The influence of the solid electrolyte on the impedance of hydrogen oxidation at patterned Ni electrodes

In order to investigate the role played by the solid electrolyte in the H₂ oxidation reaction, patterned Ni electrodes on YSZ (yttria stabilized zirconia) single crystals with different crystallographic orientations, on polycrystalline YSZ and on polycrystalline LSGM ((La,Sr)(Ga,Mg)O_{3 - x}) were studied in H ₂ + H₂O gas as a function of partial pressure of H ₂O in the temperature range of 673-823 K. Symmetrical cells using YSZ single crystals exhibit a single almost perfect electrode semicircle in the Nyquist plot. The corresponding polarisation resistance (R_p) depends on the surface orientation of the YSZ single crystals with a maximum of R _p for < 110> and a minimum for < 100>. The capacitance obtained from the impedance arc is almost temperature independent but different for the three surfaces. In the case of polycrystalline YSZ a depressed impedance arc results with the capacitance being between the extreme values measured for single crystals of different orientation. A modified situation is found for patterned Ni electrode on LSGM: the capacitance of the single arc depends on temperature and is much larger than in the case of YSZ cells. The polarisation resistance on polycrystalline LSGM is much smaller than on polycrystalline YSZ while activation energies of the polarization resistance are still similar. Capacitances and resistances are discussed in terms of possible mechanisms.