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Layered MOFs Derived Metal Oxide/Carbon Nanosheet Arrays for Catalyzing the Oxygen Evolution Reaction Jian Zhou, Yibo Dou, Awu Zhou, Lun Shu, Ya Chen, and Jian-Rong Li ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00809 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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ACS Energy Letters

Layered MOFs Derived Metal Oxide/Carbon Nanosheet Arrays for Catalyzing the Oxygen Evolution Reaction

Jian Zhou,# Yibo Dou,# Awu Zhou, Lun Shu, Ya Chen, Jian-Rong Li*

Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, P. R. China

ABSTRACT: The oxygen evolution reaction (OER) in water splitting plays critical role in some clean energy production systems. Transition metal oxides as one of the most common OER electrocatalysts have been widely explored; however their activity is limited by low electrical conductivity, slow mass transfer, and inadequate active sites. Herein, we develop a feasible strategy that layered two-dimensional metal-organic frameworks (2D MOFs) act as templates to construct metal oxide/carbon (MOx/C, M = Co, Ni, and Cu) nanosheet arrays for OER. Owning to improved conductivity and more exposed active sites afforded by their 2D structures with rich hierarchical pores and the incorporation with porous carbon, these 2D MOFs derived MOx/C arrays represent high electrocatalytic activities and good durability. Particularly, the Co3O4/CBDC, NiO/CBDC, and Cu2O/S-CTDC exhibit low overpotentials of 208, 285, and 313 mV at the current density of 10 mA cm−2, respectively, outperforming all of previously reported corresponding metal oxides based catalysts.

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Electrocatalytic water splitting provides a promising and sustainable platform for exploring and utilizing clean energy, while the overall efficiency of this process is significantly affected by the kinetically sluggish oxygen evolution reaction (OER).1-3 It is generally accepted that the OER involves multistep reactions: (I) the formation of the *OH intermediate from adsorbed H2O; (II) the decomposition of *OH to *O; (III) the transformation from *O to *OOH intermediate; and (IV) the release of O2 from *OOH.4 Thus, the elaborate design and exploration of electrocatalysts with abundant active sites and high adsorption ability toward intermediates and H2O for effective OER arouses great concern. Among various catalysts, transition metal oxides (TMOs) (M = Co, Ni, Fe, and so on) as one of the most common candidates for water splitting have been widely investigated owning to their earth abundance and high cost-efficiency.5,6 However, the catalytic activity of traditional TMOs in OER is usually limited by their low conductivity, slow mass transport, and/or inadequate catalytically active sites.7 Thus, great efforts have been devoted to design or optimize the structure of TMOs and associated catalytic performance through such as


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ACS Energy Letters

enhancing surface area or oxygen vacancies, doping heteroatoms, creating defects, as well as constructing their hybrid materials.8-10 Recently, using metal-organic frameworks (MOFs) as templates to fabricate various derivatives, particularly TMOs based ones for electrocatalytic application have been attracting great attention, due to their inherited high surface areas, rich skeleton/pore structures, and diverse atom components.11-14 However, most of reported derivatives were obtained by the pyrolysis treatment of bulk, or said three-dimensional (3D) MOFs at high temperature. The resulting materials prone to self-aggregate, thereby losing some exposed active sites and simultaneously slowing mass transfer in electrocatalysis.15,16 In addition, the interaction between the bulk MOFs-derived powder and conductive substrate is usually weak, consequently also rebating the overall efficiency and durability of the electrode.17 Therefore, developing alternative approach and/or selecting proper MOF precursors to fabricate new TMO based electrocatalysts with enhanced performance for OER are eagerly expected, but challenged. It should be noted that numerous two-dimensional (2D) materials represented fascinating catalytic performances for energy conversion, and recently a few of 2D MOFs were also used for the electrocatalytic water splitting.18-20 Nevertheless, most of 2D MOFs are indeed not suitable for this application because of their unsatisfied charge transfer resulting from the low conductivity of organic ligands.21,22 And, their



fatal structural instability under harsh conditions of strong acid/alkaline electrolytes also seriously impedes the durability.23 Inspired by the merits and drawbacks of MOF template approach to fabricate TMOs and using pure 2D MOFs for electrocatalytic water splitting, we speculate that whether layered 2D MOFs derived porous composites possess more outstanding OER performance in terms of their possible unique structures. With this in mind, herein we attempt to use different dimensional MOFs as templates to fabricate metal oxide/carbon (MOx/C, M = Co, Ni, and Cu) arrays for catalyzing OER (Figure 1). It was found that resulting highly oriented 2D MOx/C arrays display improved electrocatalytic activity with good durability, compared with the corresponding oneor three-dimensional (1D or 3D) MOFs derived MOx/C composites. Particularly, the 2D Co3O4/CBDC, NiO/CBDC, and Cu2O/S-CTDC represent the overpotentials of 208, 285, and 313 mV at the current density of 10 mA cm−2, respectively; superior to previously reported corresponding TMOs based catalysts. It should also be pointed out that more cost-effective Cu2O based catalysts were rarely used in the electrocatalysis because of their low electrical conductivity and/or instability thereby poor catalytic performance. 24

A series of obtained Cu2O/C composites herein have high-performance toward

OER not only in catalytic activity but also in durability. These results demonstrate that this 2D MOF template approach is general and superior in fabricating TMOs based catalysts for highly efficient OER.


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ACS Energy Letters

Figure 1. Schematic presentation of fabricating MOx/C (M = Co, Ni, and Cu) arrays anchored on conductor substrate, derived from 1D, 2D, and 3D MOFs, respectively for electrocatalytic OER.

Based on the above proposed strategy, Co(II)-based MOFs, 1D Co-BTC (BTC = 1,3,5-benzenetricarboxylate), 2D Co-BDC (BDC = 1,4-benzenedicarboxylate), 3D Co-DHTP (DHTP = 2,5-dihydroxyterephthalate) derived well-aligned 1D Co3O4/CBTC, 2D Co3O4/CBDC, and 3D Co3O4/CDHTP arrays were fabricated by firstly the in-situ growth of MOF arrays on substrate, and followed by the pyrolysis treatment (Figure S1). The SEM images show that 1D Co-BTC and 3D Co-DHTP arrays maintained well nanorod structures with an average diameter of ~500 nm (inset picture of Figure 2a and b). However, 2D Co-BDC array features nanosheet structure with the thickness of only ~30 nm, which is thinner than 1D Co-BTC and 3D Co-DHTP nanorods, and bulk Co-BDC (Figure 2c and S2). After the pyrolysis treatment of above Co-MOF arrays, 1D Co3O4/CBTC and 3D Co3O4/CDHTP retain the morphology of their precursors



with a rough surface. Interestingly, an open-up network structure constructed by the interconnected 2D Co3O4/CBDC nanosheets was observed (Figure 2d). And transmission electron microscopy (TEM) images show that the highly dispersed Co3O4 nanoparticles on the 2D sheet have a smaller size (~10 nm), when compared with the 3D MOF-derived Co3O4 (~100 nm) (Figure 2e and S3). The relatively smaller size is probably resulted from that the 2D interlayer space restrains the aggregation of the derived Co3O4 along the vertical direction, which would facilitate the exposure of more active sites.25 Most importantly, it was found that the Co3O4 particles are uniformly wrapped by the derived carbon, which suggests their strong connection, being favorable for improving conductivity and stability of the Co3O4/CBDC composite.13 In addition, the well-defined Co3O4, reflected by three bright rings in the selected area electron diffraction (SAED) pattern corresponding to (111), (220), and (311) planes of its spinel phase, was also revealed by the high-resolution TEM image (HRTEM) (Figure 2f). The elemental mapping analysis clearly shows the uniform distribution of Co, O, and C elements in the composite (Figure S4). Considering the content of C probably has an effect on the OER activity and stability, thus the energy dispersive X-ray (EDX) spectroscopy was performed to record the contents of C in various Co3O4/C composites (Figure S5). It was found that the 2D Co3O4/CBDC has a higher content of C among derived Co3O4/C composites. The reason is probably that the high content of C and sufficient exposure of organic


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ACS Energy Letters

ligands in 2D Co3O4/CBDC might facilitate the conversion of more C during pyrolysis.19 Meanwhile, the variation of microstructural features was investigated by the powder X-ray diffraction (PXRD). Compared with corresponding Co-MOF powder, the appearance of well-matched diffraction patterns illustrates the successful in-situ growth of each Co-MOF array directly on the substrate (Figure 2g and S6). After the pyrolysis treatment, the diffraction peaks at 19.2, 31.3, and 36.9° were observed for all derived Co3O4/C, which can be assigned to the (111), (220), and (311) lattice planes of cubic Co3O4 spinel phase, respectively (Figure 2h and S7). Simultaneously, the Raman spectra were used to identify the existence of the carbon. As expected, all of the derivatives show two intense bands at 1356 and 1569 cm−1 corresponding to the D- and G-band, respectively, which can be ascribed to the sp3 carbon and graphitic sp2 carbon (Figure 2i). The high values of IG/ID (I is the intensity of D- or G-band) manifest a distinct degree of graphitization in the MOF-derived Co3O4/C arrays, being favorable for improving conductivity and subsequently promoting charge transfer for electrocatalysis.26 In addition, the 2D Co3O4/CBDC represents a IV-type N2 adsorption/desorption isotherm with a H3-type hysteresis loop, showing its hierarchical pore structure (Figure S8). The evaluated specific surface area is 181.9 m2 g–1, much higher than those of 1D Co3O4/CBTC (75.6 m2 g–1), 3D Co3O4/CDHTP (98.7 m2 g–1), and bare 2D Co3O4 (20.1 m2 g–1) owing to its porous 2D structural nature.



Figure 2. (a) SEM image of 1D Co3O4/CBTC (inset is the corresponding Co-BTC image); (b) SEM image of 3D Co3O4/CDHTP (inset is the corresponding Co-DHTP image); (c) SEM image of 2D Co-BDC; (d) SEM image of 2D Co3O4/CBDC arrays; (e) TEM and (f) HRTEM images of the 2D Co3O4/CBDC (inset is the corresponding SAED pattern); (g) PXRD patterns of Co-MOFs arrays; (h) PXRD patterns and (i) Raman spectra of 1D Co3O4/CBTC, 2D Co3O4/CBDC, and 3D Co3O4/CDHTP arrays.

Then, the OER activities of the 1D Co3O4/CBTC, 2D Co3O4/CBDC, 3D Co3O4/CDHTP, and the control sample 2D Co3O4 arrays directly as electrodes were evaluated. The linear sweep voltammetrys (LSVs) of these catalysts were recorded at a scan rate of 10 mV s–1 (Figure 3a). Compared with the 2D Co3O4 array with a high overpotential of 370 mV at a current density of 10 mA cm–2, MOF-derived 1D Co3O4/CBTC and 3D Co3O4/CDHTP show lower overpotentials of 295 and 260 mV (Figure 3b), respectively, probably because of more active sites afforded by their


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ACS Energy Letters

larger specific surface area and improved charge transfer due to the incorporation of highly conductive carbon matrix. Most significantly, the 2D Co3O4/CBDC displays an quite low overpotential of 208 mV, lower than those of Co3O4/CBTC, Co3O4/CDHTP, all previously reported Co3O4 based electrocatalysts (Table S1), and even commercial InO2/C catalyst (265 mV). The Tafel slope of the 2D Co3O4/CBDC (50.1 mV dec−1) is also much smaller than those of 3D Co3O4/CDHTP (87.4 mV dec−1), 1D Co3O4/CBTC (104.2 mV dec−1), 2D bare Co3O4 (123.9 mV dec−1), and InO2/C (89.2 mV dec−1) (Figure 3c), in good agreement with the LSV results. The 2D arrayed structure and the incorporation with carbon can help the formation of the *OH intermediate from adsorbed H2O. This is beneficial for promoting the first step of OER reaction, thus facilitating the rapid decomposition of *OH and enhancing the OER activity.27 Moreover, the high content of carbon in 2D Co3O4/CBDC (Figure S5) could further contribute to improving the electrical conductivity and charge transport kinetic. In addition, compared with the Co3O4/CBDC powder based electrode (Figure S9), the 2D Co3O4/CBDC array indeed also shows improved catalytic activity. This might be due to that the highly oriented Co3O4/CBDC array can suppress the aggregation of active components and improve the efficient contact between active sites and substrate. Furthermore, the durability of these electrodes was evaluated by the chronoamperometric measurements. As shown in Figure 3d, the current density of the 2D Co3O4/CBDC array electrode only lost ~3% of their original current density after 36 h run, being quite lower than those of 1D Co3O4/C (~29%), 3D Co3O4/C (~16%), and



InO2/C (~39%) electrodes. The outstanding stability of the 2D Co3O4/CBDC electrode could be attributed to not only the well-wrapping of small-sized Co3O4 nanoparticles by the C matrix as mentioned above (Figure 2e), but also its strong connection with the substrate afforded by the initial in-situ growth of MOF arrays.28 Both of them can refrain the loss of active sites and the electrode corrosion.13,29

Figure 3. (a) LSV curves, (b) overpotentials at the current density of 10 mA cm–2, (c) Tafel plots, and (d) chronoamperometric curves at 1.53 V vs. reversible hydrogen electrode (RHE) over 36 h for various Co3O4/C arrays and the control samples.

Similarly, 1D NiO/CBTC, 2D NiO/CBDC, and 3D NiO/CDHTP arrays were fabricated by the in-situ growth of 1D Ni-BTC, 2D Ni-BDC, and 3D Ni-DHTP MOF arrays and then the pyrolysis treatments (Figure S10-14). Experimental results on OER show that the 2D NiO/CBDC nanosheet array has the lower overpotential and Tafel slope of 285 mV and 61.9 mV s–1, respectively, compared with those of 1D


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ACS Energy Letters

NiO/CBTC (355 mV and 94.3 mV s–1), 3D NiO/CDHTP (315 mV and 92.1 mV s–1), and other reported NiO based catalysts (Figure S15 and Table S2). The good OER performance of the 2D NiO/CBDC can be attributed to the unique 2D arrayed structure and rich mesopores, and simultaneously the relative higher contents of C compared with those of 1D NiO/CBTC and 3D NiO/CDHTP (Figure S13). As shown in Figure S16, chronopotentiometric curves demonstrate that the 2D NiO/C represents a negligible increasing of the overpotential compared with that of 1D/3D NiO/C after 36 h test. In addition, no obvious microstructure and morphology variation of the 2D Co3O4/CBDC and NiO/CBDC were observed after chronoamperometry measurements (Figure S17 and 18). Above results on Co3O4/C and NiO/C array systems demonstrate the superior electrocatalytic activity and stability of the 2D MOF derived composites in OER. In view of the advantages of the 2D Co/Ni-MOF derivatives confirmed above, we further used this 2D MOF template approach to fabricate rarely explored Cu2O based composite arrays for catalyzing OER. A series of 2D Cu(II)-MOF derivatives, Cu2O/S-doped CTDC (Cu2O/S-CTDC), Cu2O/CFDC, and Cu2O/CBDC nanosheet arrays were prepared by the pyrolysis treatments of corresponding 2D Cu-TDC (TDC = 2,5-thiophenedicarboxylate), Cu-FDC (FDC = 2,5-furandicarboxylate), and Cu-BDC arrays in-situ grown on substrate, respectively (Figure 4a and S19-22). Here we take 2D Cu2O/S-CTDC as the example for detailed structural analysis and discussion among these compositions. From Figure 4b and c, similar interconnected nanosheets consisting of carbon and smaller Cu2O nanoparticles (~50 nm) are observed, being



different from the morphology of the 3D Cu-BTC derived Cu2O/CBTC and bare Cu2O (Figure S23 and 24). The HR-TEM image shows an interplanar distance of ~0.25 nm, which can be ascribed to the (111) plane of Cu2O (inset of Figure 4c). The corresponding EDX analysis illustrates the contents of S (~7.1 wt%) and C (~8.6 wt%) in the Cu2O/S-CTDC, which are higher than those in the 3D Cu2O/CBTC (~3.8 wt%) (Figure 4d, S25, and 26). Clearly, the high contents of heteroatoms in the composite could improve the conductivity and afford more active sites for OER catalysis.30

Figure 4. (a) SEM image of 2D Cu-TDC (inset is the magnified SEM images); (b) SEM image of 2D Cu2O/S-CTDC arrays (inset is the magnified SEM images); (c) TEM image of 2D Cu2O/S-CTDC (inset is the corresponding HRTEM image); (d) elemental mappings of Cu2O/S-CTDC; (e) PXRD patterns of Cu-MOF arrays; (f) PXRD patterns and (g) Raman spectra of 2D Cu2O/S-CTDC, Cu2O/CFDC, Cu2O/CBDC, and 3D Cu2O/CBTC arrays.


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ACS Energy Letters

Furthermore, PXRD (Figure 4e and f, and S27), Raman spectra (Figure 4g), and X-ray photoelectron spectroscopy (XPS) (Figure S28) identified the variation of the 2D Cu-MOFs deriving into 2D Cu2O/C arrays. The high-resolution XPS spectra of Cu2O/S-CTDC were analyzed in detail. The peaks at 931.9 and 951.6 eV can be assigned to the Cu 2p3/2 and 2p1/2 of Cu2O, respectively (Figure S29a). And the main peak at 530.2 eV is related to the binding energy of Cu−O (Figure S29b), while another one at 531.7 eV shows the existence of –OH, probably favorable for the formation of an *OH intermediate from adsorbed H2O and the further oxidation in OER process.24,27 Moreover, the C 1s spectra (Figure S29c) reveal the high intensity of the C=C sp2 peak (61%), being coincident with the Raman result (Figure 4g). Interestingly, the peak at 285.4 eV ascribed to the binding energy of C−S was observed, demonstrating the existence of the S-doped C. And, peaks at 163.9 and 165.3 eV of the S 2p3/2 and S 2p1/2 can be assigned to the C–S–C bond, respectively, suggesting that S has been successfully doped into the graphitic C matrix (Figure S29d).31 It should be pointed out that the S doping can improve the conductivity and generate defects in the C matrix,32 being favorable for accelerating charge transfer and exposing more active sites of the catalyst for OER. All of above features including the existence of –OH groups in the Cu2O, high intensity of the graphitic C, as well as C–S–C bonds would endow the 2D Cu2O/S-CTDC array with improved rate of the charge transfer and the kinetics for OER, which will be discussed below.



On the other hand, these 2D Cu2O/S-CTDC, Cu2O/CFDC, and Cu2O/CBDC arrays also possess larger specific surface areas of 160.4, 146.2, and 150.5 m2 g–1, respectively than those of 3D Cu2O/CBTC (80.5 m2 g–1) and bare Cu2O (11.7 m2 g–1) (Figure S30 and 31a). And, the evaluated pore-size distributions lie in both 1~2 nm and 5~20 nm ranges, revealing hierarchical pore structures of these Cu2O/C composites (Figure S31b), which could create more active catalytic sites for OER correspondingly.33 Subsequently, the OER activities of these 2D Cu-MOFs derived Cu2O/C arrays were evaluated. As expected, the Cu2O/CFDC and Cu2O/CBDC display the lower overpotentials of 352 and 360 mV, than those of 3D Cu2O/CBTC (405 mV) and bare Cu2O (490 mV) (Figure 5a and b). Significantly, the Cu2O/S-CTDC represents the minimum overpotential of only 313 mV, which is lower than those of all previously reported Cu2O based catalysts (Table S3). And, the trend of the Tafel slope is Cu2O/S-CTDC (65.6 mV dec−1) < Cu2O/CBDC (84.7 mV dec−1) < Cu2O/CFDC (86.2 mV dec−1) < Cu2O/CBTC (105.4 mV dec−1) < Cu2O (125.1 mV dec−1) (Figure 5c). The lowest overpotential and Tafel slope of Cu2O/S-CTDC among these three 2D Cu2O/C arrays could be attributed to such a fact that the S doping might create defects in the C matrix and/or change its structure, which are beneficial for providing more active sites and boosting higher conductivity of the C matrix as mentioned above.34,35 Simultaneously, S doped carbon layer can also tune the adsorption of reaction intermediates, and correspondingly improving reaction activity.35


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ACS Energy Letters

Figure 5. (a) LSV curves; (b) overpotentials at the current density of 10 mA cm–2; (c) Tafel plots; and (d) capacitive currents at 1.30 V as a function of scan rates; (e) Faradaic efficiency at 1.63 V; and (f) multi-step chronoamperometric curves at varied potentials over 60 h for 2D Cu2O/S-CTDC, Cu2O/CFDC, Cu2O/CBDC, and 3D Cu2O/CBTC arrays.



The significantly enhanced OER activities of these 2D Cu2O/C arrays were further analyzed and discussed. Compared with 3D Cu2O/CBTC array, their porous 2D structure can afford high electrocatalytic active surface area (ECSA), which was herein evaluated by the corresponding double-layer capacitance (Cdl).36-38 The calculated Cdl of 2D Cu2O/S-CTDC, Cu2O/CFDC, and Cu2O/CBDC are 162.5, 121.4, and 115.1 mF cm−2, respectively all larger than that of 3D Cu2O/CBTC (89.1 mF cm−2) (Figure 5d and S32). The enhanced ECSA can benefit the rapid charge transfer and improved kinetics in catalyzing OER. The faster charge transfer in these 2D Cu2O/C arrays were also confirmed by the lower intrinsic resistance of them, compared with those of 3D Cu2O/CBTC and bare Cu2O (Figure S33). Moreover, the corresponding Faradaic efficiencies evaluated from the O2 production measurements, for the 2D Cu2O/S-CTDC, Cu2O/CFDC, and Cu2O/CBDC are 96, 94, and 95%, respectively, obviously higher than that of 3D Cu2O/CBTC (87%) (Figure 5e and S34). These high values illustrate high rate of O2 release and good energy conversion efficiency in using these 2D Cu2O/C arrays. Clearly, their excellent catalytic activities can be attributed to both the more exposed active sites and high electrical conductivity afforded by their unique 2D structures and high contents of strong combined C (Figure S25). Additionally, the thin layer and rich mesopores of the nanosheets affords more exposed Cu2O (111) surface,39 which could serve as active centers to facilitate the adsorption of intermediate and H2O to accelerate reacting kinetics of the OER process.


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ACS Energy Letters

As mentioned above, the poor stability greatly limits the development of Cu2O based electrocatalysts,24,39 the durability of these 2D Cu2O/C arrays was thus investigated. The results show that Cu2O/S-CTDC, Cu2O/CFDC, and Cu2O/CBDC arrays maintained more than 95% of their original currents at each given potential over 60 h, being superior to the 3D Cu2O/CBTC (~26% decay) and bare Cu2O array (~40% decay) (Figure 5f and S35). The inactivation of traditional Cu2O in catalyzing OER was usually attributed to the surface oxidation of it into CuO, which was further confirmed by PXRD (Figure S36a).40,41 However, the overpotentials of the 2D Cu2O/C arrays show less increase after 24 h mesurement (Figure S37). The excellent stability could be explained as that the strong connection between small Cu2O particles and C matrix (Cu2O is wrapped by C) guarantees structural integrity and good conductivity. The latter inhibits the inner electron accumulation and then suppresses the oxidation of Cu+ to Cu2+ on the surface Cu2O/C (Figure S36b and S38).30,42 Overall, the good OER performance of above obtained 2D TMO/C composite arrays from the 2D MOFs template-directed strategy can be explained based on the following advantages: (1) compared with traditionally nonporous TMOs based arrays, the 2D MOFs derived MOx/C arrays have high specific surface areas and rich hierarchical pore structures, which can efficiently facilitate the exposure of more active sites, electrolyte penetration, and release of gas bubbles for rapid redox reaction; (2) the 2D MOFs derived composites have highly oriented and arrayed structures,



being capable of restraining TMO aggregation thereby increasing active sites to some extent. Moreover, the 2D layered structure with interlayer spaces can impart the derived MOx nanoparticles with small size. Thus, the optimized structure is favorable for more exposed active sites and reduced surface energy, and can drive fast mass/charge transport along the 2D nanosheet, compared with 1D or 3D MOF derivatives; (3) heteroatoms from organic ligands of MOFs can be easily doped into composites with uniform distribution, which can create more catalytic active sites and/or boost conductivity; and (4) the strong interaction between the individual MOx particles and C matrix can ensure improved conductivity and structural integration, thus endowing an effective path for fast electron transfer and simultaneously guaranteeing high durability in the OER process.

In summary, by the judicious selection of proper 2D MOFs as templates, a series of highly oriented 2D MOx/C (M = Co, Ni, and Cu) nanosheet arrays were fabricated and used for electrocatalytic OER. This 2D MOF template strategy enables the resulting 2D MOx/C arrays fascinating features with porous 2D arrayed structures and strong connection between MOx and the C matrix. The former endows more exposed catalytic active sites and fast mass transfer, and simultaneously the latter affords increased conductivity and structural integration. As a result, the resultant 2D MOx/C composites exhibit significantly enhanced catalytic activity and good durability. Particularly, the electrocatalytic activities of several newly obtained members are superior to corresponding 1D or 3D MOF derived counterparts, as well as previously


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ACS Energy Letters

reported MOx based catalysts. It is expected that this 2D MOFs template-directed strategy can be extended to the fabrication of other derivatives/composites, such as metal sulfides, metal phosphides, and metal borides as high-performance catalysts for wide applications.

ASSOCIATED CONTENT Supporting Information SEM, TEM, EDX, XPS, BET, PXRD, LSV, and EIS images of 2D MOx/C (M = Co, Ni, and Cu). Comparison of the electrocatalytic activity of 2D MOx/C with previously reported metal oxide based electrocatalysts.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions #

Z.J. and D.Y. contributed equally

ACKNOWLEDGMENTS This work was supported by the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (Grant No. 51621003), the Natural Science Foundation of China (21576006 and 21606006), and the Beijing Natural Science Foundation (2174064).



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