Article Cite This: ACS Appl. Energy Mater. 2019, 2, 3927−3935
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Stepwise Electrochemical Construction of FeOOH/Ni(OH)2 on Ni Foam for Enhanced Electrocatalytic Oxygen Evolution Siqi Niu,† Yanchun Sun,‡ Guoji Sun,§ Dmitrii Rakov,† Yuzhi Li,† Yan Ma,† Jiayu Chu,† and Ping Xu*,†
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†
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No. 92 West Dazhi Street, Nangang District, Harbin 150001, China ‡ Heilongjiang River Fisheries Research Institute of Chinese Academy of Fishery Sciences, Laboratory of Quality & Safety Risk Assessment for Aquatic Products (Harbin), Ministry of Agriculture, Harbin 150070, China § State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Shenzhen Graduate School, Shenzhen 518055, China S Supporting Information *
ABSTRACT: As the oxygen evolution reaction (OER) is the bottleneck of electrocatalytic water splitting, it is highly imperative to develop OER catalysts with excellent activity and stability. Herein, we demonstrate a stepwise electrochemical construction of crystalline α-FeOOH/β-Ni(OH)2 composite structure supported on nickel foam (FeOOH/Ni(OH)2/NF) through cathodic electrodeposition of β-Ni(OH)2 nanosheets followed by electrophoretic deposition of α-FeOOH nanoparticles. Taking advantange of the synergistic effect of Ni and Fe as well as the formed interface, this composite structure is highly active for the OER process in alkaline media (1 M KOH), providing a very low overpotential of 207 mV versus the reversible hydrogen electrode (RHE) at a geometric catalytic current density of 40 mA cm−2 and a Tafel slope of 70 mV dec−1, which is superior to most reported (oxy)hydroxide-based OER electrocatalysts. In combination with density functional theory (DFT) calculations, it is verified that the synergistic interface effect between the real active sites NiOOH and FeOOH can facilitate the OER process. We believe this stepwise electrochemical technique for constructing Ni−Fe (oxy)hydroxide composites can provide new insights into the design and synthesis of highly efficient electrocatalysts for energy conversion applications. KEYWORDS: FeOOH, electrodeposition, electrophoretic deposition, electrocatalysis, oxygen evolution reaction
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evolution in alkaline conditions.5,21−28 As for the synthesis strategy, the coprecipitation technique is frequently used to synthesize Ni−Fe hydroxides,9 while the products have a tendency to stack on top of each other, and the morphology control is difficult to be achieved.29 Later, the homogeneous precipitation method is adopted to synthesize the Ni−Fe layered double hydroxide (LDH), and the OER performance can be further improved.30 Nevertheless, these methods are relatively time-consuming although the obtained catalysts are competitive in OER catalysis. As a contrast, the electrodeposition technique has the advantages of facile procedures and cost-effectiveness. Zhao et al. reported an electrodeposition technique of Ni−Fe hydroxide on different substrates with outstanding OER performance.31−33 Stepwise electrodeposition of Ni−Fe hydroxide film on indium tin oxide or glassy carbon electrode also displays remarkable OER response,34 where the loading of the active species can be manipulated through the electrodeposition time.
INTRODUCTION Electrocatalytic water splitting is considered to be one of the most promising techniques to obtain carbon-neutral, sustainable, and clean hydrogen energy that is prospective to replace fossil energy in the future.1,2 Overall water splitting contains two half-reactions: cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). Generally, OER is relatively sluggish in kinetics due to four sequential proton-coupled electron transfer process as well as oxygen−oxygen bond formation.3,4 Therefore, OER is the ratelimiting step of overall water splitting, and much effort has been devoted to the designed synthesis of OER catalysts with better stability and more outstanding performance. So far, some first-row transition metal oxides,5−7 (oxy)hydroxides,8−10 chalcogenides,11−14 nitrides,15 and phosphides16,17 have been confirmed as highly efficient OER electrocatalysts. Among them, (oxy)hydroxide-based OER catalysts have attracted great interest. In this regard, various (oxy)hydroxides with fascinating morphologies and relatively low overpotential have been successively explored in recent years.18−20 Specially, Ni−Fe-based materials have displayed lower overpotential to drive water splitting for oxygen © 2019 American Chemical Society
Received: April 21, 2019 Accepted: April 29, 2019 Published: April 29, 2019 3927
DOI: 10.1021/acsaem.9b00785 ACS Appl. Energy Mater. 2019, 2, 3927−3935
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ACS Applied Energy Materials
Electrophoretic deposition process was performed for 5 min under a constant current of 150 mA. The prepared cathode was FeOOH/NF, and it was rinsed with ultrapure water and dried in a vacuum drier. During the electrophoretic deposition process, hydrogen evolution reaction occurred on the surface of cathode and H+ ions were consumed, leading to an increase in pH value. Therefore, with abundant chloride ions, Fe(OH)3 was transformed into FeOOH through a phase transition process.46 Preparation of FeOOH/Ni(OH)2/NF. FeOOH/Ni(OH)2/NF was prepared with a similar method mentioned above. The asprepared Ni(OH)2/NF was used as the cathode during electrophoretic deposition process. The cathodic electrodeposition of Ni(OH)2 was performed under a constant cathodic current of 3 mA cm−2 for 550 s, and electrophoretic deposition of FeOOH was performed for 5 min under a constant current of 150 mA. To achieve optimal OER performance, an orthogonal experiment was carried out. The cathodic electrodeposition of Ni(OH)2 was operated under a constant cathodic current of 3 mA cm−2 for 350, 550, and 750 s, and electrophoretic deposition process was performed for 1, 3, 5, and 7 min under a constant current of 150 mA. The as-prepared FeOOH/ Ni(OH)2/NF was rinsed with ultrapure water and then dried in a vacuum oven for further use. The molar ratio of Ni(OH)2 and FeOOH was calculated according to the inductively coupled plasma atomic emission spectroscopy (ICP-AES, PerkinElmer (PE) Optima 8300) test result. Both the NF substrate and the catalyst were dissolved with nitric acid for ICP-AES measurement. The Ni content from NF substrate was deducted for calculating the molar ratio of Ni(OH)2 and FeOOH. DFT Calculations. In this work, all calculations were performed using the density functional theory (DFT) by the Vienna Ab-initio Simulation Package (VASP).47 The interactions between valence electrons and ions were treated with the projector augmented wave (PAW) potentials.48 The exchange-correlation interactions were described by generalized gradient approximation (GGA)49 with the Perdew−Burke−Ernzerhof (PBE) functional50 spin-polarization included through all the calculations. The electron wave functions were expanded in a plane-wave basis with cutoff energy of 520 eV. The convergence criteria for residual force and energy on each atom during structure relaxation were set to 0.01 eV/A and 10−3 eV, respectively. The vacuum space was more than 15 Å to avoid the interaction between periodical images. The Brillouin zone was sampled. The Monkhorst−Pack51 mesh for NiOOH bulk, FeOOH bulk NiOOH(001) surface, FeOOH(001) surface, and NiOOH/ FeOOH heterostructures was 8 × 8 × 10, 4 × 8 × 10, 4 × 4 × 1, 2 × 4 × 1, and 1 × 4 × 10, respectively. Owing to the high loading of catalyst on the substrate and either hydrogen adsorption or hydrogen/ hydroxyl intermediates, desorption occurs on the catalyst surface in each elementary reaction step of oxygen evolution reaction; both the substrate40,52 and transient state53,54 influences are negligible when we construct a model for DFT calculations. Characterization. X-ray diffraction (XRD) measurements were performed on a Rigaku D/MAXRC X-ray diffractometer (45.0 kV, 50.0 mA) using Cu target as the anticathode to produce corresponding X-ray radiation. Raman spectra were collected on a Renishaw inVia confocal microRaman spectroscopy system using the TE air-cooled 576 × 400 CCD array with a 532 nm excitation laser. The incident laser power was kept at 0.1 mW, and a total accumulation time of 10 s was employed. X-ray photoelectron spectra (XPS) were recorded on a PHI 5700 ESCA system with Al Kα radiation as excitation source (hν = 1486.6 eV). To eliminate the Auger peak hindering of Ni, the samples containing Fe element used Mg Kα as excitation source (hν = 1253.6 eV). Scanning electron microscopy (SEM) was performed on a ZEISS Merlin Compact with samples mounted on aluminum studs by graphite tape and sputtercoated with gold before analysis. Transmission electron microscopy (TEM) images were obtained on a Tecnai F20 operating at an accelerating voltage of 200 kV. Electrode Preparation and Electrochemical Measurements. All electrochemical measurements were performed at room temperature in a conventional three-electrode cell on an electrochemical
Notably, as pointed out in many works, the real active species of the hydroxides, phosphides, sulfides, and phosphosulfides in OER catalysis are actually the formed oxyhydroxides at the catalyst surface.3 In this respect, FeOOH has been proved to be a promising OER electrocatalyst,35−39 while the intrinsic poor conductivity and mass-transfer ability are always the hot issues that hinder its application in electrocatalysis. Therefore, FeOOH has been fabricated on Co,40 CeO2,41 and N-doped porous carbons42 to further improve its electrocatalytic performance. Herein, we demonstrate a stepwise construction of α-FeOOH/β-Ni(OH)2 hybrid materials on Ni foam (FeOOH/Ni(OH)2/NF) through a first cathodic electrodeposition of β-Ni(OH)2 nanosheets and a subsequent electrophoretic deposition of 10 nm α-FeOOH colloidal nanoparticles. The as-fabricated FeOOH/Ni(OH)2/NF hybrid system is highly active for the electrocatalytic OER process in alkaline media, which requires a very low overpotential of 207 mV versus the reversible hydroxide electrode (RHE) to achieve a current density of 40 mA cm−2. Ni(OH)2 and FeOOH are uniformly interconnected with each other at the nanoscale. Therefore, Ni(OH)2 can strongly bind FeOOH on the NF to ensure good electrical contact29 and decrease the Schottky barriers.43,44 The observation indicates that the composite structure of Ni−Fe oxyhydroxides can also facilitate a Faradaic response. Furthermore, the results of DFT calculations indicate that compared to Ni(OH) 2 and FeOOH, the composites can give the optimal free energy changes of the intermediates and the products in each stage of the OER process, leading to outstanding OER performance. This facile synthetic strategy for oxyhydroxide/hydroxide hybrid materials supported on Ni foam may open up a new way to the development of high-performance electrocatalysts for OER applications.
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EXPERIMENTAL SECTION
Reagents. Commercial Ni foam was purchased from Suzhou Jia Shi De Foam Metal Co. Ltd. All chemicals were AR grade and used without further purification. All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. Ultrapure water (from MilliQ system) was used in all experiments. Electrochemical Deposition of Ni(OH)2 on Ni Foam (Ni(OH)2/NF). Commercial Ni foam (NF, cut into an area of 1 × 2 cm2) was degreased by immersing into a mixture of ethanol and acetone (v:v = 1:1) for 30 min under ultrasonication conditions and washed with ultrapure water several times. Then, the NF was washed with 3 M HCl to remove surface nickel oxide for 15 min under ultrasonication conditions. The NF was rinsed with ultrapure water and ethanol for several times before being dried in a vacuum oven. Cathodic electrodeposition of Ni(OH)2 on NF was performed in a conventional three-electrode setup according to a previous work,45 where NF was used as the working electrode, Pt foil as the counter electrode, and the saturated calomel electrode (SCE) as the reference electrode. The electrolyte consisted of 1 M Ni(NO3)2 and 0.075 M NaNO3. The cathodic electrodeposition was performed under a constant cathodic current of 3 mA cm−2 for 550 s with NF area of 1 × 1 cm2 immersed in the electrolyte. Subsequently, the NF was immersed in 1 M KOH for 48 h. The as-prepared Ni(OH)2/NF was rinsed with ultrapure water and then dried in a vacuum oven for further use. Electrophoretic Deposition of FeOOH on Ni Foam (FeOOH/ NF). FeOOH/NF was synthesized by the following procedure. First, 0.5 g of FeCl3 was dissolved in 20 mL of ultrapure water, and the solution was dropwise added to 200 mL of boiling ultrapure water and kept for 1−2 min to prepare the Fe(OH)3 colloid. For electrophoretic deposition, the NF sheet was used as both the anode and the cathode. The NF was immersed in the electrolyte with area of 1 × 1 cm2. 3928
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ACS Applied Energy Materials workstation (CHI 660E). A graphite rod (Alfa Aesar, 99.9995%) was used as the counter electrode and Hg/HgO (1 M KOH solution) as the reference electrode. All the electrochemical measurements were operated in 1 M KOH(aq) solution. The working electrode was cycled at a scan rate of 50 mV s−1 for 30 cycles to activate the electrode in the electrolyte, and the full cyclic voltammograms (the 30th cycle) of Ni(OH)2/NF, FeOOH/NF, and FeOOH/Ni(OH)2/ NF in 1 M KOH at the scan rate of 50 mV/s after iR correction are shown in Figure S1. Linear sweep voltammetry (LSV) was measured at a scan rate of 5 mV s−1. Cyclic voltammograms (CV) at different scan rates (20, 40, 60, 80, 100, 120, 140, 160, 180, and 200 mV s−1) were collected in the range 1.031−1.131 V vs RHE to estimate the double-layer capacitance (Cdl). The electrochemical impedance spectroscopy (EIS) measurements were tested at the potential of 1.681 V vs RHE with the frequency ranging from 106 to 0.1 Hz. To better compare the true catalytic activity of the different catalysts, we used the series resistance (Rs ∼ 1 Ω) determined from EIS experiments to correct the polarization measurements and Tafel slope analysis for the Ohm losses. A multicurrent process performed in 1 M KOH was used to test the mass transport property of the FeOOH/Ni(OH)2/NF electrode. The current density was normalized from the apparent area of the NF (1 × 1 cm2). Reference Electrode Calibration. We used Hg/HgO (1 M KOH) as the reference electrode in all measurements. It was calibrated with respect to the reversible hydrogen electrode (RHE).55 The calibration was performed in the high-purity hydrogen-saturated electrolyte (1 M KOH). It should be noted that the pH value of the 1 M KOH is actually 13.6, and the room temperature is ∼20 °C when we conduct the electrochemical measurements. The potential was referred to RHE by calibrating the Hg/HgO with Pt sheets (Aldrich) as both the working and the counter electrodes. The LSV curve was swept in a cathodic direction at a scan rate of 1 mV s−1. The value at which the current crossed zero (0.881 V) was taken to be the thermodynamic potential for the hydrogen electrode reaction (Figure S2).
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Figure 1. (a) Schematic illustration for constructing FeOOH/ Ni(OH)2 on Ni foam through cathodic electrodeposition of Ni(OH)2 and subsequent electrophoretic deposition of FeOOH, TEM, and HRTEM images of (b, e) Ni(OH)2, (c, f) FeOOH, and (d, g) FeOOH/Ni(OH)2 composites (the inset shows the corresponding selected area diffraction pattern). (h) STEM image and elemental mapping of FeOOH/Ni(OH)2 composites.
RESULTS AND DISCUSSION α-FeOOH/β-Ni(OH)2 hybrid materials supported on Ni foam (FeOOH/Ni(OH)2/NF) can be successfully fabricated through a first cathodic electrodeposition of Ni(OH) 2 nanosheets and a subsequent electrophoretic deposition of FeOOH nanoparticles (Figure 1a). It is found that green βNi(OH)2 was deposited on NF with the cathodic electrodeposition method, and reddish-brown FeOOH was deposited on Ni(OH)2/NF through the following electrophoretic deposition technique (Figure S3). The morphologies of the prepared catalysts were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Ni(OH)2, FeOOH, and FeOOH/Ni(OH)2 composites are homogeneously deposited on NF through the electrochemical routes (Figure S4). It can be seen that Ni(OH)2 is actually composed of numerous nanosheets, while the spherical FeOOH is assembled by tiny nanoparticles. The TEM image of Ni(OH)2 clearly shows the nanosheet structures (Figure 1b), and a lattice fringe of 2.33 Å in the HRTEM image can be assigned to the (101) plane of the βNi(OH)2 (Figure 1e). Spherical FeOOH particles are actually composed of an assembly of FeOOH nanoparticles that are ∼10 nm in size (Figure 1c), and a lattice fringe of 2.69 Å in the HRTEM image is ascribed to the (130) plane of the α-FeOOH (Figure 1f). FeOOH nanoparticles supported on Ni(OH)2 nanosheets can be seen in the TEM image of the FeOOH/ Ni(OH)2 composites (Figure 1d), and the lattice fringes of 2.33 and 2.69 Å due to the (101) and (130) crystal planes of βNi(OH) 2 and α-FeOOH, respectively, can be clearly distinguished (Figure 1g). The selected-area electron diffrac-
tion (SAED) pattern of the composites shows the polycrystalline structure with (101) and (130) planes corresponding to Ni(OH)2 and FeOOH, respectively (inset in Figure 1g). Scanning transmission electron microscopy (STEM) and elemental mapping (Figure 1h) confirm the uniform distribution of Ni, Fe, and O in the FeOOH/Ni(OH)2 hybrid composites, an indication that α-FeOOH nanoparticles are homogeneously anchored on the β-Ni(OH)2 nanosheets. The loading mass of composite catalyst is determined to be 2.7 mg cm−2, and the molar ratio of Ni(OH)2 and FeOOH is about 4:5 from the ICP-AES measurement. To investigate the structure of the prepared catalysts, Raman spectra as well as XPS spectra were collected (Figure 2). For βNi(OH)2, the Raman bands at 307 and 447 cm−1 are identified to be translational Eg(2) and A1g(1) modes of Ni−O in βNi(OH)2, and the weak peak at 512 cm−1 is associated with structural defects.56,57 The band at 1056 cm−1 corresponds to second-order two-phonon (2P) modes of Ni−O in βNi(OH)2.58,59 Intense characteristic peaks at 541 and 675 cm−1 can be observed in the Raman spectrum of α-FeOOH due to the asymmetric stretching vibration of metal and hydroxide group.60 The Raman spectrum of the FeOOH/ Ni(OH)2 composite structure shows typical α-FeOOH bands and weak bands of β-Ni(OH)2 due to the presence of α3929
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FeOOH/NF are provided in Figure S5, which can be well indexed to the Ni(OH)2 and FeOOH species, respectively. To further verify the catalysts structure, X-ray diffraction (XRD) measurements were carried out. To avoid the influence of NF, the synthesized electrocatalysts on NF were ultrasonicated in ethanol to get powder catalysts. The XRD pattern (Figure S6) confirms that β-Ni(OH)2 (JCPDS 14-0117) is successfully synthesized through the cathodic electrodeposition. Five diffraction peaks centered at 2θ = 19.3°, 33.1°, 38.5°, 60.2°, and 62.7° can be indexed to the (001), (100), (101), (003), and (111) planes of β-Ni(OH)2.51 As for the prepared FeOOH, diffraction peaks centered at 2θ = 21.2° and 33.2° are due to the (110) and (130) crystal planes of α-FeOOH (JCPDS 81-0462).60 For FeOOH/Ni(OH)2 composites, besides the XRD peaks of α-FeOOH, weak characteristic peaks of β-Ni(OH)2 can also be detected, which may result from the coverage of α-FeOOH nanoparticles on β-Ni(OH)2 nanosheets. These results are well consistent with the XPS and Raman data, and the broadened XRD peak can well match the particle size of the as-prepared FeOOH and β-Ni(OH)2. The above structure characterizations strongly support that the material supported on NF through a first cathodic electrodeposition, and a subsequent electrophoretic deposition consists of β-Ni(OH)2 and α-FeOOH but not NiFe (oxy)hydroxide.34 The electrocatalytic OER activities of pure NF, Ni(OH)2/ NF, FeOOH/NF, FeOOH/Ni(OH)2/NF, and the benchmark IrO2 were compared in 1 M KOH aqueous solution (Figure 3). Figure 3a shows the polarization curves after iR correction. It is noted that pure NF and IrO2/NF have very limited OER activity. Ni(OH)2/NF also shows poor activity, which requires an overpotential (η) of 327 mV vs RHE to deliver a geometric current density of 40 mA cm−2. The as-prepared FeOOH/NF through electrophoretic deposition displays enhanced electrocatalytic OER activity, which achieves a current density of 40 mA cm−2 at η = 254 mV vs RHE. Impressively, FeOOH/ Ni(OH)2/NF displays even better electrocatalytic OER activity, which only requires an η of 207 mV vs RHE to
Figure 2. Structure characterizations of the obtained catalysts. (a) Raman spectra of Ni(OH)2/NF, FeOOH/NF, and FeOOH/ Ni(OH)2/NF. XPS spectra of (b) Fe 2p, (c) Ni 2p, and (d) O 1s of FeOOH/Ni(OH)2/NF composites.
FeOOH on the surface of the composites. XPS was employed to better analyze the chemical environment of the synthesized composite structure (Figure 2b−d). The binding energies of Fe 2p3/2 and Fe 2p1/2 are centered at 713.7 and 725.3 eV, respectively, indicating the existence of Fe3+ (Figure 2b).61 The Ni 2p XPS spectrum in Figure 2c has two main peaks located at 856.8 and 874.7 eV, consistent with Ni2+ 2p3/2 and Ni2+ 2p1/2. Besides, two strong satellite peaks centered at 863.3 and 881.4 eV also reveal the presence of Ni2+ ions.62 In Figure 2d, O 1s peaks located at 529.5 and 531.2 eV can be detected, ascribed to the “unprotonated O” species in the FeOOH and “protonated O” in hydroxide group, respectively.63 The above results confirm that the prepared catalyst is the hybrid of Ni(OH)2 and FeOOH. XPS spectra of Ni(OH)2/NF and
Figure 3. Electrochemical properties of Ni(OH)2/NF, FeOOH/NF, and FeOOH/Ni(OH)2/NF composites for OER catalysis in 1 M KOH. (a) Polarization curves after iR correction, in comparison to pure NF and IrO2/NF. (b) Tafel plots for the data presented in (a). (c) Plots showing the extraction of the Cdl. (d) Electrochemical impedance spectroscopy Nyquist plots. (e) Multicurrent process obtained with FeOOH/Ni(OH)2/NF electrode in 1 M KOH without iR correction. The current density started at 25 mA cm−2 and finished at 175 mA cm−2, with an increment of 25 mA cm−2 after every 400 s. (f) Long-term durability test of FeOOH/Ni(OH2/NF at a constant anodic current density of 40 mA cm−2 for 28 h. 3930
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ACS Applied Energy Materials reach a current density of 40 mA cm−2. Notably, a very low η of 277 mV vs RHE is required to provide a high current density of 350 mA cm−2 (Figure S7). The polarization curves without iR correction are provided in Figure S8, and the variation trend of the electrocatalytic activity is the same as that after iR correction, an indication that FeOOH/Ni(OH)2 composite structure indeed has the best performance. This high OER activity from FeOOH/Ni(OH)2/NF is comparable to or even better than some of the reported most promising OER electrocatalysts (Table S1).30,36,64 The improved activity of the constructed composite structure catalyst should be due to the synergistic effect of Ni and Fe species.37,65 The oxidation peak of Ni(OH)2 becomes weak after we construct the composite structure of Ni(OH)2 and FeOOH, as Fe3+ can reduce the number of electrons needed to oxidize Ni atoms, which can also reduce the energy barrier to create active Ni intermediate for water oxidation.40 Actually, this synergistic effect of Ni and Fe on OER has been witnessed in previous works.5,9,66−68 Moreover, the formed interface between Ni(OH)2 and FeOOH can ensure good electrical contact43 and decrease the Schottky barriers43,44 and thus facilitate the Faradaic response (Figure S9). From the extrapolation of the linear region of overpotential (η) vs log j (Figure 3b), Tafel slopes of 140, 100, and 97 mV dec−1 (after iR correction) can be obtained for Ni(OH)2/NF, FeOOH/NF, and IrO2/NF, respectively. In the composite structure of FeOOH/Ni(OH)2/ NF, an exceptionally low Tafel slope value of 70 mV dec−1 suggests highly efficient kinetics for oxygen evolution. Furthermore, the OER Faradaic efficiency (FE) of the FeOOH/Ni(OH)2/NF composites was performed under the current of 10 mA for 2 h, and a high FE of 99% can be calculated (Figure S10). To better understand the origin of improved activity for OER by forming FeOOH/Ni(OH)2 composites, the electrochemically active surface area (ECSA) of various samples was estimated. Cyclic voltammetry (CV) measurements were employed to extract the double-layer capacitance (Cdl) (Figure S11), and the calculated Cdl values are 2.54, 2.96, 3.73, and 5.07 mF cm−2 for Ni(OH)2/NF, FeOOH/NF, FeOOH/ Ni(OH)2/NF, and IrO 2/NF, respectively (Figure 3c), indicating that the FeOOH/Ni(OH)2/NF can expose more catalytically active sites for water oxidation. The ECSAnormalized current density was applied to compare the intrinsic activity of the catalysts (Figure S12), where the FeOOH/Ni(OH)2/NF sample exhibits the highest current density of 2.66 mA cm−2 at η = 270 mV vs RHE, suggesting the best intrinsic activity toward the OER process. Electrochemical impedance spectroscopy (EIS) was applied to provide further insights into the electrode kinetics during the OER process. The Nyquist plots (Figure 3d) were fitted with an equivalent circuit (inset in Figure 3d) to extract the charge transfer resistance (Rct) of 1.20, 0.60, 0.33, and 0.19 Ω for IrO2/NF, Ni(OH)2/NF, FeOOH/NF, and FeOOH/Ni(OH)2/NF, which can decrease the charge transfer resistance and accelerate the Faradaic response and consequently lead to superior OER kinetics. A multistep chronopotentiometric curve obtained on FeOOH/Ni(OH)2/NF in 1 M KOH (Figure 3e) reflects the excellent mass transport property (inward diffusion of OH− and outward diffusion of oxygen bubbles), electrical conductivity, and mechanical robustness of the FeOOH/Ni(OH)2/NF electrode. Stability and durability are also important parameters for OER catalysts. At a constant current density of 40 mA cm−2, the potential can be stably
maintained at around 1.46 V vs RHE after an electrolysis for 28 h (Figure 3f). Moreover, the composite structure of FeOOH/ Ni(OH)2/NF exhibits almost negligible degradation of OER performance after 1000 CV cycles (Figure S13). The above results clearly reveal that the FeOOH/Ni(OH)2/NF composite structure is a promising OER electrocatalyst with high activity, durability, and stability. Orthogonal experiments were performed to achieve optimal OER performance. The cathodic electrodeposition of Ni(OH)2 was operated under constant cathodic current of 3 mA cm−2 for 350, 550, and 750 s, and the electrophoretic deposition process was performed for 1, 3, 5, and 7 min under constant current of 150 mA (Figure S7). The electrocatalytic activity of FeOOH/Ni(OH)2/NF first increases with prolonging the electrodeposition time of Ni(OH)2 and the electrophoresis time of FeOOH because the loading of catalysts increases with long-time electrodeposition or electrophoresis. However, the catalytic activity will be deteriorated when the electrodeposition or electrophoresis time is further prolonged, as too much loading of catalysts will hinder the mass transfer process. Orthogonal experimental results show the composite structure from an electrodeposition of Ni(OH)2 for 550 s and an electrophoretic deposition of FeOOH for 5 min displays the best catalytic performance, which is the one shown in the above discussion. Despite the remarkable activity, stability, and durability, it is necessary to characterize the structures and morphologies after the OER process to understand the real active species. It is found that Ni(OH)2/NF, FeOOH/NF, and FeOOH/Ni(OH)2/NF all turned black after the OER process (Figure S14). As shown in Figure 4a, compared with initial β-
Figure 4. Structure characterizations of as-prepared catalysts after OER. (a) Raman spectra of Ni(OH)2/NF, FeOOH/NF, and FeOOH/Ni(OH)2/NF composites. XPS spectra of (b) Fe 2p, (c) Ni 2p, and (d) O 1s in FeOOH/Ni(OH)2/NF.
Ni(OH)2, the Raman spectrum of Ni(OH)2/NF after the OER process displays a pair of sharp peaks at 473 and 556 cm−1, ascribed to the characteristic Ni−O vibration of NiOOH.37,58,69 As for FeOOH/NF after OER, no obvious peak shifts were found, an indication of the stable structure of FeOOH during the OER process. The Raman spectrum of FeOOH/Ni(OH)2/NF after OER shows typical metal and hydroxide group asymmetric stretching vibration of α-FeOOH 3931
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Figure 5. Structure of (a) NiOOH, (b) FeOOH, and (c) NiOOH/FeOOH composite. (d) Calculated free energy diagram of the OER process on the surface of NiOOH (001), FeOOH (001), and NiOOH/FeOOH (001).
and the characteristic peaks caused by the Ni−O vibration of NiOOH.58 In addition, XPS spectra were also measured to better confirm the structure of catalysts after OER. Fe 2p3/2 and Fe 2p1/2 are located at 712.6 and 725.6 eV, respectively (Figure 4b), nearly at the same positions as they were before the OER process.62 The Ni 2p XPS spectrum (Figure 4c) has dramatic changes: two main peaks centered at 858.2 and 876.1 eV together with the strong satellite peaks at 864.3 and 882.1 eV are consistent with Ni 2p3/2 and Ni 2p1/2 of Ni3+ species.62 Besides, O 1s peaks (Figure 4d) centered at 529.3 and 531.4 eV can also be detected,63 and interestingly, the peak located at 529.3 eV becomes stronger due to the conversion of βNi(OH)2 into NiOOH during the OER process. The above results clearly show that FeOOH/Ni(OH)2/NF would transform into FeOOH/NiOOH/NF during the OER process. XPS spectra of Ni(OH)2/NF and FeOOH/NF after the process of oxygen evolution are also provided in Figure S15, which can be well indexed to the NiOOH62,63 and FeOOH species, respectively, confirming that oxyhydroxide species are the real catalysts for the OER. Moreover, SEM (Figure S16) and TEM images (Figure S17a−c) demonstrate no obvious change in morphology of catalysts after long time OER process. The lattice fringes of 2.08 and 2.69 Å in the HRTEM images can be assigned to the (210) plane of the NiOOH and the (130) plane of the FeOOH (Figure S17d−f), which are consistent with the XPS and Raman data. Furthermore, STEM and elemental mapping (Figure S17g) confirm the uniform distribution of Ni, Fe, and O in the FeOOH/NiOOH hybrid composites after the OER process. DFT calculations were performed to further investigate the synergistic interface effect between Ni(OH)2 and FeOOH. However, considering the formed NiOOH after OER activation process, we constructed a model using NiOOH and FeOOH (Figure 5 and Figure S18). The thermodynamic potential for oxygen evolution is 1.23 V at standard conditions. In practice, a potential above 1.23 V is required to proceed at a measurable rate for this reaction. In general, in alkaline solution, the OER evolution occurs according to the next four steps:70 ∗ + OH− → OH* + e−
(1)
OH* + OH− → O* + H 2O + e−
(2)
O* + OH− → OOH* + e−
(3)
OOH* + OH− → O2 + H 2O + ∗ + e−
(4)
where the asterisk denotes a surface site. The Gibbs free energy change for these steps can be expressed as ΔG1 = ΔGOH
(5)
ΔG2 = ΔGO − ΔGOH
(6)
ΔG3 = ΔGOOH − ΔGO
(7)
ΔG4 = 4.92 eV − ΔGOOH
(8)
The Gibbs free energies are related to the adsorption energies of OH*, O*, and OOH*. The free energy difference ΔGi is calculated by ΔGi = ΔEi + ΔZPEi − TΔSi, where ΔEi is the energy difference calculated using DFT relative to H2O and H2, ΔZPEi is the difference of zero-point energy obtained by the vibrational frequencies computed, and ΔSi (entropy) is difference of entropy. Figure 5 shows the calculated free energy of each step during the OER process on NiOOH(001), FeOOH(001), and NiOOH/FeOOH(001). It can be seen that the rate-determining step (RDS) of NiOOH and NiOOH/ FeOOH is step 2, while the RDS of FeOOH is step 3. It is obvious that the RDS energy on NiOOH/FeOOH is lower than that on FeOOH(001), while the free energy of the RDS on NiOOH(001) is the highest. Therefore, the NiOOH/ FeOOH heterostructure should exhibit the most outstanding OER catalytic performance, which is consistent with the experimental results.
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CONCLUSIONS In conclusion, a highly efficient OER electrocatalyst consisting of α-FeOOH/β-Ni(OH)2 composite structure supported on nickel foam (FeOOH/Ni(OH)2/NF) was successfully constructed by electrodeposition and subsequent electrophoretic deposition. Both the investigations of experiment and theory confirm the synergistic effect of Ni and Fe species, and the formed interface between Ni(OH)2 and FeOOH enables this composite material possessing very high OER activity, requiring an extremely low overpotential of η = 207 mV to achieve a geometric current density of 40 mA cm−2 in alkaline media. FeOOH/Ni(OH)2 would be converted into FeOOH/ NiOOH during the OER process, and this double oxyhydroxide structure accounts for the excellent OER activity and stability. This work demonstrates the value of the stepwise electrochemical deposition technique in constructing composite material, which provides new thought in electrocatalysis and other renewable energy conversion and storage systems. 3932
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00785.
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Experimental details and additional characterizations (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (P.X.). ORCID
Ping Xu: 0000-0002-1516-4986 Author Contributions
S.N. and Y.S. contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are thankful for the financial support from the National Natural Science Foundation of China (21471039, 21571043, 21671047, and 21871065), Fundamental Research Funds for the Central Universities (PIRS of HIT A201502 and HIT. BRETIII. 201223), China Postdoctoral Science Foundation (2014M560253), Postdoctoral Scientific Research Fund of Heilongjiang Province (LBH-Q14062, LBH-Z14076), and Natural Science Foundation of Heilongjiang Province (B2015001).
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