An Investigation of Fe-Based Integrated Electrode for Water Oxidation

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

An Investigation of Fe-Based Integrated Electrode for Water Oxidation in Neutral and Alkaline Solutions Jianying Wang, Shangshang Zuo, Guangfeng Wei, Yanli Niu, Lixia Guo, and Zuofeng Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01974 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 19, 2019

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The Journal of Physical Chemistry

An Investigation of Fe-Based Integrated Electrode for Water Oxidation in Neutral and Alkaline Solutions Jianying Wang,† Shangshang Zuo,† Guangfeng Wei, Yanli Niu, Lixia Guo, and Zuofeng Chen* Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China Abstract: We report here an iron-integrated transparent electrode by a scalable one-pot hydrothermal reaction and its applications as an active electrocatalyst or as a functional electrode substrate for the oxygen evolution reaction. Characterization of the electrode reveals that it is a well-defined, compact β-FeOOH film whose thickness, photoelectric properties, and catalytic performance are controllable by varying synthetic conditions. The direct-growth strategy is beneficial in its improved electrical conductivity compared to catalystpowder-coated electrodes. The integrated electrode exhibits an onset overpotential of 383 mV toward OER and a Tafel slope of 36 mV/decade at pH 7, which are impressive results for an iron-based catalyst. Calcination of β-FeOOH at 650 °C gives -Fe2O3 which allows for a direct comparison of catalysis performance between the two electrodes and for a comparison by density functional theory. The iron-integrated electrode can also serve as an excellent electrode substrate for stepwise fabrication of a high-performance NiFebased electrocatalyst for OER in alkaline media. In this line, the β-FeOOH film electrode is first calcined at a low temperature of 215 °C which produces amorphous Fe2O3. Consequently, the amorphous nature of this electrode substrate favors anodic deposition of Ni with a maximized Ni-Fe contact resulting in a high-performance NiFe electrocatalyst. Introduction Hydrogen is a promising energy carrier for replacing traditional fossil fuels. In large-scale water splitting to hydrogen and oxygen, water oxidation (OER) is the kinetic bottleneck with far higher overpotentials than electrodes for hydrogen evolution (HER). Iridium and ruthenium oxides1 have been shown to act as efficient OER catalysts but are expensive, enhancing the overall cost of H2 production. Many of the recent studies are thus focused on firstrow transition-metal-based materials for OER.2-5 Although significant progress has been made, a substantial effort is still demanded to lower costs by simplifying the preparation and improve catalytic performance. In many examples, the electrocatalytic materials are pre-prepared as solid powders and then cast as extraneous layers onto electrode surfaces using electrical-insulating binding agents, such as Nafion. The fabrication procedures typically lead to deteriorated electrical conductivity and mechanical strength of the electrocatalyst and a decrease in catalytic performance and stability. An additional complication for solid powders arises from strong light scattering which limits their applications on the surfaces of light-absorbing devices. Iron is the most abundant, environment-friendly transition metal in the Earth’s crust. It’s appearance in the OER utilizes its redox properties and its extensive biological/biomimetic activities in iron enzymes for oxygen activation.6,7 Iron oxides (i.e., Fe2O3) are known to be efficient photocatalysts for OER,8-13 but compared to cobalt and nickel, the use of iron-based electrocatalysts for OER has been far less explored.14-19 For oxide catalysts a limit to reactivity arises from low conductivity20,21 requiring a systematic approach to electrode design in which a decrease in structure reduces the contact resistance among the nanoparticles within electrocatalytic film. To enrich the list of iron-based OER electrocatalysts, one approach is to integrate the results of previous studies on the preparation of photoactive iron-based materials or of their intermediates. Identification of an apppropriate procedure may

also offer an additional benefit of enhanced photoelectric properties for an OER electrocatalyst that could be of relevance in photoelectrochemical applications.22,23 In earlier reports, several approaches have been reported for the preparation of iron-based photocatalyst films. They include photochemical metal-organic deposition,24 electrodeposition,25 pulsed-laser deposition,26 successive ionic-layer adsorption,27 and ultrasonic spray pyrolysis.28 Recently, the hydrothermal reaction has been applied as an intermediate step in the preparation iron-based photocatalyst films.29 An advantage of this approach is that it can be carried out on a large scale without a need for expensive instrumentation and is more controllable, reproducible, and versatile than other approaches. Iron is also an important co-catalyst for cobalt and nickel with NiFe-based electrocatalysts ulilized for the OER in alkaline media.30-48 The preparation of the latter has been dominated by codeposition or co-reactions between Ni and Fe precursors.33-36 Stepwise procedures for high-performance OER electrocatalysts are uncommon because of the challenge of obtaining efficient NiFe contact to exploit synergistic metal-metal interactions. We report here the fabrication of a well-defined, compact FeOOH film integrated onto an FTO (F-doped SnO2) substrate as a product of a one-pot hydrothermal synthesis. The procedure has been used to prepare a highly active electrocatalyst for OER or as an electrode substrate for preparing NiFe OER electrocatalysts (Scheme 1). The direct-growth strategy avoids the use of electrical-insulating agents or binders with enhanced electrical conductivity of the electrode. The electrode exhibits remarkable photoelectric properties and mechanical strength. It is highly active toward the OER in neutral media, far more active than a FeOOH powder-coated electrode prepared by electrophoresis or its dehydrated counterpart, -Fe2O3, formed by calcination at 650 °C. Moreover, calcination to amorphous Fe2O3 (a-Fe2O3) at a low temperature 215 °C, followed by Ni addition by simple anodic potential scans, gives a high-performance NiFe-based electrocatalyst for OER in alkaline solutions with a capacity for scale up for practical applications.

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Scheme 1. Fabrication procedure for -FeOOH/FTO and NiFeOx/FTO electrodes and their utilization for OER. aFe2O3/FTO: amorphous Fe2O3/FTO. Characterization - the nature, photoelectric properties, and mechanical strengths of integrated electrodes. The fabrication of the electrode -FeOOH/FTO was accomplished by a facile one-pot method, i.e., by treating FTO slides in an autoclave containing aqueous FeCl3 and NaNO3 at 95 °C. As shown in Figures 1A and 1B, the as-formed iron-based film is quite uniform and yellow in color. The film thickness is closely related to the concentration of FeCl3 and the reaction time, which can be seen by the change in electrode color over time. The FeOOH film was specifically developed for the FTO substrate. In a control experiment, the use of ITO (Sn-doped In2O3) revealed no film deposition on the electrode surface. In the other hydrothermal reaction with AZO (Al-doped ZnO), dissolution of the AZO layer led to a loss of electrode nonconductivity. To deconvolute the key substrate component (SnO2 or the F dopant) that governs the growth of -FeOOH on FTO, we prepared a nonconductive, F-free SnO2 film coated on a glass substrate by magnetron sputtering. The latter was then subjected to a hydrothermal treatment under the same experimental conditions. As shown in Figure S1, the -FeOOH film was also developed on the surface of the F-free SnO2 film consistent with strong cohesion between -FeOOH and SnO2 in the integrated electrode. Figures 1C and 1D show top-down and cross-sectional SEM images of -FeOOH films prepared in a 15 mM FeCl3 solution for 1 h. The films consist of shuttle-like nanoparticles that completely cover the FTO substrate with a thickness of ~80 nm. Figure S2 provides additional SEM images of the -FeOOH film prepared in a 60 mM FeCl3 solution for the same period giving films of thickness ~400 nm. For reference, the SEM images of blank FTO are shown in Figure S3. In Figure 1E, the TEM image at low magnification shows a more distinct shuttle-like structure for a sample following removal from the FTO substrate. In Figure 1F, the HRTEM image demonstrates clear lattice fringes with a d-spacing of 0.25 nm, which is in good agreement with the (211) plane of -FeOOH. The SAED pattern in Figure 1G displays well-arranged sharp diffraction spots, consistent with crystalline -FeOOH.

Figure 1. Photos of -FeOOH/FTO electrodes prepared in FeCl3 solutions at different concentrations for 1 h (A) and in a 15 mM FeCl3 solution for different time periods (B); (C) top-down and (D) cross-sectional SEM images of an -FeOOH/FTO electrode prepared in a 15 mM FeCl3 solution for 1 h. (E) TEM image, (F) HRTEM image, and (G) SAED pattern of the -FeOOH material scraped from the surface of FTO. Additional experimental evidence was obtained to characterize the in-situ -FeOOH films. Figure 2A shows an XRD pattern for a film scraped from FTO. The diffraction peaks are indexed to crystalline -FeOOH (JCPDS 75-1594). The EDX spectrum in Figure 2B and the XPS survey spectrum in Figure S4 are consistent with the existence of Fe and O in the film with Sn from the FTO substrate. In Figure 2C, the high-resolution XPS spectrum of Fe 2p includes characteristic peaks at ~711.2 and 724.8 eV with small satellite peaks.49 The energy separation between the envelope (711.2 eV) and satellite (719.3 eV) of Fe 2p3/2 is ~8.1 eV, is consistent with Fe(III) rather than Fe(II) (~5 eV).15,16 In addition, the envelope of the Fe 2p3/2 spectrum could be fit to a sequence of multiplets and the surface peak for Fe3+ 2p3/2.15,16,50 In Figure 2D, the high-resolution XPS spectrum for O 1s was deconvoluted into three peaks. The peaks at 529.6 and 531.2 eV are consistent with oxygen of O2‒ and OH‒, respectively, while the peak at the higher binding energy of 532.3 eV is characteristic for absorbed water.21,49 In Figure 2E, the FTIR spectrum of the deposited film also shows typical features for -FeOOH. The band at 3453 cm–1 arises from the OH stretching mode in -FeOOH with contribution from the stretching mode of surface-adsorbed H2O.51,52 The bands at 840 and 697 cm–1 can be assigned to the deformation vibrations of OH– groups.52 Figure S5 shows the Raman spectrum of the ironbased film. The bands at 213 and 275 correspond to the A1g(1) and Eg2 + Eg3 modes of hematite, respectively.53 The presence of these hematite bands is rationalized by the metastability of the FeOOH nanocrystals with respect to dehydration by laser power during Raman measurements.53 Indeed, the film changed from yellow to red after laser irradiation.54 Figure 2F shows the thermal decomposition of the -FeOOH sample. The peaks at 215 and 310 °C are indicative of the dehydration of -FeOOH to form Fe2O3.51,55 Together, these results are in good agreement with formation of surface-bound crystalline, -FeOOH on FTO.


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The Journal of Physical Chemistry glass cutter. The distance between grooves is ~1.5 mm with stabilization of the film strips occurring during slicing. Figures 3E and 3H illustrate images of -FeOOH/FTO electrodes after sonication in a water bath for 1 h. The results show that the film was only slightly attenuated after long-term sonication with no evidence for film loss. Both results are consistent with significant adhesion strength for the -FeOOH film on FTO and for the compact nature of the film.

Figure 2. XRD pattern (A), EDX spectrum (B), high-resolution XPS spectra of Fe 2p (C) and O 1s (D), FTIR spectrum (E), and TGA curve for the pyrolytic decomposition (F) of the -FeOOH film. The -FeOOH/FTO electrode exhibits excellent and controllable photoelectric properties and a high mechanical strength. As shown in Figures 3A, 3B, and S6, the transmittance of the film decreases with increasing film thickness with the latter controllable by varying the FeCl3 concentration and reaction time. For a -FeOOH/FTO electrode of film thickness of 80 nm, the average transmittance is 81% from 400 - 800 nm. Increasing the film thickness to 400 nm reduced the transmittance to 70% with the latter a key in photoelectrochemical device design.22,23 Film thickness can also dictate electrical conductivity which may become a limiting factor for the electrocatalytic performance. Figures 3A and 3B show how variations in sheet resistance with thickness for the -FeOOH/FTO electrode can vary with FeCl3 concentration and reaction time. In a typical experiment, the sheet resistance increased from 12 Ω sq−1 for FTO to 32 Ω sq−1 for an 80 nm -FeOOH/FTO electrode. Further increases in film thickness, of up to 400 nm, led to a sheet resistance of 78 Ω sq−1. Note that considering the low electrical conductivity of -FeOOH (~10‒5 S cm-1),21 the increase in the sheet resistance with coated -FeOOH films is insignificant which is within the background from the solution resistance. The sheet resistance change from a blank FTO electrode to a bi-layered -FeOOH/FTO electrode can be modeled and estimated (Scheme S1). Because of the small film thickness and the low electrical conductivity of -FeOOH, the current flows vertically across the film without a horizontal component. The coating of an 80-nm and 400-nm -FeOOH film would theoretically cause an additional resistance of 1.6 Ω and 8 Ω, respectively. The higher measured sheet resistance appears from two sources, uncounted nanoparticle contact resistance in the -FeOOH film and contact resistance between -FeOOH and FTO. In electrode design, the compact nature of the -FeOOH film can minimize sheet resistance improving catalytic performance (see below). The compact nature of the films was also investigated by adhesion strength measurements. As shown by the photo in Figure 3C, and the dark-field microscopic image in Figure 3F, both illustrate the external strength of the films. Figures 3D and 3G show images of the -FeOOH/FTO electrode after slicing with a

Figure 3. (A, B) Averaged transmittance between 400 - 800 nm and sheet resistance of an -FeOOH/FTO electrode. The electrodes were prepared in FeCl3 solutions at different concentrations for 1 h (A) or in 15 mM FeCl3 solutions at various times (B). Photo and dark-field microscopic images of the FeOOH/FTO electrodes after being scratched with a 2H pencil (C, F), sliced with a glass cutter (D, G), or sonicated in a water bath for 1 h (E, H). Electrocatalysis Water oxidation catalysis by -FeOOH/FTO electrodes was investigated in 0.5 M phosphate buffer solutions (PBS) at pH 7. Electrode performance was highly dependent on the experimental conditions used in film fabrication that determined the thickness. As shown by the linear sweep voltammetry (LSV) curves in Figure 4A, the catalytic current increases rapidly with an initial increase in the concentratin of FeCl3. The catalytic onset occurs at 1.2 V, and a maximum value of 35 mA cm–2 is achieved at 1.45 V with the 80-nm film (with 15 mM FeCl3). A further increase in precuror concentration results in a decrease in catalytic current with current stabilization at ~10 mA cm–2 for the 400-nm film (with 60 mM FeCl3). Figure 4B illustrates a similar effect of reaction time on catalytic performance. Although the active film is necessary for catalytic water oxidation, a gradually increased film thickness appears to block rapid electron transfer to the underlying FTO substrate giving a decrease in catalytic current. Related cyclic voltammetry (CV) data with variations in scan rates, pH, and in a CH3CN/H2O mixture at pH 7 are shown in Figures S7-S9. Figure S10 shows that the electrode is stable over multiple scans under catalytic conditions. To further test its durability during water oxidation, the electrode was subjected to controlled potential electrolysis (CPE) in 0.5 M PBS at pH 7. With a constant potential of 1.40 V, a sustained catalytic current density of 2.2 mA cm–2 was achieved for at least 10 h, Figure 4C. After electrolysis, the LSV profile for the -FeOOH/FTO electrode was nearly unchanged, Figure S11. In a related comparison, a freshly prepared -FeOOH/FTO electrode and second electrode, exposed in air for 3 months, also revealed no degradation in catalytic behavior, as shown in Figure S12.


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To probe intrinsic catalytic activity, Tafel measurements were conducted in 0.5 M PBS at pH 7. Based on the data in Figure 4D, the slope of a plot of  vs. log j was 36 mV/decade. From the Tafel plot, a current density of 10 mA cm–2 was achieved with an overpotential of 590 mV. The small overpotential and Tafel slope make this -FeOOH/FTO electrode among the most active OER catalysts based on iron in neutral to weakly basic solutions (Table 1). To ascertain the extent of iron deposition on FTO, an 80-nm film was dissolved in 0.1 M HNO3 with the latter analyzed by ICPOES. Based on the working-curve method in Figure S13, 224 nmol of iron was deposited per cm2 with 325 Fe atoms per 25 Å2. According to literature reports,55 a monolayer coverage is reached at a surface density of 1.7 -FeOOH unit cells per 25 Å2. This result is consistent with the formation of 191 layers of the FeOOH unit cell on FTO. The thickness, evaluated by modeling a 191-unit-cell-layer film (60 nm), was close to the measured value of 80 nm. Based on the number of deposited Fe atoms, the turnover frequency (TOF) was 0.7 s‒1 per deposited Fe atom at an overpotential of 630 mV, from the Tafel plot. Table 1. Comparison of the catalytic performances for a variety of iron-based OER electrocatalysts in neutral or weakly basic solutions.

Fe-based

7.0 7.0

 mV @ j (mA cm– 2) 590 @ 10 733 @ 10.5

Fe-based

7.0

Nano Fe2O3 Fe-based Fe-based

Electrocatalys t

-FeOOH

pH

Tafel slope (mV/decade)

Ref.

36 52

This work

633 @ 7

47

16

7.0

409 @ 1

66

17

9.2 9.75

600 @ 10 560 @ 10

45 34

18

15

19

To probe the high electrocatalytic activity of the FeOOH/FTO electrode, we compared its catalytic performance to a -FeOOH powder which had been cast onto FTO by electrophoresis of an as-prepared -FeOOH powder, FeOOH@FTO (see Supporting Information). Figure S14 shows photographs and SEM images of the sample. As compared with independently grown -FeOOH/FTO electrodes, the external coating on the -FeOOH@FTO is loose and less symmetrical. A weak surface cohesion is confirmed by easy hand loss of the coating. Figure S15 illustrates catalytic performance in 0.5 M PBS at pH 7. Compared with -FeOOH/FTO, its current density was decreased by a factor of ~10. The two electrodes are compositionally the same and the enhanced reactivity for in-situ film is presumably due its enhanced electrical conductivity.

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Figure 4. Variation in LSV curves (50 mV s–1) for FeOOH/FTO with different concentration of FeCl3 after 1 h (A) and with a 15 mM FeCl3 solution at different times (B). Inserts show magnified views of the LSV curves and plots of the catalytic current density at 1.45 V vs. either FeCl3 concentration or reaction time. (C) CPE of -FeOOH/FTO at 1.40 V. (D) Tafel plot, η = (Vappl − iR) – E(pH 7), where Vappl is the applied potential, iR is the solution resistance, and E(pH 7) = 0.817 V is the thermodynamic potential for OER at pH 7. As noted above, thermal decomposition of -FeOOH leads to dehydration to form Fe2O3. The XRD patterns in Figure 5A show that the decrease in -FeOOH content and formation of Fe2O3 are dependent on the calcination temperature. After calcination at 215 °C for 2 h, the disappearance of the characteristic diffraction peak for -FeOOH at 34.96° for the (211) plane, and the absence of new peaks, are consistent with low temperature dehydration to give amorphous Fe2O3 designated as a-Fe2O3/FTO in the figure. With an increased calcination temperature to 650 °C, diffraction peaks at 35.55° and 63.94°, from the (110) and (300) planes of Fe2O3, are observed consistent with conversion of amorphous aFe2O3 to crystalline -Fe2O3 at the higher temperature. In Figure 5B, the shift of Fe 2p XPS spectra toward lower binding energies is also consistent with the conversion to the iron oxide.49,50 Access to -Fe2O3/FTO by calcination of -FeOOH/FTO allows for a comparison of catalytic activity under constant experimental conditions. As shown in Figure 5C, the catalytic current decreases with a decrease in the -FeOOH phase or an increase in the -Fe2O3 phase. It is consistent with -FeOOH being the more electrocatalytically active phase toward OER than -Fe2O3, although -Fe2O3 is photocatalytically active.8-13 These observations are also reminiscent of the criticism that hematite is a slow catalyst for OER and the use of co-catalysts for the reaction.56-58 The nature of both the -FeOOH/FTO and -Fe2O3/FTO electrodes were unchanged after prolonged electrolysis. Visual inspection of both revealed no changes before and after CPE for 10 h. The XRD patterns in Figure 5D are also consistent with the preservation of both electrodes with characteristic diffraction peaks preserved and reactivities unaltered after electrolysis.


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Figure 5. (A) XRD patterns, (B) high-resolution XPS spectra of Fe 2p, and (C) LSVs (50 mV/s, pH = 7) of blank FTO (black), FeOOH/FTO (red), -FeOOH/FTO after calcination at 215 °C (green, a-Fe2O3/FTO) or 650 °C (blue, -Fe2O3/FTO) for 2 h. (D) XRD patterns of -FeOOH/FTO and -Fe2O3/FTO before and after CPE for 10 h at 1.40 V. The free energy changes for the individual steps in the reaction cycle play an important role in the OER activity of electrocatalysts.59-61 In order to investigate the stepwise oxidation of water on α-Fe2O3 and -FeOOH surfaces, we further investigated the optimal free energy changes of intermediates and products with α-Fe2O3 and -FeOOH as catalysts by density functional theory (DFT) calculations. The relevant computational methods are described in Supporting Information. The calculated free energy diagram for OER on α-Fe2O3 (blue) and -FeOOH (red) electrodes are shown in Figure 6. In the analyses the overpotential is determined by the highest free energy change step, M-OH → M-O + H+ + e–, for both α-Fe2O3 and -FeOOH. Calculated ∆G values are 1.11 eV for α-Fe2O3 and 0.99 eV for FeOOH. The results of the analysis were consistent with experimental results with -FeOOH exhibiting a lower onset overpotential by 0.1 V than α-Fe2O3. The higher catalytic activity of -FeOOH at this stage may arise from its inherent channel structure with enhanced hydrogen bonding. The latter may stabilize adsorbed O (see Figures S16 and S17 in Supporting Information) which reduces ∆G for dehydrogenation and enhances catalysis.55,62

Figure 6. (A) Calculated free energy diagram for OER on α-Fe2O3 (blue) and -FeOOH (red) at pH = 7. (B) The adsorption free energy changes for the four electron transfer steps during OER on α-Fe2O3 and -FeOOH. The four-electron reaction pathways and optimized structures for the reaction intermediates are shown in Supporting Information. NiFe-based electrocatalyst by stepwise formation. The facile one-pot synthesis allows for the high volume production of -FeOOH/FTO electrodes. The high catalytic activity of the integrated electrode at neutral pH, the film-

thickness-controlled photoelectric properties, and the surface adhesion strength are remarkable. We extended the current study to related catalyst electrodes. An obvious extension was to Fe-Ni given that combinations of the two have been shown to increase the OER through synergistic, metal-metal interactions.30-48 In our approach, decoration of -FeOOH/FTO and its derived electrodes with Ni was explored. Deposition of Ni was readily achieved by anodic CV scans (3 cycles) in 0.1 M borate buffer solutions (BBS, pH 9.2) containing 1 mM Ni(II).2 Figure 7A compares LSV curves for various Nicoated electrodes including Ni/FTO, Ni,-FeOOH/FTO, Ni,aFe2O3/FTO and Ni,-Fe2O3/FTO in 1 M KOH (pH 13.6), which were obtained by the same anodic deposition method on different electrode substrates. LSV curves for the four electrodes and backgrounds without a Ni coating are shown in Figure S18. The effect of added Ni on catalytic performance was greatly dependent on the substrate electrode. The most pronounced improvement was observed for Ni,a-Fe2O3/FTO which features a pre-wave at 0.6 V due to the surface oxidation of Ni2+ to Ni3+, followed by a catalytic onset at 0.68 V. The onset potential is at least 120 mV lower than those for the other electrodes. To evaluate the stability, CPE experiments were carried out in 1.0 M KOH at 0.93 V. As shown in Figure 7B, a sustained current density of 7.2 mA cm–2 was obtained at a Ni,a-Fe2O3/FTO electrode for an extended period. Current densities for the other electrodes were smaller with slight decreases in current over extended periods. As shown in Figure 7C, a Tafel slope of 33 mV/decade was obtained for Ni,a-Fe2O3/FTO in 1 M KOH. From the Tafel plot, a current density of 10 mA cm–2 was obtained with an overpotential of 310 mV. Although the stepwise preparation of the electrode requires only 3 CV scan cycles for Ni deposition, the catalytic performance is comparable to those reported earlier for NiFebased catalysts on 2D planar substrates.34,36,38,40,42,47 Parallel experiments with the other electrodes give larger Tafel slopes, between 57-71 mV/decade. Based on the data in Figure 7D, Nyquist plots of electrochemical impedance (EIS) measurements were obtained to evalate charge-transfer resistances. The first semicircle in the high-frequency region arises from the chargetransfer resistance from the bulk electrolyte to the electrode surface.63 The second semicircle, in the low-frequency region, arises from charge-transfer resistance within the electrode.48,64 The the latter was much lower for Ni,a-Fe2O3/FTO than for the other electrodes, consistent with its superior catalytic activity. Figure S19 shows the Nyquist plots of the Ni,a-Fe2O3/FTO electrode at various overpotentials in 1 M KOH. As expected, the charge-transfer resistance decreased as the overpotential increased. Figure S20 shows SEM images for the four electrodes discussed above. They reveal no difference in morphology before and after Ni coating. In contrast to other crystallized film electrodes, the morphology of a-Fe2O3/FTO and Ni,a-Fe2O3/FTO electrodes is less regular, consistent with their amorphous nature. Characterization of the Ni,a-Fe2O3/FTO electrode by TEM, SAED and XRD revealed no signals due to deposited Ni, Figure S21. However, XPS analysis in Figure S22 detected a significant amount of Ni on the electrode. The contrast between the SEM/TEM/SAED/XRD and XPS results are consistent with a low loading of Ni which is small in size and poorly crystalline. ICPOES analysis was used to accurately determine the amount of Ni deposited. The working-curve method in Figure S23 shows that 15.3 nmol of Ni was deposited on the FTO electrode per square centimeter, which is ~13 times less than the amount of iron. Based on the number of the deposited Ni atoms, the TOF of the Ni,aFe2O3/FTO electrode was 1.7 s–1 per deposited Ni at an


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overpotential of 310 mV, as determined from the Tafel plot. This TOF value is comparable to those reported in the literature.34,35,38,40,42,44-46

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.xxxxxxx. Experimental section and additional information as noted in the text.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions

† J.W. and S.Z. contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Figure 7. (A) LSV curves (50 mV/s), (B) CPE curves, (C) Tafel plots, and (D) Nyquist plots of Ni/FTO, Ni,-FeOOH/FTO, Ni,aFe2O3/FTO and Ni,-Fe2O3/FTO electrodes in a 1 M KOH aqueous solution. The higher catalytic activity and stability of the Ni,aFe2O3/FTO electrode is presumably due to its amorphous nature. We assume that extraneous Ni is more easily incorporated into the amorphous film which forms a relatively uniform NiFe composite. The resulting electrode gives enhanced catalytic performance through metal-metal interactions. These results are consistent with earlier reports that the most efficient NiFe-based OER electrocatalysts are amorphous composites. By contrast, a crystalline iron-based film with Ni deposition only on the surface, is insufficient to guarantee sufficient contact between Ni and Fe. Stability is also affected during long-term electrolysis because Ni is largely exposed on the crystalline electrode surface. Related strategies may be useful for the stepwise formation of other bimetallic or even trimetallic catalysts on a-Fe2O3/FTO electrodes. Conclusions In summary, we report here a -FeOOH/FTO-integrated electrode. It was prepared by a convenient one-pot hydrothermal reaction, specific to the FTO substrate arising from the strong interaction between -FeOOH and SnO2. The integrated electrode showed excellent and controllable photoelectric properties, a strong adhesion strength with high catalytic activity and stability toward OER catalysis in neutral aqueous solutions. A comparison of catalytic performance to -FeOOH powder-coated electrodes, fabricated by electrophoresis, or an -Fe2O3/FTO electrode, obtained by calcination of -FeOOH/FTO at 650 °C, has provided insight into its high catalytic activity. The electrode was also used as a novel substrate for the fabrication of more sophisticated electrodes. The experimental results reveal that calcination of FeOOH/FTO, at the relatively low temperature of 215 °C, gave amorphous Fe2O3. Further, the stepwise formation of a highperformance NiFe-based OER electrocatalyst was realized by anodic CV scans (only 3 cycles) in a Ni(II) solution. The performance of the integrated electrode was remarkable. It may open a door to additional routes to convenient and competitive catalysts for water splitting.

ASSOCIATED CONTENT Supporting Information

This work was supported by the National Natural Science Foundation of China (21573160, 21872105), the Fundamental Research Funds for the Central Universities, and Science & Technology Commission of Shanghai Municipality (14DZ2261100).

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(64) Gao, M.; Sheng, W.; Zhuang, Z.; Fang, Q.; Gu, S.; Jiang, J.; Yan, Y., Efficient Water Oxidation Using Nanostructured Alpha-Nickel-

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Hydroxide as an Electrocatalyst. J. Am. Chem. Soc. 2014, 136, 7077-7084.


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An Investigation of Fe-Based Integrated Electrode for Water Oxidation

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