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The Hybrids of Cobalt/Iron Phosphides Derived from Bimetal-Organic Frameworks as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction Ting Zhang, Jing Du, Pinxian Xi, and Cai-Ling Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12189 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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ACS Applied Materials & Interfaces

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The Hybrids of Cobalt/Iron Phosphides Derived from Bimetal-Organic

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Frameworks as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction Ting Zhang, Jing Du, Pinxian Xi, Cailing Xu*

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State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal

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Chemistry and Resources Utilization of Gansu Province, Laboratory of Special Function Materials

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and Structure Design of the Ministry of Education, College of Chemistry and Chemical

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Engineering, Lanzhou University, Lanzhou 730000, China

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*Corresponding author:

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Cailing

Xu:

Tel:

+86-931-891-2589,

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[email protected]

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Abstract

FAX:

+86-931-891-2582,

Email:

[email protected],

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The electrochemical splitting of water, as an efficient and large-scale method to

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produce H2, is still hindered by the sluggish kinetics of the oxygen evolution reaction

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(OER) at the anode. Considering the synergetic effect of the different metal sites with

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coordination on the surface of electrocatalysts, the hybrids of Co/Fe phosphides

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(denoted as Co-Fe-P) is prepared by one-step phosphorization of CoFe-MOFs for the

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first time as high-efficient electrocatalysts for OER. Benefiting from the synergistic

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effect of Co and Fe, the high valence of Co ions induced by strongly electronegative P

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and N and the large electrochemical active surface area (ECSA) originated from

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exposed nanowires on the surface of Co/Fe phosphides, the resultant Co-Fe-P-1.7

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exhibits remarkable electrocatalytic performances for OER in 1.0 M KOH, affording

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an overpotential as low as 244 mV at a current density of 10 mA/cm2, a small Tafel 1

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slope of 58 mV/dec and good stability, which is superior to that of the state-of-the-art

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OER electrocatalysts. In addition, the two-electrode cell with Co-Fe-P-1.7 modified

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Ni Foam as anode and cathode in an alkaline electrolyte, respectively, exhibits the

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decomposition potential of ca. 1.60 V at the current density of 10 mA/cm2 and

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excellent stability.

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Keywords: OER, bimetal-organic frameworks, phosphides, water splitting, HER

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Introduction

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Owing to the increasing fossil fuels crisis, global pollution and waste of natural

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resources, green and renewable hydrogen energy has been received much attention

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due to its high energy capacity and zero carbon dioxide release.1-4 Electrocatalytic

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water splitting, a method to produce hydrogen (H2) and oxygen (O2) through the

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half-reaction of the hydrogen evolution reaction (HER) and oxygen evolution reaction

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(OER), has been recognized as one of the most promising and environmentally

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friendly ways for the production of hydrogen energy.5-8 However, OER, which

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generally needs a large overpotential due to the high energy barrier for O-H bond

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breaking and attendant O-O bond formation in OER, is the key step of the

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electrochemical water splitting because of the kinetics sluggish.9-12 To date, IrO2 and

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RuO2 have been widely applied as anode materials of water splitting because of its

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high efficiency, fast response and low overpotentials.13-15 However, the actual

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application is greatly constrained by their scarcity and high costs. Consequently,

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searching for low cost, earth-abundant and highly efficient electrocatalysts for OER is

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still urgently needed.16-18 2


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Up to now, some nonprecious electrocatalysts with good electrochemical

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performances for OER such as transition-metal oxides,19-20 hydroxides,21-22 layered

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double hydroxides (LDHs),23-24 sulfides25-26 and phosphate27-30 etc. have been

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reported. Among the different electrocatalysts for OER, the recently reported

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transition-metal phosphides are very promising not only because of their high

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abundance and low cost but also owing to their great base stability in the alkaline

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solution.31-34 However, these transition metal phosphides still suffer from large

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overpotential and Tafel slopes during electrochemical operation. For example, the

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surface oxidized CoP nanorods synthesized by Chang et al. shows an overpotential

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of 320 mV at the current density of 10 mA/cm2 and a Tafel slope of 71 mV/dec.29

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The CP@FeP composites fabricated through a facile phosphorization of iron

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oxyhydroxide precursors exhibit an increasing overpotential of 350 mV at the current

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density of 10 mA/cm2 and a decreasing Tafel slope of 63.6 mV/dec toward OER.30

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The preliminary results reveal that the different metal sites with coordination on the

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surface of electrocatalysts are pivotal for OER because of their outstanding

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chemisorptions of OH- and oxygen-containing intermediates (OH*, OOH*, and

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H+).35 Therefore, an optimized integration of phosphides materials with different

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metal sites to construct novel may be an efficient method to extremely improve the

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water-oxidation activity.36-37 According to the previous reports, the catalytic

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efficiency of Co-based catalysts will highly be influenced by Fe element, which

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could enhance the breaking of the Co-O bond and the formation of O-O bond to

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promote the OER performance.38-41 Therefore, it is highly desirable to fabricate the 3



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hybrids of Co/Fe phosphides with excellent electrocatalytic performance for OER.

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Very recently, nickel or cobalt phosphides have been successfully synthesized

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via pyrolysis and a subsequent phosphating process with MOFs as a precursor.42-47

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Owing to the large specific surface area, high C content of coordinate organic

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ligands and controllable pore texture, the resultant nickel or cobalt phosphides

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exhibit remarkable catalytic performance for OER or HER in alkaline

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electrolyte.42-49 Even so, the overpotential at a current density of 10 mA/cm2 for

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OER is still greater than or equal to 300 mV. For example, You et al.42 showed that

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the overpotential of the Co-P/NC derived from ZIF-67 is 319 mV at a current density

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of 10 mA/cm2 and the mixed phases of Ni5P4 and Ni2P derived from Ni–Ni

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Prussianblue46 demonstrates an overpotential of 300 mV at a current density of 10

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mA/cm2. However, the hybrid

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using bimetal-organic frameworks as precursor, which could be easily synthesized

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by adding the second metal ion into the MOFs.

of bimetal phosphides have not been prepared by

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In this context, the hybrids of Co/Fe phosphides with the mixed phases of CoP,

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Co2P and FeP were developed for OER electrocatalysts through one-step

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phosphorization calcination of CoFe-MOFs. The optimal Co-Fe-P-1.7 exhibits

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remarkable OER activity in 1 M KOH solution, accompanied by a very low

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overpotential of 244 mV at a current density of 10 mA/cm2, a small Tafel slope of 58

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mV/dec and an excellent stability, which were extremely superior to those of the

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reported materials, such as CoMnP,50

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Co3O4 and CoP nanorod derived from MOFs29,51 et al. Furthermore, when the

(Co1-xFex)2P,41 Co2P,34 CoP43 as well as

4


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Co-Fe-P-1.7 is employed as a bifunctional electrocatalysts, the overall water splitting

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in 1 M KOH is operated at a low cell voltage of ca. 1.60 V to achieve a current

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density of 10 mA/cm2, which is comparable to those of previously reported

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electrocatalysts.42, 52-54 All these prominent results demonstrate that the Co-Fe-P-1.7

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are quite promising electrochemical catalysts in overall water splitting.

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Results and Discussion

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The synthesis procedures of CoFe-MOFs and corresponding Co-Fe-P hybrids are

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clarified in Supporting Information. Firstly, CoFe-MOFs are synthesized by

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solvothermal method. And then corresponding Co-Fe-P hybrids are obtained by

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one-step phosphorization of the CoFe-MOFs using sodium hypophosphite

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monohydrate as phosphorus source (see details in Supporting Information). In order to

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determine the Co/Fe molar ratio in CoFe-MOFs samples, the inductively coupled

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plasma atomic emission spectroscopy (ICP-AES) is performed (Table S1, Supporting

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Information). According to the results of ICP, the samples are labeled as

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CoFe-MOFs-0.67,

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CoFe-MOFs-2.0, respectively.

CoFe-MOFs-1.1,

CoFe-MOFs-1.3,

CoFe-MOFs-1.7

and

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Fig. S1 shows the steady-state polarization curves of the prepared CoFe-MOFs

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with different Co and Fe ratios. As shown in Fig. S1, the CoFe-MOFs-1.7 shows the

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lowest OER overpotential (319 mV vs. RHE) at a current density of 10 mA/cm2 and

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the smallest Tafel slope (56 mV/dec). In addition, the CoFe-MOFs-1.7 also exhibits

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the better electrochemical performance than pure Fe-MOFs or Co-MOFs from Fig. S2.

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Therefore, the CoFe-MOFs-1.7 is used as the optimal candidate for subsequent

23

experiments. 5



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1 2 3

Fig. 1. (A) Simulated data from crystal structure (a) and experimental XRD patterns of Co-MOFs (b) and CoFe-MOFs-1.7 (c) and (B) FTIR spectrum of CoFe-MOFs-1.7

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The X-ray diffraction (XRD) patterns of Co-MOFs and CoFe-MOFs-1.7 are

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shown in Fig. 1A. It can be seen that the as-prepared Co-MOFs have the similar

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diffraction peaks with the simulated data, indicating the successful synthesis of pure

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(Co(bdc)(DMF)). After incorporation of Fe into Co-MOFs, no obvious change can be

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observed in the X-ray diffraction spectrum. However, the diffraction peaks of

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CoFe-MOFs-1.7 slightly shift to the higher 2θ value after Fe incorporation, which is

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due to the partial substitution of Fe.55 In order to further testify the successful build-up

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of CoFe-MOFs, the characteristic vibrative peaks of the main functional groups in

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CoFe-MOFs-1.7 are confirmed by Fourier transform infrared (FTIR) spectroscopy

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(Fig. 1B). The peak located at 3429 cm-1 is assigned to the O-H vibration, indicating

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the presence of hydroxy in CoFe-MOFs-1.7.39 Bands at 1403.5 (C-N stretching) and

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1698 cm-1 (C=O stretching) can be attributed to the coordinated DMF and the

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carbonyl function of H2BDC.56 The peaks at ca. 1585 cm-1 and 1498 cm-1 related with

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C=C stretching of the benzene rings.57 According to the XRD and FTIR results, the

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CoFe-MOFs were successfully prepared.

6


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Fig. 2. FESEM (A), TEM (B,C), STEM images (D) and the corresponding elemental mapping (E, F, G, H) of CoFe-MOFs-1.7

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FESEM and TEM images of CoFe-MOFs-1.7 are shown in Fig. 2. As shown in

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Fig. 2A, the spindle-shaped products with rough surface are present. Meanwhile,

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some small particles can be observed. Their energy dispersive X-ray spectroscopy

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(EDS) reveals the existence of Co, Fe, C and O elements in the products (Fig. S4A).

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To further analyze the microscopic structure of CoFe-MOFs-1.7, TEM are carried out.

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In Fig. 2B, the CoFe-MOFs-1.7 shows a size being ~ 800 nm in length and ~400 nm

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in width. From Fig. 2C, it can be seen that the surface of CoFe-MOFs is comprised of

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tiny nanoparticles. The scanning transmission microscopy (STEM) and the 7



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corresponding elemental mapping demonstrate uniform distribution of Co, Fe, C and

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O elements in CoFe-MOFs-1.7 (Fig. 2D-H).

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Fig. 3. XRD pattern (A), FESEM (B), TEM(C, D), HRTEM (E) of Co-Fe-P-1.7 as well as their STEM images (F) and the corresponding elemental mapping (G, H, I, J, K)

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After phosphorization calcination of CoFe-MOFs-1.7 in N2, the obtained sample

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is firstly characterized by the powder X-ray diffraction (XRD) (Fig. 3A). The

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phosphides can be well-indexed into a complex structure of CoP phase (JCPDS No.

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65-2593), Co2P phase (JCPDS No. 32-0306) and FeP phase (JCPDS No. 65-2595). In

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details, the major diffraction peaks at 31.6 °, 36.3 °, 48.1 ° and 56.7 ° can be indexed

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to the diffraction from (011), (111), (211), and (103) planes of orthorhombic CoP,

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while the weaker peaks at 32.8°, 34.6° and 38.0° as well as at 40.7° and 43.3° are

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ascribed to FeP and Co2P, respectively. These results reveal that the successful

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conversion of CoFe-MOFs-1.7 into the hybrids of Co-Fe-P-1.7 after the

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low-temperature phosphidation reaction. Fig. 3B and C exhibit that the resulting

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Co-Fe-P-1.7 sample inherits the overall spindle-like morphology, suggesting that the

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phosphorization could successfully transform CoFe-MOFs-1.7 into Co-Fe-P-1.7 8


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without any structure damage. However, the products possess the coarser surface and

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more incompact structure than those of CoFe-MOFs-1.7. In addition, the chemical

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composition of Co-Fe-P-1.7 is confirmed by EDS analysis in Fig. S4B, which

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displays the existence of Co, Fe, C, O and P element. However, the intensity of O

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peak becomes weaker than that of CoFe-MOFs-1.7 (Fig. S4A), which may be caused

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by the change of organic ligands during phosphorization. Meanwhile, a small amount

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of Na element is detected by the EDS, which is believed to be from NaH2PO2 during

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phosphorization. Furthermore, from the enlarged TEM image, the spindle-like

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samples is intimately surrounded by abundant radial nanowires with a length of ~600

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nm (Fig. 3D). The formation of nanowires may be due to the induction of hydroxyl in

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organic ligands when they were pyrolyzed, which can promote the anisotropic growth

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of the nanowires.58 However, the authentic mechanism needs to be further studied in

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the subsequent experiments. These exposed nanowires will provide the large specific

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surface area for the products and thus benefit to their electrocatalytic performance.

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The structure of Co-Fe-P-1.7 can also be represented by HRTEM image (Fig. 3E), the

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lattice fringes of 0.268 and 0.281 nm are clearly identical with the lattice planes of

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(011) of FeP and (011) of CoP, respectively. The STEM and the corresponding

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elemental mapping show that the Co, Fe, C, O and P elements evenly exist in the

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hybrids of Co-Fe-P-1.7 (Fig. 3F-K).

9



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1 2 3 4

Fig. 4. Steady-state polarization curves (A) and the Tafel plots (B) of bare GCE, CoFe-LDH, CoFe-MOFs-1.7, Co-Fe-P-1.7 and commercial IrO2 modified RDEs in 1 M KOH solution for OER (scan rate: 10 mV s-1), respectively

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The OER activity of bare GCE, CoFe-LDH, CoFe-MOFs-1.7, Co-P, Fe-P,

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Co-Fe-P and commercial IrO2 modified RDEs with the same mass loading of 0.424

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mg/cm2 are studied in 1 M KOH (Fig. S3 and Fig. S6, Supporting Information). Fig.

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4A shows that the CoFe-MOFs-1.7 exhibits the superior activity for OER compared to

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CoFe-LDH. The polarization curve of the Co-Fe-P-1.7 shows an onset overpotential

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of ~200 mV vs. RHE and an overpotential of ~260 mV at a current density of 10

11

mA/cm2, which is obviously lower than that of CoFe-MOFs-1.7, Co-P, Fe-P, other

12

Co-Fe-P (Fig. S6) and the “golden catalyst” of IrO2 (300 mV vs. RHE). Moreover,

13

this low overpotential is much lower than most reported values of non-precious metal

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OER catalysts in the literature (Table S2, Supporting Information). As shown in Fig.

15

4B, the corresponding OER Tafel slopes of CoFe-LDH, CoFe-MOFs-1.7 and

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Co-Fe-P-1.7 are within the range of 50-60 mV/dec, which indicate the similar

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rate-determining steps of these catalysts in OER.39 Meanwhile, the Tafel slopes of

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these catalysts are lower than that of the reported highly active catalysts (Table S2,

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Supporting Information), indicating their more rapid OER kinetics.59-60 On the other 10


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1

hand, the exchange current density, as the intrinsic property of the electrode reaction,

2

reflects the ability of electron transfer and the difficulty of an electrode reaction and

3

depends on catalyst materials, electrolyte and temperature.61 The exchange current

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density can be acquired by the extrapolated linear part of Tafel plots. The large

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exchange current density of Co-Fe-P-1.7 (Fig. 4B) illustrate the less driving force

6

needed (small overpotential) in the OER reaction. In addition, the electrochemical

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impedance spectroscopy (EIS) images of CoFe-MOFs-1.7 and Co-Fe-P-1.7 (Fig. S7)

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showed

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CoFe-MOFs-1.7, which demonstrates the fast reaction kinetic of Co-Fe-P-1.7. The

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superior OER activity of Co-Fe-P-1.7 can be firstly attributed to the 1D nanowires on

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their surface. It is believed that the 1D nanowires can provide a very rough surface,

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which not only provides abundant active sites for electrocatalytic reactions, but also

13

facilitates inter-electron transport along the basal surfaces.62-63

that

Co-Fe-P-1.7

has

the

lower

charge-transfer

11


resistance

than


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1 2 3 4

Fig. 5. XPS survey spectra of CoFe-LDH, CoFe-MOFs-1.7 and Co-Fe-P-1.7 (A); High resolution XPS spectra of Fe 2p (B), Co 2p (C) for CoFe-LDH, CoFe-MOFs-1.7 and Co-Fe-P-1.7 and P 2p (D) for Co-Fe-P-1.7

5

To further understand the superior OER activity of Co-Fe-P-1.7, the chemical

6

valence state and surface composition of CoFe-LDH, CoFe-MOFs-1.7 and

7

Co-Fe-P-1.7 are characterized by XPS. Fig. 5A shows the presence of Co, Fe, C and

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O element in CoFe-LDH, CoFe-MOFs-1.7 and Co-Fe-P-1.7 samples. In addition,

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modicum of N element is detected in CoFe-MOFs-1.7 and Co-Fe-P-1.7 samples,

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respectively, which may be originated from the coordinated DMF in MOFs. This

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phenomenon is in accordance with the results of FTIR. As expected, the P element is

12

only observed in Co-Fe-P-1.7 compounds. For the high-resolution XPS spectra of Fe

13

2p (Fig. 5B), all the samples show two main peaks of Fe 2p3/2 and Fe 2p1/2 at 711.2

14

and 724.3 eV and a satellite line with a binding energy of 717.8 eV, which indicates

15

the presence of Fe3+.26, 39, 64 The Co 2p spectrum of CoFe-LDH displays two major 12


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peaks of Co 2p3/2 and Co 2p1/2 at 781.2 eV and 796.4 eV, respectively, and two

2

shakeup satellite peaks (786.0 and 802.6 eV) (Fig. 5C). It suggests the present of Co2+

3

in CoFe-LDH.65-66 For CoFe-MOFs-1.7, the peak position of Co 2p3/2 and Co 2p1/2

4

shifts to 781.7 and 796.7 eV, respectively. However, in the Co 2p XPS spectrum of

5

Co-Fe-P-1.7, the peaks of Co 2p3/2 and Co 2p1/2 are observed at 782.0 and 797.0 eV,

6

respectively. The slight shift toward the high binding energy for the Co 2p3/2 and Co

7

2p1/2 spin-orbit peaks of CoFe-MOFs-1.7 and Co-Fe-P-1.7 is ascribed to the departure

8

of Co electron cloud induced by N and P with the strong electronegativity,39, 67 which

9

caused the partial oxidation of Co2+ in CoFe-MOFs-1.7 and Co-Fe-P-1.7.68 However,

10

compared with the Co 2p spectrum of CoFe-MOFs-1.7, the Co 2p spectrum of

11

Co-Fe-P-1.7 have the larger binding energy, which indicates the high content of Co3+.

12

Meanwhile, the much stronger satellite peaks of CoFe-MOFs-1.7 centered at about

13

787 and 803 eV also indicate the lower percentage of Co3+ in CoFe-MOFs-1,7.69 As a

14

result, the O- adsorption ability of catalyst will be enhanced by the high valence of Co

15

ions, thus facilitate the formation of hydroperoxy (OOH) species in subsequent

16

steps.26 Therefore, the electrocatalytic activity of Co-Fe-P-1.7 is obviously promoted.

17

The P 2p spectrum (Fig. 5D) is deconvoluted into three peaks located at 133.0, 130.0

18

and 129.0 eV, which can be ascribed to the binding energies of P-C/P-O, P 2p3/2 and P

19

2p1/2 in phosphides and phosphorus oxide, respectively.43 The high-resolution N 1s

20

spectra of CoFe-MOFs-1.7 and Co-Fe-P-1.7 ( Fig. S8A and B) reveal that the form of

21

pyridinic-N (399.0 eV) in two catalysts.67 The C 1s spectrum of CoFe-MOFs-1.7 ( Fig.

22

S8C) is deconvoluted into three peaks located at 284.5, 285.9 and 287.9 eV, 13



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1

respectively, which can be attributed to the species of C-C=C, C=O/C-N, O-C=O,

2

respectively.26 For the C 1s spectra of Co-Fe-P-1.7 in Fig. S8D, the position of the

3

three peaks are almost the same as that of CoFe-MOFs, but the intensity of peaks at ca.

4

285.9 eV and 287.9 eV become very weak, which further proves the transformation of

5

the C matrix after the subsequent phosphorization.70

6

Furthermore, the electrochemical active surface area (ECSA) is conducted to

7

confirm the high activity of Co-Fe-P-1.7. It is well known that an increase of

8

electrochemical active surface area often leads to the enhancement of catalytic

9

activity. The ECSA can be calculated by the electrochemical double-layer capacitance

10

(Cdl) of active materials, which is generally proportional to their ECSA (ECSA =

11

Cdl/Cs, Cs is specific capacitance).26,

12

CoFe-MOFs-1.7 and Co-Fe-P-1.7 was probed by cyclic voltammogram (Fig. S9). Fig

13

S9A, B and C show the typical CV curves of CoFe-LDH, CoFe-MOFs-1.7 and

14

Co-Fe-P-1.7 at different scan rates. By plotting the ∆J= Ja-Jc at 1.36 V vs. RHE

15

against the scan rate, the slope which is twice of Cdl can be obtained. As shown in Fig.

16

S9D, the Cdl of CoFe-LDH, CoFe-MOFs-1.7 and Co-Fe-P-1.7 is 10.5 mF/cm2, 21.5

17

mF/cm2 and 23.0 mF/cm2, respectively. According to the Cs reported in previous

18

research, the EASA of Co-Fe-P-1.7 can be calculated as 27.1 cm2 with a surface

19

roughness factor (Rf) of 383.3, which is much higher than that of CoFe-LDH

20

(ECSA=12.4 cm2, Rf =174.7) and CoFe-MOFs-1.7 (ECSA=25.3 cm2, Rf = 357.8).

21

These results verify that the higher intrinsic catalytic activity of Co-Fe-P-1.7 can be

22

ascribed to the increased electroactive sites.

66-67, 71

Therefore, the Cdl of CoFe-LDH,

14


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1

Furthermore, mass activities of CoFe-LDH, CoFe-MOFs-1.7 and Co-Fe-P-1.7

2

are also estimated. At an overpotential of 280 mV, mass activity of Co-Fe-P-1.7 is

3

found to be 50.2 A/g in 1 M KOH, which is much larger than that of CoFe-LDH (2.6

4

A/g) and CoFe-MOFs-1.7 (5.9 A/g). Assuming that the Co and Fe ions are all active

5

materials, therefore, TOF value of Co-Fe-P-1.7 is calculated based on ICP results. It

6

can be found that the TOF of Co-Fe-P-1.7 is 0.036/s (Co ions) and 0.057/s (Fe ions) at

7

280 mV, which is much higher than that of CoFe-MOFs-1.7 (0.0042/s and 0.0066/s).

8

These results further suggest that the Co-Fe-P-1.7 possess the excellent catalytic

9

performance toward OER.

10 11 12 13

Fig. 6. Current density vs. time (I-t) curve of Co-Fe-P-1.7 modified RDE for OER at the constant overpotential of 260 mV (A) and polarization curves (a) before and (b) after 10 h-long electrocatalytic OER (B)

14

The operating stability of catalyst is of great significance for their practical

15

application. The stability of Co-Fe-P-1.7 was tested at 10 mA/cm2 for 10 h. As

16

displayed in Fig. 6A, the I-t curve of Co-Fe-P-1.7 show negligible decay during

17

continuous oxygen evolution of 10 h, indicating the high OER activity and long-term

18

stability. The excellent stability of the electrode is further confirmed by the

19

polarization curve (Fig. 6B) after I-t test. As can be seen, a negligible difference is 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

detected in the polarization curves after continuous I-t testing of 10 h in 1 M KOH.

2

These results indicate that Co-Fe-P-1.7 possesses the excellent stability in alkaline

3

solution. Moreover, in view of the negative effect of the binder, the Co-Fe-P-1.7 is

4

directly employed as binder-free catalysts for OER by loading it on porous nickel

5

foam. With a mass loading of 1.0 mg/cm2, the Co-Fe-P-1.7 modified Ni foam

6

electrode exhibits the overpotential of only 244 mV for OER at a current density of 10

7

mA/cm2 and a negligible degradation after chronopotentiometry operation for 30 h

8

(Fig. S10).

9 10 11 12 13

Fig. 7. Cyclic voltammogram curves of Ni foam (a) and Co-Fe-P-1.7 modified Ni foam (b) in a two-electrode system. (A) Durable operation of Co-Fe-P-1.7/Ni foam at 10 mA/cm2 in 1 M KOH (scan rate: 10 mV/s). The inset of (B) shows the hydrogen and oxygen evolution during the constant-current electrocatalysis

14

Moreover, the HER catalytic performance of Co-Fe-P-1.7 is further tested in

15

order to realize the overall water splitting at low cell potential (Fig. S11). From the

16

LSV curves in Fig. S11A, the Co-Fe-P-1.7 yields the highest HER activity with a

17

decent overpotential of 295 mV at a current density of 10 mA/cm2 compared to

18

CoFe-LDH (~309 mV), CoFe-MOFs-1.7 (~308 mV) and commercial IrO2 (~310 mV),

19

which is comparable to previous reported catalysts (320 mV vs. RHE for CoP, 277

20

mV vs. RHE for NiFeP, 331 mV vs. RHE for Ni(OH)2).72 Next, in consideration of 16


Page 16 of 24

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1

the multifunctional catalytic performances of Co-Fe-P-1.7 for both OER and HER,

2

the electrochemical performance of Co-Fe-P-1.7 modified Ni foam for overall water

3

splitting is studied in a symmetric two-electrode cell (Fig. 7A). A current density of 10

4

mA/cm2 is delivered at approximately 1.60 V, which is lower than the reported results

5

(Table S3, Supporting Information) and comparable to that of the commercial

6

catalysts (Pt/C and IrO2) modified Ni foam with the same loading (Fig. S12). In

7

addition, there is a negligible increase over 40 h V-t testing (Fig. 7B), demonstrating

8

the promising application of the Co-Fe-P-1.7 in overall water splitting because of the

9

low decomposition potential and remarkable stability.

10 11

Conclusion In summary, the hybrid

of Co/Fe phosphides has been successfully prepared by

12

a phosphorization treatment of CoFe-MOFs. The resulting Co-Fe-P-1.7 exhibit

13

excellent electrocatalytic activities and superior stability for OER due to the

14

synergistic effects of different phosphide, the high valence of Co ions induced by P

15

and N and the large electrochemical active surface area (ECSA) originated from

16

exposed nanowires on the surface of Co-Fe-P-1.7. Moreover, the as-fabricated

17

two-electrode cell with Co-Fe-P-1.7 modified Ni Foam as anode and cathode,

18

respectively, demonstrated the promising application of the Co-Fe-P-1.7 in overall

19

water splitting because of the low decomposition potential and remarkable stability.

20

Acknowledgements

21

This work was supported by grants from Natural Science Foundation of China

22

(NNSFC no. 21673105, 21503102), the Fundamental Research Funds for the Central 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

University (lzujbky-2016-K09, lzujbky-2015-274), the Science and Technology

2

Program of Gansu Province of China (145RJZA176) and Open Project of Key

3

Laboratory for Magnetism and Magnetic Materials of the Ministry of Education

4

(LZUMMM2016008).

5

ASSOCIATED CONTENT

6

Supporting Information Available: EDS, XRD patterns, XPS, polarization curves and

7

Tafel slopes of different MOFs and derived phosphides, electrochemical impedance

8

spectroscopy (EIS) images and cyclic voltammograms curves. This material is

9

available free of charge via the Internet at http://pubs.acs.org.

10

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Iron Phosphides Derived from ... - ACS Publications

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