Oxidation of hydrogen on Ni/yttria-stabilized zirconia cermet anodes

The oxidation of hydrogen on Ni/yttria-stabilized zirconia (Ni/YSZ) is studied by impedance spectroscopy. The active thickness obtained is 20 mu m or less. Conditions such as temperature, anodic overvoltage, electrode potential, H-2 and H2O partial pressure are varied. Three distinct area are identified in impedance spectra. representing at least three rate-limiting processes. One equivalent circuit of the type LR(RQ)(RQ)(RQ), where Q = Y-O(j omega)(n), is used to describe all recorded impedance in the temperature range 850 to 1000 degrees C. The n-values are held constant, allowing a direct comparison of R and Y(0)values for different structures and conditions. The high-frequency are (1 to 50 kHz) is sensitive to the cermet structure (particle size) and relatively insensitive to atmospheric composition and overvoltage. The related imperfect capacitance is suggested to be interpreted as a double-layer capacitance in the Ni/YSZ interface. The medium; (10 Hz to 1 kHz) and low-frequency are (0.1 to 10 Hz) are sensitive to atmospheric composition and overvoltage. Both reaction resistances change their dependency on H-2 partial pressure around 0.5 atm. The perfect capacitance related to the low-frequency are is in the order of 0.5 to 2.5 F/cm(2), indicating an absorbed charged species rather than surface adsorption.