Influence of Ni-YSZ Electrode Thickness on the Behaviour of Commercial Solid Oxide Electrolysis Cells by Mean of Electrochemical Impedance Spectroscopy

Our world is facing energy needs always growing, the mid-term disappearance of important energy resources such as fossil fuels and global warming issues consecutive to massive greenhouse gas production. A possible answer to produce renewable and clean energy consists in combining promising technologies such as solar photovoltaic, wind turbine and fuel cell, thus making an energy mix. Hydrogen is an energy carrier which can be part of this energy mix. High temperature electrolysis (HTE) using a solid oxide electrolyte is a clean way to produce, from water and electricity, hydrogen. However, many studies showed that the electrochemical phenomena related to the Ni-YSZ electrode have a strong influence on cell performances, including the ones associated to gas transport [1-3]. Further investigations on the hydrogen side of the cell are hence needed. In the following work, two commercial cells with different Ni-YSZ thicknesses are considered. A deep analysis is performed in a two-electrode configuration by mean of electrochemical impedance spectroscopy (EIS) with the use of electrical equivalent circuits (EEC), showing that all the main phenomena governing SOEC functioning are related to the Ni-YSZ electrode for both cells, each one characterized by a specific relaxation frequency and capacitance. Indeed, the impedance diagrams measured were deconvoluted, from the highest to the lowest frequencies, into four arcs: a high frequency arc (HF arc), a first middle frequency arc (MF1 arc), a second middle frequency arc (MF2 arc) and a low frequency arc (LF arc). These arcs were respectively associated to charge transfer at the Ni-YSZ electrode, a first H₂O diffusion phenomenon, gas conversion at the Ni-YSZ electrode and a second H₂O diffusion phenomenon. Thus, changing Ni-YSZ electrode thickness appears to mainly influence the H₂O diffusion phenomenon associated to MF1 arc and the H₂O gas conversion related to MF2 arc, while phenomena associated to HF and LF arcs are poorly sensitive to this thickness change. This result suggests that the phenomena associated to MF1 and MF2 arcs occur mainly in the volume of Ni-YSZ electrode, contrary to phenomena related to HF and LF arcs. This in the same time allows distinguishing the two H₂O diffusion phenomena identified.

[1] A. Nechache, M. Cassir, A. Ringuedé, J. Power Sources 258 164 (2014).

[2] P. Kim-Lohsoontorn, J. Bae, J. Power Sources, 196, 7161(2011).

[3] PK. Patro PK, T. Delahaye T, E. Bouyer E, PK Sinha, Int. J. Hydrogen Energy, 37, 3865 (2012).