A Highly Reliable Ni/NiO Reference Electrode for Molten Li-Na-K Carbonates

Precise control/measurement of electrode potential is significant to understanding and tailoring the electrochemical reactions in high-temperature molten carbonates. However, a highly stable and reproducible reference electrode is still absent in the electrolyte. This work reported an extremely reliable Ni/NiO reference electrode, rather than an Ag⁺/Ag electrode, for molten ternary Li-Na-K carbonates. The selection principle of redox couple and membrane material are systematically investigated by thermodynamic analysis and experimental characterization. The stability, polarization resistance, and potential repeatability of the Ni/NiO reference electrode were evaluated by electrochemical measurements. The linear sweep voltammetry (LSV) results showed that the potential change was less than 17.3 mV within 120 h. The electrode potential could be restored quickly after large-current polarization. The minimum potential difference among parallel reference electrodes is 0.81 mV, demonstrating an excellent reproducibility of the Ni/NiO reference electrode. Overall, the novel reference electrode based on the Ni/NiO redox couple and the mullite membrane possessed potential long-term stability, excellent polarization resistance, and potential repeatability in molten carbonates.