Life Testing of Ni-YSZ Fuel Electrodes under Electrolysis, Fuel Cell, and Current Switching Operation

Abstract

Most studies of solid oxide cell degradation during electrolysis or reversible current-switching operation have focused on the oxygen electrode.1 However, one recent study has shown that Nickel/yttria-stabilized-zirconia (Ni-YSZ) fuel electrodes coupled with YSZ electrolytes may also degrade when operated under applied electrolysis current.2 Most studies of Ni-YSZ degradation have focused on fuel cell operating conditions, including the effect of temperature3 and fuel gas composition.4 Currently, relatively little is known about the electrolysis operating conditions, e.g., temperature and current density, that lead to degradation, and no studies have been reported to date on current switching operation. In the present work, a degradation study of Ni-YSZ electrodes on YSZ electrolytes was carried out using a symmetric cell geometry. The cells are tape-cast co-fired structures consisting of thin YSZ electrolytes sandwiched between thick Ni-YSZ electrodes. This geometry is meant to mimic the structure of Ni-YSZ anode-supported cells, and has the advantages that: (1) the life tests are simplified compared to full cells, with no gas sealing required; (2) unlike full cells, there is no need to separate out responses from an oxygen electrode in impedance spectra; (3) in current-switched testing, both Ni-YSZ electrodes tend to evolve in the same way, such that the cells remain electrochemically symmetrical; and (4) in dc-current testing, results are obtained on both fuel cell and electrolysis operation, as one electrode operates under anodic conditions and the other operates under cathodic conditions. The Ni/YSZ|YSZ|Ni/YSZ cells were tested at 800 °C in 97% H2 – 3% H2O for up to 1000 h, with current density ranging from 0.2 to 1.2 A/cm2. Electrochemical impedance spectroscopy measurements were made periodically with the current set to zero, during short interruptions of the life tests. At 0.2 A/cm2, there was little change in the ohmic or polarization resistance during the tests, and post-test SEM microstructural evaluation showed no measurable change. At the higher current density of 0.8 A/cm2, on the other hand, substantial microstructural and electrochemical changes were observed. For current-switching operation, the ohmic resistance increased slightly, while the polarization resistance showed a decreasing tendency. The main microstructural change was the observation of nanoparticle formation in the Ni-YSZ electrode functional layers. The nanoparticles contain Ni, Yittria and zirconia, which potentially increased the triple phase boundary density, possibly explaining the decrease of polarization resistance. For dc current operation, the ohmic resistance increased relatively rapidly, while the polarization resistance dropped slowly. Nanoparticles were observed on both electrodes after the test, but with different morphologies. In addition, cross-sectional SEM images showed intergranular fracture within the YSZ electrolyte, due to grain boundary voids that formed during the dc current test, presumably explaining the observed electrolyte resistance increase. Reference 1 Call, A. V., Railsback, J. G., Wang, H. Q. & Barnett, S. A. Degradation of nano-scale cathodes: a new paradigm for selecting low-temperature solid oxide cell materials. Phys Chem Chem Phys 18, 13216-13222 (2016). 2 Chen, M. et al. Microstructural Degradation of Ni/YSZ Electrodes in Solid Oxide Electrolysis Cells under High Current. J Electrochem Soc 160, F883-F891, doi:10.1149/2.098308jes (2013). 3 Kennouche, D., Chen-Wiegart, Y. C. K., Cronin, J. S., Wang, J. & Barnett, S. A. Three-Dimensional Microstructural Evolution of Ni-Yttria-Stabilized Zirconia Solid Oxide Fuel Cell Anodes At Elevated Temperatures. J Electrochem Soc 160, F1293-F1304, doi:10.1149/2.084311jes (2013). 4 Matsui, T., Kishida, R., Muroyama, H. & Eguchi, K. Comparative Study on Performance Stability of Ni-Oxide Cermet Anodes under Humidified Atmospheres in Solid Oxide Fuel Cells. J Electrochem Soc 159, F456-F460, doi:10.1149/2.053208jes (2012).