Letter pubs.acs.org/NanoLett
One-Dimensional RuO2/Mn2O3 Hollow Architectures as Efficient Bifunctional Catalysts for Lithium−Oxygen Batteries Ki Ro Yoon, Gil Yong Lee, Ji-Won Jung, Nam-Hoon Kim, Sang Ouk Kim, and Il-Doo Kim* Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea S Supporting Information *
ABSTRACT: Rational design and massive production of bifunctional catalysts with fast oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics are critical to the realization of highly efficient lithium−oxygen (Li−O2) batteries. Here, we first exploit two types of double-walled RuO2 and Mn2O3 composite fibers, i.e., (i) phase separated RuO2/Mn2O3 fiber-in-tube (RM-FIT) and (ii) multicomposite RuO2/Mn2O3 tube-in-tube (RM-TIT), by controlling ramping rate during electrospinning process. Both RM-FIT and RM-TIT exhibited excellent bifunctional electrocatalytic activities in alkaline media. The air electrodes using RM-FIT and RM-TIT showed enhanced overpotential characteristics and stable cyclability over 100 cycles in the Li−O2 cells, demonstrating high potential as efficient OER and ORR catalysts. KEYWORDS: Bifunctional, electrocatalysts, manganese oxide, ruthenium oxide, lithium−oxygen batteries, electrospinning discussion, manganese based oxides (MnOx including α-, β-, γ-, δ-MnO2, and Mn2O3) have been considered as the outstanding ORR catalysts in aqueous media20,21 as well as in nonaqueous Li−O2 cells.9,22 However, the poor electronic conductivity and the relatively low OER kinetics of MnOx should be improved by combining with other cocatalysts with higher OER activity. Ruthenium based oxides (RuOx) have been actively explored due to their high electronic conductivity and outstanding catalytic activity especially for OER based electrochemistry.23−25 Recently, Lee et al., confirmed the catalytic activity of RuO2 for OER in acid and alkaline solutions,26 and Jung et al., also showed that Ru-based catalysts have high stability and superior OER property in nonaqueous TEGDME electrolyte Li−O2 cells.23 For these reasons, the diverse researches have been achieved by using RuO2 incorporated with various carbons, such as graphenes and carbon nanotubes, as bifunctional air electrodes.27−30 Among various nanobuilding blocks, one-dimensional (1-D) nanostructures with high aspect ratio and directional charge transport characteristics have been extensively explored for the energy conversion and storage chemistry. They include RuO2− Co3O4 core−shell fibers,31 CoMn2O4, NiCo2O4, CoFe2O4, NiMn2O4, and ZnMn2O4 tube-in-tubes,32 hollow MnCo2O4 and CoMn2O4 fibers,33 and etc. As bifunctional catalysts in air electrode, their electrochemical performances for Li−O2 cells
T
he rapid development of industry and increase of energy demands have been stimulating the intensive research on high efficiency, low cost, and environmental-friendly energy conversion and storage systems.1 In recent year, lithium− oxygen (Li−O2) battery has attracted considerable attention as the most promising energy storage system since the theoretical energy density of Li−O2 battery (∼3,500 Wh kg−1, based on the mass of Li and O2) is about 10 times larger than that of conventional lithium-ion battery.2 However, critical scientific challenges related to the inherent sluggish kinetics of the oxygen involved electrocatalysis, such as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), remain for the practical use of Li−O2 batteries.3,4 In response, the various electrocatalysts like noble metals,5−8 transition metals and their oxides,9,10 and carbon supports like carbon nanotubes and graphenes11−14 have been intensively explored to facilitate ORR and/or OER. Recent studies revealed that the ORR can be sufficiently promoted by the carbon support alone, but carbon often reacts with the main discharge products, i.e., Li2O2, or the electrolyte at high charge potential (>4.0 V).15,16 Even though there is unfavorable formation of byproducts (CO2, Li2CO3, and etc.), the carbon has still been widely used due to its crucial advantages such as low cost, high electronic conductivity (σgraphite > 103 S m−1),17 and high gravimetric surface area.18 So, the utilization of effective catalysts referring to both OER and ORR activities is highly demanded to mitigate the excessive overpotentials accompanying the superfluous reactions.18,19 Although the catalytic activities of the proposed materials are still under © XXXX American Chemical Society
Received: January 15, 2016 Revised: January 26, 2016
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DOI: 10.1021/acs.nanolett.6b00185 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 1. Scanning electron microscope (SEM) images (a,e), transmission electron microscope (TEM) images (b,f), HRTEM images (c,g), and EDS mapping (d,h) of the RM-FIT and RM-TIT; (i) XRD patterns of RM-FIT and RM-TIT; (j) the N2 adsorption−desorption isotherm curves of RM-FIT and RM-TIT. The corresponding pore size distribution curves are illustrated in the inset of each figure.
DIW and DMF solvents.34 It will be discussed in more detail later. For comparison, the RuO2/Mn2O3 nanofibers prepared by solely using DMF solvent and calcined at the same heating condition showed solid fibrous shape (Figure S2). Interestingly, the core fibers and shell tubes have different crystalline phase, where the interplanar distance of the shell represents (222) plane of Mn2O3 and those of cores represents (110) plane of RuO2 (Figure 1c). Energy dispersive spectroscopy (EDS) mapping and line profile (Figures 1d and S3) results verify that the most Ru elements were preferentially located in the core region surrounded by the Mn elements in the shells. In contrast, the double-walled tube-in-tube structures were obtained when the as-spun fibers were calcined with 5R. As shown in Figure 1e,f, the inner fibers have a void along the vertical direction in the middle as a type of the small tubes (∼100 nm) placed inside of the large outer tubes (∼240 nm). High resolution TEM image of RM-TIT reveals that the (110) plane of RuO2 and the (222) plane of Mn2O3 coexisted in both the inner and outer tubes (red dashed circle in Figure 1g). The EDS mapping clearly demonstrates that Ru and Mn components are homogeneously distributed on the overall positions in the RM-TIT (Figure 1h) without distinctive phase separation. From the X-ray diffraction (XRD) analysis (Figure 1i), we confirmed that both RM-FIT and RM-TIT consisted of pure
were summarized as Table S1 in the Supporting Information. Herein, we first report on facile synthesis of double-walled RuO2 and Mn2O3 composite fibers, i.e., RuO2/Mn2O3 fiber-intube (RM-FIT) and RuO2/Mn2O3 tube-in-tube (RM-TIT), by using single-nozzle electrospinning followed by heating speed controlled calcination. More importantly, bifunctional catalytic activities of RM-FIT and RM-TIT and their potential feasibility as air electrode catalysts for the Li−O2 batteries are also discussed. For the synthesis of RuO2/Mn2O3 fiber-in-tube and tube-intube structures, the polyvinylpyrrolidone (PVP), RuCl3, and Mn(OAc)2 (RuCl3/Mn(OAc)2 = 1:2 wt %) containing N,Ndimethylformamide (DMF) and deionized water (DIW) mixed solution (DMF/DIW = 1:1 vol %) were electrospun (Figure S1), and the as-spun fibers were calcined at 600 °C for 1 h with a different ramping rate (R), i.e., a low R (1 °C min−1, hereafter 1R) and a high R (5 °C min−1, 5R) (experimental details are available in the Supporting Information, experimental section). After calcination with 1R, the phase separation between core fibers (∼80 nm) and shell tubes (∼220 nm) distinctively occurred and the walls of outer tubes are spatially separated with core fibers, producing cylindrical pores (Figure 1a,b). The creation of open pores formed between the core fiber and shell tube can be attributed to the immiscible nature between the inorganic ions (Ru4+ and Mn3+) and PVP polymer matrix in B
DOI: 10.1021/acs.nanolett.6b00185 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 2. TG/DSC curves of as-spun RuCl3/Mn(OAc)2/PVP fibers calcined up to 800 °C with ramping rate of (a) 1R and (b) 5R. (c) Schematic illustration on the formation mechanism of RM-FIT and RM-TIT.
are initially assembled by the outer shells and gel matrix is split at certain point, giving rise to the formation of hollow structure in the procedure of subsequent calcination.32,38 After that, the several weight losses and exothermal peaks were also observed (230−440 °C) in both RM-FIT (1R) and RM-TIT (5R) cases. The appearance of stepwise weight losses can be explained by the decomposition of metal precursors such as chlorine and acetate, PVP polymers, and two distinctive exothermal peaks are relevant to the formation of bonding between oxygen and metal ions, like [Ru−Ox] and [Mn−Ox], in the sequence. However, two exothermal peaks even appeared at different temperature, specifically, 254 and 283 °C for [Ru−Ox] in RMFIT and RM-TIT, and 341 and 335 °C for [Mn−Ox] in RMFIT and RM-TIT, respectively. When the as-spun fiber heated up slowly (1R), two exothermal peaks were completely separated remaining steady state flow between two peaks (red circle in Figure 2a). Meanwhile, narrowing of the exothermal peaks occurred in RM-TIT, which induces the calcination of [Ru−Ox] and [Mn−Ox] to be overlapped (blue circle in Figure 2b). We can conclude that this gap between RM-FIT and RM-TIT is a result of the heating speed.32,38 As shown in Figure S7, total heating procedure of RM-FIT takes around 5 times longer than that of RM-TIT (even though RMFIT and RM-TIT used for electrochemical performance in this work were obtained after calcination until 600 °C, the heating rates and the difference of procedure time should be same). So, in the case of RM-FIT, the longer heating time allows the preemptive clustering of [Ru−Ox] species at the core region induced by the PVP assisted Ostwald ripening behavior,31,34 and the inner gel acts as a carrier matrix for the [Ru−Ox] diffusion before the condensation of the hollow structure. Thus, crystallized RuO2 fibers are predominantly formed at the inner region of the fibers, and the residual [Mn−Ox] species are crystallized into Mn2O3 at the outer layer. Finally, there is no significant gravimetric change (>440 °C), and the phase separated RuO2/Mn2O3 fiber-in-tubes (RM-FIT) are obtained as described in the schematics (Figure 2c). However, with a
RuO2 (PDF no. 43-1027) and Mn2O3 (PDF no. 41-1442) phases without any incorporated phases. Ring patterns also indicate that both RM-FIT and RM-TIT consisted of the polycrystalline Mn2O3 and RuO2 (Figure S4), and the binding energies with regard to Ru, Mn, and O species were detected on the surface of both specimens by X-ray photoelectron spectroscopy (XPS in Figure S5).35,36 The RM-TIT have slightly higher Brunauer−Emmett−Teller (BET) surface area (29.91 m2 g−1) than RM-FIT (27.02 m2 g−1) does, which is attributed to the inner tubes of RM-TIT (Figure 1j). The multiple exposures of the surface area would have a great benefit for the numerous catalytic reaction sites and reduction of the dead-sites in same gravimetric samples, for instance, the solid RuO2/Mn2O3 fibers exhibited lower specific surface area of 20.06 m2 g−1 (Figure S2c). The atomic ratio of Ru to Mn in both specimens was determined to be around 38/62 (by at %) by using inductively coupled plasma mass spectrometry (ICPMS), which is similar to the EDS results (27/73 by at % and 40/59 by wt %, Figure S6). We further carried out the thermogravimetry (TG) and differential scanning calorimetry (DSC) analysis to elucidate the formation mechanism of unique tubular structures in RMFIT and RM-TIT. As shown in Figure 2a,b, the small fraction of the weight loss of around 20% was observed at the low temperature range (
