Effects of polyelectrolyte hybridization on the crystal structure, physicochemical properties, and electrochemical activity of layered manganese oxide
Research highlights
► Polyelectrolyte-layered manganate nanohybrids were prepared by reassembling method. ► The effect of hybridization on the electrode activity of manganate was investigated. ► A lower charge density polymer is a better guest for improving electrode performance.
Introduction
Recently both supercapacitors and lithium rechargeable batteries have attracted intense research interest as power sources for hybrid electric vehicles and plug-in electric vehicles [1], [2], [3]. The large scale applications of these energy storage devices demand the development of highly efficient electrode materials with high energy density, high power density, excellent rate characteristics, and so on. Ruthenium- and cobalt-based oxides show very promising electrode performances for supercapacitors and lithium secondary batteries, respectively [4], [5], [6]. However, the high price and poor abundance of ruthenium and cobalt elements make these materials unsuitable for large scale applications. In addition to promising electrode performance, manganese-based oxides boast low price, rich abundance, and low toxicity of manganese element. Thus, manganese oxides have received primary attention as alternative electrode materials for supercapacitors as well as for lithium secondary batteries [7], [8], [9]. The electrochemical properties of manganese oxides are supposed to strongly depend on their crystal structure, chemical bonding nature, surface area, and so on. Understanding about the relationship between these factors and electrochemical properties would be quite informative in designing highly efficient electrode materials. In the case of layered manganese oxide, its structural, chemical, and physical properties might be tailored by the intercalative hybridization with guest species. Taking into account the versatility of polyelectrolytes in chemical bonding nature, degree of protonation, and Lewis basicity/acidity, the intercalation of polyelectrolyte would provide useful way of controlling the chemical and electrochemical properties of layered manganese oxide. It is however difficult to intercalate polyelectrolytes into the layered metal oxide via conventional ion-exchange reaction. Alternatively, an exfoliation of layered manganate into individual nanosheets can provide new effective way to synthesize intercalation compounds via the reassembling of exfoliated monolayers with oppositely charged guest species [10], [11]. The success of this method is based on an electrostatic attraction between negatively charged manganese oxide nanosheets and positively charged guest species [12], [13], [14], [15]. Using this synthetic route, we synthesized successfully the intercalative nanohybrids of ZnO-layered manganate and TiO2-layered manganate [16], [17]. Ooi et al. were successful in intercalating poly(diallydimethylammonium) (PDDA) cations into the layered manganese oxide using the same synthetic strategy [14]. But they did not investigate the electrochemical properties of the PDDA-intercalated layered manganate. Moreover, there has been no report about the intercalation of other polyelectrolytes like PEI and PAH into the bulk layered manganate. Alternatively the multilayer films of manganese oxide and polyelectrolytes were fabricated via a direct electrodeposition of Mn2+ ions and polyelectrolytes, showing the electrochemical activity of the obtained multilayer films [18]. To dates, there is however no systematic study about the relationship between the chemical nature of guest polyelectrolytes and the electrochemical/physicochemical properties of polyelectrolyte-intercalated layered manganese oxide.
In this study, we synthesized layered manganese oxides hybridized with three kinds of polyelectrolytes having different chemical natures, i.e. polyethylenimine (PEI, (C2H5N)n), poly(allylamine hydrochloride) (PAH, (C3H8ClN)n), and PDDA ((C8H16ClN)n). These intercalative nanohybrids are expected to be highly valuable model compounds to elucidate the relationship between physicochemical properties and electrode performance in layered manganese oxide. As illustrated in Fig. 1, the PDDA is a strong electrolyte with quaternary ammonium groups whereas the PAH is a weak electrolyte with primary ammonium groups. The former polymer retains its positive charge regardless of the solution pH whereas the charge of the latter is significantly dependent on the pH of solution. Another weak electrolyte adopted here, PEI, has neutral amine group and thus can act as a Lewis base. The crystal structure, crystal morphology, surface area, and chemical bonding nature of the polyelectrolyte-layered manganate nanohybrids were systematically characterized with microscopic, physical, and spectroscopic tools. The electrode performances of these hybrid-materials were studied with a reference to the chemical bonding nature of the guest species.
Section snippets
Preparation
Layered potassium manganese oxide K0.45MnO2 and its protonated derivative were prepared by the solid state reaction and the following 1 M HCl treatment, as reported previously [10], [19]. The exfoliated nanosheets of the layered manganate were prepared by reacting the protonated manganate with tetrabutylammonium hydroxide (TBA·OH) for more than 10 days. Pure colloidal suspension of exfoliated nanosheets was obtained by removing a small fraction of incompletely exfoliated particles via high speed
Powder XRD analysis
The powder XRD patterns of polyelectrolyte-layered manganate nanohybrids in wet and dry conditions are presented in Fig. 2, in comparison with those of the pristine K0.45MnO2 and its protonated derivative. An acid-treatment for the pristine K0.45MnO2 causes the shift of (0 0 l) reflections toward a lower angle side, indicating the intercalation of water molecules [23]. As plotted in the left panel of Fig. 2, all of the polyelectrolyte-layered MnO2 nanohybrids before drying show a series of sharp
Conclusion
In the present work, we investigated the effects of polyelectrolyte intercalation on the structural, physicochemical, and electrochemical properties of layered manganese oxide. It becomes certain that the crystal structure, surface area and/or Mn oxidation state of the polyelectrolyte-layered manganese oxide nanohybrids are strongly dependent on the chemical nature of guest species. As the charge density of the guest polyelectrolytes increases, the structural stability of the nanohybrid becomes
Acknowledgments
This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2010-C1AAA001-2010-0029065). This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MEST) (2010-0027517), by MEST & DGIST(10-BD-0101, Convergence Technology with New Renewable Energy and Intelligent Robot), and partly by National Research Foundation of Korea Grant funded by the Korean Government(2010-0001485). The
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