Degradation mechanism of electrolyte and air electrode in solid oxide electrolysis cells operating at high polarization Jeonghee Kim a,b , Ho-Il Ji a , Hari Prasad Dasari a , Dongwook Shin c , Huesup Song d , Jong-Ho Lee a , Byung-Kook Kim a , Hae-June Je a , Hae-Weon Lee a , Kyung Joong Yoon a, * a High-Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, South Korea b Department of Fuel Cells and Hydrogen Technology, Hanyang University, Seoul, South Korea c Division of Materials Science and Engineering, Hanyang University, Seoul, Korea d Division of Advanced Materials Engineering, Kongju National University, Chonan, South Korea article info Article history: Received 20 August 2012 Received in revised form 28 October 2012 Accepted 29 October 2012 Available online 29 November 2012 Keywords: Solid oxide electrolyzer Impedance spectroscopy Anodic current Degradation Densification abstract Degradation mechanism of the electrolyte and air electrode is reported for solid oxide electrolysis cells (SOECs). Symmetric cells composed of yttria-stabilized zirconia (YSZ) electrolyte, Sr-doped LaMnO 3d (LSM)/YSZ composite working and counter electrodes, and Pt ring-type reference electrode are used to simulate the operating conditions of the air electrode. Degradation behavior in the impedance spectra is characterized as growth of mid-frequency arc at the initial stage, gradual increase of ohmic resistance throughout the operation, and sharp rise of low frequency resistance at the final stage, followed by cata- strophic cell failure. Initial stage degradation is attributed to deactivation of LSM, resulting from reduction of oxygen vacancy concentration and/or segregation of passivation species on LSM surface under anodic current passage. Intergranular fracture, which occurs along the grain boundaries of the YSZ electrolyte, is responsible for gradual increase of ohmic resistance. Increase of low frequency arc at the final stage is caused by densification of the air electrode, leading to excessive pressure build-up and delamination of the air electrode. Cation migration, which is facilitated by oxygen excess nonstoichiometry of LSM and externally applied electric field, is considered to be the main cause of permanent damages. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction Solid oxide electrolysis cells (SOECs) represent one of the most environmentally clean technologies for efficient and economic conversion of steam into hydrogen, especially when utilizing electricity and unused heat supplied from renewable energy sources, nuclear power plants and high-temperature industrial processes [1e3]. Materials and fabrication tech- niques used for solid oxide fuel cells (SOFCs) can be directly applied to SOECs since the operating temperatures of SOECs and SOFCs are similar. High temperature operation of SOECs offers inherent advantages in thermodynamics and kinetics over the low-temperature electrolysis because the electrical energy demand and electrode overvoltage for steam decom- position decrease with increasing temperature [4e6]. Thus, initial development of SOECs started in the 1980’s as an alternative to conventional alkaline electrolysis [5,7,8], but relatively few research works were performed in the 1990’s * Corresponding author. Tel.: þ82 2 958 5515; fax: þ82 2 958 5529. E-mail address: kjyoon@kist.re.kr (K.J. Yoon). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 38 (2013) 1225 e1235 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.10.113