Mixed Transition Metal Oxide with Vacancy-Induced Lattice Distortion

Jun 27, 2019 - Mixed Transition Metal Oxide with Vacancy-Induced Lattice Distortion for Enhanced Catalytic Activity of Oxygen Evolution Reaction ...
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Research Article Cite This: ACS Catal. 2019, 9, 7099−7108

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Mixed Transition Metal Oxide with Vacancy-Induced Lattice Distortion for Enhanced Catalytic Activity of Oxygen Evolution Reaction Hyeon Jeong Lee,†,⊥ Seoin Back,‡,⊥ Ji Hoon Lee,‡ Sun Hee Choi,§ Yousung Jung,*,‡,∥ and Jang Wook Choi*,†

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†

School of Chemical and Biological Engineering and Institute of Chemical Process, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea ‡ Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea § Pohang Accelerator Laboratory (PAL), Pohang University of Science and Technology (POSTECH), 80 Jigokro-127-beongil, Nam-gu, Pohang, Gyeongbuk 37673, Republic of Korea ∥ Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea S Supporting Information *

ABSTRACT: The oxygen evolution reaction (OER) constitutes the key limiting process in water electrolysis, and various catalysts have recently been introduced to improve OER efficiency. Vacancy engineering in the crystal lattice is particularly promising in catalyst design, as vacancies could perturb the electronic properties of adjacent atoms to make them catalytically active. Noting that one of the well-adopted approaches to induce vacancies in a crystal structure is the mixing of elements with different valence states, herein, we investigate crystalline NiFe−VM−O in comparison with NiO. Vacancies are naturally generated to meet charge neutrality when Ni2+ and Fe3+ are mixed via solid solution. As a result of vacancy formation, NiFe−VM−O exhibits markedly enhanced catalytic performance for the OER. A combined in situ X-ray absorption fine structure and density functional theory analysis reveals that transition metal vacancies in NiFe−VM−O distort the adjacent Ni’s electronic structure toward weakening the interaction with the reaction intermediate *O, which is also associated with the enhanced structural flexibility of NiFe−VM−O involving the transition metal vacancies. This study demonstrates the usefulness of the “vacancy-local structure−electronic property” relationship as a tool in manipulating the catalytic properties of OER electrocatalysts. KEYWORDS: electrocatalysts, oxygen evolution reaction, transition metal vacancies, lattice distortion, in situ X-ray analysis, structure−property relationship

1. INTRODUCTION

two paired reactions, OER imposes a higher energy barrier in progressing the entire reaction, as OER involves four-electron transfer and oxygen−oxygen bond formation.8−10 Under acidic conditions, two water molecules (H2O) lose four electrons to yield four protons (H+) and one oxygen molecule (O2):11

In response to global demand for green energy cycles that involve a minimal amount of fossil fuel, cost-effective and earth-abundant energy alternatives have been continuously pursued.1−4 Hydrogen (H2) is considered to be a promising substitute to fossil fuels owing to its high energy density and environmentally benign character. Nevertheless, production of H2 is not currently well-aligned with green energy cycles, as H2 is mostly produced from the petroleum-refining process. In such a situation, water splitting by electricity or sunlight has gained considerable attention as a means to overcome the limitation of the fossil fuel-involving conventional process.5−7 Water splitting comprises a hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Between these © XXXX American Chemical Society

2H 2O → 4H+ + O2 + 4e−

Under basic conditions, four hydroxide ions (OH−) lose four electrons to yield two water molecules and one oxygen molecule:11 Received: March 29, 2019 Revised: June 21, 2019 Published: June 27, 2019 7099

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ACS Catalysis 4OH− → 2H 2O + O2 + 4e−

was transferred to an autoclave and then underwent heattreatment at 100 °C for 24 h. The suspension was filtered with DI water and ethanol to remove organic residues. Finally, the obtained powder was sintered at 350 °C for 4 h under air. NiO nanoparticles were synthesized with 20 mmol of NiCl2·6H2O via the same procedure, with the exception of the final sintering step, in which the NiO was sintered at 300 °C for 2 h. 2.2. Characterization. The crystallographic information on synthesized particles was attained by carrying out X-ray diffraction (XRD) analysis (Rigaku, SmartLab, λ = 1.54056 Å). The molar ratio of Ni to Fe was identified by conducting inductively coupled plasma-atomic emission electron microscopy (ICP-AES, OPTIMA 8300, PerkinElmer) analysis. Field emission-transmission electron microscopy (FE-TEM, JEOL, Ltd.) was used to visualize the morphologies of the samples at an acceleration voltage of 200 kV. Energy dispersive spectroscopy (EDS) elemental mapping (JSM-6700F, JEOL) was performed on the surface of NiFe−VM−O to characterize its atomic distributions with respect to various elements. 2.3. Identification of Defective Ni−Fe Oxide. According to ICP-AES analysis, the Ni:Fe molar ratio turned out to be 0.818:0.182. As described in the main text, XANES analysis indicates that the oxidation states of Ni and Fe at the pristine state are 2+ and 3+, respectively. The excessive positive oxidation state from cations needs to be compensated by the content of vacancies and can thus be quantified based on the formula: (Ni0.82+Fe0.23+)1−x VMxO. The charge neutrality yields x = 0.09, revealing the chemical formula of the Ni−Fe oxide to be Ni0.72Fe0.18VM0.09O. 2.4. Electrochemical Measurements. The NiO and NiFe−VM−O electrodes were fabricated by first dispersing the active material, carbon conductive agent (Vulcan XC-72, Cabot), and Nafion (Sigma-Aldrich, 117 solution) in ethanol at a mass ratio of 8:1:1. The slurry was then cast onto conductive carbon paper (Toray, Waterproofed, TGP-H-90) without using a rotating disk. The mass loadings of the active materials in both cells were 0.2 mg cm−2. One M sodium hydroxide (NaOH, Sigma-Aldrich, 99.99%) dissolved in DI water was used as an electrolyte. Platinum (Pt) mesh and Hg/ HgO were used for the counter and reference electrodes, respectively, in a beaker cell. Electrochemical tests were carried out mainly by linear sweep voltammogram (LSV) and cyclic voltammogram (CV) measurements in the potential range of 0.2−0.8 V (vs Hg/HgO) using a battery cycler (VMP3, Biologic, France). All electrochemical measurements were conducted after IR correction. 2.5. In Situ XRD and XAFS Analysis. In situ XRD and Xray absorption fine structure (XAFS) measurements were conducted on the 1D and 7D beamline at Pohang Accelerating Laboratory (PAL) in the Republic of Korea. A lab-made Hshape acryl cell was used for electrochemical operation during X-ray measurements. The potential was scanned at 0.01 mV s−1 under CV mode to reach different charge and discharge states. To obtain fine XAFS spectra, the mass loadings of electrodes were 1 mg cm−2. The spectra for the Fe and Ni Kedges were obtained at room temperature in fluorescence mode and transmission mode, respectively. A calibration for each spectrum was performed by recording the corresponding metal foil simultaneously. The incident beam was detuned by 15−20% to minimize the flux from higher order Bragg diffraction of Si(111) crystals in a monochromator. The intensities of incident and transmitted beams were measured using a He-filled and a N2-filled IC SPEC ionization chamber,

A variety of electrocatalysts have been proposed to overcome the sluggish reaction kinetics of the OER, but high performance electrocatalysts are based mostly on noble metals, such as Ir and Ru, with high cost. In acidic or basic conditions, IrO2 and RuO2 are known to deliver outstanding catalytic performance.12−14 In an effort to find economic and practical alternatives, nonprecious transition metal (TM)-base compounds (e.g., Mn, Fe, Co, Ni) have been lately studied.15−21 Although various strategies, such as morphology control,22−24 orbital configuration tuning,9,25 and multicomponent metal composition,10,26−28 have improved the activity of nonprecious TM-based catalysts to some extent, achieving high activity remains nontrivial. Along this vein, nickel−iron composites have exhibited promise, represented by the high activity in basic media.29−38 However, most Ni−Fe compounds are lowcrystalline hydroxides or amorphous oxides, such as Fe−NiOx synthesized via a wet synthetic process without a sufficient thermal annealing step, resulting in a limited understanding of the origin of the observed catalytic performance. Therefore, an in-depth understanding of the atomic configuration of TMs and relevant structural effect on catalytic activity is worth pursuing for Ni−Fe catalyst systems. In the present investigation, we focus on defect chemistry when Ni and Fe are copresent in the crystal structure, as previous studies demonstrated that atom vacancies could markedly enhance catalytic activity.39−43 In particular, when Fe and Ni are simultaneously incorporated into the same crystal structure in the composition of NiFe−VM−O, their different preferential oxidation states, 2+ for Ni and 3+ for Fe, would give rise to TM vacancies (VM) for charge neutrality with oxygen. Through a series of in situ and ex situ analyses focusing on the bond character around Ni atoms, it was found that TM vacancies upon the incorporation of Fe result in structural distortion, accompanied by a substantial change in Ni−O and Ni−TM bond length compared to that of the bare NiO case. As a result of the local structure distortion, the overpotential in OER was decreased by 74 mV when scanned at 5 mV s−1. Density functional theory (DFT) calculations capture the structural distortion of NiFe−VM−O and attribute its enhanced catalytic activity to vacancy-induced structural flexibility, which tunes the interaction strength with the reaction intermediate, *O. The current study offers a useful guideline in designing TM oxide-based OER catalysts, which is the incorporation of TMs with different valence states so that vacancies can be induced based on the charge neutrality requirement.

2. EXPERIMENTAL SECTION 2.1. Synthesis of NiFe−VM−O and NiO Nanoparticles. NiFe−VM−O nanoparticles were synthesized through a hydrothermal synthesis process and a subsequent sintering step. All reagents were purchased from Sigma-Aldrich and used without purification. In the typical procedure, nickel chloride hexahydrate (NiCl2·6H2O, ≥ 98%) and iron chloride hexahydrate (FeCl3·6H2O, ≥ 98%) were used as metal precursors. Sixteen mmol of NiCl2·6H2O and 4 mmol of FeCl3·6H2O were first dispersed in deionized (DI) water (50 mL) and stirred vigorously for 20 min. One M of sodium carbonate (Na2CO3, ≥ 99.5%) solution was then slowly added to the metal ion dispersed solution, followed by vigorous stirring for 2 h at room temperature. Next, the brown solution 7100

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ACS Catalysis respectively. The obtained spectra were processed by using ATHENA and ARTEMIS programs in the IFEFFIT package following the standard procedure described by Kelly et al.44 The original Extended X-ray absorption fine structure (EXAFS) patterns (χ(k)) were weighted with k2 to intensify the high-k oscillation regime and were Fourier-transformed using the Hanning window to obtain the EXAFS spectra in Rspace (Å). The goodness of fit values were evaluated based on the reliable factor (R-factor). All EXAFS fitting was performed by taking four scattering paths (Ni−O1, Ni−TM, Ni−O1−O1, and Ni−O1−TM) into account in the R-space (Å), where O1 and TM indicate the neighboring oxygen and TM atoms around the absorbing Ni, respectively. 2.6. Computational Details. All electronic structure calculations were performed using the Vienna Ab initio Structure Package (VASP).45,46 GGA-PBE exchange correlation functional47 of DFT was used along with projectoraugmented wave (PAW) pseudopotentials48 to deal with the core−valence interactions, and a cutoff energy for pseudopotential was set to 500 eV. NiO was modeled as a rock salt structure, and a (2 × 2) surface unit cell of four-layered NiO (100) was simulated with (4 × 4 × 1) Monkhorst−Pack mesh. The upper two layers and adsorbates were relaxed, while the bottom two layers were kept fixed. We added a vacuum layer of at least 15 Å to rule out an imaginary interaction between repeating slabs in the z-direction. The geometry relaxations were given until the residual force on unconstrained atoms became less than 0.05 eV/Å. For all considered systems, we set antiferromagnetic (AFM) magnetic configurations along the (111) direction as reported in the literature.49,50 We converted electronic energies into free energies by adding zero-point energies, enthalpy, and entropic contributions of adsorbates treated by harmonic approximation and gas molecules by ideal gas approximation as implemented in Atomic Simulation Environments (ASE).51 To estimate the chemical potentials of proton−electron pair under different electrode potentials, the computational hydrogen electrode (CHE) method52 was employed. At a standard condition and 0 V vs RHE, the chemical potentials of proton and electron are in equilibrium with a half of chemical potential of hydrogen gas, G°(H+ + e−) = 0.5G°(H2) One can incorporate the effect of the electrode potential (U) by shifting the chemical potential of electron by−eU. Therefore, the chemical potential of proton and electron at the potential U can be calculated as follows:

Figure 1. (a) X-ray diffraction pattern and (b) TEM image of NiFe− VM−O solid-solution. The inset in (b) shows the electron diffraction pattern of NiFe−VM−O solid solution. (c) High-resolution TEM image of NiFe−VM−O particle. (Insets) FFT pattern and lattice fringes obtained from the red box. (d) Element mapping of NiFe− VM−O with respect to O, Ni, and Fe.

electron diffraction pattern of NiFe−VM−O verifies its crystallinity (Figure 1b, bottom inset). The magnified TEM image (Figure 1c) also confirmed the crystallinity of NiFe− VM−O under the corresponding crystal structure. The fast Fourier transform (FFT) pattern (Figure 1c, left bottom inset) attained from the red box displays the spots corresponding to the noted crystal planes along the [001] zone axis. The high resolution TEM (HRTEM) image (Figure 1c, right bottom inset) from the same region showed lattice fringes with a lattice spacing of 2.16 Å assigned to the (200) plane. The solidsolution character of NiFe−VM−O was also supported by its uniform elemental distributions with respect to Ni and Fe, according to energy dispersive spectroscopy (EDS) mapping (Figure 1d). The accurate stoichiometry of NiFe−VM−O was identified to be Ni0.72Fe0.18VM0.09O based on series of analyses. See details in the Experimental Section. For simplicity, the Ni− Fe oxide is denoted as NiFe−VM−O here and onward. In order to investigate the local structure of NiFe−VM−O, XAFS analysis was conducted (Figures 2, S2, and S4). The Xray absorption near edge structure (XANES) analysis in Ni−K edge indicates that the oxidation states of the Ni in NiO and NiFe−VM−O were both 2+ at their pristine state (Figure 2a). Alternatively, the Fe in NiFe−VM−O was at the oxidation state of 3+ (Figures 2b and S2), which suggests the presence of TM vacancies in NiFe−VM−O to satisfy charge neutrality. The Xray photoemission spectroscopy (XPS) analysis of NiFe−VM− O also supports that the oxidation state of Fe is +3 (Figure S3). The EXAFS spectra of NiFe−VM−O exhibits consistent local structures around both Ni and Fe compared with those of NiO (Figure S4), which is further confirmation of the homogeneous incorporation of Fe3+ in the NiO lattice. The first peaks of the EXAFS spectra of NiFe−VM−O in Ni and Fe K-edges correspond to the coordination states between the central TM and the adjacent oxygen. The bond length of Fe− O was slightly shorter than that of Ni−O, which is ascribed to the stronger Columbic attraction of Fe−O involving the Fe’s

G(H+ + e−) = G°(H+ + e−) − eU = 0.5G°(H 2) − eU

3. RESULTS AND DISCUSSION As described in the Experimental Section, the crystalline NiFe−VM−O was synthesized based on a hydrothermal reaction followed by a sintering process. The XRD results indicate that NiFe−VM−O preserved the original NiO rocksalt structure under the space group of Fm3̅m (Figures 1a and S1a of the Supporting Information, SI). The peaks of NiFe−VM−O did not split into subpeaks corresponding to individual TM oxides, implying its solid-solution feature. The peaks of NiFe− VM−O were slightly shifted to the lower angle compared to that of NiO. In addition, there are no additional peaks originating from other phases, such as Fe2O3 or NiFe2O4, further supporting the Fe’s homogeneous substitution into the NiO lattice.34,53 According to the TEM analysis, both NiFe− VM−O and NiO showed irregular shapes and similar particle sizes of approximately 20 nm (Figures 1b and S1b). The 7101

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Figure 2. (a) Ni K-edge XANES spectra of as-synthesized NiFe−VM−O and NiO. (b) Fe K-edge XANES spectra of as-synthesized NiFe−VM−O.

VM−O. On the whole, the series of electrochemical analyses coherently indicate the superior catalytic activity of NiFe− VM−O, and we attribute this outcome to its structural feature involving TM vacancies. This rationale arises from the fact that the presence of TM vacancies is a main difference between both samples, and we therefore focus on the analysis of the bulk and local structural changes of NiFe−VM−O during the OER from here onward. In addition, a CV measurement showed the redox peaks of NiO and NiFe−VM−O (Figure S6), indicating the reversible redox process of both catalysts. To capture minute structural modifications of a catalyst, we carried out in situ analysis in real time during an LSV scan while the electrode is immersed in the electrolyte.54−56 As displayed in Figure S7, the XRD patterns of both NiFe−VM−O and NiO remained almost unchanged over the entire potential range, implying that the original rocksalt structures of both NiO and NiFe−VM−O were maintained during the OER process. The distinct catalytic properties of both samples were elucidated by their in situ and ex situ XAFS analyses under CV measurements (Figure S8). In this experiment, while all in situ XAFS spectra in Ni K-edges were obtained as a transmission signal in an effort to attain reliable local information, ex situ spectra in Fe K-edges were obtained in fluorescence mode to compensate the low content of Fe in NiFe−VM−O. We first discuss the electrochemical activity of the Fe in NiFe−VM−O. According to ex situ XAFS analysis (Figure 4), the XAFS profile of NiFe−VM−O in Fe K-edge did not change substantially, which is in contrast with recent studies30,57,58 on Ni−Fe hydroxides; the Fe in Ni−Fe hydroxides could reach a high oxidation state of 4+. The Fe4+ possesses the electronic structure of 3d4, which is subject to Jahn−Teller distortion to

higher oxidation state of 3+. Therefore, the XAFS analysis reveals the structural distortion of NiFe−VM−O that engages aliovalent species, such as Fe3+ and VM. In an attempt to investigate the electrochemical properties of NiFe−V M −O and NiO as OER catalysts, LSV and chronopotentiometry analyses were performed in 1 M NaOH (Figures 3a and S5). NiFe−VM−O exhibits a higher

Figure 3. (a) LSV curves of NiFe−VM−O and NiO at a scan rate of 5 mV s−1. (b) Tafel plots of both catalysts.

oxidation peak position at 1.47 VRHE compared to that of NiO (1.43 VRHE), which is attributed to the solid-solution feature of NiFe−VM−O with Fe incorporated. This observation is consistent with previous literature.35 In the oxygen evolution potential regime, NiFe−VM−O showed a lower onset potential (1.53 VRHE) compared with that of NiO (1.58 VRHE). The more efficient catalytic activity of NiFe−VM−O was also reflected in its lower overpotential in reaching the current density of 10 mA cm−2: 371 vs 445 mV of NiO. When the Tafel plots of both electrodes were obtained from iRcompensated LSV curves, the Tafel slopes of NiFe−VM−O and NiO were 28 and 34 mV dec−1, respectively (Figure 3b), further confirming the enhanced catalytic property of NiFe−

Figure 4. Ex situ Fe K-edge XAFS spectra at pristine and OER state (1.525 V vs RHE) of NiFe−VM−O. The inset shows the pre-edges of both profiles, enlarged from the black dotted box. 7102

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Figure 5. (a) In situ Ni K-edge XANES spectra of (a) NiFe−VM−O and (b) NiO. The bottom inset shows the enlarged white line intensity from the dotted black box. Fourier transformed EXAFS spectra of (c) NiFe−VM−O and (d) NiO.

in EXAFS amplitude occurs when the N (coordination number) value decreases or the Debye-Weller factor, which is equal to the mean square relative displacement (MSRD), increases. Figure 5c and d show in situ EXAFS spectra in Ni Kedge in the Fourier transformed space. In the case of NiFe− VM−O, the intensities corresponding to Ni−O and Ni−TM bonds decreased with increasing potential (Figure 5c). In sharp contrast, the intensities of the same bonds of NiO rarely changed during the same potential sweep (Figure 5d). Since both NiFe−VM−O and NiO did not undergo phase transition in the given potential range (Figure S7), it can be seen that according to the above equation, the change in MSRD (rather than coordination number) around the Ni atom constitutes the origin of the progressively decreased peak intensities of the Fourier transformed EXAFS profiles of NiFe−VM−O in Figure 5c. The MSRD represents static and/or thermal disorder of a given atom. Therefore, the decreased profile intensities in the in situ EXAFS analysis are reflective of a more significant change in the local bond environment of NiFe−VM−O near the Ni atom upon charging compared to that of NiO (Figure S9). Notably, in the case of NiFe−VM−O, the degree of the peak decrease was more pronounced at point 5, corresponding to the potential window of 1.464−1.476 VRHE, where the Ni was oxidized from 2+ to 3+. Hence, the Ni oxidation is the main reason for the observed interatomic distance changes with the adjacent atoms, and thus the structural distortion of NiFe−VM−O during the OER process. The peak intensities of NiFe−VM−O were reversible, as the Fourier transformed EXAFS spectra were recovered in the cathodic sweep in the CV scan (Figure S9d and e). Similar to the oxidation scan, the peak intensities increased abruptly at point 11, which corresponds to the reduction of Ni from 3+ to 2+. Overall, it is postulated that the superior catalytic activity of NiFe−

lower the crystal symmetry. In general, this Jahn−Teller distortion is reflected in the pre-edge of an XAFS spectrum, as the pre-edge profile provides information relating to 1s → 3d electron transition in Fe.59,60 The pre-edge profile of NiFe− VM−O did not change during the OER process, indicating that Fe3+ did not turn to Fe4+ and therefore is not electroactive in the OER. The structural distortion of NiFe−VM−O is reflected in its in situ XANES spectrum in Ni K-edge, where the main absorption edge profile corresponds to 1s → 4p electron transition in Ni. When XANES profiles in Ni K-edge were obtained in situ at various points of the CV profiles (Figure S8), it was found that the intensity of the main absorption edge decreased with increasing potential (Figure 5a). By contrast, the absorption edge of NiO barely changed (Figure 5b). The continuously decreased absorption intensity of NiFe−VM−O at Ni K-edge is associated with the increased oxidation state of Ni from 2+ to 2+δ during the OER process.61 The XANES analyses allow us to capture the electronic properties of the Ni and Fe atoms in NiO and NiFe−VM−O during the OER. To further elucidate the local structure variation of NiFe−VM−O in the presence of TM vacancies, we analyzed the in situ EXAFS profiles of both samples during the OER. The EXAFS spectra in the j th coordination shell can be understood using the following equation, wherein Nj, S02, and σj2 are coordination number, amplitude reduction factor, and Debye-Weller factor, respectively.62 yz −2R j / λ(k) −2k2σj 2 2 jij NS e zz jj j 0 f j (k)e j χ (k) = ∑ jj sin[2kR j + δj(k)]zzzz 2 zz jj kR j z j j { k

According to the above equation, the EXAFS amplitude is related to the coordination number (N) and Debye-Weller factor (σ2) of a central atom. It is accepted63−65 that a decrease 7103

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ACS Catalysis VM−O is related to its local structural distortion involving the oxidation state change of the Ni center as compared to that of the pristine counterpart. Consistently, the changes in the bond lengths were negligible in the case of NiO. To identify the factors that cause the difference in the EXAFS spectra of both samples, the EXAFS spectra were fitted at three points that fall in the pristine state, redox reaction, and OER, respectively (Figure 6). The fitting model applied to

both NiFe−VM−O and NiO remained near six throughout the potential scan (Figure 6a). In contrast with coordination number, the trends in Debye−Waller factor and bond length were distinct between both samples (Figure 6b and c). The Debye−Waller factors of the Ni−O and Ni−TM bonds of NiFe−VM−O gradually increased from 0.009 to 0.0134 and 0.0122 to 0.0134, respectively, whereas the Debye−Waller factors of the same two bonds of NiO barely changed at 0.008 (Figure 6b). Regarding bond length, at the pristine state, the Ni−O bond length of NiFe−VM−O was 2.07 Å, which is slightly smaller than that of NiO (2.08 Å). However, the trend is opposite in the Ni−TM bond length, as the Ni−TM bond lengths of NiFe−VM−O and NiO were 3.008 and 2.969 Å, respectively. This inversion in the bond lengths is attributed to the vacancy formation in NiFe−VM−O; the TM vacancy enhances the repulsion between adjacent oxygen atoms, consequently contracting neighboring Ni−O bonds and extending Ni−TM bonds as compared to those of NiO. During the OER, the Ni−O bond length of NiFe−VM−O changed from 2.07 to 2.05 Å, whereas that of NiO remained at 2.08 Å. Likewise, the Ni−TM bond length of NiFe−VM−O decreased from 3.008 to 2.989 Å, whereas that of NiO remained unaltered at 2.968 Å. From the EXAFS fitting results, we found that the structural disorder of NiFe−VM−O was changed more significantly where the OER occurs, which originates from the existence of TM vacancies in NiFe−VM−O. Once again, this behavior of NiFe−VM−O originates from the existence of TM vacancies, and is not thus expected from other stoichiometric compounds such as Ni−Fe hydroxides. On the basis of this finding, we conducted DFT calculation to elucidate how the distorted structure can affect catalytic performance. OER consists of four proton−electron transfer steps: * + H 2O → *OH + H+ + e−

(1)

*OH → *O + H+ + e−

(2)

*O + H 2O → *OOH + H+ + e−

(3)

*OOH → * + O2 + H+ + e−

(4)

where * denotes an active site of the catalyst surface; and *O, *OH, and *OOH indicate adsorbed species on the active site. The computational hydrogen electrode (CHE) method52 was used to calculate the free energy change of each reaction step at any pH, and thus the same methodology is applicable to an alkaline condition. We considered various slab configurations to take into account several possibilities in the experiments. To maintain charge neutrality, our four-layered (100) slabs consist of 29 Ni, 2 Fe, and 32 O atoms with one Ni vacancy. We have also tested the effect of Fe concentration by removing the bottom two layers. After this removal, the Fe concentration increases from 7% to 15%. The comparison of these two conditions indicate that the qualitative trends of the latter two models with different Fe compositions remained unchanged due to the same local environment of the active sites (Table S7). For simplicity, we only considered surface and subsurface Ni vacancy. All configurations studied in this work are presented in Figure S11. Among various configurations, we chose five most stable slabs, where the relative stability differs at most by 0.3 eV (Figure S12) to further investigate OER

Figure 6. Fitting results of EXAFS profiles. The changes in (a) coordination number, (b) disorder, and (c) bond lengths for NiFe− VM−O and NiO as a function of potential.

both NiFe−VM−O and NiO was based on the rocksalt NiO structure. In the case of NiFe−VM−O, the molar ratio of Fe was reflected in the FEFF calculation.44 The experimental and theoretical EXAFS spectra in the Fourier transformed space are displayed in Figure S10, and the results of the corresponding fitting are summarized in Tables S1∼S6. Importantly, since NiO and NiFe−VM−O undergo the redox reaction and oxygen evolution reaction at different potentials, the potentials that showed similar currents were selected for comparative analysis between both samples. The fitting of the EXAFS profiles provides useful information on the local environments near Ni for both samples. The Ni−O coordination numbers (N) of 7104

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Figure 7. Free energy diagrams for OER: (a) NiO and (b) NiFe−VM−O at 0 V (black line) and 1.23 V (blue line). Red dashed lines correspond to the free energies of the ideal catalyst with 0 V overpotential. Potential determining step (PDS) and corresponding theoretical overpotentials are marked in blue arrows. Top views of each reaction step are presented. Color code: gray (Ni), brass yellow (Fe), and red (oxygen). (c) Reorganization energy (ΔEreorg,eV) of NiO (gray) and NiFe−VM−O (red) upon the adsorption of each reaction intermediate. ΔEreorg is calculated as the energy difference of slab before and after the adsorption. (d) Degree of displacement (ΣΔd,Å) for both catalysts. Degree of displacement is defined as the sum of displacement of all slab atoms upon the adsorption of reaction intermediates.

for NiFe−VM−O. Considering that a rate of each elementary deprotonation step is proportional to the associated reaction free energy,66,67 the intermediate compound in the thermodynamically most unfavorable step (i.e, PDS) would be the most dominant species to be mainly observed in experimental measurements during the overall OER process. The most dominant adsorbed species during OER is therefore *O for NiO and *OH for NiFe−VM−O. The calculated oxidation states of active Ni atom estimated from the magnetic moments (Table S8) indeed suggest Ni2+ for *O-adsorbed NiO and Ni3+ for *OH-adsorbed NiFe−VM−O. These results agree well with the XANES interpretations described above. The effects of Fe doping may be divided into a geometric effect originated from Ni atomic vacancy and electronic structure modification of active Ni site. Figure 7c compares the reorganization energy of slab atoms for adsorption of reaction intermediates, a measure of structural flexibility. We observed no difference in ΔEreorg for *OH and *OOH adsorption on NiO and NiFe−VM−O, but for *O adsorption, ΔEreorg is much smaller in the case of NiFe−VM−O (requiring less energy to distort the catalyst to accommodate *O). In Figure 7d, the sum of atomic displacements of the slab is compared for NiO and NiFe−VM−O when different adsorbates are adsorbed. Clearly, the NiFe−VM−O slab undergoes noticeably larger distortions than the NiO slab does for all adsorbates. For *OH,

activity. The theoretical overpotential (η) is calculated as follows: η = max[ΔG1 , ΔG2 , ΔG3 , ΔG4 ] /e − 1.23V

Further details on the computational setups are provided in the Experimental Section. Figure 7a and b compare the free energy diagrams of NiO and NiFe−VM−O. Our calculated η of NiO and NiFe−VM−O are 0.35 and 0.25 V, respectively, and η of NiO is in good agreement with the previous DFT calculation.50 Considering various possibilities (Figure S13), we found that the most active reaction sites of NiFe−VM−O are the Ni sites, while Fe sites are less active with η of 0.6 V, supporting that Fe3+ is not directly responsible for the observed OER activity. The activity improvement on NiFe−VM−O over NiO can be rationalized in terms of the relative stability of *O. In NiO catalyst, the formation of *OOH from *O (ΔG3) constitutes the potential determining step (PDS). Alternatively, in the case of NiFe− VM−O, as *O is destabilized in NiFe−VM−O catalyst compared to NiO, the formation of *O from *OH (ΔG2) becomes the PDS. The latter PDS and oxidation states of Ni determined from the DFT calculations can be linked to the XANES observations (Figure 5a and b) in which the Ni oxidation state remained the same as 2+ for NiO during OER but changed from 2+ to 2+δ 7105

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Research Article

ACS Catalysis for example, the slab distortion in NiFe−VM−O is almost twice as much as in NiO (Figure 7d), although the ΔEreorg required to distort NiFe−VM−O and NiO is almost identical (Figure 7c). Overall, our geometric analysis indicates NiFe−VM−O to be quite flexible in geometric distortions to accommodate adsorbates, perhaps due to a Ni-vacancy derived by Fe-doping. The calculation results are also in good agreement with our experimental observations that the XAFS and XANES spectra of NiFe−VM−O evidently changed while that of rigid NiO remained unchanged during the OER. We then analyzed the projected density of state analysis (Figure S14) and Bader charge analysis (Table S9)68 to investigate the effect of a more flexible structure of NiFe−VM− O due to Fe-doping-derived Ni atomic vacancy. The results suggest that the electronic structure of active Ni atom in bare NiO and NiFe−VM−O surfaces does not differ significantly, as evidenced by the similar calculated d-band center of Ni atom on both surfaces (−1.49 eV (NiO) vs −1.50 eV (NiFe−VM− O)) and Bader charge of active Ni site (+1.16 (NiO) vs +1.01 (NiFe−VM−O)). Interestingly, however, we observed a shift in d-band center position when NiO and NiFe−V M −O reorganized to adsorb reaction intermediates compared to their bare surfaces. Particularly, as summarized in Table 1, the

VM−O make its surface atomic arrangement flexible in a way that the d-band center of Ni is positioned to destabilize *O. The present investigation offers a useful insight in designing OER catalysts, which is vacancy engineering that tunes the interaction with reaction intermediates toward lowering the reaction barrier.

■

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b01298.

■

NiO NiFe−VM−O

*O-slab

*OH-slab

*OOH-slab

−1.49 −1.50

−1.26 −1.42

−1.42 −1.47

−1.41 −1.50

XRD pattern and TEM image of NiO, chronopotentiometry results and in situ XRD profiles of NiFe−VM−O and NiO, in situ EXAFS profiles of NiFe−VM−O, fitting results of EXAFS profiles at different potential regimes, and supplementary theoretical calculations (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Seoin Back: 0000-0003-4682-0621 Ji Hoon Lee: 0000-0002-6459-7120 Yousung Jung: 0000-0003-2615-8394 Jang Wook Choi: 0000-0001-8783-0901

Table 1. Calculated d-Band Center (in eV) of the Active Ni Site for NiO vs NiFe−VM−O Catalystsa bare slab

ASSOCIATED CONTENT

Author Contributions ⊥

For notations, *O-slab denotes the reorganized slab geometries of NiO and NiFe−VM−O to adsorb *O (see also Figure S14).

a

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

d-band center of NiO changed from −1.49 eV to −1.26 eV, and that of NiFe−VM−O changed from −1.50 eV to −1.42 eV when adsorbing *O, suggesting that *O interacts with NiFe− VM−O more weakly than NiO since d-band distribution at lower energies in the case of NiFe−VM−O implies a higher probability of antibonding filling.69,70 This observation could explain the destabilization of *O on NiFe−VM−O. Interestingly, the d-band center values upon *OH and *OOH adsorption remained similar for both NiO and NiFe−VM−O. This observation could be linked to the ΔEreorg, where the ΔEreorg for *OH and *OOH adsorption is similar for both catalysts, while that for *O adsorption on NiO requires the largest energy. Our theoretical analysis thus suggests that the improved OER activity is attributed to Fe-doping-derived Ni atomic vacancy that improves geometric flexibility and therefore tunes the electronic structure of the active Ni site.

ACKNOWLEDGMENTS Y . J . a c k n o w l e d g e s t h e s u pp o r t b y N R F K o r e a (2017R1A2B3010176 and 2019M3E6A1064706), and generous supercomputing time from KISTI. J.W.C. acknowledges the support by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2018R1A2A1A19023146 and NRF2017M1A2A2044504) and Research Settlement Fund for the new faculty of Seoul National University. J.W.C. also acknowledges the support by Inter-university Semiconductor Research Center (ISRC) at Seoul National University. H.J.L. acknowledges the National Research Foundation of Korea for the 2016 Global Ph.D. Fellowship Program (NRF2016H1A2A1909213). The authors also acknowledge technical support with the 1D (1D-XRS) and 7D (7D-XAFS) beamlines at Pohang Accelerating Laboratory (PAL).

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4. CONCLUSIONS The activity of metal oxide catalysts for the OER can be greatly enhanced by local lattice distortions, which control the degree of interaction with key reaction intermediates. In the case of NiFe−VM−O, the solid solution mixing of Fe3+ into a NiO matrix gives rise to transition metal vacancies that alter adjacent Ni−O and Ni−TM bond strengths toward improving the catalytic activity of NiFe−VM−O compared to that of NiO. NiFe−VM−O exhibits clearly lower overpotentials and onset potentials. The advantage of transition metal vacancies is also reflected in the DFT calculation that indicates the lower reorganization energy of NiFe−VM−O upon the interaction with *O. In other words, transition metal vacancies in NiFe−

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