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Loading Pt nanoparticles on MOFs for improved oxygen evolution Jinxue Guo, Xinqun Zhang, Yanfang Sun, Lin Tang, Qingyun Liu, and Xiao Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02926 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on November 3, 2017

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ACS Sustainable Chemistry & Engineering

Loading Pt nanoparticles on MOFs for improved oxygen evolution

Jinxue Guoa, Xinqun Zhanga, Yanfang Sunb, Lin Tanga, Qingyun liuc, Xiao Zhanga,∗∗ a

Key Laboratory of Rubber-plastics, Ministry of Education/Shandong Provincial Key Laboratory of

Rubber-plastics, State Key Laboratory Base of Eco-chemical Engineering, Key Laboratory of Sensor Analysis of Tumor Marker (Ministry of Education), College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, No. 53, Zhengzhou Road, Qingdao 266042, China b

c

College of Science and Technology, Agricultural University of Hebei, Cangzhou 061100, China

College of Chemical and Environmental Engineering, Shandong University of Science and

Technology, Qingdao 266510, China ABSTRACT Electrochemical oxygen evolution reaction (OER) is the crucial and limiting reaction in several renewable energy conversion systems and metal-organic frameworks (MOFs) have triggered increasing research interests as potential catalysts towards OER. Deeper understanding of the OER activity over MOFs is extremely desired for the exploitation of robust MOFs based electrocatalysts. Herein, Pt nanoparticles are loaded on Prussian blue analogues (Co3[Fe(CN)6]2 and Ni3[Fe(CN)6]2 nanocubes) to obtain improved catalytic activity. Co3[Fe(CN)6]2 and Ni3[Fe(CN)6]2 are rationally selected because of their containing of the fascinating transition metals of Co and Ni species. Using the hybrid catalyst as modular system, we demonstrate the inspiring effect of Pt on the OER activity of MOFs. Detailed exploration conclusively demonstrates that, Pt supplies combined advantages for the enhanced OER

∗

Corresponding author. Tel.: +86 532 84022681; Fax: +86 532 84023927. E-mail address: [email protected] (X. Zhang). 1

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activity of MOFs, such as improving the intrinsic catalytic activity by tuning the valence state of transition metals (Co, Ni), increasing active sites, and enhancing charge transfer. Moreover, thorough electrochemical studies are also performed to declare the key role of Pt in the excellent catalytic activity and stability. We believe that introduction of trace amounts of Pt or other noble metals will be an effective solution to achieve a significant improvement in the OER activity of MOFs. Keywords: Co3[Fe(CN)6]2; Ni3[Fe(CN)6]2; Pt; Oxygen evolution; Energy conversion

Introduction

Sustainable and clean energy sources have captured considerable research efforts to deal with the ever-increasing energy depletion and environmental pollution. Electrolysis of water splitting is believed as a clean and competitive solution to produce hydrogen and oxygen energy, which possesses specific advantage of carbon-free emissions.1,2 This system includes two electrolysis processes, in terms of the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER). Generally, OER is believed as the rate-limiting step of water splitting due to its sluggish kinetics, which is also critical reaction involved in other energy conversion systems.3 High-performance electrocatalyst is crucial for the OER kinetics. Till now, the state-of-art catalysts towards OER are Ru and Ir based oxides, nevertheless, their widespread application is hindered by the limited supply and high cost.4,5 Therefore, development of earth abundant alternatives with sufficient output is on the research focus. Over the past years, transition metal hydroxides,6,7 carbides,8,9,10 nitrides,11,12 phophides,12-15 and chalocogenides,16,17 as well as non-metal based catalysts,18,19 have been studied as promising cost-effective catalysts for OER. Specifically, metal-organic frameworks (MOF) derived transition

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metal compounds, such as phophides,20-22 sulfides,23-25 oxides,26-28 and metals-carbon composites,29,30 have triggered special interests because of the additional advantages endowed with the structural benefits of precursor MOFs. Lou and his co-workers have prepared NiCoP/C nanobox using ZIF-67 as precursor, which shows excellent activity and stability towards OER.20 Yang et al. have synthesized FeCoNi/graphene electrocatalyst from Prussian blue analogues to achieve highly efficient OER activity.29 Recently, some literatures have reported the direct use of MOFs as electrocatalyst for OER.31-38 However, the poor conductivity of MOFs prevents their function express of OER performance, and their inherent catalytic activity needs to be further improved. In this work, the fascinating transition metals of Co and Ni are selected as active centers for the synthesis of Prussian blue analogues as novel electrocatalysts for OER, including Co3[Fe(CN)6]2 (Co-PB) and Ni3[Fe(CN)6]2 (Ni-PB) nanocubes. Thereafter, we load Pt nanoparticles on these MOFs to pursue enhanced charge transfer and OER kinetics. Using the fabricated hybrid interfaces of Co3[Fe(CN)6]2 nanocube/Pt (Co-PB/Pt) and Ni3[Fe(CN)6]2 nanocube/Pt (Ni-PB/Pt), the combined benefits of Pt on enhancing the OER activity of Prussian blue analogues are well probed. It is found that, Pt could tune the valence state of Co/Ni, introduce more active sites, and enhance the reaction kinetics, thus improving the intrinsic and extrinsic catalytic activity of MOFs simultaneously.

Experimental

Co(NO3)2·6H2O (0.6 mmol) is dissolved in 20 mL of deionized water. K3Fe(CN)6 (0.4 mmol) and sodium citrate (0.9 mmol) are dissolved in 20 mL of water. The obtained water solution is mixed under stirring and then aged for 24 h. The precipitates of Co3[Fe(CN)6]2 nanocubes are centrifugally collected and vacuum dried at 60 oC. 60 mg of the synthesized Co3[Fe(CN)6]2 nanocubes are dispersed in 30 mL

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of water. H2PtCl6·3H2O (3 mg) is dissolved in 30 mL of water. After mixed, the mixture is heated at 80 o

C for 4 h. The resultant powder is transferred in a tube furnace and heated at 120 oC for 3 h in Ar/H2

atmosphere to obtain the final Co-PB/Pt. The Ni-PB/Pt is prepared via the same strategy using Ni(NO3)2 as reagent. The Pt content in Co-PB/Pt or Ni-PB/Pt is determined with the inductively coupled plasma mass spectrometry (ICP-MS) technique and controlled as low as 4.0 wt.%. The obtained samples are characterized with transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy (EDX) from a FEI Tecnai G2 F30, X-ray photoelectron spectrum (XPS) on a RBD upgraded PHI-5000c ESCA system (Perkin Elmer), scanning electron microscope (SEM) on a JEOL JSM-7500F, and powder X-ray diffraction (XRD) from a Philips X’-pert X-ray diffactometer. The electrolyte is analyzed after catalytic test by inductively coupled plasma atomic emission spectrometry (ICP-AES, Perkin-Elmer Optima 3100XL). The electrochemical OER experiments are performed on a CHI760D (CH Instruments, Shanghai, China) with a three-electrode cell. Co-PB/Pt or Ni-PB/Pt (5 mg), 2 mL of Nafion solution (0.05 wt.%), and 2 mL of water are ultrasonically mixed. One portion of the catalyst ink (70 µg cm-2) is drop-casted onto a glassy carbon (GC) electrode. The modified GC electrode is used as the working electrode. Graphite rod and saturated calomel electrodes (SCE) serve as the counter electrode and reference electrode, respectively. The OER activity is evaluated by linear sweep voltammetry (LSV) tests in 1 M KOH at a scan rate of 2 mV s-1. The potentials are normalized by a reversible hydrogen electrode (RHE). Commercial Pt/C (10%) catalyst is employed for comparison. The overpotential at a current density of 1 mA cm-2 is defined as the onset potential. The electrochemical double-layer capacitance (Cdl) is collected between the potential range where no faradic processes at varied scan rates, which is used to calculate the ECSA of Co-PB/Pt and Ni-PB/Pt catalysts.

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Resultants and discussion

Fig. 1a shows the powder XRD pattern of Co-PB/Pt. All the diffraction peaks can be well assigned to cubic Co3[Fe(CN)6]2·10H2O (PDF No. 46-0907) and cubic Pt (PDF No. 04-0802), indicating the formation of composite catalyst. And high crystalline is observed from both Co-PB and Pt. The morphology of Co-PB/Pt is recorded with SEM (Fig. 1b), in which uniform nanocubes with size of 140-160 nm are observed. Notably, the morphology and size of Co-PB/Pt show no detectable changes in comparison with the pristine Co-PB (Fig. S1). In the TEM image (Fig. 1c), cube-like nanostructure is also observed for Co-PB/Pt. Notably, Pt nanoparticles with well dispersion can be observed on the surface of nanocubes. To observe these nanoparticles more clearly, the high-resolution TEM (HRTEM) image is presented. In Fig. 1d, monodispersed Pt nanoparticles on the surface of Co-PB show several nanometers in size. The EDX elemental mapping in Fig. 1e illustrates that, elements Co and Fe are well overlapped and fully fill the nanocubes. The well dispersed Pt element indicates the well dispersion of Pt nanoparticles on the surface of Co-PB nanocubes. The as-obtained Ni-PB/Pt is also characterized with SEM and TEM techniques. In the SEM image (Fig. S2a), nanocubes with uniform size of 60-80 nm are obtained for Ni-PB/Pt, which is in accordance with pristine Ni-PB (Fig. S2d). Observed from the TEM image (Fig. S2b), Pt nanoparticles are well dispersed on the surface of Ni-PB, possessing the size of several nanometers. The XRD pattern of Ni-PB/Pt is shown in Fig. S2c. The diffraction peaks corresponding to cubic Ni3[Fe(CN)6]2·H2O (PDF No. 82-2283) and cubic Pt (PDF No. 04-0802) are detected, and no other impurity peak is observed. It is generally accepted that the valence state of metal centers is critical for the intrinsic catalytic activity of transition metal based electrocatalyst. In the designed composite catalyst, a hybrid interface of Pt/transition metal (Co or Ni) is fabricated towards better catalytic activity. XPS measurements are 5



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performed over these interfaces of Co-PB/Pt and Ni-PB/Pt to probe the possible tune of valence state on catalytic centers. The survey XPS spectrum of Co-PB/Pt is shown in Fig. S3. Elements Pt, Co, Fe, C, N, and O are detected, which is in consistent with XRD result. The Pt 4f XPS spectrum is shown in Fig. 2a, which is deconvoluted into two pairs of peaks. One pair of peaks of 73.3 eV and 76.8 eV is assigned to Pt (0) species (blue curves), and the other pair of peaks at 74.0 eV and 77.5 eV is related to the surface Pt (II) species (red curves).39,40 The high-resolution Co 2p XPS spectrum for Co-PB/Pt is shown in Fig 2b. The Co 2p spectrum for Co-PB is also presented for comparison. For Co-PB/Pt, the peaks at 783.1 and 798.0 eV are observed, corresponding to Co 2p3/2 and Co 2p1/2 of Co2+.16,41 Interestingly, these peaks show positive shifts of ~0.5 eV than the corresponding Co 2p peaks of Co-PB, indicating that the electronic structure of Co is evidently tuned to the higher oxidation state as a result of anchored Pt specie.42-44 Nothing that, the higher oxidation state of Co centers is advantageous for OER activity.42-44 Moreover, the XPS measurement on Ni-PB/Pt shows the similar result, further confirming the tune effect of Pt on the valence state of Ni species. The Ni 2p XPS spectra of Ni-PB/Pt and Ni-PB are shown in Fig. S4. Clearly, Pt on Ni-PB surface acts as electron absorber to drive Ni to a higher oxidation state, resulting in a positive shift of 1.3 eV. Concluded from the above results, the loaded Pt nanoparticles at the MOFs interface can serve as electronegative metal to tune the chemical state of transition metal (Co or Ni) active centers, thus improving their intrinsic OER activity. The OER performances of the as-obtained Co-PB/Pt and Ni-PB/Pt are characterized using a rotating disk electrode (RDE) configuration. Fig. 3a shows the polarization curves of Co-PB/Pt, Co-PB, Pt/C (10 wt.%) and RuO2 in 1 M KOH. Pt/C catalyst delivers low OER activity, affording a current density of 1 mA cm-2 at overpotential of 480 mV. Co-PB shows relatively active OER performance, delivering a current density of 10 mA cm-2 at overpotential of 340 mV. This value is lower than that (408 mV) of

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UTSA-16 reported in Ref. 32, which is even better than the onset overpotential (potential at current density of 1 mA cm-2) of FeTPyP-Co (351 mV),33 Co-WOC-1 (390 mV),34 Co-ZIF-9 (510 mV),37 and Co-TpBpy (400).38 After introducing Pt nanoparticles on its surface, the OER activity of Co-PB/Pt shows remarkable improvement. Observed from the LSV curves of Co-PB/Pt and Co-PB, Co-PB/Pt shows appreciable negative-shift overpotential and higher current density. A much lower overpotential of 300 mV at 10 mA cm-2 is obtained for Co-PB/Pt. In comparison with RuO2 catalyst, Co-PB/Pt affords the current of 10 mA cm-2 at slightly higher overpotential, however, it delivers higher current than RuO2 when potential exceeds 1.57 V, indicating excellent OER activity. The detailed OER values of onset overpotential and η10 obtained from the reported Co/Ni based catalysts and some MOFs catalysts are shown in Table 1. Clearly, Co-PB/Pt shows better OER activity than the MOF based electrocatalysts.26,32,33,34,37,38 More importantly, the OER activity of Co-PB/Pt is superior than most of the Co-containing catalysts,7,11,16,22 which is close to the best Co-containing catalysts ever reported, 12,26,27,29 making it a promising candidate for high-performance catalyst towards OER. The corresponding Tafel plots are shown in Fig. 3b. Co-PB/Pt shows a much lower Tafel slope (68 mV dec-1) than Co-PB (83 mV dec-1), indicating that Co-PB/Pt possesses more rapid OER kinetics. In Fig. 3a, an oxidation peak is observed in the LSV curves of both Co-PB/Pt and Co-PB, which is generally ascribed to the reaction of Co(II)-to-Co(III).32,34 Interestingly, the oxidation peak of Co-PB/Pt (1.22 V versus RHE) shows appreciably negative shift than that of Co-PB (1.34 V). In the aforementioned XPS measurements, the introduced Pt species generates the higher oxidation state of Co(II), which should be responsible for the negative shift of the oxidation peak of Co(II)/Co(III) for Co-PB/Pt. The repeated OER experiments are conducted to evaluate the catalytic durability of

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Co-PB/Pt and Co-PB. In Fig. 3c, the second and 500th LSV curves of Co-PB/Pt show negligible changes, showing the good catalytic stability. On the contrary, Co-PB (Fig. 3d) delivers a detectable current decrease after 500 cycles, showing suppressed overpotential of 380 mV at 10 mA cm-2. Notably, after the initial cycle, the oxidation peak related to Co(II)-to-Co(III) disappears in the following LSV curves for Co-PB. This phenomenon is also observed in its Cyclic voltammetry (CV) characterizations (Fig. S5b). However, in Fig. 3c, this oxidation peak in the LSV curves of Co-PB/Pt exists throughout the durability test, which shifts to a lower peak position of 1.15 V after the first cycle. This shift of oxidation peak is further confirmed by the repeated CV measurements of Co-PB/Pt (Fig. S5a). It is widely believed that, Co(II) species in Co-based electrocatalysts firstly transfer into high oxidation state, which then serves as active sites for catalytic reaction, indicating that the Co(II)/Co(III) couple is crucial for the OER performance.32,34 In the hybrid catalyst, the introduced Pt assures the constant oxidation reaction of Co(II)-to-Co(III), thus maintains the OER activity during stability test. To further confirm this, the Co-PB/Pt and Co-PB after five cycles of LSV test are characterized by the CV techniques. In Fig. 3e, no obvious redox peak is detected for Co-PB, in agreement with the LSV test. For Co-PB/Pt, a pair of quasi-reversible redox peaks is observed at 1.62 V and 1.09 V in the CV curve, which is assigned to the reversible reaction between Co(II) and Co(III).34 Therefore, it can be concluded that Pt plays the key role to assure the reversible reaction of Co(II)-to-Co(III), endowing Co-PB/Pt with improved OER activity and stability. To probe the root of the OER stability, the Co-PB/Pt after catalytic test is characterized with SEM and XRD techniques. Both SEM (Fig. S6a) and XRD (Fig. S6b) show that, no detectable changes are observed from Co-PB/Pt, indicating the good stability. Moreover, the electrolyte after OER test is analyzed by ICP. Co and Pt elements are not detected, further confirming the stability of Co-PB/Pt catalyst.

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Previous literature has reported that, introducing noble metal on metal oxide catalyst can induce additional active sites at their interface.41 In our work, the introduced Pt on the surface of Co-PB nanocube possibly supplies the same effect. To verify this hypothesis, the electrochemical capacitance measurements of Co-PB/Pt (Fig. S7a) and Co-PB (Fig. S7b) are conducted to obtain their electrochemically active surface area. As shown in Fig. 3f, Co-PB/Pt shows much higher electrochemical double-layer capacitance of 57 mF cm-2 than that of 27 mF cm-2 for Co-PB. This result indicates that the introduced Pt indeed creates more active sites, which should contribute to the improved OER activity of Co-PB/Pt. EIS measurements are also performed to show the improved electrode kinetics. The Randles equivalent circuit is used to determine the ionic and charge transfer resistances of the OER process. As shown in the inset of Fig. S8, Rs corresponds to the resistance of the electrolyte. Cdl is the double-layer capacitance. And Rct is related to the charge transfer resistance. The arc in the Nyquist plots is related to the Rct of OER. Clearly, Co-PB/Pt possesses much smaller charge-transfer resistance than Co-PB, indicating that the introduced Pt greatly enhances the reaction kinetics of OER. This should be another contribution for the improved OER performance of Co-PB/Pt. Concluded from the aforementioned results, the boosting effects of Pt on Co-PB nanocubes mainly include: 1) tune the active centers of Co to a higher oxidation state that is favorable for OER activity; 2) assure the constant oxidation reaction of Co(II)-to-Co(III) to maintain the catalytic stability; 3) introduce more active sites for improved OER performance; 4) accelerate charge transfer to enhance reaction kinetics. Endowed with the combined benefits, the designed Co-PB/Pt catalyst exhibits significantly improved OER activity and stability than pristine Co-PB. The OER performances of Ni-PB/Pt are also carried out and similar inspiring effect of loaded Pt on the OER activity of Ni-PB is obtained. LSV tests (Fig. 4a) show that pristine Ni-PB delivers a

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relatively low onset overpotetnial of 300 mV for OER. After incorporation with Pt, Ni-PB/Pt exhibits remarkably improved OER performance than Ni-PB, including much lower overpotetnial of 320 mV at 10 mA cm-2 than that of 470 mV for Ni-PB, and larger catalytic current. Notably, Ni-PB/Pt affords current density as high as RuO2 when the potential exceeds 1.62 V. Observed from Table 1, this value of 320 mV is superior to most of the MOF based catalysts26,32,33,34,37,38 and Ni-containing catalysts,20,25,30 which is among the best results of Ni-containing catalysts. 13,14,17,21 Importantly, an oxidation peak is observed at 1.48 V in the LSV curve of Ni-PB/Pt, which is assigned to the reaction of Ni(II)-to-Ni(III).14,42 This oxidation reaction is also shown up in the CV curve in Fig. S9. On the contrary, this peak cannot be detected in the LSV curve of Ni-PB. Similar to the result of Co-PB/Pt, this should be ascribed to the loaded Pt nanoparticles, which could tune the oxidation state of Ni in the MOFs. This is in agreement with the XPS characterization. The corresponding Tafel plots are displayed in Fig. 4b to evaluate the reaction kinetics for OER. In the overpotential region between 200 mV and 250 mV, Ni-PB/Pt shows a low Tafel slope of 37 mV dec-1, indicating that the adsorption of oxygenated species on the interface of Pt and Ni-PB takes place very fast.14 With the overpotential increasing to the region of 300-370 mV, the Tafel slope sharply increases to 98 mV dec-1, suggesting that the formation of O=O bond should be the rate-limited step.14 Noting that, this value of 98 mV dec-1 is still much smaller than that of 144 mV dec-1 for Ni-PB, due to the fascinating effect of Pt. Fig. 4c displays the repeated LSV curves of Ni-PB/Pt. Clearly, the first and 200th LSV curves overlap very well. When the test cycles are extended to 500, the intensity of the oxidation peak fades obviously. However, in comparison with the first LSV curve, no detectable catalytic current density is obtained from the 500th cycle. All these results indicate the good catalytic stability of Ni-PB/Pt. The SEM image (Fig. S6c) and XRD pattern (Fig. S6d) of Ni-PB/Pt after OER test are obtained, which show no

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changes in comparison with freshly obtained Ni-PB/Pt, thus revealing its good stability during the catalytic process. The ICP analysis indicates that, no Ni and Pt are observed in the electrolyte after catalytic test, showing the good stability of catalyst. The Cdl of Ni-PB/Pt and Ni-PB are also collected and shown in Fig. 4d. Ni-PB/Pt shows a higher Cdl of 1.4 mF cm-2 than that (1.0 mF cm-2) of Ni-PB, due to the additional active sites introduced by loaded Pt nanoparticles. In the EIS measurements (Fig. S10), much decreased charge-transfer resistance is obtained from Ni-PB/Pt than pristine Ni-PB, assuring enhanced OER reaction kinetics of Ni-PB/Pt. Therefore, loaded Pt nanoparticles on Ni-PB also supply combined advantages for improving both intrinsic and extrinsic OER activity of Ni-PB.

Conclusions

In summary, Co-PB and Ni-PB nanocubes are developed as potential electrocatalysts for OER. Pt nanopartiles are loaded on Co-PB and Ni-PB towards improved OER activity. Moreover, the designed hybrid catalysts are used as modular systems to demonstrate the engineering improvements in MOFs towards electrochemical OER. The positive shifts in the Co and Ni XPS spectrum of designed hybrid catalysts originally reveal the change in the catalytic activity of Prussian blue analogues, which is responsible for the decreased overpotetnial and increased catalytic current for OER. Subsequent chemical characterizations further identify more advantages of Pt addition for enhancing the OER of Co-PB/Pt and Ni-PB/Pt, in terms of more active sites, enhanced charge transfer, and improved catalytic stability. A low overpotential of 300 mV is obtained for Co-PB/Pt to afford a current density of 10 mA cm-2, and 320 mV is needed for Ni-PB/Pt to generate a current of 10 mA cm-2. Our present catalyst synthesis strategy can be extended to prepare hybrid catalysts of Pt or other noble metals with MOFs for the applications in sustainable energy storage and conversion systems.

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Supporting Information

SEM image of pristine Co-PB. SEM image, TEM image, and XRD pattern of Ni-PB/Pt. SEM image of pristine Ni-PB. The survey XPS spectrum of Co-PB/Pt. The high-resolution XPS spectrum of Ni 2p in Ni-PB/Pt and Ni-PB. CV curves of Co-PB/Pt and Co-PB. SEM image and XRD pattern of Co-PB/Pt after OER test. SEM image and XRD pattern of Ni-PB/Pt after OER test. Electrochemical capacitance measurements of Co-PB/Pt and Co-PB conducted in a potential range where no faradic processes. EIS spectra of Co-PB/Pt and Co-PB. Polarization curve and CV curve of Ni-PB/Pt. EIS spectra of Ni-PB/Pt and Ni-PB.

Acknowledgements

We thank for the financial support from Natural Science Foundation (2016GGX104019, ZR2014JL015, ZR2014EMM004) of Shandong Province.

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20. He, P.; Yu, X. Y.; Lou, X. W. Carbon-incorporated nickel-cobalt mixed metal phosphide nanoboxes with enhanced electrocatalytic activity for oxygen evolution. Angew. Chem. Int. Edit. 2017, 56, 3897-3900. 21. Zou, H. H.; Yuan, C. Z.; Zou, H. Y.; Cheang, T. Y.; Zhao, S. J.; Qazi, U. Y.; Zhong, S. L.; Wang, L.; Xu, A. W. Bimetallic phosphide hollow nanocubes derived from a prussian-blue-analog used as high-performance catalysts for the oxygen evolution reaction. Catal. Sci. Technol. 2017, 7, 1549-1555. 22. Wu, R.; Wang, D. P.; Zhou, K.; Srikanth, N.; Wei, J.; Chen, Z. Porous cobalt phosphide/graphitic carbon polyhedral hybrid composites for efficient oxygen evolution reactions. J. Mater. Chem. A 2016, 4, 13742-13745. 23. Tian, T.; Huang, L.; Ai, L.; Jiang, J. Surface anion-rich NiS2 hollow microspheres derived from metal–organic frameworks as a robust electrocatalyst for the hydrogen evolution reaction. J. Mater. Chem. A 2017, 5, DOI: 10.1039/C7TA06671F. 24. Hu, H.; Han, L.; Yu, M.; Wang, Z.; Lou, X. W. Metal-organic-framework-engaged formation of Co nanoparticle-embedded carbon@Co9S8 double-shelled nanocages for efficient oxygen reduction. Energy Environ. Sci. 2016, 9, 107-111. 25. Yang, L.; Gao, M.; Dai, B.; Guo, X.; Liu, Z.; Peng, B. An efficient NiS@N/S-C hybrid oxygen evolution electrocatalyst derived from metal-organic framework. Electrochim. Acta 2016, 191, 813-820. 26. Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Metal-organic framework derived hybrid Co3O4-carbon porous nanowire arrays as reversible oxygen evolution electrodes. J. Am. Chem. Soc. 2014, 136, 13925-13931.

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Table 1 Comparison of electrocatalytic OER activity for MOFs based, Co-containing, and Ni-containing catalysts in alkaline media. Electrocatalysts

Onset potential (mV)

Etched CoSn(OH)6 S,N-Fe/N/C-CNT CoN nanowire

η10 (mV)

Ref.

274 370 290

7. Energy Environ. Sci. 2016, 9, 473 9. Angew. Chem. Int. Edit. 2017, 56, 610 11. Angew. Chem. Int. Edit. 2016, 55, 8670

CoNP@NC/NG NiP nanoplate NiP foam CoMoS3 nanotube Ni3Se2 NiCoP/C

250 240 210

390 300 220 320 290 330

12. J. Mater. Chem. A 2016, 4, 10575 13. Energy Environ. Sci. 2016, 9, 1246 14. J. Mater. Chem. A 2016, 4, 5639 16. J. Mater. Chem. A 2017, 5, 11309 17. Energy Environ. Sci. 2016, 9, 1771 20. Angew. Chem. Int. Edit. 2017, 56, 3897

NiFeP nanocube CoP@GC NiS@N/S-C Co3O4-C nanowire Co(C12H6O4)(H2O)4 CoOx@CN FeCoNi

240 310 240 320

290 345 417 290 520 260 288

21. Catal. Sci. Technol. 2017, 7, 1549 22. J. Mater. Chem. A 2016, 4, 13742 25. Electrochim. Acta 2016, 191, 813 26. J. Am. Chem. Soc. 2014, 136, 13925 26. J. Am. Chem. Soc. 2014, 136, 13925 27. J. Am. Chem. Soc. 2015, 137, 2688 29. ACS Sustainable Chem. Eng. 2017, 5, 4771

Ni@graphene

300

370

30. ACS Sustainable Chem. Eng. 2017, 5, 4771

MAF-X27-OH UTSA-16

240 320

292 408

31. J. Am. Chem. Soc. 2016, 138, 8336

32. ACS Appl. Mater. Interfaces 2017, 9, 7193

FeTPyP-Co Co-WOC-1 Co-ZIF-9 Co-TpBpy Co-PB/Pt Co-PB Ni-PB/Pt Ni-PB

351 390 510 400

300

300 340 320 470

33. J. Am. Chem. Soc. 2016, 138, 3623 34. Angew. Chem. Int. Ed. 2016, 55, 2425 37. Nanoscale 2014, 6, 9930 38. Chem. Mater. 2016, 28, 4375 This work This work This work This work

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Figures

Fig. 1 (a) Powder XRD pattern, (b) SEM image, (c) TEM image, (d) HRTEM image, (e) EDX elemental mapping image of Co-PB/Pt.

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Fig. 2 (a) The high-resolution XPS spectrum of Pt 4f of Co-PB/Pt. (b) The XPS spectra of Co 2p in Co-PB/Pt and Co-PB.

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Fig. 3 (a) Polarization curves of Co-PB/Pt, Co-PB, Pt/C and RuO2 in 1 M KOH. (b) Corresponding Tafel polts. (c) The first, second, and 500th cycles of LSV curves for Co-PB/Pt. (d) The first, second, and 500th cycles of LSV curves for Co-PB. (e) CV curves of Co-PB/Pt and Co-PB after the initial LSV test. (f) Measured capacitive currents plotted as a function of scan rate of Co-PB/Pt and Co-PB.

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Fig. 4 (a) Polarization curves of Ni-PB/Pt, Ni-PB, and RuO2 in 1 M KOH. (b) Corresponding Tafel polts of Ni-PB/Pt and Ni-PB. (c) The first, 200th, and 500th cycles of LSV curves for Ni-PB/Pt. (d) Measured capacitive currents plotted as a function of scan rate of Ni-PB/Pt and Ni-PB.

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For Table of Contents Use Only

Pt is introduced into Prussian blue analogues (Co3[Fe(CN)6]2 and Ni3[Fe(CN)6]2 nanocubes) as modular catalysts to understand the interaction between transition metals-containing MOF catalyst and noble metals in enhancing the electrochemical oxygen evolution reaction (OER), and the as-obtained hybrid catalysts exhibit significantly improved OER activity due to the promoting effect of added Pt.

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