NiOOH Exfoliation-Free Nickel Octahedra as Highly Active and

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NiOOH Exfoliation-free Nickel Octahedra as Highly Active and Durable Electrocatalysts toward the Oxygen Evolution Reaction in an Alkaline Electrolyte Byeongyoon Kim, Aram Oh, Mrinal Kanti Kabiraz, Youngmin Hong, Jinwhan Joo, Hionsuck Baik, Sang-Il Choi, and Kwangyeol Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19457 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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ACS Applied Materials & Interfaces

NiOOH Exfoliation-free Nickel Octahedra as Highly Active and Durable Electrocatalysts toward the Oxygen Evolution Reaction in an Alkaline Electrolyte Byeongyoon Kim†,#,¶, Aram Oh⊥,¶, Mrinal Kanti Kabiraz§,¶, Youngmin Hong§, Jinwhan Joo†,#, Hionsuck Baik⊥, Sang-Il Choi*,§ and Kwangyeol Lee*,†,#

†Department

#

of Chemistry, Korea University, Seoul 02841, Korea

Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science (IBS), Seoul

02841, Korea

⊥Korea

Basic Science Institute (KBSI), Seoul 02841, Korea

§Department

of Chemistry and Green-Nano Materials Research Center, Kyungpook National

University, Daegu 41566, Korea

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ABSTRACT. A layered β-NiOOH crystal with under-coordinated facets is an active and economically viable non-noble catalyst for the oxygen evolution reaction (OER) in alkaline electrolytes. However, it is extremely difficult to enclose the β-NiOOH crystal with undercoordinated facets due to its inevitable crystal transformation to γ-NiOOH, resulting in the exfoliation of the catalytic surfaces. Herein, we demonstrate {111}-faceted Ni octahedra as the parent substrates which surfaces are easily transformed to catalytically active β-NiOOH during the alkaline OER. Electron microscopic measurements demonstrate that the horizontally stacked β-NiOOH on the surfaces of Ni octahedra has resistance to further oxidation to γ-NiOOH. By contrast, significant crystal transformation and thus the exfoliation of the γ-NiOOH sheets can be observed on the surfaces of Ni cubes and rhombic dodecahedra (RDs). Electrocatalytic measurements exhibit that the β-NiOOH formed on Ni octahedra performs highly enhanced OER durability compared to the Ni cubes, Ni RDs, and the state-of-the-art Ir/C catalysts.

KEYWORDS. Oxygen evolution reaction, electrocatalyst, nickel oxyhydroxide, heteroepitaxy, phase transformation


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Oxygen evolution reaction (OER) is a crucial anodic half-reaction in aqueous electrolytes that accompanies cathodic electrowinning or hydrogen evolution reaction.1-4 Since the OER is sluggish at high overpotentials, a lot of effort has been made to develop efficient and durable OER catalysts in different pH conditions.5-14 Until recently, IrO2 has been considered as the benchmark catalyst owing to its remarkable performances toward the OER, while with RuO2 remained durability issues.15-17 However, high price and limited supply of Ir have encouraged researchers to explore alternative catalysts that are economically competitive, yet efficient and durable. In an alkaline electrolyte, oxides and hydroxides of transition metals, such as Mn, Fe, Co, and Ni, are suggested as the potential candidates showing attractive OER performances.3,18-21 Among the various metal oxides and hydroxides, β-Ni(OH)2 has been of great importance as the model OER catalyst because of its high utilization in a rechargeable alkaline battery as a similar system and well-known reversible structural transformation during redox reactions.10,20-32 β-Ni(OH)2 in alkaline media can be electrochemically oxidized to β- and γ-phases of NiOOH,33 showing a remarkable activity and corrosion resistance toward the OER.34-36 Recent debates on the issue of crystal phase-dependent OER activity challenge the long-held view, demonstrating that the βNiOOH is more efficient than γ-NiOOH.34-40 Several experimental results demonstrate a new finding that the layered γ-NiOOH sheets is more active than layered β-NiOOH sheets due to the large population of catalytically active Ni4+.35,41,42 However, calculations imply that the undercoordinated side faces of layered β-NiOOH outperform the fully coordinated faces or even under-coordinated side faces of layered γ-NiOOH as in the original view.28,43-45 While undercoordinated faces of β-NiOOH may exhibit a higher OER activity than γ-NiOOH, the β-phase is inevitably converted to γ-phase during the OER in alkaline media, making β-NiOOH unable to



act as the OER catalyst.37-40 Therefore, the prevention of phase alteration and thus maintenance of the layered β-NiOOH structure with under-coordinated side faces remain a great agenda for the development of highly active and durable Ni-based electrocatalysts. Fast advances in nanocrystal-based catalysis could address the engineering of surfacestructural features, such as surface energy, surface atomic arrangement, and catalytically active site. Therefore, the β-phase of the NiOOH might be synthesized via the control of the atom packing motif, namely the facet on a substrate nanocrystal. However, despite the potential advantage of side faces of layered β-NiOOH based nanocrystals, experimental examples are utterly rare. Only theoretical studies have been performed, likely due to the lack of synthetic routes to control the facet of β-NiOOH. In this work, we introduce the faceted Ni nanocrystals as the parent substrate for the formation of β-NiOOH with the exposure of under-coordinated faces on the surfaces. Pt impurities are added to the Ni matrix during the synthesis of the faceted Ni nanocrystal, and the segregated Pt atoms at the core to vertices aid the faceting of Ni nanocrystals with the low indexes of {111}, {100}, and {110}.46 As the result, Ni octahedral, cubic, and rhombic dodecahedral (RD) nanocrystals have been obtained. The surface oxidation of shape-controlled Ni nanocrystals occurs naturally after the synthesis and a subsequent hydroxylation in a 0.5 M KOH aqueous solution. The conversions of β-Ni(OH)2 to β-NiOOH and further to γ-NiOOH are observed by applying the electric potential over 1.35 V (vs. reversible hydrogen electrode (RHE)) during the OER. Interestingly, the formation of β- or γNiOOH on the surface of Ni nanocrystal is facet-dependent. The β-NiOOH was observed on Ni octahedra, while the γ-NiOOH was found on Ni cubes and RDs. During a long-term OER operation, the surface β-NiOOH on Ni octahedra was maintained while the γ-NiOOH was peeled off from Ni cubes and RDs, resulting in the OER deactivation. The sustainable under-


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coordinated side faces of layered β-NiOOH on the {111}-faceted Ni octahedra exhibit highly active and durable performances toward the OER compared to the state-of-the-art Ir/C. Typical synthesis of Ni nanocrystals with three different shapes was carried out in solvothermal condition. Ni nanocrystals with octahedral, cubic, and RD shapes were achieved by introducing different gases during the seed formation and further growth, and minimal use of Pt helped the shaping of Ni nanocrystals with nearly pure Ni surfaces. The detailed synthesis of nanocrystals is described in the Experimental Section of Supporting Information. In the early stage of the growth of the nanocrystals, difference in decomposition rates of Ni and Pt precursors leads to sequential formation of Pt core and Ni shell structures. During the ripening, the small amount of Pt core branched into the Ni shell along the all directions that resulted in nanocrystals with octahedral symmetry (Oh symmetry group for octahedron, cube, and RD). The shape of Ni octahedron was well-controlled in the presence of CO, a selective Ni{111} capping molecule.46-48 In the case of cube, Ni{100} was stabilized by oleylamine and stearic acid in a sequential introduction of CO and Ar gases.47,49 The shape of RD enclosed by {110} facets was achieved via collaboration of oleylamine, stearic acid, and chlorine under Ar gas.50-52 High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images, energy dispersive spectrometer (EDS) elemental mapping, and high-resolution TEM (HRTEM) images and their corresponding fast Fourier transform (FFT) patterns of Ni nanocrystals with different shapes are shown in Figure 1. The average edge lengths of Ni octahedra, cubes, and RDs are 27±4, 26±5, and 20±4 nm (Figure S1), respectively. The EDS mapping images clearly show the segregation of Pt atoms at the core and along the directional axes to the vertices (yellow dots). Nanocrystals are largely composed of Ni (blue dots) with tiny amounts of Pt; 1.2 at% for octahedron, 0.4 at% for cube, and 1.1 at% for RD, as determined by inductively coupled plasma



atomic emission spectrometry. Oxygen is also detected at the shell of all the nanocrystals as shown in EDS mapping data (red dots), indicating the surface oxidation of Ni nanocrystals after the synthesis and during the further washing process. Therefore, the distinctively segregated Ni, Pt, and NiO phases are found for all three morphologies of Ni nanocrystals in the HRTEM images. Corresponding FFT patterns reveal the core composed of Ni and Pt, the body of Ni, and the shell of NiO, respectively. With a detailed knowledge of the surface structures and compositions of the present Ni nanocrystals, we evaluated the steady-state OER performances for the activated Ni catalyst supported on carbon (Ni nanocrystals/C, Figure S2). The measurements of OER activity and long-term stability of the three different Ni catalysts were carried out according to the previous literatures and compared with the state-of-the-art Ir/C and IrO2 catalysts.53,54 The OER performances of other comparative transition metal based catalysts are listed in Table S1.33,55-63 The catalysts were electrochemically cleaned under the potential cycling for 10 times with the scanning rate of 0.10 V s-1 between 0.088 and 1.0 V vs. RHE (VRHE) in an Ar-saturated 0.5 M KOH aqueous solution. Then the cyclic voltammograms (CVs) of the all catalysts were recorded with the same scanning condition. The CO stripping of Ir/C and Ni octahedra/C was also recorded. As shown in Figure S3, the peaks for under potential deposition of hydrogen (Hupd) and CO oxidation appear for the state-of-the-art Ir/C catalyst but no responses for the Ni catalysts due to the negligible amount of surface Pt. Ni is unresponsive to the Hupd and CO stripping measurements, indicating nearly pure Ni surfaces of as-prepared Ni catalysts. The OER polarization curves of the catalysts were obtained in an O2-saturated 0.5 M KOH solution by using a rotating-disk electrode (RDE) at a rotating speed of 1600 rpm and a scan rate of 0.020 V s-1. Then the OER polarization curves were normalized with the geometric surface area of RDE,


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and the resulting geometric current densities at 1.6 VRHE show the trend of catalyst activity as follows: Ni cubes/C > IrO2 > Ni octahedra/C > commercial Ir/C > Ni RDs/C (Figure 2a). Current densities were also normalized with the total mass of metal loading on RDE, and the corresponding mass activities at 1.6 VRHE are 0.5, 0.74, 0.27, 0.58, and 0.41 A mg-1metal for Ni octahedra/C, Ni cubes/C, Ni RDs/C, IrO2, and commercial Ir/C (Figure S4), respectively. Ni octahedra/C and Ni cubes/C show enhanced geometry- and mass-based specific OER activities than the state-of-the-art Ir/C. It should be noted that the value of specific activities is a typical guide for the comparison of catalysts, but not presenting the essential feature of the catalyst. A good catalyst should be stable when OER is continued for a long-period of time. Therefore, we conducted a long-term stability of OER for the better evaluation of catalysts.53 We tested chrono-potentiometric stability for the as-obtained Ni catalysts and Ir/C instead of potential cycling in order to avoid the reduction of electrocatalytically active species.64 The catalysts were held at a constant current density of 10 mA cm-2RDE at a rotation speed of 1600 rpm in an O2-saturated 0.5 M KOH solution, while the VRHE was measured as a function of time (Figure S5). In this test, the overpotential of Ir/C increased abruptly after 1.5 h, thus we stopped the measurement after 1 h. The ηt=0 for Ir/C was increased from 0.33 to 0.45 V (ηt=1h) at the current density of 10 mA cm-2RHE. The general instability of Ir/C in alkaline OER has been previously reported.53,54,65 In the case of IrO2 in alkaline media, ηt=0 was increased from 0.34 to 0.44 V (ηt=2h) at the current density of 10 mA cm2

RHE.

For the Ni catalysts, the chrono-potentiometric test was conducted in a more severe

condition of 2 h. The Ni cubes/C showed the lowest ηt=0 of 0.32 V at 10 mA cm-2RHE among four different catalysts, however its ηt=2h increased to 0.36 V (Figure S6). In the case of Ni RDs/C, the ηt=0 of 0.36 V increased to ηt=2h of 0.38 V (Figure S7). However, the Ni octahedra/C showed a



reversal of trend that the ηt=0 of 0.34 V decreased to ηt=2h of 0.32 V (Figure 2b), revealing sustained and even improved OER activity for a long period of time. TEM images of Ni octahedra/C showed no observable changes in structures during the stability test (Figure S8). The Tafel slopes of Ni octahedra/C (84.8 mV dec−1), Ni cubes/C (71.6 mV dec−1), and Ni RDs/C (93.4 mV dec−1) are smaller than that of Ir/C (109 mV dec−1) (Figure 2c), suggesting that the OER facilitated by Ni catalysts follows a kinetically more efficient pathway than Ir/C. After 2 h of chrono-potentiometric test (1 h for Ir/C), triangle-symbolled Tafel plots in Figure 2c indicate that the Ni octahedra/C showed a decreased value to 75.2 mV dec−1 (Figure 2b). On the contrary, Ni cubes/C, Ni RDs/C, IrO2, and Ir/C showed increased in Tafel slopes of 96.3, 115.8, 82.3, and 124 mV dec−1, respectively, indicating the deactivation of the catalysts over time. With a robust carbon paper electrode (CPE), we conducted the chrono-potentiometric stability test for 24 h. The Ni octahedra/CPE maintained the OER performance over the long-term of 24 h, and no sign of catalyst deactivation was observed (Figure S9). The overpotentials at current density of 10 mA cm-2RDE were recorded to compare the OER performance before (ηt=0) and after (ηt=x, x= 1, 2 h) the stability test (Figure 2d). An overall graphical comparison of all these catalysts in terms of stability is presented in Figure 3. The abscissa signifies the ηt=0 required to achieve the specific activity of 10 mA cm-2RDE at initial stage of OER. The ordinate signifies the ηt=2h required to achieve 10 mA cm-2RDE after 2 h of stability. The diagonal dashed line is the projected response for a stable catalyst; catalysts with plots above the line show the deactivation (or passivation) over time. Highly active and durable catalyst can be positioned at lower overpotential and below the line. We conclude that the trend of catalytic stability of the following catalysts in terms of overpotential is Ni octahedra/C > Ni cubes/C > Ni RDs/C > IrO2 > Ir/C.


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The anodic onset potential observed at 1.35 VRHE and the adjacent peak for Ni octahedra/C, observed in the inset of Figure 2a, correspond to the electrochemical oxidation of β-Ni(OH)2 to β-NiOOH as confirmed from previous surface-enhanced Raman scattering (SERS) and x-ray photoelectron spectroscopy (XPS) studies.22,64 An increase in applied potential may result in the further oxidation of overcharging β-NiOOH to γ-NiOOH.64 Therefore, the formation and further exfoliation of γ-NiOOH during the stability test for Ni cubes/C and Ni RDs/C lead to an increase of OER overpotentials and thus the deactivation of catalysts. However, based on the observations of electro-catalyst stability, the {111} facet of Ni octahedra/C may prevent the β- to γ-phase transformation of NiOOH during the harsh stability test for 2 h. This phenomenon indicates the surface passivation and even the formation of more electroactive surfaces such as undercoordinated side faces of layered β-NiOOH during the stability test.66 In parallel with the electrochemistry study, we performed mechanism study to track the changes of surface structure of Ni nanocrystals along the oxidation stages by TEM, HRTEM, and X-ray diffraction (XRD). For the formation of β-Ni(OH)2 on the surfaces, the as-obtained Ni nanocrystals were kept in an Ar-saturated 0.5 M KOH solution for 10 h at room temperature. As shown in Figure 4a, the β-Ni(OH)2 shell of a hexagonal close packed (hcp) structure was formed on the {111}-faceted face centered cubic (fcc) NiO/Ni octahedron. A crystallographic model of Figure 4c indicates that slight lattice mismatches of -9.9% along hcp with fcc direction and 6.0% along hcp with fcc direction, respectively, between {100}hcp of βNi(OH)2 and {111}fcc of NiO/Ni octahedron resulted in a epitaxial growth and a exposure of side faces of layered β-Ni(OH)2 as shown by the HRTEM image in the inset of Figure 4a. However, the large lattice mismatch of 56.1% along hcp with fcc between {100}hcp of βNi(OH)2 and {100}fcc of NiO/Ni cubes rendered a poor epitaxial interaction (Table S2). The



lattice mismatch of {100}hcp of β-Ni(OH)2 with {110}fcc of NiO/Ni RDs (10.4% along hcp with fcc) surfaces is larger than that with {111}fcc of octahedra. In addition, an atomic model of β-Ni(OH)2 formed on a rock salt NiO ionic crystal suggests an ionic repulsion on the surfaces of Ni nanocrystals. The {111}fcc facet of rock salt NiO exposes Ni2+ ions on its top layer and therefore the O2- from the {100}hcp faceted β-Ni(OH)2 can bind strongly to the NiO as shown in Figure 5a. Therefore, the small lattice mismatch at the epitaxial junction of the Ni octahedron is expected to be ignored, leading to a sustainable surface βNi(OH)2. In contrast, Ni2+ and O2- ions are alternately placed on the top layers of {100}fcc and {110}fcc faceted NiO. Therefore, the lattice mismatch for Ni cubes and RDs can induce repulsive interactions of O2- ions between NiO and β-Ni(OH)2 (Figure 5, b and c).67-69 As the result, significant exfoliation of the β-Ni(OH)2 sheets can be observed for both the Ni cubes and RDs after 10 h of treatment in alkaline media due to the lattice mismatch and the ionic repulsion (Figure 5, e and f), but not for the Ni octahedra (Figure 5d). We propose that the synergy between the level of surface lattice mismatch and ionic repulsion is the determining factor for the alkaline OER stability of Ni nanocrystals. To understand the facet-dependent OER stability, the electrochemical oxidation of β-Ni(OH)2 to β-NiOOH and further oxidation to γ-NiOOH on the Ni nanocrystals was accomplished by applying oxidative potential of 1.35 VRHE for 2 h in alkaline media. The exfoliated β-Ni(OH)2 sheets on the surfaces of Ni cubes and RDs could be easily oxidized to β-NiOOH and to γ-NiOOH. Finally, the exfoliation of γ-NiOOH is observed for the Ni cubes and RDs (Figure S10). TEM and HRTEM images of the surface of Ni octahedra exhibit only the β-NiOOH shell; neither the formation nor the further exfoliation of γ-NiOOH was observed (Figure 4b and S10). A crystallographic model of Figure 4d reveals the reduced lattice mismatch (-5.4% along hcp with fcc direction)


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between the {100}hcp β-NiOOH and its sublayer of {111}fcc NiO (Table S2). The oxidation of βto γ-NiOOH requires a dramatic change of interlayer distance from 4.8 to 7.0 Å along c-axis. On the {111}fcc facet of NiO, the β- to γ-phase transformation could be greatly hindered due to the greatly increased lattice mismatch from -5.4 to 40% along hcp with fcc direction during the expansion of the interlayers (Figure 4, d to f). Therefore, the well-aligned interfaces derive not only the formation of OER active under-coordinated {100}hcp facets of layered βNi(OH)2 but also the prevention of the further oxidation to γ-NiOOH. Based on the HRTEM images and crystallographic model, we built the conceptual surface of β-NiOOH/NiO{111}fcc/Ni octahedron for the active and durable OER model catalyst as shown in Figure 4e. This illustrates the well-aligned crystal layers of β-NiOOH on the surface of NiO and exposed undercoordinated {100}hcp facets. Our finding supports the long-held view and density functional theoretical (DFT) study that the side facets of layered β-NiOOH, which have under-coordinated metal sites, are more active for the OER than the fully coordinated {001}hcp facet of γNiOOH.28,44,45,70 Figure 6 and Figure S11 show XRD patterns of the Ni nanocrystals at different oxidation conditions. As-prepared Ni nanocrystals exhibit a dominant Ni phase pattern with the minute NiO diffractions in Figure 6a and Figure S11a. After 10 h of hydroxylation in an 0.5 M KOH solution, significant diffractions of β-Ni(OH)2 are collected (Figure 6b). Then, the diffractions of β-Ni(OH)2 disappeared after 2 h of OER, but the characteristic diffractions with 7 Å of γ-NiOOH undoubtedly appeared for the Ni cubes and RDs, except for the Ni octahedra (Figure 6c). The thin layered β-NiOOH shell on the Ni octahedra might be hardly identified by XRD due to its small crystallite size. Surprisingly, the phase trans-formation of β- to γ-NiOOH was distinctively suppressed on the surfaces of Ni octahedra even in a more severe condition of 24 h of OER



(Figure 6d). XPS was collected for catalysts before and after the OER experiment. As shown in Figure S12, the surface of the Ni nanocrystals after OER is composed of Ni, NiO, Ni(OH)2, βand γ-NiOOH. Overall, the Ni metal in the surface was transformed to more oxidized forms and some fraction of Ni(II) species was further oxidized to Ni(III) species during the OER. All observations from the XRD and XPS measurement support the phase transformation during the overall oxidation stages and provide the characterization of the sustainable surface β-NiOOH on Ni octahedra. In summary, the synthesis of highly active and durable β-NiOOH nanocatalyst was achieved by obtaining the shape-controlled Ni octahedra and the subsequent hydroxylation and oxidation in an Ar-saturated 0.5 M KOH solution. The crystal structure modeling showed that the {111}fcc facet of NiO/Ni has a well-suited characteristic ionic heterepitaxy to the {100}hcp facet of βNiOOH. Experimental results showed that the {100}hcp faceted β-NiOOH was maintained on the surface of the Ni octahedra and was not oxidized to γ-NiOOH during the OER. However, the βto γ-NiOOH transformation and thus the surface exfoliation resulted in the deactivation of Ni cubes and rhombic dodecahedra toward the long-term OER. Electrochemical measurements revealed that surface β-NiOOH modified Ni octahedra/C exhibited the prominent OER activity and durability that outperform those of the state-of-the-art Ir/C catalyst. We believe that the findings in this work would offer great opportunities for understanding and approach to fabrication of layered metal(oxy)hydroxides with desired reactive surfaces as the highly active and durable alkaline OER catalyst.


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Figure 1. Structural analyses of Ni nanocrystals. HAADF-STEM images and EDS elemental mapping analyses of the Ni (a) octahedra, (b) cubes, and (c) rhombic dodecahedra (RDs). HRTEM images of the Ni (d) octahedron, (e) cube, and (f) RD and their corresponding FFT patterns of the i) core, ii) body, and iii) shell of the nanocrystal.



Figure 2. (a) OER polarization curves measured at a scan rate of 0.02 V s-1 in an O2-saturated 0.5 M KOH solution with a rotation speed of 1600 rpm where the current densities (j) were normalized against geometric surface area of RDE. (b) OER polarization curves of Ni octahedra/C catalyst obtained at a scan rate of 0.02 V s-1 before and after the stability test. (c) OER Tafel plots derived from the OER polarization curves of the corresponding catalysts before and after the stability test. (d) OER overpotentials required to achieve the current density of 10 mA cm-2RDE for the catalysts before and after the stability test.


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Figure 3. An overall graphical comparison of the overpotentials of the catalysts at a current density of 10 mA cm-2RDE before and after the stability test.



Figure 4. Structural analyses of heteroepitaxial β-Ni(OH)2 and β-NiOOH phases on {111}fcc faceted Ni octahedron. HRTEM images of the Ni octahedron (a) after hydroxylation and (b) after OER stability for 2 h. The insets showing the magnified views of the selected area in HRTEM images and their corresponding FFT patterns with zone axis of fcc. Crystallographic models showing the calculated lattice mismatches of (c) β-Ni(OH)2 and (d) β-NiOOH on {111}fcc faceted NiO with their projected view along the zone axis of fcc and fcc. (e) Conceptual perspective model of Ni@NiO@β-NiOOH surface structure. (f) Crystallographic model showing the water intercalated γ-NiOOH on {111}fcc faceted NiO. Ni and O atoms are colored grey and red, respectively. {001}hcp facet of β-Ni(OH)2 or β-NiOOH and {111}fcc facet of NiO are colored green and yellow, respectively.


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Figure 5. Atomic models of β-Ni(OH)2 on (a) {111}fcc, (b) {100}fcc, and (c) {110}fcc facets of NiO with zone axis of fcc. Ni and O atoms colored white and red, respectively. Attractive and repulsive interactions between Ni and O atoms represented with white dashed line and yellow arrow, respectively. TEM images of surface hydroxylated Ni (d) octahedra, (e) cubes, and (f) RDs in 0.5 M KOH solutions for 10 h at room temperature.



Figure 6. Magnified XRD spectra of the Ni nanocrystals with different shapes (a) of a fresh condition, (b) after 10 h of hydroxylation in 0.5 M KOH solutions, and after (c) 2 and (d) 24 h of OER stability test in 0.5 M KOH solutions. References: Ni (PDF#00-004-0850), NiO (PDF#00004-0835), β-Ni(OH)2 (PDF#00-014-0117), β-NiOOH (PDF#00-006-0141), and γ-NiOOH (PDF#00-006-0075).


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ASSOCIATED CONTENT Supporting Information. Experimental details, TEM images, CVs, OER polarization curves, chrono-pontentiometric curves, table for the lattice mismatch. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *[email protected] (S.-I.C. for electrochemical measurements) *[email protected] (K.L. for synthesis and characterization) Author Contributions ¶These

authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Research Foundation of Korea grants (NRF-2017R1A2B3005682 and NRF-2016H1D5A1910726, NRF-2017-R1A6A3A01008861), IBS-R023D1, and KBSI Project E37300. The authors thank Korea Basic Science Institute (KBSI) for the usage of their HRTEM instrument.



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NiOOH Exfoliation-Free Nickel Octahedra as Highly Active and

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