Enhancing Water Oxidation Catalysis on a Synergistic Phosphorylated

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Enhancing Water Oxidation Catalysis on a Synergistic Phosphorylated NiFe Hydroxide by Adjusting Catalyst Wettability Yibing Li, and Chuan Zhao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03497 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 1, 2017

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Enhancing Water Oxidation Catalysis on a Synergistic Phosphorylated NiFe Hydroxide by Adjusting Catalyst Wettability Yibing Li, and Chuan Zhao* School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia. Corresponding Author: [email protected] ABSTRACT Surface wettability is very important for designing and developing heterogeneous electrocatalysts that applied in an aqueous environment. Here, by adjusting the surface wettability of the catalyst using a facile two-step strategy, a porous nanosheet electrocatalyst composed of NiFe hydroxide and NiFe phosphate (denoted as NiFe/NiFe:Pi), is designed and developed. The two-step strategy not only allows us to successfully control the morphology but also provides an approach to modify the surface chemical property of a catalyst. After phosphorylation, the surface wettability of the NiFe/NiFe:Pi catalyst is significantly enhanced (contact angle: 44°± 3°) than that of NiFe hydroxide electrode without phosphorylation (contact angle: 129°± 5°). Serving as an oxygen evolution catalyst in 1 M KOH aqueous solution, the NiFe/NiFe:Pi electrode yields strong synergistic oxygen evolution activity to deliver a current density of 10 mA cm-2 with an overpotential merely of 290 mV. The catalyst also exhibits extraordinary high current densities of 300 mA cm-2 at extremely low overpotential of 340 mV, which is among the best Ni-based OER electrocatalysts to date. The well-maintained open porous nanosheet structure, enhanced surface wettability and increased charge transfer rate at the NiFe/NiFe:Pi catalyst, together with effective mass transport and ACS Paragon Plus Environment

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enlarged catalytic active sites offered by the hierarchical porous structure, contribute to the extraordinary water oxidation performance. KEYWORDS: wettability, NiFe hydroxide, NiFe phosphate, phosphorylation, oxygen evolution reaction 1. INTRODUCTION With the increasing demand for clean energy, enormous efforts have been focused on the design and development of efficient, low-cost hydrogen generation systems such as (photo) electrochemical water splitting.1-2 Oxygen evolution reaction (OER) is a half reaction of water splitting and it also plays a crucial role in other renewable energy technologies such as metalair batteries.3 However, OER involves the complex four-electron, four-proton transfer process, and it proceeds far from the equilibrium potential (1.23 V vs. RHE), causing large energy losses in the overall water splitting process.4 Therefore, the development of highly efficient oxygen evolution catalysts with low overpotential and superior stability is being actively pursued. Precious metal Ru- and Ir-based catalysts are known to exhibit excellent overall OER performances.5-6 More recent efforts have been devoted to the development of OER electrocatalysts based on earth abundant materials such as perovskite,4 chalcogenide,7 cobalt phosphate (Co-Pi),8-9 nickel borate (Ni-Bi),10 nickel iron based materials,11-14 and other metal/carbon-based catalysts15-19 due to their low cost and high catalytic activity. Amorphous Co-Pi and Ni-Bi have demonstrated exciting OER catalytic activities in neutral electrolytes.8, 10 Nevertheless, the in-situ electrodeposited Co-Pi and Ni-Bi films are generally in the form of dense films with large particle sizes and low specific surface areas. Thus, the formation of nanoporous structure and nanoparticles with larger active surface and enhanced mass transfer still remains challenging. Furthermore, electrodeposition of Co-Pi or Ni-Bi catalyst films prior to water electrolysis is time consuming owing to the poor solubility of metal salts in neutral buffer solution.20 NiFe hydroxide-based catalysts are regarded as one

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of the best nonprecious OER electrocatalysts in alkaline solutions.11-13 Different from electrodeposited Co-Pi and Ni-Bi highly dense films, nanoporous NiFe hydroxide catalysts can be synthesized via electrodeposition or hydrothermal reaction onto conductive substrate such as nickel foam.11-12 Nevertheless, despite their promising performance, currently there is a lack of effective strategies for further improving the catalytic activity of NiFe hydroxide nanosheets for OER. It is known that catalytic performance of the heterogeneous catalyst is not only related to active sites, the adsorption of the reactants and products also plays a significant role. The molecular adsorption on a catalyst is strongly influenced by the catalyst wettability. Suitable wettability in principle could improve the adsorption of reactants or products, leading to enhancement in catalytic activity.21 Inspired by the attractive electrocatalytic properties and structures of the amorphous transition metal phosphates and nanoporous NiFe-based hydroxide, here we report a novel synergistic NiFe/NiFe:Pi catalyst with enhanced wettability as an water oxidation electrocatalyst by a two-step strategy involving firstly the electrodeposition of a 3D NiFe hydroxide porous structure on carbon fiber paper (CFP) substrate followed by reaction with phosphorus vapour to partially convert the hydroxide into phosphate without altering the hierarchy porous structure (Figure 1). The two-step synthetic strategy shows advantages for morphology and surface chemical property control and enables the fabrication of highly-efficient 3D porous metal phosphate catalyst for water oxidation. The result shows that the phosphate groups not only favorable to improve the contact between electrolyte and the electrode surface as evidenced by the enhanced surface wettability, but also modify the local electronic structure of nickel and iron atoms to afford a fast electron transfer rate. Meanwhile, the unique hierarchically structured porous catalyst configuration on conductive CFP enables large working surface area and excellent gas bubble dissipation ability. 2. EXPERIMENTAL SECTION ACS Paragon Plus Environment

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Synthesis of NiFe Hydroxide (NiFe) on CFP. The NiFe hydroxide was electrodeposited onto the carbon ﬁber paper (CFP, Fuel Cell Store) using a previously established method.12 Before electrodeposition, the CFP was connected with copper wire by using conducting silver epoxy as working electrode. The conducting silver epoxy and copper wire were kept above the electrolyte bath which contains 1:1 molar ratio of Ni2+/Fe3+, that is, 21.82 mg of Ni(NO3)2•6H2O and 30.30 mg of Fe(NO3)3•9H2O in 25.0 mL MilliQ-water. The electrodeposition was carried out in a standard three-electrode electrochemical cell, containing CFP 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. After electrodeposition, the CFP was carefully withdrawn from the electrolyte, rinsed with MilliQ-water, and dried in air to use as working electrode or for further treatment. Synthesis of NiFe Hydroxide/NiFe Phosphate (NiFe/NiFe:Pi) on CFP. Briefly, the electrode deposited with NiFe on CFP were placed in a crucible boat and inserted in the centre of the tube furnace equipped with gas controllers. The crucible boat containing 100.0 mg of NaH2PO2•H2O and NiFe loaded CFP were placed in the upstream and downstream positions in the tube furnace, respectively. Subsequently, the samples were calcinated at 300 °C for 1 h with a heating speed of 3 °C/min in a flowing Ar atmosphere, followed by naturally cooling down to room temperature under Ar. To optimize the OER performance, NiFe/NiFe:Pi composites using 20.0 mg, 100.0 mg and 500.0 mg of NaH2PO2•H2O with a fixed 1:1 molar ratio of Ni2+/Fe3+ were prepared with other reaction conditions unchanged. In addition, NiFe/NiFe:Pi composites with different Ni2+/Fe3+ ratios toward OER were also fabricated with a fixed amount of NaH2PO2•H2O precursor (100.0 mg). For comparison, the NiFe/NiFe:Pi composite also can be grown onto nickel foil substrate, where CFP was replaced by a piece of nickel foil with other reaction conditions unchanged.


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Physical Characterization. . Scanning electron microscope (SEM, JSM-7001F) and Xray diffraction (XRD, Empyrean PANalytical diffractometer, CuKα radiation) were employed for characterizing the prepared samples. Transmission electron microscopy (TEM), highresolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDS) mapping images were obtained from Philips CM200. Chemical compositions of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB250i X-ray Photoelectron Spectrometer). Contact angles with 1 M KOH in Milli-Q water were measured with sessile drop method using Rame-Hart 100-00 goniometry. Three different spots per substrate on three different areas were measured. Raman spectra were recorded on a Renishaw inVia spectrometer using a 514 nm laser. Electrochemical Measurement. All electrochemical measurements were carried out with a CHI 660 electrochemical workstation. As-prepared NiFe/NiFe:Pi electrode on CFP was directly used as the working electrode without further treatments. A Pt wire and Ag/AgCl (1 M KCl) were used as 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. OER linear sweep voltammetry (LSV) polarization curves were recorded at a scan rate of 5 mV s-1. Before recording, the NiFe/NiFe:Pi was scanned for 20 cycles in 1 M KOH solution from 1.061 V 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. Chronoampermetric measurement was obtained under the same experimental setup. 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 amplitude of 10 mV in 1 M KOH electrolyte. Zview software was used to fit equivalent circuit with experimental EIS data. The reaction product was investigated by rotating ring-disk electrode (RRDE). To prepare the electrocatalytic ink, the as-prepared NiFe/NiFe:Pi compound was carefully ACS Paragon Plus Environment

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scraped off from the CFP, ultrasonically dispersed in the mixture of water, ethanol and nafion, and coated onto the RRDE electrode. To monitor the generation of H2O2, the ring potential was set at a constant potential of 1.5 V (vs. RHE). To calculate the Faradaic efficiency (ɛ) and ensure the current was from oxygen evolution, the ring electrode potential was set at 0.4 V (vs. RHE) to reduce the generated O2 with the continuous O2 evolution at a constant disk current of 120 µA. The Faradaic efficiency (ɛ) was calculated following the equation: ɛ = Ir/(Id N) where Id represents the disk current, Ir is the ring current, and N is the current collection efficiency of the RRDE, which is determined to be 0.28. Calculation of Electrochemically Active Surface Area (ECSA). The calculation of ECSA is based on the measured double layer capacitance (CDL) of the synthesized electrodes in 1 M KOH according to the method reported by McCrory et al.22 Briefly, a potential range where no apparent Faradaic process happened was determined firstly 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: ic = v CDL Thus, the ECSA can be calculated with CDL and a known Cs = 0.040 mF cm−2 in 1 M KOH based on typical reported values. 3. RESULTS AND DISCUSSION To synthesize the hybrid NiFe/NiFe:Pi catalyst with high wettability, the preformed NiFe hydroxide surface is partially transformed into NiFe phosphate via a facile vapour phase transformation approach. Specifically, the electrode was prepared by a facile, two-step method. Firstly, NiFe hydroxide was electrodeposited onto CFP substrate according to the previously established procedure.12 Briefly, the CFP working electrode was electrodeposited


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with NiFe hydroxide at a constant potential of -1.0 V (vs. Ag/AgCl) for 5 min in an electrolyte bath containing 1:1 molar ratio of Ni2+ and Fe3+ nitrates. Then, it was partially converted into NiFe phosphate via a gas phase reaction approach at 300 °C in a tube furnace under flowing argon with the NaH2PO2•H2O and NiFe hydroxide loaded CFP placed in the upstream and downstream position, respectively. As the annealing temperature elevated, the decomposition of NaH2PO2•H2O is accompanied by releasing of phosphine (PH3) and H2O vapour, which subsequently react with NiFe hydroxide to form NiFe phosphate and will adjust the surface wettability of the catalyst (see below). The scanning electron microscope (SEM) images of the NiFe hydroxide before and after phosphorylation both show nanosheet-like structure with open and interconnected pores (Figure 2a and b), thereby providing effective channels for fast mass transport and charge transfer during water oxidation, which demonstrates the significant advances of the two-step synthetic strategy for morphology control. After phosphorylation, a clearly uniform distribution of Ni, Fe and P elements over the nanosheet is observed from SEM-EDS mapping (Figure S1). The nanosheet-like structure of NiFe/NiFe:Pi was further confirmed by transmission electron microscope (TEM) as shown in Figure 2c, while the high resolution transmission electron microscope (HRTEM) result (Figure 2d) reveals no lattice fringes from the nanosheet, revealing its amorphous nature which also has been confirmed from selected area electron diffraction (SAED, inset in Figure 2c). The amorphous metal hydroxide has been reported to be more OER active than their crystalline counterparts, as a result of lattice distortion of the surface amorphous metal/metal oxide and large electrochemically active surface area with more active sites.23-24 The chemical composition of the NiFe/NiFe:Pi catalyst was measured by STEM-EDS elemental mapping (Figure 3a-e), which verified the homogeneous distribution of Ni, Fe, P and O elements across the nanosheet. X-ray diffraction (XRD) was performed to determine the crystalline structure of NiFe/NiFe:Pi as well. The XRD result of ACS Paragon Plus Environment

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NiFe/NiFe:Pi in Figure 3f shows only the characteristic peaks for CFP at the angle (2θ) of 25.5°, which is indexed as the C(002) reflection of the hexagonal graphite structure and characteristic diffraction peaks of graphite at 2θ of about 43° and 53°, associated with C(100) and C(004) diffractions of graphite, respectively, demonstrating its amorphous nature and corresponds well with the HRTEM and SAED results. Due to the amorphous phase of the NiFe/NiFe:Pi composite, X-ray photoelectron spectroscopy (XPS) was then used to investigate the oxidation states of the elements on the nanosheet surface, before and after phosphorylation. For NiFe hydroxide, the high resolution Fe2p spectra shows binding energy peaks at ~711.5 eV (Fe2p3/2) and ~725 eV (Fe2p1/2) with a satellite peak at ~719 eV. The energy separation between the Fe2p3/2 (711.5 eV) and satellite (719 eV) of Fe2p3/2 is 7.5 eV, suggesting Fe3+ is the dominant oxidation state in NiFe (Figure 4a). As for the NiFe/NiFe:Pi sample, the coexistence of Fe3+ and Fe2+ is the case, as evidenced from the broadened Fe2p1/2 and Fe2p3/2 peaks associated with a shift of them to higher binding energy.25 The partially conversion of Fe3+ to Fe2+ species is also confirmed by the formation of a new binding energy peak at ~710.0 eV which is characteristic peak for Fe2+.26 In the high resolution Ni2p spectra (Figure 4b), two main peaks at ~856.2 eV and ~873.7 eV are associated with the Ni2p3/2 and Ni2p1/2 electronic configurations, respectively, and the spin-orbit splitting value of Ni2p1/2 and Ni2p3/2 reaches 17.5 eV, indicating the oxidation state for nickel is Ni2+.27 However, the high resolution Ni2p spectrum of NiFe/NiFe:Pi, similar to the Fe2p spectrum after phosphorylation, shifts to higher binding energy compared with NiFe, indicating a modified local electronic structure of nickel cations, likely originated from the inductive effect and strong interactions of the phosphate anion groups with metal ions in the NiFe/NiFe:Pi.28 This may have important implications in modulating electronic environments of the metal centers in NiFe/NiFe:Pi nanosheets by the formation of phosphate group and thus promoting the OER catalysis. The binding energy of


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P2p core-level XPS exhibits a solely peak centered at ~134.4 eV (Figure 4c), indicating the formation of phosphate bonding.29 The formation of the synergistic composite containing NiFe hydroxide and NiFe phosphate is evidenced from the high resolution O1s XPS spectra which can be deconvoluted into two peaks (Figure 4d). The peak at ~531.8 eV is ascribed to the hydroxyl group, while the peak centered at ~533.8 eV suggests the existence of P-O bond from phosphate.30 Based on the above data, the mechanism for the formation of NiFe phosphate is proposed. As the annealing temperature elevated, the decomposition of NaH2PO2•H2O is accompanied by releasing of phosphine (PH3) and H2O vapour,31-32 which subsequently react with NiFe hydroxide to form NiFe phosphate and at the same time, PH3, as a reducing agent, reduce Fe3+ to Fe2+.33 The synthesized samples are firstly used as water oxidation catalysts under an alkaline condition and the influence of the surface wettability on the water oxidation performance is discussed afterward. To optimize the OER performance, a set of NiFe/NiFe:Pi electrodes were prepared by changing the amount of phosphorous precursor used and the Ni2+/Fe3+ concentration ratios in the electrodeposition bath. As shown in Figure S2, the highest OER performance is obtained for the NiFe/NiFe:Pi electrode prepared with 100.0 mg of NaH2PO2•H2O. The excess use of phosphorus precursor (e.g., 500.0 mg) results in a thick phosphate coating with collapsed structures (Figure S3). Consequently, lower water oxidation performance is achieved. Using 20.0 mg of NaH2PO2•H2O results in insufficient thickness coating and only slight increase in oxygen evolution current is achieved compared to NiFe (Figure S2). Besides the phosphorus precursor, the Ni2+/Fe3+ concentration ratios during electrodeposition also plays important role to the OER performance. It is found that a 1:1 molar ratio of Ni2+ and Fe3+ yields the NiFe/NiFe:Pi electrode with the highest OER catalytic activity (Figure S4). Thus, the optimal synthesis conditions using 100.0 mg of NaH2PO2•H2O with a 1:1 molar ratio of Ni2+ and Fe3+ are adopted hereafter. ACS Paragon Plus Environment

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For comparison, NiFe, Ni:Pi and CFP:Pi electrodes are also prepared and investigated under the same condition. The bare CFP substrate shows negligible OER current before and after phosphorylation within the potential range studied (CFP:Pi is shown in Figure 5a). As seen from Figure 5a, the polarization curve of NiFe/NiFe:Pi exhibits the lowest onset potential of 1.43 V (vs. RHE) and much greater catalytic current density compared to NiFe hydroxide and Ni:Pi, suggesting a strong synergistic effect by the formation of phosphate. The overpotential (η) required for delivering a current density of 10 mA cm-2, equivalent to 10% solar energy conversion efficiency for NiFe/NiFe:Pi, is merely 290 mV. In contrast, 320 mV and 340 mV are needed for NiFe and Ni:Pi, respectively. These overpotential values are comparable or smaller than recently reported Ni-based OER electrocatalysts such as NiFeOx/C (280 mV),34 Ni2P (290 mV),35 NiCo layered double hydroxide (367 mV),36 NiFe layered double hydroxide/carbon nanotube (247 mV),13 Ni5P4 (290 mV),37 NiCoS (380 mV)38 and other low-cost catalysts (Table S1). To achieve a high current density of 300 mA cm-2, a small η = 340 mV is required for NiFe/NiFe:Pi, where only 36 mA cm-2 and 13 mA cm-2 can be obtained for NiFe and Ni:Pi electrodes, respectively, at the same overpotential. Such catalysts that can achieve large current densities at low overpotentials are highly demanded in water electrolysis industry. To gain further insight into the prepared electrodes, Tafel slopes for all the samples were investigated. As shown in Figure 5b, the Tafel slope for NiFe/NiFe:Pi is as low as 38 mV dec1

, which is smaller than that of individual NiFe (48 mV dec-1) and Ni:Pi (50 mV dec-1),

respectively, indicating rapid OER rates using NiFe/NiFe:Pi electrocatalyst. In addition, the excellent mass transport and charge transfer properties, and robust stability of the NiFe/NiFe:Pi catalyst is evidenced by multi-step chronopotentiometric responses. As seen from Figure 5c, the current density is increased from 50 mA cm-2 to 300 mA cm-2 with an increment of 50 mA cm-2 per 500s. For each increment, an immediate level off OER potential is observed without detectable fluctuation. Moreover, the exactly same potential is established ACS Paragon Plus Environment

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at the same current density for the reverse process, demonstrating excellent mass transport and charge transfer, fast bubble removal, and the robustness of the NiFe/NiFe:Pi electrode.12 The water oxidation reaction product on the NiFe/NiFe:Pi electrode is investigated by using rotating ring-disk electrode (RRDE) system.4, 39 The ring potential was set at a constant potential of 1.5 V (vs. RHE) to monitor the generation of H2O2. As seen from Figure S7, no detectable ring current was observed. To determine the Faradaic efficiency, the ring electrode potential was set at 0.4 V (vs. RHE) to monitor the generated O2. With the continuous O2 evolution at a constant disk current of 120 µA and O2 reduction current of -33 µA on the ring electrode (Figure 5d), a Faradaic efficiency of 98.2% is obtained, confirming O2 is the primary product. The long-term stability of an electrocatalyst is another crucial parameter to consider for its future practical applications. As shown in Figure 5e, after 1,000 potential cycles, the LSV curve of the NiFe/NiFe:Pi electrode is almost identical to the initial one, indicating excellent stability of the electrode. Furthermore, with continuous OER at a constant current density of 20 mA cm-2 (Figure 5f), the overpotential of the NiFe/NiFe:Pi electrode remains stable around 300 mV for over 10 h. The morphology and O1s and P2p XPS spectrum remained almost unchanged after long-term stability measurements (Figure S8 and S9). The above data confirm the long-term robustness of the NiFe/NiFe:Pi catalyst, which is crucial for practical applications. To understand the origin of the enhanced OER performance of NiFe/NiFe:Pi, we investigated the electrochemical surface area (ECSA) by determining the double layer capacitance (CDL) (Figure S5 and S6). As expected, the NiFe/NiFe:Pi electrode exhibits almost doubled current than that of NiFe as shown in Figure 6a, indicating almost doubled ECSA and more accessible active sites are created after phosphorylation. The increased surface area in this synergistic catalyst is favorable for the enhanced OER activity. Furthermore, the interfacial properties of the prepared samples and the electrical exchange ACS Paragon Plus Environment

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between the solution and the electrode are evaluated by electrochemical impedance spectroscopy (EIS). All impedance spectra are ﬁtted using an equivalent RC circuit model, as shown in the inset of Figure 6b, 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. Figure 6b shows that the charge transfer resistance (Rct) of NiFe/NiFe:Pi (13.6 Ω) is much smaller than that of NiFe (209.6 Ω), revealing fast shuttling of charge transfers during OER between NiFe/NiFe:Pi and the electrolyte. This enhanced charge transfer rate will enable effective electrical integration to minimize the parasitic ohmic losses and contribute to the improved OER activity of NiFe/NiFe:Pi.40 It is known that OER in alkaline solution typically takes place via four elementary steps:41 OH- + * → OH* + e-

(1)

OH*+ OH-→ O* + H2O + e-

(2)

O* + OH-→ OOH* + e-

(3)

OOH*+ OH-→ O2 + H2O + e-

(4)

Therefore, water adsorption on the active site (*) of the electrode surface is the elementary step to water oxidation. As for the NiFe/NiFe:Pi electrocatalyst, it is found that the formation of phosphate groups is far more favorable to improve the contact between electrolyte and the electrode surface as evidenced by the enhanced surface wettability in Figure 6c and d and much smaller contact angle is observed for NiFe/NiFe:Pi (44°± 3°) than the NiFe electrode (129°± 5°), meaning an enhanced compatibility and affinity of the heterogeneous catalyst to water after phosphorylation, thus leading to an enhancement in its catalytic activity towards OER.


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It should be noted that the surface chemistry as well as the surface wettability of the catalyst may be changed under the applied polarization potential. Therefore, we further carried out the surface wettability test of NiFe/NiFe:Pi and NiFe hydroxide on carbon fiber paper after a long-term substantially high potential catalysis (1.9 V vs. RHE) and the wettability was found significantly improved after electrochemical polarization conditions. Shown in the Figure 6d for NiFe/NiFe:Pi, a contact angle of ～44° was obtained without polarization. After the long-term catalysis at 1.9 V, super hydrophilic behavior is observed and no contact angle can be measured (Figure 7b). For NiFe, a contact angle of ～45° was obtained after the long-term catalysis (Figure 7a), also much smaller than the non-polarized NiFe sample (Figure 6c). These results confirm that phosphorylation can significantly improve the catalyst surface wettability. Especially under the electrochemical polarization conditions, the catalyst surface is super hydrophilic, which subsequently results in better gas bubble dissipation ability (Figure 7d) and higher OER activity. Moreover, the electrode remains stable under a large current density as well (Figure S10), further revealing the robust property of the prepared NiFe/NiFe:Pi catalyst. The enhanced wettability and improved catalytic activity of NiFe/NiFe:Pi compared to NiFe hydroxide was also observed when the CFP substrate was replaced by a nickel foil substrate. As shown from Figure 8a and b, after phosphorylation, a smaller contact angle for NiFe/NiFe:Pi on nickel foil is obtained than the NiFe electrode without treatment, which means the universal of this two-step synthetic strategy for catalyst surface chemical property controls. The polarization performance of NiFe/NiFe:Pi on nickel foil exhibits almost doubled catalytic current density than the NiFe electrode at a potential of 1.65 V (Figure 8c), which once again suggests a strong synergistic effect by the formation of phosphate. The formation of the phosphate was further confirmed by the Raman spectra as shown in Figure 8d as an obvious band is formed near ～1,004 cm-1 in the spectrum, assigning to the symmetric PO4stretching mode in the NiFe/NiFe:Pi composite.42 The other peaks at lower frequency for both ACS Paragon Plus Environment

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NiFe and for NiFe/NiFe:Pi are assigned to the characteristic peaks observed for NiFe hydroxide catalyst.43 The exact reasons for the enhanced wettability and improved catalytic performance over the NiFe/NiFe:Pi catalyst are not sure. However, previous study by Kim et al. showed that cobalt phosphate with distorted cobalt tetrahedral geometry, induced by the phosphate/pyrophosphate groups, can favor water adsorption and enhance water oxidation activity significantly.44 More recently, our work also shows that the phosphate groups in nickel phosphate is favorable for water absorption.45 It is therefore reasonable to deduce that facile adsorption of water is originated from the distorted geometry of nickel or iron with open coordinate sites caused by the phosphate groups, which adjust the catalyst surface wettability that can facilitate water adsorption and subsequent water oxidation. 4. CONCLUSIONS In conclusion, a 3D NiFe/NiFe:Pi porous structured electrocatalyst with enhanced wettability is designed and developed as a low-cost, high efficiency catalyst for OER for the first time. The enhanced OER activities of the synergistic NiFe/NiFe:Pi catalyst are mainly attributed to: i) facile water adsorption originated from the significantly improved surface wettability caused by the phosphate groups which introduce distorted geometry of nickel or iron with open coordinate sites; ii) enhanced electrochemical surface area after phosphorylation which ( ( provide more catalytic active sites for water adsorption and oxidation; iii) 3D hierarchical porous structure which offers excellent interfacial properties for mass transport and charge transfer. The study shows the promise of NiFe/NiFe:Pi as a low-cost efficient anode catalyst ( for (photo) electrochemical water splitting devices. The material design and fabrication strategy by adjusting the wettability of a synergistic electrocatalyst without damaging the porous morphology may present a simple generic approach for phosphorylation of metal


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oxide or hydroxide catalysts with enhanced performance for use in electrochemical energy conversion and storage devices. AUTHOR CONTRIBUTIONS The manuscript was written through contributions of both authors. Both authors have given approval to the final version of the manuscript. Supporting Information Available: Tables S1, Figures S1-S10, and related references. This material is available free of charge via the Internet at http://pubs.acs.org. 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. Bin Gong from MWAC for his assistance in XPS and Dr. Sheng Chen in our group for his discussion. The study was financed by an ARC Discovery Grant (DP160103107). REFERENCES 1. Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Chem. Rev. 2010, 110, 6474-6502. 2. Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Chem. Soc. Rev. 2015, 44, 2060-2086. 3. Li, Y.; Gong, M.; Liang, Y.; Feng, J.; Kim, J. E.; Wang, H.; Hong, G.; Zhang, B.; Dai, H. Nat. Commun. 2013, 4, 1805. 4. Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. Science 2011, 334, 1383-1385. 5. Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Chem. Rev. 2010, 110, 6446-6473. 6. Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. J. Phys. Chem. Lett. 2012, 3, 399-404. 7. Tang, C.; Cheng, N.; Pu, Z.; Xing, W.; Sun, X. Angew. Chem. Int. Ed. 2015, 54, 9351-9355. 8. Kanan, M. W.; Surendranath, Y.; Nocera, D. G. Chem. Soc. Rev. 2009, 38, 109-114. 9. Esswein, A. J.; Surendranath, Y.; Reece, S. Y.; Nocera, D. G. Energy Environ. Sci. 2011, 4, 499-504. 10. Dinca, M.; Surendranath, Y.; Nocera, D. G. P. Natl. Acad. Sci. USA 2010, 107, 1033710341. 11. Chen, S.; Duan, J.; Bian, P.; Tang, Y.; Zheng, R.; Qiao, S. Z. Adv. Energy Mater. 2015, 5, 1500936. 12. Lu, X. Y.; Zhao, C. Nat. Commun. 2015, 6, 6616. 13. Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H. J. Am. Chem. Soc. 2013, 135, 8452-8455. ACS Paragon Plus Environment

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14. Xiao, C.; Li, Y.; Lu, X.; Zhao, C. Adv. Funct. Mater. 2016, 26, 3515-3523. 15. Suen, N. T.; Hung, S. F.; Quan, Q.; Zhang, N.; Xu, Y. J.; Chen, H. M. Chem. Soc. Rev. 2017, 46, 337-365. 16. Gong, M.; Dai, H. Nano Res. 2015, 8, 23-39. 17. Burke, M. S.; Enman, L. J.; Batchellor, A. S.; Zou, S.; Boettcher, S. W. Chem.Mater. 2015, 27, 7549-7558. 18. Gong, M.; Wang, D. Y.; Chen, C. C.; Hwang, B. J.; Dai, H. Nano Res. 2016, 9, 28-46. 19. Anantharaj, S.; Ede, S. R.; Sakthikumar, K.; Karthick, K.; Mishra, S.; Kundu, S. ACS Catal. 2016, 6, 8069-8097. 20. Surendranath, Y.; Kanan, M. W.; Nocera, D. G. J. Am. Chem. Soc. 2010, 132, 1650116509. 21. Wang, L.; Xiao, F. S. ChemCatChem 2014, 6, 3048-3052. 22. McCrory, C. C. L.; Jung, S. H.; Peters, J. C.; Jaramillo, T. F. J. Am. Chem. Soc. 2013, 135, 16977-16987. 23. Smith, R. D. L.; Prévot, M. S.; Fagan, R. D.; Zhang, Z.; Sedach, P. A.; Siu, M. K. J.; Trudel, S.; Berlinguette, C. P. Science 2013, 340, 60-63. 24. Yang, Y.; Fei, H.; Ruan, G.; Xiang, C.; Tour, J. M. ACS Nano 2014, 8, 9518-9523. 25. Teng, X. W.; Black, D.; Watkins, N. J.; Gao, Y. L.; Yang, H. Nano Lett. 2003, 3, 261-264. 26. Bonelli, R.; Zacchini, S.; Albonetti, S. Catalysts 2012, 2, 1-23. 27. Grosvenor, A. P.; Biesinger, M. C.; Smart, R. S.; McIntyre, N. S. Surf. Sci. 2006, 600, 1771-1779. 28. Jin, K.; Park, J.; Lee, J.; Yang, K. D.; Pradhan, G. K.; Sim, U.; Jeong, D.; Jang, H. L.; Park, S.; Kim, D.; Sung, N. E.; Kim, S. H.; Han, S.; Nam, K. T. J. Am. Chem. Soc. 2014, 136, 7435-7443. 29. Steinmiller, E. M. P.; Choi, K. S. P. Natl. Acad. Sci. 2009, 106, 20633-20636. 30. Grosseau-Poussard, J. L.; Panicaud, B.; Pedraza, F.; Renault, P. O.; Silvain, J. F. J. Appl. Phys. 2003, 94, 784-788. 31. Noisong, P.; Danvirutai, C.; Boonchom, B. J. Chem. Eng. Data 2009, 54, 871-875. 32. Hu, Z.; Shen, Z.; Yu, J. C. Chem. Mater. 2015, 28, 564–572. 33. Cha'on, U.; Valmas, N.; Collins, P. J.; Reilly, P. E. B.; Hammock, B. D.; Ebert, P. R. Toxicol. Sci. 2007, 96, 194-201. 34. Qiu, Y.; Xin, L.; Li, W. Langmuir 2014, 30, 7893-7901. 35. Stern, L.-A.; Feng, L.; Song, F.; Hu, X. Energy Environ. Sci. 2015, 8, 2347-2351. 36. Liang, H.; Meng, F.; Cabán-Acevedo, M.; Li, L.; Forticaux, A.; Xiu, L.; Wang, Z.; Jin, S., Nano Lett. 2015, 15, 1421-1427. 37. Ledendecker, M.; Calderon, S. K.; Papp, C.; Steinruck, H. P.; Antonietti, M.; Shalom, M., Angew. Chem. Int. Edit. 2015, 54, 12361-12365. 38. Zhu, Y. P.; Liu, Y. P.; Ren, T. Z.; Yuan, Z. Y. Adv. Funct. Mater. 2015, 25, 7337-7347. 39. Luo, J. S.; Im, J. H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N. G.; Tilley, S. D.; Fan, H. J.; Gratzel, M. Science 2014, 345, 1593-1596. 40. Peng, Z.; Jia, D.; Al-Enizi, A. M.; Elzatahry, A. A.; Zheng, G. Adv. Energy Mater. 2015, 5, 1402031. 41. Bajdich, M.; García-Mota, M.; Vojvodic, A.; Nørskov, J. K.; Bell, A. T. J. Am. Chem. Soc. 2013, 135, 13521-13530. 42. Zhang, L.; Brow, R. K. J. Am. Ceram. Soc. 2011, 94, 3123-3130. 43. Louie, M. W.; Bell, A. T. J. Am. Chem. Soc. 2013, 135, 12329-12337. 44. Kim, H.; Park, J.; Park, I.; Jin, K.; Jerng, S. E.; Kim, S. H.; Nam, K. T.; Kang, K. Nat. Commun. 2015, 6, 8253. 45. Li, Y.; Zhao, C. Chem. Mater. 2016, 28, 5659-5666.


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Figure 1. Schematics of NiFe/NiFe:Pi 3D-hierarchical nanostructures fabricated by first step electrodeposition followed by the second step phosphorylation process.


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Figure 2. SEM images of (a) NiFe hydroxide and (b) NiFe/NiFe:Pi. (c) TEM (inset: corresponding SAED pattern) and (d) HRTEM image of NiFe/NiFe:Pi, respectively.


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Figure 3. (a) High-angle annular dark-field scanning TEM and elemental mapping images of (b) Ni, (c) Fe, (d) P and (e) O. (f) XRD spectra of CFP and NiFe/NiFe:Pi on CFP.


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Figure 4. High resolution XPS spectra of NiFe and NiFe/NiFe:Pi. (a) Fe2p, (b) Ni2p, (c) P2p and (d) O1s.


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Figure 5. (a) OER polarization curves of NiFe, NiFe/NiFe:Pi, Ni:Pi and CFP:Pi and (b) Corresponding Tafel plots. (c) Multi-current chronopotentiometry response for NiFe/NiFe:Pi without iR compensation. (d) Ring current of NiFe/NiFe:Pi (potential at 0.5 V vs. RHE) on RRDE with a constant disk current of 120 µA. (e) LSV curves for NiFe/NiFe:Pi before and after 1,000 CV cycles. (f) Chronopotentiometric curve of NiFe/NiFe:Pi electrode with constant current density of 20 mA cm-2.


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Figure 6. (a) Comparison of the CVs measured in a non-Faradaic region on NiFe and NiFe/NiFe:Pi electrode at scan rate of 200 mV s-1. (b) Electrochemical impedance spectra recorded at overpotential of 300 mV (vs. RHE) on NiFe and NiFe/NiFe:Pi (inset: equivalent RC circuit model). Contact angle measurement of (c) NiFe and (d) NiFe/NiFe:Pi electrode on CFP.

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Figure 7. (a), (b) Contact angle measurement of NiFe and NiFe/NiFe:Pi electrode after longterm stability at an applied potential of 1.9 V (vs. RHE) on CFP. OER gas-bubble effect on (c) NiFe and (d) NiFe/NiFe:Pi electrode, respectively, at an applied potential of 1.9 V.


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Figure 8. Contact angle measurement of (a) NiFe and (b) NiFe/NiFe:Pi electrode on nickel foil. (c), (d) OER polarization curves on nickel foil and Raman spectra of NiFe, NiFe/NiFe:Pi, respectively.


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Table of Contents Graphic

CFP NiFe Hydroxide 300 -2

250

j / mA cm

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200 150 100

NiFe NiFe/NiFe:Pi Ni:Pi CFP:Pi

50 0 1.2

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1.4 1.5 1.6 E / V vs. RHE

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NiFe/NiFe:Pi

By adjusting the surface wettability of the catalyst, a novel porous nanosheet electrocatalyst composed of NiFe hydroxide and NiFe phosphate (denoted as NiFe/NiFe:Pi), is designed and developed. The enhanced surface wettability and increased charge transfer rate at the NiFe/NiFe:Pi catalyst, together with effective mass transport and enlarged catalytic sites offered by the hierarchical porous structure, contribute to the extraordinary water oxidation performance.


Enhancing Water Oxidation Catalysis on a Synergistic Phosphorylated

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