80,000 current on/off cycles in a one year long steam electrolysis test with a solid oxide cell

Abstract

The current ON/OFF switching of a solid oxide electrolysis cell is treated as elementary step for power variation in a steam electrolyser system. If the cell voltage in the ON mode is adjusted to the thermal neutral voltage, heat generation remains zero in both modes, which largely facilitates the thermal management. To verify whether the cells withstand the switching, an electrolysis durability test with an electrolyte supported solid oxide cell was performed during one year at about 850 °C. The cell consisted of a 3YSZ electrolyte, CGO diffusion-barrier/adhesion layers, a lanthanum strontium cobaltite ferrite (LSCF) oxygen electrode, and a nickel/gadolinia-doped ceria (Ni/GDC) steam/hydrogen electrode. The test included two operation blocks with each 40,000 cycles of 2 min duration and a current density of −0.7 Acm−2 in the ON mode (−0.07 Acm−2 in OFF mode), as well as steady-state ON periods with 5800 h duration. Voltage degradation was 5 mV/1000 h (0.4%/1000 h) and the increase in the area specific resistance 7 mΩcm2/1000 h, without notable dependence on current cycling. Impedance spectroscopic results were in agreement with the only small switching transients seen in the cell voltage; moreover, they confirmed a dominating ohmic degradation together with minor contributions from gas conversion and reaction, respectively. No electrode delamination was detectable after scheduled test completion.