Long-term operation of a solid oxide cell stack for coelectrolysis of steam and CO₂

High temperature electrolysis based on solid oxide electrolysis cells (SOECs) is a promising technology for production of synthetic fuels. The SOEC units can be used for co-electrolysis of steam and CO2 to produce synthesis gas (syngas, CO+H2), which can be further processed to a variety of synthetic fuels such as methane, methanol or DME.

Previously we have reported electrolysis operation of solid oxide cell stacks for periods up to about 1000 hours. In this work, operation of a Haldor Topsoe 8-cell stack (stack design of 2014) in co-electrolysis mode for 6000 hours is reported. The stack consists of Ni/YSZ electrode supported SOEC cells with a footprint of 12X12 cm2.

The co-electrolysis operation was carried out by supplying a mixture of 45 % CO2 + 45 % H2O + 10 % H2 to the stack operating with a fixed conversion of 39 % for steam and CO2. The stack was operated at different conditions. Initial operation at 700 oC and -0.25 A/cm2 lasted for only 120 hours due to severe degradation of the bottom cell.

Regaining the stack performance was realized by increasing the operation temperature to 750 oC. After reactivation, the stack showed negligible degradation at 750 oC and -0.25 A/cm2 and about 1.4 %/1000 h performance degradation at 750 oC and -0.5 A/cm2. This study demonstrates feasibility of long-term co-electrolysis operation via SOEC stacks and of careful temperature variation as a tool to regain the stack performance.