FeP Hierarchical Superstructure

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Energy, Environmental, and Catalysis Applications

Interface Engineering of Co(OH)2/Ag/FeP Hierarchical Superstructure as Efficient and Robust Electrocatalyst for Overall Water Splitting Xiaotong Ding, Yuguo Xia, Qiannan Li, Shun Dong, Xiuling Jiao, and Dairong Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19623 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 9, 2019

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ACS Applied Materials & Interfaces

Interface

Engineering

of

Co(OH)2/Ag/FeP

Hierarchical Superstructure as Efficient and Robust Electrocatalyst for Overall Water Splitting Xiaotong Ding†, Yuguo Xia*‡, Qiannan Li†, Shun Dong†, Xiuling Jiao† and Dairong Chen*† †School

of Chemistry & Chemical Engineering, Shandong University, Jinan 250100, P.

R. China ‡National

Engineering Research Center for Colloidal Materials, Shandong University,

Jinan 250100, P. R. China

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ACS Paragon Plus Environment

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ABSTRACT: Rational design and preparation of electrocatalyst with optimal component and interfaces, which can work well for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline media, is of great importance in practical water splitting. Herein, a multiscale structure surface engineering approach to construct Co(OH)2/Ag/FeP hybrid as efficient electrocatalysis for water splitting in alkaline media is reported. By optimizing the component ratio and engineering interfacial structure, the Co(OH)2/Ag/FeP hybrid eletrocatalyst exhibits promoted HER and OER activity as well as stability in alkaline media, achieving an overpotential of 118 mV and 236 mV at a current density of 10 mA cm-2, respectively. Further experimental characterizations demonstrate the electron structure changes in Co(OH)2/Ag/FeP hybrid after constructing the interfaces, which is benefit to generate low charge state Fe2+ and high-oxidized Co3+/4+. The first-principle calculations reveal the dissociation of H2O at the interface region is energetically favourable, which is responsible for the enhanced HER and OER activity. Furthermore, two-electrode alkaline water electrolyzer constructed by Co(OH)2/Ag/FeP hybrid electrocatalysts only requires a voltage of 1.56 V to afford a current density of 10 mA cm-2, which is superior to the commercial Pt/C-IrO2 catalytic couple and makes it a promising material to be employed as effective bifunctional catalysts for overall water splitting.

KEYWORDS: interface engineering, bifunctional electrocatalysts, electrocatalysis, water splitting, water dissolution

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1. INTRODUCTION Electrochemical water splitting is regarded as a promising and sustainable strategy to solve the continuous growth of energy crises and environmental pollution,1-3 and electrocatalysts are particular vital to the two half-reactions involved in water splitting, that is, the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).4-6 At present, noble metal based catalysts, Pt and IrO2/RuO2 hold the benchmark for HER and OER, respectively.7,8 But the scarcity and high cost greatly impede their scalable utilization.9,10 Besides, integrating HER and OER catalysts in different electrolytes may bring about inferior overall performance or instability due to corrosion or dissolution.11,12 Thus, it is of great necessity to develop non-precious and sustainable bifunctional electrocatalyst that can efficiently catalyze both HER and OER in the same electrolyte. Given the low ionic conductivity in neutral media and the structural instability of OER catalyst in acidic media, developing bifunctional electrocatalysts are mainly focused in alkaline electrolyte.13,14 However, suffering from the sluggish reaction kinetics due to additional water dissociation,15,16 the state-of-the-art electrocatalysts generally reveals decreased HER activity in alkaline media relative to that in acidic media.17,18 Therefore, to find bifunctional electrocatalyst that simultaneously reveals efficient HER and OER performances in alkaline media is still a great challenge. Constructing heterogeneous nanostructures, which reveals synergistically enhanced kinetics relative to their single component due to the increase of active sites and electron redistributed interfaces, is considered to be a feasible approach to prepare bifunctional 3



electrocatalyst.19-21 To date, some recent reports of non-precious bifunctional electrocatalysts constructed by alternative catalytic materials, such as transition metal hydroxides,22 sulfides,23 phosphides,24 and nitrides,25 etc. reveal good catalytic activity for HER or OER. However, only few of them have competitive catalytic property comparing with Pt/C-IrO2 catalytic couple.26,27 In addition, controlling over the morphology of the heterostructures is also required,28,29 and one-dimensional nanostructure is regarded as promising morphology to prepare highly efficient electrocatalyst due to the abundant active sites in a radial dimension and favourable charge transfer rate along the axial direction.30,31 Thus, rational engineering of component and interfaces as well as morphology to guarantee the synergy involved in the heterostructure and achieve good catalytic activity for both HER and OER is prerequisite. Herein, considering the low-cost and efficient characteristics of FeP and Co(OH)2 separately for HER and OER, as well as the good conductivity of Ag, hierarchical Co(OH)2/Ag/FeP heterostructure composed of zero-dimensional Ag nanoparticle, onedimension FeP nanorod and two-dimension Co(OH)2 nanosheet grown on Ti foils is fabricated via a facile route. This hierarchical superstructure not only reveals increased well-exposed active sites but also generates new bonding mode, resulting in a changed electronic and interfacial structure. The optimal Co(OH)2/Ag/FeP heterostructure shows low overpotentials of 118 and 236 mV to reach a current density of 10 mA cm-2 for HER and OER in 1.0 M KOH, respectively. Furthermore, the overall water splitting can be delivered with Co(OH)2/Ag/FeP-Co(OH)2/Ag/FeP hybrid catalytic couple at a 4


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low operation η10 voltage of 1.56 V with approving durability for 50 h, which is superior to the commercial Pt/C-IrO2 couple and most of up-to-date nonprecious electrocatalysts. In addition, synergistic interaction and the reasonable mechanism for the enhanced water-splitting activity are also discussed. 2. RESULTS AND DISCUSSION 2.1. Physicochemical Characterization of the Hybrid Electrocatalyst

Scheme 1. Schematic diagram for the synthetic protocol for Co(OH)2/Ag/FeP hybrid electrocatalyst The synthetic protocol of hierarchical Co(OH)2/Ag/FeP hybrid electrocatalyst on Ti foil is illustrated in Scheme 1. Briefly, FeP nanorod arrays are fabricated according to previous report,32 and Ag nanoparticles are loaded by a modified UV-assisted reduction method.33 Subsequently, ultrathin Co(OH)2 nanosheets are further grown in the surface of Ag/FeP through electrodeposition method.34 As shown in Figure 1a, power X-ray diffraction (pXRD) analysis confirms above phase evolution of synthetic procedures (FeP, JCPDS: 65-2595; Co(OH)2, JCPDS: 46-0605), and no characteristic peaks of impurity are observed. Morphologies of representative samples in the synthetic 5



procedure are investigated by scanning electron microscopy (SEM). As revealed in Figure 1b, FeP nanorod arrays (NAs) reveals flower-like hierarchical morphology, which is consisted of nanorods in a diameter of ca.50-100 nm. The morphology of FeP NAs is nearly unchanged after loading Ag nanoparticles (Figure 1c), while ultrathin Co(OH)2 nanosheets with size in the range of 20-30 nm are uniformly coated on the FeP NAs (Figure 1d). The EDS spectrum as shown in Figure S1 verified the presence of Fe, Co, Ag, P, and O. The optimal nanostructure was obtained by regulating the deposition time of Co(OH)2. With the increase of the deposition time, the content of Co(OH)2 was increasing (Figure S2 and Table S1). HRTEM image (Figure 1e) further confirms the existence of FeP, Ag and Co(OH)2 phase. The lattice spacing of 0.273 nm and 0.289 nm are separately ascribed to the (011) and (002) plane of FeP, which is further confirmed by their dihedral angle, indicating the exposed plane of FeP nanorod is (100) plane (Figure S3). The EDX elemental mapping image (Figure 1f) reveals the spatial elemental distribution in Co(OH)2/Ag/FeP hybrid, in which the signals of Fe and P, Co and O are separately concentrated on the interior zone and outer shell, and no obvious Ag elemental signals are observed because of its low content (ICP-AES result reveals the content of Ag is ca. 6.8 wt%), illustrative of the successful preparation of Co(OH)2/Ag/FeP hybrid hierarchical superstructure.

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Figure 1. (a) pXRD patterns of as-prepared samples, (b-d) SEM images of FeP NAs, Ag/FeP and Co(OH)2/Ag/FeP on Ti foil, (e) HRTEM image and (f) EDX elemental mapping of Co(OH)2/Ag/FeP/Ti. To further demonstrate the surface elemental charge state changes in Co(OH)2/Ag/FeP hybrid, X-ray photoelectron spectroscopy (XPS) is conducted as revealed in Figure 2. The survey spectrum (Figure 2a) for Co(OH)2/Ag/FeP/Ti confirms the presence of Fe, Co, P, O and Ag elements. Typical Fe 2p, Co 2p, P 2p and O 1s peaks are found as shown in Figure 2b-e.35,36 Generally, the position of binding energy for certain atom is affected by its coordination environment or valence state. Herein, the Fe 2p peaks successively shifts to the lower energy while Co 2p, P 2p and O 1s peaks shifts to the higher energy from that in pristine FeP or Co(OH)2 to Co(OH)2/Ag/FeP hybrid, which illustrates the valence state change of these atoms and indicates the strong electron interactions formed among FeP, Ag and Co(OH)2. The increase binding energy of Co 2p Co(OH)2/Ag/FeP indicates the more beneficial to

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generate higher-oxidized Co3+/Co4+ species during OER process.37

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while the decrease

binding energy of Fe 2p in Co(OH)2/Ag/FeP hybrid illustrates the tendency to generate low valence state Fe2+, which is more benefit for adsorption of H+ or H2O molecule.38 In addition, compared with the peak position in Ag/Co(OH)2, the Ag 3d peaks in Co(OH)2/Ag/FeP hybrid also upshift to higher energy, which illustrates that the valence electrons of Ag have tendency to be transferred to FeP. Except for involving in the electronic redistribution process in Co(OH)2/Ag/FeP hybrid, Ag NPs are considered to increase the energy of Co 3d electrons in valence state, resulting in easier conversion from Co2+ to Co3+/4+, which is similar to the effect of Ag NPs in our previous study of Ni(OH)2/Ag/RGO hybrid.33 Overall, strong electron interactions are formed in Co(OH)2/Ag/FeP hybrid and electrons are transferred from Co to Ag then to Fe after charge redistribution.

Figure 2. (a) XPS full spectrum survey for Co(OH)2/Ag/FeP/Ti. Core level XPS spectra comparison of (b) Fe 2p (c) Co 2p (d) P 2p (e) O 1s and (f) Ag 3d.

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2.2. Enhanced Electrocatalytic HER and OER Performance for the Hybrid Electrocatalyst The electrocatalytic performances of Co(OH)2/Ag/FeP/Ti hybrid for HER are investigated in a conventional three-electrode configuration in 1.0 M oxygen-saturated KOH aqueous electrolyte with Pt/C/Ti (loading: 3.5 mg cm-2) as comparisons. Figure 3a reveals the linear sweep voltammetry (LSV) curves of as-prepared electrodes, and Co(OH)2/Ag/FeP/Ti hybrid exhibits a lower onset potential and overpotentials, achieving an overpotential of 118 mV and 338 mV versus RHE at j = -10 mA cm-2 (η10) and j = -100 mA cm-2 (η100) without iR compensation, respectively, which is smaller than that of Ti foil (279 mV), FeP/Ti (202 mV), Ag/FeP/Ti (169 mV) and Co(OH)2/FeP/Ti (157 mV) at η10, indicative of the enhanced electrocatalytic activity for FeP NAs after compositing with Ag nanoparticles and Co(OH)2 nanosheets. In addition, the effects of loading amount of Ag nanoparticles and electrodeposition time of Co(OH)2 nanosheets are also investigated by LSV curves (Figure S4), which reveals that 6.8 wt% and 4 min are the optimal values for the HER activity of Co(OH)2/Ag/FeP/Ti hybrid. The Tafel slopes derived from LSV curves are further calculated to illustrate the kinetic reaction routes, and Co(OH)2/Ag/FeP/Ti hybrid electrode reveals the smallest value of 79 mV dec-1 compared with those of bare Ti foil (265 mV dec-1), FeP/Ti (131 mV dec-1), Ag/FeP/Ti (98 mV dec-1) and Co(OH)2/FeP/Ti (101 mV dec-1). Herein, despite of the inferior HER catalytic property relative to benchmark Pt/C, the as-synthesized Co(OH)2/Ag/FeP/Ti hybrid still reveals better OER catalytic property in alkaline media comparing with other up-to-date nonprecious 9



metal-based electrocatalysts in overpotentials and Tafel slope as revealed in Figure S5. Besides, the HER of Co(OH)2/Ag/FeP/Ti hybrid in alkaline media is considered to undergo Volmer-Heyrovsky pathways, and all of these electrodes comprising FeP NAs exhibits Tafel slopes in the range of 79-129 mV dec-1, approaching to the ideal value for the Volmer step (120 mV dec-1),39 which illustrates the dissociation of H2O molecular on the surface of these electrodes is the rate-limiting step for HER. In order to further illustrate the enhanced HER activity for Co(OH)2/Ag/FeP/Ti hybrid, the turnover frequency (TOF) and mass activity are evaluated as shown in Figure 3c-d in which Co(OH)2/Ag/FeP/Ti hybrid electrodes both reveal better HER activity than other electrodes. Except for the electrocatalytic activity, electrochemical stability is also a crucial index to evaluate its practical application. Here, long-term chronoamperometry and accelerated degradation test (ADT) are adopted as the evaluation methods to investigate the stability of Co(OH)2/Ag/FeP/Ti hybrid. As shown in Figure 3e, HER activity retains nearly unchanged for 50 h of continuous hydrogen release at -240 mV and -320 mV, and no obvious current density decay is observed. Moreover, the representative Co(OH)2/Ag/FeP/Ti hybrid electrocatalyst reveals excellent durability with a small increase in overpotential of only 7 mV and 13 mV at 100 mA cm-2 after 5000 cycles and 10000 cycles from the ADT results, respectively (Figure 3f). The phase and microstructure of the Co(OH)2/Ag/FeP/Ti hybrid after I-t test is also measured as shown in Figure S6, and no structural and morphological changes are observed, indicative of its robust electrochemical stability. In addition, the gas generated during the HER test is measured by gas chromatography to further calculate 10


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the Faradaic efficiency (Figure S7), and the Co(OH)2/Ag/FeP/Ti hybrid approaches 100% in efficiencies, which indicates the observed current is solely consumed to hydrogen production during the HER process.

Figure 3. Electrocatalytic properties of the electrodes for HER. (a) LSV curves (no iRcorrection), (b) Tafel slope, (c) TOF values, (d) mass activities of Co(OH)2/Ag/FeP/Ti, Co(OH)2/FeP/Ti, Ag/FeP/Ti, FeP/Ti, bare Ti and Pt/C, (e) chronoamperometry curves of the Co(OH)2/Ag/FeP/Ti electrode at overpotential of -240 mV and -320 mV, and (f) Polarization curves of Co(OH)2/Ag/FeP/Ti electrode recorded during the ADT test. LSV scan rate: 10 mV s-1. The electrocatalytic OER properties of Co(OH)2/Ag/FeP/Ti hybrid are also investigated in detail considering on its kinetically slow characteristics due to typically higher reaction barrier in the formation of the O−O bond.40 The effect of loading amount of Ag and electrodeposition time on OER property are similarly investigated by LSV curves (Figure S8), which reveals the change of OER activity along with

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above-mentioned parameters is not exclusive and best OER activity is also achieved in the same synthetic conditions as in HER. Figure 4a reveals that Co(OH)2/Ag/FeP/Ti electrode exhibits the lowest onset overpotential and only requires 236 mV versus RHE to achieve current density of 10 mA cm-2, which is superior to that of Co(OH)2/Ti (315 mV), Ag/Co(OH)2/Ti (288 mV), Co(OH)2/FeP/Ti (277 mV), commercial IrO2 (271 mV) and most of the other up-to-date nonprecious metal-based electrocatalysts (Figure S9). Meanwhile, the Co(OH)2/Ag/FeP/Ti exhibits the smallest Tafel slope of 56 mV dec-1, which indicates its improved charge transfer kinetics. In the classic Bulter-Volmer mechanistic kinetic model, for a chemically reversible multistep reactions with a single rate-determining step, the Tafel slope follows the principle of (59 𝑚𝑉 𝑑𝑒𝑐 -1)/(𝑛' + 𝛼), where n′ is the number of single-electron transfer steps prior to the rate-determining step and α is the symmetry/transfer coefficient (typically taken as 0.5).41 Herein, the Tafel slopes of electrodes comprising Co(OH)2 component varying from 118 to 56 mV dec-1 indicate their different OER kinetic mechanism. The TOF and mass activity are further measured as shown in Figure 4c-d which also illustrate the enhanced OER activity in Co(OH)2/Ag/FeP/Ti hybrid. Moreover, to evaluate the electrochemical OER stability of the Co(OH)2/Ag/FeP/Ti electrode, a long-term water oxidation and ADT are conducted. As shown in Figure 4e, Co(OH)2/Ag/FeP/Ti electrode exhibits excellent stability with negligible decrease of current density at 320 mV and 350 mV. The durability of Co(OH)2/Ag/FeP/Ti electrode investigated by ADT also reveals impressive results with only 6 mV and 13 mV increase at 100 mA cm-2 after 5000 and 10000 cycles (Figure 4f). The phase and microstructure of Co(OH)2/Ag/FeP/Ti hybrid 12


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after I-t test is also measured as shown in Figure S10, and no structural and morphological changes are found. The Faradaic efficiency proceeded on Co(OH)2/Ag/FeP/Ti hybrid is calculated to be approaching 100% on the base of oxygen evolution (Figure S11), which illustrates its high electron utilization efficiency in the OER process.

Figure 4. Electrocatalytic properties of the electrodes for OER. (a) LSV curves without iR-correction,

(b)

Tafel

slope,

(c)

TOF

values,

(d)

mass

activities

of

Co(OH)2/Ag/FeP/Ti, Co(OH)2/FeP/Ti, Ag/Co(OH)2/Ti, Co(OH)2/Ti, bare Ti and Pt/C, (e) chronoamperometry curves of the Co(OH)2/Ag/FeP/Ti electrode at overpotential of 320 mV and 350 mV, and (f) Polarization curves of Co(OH)2/Ag/FeP/Ti electrode recorded during the ADT test. LSV scan rate: 10 mV s-1. 2.3. Two-electrode Water Splitting Given the excellent bifunctional electrocatalytic ability for HER and OER of Co(OH)2/Ag/FeP/Ti electrode in alkaline media, a two-electrode water splitting cell is assembled in an alkaline electrolyte using Co(OH)2/Ag/FeP/Ti electrode as both the 13



anode and cathode as revealed in the digital photograph (Figure 5a). Meanwhile, two alkaline electrodes based on commercial Pt/C-IrO2 electrocatalysts is also built for comparison. As shown in Figure 5b, Co(OH)2/Ag/FeP/Ti catalytic couple affords current densities of 10 and 20 mA cm-2 at ca. 1.56 and 1.61 V, respectively, whereas the commercial Pt/C-IrO2 electrodes achieves the same current density at ca. 1.60 and 1.66 V. Besides, Co(OH)2/Ag/FeP/Ti hybrid catalytic couple also affords high output (compared with commercial Pt/C-IrO2 couple) at same applied voltage (1.7 V). Therefore, the catalytic output of the electrolyzer assembled by Co(OH)2/Ag/FeP/Ti hybrid is superior to the commercial noble metal-based Pt/C-IrO2 couple. In addition, compared with other up-to-date bifunctional electrocatalysts, the Co(OH)2/Ag/FeP/Ti hybrid also reveals comparable activity as shown in Figure S12. The electrochemical stability is measured by long-term overall water splitting (Figure 5c), and voltages of 1.70 and 1.87 V are applied to achieve current density of ca.50 and 100 mA cm-2 respectively, which reveals no obvious decay in the 50 h continues operation, illustrative its robust structural stability. In addition, the practical gas evolutions in the cathode and anode are measured by gas chromatography (Figure 5d), and the molar amounts of gas released approach the theoretical values with a hydrogen and oxygen release rates of 0.6373 and 0.3186 mol g-1 h-1, respectively, which illustrates the solely conversion of hydrogen and oxygen in HER and OER, respectively, and high electron utilization efficiency in the overall water splitting process.

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Figure 5. (a) Digital photograph of two-electrode configuration, (b) LSV curves of the electrode pairs for overall water splitting, (c) current density versus time curves at an applied voltage of 1.70 V and 1.86 V, respectively. and (d) the experimental and theoretical gas evolution of Co(OH)2/Ag/FeP/Ti electrode pair versus time Electrolyte: 1M KOH solution; LSV scan rate: 10 mV s-1. Temperature: 25°C. 2.4. Mechanism for the Interface Engineering of the Hybrid Electrocatalyst To illustrate the coupling mechanism involved in the Co(OH)2/Ag/FeP/Ti hybrid for the enhanced HER and OER activity, characterizations related to electrochemical water splitting are further conducted. Generally, electrochemical activity is highly depended on the nature and amount of uncoordinated metal sites.42 Thus, the electrochemical surface areas (ECSA) are measured as shown in Figure 6a. The ECSA values are estimated by the double electric layer capacitances (Cdl) derived from cyclic voltammograms at different scan rate (Figure S13) and the value of Co(OH)2/Ag/FeP/Ti hybrid is larger than other catalysts, demonstrating its higher density of catalytically active sites. The Cdl values of Pt/C/Ti and IrO2/Ti were shown 15



in Figure S14. Of note, the specific activity calculated by ECSA for Co(OH)2/Ag/FeP/Ti hybrid is not the optimum (Figure S15), which indicates the enhanced water splitting activity is not mainly attributed to the active surface areas but the constructed interfaces.43 The electrochemical impedance spectroscopy (EIS) is further measured to illustrate the interfacial properties between the electrode and the electrolyte as shown in Figure 6b. The smallest arc radius of Co(OH)2/Ag/FeP/Ti hybrid in the high-frequency region of the Nyquist plots illustrated its smallest charge transfer resistance as compared with other catalysts. Moreover, room-temperature electron spin resonance (ESR) spectra are further measured to investigate the unpaired electrons involved in as-prepared catalysts to explore the changes of coordination environment of central metal ion, which is highly related to the electrochemical process.44 As shown in Figure 6c, one single Lorentzian line centered at g = 2.02 is detected, which is assigned to unpaired electron of Co3+ ion.45 The ESR intensity of Co(OH)2/Ag/FeP hybrid reveals remarkably increase comparing with those of pristine Co(OH)2 and Co(OH)2/FeP, which illustrates its proportion increase of Co3+, and is consistent with the analysis results of XPS that electrons transferred from Co to Ag then to Fe to create high-oxidized Co3+/4+. Besides, the charge-carrier density (Nd) deduced from the Mott-Schottky plots to evaluate the change of carrier concentration. As shown in Figure 6d and Figure S16, the Nd value for Co(OH)2/Ag/FeP/Ti (5.6×1022) is significantly enhanced compared with that of FeP/Ti (1.4×1021), Ag/FeP/Ti (3.7×1022), Co(OH)2/FeP/Ti (3.3×1022), Co(OH)2/Ti (7.8×1021), and Ag/Co(OH)2/Ti (1.9×1022), which also illustrates a faster electron transfer rate in Co(OH)2/Ag/FeP/Ti hybrid. To 16


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further investigate the effect of constructed interfaces on the electrochemical HER and OER, high resolution XPS spectra are measured to check the difference of FeP/Ti, Co(OH)2/Ti and Co(OH)2/Ag/FeP/Ti after HER or OER cycling for 5000 times, in which the HER tests for FeP and Co(OH)2/Ag/FeP are conducted in the potential range of -0.6 V to 0 V vs RHE, while the OER activities for Co(OH)2 and Co(OH)2/Ag/FeP were measured in the potential range of 1 V to 1.9 V vs RHE. All above electrocatalytic properties are measured with a scan rate of 10 mV s-1. As shown in Figure 6e-f，the Fe 2p3/2 spectrum and Co 2p3/2 spectrum exhibit predominant peak centered at 711.8 eV and 780.3 eV which can be ascribed to the binding energy of Fe in FeOOH and Co in CoOOH,46,47 respectively. No phosphates and Ag oxide generate after OER process as proven by the XPS spectra (Figure S17). Relative to the peak positions of Fe 2p and Co 2p in pristine FeP and Co(OH)2 after HER or OER cycling for 5000 times, the peak positions of Fe 2p and Co 2p in Co(OH)2/Ag/FeP hybrid separately locate in the lower and higher energy regions, which is benefit to generate low charge state Fe2+ and highoxidized Co3+/4+, respectively, demonstrating the enhanced HER and OER activities are caused by the structural and electronic changes in Co(OH)2/Ag/FeP hybrid comparing to pristine FeP and Co(OH)2.

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Figure 6. (a)The current density difference values (Ja-Jc) versus scan rate, (b) Nyquist plots and corresponding equivalent circuit, (c) ESR spectra, (d) Mott-Schottky plots, high resolution XPS spectra of (e) Fe 2p and (f) Co 2p regions after HER and OER cycling for 5000 times. Given the main contribution of the enhanced electrocatalytic activity arising from the construction interface of Co(OH)2/FeP other than Ag composition as proven by HER, OER and overall splitting property (Figure S18) and inaccurate description to practical interface configuration in Co(OH)2/Ag/FeP atomic model (Figure S19) due to the isolated distribution of Ag NPs between FeP and Co(OH)2, herein, only Co(OH)2/FeP interfaces and corresponding Gibbs free energy evolution are discussed. To further clarify the mechanism of the constructed interfaces on the electrochemical HER and OER in alkaline media, chemisorption free energies of hydroxyl ( GOH ) and H2O molecule ( GH O ) on the (100) surface of FeP, (001) surface of Co(OH)2 and the 2

constructed interface of Co(OH)2/FeP are calculated. The optimal chemisorption

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species and corresponding Gibbs energy on FeP, Co(OH)2 and interface of Co(OH)2/FeP heterojunction are revealed in Figure 7a. The preferential configuration for H2O is molecularly adsorbed on the (100) surface of FeP (ΔG = -1.186 eV) instead of dissociating into hydroxyl and Had (ΔG = -0.267 eV, Figure S20), which illustrates the sluggish Volmer reaction acts as the rate-limiting step in FeP catalyzed HER. On the contrary, the optimal configuration for H2O adsorbing on the interface of Co(OH)2/FeP is dissociated into hydroxyl and Had (ΔG = -2.573eV), which exhibits superior binding activity comparing with other absorption configurations (Figure S20), indicating an improved kinetics and enhanced HER/OER processes. In addition, strong orbitals hybridization between Fe and Co species can be found in the density of states (Figure 7b) ranging from -1.5 eV to 0 eV, which indicates the strong interaction between Fe and Co atoms. The hydrated alkaline-metal cations can be stabilized by the surface electron accumulated region of Fe atoms through electrostatic interaction,16 resulting in an enhanced interaction between water and the surface of hybrid. To give a more visual image about the interaction between FeP and Co(OH)2, the difference charge density is plotted as shown in Figure 7c, and reveals a strong charge redistribution appeared at the interface region of Co(OH)2/FeP, which is considered to be the essential reason for the enhanced water splitting in alkaline media.

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Figure 7. (a) Optimal chemisorption configurations and corresponding Gibbs free energy of H2O or hydroxyl adsorbing on the surface of FeP, Co(OH)2 and Co(OH)2/FeP heterostructure; the adsorbing hydroxyls or H2O are labeled by ellipse. (b) density of states of FeP, Co(OH)2 and Co(OH)2/FeP. (c) difference charge density image for the interface of Co(OH)2/FeP; yellow and cyan regions represent electron accumulation and depletion, respectively. According to all the experimental and theoretical results presented above, the reasons for the enhanced water splitting activity as well as durability can be concluded. First, theperformances of electrocatalysts are predominantly dominated by water dissociation and hydrogen binding energy, especially for the HER process. Provided that the catalysts have better capability to dissociate the water molecule adsorbed on the surface and facilitate the chemisorption of both OH- and H* intermediates,48,49 the HER and 20


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OER performances in alkaline media will be greatly improved. Here, the HER and OER initial kinetic step involved in Co(OH)2/FeP is changed due to the formation of interface compared with pristine FeP and Co(OH)2, which results in an acceleration in dissociation of H2O molecular in the interfaces. Second, despite the ECSA is not the dominant factor for the enhanced water splitting activity, the enhanced ECSA in Co(OH)2/FeP is still benefit to the HER/OER processes due to the increased active sites. Third, although Ag nanoparticle does not directly participate the electrochemical reaction, the charge transfer rate and carrier concentration is indeed greatly enhanced. Finally, the decorated Co(OH)2 sheets in the Co(OH)2/Ag/FeP superstructure prevent the oxidation of FeP in alkaline, which makes it a robust electrocatalyst for overall water splitting. 3. CONCLUSIONS In summary, a hierarchical Co(OH)2/Ag/FeP hybrid electrode is rationally designed and prepared by interface engineering strategy. The Ag nanoparticle are uniformed distributed on the surface of FeP nanorod arrays, and the following decorated Co(OH)2 nanosheet provides increased number of catalytically active sites. Besides, the changed electron structure after constructing interfaces involved in Co(OH)2/Ag/FeP hybrid is considered to be benefit to generate low charge state Fe2+ and high-oxidized Co3+/4+, which should be the primary reason for the enhanced electrochemical water splitting activity. In addition, DFT calculations also demonstrate the strong charge redistribution and water dissociation is energetically favorable at the interface region of

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Co(OH)2/Ag/FeP. By evaluating as-prepared Co(OH)2/Ag/FeP hybrid in a twoelectrode system, a cell voltage of only 1.56 V is required to achieve a current density of 10 mA cm-2, superior to most current noble-metal-free electrocatalysts and the commercial Pt/C-IrO2 couple, which makes it a promising material to be employed as effective bifunctional catalysts for overall water splitting. 4. EXPERIMENTAL SECTION 4.1. Chemicals and Materials. Iron chloride hexahydrate (FeCl3·6H2O, ≥99%), cobalt nitrate hexahydrate (Co(NO)3·6H2O, ≥99%) and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. Silver nitrate (AgNO3, ≥99.8%) was purchased from Xilong Chemical Co., Ltd. Sodium hypophosphite (NaH2PO2, ≥99%), 20 wt% Pt/C, Iridium(IV) oxide (IrO2, 99%) and Ti foil were purchased from Alfa Aesar. All chemicals were used as received. The water used in the experiments was ultrapure water. 4.2. Materials Preparation. Synthesis of FeP Nanorods on Ti foil (FeP/Ti): The synthesis of FeP/Ti was processed according to a reported method.32 The sample was prepared and stored for further experiments. Synthesis of Ag/FeP Nanocomposite on Ti foil (Ag/FeP/Ti): Ag nanoparticles were loaded on the FeP nanorods through UV-assisted reduction.33 Typically, a piece of the obtained FeP/Ti plate was immersed in 5 mL of aqueous solution in a sealed bottle and degassed with nitrogen gas for 15 min. Subsequently, the sealed bottle was irradiated

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under 254 nm UV light for 50 min, and then 300 μL of degassed AgNO3 solution (0.0647 mol L-1) was immediately added. After reacted at room temperature for 12 h under dark, the substrate was taken out, washed with ethanol, and dried at 50 °C under vacuum. Synthesis of Co(OH)2/Ag/FeP Heterostructure on Ti foil (Co(OH)2/Ag/FeP/Ti): The Co(OH)2 nanosheets were vertically grown on the surface of Ag/FeP/Ti by electrodeposition.34 The electrodeposition was carried out in a standard three-electrode system, using the obtained Ag/FeP /Ti, a Pt wire, and Ag/AgCl (3 M KCl) reference electrode as the working electrodes, the counter electrode, and the reference electrode. The electrolyte was 6×10-3 M Co(NO)3 aqueous solution (30 mL). The constant potential electrodeposition was carried out at -1.0 V (versus Ag/AgCl) for 1, 2, 3, 4, 5, and 6 min. As contrast, other composites were also prepared through the above procedures. Preparation of 20 wt% Pt/C/Ti and IrO2/Ti: Typically, 20 wt% Pt/C and IrO2 were selected as the benchmark electrocatalyst for the HER and OER. To prepare the electrode, 5 mg catalyst was dispersed in a solution containing 250 μL water, 250 μL alcohol, and 10 μL 5% Nafion by sonication for 30 min. After the sonication, 350 μL of the above suspension was spread on the Ti foil (1 cm×1 cm) and dried at 60 ℃. The catalyst loading was ≈3.5 mg cm-2. 4.3. Characterization. The morphology of samples and energy dispersive spectroscopy were performed from

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SEM (Hitachi-SU8010 field emission scanning electron microscope). HRTEM and elemental mapping investigations were examined by a JEOL JEM-2100 instrument. XRD patterns obtained through a Rigaku D/Max 2200 PC diffractometer with a graphite-monochromatized Cu Kα radiation source (λ = 0.15418 nm). XPS spectra were processed on a Perkin-Elmer PHI X-tool XPS instrument with a monochromatic Al Kα X-ray source, using C 1s (binding energy of 284.8 eV) as a reference. Metal elemental analysis of samples was measured by ICP-AES. The ESR spectra were recorded on a EMXmicro electron spin resonance spectrometer under room temperature. The evolving H2 and O2 were analyzed by gas chromatography (TCD, GC-7620, Agilent Technologies). 4.4. Electrochemical Measurements. All the electrochemical measurements were carried out using a CHI 760E electrochemical workstation (CH Instruments, China) in a standard three-electrode system in a solution of 1 M KOH. Considering the dissolution of Pt in alkaline media which may affect the HER test, carbon rod electrode and Hg/HgO electrode were used as counter and reference electrode, respectively. The potential measured in this study was calibrated to reversible hydrogen electrode (RHE) by the equation ERHE = EHg/HgO +0.098 + 0.059 × pH. The working electrodes were preconditioned by 10 cycles cyclic voltammetry scans from -1.5 to 1.0 V versus Hg/HgO reference electrode at 50 mV s-1. The polarization curves were tested by linear sweep voltammetry at a scan rate of 10 mV s-1.

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4.5. Measurements of Electrochemical Impedance Spectroscopy (EIS) and MottSchottky Plots. EIS spectra and Mott-Schottky plots were also measured using a three-electrode system on CHI 760E electrochemical workstation. EIS spectra were recorded in 1 M KOH solution at open circuit potential with the frequency range from 100000 Hz to 0.1 Hz. Mott-Schottky plots were recorded from 0 to 1.5 V versus Hg/HgO reference electrode with the frequency of 3000 Hz. 4.6. Methods for Faradic Efficiency and TOF Calculation. Details about the calculations of Faradic efficiency and turnover frequency (TOF) were shown below: 𝜂 = (𝑚 × 𝑛 × 𝐹)/(𝐼 × 𝑡) Where η is the Faradic efficiency, m is the actual molar number of H2 or O2, n is the number of reactive electrons, F is Faraday's constant (96485.3 C mol-1), I is the current and t is time. 𝑇𝑂𝐹 = (𝑗 × 𝑆)/(𝑁 × 𝐹 × 𝑛) The values of the TOF (S-1) were calculated from the measured current density j (mA cm-2), the surface area of electrode S (1 cm2), the number of electrons required for per mole of gas (H2 or O2), Faraday's constant F (96485.3 C mol-1), and the moles of metal atoms on the electrode n (mol). 4.7. Theoretical Calculation DFT calculations corrected by on-site Coulomb interaction were carried out using the 25



Vienna ab initio simulation package (VASP).50,51 The exchange-correction function was treated by the Perdew-Burke-Ernzerhof for solid (PBEsol)52 generalized gradient approximation (GGA) and the wave functions were expanded in a plane wave basis with an energy cutoff of 400 eV. The effective U-J values of 3.5 for Co and Fe were introduced to account for the strong on-site Coulomb repulsion of Co and Fe atoms.53 To simulate the catalytic process on the surface, slab models with vacuum thickness larger than 15 Å were constructed. The Brillouin zone was sampled by a Γ-centered method. For all the calculations, the convergence criteria for the electronic and ionic relaxation are 10-5 eV and 0.02 eV/Å, respectively. Considering the conversion of Co(OH)2 to CoOOH during OER process and their structural similarity, the partial surface H atoms in the first layer of Co(OH)2 were removed to simulate CoOOH as in other reference. 53 Given the main phase is still Co(OH)2, the interface structure was still named as Co(OH)2/FeP. The adsorption energy and Gibbs free energy of hydroxyl and H2O on the surface of catalyst were calculated as using following equations. 1 Δ𝐸(𝑂𝐻 ∗ ) = 𝐸(𝑂𝐻 ∗ ) ― 𝐸 ∗ ― (𝐸𝐻2𝑂 ― 𝐸𝐻2) 2 Δ𝐸(𝐻2𝑂 ∗ ) = 𝐸(𝐻2𝑂 ∗ ) ― 𝐸( ∗ ) ― 𝐸𝐻2𝑂 Δ𝐺 = Δ𝐸 + Δ𝑍𝑃𝐸 ― 𝑇Δ𝑆 Where * denoted as the adsorbing substrate. E, ZPE and S were the ground state energies calculated by DFT, zero-point energy and entropy correction values, respectively. Parameters adopted to calculated the ZPE correction were referenced to Nørskov’s work.54 26


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ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website. SEM images of Co(OH)2/Ag/FeP/Ti nanostructures, linear voltammetry curves, Faradic efficiency curves, cyclic voltammograms, specific activity, Gibbs free energy calculation, and ICP analysis. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y.X.) *E-mail: [email protected] (D.C.) ORCID Yuguo Xia: 0000-0002-0405-3439 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant 21271118), Natural Science Foundation of Shandong Province (Grant ZR2016BQ22) and the Taishan Scholars Climbing Program of Shandong Province (tspd20150201). The numerical calculations in this paper have been done on the supercomputing system in the Supercomputing Center, Shandong University, Weihai. REFERENCES (1) Caban-Acevedo, M.; Stone, M. L.; Schmidt, J. R.; Thomas, J. G.; Ding, Q.; Chang, 27



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FeP Hierarchical Superstructure

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