Metal-organic frameworks-derived hollow-structured iron-cobalt bimetallic phosphide electrocatalysts for efficient oxygen evolution reaction

Journal of Alloys and Compounds 821 (2020) 153463

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Metal-organic frameworks-derived hollow-structured iron-cobalt bimetallic phosphide electrocatalysts for efficient oxygen evolution reaction Jingyun Wang a, b, Jin Wang a, *, Meng Zhang a, Shumin Li a, Rui Liu b, **, Zhengquan Li a, *** a

Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Zhejiang Normal University, Jinhua, Zhejiang, 321004, PR China Ministry of Education Key Laboratory of Advanced Civil Engineering Materials, School of Materials Science and Engineering, Institute for Advanced Study, Tongji University, Shanghai, 201804, PR China

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 September 2019 Received in revised form 16 December 2019 Accepted 19 December 2019 Available online 21 December 2019

The oxygen evolution reaction (OER) is a cornerstone reaction for many renewable energy technologies. Developing low-cost and durable alternatives to precious-metal catalysts with high activity for OER are highly desirable to reduce the processing cost and complexity of renewable energy systems. Recently, metal phosphides based on earth-abundant transition metals, especially bimetallic phosphates have emerged as promising candidates for efficient OER catalysts. The rational design of nanostructured bimetallic phosphates is critical for the practical applications of these electrocatalysts, which requires a well understanding of the composition control and nanocrystal growth process. Herein, we report a facile strategy for the synthesis of novel iron-cobalt bimetallic phosphides (FeCoP) with hollow structures by phosphating Fe (Co)-centered metal-organic frameworks (MOFs). The resultant FeCoP hollow polyhedron, which have large specific surface area providing rich catalytic active sites, show excellent electrocatalytic activity towards OER with low overpotential, small Tafel slope, and remarkable stability. This work provides a new strategy for exploring efficient phosphide catalysts and opens up new avenues for the development of OER catalysts. © 2019 Elsevier B.V. All rights reserved.

Keywords: MOFs Hollow structures FeeCoeP electrocatalysts Phosphide OER

1. Introduction With the depletion of fossil fuels and worsening of the environment, it is urgent to develop clean and renewable energy conversion systems with high efficiency and low cost [1e3]. Hydrogen (H2) is considered as a promising candidate due to it has a high energy density and does not generate pollutants during combustion [4,5], and water electrolysis provides an appealing approach to produce H2 fuels [6,7]. However, large-scale water electrolysis is greatly limited by the sluggish anodic oxygen evolution reaction (OER), which generally has slow kinetics and requires large overpotentials [8e10]. Therefore, an appropriate catalyst for OER is essential to accelerate the overall reaction rate and lower the overpotential. To date, the state-of-the-art OER technique requires

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (J. Wang), [email protected] (R. Liu), [email protected] (Z. Li). https://doi.org/10.1016/j.jallcom.2019.153463 0925-8388/© 2019 Elsevier B.V. All rights reserved.

the use of Ir- and Ru-based electrocatalyst. The high cost and scarcity of the precious metals greatly impede their applications [11,12]. To solve these problems, extensive efforts have been devoted to exploring highly active, low-cost and durable OER catalysts [13e15]. In the past few years, earth-abundant transition metal compounds (e.g., oxides [16,17], hydroxides [18,19], nitrides [20,21], phosphides [22,23], etc.) have been demonstrated to be promising electrocatalytic materials. Particularly, transition metal phosphides (TMPs, M ¼ Fe, Co, Ni, Cu) have received considerable attention due to their good catalytic activity and long stability for electrochemical OER in alkaline media [24,25]. The performance of the electrochemical catalysts is largely dependent on its composition and morphology. On one hand, the electrocatalytic performance of bimetallic TMPs are usually superior to their single component counterparts due to tunable electronic structure and strong synergistic effect [26]. For example, Xu et al. [27] reported a series of phosphides electrocatalysts containing different equimolar metal (M ¼ Fe, Co, Ni) components, Ding et al. [28] found that the FeCoP400 exhibited excellent and stable electrocatalytic activities in

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alkaline media, and Du et al. [29] demonstrated the CoFeP for efficient OER. On the other hand, the rational design of nanostructured TMPs (e.g., porous and/or hollow structured) is also critical to improving their electrocatalytic performance, because of the porous and/or hollow structures in TMPs can provide a large surface area with a large number of active sites, and an open framework for better reaction kinetics and stability [30,31]. Therefore, it is necessary to find an effective strategy that can regulate both the composition and morphology of bimetallic TMPs for efficient electrocatalytic OER. Recently, MOFs have attracted great attention because of their many inherently superior properties, including versatile catalytic activities, high surface areas, tunable pore sizes and remarkable structural diversity [32,33]. Despite the pristine MOFs are difficult to be used directly for OER due to their poor electrical conductivities (about 1010 S m1) [34], they are ideal precursors for the preparation of metal-based electrocatalysts [35,36]. The components of the resulted bimetallic electrocatalysts can be easily controlled by modulating the molar ratios of the metallic components in MOFs. Importantly, the unique structural properties of MOFs would probably induce the formation of various porous or hollow structures in these derived catalysts. In the past few years, many hollow-structured TMPs derived from MOFs have been reported, and they exhibit excellent OER performance [37e39]. For example, Huang et.al. have prepared hollow barrel-like FeNiP electrocatalysts through a novel strategy using P-containing MOFs [37]; Li et.al. have designed CoP nanoparticles embedded in Ndoped carbon nanotubes derived from core-shell ZIF-8@ZIF-67 [25]; Lou et.al. have synthesized highly crystalline Ni-doped FeP/ carbon hollow nanorods derived from Ni-doped MIL-88A [40]. Despite great successes have been made on MOF-derived hollow electrocatalysts, controlling both morphology and composition of bimetallic TMPs is still challenging. Herein, we report a facile strategy to synthesize MOF-derived FexCoyP bimetallic phosphides electrocatalysts for OER (where x/y represents the molar ratios of Fe to Co). By continuously controlling the components of Fe and Co in Fe-based MOF precursor (Fe-MIL88), bimetallic phosphide electrocatalysts with hollow/porous structures are successfully prepared after phosphating. Benefitting from the synergistic effect between Fe and Co and unique hollow structure, these FexCoyP electrocatalysts exhibited remarkable catalytic performance for OER in an alkaline electrolyte, among which FeCo2P displays an overpotential of 320 mV at the current density of 10 mA cm2 and a small Tafel slope of 55 mV dec1. In addition, the hollow-structured FeCo2P demonstrates an excellent durability after 12 h continuous operation. This work may provide a new strategy to develop efficient phosphide electrocatalysts with tunable morphologies and compositions for OER application.

2. Experimental section 2.1. Chemicals Iron (ІІІ) chloride hexahydrate (FeCl3$6H2O, 98%), Cobalt (ІІ) nitrate hexahydrate (Co(NO3)2$6H2O, 98%), terephthalic acid (1,4BDC), N, N-Dimethylformamide (DMA), Sodium hypophosphite (NaH2PO2$H2O, 99 wt%), ethanol (99.7 wt%), potassium hydroxide (KOH,  85.0 wt%) were purchased from Sinopharm Chemical Reagent Co. Ltd. Nafion solution (5 wt%) was purchased from SigmaAldrich Co. LLC. The ultrapure water (18.2 MU. cm1, 25  C) used in all experiments was obtained from milli-Q system. All reagents were used without further purification.

2.2. Synthesis of Fe-MIL-88 Fe-MIL-88 was synthesized according to the previous work with a slightly modification [41]. Briefly, 1 mmol FeCl3$6H2O and 1 mmol 1, 4-BDC were dissolved in 30 mL DMA and magnetically stirred for 1 h at room temperature. The obtained solution was transferred into a vial and sealed in Teflon-lined stainless-steel autoclave. After heating treatment at 150  C for 3 h, the above solution was cooled to room temperature. The powder obtained through high speed centrifugation (5000 rpm, 10 min) was washed with DMA and ethanol to remove organic residue. The process was repeated twice, then the samples were dried overnight in a vacuum oven at 60  C. 2.3. Synthesis of FeCo2-MIL-88 The FeCo2-MIL-88 was synthesized using the following procedure, 0.333 mmol FeCl3$6H2O, 0.666 mmol Co(NO3)2$6H2O, and 1 mmol 1, 4-BDC were dissolved in 30 mL DMA and magnetically stirred for 1 h at room temperature. The obtained solution was transferred into a vial and sealed in teflon-lined stainless-steel autoclave. After heating treatment at 150  C for 3 h, the above solution was cooled to room temperature. The powder obtained by high speed centrifugation (5000 rpm, 10 min) was washed with DMA and ethanol to remove organic residue. The process was repeated twice, then the samples were dried overnight in a vacuum oven at 60  C. Besides, Fe2Co-MIL-88 and FeCo-MIL-88 were prepared in a similar procedure, except that the molar ratios of Fe to Co were 2:1 and 1:1, respectively. 2.4. Synthesis of FexCoyP In a typical procedure, 20 mg as-prepared FexCoy-MIL-88 and 1000 mg NaH2PO2$H2O were loaded into two porcelain boats, respectively. The porcelain boats were transferred into a tube furnace with the NaH2PO2$H2O at the upstream side. The samples were then annealing at 350  C for 3 h under N2 atmosphere with a ramp rate of 5  C min1. 2.5. Characterization The morphologies of the materials were characterized by Transmission electron microscope (TEM, JEM-2100F) equipped with Energy Dispersive X-ray (EDX) spectroscopy and Fieldemission scanning electron microscopy (SEM). Elemental mapping images were recorded by EDX spectroscopy attached to TEM (JEM-2100F). Inductively coupled plasma-optical emission spectrometer (ICP-OES) analyses were performed in a Varian VISTAMPX instrument, the samples were dissolved with a mixture of HCl and HNO3 (3:1, volume ratio) which was then diluted with deionized water. Powder X-ray diffraction (XRD) was recorded on a Philips PW3040/60 X-ray diffractometer using Cu-Ka as radiation source. X-ray photoelectron spectroscopy (XPS) measurements were measured on an ESCALab MKII X-ray photoelectron spectrometer equipped with a Mg-Ka X-ray radiation. N2 adsorptiondesorption experiments were carried out at 77 K on a Quadrachrome adsorption instrument (Autosorb-iQ3; Quantachrome, America). The specific surface area was calculated by using the Brunauer-Emmett-Teller (BET) method. The pore size distribution was calculated from the adsorption curves with the QSDFT model. 2.6. Electrochemical measurements All the electrochemical measurements were carried out on an electrochemical workstation (CHI 760) with a standard threeelectrode system. The standard calomel electrode (SCE) and a

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carbon electrode were selected as the reference electrode and the counter electrode, respectively. The preparation of the working electrode is as follows: first, 3 mg of the as-prepared catalyst was dispersed in 1 mL mixture of water and ethanol (v/v ¼ 1:1) followed by the addition of 20 mL nafion solution (5 wt%), and the solution was sonicated for 20 min to form the homogeneous ink. Then, 20 mL of the ink was dropped onto a piece of clean carbon fiber paper (CFP) substrate (1 cm  1 cm) and dried at room temperature. The mass loading of catalysts was 0.5 mg cm2. The working electrode was activated before electrochemical test by scanning for 50 cycles from 0.05 V to 0.05 V (versus SCE electrode) at a scanning rate of 50 mV s1. The OER performance was evaluated in O2-saturated 0.1 M KOH solution. A scan rate of 5 mV s1 was applied to the linear sweep voltammetry (LSV) of the samples. Electrochemical impedance spectroscopy (EIS) measurements were conducted at frequency ranging from 0.01 Hz to 100 kHz with an amplitude of 5 mV, and a 0.5 V of potential was applied. Cyclic voltammetry (CV) method was used to determine the electrochemical double-layer capacitances (Cdl). Electrochemical surface area (ECSA) of samples was evaluated by measuring their (Cdl) according to the equation of ECSA ¼ Cdl/Cs, where Cs is the specific capacitance of samples (generally 0.040 mF cm2 for hydroxide materials in alkali condition). The Cdl were collected through CV by varying the scan rate (10, 20, 40, 60, 80 and 100 mV s1) in the non-Faradaic region from 0.05 to 0.05 V (Cdl ¼ charging current/scan rate). All the polarization measured in this work were conducted with iR compensation and were converted to reverse hydrogen electrode (RHE) by the following equation: ERHE ¼ ESCE þ 0.059 pH þ 0.241 [42]. 3. Results and discussion Previous studies have shown that MOFs are ideal precursors for the preparation of structurally diverse TMPs for energy conversion [43]. To obtain the hollow-structured bimetallic TMPs and study their OER performance, we took the Fe-MIL-88 and FexCoy-MIL-88 as precursors and phosphatized them to FexCoyP for electrocatalytic OER. 3.1. Synthesis and morphological characterization of Fe-MIL-88 and FexCoy-MIL-88 As our initial target, Fe-MIL-88 were prepared by heating the DMA solution containing FeCl3$6H2O and 1, 4-BDC at 150  C for 3 h, see Experimental section for detailed procedures. The SEM and TEM images in Fig. 1 (a, e) clearly indicate that the synthesized pristine Fe-MIL-88 has uniform hexagonal rod-shape with a diameter of about 100 nm and a length of about 1 mm. The rod-liked Fe-MIL-88 is match well with the results in previous work [44]. Since the bimetallic TMPs usually have higher catalytic activity than their single component counterparts, we try to introduce some cobalt (which have similar ion radius and electronic structure with iron) to replace the iron in Fe-MIL-88, and study the effect of components on the morphology. When the initial molar ratio of Co to Fe reaches 1:2 (Fe2Co-MIL-88), the average length of these hexagonal nanorods is reduced to about 800 nm (Fig. 1b, f), while the rod-like morphology of the sample is well maintained. As the molar ratio of Co to Fe increases to 1:1, the average length of the spindle-shaped FeCo-MIL-88 becomes 600 nm. (Fig. 1c, g). When the molar ratio of Co to Fe reaches 2:1 (FeCo2-MIL-88), the sample remain a spindle-like structure, but the length is reduced to about 400 nm (Fig. 1d, h). The above changes in morphology indicate that cobalt incorporation will reduce the growth rate of Fe-MIL-88 and adjust its orientation. As the amount of cobalt increases, the size of the resulting rod becomes smaller, perhaps because cobalt has a

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smaller atomic radius than iron, and it contributes to the formation of smaller particles [45]. Besides, to assess the phase transformation during growth, XRD characterization was conducted. As shown in Fig. S1, the diffraction patterns of the pristine Fe-based MOFs could be assigned to the hexagonal space group of MIL-88B (P62c) [44], suggesting the obtained MOFs are the Fe-MIL-88. After the introduction of cobalt, there are no significant changes in their XRD patterns, indicating that the crystal structure of Fe-MIL88 was well maintained. 3.2. Transformation of FexCoy-MIL-88 to FexCoyP Although the FexCoy-MIL-88 has a specific morphology and a large number of metal centers, the presence of the organic ligands can reduce its electrocatalytic activity due to their low conductivity. It is therefore necessary to remove these organic ligands. Hightemperature phosphating has proven to be an effective route to achieve this goal, and this process can yield metal phosphides with good OER properties [46]. We therefore phosphating the obtained FexCoy-MIL-88 to prepare the electrocatalysts with a certain morphology. After the phosphating treatment, the morphologies of the Fe-MIL-88 and its derivatives are performed by SEM and TEM, as displayed in Fig. 2. For the Fe2P (phosphating products of Fe-MIL88), they exhibit uniform rod-like structures. Although the size and shape of Fe2P coincide with the Fe-MIL-88, their surfaces become rough (Fig. 2a). Further observation of Fe2P polyhedron by TEM reveals that the nanorods are composed of a large number of nanoparticles (NPs) (Fig. 2e), which are highly uniform and dispersed throughout the polyhedron, confirming that the uniform distribution of metal ions in the Fe-MIL-88 can effectively prevent the agglomeration of these NPs after phosphating. For derivatives of Fe-MIL-88, with the increase of cobalt content, the size of the corresponding phosphide decreases gradually (Fig. 2b, c, f, g and d, h), and the tendency is consistent with the evolution of the nonphosphated derivatives in Fig. 1. In addition, for the FeCo2P polyhedron, although a uniform spindle-like morphology is maintained, a well-defined hollow structure appears inside (Fig. 2d, h). This can be confirmed by the SEM image in Fig. 2d, the hollow structures can be clearly seen in some rod-shaped samples with damaged outer layers. The hollow structure of these catalysts can supply a large specific surface area and provide more catalytic sites for catalytic reactions. 3.3. Formation mechanism of FexCoyP On basis of the evolution of the morphology of phosphides, we can infer the formation mechanism of FexCoyP electrocatalysts. Scheme 1 describes the synthesis of metal phosphides with different morphologies through a facile one-step phosphating process. First, FexCoy-MIL-88 is synthesized by self-assembly of metal ions (Fe3þ and Co2þ) and organic ligands (H2BDC) in DMA. By controlling the molar ratios of Co to Fe, a series of rod-shaped or spindle-shaped FexCoy-MIL-88 can be obtained. The evolution in morphology and size of FexCoy-MIL-88 may be due to the fact that the incorporation of cobalt changes the coordination environment of Fe3þ and H2BDC. After that, these FexCoy-MIL-88 nanorods are chemically converted into FeeCo mixed metal phosphides by treatment with NaH2PO2 at 350  C for 3 h in a N2 atmosphere. At the same time, the organic ligands in FexCoy-MIL-88 are converted to amorphous carbon (as discussed below in Fig. 3). The resulted FexCoyP/C (referred to as FexCoyP) nanorods composed of FeeCo bimetallic phosphides and amorphous carbon. Furthermore, the obtained FexCoyP phosphides retain the shape of their precursors (FexCoy-MIL-88), but have a porous or hollow structure. With the increase of cobalt contents, the morphology of FexCoyP is

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Fig. 1. Morphologies of Fe-MIL-88 and its derivatives. (aed) SEM images and (eeh) TEM images of the obtained (a, e) Fe-MIL-88, (b, f) Fe2Co-MIL-88, (c, g) FeCo-MIL-88, (d, h) FeCo2MIL-88.

Fig. 2. Morphologies of FexCoyP. (aed) SEM images and (eeh) TEM images of (a, e) Fe2P, (b, f) Fe2CoP, (c, g) FeCoP, (d, h) FeCo2P.

transformed from a rod-shaped structure to a hollow spindleshaped polyhedron. The molar ratios of Fe to Co in FexCoyP are also listed in Table S1. The formation of FeCo2P in hollow structure at a high Co/Fe ratio may be related to the different rates of phosphate formation by Co2þ and Fe3þ ions. Under the identical conditions, Co2þ ions are more reactive to phosphate formation than Fe3þ ions. When the Co2þ ions dominate (Co/Fe ratio of 2), the phosphorus precursors preferentially react with Co2þ on the surface of the MOF precursors. These rapidly formed metal phosphide particles on their surfaces will act as the reaction center, prompting the inner metal ions to diffuse to the surface and react with phosphorus precursors to form phosphates. The continuous outward spread of metal ions in the MOF precursors results in the emergence of voids, forming a hollow structured FeCoP. However, when the Fe3þ ions dominate, due to the slow reaction rate between Fe3þ ions and phosphorus precursors, the phosphorus precursors will spread to the whole MOF precursors thus reacting with metal ions in a homogeneous manner. Finally, a FeCoP with relatively uniform structure is formed.

3.4. Evidence for the formation of hollow structured FexCoyP polyhedron In order to gain insight into the formation of FexCoyP, we conduct further studies using techniques such as XRD, HRTEM and XPS. Fig. 3a shows the powder X-ray diffraction (XRD) patterns of samples with different compositions, along with Fe2P (51e0943) and Co2P (32e0306) for comparison. All samples exhibit singlephase diffraction patterns similar to the Fe2P and Co2P. It is concluded that the samples prepared from the FexCoy-MIL-88 through phosphating are solid solutions of Fe2P and Co2P. Therefore, the ideal composition of the present samples can be expressed as FexCoyP. Moreover, no diffraction peaks of other species exist (such as the amorphous metal-(oxy)hydroxide species). Meanwhile, the presence of a broad diffraction pattern between 2q diffraction angles of 20e40 demonstrates the presence of amorphous carbon in FexCoyP. In addition, the corresponding HRTEM image of hollow FeCo2P polyhedron displays a low crystallinity (Fig. 3b), which is also consistent with XRD patterns. It is obvious that the hollow FeCo2P polyhedron is surrounded by continuous amorphous carbon species, which can effectively prevent the

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compositions and chemical states of the FeCo2P. As shown in Fig. 4, Fe, Co, P and O elements can be detected in their survey spectrum, the existence of O elements may originate from partial oxidation of the sample exposed to oxygen [47]. Moreover, as for the high resolution XPS spectrum of Fe 2p (Fig. 4b), the peak located at 706.9 eV is the characteristic peak of the FeeP bonding, and the double peaks at 711.4 and 724.4 eV correspond to Fe2þ 2p3/2 and Fe2þ 2p1/2, respectively. The second double peaks at 715.6 and 728 eV can be assigned to the splitting peaks of Fe3þ 2p3/2 and Fe3þ 2p1/2, respectively [48,49]. At the same time, Fig. 4c gives high-resolution XPS spectrum of Co 2p. The peaks located at 778.5 eV can be assigned to CoeP in FeCo2P, and two peaks at 782 and 798.1 eV are characteristic peaks of Co2þ ions, respectively [50e52]. For P 2p spectrum (Fig. 4d), there are two main peaks at 129.5 and 133.6 eV, which are attributed to metal phosphide and oxidized P species due to the exposure to air [53,54]. The XPS results demonstrate the successful chemical conversion from FeCo2-MIL-88 to FeCo2P via the phosphorization process. 3.5. Electrocatalytic properties of FexCoyP Scheme 1. Schematic illustration of the formation of FexCoyP.

collapse of MOF skeleton (Fig. 3c), thereby maintaining structural stability during pyrolysis. As examined by STEM, the corresponding elemental mappings demonstrate the uniform distribution of Fe, Co, P and C elements in the hollow-structured FeCo2P polyhedron (Fig. 3d and e). The corresponding EDX spectrum in Fig. S2 also confirms the existence of Fe, Co, P and C elements. The XPS spectroscopy measurements are used to assess the

The electrochemical performance of the samples towards OER was tested using a typical three-electrode system in 0.1 M KOH solution. As shown in LSV curves (Fig. 5a and Fig. S3), the Fe2P NPs and Co2P display a large overpotential of 430 mV and 420 mV to reach the current density of 10 mA cm2, respectively. When the initial molar ratio of Co/Fe reaches 1:2, the overpotential of Fe2CoP NPs reduces to 410 mV (at 10 mA cm2). This value is still larger than that of FeCoP NPs (390 mV at 10 mA cm2), confirming the presence of cobalt can effectively improve their OER performance. With the further increase of Co amount (the initial molar ratio of Co to Fe is 2:1), the hollow-structured FeCo2P polyhedron shows the

Fig. 3. Morphologies and structure characterizations of the hollow-structured FeCo2P polyhedron. (a) XRD patterns FeCo2P and FexCoyP, (b) TEM images, (c) HRTEM images, (d) STEM and (e) elemental mapping images.

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Fig. 4. XPS spectra of the prepared hollow-structured FeCo2P polyhedron. (a) survey spectrum, (b) Fe 2p, (c) Co 2p, and (d) P 2p.

Fig. 5. OER performance of the samples measured in 0.1 M KOH. (a) LSV curves, (b) corresponding Tafel slops, (c) Overpotentials derived from OER polarization at j ¼ 10 mA cm2, (d) EIS spectra.

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lowest overpotential of (320 mV at 10 mA cm2) (Fig. 5c). For comparison, the LSV of the commercial RuO2 was also conducted, the overpotential is about 270 mV at 10 mA cm2 (see Fig S4), the OER performance of the FeCo2P is very close to that of the RuO2. The performance of hollow FeCo2P polyhedron outperforms most nonnoble metal bifunctional catalysts for OER (see Table S2-Table S4). In order to investigate the influence of carbon on the OER activity, the FeCo2P/C composite are etched with diluted HCl (0.5 M) to obtain the P-doped carbon (denoted as P-doped C). As shown in Fig S5, the P-doped C show an overpotential of about 440 mV at 10 mA cm2, revealing that the P-doped C contributes to the OER performance. However, the overpotential of P-doped C is inferior to that of the FeCo2P/C composite (320 mV at 10 mA cm2), indicating that the co-existence of Fe and Co can significantly improve the overall OER performance of the electrode. To further study the possible electrocatalytic mechanism for OER, the Tafel plots are presented in Fig. 5b. A smaller Tafel slope is conductive to the reaction kinetics in the OER process. Obviously, the hollow FeCo2P polyhedron gives a slope of 55 mV dec1, which is much lower than that of Fe2P NPs (150 mV dec1), Fe2CoP NPs (70 mV dec1) and FeCoP NPs (60 mV dec1). Confirming that the hollow-structured FeCo2P polyhedron has a significant fast OER kinetics. To investigate the kinetics of electrochemical reaction process, the EIS measurements of samples were further conducted. As shown in Fig. 5d, all the metal phosphides show the similar series resistance (Rs), which can be ascribed to the same electrolyte medium. The hollow-structured FeCo2P polyhedron shows a low charge transfer resistance (Rct) value of 21 U, which is much smaller than those of Fe2P NPs (72 U), Fe2CoP NPs (66 U) and FeCoP NPs (40 U), suggesting a faster charge transfer process in FeCo2P, this result is also in accordance with their fast OER kinetic shown in their Tafel plots. Besides, the EIS value we measured for the pristine CFP is about 80 U (Fig. S6), which is higher than those of the FexCoyP/CFP electrodes. The decrease of the overall EIS values may attribute to the metallic properties of FexCoyP, which improves the conductivity of the electrodes. The ECSA is another important factor responsible for the activity of catalysts, which are investigated by measuring their Cdl [55]. The CV curves of Fe2P NPs, Fe2CoP NPs, FeCoP NPs and the hollow FeCo2P polyhedron with different scan rates are shown in Fig. S7. As presented in Fig. 6a., the hollow FeCo2P polyhedron exhibits a slope value of 30.5 mF cm2, much larger than those of Fe2P NPs (Cdl ¼ 8.5 mF cm2), Fe2CoP NPs (16.5 mF cm2) and FeCoP NPs (25.8 mF cm2), implying much more electrocatalytic active sites in FeCo2P. The high electrocatalytic activity of FeCo2P composite mainly originates from its high surface area of the hollow structure.

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In addition, the N2 adsorption-desorption investigation are further performed to characterize the structure of the samples. As shown in Fig. S8, compared with Fe2CoP NPs and FeCoP NPs, the hollow FeCo2P polyhedron catalyst shows the largest BET surface area of 80 m2 g1 with an average pore size of about 3 nm. The large surface area of hollow FeCo2P polyhedron is beneficial for charge and mass transfer for electrocatalysts. High durability of electrocatalyst is also vital for energy conversion systems. The electrochemical stability of hollow FeCo2P polyhedron is evaluated by LSV tests and chronoamperometry method, respectively. Continuous CV scanning were conducted with the scan rate of 50 mV s1, and there is no apparent change in their LSV curves after 1000 cycles (Fig. 6b). In addition, chronoamperometry curve revealed that the hollow-structured FeCo2P polyhedron catalyst can keep excellent catalytic stability during long-time testing and only 10% drops in the current density after a 12 h test (Fig. 6c). The initial potential drop indicates that the FeCo2P catalyst may undergo a typical activation process through partially oxidation on its surface. In fact, the in-situ oxidation or hydrogenation behavior has recently been observed in many other non-oxide based OER catalysts [56e59]. The presence of metal-(oxy)hydroxide species help to bind OOH to the surface and act as the active site for promoting the OER performance [60]. We investigated the chemical states of FeCo2P precatalysts after the OER electrolysis using XPS. Changes in the chemical composition of the surface can be seen from the XPS of FeCo2P after the OER stability test. As shown in Fig. S9, the peaks of Fe 2p and Co 2p for the post-OER FeCo2P have a slight shift of 0.1 eV, implying that the initial phosphide was primarily oxidized to (oxy) hydroxide during the OER. The phenomenon agrees well with other transition metal phosphides [61]. Of particular note is that there is no obvious oxide phase from the XRD patterns (Fig. S10), confirming that the resulting (oxy)hydroxides on the surface is amorphous and the major phase of the sample is still FeCo2P. These results indicate the FeCo2P catalyst exhibits only partial oxidation on its surface without oxidation of the bulk. Notwithstanding the surface composition change, the OER performance remained excellent during the whole long-term stability test, which are usually ascribed to increased active sites and electronic conductivity through the metallic FeCo2P [62]. Overall, the superior OER performance of FeCo2P can be attributed to the following reasons: first, the unique hollow structure supplies a large surface area, which providing more active sites, enhancing the mass transport and increasing the contact between the electrode and the electrolyte. Meanwhile, the introduction of Co in the Fe2P can promote their electrocatalytic activity through synergistic effect between Fe and Co. In addition, the existence of

Fig. 6. (a) Plots of the capacitive current against the scan rate, (b) Polarization curves of the hollow FeCo2P polyhedron at initial and after 1000 CV cycles, (c) The chronoamperometric test of hollow FeCo2P polyhedron for 12 h.

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amorphous carbon can serve as the framework for maintaining the structure of MOFs and enabling a more stable structure after calcination treatment. 4. Conclusions In summary, we developed a facile strategy for synthesizing a series of FexCoyP with controllable morphologies and compositions towards electrocatalytic OER. By controlling the molar ratios of Co to Fe, the hollow-structured FeCo2P exhibit excellent electrocatalytic activity for OER. Specifically, the hollow FeCo2P polyhedron requires only an overpotential of 320 mV to achieve a current density of 10 mA cm2 and a high electrochemical stability. The facile method can be used for the design and synthesis of many other functional materials with controllable morphologies and compositions for various energy-related applications. This work provides a new strategy for exploring efficient phosphide catalysts and opens up new avenues for the development of noble metal free electrocatalysts. Author contributions section Jingyun Wang and Jin Wang contributed to analysis of the data and writing the manuscript. Meng Zhang and Shumin Li carried out the synthesis of materials, the characterizations of the assynthesized samples. Rui Liu and Zhengquan Li contributed to the conception and design of the experiment. All authors reviewed the manuscript. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge financial support from National Natural Science Foundation of China (21975223, 21701143), Natural Science Foundation of Zhejiang Province (LGG19B010002and R15B010001), and the Shanghai Municipal Natural Science Foundation (17ZR1432200). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.153463. References [1] H. Dotan, S.W. Sheehan, K.D. Malviya, D.G. Ziv Arzi, A. Rothschild, G.S. Grader, Decoupled hydrogen and oxygen evolution by a two-step electrochemicalchemical cycle for efficient overall water splitting, Nat. Energy 4 (2019) 786e795. [2] T. He, H.J. Cao, P. Chen, Complex hydrides for energy storage, conversion, and utilization, Adv. Mater. 31 (2019) 1902757e1902775. [3] P. Zhang, X.W. Lou, Design of heterostructured hollow photocatalysts for solar-to-chemical energy conversion, Adv. Mater. 31 (2019) 1900281. [4] D.B. Liu, X.Y. Li, S.M. Chen, S. Duan, J.L. Lu, B.H. Ge, P.M. Ajayan, J. Jiang, L. Song, et al., Atomically dispersed platinum supported on curved carbon supports for efficient electrocatalytic hydrogen evolution, Nat. Energy 4 (2019) 512e518. [5] A. Schneemann, J.L. White, S.Y. Kang, S. Jeong, M.D. Allendorf, V. Stavila, Nanostructured metal hydrides for hydrogen storage, Chem. Rev. 11 (2018) 10775e10839. [6] Z.L. Chen, B. Fei, M.L. Hou, H.L. Qing, R.B. Wu, et al., Ultrathin prussian blue analogue nanosheet arrays with open bimetal centers for efficient overall water splitting, Nano Energy 68 (2020) 104371e104380. [7] W. Zhang, Y.F. Sun, Q.Y. Liu, J.X. Guo, X. Zhang, Vanadium and nitrogen codoped CoP nanoleaf array as pH-universal electrocatalyst for efficient

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