Degradation of solid oxide electrolysis cells (SOECs) is probably the biggest concern in the field of high temperature steam electrolysis (HTSE). Anode supported, YSZ-based microtubular solid oxide fuel cells (SOFC) have been tested in fuel cell mode and also at high voltages (up to 2.8V) under electrolysis mode. At high steam conversion rates the cell voltage tends to saturate. Our hypothesis is that this effect is caused by the electroreduction of the thin YSZ electrolyte which induces electronic conduction losses.YSZ reduction increases the cathode activity and reduces cathode overpotential.Operation of the cell in severe electrolyte reduction conditions induces irreversible damage at the YSZ electrolyte as observed in SEM experiments by the formation of 1 Presented at Fuel Cells Science & Technology 2010, Zaragoza, Spain, 6-7 October 2010 2 voids at the grain boundaries of the dense YSZ electrolyte. Evidence of this damage was also given by the increase of the ohmic resistance measured by AC impedance. Signs of electrolyte degradation were also found by both EDX analysis and micro-Raman spectroscopy performed along a transverse-cross section of the cell. The observed oxygen electrode delamination is associated to the high oxygen partial pressures gradients that take place at the electrolyte/oxygen electrode interface.

Reversible operation of a microtubular solid oxide fuel cell (SOFC) with high electrochemical efficiency is reported. These devices can ideally produce hydrogen from electricity and steam [solid oxide electrolyser (SOE)] and then use the stored hydrogen to generate electricity and heat (SOFC), acting as a storage device for the electrical energy. A fuel-electrode-supported Ni–yttria-stabilized zirconia (YSZ)/YSZ/ (La0.8Sr0.2)0.98MnO3 cell, 2.4 mm in diameter and 20μm of electrolyte thickness, was evaluated in an electrolysis mode as a function of the steam concentration supplied to the Ni/YSZ electrode. A good cell performance was obtained at temperatures as high as 950°C for the electrolysis operation. At 850°C , the cell withstood current densities of −1A/cm2 at 1.3 V with steam utilization of 16.5%. The production of hydrogen in the electrolyzer was tested by mass spectrometry. Their performance, especially in the SOE mode, is very promising for high temperature electrolysis applications. Voltage–current curves present an S-shaped nonlinear behavior in the electrolysis mode with a tendency to saturate at high current density values. The cell could sustain current densities as high as −6A/cm2 at 1.5 V, using 70%normalH2O/15%normalH2/15%normalN2 as a fuel with an area-specific resistance of the cell of 0.26Ωcm2 . The origin of this effect is discussed.