C nanosheet arrays for cost-effective electrocatalytic water splitting

Accepted Manuscript Self-standing FeCo Prussian blue analogue derived FeCo/C and FeCoP/C nanosheet arrays for cost-effective electrocatalytic water splitting Rui Xiang, Yijun Duan, Cheng Tong, Lishan Peng, Jian Wang, Syed Shoaib Ahmad Shah, Tayyaba Najam, Xun Huang, Zidong Wei PII:

S0013-4686(19)30197-5

DOI:

https://doi.org/10.1016/j.electacta.2019.01.170

Reference:

EA 33573

To appear in:

Electrochimica Acta

Received Date: 2 November 2018 Revised Date:

16 January 2019

Accepted Date: 27 January 2019

Please cite this article as: R. Xiang, Y. Duan, C. Tong, L. Peng, J. Wang, S.S.A. Shah, T. Najam, X. Huang, Z. Wei, Self-standing FeCo Prussian blue analogue derived FeCo/C and FeCoP/C nanosheet arrays for cost-effective electrocatalytic water splitting, Electrochimica Acta (2019), doi: https:// doi.org/10.1016/j.electacta.2019.01.170. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

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Graphic abstract

FeCo/C and FeCoP/C nanosheet arrays for highly efficient water splitting are

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prepared by facile transformation of self-standing FeCo Prussian Blue Analogous.

ACCEPTED MANUSCRIPT Self-standing FeCo Prussian Blue Analogue Derived FeCo/C and FeCoP/C Nanosheet Arrays for Cost-Effective Electrocatalytic Water Splitting Rui Xiang, Yijun Duan, Cheng Tong, Lishan Peng, Jian Wang, Syed Shoaib Ahmad

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Shah, Tayyaba Najam, Xun Huang* and Zidong Wei* Chongqing Key Laboratory of Chemical Process for Clean Energy and Resource Utilization, School of Chemistry and Chemical Engineering, Chongqing University,

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Chongqing 400044, China.

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email address: [email protected], [email protected]. Abstract

Low-cost and efficient electrocatalysts for water splitting hold a significant position in future renewable energy system. Herein, we first fabricated a self-standing bimetallic

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FeCo Prussian blue analogue nanosheet array (FeCo PBA) on Ni foam through ion exchange between K3[Fe(CN)6] and hydrothermal synthesized Co3(PO4)2·8H2O nanosheet arrays. Then the obtained FeCo PBA were facilely transformed into

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FeCo/C and FeCoP/C nanosheet arrays through hydrogenation and phosphidation,

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respectively. Benefiting from the enhanced mass transfer in porous structure, the intimate contact with Ni framework and the synergistic effect of bimetal sites, the resultant FeCo/C NS and FeCoP/C NS demonstrate superior oxygen and hydrogen evolution activity in alkaline media, respectively. Impressively, a low cell voltage of 1.55 V is sufficient to afford a current density of 10 mA cm-2 by coupling FeCo/C NS and FeCoP/C NS in a two-electrode water splitting electrolyzer, surpassing the performance of PtC based couple (Vcell,10 =1.60 V). This work provides a new 1

ACCEPTED MANUSCRIPT approach to construct highly efficient and cost-effective water splitting electrodes. Key words: Prussian blue analogue, Electro-catalysis, Oxygen evolution reaction, Hydrogen evolution reaction, Overall water splitting

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1. Introduction Electrocatalytic water splitting to produce H2 and O2 is believed to be a promising and reliable strategy to provide the human community with renewable

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energy in the future [1]. However, high overpotential is usually required to drive

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water splitting reaction effectively, due to the sluggish kinetics of both the oxygen evolution reaction (OER) at anode and the hydrogen evolution reaction (HER) at cathode [2, 3]. Currently, the state-of-the-art catalysts with high OER and HER activity are mainly precious metal (e.g. Ru, Ir and Pt) based materials, whose

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large-scale utilization is greatly hampered by their high cost and scarce reserves [4-6]. Therefore, massive research interests have been concentrated on the development of non-precious metal based catalysts, among which 3d transitional metals (e.g. Fe, Co

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and Ni) have become attractive candidates owing to their low cost, earth abundance

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and low toxicity [7-9].

Nonoxides of 3d transitional metals have been extensively explored as OER and

HER catalysts, including phosphides [10], carbides [11], sulfides [12], alloys [13], etc. To prepare such compounds, metal-organic frameworks (MOFs) are widely used as precursors, owing to their porous and tunable carbon-rich structures [14]. For example, He et al. fabricated carbon-incorporated NiCoP/C nanoboxes by taking ZIF-67 as both structure-directing agent and cobalt resources [15]. Liu et al. loaded cobalt 2

ACCEPTED MANUSCRIPT nanoparticles on nitrogen and boron co-doped graphitic carbon (Co/NBC) by direct carbonization of a new cobalt-based boron imidazolate framework (BIF-82-Co) [16]. Nevertheless, in most cases, expensive organic linkers were employed during the

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preparation of these MOFs, making the derived catalysts less economical as expected. Therefore, Prussian blue analogues (PBA), a class of affordable MOFs, have attracted academic attentions for their potential applications in biosensors [17], battery

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electrode materials [18], electrocatalysts [19] and so on. As an example, Fan et al.

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synthesized Fe doped Ni3C dispersed in carbon nanosheets as bifunctional overall water splitting catalyst by calcinating Fe doped Ni[Ni(CN)4] complex [20]. However, as limited by synthetic methods, most of these catalysts are in powder form and their binding to gas evolution electrode suffers from the problems of limited mass and

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charge transfer, low loading and poor mechanical stability. As a result, construction of self-standing integrate electrode derived from PBA nanostructures should be an effective method to alleviate the problems of powder catalysts, which has been

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seldom reported in the literature.

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Herein, based on the above analysis, we developed a sacrificial template strategy to fabricate self-standing petaloid bimetallic FeCo PBA nanosheet arrays on Ni foam by employing Co3(PO4)2·8H2O as template, which were further transformed to FeCo/C NS as OER electrode and FeCoP/C NS as HER electrode by facile hydrogenation and phosphorization, respectively. Benefiting from the open porous structure and intimate contact with Ni foam, a two-electrode alkaline electrolyser exhibits a low cell voltage of 1.55 V at 10 mA cm-2. This work demonstrates an 3

ACCEPTED MANUSCRIPT effective approach for the fabrication of well-structured self-standing FeCo PBA nanosheet arrays, which provides new opportunities for the development of cost-effective electrocatalytic water splitting devices.

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2. Experimental 2.1 Material preparation

Synthesis of Co3(PO4)2·8H2O nanosheet arrays (CoPO NS): The preparation of

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CoPO NS follows the published procedure with modifications [21]. Briefly,

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commercial Ni foam (1×4 cm2) was successively washed with 3 M HCl, ethanol, water and ethanol under sonication, and dried under N2 flow. 0.0872 g Co(NO3)2·6H2O and 0.0344 g NH4H2PO4 were added into 60 mL pure water, and dissolved with magnetic stirring to form a clear solution. The pretreated Ni foam and

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the above-mentioned solution was transferred into a 100 mL autoclave to undergo a hydrothermal treatment at 120 oC in electronic oven for 6 h. After cooling to room temperature, the as obtained Ni foam was thoroughly washed with water and dried at

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60 oC in electronic oven.

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Synthesis of cobalt hexacyanoferrate nanosheet arrays (FeCo PBA): To prepare FeCo PBA, the as-obtained CoPO NS nanosheet arrays were immersed into K3[Fe(CN)6] solution with different concentrations (0.02, 0.1 and 0.2 M) and heated by water bath under varying temperatures (30, 60 and 90 oC) and time periods (30, 60, 90, 120 and 150 min). Subsequently, the Ni foam loaded with FeCo PBA was washed with copious water and dried at 60 oC. Synthesis of FeCo/C NS and FeCoP/C NS: Hydrogenation treatment led to the 4

ACCEPTED MANUSCRIPT formation of FeCo/C NS. Specifically, the as-obtained FeCo PBA nanosheet arrays were loaded into a tube furnace and heated under H2/N2 (10 : 90) atmosphere at 400 o

C for 60 min with a heating rate of 5 oC min-1 (denoted as FeCo/C NS). To obtain

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FeCoP/C NS, FeCo PBA nanosheet arrays and 1.2 g NaH2PO2·H2O were loaded into a tube furnace and heated under N2 atmosphere at 350 oC for 120 min with a heating rate of 2 oC min-1.

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2.2 General characterizations

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The surface morphology and the microstructure of the catalysts were analyzed by X-ray diffraction (XRD-6000, Shimadzu), X-ray photoelectron spectroscopy (XPS, PHI 550 ESCA/SAM), field-emission scanning electron microscopy (FE-SEM, JSM-7800, Japan), and energy dispersive X-ray spectra (EDS, OXFORD respectively.

Transmission

electron

microscopy

(TEM)

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Link-ISIS-300),

measurements were conducted on a Hitachi H-8100 electron microscope (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. Fourier Transform Infrared

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Spectrometry (FTIR) was detected on Nicolet iS50 FTIR spectrometer. Raman spectra

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were obtained with a LabRAM HR Evolution spectrophotometer coupled with an optical microscope of 1 µm resolution. 2.3 Electrochemical measurements Electrochemical measurements were conducted by a Princeton Applied Research

Parstat 4000 potentiostat at room temperature. A standard three electrode cell with 1.0 M KOH as electrolyte was used for all the electrochemical tests unless otherwise stated. The as-synthesized electrodes were directly used as working electrode, and a 5

ACCEPTED MANUSCRIPT carbon rod was used as counter electrode. Ag/AgCl (3 M KCl) was used as reference electrode and calibrated in high purity H2 saturated electrolyte with a Pt wire as the working electrode. Unless otherwise stated, potentials were converted to overpotential

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(η) according to equations: ηOER = E (vs. RHE) -1.23 V, ηHER = E (vs. RHE) -0 V, E (vs. RHE) = E (Ag/AgCl) +1.016 V. Prior to data collection, 20 cycles of cyclic voltammetry (CV) at a scan rate of 50 mV s-1 was conducted between 0.2 ~ -0.2 V (vs.

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RHE) for HER or 1.0 ~ 1.6 V (vs. RHE) for OER. Linear sweep voltammetry (LSV)

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was recorded at a scan rate of 1 mV s-1 between 0.2 ~ -0.5 V (vs. RHE) for HER or 1.0 ~ 1.8 V (vs. RHE) for OER. Polarization curves for OER and HER were tested without iR correction, and compensated manually with the calculated solution resistance derived from impedance spectroscopy at a compensation rate of 85%. AC

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impedance measurements were carried out at overpotential of -50 mV (for HER) or 250 mV (for OER) between frequency ranges from 10 kHz to 0.01 Hz. The data was analyzed and fitted using ZSimpWin software. For the determination of double layer

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capacitance (Cdl), CV scans were performed in the non-Faradaic region at scan rates

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of 10, 30, 50, 70 and 90 mV s-1 (ESI for more information). To test stability, chronopotentiometry method was conducted at fixed currents. 3. Results and discussion 3.1 Catalyst preparation The proposed fabrication approach is schematically illustrated in Figure 1. 3D Ni foam serves as the conductive substrate, providing macro-pore system for efficient mass transfer. During the first step, hydrothermal reaction is conducted to grow 6

ACCEPTED MANUSCRIPT Co3(PO4)2·8H2O nanosheet arrays (CoPO NS) on Ni foam. Then, through an ion exchange reaction with the participance of K3[Fe(CN)6], CoPO NS are conformally transformed into bimetallic Fe/Co Prussian blue analog (FeCo PBA) nanosheets

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arrays. Finally, facile transformation of FeCo PBA to be functional water splitting catalysts can be achieved with different heating recipes. To be specific, FeCo/C NS

NaH2PO2 produce FeCoP/C NS for HER.

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3.1.1 Step I: Hydrothermal synthesis of CoPO NS

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OER catalysts can be obtained by calcination in H2/N2 atmosphere, while heating with

Co3(PO4)2·8H2O is a strongly hydrated compound. It contains chains of metal coordinated octahedra and tetrahedra that form sheets perpendicular to the crystallographic a axis with water layers confined between them (Figure S1) [22].

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Two distinct Co2+ sites can be found: one site is coordinated by four water ligands and two phosphate groups, while the other site is surrounded by two water ligands and four phosphate groups. Such a structure makes the exchange of water ligand relatively

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facile [22]. CoPO NS are firstly grown on Ni foam with a well-defined hydrothermal

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route [21]. Under the optimized reaction condition, petaloid CoPO NS with micrometer size cover the entire Ni foam back bone (Figure 2 a2), in which relatively smooth surface is revealed under high magnifications (Figure 2 a1). The intrinsic crystal structure and morphology characters make CoPO NS to be an ideal precursor for ion exchange reaction and conformal transformation. Previous studies demonstrated that cobalt carbonate hydroxide (CCH) can serve as sacrificial template in the synthesis of ZIF-67 [23] or Fe/Co PBA [24], but its needle-like structure always 7

ACCEPTED MANUSCRIPT leads to ill-developed morphologies even under rigid reaction conditions. 3.1.2 Step II: Synthesis of FeCo PBA through ion exchange Figure 2b-d illustrate the SEM images of FeCo PBA samples obtained from

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CoPO NS with different ion exchange time. The concentration of K3Fe(CN)6 and reaction temperature were kept constant at 0.1 M and 60 oC, respectively. At early stage of ion exchange reaction with 0.1 M K3Fe(CN)6 (0.5 h, Figure 2 b1), small

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crystals are selectively formed on the surface of CoPO NS. As reaction time extended

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to 1.5 h, those small crystals gradually grow into bigger ones (Figure 2 c1) along the lengthwise direction rather than perpendicular to the CoPO NS surface. After reaction for 2.5 h, the surface become relatively smooth again (Figure 2 d1). In general, the overall nanosheet arrays morphology is retained and no decomposition of the

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precursor structure is observed, which is different from the previous studies [24]. Because of the unique crystal structure of Co3(PO4)2·8H2O, Fe(CN)63- can easily get through the interlayers and exchange with water ligands. The newly formed FeCo

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PBA crystal hampers the further transport of Fe(CN)63-, thus it tends to grow along the

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lengthwise direction. Based on these analyses, the entire evolution process is schematically illustrated in the insets of Figure 2 a1-d1. Besides, as shown in Figure S2 and S3, both K3Fe(CN)6 concentration and reaction temperature influence the nucleation process of FeCo PBA. Specifically, the nucleation of FeCo PBA is much slower under lower K3Fe(CN)6 concentration (0.02 M), and FeCo PBA crystals dispersed on the surface of CoPO NS are observed after 2.5 h of heating with 60 oC water bath (Figure S2). In contrast, high K3Fe(CN)6 concentration (0.2 M) is also 8

ACCEPTED MANUSCRIPT disadvantageous for the growth of FeCo PBA, and irregular particles are evolved (Figure S2). Quite similar morphology evolution trend is also observed under different reaction temperatures (Figure S3). Therefore, we speculate that the exchange

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of water ligands by Fe(CN)63- induces the decomposition of CoPO crystal structure, and free Co2+ is released. At the meantime, crystallization of FeCo PBA happen on the CoPO surface under appropriate K3Fe(CN)6 concentration and temperature. The

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degradation of nanosheet architecture is observed only under relatively harsh

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conditions (90 oC, Figure S3 d1-d2), demonstrating the high tolerance of CoPO NS to reaction conditions.

XRD, FT-IR and XPS characterizations were conducted to elucidate the phase and composition information. As shown in Figure 3a, the phase purity of the

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as-obtained CoPO NS was checked by powder XRD, and is indexed to phase pure Co3(PO4)2·8H2O (JCPDS No. 33-0432). The XRD pattern of the resultant FeCo PBA shows the characteristic Prussian blue analogues with a face centered cubic structure

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(Co3[Fe(CN)6]·10H2O, JCPDS No. 46-0907) [25]. However, due to the well-known

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internal redox reaction, two common oxidation states (II, III) for both cobalt and iron are usually coexisted in Cobalt hexacyanoferrate, leading to a multitude of compound stoichiometries [26-29]. Therefore, the stoichiometric formula of the resultant FeCo PBA can only be determined to be K1.2Co[Fe(CN)6] by XPS. This is proved by the smaller cell parameter of the as-obtained FeCo PBA (a = 9.99 Å) as compared with that of Co3[Fe(CN)6]2·10H2O (a = 10.29 Å), since such a variation reflects a contraction of the cobalt coordination sphere accompanying the oxidation state 9

ACCEPTED MANUSCRIPT change from +II to +III [26]. FTIR spectrum of CoPO NS in Figure 3b shows the characteristic stretching vibration bands of P-O in the region of 900-1200 cm-1 and P-O-P in the region of 400 - 900 cm-1 [22, 30, 31]. After ion exchange reaction, an

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intense peak centered at 2105 cm-1 was observed in FeCo PBA (Figure3b), which can be indexed to the stretching of -CN group in the Co-CN-Fe chain [32, 33]. In addition, the characteristic absorption peak of M-C stretching can also be observed at 592 cm-1

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[34]. The C 1s spectrum of adventitious carbon centered at 284.8 eV was used as the

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reference (Figure S4) for XPS analysis, and Survey XPS spectra of CoPO NS shown in Figure S4a reveals the existence of Co, P, O and C elements. The stoichiometric ratio of Co and P is 1.3, close to the theoretical value of Co3(PO4)2·8H2O. High-resolution Co 2p XPS spectrum of CoPO NS illustrates the characteristic Co

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2p3/2 and Co 2p1/2 peaks at binding energies (BE) of 781.3 eV and 797.5 eV, respectively (Figure 3c), which along with their satellite peaks demonstrate the chemical state of Co2+ in CoPO NS [21]. As shown in Figure S4b, a single peak

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assigned to P-O bonding is observed at 133.4 eV [35]. The O 1s spectrum of CoPO

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NS in Figure S4c reveals two peaks at 531.1 eV and 532.8 eV, which corresponds to lattice oxygen in P-O group and the absorbed water, respectively [21]. The survey XPS spectra of FeCo PBA show the coexistence of Fe, Co, O, N, K

and C elements (Figure S5a) with the stoichiometric ratio of K:Co:Fe to be 1.2:1:1. Consistent with previous reports [28, 34], the Fe 2p spectrum in Figure 3d shows the major Fe2+ species with BEs of 708.4 eV (2p3/2) and 721.3 eV (2p1/2), while the minor peak at 712.7 eV with satellite peaks indicated the existence of Fe3+ species, 10

ACCEPTED MANUSCRIPT which is a common phenomenon for PBAs [28, 36, 37]. The Co 2p spectrum of FeCo PBA in Figure 3e shows three characteristic peaks at 781.4 eV (2p3/2), 782.6 eV (2p3/2) and 797.5 eV (2p1/2), indicating the coexistence of +II and +III oxidation

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state of Co [28]. A single peak at 397.9 eV belonging to -CN is observed in the N 1s spectrum in Figure 3f [38]. Beside the C 1s peak at 284.8 eV, the peaks at binding energies of 294.0 and 296.7 eV in Figure S5b can be indexed to K 2p3/2 and K 2p1/2,

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respectively [27]. Two different states of oxygen species were observed in the

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high-resolution O 1s spectra (Figure S5c). The peak at binding energy of 533.5 eV can be assigned to the adsorbed water, while the other with a lower energy (531.9 eV) can be attributed to coordinated water [27]. With the above-mentioned evidences, it can be deduced that Co3(PO4)2·8H2O nanosheets were synthesized with the proposed

reaction.

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hydrothermal route, and conformally transferred to FeCo PBA by ion exchange

3.1.3 Step III: Transformation of FeCo PBA to FeCo/C NS and FeCoP/C NS

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Previous studies have demonstrated that MOFs are ideal precursors for the

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fabrication of structure-diversified materials for energy conversion and storage [14]. In this work, the as obtained FeCo PBA were calcinated with different heating recipes to fabricate integrate electrodes for water splitting. To be specific, FeCo PBA was transformed to FeCo/C NS for OER by being heated in H2/N2 atmosphere, and to FeCoP/C NS for HER by being calcinated with NaH2PO2. To assess the phase transformation during calcination, XRD characterization was conducted. As illustrated in Figure 4a, the diffraction peaks of FeCo PBA disappeared, 11

ACCEPTED MANUSCRIPT and new peaks corresponding to cubic FeCo alloy (JCPDS No. 49-1568) can be observed after hydrogenation treatment. In addition, cubic Co (JCPDS No. 15-0806) was also detected by XRD. Figure S6a shows the survey XPS spectrum of FeCo/C NS,

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which indicates the coexistence of Co, Fe, O, K and C elements. High-resolution Fe 2p XPS spectrum in Figure 4b illustrates the Fe 2p3/2 and Fe 2p1/2 peaks at 708.7 eV and 721.4 eV, respectively, demonstrating the existence of Fe0 [39]. In addition, other

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peaks centered at 712.5 eV (Fe 2p3/2) and 724.9 eV (Fe 2p1/2) indicate the presence

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of Fe3+, possibly resulted from the surface oxidation [40, 41]. Similarly, Co0 species is also evidenced by peak at 778.1 eV in the high-resolution Co 2p spectrum in Figure 4c. Meanwhile, several peaks at higher BEs (i.e. 780.9 eV, 797.0 eV and the corresponding satellites 786.5 eV and 802.7 eV ) are indexed to ionic state Co species

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[42-44]. Consistent with the above-mentioned results, high-resolution O 1s XPS spectrum in Figure 4d suggests three categories of oxygen species: lattice oxygen (529.3 eV), O-vacancies (531.1 eV) and adsorbed water molecules (532.9 eV) [45,

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46]. The deconvoluted high-resolution C1s spectrum in Figure S6b exhibits three

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bands relating to the C-C (284.8 eV), C=N (285.3 eV), and O=C-O (288.6 eV) functional groups [47]. To further probe the generated carbon species, Raman spectroscopy were carried out. As shown in Figure S7, the characteristic peak belonging to the stretching modes of cyanide ligands in FeCo PBA was observed between 2000-2200 cm-1[48]. In addition, the broadening feature of this peak also indicates the co-existence of CoII-NC-FeIII and CoIII-NC-FeII [48]. As expected, the Raman spectrum (Figure S7) of FeCo/C shows prominent peaks at ca. 1326 cm-1 and 12

ACCEPTED MANUSCRIPT ca. 1588 cm-1, which can be assigned to D and G bands of carbon, respectively [49]. The above results prove that slightly surface oxidized FeCo alloy particles composited with carbon sheets (FeCo/C NS) are obtained by hydrogenation treatment of FeCo

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PBA. The XRD pattern of FeCoP/C NS in Figure 4a suggests FeCo PBA is successfully transformed into barringerite Fe2P (JCPSD No. 51-0943), while the

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absence of signals corresponding to Co2P indicates its amorphous nature. XPS

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measurements further disclosed the surface characters of FeCoP/C NS. As shown in Figure S8a, Fe, Co, O, K, C and P elements were recorded in the survey spectrum of FeCoP/C NS. The Fe 2p high resolution XPS spectrum is deconvoluted into two spin−orbit doublets: one doublet at 709.9 eV and 724.5 eV corresponds to Fe2+ and

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another doublet at 712.8 eV and 728.1 eV belongs to Fe3+ (Figure 4b), indicating that ternary Fe-Co-P compound was obtained [50]. Notably, no characteristic signals of Fe2P is observed [51], which might be due to the limited depth that can be probed by

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XPS (several nm). In Figure 4c, two spin−orbit doublets are illustrated in the Co 2p

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high-resolution XPS spectrum. The first one at 779.0 and 793.9 eV is assigned to Co 2p3/2 and Co 2p1/2 in Co2P [52], and the other one at 781.9 and 797.7 eV is indexed to CoOx species derived from surface oxidation of cobalt phosphide [53, 54]. Different from the O 1s spectrum of FeCo/C NS, only two overlapping peaks at 531.7 and 533.3 eV are observed in FeCoP/C NS in Figure 4d, which can be assigned to a combination of orthophosphate and surface carbonate species, respectively [52]. The P 2p high-resolution XPS spectrum illustrated in Figure S8b shows the characteristic 13

ACCEPTED MANUSCRIPT peaks of phosphide (130.8 and 129.8 eV for P 2p1/2 and P 2p3/2, respectively) and oxidized P species (134.5 eV) [50]. Moreover, the M/P (M = Co + Fe) ratio was determined to be ca. 0.21, indicating a P rich nature of the freshly prepared sample. In

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the deconvoluted high-resolution C1s spectrum in Figure S8c, two bands relating to the C-C (284.8 eV), and C=N (285.3 eV) functional groups were observed [47]. Similar to FeCo/C NS, the Raman spectrum of FeCoP/C NS in Figure S7 shows

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similar peaks at ca. 1326 cm-1 and ca. 1588 cm-1, which can also be assigned to D and

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G bands of carbon, respectively [49]. The above analysis proves the successful synthesis of slightly surface-oxidized CoFeP/C composite nanosheets (FeCoP/C NS) by the phosphorization of FeCo PBA.

SEM and TEM were conducted to elucidate the morphology and composition

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characters of the as-obtained FeCo/C NS and FeCoP/C NS. As shown in Figure 5 a1 and a2, a relatively rough surface is observed over FeCo/C NS, but the nanosheets array morphology of FeCo PBA is generally retained after hydrogenation treatment,

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which is quite beneficial to the exposure of active sites. Moreover, numerous of tiny

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spots are found to be evenly dispersed in the nanosheets by TEM (Figure 5 a3), which are mainly FeCo alloy particles as revealed by high-resolution TEM. As illustrated in Figure 5 a4, nanoparticle around 10 nm with clear lattice fringes can be found, and the measured interplanar spacing of 0.195 nm can be indexed to the (110) plane of cubic FeCo alloy. In addition, consistent with the XRD results, small Co nanoparticles with an interplanar spacing of 0.203 nm corresponding to (111) plane of cubic Co can also be observed. These nanoparticles are imbedded in the carbon matrix and surrounded 14

ACCEPTED MANUSCRIPT by amorphous carbon layers. Elemental mapping clearly reveals the uniform distribution of Fe and Co in the nanosheets (Figure S9 a-f), with Fe/Co ratio determined by EDX to be roughly 1:1 (Figure S9g).

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As illustrated by the SEM images of FeCoP/C NS in Figure 5 b1-b2, no prominent morphology changes can be observed after the phosphorization treatment, demonstrating the high durability of the FeCo PBA precursor. As revealed in Figure 5

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b3-b4, tiny nanoparticles with an interplanar spacing of 0.239 nm can be observed,

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which indicates the formation of Fe2P. Beside these sporadic crystallites, prominent amorphous regions are illustrated in the high-resolution TEM images, revealing the poor crystallinity nature of CoFeP/C nanosheets, in accordance with the relatively weak powder XRD diffractions shown in Figure 4a. As shown in Figure S10 a-f,

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elemental mapping reveals the homogeneous distribution of Fe, Co and P elements throughout the nanosheets, suggesting solid solution has been obtained practically [55]. Lastly, similar to that of FeCo/C NS, the Fe/Co ratio determined by EDX is

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roughly 1:1 (Figure S10g).

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In a word, bimetallic FeCo/C NS and FeCoP/C NS are produced through the facile transformation of bimetallic FeCo PBA nanosheet arrays. We speculate FeCo PBA can also serve as a good precursor to produce other nanosheet arrays such as FeCo sulphide and nitride through a similar low temperature sulfurization/nitridation process [56]. 3.2 Electrochemical performance To evaluate the OER and HER catalytic performance, the corresponding 15

ACCEPTED MANUSCRIPT electrodes were directly used as binder-free working electrodes with a fixed geometric area of 1 cm2. All the electrochemical tests were performed in 1.0 M KOH solution, and LSV curves were manually compensated based on the electrochemical impedance

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spectroscopy measurements. As shown in Figure 6a, FeCo/C NS presents a quite low overpotential of 219 mV at the benchmark current density of 10 mA cm-2 in OER process, indicating a superior activity to that of CoPO NS (η10=270 mV) and bare Ni

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foam (η10=372 mV). In addition, the observed catalytic activity also outperforms most

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of recently reported Fe/Co based catalysts, including FeCoOOH PNSAs/CFC (η10=266 mV) [57], CoFe LDHs-Ar/NF (η10=237 mV) [58], a-Co4Fe(OH)x (η10=295 mV) [59], Fe-Co-P hollow sphere (η10=252 mV) [60], and NF@NC-CoFe2O4/C NRAs (η10=240 mV) [61] (Table S1).

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The catalytic kinetics for OER are revealed by the Tafel slopes derived from the corresponding LSV curves. As shown in Figure 6b, the smallest Tafel slope value of 74 mV dec-1 is recorded with FeCo/C NS, demonstrating a much more favorable

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catalytic kinetics. Moreover, according to previous reports, this Tafel slope value

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(close to 60 mV dec-1) might also indicate that the rate determining step of the overall OER reaction on FeCo/C NS is a surface chemical rearrangement step, while the discharge of OH- step is the rate determining step on Ni foam and CoPO NS (Tafel values close to 120 mV dec-1) [62, 63]. Although the calculated double layer capacitance (Cdl) of FeCo/C NS (2.8 mF) in Figure S11 is higher than that of CoPO NS (0.6 mF), the normalized LSV curves (Figure S12a) demonstrate the same catalytic activity trend (i.e. FeCo/C NS > CoPO NS > Ni foam), indicating that the 16

ACCEPTED MANUSCRIPT better performance of FeCo/C NS is mainly due to the higher intrinsic activity. At first, the alloy nature of FeCo nanoparticle and the carbon residues endow the FeCo/C NS composite with superior conductivity. As illustrated in Figure S13, the smallest charge

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transfer resistance of 0.6 Ω for FeCo/C NS was calculated by simulating the experimental data, demonstrating an enhanced charge transfer. Second, as proved by previous reports [60, 64, 65], the introduction of Fe induces the partial charge transfer

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between Fe and Co, resulting in a modified electronic structure of the catalyst. As the

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oxidation peak of Co in FeCo/C NS was greatly suppressed (Figure 6a), a similar phenomenon might also exist in FeCo/C NS, thus exhibits higher intrinsic activity than that of CoPO NS [64].

The HER activity of FeCoP/C NS was also evaluated in 1.0 M KOH solution. By

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taking a carbon rod as counter electrode, the LSV curves were measured with a scan rate of 1 mV s-1. As depicted in Figure 6c, FeCoP/C NS shows remarkable HER catalytic activity with overpotential of -55 mV at -10 mA cm-2, whereas negligible

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activity of CoPO NS (η-10 = -240 mV) and bare Ni foam (η-10 = -254 mV) are revealed

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by LSV plots. Although the observed performance of FeCoP/C NS is slightly inferior to that of commercial PtC in low overpotential range (η-10 = -22 mV), its activity matches or even surpasses that of PtC in higher overpotential range, highlighting the promising industrial usage. Moreover, the HER performance of FeCoP/C NS is also comparable to most recently reported metal phosphides, such as O-Co2P-3 (η-10 = -160 mV) [66], FeP (η-10 = -84 mV) [67], iron phosphide nanotube (η-10 = -120 mV) [68] and (CoP)x–(FeP)1–x NRs/G (η-10 = -97 mV) [69] (Table S2). As shown in Figure 6d, 17

ACCEPTED MANUSCRIPT similar to LSV results, the Tafel slopes of the above-mentioned catalysts follow a trend of PtC (-46 mV dec-1) < FeCoP/C NS (-107 mV dec-1) < bare Ni foam (-142 mV dec-1) < CoPO NS (-155 mV dec-1). Meanwhile, the measured Tafel slope value of

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-107 mV dec-1 indicates that HER over FeCoP/C NS follows the Volmer-Heyrovsky mechanism [70].

The observed magnificent catalytic activity of FeCoP/C NS can be attributed to

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several aspects. Firstly, as demonstrated by previous reports, the binding energy (BE)

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between metal phosphide and H is always lower than that between metal and H, thus facilitating the evolution of H2.[71] Second, incorporation of Fe induces partial charge transfer between Co and Fe in addition to that between Co and P, which leads to enhanced intrinsic catalytic activity of FeCo mixed phosphide (evidenced by the

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normalized LSV curves in Figure S12b) [50, 72, 73]. Third, as demonstrated in Figure S11, the larger Cdl value of 3.5 mF is determined for FeCoP/C NS, indicating a more efficient exposure of active sites in comparison with bare Ni foam (0.9 mF). Lastly,

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quite good conductivity of metal phosphides contributes greatly for improved charge

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transfer efficiency, which is proved by the much smaller charge transfer resistance of FeCoP/C NS (4.7 Ω) in Figure S14. Stability is a pivotal parameter for practical usage purpose, and thus

chronopotentiometry was conducted to investigate the stability of prepared catalysts under corresponding electrochemical reaction conditions. As shown in Figure S15, overpotential of FeCo/C NS level off quickly and maintain almost constant over 20 h, indicating a reasonable durability. In addition, the sample obtained after stability test 18

ACCEPTED MANUSCRIPT was subjected to XPS analysis (Figure S16), and FeCo oxyhydroxide species were detected. This demonstrate that surface reconstruction happened during OER, and the in situ generated FeCo oxyhydroxides should be responsible for the superb activity.

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As shown in Figure S17, the stability of FeCoP/C NS in HER is not as satisfactory as FeCo/C NS in OER, as the overpotential increases gradually to maintain the constant current density, which can be ascribed to the preferential dissolution of P in Co2P

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under alkaline environment. This is proved by the XPS results of the post reaction

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sample. Consistent with the observation of Zhang et al [52], the surface M/P (M = Co+Fe) ratio increased from ca. 0.21 to Ca. 2.2 (Figure S18), which should be responsible for the degradation of the HER catalytic performance. The multicurrent density chronopotentiometry curves in the inset of Figure S15 and Figure S17 both

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reveal a quick response to current changes, further highlighting the potential practical usage of the as-obtained FeCo/C NS and FeCoP/C NS. The application prospect of the as-synthesized FeCo/C NS and FeCoP/C NS in

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overall water splitting was briefly investigated with a standard two-electrode system.

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FeCo/C NS and FeCoP/C NS were directly used as the anode and cathode with a fixed geometric area of 1 cm2, respectively (Figure 7a). After an activation process of 20 CV scan cycles, polarization curves were recorded with a scan rate of 1 mV s-1. As illustrated in Figure 7b, a cell voltage of 1.55 V is sufficient to afford a current density of 10 mA cm-2, surpassing the noble metal-based PtC || PtC couple (Vcell,10 =1.60 V) and among the top of other reports summarized in Table S3. Finally, multi-current chronopotentiometry plots in Figure 7c demonstrate the fast reaction of current 19

ACCEPTED MANUSCRIPT density to potential switch and favorable stability, highlighting the promising practical usage. 4. Conclusions

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In summary, Co3(PO4)2·8H2O nanosheet arrays (CoPO NS) constructed by chains of metal coordinated octahedra and tetrahedra have been proved to be an ideal precusor for the fabrication of FeCo PBA through an ion exchange approach. As a

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proof of concept, the resultant FeCo PBA can be transformed to be FeCo/C NS as

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OER catalyst and FeCoP/C NS as HER catalyst by facile hydrogenation and phosphidation, respectively. With FeCo/C NS and FeCoP/C NS coupled in a two-electrode water splitting electrolyser, the cell voltage is as small as 1.55 V at 10 mA cm-2. Benefiting from the relatively fragile coordination environment and lamellar

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crystal structure, CoPO NS are highly tolerant to the ion exchange reaction conditions and may also be a promising candidate in other energy conversion and storage fields, including sodium-ion batteries, metal-air batteries and fuel cells. Therefore, this work

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is meaningful both in the scope of fundamental research and practical applications.

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Acknowledgements

This research work was financially sponsored by National Natural Science

Foundation of China (Grant No.: 21436003 and 21576032). Zidong Wei supervised the project. Rui Xiang, Yijun Duan, Cheng Tong conceived and designed the experiments. All authors discussed the results and Zidong Wei, Xun Huang, Lishan Peng, Jian Wang, Syed Shoaib Ahmad Shah and Tayyaba Najam co-wrote the paper. The authors declare no competing financial interests. Correspondence and requests for 20

ACCEPTED MANUSCRIPT materials should be addressed to Xun Huang and Zidong Wei. Notes and references [1] J. Chi, H. Yu, Water electrolysis based on renewable energy for hydrogen production,

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Figure caption Figure 1. Schematic illustration of the preparation process of FeCo/C NS and FeCoP/C NS on Ni foam. 30

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Figure 2. SEM images of the transformation process from CoPO NS to FeCo PBA at different interval of ion exchange reaction: 0 h (a1, a2), 0.5 h (b1, b2), 1.5 h (c1, c2)

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and 2.5 h (d1, d2); Inset in a1-d1: Diagrams showing the formation of FeCo PBA depending upon the time of ion exchange reaction. The concentration of K3Fe(CN)6

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and reaction temperature were kept constant at 0.1 M and 60 oC, respectively.

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Figure 3. (a) XRD spectra and (b) FTIR spectra of CoPO NS and FeCo PBA; (c) High-resolution Co 2p spectra of CoPO NS; (d) High-resolution Co 2p, (e) Fe 2p and (f) N 1s spectra of FeCo PBA.

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Figure 4. (a) XRD patterns of FeCo PBA (black), FeCo/C NS (red) and FeCoP/C NS (green); High-resolution Fe 2p (b), Co 2p (c) and O 1s (d) XPS spectra of FeCo/C NS

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(1) and FeCoP/C NS (2).

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Figure 5. SEM images (a1, a2), TEM image (a3), and High-resolution TEM image (a4) of FeCo/C NS; SEM images (b1, b2), TEM image (b3), and High-resolution TEM image (b4) of FeCoP/C NS.

Figure 6. (a) LSV, (b) corresponding Tafel slopes of Bare Ni foam (black), FeCo/C NS (red) and CoPO NS (green) in 1.0 M KOH solution for OER; (c) LSV, (d) corresponding Tafel slopes of Bare Ni foam (black), FeCoP/C NS (red), CoPO NS 31

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Figure 7. (a) digital photo graph of the two-electrode system with FeCo/C NS and

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FeCoP/C NS as the anode and cathode, respectively; (b) polarization curves of FeCo/C NS || FeCoP/C NS (red) and PtC || PtC (black); (c) multi-current

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chronopotentiometry plots of FeCo/C NS || FeCoP/C NS in 1.0 M KOH solution.

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ACCEPTED MANUSCRIPT Highlights: 1. Petaloid self-standing bimetallic FeCo Prussian blue analogue nanosheet array (FeCo PBA) on Ni foam is fabricated through ion exchange reaction.

achieved by hydrogenation and phosphorization.

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2. Facile transformation of FeCo PBA into functional water splitting electrodes is

3. Cost-effective and highly efficient two-electrode water splitting electrolysor with

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a low cell voltage (1.55V) is constructed with the as-obtained FeCo/C NS and

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FeCoP/C NS.