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Crucial roles of interfacial coupling and oxygen defect in multifunctional 2D inorganic nanosheets
Journal Pre-proof Crucial roles of interfacial coupling and oxygen defect in multifunctional 2D inorganic nanosheets Xiaoyan Jin, Daniel Adjei Agyeman, Saeyoung Kim, Yeon Hwa Kim, Min Gyu Kim, Yong-Mook Kang, Seong-Ju Hwang PII:
S2211-2855(19)30899-7
DOI:
https://doi.org/10.1016/j.nanoen.2019.104192
Reference:
NANOEN 104192
To appear in:
Nano Energy
Received Date: 15 July 2019 Revised Date:
30 September 2019
Accepted Date: 11 October 2019
Please cite this article as: X. Jin, D.A. Agyeman, S. Kim, Y.H. Kim, M.G. Kim, Y.-M. Kang, S.-J. Hwang, Crucial roles of interfacial coupling and oxygen defect in multifunctional 2D inorganic nanosheets, Nano Energy (2019), doi: https://doi.org/10.1016/j.nanoen.2019.104192. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Graphical abstract
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Crucial roles of interfacial coupling and oxygen defect in multifunctional 2D inorganic nanosheets
Xiaoyan Jin,a,1 Daniel Adjei Agyeman,b,1 Saeyoung Kim,c,1 Yeon Hwa Kim,c Min Gyu Kim,d Yong-Mook Kang,e,* and Seong-Ju Hwanga,*
a
Center for Hybrid Interfacial Chemical Structure (CICS), Department of Materials Science
and Engineering, Yonsei University, Seoul 03722, Republic of Korea b
Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul
04620, Republic of Korea c
Center for Hybrid Interfacial Chemical Structure (CICS), Department of Chemistry and
Nanoscience, Ewha Womans University, Seoul 03760, Republic of Korea d
Beamline Research Division, Pohang Accelerator Laboratory (PAL), Pohang 37673,
Republic of Korea e
Department of Materials Science and Engineering, Korea University, Seoul 02841,
Republic of Korea 1
These authors contributed equally to this work.
* Corresponding author Tel) +82-2-2123-2852 E-mail) [email protected] (S.-J.H.), [email protected] (Y.-M.K.)
Abstract
An effective way to optimize the functionality of inorganic 2D nanosheets can be developed by tailoring their interfacial electronic coupling and crystal defect in the hybrid structure. The heterolayer hybridization between exfoliated Co−Fe-layered double hydroxide (LDH) and RuO2 nanosheets can provide an efficient way of optimizing the interfacial coupling and oxygen vacancy of restacked nanosheets. The obtained Co−Fe-LDH−RuO2 nanohybrid shows outstanding electrode performance for Li−O2 batteries with excellent bifunctional oxygen electrocatalytic activity, which is much superior to those of the precursor materials. In-situ X-ray absorption spectroscopic and electrochemical characterizations highlight the remarkable improvement of electrocatalysis kinetics and electrochemical stability upon hybridization, which is attributable to the intimate interfacial interaction and oxygen vacancy formation of restacked 2D inorganic nanosheets. This study underscores that a fine-control of electronic coupling and defect structure via heterolayer hybridization is quite effective in exploring high-performance bifuntional electrocatalysts applicable as Li−O2 cathode.
Keywords: Exfoliated 2D nanosheet; interfacial electron coupling; heterogeneous catalysis; oxygen electrocatalyst; Li−O2 batteries
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1. Introduction As emerging alternatives to graphene, exfoliated 2D nanosheets (NSs) of inorganic solids attract great deal of research activities because of their diverse tunable physicochemical properties and valuable functionalities [1−3]. The subnanometer-level thickness and hydrophilic surface nature of exfoliated 2D inorganic NSs render these materials effective building blocks for synthesizing strongly-coupled hybrid materials with various inorganic species [4,5]. The layer-by-layer hybridization between exfoliated inorganic NSs with strong interfacial coupling allows not only to finely tailor the chemical bonding natures and physicochemical properties of component NSs but also to merge two different functionalities into single solid lattice [6,7]. The resulting NS-based hybrid materials show promising multifunctionality, which plays a crucial role in many emerging energy-storage and production devices [6−9]. In one instance, the intercalative hybridization between electrocatalyst NSs active for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is supposed to provide a novel synthetic strategy to efficient electrode for Li−O2 batteries [10], since bifunctional electrocatalysts with OER/ORR activity are demanded for the electrode application of Li−O2 batteries [11,12]. In contrast to the hybridtype materials, only a few single-component materials display bifunctional electrocatalyst performance for OER and ORR. Of prime importance is that a fine-control of the interfacial electronic coupling and defect structure of component 2D inorganic NSs can offer additional opportunity to further optimize the electrocatalyst functionality of the NS-based hybrid materials [6]. One of the promising candidate NSs for the exploration of Li−O2 hybrid electrodes is a couple of conductive OER-active RuO2 NS and ORR/OER-active Cocontaining LDH NS. Although many studies have reported the layer-by-layer assembly of 2D heterolayered inorganic nanohybrid, most of researches about the 2D heterolayered inorganic
3
nanohybrids have employed graphene as a building block [13−15]. There are only a limited number of papers about the 2D heterolayered nanohybrids composed of two kinds of inorganic NSs including RuO2 NS [16−18]. At the time of this submission, we are unaware of
the
synthesis
of
heterolayered
2D
LDH−metal
oxide
hybrid
bifunctional
electrocatalysts/electrodes for Li−O2 batteries via the layer-by-layer hybridization between exfoliated LDH and transition metal oxide NSs. In this study, strongly-coupled 2D heterolayered nanohybrids are synthesized by the layerby-layer hybridization of exfoliated Co−Fe-LDH and RuO2 NSs, as illustrated in Fig. 1a. The resulting Co−Fe-LDH−RuO2 nanohybrids are employed as oxygen electrocatalysts and electrodes for Li−O2 batteries to elucidate the effects of interfacial coupling and oxygen vacancy of component 2D NSs on these functionalities. The chemical bonding nature, oxygen defect structure, and internal electronic interaction of heterolayered Co−FeLDH−RuO2 nanohybrids are systematically investigated with various in-situ spectroscopic and electrochemical techniques. The effect of the surface charge of component NSs on the interlayer coupling and oxygen vacancy of heterolayered Co−Fe-LDH−RuO2 nanohybrids is also studied with surface-charged-controlled LDH NSs.
2. Experimental 2.1. Synthesis The pristine Co−Fe-LDH was synthesized by the co-precipitation method, as reported previously [19]. The molar ratio of Co and Fe in Co−Fe-LDH was adjusted to 3:1. The monolayer Co−Fe-LDH NS was obtained by exfoliation of bulk LDH material in formamide under N2 flow. The colloidal suspension of RuO2 NS was prepared by exfoliation of layered H0.2RuO2 with tetrabutylammonium (TBA+) ions, as reported previously [20]. The
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exfoliation yields of the Co−Fe-LDH and RuO2 NSs were estimated as ~100% and ~90%, respectively. Both the concentrations of exfoliated LDH and RuO2 NSs were adjusted to 1 mg mL−1. The resulting colloidal suspensions showed a positive zeta potential of +39 mV for LDH NS and a negative zeta potential of −42 mV for RuO2 NS, see Fig. S1a of Supporting information, confirming their opposite surface charges. Transmission electron microscopy (TEM, JEOL JEM-2100F, 200 kV) analysis clearly demonstrated the formation of highly anisotropic 2D NSs with the lateral dimensions of ~20−40 nm for Co−Fe-LDH and ~300−1000 nm for RuO2, see Figs. S1b and S1c of Supporting information. The heterolayered nanohybrids of Co−Fe-LDH−RuO2 were synthesized by mixing the colloidal suspensions of exfoliated Co−Fe-LDH and RuO2 NSs at room temperature. Since both the exfoliated LDH and RuO2 NSs possess quite large surface charges of +39 mV and −42 mV, there must be prominent electrostatic interaction between these oppositely-charged NSs, which is mainly responsible for the homogeneous hybridization between these NSs. To make sure the homogeneous hybridization between two kinds of NSs, a very slow mixing rate of 1 mL min−1 was employed, which was effective in avoiding the fast self-agglomeration of colloidal NSs. The obtained precipitates were washed thoroughly with ethanol and distilled water, and then freeze-dried. Several molar ratios of RuO2/Co−Fe-LDH (0.25, 0.5, and 0.75) were employed for the synthesis of nanohybrids to study the effect of chemical composition on their physicochemical properties. The resulting Co−Fe-LDH−RuO2 nanohybrids are denoted as CFR0.25, CFR0.5, and CFR0.75, respectively.
2.2. Characterization The crystal structures of the present materials were examined with powder X-ray diffraction (XRD) analysis (Rigaku D/Max-2000/PC, Ni-filtered Cu Kα radiation, λ =
5
1.54184 Å, 298 K). The crystal morphologies of the present materials were probed with field emission-scanning electron microscopy (FE-SEM, JEOL JSM-6700F) and TEM analyses. The size distributions of exfoliated LDH and RuO2 NSs were determined by a standard dynamic light scattering (DLS) analysis (Malvern Zetasizer Nano ZS). The internal charge transfer in the present materials was studied with photoluminescence (PL) spectroscopy using Perkin-Elmer LS55 fluorescence spectrometer. The N2 adsorption−desorption isotherms of present materials were measured with Micromeritics ASAP 2020 at 77 K. Micro-Raman spectra were collected with a Horiba Jobin Yvon LabRam Aramis spectrometer, in which an Ar-ion laser beam (λ = 514.5 nm) was used as an excitation source. Fourier transformed-infrared (FT-IR) spectroscopy (Varian FTS800) was used to probe the functional groups of nanohybrids. The electronic configurations and local crystal structures of the present materials were examined with X-ray absorption near-edge structure/extended X-ray absorption fine structure (XANES/EXAFS) spectroscopic analysis at the beamline 10C of the Pohang Accelerator Laboratory (PAL) in Korea. The oxidation states of the component ions in the present materials were studied with X-ray photoelectron spectroscopy (XPS, Thermo VG, UK, Al Kα).
2.3. Electrocatalytic activity tests The catalyst ink was obtained by dispersing 7 mg of catalyst material, 3 mg of carbon black (Vulcan-XC72R), and 20 µL of 5 wt% Nafion solution into a 5 mL of Milli-Q water/isopropanol (4/1, v/v) mixed solution, which was followed by ultra-sonication for 1 h. To prepare the working electrode, 10 µL catalyst ink was dropped onto the glassy carbon electrode (3 mm diameter, ALS Co.) and dried at 50 °C. A platinum wire, a saturated calomel electrode (SCE), and 1.0 M KOH solution (OER)/0.1 M KOH solution (ORR) were
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used as a counter electrode, a reference electrode, and an electrolyte, respectively. Before the measurement, O2 gas was bubbled into the electrolyte for 30 min. The measured potentials were normalized to reversible hydrogen electrode (RHE) as the reference. All the experiments of linear sweep voltammogram (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) were carried out by using a RRDE-3A (ALS Co.) as a rotator and the IVIUM analyzer. The LSV curves were measured with a scan rate of 5 mV s−1 and a rotating speed of 1600 rpm. The EIS data were collected in the frequency range of 0.1−100000 Hz at 1.64 V vs. RHE (OER) /0.76 V vs. RHE (ORR).
2.4. Li−O2 battery test The prepared catalyst materials (0.02 g for each), conducting agent (ketjen black), and binder (HFP-PVDF) were dispersed with a mass ratio of 40:45:15 in N-methyl-2-pyrrolidone. The mixture was cast onto a GDL current collector (JNTG Co. Ltd., JNT30) and dried in vacuum at 120 °C for 5 h. The cathode was punched into disk having a diameter of 1.3 cm and an average catalyst loading density of 1.8−2.0 mg cm−2. Lithium foil and one sheet of glass fiber were used for anode and separator, respectively. Lithium metal, separator, and asprepared air electrode were stacked into a Swagelok-type Li−O2 cell in sequence. The electrolyte was made up of 1 M lithium bis(trifluoromethane) sulphonamide in tetramethylene glycol dimethyl ether. The cell was galvanostatically discharged and charged at a current density of 200 mA g−1 over a range of 2.0−4.5 V. For the stabilization of cycling, the cell was also tested using a constant current−constant voltage (CC−CV) mode with potentiostatic steps at 4.2 V under limited capacity condition (1000 mAh g−1). The EIS measurement was performed with an AC voltage amplitude of 10 mV s−1 in the frequency range of 100 MHz to 10 KHz using an impedance analyzer. Ex-situ FE-SEM studies of the
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discharge products were carried out by disassembling the cells in glove box under argon atmosphere to avoid the oxidation of the product formed. The cathode was washed in dimethyl carbonate (DMC) to remove residual electrolyte and dried under argon.
3. Results and discussion 3.1. Microscopic, diffraction, and spectroscopic analyses Fig. 1b and 1c present FE-SEM and TEM images of the CFR nanohybrids, respectively. All the CFR nanohybrids show the house-of-cards-type stacking of exfoliated LDH and RuO2 NSs, reflecting their mesoporous nature. The layer-by-layer stacking of Co−Fe-LDH and RuO2 NSs is confirmed by selected area electron diffraction (SAED) pattern of CFR0.5 showing diffraction rings corresponding to LDH and RuO2 components (Fig. 1d). FT-IR spectroscopic analysis for the CFR nanohybrids demonstrates characteristic IR bands of (Ru−O) and (Co/Fe−O) bonds, indicating the co-existence of LDH and RuO2 NSs (Fig. S2 of Supporting information). The homogeneous hybridization between LDH and RuO2 NSs in nanoscale is further evidenced by energy dispersive spectrometry (EDS)−elemental mapping analysis exhibiting uniform distribution of Co, Fe, Ru, and O elements in the entire region of CFR nanohybrids (Fig. S3 of Supporting information). According to powder XRD analysis (Fig. 1e), all the CFR nanohybrids exhibit equally-spaced (00l) Bragg reflections in low 2θ region, indicating the formation of the layer-by-layer-ordered heterostructure of Co−Fe-LDH and RuO2 NSs. An increase of RuO2 content remarkably enhances the intensities of (00l) reflections, highlighting the incorporation of RuO2 NS with higher electron density into the restacked LDH NSs [18]. Since the present nanohybrids are synthesized by restacking of exfoliated LDH and RuO2 NSs, their crystal lattices are composed of layer-by-layer-stacked Co−Fe-LDH and RuO2 NSs along the c-axis with random in-plane orientation. No
8
observation of (hkl) reflections makes it impossible to specify the in-plane stacking pattern of component NSs. Since the layer thicknesses of both Co−Fe-LDH and RuO2 NSs are fairly similar to one another, the c-axis lattice parameter of the resulting CFR nanohybrid can be defined as the average interplanar distance of restacked NSs. Thus, the unit cell of the CFR nanohybrids can be assumed to contain a single layer of LDH/RuO2 NS along the c-axis. With this structural model, the observed intense (00l) diffraction peak at the lowest angle can be assigned as (001) reflection, like many other reports about NS-restacked materials [5,6,21]. The enhanced interfacial interaction and oxygen vacancy formation in the CFR nanohybrids are evidenced by a series of spectroscopic analyses. As plotted in the XANES spectra of Fig. 1f−1h, the hybridization of Co−Fe-LDH with RuO2 NS results in the lowering of Co K-edge and Fe K-edge energies, whereas the CFR nanohybrids exhibit higher Ru Kedge energy than does the RuO2 NS, clearly demonstrating significant interfacial charge transfer from LDH NS to RuO2 NS. Such a strong interfacial electronic coupling is crossconfirmed by XPS analysis showing the distinct blue-shifts of Co 2p and Fe 2p peaks and the red-shift of Ru 3p peaks upon the hybridization (Fig. S4 of Supporting information). The significant interfacial interaction between restacked Co−Fe-LDH and RuO2 NSs is further confirmed by PL spectroscopy. As presented in Fig. S5 of Supporting information, the CFR0.5 nanohybrid shows much weaker PL intensity than does the pristine Co−Fe-LDH. The observed lowering of PL intensity upon hybridization can be regarded as another evidence for significant interfacial charge transfer between LDH and RuO2, leading to the depression of charge recombination in LDH layer. All the XANES, XPS, and PL results presented here underscore the occurrence of strong interfacial interaction between Co−FeLDH and RuO2 NSs. The maintenance of the original crystal structures of Co−Fe-LDH and
9
RuO2 NSs upon the hybridization is clearly confirmed by EXAFS analysis (Fig. 1i−1k and S6 of Supporting information), exhibiting nearly identical spectral features for both CFR and Co−Fe-LDH/layered RuO2 [22]. According to the curve fitting analysis (Table S1 of Supporting information), the hybridization of Co−Fe-LDH NS with RuO2 NS induces a gradual decrease of the coordination numbers (CNs) with an increase of Debye-Waller (σ2) factors for both (Co−O) and (Fe−O) coordination shells, underscoring the creation of oxygen defects and the increase of structural disorder in Co−Fe-LDH NS caused by elastic deformation occurring during intercalative hybridization [23]. To understand the origin of the formation of oxygen defects upon the hybridization process, the EXAFS analysis is also carried out for the exfoliated Co−Fe-LDH. According to the curve fitting analysis (Table S1 of Supporting Information), the CNs of (Co−O) and (Fe−O) coordination shells for the exfoliated Co−Fe-LDH are smaller than those for the pristine Co−Fe-LDH but larger than those for the CFR nanohybrids, underscoring the gradual increase of oxygen defect content during the exfoliation and the following hybridization processes. Since both the exfoliation process and the subsequent hybridization with heterogeneous RuO2 NS cause significant lattice deformation of LDH layer, the present EXAFS result can provide strong evidence for the crucial role of the elastic deformation of LDH layer in the formation of the oxygen defect in LDH layers. The presence of oxygen vacancies in Co−Fe-LDH NS is beneficial in promoting the adsorption of H2O on the surface of catalysts, thus enhancing the electrocatalytic activities of Co−Fe-LDH [24]. The larger σ2 values for Co K-edge than for Fe K-edge are attributable to the enhanced structural distortions caused by the Jahn-Teller active nature of Co2+ ion (d7) [25]. In contrast to Co K- and Fe K-edge data, the curve fitting analysis for Ru K-edge data indicates negligible change of CNs and increase of σ2 values upon the hybridization (Table S1 of Supporting information), highlighting higher structural
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stability of layered metal oxide lattice than LDH lattice. Generally the relative structural stabilities of inorganic solids can be well understood from the difference in their lattice energies. While the RuO2 lattice consists of RuO6 octahedra with tetravalent Ru4+ cations and divalent O2− anions, the Co−Fe-LDH lattice is composed of Co(OH)6/Fe(OH)6 octahedra with divalent Co2+/trivalent Fe3+ cations and monovalent OH− anions. Since the lattice energy of inorganic solid is proportional to the product of cationic charge and anionic charge, the layered RuO2 lattice is supposed to have much greater lattice energy than the Co−FeLDH lattice. The greater lattice energy of RuO2 than Co−Fe-LDH is responsible for the higher lattice stability of the former.
3.2. N2 adsorption−desorption analysis N2 adsorption−desorption isotherm measurement clearly demonstrates the formation of mesoporous structure with expanded surface area upon the hybridization of LDH and RuO2 NSs, see Fig. S7 of Supporting information. Based on the Brunauer−Emmett−Teller (BET) equation, the surface areas of the present materials are calculated as 61, 97, and 88 m2 g−1 for CFR0.25, CFR0.5, and CFR0.75, respectively, which are much larger than those of the pristine Co−Fe-LDH (19 m2 g−1) and RuO2 (13 m2 g−1). Such an increase of porosity upon hybridization implies the provision of more electrocatalytically active sites for ORR and OER as well as more deposition sites for the discharge product of Li−O2 battery. The presence of mesopores with average diameter of ~3−4 nm in the present nanohybrids is confirmed by the pore size calculation based on Barrett−Joyner−Halenda (BJH) equation (Fig. S8 of Supporting information), which is in good agreement with the FE-SEM results.
3.3. Electrocatalytic activity measurement
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The present CFR nanohybrids are tested as electrocatalysts for OER and ORR to probe the effect of heterolayer hybridization on the electrocatalytic activity. As shown in Fig. 2a−2e, all the CFR nanohybrids commonly demonstrate much smaller overpotentials at a current density of 10 mA cm−2 for OER and greater half-wave potentials for ORR than those of the precursors Co−Fe-LDH and RuO2, highlighting the improvement of oxygen electrocatalyst performance upon the intercalative hybridization of NSs. Among the present nanohybrids, CFR0.5 exhibits much higher oxygen electrocatalyst activity, indicating the optimal chemical composition of this material. The improvement of OER and ORR kinetics upon the hybridization is verified by much smaller Tafel slope of CFR0.5 as compared with those of the precursors Co−Fe-LDH and RuO2 [26,27]. The beneficial effect of nanoscale hybridization on the bifunctional electrocatalyst performance of component NSs is further verified by the inferior functionality of the physical mixture of Co−Fe-LDH and RuO2 with the same composition as CFR0.5 (Fig. S9 of Supporting information). The CFR0.5 nanohybrid shows highly stable electrocatalyst performance without any notable degrading by 25,000 s, which is in stark contrast to the precursor Co−Fe-LDH exhibiting a notable increase of operation potential at ~5,000 s (Fig. 2f). This result underscores the positive effect of hybridization on electrocatalyst durability.
3.4. ECSA, EIS, and in-situ XANES analyses The evolutions of electrochemical surface area (ECSA), charge transfer kinetics, and chemical bonding nature upon hybridization and electrocatalysis process are investigated to understand the origin of the improved electrocatalyst performances of CFR nanohybrids. The ECSA can be estimated from the linear slope in the plot of ∆j (= ja − jc) vs. RHE against the scan rate, which corresponds to twice of the double layer capacitance (Cdl) [28]. As
12
depicted in Fig. 3a and S10 of Supporting information, all the CFR nanohybrids have markedly larger ECSAs than do the precursors Co−Fe-LDH and RuO2 NSs, which can be ascribed to the surface expansion and the formation of oxygen defects in LDH upon hybridization [29]. According to the EIS data of Fig. 3b and 3c, the diameter of semicircle is commonly smaller for the CFR nanohybrids at potentials of 0.76 and 1.64 V compared with those of the precursors Co−Fe-LDH and RuO2, underscoring the depression of charge transfer resistance (Rct) upon the hybridization [30,31]. Among the present materials, CFR0.5 displays the smallest radius of semicircle, indicating the best charge transfer kinetics of this material. The observed improvement of ECSA and charge transfer kinetics upon hybridization can be ascribed to a strong interfacial coupling between Co−Fe-LDH and RuO2 NSs [32]. The variation of the chemical bonding nature of CFR nanohybrid during the OER process is also investigated with in-situ XANES analysis. As depicted in Figs. 3d−3f, both CFR0.5 and Co−Fe-LDH commonly exhibit a distinct blue-shift of Co K-edge energy with increasing the applied potential from 1.2 to 1.7 V, indicating the oxidation of Co ion with the bond formation of Co−OH and/or Co−OOH under the oxidative potential [33]. In comparison with the pristine Co−Fe-LDH, CFR0.5 displays much weaker change of edge energy during the OER process, emphasizing the depression of charge accumulation by a promoted hole injection into the adsorbed reactant upon the hybridization with conductive RuO2 NS. This finding provides strong evidence for the improvement of interfacial charge transfer between LDH and adsorbed reactant due to intimate electronic coupling with conductive RuO2 NS (Fig. 3g). This makes crucial contribution to the enhanced electrocatalyst functionality of CFR. In contrast to the Co K-edge XANES spectra, the Fe K-edge and Ru K-edge XANES spectra of CFR0.5 show only negligible alterations of edge position with increasing
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oxidative potential from 1.2 to 1.7 V. This observation highlights that, among many metal component ions, the Co ion of Co−Fe-LDH makes the most crucial contribution to the excellent electrocatalyst activity of CFR [34]. As evidenced by the EXAFS analysis (Table S2 of Supporting information), the hybridization with RuO2 NS gives rise to the decrease of oxidation states of Co/Fe ions with the elongation of (Co/Fe−O) bond distance and the creation of oxygen defects in the Co−Fe-LDH NS (Fig. 3h). Taking into the Jahn-Teller active nature of Co2+ (d7) ion, an elongated axial (Co−O) bond distance of CFR is helpful in promoting the adsorption of oxygen species (i.e. reactant for OER) on Co sites, which is responsible for the major role of Co ion as OER active site (Fig. 3h) [35]. Also, the formation of oxygen defects in Co−Fe-LDH upon the hybridization makes significant contribution to the enhanced adsorption of H2O and O2 (i.e. reactant for ORR) and thus to the improved ORR activity of CFR [22]. It is worthwhile to mention that, in contrast to CFR exhibiting negligible spectral change, a significant modification of Ru K-edge data with notable edge shift occurs for the precursor RuO2 NS during the OER process, highlighting the remarkable improvement of the electrochemical stability of RuO2 NS upon the intimate hybridization with LDH.
3.5. Li−O2 battery test Based on the bifunctional oxygen electrocatalyst performance of the present CFR nanohybrids, these materials are employed as cathode catalysts for Li−O2 batteries. As depicted in Fig. 4a, the CRF0.5 nanohybrid exhibits much larger discharge capacity with much lower charge overpotential of ~3.8 V for OER and relatively stable potential of ~2.7 V for ORR than do the pristine RuO2 and Co−Fe-LDH, underscoring the beneficial effect of hybridization on the cathode performance. Significant similar shapes in the initial discharge
14
curves of CFR0.5 and Co−Fe-LDH strongly suggest a major role of LDH component as initial active sites for the adsorption of oxide species on the nanohybrid. To further verify the superior performance of the CFR nanohybrids over the precursors, the capacity limitation is adopted to suppress the excessive formation of insulating discharge product (Fig. 4b−d). The CFR0.5 nanohybrid shows the most stable cycling for more than 100 cycles, which is in stark contrast to RuO2 and Co−Fe-LDH exhibiting drastic capacity fading before 70 cycles (Fig. 4c), confirming the remarkable advantage of hybridization on electrode functionality. Such an excellent cyclic stability of CFR0.5 can be associated with its relatively lower charge−discharge overpotentials than the pristine materials (Fig. 4d). The observed excellent electrode performance of CFR0.5 is attributable to the creation of oxygen defects in LDH and the occurrence of strong interfacial electronic coupling upon the hybridization. Also, the expanded surface area of CFR0.5 cathode material makes additional contribution to its improved electrode functionality via the enhancement of the mass transfer of oxygen and solvated Li ions as well as electron transfer. Such evolutions cause the enhanced surfaceexposure of electrocatalytically-active sites, the provision of efficient deposition sites for discharge product, and the improvement of transport pathways for electron and mass [36]. According to CV analysis (Fig. 4e), the ORR activities of CFR and Co−Fe-LDH are confirmed by the observation of a cathodic peak at an onset potential ~2.7 V. For the OER process, the CRF0.5 nanohybrid displays a much higher activity with a broader anodic peak at an onset potential of ~3.25 V than the other materials, indicating the promoted decomposition of discharge products. As probed by XANES/EXAFS analyses (Fig. 1f−1k), a strong interfacial coupling between RuO2 and Co−Fe-LDH induces net RuO2 to LDH charge redistribution with creation of oxygen defects, thus facilitating the adsorption of the oxide species (LiO2 or Li2O2) with a moderate bond strength for enhanced OER activities [37,38].
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The beneficial effect of hybridization on charge transfer kinetics is evidenced by the EIS analysis conducted after the 1st discharging without the capacity limitation demonstrating a significant decrease of Rct upon hybridization (Fig. 4f and S11 of Supporting information). Also, the discharge products on CFR0.5, RuO2, and Co−Fe-LDH are analyzed with FE-SEM, see Fig. S12 of Supporting information. The CFR0.5 cathode after the discharge process displays a homogenous deposition of sheet-like discharge products with 2D structure over the entire electrode, which must be beneficial for mass transfer and electron transport for improved ORR and OER activities. Conversely, the unhybridized RuO2 and Co−Fe-LDH cathodes demonstrate the formation of a film-like and typical toroidal discharge products, respectively, leading to the significant impeding of the following OER process. The homogenous charge redistribution of electrocatalytically-active LDH NS caused by the effective electronic coupling with highly-conductive RuO2 NS is responsible for the optimization of the microstructure of discharge products in the CFR nanohybrid and the following efficient facilitation of its decomposition during OER.
3.6. Effect of the surface charge of component NS on interlayer coupling and defect structure of nanohybrid To study the effect of the surface charge of component NS on the interfacial coupling and oxygen defect of nanohybrids, two kinds of surface-charge-controlled Co−Fe-LDH materials are synthesized with different Co2+/Fe3+ molar ratios of 2/1 and 4/1, since the surface charge of LDH layer can be easily controlled by altering the molar ratios of divalent and trivalent metal ions [39]. According to the zeta potential measurements, the surface charges of exfoliated Co−Fe-LDH NSs with the Co/Fe molar ratios of 2/1, 3/1, and 4/1 show gradually changing surface charges of +48, +39, and +29 mV, clearly demonstrating the successful
16
control of the surface charge of Co−Fe-LDH NS with alteration of Co/Fe ratios. The exfoliated NSs of these surface-charge-controlled LDHs are employed for the synthesis of Co−Fe-LDH−RuO2 hybrid NSs with different interfacial interaction. The obtained nanohybrids composed of RuO2 and Co−Fe-LDH with Co/Fe molar ratio of 2/1, 3/1, and 4/1 are denoted as CFR0.5-2/1, CFR0.5, and CFR0.5-4/1, respectively. According to the curve fitting EXAFS analyses for these nanohybrids (Fig. S13 and Table S2 of Supporting information), the CNs of (Co/Fe−O) shells becomes smaller in the order of CFR0.5-4/1 > CFR0.5 > CFR0.5-2/1, indicating a gradual increase of interfacial coupling and oxygen vacancy with increasing the surface charge of LDH NS. The present EXAFS results provide strong evidence for the enhancement of interfacial interaction between RuO2 and LDH NSs with the increase of LDH surface charge and the resulting increase of oxygen defect content. Additionally we carried out EIS analyses for these nanohybrids to probe the evolution of interfacial electronic coupling between LDH and RuO2 NSs upon the change of LDH surface charge (Fig. S14 of Supporting information). The charge transfer resistance of the nanohybrids becomes smaller in the order of CFR0.5-4/1 > CFR0.5 > CFR0.5-2/1, highlighting the stronger electronic coupling of LDH NS having higher surface charge with highly conductive RuO2 NS. All the experimental findings presented here underscore the feasibility of surface charge as an effective tool for tailoring the interfacial interaction and defect structure of restacked NSs.
4. Conclusions In conclusion, a fine-control of interfacial electronic coupling and crystal defect structure of 2D inorganic NSs provides an effective way of exploring high-performance hybrid electrocatalysts and cathodes for Li−O2 batteries via the layer-by-layer hybridization of
17
exfoliated NSs. The intimately-coupled 2D nanohybrid composed of exfoliated polar Co−FeLDH and RuO2 NSs with notable oxygen deficiency displays excellent catalyst performance for Li−O2 batteries with markedly lowered overpotential, outstanding cyclability, and reversible formation/decomposition of discharge product. The improved electrode and electrocatalyst performances of heterolayered CFR nanohybrids are attributable to strong interfacial electronic coupling between LDH and RuO2 NSs as well as the creation of significant oxygen vacancies, resulting in the enhancement of the diffusion of Li ions and reactants, and the improvement of OER/ORR kinetics and charge transfer kinetics upon hybridization. Moreover, since the LDH NS has smaller lateral dimension than does RuO2 NSs, the restacked CFR nanohybrids contain large numbers of edge-sites of LDH with high catalytic activity, which makes significant contribution to the excellent electrocatalyst performance of these materials. Additionally, marked difference in the lateral sizes of LDH and RuO2 NSs would be effective in forming loosely-restacked materials with expanded surface area, which is beneficial in optimizing the electrocatalyst and electrode performance of restacked nanohybrid. Considering the crucial roles of surface nature and defect structure of inorganic NS in interfacial electronic coupling, there are lots of opportunities to optimize multifunctionalities of heterolayered nanohybrids via the tailoring of defect structure, surface roughness, surface structure, and surface composition of component NSs [5,8,40,41]. In fact, the effective controllability of defect structure and interfacial coupling with the change of the surface charge of component NS is obviously evidenced from the present study. Our current project is to explore efficient multifunctional materials applicable for fuel cells and water electrolyzers via the intimate heterolayer hybridization between diverse couples of inorganic NSs with controlled surface natures, surface charges, and crystal defects.
18
Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2017R1A2A1A17069463) and by the Korea government (MSIT) (No. NRF-2017R1A5A1015365). The experiments at PAL were supported in part by MOST and POSTECH.
Supporting Information Zeta potential data, TEM images, and size distribution data of the exfoliated Co−Fe-LDH and RuO2 nanosheets, FT-IR spectra, EDS−elemental maps, PL data, XPS data, k3-weighted EXAFS oscillations, EXAFS fitting results, N2 adsorption−desorption data, pore size distributions, LSV curves, CV curves, EIS data, and FE-SEM images of CFR nanohybrids.
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Figure captions
Fig. 1. (a) Synthesis scheme to CFR nanohybrid. (b) FE-SEM images, (c) TEM images, (d) SAED pattern, (e) powder XRD patterns, (f,g,h) XANES and (i,j,k) Fourier transform (FT)EXAFS spectra of CFR nanohybrids and references.
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Fig. 2. (a) LSV curves for OER, (b) Tafel plots for OER, (c) LSV curves for ORR, (d) Tafel plots for ORR, (e) overpotentials/half-wave potentials, and (f) stability data for OER at 10 mA cm−2 for CFR nanohybrids and references.
26
Fig. 3. (a) Plot of charging current density differences vs. scan rates, (b,c) EIS curves, and (d,e,f) in-situ XANES spectra for CFR nanohybrids and references. (g) Interfacial charge transfer scheme and (h) schematic illustration of introducing oxygen defects to Co−Fe-LDH.
27
Fig. 4. (a) Initial galvanostatic discharge/charge profiles, (b) galvanostatic discharge/charge curves with the capacity limitation, (c) discharge capacity retentions, (d) cyclic variation of terminal voltages, (e) CV data, and (d) EIS curves for CFR nanohybrids and references.
28
Highlights • The interfacial coupling and crystal defect play crucial role in inorganic nanosheet. • Hybridization between 2D inorganic nanosheets yields strongly-coupled nanohybrid. • The exfoliation-hybridization process creates oxygen defects in inorganic nanosheet. • Heterolayered nanohybrids show excellent electrocatalyst/Li−O2 electrode activity. • Electrocatalysis kinetics of nanosheet can be improved by strong interfacial coupling.
1
Dr. Xiaoyan Jin is currently a postdoctoral researcher in the Department of Materials Science and Engineering at Yonsei University under Prof. Seong-Ju Hwang. She received her Ph.D. degree in inorganic chemistry (2018) from Ewha Womans University. Her research focuses on synthesis and characterization of layered metal oxides, layered double hydroxides, and metal chalcogenides, and their applications in electrocatalyst, supercapacitor, photocatalyst, and Na/Li-ion batteries.
Dr. Daniel Adjei Agyeman is currently an Assistant Professor in the Department of Energy and Materials at Dongguk University. He received his Ph.D. degree in Energy Materials Engineering (2018) from Dongguk University under Prof. Yong-Mook Kang’s guidance. His research focuses on synthesis and characterization of transition metal compounds applicable for energy storage (Li/Na ion battery), and electro-catalysis.
Saeyoung Kim received a B.S. degree in chemistry (2018) from Ewha Womans University. She is currently a M.S. Student in the Department of Chemistry and Nanoscience at Ewha Womans University (Supervisor: Prof. Seong-Ju Hwang). Her research focuses on synthesis and characterization of layered metal oxides and layered double hydroxides, and their applications in electrocatalyst.
Yeon Hwa Kim received a B.S. degree in chemistry (2018) from Ewha Womans University. She is currently a M.S. Student in the Department of Chemistry and Nanoscience at Ewha Womans University (Supervisor: Prof. Seong-Ju Hwang). Her research focuses on synthesis and characterization of the 2D inorganic nanosheet-based nanohybrid for energy-related applications.
Dr. Min Gyu Kim is chief staff scientist of Pohang Accelerator Laboratory (PAL), Korea. He received B.S. (1993), M.S. (1995), Ph. D (2002) in Department of Chemistry of Yonsei University (Korea). He joined at beamline research division of PAL in 2002. He is currently in charge of BL10C (Wide-XAFS) beamline with Wiggler insertion device and. His research interests are focused on a fundamental solid state chemistry combined with synchrotron radiation-based in situ characterization including XAFS/X-ray scattering/Imaging, covering synthesis, functionalities and real-time characterization for energy storage-conversion materials, organic-inorganic hybrid materials, and environmental materials.
Prof. Yong-Mook Kang completed his B.S. (1999), M.S. (2001), and Ph.D. (2004) in Korea Advanced Institute of Science and Technology. He has been a senior researcher in Samsung SDI Co., LTD. He was a professor at Department of Energy and Materials Engineering in Dongguk University and then now a full professor at Department of Materials Science and Engineering in Korea University. His research area covers electrode or catalyst materials for Li rechargeable batteries and various post Li batteries, such as Li-air battery, Na rechargeable battery and so on.
Prof. Seong-Ju Hwang is currently a full professor in the Department of Materials Science and Engineering at Yonsei University. He received a B.S. degree in chemistry (1992) and a M.S. degree in inorganic chemistry (1994) from Seoul National University (Korea), a Ph.D. degree in inorganic chemistry from Université Bordeaux I (France) in 2001. He worked as a full professor in the Department of Chemistry and Nanoscience at Ewha Womans University from 2005 to 2019. His research focuses on the synthesis and characterization of lowdimensional nanostructured transition metal compounds applicable for energy production, energy storage, and environmental purification.
S2211-2855(19)30899-7
DOI:
https://doi.org/10.1016/j.nanoen.2019.104192
Reference:
NANOEN 104192
To appear in:
Nano Energy
Received Date: 15 July 2019 Revised Date:
30 September 2019
Accepted Date: 11 October 2019
Please cite this article as: X. Jin, D.A. Agyeman, S. Kim, Y.H. Kim, M.G. Kim, Y.-M. Kang, S.-J. Hwang, Crucial roles of interfacial coupling and oxygen defect in multifunctional 2D inorganic nanosheets, Nano Energy (2019), doi: https://doi.org/10.1016/j.nanoen.2019.104192. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Graphical abstract
1
Crucial roles of interfacial coupling and oxygen defect in multifunctional 2D inorganic nanosheets
Xiaoyan Jin,a,1 Daniel Adjei Agyeman,b,1 Saeyoung Kim,c,1 Yeon Hwa Kim,c Min Gyu Kim,d Yong-Mook Kang,e,* and Seong-Ju Hwanga,*
a
Center for Hybrid Interfacial Chemical Structure (CICS), Department of Materials Science
and Engineering, Yonsei University, Seoul 03722, Republic of Korea b
Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul
04620, Republic of Korea c
Center for Hybrid Interfacial Chemical Structure (CICS), Department of Chemistry and
Nanoscience, Ewha Womans University, Seoul 03760, Republic of Korea d
Beamline Research Division, Pohang Accelerator Laboratory (PAL), Pohang 37673,
Republic of Korea e
Department of Materials Science and Engineering, Korea University, Seoul 02841,
Republic of Korea 1
These authors contributed equally to this work.
* Corresponding author Tel) +82-2-2123-2852 E-mail) [email protected] (S.-J.H.), [email protected] (Y.-M.K.)
Abstract
An effective way to optimize the functionality of inorganic 2D nanosheets can be developed by tailoring their interfacial electronic coupling and crystal defect in the hybrid structure. The heterolayer hybridization between exfoliated Co−Fe-layered double hydroxide (LDH) and RuO2 nanosheets can provide an efficient way of optimizing the interfacial coupling and oxygen vacancy of restacked nanosheets. The obtained Co−Fe-LDH−RuO2 nanohybrid shows outstanding electrode performance for Li−O2 batteries with excellent bifunctional oxygen electrocatalytic activity, which is much superior to those of the precursor materials. In-situ X-ray absorption spectroscopic and electrochemical characterizations highlight the remarkable improvement of electrocatalysis kinetics and electrochemical stability upon hybridization, which is attributable to the intimate interfacial interaction and oxygen vacancy formation of restacked 2D inorganic nanosheets. This study underscores that a fine-control of electronic coupling and defect structure via heterolayer hybridization is quite effective in exploring high-performance bifuntional electrocatalysts applicable as Li−O2 cathode.
Keywords: Exfoliated 2D nanosheet; interfacial electron coupling; heterogeneous catalysis; oxygen electrocatalyst; Li−O2 batteries
2
1. Introduction As emerging alternatives to graphene, exfoliated 2D nanosheets (NSs) of inorganic solids attract great deal of research activities because of their diverse tunable physicochemical properties and valuable functionalities [1−3]. The subnanometer-level thickness and hydrophilic surface nature of exfoliated 2D inorganic NSs render these materials effective building blocks for synthesizing strongly-coupled hybrid materials with various inorganic species [4,5]. The layer-by-layer hybridization between exfoliated inorganic NSs with strong interfacial coupling allows not only to finely tailor the chemical bonding natures and physicochemical properties of component NSs but also to merge two different functionalities into single solid lattice [6,7]. The resulting NS-based hybrid materials show promising multifunctionality, which plays a crucial role in many emerging energy-storage and production devices [6−9]. In one instance, the intercalative hybridization between electrocatalyst NSs active for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is supposed to provide a novel synthetic strategy to efficient electrode for Li−O2 batteries [10], since bifunctional electrocatalysts with OER/ORR activity are demanded for the electrode application of Li−O2 batteries [11,12]. In contrast to the hybridtype materials, only a few single-component materials display bifunctional electrocatalyst performance for OER and ORR. Of prime importance is that a fine-control of the interfacial electronic coupling and defect structure of component 2D inorganic NSs can offer additional opportunity to further optimize the electrocatalyst functionality of the NS-based hybrid materials [6]. One of the promising candidate NSs for the exploration of Li−O2 hybrid electrodes is a couple of conductive OER-active RuO2 NS and ORR/OER-active Cocontaining LDH NS. Although many studies have reported the layer-by-layer assembly of 2D heterolayered inorganic nanohybrid, most of researches about the 2D heterolayered inorganic
3
nanohybrids have employed graphene as a building block [13−15]. There are only a limited number of papers about the 2D heterolayered nanohybrids composed of two kinds of inorganic NSs including RuO2 NS [16−18]. At the time of this submission, we are unaware of
the
synthesis
of
heterolayered
2D
LDH−metal
oxide
hybrid
bifunctional
electrocatalysts/electrodes for Li−O2 batteries via the layer-by-layer hybridization between exfoliated LDH and transition metal oxide NSs. In this study, strongly-coupled 2D heterolayered nanohybrids are synthesized by the layerby-layer hybridization of exfoliated Co−Fe-LDH and RuO2 NSs, as illustrated in Fig. 1a. The resulting Co−Fe-LDH−RuO2 nanohybrids are employed as oxygen electrocatalysts and electrodes for Li−O2 batteries to elucidate the effects of interfacial coupling and oxygen vacancy of component 2D NSs on these functionalities. The chemical bonding nature, oxygen defect structure, and internal electronic interaction of heterolayered Co−FeLDH−RuO2 nanohybrids are systematically investigated with various in-situ spectroscopic and electrochemical techniques. The effect of the surface charge of component NSs on the interlayer coupling and oxygen vacancy of heterolayered Co−Fe-LDH−RuO2 nanohybrids is also studied with surface-charged-controlled LDH NSs.
2. Experimental 2.1. Synthesis The pristine Co−Fe-LDH was synthesized by the co-precipitation method, as reported previously [19]. The molar ratio of Co and Fe in Co−Fe-LDH was adjusted to 3:1. The monolayer Co−Fe-LDH NS was obtained by exfoliation of bulk LDH material in formamide under N2 flow. The colloidal suspension of RuO2 NS was prepared by exfoliation of layered H0.2RuO2 with tetrabutylammonium (TBA+) ions, as reported previously [20]. The
4
exfoliation yields of the Co−Fe-LDH and RuO2 NSs were estimated as ~100% and ~90%, respectively. Both the concentrations of exfoliated LDH and RuO2 NSs were adjusted to 1 mg mL−1. The resulting colloidal suspensions showed a positive zeta potential of +39 mV for LDH NS and a negative zeta potential of −42 mV for RuO2 NS, see Fig. S1a of Supporting information, confirming their opposite surface charges. Transmission electron microscopy (TEM, JEOL JEM-2100F, 200 kV) analysis clearly demonstrated the formation of highly anisotropic 2D NSs with the lateral dimensions of ~20−40 nm for Co−Fe-LDH and ~300−1000 nm for RuO2, see Figs. S1b and S1c of Supporting information. The heterolayered nanohybrids of Co−Fe-LDH−RuO2 were synthesized by mixing the colloidal suspensions of exfoliated Co−Fe-LDH and RuO2 NSs at room temperature. Since both the exfoliated LDH and RuO2 NSs possess quite large surface charges of +39 mV and −42 mV, there must be prominent electrostatic interaction between these oppositely-charged NSs, which is mainly responsible for the homogeneous hybridization between these NSs. To make sure the homogeneous hybridization between two kinds of NSs, a very slow mixing rate of 1 mL min−1 was employed, which was effective in avoiding the fast self-agglomeration of colloidal NSs. The obtained precipitates were washed thoroughly with ethanol and distilled water, and then freeze-dried. Several molar ratios of RuO2/Co−Fe-LDH (0.25, 0.5, and 0.75) were employed for the synthesis of nanohybrids to study the effect of chemical composition on their physicochemical properties. The resulting Co−Fe-LDH−RuO2 nanohybrids are denoted as CFR0.25, CFR0.5, and CFR0.75, respectively.
2.2. Characterization The crystal structures of the present materials were examined with powder X-ray diffraction (XRD) analysis (Rigaku D/Max-2000/PC, Ni-filtered Cu Kα radiation, λ =
5
1.54184 Å, 298 K). The crystal morphologies of the present materials were probed with field emission-scanning electron microscopy (FE-SEM, JEOL JSM-6700F) and TEM analyses. The size distributions of exfoliated LDH and RuO2 NSs were determined by a standard dynamic light scattering (DLS) analysis (Malvern Zetasizer Nano ZS). The internal charge transfer in the present materials was studied with photoluminescence (PL) spectroscopy using Perkin-Elmer LS55 fluorescence spectrometer. The N2 adsorption−desorption isotherms of present materials were measured with Micromeritics ASAP 2020 at 77 K. Micro-Raman spectra were collected with a Horiba Jobin Yvon LabRam Aramis spectrometer, in which an Ar-ion laser beam (λ = 514.5 nm) was used as an excitation source. Fourier transformed-infrared (FT-IR) spectroscopy (Varian FTS800) was used to probe the functional groups of nanohybrids. The electronic configurations and local crystal structures of the present materials were examined with X-ray absorption near-edge structure/extended X-ray absorption fine structure (XANES/EXAFS) spectroscopic analysis at the beamline 10C of the Pohang Accelerator Laboratory (PAL) in Korea. The oxidation states of the component ions in the present materials were studied with X-ray photoelectron spectroscopy (XPS, Thermo VG, UK, Al Kα).
2.3. Electrocatalytic activity tests The catalyst ink was obtained by dispersing 7 mg of catalyst material, 3 mg of carbon black (Vulcan-XC72R), and 20 µL of 5 wt% Nafion solution into a 5 mL of Milli-Q water/isopropanol (4/1, v/v) mixed solution, which was followed by ultra-sonication for 1 h. To prepare the working electrode, 10 µL catalyst ink was dropped onto the glassy carbon electrode (3 mm diameter, ALS Co.) and dried at 50 °C. A platinum wire, a saturated calomel electrode (SCE), and 1.0 M KOH solution (OER)/0.1 M KOH solution (ORR) were
6
used as a counter electrode, a reference electrode, and an electrolyte, respectively. Before the measurement, O2 gas was bubbled into the electrolyte for 30 min. The measured potentials were normalized to reversible hydrogen electrode (RHE) as the reference. All the experiments of linear sweep voltammogram (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) were carried out by using a RRDE-3A (ALS Co.) as a rotator and the IVIUM analyzer. The LSV curves were measured with a scan rate of 5 mV s−1 and a rotating speed of 1600 rpm. The EIS data were collected in the frequency range of 0.1−100000 Hz at 1.64 V vs. RHE (OER) /0.76 V vs. RHE (ORR).
2.4. Li−O2 battery test The prepared catalyst materials (0.02 g for each), conducting agent (ketjen black), and binder (HFP-PVDF) were dispersed with a mass ratio of 40:45:15 in N-methyl-2-pyrrolidone. The mixture was cast onto a GDL current collector (JNTG Co. Ltd., JNT30) and dried in vacuum at 120 °C for 5 h. The cathode was punched into disk having a diameter of 1.3 cm and an average catalyst loading density of 1.8−2.0 mg cm−2. Lithium foil and one sheet of glass fiber were used for anode and separator, respectively. Lithium metal, separator, and asprepared air electrode were stacked into a Swagelok-type Li−O2 cell in sequence. The electrolyte was made up of 1 M lithium bis(trifluoromethane) sulphonamide in tetramethylene glycol dimethyl ether. The cell was galvanostatically discharged and charged at a current density of 200 mA g−1 over a range of 2.0−4.5 V. For the stabilization of cycling, the cell was also tested using a constant current−constant voltage (CC−CV) mode with potentiostatic steps at 4.2 V under limited capacity condition (1000 mAh g−1). The EIS measurement was performed with an AC voltage amplitude of 10 mV s−1 in the frequency range of 100 MHz to 10 KHz using an impedance analyzer. Ex-situ FE-SEM studies of the
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discharge products were carried out by disassembling the cells in glove box under argon atmosphere to avoid the oxidation of the product formed. The cathode was washed in dimethyl carbonate (DMC) to remove residual electrolyte and dried under argon.
3. Results and discussion 3.1. Microscopic, diffraction, and spectroscopic analyses Fig. 1b and 1c present FE-SEM and TEM images of the CFR nanohybrids, respectively. All the CFR nanohybrids show the house-of-cards-type stacking of exfoliated LDH and RuO2 NSs, reflecting their mesoporous nature. The layer-by-layer stacking of Co−Fe-LDH and RuO2 NSs is confirmed by selected area electron diffraction (SAED) pattern of CFR0.5 showing diffraction rings corresponding to LDH and RuO2 components (Fig. 1d). FT-IR spectroscopic analysis for the CFR nanohybrids demonstrates characteristic IR bands of (Ru−O) and (Co/Fe−O) bonds, indicating the co-existence of LDH and RuO2 NSs (Fig. S2 of Supporting information). The homogeneous hybridization between LDH and RuO2 NSs in nanoscale is further evidenced by energy dispersive spectrometry (EDS)−elemental mapping analysis exhibiting uniform distribution of Co, Fe, Ru, and O elements in the entire region of CFR nanohybrids (Fig. S3 of Supporting information). According to powder XRD analysis (Fig. 1e), all the CFR nanohybrids exhibit equally-spaced (00l) Bragg reflections in low 2θ region, indicating the formation of the layer-by-layer-ordered heterostructure of Co−Fe-LDH and RuO2 NSs. An increase of RuO2 content remarkably enhances the intensities of (00l) reflections, highlighting the incorporation of RuO2 NS with higher electron density into the restacked LDH NSs [18]. Since the present nanohybrids are synthesized by restacking of exfoliated LDH and RuO2 NSs, their crystal lattices are composed of layer-by-layer-stacked Co−Fe-LDH and RuO2 NSs along the c-axis with random in-plane orientation. No
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observation of (hkl) reflections makes it impossible to specify the in-plane stacking pattern of component NSs. Since the layer thicknesses of both Co−Fe-LDH and RuO2 NSs are fairly similar to one another, the c-axis lattice parameter of the resulting CFR nanohybrid can be defined as the average interplanar distance of restacked NSs. Thus, the unit cell of the CFR nanohybrids can be assumed to contain a single layer of LDH/RuO2 NS along the c-axis. With this structural model, the observed intense (00l) diffraction peak at the lowest angle can be assigned as (001) reflection, like many other reports about NS-restacked materials [5,6,21]. The enhanced interfacial interaction and oxygen vacancy formation in the CFR nanohybrids are evidenced by a series of spectroscopic analyses. As plotted in the XANES spectra of Fig. 1f−1h, the hybridization of Co−Fe-LDH with RuO2 NS results in the lowering of Co K-edge and Fe K-edge energies, whereas the CFR nanohybrids exhibit higher Ru Kedge energy than does the RuO2 NS, clearly demonstrating significant interfacial charge transfer from LDH NS to RuO2 NS. Such a strong interfacial electronic coupling is crossconfirmed by XPS analysis showing the distinct blue-shifts of Co 2p and Fe 2p peaks and the red-shift of Ru 3p peaks upon the hybridization (Fig. S4 of Supporting information). The significant interfacial interaction between restacked Co−Fe-LDH and RuO2 NSs is further confirmed by PL spectroscopy. As presented in Fig. S5 of Supporting information, the CFR0.5 nanohybrid shows much weaker PL intensity than does the pristine Co−Fe-LDH. The observed lowering of PL intensity upon hybridization can be regarded as another evidence for significant interfacial charge transfer between LDH and RuO2, leading to the depression of charge recombination in LDH layer. All the XANES, XPS, and PL results presented here underscore the occurrence of strong interfacial interaction between Co−FeLDH and RuO2 NSs. The maintenance of the original crystal structures of Co−Fe-LDH and
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RuO2 NSs upon the hybridization is clearly confirmed by EXAFS analysis (Fig. 1i−1k and S6 of Supporting information), exhibiting nearly identical spectral features for both CFR and Co−Fe-LDH/layered RuO2 [22]. According to the curve fitting analysis (Table S1 of Supporting information), the hybridization of Co−Fe-LDH NS with RuO2 NS induces a gradual decrease of the coordination numbers (CNs) with an increase of Debye-Waller (σ2) factors for both (Co−O) and (Fe−O) coordination shells, underscoring the creation of oxygen defects and the increase of structural disorder in Co−Fe-LDH NS caused by elastic deformation occurring during intercalative hybridization [23]. To understand the origin of the formation of oxygen defects upon the hybridization process, the EXAFS analysis is also carried out for the exfoliated Co−Fe-LDH. According to the curve fitting analysis (Table S1 of Supporting Information), the CNs of (Co−O) and (Fe−O) coordination shells for the exfoliated Co−Fe-LDH are smaller than those for the pristine Co−Fe-LDH but larger than those for the CFR nanohybrids, underscoring the gradual increase of oxygen defect content during the exfoliation and the following hybridization processes. Since both the exfoliation process and the subsequent hybridization with heterogeneous RuO2 NS cause significant lattice deformation of LDH layer, the present EXAFS result can provide strong evidence for the crucial role of the elastic deformation of LDH layer in the formation of the oxygen defect in LDH layers. The presence of oxygen vacancies in Co−Fe-LDH NS is beneficial in promoting the adsorption of H2O on the surface of catalysts, thus enhancing the electrocatalytic activities of Co−Fe-LDH [24]. The larger σ2 values for Co K-edge than for Fe K-edge are attributable to the enhanced structural distortions caused by the Jahn-Teller active nature of Co2+ ion (d7) [25]. In contrast to Co K- and Fe K-edge data, the curve fitting analysis for Ru K-edge data indicates negligible change of CNs and increase of σ2 values upon the hybridization (Table S1 of Supporting information), highlighting higher structural
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stability of layered metal oxide lattice than LDH lattice. Generally the relative structural stabilities of inorganic solids can be well understood from the difference in their lattice energies. While the RuO2 lattice consists of RuO6 octahedra with tetravalent Ru4+ cations and divalent O2− anions, the Co−Fe-LDH lattice is composed of Co(OH)6/Fe(OH)6 octahedra with divalent Co2+/trivalent Fe3+ cations and monovalent OH− anions. Since the lattice energy of inorganic solid is proportional to the product of cationic charge and anionic charge, the layered RuO2 lattice is supposed to have much greater lattice energy than the Co−FeLDH lattice. The greater lattice energy of RuO2 than Co−Fe-LDH is responsible for the higher lattice stability of the former.
3.2. N2 adsorption−desorption analysis N2 adsorption−desorption isotherm measurement clearly demonstrates the formation of mesoporous structure with expanded surface area upon the hybridization of LDH and RuO2 NSs, see Fig. S7 of Supporting information. Based on the Brunauer−Emmett−Teller (BET) equation, the surface areas of the present materials are calculated as 61, 97, and 88 m2 g−1 for CFR0.25, CFR0.5, and CFR0.75, respectively, which are much larger than those of the pristine Co−Fe-LDH (19 m2 g−1) and RuO2 (13 m2 g−1). Such an increase of porosity upon hybridization implies the provision of more electrocatalytically active sites for ORR and OER as well as more deposition sites for the discharge product of Li−O2 battery. The presence of mesopores with average diameter of ~3−4 nm in the present nanohybrids is confirmed by the pore size calculation based on Barrett−Joyner−Halenda (BJH) equation (Fig. S8 of Supporting information), which is in good agreement with the FE-SEM results.
3.3. Electrocatalytic activity measurement
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The present CFR nanohybrids are tested as electrocatalysts for OER and ORR to probe the effect of heterolayer hybridization on the electrocatalytic activity. As shown in Fig. 2a−2e, all the CFR nanohybrids commonly demonstrate much smaller overpotentials at a current density of 10 mA cm−2 for OER and greater half-wave potentials for ORR than those of the precursors Co−Fe-LDH and RuO2, highlighting the improvement of oxygen electrocatalyst performance upon the intercalative hybridization of NSs. Among the present nanohybrids, CFR0.5 exhibits much higher oxygen electrocatalyst activity, indicating the optimal chemical composition of this material. The improvement of OER and ORR kinetics upon the hybridization is verified by much smaller Tafel slope of CFR0.5 as compared with those of the precursors Co−Fe-LDH and RuO2 [26,27]. The beneficial effect of nanoscale hybridization on the bifunctional electrocatalyst performance of component NSs is further verified by the inferior functionality of the physical mixture of Co−Fe-LDH and RuO2 with the same composition as CFR0.5 (Fig. S9 of Supporting information). The CFR0.5 nanohybrid shows highly stable electrocatalyst performance without any notable degrading by 25,000 s, which is in stark contrast to the precursor Co−Fe-LDH exhibiting a notable increase of operation potential at ~5,000 s (Fig. 2f). This result underscores the positive effect of hybridization on electrocatalyst durability.
3.4. ECSA, EIS, and in-situ XANES analyses The evolutions of electrochemical surface area (ECSA), charge transfer kinetics, and chemical bonding nature upon hybridization and electrocatalysis process are investigated to understand the origin of the improved electrocatalyst performances of CFR nanohybrids. The ECSA can be estimated from the linear slope in the plot of ∆j (= ja − jc) vs. RHE against the scan rate, which corresponds to twice of the double layer capacitance (Cdl) [28]. As
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depicted in Fig. 3a and S10 of Supporting information, all the CFR nanohybrids have markedly larger ECSAs than do the precursors Co−Fe-LDH and RuO2 NSs, which can be ascribed to the surface expansion and the formation of oxygen defects in LDH upon hybridization [29]. According to the EIS data of Fig. 3b and 3c, the diameter of semicircle is commonly smaller for the CFR nanohybrids at potentials of 0.76 and 1.64 V compared with those of the precursors Co−Fe-LDH and RuO2, underscoring the depression of charge transfer resistance (Rct) upon the hybridization [30,31]. Among the present materials, CFR0.5 displays the smallest radius of semicircle, indicating the best charge transfer kinetics of this material. The observed improvement of ECSA and charge transfer kinetics upon hybridization can be ascribed to a strong interfacial coupling between Co−Fe-LDH and RuO2 NSs [32]. The variation of the chemical bonding nature of CFR nanohybrid during the OER process is also investigated with in-situ XANES analysis. As depicted in Figs. 3d−3f, both CFR0.5 and Co−Fe-LDH commonly exhibit a distinct blue-shift of Co K-edge energy with increasing the applied potential from 1.2 to 1.7 V, indicating the oxidation of Co ion with the bond formation of Co−OH and/or Co−OOH under the oxidative potential [33]. In comparison with the pristine Co−Fe-LDH, CFR0.5 displays much weaker change of edge energy during the OER process, emphasizing the depression of charge accumulation by a promoted hole injection into the adsorbed reactant upon the hybridization with conductive RuO2 NS. This finding provides strong evidence for the improvement of interfacial charge transfer between LDH and adsorbed reactant due to intimate electronic coupling with conductive RuO2 NS (Fig. 3g). This makes crucial contribution to the enhanced electrocatalyst functionality of CFR. In contrast to the Co K-edge XANES spectra, the Fe K-edge and Ru K-edge XANES spectra of CFR0.5 show only negligible alterations of edge position with increasing
13
oxidative potential from 1.2 to 1.7 V. This observation highlights that, among many metal component ions, the Co ion of Co−Fe-LDH makes the most crucial contribution to the excellent electrocatalyst activity of CFR [34]. As evidenced by the EXAFS analysis (Table S2 of Supporting information), the hybridization with RuO2 NS gives rise to the decrease of oxidation states of Co/Fe ions with the elongation of (Co/Fe−O) bond distance and the creation of oxygen defects in the Co−Fe-LDH NS (Fig. 3h). Taking into the Jahn-Teller active nature of Co2+ (d7) ion, an elongated axial (Co−O) bond distance of CFR is helpful in promoting the adsorption of oxygen species (i.e. reactant for OER) on Co sites, which is responsible for the major role of Co ion as OER active site (Fig. 3h) [35]. Also, the formation of oxygen defects in Co−Fe-LDH upon the hybridization makes significant contribution to the enhanced adsorption of H2O and O2 (i.e. reactant for ORR) and thus to the improved ORR activity of CFR [22]. It is worthwhile to mention that, in contrast to CFR exhibiting negligible spectral change, a significant modification of Ru K-edge data with notable edge shift occurs for the precursor RuO2 NS during the OER process, highlighting the remarkable improvement of the electrochemical stability of RuO2 NS upon the intimate hybridization with LDH.
3.5. Li−O2 battery test Based on the bifunctional oxygen electrocatalyst performance of the present CFR nanohybrids, these materials are employed as cathode catalysts for Li−O2 batteries. As depicted in Fig. 4a, the CRF0.5 nanohybrid exhibits much larger discharge capacity with much lower charge overpotential of ~3.8 V for OER and relatively stable potential of ~2.7 V for ORR than do the pristine RuO2 and Co−Fe-LDH, underscoring the beneficial effect of hybridization on the cathode performance. Significant similar shapes in the initial discharge
14
curves of CFR0.5 and Co−Fe-LDH strongly suggest a major role of LDH component as initial active sites for the adsorption of oxide species on the nanohybrid. To further verify the superior performance of the CFR nanohybrids over the precursors, the capacity limitation is adopted to suppress the excessive formation of insulating discharge product (Fig. 4b−d). The CFR0.5 nanohybrid shows the most stable cycling for more than 100 cycles, which is in stark contrast to RuO2 and Co−Fe-LDH exhibiting drastic capacity fading before 70 cycles (Fig. 4c), confirming the remarkable advantage of hybridization on electrode functionality. Such an excellent cyclic stability of CFR0.5 can be associated with its relatively lower charge−discharge overpotentials than the pristine materials (Fig. 4d). The observed excellent electrode performance of CFR0.5 is attributable to the creation of oxygen defects in LDH and the occurrence of strong interfacial electronic coupling upon the hybridization. Also, the expanded surface area of CFR0.5 cathode material makes additional contribution to its improved electrode functionality via the enhancement of the mass transfer of oxygen and solvated Li ions as well as electron transfer. Such evolutions cause the enhanced surfaceexposure of electrocatalytically-active sites, the provision of efficient deposition sites for discharge product, and the improvement of transport pathways for electron and mass [36]. According to CV analysis (Fig. 4e), the ORR activities of CFR and Co−Fe-LDH are confirmed by the observation of a cathodic peak at an onset potential ~2.7 V. For the OER process, the CRF0.5 nanohybrid displays a much higher activity with a broader anodic peak at an onset potential of ~3.25 V than the other materials, indicating the promoted decomposition of discharge products. As probed by XANES/EXAFS analyses (Fig. 1f−1k), a strong interfacial coupling between RuO2 and Co−Fe-LDH induces net RuO2 to LDH charge redistribution with creation of oxygen defects, thus facilitating the adsorption of the oxide species (LiO2 or Li2O2) with a moderate bond strength for enhanced OER activities [37,38].
15
The beneficial effect of hybridization on charge transfer kinetics is evidenced by the EIS analysis conducted after the 1st discharging without the capacity limitation demonstrating a significant decrease of Rct upon hybridization (Fig. 4f and S11 of Supporting information). Also, the discharge products on CFR0.5, RuO2, and Co−Fe-LDH are analyzed with FE-SEM, see Fig. S12 of Supporting information. The CFR0.5 cathode after the discharge process displays a homogenous deposition of sheet-like discharge products with 2D structure over the entire electrode, which must be beneficial for mass transfer and electron transport for improved ORR and OER activities. Conversely, the unhybridized RuO2 and Co−Fe-LDH cathodes demonstrate the formation of a film-like and typical toroidal discharge products, respectively, leading to the significant impeding of the following OER process. The homogenous charge redistribution of electrocatalytically-active LDH NS caused by the effective electronic coupling with highly-conductive RuO2 NS is responsible for the optimization of the microstructure of discharge products in the CFR nanohybrid and the following efficient facilitation of its decomposition during OER.
3.6. Effect of the surface charge of component NS on interlayer coupling and defect structure of nanohybrid To study the effect of the surface charge of component NS on the interfacial coupling and oxygen defect of nanohybrids, two kinds of surface-charge-controlled Co−Fe-LDH materials are synthesized with different Co2+/Fe3+ molar ratios of 2/1 and 4/1, since the surface charge of LDH layer can be easily controlled by altering the molar ratios of divalent and trivalent metal ions [39]. According to the zeta potential measurements, the surface charges of exfoliated Co−Fe-LDH NSs with the Co/Fe molar ratios of 2/1, 3/1, and 4/1 show gradually changing surface charges of +48, +39, and +29 mV, clearly demonstrating the successful
16
control of the surface charge of Co−Fe-LDH NS with alteration of Co/Fe ratios. The exfoliated NSs of these surface-charge-controlled LDHs are employed for the synthesis of Co−Fe-LDH−RuO2 hybrid NSs with different interfacial interaction. The obtained nanohybrids composed of RuO2 and Co−Fe-LDH with Co/Fe molar ratio of 2/1, 3/1, and 4/1 are denoted as CFR0.5-2/1, CFR0.5, and CFR0.5-4/1, respectively. According to the curve fitting EXAFS analyses for these nanohybrids (Fig. S13 and Table S2 of Supporting information), the CNs of (Co/Fe−O) shells becomes smaller in the order of CFR0.5-4/1 > CFR0.5 > CFR0.5-2/1, indicating a gradual increase of interfacial coupling and oxygen vacancy with increasing the surface charge of LDH NS. The present EXAFS results provide strong evidence for the enhancement of interfacial interaction between RuO2 and LDH NSs with the increase of LDH surface charge and the resulting increase of oxygen defect content. Additionally we carried out EIS analyses for these nanohybrids to probe the evolution of interfacial electronic coupling between LDH and RuO2 NSs upon the change of LDH surface charge (Fig. S14 of Supporting information). The charge transfer resistance of the nanohybrids becomes smaller in the order of CFR0.5-4/1 > CFR0.5 > CFR0.5-2/1, highlighting the stronger electronic coupling of LDH NS having higher surface charge with highly conductive RuO2 NS. All the experimental findings presented here underscore the feasibility of surface charge as an effective tool for tailoring the interfacial interaction and defect structure of restacked NSs.
4. Conclusions In conclusion, a fine-control of interfacial electronic coupling and crystal defect structure of 2D inorganic NSs provides an effective way of exploring high-performance hybrid electrocatalysts and cathodes for Li−O2 batteries via the layer-by-layer hybridization of
17
exfoliated NSs. The intimately-coupled 2D nanohybrid composed of exfoliated polar Co−FeLDH and RuO2 NSs with notable oxygen deficiency displays excellent catalyst performance for Li−O2 batteries with markedly lowered overpotential, outstanding cyclability, and reversible formation/decomposition of discharge product. The improved electrode and electrocatalyst performances of heterolayered CFR nanohybrids are attributable to strong interfacial electronic coupling between LDH and RuO2 NSs as well as the creation of significant oxygen vacancies, resulting in the enhancement of the diffusion of Li ions and reactants, and the improvement of OER/ORR kinetics and charge transfer kinetics upon hybridization. Moreover, since the LDH NS has smaller lateral dimension than does RuO2 NSs, the restacked CFR nanohybrids contain large numbers of edge-sites of LDH with high catalytic activity, which makes significant contribution to the excellent electrocatalyst performance of these materials. Additionally, marked difference in the lateral sizes of LDH and RuO2 NSs would be effective in forming loosely-restacked materials with expanded surface area, which is beneficial in optimizing the electrocatalyst and electrode performance of restacked nanohybrid. Considering the crucial roles of surface nature and defect structure of inorganic NS in interfacial electronic coupling, there are lots of opportunities to optimize multifunctionalities of heterolayered nanohybrids via the tailoring of defect structure, surface roughness, surface structure, and surface composition of component NSs [5,8,40,41]. In fact, the effective controllability of defect structure and interfacial coupling with the change of the surface charge of component NS is obviously evidenced from the present study. Our current project is to explore efficient multifunctional materials applicable for fuel cells and water electrolyzers via the intimate heterolayer hybridization between diverse couples of inorganic NSs with controlled surface natures, surface charges, and crystal defects.
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Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2017R1A2A1A17069463) and by the Korea government (MSIT) (No. NRF-2017R1A5A1015365). The experiments at PAL were supported in part by MOST and POSTECH.
Supporting Information Zeta potential data, TEM images, and size distribution data of the exfoliated Co−Fe-LDH and RuO2 nanosheets, FT-IR spectra, EDS−elemental maps, PL data, XPS data, k3-weighted EXAFS oscillations, EXAFS fitting results, N2 adsorption−desorption data, pore size distributions, LSV curves, CV curves, EIS data, and FE-SEM images of CFR nanohybrids.
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Figure captions
Fig. 1. (a) Synthesis scheme to CFR nanohybrid. (b) FE-SEM images, (c) TEM images, (d) SAED pattern, (e) powder XRD patterns, (f,g,h) XANES and (i,j,k) Fourier transform (FT)EXAFS spectra of CFR nanohybrids and references.
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Fig. 2. (a) LSV curves for OER, (b) Tafel plots for OER, (c) LSV curves for ORR, (d) Tafel plots for ORR, (e) overpotentials/half-wave potentials, and (f) stability data for OER at 10 mA cm−2 for CFR nanohybrids and references.
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Fig. 3. (a) Plot of charging current density differences vs. scan rates, (b,c) EIS curves, and (d,e,f) in-situ XANES spectra for CFR nanohybrids and references. (g) Interfacial charge transfer scheme and (h) schematic illustration of introducing oxygen defects to Co−Fe-LDH.
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Fig. 4. (a) Initial galvanostatic discharge/charge profiles, (b) galvanostatic discharge/charge curves with the capacity limitation, (c) discharge capacity retentions, (d) cyclic variation of terminal voltages, (e) CV data, and (d) EIS curves for CFR nanohybrids and references.
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Highlights • The interfacial coupling and crystal defect play crucial role in inorganic nanosheet. • Hybridization between 2D inorganic nanosheets yields strongly-coupled nanohybrid. • The exfoliation-hybridization process creates oxygen defects in inorganic nanosheet. • Heterolayered nanohybrids show excellent electrocatalyst/Li−O2 electrode activity. • Electrocatalysis kinetics of nanosheet can be improved by strong interfacial coupling.
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Dr. Xiaoyan Jin is currently a postdoctoral researcher in the Department of Materials Science and Engineering at Yonsei University under Prof. Seong-Ju Hwang. She received her Ph.D. degree in inorganic chemistry (2018) from Ewha Womans University. Her research focuses on synthesis and characterization of layered metal oxides, layered double hydroxides, and metal chalcogenides, and their applications in electrocatalyst, supercapacitor, photocatalyst, and Na/Li-ion batteries.
Dr. Daniel Adjei Agyeman is currently an Assistant Professor in the Department of Energy and Materials at Dongguk University. He received his Ph.D. degree in Energy Materials Engineering (2018) from Dongguk University under Prof. Yong-Mook Kang’s guidance. His research focuses on synthesis and characterization of transition metal compounds applicable for energy storage (Li/Na ion battery), and electro-catalysis.
Saeyoung Kim received a B.S. degree in chemistry (2018) from Ewha Womans University. She is currently a M.S. Student in the Department of Chemistry and Nanoscience at Ewha Womans University (Supervisor: Prof. Seong-Ju Hwang). Her research focuses on synthesis and characterization of layered metal oxides and layered double hydroxides, and their applications in electrocatalyst.
Yeon Hwa Kim received a B.S. degree in chemistry (2018) from Ewha Womans University. She is currently a M.S. Student in the Department of Chemistry and Nanoscience at Ewha Womans University (Supervisor: Prof. Seong-Ju Hwang). Her research focuses on synthesis and characterization of the 2D inorganic nanosheet-based nanohybrid for energy-related applications.
Dr. Min Gyu Kim is chief staff scientist of Pohang Accelerator Laboratory (PAL), Korea. He received B.S. (1993), M.S. (1995), Ph. D (2002) in Department of Chemistry of Yonsei University (Korea). He joined at beamline research division of PAL in 2002. He is currently in charge of BL10C (Wide-XAFS) beamline with Wiggler insertion device and. His research interests are focused on a fundamental solid state chemistry combined with synchrotron radiation-based in situ characterization including XAFS/X-ray scattering/Imaging, covering synthesis, functionalities and real-time characterization for energy storage-conversion materials, organic-inorganic hybrid materials, and environmental materials.
Prof. Yong-Mook Kang completed his B.S. (1999), M.S. (2001), and Ph.D. (2004) in Korea Advanced Institute of Science and Technology. He has been a senior researcher in Samsung SDI Co., LTD. He was a professor at Department of Energy and Materials Engineering in Dongguk University and then now a full professor at Department of Materials Science and Engineering in Korea University. His research area covers electrode or catalyst materials for Li rechargeable batteries and various post Li batteries, such as Li-air battery, Na rechargeable battery and so on.
Prof. Seong-Ju Hwang is currently a full professor in the Department of Materials Science and Engineering at Yonsei University. He received a B.S. degree in chemistry (1992) and a M.S. degree in inorganic chemistry (1994) from Seoul National University (Korea), a Ph.D. degree in inorganic chemistry from Université Bordeaux I (France) in 2001. He worked as a full professor in the Department of Chemistry and Nanoscience at Ewha Womans University from 2005 to 2019. His research focuses on the synthesis and characterization of lowdimensional nanostructured transition metal compounds applicable for energy production, energy storage, and environmental purification.