Elsevier

Nano Energy

Volume 67, January 2020, 104192
Nano Energy

Full paper
Crucial roles of interfacial coupling and oxygen defect in multifunctional 2D inorganic nanosheets

https://doi.org/10.1016/j.nanoen.2019.104192Get rights and content

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.

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 bifunctional electrocatalysts applicable as Li−O2 cathode.

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], [2], [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], [7], [8], [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 hybrid-type 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 Co-containing 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 nanohybrids have employed graphene as a building block [[13], [14], [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], [17], [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 layer-by-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−Fe-LDH−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.

Section snippets

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 exfoliation yields of the Co−Fe-LDH and RuO2 NSs were estimated as ~100% and

Microscopic, diffraction, and spectroscopic analyses

Fig. 1b and c 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

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 exfoliated NSs. The intimately-coupled 2D nanohybrid composed of exfoliated polar Co−Fe-LDH and RuO2 NSs with notable oxygen deficiency displays excellent catalyst performance for Li−O2 batteries with markedly lowered overpotential,

Acknowledgments

This work was supported by the National Research Foundation of Korea 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.

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.

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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 low-dimensional nanostructured transition metal compounds applicable for energy production, energy storage, and environmental purification.

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    These authors contributed equally to this work.

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