Publication: Research › Ph.D. thesis – Annual report year: 2007
Solid oxide fuel cells (SOFCs) produced at Risø National Laboratory was tested as steam electrolysers under various current densities, operating temperatures and steam partial pressures. At 950 °C and a cell voltage of 1.48V the current density was -3.6A/cm2 with app. 30% H2 + 70% H2O in the inlet gas and a H2O utilization of app. 40%. The tested SOECs were also used for CO2 electrolysis. Economy studies of CO and H2 production show that especially H2 production can be competitive in areas with cheapelectricity. Assuming the above described initial performance and a lifetime of 10 years it is possible to achieve a production price of 0.7 US$/kg H2 with an electricity price of 1.3 US¢/kWh. The cell voltage was measured as function of time. In test ofabout two month of duration a long-term degradation was observed. At 850 °C, -0.5 A/cm2 with 50 vol% H2 the degradation rate was app. 20 mV/1000h. It was shown that the degradation happens at Ni/YSZ-electrode. The long term degradation is probably causedby coarsening of the Ni-particles. After onset of electrolysis operation a transient passivation1/reactivation phenomena with duration of several days was observed. It was shown that the phenomenon is attributed to the SiO2 contamination at the Ni/YSZelectrode-electrolyte interface. The SiO2 arises from the albite glass sealing (NaAlSi3O8) that surrounds the electrode. Si may enter the Ni/YSZ electrode via the reaction Si(OH)4(g) « SiO2(l) +2H2O(g) . At the active sites of the Ni/YSZ electrode steamis reduced via the reaction H2O + 2e- ® H2 + O2-. This shifts the equilibrium of the first reaction to form SiO2(l) at the active sites. After a certain time the sealing crystallizes and the SiO2(l) evaporates from the active sites and the cellreactivates. The passivation is shown to relate to a build up of a diffusion-type impedance arc that converge towards (j?)-½ for the frequency ? converging towards infinity.
|Publication date||Feb 2007|
|Place of publication||Kgs. Lyngby, Denmark|
|Publisher||Technical University of Denmark (DTU)|
|Number of pages||135|
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