Self-supported bimetallic phosphide-carbon nanostructures derived from metal-organic frameworks as bifunctional catalysts for highly efficient water splitting

Electrochimica Acta 318 (2019) 244e251

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Self-supported bimetallic phosphide-carbon nanostructures derived from metal-organic frameworks as bifunctional catalysts for highly efficient water splitting Qianqian Zhou a, Jiayan Wang a, Fenya Guo a, Hongwei Li a, Mengzhe Zhou a, Jinjie Qian b, Ting-Ting Li a, **, Yue-Qing Zheng a, * a Chemistry Institute for Synthesis and Green Application, School of Materials Science and Chemical Engineering, Ningbo University, 818 Fenghua Road, Ningbo, 315211, PR China b College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, 325035, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 April 2019 Received in revised form 26 May 2019 Accepted 14 June 2019 Available online 14 June 2019

Nanostructured transition-metal phosphides (TMPs) have recently emerged as a new family of nonnoble-metal catalysts to drive water splitting due to their unique electronic and redox properties. However, most progress focused on developing mono-metal phosphide nanostructures. In this work, a facile template-based method and low-temperature phosphorization process are proposed to fabricate self-supported Ni-based bimetallic phosphide encapsulated in amorphous carbon by using metal-organic framework (MOF) as the precursor and three-dimensional nickel foam (NF) as the support, which is termed as Ni2P-Co2P@C/NF. This composite demonstrates remarkable electrocatalytic activities towards both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline electrolyte (1 M KOH, pH 13.6), affording low overpotentials of 290 and 167 mV to deliver the current density of 50 mA cm
2 for OER and HER, respectively, preceding the majority of recently reported MOFs-derived TMPs. This excellent performance is considered as the results of its large catalytic surface area, concerted synergy from composited structure as well as the increased electrical conductivity. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Bimetallic phosphides Carbon encapsulating Bifunctional catalyst Water splitting Synergy

1. Introduction The environmental pollution and energy crisis caused by the ongoing depletion of fossil fuels have triggered urgent quest for new energy conversion and storage approach with high efficiency and environmentally friendliness. Hydrogen is considered as one of the most ideal alternative energy carriers to replace traditional fossil fuel energy because of its high mass energy density, sustainable and regenerative nature [1,2]. Electrolysis of water for large-scale production of high purity hydrogen includes two halfreactions, OER (2H2O / 4Hþ þ O2 þ4e
, E0 ¼ 1.23 V vs reversible hydrogen electrode, RHE) and HER (4Hþ þ 4e
/ 2H2, E0 ¼ 0.00 V vs RHE) [3]. Due to the sluggish kinetics intrinsically associated with both reactions, the efficient electrocatalysts are generally

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T.-T. Li), [email protected] (Y.-Q. Zheng). https://doi.org/10.1016/j.electacta.2019.06.082 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

demanded. Currently, the precious metal included state-of-the-art catalysts such as IrO2, RuO2 and Pt/C are still the benchmark catalysts for OER and HER, respectively. However, the large-scale employment of these Pt-group catalysts is greatly impeded because of the exorbitant cost and low natural abundance [4,5]. Consequently, the developing of efficient nonprecious metal catalysts for water splitting and especially bifunctional catalysts pairing both OER and HER active sites has sparked extensive research interest and significant progress has been achieved based on numerous transition metal-containing materials, such as selenides [6], sulfides [7], oxides [8], phosphides [9,10] as well as alloys [11]. Among them, TMPs have shown great promise as bifunctional catalysts due to their excellent electrical conductivity and diversity in term of composition and morphology. However, most progress focus on developing mono-metal phosphide nanostructures, and their activities are still far from satisfactory [12e15]. The incorporation of foreign metal atoms into the crystal lattice and construction of bimetallic phosphides is demonstrated as a promising approach to optimize the catalytic activities of TMPs as

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their local coordination environment and electronic structure can be expediently regulated [16,17]. For instance, Ni2P-CoP material exhibits superior HER and OER activity to mono-metal Ni2P and CoP counterparts in 1 M KOH electrolyte [18]. Ni2(1-x)Mo2xP nanowire arrays also show better HER activity when compared with their Ni2P reference material [19]. These results verified that the strong synergistic effects caused by the redistribution of valence electrons in bimetallic phosphides result in the improved catalytic efficiency. Additionally, the fabrication of TMPs and carbonaceous material based composites by introducing carbon layers as the favorable support is an attractive strategy to further enhance the electrocatalytic performance of TMPs, because it can not only improve their electrical conductivity and mechanical robustness, but also lead to more active sites simultaneously [20]. Some groups have reported that hierarchical TMPs@carbon composites can be fabricated by directly annealing MOFs precursors at high temperatures in an inert atmosphere by using NaH2PO2 as the phosphorus sources, the inherited porous architecture and high surface area endow these phosphides with rich active sites and remarkable catalytic activity [21e24]. For example, optimized ZIF-67 derived CoP@BCN (BCN ¼ B/N co-doped graphene) nanotubes show notable electrocatalytic HER activity in wide pH range along with excellent long-term stability [23]. More recently, Li's group reported CoP encapsuled in N-doped carbon nanotube hollow polyhedron derived from ZIF-8@ZIF-67 core-shell structure as a bifunctional electrocatalyst for overall water splitting, affording a current density of 10 mA cm
2 at fixed potential of 1.64 V [24]. The outstanding catalytic activity can be ascribed to the synergistic effects between CoP and N-doped carbon nanotubes. Herein, a self-supported homobimetallic phosphide-carbon composite composed of Ni2P-Co2P microsheets on a porous nickel foam with amorphous carbon layers (marked as Ni2P-Co2P@C/NF) was fabricated through a facile hydrolysis-etching technique followed by a low-temperature phosphorization process by using the well-aligned ZIF-67 microsheets as the precursors. As a bifunctional electrocatalyst, the resultant Ni2P-Co2P@C/NF composite demonstrates remarkable electrocatalytic performance for both OER and HER in 1 M KOH electrolyte, featuring low overpotentials of 290 and 167 mV to deliver the current density of 50 mA cm
2, respectively, outperforms the performances of Co2P/NF and Ni2P-Co2P/NF counterparts, as well as the state-of-the-art precious metal included catalysts. The enhanced catalytic activity of Ni2P-Co2P@C/ NF results from the highly open space with rich active sites of the obtained delicate structure and the synergetic effects between Ni2P-Co2P and amorphous carbon layers.

2. Experimental section 2.1. Materials All chemicals such as cobaltous nitrate hexahydrate (Co(NO3)2$6H2O), 2-methylimidazole (C4H6N2), nickel nitrate hexahydrate (Ni(NO3)2$6H2O), sodium hypophosphite (NaH2PO2), IrO2, 20% Pt/C, potassium hydroxide (KOH), glucose (C6H12O6), absolute ethanol, Nafion solution (5 wt%) were purchased commercially and without further purification. The aqueous solutions used for synthesis and electrochemical measurements were prepared by MilliQ water (R > 18.2 MU cm
1). Other organic solvents were of analytical grade quality. Prior to use, porous nickel foam (NF, 1  3 cm2) was sequentially ultrasonicated in diluted HCl solution and absolute ethanol for 10 min, then washed with Milli-Q water for several times.

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2.2. Synthesis of ZIF-67/NF ZIF-67/NF was prepared according to previous literatures with some modification [25,26]. 2-methylimidazole was dispersed in 40 mL Milli-Q water to obtain solution A with 2-methylimidazole concentration of 0.4 M. Co(NO3)2$6H2O was dissolved into another 40 mL Milli-Q water to obtain solution B with concentration of 50 mM. Both aqueous solutions were mixed quickly, then a piece of porous NF was immersed into the above reaction solution. After 4 h, NF was taken out and washed with water and then dried at 60  C in vacuum. The deposited ZIF-67 with color of purple on NF was used for further characterization. 2.3. Synthesis of NiCo(OH)x@glucose/NF [27]. ZIF-67/NF and 100 mL ethanol/water mixed solution (v/v, 1:4) with 0.05 g Ni(NO3)2$6H2O were placed to a beaker, the solution temperature was kept at about 85  C by water-bath heater. ZIF-67/ NF was taken out and rinsed with water until its purple color disappeared. Subsequently, the obtained NiCo(OH)x/NF sample was immersed into 50 mL 0.1 M glucose solution and kept it for 24 h at room temperature and finally dried at 60  C to obtain NiCo(OH)x@glucose/NF precursor. Co(OH)2@glucose/NF was prepared according to the similar procedure but without the addition of Ni(NO3)2$6H2O. 2.4. Synthesis of Ni2P-Co2P@C/NF To obtain Ni2P-Co2P@C/NF, a piece of NiCo(OH)x@glucose/NF precursor and 1 g NaH2PO2 were placed in two separated quartz boats, respectively. The quartz boat with NaH2PO2 was placed at the upstream side of the tube furnace, and then the tube furnace was heated to 350  C with a ramping speed of 1  C min
1 and kept for 2 h under N2 atmosphere. The loading mass of Ni2P-Co2P@C on NF was weighed by an analytical balance, which is equal to 4 mg cm
2. For comparison, Ni2P-Co2P/NF and Co2P@C/NF counterparts were prepared by directly phosphorization of NiCo(OH)x/NF and Co(OH)2@glucose/NF. NiCoOx/NF and NiCoOx@C/NF samples were obtained through directly calcining NiCo(OH)x and NiCo(OH)x@glucose in tube furnace under N2 gas flow without the addition of NaH2PO2. The loading mass of Ni2P-Co2P/NF, Co2P@C/ NF, NiCoOx/NF and NiCoOx@C/NF comparative samples were about 4.0, 3.9, 3.8 and 3.9 mg cm
2, respectively. 2.5. Characterization Powder X-ray diffraction (XRD) patterns and element composition of all samples were obtained on a Bruker AXS D8 Advance Xray diffractometer (Cu Ka radiation, l ¼ 0.1542 nm, 6 min
1, 2q range from 5 to 80 ) and an inductively coupled plasma atomic emission spectrometer (ICP-AES, PerkinElmer 2100DV), respectively. The morphologic information was recorded by a fieldemission scanning electron microscope (SEM, Hitachi S-4800) and a transmission electron microscope (TEM, Tecnai G2 F20 STWIN). X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 instrument. 2.6. Electrochemical measurements The electrochemical measurements were performed on a CHI 760E electrochemical workstation with a standard three-electrode setup, Ni2P-Co2P@C/NF with an active area of 1  1 cm2 was employed as the working electrode, an Ag/AgCl electrode and a Pt foil were employed as reference and the counter electrode, respectively. Polarization curves were recorded by linear sweep

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voltammetry (LSV) with a scan rate of 1 mV s
1 with iR-compensation. Tafel slope was derived from the corresponding LSV curve. Cyclic voltammetry (CV) was performed in a non-Faradaic potential range with various scan rates (10e50 mV s
1) to estimate the double layer capacitance (Cdl) and electrochemically active surface area (ECSA). The slope of current density vs scan rates is equal to Cdl, the ECSA was calculated according to the following equation: ECSA ¼ Cdl/Cs [28]. Electrochemical impedance spectroscopy (EIS) was carried out in a range from 1  105 to 1  10
2 Hz with an amplitude of 5 mV. Long-term electrolysis includes chronopotentiometry and chronoamperometry was performed to evaluate the durability of the samples. The potentials reported in this work were converted to RHE scale via calibration with the following equation: ERHE ¼ EAg/AgCl þ 0.197 þ 0.0592  pH. Faradaic efficiency (FE) was derived based on the calculated and experimental amounts of products (O2 or H2). 3. Results and discussion The preparation procedure of Ni2P-Co2P@C/NF is diagrammatically demonstrated in Fig. 1 [25e27]. Firstly, two dimensional ZIF67 microsheets were in-situ grown on porous NF through a simple liquid-phase deposition process by using Co(NO3)2 as the metal source and 2-methylimidazole as the ligand. The XRD patterns of the obtained sample match well with the simulated signals of ZIF67 (Fig. S1), indicating the successful formation of ZIF-67/NF [29]. ZIF-67/NF is composed of numerous well-aligned microsheets with smooth surfaces, the thickness of each microsheet is about 0.5 mm. NiCo(OH)x/NF was prepared in the subsequent ion-exchange and etching reaction in the Ni2þ containing ethanol solution, the morphology was retained but with much thinner thickness (Fig. S2). The XRD results verified it contains Ni(OH)2 and Co(OH)2 components (Fig. S3). Then, NiCo(OH)x/NF was immersed into the glucose solution for 24 h to obtain NiCo(OH)x@glucose/NF. Finally, this substrate was convert to the target product Ni2P-Co2P@C/NF through a low-temperature phosphorization process by using NaH2PO2 as the phosphorus source under N2 atmosphere, in which the interior NiCo(OH)x was converted to Ni2P-Co2P and the outer glucose was carbonized and carbon layers formed. The XRD patterns given in Fig. 2a demonstrate that Ni2P-Co2P@C/NF mainly contains Ni2P and Co2P, the diffraction peaks located at 47.7, 54.2 and 55 can be indexed to (210), (002), and (211) planes of Ni2P (JCPDS: 65-1989), respectively [29]. As for Co2P, the characteristic peaks at 31.6, 40.7, 42 and 48.7 correspond well to (200), (121), (220) and (031) planes (JCPDS: 32-0306), respectively, no diffraction information from carbon was detected, suggesting its amorphous nature. Besides, two small peaks at 35.8 and 38.6 belong to (200) lattice plane of CoP2 (JCPDS 26-0481) and (101) lattice plane of Ni(OH)2 (JCPDS 14-0117) respectively, indicating a small quantity of CoP2 was produced and partial oxidation of Ni2P-Co2P on the

electrode surface when exposed in the air. Raman spectroscopy was employed to further investigate the component of Ni2P-Co2P@C, Fig. S5 shows two peaks at 1580 and 1380 cm
1, which contribute to G-band of sp2 graphite and D band of sp3-disordered carbon [27,30], confirming the successful carbonization of glucose. The inductively coupled plasmas atomic emissive spectrometry (ICPAES) reveals that the atomic ratio of Ni and Co elements in Ni2PCo2P@C composite is about 7:2. The SEM image of Ni2P-Co2P@C/NF is presented in Fig. 2b, the well-defined microsheets are uniformly grown the NF skeleton, no collapse and aggregate were observed. Such an exquisite two dimensional morphology could provide high surface area and abundant active sites, which would promote the electrocatalytic activity. To obtain more insights into the morphology and component of Ni2P-Co2P@C, TEM analysis was further conducted. The highresolution TEM image in Fig. 2d reveals two discernable lattice fringes with d-spacing of 0.22 and 0.19 nm corresponding well to (121) and (210) lattice planes of Co2P and Ni2P, respectively. The selected area electron diffraction (SAED) image of Ni2P-Co2P@C (Fig. 2e) indicates the typical polycrystalline character, the main diffraction rings can be indexed to (422), (420) planes of Ni2P and (232), (040) planes of Co2P. The elemental mapping image of Co, Ni, P and C elements in Fig. 2f illustrates the homogeneous distribution of each element throughout the entire microsheet, which further verify the successful transformation of NiCo(OH)x@glucose precursor to Ni2P-Co2P@C. The detailed electronic state of each element in Ni2P-Co2P@C was investigated by XPS. The survey XPS spectrum in Fig. S6 indicates the coexistence of P, C, Ni, Co and O elements. The magnified XPS spectrum for Ni 2p region was exhibited in Fig. 3a. Two spinorbit doublets are observed, in which the peaks located at 852.8 and 856.8 eV belong to the Ni 2p3/2, the peaks with binding energies of 870.3 and 874.7 eV are assigned to the Ni 2p1/2. The peaks at 852.8 and 870.3 eV can be ascribed to Ni2þ species, while peaks at 856.8 and 847.7 eV indexed to Ni2þ in phosphate, caused by surface oxidation. The satellite peaks of Ni 2p are located at 861.6 and 880.7 eV [31,32]. As for the high-resolution XPS spectrum of Co 2p (Fig. 3b), the binding energies of Co 2p3/2 and Co 2p1/2 ascribed to Co-P can be found at 778.8 and 793.8 eV, respectively. The satellite peaks of Co 2p were demonstrated at 786.3 and 803.4 eV [24,33]. Compared to Co 2p in Co2P@C/NF, the binding energies of Co 2p in Ni2P-Co2P@C show a positive shift by about 0.3 eV (Fig. S7), indicating the introduction of Ni2P alters the electronic structure of Co2P, which leads to strong synergistic effects between Ni2P/Co2P interfaces and thus promotes the adsorption and dissociation of intermediates during water electrolysis [31,34]. The XPS spectrum of P 2p in Fig. 3c shows three peaks at 129.6, 130.4 and 134.2 eV, corresponding to P 2p3/2, P 2p1/2 in metal phosphides and the partially oxidized P species, respectively [35]. The C 1s peak of Ni2PCo2P@C shows four deconvoluted peaks at 284.2, 284.9, 286.2 and

Fig. 1. Schematic illustration of the synthetic procedure of Ni2P-Co2P@C/CF.

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Fig. 2. (a) XRD patterns, (b) SEM image, (c) TEM image, (d) HR-TEM image, (e) SAED image and (f) elemental mapping of the obtained Ni2P-Co2P@C/NF.

288.7 eV (Fig. 3d), in which the former two peaks are assigned to sp2 and sp3 C species, and the others can be ascribed to C-O and C¼C [36]. The electrocatalytic performance of Ni2P-Co2P@C/NF towards OER was first investigated by LSV in 1 M KOH electrolyte. For comparison, the electrocatalytic activities of Ni2P-Co2P/NF, Co2P@C/ NF, NiCoOx/NF, NiCoOx@C/NF and commercial IrO2 were also investigated. The polarization curves of all samples were presented in Fig. 4a. The corresponding overpotentials of Ni2P-Co2P/NF, Co2P@C/NF, NiCoOx/NF and NiCoOx@C/NF to obtain the current density of 50 mA cm
2 are 300, 299, 390 and 330 mV, respectively, while Ni2P-Co2P@C/NF only requires 290 mV to deliver the same current density. It is worth noting that the obtained Ni2P-Co2P@C/ NF also shows better OER performance than state-of-the-art IrO2 material and the majority of recently reported MOFs-derived metal phosphides (Table S1), such as Fe-CoP hollow triangle plate arrays [35], Mn-CoP [37] and FeCo-P/C [38]. To prove the reliability of the results, we provided the LSV curves of three fresh Ni2P-Co2P@C/NF samples (Fig. S8). The OER activity of these electrodes was roughly the same, which identify the excellent activity for water oxidation. Fig. 4b demonstrates the Tafel slopes estimated from LSV curves, a Tafel slope of 64 mV dec
1 was obtained for Ni2P-Co2P@C/NF, which is the lowest value among all samples, revealing its favorable OER kinetics. These results demonstrate that the synergistic effects between Ni2P-Co2P and amorphous carbon layers enhance the OER

activity efficiently. The Nyquist curves obtained from EIS along with an equivalent circuit are shown in Fig. 4d, in which CPE represents a constant phase element originating from the double-layer capacitance, Rs and Rct are the electrolyte resistance and the chargetransfer resistance, respectively [39]. Rs and Rct values of 1.28 and 2.06 U for Ni2P-Co2P@C/NF are obtained at 1.5 V vs RHE, much smaller than those of other samples under the same operating condition, indicating smaller internal resistance and faster charge transfer rate. The increased electrical conductivity of Ni2P-Co2P@C/ NF can be contributed to the self-supported synthesis method and the introduction of amorphous carbon layers. To estimate the ECSA, Cdl was obtained by CV at a non-faradic potential range with various scan rates (Fig. S9). Cdl value of Ni2P-Co2P@C/NF is 287 mF cm
2, which is much higher than those of other samples (Fig. 4e), suggesting the high active surface area of Ni2P-Co2P@C/NF. The long-term stability of Ni2P-Co2P@C/NF towards OER was also studied. Fig. 4f shows the amperometric i-t curve of Ni2PCo2P@C/NF recorded at 1.55 V vs RHE, the current density decreased slightly after 20 h electrolysis (from 34 to 31 mA cm
2). The Faradic efficiency for initial one-hour electrolysis was also calculated, about 0.33 mmol of O2 was formed and a Faradic efficiency of 97% was obtained, indicating there was hardly any other reaction (Fig. S10). The chronopotentiometric curve recorded at a fixed current density of 30 mA cm
2 for Ni2P-Co2P@C/NF was shown in Fig. S11, the required overpotential of about 330 mV was

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Fig. 3. (a) Ni 2p, (b) Co 2p, (c) P 2p and (d) C 1s XPS spectra of Ni2P-Co2P@C/NF.

Fig. 4. (a) LSV plots for OER and (b) corresponding Tafel slopes of all samples in 1 M KOH electrolyte. (c) Histogram of overpotentials (h) at j ¼ 50 mA cm
2 (blue column) and the Tafel slopes (orange column) for each sample. (d) Nyquist plots and (e) Cdl plots for different samples. (f) Stability test of Ni2P-Co2P@C/NF at 1.55 V vs RHE. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

maintained for over 20 h, manifesting its excellent durability. The changes in morphology and composition of Ni2P-Co2P@C/NF after the long-term electrolysis were presented in Figs. S12e14. The microsheet morphology was almost preserved, while the XPS analysis indicates the slight negative shift of Ni 2p region, the new

peaks at around 855.7 and 873.2 eV can be ascribed to Ni2þ 2p signals in Ni(OH)2 or NiO. The formation of oxidized Co species with higher oxidation state was also observed and the P 2p3/2 signal of metal phosphide nearly disappeared. The signal of O 1s was also provided. As demonstrated in Fig. S15, the high-resolution XPS

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spectra of O 1s after 20 h of long-term electrolysis exhibited the peak at 531.1 eV which result from the hydroxides hydroxylated from Ni2P-Co2P [40]. The elemental mapping of Ni2P-Co2P@C/NF after OER was demonstrated in Fig. S16, the obvious signal of O further support the inference of oxide/hydroxide formation. These results demonstrate that the continuous OER in alkaline media would produce transition metal hydroxides or oxides and act as real active species for OER, which is consistent with previous studies [41,42]. The HER activities of Ni2P-Co2P@C/NF and other samples were also evaluated in alkaline solution. As expected, the commercial Pt/ C electrode exhibits the outstanding HER performance with the smallest catalytic onset potential and the highest current density (Fig. 5a). Ni2P-Co2P@C/NF also shows superior HER activity to other samples, the required overpotential for Ni2P-Co2P@C/NF to reach 50 mA cm
2 is as low as 167 mV, while Ni2P-Co2P/NF, Co2P@C/NF, NiCoOx/NF and NiCoOx@C/NF require 187, 240, 251 and 267 mV to deliver the same current density, respectively. Furthermore, the HER performance of Ni2P-Co2P@C/NF is also comparable to most recently reported bimetallic phosphides derived from MOFs, including Co-Fe-P [18], Ni2P/NiCoP@NCCs (NCCs ¼ nitrogen-doped carbon nanocones) [43], Cu0.3Co2.7P/NC (NC ¼ nitrogen-doped carbon) [44] and Ni-Fe-P [45] (Table S1). The Tafel slopes were then calculated to obtain more information about the HER mechanism (Fig. 5b), Ni2P-Co2P@C/NF demonstrates a low Tafel slope of 68 mV dec
1, which is in the range of 40e120 mV dec
1, indicating the HER process of Ni2P-Co2P@C/NF adopts the Volmer-Heyrovsky mechanism [46]. The Nyquist plots shown in Fig. 5d indicate a much smaller semicircle diameter of Ni2P-Co2P@C/NF than other samples, implying the much faster charge transfer rate and favorable reaction kinetics. The superior HER activity of Ni2P-Co2P@C/NF was further supported by its large active surface area. Ni2P-Co2P@C/ NF demonstrates the highest Cdl value, indicating the larger ECSA than others and lead to more exposed active sites (Fig. 5e). The durability of Ni2P-Co2P@C/NF toward HER was assessed by chronoamperometry at a fixed potential of
0.19 V vs RHE. As shown in Fig. 5f, the current density was remained at 24.5 mA cm
2 for at least 20 h, which indicates outstanding HER stability of Ni2P-

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Co2P@C/NF. The Faradic efficiency for HER was calculated to be 96% (Fig. S18). Ni2P-Co2P@C/NF was also characterized by SEM, XRD and XPS (Figs. S12e14) after the long-term electrolysis. The morphology of Ni2P-Co2P@C/NF was well maintained, but a portion of the sample was transformed into their hydroxides or oxides in strong alkaline media, which is similar to that of OER. The XPS results also eliminate the possibility of Pt impurities deposited on Ni2P-Co2P@C surface to contribute to the enhanced catalytic activity, demonstrating that the high HER activity is attributed to Ni2P-Co2P@C/NF itself [47]. Above all, the excellent electrocatalytic activity of Ni2P-Co2P@C/ NF arises from the following reasons. Firstly, the smart porous skeleton of nickel foam and the two dimensional architecture of Ni2P-Co2P@C could provide large surface area, which is helpful to expose a large number of active sites and facilitate the electrolyte penetration. Secondly, the electron interaction and synergistic effects between Ni2P-Co2P and amorphous carbon layers greatly improve the intrinsic activity of Ni2P-Co2P@C/NF. Thirdly, the Ni2PCo2P@C microsheets in-situ grown on porous nickel foam substrate and the introduce carbon species to the composite obviously increase the conductivity and mechanical stability and provide additional active sites simultaneously. 4. Conclusions Self-supported Ni2P-Co2P@C with two dimensional microsheet morphology on porous nickel foam was fabricated by using ZIF-67 as the precursor and glucose as the carbon source. When employed as bifunctional catalyst, Ni2P-Co2P@C/NF demonstrates notable electrocatalytic activity and stability, which only needs low overpotential of 290 and 167 mV to approach the current density of 50 mA cm
2 towards OER and HER in 1 M KOH electrolyte, respectively, outperforming the state-of-the-art precious metal included catalysts and most recently reported MOFs-derived metal phosphides. The impressive catalytic efficiency could be ascribed to the large surface area provided by the well-defined microsheet morphology, strong synergistic effects as well as the improved electrical conductivity. This work not only presents an attractive

Fig. 5. (a) LSV plots for HER and (b) corresponding Tafel slopes of all samples in 1 M KOH electrolyte. (c) Histogram of overpotentials (h) at j ¼ 50 mA cm
2 (blue column) and the Tafel slopes (orange column) for each sample. (d) Nyquist plots and (e) Cdl plots for different samples. (f) Stability test of Ni2P-Co2P@C/NF under
0.19 V vs RHE. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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catalyst for water splitting but paves a promising way to design and develop low-cost multifunctional catalysts derived from MOFs for other energy conversion applications. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant No. 21603110) and K. C. Wong Magna Fund in Ningbo University. M.Z thanks the supports from Student Research and Innovation Program in Ningbo University (Grant No. 2019SRIP2507). Appendix A. Supplementary data

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Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.06.082.

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