A dual-electrode oxide ion sensor for molten carbonates

The concentration of oxide ions (O^2–) determines the physicochemical properties of molten carbonates and modulates (electro)chemical reactions. However, it still lacks an efficient and reliable method to measure O^2– concentration in molten carbonates. Herein, we design a dual-electrode sensor to monitor the O^2– content in real time. For sensing O^2– in the target molten medium, we deliberately selected a Ni/NiO redox couple to serve as an O^2–-responding internal reference membrane electrode (RE_in) assembled in an yttrium-stabilized zirconia tube, which is accessible to achieve a stable and excellent response of O^2– from the bulk melt. To measure the membrane potential of RE_in that is associated with the O^2– concentration in the bulk melt, we paired a Ag/Ag₂SO₄ external reference electrode (RE_ex), which can indicate the potential difference between RE_in and RE_ex by measuring the open circuit potential (OCP), therefore figuring out the O^2– concentration based on Nernst equation. Taking molten Li₂CO₃-Na₂CO₃-K₂CO₃ for example, the homemade dual-electrode sensor is well-responsive with a reliable linear correlation coefficient of R² = 0.9992 even the O^2– content is as low as 1.27 × 10⁻⁴ mol kg⁻¹. The proposed dual-electrode gives clues to the rational design of anion sensor for relatively high-temperature systems.