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SILAR deposited iron phosphate as a bifunctional electrocatalyst for efficient water splitting
Accepted Manuscript SILAR deposited iron phosphate as a bifunctional electrocatalyst for efficient water splitting P.T. Babar, A.C. Lokhande, H.J. shim, M.G. Gang, B.S. Pawar, S.M. Pawar, Jin Hyeok Kim PII: DOI: Reference:
S0021-9797(18)31086-5 https://doi.org/10.1016/j.jcis.2018.09.015 YJCIS 24066
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
1 July 2018 2 September 2018 5 September 2018
Please cite this article as: P.T. Babar, A.C. Lokhande, H.J. shim, M.G. Gang, B.S. Pawar, S.M. Pawar, J. Hyeok Kim, SILAR deposited iron phosphate as a bifunctional electrocatalyst for efficient water splitting, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.09.015
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SILAR deposited iron phosphate as a bifunctional electrocatalyst for efficient water splitting a
P. T. Babara, A. C. Lokhandea, H. J. shima, M. G. Gang , B. S. Pawarb, S. M. Pawarb and Jin Hyeok Kima,* a
Optoelectronic Convergence Research Center, Department of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, South Korea
b
Division of Physics and Semiconductor Science, Dongguk University, Seoul 100-715, South Korea
Abstract The development of efficient and earth-abundant electrocatalysts for overall water splitting is important but still challenging. Herein, iron phosphate (FePi) electrode is synthesized using a successive ionic layer deposition and reaction (SILAR) method on a nickel foam substrate at room temperature and is used as a bifunctional electrocatalyst for water splitting. The prepared FePi electrodes show excellent electrocatalytic activity and stability for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The FePi electrode exhibits low overpotential of 230 mV and 157 mV towards the OER and HER, respectively, with superior long-term stability. As a result, an electrolyzer that exploits FePi as both the anode and the cathode is constructed, which requires a cell potential of 1.67 V to deliver a 10 mA cm-2 current density in 1 M KOH solution. The exceptional features of the catalyst lie in its structure and active metal sites, increasing surface area, accelerated electron transport and promoted reaction kinetics. This study may provide a facile and scalable approach to design a highefficiency, earth-abundant electrocatalyst for water splitting. 1
Keywords: electrocatalyst; iron phosphate; SILAR method; water splitting. *Corresponding author E-mail: [email protected] (Jin Hyeok Kim)
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1. Introduction Hydrogen energy is likely to be a promising solution for fossil fuel depletion and environmental pollution [1]. Hydrogen, a carbon-free source with a high energy density and an environmentally friendly nature, can be used as an ideal energy source to replace the traditional role of fossil fuels provided there is an efficient and economical process for hydrogen production [2]. Currently, approximately 96% hydrogen is produced from a methane reforming reaction, which still depends on fossil fuels. In contrast, hydrogen generation from an electrochemical water splitting reaction, when the process is coupled to renewable resources such as solar or wind, can provide a clean method for hydrogen production [3,4]. Therefore, water splitting is one the most attractive and favourable technologies to obtain clean and sustainable energy [5]. Despite this, the implementation of this technology has been hampered because an active electrocatalyst is required to drive the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER), making the process inefficient [6]. Among them, the OER is kinetically more sluggish due to a complex pathway involving OH bond splitting followed by OO and O=O bond formation [7]. Precious metals such as Ru or Ir and Pt are the benchmarks for OER and HER catalysts, however, practical applications of such catalyst are constrained by their high cost and scarcity as well as stability [8]. These limitations have encouraged the development of earth-abundant transition metal-based materials including hydroxide, oxide, nitride, phosphide and chalcogenide electrocatalysts for the OER and HER [9–11]. Thus, an economical substitute for the high-efficiency OER and HER electrocatalysts is needed for practical applications. Additionally, electrocatalyst durability is another important parameter because a direct electrochemical reaction on an electrocatalyst may cause the degradation of the catalyst itself [12]. 3
Currently, transition metal phosphate/ Phosphide (TMP) have been used as a new alternative non-noble metal catalyst for water splitting, whose performance was reported to be as proficient as noble metals [13]. TMP can be described as a doping of P atoms into a crystal lattice of transition metals (Fe, Co, Ni, Cu, and Mo) [14]. Among the various transition metals, Fe is economical with a high crust abundance and has low toxicity [15,16]. To date, remarkable progress has been made regarding the use of iron phosphate as HER electrocatalyst, however, its electrocatalytic performance towards the OER has been rarely investigated [17,18]. The most common chemical method for the synthesis of TMPs requires a high temperature (300 to 700 °C), an organic solvent and a long reaction time [19]. For this purpose, a scalable fabrication process for an electrocatalyst at room temperature must be established. The successive ionic layer deposition and reaction (SILAR) method enables the rapid synthesis of phosphides at room temperature. The deposition of thin films by the SILAR method has a number of advantages including (i) simple, quick, economical and suitable large area depositions, (ii) easily controllable deposition rates and thicknesses, (ii) absence of sophisticated instruments and (iv) room temperature operation [20]. In this manuscript, iron phosphate (labeled as FePi) supported on a nickel foam (NF) is synthesized using SILAR method at room temperature and shows excellent electrocatalytic properties towards the OER and HER in an alkaline solution along with good durability. Specifically, the FePi electrode achieves low overpotentials of 230 and 157 mv for OER and HER respectively, to reach 10 mA cm-2 current density. When alkaline electrolyzers are fabricated based on FePi electrocatalysts, a voltage of 1.67 V at 10 mA cm-2 current density is demonstrated. 2. Experimental Section 4
2.1 Preparation of three-dimensional porous FePi The FePi catalytic electrode was fabricated by a simple, low cost, and rapid SILAR process at room temperature. Prior to deposition, a piece of NF (1 x 4 cm2) was first cleaned with 1 M HCl, followed by successive sonication in ethanol, acetone, and distilled water to remove surface contaminants and then dried in an air atmosphere. In a typical synthesis of FePi/NF, 0.1 M iron chloride (FeCl3·6H2O), and 0.1 M sodium hypophosphite (NaH2PO4·H2O) were used and dissolved separately in double distilled water to form cation and anion solutions, respectively. In the present work, the FePi films were deposited on NF substrate by alternate dipping of the NF substrate in cation and anion precursor solutions for 10 s. After 25 cycles, the obtained NF substrate coated with FePi was rinsed with DI water and then dried in air. The obtained FePi on NF substrate is referred as FePi/NF. For a meaningful comparison, Fe(OH)2/NF was prepared using the same procedure except for the phosphate precursor solution. 2.2 Measurement of structural and electrochemical properties The crystal structure of the FePi/NF electrode was analysed by X-ray diffraction (XRD, X’Pert PRO, Philips, Eindhoven, Netherlands) operated at 40 kV and 30 mA with Ni-filtered Cu Kα radiation [k=1.54056 Å]. The chemical states of the films were investigated using X-ray photoelectron spectroscopy (XPS, VG Multi lab 2000, Thermo VG Scientific, U.K.) with a monochromatic Mg Kα (1253.6 eV) radiation source. The surface morphologies of the films were examined by field-emission scanning electron microscopy (FE-SEM, S4800, HITACHI Inc.) operating at 10 kV and 20 mA, and the compositional distribution of the elements were examined by energy dispersive X-ray spectroscopy (EDX). All the electrochemical properties of the films were investigated with a WonATech, WMPG1000 multichannel potentiostat in a threeelectrode system. A typical three-electrode system was equipped with the as-prepared FePi as the 5
working electrode, platinum as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. Polarization curves were obtained by linear sweep voltammetry (LSV) with a scan rate of 5 mV s1 in a 1 M KOH solution. All the potentials were measured against and converted to a reversible hydrogen electrode (RHE) according to Nernst equation [21]:
The overpotentials (ɳ) were calculated based on the formula: . The turnover frequency (TOF) values were calculated by assuming that all the metal atoms are involved in the electrolysis [22]
where j (mA cm-2) is the current density at a fixed overpotential, S is the surface area, 4 is the constant that means 4 electrons per mole of O2, F is Faraday’s constant (96485.3 C mol-1) and n is a mole of coated metal atoms on the electrode, which is calculated from catalyst loading m, the molecular weight of the coated catalyst. 3. Results and Discussion The XRD pattern of the FePi/NF electrode is shown in Figure S1a (Supporting Information). The results indicate the absence of a peak pertaining to FePi/NF, implying that the FePi/NF film is amorphous in nature. The XRD pattern shows only three diffraction peaks at 44.5 °, 55.8 °, and 76.4 ° derived from the NF (indicated by #). We also checked the XRD pattern of the FePi powder (Figure S1b). This pattern shows the absence of diffraction peaks, indicating the amorphous structure of FePi. Figure 1 shows the surface morphologies of the SILAR deposited FePi on the NF substrate. The FE-SEM images revealed that the NF substrate
6
to be completely and uniformly covered by FePi and shows a rippled porous nanosheets structure and these nanosheets were interconnected to each other (Figure 1b). The detailed morphology of FePi revealed by transmission electron microscopy (TEM) analysis. In accordance with FE-SEM images, nanosheets like morphology observed in Figure 1c. The
FePi/NF nanosheets are
interconnected to each other to form porous architecture. The high-resolution TEM image (HRTEM) show not clearly lattice fringes for FePi/NF, further confirming FePi/NF is amorphous in nature (Figure 1d). Elemental mapping was used to determine the uniformity of the elements, and the result shows the uniform distributions of Fe and P in the film (Figure 1 (e-h)). The chemical state of the surface of the film was investigated by X-ray photoelectron spectroscopy (XPS) analysis, and the results are shown in Figure 2. The XPS survey spectrum in Figure 2a reveals the existence of Fe, P, and O in the FePi/NF catalyst. The deconvolution of the Fe 2p spectrum (Figure 2b) shows the two peaks at approximately 711.3 eV and 724.8 eV, corresponding to Fe 2p3/2 and Fe 2p1/2 with their satellite peaks, respectively, which can be assigned to the Fe3+ state [23,24]. In the XPS spectrum of P (Figure 2c), the peak centred at approximately 133 eV can be attributed to phosphate [25]. As shown in Figure 3d, the O 1s spectrum peak at a binding energy 530.8 eV can also be attributed to the phosphate species [26]. Additionally, we also check the XPS spectra of Fe(OH) 2/NF (Figure S5). The electrocatalytic activities of all electrodes were evaluated using a standard threeelectrode cell in a 1 M KOH electrolyte. The OER performance of FePi with different SILAR cycles was also studied and shown in Figure S2, 25 cycles (labeled as FePi/NF) exhibiting better OER activity. Figure 3a shows the polarization curves of all the electrodes at a scan rate of 5 mV s-1. The linear sweep voltammetry (LSV) curves revealed that FePi/NF exhibits a higher current density with a smaller overpotential than that of Fe(OH)2/NF, RuO2/NF and bare NF. The 7
FePi/NF electrode could achieve an OER overpotential as low as 230 mV compared with that of Fe(OH)2/NF (263 mV), RuO2/NF (270 mV)and bare NF (400 mV) at a current density of 10 mA cm-2. The FePi/NF electrode showed a much lower overpotential suggesting that the OER activity of FePi/NF is much better than that of the other catalyst. A Tafel slope is an essential parameter for the evaluation of the reaction kinetics, which relates to the effect of an overpotential on the current density. As shown in Figure 3b, the Tafel slope of the FePi/NF electrocatalyst is 70 mV dec-1, which is much smaller than that of Fe(OH)2/NF (80 mV dec-1), RuO2/NF (84 mV dec-1) and bare NF (105 mV dec-1), suggesting superior OER kinetics for the FePi/NF electrode. The small Tafel slope and lower overpotential of FePi/NF are comparable to other reported catalysts in the literature (Table S1). Durability is also an important criterion to assess the performance of the catalyst in practical applications. Herein, the long-term electrochemical durability of FePi/NF for the electrolysis of water oxidation was measured at a constant current density of 10 mA cm-2 (Figure 3c). As shown in Figure 3d, the FePi/NF electrode displays a negligible change in OER polarization curve after 24 h of continuous operation, indicating the good durability towards the OER. The robust nature of FePi on NF substrate was indicated by an FE-SEM analysis of the catalyst after the 24 h durability test in Figure S3. The SEM images indicate that morphology undergoes substantial aggregation during the continuous evolution of gas for 24 h. The electrocatalytic activity of the catalyst towards the HER was also assessed in a 1 M KOH electrolyte solution. Figure 4a shows the HER polarization curves for FePi/NF, Fe(OH)2/NF, Pt/NF and bare NF. As shown in Figure 4a, the bare NF shows weak electrocatalytic activity towards the HER, and FePi/NF exhibits a higher HER activity. Generally, Pt/NF (45 mV) exhibits higher HER activity. Impressively, FePi/NF can deliver a catalytic 8
current density of -10 mA cm-2 at an overpotential of 157 mV. In contrast, Fe(OH)2/NF and bare NF require higher overpotentials of 196, 275 mV respectively, to achieve the same current density. Furthermore, FePi/NF also exhibited the smaller Tafel slope (47 mV dec-1) than those of Fe(OH)2/NF (68 mV dec-1), and bare NF (95 mV dec-1), indicating more favourable catalytic kinetics for HER on FePi/NF (Figure 4b). The FePi/NF electrode exhibits good catalytic properties with other reported HER catalysts (Table S2). Additionally, long-term HER durability test was also conducted at a constant current density of -10 mA cm-2 for 24 h (Figure 4c). The FePi/NF electrode achieved a stable current density for 24 h and retained 90% of its catalytic activity. The overpotential was negatively shifted (5 mV), even after the 24 h durability test, suggesting the superior stability of FePi/NF during HER process (Figure 4d). Inspired by the high catalytic activity of FePi/NF towards the OER and HER in 1 M KOH, we then assembled a two-electrode configuration using FePi/NF as both the positive and negative electrodes for full water splitting. This bifunctional characteristic of FePi/NF is demonstrated, and corresponding results are shown in Figure 5a. The FePi/NF catalyst achieved a current density of 10 mA cm-2 at a cell voltage of 1.67 V (Figure 5a). This performance is comparable to or outperforms the non-precious metal electrolyzers based on FePi nanotubes (1.69 V) [27], Ni5P4 (1.7 V) [28], and NESS/P (1.74 V) [29]. A chronopotentiometric durability test was also performed to confirm the long-term stability of FePi/NF at a current density of 10 mA cm-2 for 24 h. As seen in Figure 5b, the FePi/NF based water splitting system shows excellent stability with negligible loss occurs in the static current. It is well known that the electrochemical water splitting process involves three major steps: i) the adsorption of water molecules onto the surface of the electrode; ii) the water oxidation and reduction on the active sites and iii) the evolution of gas products [30]. In 9
consideration of the above results, we would like to discuss the causes of the superior catalytic activity of FePi/NF. FePi/NF nanosheets have a porous structure and offer a large surface area and expose more active sites, which are favorable for water molecule adsorption. The effective electrochemical active surface area (ECSA) can be obtained by the following equation [31]: (1) where Cs is the specific capacitance in an alkaline electrolyte (Cs = 0.040 mF cm-2) and Cdl is the electrochemical double layer capacitance, measured using cyclic voltammetry curves (CV) at different scan rates (Figure S4). Figure 6a shows a plot of current density versus scan rate, whereby the linear slope is Cdl. The ECSA values of FePi/NF, and Fe(OH)2/NF were 122.5 cm-2 and 102.5 cm2 respectively. The high ECSA value of FePi/NF indicates a higher surface area and higher catalytic activity. During the second step of water oxidation and reduction, the hierarchical nanostructure provides more access to the active sites, and the high electrical conductivity facilitates fast electron transport, which eases the reaction kinetics [32]. This viewpoint is substantiated by electrochemical impedance spectroscopy (EIS). The Nyquist plot obtained for all the films is shown in Figure 6b, and the inset shows the corresponding fitted equivalent circuit. The Nyquist plot further proves that FePi/NF shows the lowest real axis intercept value (0.92 Ω), indicating the lowest charge transfer resistance and diffusion of the electrolyte within the electrode [33]. The low charge transfer resistance and diffusion of the electrolyte within the FePi/NF electrode reveal much faster electron transport and better catalytic kinetics [34]. Additionally, the catalytic activity of all the electrodes in terms of turnover frequency (TOF) value can be derived from LSV curve for the OER (see experimental section). The TOF is an indicator of the intrinsic activity for a catalyst, which refers to the rate of electron delivery per surface metal atom per second [35]. The TOF values of FePi/NF and Fe(OH)2/NF 10
are 0.031 s-1 and 0.022 s-1 respectively at an overpotential of 320 mV. Apart from low cost, and good robustness the FePi catalyst showed high catalytic activity because i) FePi enables electron transfer and lowers the charge transfer resistance; ii) the porous structure greatly facilitates more active sites and close contact with the electrolyte; iii) the amorphous materials are more active than their crystalline form and shows better catalytic activity [21]; iv) NF offers a high surface area, excellent stability, a high conductivity and a porous structure for the easy detachment of gas bubbles generated during water splitting without damaging the electrocatalyst [36]. 4. Conclusion In summary, the NF supported FePi was prepared via a simple and fast SILAR method at room temperature. The optimized FePi electrode can be used as an efficient and durable electrocatalyst for the OER and HER. Small overpotentials of 230 mV and 157 mV for the OER and HER, respectively, is needed to reach a current density of 10 mA cm-2. As a result, using the FePi electrode in a two-electrode electrolyzer, a cell voltage of 1.67 is achieved at a current density of 10 mA cm-2. The excellent electrocatalytic activity of FePi on NF can be attributed to the 3D binder-free nanostructure with a high number of active sites, an efficient electron transfer process, and high mechanical stability. This work provides an alternative approach to design and explores cost-effective bifunctional electrodes for OER and HER electrocatalysis. Acknowledgment This work was supported by the Human Resources Development program (No. 20164030201310) Of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea government Ministry of Trade, Industry and Energy and supported by supported by the Technology Development Program to solve climate changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (2016936784). 11
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References [1]
M.S. Dresselhaus, I.L. Thomas, Alternative energy technologies, Nature. 414 (2001) 332– 337. doi:10.1038/35104599.
[2]
T.E. Mallouk, Divide and conquer, Nat. Chem. 5 (2013) 362–363. doi:10.1038/nchem.1634.
[3]
Y. Cheng, S.P. Jiang, Advances in electrocatalysts for oxygen evolution reaction of water electrolysis-from metal oxides to carbon nanotubes, Prog. Nat. Sci. Mater. Int. 25 (2015) 545–553. doi:10.1016/J.PNSC.2015.11.008.
[4]
X. Zou, Y. Zhang, Noble metal-free hydrogen evolution catalysts for water splitting, Chem. Soc. Rev. 44 (2015) 5148–5180. doi:10.1039/C4CS00448E.
[5]
E. Fabbri, A. Habereder, K. Waltar, R. Kötz, T.J. Schmidt, Developments and perspectives of oxide-based catalysts for the oxygen evolution reaction, Catal. Sci. Technol. 4 (2014) 3800–3821. doi:10.1039/C4CY00669K.
[6]
L. Zhang, L. Xie, M. Ma, F. Qu, G. Du, A.M. Asiri, L. Chen, X. Sun, Co-based nanowire films as complementary hydrogen- and oxygen-evolving electrocatalysts in neutral electrolyte, Catal. Sci. Technol. 7 (2017) 2689–2694. doi:10.1039/C7CY00703E.
[7]
B. Zhang, X. Zheng, O. Voznyy, R. Comin, M. Bajdich, M. García-Melchor, L. Han, J. Xu, M. Liu, L. Zheng, F.P. García de Arquer, C.T. Dinh, F. Fan, M. Yuan, E. Yassitepe, N. Chen, T. Regier, P. Liu, Y. Li, P. De Luna, A. Janmohamed, H.L. Xin, H. Yang, A. Vojvodic, E.H. Sargent, Homogeneously dispersed multimetal oxygen-evolving catalysts., Science. 352 (2016) 333–7. doi:10.1126/science.aaf1525.
[8]
Y. Jiao, Y. Zheng, M. Jaroniec, S.Z. Qiao, Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions., Chem. Soc. Rev. 44 (2015) 2060–86. 13
doi:10.1039/c4cs00470a. [9]
X.Y. Yu, X.W. David Lou, Mixed Metal Sulfides for Electrochemical Energy Storage and Conversion, Adv. Energy Mater. 8 (2018) 1701592. doi:10.1002/aenm.201701592.
[10] Y. Shi, B. Zhang, Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction, Chem. Soc. Rev. 45 (2016) 1529–1541. doi:10.1039/C5CS00434A. [11] Y. Zhang, Q. Zhou, J. Zhu, Q. Yan, S.X. Dou, W. Sun, Nanostructured Metal Chalcogenides for Energy Storage and Electrocatalysis, Adv. Funct. Mater. 27 (2017) 1702317. doi:10.1002/adfm.201702317. [12] X. Zhou, Z. Xia, Z. Zhang, Y. Ma, Y. Qu, One-step synthesis of multi-walled carbon nanotubes/ultra-thin Ni(OH) 2 nanoplate composite as efficient catalysts for water oxidation, J. Mater. Chem. A. 2 (2014) 11799–11806. doi:10.1039/C4TA01952K. [13] D. Xiong, X. Wang, W. Li, L. Liu, Facile synthesis of iron phosphide nanorods for efficient and durable electrochemical oxygen evolution, Chem. Commun. 52 (2016) 8711–8714. doi:10.1039/C6CC04151E. [14] H. Zhao, Z.-Y. Yuan, Transition metal–phosphorus-based materials for electrocatalytic energy conversion reactions, Catal. Sci. Technol. 7 (2017) 330–347. doi:10.1039/C6CY01719C. [15] J. Saha, T.P. Radhakrishnan, Soft Chemical Fabrication of Iron-Based Thin Film Electrocatalyst for Water Oxidation under Neutral pH and Structure–Activity Tuning by Cerium Incorporation, Langmuir. 33 (2017) 8372–8382. doi:10.1021/acs.langmuir.7b01647. [16] P.T. Babar, B.S. Pawar, A.C. Lokhande, M.G. Gang, J.S. Jang, M.P. Suryawanshi, S.M. 14
Pawar, J.H. Kim, Annealing temperature dependent catalytic water oxidation activity of iron oxyhydroxide thin films, J. Energy Chem. 26 (2017) 757–761. doi:10.1016/J.JECHEM.2017.04.012. [17] P. Xiao, W. Chen, X. Wang, A Review of Phosphide-Based Materials for Electrocatalytic Hydrogen Evolution, Adv. Energy Mater. 5 (2015) 1500985. doi:10.1002/aenm.201500985. [18] J.F. Callejas, C.G. Read, C.W. Roske, N.S. Lewis, R.E. Schaak, Synthesis, Characterization, and Properties of Metal Phosphide Catalysts for the Hydrogen-Evolution Reaction, Chem. Mater. 28 (2016) 6017–6044. doi:10.1021/acs.chemmater.6b02148. [19] H. Liang, A.N. Gandi, C. Xia, M.N. Hedhili, D.H. Anjum, U. Schwingenschlögl, H.N. Alshareef, Amorphous NiFe-OH/NiFeP Electrocatalyst Fabricated at Low Temperature for Water Oxidation Applications, ACS Energy Lett. 2 (2017) 1035–1042. doi:10.1021/acsenergylett.7b00206. [20] H.M. Pathan, C.D. Lokhande, Deposition of metal chalcogenide thin films by successive ionic layer adsorption and reaction (SILAR) method, Bull. Mater. Sci. 27 (2004) 85–111. doi:10.1007/BF02708491. [21] P. Babar, A. Lokhande, H.H. Shin, B. Pawar, M.G. Gang, S. Pawar, J.H. Kim, Cobalt Iron Hydroxide as a Precious Metal-Free Bifunctional Electrocatalyst for Efficient Overall Water Splitting, Small. 14 (2018) 1702568. doi:10.1002/smll.201702568. [22] M.-R. Gao, X. Cao, Q. Gao, Y.-F. Xu, Y.-R. Zheng, J. Jiang, S.-H. Yu, Nitrogen-Doped Graphene Supported CoSe 2 Nanobelt Composite Catalyst for Efficient Water Oxidation, ACS Nano. 8 (2014) 3970–3978. doi:10.1021/nn500880v. [23] A.P. Grosvenor, B.A. Kobe, M.C. Biesinger, N.S. McIntyre, Investigation of multiplet 15
splitting of Fe 2p XPS spectra and bonding in iron compounds, Surf. Interface Anal. 36 (2004) 1564–1574. doi:10.1002/sia.1984. [24] B. Lee, C. Kim, Y. Park, T.-G. Kim, B. Park, Nanostructured Platinum/Iron Phosphate Thin-Film Electrodes for Methanol Oxidation, Electrochem. Solid-State Lett. 9 (2006) E27. doi:10.1149/1.2256688. [25] D. Yang, Z. Lu, X. Rui, X. Huang, H. Li, J. Zhu, W. Zhang, Y.M. Lam, H.H. Hng, H. Zhang, Q. Yan, Synthesis of Two-Dimensional Transition-Metal Phosphates with Highly Ordered Mesoporous Structures for Lithium-Ion Battery Applications, Angew. Chemie Int. Ed. 53 (2014) 9352–9355. doi:10.1002/anie.201404615. [26] D. Zhong, L. Liu, D. Li, C. Wei, Q. Wang, G. Hao, Q. Zhao, J. Li, Facile and fast fabrication of iron-phosphate supported on nickel foam as a highly efficient and stable oxygen evolution catalyst, J. Mater. Chem. A. 5 (2017) 18627–18633. doi:10.1039/C7TA05580C. [27] Y. Yan, B.Y. Xia, X. Ge, Z. Liu, A. Fisher, X. Wang, A Flexible Electrode Based on Iron Phosphide Nanotubes for Overall Water Splitting, Chem. - A Eur. J. 21 (2015) 18062– 18067. doi:10.1002/chem.201503777. [28] M. Ledendecker, S. Krick Calderón, C. Papp, H.-P. Steinrück, M. Antonietti, M. Shalom, The Synthesis of Nanostructured Ni 5 P 4 Films and their Use as a Non-Noble Bifunctional Electrocatalyst for Full Water Splitting, Angew. Chemie Int. Ed. 54 (2015) 12361–12365. doi:10.1002/anie.201502438. [29] M.-S. Balogun, W. Qiu, Y. Huang, H. Yang, R. Xu, W. Zhao, G.-R. Li, H. Ji, Y. Tong, Cost-Effective Alkaline Water Electrolysis Based on Nitrogen- and Phosphorus-Doped Self-Supportive Electrocatalysts, Adv. Mater. 29 (2017) 1702095. 16
doi:10.1002/adma.201702095. [30] J. Jiang, A. Zhang, L. Li, L. Ai, Nickel–cobalt layered double hydroxide nanosheets as high-performance electrocatalyst for oxygen evolution reaction, J. Power Sources. 278 (2015) 445–451. doi:10.1016/J.JPOWSOUR.2014.12.085. [31] P.T. Babar, A.C. Lokhande, B.S. Pawar, M.G. Gang, E. Jo, C. Go, M.P. Suryawanshi, S.M. Pawar, J.H. Kim, Electrocatalytic performance evaluation of cobalt hydroxide and cobalt oxide thin films for oxygen evolution reaction, Appl. Surf. Sci. 427 (2018) 253–259. doi:10.1016/J.APSUSC.2017.07.142. [32] M.-Q. Yang, J. Dan, S.J. Pennycook, X. Lu, H. Zhu, Q.-H. Xu, H.J. Fan, G.W. Ho, Ultrathin nickel boron oxide nanosheets assembled vertically on graphene: a new hybrid 2D material for enhanced photo/electro-catalysis, Mater. Horizons. 4 (2017) 885–894. doi:10.1039/C7MH00314E. [33] S.M. Pawar, B.S. Pawar, B. Hou, J. Kim, A.T. Aqueel Ahmed, H.S. Chavan, Y. Jo, S. Cho, A.I. Inamdar, J.L. Gunjakar, H. Kim, S. Cha, H. Im, Self-assembled two-dimensional copper oxide nanosheet bundles as an efficient oxygen evolution reaction (OER) electrocatalyst for water splitting applications, J. Mater. Chem. A. 5 (2017) 12747–12751. doi:10.1039/C7TA02835K. [34] J. Wang, C.F. Tan, T. Zhu, G.W. Ho, Topotactic Consolidation of Monocrystalline CoZn Hydroxides for Advanced Oxygen Evolution Electrodes, Angew. Chemie Int. Ed. 55 (2016) 10326–10330. doi:10.1002/anie.201605096. [35] H. Wu, X. Lu, G. Zheng, G.W. Ho, Topotactic Engineering of Ultrathin 2D Nonlayered Nickel Selenides for Full Water Electrolysis, Adv. Energy Mater. 8 (2018) 1702704. doi:10.1002/aenm.201702704. 17
[36] P.T. Babar, A.C. Lokhande, M.G. Gang, B.S. Pawar, S.M. Pawar, J.H. Kim, Thermally oxidized porous NiO as an efficient oxygen evolution reaction (OER) electrocatalyst for electrochemical water splitting application, J. Ind. Eng. Chem. 60 (2018) 493–497. doi:10.1016/J.JIEC.2017.11.037.
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Figure Captions Figure 1 FE-SEM images of the (a) bare NF , (b) FePi/NF film at different magnifications, (c) TEM image of FePi/NF, d) HR-TEM image of the FePi/NF and (e-h) the elemental mappings confirming the uniform distributions of Fe and P on NF substrate. Figure 2(a) High-resolution XPS survey spectrum of FePi/NF and the (b) Fe 2p, (c) spectra of P 2p and (d) O 1s spectra of the FePi/NF film. Figure 3(a) OER LSV polarization curves, (b) OER Tafel plots, (c) long-term stability of FePi/NF at 10 mA cm-2 for 24 h and (d) OER polarization curve of FePi/NF before and after stabilization. Figure 4(a) HER LSV polarization curves, (b) HER Tafel plots, (c) long-term stability of FePi/NF at -10 mA cm-2 for 24 h and (d) HER polarization curve before and after stabilization. Figure 5(a) Polarization curve obtained in a two electrode configuration. (b) Long-term stability test at 10 mA cm-2 for the overall water splitting performance of the bifunctional FePi/NF electrocatalyst (inset polarization curve after stabilization). Figure 6(a) Plot of the capacitive current densities versus scan rate of the FePi/NF and Fe(OH)2/NF electrodes and (b) the EIS spectra of FePi/NF, and Fe(OH)2/NF, inset is an equivalent circuit used for fitting Nyquist plots.
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S0021-9797(18)31086-5 https://doi.org/10.1016/j.jcis.2018.09.015 YJCIS 24066
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
1 July 2018 2 September 2018 5 September 2018
Please cite this article as: P.T. Babar, A.C. Lokhande, H.J. shim, M.G. Gang, B.S. Pawar, S.M. Pawar, J. Hyeok Kim, SILAR deposited iron phosphate as a bifunctional electrocatalyst for efficient water splitting, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.09.015
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SILAR deposited iron phosphate as a bifunctional electrocatalyst for efficient water splitting a
P. T. Babara, A. C. Lokhandea, H. J. shima, M. G. Gang , B. S. Pawarb, S. M. Pawarb and Jin Hyeok Kima,* a
Optoelectronic Convergence Research Center, Department of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, South Korea
b
Division of Physics and Semiconductor Science, Dongguk University, Seoul 100-715, South Korea
Abstract The development of efficient and earth-abundant electrocatalysts for overall water splitting is important but still challenging. Herein, iron phosphate (FePi) electrode is synthesized using a successive ionic layer deposition and reaction (SILAR) method on a nickel foam substrate at room temperature and is used as a bifunctional electrocatalyst for water splitting. The prepared FePi electrodes show excellent electrocatalytic activity and stability for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The FePi electrode exhibits low overpotential of 230 mV and 157 mV towards the OER and HER, respectively, with superior long-term stability. As a result, an electrolyzer that exploits FePi as both the anode and the cathode is constructed, which requires a cell potential of 1.67 V to deliver a 10 mA cm-2 current density in 1 M KOH solution. The exceptional features of the catalyst lie in its structure and active metal sites, increasing surface area, accelerated electron transport and promoted reaction kinetics. This study may provide a facile and scalable approach to design a highefficiency, earth-abundant electrocatalyst for water splitting. 1
Keywords: electrocatalyst; iron phosphate; SILAR method; water splitting. *Corresponding author E-mail: [email protected] (Jin Hyeok Kim)
2
1. Introduction Hydrogen energy is likely to be a promising solution for fossil fuel depletion and environmental pollution [1]. Hydrogen, a carbon-free source with a high energy density and an environmentally friendly nature, can be used as an ideal energy source to replace the traditional role of fossil fuels provided there is an efficient and economical process for hydrogen production [2]. Currently, approximately 96% hydrogen is produced from a methane reforming reaction, which still depends on fossil fuels. In contrast, hydrogen generation from an electrochemical water splitting reaction, when the process is coupled to renewable resources such as solar or wind, can provide a clean method for hydrogen production [3,4]. Therefore, water splitting is one the most attractive and favourable technologies to obtain clean and sustainable energy [5]. Despite this, the implementation of this technology has been hampered because an active electrocatalyst is required to drive the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER), making the process inefficient [6]. Among them, the OER is kinetically more sluggish due to a complex pathway involving OH bond splitting followed by OO and O=O bond formation [7]. Precious metals such as Ru or Ir and Pt are the benchmarks for OER and HER catalysts, however, practical applications of such catalyst are constrained by their high cost and scarcity as well as stability [8]. These limitations have encouraged the development of earth-abundant transition metal-based materials including hydroxide, oxide, nitride, phosphide and chalcogenide electrocatalysts for the OER and HER [9–11]. Thus, an economical substitute for the high-efficiency OER and HER electrocatalysts is needed for practical applications. Additionally, electrocatalyst durability is another important parameter because a direct electrochemical reaction on an electrocatalyst may cause the degradation of the catalyst itself [12]. 3
Currently, transition metal phosphate/ Phosphide (TMP) have been used as a new alternative non-noble metal catalyst for water splitting, whose performance was reported to be as proficient as noble metals [13]. TMP can be described as a doping of P atoms into a crystal lattice of transition metals (Fe, Co, Ni, Cu, and Mo) [14]. Among the various transition metals, Fe is economical with a high crust abundance and has low toxicity [15,16]. To date, remarkable progress has been made regarding the use of iron phosphate as HER electrocatalyst, however, its electrocatalytic performance towards the OER has been rarely investigated [17,18]. The most common chemical method for the synthesis of TMPs requires a high temperature (300 to 700 °C), an organic solvent and a long reaction time [19]. For this purpose, a scalable fabrication process for an electrocatalyst at room temperature must be established. The successive ionic layer deposition and reaction (SILAR) method enables the rapid synthesis of phosphides at room temperature. The deposition of thin films by the SILAR method has a number of advantages including (i) simple, quick, economical and suitable large area depositions, (ii) easily controllable deposition rates and thicknesses, (ii) absence of sophisticated instruments and (iv) room temperature operation [20]. In this manuscript, iron phosphate (labeled as FePi) supported on a nickel foam (NF) is synthesized using SILAR method at room temperature and shows excellent electrocatalytic properties towards the OER and HER in an alkaline solution along with good durability. Specifically, the FePi electrode achieves low overpotentials of 230 and 157 mv for OER and HER respectively, to reach 10 mA cm-2 current density. When alkaline electrolyzers are fabricated based on FePi electrocatalysts, a voltage of 1.67 V at 10 mA cm-2 current density is demonstrated. 2. Experimental Section 4
2.1 Preparation of three-dimensional porous FePi The FePi catalytic electrode was fabricated by a simple, low cost, and rapid SILAR process at room temperature. Prior to deposition, a piece of NF (1 x 4 cm2) was first cleaned with 1 M HCl, followed by successive sonication in ethanol, acetone, and distilled water to remove surface contaminants and then dried in an air atmosphere. In a typical synthesis of FePi/NF, 0.1 M iron chloride (FeCl3·6H2O), and 0.1 M sodium hypophosphite (NaH2PO4·H2O) were used and dissolved separately in double distilled water to form cation and anion solutions, respectively. In the present work, the FePi films were deposited on NF substrate by alternate dipping of the NF substrate in cation and anion precursor solutions for 10 s. After 25 cycles, the obtained NF substrate coated with FePi was rinsed with DI water and then dried in air. The obtained FePi on NF substrate is referred as FePi/NF. For a meaningful comparison, Fe(OH)2/NF was prepared using the same procedure except for the phosphate precursor solution. 2.2 Measurement of structural and electrochemical properties The crystal structure of the FePi/NF electrode was analysed by X-ray diffraction (XRD, X’Pert PRO, Philips, Eindhoven, Netherlands) operated at 40 kV and 30 mA with Ni-filtered Cu Kα radiation [k=1.54056 Å]. The chemical states of the films were investigated using X-ray photoelectron spectroscopy (XPS, VG Multi lab 2000, Thermo VG Scientific, U.K.) with a monochromatic Mg Kα (1253.6 eV) radiation source. The surface morphologies of the films were examined by field-emission scanning electron microscopy (FE-SEM, S4800, HITACHI Inc.) operating at 10 kV and 20 mA, and the compositional distribution of the elements were examined by energy dispersive X-ray spectroscopy (EDX). All the electrochemical properties of the films were investigated with a WonATech, WMPG1000 multichannel potentiostat in a threeelectrode system. A typical three-electrode system was equipped with the as-prepared FePi as the 5
working electrode, platinum as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. Polarization curves were obtained by linear sweep voltammetry (LSV) with a scan rate of 5 mV s1 in a 1 M KOH solution. All the potentials were measured against and converted to a reversible hydrogen electrode (RHE) according to Nernst equation [21]:
The overpotentials (ɳ) were calculated based on the formula: . The turnover frequency (TOF) values were calculated by assuming that all the metal atoms are involved in the electrolysis [22]
where j (mA cm-2) is the current density at a fixed overpotential, S is the surface area, 4 is the constant that means 4 electrons per mole of O2, F is Faraday’s constant (96485.3 C mol-1) and n is a mole of coated metal atoms on the electrode, which is calculated from catalyst loading m, the molecular weight of the coated catalyst. 3. Results and Discussion The XRD pattern of the FePi/NF electrode is shown in Figure S1a (Supporting Information). The results indicate the absence of a peak pertaining to FePi/NF, implying that the FePi/NF film is amorphous in nature. The XRD pattern shows only three diffraction peaks at 44.5 °, 55.8 °, and 76.4 ° derived from the NF (indicated by #). We also checked the XRD pattern of the FePi powder (Figure S1b). This pattern shows the absence of diffraction peaks, indicating the amorphous structure of FePi. Figure 1 shows the surface morphologies of the SILAR deposited FePi on the NF substrate. The FE-SEM images revealed that the NF substrate
6
to be completely and uniformly covered by FePi and shows a rippled porous nanosheets structure and these nanosheets were interconnected to each other (Figure 1b). The detailed morphology of FePi revealed by transmission electron microscopy (TEM) analysis. In accordance with FE-SEM images, nanosheets like morphology observed in Figure 1c. The
FePi/NF nanosheets are
interconnected to each other to form porous architecture. The high-resolution TEM image (HRTEM) show not clearly lattice fringes for FePi/NF, further confirming FePi/NF is amorphous in nature (Figure 1d). Elemental mapping was used to determine the uniformity of the elements, and the result shows the uniform distributions of Fe and P in the film (Figure 1 (e-h)). The chemical state of the surface of the film was investigated by X-ray photoelectron spectroscopy (XPS) analysis, and the results are shown in Figure 2. The XPS survey spectrum in Figure 2a reveals the existence of Fe, P, and O in the FePi/NF catalyst. The deconvolution of the Fe 2p spectrum (Figure 2b) shows the two peaks at approximately 711.3 eV and 724.8 eV, corresponding to Fe 2p3/2 and Fe 2p1/2 with their satellite peaks, respectively, which can be assigned to the Fe3+ state [23,24]. In the XPS spectrum of P (Figure 2c), the peak centred at approximately 133 eV can be attributed to phosphate [25]. As shown in Figure 3d, the O 1s spectrum peak at a binding energy 530.8 eV can also be attributed to the phosphate species [26]. Additionally, we also check the XPS spectra of Fe(OH) 2/NF (Figure S5). The electrocatalytic activities of all electrodes were evaluated using a standard threeelectrode cell in a 1 M KOH electrolyte. The OER performance of FePi with different SILAR cycles was also studied and shown in Figure S2, 25 cycles (labeled as FePi/NF) exhibiting better OER activity. Figure 3a shows the polarization curves of all the electrodes at a scan rate of 5 mV s-1. The linear sweep voltammetry (LSV) curves revealed that FePi/NF exhibits a higher current density with a smaller overpotential than that of Fe(OH)2/NF, RuO2/NF and bare NF. The 7
FePi/NF electrode could achieve an OER overpotential as low as 230 mV compared with that of Fe(OH)2/NF (263 mV), RuO2/NF (270 mV)and bare NF (400 mV) at a current density of 10 mA cm-2. The FePi/NF electrode showed a much lower overpotential suggesting that the OER activity of FePi/NF is much better than that of the other catalyst. A Tafel slope is an essential parameter for the evaluation of the reaction kinetics, which relates to the effect of an overpotential on the current density. As shown in Figure 3b, the Tafel slope of the FePi/NF electrocatalyst is 70 mV dec-1, which is much smaller than that of Fe(OH)2/NF (80 mV dec-1), RuO2/NF (84 mV dec-1) and bare NF (105 mV dec-1), suggesting superior OER kinetics for the FePi/NF electrode. The small Tafel slope and lower overpotential of FePi/NF are comparable to other reported catalysts in the literature (Table S1). Durability is also an important criterion to assess the performance of the catalyst in practical applications. Herein, the long-term electrochemical durability of FePi/NF for the electrolysis of water oxidation was measured at a constant current density of 10 mA cm-2 (Figure 3c). As shown in Figure 3d, the FePi/NF electrode displays a negligible change in OER polarization curve after 24 h of continuous operation, indicating the good durability towards the OER. The robust nature of FePi on NF substrate was indicated by an FE-SEM analysis of the catalyst after the 24 h durability test in Figure S3. The SEM images indicate that morphology undergoes substantial aggregation during the continuous evolution of gas for 24 h. The electrocatalytic activity of the catalyst towards the HER was also assessed in a 1 M KOH electrolyte solution. Figure 4a shows the HER polarization curves for FePi/NF, Fe(OH)2/NF, Pt/NF and bare NF. As shown in Figure 4a, the bare NF shows weak electrocatalytic activity towards the HER, and FePi/NF exhibits a higher HER activity. Generally, Pt/NF (45 mV) exhibits higher HER activity. Impressively, FePi/NF can deliver a catalytic 8
current density of -10 mA cm-2 at an overpotential of 157 mV. In contrast, Fe(OH)2/NF and bare NF require higher overpotentials of 196, 275 mV respectively, to achieve the same current density. Furthermore, FePi/NF also exhibited the smaller Tafel slope (47 mV dec-1) than those of Fe(OH)2/NF (68 mV dec-1), and bare NF (95 mV dec-1), indicating more favourable catalytic kinetics for HER on FePi/NF (Figure 4b). The FePi/NF electrode exhibits good catalytic properties with other reported HER catalysts (Table S2). Additionally, long-term HER durability test was also conducted at a constant current density of -10 mA cm-2 for 24 h (Figure 4c). The FePi/NF electrode achieved a stable current density for 24 h and retained 90% of its catalytic activity. The overpotential was negatively shifted (5 mV), even after the 24 h durability test, suggesting the superior stability of FePi/NF during HER process (Figure 4d). Inspired by the high catalytic activity of FePi/NF towards the OER and HER in 1 M KOH, we then assembled a two-electrode configuration using FePi/NF as both the positive and negative electrodes for full water splitting. This bifunctional characteristic of FePi/NF is demonstrated, and corresponding results are shown in Figure 5a. The FePi/NF catalyst achieved a current density of 10 mA cm-2 at a cell voltage of 1.67 V (Figure 5a). This performance is comparable to or outperforms the non-precious metal electrolyzers based on FePi nanotubes (1.69 V) [27], Ni5P4 (1.7 V) [28], and NESS/P (1.74 V) [29]. A chronopotentiometric durability test was also performed to confirm the long-term stability of FePi/NF at a current density of 10 mA cm-2 for 24 h. As seen in Figure 5b, the FePi/NF based water splitting system shows excellent stability with negligible loss occurs in the static current. It is well known that the electrochemical water splitting process involves three major steps: i) the adsorption of water molecules onto the surface of the electrode; ii) the water oxidation and reduction on the active sites and iii) the evolution of gas products [30]. In 9
consideration of the above results, we would like to discuss the causes of the superior catalytic activity of FePi/NF. FePi/NF nanosheets have a porous structure and offer a large surface area and expose more active sites, which are favorable for water molecule adsorption. The effective electrochemical active surface area (ECSA) can be obtained by the following equation [31]: (1) where Cs is the specific capacitance in an alkaline electrolyte (Cs = 0.040 mF cm-2) and Cdl is the electrochemical double layer capacitance, measured using cyclic voltammetry curves (CV) at different scan rates (Figure S4). Figure 6a shows a plot of current density versus scan rate, whereby the linear slope is Cdl. The ECSA values of FePi/NF, and Fe(OH)2/NF were 122.5 cm-2 and 102.5 cm2 respectively. The high ECSA value of FePi/NF indicates a higher surface area and higher catalytic activity. During the second step of water oxidation and reduction, the hierarchical nanostructure provides more access to the active sites, and the high electrical conductivity facilitates fast electron transport, which eases the reaction kinetics [32]. This viewpoint is substantiated by electrochemical impedance spectroscopy (EIS). The Nyquist plot obtained for all the films is shown in Figure 6b, and the inset shows the corresponding fitted equivalent circuit. The Nyquist plot further proves that FePi/NF shows the lowest real axis intercept value (0.92 Ω), indicating the lowest charge transfer resistance and diffusion of the electrolyte within the electrode [33]. The low charge transfer resistance and diffusion of the electrolyte within the FePi/NF electrode reveal much faster electron transport and better catalytic kinetics [34]. Additionally, the catalytic activity of all the electrodes in terms of turnover frequency (TOF) value can be derived from LSV curve for the OER (see experimental section). The TOF is an indicator of the intrinsic activity for a catalyst, which refers to the rate of electron delivery per surface metal atom per second [35]. The TOF values of FePi/NF and Fe(OH)2/NF 10
are 0.031 s-1 and 0.022 s-1 respectively at an overpotential of 320 mV. Apart from low cost, and good robustness the FePi catalyst showed high catalytic activity because i) FePi enables electron transfer and lowers the charge transfer resistance; ii) the porous structure greatly facilitates more active sites and close contact with the electrolyte; iii) the amorphous materials are more active than their crystalline form and shows better catalytic activity [21]; iv) NF offers a high surface area, excellent stability, a high conductivity and a porous structure for the easy detachment of gas bubbles generated during water splitting without damaging the electrocatalyst [36]. 4. Conclusion In summary, the NF supported FePi was prepared via a simple and fast SILAR method at room temperature. The optimized FePi electrode can be used as an efficient and durable electrocatalyst for the OER and HER. Small overpotentials of 230 mV and 157 mV for the OER and HER, respectively, is needed to reach a current density of 10 mA cm-2. As a result, using the FePi electrode in a two-electrode electrolyzer, a cell voltage of 1.67 is achieved at a current density of 10 mA cm-2. The excellent electrocatalytic activity of FePi on NF can be attributed to the 3D binder-free nanostructure with a high number of active sites, an efficient electron transfer process, and high mechanical stability. This work provides an alternative approach to design and explores cost-effective bifunctional electrodes for OER and HER electrocatalysis. Acknowledgment This work was supported by the Human Resources Development program (No. 20164030201310) Of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea government Ministry of Trade, Industry and Energy and supported by supported by the Technology Development Program to solve climate changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (2016936784). 11
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References [1]
M.S. Dresselhaus, I.L. Thomas, Alternative energy technologies, Nature. 414 (2001) 332– 337. doi:10.1038/35104599.
[2]
T.E. Mallouk, Divide and conquer, Nat. Chem. 5 (2013) 362–363. doi:10.1038/nchem.1634.
[3]
Y. Cheng, S.P. Jiang, Advances in electrocatalysts for oxygen evolution reaction of water electrolysis-from metal oxides to carbon nanotubes, Prog. Nat. Sci. Mater. Int. 25 (2015) 545–553. doi:10.1016/J.PNSC.2015.11.008.
[4]
X. Zou, Y. Zhang, Noble metal-free hydrogen evolution catalysts for water splitting, Chem. Soc. Rev. 44 (2015) 5148–5180. doi:10.1039/C4CS00448E.
[5]
E. Fabbri, A. Habereder, K. Waltar, R. Kötz, T.J. Schmidt, Developments and perspectives of oxide-based catalysts for the oxygen evolution reaction, Catal. Sci. Technol. 4 (2014) 3800–3821. doi:10.1039/C4CY00669K.
[6]
L. Zhang, L. Xie, M. Ma, F. Qu, G. Du, A.M. Asiri, L. Chen, X. Sun, Co-based nanowire films as complementary hydrogen- and oxygen-evolving electrocatalysts in neutral electrolyte, Catal. Sci. Technol. 7 (2017) 2689–2694. doi:10.1039/C7CY00703E.
[7]
B. Zhang, X. Zheng, O. Voznyy, R. Comin, M. Bajdich, M. García-Melchor, L. Han, J. Xu, M. Liu, L. Zheng, F.P. García de Arquer, C.T. Dinh, F. Fan, M. Yuan, E. Yassitepe, N. Chen, T. Regier, P. Liu, Y. Li, P. De Luna, A. Janmohamed, H.L. Xin, H. Yang, A. Vojvodic, E.H. Sargent, Homogeneously dispersed multimetal oxygen-evolving catalysts., Science. 352 (2016) 333–7. doi:10.1126/science.aaf1525.
[8]
Y. Jiao, Y. Zheng, M. Jaroniec, S.Z. Qiao, Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions., Chem. Soc. Rev. 44 (2015) 2060–86. 13
doi:10.1039/c4cs00470a. [9]
X.Y. Yu, X.W. David Lou, Mixed Metal Sulfides for Electrochemical Energy Storage and Conversion, Adv. Energy Mater. 8 (2018) 1701592. doi:10.1002/aenm.201701592.
[10] Y. Shi, B. Zhang, Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction, Chem. Soc. Rev. 45 (2016) 1529–1541. doi:10.1039/C5CS00434A. [11] Y. Zhang, Q. Zhou, J. Zhu, Q. Yan, S.X. Dou, W. Sun, Nanostructured Metal Chalcogenides for Energy Storage and Electrocatalysis, Adv. Funct. Mater. 27 (2017) 1702317. doi:10.1002/adfm.201702317. [12] X. Zhou, Z. Xia, Z. Zhang, Y. Ma, Y. Qu, One-step synthesis of multi-walled carbon nanotubes/ultra-thin Ni(OH) 2 nanoplate composite as efficient catalysts for water oxidation, J. Mater. Chem. A. 2 (2014) 11799–11806. doi:10.1039/C4TA01952K. [13] D. Xiong, X. Wang, W. Li, L. Liu, Facile synthesis of iron phosphide nanorods for efficient and durable electrochemical oxygen evolution, Chem. Commun. 52 (2016) 8711–8714. doi:10.1039/C6CC04151E. [14] H. Zhao, Z.-Y. Yuan, Transition metal–phosphorus-based materials for electrocatalytic energy conversion reactions, Catal. Sci. Technol. 7 (2017) 330–347. doi:10.1039/C6CY01719C. [15] J. Saha, T.P. Radhakrishnan, Soft Chemical Fabrication of Iron-Based Thin Film Electrocatalyst for Water Oxidation under Neutral pH and Structure–Activity Tuning by Cerium Incorporation, Langmuir. 33 (2017) 8372–8382. doi:10.1021/acs.langmuir.7b01647. [16] P.T. Babar, B.S. Pawar, A.C. Lokhande, M.G. Gang, J.S. Jang, M.P. Suryawanshi, S.M. 14
Pawar, J.H. Kim, Annealing temperature dependent catalytic water oxidation activity of iron oxyhydroxide thin films, J. Energy Chem. 26 (2017) 757–761. doi:10.1016/J.JECHEM.2017.04.012. [17] P. Xiao, W. Chen, X. Wang, A Review of Phosphide-Based Materials for Electrocatalytic Hydrogen Evolution, Adv. Energy Mater. 5 (2015) 1500985. doi:10.1002/aenm.201500985. [18] J.F. Callejas, C.G. Read, C.W. Roske, N.S. Lewis, R.E. Schaak, Synthesis, Characterization, and Properties of Metal Phosphide Catalysts for the Hydrogen-Evolution Reaction, Chem. Mater. 28 (2016) 6017–6044. doi:10.1021/acs.chemmater.6b02148. [19] H. Liang, A.N. Gandi, C. Xia, M.N. Hedhili, D.H. Anjum, U. Schwingenschlögl, H.N. Alshareef, Amorphous NiFe-OH/NiFeP Electrocatalyst Fabricated at Low Temperature for Water Oxidation Applications, ACS Energy Lett. 2 (2017) 1035–1042. doi:10.1021/acsenergylett.7b00206. [20] H.M. Pathan, C.D. Lokhande, Deposition of metal chalcogenide thin films by successive ionic layer adsorption and reaction (SILAR) method, Bull. Mater. Sci. 27 (2004) 85–111. doi:10.1007/BF02708491. [21] P. Babar, A. Lokhande, H.H. Shin, B. Pawar, M.G. Gang, S. Pawar, J.H. Kim, Cobalt Iron Hydroxide as a Precious Metal-Free Bifunctional Electrocatalyst for Efficient Overall Water Splitting, Small. 14 (2018) 1702568. doi:10.1002/smll.201702568. [22] M.-R. Gao, X. Cao, Q. Gao, Y.-F. Xu, Y.-R. Zheng, J. Jiang, S.-H. Yu, Nitrogen-Doped Graphene Supported CoSe 2 Nanobelt Composite Catalyst for Efficient Water Oxidation, ACS Nano. 8 (2014) 3970–3978. doi:10.1021/nn500880v. [23] A.P. Grosvenor, B.A. Kobe, M.C. Biesinger, N.S. McIntyre, Investigation of multiplet 15
splitting of Fe 2p XPS spectra and bonding in iron compounds, Surf. Interface Anal. 36 (2004) 1564–1574. doi:10.1002/sia.1984. [24] B. Lee, C. Kim, Y. Park, T.-G. Kim, B. Park, Nanostructured Platinum/Iron Phosphate Thin-Film Electrodes for Methanol Oxidation, Electrochem. Solid-State Lett. 9 (2006) E27. doi:10.1149/1.2256688. [25] D. Yang, Z. Lu, X. Rui, X. Huang, H. Li, J. Zhu, W. Zhang, Y.M. Lam, H.H. Hng, H. Zhang, Q. Yan, Synthesis of Two-Dimensional Transition-Metal Phosphates with Highly Ordered Mesoporous Structures for Lithium-Ion Battery Applications, Angew. Chemie Int. Ed. 53 (2014) 9352–9355. doi:10.1002/anie.201404615. [26] D. Zhong, L. Liu, D. Li, C. Wei, Q. Wang, G. Hao, Q. Zhao, J. Li, Facile and fast fabrication of iron-phosphate supported on nickel foam as a highly efficient and stable oxygen evolution catalyst, J. Mater. Chem. A. 5 (2017) 18627–18633. doi:10.1039/C7TA05580C. [27] Y. Yan, B.Y. Xia, X. Ge, Z. Liu, A. Fisher, X. Wang, A Flexible Electrode Based on Iron Phosphide Nanotubes for Overall Water Splitting, Chem. - A Eur. J. 21 (2015) 18062– 18067. doi:10.1002/chem.201503777. [28] M. Ledendecker, S. Krick Calderón, C. Papp, H.-P. Steinrück, M. Antonietti, M. Shalom, The Synthesis of Nanostructured Ni 5 P 4 Films and their Use as a Non-Noble Bifunctional Electrocatalyst for Full Water Splitting, Angew. Chemie Int. Ed. 54 (2015) 12361–12365. doi:10.1002/anie.201502438. [29] M.-S. Balogun, W. Qiu, Y. Huang, H. Yang, R. Xu, W. Zhao, G.-R. Li, H. Ji, Y. Tong, Cost-Effective Alkaline Water Electrolysis Based on Nitrogen- and Phosphorus-Doped Self-Supportive Electrocatalysts, Adv. Mater. 29 (2017) 1702095. 16
doi:10.1002/adma.201702095. [30] J. Jiang, A. Zhang, L. Li, L. Ai, Nickel–cobalt layered double hydroxide nanosheets as high-performance electrocatalyst for oxygen evolution reaction, J. Power Sources. 278 (2015) 445–451. doi:10.1016/J.JPOWSOUR.2014.12.085. [31] P.T. Babar, A.C. Lokhande, B.S. Pawar, M.G. Gang, E. Jo, C. Go, M.P. Suryawanshi, S.M. Pawar, J.H. Kim, Electrocatalytic performance evaluation of cobalt hydroxide and cobalt oxide thin films for oxygen evolution reaction, Appl. Surf. Sci. 427 (2018) 253–259. doi:10.1016/J.APSUSC.2017.07.142. [32] M.-Q. Yang, J. Dan, S.J. Pennycook, X. Lu, H. Zhu, Q.-H. Xu, H.J. Fan, G.W. Ho, Ultrathin nickel boron oxide nanosheets assembled vertically on graphene: a new hybrid 2D material for enhanced photo/electro-catalysis, Mater. Horizons. 4 (2017) 885–894. doi:10.1039/C7MH00314E. [33] S.M. Pawar, B.S. Pawar, B. Hou, J. Kim, A.T. Aqueel Ahmed, H.S. Chavan, Y. Jo, S. Cho, A.I. Inamdar, J.L. Gunjakar, H. Kim, S. Cha, H. Im, Self-assembled two-dimensional copper oxide nanosheet bundles as an efficient oxygen evolution reaction (OER) electrocatalyst for water splitting applications, J. Mater. Chem. A. 5 (2017) 12747–12751. doi:10.1039/C7TA02835K. [34] J. Wang, C.F. Tan, T. Zhu, G.W. Ho, Topotactic Consolidation of Monocrystalline CoZn Hydroxides for Advanced Oxygen Evolution Electrodes, Angew. Chemie Int. Ed. 55 (2016) 10326–10330. doi:10.1002/anie.201605096. [35] H. Wu, X. Lu, G. Zheng, G.W. Ho, Topotactic Engineering of Ultrathin 2D Nonlayered Nickel Selenides for Full Water Electrolysis, Adv. Energy Mater. 8 (2018) 1702704. doi:10.1002/aenm.201702704. 17
[36] P.T. Babar, A.C. Lokhande, M.G. Gang, B.S. Pawar, S.M. Pawar, J.H. Kim, Thermally oxidized porous NiO as an efficient oxygen evolution reaction (OER) electrocatalyst for electrochemical water splitting application, J. Ind. Eng. Chem. 60 (2018) 493–497. doi:10.1016/J.JIEC.2017.11.037.
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Figure Captions Figure 1 FE-SEM images of the (a) bare NF , (b) FePi/NF film at different magnifications, (c) TEM image of FePi/NF, d) HR-TEM image of the FePi/NF and (e-h) the elemental mappings confirming the uniform distributions of Fe and P on NF substrate. Figure 2(a) High-resolution XPS survey spectrum of FePi/NF and the (b) Fe 2p, (c) spectra of P 2p and (d) O 1s spectra of the FePi/NF film. Figure 3(a) OER LSV polarization curves, (b) OER Tafel plots, (c) long-term stability of FePi/NF at 10 mA cm-2 for 24 h and (d) OER polarization curve of FePi/NF before and after stabilization. Figure 4(a) HER LSV polarization curves, (b) HER Tafel plots, (c) long-term stability of FePi/NF at -10 mA cm-2 for 24 h and (d) HER polarization curve before and after stabilization. Figure 5(a) Polarization curve obtained in a two electrode configuration. (b) Long-term stability test at 10 mA cm-2 for the overall water splitting performance of the bifunctional FePi/NF electrocatalyst (inset polarization curve after stabilization). Figure 6(a) Plot of the capacitive current densities versus scan rate of the FePi/NF and Fe(OH)2/NF electrodes and (b) the EIS spectra of FePi/NF, and Fe(OH)2/NF, inset is an equivalent circuit used for fitting Nyquist plots.
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Graphical abstract-
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