Typical anode materials for solid oxide fuel cells (SOFC) are Ni-cermets, either Ni/yttria stabilized zirconia (Ni-YSZ) or Ni/gadolinia doped ceria (Ni-GDC) [1]. In the case of Ni-cermet materials before the anode is ready for work NiO is reduced to Ni. During reduction process the solid phase volume of an anode decreases approximately 40%. Due to either system malfunction or contamination, air (oxygen) might diffuse into anode compartment, which causes Ni reoxidation to NiO. During this process the solid phase volume of an anode increases 66% [1,2]. Reoxidation of Ni might take place if after cell operation fuel gas flow is stopped in order to decrease fuel cell upkeep costs [3]. Reoxidation of Ni might also occur if electric load for the cell is too high and oxide ion flux through the membrane creates too oxidative environment at Ni catalyst.

In order to understand the dynamics of redox processes and to design the Ni-GDC electrodes with optimal microstructure the information related to simultaneous structural and electrochemical changes during SOFC operation would be very useful. In this work a novel approach for simultaneous monitoring of electrochemical (EC) properties and crystallographic structure in operating SOFC i.e. in operando SOFC EC-XRD measurement cell has been proposed and applied to understand the redox dynamics in Ni-GDC anode.

The impact of polarization on structure of Ce_0.9 Gd_0.1O_2-d (GDC) in Ni-GDC cermet anode as well as impact of NiO ⇌ Ni redox cycles on the size and structure of Ni particles and electrical contact between particles has been studied with the in house developed in operando EC-HTXRD cell. Changes in the lattice parameters of Ni and GDC as well as electrochemical properties of Pt|Ni-GDC|ScCeSZ|GDC|LSC were monitored as a function of temperature (T), electrode polarization (E) and oxygen partial pressure (pO₂). Influence of temperature, pO₂ and polarization on GDC lattice parameters was observed. O^2- current through the membrane lead to decrease of unit cell volume of GDC lattice. The observed change in lattice parameter upon polarization change from open circuit voltage (OCV) (depending on temperature, about 1.1 V) to 0.9V (potential of 3-electrode setup) was equal to 50 degree temperature change.

[1] A. Faes, A. Hessler-Wyser, A. Zryd, J. Van Herle, A review of RedOx cycling of solid oxide fuel cells anode, Membranes (Basel). 2 (2012) 585–664. doi:10.3390/membranes2030585.

[2] T. Klemenso̸, C. Chung, P.H. Larsen, M. Mogensen, The Mechanism Behind Redox Instability of Anodes in High-Temperature SOFCs, J. Electrochem. Soc. 152 (2005) A2186. doi:10.1149/1.2048228.

[3] D. Waldbillig, A. Wood, D.G. Ivey, Thermal analysis of the cyclic reduction and oxidation behaviour of SOFC anodes, Solid State Ionics. 176 (2005) 847–859. doi:10.1016/j.ssi.2004.12.002.