Iron-Doped Nickel Phosphate as Synergistic Electrocatalyst for Water

Article pubs.acs.org/cm

Iron-Doped Nickel Phosphate as Synergistic Electrocatalyst for Water Oxidation Yibing Li and Chuan Zhao* School of Chemistry, The University of New South Wales, Sydney, New South Wales 2052, Australia S Supporting Information *

ABSTRACT: Electrochemical water splitting into hydrogen and oxygen has been regarded as one of the most promising approaches to produce clean hydrogen fuel using electricity generated from renewable energy sources such as solar energy, wind power, or hydropower etc. Recent findings have demonstrated significant potential of nonprecious, nickel-based electrocatalysts as efficient oxygen evolution reaction (OER) to replace traditional ruthenium (Ru) and iridium (Ir)-based precious metal catalysts. Here, for the first time, we report a novel three-dimensional iron-doped nickel phosphate catalyst by stepwise autologous hydrothermal growth of nickel phosphate (Ni:Pi) spontaneously from nickel foam (NF) followed by electrodeposition of iron hydroxide (denoted as Ni:Pi-Fe/NF). Our findings reveal that the incorporated iron could play strong synergistic effects on the OER activities of nickel phosphate in alkaline solution, delivering a current density of 10 mA cm−2 at an extremely small overpotential (η) of 220 mV and extraordinary high current density of 500 mA cm−2 at η = 290 mV in 1 M KOH, which is among the best Ni-based OER electrocatalysts to date. Furthermore, in a concentrated alkaline electrolyte (5 M KOH), the Ni:Pi-Fe/NF electrode can reach a high current density of 1 600 mA cm−2 at an overpotential merely of 332 mV and shows excellent electrocatalytic stability in prolonged bulk water electrolysis, meaning it could highly meet the requirement of the industry alkaline electrolysis system. Mechanism investigations employing X-ray photoelectron spectroscopy (XPS), electrochemical polarization, contact angle measurement, and Raman spectra suggest strong interactions between Ni:Pi and Fe, with nickel oxyhydroxide (NiOOH) being the primary catalytic active site and nickel phosphate facilitating water adsorption. The iron doping changes the local Ni−O environments which synergistically enhance the Ni:Pi-Fe/NF catalytic activity toward OER.

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INTRODUCTION Electrolytic water splitting to generate hydrogen provides a promising alternative approach to address the global energy issues and has attracted much attention in the last few decades.1−3 For grid scale application of the water splitting technology, the efficiency is the key. The overall water splitting efficiency is largely limited by the sluggish oxygen evolution reaction (OER), which involves the transfer of four protons and four electrons proceeding far from its equilibrium potential (1.23 V vs reversible hydrogen electrode (RHE)), causing significant energy losses. Precious metal oxides, such as ruthenium and iridium oxide, have been used as benchmark OER catalysts to lower the activation energy and thus enhance conversion rate of water oxidation. Nevertheless, they are not suitable for large-scale applications because of their scarcity and high costs.4,5 It is therefore highly desirable to seek efficient low-cost, earth-abundant OER catalysts to replace expensive precious metal/metal oxide catalysts. Recently, inexpensive cobalt and nickel-based materials such as cobalt phosphate (Co:Pi),6,7 nickel borate (Ni:Bi),8 surface oxidized steels,9−11 and Co3O4 based composites,12−14 etc. have been intensively investigated as catalysts for water electrolysis in neutral or alkaline electrolytes. Transitional metal-based oxide/hydroxide such as NiFe-hydroxides as effective OER catalysts in alkaline © 2016 American Chemical Society

conditions with >12% solar-to-hydrogen efficiency have been created using these catalysts.3,15 Despite promising performances, the understanding of the origin of the OER catalytic activity in these transition metalbased catalysts is usually insufficient and remains an active area of research. Nevertheless, mechanistic studies have suggested that the presence of Co4+ species in Co:Pi16 and the change of average valence state of the nickel species in Ni:Bi8 in response to an applied positive voltage are important for efficient oxygen evolution. Furthermore, recent researches have showed that the existence of the phosphate groups in transitional metal compounds not only acts as proton acceptors that facilitate the oxidation of Mn atoms in Mn3(PO4)2·H2O17 but also induces distorted local cobalt geometry in Na2CoP2O7 that favorite water adsorption and further water oxidation.18 Recent studies also indicate that iron incorporation could significantly enhance the OER activity of Ni:Bi and cobalt iron (oxy) hydroxide.19,20 There is no story without coincidences, and other discoveries showed iron in nickel oxide (hydroxide) electrodes not only causing improvement in electrical Received: April 19, 2016 Revised: July 29, 2016 Published: July 31, 2016 5659

DOI: 10.1021/acs.chemmater.6b01522 Chem. Mater. 2016, 28, 5659−5666

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Chemistry of Materials

X-ray spectroscopy (EDS) mapping images were obtained from Philips CM200. TEM sample was prepared by strong sonication of the catalysts directly from NF using the Branson 400 W digital sonifier for 20 s and drop casting their ethanol solution onto copper grids. Raman spectra were recorded on a Renishaw inVia spectrometer using a 514 nm laser. Electrochemical Evaluation. All electrochemical measurements were carried out with a CHI760 electrochemical workstation. Asprepared Ni:Pi-Fe electrode on NF or nickel foil was directly used as the working electrode after electrodeposition without further treatments. A Pt wire and Ag/AgCl (1 M KCl) were used as the counter electrode and reference electrode, respectively. All potentials measured were calibrated to reversible hydrogen electrode (RHE) using the following equation: ERHE = EAg/AgCl + 0.235 V + 0.059 pH. Cyclic voltammetry (CV) and OER linear sweep voltammetry (LSV) polarization curves were recorded at a scan rate of 5 mV s−1. Before recording, the Ni:Pi-Fe/NF was scanned for 20 cycles in 1 M KOH solutions from 1.061 to 1.661 V (vs RHE) until a stable cyclic voltammogram was recorded. All the OER polarization curves were measured in 1 M KOH with 95% iR compensation. Cyclic voltammetry and chronoampermetric measurement were obtained under the same experimental setup without iR compensation. Electrochemical impedance spectra (EIS) of samples were measured at 1.53 V (vs RHE) in the frequency range of 0.1−100 000 Hz with an amplitude of 10 mV in 1 M KOH electrolyte. Zview software is used to fit equivalent circuit with experimental EIS data. The electrochemically active surface area (ECSA) is calculated based on the measured double layer capacitance of the synthesized electrodes in 1 M KOH according to the method reported by McCrory et al.23 Briefly, a potential range where no apparent Faradaic process happened was determined first using the static CVs. The charging current ic which equals to the product of the scan rate (v), and the electrochemical double-layer capacitance (CDL), was measured from the CVs at different scan rates and follows the equation:

conductivity but also enhancing the activity of NiOOH through a Ni−Fe partial-charge-transfer activation process which greatly lowers the OER overpotential and delivers high current density.21,22 Inspired by the important roles of the phosphate groups in cobalt phosphate and the strong synergistic effect of iron in NiFe based hydroxide OER catalysts, we anticipate a similar effect of iron on the OER catalytic activity of nickel phosphate. Herein, we report a novel hydrothermal approach for the synthesis of nickel phosphate directly from nickel foam (NF) followed by iron incorporation via electrodeposition. Using NF substrate as the source of nickel can enable the best ohmic contact and mechanical robustness between the nickel phosphate catalyst and NF support, forming a fully integrated catalyst for efficient OER under stringent high current density and strong gas evolution conditions. Most of current reported OER electrodes are prepared via deposition or casting of foreign catalysts onto the catalyst support, which offers intrinsically limited charge transport properties and mechanical strength. In this work, incorporation of iron into the nickel phosphate (Ni:Pi) induces a strong synergistic effect which has been confirmed from various characteristic techniques including X-ray photoelectron spectroscopy (XPS), electrochemical polarization, contact angle measurement, etc., and significantly enhanced OER catalytic activity is observed for the iron-doped nickel phosphate on NF (Ni:Pi-Fe/NF) electrode, compared to the nickel phosphate on NF (Ni:Pi/NF) electrode without iron and iron hydroxide electrodeposited on NF (Fe/NF) electrode without phosphate, which outperforms most of the known Nibased OER catalysts in alkaline media.

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EXPERIMENTAL SECTION

ic = vC DL

Materials and Methods. A typical two-step synthetic process of Ni:Pi-Fe/NF is shown as follows: (1) nickel foam (NF) (thickness, 1.6 mm; bulk density, 0.45 g cm−3; Goodfellow) was sonicated in 5 M HCl solution for 15 min to remove the nickel oxide layer on the surface, rinsed subsequently with water, and then immersed into 20 mL of 6 mM NaH2PO2·H2O solution. The resulting solution containing the cleaned NF was transferred into a Teflon-lined stainless steel autoclave with a total volume of 50 mL. Then, the hydrothermal reaction was performed at 150 °C for 5 h. After hydrothermal reaction, the autoclave was cooled down to room temperature and then NF was taken out for electrodeposition. (2) The cathodic electrodeposition of iron hydroxide was carried out in a standard three-electrode electrochemical cell, containing Ni:Pi/NF as the working electrode, a platinum wire as the auxiliary electrode, and an Ag/AgCl (1 M KCl) as the reference electrode. The constant potential electrodeposition was then carried out at −1.0 V (vs Ag/ AgCl) for 5 min in 3 mM Fe(NO3)3·9H2O solution. After electrodeposition, the electrode was withdrawn from the electrolyte, rinsed with Milli-Q-water, and directly used as working electrode. For the electrode grown on nickel foil, NF was replaced by a piece of nickel foil with other reaction conditions unchanged. Physical Characterization. Scanning electron microscopy (SEM, JSM-7001F) and X-ray diffraction (XRD) using the Empyrean PANalytical diffractometer in the grazing incidence measurement mode (GIXRD, CuKα radiation), which is specially used for analyzing thin films grown on substrate, were employed to characterize our samples instead of using the normal powder XRD technique. Chemical compositions of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB250i X-ray photoelectron spectrometer). Contact angles with Milli-Q water were measured with the sessile drop method using Rame-Hart 100-00 goniometry. Three different spots per substrate on three different areas were measured. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive

Thus, the ECSA can be calculated with CDL and a known Cs = 0.04 mF cm−2 in 1 M KOH based on typical reported values. The Faradaic efficiency was obtained by using a fluorescence-based oxygen sensor (Oceanic Optical) to determine the generated oxygen gas and calculated using the procedure described previously.24−26

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RESULTS AND DISCUSSION A two-step process was developed to synthesize the Ni:Pi-Fe/ NF electrocatalyst. First, nickel phosphate was grown on NF (Ni:Pi/NF) via a hydrothermal approach by using NaH2PO2· H2O as the phosphorus source. Nickel foam is directly used as the source of nickel without adding any nickel salts, templates, or surfactants. Then, electrodeposition of iron hydroxide nanoparticles were carried out at −1.0 V (vs Ag/AgCl) for 5 min in 3 mM Fe(NO3)3·9H2O solution onto the as-prepared Ni:Pi/NF electrode. The cathodic deposition process involves the reduction of nitrate to generate OH− ions which subsequently react with Fe3+ to form ferric hydroxide on the working electrode surface.3,23,27 As shown in Figure 1a, the surface morphology of Ni:Pi/NF composite is mainly consists of roughened nanoparticle with open pores between them which could serve as the roles of the template and facilitate the afterward iron nitrate penetrating into the pores then uniformly depositing iron hydroxide onto it. As seen from Figure 1b, the electrodeposited Ni:Pi-Fe/NF film consists of interconnected nanoparticles with void space between large aggregates, allowing the electrolyte to diffuse into the pores and reach the catalysts underneath. The cross-section analysis of the catalyst by SEM (Figure S1a,b) suggests the thickness of the catalyst layer is ∼300 nm. 5660


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Figure 2. (a) XRD of Ni:Pi-Fe/NF, (b) Ni 2p XPS of Ni:Pi/NF and Ni:Pi-Fe/NF (before and after OER), (c) P 2p XPS spectra, (d) Fe 2p XPS of Ni:Pi-Fe/NF, and (e) O 1s XPS spectra before and after OER for Ni:Pi-Fe/NF.

The composition of the Ni:Pi-Fe film and the oxidation state of each element are further investigated by X-ray photoelectron spectroscopy (XPS). Figure 2b shows the core level XPS spectra of Ni 2p. The peak at ∼852.5 eV corresponds to Ni metal in the oxidation state of zero for both Ni:Pi/NF and Ni:Pi-Fe/NF, originating from the underlying NF substrate. The binding energies of Ni 2p at ∼855.5 eV (2p3/2) and ∼873.0 eV (2p1/2) as well as their corresponding shakeup resonances at ∼861.1 eV and ∼873.7 eV are assigned to the oxidized nickel.29 After iron incorporation, the Ni 2p profile shifts to higher binding energies at a value around 0.33 eV, indicating the interaction between iron and Ni:Pi. As for the P 2p profiles for both samples, the P 2p3/2 and P 2p1/2 peaks at ∼133.2 eV and ∼138.7 eV can be assigned to P−O bonding (Figure 2c), indicating the formation of phosphate species.30 These results confirm the formation of nickel phosphate directly from NF without using extra nickel precursors. However, compared to Ni:Pi/NF, the intensity of the P 2p and Ni 2p XPS patterns for Ni:Pi-Fe/NF is weakened, attributing to the deposited iron hydroxide and thereby limit the measuring depth of the XPS. The high-resolution Fe 2p XPS in Figure 2d after iron deposition shows binding energy values located at ∼710.9 eV (Fe 2p3/2) and ∼724.6 eV (Fe 2p1/2), respectively, and shake up the satellite at ∼719.0 eV, demonstrating the oxidation state of iron is primarily Fe3+ in Ni:Pi-Fe/NF. Furthermore, a clear iron peak is observed in the XPS survey spectra after electrodeposition, confirming the existence of iron in the sample (Figure S2). For the O 1s XPS spectra (Figure 2e), the peak at ∼531.3 eV is assigned to the phosphate and hydroxyl species, while the peak at ∼529.7 eV is attributed to the metal−O bond. It is worth noting that the different Ni 2p and O 1s XPS before and after OER suggests

Figure 1. (a) SEM image of Ni:Pi/NF from the marked area of the NF (inset). (b) SEM image of Ni:Pi-Fe/NF. (c) TEM, (d) HRTEM, and (e−h) TEM-EDS elemental mapping images of a fragment of the Ni:Pi-Fe/NF catalyst.

The TEM and HRTEM images of a fragment of the catalyst are shown in Figure 1c and d, respectively. The lack of a clear lattice fringes in the HRTEM image indicates the amorphous nature of the product. Elemental mapping in Figure 1e−h reveals the successful doping of Fe and the uniform distribution of Ni, Fe, P, and O in the Ni:Pi-Fe hybrid. It should be noted that the introduction of iron, in the form of ferric hydroxide, into the Ni:Pi/NF structure is not simply coated onto the preformed Ni:Pi surface but indeed have a strong interaction between the two phases, modify the electronic environment of Ni:Pi, and boost the OER performance, which will be demonstrated in the sections that follow. Figure 2a displays the X-ray diffraction (XRD) pattern of the Ni:Pi-Fe/NF electrode. The XRD pattern shows no new peaks other than that from the apparent metallic nickel substrate, suggesting the formed Ni:Pi-Fe film is amorphous in nature, which corresponds with the HRTEM result. Amorphous structured material has been reported to be more active than their crystalline counterparts to obtain high activity with large electrochemically active surface area and more active sites.28 5661


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Ni-based electrocatalysts reported to date at high current densities.40 These results faithfully satisfy the requirements for commercial water electrolyzers (for example, j ≥ 500 mA cm−2 at η ≤ 300 mV).28 The Faradaic efficiency of the catalyst was measured using a fluorescence-based oxygen sensor in 1 M KOH, and a high Faradaic efficiency of 97% was obtained (Figure S3). Furthermore, the outstanding catalytic activity of Ni:Pi-Fe/ NF for OER is further confirmed by examining the Tafel slopes according to the Tafel equation η = b log(j/j0), where η is the overpotential, b the Tafel slope, j the current density, and j0 the exchange current density. As shown in Figure 3c, the Tafel slopes obtained from low overpotential region reveal small Tafel slopes of 37, 47, and 54 mV dec−1 for Ni:Pi-Fe/NF, Fe/ NF, and Ni:Pi/NF, respectively. The smallest Tafel slope is observed for the Ni:Pi-Fe/NF electrode, in accordance with the LSV curves (Figure 3b), indicating more rapid OER rates obtained using Ni:Pi-Fe/NF electrocatalyst. The above results suggest the Ni:Pi-Fe/NF is among the most active nonprecious catalysts for alkaline water oxidation. With the superior performance of the Ni:Pi-Fe/NF electrode in hand, the robustness and high-efficiency of the electrocatalyst is further evaluated under extreme conditions such as highly concentrated alkaline solution (e.g., 5 M KOH) and at high current density (e.g., j > 1 000 mA cm−2). Figure 3d represents a multistep chronopotentiometric curve obtained at Ni:Pi-Fe/ NF electrode in 5 M KOH solution. In the experiment, the current density is increased stepwise from 200 mA cm−2 to 1 600 mA cm−2 with an increment of 200 mA cm−2 per 200 s, and the corresponding changes of potential is recorded. At the start of 200 mA cm−2, the potential immediately levels off at around 1.60 V (vs RHE) and remains constant for the remaining 200 s. Similar results are also obtained for all current densities tested herein up to 1 600 mA cm−2, indicating the excellent mass transport properties and mechanical robustness of the Ni:Pi-Fe/NF electrode. Take into the consideration of the 1.9 Ω resistance measured for the electrode in 5 M KOH electrolyte, it only needs a η = 332 mV to deliver such a high current density of 1 600 mA cm−2, suggesting the electrode is practical for industrial operations. The robustness and durability of the as-prepared Ni:Pi-Fe/ NF is first studied by CV. Figure 4a exhibits a better activity retaining ability after 1 000 CV cycles. Furthermore, the robustness of the Ni:Pi-Fe/NF electrode is confirmed by long-term water electrolysis under an extremely high current density of 1 000 mA cm−2 in 5 M KOH solution. As shown in Figure 4b, the electrolysis voltage of the Ni:Pi-Fe/NF electrode remain stable during continuous oxygen evolution for 10 h, indicating the excellent OER activity and long-term stability. Very recent study has reported that phosphates can be converted to hydroxides after cycling in an alkaline electrolyte.41 However, our results show this conversion process is relatively minor as the bulk nickel phosphate before and after prolonged OER is almost unchanged (Figure S4). Interestingly, the occurrence of OER is accompanied by the color changes of the Ni:Pi-Fe/NF electrode from silver gray to dark black (Figure 4c,d), attributed to the electrochromic behavior of Ni:Pi as a result of in situ oxidation of silver Ni2+ to higher oxidized state nickel during the OER process.42,43 This property can offer a “smart” tool for visual monitoring the activity of the OER electrode from its color. To understand the origins of the outstanding OER activity of Ni:Pi-Fe/NF, we detect the electrochemical active surface areas

the changes of the local Ni−O environment and which will be further investigated in the following sections. The electrocatalytic performances of the fabricated electrodes are first evaluated by using cyclic voltammetry (CV) and linear sweep voltammetry (LSV) in a three-electrode cell configuration in 1 M KOH solution. For comparison, the NF electrode with directly deposited iron (Fe/NF) and the Ni:Pi electrode without iron incorporation (Ni:Pi/NF) are also prepared. The CV studies in Figure 3a reveals that Ni:Pi-Fe/

Figure 3. (a) CV scans for Ni:Pi-Fe/NF, Ni:Pi/NF, and Fe/NF electrodes in 1 M KOH at a sweeping rate of 5 mV s−1 without iR compensation. (b) OER polarization curves and (c) corresponding Tafel plots of different electrodes with 95% iR compensation. (d) Multicurrent process obtained with the Ni:Pi-Fe/NF oxygen electrode in 5 M KOH without iR correction.

NF exhibits the best OER performance among the three electrodes, with higher current density and lower overpotential than Ni:Pi/NF and Fe/NF, respectively. The anodic peaks in the potential region of 1.3−1.5 V (vs RHE), prior to the OER process, are attributed to the formation of NiOOH, which is similar to previous studies on nickel borate and nickel phosphate and is generally believed to be the catalytic active sites for OER.21,31−33 After Fe-doping, the positive shift of this redox potential indicates the alteration of the redox properties of Ni in the Ni:Pi and such synergistic interaction between Ni:Pi and Fe results in enhanced oxygen evolution. This phenomenon of the positive shift of Ni2+/Ni3+ after iron incorporation has also been reported by other groups previously.21,34 The LSV curves obtained for the three electrodes confirm the strong synergistic effect with iron incorporation (Figure 3b). The overpotential (η) of Ni:Pi-Fe/ NF required to deliver a current density of 10 mA cm−2 is merely η = 220 mV, which is comparable to other reported active Ni based electrocatalysts such as activated stainless steel with the formation of a Ni, Fe-oxide layer (212 mV at 12 mA cm−2),10 NiFe hydroxide/NF (215 mV),3 NiFe layered double hydroxide(LDH)/NF (240 mV),15 NiFe LDH/carbon nanotube (247 mV),35 Ni2P (290 mV),36 NiFeOx/C (280 mV),37 NiCo LDH (367 mV),38 and Ni5P4 (290 mV)39 (Table S1). For industrial applications, it is also highly desired for the electrode to deliver very high current densities (e.g., 500 mA cm−2) with lower overpotentials (e.g., 300 mV). For the Ni:PiFe/NF electrode, it merely requires a η = 290 mV to deliver a high current density of 500 mA cm−2, outperforming Ni:Pi/NF (η = 370 mV) and Fe/NF (η = 350 mV), and most, if not all, of 5662


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Figure 4. (a) Polarization curves before and after 1 000 CV cycles with the Ni:Pi-Fe/NF electrode. (b) Chronopotentiometric curve obtained with the Ni:Pi-Fe/NF oxygen electrode in 5 M KOH with constant current densities of 1 000 mA cm−2 without iR compensation. Electrochromic behavior of Ni:Pi-Fe/NF under (c) discharged state at 1.3 V and (d) charged state at 1.63 V.

(ECSA) (Figures S5−S7) for Ni:Pi-Fe/NF, Ni:Pi/NF, and Fe/ NF, respectively, through the measured double layer capacitance.3 The results show that the ECSA measured for the three electrodes are very similar, suggesting the high OER activity of Ni:Pi-Fe/NF is more due to the synergistic interaction between Ni:Pi and iron, rather than the enlarged surface area. It is further found that the catalytic activity of Ni:Pi-Fe/NF degrades when the amount of electrodeposited iron is either too high or too low (Figure 5a). Figure S8 shows the SEM images of Ni:Pi-Fe/NF obtained from electrodeposition bath containing 1.5 mM and 6 mM Fe(NO3)3· 9H2O. The electrodeposited iron from 1.5 mM Fe(NO3)3· 9H2O bath shows morphologies of small fine nanoparticles (Figure S8a), while using 6 mM Fe(NO3)3·9H2O precursor leads to the formation of large and aggregated nanoparticles (Figure S8b). This result shows that low iron loading is not enough to induce sufficient synergistic interaction for enhancing OER activity, while high iron loading can form a thick iron hydroxide layer, which is less active for OER and block the exposed active site in the Ni:Pi/NF electrode during OER. To obtain high OER activity, an optimal level of iron doping is needed. To elucidate the OER reaction kinetics and charge transfer process, AC electrochemical impedance spectroscopy (EIS) is performed. Figure 5b shows typical Nyquist plots of the Ni:PiFe/NF, Fe/NF, and Ni:Pi/NF electrodes at η = 300 mV. All impedance spectra are fitted using an equivalent RC circuit model, as shown in the inset of Figure 5b, which consists of a resistor (Rs), representing the resistivity of the electrolyte between the working and reference electrode; a charge transfer resistance (Rct), representing the charge transfer resistivity of the redox reaction; and a capacitance (C) in parallel with the Rct, analogous to the double layer charging capacity of the solid−liquid junction. The EIS data reveal a smaller charge transfer resistance for the Ni:Pi-Fe/NF electrode (5.5 Ω) than that of Fe/NF (10.3 Ω) and Ni:Pi/NF (15.9 Ω), suggesting faster electron transfer kinetics of OER at Ni:Pi-Fe/NF electrode. This is originated from the unique catalyst design by growing Ni:Pi structure directly from NF substrate, which assures excellent electrical contact and mechanical strength of Ni:Pi catalysts to the underneath NF substrate, as well as the

Figure 5. (a) OER polarization curves of Ni:Pi-Fe/NF deposited in different Fe(NO3)3·9H2O concentrations with 95% iR compensation. (b) Electrochemical impedance spectra (EIS) of different electrodes recorded at an overpotential of 300 mV (vs RHE). (c) OER polarization curves of Ni:Pi-Fe/Ni foil, Ni:Pi/Ni foil, and Fe/Ni foil with 95% iR compensation. (d) OER polarization curves of Ni:Pi-Fe/ NF and Ni:Pi-Fe/Ni foil with 95% iR compensation. Contact angle measurement on the electrodes treated with (e) and without (f) phosphorylation process.

iron incorporation which modify the electronic properties of Ni:Pi. Besides having a highly active catalyst, the 3D interconnected macroporous structure of the nickel foam support play a critical role in enlarging the reactive surface area and enhancing the mass transport, particularly at the large current densities.3 To elucidate this point, we prepared Ni:Pi-Fe catalysts using Ni foil using the same approach (denoted as Ni:Pi-Fe/Ni foil). Again, the Ni:Pi-Fe/Ni foil electrode shows higher current density and lower overpotential compared to Ni:Pi/Ni foil and Fe/Ni foil (Figure 5c). Comparing the effect of NF and Ni foil as support on OER (Figure 5d), it is evident that NF is a better catalyst support for delivering much higher OER current density at the same applied potential. The 3D macroporous structure NF offer much higher surface area than Ni foil for the growth of Ni:Pi-Fe. Studies also have shown that NF is effective in removing the gas bubbles produced on electrode surface, thus reducing significantly the so-called “bubble overpotential” which becomes significant at higher current densities. All these properties of NF support contribute to the superior performance of the electrode, particularly at high current densities. To further understand the catalytic mechanism of Ni:Pi-Fe, contact angle measurements were carried out on the samples grown on nickel foil with or without NaH2PO2·H2O treatment. It is found that smaller water contact angle is obtained for Ni:Pi-Fe than the sample without phosphorylation treatment (Figure 5e,f). It has been reported that the phosphate groups in the Co:Pi system could induce highly distorted local cobalt geometry, which has an open coordinate sites and facilitate the uptake of oxygen adsorbate, contributing to a favorable water 5663


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Chemistry of Materials adsorption and further water oxidation.18 It is reasonable to deduce that the Ni:Pi-Fe composite OER catalysts reported here follows a similar reaction mechanism, where phosphorylation of nickel is favorable for water adsorption onto the active sites due to the distorted local nickel environment caused by phosphate groups. The incorporation of iron not only significantly improve the electron transfer rate as evidenced by EIS (Figure 5b) but also modify the electronic environment of nickel as observed from the high resolution Ni 2p XPS spectra (XPS in Figure 2b). As shown in Figure 2b, the Ni 2p XPS spectra of Ni:Pi-Fe shift to higher binding energy region in comparison to that of Ni:Pi, revealing that the interaction of iron with Ni:Pi impairs the electron density of nickel atom and may also results a partial electron transfer between Ni and Fe, thus promoting the electron transfer rate for OER reactions.44,45 This could be further confirmed by the CV shown in Figure 3a. After iron doping, the positive shift of Ni2+/Ni3+ redox potential indicates the alteration of the redox properties of Ni in the Ni:Pi, a phenomenon that also has been reported by other groups previously.21,34 The results suggest the Ni:PiFe catalyst is not a simple mixture of the two phases, rather strong synergistic interactions exist between Ni:Pi and Fe resulting the enhanced oxygen evolution. More evidence could be seen from the O 1s XPS spectra, where the peak at ∼531.3 eV is assigned to the phosphate and hydroxyl species, the peak at ∼529.7 eV is attribute to the Ni−O bond. After OER, the relative intensity of the Ni−O bond is increased, indicating a stronger Ni−O bond (short Ni−O distance) is formed, attributed to the in situ formation of species of NiOOH at the surface in the hybrid during OER.46 To further understand the interaction of the nickel phosphate with iron and the formation of the NiOOH catalytic active sites, Raman spectra were collected on the synthesized electrodes. Raman spectroscopy is sensitive to local structure and can detect the metal−O vibration band changes effectively. Shown in Figure 6, for the Ni:Pi/NF or Fe/NF electrodes,

to the excellent OER catalytic activities of Ni:Pi-Fe/NF. The nickel phosphate plays a major role in facilitating water adsorption, while the synergistic iron doping alters the local Ni−O environment, leading to the enhanced catalytic activity toward OER.

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CONCLUSIONS In conclusion, an amorphous iron-doped nickel phosphate catalyst has been prepared via a facile stepwise approach onto NF for highly efficient oxygen evolution reactions in alkaline electrolytes. The NF not only serves as a 3D catalyst support and electron collectors but also provides the nickel source for the hydrothermal autologous growth of the catalyst. Incorporation iron into Ni:Pi compounds further enhances the catalytic activity for OER. Collectively, the outstanding performance of the Ni:Pi-Fe/NF for alkaline water oxidation can be attributed to (i) the synergistic effect between nickel phosphate formation and iron incorporation, (ii) excellent charge transport and mechanical robustness as a result of the catalyst design, and (iii) the 3D macroporous structure of NF support which offer abundant active sites, effective mass transport, and fast gas bubble dissipation, particularly at high current densities. The excellent catalytic activity and stability suggest the Ni:Pi-Fe/NF as a promising catalyst candidate for electrolytic water splitting and also highlight the autologous growth of catalysts directly from the catalyst support as a promising strategy for developing fully integrated electrodes. Such catalyst/electrode architecture is shown to be advantageous for achieving optimal electrical contact between catalyst and support and high mechanical robustness and will be useful for a range of electrochemical energy devices such as water spitting and fuel cells.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01522. Additional XPS spectra, double-layer capacitance measurement for determining electrochemically active surface area, SEM images, EDS, Faradaic efficiency, and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

Figure 6. Raman spectra of NF, Ni:Pi/NF, Fe/NF, and Ni:Pi-Fe/NF before and after OER polarization.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

there is no obvious peaks detected except that from nickel foam. However, five apparent peaks can be observed for Ni:PiFe/NF, similar to the characteristic peaks observed for Ni−Fe oxyhydroxide catalyst.34 The significant difference between the Raman spectra for Ni:Pi/NF, Fe/NF, and Ni:Pi-Fe/NF suggests that the hybrid catalyst is not a simple mixture of the two materials. Interestingly, after OER, the observed Raman bands for the Ni:Pi-Fe/NF appear to merge into one strong and broad band, suggesting the formation of higher oxidation state nickel species.47 In contrast, such change in the Raman spectra cannot be observed without iron doping. In brief, both the formation of nickel phosphate and iron doping contribute

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS All the physical characterizations were carried out in Mark Wainwright Analytical Centre (MWAC) at the University of New South Wales (UNSW). We thank Dr. Bing Gong and Dr. Yu Wang from MWAC for their assistance in XPS and XRD, respectively. The study was financed by an ARC Discovery Grant (Grant DP160103107). 5664


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Iron-Doped Nickel Phosphate as Synergistic Electrocatalyst for Water

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