Highly Ordered Mesoporous Bimetallic Phosphides as Efficient

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Highly Ordered Mesoporous Bimetallic Phosphides as Efficient Oxygen Evolution Electrocatalysts Shaofang Fu, Chengzhou Zhu, Junhua Song, Mark Engelhard, Xiaolin Li, Dan Du, and Yuehe Lin ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00408 • Publication Date (Web): 15 Sep 2016 Downloaded from http://pubs.acs.org on September 18, 2016

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Highly Ordered Mesoporous Bimetallic Phosphides as Efficient Oxygen Evolution Electrocatalysts Shaofang Fu,1 Chengzhou Zhu,1* Junhua Song,1 Mark H. Engelhard,2 Xiaolin Li,2 Dan Du,1 Yuehe Lin1,2* 1

School of Mechanical and Materials Engineering, Washington State University, WA 99164,

USA 2

Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory,

Richland, WA 99354, USA AUTHOR INFORMATION Corresponding Author *[email protected] * [email protected]

ABSTRACT

Oxygen evolution from water using earth abundant transition metal based catalysts is of importance for the commercialization of water electrolyzer. Herein, we report a hard templating method to synthesize transition metal phosphides with uniform shape and size. By virtue of the

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structural feature, synergistic effect among metals and the in situ formed active species, the asprepared phosphides with optimized composition present enhanced electrocatalytic performance towards oxygen evolution reaction in alkaline solution. In details, the catalyst with optimized composition reaches a current density of 10 mA/cm2 at a potential of 1.511 V vs reversible hydrogen electrode, which is much lower than that of commercial RuO2 catalyst. Our work offers a new strategy to optimize the catalysts for water splitting by controlling the morphology and composition.

TOC GRAPHICS

Global energy consumption is gradually shifting from fossil fuels to sustainable and clean energy sources because of the increasing energy demands as well as environmental issues.1-4 The renewable hydrogen, produced by electrochemical splitting of water, is considered as a promising alternative energy source. However, there are still some obstacles hindering the widespread application of water splitting, which include the low electrocatalytic activity, poor stability and high cost of precious metal catalysts.5-7 As one critical half reaction of water splitting, oxygen evolution reaction (OER) is a kinetically slow process, which limits electrocatalysis of oxygen evolution on industrial scale.8-10 Even though catalysts like Pt, Ru, Ir


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have exhibited desirable electrocatalytic activity, their scarcity and high cost render the impracticality of their widespread use.11-13 To overcome these drawbacks, a multitude of efforts have been devoted to develop earth-abundant and highly active OER catalysts, such as transition metal nanomaterials.14-16 Several strategies have been demonstrated to be effective to improve the catalytic activity of transition metal-based materials, which include introducing dopant, surface states tuning, and formation of hybrid catalysts.17-20 For instance, Zhang and co-workers developed a facile procedure to synthesize gelled FeCoW oxy-hydroxide with homogenous metal distribution under room-temperature, which presents the lowest overpotential (0.191 mV) reported at 10 mA/cm2 in alkaline solution and excellent long term stability.21 The authors argued that the enhanced performance was attributed to the favorable local coordination environment and electronic structure produced by the synergistic interplay between W, Fe and Co. Due to the complementary nature of defect-free single-crystal electrocatalyst and the reversible adapting, single-crystal Co3O4@CoO core-shell nanocube was also reported as robust OER catalysts.22 Recently, transition metal phosphides (TMPs) have been extensively investigated as new earth-abundant and highly active OER catalysts. 23-25 However, bimetallic phosphides are rarely reported even though they might present superior catalytic performance compared with their monometallic counterparts. Arise from tailoring the intrinsic nature of the nanomaterials by tuning the elemental composition, precise control of the morphology is considered as another effective strategy to improve the electrocatalytic properties of nanocatalysts. Particularly, mesoporous nanomaterials with large pores are of particular interest in electrocatalysis. Typical example is that Yamauchi’s group has successfully synthesized various mesoporous nanostructures using different templates by electrochemical or chemical reduction method.26-28 These well-defined mesoporous structures


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with large internal surface area and high porosity are expected to provide large numbers of active sites which are favorable for mass transport and electron transfer, thus improving their catalytic performance.29-31 Based on these advantages, it is expected that the combination of structural and compositional effect could provide more opportunities to tailor the electrocatalytic properties. Herein, we for the first time synthesized a family of Co-Ni phosphides (CoNiP) with highly ordered mesoporous nanostructures with large mesopores and controllable composition using a facile hard templating method. Because of the large internal surface area, nanosized wall, and optimized composition, the as-prepared CoNiP nanocatalysts (CoNiP NCs) exhibit enhanced catalytic performance toward OER in alkaline solution.

Figure 1. TEM images of KIT-6 (A), CoP (B), and NiP (C). In this work, cubic ordered KIT-6 was first prepared according to the previous report.4 The obtained KIT-6 (Figure 1A) was then used as hard template for the further synthesis of highly ordered mesoporous CoP and NiP NCs. The transmission electron microscopy (TEM) images in Figure 1B and C reveal that both CoP and NiP NCs possess highly ordered mesopores after the removal of silica template. More precisely, the pore size is ~8 nm while the wall thickness ~5 nm


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in both nanostructures. The TEM images indicate that the composition does not have much effect on the final morphology of the as-prepared CoP and NiP NCs.

Figure 2. (A) XRD patterns of CoP and NiP NCs. (black: CoP2 PDF#26-0481, red: CoP PDF#29-0497, green: Ni5P4 PDF#18-0883, blue: Ni2P PDF#03-0953). High resolution XPS spectra of P (B), Co (C), and Ni (D) in CoP and NiP NCs. The crystallinity and phase information of the highly ordered mesoporous CoP and NiP NCs are confirmed by X-ray diffraction (XRD) patterns. As shown in Figure 2, all samples display highly crystallized structure with distinct characteristic peaks. For CoP NCs, CoP2 and CoP are the main species, while Ni5P4, and Ni2P are dominant in NiP NCs, indicating the successful phosphorization. X-ray photoelectron spectroscopy (XPS) measurements further confirm the existence of Co, P and Ni, P in highly ordered mesoporous CoP and NiP NCs, respectively


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(Figure S1). More precisely, the high resolution XPS spectra of P in Figure 2B reveal two peaks at ~ 129 eV and ~ 133 eV, reflecting the binding energy of P 2p and oxidized P species due to air exposure.32,33 The P 2p binding energy of 129 eV is negatively shifted from that of elemental P (130.2 eV).32 This result suggests that P has a partial negative charge due to the electron transfer from Co or Ni to P. For the Co 2p spectrum, the peak located at 780.88 eV corresponds to Co 2p3/2.34 As shown in Figure 2D, the peak at 852.77 eV for Ni 2p can be assigned to Niδ+ in Ni-P compound, while the peak at 856.23 eV indicates the oxidized Ni species.33,35

Figure 3. TEM images of highly ordered mesoporous Co3Ni1P (A), Co1Ni1P (B), and Co1Ni3P (C). Upon the successful synthesis of highly ordered mesoporous CoP and NiP NCs, we further demonstrate that this strategy is also applicable for the synthesis of bimetallic phosphides (CoNiP NCs) by introducing Co and Ni precursors simultaneously. As shown in Figure 3, the TEM images clearly show the highly ordered mesopores in Co3Ni1P, Co1Ni1P, Co1Ni3P NCs, which are similar as the morphology observed on CoP and NiP NCs. It is worth noting that the composition of Co and Ni precursors does not affect the final structure of the phosphides, indicating the universality of this synthesis procedure. Similar as CoP and NiP NCs, multiple


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phases are observed in CoNiP NCs according to the XRD patterns (Figure S2), indicating the existence of different phosphides in the as-prepared NCs.

Figure 4. LSV curves (A) and corresponding Tafel plots (B) of CoP, Co3Ni1P, NiP NCs and RuO2 in 1 M KOH. (C) Scan rate dependence of the current density of CoP, Co3Ni1P, NiP NCs and RuO2 at 1.025 V vs RHE. (D) Chronoamperometry curves recorded on CoP, Co3Ni1P, NiP NCs and RuO2 catalyst at specific potential E (V) vs RHE where the initial current density for each catalyst was 10 mA/cm2 in 1 M KOH (CoP: 1.53 V, Co3NiP: 1.51 V, Co1Ni1P: 1.52 V, Co1Ni3P: 1.53 V, NiP: 1.56 V, RuO2: 1.54 V). Table 1. Electrochemical parameters for OER on CoNiP NCs and commercial RuO2 catalysts in 1 M KOH.


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CoP

Co3Ni1P

Co1Ni1P

Co1Ni3P

NiP

RuO2

Eonset vs. RHE (V)

1.49

1.45

1.48

1.48

1.49

1.49

j at 1.65 V vs. RHE (mA/cm2)

151.74

225.49

133.01

71.38

54.74

119.03

E vs RHE at j = 10 mA/cm2 (V)

1.53

1.51

1.52

1.53

1.56

1.54

Tafel slope (mV/dec)

59.6

66.5

77.5

89.3

92.8

74.4

The electrocatatlytic activity of as-prepared CoP and NiP NCs for OER was investigated and compared with commercial RuO2 catalyst by linear sweep voltammetry (LSV) using a glassy carbon rotating disk electrode (RDE) in 1 M KOH solution. Both CoP and NiP NCs present characteristic OER catalytic behavior. Particularly, the electrocatalytic activity of CoP NCs is even better than that of commercial RuO2 catalyst, as illustrated in Figure 4A and Table 1. In order to explore the effect of composition on the catalytic properties, we further studied the OER catalytic performance of CoNiP NCs. Figure 4A and S2 reveal that composition has a significant effect on the corresponding activity. When the molar ratio of Co and Ni precursors is 3:1, that is, Co3Ni1P NCs, the best OER performance is achieved with the lowest onset potential of 1.45 vs RHE, which is lower than those of other CoNiP NCs and commercial RuO2 catalyst. We then compared the potential required for CoNiP NCs and RuO2 catalyst to deliver a current density of10 mA/cm2. As shown in Figure 4A inset and Table 1, when the current density reaches to 10 mA/cm2, CoP, Co3Ni1P, Co1Ni1P, Co1Ni3P, and NiP NCs shows an operating potential of 1.53, 1.51, 1.52, 1.53, 1.56 V vs RHE, respectively. Among all the CoNiP NCs, Co3Ni1P NCs presents the lowest potential, which is even lower than that of commercial RuO2 catalyst (1.54 V vs RHE). Moreover, the highest current density of 225.49 mA/cm2 among all the samples can be


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observed on Co3Ni1P at 1.65 V. The OER kinetics of as-prepared CoNiP NCs and commercial RuO2 were next evaluated by Tafel plots, as shown in Figure 4B. As expected, Co3Ni1P NCs exhibits a much smaller Tafel slope (66.5 mV/dec) compared with RuO2 electrocatalyst (74.4 mV/dec). In addition, the exchange current density can be estimated on the basis of Tafel plots using equation η/b = log (j/j0), where η is the overpotential, b is the Tafel slope, j is the current density and j0 is the exchange current density. The calculated j0 of Co3Ni1P NCs is 1.1×10-3 mA/cm2, which is higher than that of commercial RuO2 (7.5×10-4 mA/cm2). The lower Tafel slope and higher exchange current density imply the facile kinetics of OER on Co3Ni1P NCs. To shed light on the high catalytic activity of Co3Ni1P NCs, the electrochemical surface area (ECSA) of all samples, an important factor for electrocatalysts, was estimated from the CV curves in 1 M KOH solution. According to the CV curves obtained under different scan rate (Figure S3), linear relationship between scan rate and current density can be observed on all samples, as illustrated in Figure 4C. The electrochemical double-layer capacitance (Cdl), which is half of the linear slope, is normally used to represent the ECSA. The calculated Cdl value of CoP, Co3Ni1P, NiP NCs and RuO2 is 24, 15, 0.3, and 3.6 mF/cm2, respectively. The larger ECSA of Co3Ni1P NCs, caused by the unique mesoporous structure and the optimized composition, can provide more active sites and facilitate the gas diffusion, thus contributing to their enhanced catalytic activity. Moreover, fundamental studies have demonstrated that the OER overpotential is related to the free energy of the reaction.36,37 In our case, we speculate that the superior catalytic activity of Co3Ni1P NCs might be partially attributed to the lower reaction free energy by the introduction of bimetallic phosphides and the proper composition, which gives a catalytically favorable environment for OER.


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The stability of catalyst is another important factor for OER. The chronoamperometric test of Co3Ni1P NCs was carried out at designed potential (j=10 mA/cm2) in 1 M KOH for 12 h and compared with commercial RuO2 (Figure 4D). In contrast to the substantial current density loss of RuO2, CoP, Co3Ni1P and NiP NCs show enhanced catalytic activity during the stability test. In detail, the current density of Co3Ni1P increases during the first 6 h and then decreases slowly. Finally, a 60% current enhancement is remained, while RuO2 shows 37% loss on the current density. The morphology Co3Ni1P NCs after the stability test was also characterized as shown in Figure S5. The TEM image reveals that porous structure with large pores was well maintained after 12h. The high catalytic activity, favorable kinetics, as well as the good durability indicate that the as-prepared mesoporous CoNiP NCs with controllable shape and composition is promising for OER catalysis. To gain an insight into the mechanism of the catalytic activity on CoNiP NCs, the XPS analysis of CoP, Co3Ni1P, and NiP NCs was carried out after the OER test for 1h at specific potential (j=10 mV/cm2). As shown in Figure S6, the high resolution P XPS spectra of all the three samples after OER test reveal that only the oxidized phosphate species (~133 eV) can be observed after the test. The disappearance of phosphide peaks at ~129 eV indicates the oxidation on the surface of Co-P and Ni-P.38 For the high resolution XPS spectra of O on the samples after OER test (Figure S7), two intense peaks at 530 and 531.2 eV can be detected, indicating the presence of surface hydroxyls and oxygen from oxides/hydroxides.39,40 The outstanding OER activity and superior stability of the as-prepared Co3Ni1P NCs might be attributed to the following factors: (i) the unique porous structure with large pores and the presence of phosphates facilitate the mass transport and electron/proton transfer, thus enhancing the OER kinetics.41 (ii) The in situ formed oxidized Co and Ni species on the surface play an important role on the


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enhanced OER performance. (iii) The favorable composition factor plays an important role in improving the catalytic activity. This composition-dependent electrocatalytic activity is closely related to the surface reactivity of different catalysts toward the oxygen radical intermediates. therefore, Co3Ni1P NCs with optimized composition show the highest OER catalytic activity compared with other catalysts.42,43 It is worth noting that there are Fe impurities in the electrolyte even with the highest-purity product commercially available. It has been demonstrated that these impurities can affect the activity of the Ni-Co-based catalysts. The enhanced OER catalytic activity of NiCoP NCs might be partially attributed to the Fe impurities in the electrolyte.44-46 Further investigation and discussions are needed to specify the influence of the impurities in the future. In summary, we successfully developed a facile procedure to synthesize highly ordered CoP and NiP NCs with mesoporous pores and nanosized walls using a hard templating method. This method was further demonstrated to be applicable in the formation of highly ordered mesoporous CoNiP NCs. Combining the structural feature, compositional advantages and the in situ formed active species together, the as-prepared Co3Ni1P NCs show superior catalytic activity and longterm stability toward OER in alkaline solution. This method provides a promising strategy to synthesize novel catalysts with controllable shape and composition in electrolysis applications. ASSOCIATED CONTENT Supporting Information. Experimental section, XPS spectra of CoP and NiP NCs, LSV curves and corresponding Tafel plots of as-prepared CoP, Co3Ni1P, Co1Ni1P, Co1Ni3P and NiP NCs in 1 M KOH solution, CV curves of Co3Ni1P NCs at scan rate from 2 to 10 mV/s, TEM image of Co3Ni1P NCs after OER.


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AUTHOR INFORMATION *[email protected] * [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by a start-up fund of Washington State University, USA. We thank Franceschi Microscopy & Image Center at Washington State University for TEM measurements. The XPS analysis was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. PNNL is a multi-program national laboratory operated for DOE by Battelle under Contract DE-AC05-76RL0183. REFERENCES (1) Rosen, J.; Hutchings, G. S.; Jiao, F. Ordered Mesoporous Cobalt Oxide as Highly Efficient Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2013, 135, 4516-4521. (2) Shi, Y. M.; Zhang, B. Recent Advances in Transition Metal Phosphide Nanomaterials: Synthesis and Applications in Hydrogen Evolution Reaction. Chem. Soc. Rev. 2016, 45, 15291541.


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(3) Zhu, C. Z.; Fu, S. F.; Du, D.; Lin, Y. H. Facilely Tuning Porous NiCo2O4 Nanosheets with Metal Valence-State Alteration and Abundant Oxygen Vacancies as Robust Electrocatalysts Towards Water Splitting. Chem. Eur. J. 2016, 22, 4000-4007. (4) Tuysuz, H.; Hwang, Y. J.; Khan, S. B.; Asiri, A. M.; Yang, P. D. Mesoporous Co3O4 as an Electrocatalyst for Water Oxidation. Nano Res. 2013, 6, 47-54. (5) Chang, J. F.; Xiao, Y.; Xiao, M. L.; Ge, J. J.; Liu, C. P.; Xing, W. Surface Oxidized CobaltPhosphide Nanorods as an Advanced Oxygen Evolution Catalyst in Alkaline Solution. ACS. Catal. 2015, 5, 6874-6878. (6) Shan, Z. C.; Archana, P. S.; Shen, G.; Gupta, A.; Bakker, M. G.; Pan, S. L. NanoCOT: LowCost Nanostructured Electrode Containing Carbon, Oxygen, and Titanium for Efficient Oxygen Evolution Reaction. J. Am. Chem. Soc. 2015, 137, 11996-12005. (7) Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W. Cobalt-Iron (Oxy)hydroxide Oxygen Evolution Electrocatalysts: The Role of Structure and Composition on Activity, Stability, and Mechanism. J. Am. Chem. Soc. 2015, 137, 3638-3648. (8) Wu, L. H.; Li, Q.; Wu, C. H.; Zhu, H. Y.; Mendoza-Garcia, A.; Shen, B.; Guo, J. H.; Sun, S. H. Stable Cobalt Nanoparticles and Their Monolayer Array as an Efficient Electrocatalyst for Oxygen Evolution Reaction. J. Am. Chem. Soc. 2015, 137, 7071-7074. (9) Zhu, Y. P.; Liu, Y. P.; Ren, T. Z.; Yuan, Z. Y. Self-Supported Cobalt Phosphide Mesoporous Nanorod Arrays: A Flexible and Bifunctional Electrode for Highly Active Electrocatalytic Water Reduction and Oxidation. Adv. Funct. Mater. 2015, 25, 7337-7347.


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(10) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. Nickel-Iron Oxyhydroxide Oxygen-Evolution Electrocatalysts: The Role of Intentional and Incidental Iron Incorporation. J. Am. Chem. Soc. 2014, 136, 6744-6753. (11) 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. (12) Hou, Y.; Cui, S. M.; Wen, Z. H.; Guo, X. R.; Feng, X. L.; Chen, J. H. Strongly Coupled 3D Hybrids of N-doped Porous Carbon Nanosheet/CoNi Alloy-Encapsulated Carbon Nanotubes for Enhanced Electrocatalysis. Small 2015, 11, 5940-5948. (13) Han, L.; Yu, X. Y.; Lou, X. W. Formation of Prussian-Blue-Analog Nanocages via a Direct Etching Method and their Conversion into Ni–Co-Mixed Oxide for Enhanced Oxygen Evolution. Adv. Mater. 2016, 28, 4601-4605. (14) Gorlin, M.; Chernev, P.; Araujo, J.; Reier, T.; Dresp, S.; Paul, B.; Krahnert, R.; Dau, H.; Strasser, P. Oxygen Evolution Reaction Dynamics, Faradaic Charge Efficiency, and the Active Metal Redox States of Ni-Fe Oxide Water Splitting Electrocatalysts. J. Am. Chem. Soc. 2016, 138, 5603-5614. (15) Zou, S. H.; Burke, M. S.; Kast, M. G.; Fan, J.; Danilovic, N.; Boettcher, S. W. Fe (Oxy)hydroxide Oxygen Evolution Reaction Electrocatalysis: Intrinsic Activity and the Roles of Electrical Conductivity, Substrate, and Dissolution. Chem. Mater. 2015, 27, 8011-8020. (16) Swesi, A. T.; Masud, J.; Nath, M. Nickel Selenide as a High-Efficiency Catalyst for Oxygen Evolution Reaction. Energ. Environ Sci. 2016, 9, 1771-1782.


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(17) Bao, J.; Zhang, X. D.; Fan, B.; Zhang, J. J.; Zhou, M.; Yang, W. L.; Hu, X.; Wang, H.; Pan, B. C.; Xie, Y. Ultrathin Spinel-Structured Nanosheets Rich in Oxygen Deficiencies for Enhanced Electrocatalytic Water Oxidation. Angew. Chem. Int. Ed. 2015, 127, 7507-7512. (18) Song, J. H.; Zhu, C. Z.; Fu, S. F.; Song, Y.; Du, D.; Lin, Y. H. Optimization of Cobalt/Nitrogen Embedded Carbon Nanotubes as an Efficient Bifunctional Oxygen Electrode for Rchargeable Zinc-Air Batteries. J. Mater. Chem. A 2016, 4, 4864-4870. (19) Zhu, C. Z.; Wen, D.; Leubner, S.; Oschatz, M.; Liu, W.; Holzschuh, M.; Simon, F.; Kaskel, S.; Eychmüller, A. Nickel Cobalt Oxide Hollow Nanosponges as Advanced Electrocatalysts for the Oxygen Evolution Reaction. Chem. Commun. 2015, 51, 7851-7854. (20) Tang, C.; Asirib, A. M.; Sun, X. P. Highly-Active Oxygen Evolution Electrocatalyzed by a Fe-Doped NiSe Nanoflake Array Electrode. Chem. Commun. 2016, 52, 4529-4532. (21) Zhang, B.; Zheng, X. L.; Voznyy, O.; Comin, R.; Bajdich, M.; Garcia-Melchor, M.; Han, L. L.; Xu, J. X.; Liu, M.; Zheng, L. R.; et al. Homogeneously Dispersed Multimetal OxygenEvolving Catalysts. Science 2016, 352, 333-337. (22) Tung, C. W.; Hsu, Y. Y.; Shen, Y. P.; Zheng, Y. X.; Chan, T. S.; Sheu, H. S.; Cheng, Y. C.; Chen, H. M. Reversible Adapting Layer Produces Robust Single-Crystal Electrocatalyst for Oxygen Evolution. Nat. Commun. 2015, 6, 8106. (23) You, B.; Jiang, N.; Sheng, M. L.; Bhushan, M. W.; Sun, Y. J. Hierarchically Porous UrchinLike Ni2P Superstructures Supported on Nickel Foam as Efficient Bifunctional Electrocatalysts for Overall Water Splitting. ACS Catal. 2016, 6, 714-721.


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Highly Ordered Mesoporous Bimetallic Phosphides as Efficient

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