Facile Synthesis of 3D NiCoP@ NiCoPOx Core-Shelled

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Facile Synthesis of 3D NiCoP@NiCoPO Core-Shelled Nanostructures with Boosting Catalytical Activity Towards Oxygen Evolution Reaction Mi-Xue Jin, Yu-Lu Pu, Zi-Juan Wang, Zheng Zhang, Lu Zhang, Ai-Jun Wang, and Jiu-Ju Feng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00431 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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ACS Applied Energy Materials

Facile Synthesis of 3D NiCoP@NiCoPOx Core-Shelled Nanostructures with Boosting Catalytical Activity towards Oxygen Evolution Reaction Mi-Xue Jin, Yu-Lu Pu, Zi-Juan Wang, Zheng Zhang, Lu Zhang,* Ai-Jun Wang, and Jiu-Ju Feng * Key laboratory of the Ministry of Education for Advanced Catalysis Materials, College of Chemistry and Life Sciences, College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua, 321004, China KEYWORDS: Cyanogel, Transition-metal-based nanocatalysts, Three dimensional, Core-shelled structures, Oxygen evolution reaction ABSTRACT As a half-reaction, the intrinsically sluggish oxygen evolution reaction (OER) kinetics severely affects the overall water splitting efficiency. Therefore, we synthesized porous three-dimensional core-shelled NiCoP@NiCoPOx nanostructures (3D CS-NiCoP@NiCoPOx) at room temperature via a one-step cyanogel-reduction strategy, using NaBH4/NaH2PO2 as mixed reductants. Specifically, the inner NiCoP core acts as the conductive support for active catalytic NiCoPOx shell, facilitating the charge transport during OER. With the merits of abundant active sites accessible, fast electron transport and gas diffusion, CS-NiCoP@NiCoPOx exhibited outstanding 1

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electrocatalytic activity and stability towards OER in 1 M KOH electrolyte (e.g. the overpotentials are only 313 mV at 10 mA cm-2 and 398 mV at 100 mA cm-2), outperforming home-made NiCo nanosheet (NiCo-NS) and commercial available RuO2 catalysts. The developed one-step NiCl2/K3Co(CN)6 cyanogel-reduction strategy provides some insights into synthesis of novel earth-abundant and cost-efficient 3d transition-metal-based nanocatalysts for commercial applications in OER.

INTRODUCTION As a half-reaction of electrochemical water splitting, oxygen evolution reaction (OER) (2H2O → 4H+ + O2 + 4e− in the acid solution or 4OH− → 2H2O + O2 + 4e− in the alkaline electrolyte) seriously affects the overall reaction efficiency because of the intrinsically sluggish OER kinetics occurred in the multi-stepped electron transfer process.1-3 Therefore, it is of great importance to design and synthesize efficient electro-catalysts for OER to lower the over-potential and improve the overall conversion efficiency. Meanwhile, the high price and rarity of Ru- and Ir-based nanomaterials have severely hindered their further practical applications, albeit with their recognition as the most efficient OER catalysts.4-5 To this end, it is imperative to search novel OER catalysts with higher activity, superior stability and cost effectiveness. Most recently, earth-abundant and cost-efficient 3d transition-metal (e.g. Fe, Co, and Ni) based materials (TMM), especially their oxides and (oxy)hydroxides with various structures, have been regarded as highly promising Ru and Ir alternative 2


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candidates for OER under neutral or alkaline conditions.6-9 Noticeably, for an effective OER, the outer (hydr)oxide layer is the real functional catalytic species of the TMM.10-12 However, the poor intrinsic conductivity of oxide layer seriously impedes electron transport during OER process. As we know, metal compounds (such as metal chalcogenides13-14 and phosphides15-16) are much more conductive than their oxides, which serve as conductive scaffolds for the active oxide species. Therefore, the assembly of core-shell structured TMM/ oxides (termed as CS-TMM/OXs) is a key factor for enhancing the OER activity, in which the TMM core works as the conductive support for active catalytic oxides shell. Xu’s group17 fabricated core-shelled carbon-embedded NiCo@NiCoO2 by the formation of core NiCo alloy micro-rod arrays and further calcination in air, showing significantly enhanced OER catalytic activity in alkaline media. Bae’s group4 obtained 3D carbon shelled Ni-Co nanowires (CCS Ni-Co NWs) by further carbon shell decorating on Ni-Co nanowire cores, displaying high activity for OER. Although the core-shell structured TMM/OXs is one kind of promising OER catalysts, it is difficult to obtain by facile synthesis.18 As stated above, except for the core synthesis, almost all of them need second or even third treatment such as further surface oxidation of core template17-18 and outer carbon shell coating,4, 19 which makes the experiment process complicated and time-consuming.

3



Figure 1. (A) Reaction formula and (B) 3D geometrical unit of the cyanogel.

Cyanogel, a special class of 3D cyano-bridged bimetallic coordination polymer, can be easily obtained from a complexing reaction between the chlorometalates (K2PtCl4, K2PdCl4, InCl3, CoCl2, CuCl2, etc.) and transition-metal cyanometalates (K4Fe(CN)6, K3Co(CN)6, K2Ni(CN)4, etc.), as illustrated in Figure 1.20-21 By taking advantages of the intrinsic structural features of cyanogels, such as the solid nature, 3D intimately interconnected backbones and uniform distribution of the metal ions, versatile cyanogel-based approaches are well developed to fabricate various 3D metal-based nanostructures with enhanced catalytic properties.22-23 Taken together, we originally developed an easy one-step NiCl2/K3Co(CN)6 cyanogel-reduction method for synthesis of 3D core-shelled NiCoP@NiCoPOx (termed as 3D CS- NiCoP@NiCoPOx) at room temperature by employing NaBH4 and NaH2PO2 as the co-reductants. By virtue of the specific 3D porous core-shelled structure, OER features of the as-synthesized NiCoP@NiCoPOx were critically explored as benchmarked system in 1 M KOH electrolyte. 4


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RESULTS AND DISCUSSION

Scheme 1. Schematic representation of synthetic procedure of CS-NiCoP@NiCoPOx. (A) Photograph of light blue NiCl2/K3Co(CN)6 cyanogel. (B) SEM image of CS-NiCoP@NiCoPOx.

In a typical synthesis, light blue NiCl2/K3Co(CN)6 cyanogel was readily achieved by mixing NiCl2 and K3Co(CN)6 aqueous solutions at room temperature under ultrasonication (Scheme 1A). Afterward, the NaBH4 and NaH2PO2 mixed solution was added to the as-synthesized NiCl2/K3Co(CN)6 cyanogel, in which the molar ratio of NaH2PO2/NaBH4 is set to be 0.42:1.00 as the optimal choice according to our previous study.24 The black powder was eventually obtained by thoroughly washing and centrifugation (Experimental details are given in ESI†). Scanning electron microscopy (SEM) image shows the black product with the unique interconnected 3D porous architectures (Scheme 1B), very close to those obtained by noble-metal cyanogel-reduction.20,

22

This can be explained by the fact

that metal crystal nuclei grow one-by-one along the 3D characteristic backbones by the aid of the solid properties of the cyanogel during the synthesis. In the contrast experiments, the absence of NaH2PO2 yields the thick sheet-like 5



nanostructures, accompanied with the large nanoparticles with severe aggregation, as the SEM and TEM images illustrate (defined as NiCo-NS for simplicity, Figure S1). It indicates the indispensable role of NaH2PO2 for constructing the unique 3D interconnected structures. The much easier oxidation of P itself would effectively prevent the fresehly-formed metal backbones from extensive oxidation and even the collapse of the 3D structure.

Figure 2. Structural characterizations of the CS-NiCoP@NiCoPOx: (A) Low-, (B) magnified-, and (C) high-resolution TEM images; (D) XRD pattern; and (E) EDS mappings of Co (red), Ni (green), O (purple), P (brown) and their overlap.

Transmission electron microscopy (TEM) was employed to further examine the shapes and crystal structures. As shown by the TEM image (Figure 2A), small 6


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nanoparticles are interconnected with each other to form 3D nanonetworks with abundant pores, in agreement with the above SEM analysis. Such unique structure would have large active surface area, which can provide more active sites exposed to target molecules. Besides, it would effectively restrain the dissolution and aggregation effect of the metal nanoparticles driven by Ostwald ripening, and facilitate the electron transfer, gas diffusion and mass exchange rates during the electrochemical reactions,25-26 endowing the typical sample with an excellent OER performance. As illustrated by the magnified TEM image (Figure 2B), each particle exhibits a black inner core covered by an external layer with gray color. The average layer thickness is around 3-5 nm, showing the formation of the charming 3D core-shell-like structures. Moreover, high-resolution TEM image (Figure 2C) legibly exhibits well-defined lattice fringes in the marked regions. For the outer shell and inner core, the inter-planar lattice spacing distances are estimated to be 0.246 nm and 0.202 nm, which are responsible for the (111) crystal planes of oxidized NiCoOx27 and the (201) planes of NiCoP alloy,16, 28 respectively. Figure 2D exhibits the XRD pattern of the CS-NiCoP@NiCoPOx. The main broaden diffraction peak at 44o matches well with XRD description of NiCo/NiCoOx synthesized by Yan29, along with the weaken peaks at around 53o and 70o albeit with their blurring. These observations indicate the coexistence of NiCoP alloy and oxidized NiCoPOx, in good consistence with the TEM analysis. In order to depict the elemental distributions, Figure 2E provides the nanoscaled elemental mappings. Clearly, Ni, Co, P and O elements are well distributed through the whole 7



nanostructures. Additionally, O element covers the entire surface of the primary nanoparticle, as carefully checked from the mapping images, which provides a strong evidence for the formation of the oxidized NiCoPOx shell. Therefore, the black powder obtained by the current cyanogel-reduction strategy owns hierarchically interconnected core-shelled structures, in which the inner NiCoP alloy is the core, while the external oxidized NiCoPOx layer acts as the shell. The existence of the NiCoP core would dramatically promote the electron transfer for the real active NiCoPOx shell during OER.

Figure 3. XPS spectra of the CS-NiCoP@NiCoPOx and NiCo-NS in the (A) Ni 2p, (B) Co 2p, (C) O 1s and (D) P 2p regions.

8


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X-ray photoelectron spectroscopy (XPS) measurements were performed to further identify the surface chemical composition and electronic structure. For high-resolution Ni 2p3/2 XPS section, the two predominant peaks at 855.8 eV and 861.8 eV are well assigned to Ni2+ species, and the adjacent satellite peak indicates the existence of oxidized Ni species in both NiCo-NS and CS-NiCoP@NiCoPOx (Figure 3A).10 It worth noting that a peak emerges at 852.8 eV in CS-NiCoP@NiCoPOx, which is attributed to pure Ni.15 However, a similar peak appears in NiCo-NS, which is relatively small and even negligible as compared to that of CS-NiCoP@NiCoPOx under the same operation conditions. These observations demonstrate the key role of NaH2PO2 as the co-reductant for effective reduction of the Ni precursor in this synthesis. Similarly, there is a new peak detected at 778.0 eV in the Co 2p3/2 XPS segment for CS-NiCoP@NiCoPOx, which is indicative of the presence of metallic Co as a result of the formation of Co-P.10, 15 Furthermore, the peaks at 781.1 eV and 785.9 eV are ascribed to the oxidized Co state and its satellite state (Figure 3B), which are associated with the Co-POx. The coexistences of pure Ni and Co indicates the possibility of the emergence of metallic NiCoP. Figure 3C exhibits the XPS comparison of high-resolution O1s XPS region between CS-NiCoP@NiCoPOx and NiCo samples. Apart from the existence of the H-O (531.8 eV) and M-O (531.1 eV) bonds in both cases, a new peak appears at 532.6 eV (as assigned to P-O bond) for CS-NiCoP@NiCoPOx, further showing the generation of NiCoPOx. As seen in the high-resolution P2p XPS region for 9



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CS-NiCoP@NiCoPOx (Figure 3D), there are two peaks at 129.5 and 133.0 eV, which are assigned to metallic state phosphorus (P-M) and phosphate species (P-O),30-31 respectively. These observations certify the efficient co-reduction of NaBH4 and NaH2PO2 and the incorporation of P0 into the NiCo matrix, as supported by the previous work.24 That is to say, NaH2PO2 would also act as effective phosphorus (P) resource when NaBH4 and NaH2PO2 work as the co-reductants. The elemental phosphorus is generated via the hydrogenous radical anion as depicted in equation (eq. 1) and/or hydrogenous free radical as described in eq. (2), accompanied by the reduction of NiCo cyanogel to form the NiCoP core. Moreover, the as-generated H2 in eq. (2) also serves as extra reducing agent, resulting in highly enhanced reduction efficiency for NiCo cyanogel. This assumption is in good consistence with the XPS analysis of Ni, Co and P for CS-NiCoP@NiCoPOx as discussed above. To this end, the evidences obtained by the XPS and TEM analysis well identify the generation of the NiCoP core and NiCoPOx shell, confirming the formation of the unique core-shelled configuration of CS-NiCoP@NiCoPOx. H+ + H∙ + H2 PO2 - → 2H2 O + P

(1)

2H+ + H- + H2 PO2 - → 2H2 O + 1/2H2 + P

(2)

10


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Figure 4. Electrochemical measurement analysis: (A) 90% iR-compensated LSV curves of the catalysts in O2-saturated 1 M KOH at a scan rate of 10 mV s-1. Inset: LSV curves of CS-NiCoP@NiCoPOx and NiCo-NS. (B) The overpotentials required at the current densities of 10 and 100 mA cm-2. (C) Tafel plots. (D) Nyquist plots at their open circuit potential. Inset: The equivalent circuit diagram.

To evaluate the OER electrocatalytic activity of the CS-NiCoP@NiCoPOx catalyst, linear sweep voltammetry (LSV) measurements were carried out in the O2-saturated KOH solution, where the home-made NiCo-NS catalyst works as the contrast. As described by the inset in Figure 4A, both catalysts exhibit distinct anodic peaks in the potential range of 1.25 ~ 1.45 V, which are originated from the oxidation 11



states of Ni and/or Co species.15,

32-33

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For CS-NiCoP@NiCoPOx, there are two

oxidation peaks detected, whose onset oxidation potential starts earlier than that of NiCo-NS under the identical circumstance. These scenarios reveal the changed electronic structures of NiCo-NS by efficient incorporation of P, which would facilitate the formation of metal oxide intermediates in the alkaline electrolyte. Namely, the introduced P would weaken the thermodynamic barrier for electron transfer and/or promote the formation of the O−O bond, consequently lowering the extra activation energy for OER, finally enhancing the catalytic performance of [email protected],

34

Subsequently,

a

sharply

enhanced

current

of

CS-NiCoP@NiCoPOx is observed at an onset potential (Eonset) of 1.45 V, which is much smaller than that of NiCo-NS (1.52 V), indicating the dramatically improved OER activity.1 Meanwhile, the operational overpotential (η) required at 10 mA cm-2 is also another important index to evaluate the OER performance.35 As seen in Figure 4B, an overpotential of 313 mV at 10 mA cm-2 (where thermodynamic OER potential (E0H O/O = 1.23 V) acts as a reference) is needed for OER catalyzed by 2

2

CS-NiCoP@NiCoPOx, which is greatly lower than that of NiCo-NS (358 mV). Furthermore, to reach the current density of 100 mA cm-2, the overpotentials of 398 mV and 510 mV are required for CS-NiCoP@NiCoPOx and NiCo-NS, respectively. It means that a dramatically lower overpotential is required for CS-NiCoP@NiCoPOx. This is attributed to their unique core-shelled configuration in which the NiCoP core effectively behaves as conductive support for the real active catalytic NiCoPOx shell. 12


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Consistently, this speculation is confirmed by electrochemical impedance spectroscopy (EIS, Figure 4D). The equivalent circuit inserted in Figure 4D consists of the ohmic resistance of the electrolyte (Rs), the charge transfer resistance of the electrode (Rt), Warburg impedance (W), and the constant phase angle element (CPE). As calculated according to the equivalent circuit, the Rt of CS-NiCoP@NiCoPOx (10.0 ) is much smaller than that of NiCo-NS (18.2 ). As we all know, a low Rt represents a rapid charge transfer rate. To this point, the CS-NiCoP@NiCoPOx catalyst has a relatively faster charge transfer rate than that of NiCo-NS under the same conditions, due to the existence of the inner NiCoP core. In the end, the OER characters of CS-NiCoP@NiCoPOx were highly enhanced. More impressively, such low overpotentials (313 mV at 10 mA cm-2 and 398 mV at 100 mA cm-2) for the same catalyst loading (0.18 mg cm-2) are competitive to most Ni- and/or Co-based catalysts or even conductive substrate-supported OER electrocatalysts reported previously (Table 1). For example, Co nanosheets own a similar overpotential at 10 mA cm-2 when its catalyst loading is 0.34 mg cm-2, which is almost twice alternative to our samples in this research.21 The electrochemical properties of a catalyst often depend on its active surface area. To make better comprehension of the increased OER activity, the electrochemically active surface area (ECSA) of the CS-NiCoP@NiCoPOx was measured based on the double-layer capacitance measurements,36-37 using NiCo-NS as the reference. The capacitances of the double layer were derived by sweeping cyclic voltammetry (CV) curves at different scan rates in a potential window from 1.21 to 13



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1.26 V where no faradic processes occur (Figure S2). The specific capacitances are calculated to be roughly 32 and 14 mF cm-2 for CS-NiCoP@NiCoPOx and NiCo-NS, respectively. The steeply high capacitance of the former indicates a larger electrochemical surface area. Assuming 40 μF cm-2 as a moderate value for specific capacitance of a flat surface, the ECSA of CS-NiCoP@NiCoPOx is roughly 444 m2 g-1, almost one time larger than that of NiCo-NS (194 m2 g-1). The greater ECSA is comparable to most of the Ni- and/or Co-based electrocatalysts, mainly ascribed to the unique 3D interconnected structure of [email protected], 21 Table 1. Comparison of OER activities of Ni- and/or Co-based electrocatalysts in 1 M KOH.

Loading

η (mV) at

η (mV) at

Tafel slope

(mg cm-2)

10 mA cm-2

100 mA cm-2

(mV dec-1)

CS-NiCoP@NiCoPOx

~0.18

313

398

70.2

This work

NiCo-NS

~0.18

358

510

103.8

This work

Co-nanosheet

~0.34

307

407

76

21

NiCo/NiCoOx

~0.7

361

-

80

29

NiCo–NiCoO2/Carbon

~3

318

-

76

27

CoP3 nanoneedles/CFP

~1

334

-

62

2

CoP@graphene

~0.28

280

440

75

11

NiCoP/Ni foam

~1.8

280

-

87

15

NiCo-LDH/CFP*

~0.8

307

367

64

38

NiCo-LDH-P/Ni foam

~1

271

-

72

32

Material

Ref.

* CFP refers to carbon fiber paper while LDH is layered double hydroxides. 14


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The Tafel slope is an important indicator to assess the intrinsic catalytic kinetics of OER, which can be obtained from the relationship between the overpotential and the corresponding current density. When the Tafel slope is lower, the catalyst would exhibit the higher activity for OER.39 For the as-fabricated CS-NiCoP@NiCoPOx, the Tafel slope is roughly 70.2 mV dec-1, which is much smaller than that of NiCo-NS (103.8 mV dec-1), reflecting the superior OER kinetics on CS-NiCoP@NiCoPOx (Figure 4C). In general, RuO2 is a commercially available standard catalyst for effective OER. In consideration of the practical applications, we also compared the electrocatalytic performances of CS-NiCoP@NiCoPOx with RuO2 for OER under the same environment (Figure 4A). When the current density is 100 mA cm-2, an overpotential of 398 mV is required for the as-constructed CS-NiCoP@NiCoPOx catalyst, which is smaller than that of RuO2 (408 mV), albeit with a lower overpotential for RuO2 (275 mV) relative to that of CS-NiCoP@NiCoPOx (313 mV) at 10 mA cm-2. These scenarios demonstrate that the superior OER catalytic kinetics occurred on CS-NiCoP@NiCoPOx, in good agreement with their Tafel slopes. Specifically, the Tafel slope of CS-NiCoP@NiCoPOx (70.2 mV dec-1) is much smaller than that of RuO2 (81.8 mV dec-1), revealing that CS-NiCoP@NiCoPOx is a promising candidate for OER.

15



Figure 5. Durability tests of the studied catalysts: (A) Chronopotentiometry curves in O2-saturated 1 M KOH at a constant current density of 10 mA cm-2. (B) LSV curves before (solid line) and after (dash line) 500 cycles at 10 mV s-1.

As it is known, the durability is a very important factor to investigate the catalytic properties of electrocatalysts in practical applications. Herein, the stabilities of the electrocatalysts were further investigated by chronopotentiometry (Figure 5A). Since O2 is oxidation product of OER, the rotating disk electrode is employed here to avoid accumulation of O2 bubbles generated on the electrode and obtain better mass transport.24 After 5 h of testing, the overpotentials of CS-NiCoP@NiCoPOx and NiCo-NS catalysts almost keep constant, while the overpotential of RuO2 is greatly increased by the value of 105 mV to achieve the same current density. Moreover, the morphology and structure of CS-NiCoP@NiCoPOx have scare change after the chronopotentiometry run (SEM and TEM images shown in Figure S3). It indicates that the formation of the cyanogels is a critical factor accounted for the outstanding stabilities of the current electrocatalysts. Similarly, the LSV multi-turn scan tests also give strong evidence for the highly improved durability (Figure 5B). The LSV curves 16


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have slight changes before and after 500 cycles for the as-constructed CS-NiCoP@NiCoPOx, whereas significant deviation is noticed for commercial RuO2 before and after the test. All the results reveal that 3D core-shelled CS-NiCoP@NiCoPOx catalyst exhibits superior electrocatalytic ability and stability for OER. The reasons for the enhanced OER behaviors are mainly attributed to the following aspects: (1) The unique porous 3D structure owns a very large specific surface area, providing enriched active sites exposed to target reactants and facilitating charge transfer, mass and gas diffusion rates during the reactions; (2) The metallic NiCoP core, surrounded by the outer NiCoPOx shell, acts as the conductive scaffold for the active oxide species in the shell to promote the charge transfer; (3) The incorporation of P strongly alters the electronic structures of Ni and Co, and reduces the energy barriers for oxidizing the freshly-formed intermediates; (4) The 3D interconnected structure would greatly prevent the dissolution and aggregation effects of metal nanoparticles, effectively improving the stability of the electrocatalysts.

CONCLUSIONS Herein, we have developed a facile and efficient approach for one-step wet-chemical synthesis of porous 3D core-shelled CS-NiCoP@NiCoPOx architectures through a surfactant-free cyanogel-reduction route at room temperature, using the NaBH4/NaH2PO2 mixture as the co-reductants. The introduction of NaH2PO2 plays the vital role in the formation of the 3D core-shelled structure and effectively tune the 17



electronic structures. As a result, the CS-NiCoP@NiCoPOx exhibited the superior catalytic activity and durable ability towards OER when compared to its counterpart (NiCo-NS), due to the merits of the unique structure including numerous active sites available, fast electron transport, easy mass and gas diffusion. Moreover, the electrocatalytical performance of CS-NiCoP@NiCoPOx is even comparable to state-of-the-art RuO2 catalyst, showing the potential application in OER. We believe that the cost-efficient and earth-abundant CS-NiCoP@NiCoPOx with excellent catalytical features would broaden the practical applications of future water splitting devices.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: *****. Supporting Information includes Experimental section, SEM, TEM images and the double-layer capacitance measurement curves of the catalysts

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (L. Zhang) [email protected] (J.J. Feng) ORCID: 0000-0002-8855-8492 (L. Zhang) 0000-0002-7954-0573 (J.J. Feng) 18


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Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by National Natural Science Foundation of China (No. 21805245), and Zhejiang Public Welfare Technology Application Research Project (LGG19B050001), and the National Students’ Innovation and Entrepreneurship Training Program of Zhejiang Normal University (201810345013, M.X. Jin).

References (1) Zhao, S.; Wang, Y.; Dong, J.; He, C. T.; Yin, H.; An, P.; Zhao, K.; Zhang, X.; Gao, C.; Zhang, L.; Lv, J.; Wang, J.; Zhang, J.; Khattak, A. M.; Khan, N. A.; Wei, Z.; Zhang, J.; Liu, S.; Zhao, H.; Tang, Z. Ultrathin metal-organic framework nanosheets for electrocatalytic oxygen evolution. Nat. Energy 2016, 1, 16184-16183. (2) Wu, T.; Pi, M.; Zhang, D.; Chen, S. 3D structured porous CoP3 nanoneedle arrays as an efficient bifunctional electrocatalyst for the evolution reaction of hydrogen and oxygen. J. Mater. Chem. A 2016, 4, 14539-14544. (3) Anantharaj, S.; Ede, S. R.; Sakthikumar, K.; Karthick, K.; Mishra, S.; Kundu, S. Recent Trends and Perspectives in Electrochemical Water Splitting with an Emphasis on Sulfide, Selenide, and Phosphide Catalysts of Fe, Co, and Ni: A Review. ACS Catal. 2016, 6, 8069-8097. (4) Bae, S. H.; Kim, J. E.; Randriamahazaka, H.; Moon, S. Y.; Park, J. Y.; Oh, I. K. 19



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Seamlessly Conductive 3D Nanoarchitecture of Core-Shell Ni-Co Nanowire Network for

Highly

Efficient

Oxygen

Evolution.

Adv.

Energy

Mater.

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