Close Contact without Mixing: All-Aqueous Membrane-Free Flow Battery
Authors: Ibanez Santiago Enrique , Navalpotro Paula , Almonacid Ignacio , Pedraza Eduardo , Marcilla Rebeca 
Published: July 2022

The accumulation of energy produced by renewable sources to be used when demand is high is a key aspect of a truly decarbonized grid. Among all energy accumulation technologies, Redox Flow Batteries (RFBs) are specially promising due to its safety, flexibility of design and the ability to decouple capacity and power of the system. In a standard configuration, both redox electrolytes that are stored in external tanks are pumped in the electrochemical reactor separated only by an ion-selective membrane. This membrane is a critical part of the battery as it allows the selective pass of certain ions that compensate the charge imbalances while maintaining separated the active species in their respective compartments. Membranes used in RFB are expensive (up to 25% of the total cost in vanadium RFB), contribute greatly to the ohmic resistance of the battery, and the undesired crossover through it is never fully avoided which produces a performance decay of the battery over time. These issues are addressed by membrane-free RFB designs developed in our group.

The idea behind MFreeB ERC project is to take advantage of the immiscibility of certain liquids to naturally separate redox active species in a membrane-free battery design [1]. Aqueous Biphasic Systems (ABSs) are a special example of all-aqueous natural occurring immiscible phases made by mixing water with two compounds that can be either polymers, inorganic salts or both. The two redox active species added to the biphasic system can be rationally selected and optimized so that they partition specifically to one of the two phases used as anolyte and catholyte [2]. These types of batteries have been proposed by several groups, including us, mostly as “static batteries” because filter-press reactors, typically used in conventional RFB, cannot be easily adapted to this membrane-free concept. In order to develop truly flowing systems, a totally different electrochemical reactor should be designed.

Here, we report on the development of an electrochemical reactor that allows the formation of the two immiscible phases making possible the testing of immiscible batteries under flowing conditions for the first time. A counterflow flow-through configuration was used since it reduces flow instabilities while at the same time it allows high conversion rates in the carbon felt electrodes avoiding mass transport issues [3]. Several tests were performed in the battery; e.g. polarization curves, charge and discharge cycling and efficiency over time were measured. Moreover, the self-discharge phenomena, occurring near the interface when charged species enter in contact and provoking a loss of efficiency, was evaluated as a function of operating conditions. Special attention was given to the study of the different processes occurring in the region near the interface in order to mitigate losses and further optimize the design.

Acknowledgments

This work has been partially funded by project MFreeB which have received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 726217).

References

[1] P. Navalpotro et al., Angew.
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56 (2017), 1–6.

[2] P. Navalpotro et al., Energy Storage Materials. 26 (2020) 400-407.

[3] International patent WO 2021/209585 A1. Redox Flow Battery with Immiscible Electrolyte and Flow Through Electrode.