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Interlayer Expansion of Layered Cobalt Hydroxide Nanobelts to Highly Improve Oxygen Evolution Electrocatalysis Lan Huang, Jing Jiang, and Lunhong Ai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14479 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017

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ACS Applied Materials & Interfaces

Interlayer Expansion of Layered Cobalt Hydroxide Nanobelts to Highly Improve Oxygen Evolution Electrocatalysis Lan Huang, Jing Jiang*, and Lunhong Ai* Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, Shida Road 1#, Nanchong 637002, P.R. China

ABSTRACT

Water oxidation reaction is known to be energy-inefficient and generally

considered as a major bottleneck for water splitting. Exploring electrocatalysts with highefficiency and at low cost is vital to widespread utilization of this technology, but is still of big challenge. Here we report an effective strategy based on expanding interlayer of layered structures to realize a great enhancement of the OER catalytic performance from water splitting. Well-defined nanobelts of layer-structured cobalt benzoate hydroxide (Co(OH)(C6H5COO)·H2O) are successfully prepared in terms of a simple hydrothermal process. Intercalation with benzoate ions induces the interlayer expansion of the cobalt hydroxide, which is useful for the accommodation of more electrolyte ions and favorable for their diffusion and transport. The asprepared Co(OH)(C6H5COO)·H2O nanobelts need significantly smaller overpotential (~0.36 V) to reach 10 mA·cm-2 of current density compared with that of Co(OH)2 (~0.44 V) and Co3O4

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(~0.387 V) counterparts, and even favorably compare with most of the layered hydroxide-based electrocatalysts. Moreover, the Co(OH)(C6H5COO)·H2O nanobelts keeps much higher stability than that of RuO2 reference in alkaline solution. This approach would be utilized to the design and development of high-performance layered hydroxide-based electrocatalysts.

KEYWORDS

Cobalt hydroxide; Oxygen evolution; Electrocatalysis; Layered structure;

Nanobelts

1 INTRODUCTION

The gradually diminishing fossil fuels and rapidly growing energy demands have prompted tremendous efforts in the exploration of renewable and sustainable energy sources. Electricitydriven reaction splits H2O into clean fuel H2 that provides an attractive approach to achieve this purpose.1-2 As a critical half-reaction of water splitting, the oxygen evolution reaction (OER) is slow kinetics naturally, and thus it is efficiency-limited and becomes the bottleneck of water splitting. It is obvious that an effective oxygen evolution catalyst is particularly critical to speed up rate and decrease energy loss of OER process. At present, noble metal oxides (RuO2 and IrO2) are widely accepted as the most famous materials for electrochemically catalyzing OER, but the high price and shortage unavoidably obstruct their broad applications in practice.3 As a result, enormous efforts have been devoted to develop cost-effective alternatives based on earthabundant materials. The layer-structured metal (oxy)hydroxides containing stacked edge-shared MO6-octahedron layers have been proposed as in situ transformed reactive species of metal oxides and chalcogenides for catalyzing OER.4-8 Particularly, layered double hydroxides (LDHs) owned analogous layers are known to electrocatalyze water oxidation with excellent efficiency.9-11


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Their large interlayer space would endow them with rich electroactive sites and notable catalytic behaviors. Various strategies have been adopted efficiently either by hybridizing them with conductive substrates12-15 or by nanostructure-engineering to further improve their OER performance.16-17 For instance, Hu’s group made important progress by liquid phase exfoliation method to form monolayered LDHs, achieving the superior activity to their bulk counterpart because of the greatly increased electrochemical reactive sites.18 Besides the layer factor, the intercalated anions would give great impact on OER activity of LDHs as well, which determines the interlayer space to allow the favorable diffusion of reactants and boosts partial charge transfer activation effect in LDHs.19-20 Unfortunately, limited examples have been established on expanding interlayers of layered monometal hydroxides to achieve efficiently electrocatalytic OER so far. Layered metal hydroxides have an analogous structure to LDHs and are comprised by brucitelike host layers (positively charged sheets) with interlayer counter anions. Herein, we report an incorporation of benzoate ions with the length of about 0.70 nm as charge-balancing species to expand the interlayer distances of layered cobalt hydroxides, and result in the formation of onedimensional nanobelt structures. The one-dimensional nanobelt structures have specific advantage for OER catalysis, which can shorten the ion diffusion and electron transportation distance and enlarge contact area between electrode and electrolyte.21 The as-prepared Co(OH)(C6H5COO)·H2O nanobelts have a larger interlayer distance than those of the conventional LDHs and metal hydroxides (Figure 1), which are greatly capable of electrocatalyzing water oxidation. Further, the OER activities of these cobalt hydroxide nanobelts are largely enhanced in comparison with layered cobalt hydroxides and other typical


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cobalt oxides. This approach would be utilized to the design and development of highperformance layered hydroxide-based electrocatalysts.

Figure 1. Crystal structure of layered cobalt hydroxide. 2 EXPERIMENTAL SECTIONS 2.1 Synthesis of Co(OH)(C6H5COO)·H2O nanobelts Co(NO3)2·6H2O (5 mmol) and C6H5COONa (10 mmol) were dissolved in distilled water (65 mL) under stirring at ambient condition. The mixture solution was put into a 80 mL Teflon-lined stainless-steel-autoclave. The sealed autoclave was then heated to 95 °C and kept at this temperature for 48 h. Thereafter, the resulting pink precipitates were collected by centrifugation, rinsed with distilled water several times, and dried overnight at 60 °C. The samples were also synthesized at different reaction times or by using different cobalt salts. The Co3O4 counterpart was synthesized by calcination of corresponding Co(OH)(C6H5COO)·H2O at 300 °C for 2 h in air, while Co(OH)2 counterpart was synthesized according to previous study.18 2.2 Characterization


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The phase determination was conducted on Rigaku-Dmax/Ultima(IV) X-ray diffractometer with monochromatized Cu Kα radiation. The microscopic morphologies were examined by Hitachi-S4800 and JEOL-JSM-6510LV scanning electron microscopes (SEM) combined with energy dispersive X-ray spectroscopy (EDS). The detailed structures were studied by using a FEI Tecnai(G20) transmission electron microscope (TEM). The surface compositions were probed by a Perkin-Elmer PHI(5000C) X-ray photoelectron spectrometer (XPS) with Al Kα source. Infrared spectroscopy was collected on a Nicolet-6700 Fourier transform infrared (FTIR) spectrometer using potassium bromide pellet method. UV-vis diffuse reflectance spectroscopy of the samples was obtained using a Shimadzu-UV-3600 spectrophotometer. 2.3 Electrochemical measurements The evaluation on electrocatalytic OER activities were performed in a three-electrode system controlled by CHI(660E) electrochemical workstation using 1.0 M KOH solutions as electrolytes, in which 3.0 M KCl-Ag/AgCl, Pt foil and glassy carbon electrode (GCE) coated with Co(OH)(C6H5COO)·H2O were employed as reference, auxiliary and working electrodes, respectively. For the fabrication of working electrode, five microliter of catalyst dispersion (mixture of Co(OH)(C6H5COO)·H2O (5.0 mg) and Nafion solution (5.0 wt%, 10.0 µL) in water/alcohol solution (1.0 mL, 3:1 v/v)) was dropped onto GCE (geometric area: 0.07 cm2) to obtain a loading amount of 0.35 mg·cm-2. All the potentials reported as the form of reversible hydrogen electrode (RHE) based on the equation: ERHE=EAg/AgCl+0.059pH+0.197V. Cyclic voltammograms (CVs) were performed at a scan rate of 100 mV s-1 between 1.023 V and 2.023 V (vs. RHE). The working electrode was scanned for ten potential cycles before the LSV test to reach a stable state. The OER activities of Co(OH)(C6H5COO)·H2O were investigated by linear sweep voltammetry (LSV) at a scan rate of 5 mV·s-1 and the LSV data were made with an ohmic


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drop (iR) correction tested by impedance spectroscopy. Electrochemical impedance spectroscopy (EIS) were obtained under OER condition (1.643 V vs. RHE) with an AC amplitude of 5 mV by varying frequency from 0.01 to 106 Hz. Durability tests for OER catalysis were performed by means of chronoamperometry at current density of 10 mA cm-2 and chronoamperometry at potentials of 1.67 V and 1.72 V (vs. RHE) without iR compensation. RESULTS AND DISCUSSION Figure 2a is the SEM observation on as-prepared Co(OH)(C6H5COO)·H2O sample at lowmagnification, which consists of large quantities of randomly oriented one-dimensional ultralong nanostructured belts. They are intertwined with each other to form the network architecture. The SEM image at high magnification (Figure 2b) shows that these nanobelts have smooth surfaces with the lengths of several micrometers. Figure 2c and Figure 2d are representative EDS analysis for Co(OH)(C6H5COO)·H2O sample. The signals from constituted elements including cobalt, oxygen and carbon can be clearly detected, and these elements seem to be evenly distributed through nanobelts.


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Figure 2. SEM images of Co(OH)(C6H5COO)·H2O nanobelts with different magnification (a,b). EDS analysis for Co(OH)(C6H5COO)·H2O nanobelts: EDS spectrum (c) and elemental mapping (d). The detailed microstructures of Co(OH)(C6H5COO)·H2O nanobelts are further determined by TEM observations. Figure 3a displays a representative TEM image for Co(OH)(C6H5COO)·H2O sample, which confirms the typical belt-like geometry of the products. It is clear that many of the nanobelts are curved and highly flexible. The close TEM observation on an individual nanobelt (Figure 3b) confirms the rectangular cross section for the nanobelt where the cross section profile of nanobelt in its entire length is exactly the same. A typical HRTEM image (Figure 3c) of the nanobelt shows a layered stacked structure with a clear continuous lattice fringes, revealing the crystalline characteristic of nanobelts. The interplanar d-spacing of nanobelts (see the line profile in Figure 3d) is ca. 1.48 nm, matches well with the (001) lattice plane of Co(OH)(C6H5COO)·H2O. The corresponding fast Fourier transform (FFT) pattern (Figure 3e) also confirms the growth orientation of the nanobelts along (00l) direction.


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Figure 3. TEM (a,b) and HRTEM (c) images of Co(OH)(C6H5COO)·H2O nanobelts. The line profile (d) and fast Fourier transformation (FFT) image (e) of Co(OH)(C6H5COO)·H2O nanobelts. Figure 4a shows a typical X-ray diffraction (XRD) pattern of Co(OH)(C6H5COO)·H2O nanobelts, which exhibits the layer-structured characteristic diffractions and agrees well with the previously reported patterns of Co(OH)(C6H5COO)·H2O.22-23 A series of (00l) peaks at 5.95° (1.48 nm), 11.85° (0.75 nm), 17.78° (0.50 nm) indexed as (001), (002), and (003) reflections. In the light of larger interlayer spacing of 1.48 nm (estimated from low angle) for Co(OH)(C6H5COO)·H2O and the length of the benzoate anion (0.70 nm), the stacked double layers of benzoate anions in layered Co(OH)(C6H5COO)·H2O nanobelts can be formed.24 Indeed, the Co(OH)(C6H5COO)·H2O nanobelts exhibit a significant interlayer expansion in comparison with the normal Co(OH)225-26 and anion-intercalated LDHs.27-28


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The structure of the Co(OH)(C6H5COO)·H2O nanobelts was further analyzed by FTIR spectroscopy, which is a useful tool for characterization of layered metal hydroxides. Figure 4b gives the FTIR spectra of the Co(OH)(C6H5COO)·H2O nanobelts, along with C6H5COONa as comparison.

The

spectrum

clearly

demonstrates

the

hydroxyhydrate

type

of

the

Co(OH)(C6H5COO)·H2O nanobelts. The broad band at 3440 cm-1 corresponds to hydroxyl stretching vibration for interlayer water molecules, while narrow one at 3600 cm-1 can be assigned to nonhydrogen bonded Co−OH groups that is characteristic of the brucite-like structure.25 Meanwhile, a new band appears at 450 cm-1, confirming the typical Co−O stretching vibrations of brucite-like octahedral CoO6 units. As for the interlayer benzoate anions, the bands at 1608 (stretching vibrations of C=C), 712 and 682 cm-1 (bending vibrations of C−H) are characteristic of benzene rings.29-30 The two strong absorption bands at 1390 and 1565 cm-1 can be associated with symmetric and antisymmetric stretching vibrations for carboxylate groups from intercalated benzoate anions, respectively.31


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Figure 4. Structural characterization of Co(OH)(C6H5COO)·H2O nanobelts: XRD pattern (a) and FTIR spectrum (b). The composition and valence states of Co(OH)(C6H5COO)·H2O nanobelts were determined by XPS. The XPS survey scan (Figure 5a) confirms that cobalt, carbon, and oxygen elements coexist in Co(OH)(C6H5COO)·H2O nanobelts. The high-resolution C 1s XPS spectrum (Figure 5b) reveals main carbon species of aromatic ring (284.5 eV) and carboxylate groups from benzoate anions (288.2 eV) in Co(OH)(C6H5COO)·H2O nanobelts. The signals for O 1s (Figure 5c) were observed at 531.2 and 532.1 eV, matching with carboxylate groups of benzoate anions and Co–O bonds of brucite-like octahedral CoO6 in the Co(OH)(C6H5COO)·H2O nanobelts.32 In Figure 5d, the signals of Co 2p3/2 and Co 2p1/2 are observed at 781.2 and 797.3 eV, which are characteristic of Co(II). Meanwhile, two distinct satellite peaks are located at ~5 eV above their


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main peaks, evidencing surface Co(II) in Co(OH)(C6H5COO)·H2O nanobelts.33-34 Furthermore, the Co(II) feature in Co(OH)(C6H5COO)·H2O nanobelts can be verified by the UV-vis spectrum (Figure S1, Supporting Information). The Co(OH)(C6H5COO)·H2O nanobelts present three absorption bands at 490, 526, and 619 nm, which are characteristic of Co(II) species in an octahedral geometry.22

Figure 5. (a) Survey XPS spectrum of Co(OH)(C6H5COO)·H2O nanobelts. (b) C1s spectrum of Co(OH)(C6H5COO)·H2O nanobelts. (c) O1s spectrum of Co(OH)(C6H5COO)·H2O nanobelts. (d) Co2p spectrum of Co(OH)(C6H5COO)·H2O nanobelts. The LSV tests were recorded at a scan rate of 5 mV·s-1 to determine OER activities of the Co(OH)(C6H5COO)·H2O nanobelts. As comparison, the blank GCE, benchmark RuO2, Pt/C references, and other Co-based OER catalysts, such Co(OH)2 and Co3O4 counterparts (see


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detailed characterization in Figure S2-S3) are also measured in the same conditions. Notably, we particularly used Co-based OER catalysts as reference catalysts for assessing the OER activity of the Co(OH)(C6H5COO)·H2O nanobelts because they have been previously demonstrated to be excellent OER catalysts.35 As shown in Figure 6a, blank GCE presents no any activity, while Pt/C performs poorly for OER, similar to previous observations in alkaline media.36-37 As observed, the Co(OH)(C6H5COO)·H2O nanobelts are highly active for electrochemically catalyzing OER, evidenced by its relatively earlier onset potential and larger OER current. Such performance

is

superior

to

that

of

Co(OH)2

and

Co3O4

counterparts.

The

Co(OH)(C6H5COO)·H2O nanobelts need significantly smaller overpotential (~0.36 V) to reach 10 mA·cm-2 of current density compared with that of Co(OH)2 (~0.44 V) and Co3O4 (~0.387 V) counterparts, and is observed to be only 90 mV larger than that of the benchmark catalyst RuO2. It is noteworthy that the value of η10 for the Co(OH)(C6H5COO)·H2O nanobelts compares favorably with the previously reported Co-based OER catalysts38-39 and other LDH-based OER catalysts (Table S1, Supporting Information), further featuring the excellent OER performance of the

Co(OH)(C6H5COO)·H2O

nanobelts.

To

optimize

the

OER

activity

of

the

Co(OH)(C6H5COO)·H2O nanobelts, we further prepared other Co(OH)(C6H5COO)·H2O samples by varying the reaction times (Figure S4, Supporting Information) and using different cobalt sources (Figure S5, Supporting Information). Clearly, both the initial cobalt source and reaction time could affect the OER activities of the Co(OH)(C6H5COO)·H2O catalysts. The Tafel plots of the above catalysts were compared to probe OER kinetics. Figure 6b presents corresponding Tafel plots derived from the LSV curves. Co(OH)(C6H5COO)·H2O nanobelts yield a Tafel slope of 76 mV·dec-1, which is slightly bigger than the benchmark catalyst RuO2 (58 mV·dec-1). However, this value is remarkably lower compared with that of


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commercial Pt/C, Co(OH)2 and Co3O4 counterpart, revealing the favorable kinetics of Co(OH)(C6H5COO)·H2O nanobelts for catalyzing OER.

Figure 6. (a) OER polarization curves of Co(OH)(C6H5COO)·H2O nanobelts, benchmark RuO2, Pt/C, Co(OH)2 and Co3O4 counterparts. (b) LSV-derived Tafel plots of above catalysts. The electrochemical surface area (ECSA) of above catalysts was measured to shed light on the as-observed OER behaviors, which can be reflected by electrochemical double layer capacitances (Cdl). On the basis of CV curves recorded at different scan rates (Figure 7a-c), the plots of scan rate against current density are illustrated (Figure 7d), whose linear slope (equivalent to twice the Cdl) can be reflected the ECSA since the ECSA of an electrocatalyst is proportional to its Cdl value. The calculated Cdl values of the Co(OH)(C6H5COO)·H2O nanobelts, Co(OH)2, and Co3O4 counterparts are 19.5, 13.0, and 14.0 µF·cm-2, respectively, implying that


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Co(OH)(C6H5COO)·H2O nanobelts possess a largest active surface area among all the samples. Furthermore, EIS measurements of Co(OH)(C6H5COO)·H2O nanobelts were conducted to study electrode kinetics. Figure S6 shows the corresponding Nyquist plots, which suggests that Co(OH)(C6H5COO)·H2O nanobelts achieve a smallest charge transfer resistance among all the catalysts. This result implies the faster charge transfer of Co(OH)(C6H5COO)·H2O nanobelts during electrochemical OER process.

Figure 7. CVs of the Co(OH)(C6H5COO)·H2O (a), Co(OH)2 (b) and Co3O4 (c). Estimated ECSA of the Co(OH)(C6H5COO)·H2O, Co(OH)2 and Co3O4 (d). We further investigated the long-term durability of Co(OH)(C6H5COO)·H2O nanobelts. Figure 8a shows the chronopotentiometric response curve of Co(OH)(C6H5COO)·H2O nanobelts. It can


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be found that the Co(OH)(C6H5COO)·H2O nanobelts afford an almost unchanged operating potential within 14 h, whereas the potential increases significantly with reaction time for the RuO2 catalyst under the same condition, suggesting the deactivation of RuO2 and excellent durability of the Co(OH)(C6H5COO)·H2O nanobelts for OER catalysis. Figure 8b shows the chronoamperometric durability of the Co(OH)(C6H5COO)·H2O nanobelts at different applied potentials. The OER catalytic current is almost retained at 1.67 V. When the applied potential was fixed at 1.72 V, a slight decrease of current density for this potential was observed. This observation may be due to plenty of gas bubbles generated on catalyst surface under a much higher current density and overpotential, which will slowly damage the catalyst surface.

Figure 8. (a) Chronopotentiometric and (b) chronoamperometric durability tests of the Co(OH)(C6H5COO)·H2O nanobelts in 1.0 M KOH solution without iR correction. It is generally accepted that the Co(II) species in Co-based electrocatalysts tend to be oxidized and transform into high valent ones with OER-activity, which is necessary to proceed electrocatalytic OER.40-42 Co(OH)(C6H5COO)·H2O is a typical Co(II)-containing compound, whose Co species are expected to behave as OER-active sites. The CVs of the Co(OH)(C6H5COO)·H2O nanobelts in alkaline medium (Figure 9a) reveal certain oxidative


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transition between cobalt species. Clearly, Co(OH)(C6H5COO)·H2O nanobelts experiences an oxidation process with a prominent anodic peak at about 1.41 V prior to the onset potential at its first scan. Such oxidation peak then diminishes even in the second scan, signifying the irreversible surface oxidation of the Co(OH)(C6H5COO)·H2O nanobelts,5,

43

which coincides

with results observed for electrochemical oxidation of Co(OH)2 to CoOOH.6,

44

To further

support this result, XPS analysis of the Co(OH)(C6H5COO)·H2O nanobelts after OER catalysis was

performed.

In

high-resolution

Co

2p

spectrum

(Figure

9b)

of

post-OER

Co(OH)(C6H5COO)·H2O nanobelts, two peaks located at 781.0 eV and 796.3 eV are assigned to Co2p3/2 and Co2p1/2 of the Co(III) species, respectively, which have a significant shift (∼1 eV) to low binding energy compared to the Co(OH)(C6H5COO)·H2O nanobelts prior to OER catalysis. Moreover, the disappearance of characteristic shake-up satellites relevant to Co(II) further support the effective transformation of Co(II) to catalytically active Co(III) centers in the Co(OH)(C6H5COO)·H2O nanobelts.33,

45

Correspondingly, the Co(OH)(C6H5COO)·H2O

nanobelts are nearly converted to CoOOH after OER (Figure 9c). The SEM images (Figure S7, Supporting

Information)

also

reveal

that

the

significant

morphology

change

of

Co(OH)(C6H5COO)·H2O after OER. Furthermore, Raman spectrum (Figure 9e) of post-OER Co(OH)(C6H5COO)·H2O nanobelts presents a characteristic of CoOOH,46-47 where the [CoO6] unit (697 cm-1) and Co−O (524 cm-1) stretching vibration in CoOOH can be obviously seen. In fact, this in situ transformation during OER catalysis in the Co(OH)(C6H5COO)·H2O nanobelts is also indirectly reflected by its color change. The photographs of the Co(OH)(C6H5COO)·H2O nanobelts deposited on a Ti plate are shown in Figure 9d. The light pink pristine Co(OH)(C6H5COO)·H2O nanobelts turn browneblack during OER process. This phenomenon is similar to the OER process of Ni(OH)2/FTO48 and NiCo-LDH/Ni foam electrocatalysts.12 Based


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on these experimental results, we ascribe the excellent OER performance of the Co(OH)(C6H5COO)·H2O nanobelts its advantageous structures. The Co(OH)(C6H5COO)·H2O nanobelts own an open layer structure and present a favorable interlayer expansion compared to conventional metal hydroxides. By expanding interlayer, Co(OH)(C6H5COO)·H2O nanobelts gain more space for accommodation of reactants, thus providing more chance to active sites for close contact with reactants. Moreover, the enlarged interlayer distance in Co(OH)(C6H5COO)·H2O nanobelts can also assist the reactant transport and generated gas evolution. This structure feature would be utilized to the design of other layered hydroxide electrocatalysts. We therefore synthesize the layered cobalt hydroxides by using terephthalate to replace benzoate to achieve interlayer expansion, which also exhibit a significantly enhanced activity compared with that of Co(OH)2 for OER catalysis (Figure S8, Supporting Information).


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Figure 9 (a) Cyclic voltammograms of Co(OH)(C6H5COO)·H2O nanobelts without iR correction. Co2p XPS spectra (b), XRD patterns (c), optical photographs (d) and Raman spectrum (e) of Co(OH)(C6H5COO)·H2O nanobelts after chronopotentiometric tests in 1.0 M KOH solution. 4 CONCLUSIONS In summary, we describe a strategy through interlayer expansion of layered structures to realize the great enhancement of the OER catalytic performance of layered metal hydroxides. Taking Co(OH)(C6H5COO)·H2O nanobelts as an example, we successfully synthesized the benzoate ions intercalated layered cobalt hydroxide nanobelts by a facile hydrothermal process. By means of a series of characterizations, the benzoate ions intercalation induced interlayer expanding is confirmed experimentally. The as-prepared Co(OH)(C6H5COO)·H2O nanobelts


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exhibit the greatly enhanced OER catalytic performance, compared to those of Co(OH)2 and Co3O4 counterparts. Moreover, the Co(OH)(C6H5COO)·H2O nanobelts also keeps much higher stability than that of RuO2 reference in alkaline solution. This approach would be utilized to the design and development of high-performance layered hydroxide-based electrocatalysts. ASSOCIATED CONTENT Supporting

Information:

UV-vis

absorbance

spectrum,

OER

activity

of

Co(OH)(C6H5COO)·H2O nanobelts, XRD patterns and SEM images of catalysts, supplementary tables of OER catalysis. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (J. Jiang), [email protected] (L. Ai); Tel/Fax: +86-08172568081 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51572227), and Sichuan Youth Science and Technology Foundation (2013JQ0012). REFERENCES (1)

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Interlayer Expansion of Layered Cobalt Hydroxide ... - ACS Publications

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