MNC-Co. X-ray photoelectron spectroscopic (XPS) measurements were conducted to confirm the composition and chemical state of all samples. As can be seen from Figure S9a, Co, O, N, and C elements are identified in all samples, except for a Fe elemental signal appearing in MNG-CoFe. In the highresolution XPS spectra of N 1s (Figure S9b), the N 1s peak can be deconvoluted well into three peaks at 398.4, 400.1, and 401.0 eV, corresponding to pyridinic N (N3), pyrrolic N (N2), and graphitic N (N1), respectively, indicative of nitrogen doping into the carbon molecular skeletons. 21 Recent experimental and theoretical observations show that both pyridinic and graphitic nitrogen could disturb the charge distribution in the π conjugation and make adjacent C active for O2 adsorption/desorption, facilitating ORR/OER.1,20 Thus, the nitrogen dopant contents are presented in Table S3, where the total content of pyridinic N and graphitic N is the highest at 6.4 at. % with MNG-CoFe, which is higher than 6.0 at. % with MNG-Co and 4.7 at. % with MNC-Co. Furthermore, the Co contents were calculated to be 1.1 at. % for MNG-CoFe, 0.3 at.
% for MNG-Co, and 0.4 at. % for MNC-Co, respectively. In addition, the Co 2p orbital of MNG-CoFe (Figure S10) can be identified at 790−800 eV (Co02p 3/2) and 775−785 eV (Co02p 1/2), which agrees well with Co 2p with zero valences. In contrast, the binding energy of the Co 2p peak in MNGCoFe is slightly higher than that of the Co 2p peak in MNGCo, meaning that there is a decrease of electron density in the Co 2p orbital after carbon-encapsulated Co NPs alloying with trace Fe (doping content of 0.8 at. %). To the best of our knowledge, encapsulating transition metal particles into carbon layers could lead to the electron transfer from metal particles to carbon surfaces and the reduction of local work function and further enhance the catalytic activity of the surface carbon.22 In our case, it is undoubtedly evident that the introduction of trace Fe element into carbon-encapsulated Co particles results in the electron deficiency of Co 2p orbital and provides more electrons to surface carbon, possibly improving catalytic activity coupled with the contribution from nitrogen doping, even though the CoFe particles are rare in MNG-CoFe (Figure S11). To study the influence of composition and structure on the electrocatalytic activity, linear sweep voltammogram (LSV) and cyclic voltammogram (CV) measurements were performed to investigate the ORR activity for all samples. As can be seen from Figure S12, the CV curve of MNG-CoFe electrode stands out as the best ORR performance in series with an apparent oxygen reduction peak at 0.84 V (vs RHE). Meanwhile, the onset potential is observed at 0.98 V (vs RHE) in LSV curves C
DOI: 10.1021/acsaem.8b00594 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials
Figure 4. (a) Photographs of a timer powered by one rechargeable zinc−air coin battery with the MNG-CoFe electrode. (b) Polarization curves and power density curves of the zinc−air coin batteries using MNG-CoFe electrode and Pt/C+RuO2 electrode. (c) Discharging and charging polarization curves and (d) galvanostatic charge−discharge cycling (10 mA cm−2, 1 h for each cycle) of the ZAB coin batteries using different air electrodes. (e) CVs of all electrodes for OER with iR compensation. (f) Voltage−time test for all electrodes under 10 mA cm−2. (The inset of Figure 4e shows the onset potentials of LSVs for all samples; the inset of Figure 4f represents a two-electrode configuration for a water splitting device.)
(Figure 3a) of the MNG-CoFe electrode, which is much higher than 0.94 V of MNG-Co, 0.92 V of MNC-Co, and 0.97 V of Pt/C. This coincides with the highest active nitrogen content (graphitic N and pyridinic N) of the MNG-CoFe sample in series (Table S1), suggesting that both graphitic N and pyridinic N likely play a major role in the determination of the ORR. From the LSV curves in Figure S12, it can be seen that the limiting currents of all electrodes increased with increasing rotation speed (from 225 to 2025 rpm). The corresponding Koutecky−Levich (K−L) plots within the potential range from 0.65 to 0.80 V exhibit a good linearity with consistent slopes, indicating that the first order reaction kinetics for ORR is related to the oxygen concentration in the KOH electrolyte. The ECSA-normalized kinetic current densities further confirm the superior ORR activity of the MNG-CoFe electrode, suggesting that the MNG-CoFe electrode offers a high turnover frequency per ECSA toward ORR (Figure 3b and Figure S12). As depicted in Figure 3b, the MNG-CoFe electrode delivers a larger jk at both the half-wave potential and 0.70 V, in comparison with those values of other electrodes. It is notable that both the MNG-CoFe and MNG-Co electrodes have larger ECSAs than that of the MNC-Co electrode (Figure S13), highlighting the unique strategy for converting MOF materials into mesoporous graphene electrocatalysts with a large ECSA. As a result, the electrochemical impedance spectroscopy (EIS) in Figure S14 identifies a faster electron transfer system in
MNG-CoFe than in other electrodes. Note that the MNGCoFe electrode shows a similar ECSA and active N dopant (graphitic N and pyridinic N) concentration with that of the MNG-Co electrode, but with a better ORR performance, which could be attributed to the fact that the graphene-encapsulated CoFe alloy NPs are more favorable for ORR processing on the outermost layer carbons with an optimal electron cloud density or local work function. The SCN− poisoning test confirmed that M−N/C (M = Co or Fe) active sites were not involved in catalyzing the ORR on the MNG-CoFe electrode (Figure S15). Thus, nitrogen dopants and graphene-encapsulated CoFe as the active sites would be responsible for ORR. In addition, the MNG-CoFe electrode also showed excellent stability and poison resistance during ORR (Figures S16 and 17). For OER activity, the overpotential (Ej=10) at 10 mA cm−2 was measured to be 0.39 V for the MNG-CoFe electrode, which was lower than 0.41 V for MNG-Co, 0.50 V for MNCCo, and 0.45 V for RuO2/C. Meanwhile, the corresponding potential gap (ΔE) between the overpotential at Ej=10 in OER and the Eonset in ORR is an important parameter for evaluating catalytic performance with bifunctional properties. The MNGCoFe achieved the smallest value of 0.64 V, as compared to 0.71 V of MNG-Co, 0.80 V of MNC-Co, and 0.75 V of Pt/C +RuO2. In fact, the ORR and OER performance of MNG-CoFe are even better than that of leading carbon-based electrocatalysts that were recently reported (Table S2).23−26 We also D
DOI: 10.1021/acsaem.8b00594 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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surfaces. The MNG-CoFe exhibited an impressive rechargeability when used for a ZAB and water splitting device, which provided an avenue for the rational design of nanocarbon catalysts for renewable energy systems.
investigated the HER performance for these samples. The LSV curves in Figure 3e show increased cathodic current densities with lowering overpotential during negative scans in N2saturated 0.1 M KOH solution, implying that it is feasible to catalyze HER using nitrogen-doped carbon-based electrocatalysts. For comparison, the overpotentials at different current densities were calculated for all electrodes. As depicted in Figure 3f, the MNG-CoFe electrode shows smaller overpotentials than those of the MNC-Co electrode, and even closer to those of the state-of-the-art Pt/C electrodes. These results infer that the excellent HER performance of MNGCoFe could be attributed to the sufficient exposure of active sites derived from nitrogen dopants and graphene-encapsulated CoFe via the formation of mesoporous structure. Considering the superior performance in ORR and OER, the MNG-CoFe is potentially applicable for a rechargeable ZAB system. Figure S18 schematically shows construction of a Zn− air battery coin cell using the MNG-CoFe catalyst. As shown in Figure 4a, a timer (∼1.5 V) is powered by a single zinc−air coin battery using an MNG-CoFe electrode. Furthermore, a maximum power density of 97.7 mW cm−2 is observed for the battery using an MNG-CoFe electrode at 120 mA cm−2 (Figure 4b), which is much higher than 54.5 mW cm−2 for the zinc−air coin battery at 80 mA cm−2 using benchmark Pt/C and RuO2 values and those of other reported ZAB tested under air conditions (Table S3). These results indicate the excellent ORR activity of MNG-CoFe in a ZAB system, which is ascribed to the high electrochemical surface area with sufficient exposure of active sites to the electrolyte, which promotes O2, e−, and H2O diffusion and thus can facilitate the ORR process. Additionally, the bifunctional electrocatalytic activities in ZAB were also studied for MNG-CoFe and benchmark Pt/C+RuO2 electrocatalysts. As shown in Figure 4c, the polarization curves show that both discharging and charging curves of the MNGCoFe electrode in ZAB render lower overpotentials at varied current densities than those of the ZAB comprising the Pt/C +RuO2 electrode. In addition, the excellent rechargeability of the MNG-CoFe electrode was also demonstrated by the discharge/charge cycling for 18 h, outperforming that of commercial Pt/C+RuO2. For a closer observation, the discharge/charge currents are maintained at 1.25/1.86 V, respectively, for the MNG-CoFe electrode at 10 mA cm−2. These values are superior to the 1.1/2.0 V value for the Pt/C +RuO2 electrodes, demonstrating that outstanding and stable electrocatalytic activity enabled the use of an MNG-CoFe electrode in ZAB. Similarly, the MNG-CoFe electrode also shows a good water splitting capability with a potential of ∼1.66 V at 10 mA cm−2 (Figure 4e), and a good stability with 70 h operation without attenuation (Figure 4f). In contrast, these results are much better than those of a water splitting device using Pt/C and RuO2/C. The high stability of MNGCoFe is attributed to the high graphitization and crystallization of nitrogen-doped graphene enabling strong resistance to oxidation in the electrolyte. In summary, a strategy was rationally designed to enhance the electrocatalytic activity of MOF-derived porous carbon by replacing the typical ZIF-67 with the amorphous Fe-ZIF-67 fibers prepared in the solid phase. The resulting MNG-CoFe catalyst derived from amorphous Fe-ZIF-67 fibers exhibited a remarkable activity and stability for OER, ORR, and HER, due to high ECSA, active site density, and electron conductivity. Notably, nitrogen dopants and carbon-enclosed CoFe NPs can synergistically promote catalytic performance on carbon
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00594. Detailed method and characterization (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yang Yang: 0000-0002-4410-6021 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the University of Central Florida through a start-up grant (no. 20080741). REFERENCES
(1) Guo, D. H.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J. Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science 2016, 351, 361−365. (2) Xia, W.; Mahmood, A.; Liang, Z. B.; Zou, R. Q.; Guo, S. J. EarthAbundant Nanomaterials for Oxygen Reduction. Angew. Chem., Int. Ed. 2016, 55, 2650−2676. (3) Yang, Y.; Fei, H. L.; Ruan, G. D.; Tour, J. M. Porous CobaltBased Thin Film as a Bifunctional Catalyst for Hydrogen Generation and Oxygen Generation. Adv. Mater. 2015, 27, 3175−3180. (4) Yang, Y.; Fei, H. L.; Ruan, G. D.; Xiang, C. S.; Tour, J. M. Efficient Electrocatalytic Oxygen Evolution on Amorphous NickelCobalt Binary Oxide Nanoporous Layers. ACS Nano 2014, 8, 9518− 9523. (5) Niu, W. H.; Li, L. G.; Liu, X. J.; Wang, N.; Liu, J.; Zhou, W. J.; Tang, Z. H.; Chen, S. W. Mesoporous N-Doped Carbons Prepared with Thermally Removable Nanoparticle Templates: An Efficient Electrocatalyst for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2015, 137, 5555−5562. (6) Tong, Y.; Chen, P. Z.; Zhou, T. P.; Xu, K.; Chu, W. S.; Wu, C. Z.; Xie, Y. A Bifunctional Hybrid Electrocatalyst for Oxygen Reduction and Evolution: Cobalt Oxide Nanoparticles Strongly Coupled to B,NDecorated Graphene. Angew. Chem., Int. Ed. 2017, 56, 7121−7125. (7) Tackett, B. M.; Sheng, W. C.; Chen, J. G. G. Opportunities and Challenges in Utilizing Metal-Modified Transition Metal Carbides as Low-Cost Electrocatalysts. Joule 2017, 1, 253−263. (8) Liu, S. H.; Wang, Z. Y.; Zhou, S.; Yu, F. J.; Yu, M. Z.; Chiang, C. Y.; Zhou, W. Z.; Zhao, J. J.; Qiu, J. S. Metal-Organic-FrameworkDerived Hybrid Carbon Nanocages as a Bifunctional Electrocatalyst for Oxygen Reduction and Evolution. Adv. Mater. 2017, 29, 1700874. (9) Shang, L.; Yu, H. J.; Huang, X.; Bian, T.; Shi, R.; Zhao, Y. F.; Waterhouse, G. I. N.; Wu, L. Z.; Tung, C. H.; Zhang, T. R. WellDispersed ZIF-Derived Co,N-Co-doped Carbon Nanoframes through Mesoporous-Silica-Protected Calcination as Efficient Oxygen Reduction Electrocatalysts. Adv. Mater. 2016, 28, 1668−1674. (10) Torad, N. L.; Salunkhe, R. R.; Li, Y. Q.; Hamoudi, H.; Imura, M.; Sakka, Y.; Hu, C. C.; Yamauchi, Y. Electric Double-Layer Capacitors Based on Highly Graphitized Nanoporous Carbons Derived from ZIF-67. Chem. - Eur. J. 2014, 20, 7895−7900. (11) You, B.; Jiang, N.; Sheng, M. L.; Drisdell, W. S.; Yano, J.; Sun, Y. J. Bimetal-Organic Framework Self-Adjusted Synthesis of Support-
E
DOI: 10.1021/acsaem.8b00594 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials Free Nonprecious Electrocatalysts for Efficient Oxygen Reduction. ACS Catal. 2015, 5, 7068−7076. (12) Fu, S. F.; Zhu, C. Z.; Song, J. H.; Du, D.; Lin, Y. H. MetalOrganic Framework-Derived Non-Precious Metal Nanocatalysts for Oxygen Reduction Reaction. Adv. Energy Mater. 2017, 7, 1700363. (13) Kuang, M.; Wang, Q. H.; Han, P.; Zheng, G. F. Cu, CoEmbedded N-Enriched Mesoporous Carbon for Efficient Oxygen Reduction and Hydrogen Evolution Reactions. Adv. Energy Mater. 2017, 7, 1700193. (14) Zhang, W.; Jiang, X. F.; Wang, X. B.; Kaneti, Y. V.; Chen, Y. X.; Liu, J.; Jiang, J. S.; Yamauchi, Y.; Hu, M. Spontaneous Weaving of Graphitic Carbon Networks Synthesized by Pyrolysis of ZIF-67 Crystals. Angew. Chem., Int. Ed. 2017, 56, 8435−8440. (15) Hu, H.; Han, L.; Yu, M. Z.; Wang, Z. Y.; Lou, X. W. Metalorganic-framework-engaged formation of Co nanoparticle-embedded carbon@Co9S8 double-shelled nanocages for efficient oxygen reduction. Energy Environ. Sci. 2016, 9, 107−111. (16) Xia, B. Y.; Yan, Y.; Li, N.; Wu, H. B.; Lou, X. W.; Wang, X. A metal-organic framework-derived bifunctional oxygen electrocatalyst. Nat. Energy 2016, 1, 15006. (17) Niu, W.; Marcus, K.; Zhou, L.; Li, Z.; Shi, L.; Liang, K.; Yang, Y. Enhancing Electron Transfer and Electrocatalytic Activity on Crystalline Carbon-Conjugated g-C3N4. ACS Catal. 2018, 8, 1926− 1931. (18) Niu, W. H.; Li, L. G.; Wang, N.; Zeng, S. B.; Liu, J.; Zhao, D. K.; Chen, S. W. Volatilizable template-assisted scalable preparation of honeycomb-like porous carbons for efficient oxygen electroreduction. J. Mater. Chem. A 2016, 4, 10820−10827. (19) Liang, J.; Zhou, R. F.; Chen, X. M.; Tang, Y. H.; Qiao, S. Z. FeN Decorated Hybrids of CNTs Grown on Hierarchically Porous Carbon for High-Performance Oxygen Reduction. Adv. Mater. 2014, 26, 6074−6079. (20) Yang, H. B.; Miao, J. W.; Hung, S. F.; Chen, J. Z.; Tao, H. B.; Wang, X. Z.; Zhang, L. P.; Chen, R.; Gao, J. J.; Chen, H. M.; Dai, L. M.; Liu, B. Identification of catalytic sites for oxygen reduction and oxygen evolution in N-doped graphene materials: Development of highly efficient metal-free bifunctional electrocatalyst. Sci. Adv. 2016, 2, e1501122. (21) Niu, W. H.; Li, L. G.; Liu, J.; Wang, N.; Li, W.; Tang, Z. H.; Zhou, W. J.; Chen, S. W. Graphene-Supported Mesoporous Carbons Prepared with Thermally Removable Templates as Efficient Catalysts for Oxygen Electroreduction. Small 2016, 12, 1900−1908. (22) Deng, J.; Ren, P. J.; Deng, D. H.; Bao, X. H. Enhanced Electron Penetration through an Ultrathin Graphene Layer for Highly Efficient Catalysis of the Hydrogen Evolution Reaction. Angew. Chem., Int. Ed. 2015, 54, 2100−2104. (23) Zhang, J. T.; Zhao, Z. H.; Xia, Z. H.; Dai, L. M. A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions. Nat. Nanotechnol. 2015, 10, 444−452. (24) Tian, G. L.; Zhao, M. Q.; Yu, D. S.; Kong, X. Y.; Huang, J. Q.; Zhang, Q.; Wei, F. Nitrogen-Doped Graphene/Carbon Nanotube Hybrids: In Situ Formation on Bifunctional Catalysts and Their Superior Electrocatalytic Activity for Oxygen Evolution/Reduction Reaction. Small 2014, 10, 2251−2259. (25) Aijaz, A.; Masa, J.; Rosler, C.; Xia, W.; Weide, P.; Botz, A. J. R.; Fischer, R. A.; Schuhmann, W.; Muhler, M. Co@Co3O4 Encapsulated in Carbon Nanotube-Grafted Nitrogen-Doped Carbon Polyhedra as an Advanced Bifunctional Oxygen Electrode. Angew. Chem., Int. Ed. 2016, 55, 4087−4091. (26) Hu, C. G.; Dai, L. M. Multifunctional Carbon-Based Metal-Free Electrocatalysts for Simultaneous Oxygen Reduction, Oxygen Evolution, and Hydrogen Evolution. Adv. Mater. 2017, 29, 1604942.
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