MOF-derived nitrogen-doped [email protected] arrays as bifunctional electrocatalysts for efficient overall water splitting

Journal Pre-proof MOF-derived nitrogen-doped CoO@CoP arrays as bifunctional electrocatalysts for efficient overall water splitting Mengjie Lu, La Li, Duo Chen, Junzhi Li, N.I. Klyui, Wei Han PII:

S0013-4686(19)32081-X

DOI:

https://doi.org/10.1016/j.electacta.2019.135210

Reference:

EA 135210

To appear in:

Electrochimica Acta

Received Date: 27 August 2019 Revised Date:

11 October 2019

Accepted Date: 3 November 2019

Please cite this article as: M. Lu, L. Li, D. Chen, J. Li, N.I. Klyui, W. Han, MOF-derived nitrogen-doped CoO@CoP arrays as bifunctional electrocatalysts for efficient overall water splitting, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135210. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

MOF-Derived

Nitrogen-Doped

CoO@CoP

Arrays

as

Bifunctional Electrocatalysts for efficient overall water splitting Mengjie Lu,a La Li,*a Duo Chen,a Junzhi Lia, N. I. Klyuiab and Wei Han*ac a

Sino-Russian International Joint Laboratory for Clean Energy and Energy

Conversion Technology, College of Physics, Jilin University, Changchun 130012, P. R. China. E-mail: [email protected] b

V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, 41 pr. Nauki, 03028 Kyiv, Ukraine c

International Center of Future Science, Jilin University, Changchun, 130012, P.R. China. E-mail: [email protected]

*Corresponding author: E-mail address: [email protected] (L. Li), [email protected] (W. Han).

Abstract In order to keep up with the increasing demands of renewable energy technologies, it is urgent to develop highly efficient bifunctional electrocatalysts for overall water splitting, which could produce hydrogen and oxygen at the same time. Herein, we report a self-supported bead string-like nitrogen-doped CoO@CoP (N-CoO@CoP) arrays derived from MOF materials, serving as a bifunctional catalyst for overall water splitting. The unique 3D bead string-like arrays endow unimpeded electronic transport path, abundant bubble release channels and expose more active sites, which can easily achieve high current density in the electrocatalytic reaction process. The doped atom N coming from nitrogen-contained MOF can tune the electronic structure of CoP, which enhances the electrocatalytic performance further. The electrocatalysts display superb oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) performance with a small overpotential of 332 mV and 201 mV at the current density of 100 mA cm-2, respectively. Moreover, an alkaline electrolyzer employing the N-CoO@CoP as the cathode and anode exhibits a cell voltage of 1.79V at the current density of 100 mA cm-2 as well as robust durability. Keywords:

Nitrogen-Doped,

Transition

Electrocatalysts, Overall Water Splitting.

metal

phosphide,

MOF,

stable

Introduction Hydrogen (H2) and Oxygen (O2) are of vital importance to the development of clean energy and energy conversion such as solar water-splitting devices, hydrogen fuel cell, CO2 reduction technology and metal-air batteries [1-4]. The efficient and sustainable electrolysis of water is considered as the most potential way to obtain H2 and O2 with zero carbon emissions. Electrocatalysts can promote the two half reactions in the electrochemical conversion of H2O to H2 and 1/2 O2, namely HER (hydrogen evolution reaction) and OER (oxygen evolution reaction) [5-9]. The HER occurs via 2H2O+2e-=H2+2OH- in the cathode and the OER occurs via 4OH-=2H2O+O2+4e- in the anode under alkaline media [10]. To date, the most efficient and robust catalysts for the HER and OER are precious metal/metal oxides, which restrict the large-scale commercial application in the overall water splitting owing to the high cost, scarcity and instability performance. Recently, various catalysts of transition metal based materials as alternatives to these expensive noble metals have been the subject of intense research. However, the reported catalysts are more or less faced with the following challenges: (1) the slow kinetics of water dissociation in HER processes and the electrocatalysts are more sensitive to the surface structure under alkaline media; (2) the alkaline OER involves a 4-electron-proton transfer process resulting in high overpotential and limited kinetics in reaction process. Thus, to satisfy the requirement of efficient overall water splitting, the explorations of novel electrocatalysts with advantages of morphologies

optimization, fast reaction rate are one of the current research hot topics in the field of electrocatalysis. The existing earth-abundant metal-based catalysts include chalcogenides [11-12], nitrides [13], phosphides [14], (oxy)hydroxides [15], metal oxides [16], metal [17], perovskite [18], etc.. Co-based materials such as CoO [19], Co3O4 [20], Co4N [21], CoP [22], and CoS [23] as earth abundant and environmentally friendly materials, have been widely investigated. Transition metal phosphides have been also introduced in recent years as electrocatalysts for water splitting. Unfortunately, most catalysts suffer from the poor electrocatalytic properties for HER and OER due to the limit of its microtopography and single materials [24]. In consequence, considerable efforts have been focused on interface engineering, composition optimization and morphologies design, thus achieving the enhanced catalytic performance. For instance, cobalt metal powder exhibited poor HER activity, after modification with nanosheet arrays (Co NSs), it was superior to most of the non-noble metal catalysts for the HER in alkaline electrolytes [25], the graphene supported Co9S8 nanoparticles(Co9S8/G) realized a voltage of 139 mV at the current density of 10 mA cm-2 in alkaline media during the OER test, which is much lower than Co9S8 nanoparticles [26], the HER performance of CoP3 NAs/CFP nanoneedle arrays enhanced 50% compared with pure CoP3 NPs nanoparticles at the current density of 10 mA cm-2 in acidic media [27]. Over the past few decades, metal-organic frameworks (MOFs), a burgeoning kind of attractive materials known as porous crystalline structures with customizable electronic and chemical properties, have been widely used as superior naturally

nitrogenous templates to synthesize various electrocatalysts with array morphology, for example, Co3O4@X(X=Co3O4, CoS, C, and CoP) derived from MOF template [28], Nickel-cobalt bimetal phosphides nanotube derived from MOF [29], and NiCo-UMOFNs (ultrathin metal-organic framework nanosheets) [30]. So, it is worth to design a Co-based array catalyst with fast kinetics and morphologies optimization at the aid of MOF template. Herein, we design a highly efficient 3D bead string-like Nitrogen-Doped CoO@CoP arrays (N-CoO@CoP) on Ni foam derived from the self-sacrificial MOF template as bifunctional electrocatalysts. The facile and controllable synthesis strategy ensures a 3D porous heterostructure of the cobaltous oxide and cobalt phosphide nanoarrays that possess more accessible activated sites, fast gas bubbles diffusion channel and unimpeded electronic transport path. Benefiting from the unique morphology and heterostructure design, the obtained N-CoO@CoP reveal achieves admirable electrocatalytic properties with low overpotentials of 332 mV for OER and 201 mV for HER at the current density of 100 mA cm-2 in alkaline electrolyte. When the N-CoO@CoP is used as both the cathode and anode for overall water splitting, it shows a voltage of 1.79 V at the current density of 100 mA cm-2 and robust stability. Such high performance endows the N-CoO@CoP catalyst with the capability for commercial applications. Experimental Section Materials: All chemicals and reagents were purchased from commercial suppliers without further purification. Cobaltous Nitrate (Co(NO3)2·6H2O), Ammonium fluoride (NH4F), Sodium hypophosphite (NaH2PO2), Urea (CO(NH2)2) and

2-Metheylimidazole were purchased from Sinopharm Chemical Reagent Co., Ltd. Absolute ethanol and HCl solution were purchased from Beijing Chemical Factory. Synthesis of CoO nanoneedle arrays on Ni foam substrate: The nickel foam (1×4 cm2) was treated ultrasonically in 2 M HCl, acetone and ethanol for 10 min respectively to remove the surface NiO layer and subsequently washed thoroughly with deionized water several times. In a typical process [31], Co(NO3)2·6H2O (1 mmol), NH4F (4 mmol), and CO(NH2)2 (5 mmol) were added into 40 mL deionized water under ultrasonication for 30 min to get the homogeneous solution. Then the solution with treated Ni foam was transferred into 80 mL Teflon-lined stainless-steel autoclave maintained at 120℃ for 10 h. After cooling down, the pink foam was ultrasonic washed thoroughly several times to remove residues and dried in vacuum at 60℃ overnight. After annealing at 350℃ for 80 min in Ar atmosphere, the CoO nanoneedle arrays on Ni foam was obtained. Synthesis of CoO@ZIF-67: Typically [28,32], 10mmol 2-methylimidazole was dissolved in the mixture of 5 mL deionized water and 5 mL absolute ethanol, then the obtained CoO film was immersed into the solution. The beaker was sealed and placed at room temperature for 10 h. Then the purple film was rinsed with absolute ethanol and deionized several times and dried in an oven at 85 ℃ for 12 h to give CoO@ZIF-67 bead string-like arrays on Ni foam. Fabrication of N-CoO@CoP: The purple CoO@ZIF-67 array sample was put into a porcelain boat and placed in a muffle furnace at 350℃ for 2 h. Then the obtained sample and 2 g NaH2PO2 were placed in tube furnace (the porcelain boat with NaH2PO2 sample was placed on the top of the porcelain boat with the obtained sample) and heated at 300℃ for 2 h in Ar atmosphere. After cooling down, the N-CoO@CoP bead string-like arrays on Ni foam was obtained. The mass loading of the N-CoO@CoP electrocatalyst is approximately 5.2 mg cm-2. Electrochemical measurement: The electrocatalytic measurements were performed with a CHI 760E electrochemical workstation. The electrochemical experiments of HER and OER were investigated using a typical three-electrode system with a

graphite rod as the counter electrode, Hg/HgO as the reference electrode , and the self-supporting arrays grown on Ni foam (0.3×0.3 cm2) as the working electrode in 1M KOH (pH = 14). The Pt/C and Ir/C electrode were made by adding 2 mg corresponding catalyst and 15 µL Nafion in 1 mL ethanol to form a homogeneous solution, which were loaded on 0.3×0.3 cm2 Ni foam. Before the test, cyclic voltammetry was operated for 100 cycles at 50 mV/s to get stable curves. Electrochemical impedance spectroscopy (EIS) was carried out in the frequency range of 0.01 to 100000 Hz with an amplitude of 5 mV. The equivalent circuit of the EIS data was fitted with Z View software. All the potentials versus to Hg/HgO were converted to versus reversible hydrogen electrode (RHE), according the Nernst equation (ERHE = EHg/HgO + 0.098 + 0.059 × pH). The polarization curves were recorded with iR compensation. Overall water splitting measurements were performed in a two-electrode system using the self-supporting arrays grown on Ni foam (1×1 cm2) as both the anode and cathode in 1M KOH. Results and discussion The 3D self-supported N-CoO@CoP bead string-like arrays are fabricated via the hydrothermal method and in situ phosphorization treatment. As shown in Scheme 1, Ni foam is chosen as a support material because of its 3D porosity structures and good electrical conductivity. First, CoO nanowire arrays (CoO@NF) are grown vertically on Ni foam through hydrothermal reaction and calcination process in the Ar atmosphere. Then CoO@NF acts as self-sacrificial precursors and templates for proving Co2+ ions and skeletons for the growth of cobalt-based metal zeolitic

imidazolate framework polyhedral (ZIF-67) with the assistance of solvents and ligands. The ZIF-67 shell becomes thicker via the ligand penetrating the pores of ZIF-67 and reacting with the inner CoO species while deprotonation produced H+. Then, the bead string-like CoO@ZIF@NF are formed by Co2+ ions dissolved from CoO diffusing outward to react with the ligand at the surface of the ZIF-67 layer for continuous growth [32]. Finally, the black product of N-CoO@CoP@NF is obtained by one step in-situ phosphorization. Nitrogen doping improves the interface charge transfer kinetics, because the electronegativity of nitrogen is stronger than phosphorus, which lowers the d-band of CoP and weakens the H-adsorption on the surface of CoP [33]. All the preparation steps are simple to complete and feasible for large-scale practical applications. The as-prepared samples in Scheme 1 can be observed through their optical photographs (Fig. S1), which display the homogeneity of the preparation. The uniform arrays of both precursors and end-product can be distinctly observed by the scanning electron microscopy (SEM) and the transmission electron microscopy (TEM) images as shown in Fig. 1. The neat Ni foam is demonstrated of 3D porous structure by SEM image in Fig. S2, which is then immersed in Co2+ solution for the preparation of the CoO nanowire arrays on it through the hydrothermal process. Fig. 1a demonstrates the successful synthesis of the homogenous CoO nanowire arrays on the 3D Ni skeleton. After the in-situ growth of ZIF-67 on the CoO arrays by corrosion of the CoO as the Co2+ source, the regular octahedrons of MOF are strung by CoO nanowire and the morphology is changed to bead string-like arrays that seem like Chinese tomatoes on sticks (Fig. 1b). After the phosphorization treatment, the final

N-CoO@CoP have kept the original microstructure of the precursor, presenting bead string-like arrays perpendicularly towards the substrate, which can expose plenty of activated sites and supply unimpeded electric conductive path to enhance the electrocatalytic performances (Fig. 1c and 1d). Furthermore, the TEM image in Fig.1e shows the bead string-like nanorods array with a diameter of 80 nm. From the high-resolution TEM (HRTEM) image (Fig. 1f), the lattice fringes with interplanar distance of 0.24 nm can be observed, indexing to the (1 1 1) plane of CoO, and the interplanar distances of 0.24 nm, 0.28 nm, corresponding to the (1 1 1), (0 1 1) planes of CoP, which indicates that the phosphide is polycrystal instead of single crystal. The EDS elemental mapping of Co, O, N, and P are also conducted for the as-prepared N-CoO@CoP (Fig. 1e), showing the uniform distribution of these elements on the surface of the 3D Ni skeleton. All these images prove the successful preparation of 3D-bead string-like N-CoO@CoP array via a feasible hydrothermal, MOF morphology optimization and phosphating method. The X-ray diffraction (XRD) measurement is carried out to investigate the crystalline structures and phase purity of precursors and as-obtained N-CoO@CoP (Fig. 2a). XRD patterns of the CoO, CoO@ZIF-67 and N-CoO@CoP are presented in Fig. 2a, which suggests the successful preparation of the different samples. The peaks of CoO can be clearly observed in the XRD patterns of each sample and the characteristic peak of ZIF-67 can also correspond to the simulated peak of ZIF-67, which illustrates the successful synthesis of CoO and ZIF-67. For the XRD pattern of N-CoO@CoP, except for three strong peaks originating from Ni foam, the

characteristic peaks of CoO (PDF#75-0418) and CoP (PDF#85-2958) are observed simultaneously, confirming the hybrid phase of the N-CoO@CoP composite. The small peak appearing at about 41 degree corresponds to the superficial oxidation of metal phosphide after exposure to air. All of the patterns are well consistent with the corresponding standard cards. The X-ray photoelectron spectroscopy (XPS) measurements are further employed to probe the near-surface elemental composition and the chemical valence states of N-CoO@CoP samples. The full spectrum shows the existence of Co, O, N, and P elements in the composite (Fig. 2b). As displayed in Fig. 2c, the appearance of peak at 797.7 eV and 796.5 eV with one satellite peak at 803.3 eV can correspond to the spin-orbit splitting value of Co 2p1/2. The Co 2p spectrum shows two prominent peaks at 778.9 eV and 781.3 eV, and one satellite at 786.1 eV, corresponding to spin-orbit splitting values of Co 2p3/2, which is shifted to higher energy than that of metallic Co (778.2 eV for Co 2P3/2), indicating that the oxidation state of Co increase slightly in composite [33]. The high-resolution XPS spectrum of P 2p (Fig. 2d) presents two wide peaks (P 2p3/2 at 130.5 eV and P 2p1/2 at 129.6 eV), which can be attributed to phosphide and phosphate, respectively. The peak at 133.4 eV is likely to ascribe to the formation of phosphate caused by superficial oxidation of metal phosphide after exposure to air [34]. This is confirmed by the O 1s spectrum (Fig. 2e), where the peak at 531.2 eV may appear on account of the P-O and Co-O bands. The peak at 532.2eV can be explained as the O atoms of hydroxyl species of surface-adsorbed water molecules. The typical peak at 529.8 eV corresponds well to the metal-oxygen bonds. The N 1s spectrum shows the peak

centered at the binding energy of 400.2 eV, which is assigned to Co-N bonding [12]. The nitrogen source comes from the ligands of ZIF-67 (Fig. S5). The appearance of Co-N bonding indicated the doping of nitrogen. N doping can increase the binding energy of Co 2p3/2 and P 2p because N have a stronger electronegativity than Co and P. The dopant N attracts electrons from Co, P to alter the electronic structure of CoP, which provide an approach to tune the electrocatalytic properties and enhance the electrocatalytic performance [35]. The approximate percentage of N is 5.4% and the CoO/CoP ratios on the electrocatalytic performances are about 1:4 in the obtained N-CoO@CoP, which can be inferred from the results of the XPS test. The electrocatalytic activity of the N-CoO@CoP toward OER is measured in 1 M KOH solution by a typical three-electrode electrochemical system. The Ni foam and CoO electrodes (0.3 × 0.3 cm2) are tested for comparison. Fig. 3a shows the polarization curves obtained from linear sweep voltammograms (LSV) at a scan rate of 1 mV/s. The N-CoO@CoP catalyst exhibits the low overpotential η=332 mV at the current density of 100 mA cm-2, which is more competitive than many previous reported in alkaline electrolytes (Table S1). At the same current density of 100 mA cm-2, NF, CoO and IrO2 exhibit overpotentials of 474, 420 and 280 mV, respectively. The bare Ni foam shows very poor OER activity, so the high OER catalytic performance of N-CoO@CoP on Ni foam comes from the presence of N-CoO@CoP. Moreover, OER activity is also tested by Tafel plots. As shown in Fig. 3b, the Tafel plot of N-CoO@CoP, Ni foam, CoO and IrO2 are 81.5, 184, 150 and 85.4 mV dec-1. The Tafel slope is an important kinetic parameter for revealing changes in the

apparent OER mechanism. The lower the Tafel slope, the faster the OER kinetics. The Tafel slope of N-CoO@CoP is lower than others, indicating that the N-CoO@CoP is a better electrocatalyst for OER. The lowest Tafel slope together with the smallest resistance (Fig. 3c) of the N-CoO@CoP demonstrates good performance. To evaluate the

electrocatalytic

stability

of

the

material

during

OER,

multi-step

chronoamperometric tests of N-CoO@CoP are performed (Fig. 3d). The overpotential is started at 260 mV and ended at 480 mV with an increment of 20 mV at every 500 s. The current density remains quite stable at each potential or overpotential in the entire test range, demonstrating that the high stability of the obtained catalyst in a wide current density range (

800 mA cm-2), which can contribute to the unique3D bead

string-like arrays structures. The gas is released so fast that the electrodes vibrate under the pressure of the bubbles, causing the curve to fluctuate.

Furthermore, the

great stability of N-CoO@CoP catalyst can also be confirmed by comparing polarization curves (Fig. 3e), which display a slight change even after 3000 CV cycles. In addition, the results of current density versus time (I-t) at 100 mA cm-2 (Fig. 3f) over 24 h indicate once again that the material retains its electrocatalytic activity for a long time. No obvious activity attenuation proves the excellent stability of the as-prepared samples. According to previous reports, the initial transition-metal phosphide is usually oxidized to metal phosphates during OER, which plays an important role in OER catalysis. Co-phosphate easily generates on the surface of CoP, which contributes to

the robust stability of the catalytic system in the OER process. As we know, the OER consists of four consecutive stages based on the alkaline solution mechanism [36,37]: * + OH- → *OH + e-

(1)

*OH +OH- → *O+ e- + H2O

(2)

*O + OH- → *OOH + e-

(3)

*OOH + OH- → *O2 + e-

(4)

*O2 → * + O2 The above * indicates the catalytic activity site currently. This mechanism is most probable to phosphate catalysts, and catalytically active centers are usually transition metal sites. OH- is adsorbed on the active site to form *OH via the 1-electron oxidation of OH- according to eqn (1) in the first stage. Then *O forms by the removal of a second electron (eqn (2)), the oxygen atom recombines with OH- to transform into *OOH under the oxidation of a tertiary electron (eqn (3)). After the final stage, *OOH converts into *O2 after a fourth electron transfer process (eqn (4)), then O2 and the initial active site are. The adsorbed intermediates and electronic structure optimization are important for OER performance improvement. These stages are accompanied by changes in the oxidation state of polyvalent metals. For N-CoO@CoP, the changes in oxidation state of Co is Co2+ ↔ Co3+ ↔ Co4+ [38,39]. The electrocatalytic activity of the N-CoO@CoP for HER is also investigated in 1 M KOH solution utilizing a typical three-electrode electrochemical system. For comparison, the 0.3×0.3 cm2 electrode of pure Ni foam, CoO@ZIF-67 and Pt/C are tested in the same condition as well. The polarization curves obtained from linear

sweep voltammograms (LSV) at a scan rate of 2 mV/s (Fig. 4a) show that at a current density of 100 mA cm-2, Ni foam, CoO@ZIF-67, N-CoO@CoP and Pt/C require the overpotentials of 380, 318, 201 and 126 mV, respectively. N-CoO@CoP sample exhibits the lower overpotential η=201 mV at the current density of 100 mA cm-2, which is much better than that of many reported values for HER electrocatalysts performance in alkaline electrolytes (Table S2). Due to the inherent low HER capability of Ni foam, it can be concluded that the high catalytic performances stem from as-prepared bead string-like N-CoO@CoP materials. The Tafel slope of N-CoO@CoP is 37 mV dec-1 (Fig. 4b), remarkably, this value is much smaller in comparison with 154 mV dec-1 for Ni foam and 109 mV dec-1 for CoO@ZIF-67, respectively, suggesting that a two-electron transfer process occurs upon N-CoO@CoP catalyst following the Volmer-Tafel mechanism. The impedance analysis of materials is carried out with the help of electrochemical impedance spectroscopy (EIS). Compared to CoO@ZIF-67, as prepared N-CoO@CoP sample exhibits a smaller semicircle (Figure 4c), indicating the lower charge-transfer resistance and fast electronic transport during the electrochemical reaction, which is benefited from the 3D bead string-like arrays structure that provides an unimpeded electronic conductive path and numerous surface reaction sites. In addition, the results of current density versus time (I-t) at 100 mA cm-2 (Fig. 4d) over 24 h indicate that the material retains its electrocatalytic activity for a long time. Ignoring the slight change of overpotential in the initial 2 h, the N-CoO@CoP samples exhibit stable catalytic performance by and large.

It is widely accepted that HER proceeds by the Tafel-Volmer-Heyrovsky mechanisms in alkaline solution, according to three principal equations [40,41]: H2O + e- → H* + OH- (Volmer)

(5)

2H* → H2 (Tafel)

(6)

H2O + H* + e- → H2 + OH- (Heyrovsky)

(7)

The above * indicates the catalytic activity site currently. In alkaline media, to trade hydrogen adsorption, hydroxyl adsorption as well as water dissociation is vital during the HER processes. There are two widely accepted mechanisms of HER, namely the Volmer-Tafel mechanism or the Volmer-Heyrovsky mechanism. The first step of both reaction mechanisms is Volmer reaction, that is, water adsorption on active sites of the cathode electrode and then the strong covalent H-O-H breaking to form an adsorbed hydrogen atom (H*). Subsequently, there are two ways to produce hydrogen: two adsorbed hydrogen atoms combine to form hydrogen, Tafel reaction occurs (2H*→ H2); Adsorbed hydrogen atoms adsorb water and electron to form hydrogen, Heyrovsky reaction occurs. According to the different Tafel values, the speed control step of HER can be roughly determined. For N-CoO@CoP, the Tafel value of HER is 37 mV dec-1, which indicates that the Tafel process is the speed control step [42,43]. In the HER reaction process, the unique microstructure of 3D bead string-like N-doped CoO@CoP arrays and the N doping are crucial for the promotion of HER catalytic activity.

Based on the excellent HER electrocatalytic property, as well as satisfactory performance for OER of the N-CoO@CoP arrays catalyst, an overall water splitting system is designed using N-CoO@CoP arrays as bifunctional electrocatalysts. The N-CoO@CoP (1×1 cm2) works as both the anode and cathode in 1.0 M KOH. During the electrolysis process, electric power was converted into chemical energy: H2 is generated at the cathode and O2 is released from the anode (Fig. 5a). In Fig. 5b, the polarization curves of N-CoO@CoP show that the voltage difference (ΔV) between HER and OER are 1.763 V (vs RHE) at the current density of 100 mA cm-2. The value is very close to the result of the actual test. The N-CoO@CoP//N-CoO@CoP electrolyzer in alkaline electrolyte exhibits outstanding performance for overall water splitting with a voltage of 1.79 V at the current density of 100 mA cm-2 (Fig. 5c), which is much lower than CoO//CoO, CoO@ZIF-67//CoO@ZIF-67 and other phosphides according to previously reported (Table S3). The superior performance is associated with the unique bead string-like arrays structure that provides a better pathway for gas transport and releases the surface of the electrode, ensuring the fast and effective work of the catalyst. In addition, the N-CoO@CoP//N-CoO@CoP electrolyzer shows excellent stability without obvious current density lose with a cell voltage of 1.57 V at 50 mA cm-2 for 50 h (Fig. 5d). After overall water splitting for 50 h, both the cathode and anode electrodes are measured by SEM to investigate their morphology for auxiliary instruction of the excellent stability of electrolyzer (Fig. S7). The HER electrode shows negligible change compared to intrinsic morphology. Although there is a nuance caused by peroxidation during OER process, the durability

of the N-CoO@CoP//N-CoO@CoP electrolyzer is still stronger than other phosphides reported previously. After HER and OER stability tests, the XRD and XPS are conducted to analyze the electrode materials. In Fig. S8, the XRD patterns and the high-resolution XPS spectra of Co 2p and P 2p after the HER and OER stability test are almost identical to the fresh sample, except that after OER stability test, two new peaks of phosphate are generated in the XRD pattern, which can be confirmed by the high-resolution XPS spectra of Co 2p and P 2p. After OER stability test, the peak of Co 2p3/2 shifts to higher energies and the peak of P-O becomes stronger with the disappearance of P 2p1/2 and P 2p3/2, suggesting the elements of Co and P are oxidized to cobalt phosphate. The oxidized cobalt phosphate also could provide the active sites, which promote the OER reaction and maintain its stable catalytic activity. All the above results suggest that the bead string-like N-CoO@CoP arrays electrode possess excellent performance for overall water splitting as well as satisfactory durability. Conclusion In conclusion, 3D bead string-like N-doped CoO@CoP arrays have been successfully prepared by a facile controllable synthesis method. The outstanding electrocatalytic property benefit from the rational morphologies design and composition optimization, which furnish more deliverable passways and fast electron transfer channels so as to achieve fast kinetics. The as-obtained sample could also serve as a bifunctional electrocatalyst for effective and durable overall water splitting. The simple and controllable synthesis process to directionally grow of MOF temple was a great design for reference of other materials preparation, simultaneously the large-scale

current density and outstanding performance may make the N-CoO@CoP catalyst possible for broad commercial applications. Acknowledgements This work was supported by the National Natural Science Foundation of China (21571080). REFERENCES [1] N. Armaroli, V. Balzani, Chem. Int. Ed. 46 (2007) 52-66. [2] W. Stedman, H. Kang, S. Lin, J. L. Kissil, M. S. Bartolomei, P. M. Lieberman, EMBO J. 27 (2008) 654-66. [3] J. D. Figueroa, T. Fout, S. Plasynski, H. McIlvried, R. D. Srivastava, Int. J. Greenhouse Gas Control 2 (2008) 9-20. [4] F. Cheng, J. Chen, Chem. Soc. Rev. 41 (2012) 2172-92. [5] P. W. Menezes, C. Panda, S. Loos, F. Bunschei-Bruns, C. Walter, M. Schwarze, X. Deng, H. Dau, M. Driess, Energy Environ. Sci. 11 (2018) 1287-1298. [6] Z. Niu, C. Qiu, J. Jiang, L. Ai, ACS Sustainable Chem. Eng. 7 (2018) 2335-2342. [7] R.-Q. Li, B.-L. Wang, T. Gao, R. Zhang, C. Xu, X. Jiang, J. Zeng, Y. Bando, P. Hu, Y. Li, X.-B. Wang, Nano Energy, 58 (2019) 870-876. [8] Y. Liu, N. Han, J. Jiang, L. Ai, Int. J. Hydrogen Energy 44 (2019) 10627-10636. [9] L. Huang, L. Ai, M. Wang, J. Jiang, S. Wang, Int. J. Hydrogen Energy 44 (2019) 965-976. [10] M. P. Browne, Z. Sofer, M. Pumera, Energy Environ. Sci. 12 (2019) 41-58. [11] F. Yu, L. Yu, I.K. Mishra, Y. Yu, Z.F. Ren, H.Q. Zhou, Mater. Materials Today Phys. 7 (2018) 121-138. [12] J. Li, Y. Zhang, X. Zhang, J. Huang, J. Han, Z. Zhang, X. Han, P. Xu, B. Song, ACS Appl. Mater. Interfaces 9 (2017) 398-405. [13] R.-Q. Li, P. Hu, M. Miao, Y. Li, X.-F. Jiang, Q. Wu, Z. Meng, Z. Hu, Y. Bando, X.-B. Wang, J. Mater. Chem. A 6 (2018) 24767-24772.

[14] F. Yu, H. Zhou, Y. Huang, J. Sun, F. Qin, J. Bao, W.A. Goddard, 3rd, S. Chen, Z. Ren, Nat. Commun. 9 (2018) 2551. [15] H. Zhou, F. Yu, Q. Zhu, J. Sun, F. Qin, L. Yu, J. Bao, Y. Yu, S. Chen, Z. Ren, Energy Environ. Sci. 11 (2018) 2858-2864. [16] J. Meng, J. Fu, X. Yang, M. Wei, S. Liang, H.-Y. Zang, H. Tan, Y. Wang, Y. Li, Inorg. Chem. Front. 4 (2017) 1791-1797. [17] B. Zhang, X. Zheng, O. Voznyy, R. Comin, M. Bajdich, M. García-Melchor, L. Han, J. Xu, M. Liu, L. Zheng, F.P. García de Arquer, C.T. Dinh, F. Fan, M. Yuan, E. Yassitepe, N. Chen, T. Regier, P. Liu, Y. Li, P. De Luna, A. Janmohamed, H.L. Xin, H. Yang, A. Vojvodic, E.H. Sargent, Science 352 (2016) 333–337. [18] J. T. Mefford, X. Rong, A. M. Abakumov, W. G. Hardin, S. Dai, A. M. Kolpak, K. P. Johnston, K. J. Stevenson, Nat. Commun. 7 (2016) 11053. [19] S. Dou, C.-L. Dong, Z. Hu, Y.-C. Huang, J.-l. Chen, L. Tao, D. Yan, D. Chen, S. Shen, S. Chou, S. Wang, Adv. Funct. Mater. 27 (2017) 1702546. [20] T.Y. Ma, S. Dai, M. Jaroniec, S.Z. Qiao, J. Am. Chem. Soc. 136 (2014) 13925-31. [21] N. Han, P. Liu, J. Jiang, L. Ai, Z. Shao, S. Liu, J. Mater. Chem. A

6

(2018)

19912-19933. [22] I.K. Mishra, H. Zhou, J. Sun, F. Qin, K. Dahal, J. Bao, S. Chen, Z. Ren, Energy Environ. Sci. 11 (2018) 2246-2252. [23] J. Sun, M. Ren, L. Yu, Z. Yang, L. Xie, F. Tian, Y. Yu, Z. Ren, S. Chen, H. Zhou, Small 15 (2019) e1804272. [24] X. Xiao, C.-T. He, S. Zhao, J. Li, W. Lin, Z. Yuan, Q. Zhang, S. Wang, L. Dai, D. Yu, Energy Environ. Sci. 10 (2017) 893-899. [25] B. Liu, N. Zhang, M. Ma, J. Mater. Chem. A 5 (2017) 17640-17646. [26] S. Dou, L. Tao, J. Huo, S. Wang, L. Dai, Energy Environ. Sci. 9 (2016) 1320-1326. [27] T. Wu, M. Pi, D. Zhang, S. Chen, J. Mater. Chem. A 4 (2016) 14539-14544. [28] J. Zhou, Y. Dou, A. Zhou, R.-M. Guo, M.-J. Zhao, J.-R. Li, Adv. Energy Mater. 7 (2017) 1602643.

[29] L. Yan, L. Cao, P. Dai, X. Gu, D. Liu, L. Li, Y. Wang, X. Zhao, Adv. Funct. Mater. 27 (2017) 1703455. [30] S. Zhao, Y. Wang, J. Dong, C.-T. He, H. Yin, P. An, K. Zhao, X. Zhang, C. Gao, L. Zhang, J. Lv, J. Wang, J. Zhang, A. M. Khattak, N. A. Khan, Z. Wei, J. Zhang, S. Liu, H. Zhao, Z. Tang, Nat. Energy 1 (2016) 16184. [31] J. Li, G. Wei, Y. Zhu, Y. Xi, X. Pan, Y. Ji, I. V. Zatovsky and W. Han, J. Mater. Chem. A 5 (2017) 14828-14837. [32] Y. Zheng, S.-Z. Qiao, Chem 2 (2017) 751-752 [33] Q. Zhou, Z. Shen, C. Zhu, J. Li, Z. Ding, P. Wang, F. Pan, Z. Zhang, H. Ma, S. Wang, H. Zhang, Adv. Mater. 30 (2018) e1800140. [34] A.-L. Wang, J. Lin, H. Xu, Y.-X. Tong, G.-R. Li, J. Mater. Chem. A 4 (2016) 16992-16999. [35] H. Zhang, T. Wang, A. Sumboja, W. Zang, J. Xie, D. Gao, S. J. Pennycook, Z. Liu, C. Guan, J. Wang, Adv. Funct. Mater. 28 (2018) 1804846. [36] E. C. M. Tse, T. T. H. Hoang, J. A. Varnell, A. A. Gewirth, ACS Catal. 6 (2016) 5706-5714. [37] L.-A. Stern, L. Feng, F. Song, X. Hu, Energy Environ. Sci. 8 (2015) 2347-2351. [38] Y. Liu, H. Wang, D. Lin, C. Liu, P.-C. Hsu, W. Liu, W. Chen, Y. Cui, Energy Environ. Sci. 8 (2015) 1719-1724. [39] Y. Zhan, M. Lu, S. Yang, C. Xu, Z. Liu, J. Y. Lee, ChemCatChem 8 (2016) 372-379. [40] X. Zou, Y. Zhang, Chem. Soc. Rev. 44 (2015) 5148-5180. [41] R. Parsons, Trans. Faraday Soc. 54 (1958) 1053. [42] C. Du, M. Shang, J. Mao, W. Song, J. Mater. Chem. A 5 (2017) 15940-15949. [43] Y. Lv, X. Wang, Catal. Sci. Technol. 7 (2017) 3676-3691.

Figure Captions Scheme 1. Schematic illustration of the synthesis process of bead string-like N-CoO@CoP arrays. Fig. 1. SEM images of a) CoO, b) CoO@ZIF-67, c) CoO@Co3O4 and d) N-CoO@CoP, e) TEM images of N-CoO@CoP, f) HRTEM images of the N-CoO@CoP, g) EDS elemental mapping images (Co, O, N, P) of the N-CoO@CoP. Fig. 2. a) XRD patterns of the CoO, CoO@ZIF-67 and N-CoO@CoP. b) XPS survey scan spectrum of the N-CoO@CoP and high-resolution spectra of c) Co 2p, d) p 2p, e) O 1s, and f) N 1s. Fig. 3. a) OER polarization curves of Ni foam, CoO, N-CoO@CoP and IrO2. b) OER Tafel plots of Ni foam, CoO, N-CoO@CoP and IrO2 obtained from the OER polarization curves. c) EIS Nyquist plots of CoO, N-CoO@CoP at 200 mV. d) Multi-step chronoamperometric curve obtained with N-CoO@CoP at different overpotential. e) OER polarization curves of N-CoO@CoP before and after 3000 CV cycles. f) The current-time curves of the N-CoO@CoP at the current density of 100 mA cm-2, the insert is optical pictures of the current-time test. Fig. 4. a) HER polarization curves of Ni foam, CoO@ZIF-67, N-CoO@CoP and Pt/C. b) HER Tafel plots of Ni foam, CoO@ZIF-67, N-CoO@CoP and Pt/C obtained from the HER polarization curves. c) Nyquist fitting plots of Ni foam, CoO@ZIF-67, N-CoO@CoP at 200 mV. d) The current-time curves of the N-CoO@CoP at the current density of 100 mA cm-2, the insert is optical pictures of the current-time test.

Fig. 5. a) Schematic illustration of the two-electrode electrolyzer. b) The polarization curves of N-CoO@CoP for HER and OER. c) The overall water splitting performance of the CoO, CoO@ZIF-67 and N-CoO@CoP couple with a scan rate of 5 mV s-1 in 1.0 M KOH (inset: Photograph of the two-electrode electrolyzer using N-CoO@CoP as both anode and cathode at a current density of 50 mA cm-2). d) Time-dependent current density curve for N-CoO@CoP in a two-electrode configuration at the fixed potential to produce a current density of 50 mA cm-2.

There are no interest conflicts to declare.