In-situ electrochemical fingerprinting of soluble impurities in molten solar salt for CSP thermal energy storage monitoring

Molten solar salt (60 wt% NaNO₃–40 wt% KNO₃) is the leading heat transfer and storage medium for concentrated solar power (CSP) systems, but its long-term performance can be affected by soluble impurities from moisture ingress, corrosion, and raw material contamination. Here we develop a laboratory-scale in-situ electrochemical fingerprinting approach based on cyclic voltammetry to identify and calibrate the response of three key soluble impurities, chloride (Cl⁻), hydroxide (OH⁻), and chromate (CrO₄²⁻), in molten solar salt at 290 °C under inert atmosphere. Each impurity exhibits a distinct dominant electrochemical signature, characterized by an irreversible anodic oxidation of Cl⁻ at +1.5 V, a quasi-reversible anodic response of OH⁻ near +0.5 V, and an irreversible cathodic reduction of CrO₄²⁻ around −2.0 V. For all impurities, peak current densities scale linearly with the square root of scan rate, supporting predominantly diffusion-controlled behavior and enabling apparent transport analysis using the Randles-Ševčík framework. Linear calibration relationships between current response and concentration are established (R² ' 0.97), and apparent diffusion coefficients are determined, in the range of 10⁻⁷–10⁻⁵ cm² s⁻¹. Crucially, the electrochemical fingerprints remain identifiable and concentration-dependent under selected mixed-impurity conditions, indicating its capability in representative interference backgrounds. This work establishes a controlled electrochemical basis for the future development of real-time molten salt monitoring strategies, with potential relevance to early warning of moisture ingress (via OH⁻), tracking of corrosion progression (via CrO₄²⁻), and quality control of raw materials (via Cl⁻) for next-generation CSP thermal energy storage systems.