In-Situ Grown, Passivator-Modulated Anodization Derived

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In-Situ Grown, Passivator-Modulated Anodization derived Synergistically Well Mixed Ni-Fe Oxides from Ni Foam as High Performance Oxygen Evolution Reaction Electrocatalyst Xui Fang Chuah, Cheng-Ting Hsieh, Chun-Lung Huang, Duraisamy Senthil Raja, Hao-Wei Lin, and Shih-Yuan Lu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01794 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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ACS Applied Energy Materials

In-Situ Grown, Passivator-Modulated Anodization derived Synergistically Well Mixed Ni-Fe Oxides from Ni Foam as High Performance Oxygen Evolution Reaction Electrocatalyst Xui-Fang Chuah, Cheng-Ting Hsieh, Chun-Lung Huang, Duraisamy Senthil Raja, Hao-Wei Lin, and Shih-Yuan Lu* Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan. KEYWORDS: overall water splitting, electrocatalysis, anodization, nickel oxide, hematite

ABSTRACT

A simple one-step passivator-modulated nickel foam (NF) anodization process was developed to fabricate synergistically well mixed NiO/α-Fe2O3@NF, which was demonstrated a high efficiency electrocatalyst for oxygen evolution reaction (OER), exhibiting ultralow overpotentials and excellent stability at high current densities. The NiO/α-Fe2O3@NF composite electrode, with NiO and α-Fe2O3 well-mixed at nanoscale for much enhanced synergistic effects, exhibited extraordinary electrochemical activities with ultralow overpotentials of 244 and 334 mV at current densities of 50 and 500 mA/cm2, respectively for the OER in 1M KOH. The NiO/α-Fe2O3@NF

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composite electrode was further tested as both the cathode and anode for overall water splitting, and achieved low working cell voltages of 1.76 and 1.99 V at the current densities of 50 and 500 mA/cm2, respectively, outperforming the pairing of Pt/C@NF and IrO2@NF, 1.79 and 2.08 V correspondingly. The stability of the NiO/α-Fe2O3@NF electrode was also outstanding, experiencing only a minor chronoamperometric decay of 5.6 % after a continuous operation at 500 mA/cm2 for 24 hr. The success of the present catalyst design was attributed to the strong positive synergistic effects between the well mixed NiO and α-Fe2O3 at nanoscale and between the nonconductive mixed oxide catalyst and the conductive nickel foam support.

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INTRODUCTION The global energy demand has been increasing continuously and rapidly because of the

tremendous growth of human population and industrialization. Consequently, there has been a significant increase in the utilization of traditional fossil fuels, leading to severe environmental problems, such as the greenhouse effects and air pollution. The quickly expanding energy consumption has resulted in major concerns about energy crisis because of the limited fossil fuel resources. This scenario has triggered extensive research efforts to develop clean and sustainable alternative energy sources. For instance, converting excessive amounts of intermittent renewable energies into chemical fuels that can be stored and transported efficiently is a prospective solution. Among the many energy carriers under development, hydrogen is one of the most promising candidates for the clean energy infrastructure owing to its high gravimetric energy density (120 vs. 44 MJ/kg for gasoline), zero CO2 emission during consumption, high combustion efficiency, and non-toxicity. 1-3 Electrolytic water splitting, composed of hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode, is considered one of the most


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promising approaches for hydrogen production because of its sustainability and simplicity.4-6 Although water is inexhaustible, electrolysis of water has always been economically limited by its high energy consumption and its use of noble metal catalysts further increases the production cost, both hindering the large-scale production application.7-10 Therefore, development of cost-effective and highly efficient electrocatalysts, that can function at low working voltages to drive electrolytic water splitting, is critically demanded. At present, noble metals based catalysts such as Pt and IrO2 remain the most efficient electrocatalysts for the HER and OER, respectively.7, 9-10 Unfortunately, the high cost and relative scarcity of these noble metals hinders the large-scale application of such catalysts.11-13 To circumvent these shortcomings, it is of great importance to seek Earth-abundant alternatives of excellent activities and stability to replace noble metal-based catalysts, particularly for the OER catalyst since OER is the bottleneck of the electrolytic water splitting process. Non-precious transition metal oxides, particularly those of Ni, Fe, and Co, exhibit promising electrolytic performances towards the OER.6,

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Nevertheless, the full potential of these oxide based

electrocatalysts is greatly restricted by their generally poor electrical conductivities, which slow down the critical charge transport involved. To solve this problem, utilization of highly electrically conductive metal foams as the substrate to accommodate the catalyst proves to be an effective way to enhance the electrolytic performances of the oxide based electrocatalysts. Here, nickel foam (NF), a three-dimensional porous material, was employed as the conductive substrate because of its good mechanical strength, flexibility, and high electrical conductivities. The porous structure of the NF provides enhanced mass transport involved in the electrochemical process.23 Most importantly, in situ growth of electrocatalysts on the backbone of the NF ensures intimate contact


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between the electrocatalyst and substrate for efficient charge transport and improved mechanical resistances to bubbling generated stresses.24 On the other hand, the combination of Fe and Ni-based materials has been shown excellent performances for electrocatalytic water splitting, particularly for the OER.25-28 According to recent literature, incorporation of Fe3+ into Ni(OH)2 enhances the OER activity.28-32 Two different explanations have been offered. First, based on modified electronic environment of the NiOOH intermediate, the Fe3+ substituted sites become more active for the OER. Second, the activity of the Ni3+ sites of the NiOOH intermediate is enhanced as a consequence of the electronic property modified by the Fe3+ substitution.30-32 Although the underlying mechanism remains debatable, both views emphasize the role played by Fe3+ substitution in altering the electronic environment of the active intermediate. Thus, the improvement in OER activity can be well-rationalized by modulation of Fe and Ni arrangements at the atomic level to engineer the differences in electron affinity between Ni2+ and Fe3+. Consequently, Ni-Fe based electrocatalysts receive great research attention for OER catalyst development. Nevertheless, fabrication of Ni-Fe based electrocatalysts often involves complex procedures in which multiple steps are required.30, 33-35 Also, solvothermal or hydrothermal reactions at high pressures are often adopted, increasing the danger and cost of the catalyst synthesis.36-40 More importantly, traditional Ni-Fe based electrocatalysts are often composed of separate Ni-based and Fe-based materials from which the synergistic effects of the two materials cannot be fully utilized to enhance the activity of the electrocatalyst. Here, a simple one-step passivator-modulated nickel foam anodization process was developed to fabricate synergistically well-mixed NiO/α-Fe2O3@NF composites as an outstanding OER catalyst. The NiO and α-Fe2O3 domains are well-mixed at nanoscale, greatly boosting the synergistic effects because of the high NiO/α-Fe2O3 interfacial grain boundary density. The anodization proceeds


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through competition of nickel foam dissolution and oxide formation.41-43 By adding Fe-containing precursors into the anodization electrolyte, well-mixed NiO and α-Fe2O3 in-situ grown on the skeleton of the nickel foam was obtained. The in-situ growth approach promises excellent electrode stability over long term water splitting operations as the adhesion of the metal oxides on the metal foam is strong. Moreover, the interaction of Ni2+ of the NiO and Fe3+ of the α-Fe2O3 at the interfacial grain boundary brings positive synergistic effects towards the OER. It is important to note that the overpotentials and stability of most of the previously reported electrocatalysts were assessed at low current densities, which are not appropriate for practical applications.44-45 Measurements of the overpotentials and stability at practically useful high current densities (e.g., above 250 mA/cm2) are absolutely necessary to assess the real merits of the electrocatalysts for large scale applications. To this end, we reported in recent years several efficient and durable OER electrocatalysts functioning at high current densities, for example, Fe and Ni-based mixed metal phosphides dispersed in N-doped carbon,46 mixed NiO/NiCo2O4 on NF,47 N-doped graphene layer coated Fe-Ni alloy nanoparticles encapsulated within an N-doped carbon hollow nanobox,48 and bimetallic metal organic framework on NF.25 The in-situ grown, one-step passivator-modulated anodization derived well-mixed NiO/αFe2O3@NF composite electrode exhibited extraordinary electrocatalytic activities with an ultralow overpotential of 334 mV at 500 mA/cm2 for the OER in 1M KOH. Furthermore, the alkaline water electrolyser, made up of the NiO/α-Fe2O3@NF electrode as both the cathode and anode, required only a low working voltage of 1.99 V to achieve the current density of 500 mA/cm2, outperforming the pairing of Pt/C on NF as the cathode and IrO2 on NF as the anode. The NiO/α-Fe2O3@NF composite thus is an excellent bifunctional electrocatalyst, functioning well for both HER and OER. Moreover, it also showed outstanding mechanical and electrochemical robustness in long


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term stability tests at high current densities, indicating its promising practical application potential. The new synthetic approach developed in this work for synergistically well-mixed composite catalysts can be readily extended to a wide range of catalyst design.

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EXPERIMENTAL SECTION

Pretreatment of NF. Commercial NF with thickness of 1.8 mm was purchased from Changsa Liyuan Port (Changsa, China) and cut into pieces of 1 cm × 3 cm. The NF piece was first ultrasonicated in 3M HCl for 20 min to remove the native surface oxide layer, followed by washing sequentially with DI water, acetone, and ethanol and drying in a vacuum oven at 40 °C for 4 hr. Electropolishing of NF. Prior to anodization, the pretreated NF was subjected to electropolishing, in an acidic medium (6 g CrO3 in 15 mL H2SO4, 70 mL H3PO4 and 14 mL H2O) at 4 V/cm2 for 2 min, to achieve better surface smoothness for later anodic oxidation. The electropolished NF was ultrasonicated in DI water and ethanol to remove the residual electrolyte, followed by drying in a vacuum oven at 40 °C for 4 hr. Anodic Oxidation of NF. The anodic oxidation was conducted in a standard two electrode setup, where the NF acted as the anode and Pt wire as the cathode. The anodization was carried out at 10 V for 30 min using a power supply (Keithley 2000). The electrolyte was composed of an ethylene glycol solution containing 0.15M KOH, 0.01M NH4F, and 3% v/v H2O of different concentrations of FeCl2•4H2O. The electrodes were termed A-NF, 0.04M Fe/A-NF, 0.08M Fe/A-NF, 0.1M Fe/ANF, 0.12M Fe/A-NF, 0.14M Fe/A-NF, 0.16M Fe/A-NF, and 0.2M Fe/A-NF for products obtained from using a Fe precursor concentration of 0, 0.04, 0.08, 0.1, 0.12, 0.14, 0.16, or 0.2M, respectively. Immediately after the anodization, the electrodes were carefully washed with DI water and ethanol and dried in a vacuum oven at 40 °C for 2 hr. The anodization reaction was


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repeated without the participation of NH4F to investigate the role of F- as a passivator. The electrodes were termed 0.04M Fe/NF, 0.1M Fe/NF, 0.14M Fe/NF, and 0.2M Fe/NF for products obtained from using a Fe precursor concentration of 0.04, 0.1, 0.14, and 0.2M, respectively. Thermal Annealing of Electrode. To enhance the crystallinity of the oxide layer formed at anodization, thermal annealing was conducted in a tube furnace at 500 °C with a heating ramp of 5 °C/min for 4 hr under the continuous flow of Argon. Preparation of Pt-C/NF and IrO2/NF. For comparison purposes, similar nickel foam-based benchmark electrodes were prepared by loading nickel foam with 20 wt% Pt/C and IrO2 at 0.5 mg/cm2 for the HER and OER, respectively. An amount of 2 mg catalyst was dispersed in 280 μL of 50% aqueous ethanol and 120 μL of polyvinylidene fluoride (0.3 wt% in N-methylpyrrolidone as binder) with ultrasonication for 30 min to form a homogenous ink solution. 120 μL of the ink solution was drop-casted onto nickel foam, followed by drying at 60 °C in a vacuum oven to afford the benchmark electrodes for comparison with anodized NF. The Pt/C and IrO2 powders were purchased from Alfa Aesar. Characterizations. The crystalline structure of the electrodes was examined with a powder X-ray diffractometer (Shimadzu XRD-6000, Japan) with X-ray source of 3 kW copper target. The morphology of the electrodes was characterized by using a field emission scanning electron microscope (FESEM) (Hitachi SU8010, Japan). EDX (Oxford 6857, Oxford Instruments) spectroscopy was conducted to determine the elemental composition of the anodized electrode with an accelerating voltage of 15 kV. The atomic structure and selected area electron diffraction patterns were characterized by using a high-resolution transmission electron spectroscopy (HRTEM) (JEM-3000F, Japan) with an accelerating voltage of 300 kV. The XPS measurements


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were conducted to study the chemical state of the elements in the electrodes using a monochromatized Al Kα X-ray as the excitation source (XPS, Thermo ESCALAB 250XI, USA). Electrochemical Measurement. Both HER and OER performances were investigated on a CHI6275D electrochemical station with a three-electrode system where nickel foam served as the working electrode, 1 cm × 1 cm Pt foil and graphite served as the counter electrode for the OER and HER, respectively, and Hg/HgO (RE-61AP) served as the reference electrode. The electrolyte was 1M KOH solution with a pH value of 13.85 as measured with a pH meter. Prior to electrochemical measurements, the electrode was conditioned by cycling through the potential window of 0 to 1.0 V vs. Hg/HgO and -0.8 to -1.5 V vs Hg/HgO, respectively for the OER and HER thirty times with a scanning rate of 0.1 V/s. Linear sweep voltammetry (LSV) in a threeelectrode system was performed to determine the overpotential needed to drive the HER, OER, and overall water splitting. The potential windows of the measurements were set at 0 to 1.0 V vs. Hg/HgO and -0.8 to -1.5 V vs Hg/HgO, respectively for the OER and HER with a scanning rate of 0.001 V/s. The potential values recorded for the OER and HER were reported referring to the reverse hydrogen electrode (RHE) using the Nernst equation ERHE = EHg/HgO + 0.118 + 0.0591 pH, where EHg/HgO is the experimentally measured potential against the Hg/HgO reference electrode. The overpotential (ƞ) was calculated with the equation: ƞ = ERHE -1.23 for the OER. The current densities recorded was iR-compensated. The iR-compensation was conducted based on the open circuit potential determined from the CHI workstation. EIS was conducted over a frequency range of 100 kHz to 100 mHz with the sample electrodes as the working electrode and the potential was set at a value high enough to ensure the occurrence of the OER and HER. The measured EIS data were fitted to an equivalent circuit model to extract the charge transfer resistance. The overall water splitting measurement was performed in a two-electrode system. Prior to the overall water


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splitting test, the electrode was conditioned by cycling through the potential window of 1.0 to 2.2 V thirty times with a scanning rate of 0.1 V/s. The potential window of this measurement was set at 1 to 2.2 V with a scanning rate of 0.001 V/s.

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RESULTS AND DISCUSSIONS

Anodization of NF, as illustrated in Figure 1, was carried out using NH4F as the basic electrolyte with FeCl2•4H2O (with increasing concentrations of 0, 0.04, 0.08, 0.1, 0.12, 0.14, 0.16 and 0.2M) as an additive to achieve the one-step in-situ growth of well-mixed Ni-Fe oxides on the skeleton of NF.41 The electrodes thus obtained were termed A-NF, 0.04M Fe/A-NF, 0.08M Fe/A-NF, 0.1M Fe/A-NF, 0.12M Fe/A-NF, 0.14M Fe/A-NF, 0.16M Fe/A-NF, and 0.2M Fe/A-NF, respectively. Figure S1(a) shows the XRD patterns of the anodized samples. The anodized samples exhibit no characteristic diffraction peaks attributable to NiO and α-Fe2O3, and only three sharp diffraction peaks located at 44.5, 51.9, and 76.4 ° for the crystalline planes of (111), (200), and (220), respectively of Ni (JCPDS: 04-0850) appear. It is maybe because of the amorphous nature, poor crystallinity, and/or limited amount of crystalline phases of the product oxide layer. To confirm formation of the expected oxides layer, the anodized samples were thermally annealed in Argon atmosphere to improve the crystallinity and crystalline amount of the oxides layer. Annealing under the Argon atmosphere ensures that the showing of the oxides in the XRD pattern is derived solely from the anodization, instead of from the latter thermal annealing. Note that here thermal annealing is used solely for the purpose of material characterizations, and the electrocatalysts used in this study are all anodized samples without thermal annealing. Figure S1(b) and Figure 2(a) show the XRD patterns of the annealed samples, with the diffraction peaks of NiO (JCPDS 471049) evident after the thermal annealing. Three diffraction peaks observed at 37.3, 43.3, and 62.9


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° belong to the (111), (200), and (220) crystalline planes, respectively of cubic NiO.41, 43 The amount of α-Fe2O3 however was too little to show in the XRD pattern. Note that sample 0.14M Fe/A-NF was chosen as the representative sample for characterizations since it gives the best electrolytic performances as discussed in latter sections. In this anodic oxidation system, the Fions from NH4F act as the passivator promoting oxide formation,42 whereas the Cl- ions from FeCl2 • 4H2O promote the dissolution of Ni of the NF to form Ni2+ for subsequent NiO formation.49-50 It is well-documented that metals suffer from pitting corrosion caused by halide ions and the corrosion power of the halide ions toward metals follows the order of Cl->Br->F->I-.51 It can be observed that the intensity of the diffraction peaks of NiO increases as the concentration of the Fe precursor, thus the Cl- ion concentration, increases (Figure S1(b)). The roles of Cl- and F- in the anodic oxidation were further investigated by repeating the experiments without the presence of NH4F. Figure S2(a) shows the XRD patterns of samples thus obtained, revealing that the peak intensity of NiO remains unchanged regardless of the increasing Cl- ion concentration. The outcome of Figure S2(a) and S2(b) implies that F- promotes the formation of NiO. Without the presence of F-, the extent of NiO formation remains the same even though the dissolution of Ni and thus the availability of Ni2+ increases with increasing Fe precursor concentration. Furthermore, from SEM images, it is evident that anodized samples without the presence of the Fe precursor (A-NF) shows only minor surface roughness on the skeleton of the NF (Figure 2(b)), whereas the surface roughness of sample 0.14M Fe/A-NF is much more pronounced (Figure 2(c)), further proving the role of F- as a passivator promoting oxide formation to increase surface roughness.41, 43

The surface morphologies of the samples prepared at increasing Fe precursor concentrations are

shown in Figures S3-S5 at increasing SEM resolutions. To have a better understanding on the roles played by F- and Cl-, the surface morphology of the samples prepared without the


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participation of NH4F was examined (Figure S6). It is evident that the dissolution of nickel becomes more intense in the absence of the passivator and large holes were observed on the skeleton of the NF. The results further confirm the role of F- as the passivator in anodization.

Figure 1. Schematic illustration of synthetic process for in-situ grown, passivator-modulated anodization derived well mixed NiO/α-Fe2O3@NF.

Figure 2. (a) XRD patterns of annealed A-NF and 0.14M Fe/A-NF. SEM images of samples (b) A-NF and (c) 0.14M Fe/A-NF. (d) HRTEM of sample 0.14M Fe/A-NF. (e) Locally enlarged


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HRTEM image from white region boxed in (d), (f) FFT pattern obtained from white region boxed in (d). (g) Locally enlarged HRTEM image from red boxed region boxed in (d), (h) FFT pattern obtained from red region boxed in (d). (i) SAED patterns of sample 0.14M Fe/A-NF. To further confirm the formation of oxides after the anodization, the samples were investigated with HRTEM. Samples A-NF and 0.14M Fe/A-NF were immersed in an ethanol solution and subjected to continuous ultrasonification for an hour to release the NiO and α-Fe2O3 from the skeleton surface of the anodized NF. Figure S7 shows the HRTEM image of sample A-NF. Interlayer distances of 0.21 and 0.24 nm, corresponding to the d-spacing of crystalline planes (200) and (111) of cubic NiO, respectively were determined. Furthermore, from the SAED pattern, rings corresponding to the (111), (200), (220), (311) and (222) crystalline planes of NiO were observed. Both results prove the formation of the NiO from the anodic oxidation of the NF, in good accordance with the XRD finding. Figure 2(d) shows the HRTEM image of sample 0.14M Fe/ANF, exhibiting both NiO and α-Fe2O3 phases. The two oxide domains are well-mixed at nanoscale and the grain boundaries can be readily identified. The white enclosed region indicates the NiO domain, whereas the red one for the α-Fe2O3 domain. Interlayer distances of 0.21 and 0.184 nm were determined, corresponding to the d-spacing of the crystalline planes (200) of cubic NiO and (024) of rhombohedral α-Fe2O3, respectively (Figure 2(e)-2(h)). Besides, two sets of diffraction rings were identified from the corresponding SAED patterns (Figure 2(i)), one for NiO and the other for α-Fe2O3,52,53 again proving the co-existence of the two phases. To further investigate the surface chemical states of the samples, X-Ray photoelectron spectroscopy (XPS) was conducted. It is to be noted that the substrate NF is present during the XPS measurement. The full spectra of samples A-NF and 0.14M Fe/A-NF are presented in Figure 3(a) for comparison, both exhibiting signals from C 1s, O 1s, and Ni 2p with that of Fe 2p


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appearing only for sample 0.14M Fe/A-NF as expected. The details on the chemical states of Ni 2p and Fe 2p are discussed using the high-resolution XPS spectra (Figure 3(b) and 3(c)). The spectra of Ni 2p can be deconvoluted to exhibit binding energy peaks of 856 and 874 eV for Ni 2p3/2 and Ni 2p1/2, respectively, and associated satellite peaks of 862 and 880 eV. The binding energy difference between Ni 2p3/2 and Ni 2p1/2 is 18 eV, in good agreement with previously reported data.25, 42, 47 These peaks were indexed to Ni2+, further confirming the oxidation state of NiO. As for the spectra of Fe 2p, binding energy peaks located at around 711 and 725 eV are identified for Fe 2p3/2 and Fe 2p1/2 for sample 0.14M Fe/A-NF, indicating the 3+ oxidation state of α-Fe2O3.25,53 The binding energy difference between these two peaks is 14 eV, arising from the spin-orbit splitting effect.53 As for sample A-NF, no signals can be detected for Fe 2p as expected. Besides, the presence of Fe in sample 0.14M Fe/A-NF as evidenced from the EDX spectrum further supports the formation of α-Fe2O3 (Figure S8). The atomic ratio of Fe increases as the Fe precursor concentration increases, as there are more Fe2+ ions available for formation of α-Fe2O3 (Figure 3(d)). Note that the atomic ratios of Fe are relatively small, and this explains the absence of the diffraction peaks of α-Fe2O3 peaks in the XRD patterns, even after the thermal annealing treatment. Furthermore, the elemental mapping of SEM-EDX shows the uniform distributions of Fe, Ni, and O elements, further implying uniformly well-mixed NiO and α-Fe2O3 (Figure S9). From the outcomes of XRD, HRTEM, SAED, XPS, EDX, and SEM, it can be concluded that uniformly well-mixed NiO and α-Fe2O3 is successfully created via the proposed one-step, passivator-modulated anodization.


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Figure 3. (a) Full survey XPS spectra of samples A-NF and 0.14M Fe/A-NF. High resolution XPS spectra of samples A-NF and 0.14M Fe/A-NF: (b) Ni 2p and (c) Fe 2p. (d) Fe atomic % of anodized samples from EDX measurements. The electrocatalytic activities of the anodized electrodes towards OER were evaluated by using the anodized electrodes as the working electrode in 1M KOH electrolyte. For comparison, the electrocatalytic activities of blank NF, and well-known benchmark catalyst, IrO2 on NF (termed as IrO2/NF) were also measured. The linear sweep voltammetry (LSV) polarization curves (Figure 4(a)) show an oxidation peak at around 1.37 V (vs. RHE) before the sharp increase in current density caused by the water oxidation reaction for oxygen evolution. The oxidation peak is contributed by formation of the active species NiOOH through oxidation of Ni2+ to Ni3+, and


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NiOOH has been identified as the active species to catalyze the OER.24,54 Consequently, the intensity of this characteristic peak, or more precisely the area enclosed by the peak, signifies the amount of NiOOH formed in the system and correlates positively with the efficiency of the electrocatalyst.28 Accordingly, sample 0.14M Fe/A-NF exhibits remarkable catalytic activities towards the OER, with an ultralow overpotential of 244 mV at 50 mA/cm2, which is much lower than those of the blank NF (433 mV) and IrO2/NF (364 mV). As for the sample A-NF, an overpotential of 340 mV is needed to deliver a current density of 50 mA/cm2, much higher than that of sample 0.14M Fe/A-NF, showing the advantage associated with the synergistically wellmixed NiO and α-Fe2O3. Note that, for sample 0.14M Fe/A-NF, the oxidation peak overlaps with the OER curve at current densities lower than 25 mA/cm2 and thus we determine the overpotentials at 50 mA/cm2 to avoid the possible interference from the oxidation peak. Among all anodized electrodes, it is evident that η50 is the highest for sample A-NF, decreases with increasing Fe precursor concentration, reaches the lowest level for sample 0.14M Fe/A-NF, and bounces back up for sample 0.16M Fe/A-NF and 0.2M Fe/A-NF (Figure S10(a)). The increase in Fe precursor concentration gives thicker NiO layers. There are two competing effects associated with the increase of the NiO layer thickness. First, the increase in NiO layer thickness increases the charge transport resistance involved in the oxidation of NiO to form the active intermediates, resulting in less oxidation. Second, the increase in NiO layer thickness also increases the availability of NiO for NiO oxidation, leading to more oxidation. Competition of the above two opposite effects leads to an optimal NiO thickness and thus an optimal Fe loading, sample 0.14M Fe/A-NF, to give the most intensive oxidation peak. Furthermore, the state of the optimal NiO thickness and Fe loading leads to the strongest synergistic effect between NiO and α-Fe2O3 toward the electrochemical water splitting performances, thus the lowest overpotentials. The lowest overpotentials are


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achieved with the significant shift of the oxidation peak toward the less positive region. Although overpotentials at low current densities (50 mA/cm2 in this study) have been commonly used to quantify the electrocatalytic activity of the sample, it is inappropriate for commercial applications of the electrocatalyst. For commercial applications, the electrolyzers ought to be operated at high current densities to be economically useful and competitive. Therefore, the overpotentials at an ultrahigh current density of 500 mA/cm2 (ƞ500), essential for large-scale production but rarely reported and discussed in literature, were also determined. Again, sample 0.14M Fe/A-NF achieves the lowest ƞ500 of 334 mV. The trend of ƞ500 follows that of ƞ50, with 0.14M Fe/A-NF sample achieving the lowest overpotentials. These data were summarized in Table 1 for comparison.

Figure 4. OER activities of samples A-NF, 0.14M Fe/A-NF, IrO2/NF, and blank NF. (a) LSV polarization curves. (b) Tafel slopes. (c) Nyquist plots recorded at 1.64 V (vs. RHE), inset: equivalent circuit model. The catalytic kinetic characteristics of the electrodes are often investigated and quantified by Tafel slopes. The Tafel slope gives the information on how effectively the electrocatalyst can boost the current density at the expense of increasing overpotential, signifying the potential increase needed to achieve a tenfold boost in current density. Smaller Tafel slopes are desired so that high current densities can be achieved at relatively low overpotentials, leading to less electricity


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consumption. The results are shown in Figure 4(b) and S10(c) and summarized in Table 1 for comparison. Sample 0.14M Fe/A-NF again outperforms all other electrodes with a Tafel slope of 69.6 mV/dec. Tafel slopes of 87.0, 87.3, and 105 mV/dec were achieved by the A-NF, IrO2/NF and blank NF electrodes, respectively. The small Tafel slope of sample 0.14M Fe/A-NF leads to the low overpotentials at high current densities. The trend of the Tafel slope again is in consistence with those of ƞ50 and ƞ500. To further investigate the electrocatalytic kinetic characterizations of the samples, electrochemical impedance spectroscopy (EIS) was conducted. The outcomes from the EIS spectra provide the information on charge transfer resistances at the electrode/electrolyte interface, which is the key parameter to evaluate the OER performance of the electrode. The EIS was conducted at 1.64 V (vs. RHE) over a frequency range of 100 kHz to 100 mHz in 1M KOH. The setting of 1.64 V (vs. RHE) is to ensure the occurrence of the OER for all tested electrodes. The resulting Nyquist plots are presented in Figure 4(c) and S10(d) and fitted with an equivalent circuit model to extract the charge transfer resistance. In the equivalent circuit model, Rs, Rp, and Rct denote the system, electrode porosity, and charge transfer resistances, respectively, whereas CPE and Cdl represent the constant phase element and double-layer capacitance, respectively.25 Rapid charge transfer at the electrode/electrolyte interface is required for fast OER. Therefore, small Rct is desired for good OER catalysts. The small Rct value of 0.238 Ω for sample 0.14M Fe/A-NF, significantly lower than 1.084, 1.918, and 3.862 Ω of the A-NF, IrO2/NF, and blank NF electrodes, respectively, indicates the high electrolytic activity of sample 0.14M Fe/A-NF.25 The Rct values are also tabulated in Table 1 for comparison. The trend of Rct also is in good accordance with those of ƞ50, ƞ500, and Tafel slope.


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Owing to the outstanding OER activities, the HER activities of the present anodized electrodes were also investigated. The HER performances of the anodized electrodes along with the blank NF electrode and benchmark HER catalyst, 20% Pt/C, on NF (termed as Pt-C/NF), were evaluated in 1M KOH. Here, to be consistent with the OER measurements, η50 is determined for the HER to assess the low current density activities. The relevant polarization curves are presented and compared in Figure 5(a) and S11(a). Interestingly, sample 0.14M Fe/A-NF also exhibits the best HER activity with a low overpotential of 243 mV at 50 mA/cm2, which is much lower than those of the A-NF (330 mV) and blank NF (317 mV) electrodes, and comparable with that of the PtC/NF (236 mV) electrode. Moreover, the low ƞ500 value of only 354 mV achieved by sample 0.14M Fe/A-NF indicates its great potential as a bifunctional electrocatalyst for commercial electrolyzers. Although the ƞ50 of sample 0.14M Fe/A-NF is slightly larger than that of the PtC/NF electrode, the ƞ500 of sample 0.14M Fe/A-NF (354 mV) is much lower than that of the PtC/NF (504 mV) electrode, making sample 0.14M Fe/A-NF a much more cost-effective candidate for large-scale hydrogen production. The ƞ50 and ƞ500 values were summarized in Table 1 for comparison.

Figure 5. HER activities of A-NF, 0.14M Fe/A-NF, Pt-C/NF and blank NF. (a) LSV polarization curves. (b) Tafel slopes. (c) Nyquist plot recorded at -0.31 V (vs. RHE), inset: equivalent circuit model.


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The HER kinetic characteristics are next studied with Tafel plots, as shown in Figure 5(b) and S11(c). Sample 0.14M Fe/A-NF exhibits the lowest Tafel slope of 93 mV/dec among all anodized electrodes, lower than those of the comparison electrodes including the A-NF (121 mV/dec), PtC/NF (168 mV/dec), and blank NF (179 mV/dec) electrodes. The small Tafel slope of sample 0.14M Fe/A-NF leads to low overpotentials at high current densities. The Tafel slope values are tabulated in Table 1 for comparison. The trend of the Tafel slope follows those of the ƞ50 and ƞ500. To further investigate the kinetic characteristics of the electrodes, EIS was conducted at -0.31 V (vs. RHE) over a frequency range of 100 kHz to 100 mHz in 1M KOH (Figure 5(c) and S11(d)). The Nyquist plots were fitted with an equivalent circuit model same as the one used for the OER. Sample 0.14M Fe/A-NF again achieves the smallest Rct value of 1.857 Ω among all anodized electrodes and the blank NF electrode. The Rct of sample 0.14M Fe/A-NF is comparable with that of the Pt-C/NF electrode, the commonly used benchmark electrode for the HER.7-8 The Rct values are summarized Table 1 for comparison. As evident from Table 1, optimal OER and HER performances were achieved by sample 0.14M Fe/A-NF, clearly showing the existence of an optimal Fe concentration to achieve the optimal electrochemical performances according to Fig. 3(d), a strong implication of the synergistic effect between NiO and α-Fe2O3.

Table 1 Summary of OER and HER performances. OER Electrode

ƞ50

ƞ500

(mV) A-NF 0.04M Fe/A-NF

HER Rct

ƞ50

ƞ500

(mV)

Tafel slope (mV/dec)

(Ω)

(mV)

340

423

87.0

1.084

323

413

83.2

0.868

Rct

(mV)

Tafel slope (mV/dec)

(Ω)

330

479

121

3.629

338

464

120

3.464


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0.08M Fe/A-NF

317

395

81.7

0.828

316

466

116

3.42

0.1M Fe/A-NF

310

391

78.0

0.8228

316

453

114

3.149

0.12M Fe/A-NF

303

391

76.3

0.5591

303

442

112

3.126

0.14M Fe/A-NF

244

334

69.6

0.238

243

354

93

1.857

0.16M Fe/A-NF

268

370

74.4

0.408

295

421

100

2.018

0.2M Fe/A-NF

298

377

75.7

0.5164

300

432

113

3.072

IrO2/NF

364

504

87.3

1.918

-

-

-

-

Pt-C/NF

-

-

-

-

236

504

168

1.862

Blank NF

433

804

105

3.862

317

523

179

4.633

To further explore the contributing factors to the electrocatalytic activity of the samples, electrochemical active surface areas (ECSA) were determined. It is well known that ECSA is directly proportional to the amounts of active sites of the electrocatalyst, and the ECSA can be determined from the electrochemical double-layer capacitance (Cdl).47-48 The Cdl values of the samples were determined based on the cyclic voltammograms recorded at a non-Faradaic potential window (0.74-0.83 V vs. RHE) at increasing scan rates (10-100 mV/s). The results are shown in Figure S12. The Cdl was obtained as the slope of the capacitive current density achieved at 0.812 V (vs. RHE), a suitable potential within the non-Faradaic potential window, versus scan rate curve (Figure 6(a)). As expected, sample 0.14M Fe/A-NF gave the highest Cdl value of 2.09 mF/cm2, 3fold of that of the blank NF electrode. Figure 6(b) presented the Cdl values of all anodized electrodes and the blank NF electrode. The trend of the Cdl value is the same as those of the ƞ50, ƞ500, and Tafel slopes of both OER and HER. Further examination of Table 1 and Fig. 6 reveals that there exists a critical Fe concentration, above which the synergistic effect imparted by the incorporation of α-Fe2O3 becomes pronounced.


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Figure 6. (a) Linear fitting of capacitive current density achieved at 0.812 V (vs. RHE) vs scan rates in 1M KOH. (b) Cdl values of all the electrodes. Sample 0.14M Fe/A-NF exhibits outstanding OER performances and considerable HER performances, making it a promising bifunctional electrocatalyst for overall water splitting. Therefore, full cell overall water splitting was conducted in a two-electrode system using 1M KOH as the electrolyte and the 0.14M Fe/A-NF electrode as both the anode and cathode. Only 1.76 V of potential was needed to deliver a current density of 50 mA/cm2. This working cell voltage is lower than that of the Pt-C/NF//IrO2/NF couple (1.79 V) and much lower than the blank NF//blank NF couple (2.05 V), as shown in Figure 7a. Large-scale production requires operations at high current densities. The 0.14M Fe/A-NF//0.14M Fe/A-NF couple outperforms the comparison couples at 500 mA/cm2, with a cell voltage of only 1.99 V versus 2.08 and 2.92 V for the PtC/NF//IrO2/NF and blank NF//blank NF couples, respectively, making it an outstanding candidate for large-scale hydrogen production.


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Figure 7. (a) LSV polarization curves of 0.14M Fe/A-NF//0.14M Fe/A-NF, Pt-C/NF//IrO2/NF and Blank NF//Blank NF couples. Stability tests for overall water splitting of 0.14M Fe/A-NF//0.14M Fe/A-NF couple, normalized chronoamperometric curve (i-t) curves at (b) 50 and (c) 500 mA/cm2 for 24 hr. Inset of (c): enlarged curves for last 30 min. The stability of the electrocatalyst at high current densities is one of the major concerns for largescale production. The stability of sample 0.14M Fe/A-NF was assessed at current densities of 50 and 500 mA/cm2 with chronoamperometric measurements for 24 hrs. Figure 7b and 7c show the chronoamperometric (i-t) curves of the 0.14M Fe/A-NF//0.14M Fe/A-NF couple at 50 and 500 mA/cm2, respectively. The 0.14M Fe/A-NF//0.14M Fe/A-NF couple shows excellent stability,


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with only 3.06 and 5.6% decay in current densities from the initial 50 and 500 mA/cm2 after a 24hour continuous operation. Figure S13 shows the LSV polarization curves before and after the long-term stability tests at 50 and 500 mA/cm2. It shows almost no decay in current density after a 24-hour continuous operation at 50 mA/cm2. However, the decay increases with increasing the current density because of the more intense electrochemical reaction and bubbling involved. It is to be noted that the electrochemical reaction is extremely intense and large amounts of bubbles are generated and released inducing large stresses at the high current density of 500 mA/cm2. This imposes an extremely severe stability test on the electrode both electrochemically and mechanically. The ultra-durability of sample 0.14M Fe/A-NF at high current densities was further investigated by performing SEM (Figure S14 and S15), EDX (Figure S16), elemental mapping (Figure S16), and XPS measurements (Figure S17 and S18) on the post-water splitting sample. The outcomes of the above characterizations confirm the ultra-stability of sample 0.14M Fe/A-NF. Finally, the amounts of the hydrogen and oxygen production by overall water splitting of the 0.14M Fe/A-NF//0.14M Fe/NF couple at 80 mA/cm2 for 60 minutes were quantified through gas chromatography measurements. The results are compared with the theoretical values calculated from the measured current densities. As shown in Figure S19, the agreement between the measured and calculated values is excellent and the ratio of the hydrogen versus oxygen production agrees well with the theoretical value of 2, confirming the water splitting as the sole electrochemical reaction occurring during the process. Table S1 was constructed to compare the OER and overall water splitting performances of sample 0.14M Fe/NF with those of recently reported state-of-the-art nickel foam-based bifunctional electrocatalysts. It is evident that the present passivator-modulated anodization


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derived composite electrode outperforms the comparison electrodes, especially at high current densities of 500 mA/cm2, which is essential in commercialization. The highest current density reported in the literature is 100 mA/cm2, one fifth of that investigated in this work. Furthermore, the long-term stability test from literature are mostly conducted at low current densities of 20 and 100 mA/cm2, making sample 0.14M Fe/A-NF an outstanding candidate for large scale application and commercialization. The success of the present development may be attributed to the following factors. (i) The onestep anodization approach enables the in-situ growth of uniformly well-mixed NiO and α-Fe2O3 on the surface of the skeleton of the NF, without use of non-conductive polymeric binders. Binderfree systems offer low electrical resistances for the critical charge transport. Furthermore, in-situ grown of NiO and α-Fe2O3 promotes intimate contact between the electrocatalyst and the substrate. This ensures the ultra-durability of the electrode at high current densities, where it strengthens the mechanical resistance to bubbling-induced stresses. In addition, the strong contact between the catalyst and the conducting support lessens the contact resistances to charge transport at the interface.25,47,55,56 (ii) The porous structure of the NF enhances the mass transfer of the electrolyte and prevent the overgrowth of bubbles. Gas bubbles accumulating on the surface of the electrode block the active sites and thus hamper the OER and HER. The three- dimensional through-pore structure of the NF serves as open channels for effective gas release, promising continuously available active sites for water splitting. (iii) The surface roughness created on the backbone of the anodized NF is useful to expose high percentages of active sites to the electrolyte, enhancing the utilization of the electrocatalyst for the OER and HER.55,56 The results from the ECSA study serves as a solid support of this point. (iv) The relatively more hydrophilic nature of the Ni-Fe oxides layer formed on the surface of the skeleton of the NF improves the mass transfer of the electrolyte,


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enhances the contact between the electrolyte and electrocatalyst, and thus boosts the OER, HER, and overall water splitting performances. The outcomes of the EIS study support the above argument. Furthermore, static contact angle measurements were performed to investigate the hydrophilic nature of the anodized and NF electrode. Figure S20 shows that sample 0.14M Fe/ANF exhibits much better hydrophilicity than the blank NF electrode does. (v) The synergistic effect between the well-mixed NiO and α-Fe2O3 boosts the overall water splitting performance. It is believed that the interaction of Fe3+ of α-Fe2O3 with Ni2+ of NiO can increase the intrinsic electrical conductivity of the electrode, benefiting the charge transport involved. Besides, the enhanced NiOOH formation with the interaction of Fe3+ with Ni2+ promotes the water splitting.29-32 Furthermore, the NiO and α-Fe2O3 domains are mixed at nanoscale, creating large amounts of interfacial grain boundaries for the synergistical effects to take place.



CONCLUSIONS

In summary, uniformly well-mixed Ni-Fe oxides/NF composite electrode was successfully fabricated through a one-step, passivator-modulated anodization by simply introducing Fe precursors into the electrolyte. With the participation of the passivator F- and dissolution promoter Cl- in the anodic oxidation reaction, the backbone of the NF was roughened, leading to more exposures of the active sites. Because of the advantageous composition of catalysts with the 3D porous conducting NF, the fabricated NiO-α-Fe2O3/NF composite is demonstrated to be a cost effective and highly efficient oxygen evolution electrocatalyst for overall water splitting reaction, with promising stability at high current densities of 500 mA/cm2. This newly developed 0.14M Fe/A-NF proves to be an outstanding electrocatalyst candidate for large-scale electrolytic


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hydrogen production. The design concept developed herein can be readily extended to other electrocatalytic applications.



ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx. In includes additional material characterizations of prepared samples (XRD patterns, SEM images, HRTEM, SAED, EDX spectra, and elemental mapping); additional electrochemical performances of prepared samples; material characterizations and electrochemical performances of 0.14M Fe/A-NF electrode after long-term stability; comparison of OER and overall water splitting between present work and literatures.



AUTHOR INFORMATION

Corresponding Author *Email: [email protected] ORCID: 0000-0003-3217-8199



ACKNOWLEDGMENT

The authors acknowledge the financial support offered by the Ministry of Science and Technology of Taiwan, R.O.C. and Chang Chun Petrochemical Corporation and Swancor Ind. Co., Ltd. under grant MOST 106-2622-8-007-017.


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

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In-Situ Grown, Passivator-Modulated Anodization Derived

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