Characterization of Doped Yttrium Chromites as Electrodes for Solid Oxide Fuel Cell by Impedance Method

Abstract

Redox-stable ceramic electrodes, Co and Ni doped yttrium chromites, for solid oxide fuel cell (SOFC) are characterized by electrochemical impedance spectroscopy (EIS) in various P O2 and P H2 atmosphere. The polarization resistances for Y 0.8 Ca 0.2 Cr 0.8 Co 0.2 O 3-δ (YCCC) and Y 0.8 Ca 0.2 Cr 0.9 Ni 0.1 O 3-δ (YCCN) on yttrium stabilized zirconia (YSZ) electrolyte are 0.96 and 8.4 cm 2 in wet H 2 , 1.2 and 36.7 cm 2 in air at 850 • C, respectively. For the anode application, the rate-limiting steps (RDS) are identified as charge transfer and surface diffusion for both YCCC and YCCN. The primary active zone in YCCN is three-phase boundary (3PB) but extends to a small portion of the electrode bulk in the YCCC anode. For the cathode application, O 2 dissociative adsorption or diffusion is one of the RDS for both electrodes, and the active zone is limited to the 3PB area. The influence of electrolyte to electrode performance is investigated by replacing YSZ electrolyte with scandium stabilized zirconium (SSZ). Smaller polarization resistances are observed on each electrode in both wet H 2 and air atmospheres. Replacement of electrolyte can alter not only the rate of charge transfer process but also in some cases other surface processes not related to the electrolyte directly. It is proposed that the impact of the electrolyte on each electrode process is passed down as in a chain and the charge transfer step functions as the first ring in the chain. The best performance is obtained with the YCCC/SSZ combination, 0.49 and 1 cm 2 in wet H 2 and air at 850 • C, respectively, making YCCC a promising electrode.Yttrium chromites were proposed as alternative interconnect materials in SOFC applications due to the advantages found in its chemical compatibility with YSZ electrolyte and smaller chemical expansion. [1][2][3][4] With high tolerance to the reducing atmosphere, they can also work as excellent parent materials so that different transition metals whose oxides are otherwise not stable in the typical anode atmosphere can be incorporated into the chromium site. By substitution of yttrium and/or chromium site elements, the thermal expansion coefficient, electrical conductivity and catalytic property could be tailored to meet the anode requirements. 5-7 Recently, Ca & Co co-doped YCrO 3 (Y 0.8 Ca 0.2 Cr 0.8 Co 0.2 O 3-δ ) was developed successfully as an anode candidate by Yoon et al. 7 Decent performance and good tolerance toward S-poisoning were demonstrated: a 0.3 cm 2 polarization resistance for the fuel cell operated at 850 • C in H 2 with 20 ppm H 2 S.Besides this pioneering study on the performance of this material system, investigations regarding other electrochemical properties of such materials are still limited in literature. The catalytic mechanism study among these investigations is crucial to understand the reaction pathway and guide the optimization of the materials. For instance, the oxygen reduction reaction (ORR) of typical perovskite cathodes like (LaSr)MnO 3 (LSM) 8-12 and (LaSr)(CoFe)O 3 (LSCF) 13-1...

Oxygen Reduction Reaction Kinetics in Sr-Doped La2NiO4+δRuddlesden-Popper Phase as Cathode for Solid Oxide Fuel Cells

“…Those results are consistent with the ones reported by Aguadero et al 24 According to the fitting results, the small capacitances and the large characteristic frequencies for the HF arcs of Sr30 and Sr40 indicate that those arcs can be reasonably associated to a charge transfer process. [34][35][36] The incorporation of oxygen into cathode lattice has been taken into account in the ALS element. Meanwhile, the electron transfer from current collector to the cathode must not be the case because Sr30 and Sr40 possess higher electrical conductivities than LNO.…”

Positive Effects of H2O on the Hydrogen Oxidation Reaction on Sr2Fe1.5Mo0.5O6−δ-Based Perovskite Anodes for Solid Oxide Fuel Cells

“…Two kinds of anodes are widely used in the solid oxide fuel cells (SOFCs), including the cermet anodes (e.g., Ni–yttrium-stabilized zirconia (YSZ)) − and all-ceramic anodes such as perovskite structure oxides. − The steam is commonly supplied together with H 2 fuel to the Ni/YSZ anodes in the SOFCs, demonstrating apparent improvement of their electrochemical performance in humidified H 2 compared to that in dry H 2 . − Two categories of mechanisms have been proposed to elucidate the underlying insights into the enhanced hydrogen oxidation reaction (HOR) kinetics under the H 2 flow with steam. On the one hand, Bessler et al reported that the activating effect of water on the HOR kinetics can be only ascribed to equilibrium-potential effect at the Ni/YSZ anodes, without the necessity of assuming additional kinetic or catalytic effect .…”

Role of Adsorbate Coverage on the Oxygen Dissociation Rate on Sr-Doped LaMnO3 Surfaces in the Presence of H2O and CO2

“…Sr-doped LaMnO 3 (LSM) is one of the most studied perovskite materials for solid oxide fuel cell (SOFC) cathodes because of its high electronic conductivity, good oxygen reduction kinetics, and thermal and chemical stability with yttria-stabilized zirconia. − For the same reasons, LSM has also been applied in composite electrodes in solid oxide electrolyser cells and catalytic layers and/or electronically conducting phases for mixed conducting membranes and catalytic membrane reactors . At intermediate temperatures (600–800 °C), the main limiting factor for the performance of SOFCs has been ascribed to the oxygen reduction reaction (ORR) at the cathode. , Oxygen surface exchange and surface diffusion of oxide ions have been identified as two key processes that control the ORR . Surface exchange kinetics remains an elusive property because it involves several steps including adsorption and dissociation of molecular oxygen, reduction (charge transfer), and incorporation of oxygen species into the cathode or electrolyte.…”