No abstract

0.1 O 3 δ (M = Fe, Ni, Co, and Yb) (BZCY-M) oxides were − synthesized using the conventional solid-state reaction method at 1350-1550°C in air in order to investigate the effect of dopants on sintering, crystal structure, chemical stability under CO 2 and H 2 S, and electrical transport properties. The formation of the single-phase perovskite-type structure with an orthorhombic space group Imam was confirmed by Rietveld refinement using powder X-ray diffraction for the Fe, Co, Ni, and Yb-doped samples. The BZCY-Co and BZCY-Ni oxides show a total electrical conductivity of 0.01 and 8 × 10 −3 S cm −1 at 600°C in wet H 2 with an activation energy of 0.36 and 0.41 eV, respectively. Scanning electron microscope and energy-dispersive X-ray analysis revealed Ba and Co-rich secondary phase at the grain-boundaries, which may explain the enhancement in the total conductivity of the BZCY-Co. However, ex-solution of Ni at higher sintering temperatures, especially at 1550°C, decreases the total conductivity of the BZCY-Ni material. The Co and Ni dopants act as a sintering aid and form dense pellets at a lower sintering temperature of 1250°C. The Fe, Co, and Ni-doped BZCY-M samples synthesized at 1350°C show stability in 30 ppm H 2 S/H 2 at 800°C, and increasing the firing temperature to 1550°C, enhanced the chemical stability in CO 2 /N 2 (1:2) at 25-900°C. The BZCY-Co and BZCY-Ni compounds with high conductivity in wet H 2 could be considered as possible anodes for intermediate temperature solid oxide fuel cells. Citation: Mirfakhraei B, Ramezanipour F, Paulson S, Birss V and Thangadurai V (2014) Effect of sintering temperature on microstructure, chemical stability, and electrical properties of transition metal or Yb-doped BaZr 0.1 Ce 0.7 Y 0.1 M 0.1 O 3−δ (M = Fe, Ni, Co, and Yb). Front. Energy Res. 2:9.

In this paper, we report our steps forward in the search of proton conducting metal oxides possessing disordered perovskite-type and B-site ordered double perovskites for application in solid oxide fuel cells (SOFCs) and mixed proton-electron conductors as anodes for SOFCs. Role of A- and B- site cations in the perovskite structures on electrical properties and their corresponding chemical stability in CO2, H2S and H2O have been studied. Partial substitution of the parent phase is one of the most effective and popular approaches in which certain ions in the lattice are replaced by foreign species. However, "the right recipes" are not usually obvious; optimization on the functional physical and chemical properties is often based upon "the trial and error" strategy. Here, we review our progress since 2005 in the development of the proton conducting solid oxide fuel cells (H-SOFCs).

Here we compare the electrochemical properties of a porous 10 mol. % Gd-doped CeO2 (GDC) anode with those of the same material, but infiltrated with a new plasma spray synthesized 10 mol. % Mo-doped CeO2 nano material (nCMO), all at 600-800 oC in H2, with and without added 10 ppm H2S. While both of these anode materials exhibit very good H2 oxidation activity, their performance does deteriorate when exposed to H2S. However, the nCMO-infiltrated GDC anode is significantly more stable and shows much better sulfur tolerance than the single-phase GDC anode. Furthermore, the response of both anodes is reversible, at least in early exposures, and the original H2 oxidation activity is regained fully when H2S is removed from the fuel stream. At low temperatures (ca. 500oC), these anode materials, contacted to a gold paste current collector, exhibit a dramatic response to H2S, making them of potential use as a H2S sensor.

Solid oxide fuel cells (SOFCs) are alternative energy conversion devices, which deliver high quality power with a high efficiency using variety of fuels, including H2 and hydrocarbons. At present, composite Ni with Y-stabilised zirconia (YSZ) is widely used as an anode for SOFCs [1]. However, technical challenges due to coking and sulfur poisoning are yet to overcome when direct hydrocarbon-containing fuels are used. Ni not only possess good catalytic activity towards H2 oxidation and hydrocarbon reforming, it also catalyses carbon formation and poorly resists sulfur poisoning [2]. To overcome these problems, several alternatives such as Cu/CeO2/YSZ [3] and LSCM (La1-xSrxCr1-xMnxO3) [4] have been investigated. Though, these materials were able to resist coke and sulfur poisoning, they were not able to satisfy all the requirements of ideal SOFC anodes. In the present study, mixed ionic electronic conducting (MIECs) Ce0.8Y0.1Mn0.1O2- d and BaZr0.1Ce0.7Y0.1M0.1O3−δ (M = Ni and Co), and composite Ni and Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3-δ (BSCZGY) have been investigated as SOFC anodes. The cell performance of a button cell: Ce0.8Y0.1Mn0.1O2- d/Zr0.84Y0.16O2- d/La0.6Sr0.4MnO3+Zr0.84Y0.16O2- d was tested at 800 oC in 3 % H2O-H2 as a fuel and air as an oxidant. There was an obvious enhancement of the cell performance upon exposure to 10 ppm H2S-H2 for 48 h. Another area of the MIEC anodes development involves investigation of perovskite-type BaZr0.1Ce0.7Y0.1M0.1O3−δ (M = Ni and Co). The BaZr0.1Ce0.7Y0.1Ni0.1O3−δ compound shows a promising low polarization resistance of 0.4 Ω.cm2 at 800oC in H2. Low polarization resistance of the BaZr0.1Ce0.7Y0.1Ni0.1O3−δ perovskite, as a non-composite anode without any additional electron conductive phase, makes it a promising material for SOFC anodes. Proton conducting doped BaCeO3 materials is the frontrunners as electrolyte in the field of low temperature (400-700 oC) proton-SOFCs. A and B site co-doped Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3-δ has shown good chemical stability in CO2 and humid atmospheres at elevated temperature [5]. In the present work, anode performance of Ni-BSCZGY composites (with Ni: BSCZGY ratios as 30:70, 40:60 and 50:50) has been studied. Screen-printing and co-firing processes were employed to make symmetrical cell: Ni-BSCZGY/BSCZGY/Ni-BSCZGY. Area specific resistance (ASR), capacitance and activation energy values were studied to evaluate the electrochemical performance of the anode composites. 50:50 composite shows the lowest ASR value among the current investigated anode compositions (30:70 and 50:50). References 1)     M. Ihara, T. Kusano and C. Yokoyama, J. Electrochem. Soc., 148, A209 (2001). 2)     Zhe Cheng and Meilin Liu, Solid State Ionics, 178, 925 (2007). 3)     H. Kim, S. Park, J.M. Vohs and R.J. Gorte, J. Electrochem. Soc., 148, A693 (2001). 4)     S. Tao and J.T.S. Irvine, Nature Letters, 2, 320 (2003). 5)    R. Kannan, S. Gill, K. Singh, T. Fürstenhaupt and V. Thangadurai, Sci. Report, 3, 2138 (2013).