Article pubs.acs.org/cm
Oxygen Storage Capacity and Phase Stability of Variously Substituted YBaCo4O7+δ Outi Parkkima, Hisao Yamauchi, and Maarit Karppinen* Laboratory of Inorganic Chemistry, Department of Chemistry, Aalto University, FI-00076 Aalto, Finland ABSTRACT: Here, we investigate various approaches of chemical substitution to enhance the phase stability of the oxygen storage material candidate, YBaCo4O7+δ, under oxygen-containing conditions. From the results of Sr-for-Ba and R-for-Y (where R is a rare-earth element) substitutions, it can be concluded that the smaller the unit-cell volume, the less the phase absorbs oxygen but the higher the phasedecomposition temperature. Studies on the YBa(Co0.95M0.05)4O7+δ samples with M = Mn, Fe, Ni, Cu, Zn, Al, and Ga, then reveal that partial substitution of Co with Al, Ga, or Zn efficiently suppresses the phase decomposition. Most importantly, through cosubstitution with Al and Ga, the phase decomposition can be completely avoided for the YBa(Co0.85Al0.075Ga0.075)4O7+δ sample without markedly sacrificing the oxygen storage capability. Finally, it is shown that, among the RBa(Co0.85Al0.075Ga0.075)4O7+δ samples with R = Y, Dy, Ho, and Lu, only those with R = Y and Lu remain stable under oxidizing conditions at high temperatures. Possible factors affecting the phase stability of YBaCo4O7+δ and its derivatives are discussed. KEYWORDS: complex cobalt oxide, oxygen storage, chemical substitution
1. INTRODUCTION Oxygen storage materials are capable in reversibly storing and releasing large amounts of oxygen, depending on the temperature and oxygen partial pressure of the surrounding atmosphere. Such materials are important components in, e.g., automotive exhaust gas and other redox catalysts.1 Because of the huge application potential, there is an intensive search for new oxygen storage material candidates.2−5 An efficient oxygen storage material should have (i) high oxygen storage capacity (OSC), (ii) sufficiently low operation temperature, and (iii) adequate thermal stability.3 Based on these criteria, the complex cobalt oxide phase, YBaCo4O7+δ, appears to be a highly prominent material candidate, particularly for low-temperature applications.2,6 Its oxygen storage capacity (∼2700 μmol-O/g)5 is markedly larger than that of the conventional oxygen storage material CeO2−ZrO2 (∼1500 μmol-O/g).6 Moreover, it works at appreciably low temperatures (200−400 °C). The material has already been investigated for several applications, e.g., solidoxide fuel cells,9−13 oxygen separation membranes,14 and ceramic sorbents to enhance the oxyfuel combustion process.15 The weakest point of YBaCo4O7+δ is that it decomposes upon heating in an oxygen-containing atmosphere at a relatively low temperature, just above 600 °C.2 This is believed to be a major problem in applications that involve, e.g., processing steps at elevated temperatures. Fortunately, because of the flexibility of the YBaCo4O7+δ structure, it is possible to employ various chemical substitutions to enhance the phase stability of YBaCo4O7+δ at high temperatures.16−18 The parent oxygen-depleted YBaCo4O7 structure has orthorhombic symmetry and consists of two © 2013 American Chemical Society
types of corner-sharing CoO4 tetrahedra (with a ratio of 1:3) located in separate, alternately stacked layers of triangular and Kagomé types. Yttrium and barium provide the proper spacing in the structure. In this study, we demonstrate, with Sr-for-Ba and R-for-Y (where R is a rare-earth element) substitutions, that the smaller the unit-cell volume, the higher the phasedecomposition temperature. The drawback related to such substitutions is that, along with the enhanced phase stability (and decreased cell volume), both the oxygen storage capacity and the oxygen diffusion rate are reduced. As another interesting option, we also report our systematic efforts to enhance the performance of YBaCo4O7+δ through various Cosite cation substitutions. Such substitutions have been employed by other research groups to shed light on the exciting low-temperature magnetic properties of the parent YBaCo4O7 phase with a Kagome-type CoO2 framework,19−21 but not from the viewpoints of oxygen storage capability and phase stability. From the previous studies, it is known that cobalt in YBaCo4O7+δ can be partly or fully substituted at least with Zn, Al, Ga, Fe, and Ni.22−24 Very recently, we revealed that the phase stability of YBaCo4O7+δ is markedly enhanced by partial Al-for-Co17 and Ga-for-Co18 substitutions. Here, we demonstrate that the phase decomposition can be completely prevented without significantly sacrificing the oxygen storage capacity of YBaCo4O7+δ through cosubstituting ∼15% of the Co with Al and Ga. We foresee that our cosubstituted Received: November 30, 2012 Revised: January 25, 2013 Published: January 25, 2013 599
dx.doi.org/10.1021/cm3038729 | Chem. Mater. 2013, 25, 599−604
Chemistry of Materials
Article
Figure 1. TG curves recorded for YBaCo4O7+δ in an O2 gas flow: (a) up to 1000 °C with a heating rate of 1 °C/min and (b) up to 500 °C with six different heating rates of 0.5, 1, 2, 5, 10, and 20 °C/min.
2. EXPERIMENTAL SECTION
The very maximum amount of oxygen that the samples were able to absorb was investigated under high-pressure conditions; the highpressure oxygenation was carried out in an autoclave. The pressure vessel was first rinsed with oxygen to remove all air and then filled with O2 gas. The system then was filled with oxygen (∼75 bar) and heated to a temperature of 280 °C, at which point the target pressure of ∼110 atm was reached. The temperature was kept at 280 °C for 10 h, and the resultant oxygen content was determined by iodometric titration.
Three sample series, i.e., RBaCo4O7+δ (R = Y, Dy, Er, Yb, Lu), Y(Ba1‑xSrx)Co4O7+δ (x = 0−0.2), and YBa(Co0.95M0.05)4O7+δ (M = Al, Mn, Fe, Ni, Cu, Zn, Ga), were synthesized using appropriate amounts of the starting material powders, R2O3 (99.9%), BaCO3 (>99%), SrCO3 (99.9%), Co3O4 (99.7%), Al2O3 (99.9%), MnCO3 (99.9%), Fe(C2O4) (99%), Ni(CH3COO)2·4H2O (>99%), CuO (99.6%), ZnO (99.999%) bubbling. The iodine formed in the redox reaction between trivalent cobalt and iodide was titrated with standard 0.010 M Na2S2O3 solution (Merck) in the presence of a starch indicator. Each experiment was repeated three times with excellent reproducibility. The oxygen absorption/desorption properties and the phasedecomposition characteristics were investigated by means of thermogravimetry (TG) (Perkin−Elmer, Model Pyris 1 TGA). In these experiments, a sample specimen of ∼20 mg was slowly (1 °C/ min) heated from room temperature to 1000 °C in an O2 gas flow of 40 mL/min. Although all samples were annealed in argon for x ≈ 0 before the XRD measurements, this was not done before all the TG measurements. To overcome this, we normalized the TG data to δ = 0 at 600 °C. Here, it should be emphasized that this operation ended up having room-temperature oxygen content values, which agreed very well with the values determined by iodometric titration for the same samples.
3. RESULTS AND DISCUSSION Establishing the Proper Heating Rate in TG Measurements. In Figure 1a, we show a representative TG curve for the YBaCo4O7+δ compound recorded upon heating oxygendepleted (δ ≈ 0) powder in O2 gas flow (here, up to 1000 °C, using a heating rate of 1 °C/min). The curve exhibits two distinct humps: the hump in the lower-temperature region (200−400 °C) reflects the unique ability of YBaCo4O7+δ to absorb and desorb large amounts of oxygen at appreciably low temperatures, while the hump in the high-temperature region (above 600 °C) is due to the oxidative phase decomposition of YBaCo4O7+δ to BaCoO3‑δ and other binary/ternary decomposition products.2 The goal in the present study is to determine a way to suppress the second hump, i.e., to avoid the phase decomposition or at least to shift the decomposition to a higher temperature, without markedly sacrificing the lowtemperature oxygen-storage capability. A TG measurement carried out in an O2 atmosphere is an easy tool to judge the impacts of various cation substitutions on the oxygen storage characteristics and phase stability of YBaCo4O7+δ. In order to find a proper heating rate for the measurements, we recorded TG curves for YBaCo4O7+δ with various heating rates from 0.5 to 20 °C/min up to 500 °C (see Figure 1b). It is evident that, as the heating rate increases, the onset temperature for the oxygen absorption increases while the oxygen-desorption temperature remains less affected. Hence, with the faster heating rates, oxygen loading remains incomplete and the maximum (thermodynamic) oxygen content is not reached before the oxygen desorption starts. Only the two TG curves recorded with the slowest heating rates of 0.5 and 1 °C/min are essentially identical; with both of these heating rates, a clear plateau or saturation exists for the oxygen content at 7 + δ ≈ 8.2 in the temperature range of 280−360 °C. Hence, to be able to estimate the oxygen-absorption capacity of YBaCo4O7+δ-type materials (in powder form) properly from a dynamic TG experiment, the heating rate should not be higher than 1 °C/ min. Based on this observation, all of our further TG experiments for the variously cation-substituted samples were carried out with a fixed heating rate of 1 °C/min.
YBa(Co,Al,Ga)4O7+δ materials are highly potential candidates for next-generation oxygen storage materials that may be utilized in various applications requiring high OSC values, high thermal stability, and the possibility for efficient operation at low temperatures.
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dx.doi.org/10.1021/cm3038729 | Chem. Mater. 2013, 25, 599−604
Chemistry of Materials
Article
Solubility Limits of Various Cation Substituents. Synthesis of the parent phase, YBaCo4O7+δ, from stoichiometric quantities of Y2O3, BaCO3, and Co3O4 under the selected conditions (in air at 1050−1200 °C) yields samples of high quality, with typically
