Operando near-infrared spectroscopy monitoring of state-of-charge for tailored protocols in all‑copper redox flow cells

Among electrochemical storage systems, redox flow batteries are particularly attractive for large-scale applications due to the decoupling of energy and power and their flexible design. In this context, all‑copper redox flow batteries represent a promising alternative to conventional chemistries owing to the low cost, high abundance, and European supply-chain of copper. However, performance limitations related to electrolyte imbalance and species crossover require advanced monitoring strategies to ensure stable operation. Here, we report the development of an operando near-infrared spectroscopical approach for state-of-charge (SoC) monitoring in an all‑copper redox flow cell. In chloride-rich media, both Cu + and Cu 2+ exist as chloro-complexes, with Cu 2+ exhibiting a characteristic near-infrared absorption band at ≈ 920 nm. By exploiting the linear relationship between absorbance and concentration, we demonstrate accurate quantification of Cu 2+ concentration and dynamic SoC tracking under operating conditions. The spectroscopic method provides temperature-independent and non-invasive monitoring, enabling direct insight into the evolution of redox speciation during cycling. Furthermore, rotating disk electrode kinetic analysis of the Cu + /Cu 2+ redox couple allowed us to rationally redesign the charge/discharge protocol. This work proves operando spectroscopy as a powerful diagnostic tool for redox flow batteries and highlights the role of integrated kinetic approaches in advancing next-generation, cost-effective grid-scale energy storage systems.