A Stable Ag/Ag₂S Reference Electrode for Molten Chloride Salts Based on Novel Design Strategies

The stability of reference electrodes (REs) is paramount for accurate electrochemical measurements in molten salts, yet traditional designs often fall short. Here we introduce an innovative Ag/Ag2S RE with superior stability and reusability, which is constructed with a robust Ag2S-coated Ag wire and a single-pore quartz tube (30 μm). Our approach involves two key strategies: the use of Ag2S, characterized by its high melting point and low solubility, to regulate Ag+ saturation, and the incorporation of a free-flowing liquid salt bridge to surpass the limitations of ceramic membranes in conductivity. Utilizing the reversibility of the Sm3+/Sm2+ couple, the performance of the Ag/Ag2S RE is evaluated through a series of electrochemical tests, which demonstrates that the maximum difference in the half-wave potential (E1/2) of Sm3+/Sm2+ is only 3 mV over 30 h. Besides the more stable potential, Ag/Ag2S RE exhibits better reusability and reproducibility compared with the REs using mullite membranes, suggesting that the liquid salt bridge significantly mitigates potential drift, while preserving the integrity of the Ag2S coating and preventing melt contamination. Furthermore, the Ag/Ag2S RE potential is calibrated to the potential versus Cl2/Cl– for standardization.