Engineering Bilayer Electrolytes in Solid Oxide Electrolysis Cells

Solid Oxide Electrolysis Cells (SOECs) are solid-state electrochemical devices that under an applied potential can convert low-energy molecules (e.g., CO₂ and H₂O) into high-energy molecules (e.g., CO and H₂, respectively) and gas phase oxygen. In SOECs, the cathode compartment is responsible for the reduction reaction, which in the case of CO₂ electrolysis, leads to formation of CO and oxygen ions. Oxygen ions then diffuse through an oxygen-conducting solid oxide electrolyte, (traditionally yttria stabilized zirconia (YSZ)) to the SOEC anode, where oxygen evolution occurs forming gas-phase oxygen. The standard SOEC cathode is usually composed of a metal electrocatalyst (e.g., Ni) over a ceramic oxide (e.g., YSZ), while the anode is composed of strontium-doped lanthanum manganite (LSM) over the same ceramic oxide. YSZ is traditionally used as the solid oxide electrolyte, yielding viable and durable SOECs, but limits the device operation to temperatures above 800°C due to its low oxygen conductivity. One way to overcome this challenge is to replace YSZ with gadolinium doped ceria (GDC), which exhibits higher ionic conductivity, thus promoting lower operating temperatures. However, in a reducing atmosphere GDC shows mixed ionic and electronic conductivity, yielding open circuit potential values that are generally lower than the theoretically calculated voltage, thus reducing the cell performance. We propose a method for eliminating electron leakage in GDC by applying a very thin layer of YSZ over the GDC electrolyte that acts as an electron insulator, allowing only the oxide ions to pass through the electrolyte. We show that the coated GDC SOEC yields improved electrochemical performance over both the YSZ and GDC only electrolyte cells.