Correlation between hydrogen production rate, current, and electrode overpotential in a solid oxide electrolysis cell with La_(0.6)Sr_(0.4)FeO_(3-δ) thin-film cathode

摘要

A solid oxide electrolysis cell (SOEC) with a model-type La_(0.6)Sr_(0.4)FeO_(3-δ) thin-film cathode (working electrode) on an yttria-stabilized zirconia electrolyte and a porous La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O_(3-δ) counterelectrode was operated in wet argon gas at the cathode. The hydrogen formation rate in the cathode compartment was quantified by mass spectrometry. Determination of the current as well as outlet gas composition revealed the electrochemical reduction of some residual oxygen in the cathodic compartment. Quantitative correlation between gas composition changes and current flow was possible. At 640 °C a water-to-hydrogen conversion rate of ca. 4 % was found at —1.5 V versus a reversible counterelectrode in 1 % oxygen. Onset of hydrogen formation could already be detected at voltages as low as —0.3 V. This reflects a fundamental difference between steam electrolysis and electrolysis of liquid water: substantial hydrogen production in a SOEC is already possible at pressures much below ambient. This causes difficulties in determining the cathodic overpotential of such a cell.