Raman In Situ Monitoring of Concentrated Solutions for Copper-Based Redox Flow Batteries

Nowadays the scientific community is at the forefront of the fight against climate change, exemplified by the targets set in the Paris Accord, and the goal of a zero-carbon, sustainable economy. These goals can be only achieved with a higher share of renewable energy, which being mostly of an intermittent nature, require innovative energy storage solutions deployable at large scale. A vast capacity of stationary energy storage must be created, and redox flow batteries (RFB) are one of the best technological solutions to provide it. However, multiple hurdles have to be overcome to make such deployment possible; they are of different nature, ranging from technical aspects related to performance (e.g. long cycle life or electrolyte composition optimization) to economic ones (e.g., the need to reach a competitive levelized cost of energy and to raise vast capital sums), and from environmental issues (e.g., toxicity and recyclability) to the security of supply (e.g., dependence on critical raw materials from outside of the EU or on a shallow market dependent on the production of a by-product of a non-related material).<br/>Within the project CUBER (financed by the EU community grant number 875605) we are developing to validate in a relevant environment, a low-cost and scalable stationary energy storage technology, based on redox flow battery systems, with a proven superior environmental performance based on a non-critical and earth-abundant material (copper).<br/>One of the main problems related to the developing of this new class of battery is the solubility problem of the high concentrated solutions.<br/>In this regards the spectroscopic characterization and Raman measurement, in particular, can be very useful to deep understand the presence and the nature of the residues.<br/>The stability of the solution was studied and verified with in-situ monitoring of the solution as a function of time and temperature. The measurements were acquired with BW-TEK: i-Raman® Plus, fiber-coupled Raman system with excitation at 1064 nm to avoid luminescence. The system permits to monitor of the presence of CuCl2 Raman features in the solution and helps in the definition of the temperature at which the first residues are observed. The samples were kept at 60 °C for 12 hours, then it is cooled down with different cooling rates. Remarkably, the highest temperature before the Raman feature of CuCl2 was observed depending on the cooling rate. If a slow rate is utilized, it seems that the stability is preserved for a wider temperature range, while for a faster cooling rate the precipitate was observed for higher temperatures.<br/>The laboratory setup consists of a controlled thermal bath (40 °C) and a continuous flux obtained with a peristaltic pump at room temperature. We utilize two Raman probes, one at the beginning of the path and a second probe that monitors the effect of thermal shock.<br/>The results confirm the possibility to verify the presence of residues and monitoring the system without the necessity to stop the flow and /or sample the solution.