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Oct 30, 2017 - College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266510, China...
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Loading Pt Nanoparticles on Metal−Organic Frameworks for Improved Oxygen Evolution Jinxue Guo,† Xinqun Zhang,† Yanfang Sun,‡ Lin Tang,† Qingyun Liu,§ and Xiao Zhang*,† †

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 ‡ 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 S Supporting Information *

ABSTRACT: An 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 toward 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 the 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 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

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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 to be a clean and competitive solution to produce hydrogen and oxygen energy, which possesses the specific advantage of carbon-free emissions.1,2 This system includes two electrolysis processes: anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER). Generally, OER is believed to be the rate-limiting step of water splitting due to its sluggish kinetics, which is also a critical reaction involved in other energy conversion systems.3 A high-performance electrocatalyst is crucial for the OER kinetics. Until now, state-of-the-art catalysts toward OER are Ru- and Ir-based oxides; nevertheless, their widespread application is hindered by limited supply and high cost.4,5 Therefore, development of Earth-abundant alternatives with sufficient output is the research focus. Over the past years, transition metal hydroxides, 6,7 carbides,8−10 nitrides,11,12 phophides,12−15 and chalocogenides,16,17 as well as nonmetal-based catalysts,18,19 have been studied as promising cost-effective catalysts for OER. Specifically, © 2017 American Chemical Society

metal−organic frameworks (MOF)-derived transition metal compounds, such as phophides,20−22 sulfides,23−25 oxides,26−28 and metal-carbon composites,29,30 have triggered special interests because of the additional advantages endowed with the structural benefits of precursor MOFs. Lou and his coworkers have prepared an NiCoP/C nanobox using ZIF-67 as the precursor, which shows excellent activity and stability toward OER.20 Yang et al. have synthesized an 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 functioning for OER performance, and their inherent catalytic activity needs to be further improved. In this work, the 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 (CoReceived: August 23, 2017 Revised: October 9, 2017 Published: October 30, 2017 11577

DOI: 10.1021/acssuschemeng.7b02926 ACS Sustainable Chem. Eng. 2017, 5, 11577−11583

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) Powder XRD pattern, (b) SEM image, (c) TEM image, (d) HRTEM image, and (e) EDX elemental mapping image of Co-PB/Pt. atomic emission spectrometry (ICP-AES, PerkinElmer 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. A 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). A 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 with no faradic processes at varied scan rates, which is used to calculate the ECSA of Co-PB/Pt and Ni-PB/ Pt catalysts.

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 studied. It is found that Pt could tune the valence state of Co/Ni, introduce more active sites, and enhance reaction kinetics, thus improving the intrinsic and extrinsic catalytic activity of MOFs simultaneously.

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EXPERIMENTAL SECTION

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 °C. Here, 60 mg of the synthesized Co3[Fe(CN)6]2 nanocubes are dispersed in 30 mL of water. H2PtCl6·3H2O (3 mg) is dissolved in 30 mL of water. After being mixed, the mixture is heated at 80 °C for 4 h. The resultant powder is transferred in a tube furnace and heated at 120 °C for 3 h in an Ar/H2 atmosphere to obtain the final Co-PB/Pt. Ni-PB/Pt is prepared via the same strategy using Ni(NO3)2 as the 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 microscopy (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 (PerkinElmer), 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 a catalytic test by inductively coupled plasma

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RESULT AND DISCUSSION Figure 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 a composite catalyst, and high crystallinity is observed from both Co-PB and Pt. The morphology of Co-PB/Pt is recorded with SEM (Figure 1b), in which uniform nanocubes the 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 (Figure S1). In the TEM image (Figure 1c), a cube-like nanostructure is also observed for Co-PB/Pt. Notably, Pt nanoparticles with good dispersion can be observed on the 11578

DOI: 10.1021/acssuschemeng.7b02926 ACS Sustainable Chem. Eng. 2017, 5, 11577−11583

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

Figure 3. (a) Polarization curves of Co-PB/Pt, Co-PB, Pt/C, and RuO2 in 1 M KOH. (b) Corresponding Tafel polts. (c) First, second, and 500th cycles of LSV curves for Co-PB/Pt. (d) 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.

surface of nanocubes. To observe these nanoparticles more clearly, the high-resolution TEM (HRTEM) image is presented. In Figure 1d, monodispersed Pt nanoparticles on the surface of Co-PB are shown several nanometers in size. The EDX elemental mapping in Figure 1e illustrates that elements Co and Fe are well overlapped and fully fill the nanocubes. The well-dispersed Pt element indicates the good 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

(Figure S2a), nanocubes with uniform sizes of 60−80 nm are obtained for Ni-PB/Pt, which is in accordance with pristine NiPB (Figure S2d). Observed from the TEM image (Figure 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 Figure S2c. The diffraction peaks corresponding to cubic Ni3[Fe(CN)6]2·H2O (PDF No. 822283) and cubic Pt (PDF No. 04-0802) are detected, and no other impurity peak is observed. 11579

DOI: 10.1021/acssuschemeng.7b02926 ACS Sustainable Chem. Eng. 2017, 5, 11577−11583

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ACS Sustainable Chemistry & Engineering It is generally accepted that the valence state of metal centers is critical for the intrinsic catalytic activity of the transition metalbased electrocatalyst. In the designed composite catalyst, a hybrid interface of Pt/transition metal (Co or Ni) is fabricated toward better catalytic activity. XPS measurements are 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 Figure S3. Elements Pt, Co, Fe, C, N, and O are detected, which is consistent with XRD results. The Pt 4f XPS spectrum is shown in Figure 2a, which is deconvoluted into two pairs of peaks. One pair of peaks at 73.3 and 76.8 eV is assigned to Pt (0) species (blue curves), and the other pair of peaks at 74.0 and 77.5 eV is related to the surface Pt (II) species (red curves).39,40 The highresolution Co 2p XPS spectrum for Co-PB/Pt is shown in Figure 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 compared to 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 species.42−44 Note that the higher oxidation state of Co centers is advantageous for OER activity.42−44 Moreover, the XPS measurement on Ni-PB/Pt shows a similar result, further confirming the tuning 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 Figure S4. Clearly, Pt on the Ni-PB surface acts as an 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 an 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 NiPB/Pt are characterized using a rotating disk electrode (RDE) configuration. Figure 3a shows the polarization curves of Co-PB/ Pt, Co-PB, Pt/C (10 wt %), and RuO2 in 1 M KOH. The Pt/C catalyst delivers low OER activity, affording a current density of 1 mA cm−2 at an overpotential of 480 mV. Co-PB shows relatively active OER performance, delivering a current density of 10 mA cm−2 at an overpotential of 340 mV. This value is lower than that (408 mV) of UTSA-16 reported in ref 32, which is even better than the onset overpotentials (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 an appreciable negative-shift overpotential and higher current density. A much lower overpotential of 300 mV at 10 mA cm−2 is obtained for CoPB/Pt. In comparison with the RuO2 catalyst, Co-PB/Pt affords a current of 10 mA cm−2 at slightly higher overpotential; however, it delivers a higher current than RuO2 when the potential exceeds 1.57 V, indicating excellent OER activity. The detailed OER values of the 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 MOFs-based electrocatalysts.26,32−34,37,38 More importantly, the OER activity of Co-PB/Pt is superior to 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 a high-performance catalyst toward

Table 1. Comparison of Electrocatalytic OER Activity for MOFs-based, Co-Containing, and Ni-Containing Catalysts in Alkaline Media Electrocatalysts Etched CoSn(OH)6 S,N-Fe/N/C-CNT CoN nanowire CoNP@NC/NG NiP nanoplate NiP foam CoMoS3 nanotube Ni3Se2 NiCoP/C NiFeP nanocube CoP@GC NiS@N/S-C Co3O4-C nanowire Co(C12H6O4)(H2O)4 CoOx@CN FeCoNi Ni@graphene MAF-X27-OH UTSA-16 FeTPyP-Co Co-WOC-1 Co-ZIF-9 Co-TpBpy Co-PB/Pt Co-PB Ni-PB/Pt Ni-PB

Onset potential (mV)

250 240 210 240 310 240 320

300 240 320 351 390 510 400

300

η10 (mV)

ref.

274 370 290 390 300 220 320 290 330 290 345 417 290 520 260 288 370 292 408

7 9 11 12 13 14 16 17 20 21 22 25 26 26 27 29 30 31 32 33 34 37 38 This work This work This work This work

300 340 320 470

OER. The corresponding Tafel plots are shown in Figure 3b. CoPB/Pt shows a much lower Tafel slope (68 mV dec−1) than CoPB (83 mV dec−1), indicating that Co-PB/Pt possesses more rapid OER kinetics. In Figure 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 an appreciable negative shift compared to 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 Co-PB/Pt and Co-PB. In Figure 3c, the second and 500th LSV curves of CoPB/Pt show negligible changes, showing good catalytic stability. On the contrary, Co-PB (Figure 3d) delivers a detectable current decrease after 500 cycles, showing a 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 (Figure S5b). However, in Figure 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 the oxidation peak is further confirmed by the repeated CV measurements of Co-PB/Pt (Figure S5a). It is widely believed that Co(II) species in Co-based electrocatalysts first transfer into high oxidation states, which then serves as active sites for catalytic reactions, indicating that the Co(II)/Co(III) 11580

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Figure 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) First, 200th, and 500th cycles of LSV curves for Ni-PB/Pt. (d) Measured capacitive currents plotted as a function of scan rates of Ni-PB/Pt and Ni-PB.

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 maintaining the OER activity during the stability test. To further confirm this, Co-PB/Pt and Co-PB after five cycles of an LSV test are characterized by CV techniques. In Figure 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 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 a key role to ensure the reversible reaction of Co(II)-toCo(III), endowing Co-PB/Pt with improved OER activity and stability. To probe the root of OER stability, Co-PB/Pt after the catalytic test is characterized with SEM and XRD techniques. Both SEM (Figure S6a) and XRD (Figure S6b) show that no detectable changes are observed from Co-PB/Pt, indicating good stability. Moreover, the electrolyte after the OER test is analyzed by ICP. Co and Pt elements are not detected, further confirming the stability of the Co-PB/Pt catalyst. Previous literature has reported that introducing a noble metal on a metal oxide catalyst can induce additional active sites at their interface.41 In our work, the introduced Pt on the surface of a CoPB nanocube possibly supplies the same effect. To verify this hypothesis, the electrochemical capacitance measurements of Co-PB/Pt (Figure S7a) and Co-PB (Figure S7b) are conducted to obtain their electrochemically active surface areas. As shown in Figure 3f, Co-PB/Pt shows a much higher electrochemical double-layer capacitance of 57 mF cm−2 compared to 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 Figure S8, Rs corresponds to the resistance of the electrolyte. Here, 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 a 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 include mainly the following: (1) tune the active centers of Co to a higher oxidation state that is favorable for OER activity, (2) ensure the constant oxidation reaction of Co(II)-to-Co(III) to maintain the catalytic stability, (3) introduce more active sites for improved OER performance, and (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 a similar inspiring effect of loaded Pt on the OER activity of Ni-PB is obtained. LSV tests (Figure 4a) show that pristine Ni-PB delivers a relatively low onset overpotential of 300 mV for OER. After incorporation with Pt, Ni-PB/Pt exhibits remarkably improved OER performance compared to Ni-PB, including a much lower overpotential of 320 mV at 10 mA cm−2 compared to 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 MOFs-based catalysts26,32−34,37,38 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 in the CV curve in Figure S9. On the contrary, this peak cannot be detected in the LSV curve of Ni-PB. Similar to the result of CoPB/Pt, this should be ascribed to the loaded Pt nanoparticles, which could tune the oxidation state of Ni in the MOFs. This is in 11581

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ACS Sustainable Chemistry & Engineering agreement with the XPS characterization. The corresponding Tafel plots are displayed in Figure 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 the 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 the OO bond should be the rate-limiting 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 effect of Pt. Figure 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 NiPB/Pt. The SEM image (Figure S6c) and XRD pattern (Figure S6d) of Ni-PB/Pt after the OER test are obtained, which show no 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 the catalytic test, showing the good stability of the catalyst. The Cdl of Ni-PB/Pt and Ni-PB are also collected and shown in Figure 4d. Ni-PB/Pt shows a higher Cdl of 1.4 mF cm−2 compared to that (1.0 mF cm−2) of Ni-PB due to the additional active sites introduced by loaded Pt nanoparticles. In the EIS measurements (Figure S10), much decreased charge-transfer resistance is obtained from Ni-PB/Pt compared to pristine NiPB, 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.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 532 84022681. Fax: +86 532 84023927. E-mail: [email protected] (X. Zhang). ORCID

Xiao Zhang: 0000-0003-0165-1561 Notes

The authors declare no competing financial interest.

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

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REFERENCES

(1) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 2015, 44, 2060−2086. (2) Li, X.; Hao, X.; Abudula, A.; Guan, G. Nanostructured catalysts for electrochemical water splitting: current state and prospects. J. Mater. Chem. A 2016, 4, 11973−12000. (3) Cao, R.; Lai, W.; Du, P. Catalytic water oxidation at single metal sites. Energy Environ. Sci. 2012, 5, 8134−8157. (4) Stoerzinger, K. A.; Qiao, L.; Biegalski, M. D.; Shao-Horn, Y. Orientation-dependent oxygen evolution activities of rutile IrO2 and RuO2. J. Phys. Chem. Lett. 2014, 5, 1636−1641. (5) Ping, Y.; Nielsen, R. J.; Goddard, W. A., III The reaction mechanism with free energy barriers at constant potentials for the oxygen evolution reaction at the IrO2 (110) surface. J. Am. Chem. Soc. 2017, 139, 149−155. (6) Dionigi, F.; Strasser, P. NiFe-based (oxy)hydroxide catalysts for oxygen evolution reaction in non-acidic electrolytes. Adv. Energy Mater. 2016, 6, 1600621. (7) Song, F.; Schenk, K.; Hu, X. A. nanoporous oxygen evolution catalyst synthesized by selective electrochemical etching of perovskite hydroxide CoSn(OH)6 nanocubes. Energy Environ. Sci. 2016, 9, 473− 477. (8) Seh, Z. W.; Fredrickson, K. D.; Anasori, B.; Kibsgaard, J.; Strickler, A. L.; Lukatskaya, M. R.; Gogotsi, Y.; Jaramillo, T. F.; Vojvodic, A. Twodimensional molybdenum carbide (MXene) as an efficient electrocatalyst for hydrogen evolution. ACS Energy Lett. 2016, 1, 589−594. (9) Chen, P.; Zhou, T.; Xing, L.; Xu, K.; Tong, Y.; Xie, H.; Zhang, L.; Yan, W.; Chu, W.; Wu, C.; Xie, Y. Atomically dispersed iron-nitrogen species as electrocatalysts for bifunctional oxygen evolution and reduction reactions. Angew. Chem., Int. Ed. 2017, 56, 610−614. (10) Jiang, J.; Liu, Q.; Zeng, C.; Ai, L. Cobalt/molybdenum carbide@ N-doped carbon as a bifunctional electrocatalyst for hydrogen and oxygen evolution reactions. J. Mater. Chem. A 2017, 5, 16929−16935. (11) Zhang, Y.; Ouyang, B.; Xu, J.; Jia, G.; Chen, S.; Rawat, R. S.; Fan, H. J. Rapid synthesis of cobalt nitride nanowires: highly efficient and low-cost catalysts for oxygen evolution. Angew. Chem., Int. Ed. 2016, 55, 8670−8674. (12) Zhong, X.; Jiang, Y.; Chen, X.; Wang, L.; Zhuang, G.; Li, X.; Wang, J. G. Integrating cobalt phosphide and cobalt nitride-embedded nitrogen-rich nanocarbons: high-performance bifunctional electro-

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CONCLUSIONS In summary, Co-PB and Ni-PB nanocubes are developed as potential electrocatalysts for OER. Pt nanoparticles are loaded on Co-PB and Ni-PB toward improved OER activity. Moreover, the designed hybrid catalysts are used as modular systems to demonstrate the engineering improvements in MOFs toward 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 overpotential 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 applications in sustainable energy storage and conversion systems.

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spectrum of Co-PB/Pt. High-resolution XPS spectra of Ni 2p in Ni-PB/Pt and Ni-PB. CV curves of Co-PB/Pt and Co-PB. SEM images and XRD patterns of Co-PB/Pt and Ni-PB/Pt after OER tests. Electrochemical capacitance measurements of Co-PB/Pt and Co-PB conducted in a potential range with 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. (PDF)

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02926. SEM images of pristine Co-PB and Ni-PB. SEM image, TEM image, and XRD pattern of Ni-PB/Pt. Survey XPS 11582

DOI: 10.1021/acssuschemeng.7b02926 ACS Sustainable Chem. Eng. 2017, 5, 11577−11583

Research Article

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DOI: 10.1021/acssuschemeng.7b02926 ACS Sustainable Chem. Eng. 2017, 5, 11577−11583