In Situ Growth of Nanoparticles in Layered Perovskite La0.8Sr1.2Fe0

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In situ Growth of Nanoparticles in Layered Perovskite La0.8Sr1.2Fe0.9Co0.1O4-# as an Active and Stable Electrode for Symmetrical Solid Oxide Fuel Cells Jun Zhou, Tae-Ho Shin, Chengsheng Ni, Gang Chen, Kai Wu, Yonghong Cheng, and John T S Irvine Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00071 • Publication Date (Web): 11 Apr 2016 Downloaded from http://pubs.acs.org on April 14, 2016

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Chemistry of Materials

In situ Growth of Nanoparticles in Layered Perovskite La0.8Sr1.2Fe0.9Co0.1O4-δ as an Active and Stable Electrode for Symmetrical Solid Oxide Fuel Cells

Jun Zhou,* †,‡ Tae-Ho Shin,‡ Chengsheng Ni,‡ Gang Chen,† Kai Wu,† Yonghong Cheng, † John T. S. Irvine*,‡ †

Center of Nanomaterials for Renewable Energy, State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an

Jiaotong University, Xi’an 710049, People’s Republic of China ‡

School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9ST, Scotland, United Kingdom

*Address correspondence to: [email protected] (J. Zhou) and [email protected] (J. TS. Irvine)

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Abstract: Compared to traditional deposition techniques, in situ growth of nanoparticles on material surfaces is one of more time- and cost-effective way to design new catalysts. The B-site transition metal cations in perovskite lattice could be partially exsolved as nanoparticles upon reducing condition, greatly enhancing catalytic activity. Here, we demonstrate that growing nanoparticles on the surface of a layered perovskite La0.8Sr1.2Fe0.9Co0.1O4±δ (LSFC), which could be applied as a redox stable and active electrode for intermediate-temperature symmetrical solid oxide fuel cells (IT-SSOFCs). Substitution of a proper amount of Co into the layered perovskite can thus optimize cathode and anode performance simultaneously. For example, the polarization resistances (Rp) of LSFC electrode at 800 oC are 0.29 and 1.14 Ω cm2 in air and in 5% H2/N2 respectively, which are much smaller compared with the Rp of Co-free La0.8Sr1.2FeO4±δ. The lower polarization resistance for LSFC in air can be mainly attributed to the enhanced electrical conductivity through the partial substitution of iron by cobalt in La0.8Sr1.2FeO4±δ. Meanwhile, the electrocatalytic activity of H2 greatly improved due to the formation of exsolved homogeneous Co0 nanoparticles on the surface of LSFC, which appears to promote hydrogen oxidation reaction. Lower polarization resistance of 0.21 Ω cm2 in air and 0.93 Ω cm2 in 5% H2/N2 at 800 oC could be obtained further by examined LSFC-Gd0.1Ce0.9O2-δ (CGO) composite as electrode for IT-SSOFCs.

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INTRODUCTION In the coming decades, more and more serious energy issues will emerge due to the lack of efficient renewable energy sources and the likely depletion of fossil fuel reserves. Solid oxide fuel cells (SOFCs), which were considered as promising electrochemical devices of energy conversion,1-5 offer an alternative solution to the looming energy crisis. A significant advantages of SOFCs is fuel flexibility because of their high efficiency in energy conversion when multiple gas fuels were used, such as hydrogen, hydrocarbons, and biofuels et. al.6-10 Generally, SOFCs have traditionally been comprised of four elements, with ytrria-stabilised zirconia (YSZ) used as the electrolyte, porous Ni/YSZ cermet as the anode, porous (La,Sr)MnO3 (LSM) as the cathode, and La1-xSrxCrO3, Cr alloys or steels as interconnect materials.11,12 However, a new configuration of symmetrical SOFC may replace the traditional SOFC configuration because the same material could be applied as both anode and cathode simultaneously.13,14 There are several advantages to using this symmetrical SOFC configuration. First, the production of SOFCs can be simplified significantly, because only one thermal step is required during the fabrication of the dense electrolyte and the porous symmetrical electrodes. In addition, the application of the same material as symmetrical electrode can also minimize the problem of compatibility.15 Moreover, the problems of possible sulphur poisoning and coke formation on the anode can potentially be addressed by simply reversing the flow of gas, operating the anode as a cathode. Thus, this state-of-the-art approach might allow for a higher tolerance of sulfur and carbon containing fuels. Despite their operating in different atmospheric conditions, anode and cathode require high electronic conductivity, thermal and phase stability, and proper catalytic activity et .al.14 3



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Recently, a number of electrodes13,16-24 indicate that symmetrical SOFCs are a valuable alternative to traditional SOFCs. However, a lower temperatures operating range of 500-800 o

C is required in order to make symmetrical SOFCs economically competitive with existing

technology, such as the conventional high temperature SOFC. It is a challenge to find proper electrode materials for symmetrical SOFC which have good electrochemical performances at lower temperatures. Thus, novel electrode materials require to be developed to improve the properties of symmetrical SOFCs at intermediate temperatures. K2NiF4-type structural perovskite oxides have shown a proper stability in reducing conditions25 because of the accommodation ability in a wide range of coordination environments. Several typical Ruddlesden-Popper oxides, such as Sr3-xFe2-yCoyO7-δ26 La0.8Sr1.2Co0.5M0.5O4-δ (M=Fe, Mn),27 and Sr3Fe2O7-δ28 have shown reasonably good structural stability and proper conductivities in reducing atmospheres at elevated temperatures, which make these oxides alternative materials for SOFCs and oxygen separation membranes. Moreover, it has been reported that LaSrCo0.5Fe0.5O4 is stable under reducing conditions (10% H2/N2) up to 800 oC by decreasing the oxidation states of cobalt from +3 to +2 and formation of the nonstoichiometric phase LaSrCo0.5Fe0.5O3.75.27 In addition, due to various electrical and catalytic applications, K2NiF4-type oxides such as La2MO4 (M=Ni, Cu, Fe, Co, Mn) have recently attracted much attention as possible cathode materials for SOFCs.29-39 Our previous works also showed that La0.6Sr1.4MnO4 and LaxSr2-xFeO4 are stable under reducing conditions and could be used as electrodes for symmetrical SOFCs. However, the electrochemical properties need be improved in further.40, 41 Thus, it is of great significance and interest to develop active electrode materials that 4


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possess combined properties with K2NiF4-type phase applied as both anode and cathode. Previous studies have demonstrated that enhancement of an electrode’s electrochemical properties for a traditional SOFC has been achieved through a surface decorated with uniformly dispersed, catalytically active nanoparticles.42,

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Generally, several deposition

techniques such as physical vapour deposition44, 45 and chemical impregnation46 were used to grow catalytic nanoparticles on external surfaces. However, it is difficult to control over the distribution, size, and anchorage of the deposited materials during synthesis and ageing through these traditional methods. Recently, an alternative method has been to incorporate transition metal atoms into a host perovskite lattice during preparation in oxidizing atmospheres and exsolved at the surface as nano-sized metallic particles under reducing conditions (i.e., in situ growth).47-49 Although this strategy has been demonstrated mainly for perovskite structural oxides, until now few similar phenomena in K2NiF4 structural compounds have been reported. Here, we describe a new layered perovksite La0.8Sr1.2Fe0.9Co0.1O4±δ (LSFC), designed to exolve B-site transition metal (Co0) in the form of electrocatalytically active nano-sized metallic particles under reducing conditions. The objective of this study was also to understand lanthanum strontium ferrites with low contents of cobalt-doping by evaluation of the alternative symmetrical SOFC electrode material with better redox stability and promising properties. EXPERIMENTAL SECTION La0.8Sr1.2Fe0.9Co0.1O4-δ (marked as LSFC) powders were synthesized by the solid-state reaction method. In this work, starting materials were La2O3 (AR), SrCO3 (AR), Co3O4 (AR) 5



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and Fe2O3 (AR). In order to remove residual water or carbonate, all metal-oxides were calcined in air at 700 oC for 8 h. Stoichiometric quantities of pre-fired materials were ball milled for 2 h at 160 rpm and pellets were pressed by a hydraulic press. Then, the pellets were calcined in Ar at 1380 oC for 10 h. LSFC powders were obtained finally by breaking up the pellets and an intensive milling step at 400 rpm for 1 h. The powders were transferred into an alumina boat, and calcined in a tube furnace at 850 °C for 20 h in a flow of 5% H2/Ar, so that the phase stability of the electrode material was investigated. The final powders were ball-milled and marked as LSFCR. Furthermore, LSFCR was calcined in air at 850 oC for 10 h and 20 h respectively to prepare the re-oxidized samples (marked as LSFCO). The phases of these samples were identified by a Philips Model PW1050 X-ray diffractmeter (XRD), operated at 40 kV and 30 mA using Cu Kα radiation. Diffraction peaks were refined using FullProf software.50 Thermogravimetric analysis (TGA) was carried out by a Netzsch instrument (Model STA 449C) which was equipped with Proteus thermal analysis software. The oxygen stoichiometry of these three samples was determined by the iodometric titration method.51 The surface morphology of the powders and the cross-section of single cells were determined by a scanning electron microscopy (SEM) (JEOL Model JSM 6700F). Transmission electron microscopy (TEM) was performed by a TEM microscope (JEOL Model JEM-2011) equipped with an Oxford Link Isis X-ray EDS system. The elemental compositions were investigated by an Inductively Coupled Plasma-Atomic Emission Spectrometer (IRIS, Advantage ER/S, ICP-AES). X-ray photoelectron spectroscopy (XPS) was characterized by Thermo Scientific K-Alpha (USA) to analyze the oxidation states and relative abundance of the elements at the surface of particles. The binding energy scale 6


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was internally calibrated with respect to the carbon contamination using C 1s peak at 284.5 eV. A nonlinear Shirley-type background method was used to ananyze the core peaks. In the fitting, the peak positions and areas were optimized by Lorentzian-Gaussian method. The data of the surface atomic ratio of each metal was estimated by calculating the ratio between peak area and relative sensitivity factor (R.S.F). The pressed pellets of LSFC were sintered at 1500 oC for 10 h in order to measure the electrical conductivity. The sintered pellets have shown relative densities larger than 90%. The four-probe method was applied to test the conductivity of pellets and gold pastes and wires were used as current collector. Samples were characterized in a furnace and the oxygen partial pressure (PO2) was determined by a zirconia PO2 sensor. During the measurements, the isothermal PO2-dependency of electrical conductivity was determined by starting at low PO2 in 5% H2/N2 (3 vol.% H2O) gas flow. The 5% H2/N2 (3 vol.% H2O) was then replaced by N2 and changing slowly toward ambient PO2 by switching off the N2 gas flow and following the entrance of air into the furnace. Measurement of electrical conductivity after each redox cycle was performed and obtained until equilibrium was achieved by using the same procedure. La0.9Sr0.1Ga0.8Mg0.2O3-δ (LSGM) powders were synthesized by solid-state reaction method. Stoichiometric quantities of La2O3 (AR), SrCO3 (AR), Ga2O3 (AR), and MgO (AR) were mixed and sintered in air at 1350 oC for 10 h. The XRD analyzes showed that a single LSGM phase was obtained. Then, the mixture was pressed into pellets (thickness:1～1.2 mm) and sintered at 1500 oC for 10 h in air. Generally, significant decrease of series and polarization resistance of anode was obtained when a multilayer anode was fabricated.17 In this work, the electrodes were graded: first and 7



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second layer 75 wt.% LSFC+25 wt.% LSGM; third, fourth, and fifth layer pure LSFC, or 70 wt.% LSFC-30 wt.% CGO mixed powder. The powders of LSFC, LSGM, or CGO were mixed thoroughly for the preparation of inks. The mixtures including 2.0 wt.% HypermerTM D1 and a vehicle consisting of 5 wt.% poly (vinyl butyrate) in Terpineol were used as dispersant in this work. The final electrode inks were printed by a semi-automatic screen-printer (DEK 248) (325 mesh) on both sides of LSGM pellets. The multilayer electrodes were dried at 110 oC between coating each layer and finally fired at 900 oC, resulting in a graded electrode instead of multilayer. The area of the working electrode is about 1.02 cm2. Gold paste was then used for current collection in impedance tests, while silver mesh together silver paste was used in fuel cell tests. The electrochemical impedance spectroscopies (EIS) were performed by a Solartron 1255 (Schlumberger) frequency response analyser (0.01 Hz～1 MHz) coupled with a 1287 electrochemical interface. EIS data were also fitted by the ZView software (Scribner) with an equivalent circuit. To investigate PO2 or PH2 dependence of the polarization processes, the cathode was operated under ambient pressure with different N2/O2 mixtures and varying N2/H2 (3 vol% H2O) mixtures at the anode side. When testing the I-V curves of single cells, the fabricated symmetrical cells were sealed onto a ceramic tube using high temperature resistant sealant. The cathode was exposed to ambient air, while the humidified H2 (3 vol.% H2O) gas flow was the fuel at anode with flow rate of 25 mL min-1. RESULTS AND DISCUSSION Chemical stability under oxidizing and reducing atmospheres are significant requirements for an electrode material applied in symmetrical SOFCs. Thus, we assessed the chemical stability 8


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of LSFC in 5% H2/Ar at 850 oC for 20 h (LSFCR). Then, LSFCR was re-oxidized in air at 850 o

C for 20 h (LSFCO). The refined room temperature XRD patterns of LSFC, LSFCR, and

LSFCO powders are depicted in Figure 1(a-c). The results showed that all phases exhibit similar K2NiF4 layered-perovskite structure, although a small Co metal peak was observed in Figure 1d. Furthermore, the lattice parameter a decreases slightly while c increases significantly after reduction (Table 1). The lowering of the coordination number could lead to a decrease of the parameter a and loss of lattice oxygen can cause such a lattice contraction.52 It noted that the cobalt-containing K2NiF4 structural oxides with oxygen deficiency have various types of unusual geometries with lower coordination number (

In Situ Growth of Nanoparticles in Layered Perovskite La0.8Sr1.2Fe0

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