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 2 0 mu m or less. Conditions such as temperature, anodic overvoltage, el ectrode potential, H-2 and H2O partial pressure are varied. Three dist inct area are identified in impedance spectra. representing at least t hree 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-value s 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 relat ively insensitive to atmospheric composition and overvoltage. The rela ted imperfect capacitance is suggested to be interpreted as a double-l ayer 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 com position and overvoltage. Both reaction resistances change their depen dency 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 adsorptio n.