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Article Cite This: ACS Appl. Energy Mater. 2019, 2, 4188−4194

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Facile Synthesis of 3D NiCoP@NiCoPOx Core−Shell Nanostructures with Boosted Catalytic Activity toward 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

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S Supporting Information *

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−shell NiCoP@NiCoPOx nanostructures (3D CS-NiCoP@NiCoPOx) at room temperature via a one-step cyanogel reduction strategy, using NaBH4/NaH2PO2 as a mixed reductant. Specifically, the inner NiCoP core acts as the conductive support for the active catalytic NiCoPOx shell, facilitating the charge transport during OER. With the merits of abundant accessible active sites, fast electron transport, and gas diffusion, CS-NiCoP@ NiCoPOx exhibited outstanding electrocatalytic activity and stability toward 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 a homemade NiCo nanosheet (NiCo-NS) and commercially available RuO2 catalysts. The developed one-step NiCl2/K3Co(CN)6 cyanogel reduction strategy provides some insights into the synthesis of novel earth-abundant and cost-efficient 3D transition-metal-based nanocatalysts for commercial applications in OER. KEYWORDS: cyanogel, transition-metal-based nanocatalysts, three-dimensional, core−shell structures, oxygen evolution reaction

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nides13,14 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−shell carbon-embedded NiCo@NiCoO2 by formation of core NiCo alloy microrod 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 decoration on the 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 treatments, such as further surface oxidation of the core template17,18 and outer carbon shell coating,4,19 which makes the experiment process complicated and time-consuming. Cyanogel, a special class of 3D cyano-bridged bimetallic coordination polymer, can be easily obtained from a

INTRODUCTION As a half-reaction of electrochemical water splitting, the oxygen evolution reaction (OER) (2H2O → 4H+ + O2 + 4e− in acid solution or 4OH− → 2H2O + O2 + 4e− in alkaline electrolyte) seriously affects the overall reaction efficiency because of the intrinsically sluggish OER kinetics that occur in the multistep electron transfer process.1−3 Therefore, it is of great importance to design and synthesize efficient electrocatalysts for OER to lower the overpotential 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 for novel OER catalysts with higher activity, superior stability, and cost effectiveness. Most recently, earth-abundant and cost-efficient 3D transition-metal-based (e.g., Fe, Co, and Ni) materials (TMM), especially their oxides and (oxy)hydroxides with various structures, have been regarded as highly promising Ru and Ir alternative 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 the oxide layer seriously impedes electron transport during the OER process. As we know, metal compounds (such as metal chalcoge© 2019 American Chemical Society

Received: February 28, 2019 Accepted: May 15, 2019 Published: May 15, 2019 4188

DOI: 10.1021/acsaem.9b00431 ACS Appl. Energy Mater. 2019, 2, 4188−4194

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ACS Applied Energy Materials

thorough washing and centrifugation (experimental details are given in the Supporting Information). A scanning electron microscopy (SEM) image shows the black product with 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 with the aid of solid properties of the cyanogel during the synthesis. In the contrast experiments, the absence of NaH2PO2 yields thick sheet-like nanostructures, accompanied by large nanoparticles with severe aggregation, as illustrated by the SEM and transmission electron microscopy (TEM) images (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 freshly formed metal backbones from extensive oxidation and even the collapse of the 3D structure. Transmission electron microscopy was employed to further examine the shapes and crystal structures. As shown by the TEM image (Figure 2A), small nanoparticles are intercon-

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 Due to the advantages of the

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

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 metalbased nanostructures with enhanced catalytic properties.22,23 Taken together, we originally developed an easy one-step NiCl2/K3Co(CN)6 cyanogel reduction method for the synthesis of 3D core−shell NiCoP@NiCoPOx (termed as 3D CSNiCoP@NiCoPOx) at room temperature by employing NaBH4 and NaH2PO2 as the co-reductants. By virtue of the specific 3D porous core−shell structure, OER features of the as-synthesized NiCoP@NiCoPOx were critically explored as a benchmark system in 1 M KOH electrolyte.

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RESULTS AND DISCUSSION 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 Figure 2. Structural characterizations of the CS-NiCoP@NiCoPOx: (A) low-resolution, (B) magnified-resolution, and (C) high-resolution TEM images; (D) X-ray diffraction pattern; and (E) energy-dispersive spectroscopy mappings of Co (red), Ni (green), O (purple), P (brown), and their overlap.

Scheme 1. Schematic Representation of the Synthetic Procedure of CS-NiCoP@NiCoPOx: (A) Photograph of Light Blue NiCl2/K3Co(CN)6 Cyanogel and (B) SEM Image of CS-NiCoP@NiCoPOx

nected with each other to form 3D nanonetworks with abundant pores, in agreement with the above SEM analysis. Such a unique structure would have large active surface area, which can provide more active sites exposed to target molecules. In addition, it would effectively restrain the dissolution and aggregation effects 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. 4189

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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.

X-ray photoelectron spectroscopy (XPS) measurements were performed to further identify the surface chemical composition and electronic structure. For the high-resolution Ni 2p3/2 XPS section, the two predominant peaks at 855.8 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 is 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 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 indicate the possibility of the emergence of metallic NiCoP. Figure 3C exhibits the XPS comparison of high-resolution O 1s 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 P 2p XPS region for 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

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−shelllike structures. Moreover, the high-resolution TEM image (Figure 2C) legibly exhibits well-defined lattice fringes in the marked regions. For the outer shell and inner core, the interplanar lattice spacing distances are estimated to be 0.246 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 X-ray diffraction (XRD) pattern of the CS-NiCoP@NiCoPOx. The main broadened diffraction peak at 44° matches well with the XRD description of NiCo/ NiCoOx synthesized by Yan,29 along with the weakened peaks at around 53 and 70°, albeit with their blurring. These observations indicate the coexistence of a NiCoP alloy and oxidized NiCoPOx, consistent 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 nanostructures. Additionally, the O element covers the entire surface of the primary nanoparticle, as carefully checked from the mapping images, which provides strong evidence for the formation of the oxidized NiCoPOx shell. Therefore, the black powder obtained by the current cyanogel reduction strategy has hierarchically interconnected core−shell structures, in which the inner NiCoP alloy is the core, whereas 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. 4190

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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) 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.

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 (E0H2O/O2 = 1.23 V) acts as a reference) is needed for OER catalyzed by 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 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−shell configuration in which the NiCoP core effectively behaves as conductive support for the real active catalytic NiCoPOx shell. Consistently, this speculation is confirmed by electrochemical impedance spectroscopy (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 Ω) is

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 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 asgenerated H2 in eq 2 also serves as an extra reducing agent, resulting in highly enhanced reduction efficiency for the NiCo cyanogel. This assumption is very consistent with the XPS analysis of Ni, Co, and P for CS-NiCoP@NiCoPOx as discussed above. To this end, the evidence obtained by the XPS and TEM analysis identifies the generation of the NiCoP core and the NiCoPOx shell, confirming the formation of the unique core−shell configuration of CS-NiCoP@NiCoPOx. H+ + H·+H 2PO2− → 2H 2O + P

(1)

2H+ + H− + H 2PO2− → 2H 2O + 1/2H 2 + P

(2)

To evaluate the OER electrocatalytic activity of the CSNiCoP@NiCoPOx catalyst, linear sweep voltammetry (LSV) measurements were carried out in the O2-saturated KOH solution, where the homemade 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 originate from the oxidation states of Ni and/or Co species.15,32,33 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 4191

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ACS Applied Energy Materials Table 1. Comparison of OER Activities of Ni- and/or Co-Based Electrocatalysts in 1 M KOH material

loading (mg cm−2)

η (mV) at 10 mA cm−2

η (mV) at 100 mA cm−2

Tafel slope (mV dec−1)

ref

CS-NiCoP@NiCoPOx NiCo-NS Co nanosheet NiCo/NiCoOx NiCo−NiCoO2/carbon CoP3 nanoneedles/CFP CoP@graphene NiCoP/Ni foam NiCo-LDH/CFPa NiCo-LDH-P/Ni foam

∼0.18 ∼0.18 ∼0.34 ∼0.7 ∼3 ∼1 ∼0.28 ∼1.8 ∼0.8 ∼1

313 358 307 361 318 334 280 280 307 271

398 510 407

70.2 103.8 76 80 76 62 75 87 64 72

this work this work 21 29 27 2 11 15 38 32

440 367

CFP refers to carbon fiber paper, and LDH is layered double hydroxides.

a

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 (dashed line) 500 cycles at 10 mV s−1.

much smaller than that of NiCo-NS (18.2 Ω). As known, a low Rt represents a rapid charge transfer rate. To this point, the CSNiCoP@NiCoPOx catalyst has a charge transfer rate relatively faster than that of NiCo-NS under the same conditions, due to the existence of the inner NiCoP core. In the end, the OER characteristics 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 have a similar overpotential at 10 mA cm−2 when its catalyst loading is 0.34 mg cm−2, which is almost twice than that of our samples in this research.21 The electrochemical properties of a catalyst often depend on its active surface area. To better comprehend 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 curves at different scan rates in a potential window from 1.21 to 1.26 V where no faradaic processes occur (Figure S2). The specific capacitances are calculated to be roughly 32 and 14 mF cm−2 for CSNiCoP@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 CSNiCoP@NiCoPOx is roughly 444 m2 g−1, almost 1-fold 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 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 asfabricated 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 CSNiCoP@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 is 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. As known, the durability is a very important factor to investigate the catalytic properties of electrocatalysts in 4192

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ACS Applied Energy Materials practical applications. Herein, the stabilities of the electrocatalysts were further investigated by chronopotentiometry (Figure 5A). As O2 is an 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 CSNiCoP@NiCoPOx and NiCo-NS catalysts almost keep constant, whereas 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 CSNiCoP@NiCoPOx scarcely 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 accounting for the outstanding stabilities of the current electrocatalysts. Similarly, the LSV multiturn scan tests also give strong evidence for the highly improved durability (Figure 5B). The LSV curves 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 of the results reveal that 3D core−shell 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 has 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.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Lu Zhang: 0000-0002-8855-8492 Jiu-Ju Feng: 0000-0002-7954-0573 Notes

The authors declare no competing financial interest.

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

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(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, 16184−16193. (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. Seamlessly Conductive 3D Nanoarchitecture of CoreShell Ni-Co Nanowire Network for Highly Efficient Oxygen Evolution. Adv. Energy Mater. 2017, 7, 1601492−1601502. (5) Sun, F.; Wang, G.; Ding, Y.; Wang, C.; Yuan, B.; Lin, Y. NiFeBased Metal-Organic Framework Nanosheets Directly Supported on Nickel Foam Acting as Robust Electrodes for Electrochemical Oxygen Evolution Reaction. Adv. Energy Mater. 2018, 8, 1800584−1800594. (6) Qiu, C.; Jiang, J.; Ai, L. When Layered Nickel-Cobalt Silicate Hydroxide Nanosheets Meet Carbon Nanotubes: A Synergetic Coaxial Nanocable Structure for Enhanced Electrocatalytic Water Oxidation. ACS Appl. Mater. Interfaces 2016, 8, 945−951. (7) Wang, J. G.; Hua, W.; Li, M.; Liu, H.; Shao, M.; Wei, B. Structurally Engineered Hyperbranched NiCoP Arrays with Superior Electrocatalytic Activities toward Highly Efficient Overall Water Splitting. ACS Appl. Mater. Interfaces 2018, 10, 41237−41245. (8) Hu, E.; Feng, Y.; Nai, J.; Zhao, D.; Hu, Y.; Lou, X. W. Construction of hierarchical Ni-Co-P hollow nanobricks with oriented nanosheets for efficient overall water splitting. Energy Environ. Sci. 2018, 11, 872−880. (9) Du, C.; Yang, L.; Yang, F.; Cheng, G.; Luo, W. Nest-like NiCoP for Highly Efficient Overall Water Splitting. ACS Catal. 2017, 7, 4131−4137. (10) Bai, X.; Ren, Z.; Du, S.; Meng, H.; Wu, J.; Xue, Y.; Zhao, X.; Fu, H. In-situ structure reconstitution of NiCo2Px for enhanced electrochemical water oxidation. Sci. Bull. 2017, 62, 1510−1518. (11) Zhang, G.; Wang, G.; Liu, Y.; Liu, H.; Qu, J.; Li, J. Highly Active and Stable Catalysts of Phytic Acid-Derivative Transition

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CONCLUSIONS Herein, we have developed a facile and efficient approach for one-step wet-chemical synthesis of porous 3D core−shell CSNiCoP@NiCoPOx architectures through a surfactant-free cyanogel reduction route at room temperature, using the NaBH4/NaH2PO2 mixture as the co-reductant. The introduction of NaH2PO2 plays a vital role in the formation of the 3D core−shell structure and effectively tune the electronic structures. As a result, the CS-NiCoP@NiCoPOx exhibited superior catalytic activity and durable ability toward OER when compared to its counterpart (NiCo-NS), due to the merits of the unique structure including numerous available active sites, fast electron transport, mass, and gas diffusion. Moreover, the electrocatalytic performance of CS-NiCoP@ NiCoPOx is even comparable to that of the 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 catalytic features would broaden the practical applications of future water splitting devices.

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Experimental section, SEM, TEM images, and the double-layer capacitance measurement curves of the catalysts (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00431. 4193

DOI: 10.1021/acsaem.9b00431 ACS Appl. Energy Mater. 2019, 2, 4188−4194

Article

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DOI: 10.1021/acsaem.9b00431 ACS Appl. Energy Mater. 2019, 2, 4188−4194