Porous transition metal phosphides derived from Fe-based Prussian blue analogue for oxygen evolution reaction

Journal of Alloys and Compounds 814 (2020) 152332

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Porous transition metal phosphides derived from Fe-based Prussian blue analogue for oxygen evolution reaction Xin Ding, Waqar Uddin, Hongting Sheng, Peng Li, Yuanxin Du**, Manzhou Zhu* Department of Chemistry and Centre for Atomic Engineering of Advanced Materials, Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Anhui University, Hefei, Anhui, 230601, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 May 2019 Received in revised form 25 August 2019 Accepted 17 September 2019 Available online 18 September 2019

Recently, transition metal phosphides (TMPs) have been reported as a new kind of anode catalyst for oxygen evolution reaction (OER), and the composition of the material largely determines the performance of the catalysts. However, few reports explore how electrocatalytic performance of TMPs changed with the composition of the catalysts. We selected three different component Fe-based Prussian blue analogues (PBA) as precursors and transformed them into corresponding metal phosphides by simple heat treatment. By tuning the phosphidation temperature, a series of FeCoP, FeNiP, and FeMnP were obtained, in which FeCoP under a suitable phosphidation temperature at 400
C exhibits most obviously porous structure and broadest distribution of pore size, which benefits for the mass transfer and oxygen release during OER. Besides, the charge-transfer resistance (Rct) of TMPs has greatly decreased by introducing of Co in comparison of Ni and Mn, which accelerate the electron transport in OER. Due to the porous geometric structure and unique electronic structure, FeCoP-400 shows excellent and stable electrocatalytic activities of OER in 1 M KOH, with overpotentials of 261 mV at a current density of 10 mA cm2, superior to commercial RuO2 and most OER electrocatalysts. Furthermore, FeCoP-400 exhibits outstanding stability with only 4% increase in potential during 24 h chronopotentiometry. © 2019 Elsevier B.V. All rights reserved.

Keywords: Prussian blue analogues Nanocubes Binary transition metal phosphides Oxygen evolution reaction

1. Introduction Nowadays, searching clean and sustainable new energy has become the focus of solving the energy crisis and environmental pollution. As a new energy source with huge potential, hydrogen (H2) exhibits many advantages such as wide source, renewable, clean and pollution-free, high energy density, high conversion density and easy to storage [1]. Electrolytic water in the alkaline electrolytic cell is a technology that can realize large-scale hydrogen production, as an important half-reaction in electrolytic water, oxygen evolution reaction (OER) limits the efficiency of the water splitting because of its slow chemical kinetics. Due to the four independent electron transfer processes in OER which involves multiple transitions of proton and electron, resulting in a large overpotential and energy loss without suitable catalysts [2e5]. Therefore, reducing OER overpotential is an important factor to save the cost of hydrogen production in electrolysis water [6,7]. At

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Du), [email protected] (M. Zhu). https://doi.org/10.1016/j.jallcom.2019.152332 0925-8388/© 2019 Elsevier B.V. All rights reserved.

present, noble metal oxides such as IrO2 and RuO2 were widely used as commercial OER catalysts in alkaline solution. However, the expensive price and scarcity of noble metal catalysts limit their application, it is urgent to find cheap and abundant non-noble metal materials to replace them. Transition metal compounds (Fe, Co, Ni, Mn, etc.) including nitrides [8,9], sulfides [10,11], phosphides [12e15] and hydroxides [16,17] show fine OER activity in alkaline media, which are promising alternatives to noble metal materials, especially the transition metal phosphides (TMPs) due to their abundant active sites, good electrical conductivity, structural stability and thermal stability. The performance of the catalysts is largely dependent on its composition, in previous reports, performance of electrocatalysts of mixed metal phosphides are superior to single ones. For example, Li et al. [18] reported Mn doped porous CoP nanosheets, Yu et al. [19] grown FeP on Ni2P/Ni foam, and Du et al. [20] synthesized CoFeP hollow microspheres, all of them showed the effectively improved OER performance compared with the single component phosphide due to controllable electronic structure and strong synergistic effect. However, the influence of the metal type in the composites with the same geometric structure on the electrocatalytic OER

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activity has rarely been compared. As a classical transitional metal-organic framework (MOF) material, Prussian blue analogues (PBA) exhibits similar structure to Prussian blue (PB) which can be prepared by substituting Fe2þ and Fe3þ in PB with appropriate transition metal ions, they all retain the framework of PB. Due to the advantages of controllable pore structure, large surface area, good thermal stability, low cost and easy to prepare, PBA has broad application prospects in various fields. In recent years, PBA was widely used as a three-dimensional nanostructure precursor in energy storage and conversion, there are many reports about high activity TMPs electrocatalysts derived from PBA [21e27], but how electrocatalytic performance of TMPs varies with the composition of the catalysts derived from same geometric structure precursors are rarely reported. In this paper, three different Fe-based PBAs (FeCo-PBA, FeNi-PBA, FeMn-PBA) were used as precursors to prepare a series of TMPs with different components in the Ar atmosphere by simple heat treatment. The obtained mixed metal phosphides have similar geometric structures which preserve the cubic shape of precursor, among them FeCoP-400 (under 400
C phosphidation) shows more obvious porous structure, lower Rct and better OER activity. By comparing the electrochemical surface area (ECSA) and normalizing the current density with BET surface area of phosphidation products, the results showed that FeCoP-400 can exposes more active sites, and FeCoP-400 exhibits widely distributed pore size, which is beneficial to improving the electrocatalytic performance, superior to FeNiP400, FeMnP-400 and commercial RuO2. We hope the experimental results can provide inspiration for similar research of OER electrocatalysts. 2. Experimental section 2.1. Samples preparation 2.1.1. Materials Cobalt chloride hexahydrate (CoCl2$6H2O,  99.0%, Macklin), nickel chloride hexahydrate (NiCl2$6H2O,  99.0%, Aladdin), manganese sulfate monohydrate (MnSO4$H2O,  99.99%, Aladdin), sodium citrate dihydrate (C6H5Na3O7$2H2O,  99.0%, Aladdin), potassium ferricyanide (K3FeC6N6, 99.5%, Aladdin), polyvinylpyrrolidone (MW ~ 40000, 99.0%, Solarbio), sodium hypophosphite monohydrate (99.0%, NaH2PO2$H2O, Aladdin). 2.1.2. Synthesis of Prussian blue analogues PBA precursors were synthesized by reference to previously reported literatures with revision [28,29]. For FeCo-PBA, 0.6 mmol cobalt chloride hexahydrate and 0.6 mmol sodium citrate dihydrate were dissolved in 20 mL deionized water, 0.4 mmol K3 [Fe(CN)6] was dissolved in 20 mL deionized water, then these two aqueous solutions were mixed under magnetic stirred for 3 min, the obtained mixed solution was aged for 24 h at room temperature, the precipitates were collected by centrifugation and washed by water and ethanol, then the precipitates were dried at 60
C overnight. To synthesize the FeNi-PBA, nickel chloride hexahydrate was used to replace cobalt chloride hexahydrate, the obtained mixed solution was aged for 12 h with other conditions remain unchanged. To prepare FeMn-PBA, 1.5 g of polyvinylpyrrolidone was dissolved in 100 mL mixed deionized water and ethanol at the volume ratio of 1:1 and stirring for 30 min until the solution was clear and transparent, then immediately add 0.225 g manganese sulfate monohydrate into the solution and stir constantly until manganese sulfate was complete dissolved, this mixed solution was called solution A. Solution B was prepared by dissolving 0.33 g of K3 [Fe(CN)6] in 50 mL deionized water at the same time, the solution B was slowly added into solution A in 5 min. The obtained mixture

was stirred constantly for 30 min and then aged for 12 h at room temperature. Finally, the post-processing method is the same as the previous two samples. 2.1.3. Synthesis of Prussian blue analogues phosphides 0.05 g PBA and 1 g NaH2PO2 were put at both the ends of the porcelain boat and placed into the tube furnace, the side with NaH2PO2 was placed at the upstream side. Then the furnace was heated to different temperatures with a heating rate of 2
C min1 and kept for 2 h under Ar atmosphere, the phosphidation samples were cooling to room temperature under Ar atmosphere. 2.2. Material characterization Scanning electron microscopy (SEM) was performed on an */S4800 scanning electron microscope. Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and element mapping analysis were conducted on a JEM-2100 F. Thermogravimetric analysis (TGA) was acquired from room temperature to 800
C at a heating rate of 10
C min1 on a *TGA5500 instrument. FT-IR spectra were recorded with Vertex 80 þ Hyperion 2000. X-ray diffraction (XRD) patterns were obtained on SmartLab 9 KW with Cu Ka radiation. X-ray photoelectron spectroscopy (XPS) measurements were on ESCALAB 250Xi instrument. Such as the specific surface area and pore size distribution were calculated from each corresponding nitrogen adsorption-desorption isotherm by applying the Brunauer-Emmett-Teller (BET) equation on ASAP2020 M þ C. The bulk compositions were evaluated using an inductively coupled plasma mass spectrometry (ICP-MS) on iCAP 7400 Duo. 2.3. Electrochemical measurement The electrochemistry performance was tested on CHI660E electrochemical workstation in a three-electrode system. The electrocatalysts were loaded on a rotating disk electrode (RDE) used as working electrode. An Ag/AgCl electrode (in saturated KCl solution) and a carbon rod were used as the reference electrode and the counter electrode, respectively. To prepare working electrodes, 5 mg of catalysts and 20 mL 5 wt% Nafion were dispersed in 1 mL absolute ethanol and ultrasound treated for 30 min to obtain inklike mixture, then 16 mL ink was dropped onto the working electrode with a loading amount of about 0.40 mg cm2. The HER and OER performances were all tested in 1 M KOH solution. Linear sweep voltammetry (LSV) curves were tested at the potential range of 1.0 Ve0 V vs. RHE for HER and 1.0 Ve1.8 V vs. RHE for OER at a scan rate of 5 mV s1, the rotation rate of RDE was 1600 rpm. The potentials were calculated relative to the RHE according to the following equation: ERHE ¼ EAg/AgCl þ 0.059 pH þ 0.1988 (pH ¼ 13.8). The electrochemical double-layer capacitance was performed by cyclic voltammograms at different scan rates with a narrow potential ranging from 0.9 V to 0.8 V vs. RHE for HER and 1.1 Ve1.2 V vs. RHE for OER. Electrochemical impedance spectroscopy (EIS) was tested in the frequency range from 100 kHz to 0.01 Hz at the potential of 0.2 V and 1.5 V vs. RHE for HER and OER. Stability of electrocatalysts were tested by chronopotentiometry with a constant current density of 10 mA cm2. The overall water splitting test were measured in a two-electrode system with electrocatalysts loaded on Ni foam as both cathode and anode in 1 M KOH at a scan rate of 2 mV s1. Nitrogen was continuously injected into 1 M KOH solution for 30 min before the electrochemical test, all the above experiments were tested without IR compensation.

X. Ding et al. / Journal of Alloys and Compounds 814 (2020) 152332

3. Results and discussions The synthetic steps of Fe-based PBAs and their corresponding phosphidation products are depicted in Scheme 1. The synthesized PBAs exhibits cubic structure, smooth surface, and different diameters, with the side length of about 200 nm for FeCo-PBA, 100 nm for FeNi-PBA and 700 nm for FeMn-PBA (Fig. S1). XRD pattern confirms the crystalline structure of PBAs, show the diffraction peaks of FeCo-PBA (JCPDS no. 46e0907), FeNi-PBA (JCPDS no. 46e0906) and FeMn-PBA (ICSD no. 151693) without impurities observed, indicating that pure PBA precursors were obtained (Fig. S2). Three kinds of PBA were tested by TGA (Fig. S3), the mass loss below 200
C corresponds to the decomposition of crystalline water in PBA, and the mass loss after 300
C can be attributed to the gradual decomposition of PBA crystal structure, three different phosphidation temperatures 300
C, 350
C, 400
C were selected through the results of TGA. Phosphidation products of PBAs at different temperatures were characterized by XRD (Fig. 1). It can be observed that the characteristic diffraction peaks of FeCo-PBA still exist at 300
C and disappear gradually with the increase of temperature, and finally almost disappeared at 400
C, suggesting the FeCo-PBA gradual transform to corresponding phosphides. FeCoP-400 is composed of a mixture of CoP (JCPDS no. 29e0497) and FeP (JCPDS no. 71e2262). The diffraction peaks of FeNi-PBA are disappeared at 300
C, and the characteristic peaks of phosphidation products NiP2 (JCPDS no. 13e0213) and FeP (JCPDS no. 71e2262) became obvious with the increase of temperature. FeMnP shows week XRD peak strength at the temperature of 300
C, 350
C, and little characteristic peaks become appeared at 400
C, it may be attributed to the mixture of FeP (JCPDS no. 71e2262), FeP2 (JCPDS no. 89e2261), MnP (JCPDS no. 89e4841). XPS analysis was used to characterize the phosphidation products to further determine their composition and valence states. XPS spectra of FeCoP-400 reveal the existence of Co, Fe and P elements (Fig. 2). The high-resolution of Co 2p spectrum (Fig. 2a) shows the peaks at 779.0 eV and 793.9 eV were derived from Co 2p3/2 and Co 2p1/2 ascribed to CoeP bonds, the peaks at 781.5 eV and 784.3 eV

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were both derived from Co 2p3/2 attributed to Co oxides of Co2þ and Co3þ [30,31,36]. The peaks located at 707.3 eV and 720.1 eV, 710.7 eV and 714.0 eV in Fe 2p high-resolution spectrum (Fig. 2b) were derived from Fe 2p3/2 and 2p1/2 corresponding to FeeP, and Fe oxides of Fe3þ, Fe2þ [19,32,33]. The formation interface of metal phosphide-metal oxide can enhance the synergistic effect and mass transfer process between the two substances, which is beneficial to OER activity [34,35]. In the high-resolution XPS spectrum of P 2p (Fig. 2c), the peaks located at 133.5 eV and 134.2 eV were corresponded to P 2p1/2 and P 2p3/2 of POx due to the surface oxidation, the peaks at 129.3 eV and 130.1 eV are corresponded to P 2p3/2 and P 2p1/2 of M  P, respectively [19,30e33]. In high resolution Ni 2p spectrum (Fig. S4a) of FeNiP-400, the peaks at 853.6 eV and 870.8 eV correspond to Ni 2p3/2 and Ni 2p1/2 bonded by NieP, the peaks at 856.6 eV and 874.6 eV correspond to the oxidation state of Ni, and the peak at 861.5 eV is the satellite peak [19,33,36]. The Fe 2p spectrum of FeNiP-400 (Fig. S4b) is similar to that of FeCoP-400, except a new peak at 723.9 eV is observed which can be attributed to the Fe3þ oxides. The P 2p spectra of FeNiP-400 (Fig. S4c) and FeCoP-400 are similar, showing the characteristics of POx and M-P. In the high-resolution Mn 2p XPS spectrum (Fig. S5a) of FeMnP-400, the peaks located at 641.5 eV, 642.4 eV and peaks of 654.1 eV are the characteristic peaks of Mn2þ, the peak of 646.8 eV ascribed to Mn3þ [11,18,36]. For the Fe 2p spectrum, the peaks at 708.8 eV and 721.7 eV are characteristic of Fe 2p3/2 and Fe 2p1/2 correspond to FeeP bonds (Fig. S5b). In high-resolution P 2p spectrum, the peaks located at 133.0 eV and 133.7 eV were derived from P 2p3/2 and P 2p1/2 assigned to POx, and the peak at 129.5 eV was derived from P 2p1/2 and attributed to M  P (Fig. S5c). The high resolution of C 1s spectrum (Fig. S6a) of FeCoP-400, FeNiP-400 and FeMnP-400 can be only fitted to one peak with binding energy at 284.6 eV, which can be assigned to CeC bonds. Similarly, in high resolution N 1s spectrum (Fig. S6b), the only peak located at 397.8 eV can be attributed to pyridinic N [8,9]. Raman spectra (Fig. S7) of FeCoP400, FeNiP-400 and FeMnP-400 show weak characteristic peak of D and G bands at 1338 cm1 and 1554 cm1. Comparing the XPS spectra of the precursor PBA with those of phosphidation products

Scheme 1. Schematic illustration of the preparation of three kinds of Fe-based PBA and corresponding phosphidation products.

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Fig. 1. XRD patterns of (a) FeCo-PBA, (b) FeNi-PBA, (c) FeMn-PBA and as-prepared corresponding phosphides products at different temperatures. XRD patterns of (d) FeCoP-400, standard PDF card of CoP and FeP, (e) FeNiP-400, standard PDF card of NiP2 and FeP (f) FeMnP-400, standard PDF card of FeP, FeP2, MnP.

Fig. 2. High-resolution XPS spectra of FeCoP-400 (a) Co 2p, (b) Fe 2p, and (c) P 2p spectrum. XPS spectra comparison of FeCo-PBA and as-prepared FeCoP at different temperatures (d) Co 2p, (e) Fe 2p, and (f) P 2p spectrum.

X. Ding et al. / Journal of Alloys and Compounds 814 (2020) 152332

at different temperatures (Fig. 2def), the result demonstrates FeCoPBA cannot completely transform to phosphides at 300
C, the characteristic peaks of the precursor disappear gradually with the increase of temperature, and thoroughly disappear until 400
C, which is consistent with the results of XRD. For FeNi-PBA, the characteristic peaks of FeNi-PBA disappeared at 300
C, and the characteristic peaks of phosphidation products become obviously with the increase of temperature (Figs. S4def), and the similar phenomena also exists in FeMn-PBA transformation (Figs. S5def). Three PBAs were phosphidated at three different temperatures under Ar atmosphere, the electron microscopic images show all the phosphidation products maintained cubic framework (Fig. 3, Figs. S8 and S9). The element content in phosphidation products at 400
C were characterized by energy-dispersive spectroscopy (EDX) (Fig. S10) and ICP-MS (Table S1). It is worth noting the atomic ratios of Fe and Co, Fe and Ni, Fe and Mn in the three phosphidation products are all close to 1:1.5, consistent with the atomic ratios of the precursors. That's why we selected the serial Fe-based PBA as template, it can not only maintain the same framework structure but also effectively control the composition ratio of metal and keep them consistent during the phosphidation process. It can help us exclude variable factors, which is beneficial to study the influence of metal kinds on catalytic performance. FT-IR spectra were used to further confirm the change from PBA to phosphidation products (Fig. S11). For PBA precursors, the peaks located at ~3600 cm1 and ~1611 cm1 correspond to the OH bending vibration, the peak at ~3408 cm1 is ascribed to the stretching vibration of OH, they are all corresponded to the water molecules in PBAs [37]. Two peaks at ~2158 cm1 and ~2113 cm1 are derived from the stretching vibration of CN group [38]. It can be observed that the characteristic peaks of PBA precursors disappear with the increase of temperature, indicating the decomposition of PBA precursor and the generation of corresponding phosphidation products, which is consistent with the results of XRD and XPS. The SEM and TEM images of FeCoP (Fig. 3) showed the homogeneous distribution of pore and rough surface replacing the smooth surface of FeCo-PBA precursors. With the increase of temperature, the porous structure of FeCoP exhibits more obviously, suggesting the gradual collapse of the FeCo-PBA. The formation of these pores may be due to the decomposition of organic ligands and the release of gases. Benefiting from the porous structure of the phosphidation products, it effectively increases the surface area of the catalyst and exposes more active sites, thus will accelerate mass transfer and improve electrocatalytic activity. The lattice fringes in HRTEM images are consistent with lattice spacing's of 0.278 nm, 0.283 nm, 0.244 nm, 0.249 nm and 0.194 nm corresponding to the (002), (011), (102), (111), (112) planes of CoP (Fig. 3j and k). The selected-area electronic diffraction (SAED) pattern is connected with the dots and rings, indicating the polycrystalline structure of CoP, the diffraction rings from outside to inside correspond to the (011), (111), (112), (301) planes of CoP (Fig. 3l). The morphology of FeNiP is similar to that of FeCoP, both of them have a rough surface and porous structure, FeNiP particles still retain the cubic morphology after phosphidation, but they become aggregation, which may affect their electrocatalytic activity (Fig. S8). HRTEM image of FeNiP-400 showed the lattice fringes with a distance of 0.160 nm, 0.189 nm and 0.201 nm ascribe to exposed planes of (222), (220), (310) of NiP2 and FeP was exposed to (111) planes with lattice distance of 0.246 nm (Figs. S6j and k), and the SAED pattern also confirmed its polycrystalline structure with (020), (220), (222), (310) planes from the outside diffraction rings to inside diffraction rings (Fig. S6l). For FeMnP, it can be observed that with the increase of temperature, adherent small particles gradually formed on its surface (Fig. S9), and there is no

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obvious porous structure in FeMnP. From TEM images, it is difficult to phosphidate the FeMn-PBA to its interior possibly because of its large diameter, and the surface was covered by phosphidation products which hindered the trend of further phosphidate to the particle center. The phosphidation products at 400
C were characterized by element mapping (Fig. 3gei, S8g-i, S9g-i). The results show that the elements of FeCoP-400 and FeNiP-400 are uniformly distributed which shows the uniform chemical composition of the whole catalysts. However, the surface distribution of phosphides on the surface of FeMnP-400 can be seen from the elemental distribution images of P element with low phosphidation degree of the center part. The OER performance of these phosphidation products was tested in 1 M KOH by linear sweep voltammograms (LSV) using commercial RuO2 as standard oxygen evolution reaction contrast material. LSV of PBAs and their corresponding phosphidation products at different temperatures were tested firstly, and the phosphidation products at 400
C exhibit the best OER performance (Fig. S12). It was observed that both FeCoP-400 and FeNiP400 show good OER performance with similar overpotential at low current density, and both of them was superior to commercial RuO2 and most of OER catalysts (Fig. 4a and Table S2). At the current density of 10 mA cm2, the overpotential of FeCoP-400, FeNiP-400 and RuO2 is 261 mV, 264 mV and 313 mV, respectively. It is worth noting that the overpotential of FeCoP-400 is lower than that of FeNiP-400 at higher current density. For example, the overpotential of FeCoP-400 at the current density of 50 mA cm2 is 317 mV, while for FeNiP-400 is 372 mV; and at the same potential, the current density of FeCoP-400 can reach 100 mA cm2, while for FeNiP-400, it can only obtain 80 mA cm2. The relatively worse OER performance of FeMnP-400 may be attributed to its larger particle size, incompletely phosphidation of the inner part and the solid structure, which decrease the exposure of active sites, and not conducive to charge transfer. The Tafel slope of FeCoP-400 (50 mV dec1) is much lower than those of FeNiP-400 (72 mV dec1) and RuO2 (90 mV dec1) (Fig. 4b), indicating that FeCoP-400 exhibits the fastest kinetics. Then, the stability of three phosphidation products was tested by chronopotentiometry at 10 mA cm2 current density for 24 h, the results show that FeCoP-400 exhibit better stability than FeNiP-400 and FeMnP-400 in alkaline solution with only 4% increase in potential (Fig. 4c). In order to further explore the differences of OER performance among FeCoP-400, FeNiP-400 and FeMnP-400, electrochemical surface area (ECSA) and specific surface area of three electrocatalysts were measured using electrochemical double layer capacitance (Cdl) (Fig. S13) and nitrogen adsorption desorption test (Fig. 5). Cdl of FeCoP-400 (18.5 mF cm2) is larger than that of FeNiP400 (13.6 mF cm2) (Fig. 4d), while FeCoP-400 has smaller specific surface area (30.0159 m2/g) than those of FeNiP-400 (42.5281 m2/ g) and FeMnP-400 (52.9273 m2/g) (Fig. 5aec). After normalizing the current density by BET surface area, FeCoP-400 still exhibits best performance (Fig. 5d), indicating FeCoP-400 can expose more active sites under the same surface area than FeNiP-400 and FeMnP-400 in OER process. The analysis of pore size distribution confirms that FeCoP-400 exhibits wider pore distribution and larger pore size which is beneficial to mass transfer process and improve the electrochemical performance. Besides, the electrochemical impedance spectra (EIS) were tested to compare the Rct. The corresponding equivalent circuit of the EIS curves was shown in Fig. S14 [8,9,45]. In EIS diagrams (Fig. S15), FeCoP-400 (~6.8 U cm2) shows lower Rct than that of FeNiP-400 (~11.8 U cm2), indicating that FeCoP-400 exhibits better electrical conductivity and faster electron transmission rate which can accelerate the charge transfer rate in the electrocatalytic process. In addition, XPS spectrum shows that both FeCoP-400 and

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Fig. 3. SEM and TEM images of FeCoP at (a, d) 300
C, (b, e) 350
C, (c, f) 400
C phosphidation temperature. EDX element mapping of FeCoP-400, (g) Fe, (h) Co, (i) P. (j, k) HR-TEM images and (l) SAED patterns of FeCoP-400.

FeNiP-400 contain a certain amount of M-OOH after OER (Figs. S16, 17, 18), which is beneficial to the four-electron transfer step in OER to enhance the activity of electrocatalytic performance. The peaks at 780.7 eV and 782.2 eV in high resolution Co 2p spectrum of FeCo400 can be attributed to Co2þ and Co3þ in CoOOH [39,40] (Fig. S16a). In Ni 2p high-resolution spectrum of FeNi-400 (Fig. S17a), the peaks located at 862.3 eV and 852.0 eV attributed to Ni 2p3/2 peaks of Ni3þ and NiOOH bonding [19,41,42]. The characteristic peaks in Mn 2p spectrum (Fig. S18a) almost remain unchanged, indicating there no MnOOH formed during OER, it may be one of the reasons which attribute to the low OER performance of FeMnP-400. Besides, the XPS spectra of other elements in FeCo400, FeNi-400 and FeMn-400, such as Fe 2p, P 2p, and O 1s spectrum are almost same (Figs. S16e18, b-d). The peaks at 711.2 eV and 713.4 eV in Fe 2p spectrum correspond to Fe3þ and Fe2þ in FeOOH [43] (Fig. S16e18, b). In P 2p spectrum, the peak belongs to M  P bond disappeared after OER, only with the characteristic peak of

POx (Fig. S16e18, c). And in O 1s spectrum, the peak at about 535.6 eV attributed to O element of Nafion (Fig. S16e18, d). Furthermore, the TEM images of FeCoP-400, FeNiP-400, FeMnP-400 after OER shows that the edge of the catalysts is rough and loose, also demonstrating the formation of M-OOH (Fig. S19). Except the above reasons, the FeCoP showed the higher activity than FeNiP is because iron and cobalt ions can form wellcoordinated electronic structures and show lower adsorption strength between active surface and oxygen intermediates than FeNiP, which is conducive to reduce OER overpotential [22]. On the other hand, due to the difference in the internal electronic structures, the addition of Co can form high valence TMPs with stronger oxidation ability than that of Ni, thus in FeCoP the interaction between metal cations and OH groups are enhanced, which leads to a faster OER kinetics [44]. We further tested the HER properties of each catalyst in 1 M KOH, and compared with 20% commercial Pt/C, in which the

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Fig. 4. (a) LSV curves, (b) Tafel plots, (c) Chronoamperometry measurements for stability tests of FeCoP-400, FeNiP-400, FeMnP-400 and RuO2, (d) capacitive current at 1.15 V vs. RHE as a function of scan rate for FeCoP-400, FeNiP-400 and FeMnP-400.

Fig. 5. Nitrogen adsorption-desorption isotherm of (a) FeCoP-400, (b) FeNiP-400, (c) FeMnP-400, inset figure is corresponding pore-size distribution, (d) LSV curves normalized by the BET surface area of FeCoP-400, FeNiP-400, FeMnP-400 for OER.

products at 400
C phosphidation show the better HER performance than PBA precursors and those phosphides under other heat-treatment temperature (Figs. S20aec). Especially, FeCoP-400 exhibits the best HER performance, with an overpotential of 204 mV at 10 mA cm2 current density and a Tafel slope of 102 mV dec1, representing the Volmer-Heyrovsky mechanism in HER. The stability of the electrocatalyst for 12 h was measured by chronopotentiometry (Figs. S20def). The results showed that FeCoP400 exhibits well HER stability in alkaline solution. In addition, three electrocatalysts were loaded on nickel foam to test the

performance of the electrocatalytic overall water splitting. The voltage of FeCoP-400 jj FeCoP-400 and FeNiP-400 jj FeNiP-400 at 10 mA cm2 is close to 1.69 V (Fig. S21a), and the electrocatalysts is durable for 24 h chronoamperometry measurements (Fig. S21b).

4. Conclusion In summary, three kinds of Fe-based PBA were used as precursors and transformed into corresponding metal phosphides by a simple heat treatment. The optimum phosphidation temperature is

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400
C, and FeCoP-400 was regarded as the most excellent OER electrocatalysts with outstanding long-term stability in alkaline in comparison to FeNiP-400 and FeMnP-400. After phosphidation, FeCoP-400 not only retain the cubic framework of precursors but also exhibit rough surface and a porous structure with larger surface area, which can expose more active sites in electrocatalysis to accelerate mass transfer and enhance electrocatalytic performance. In addition, FeCoP-400 exhibits lower charge-transfer resistance compared with two other electrocatalysts, which benefit for electron transfer and lead to faster kinetics during the OER process. This work not only prepares high performance electrocatalysts FeCoP400 for OER but also investigates the effects of the composition of electrocatalysts with the same geometrical structure on the electrocatalytic performance, and further reflects enormous potential of PBA for the synthesis of functional materials for energy conversion, storage and electrocatalysis applications. Acknowledgements The work is supported by National Natural Science Foundation of China (61601001, 21871001, 21631001), Anhui Provincial Natural Science Foundation (1708085QB37), the Anhui province key research and development program project (201904d07020001).

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Appendix A. Supplementary data [24]

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152332.

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