A Membrane‐Free Redox Flow Battery with Two Immiscible Redox Electrolytes

Abstract

Flexible and scalable energy storage solutions are necessary for mitigating fluctuations of renewable energy sources. The main advantage of redox flow batteries is their ability to decouple power and energy. However, they present some limitations including poor performance, short‐lifetimes, and expensive ion‐selective membranes as well as high price, toxicity, and scarcity of vanadium compounds. We report a membrane‐free battery that relies on the immiscibility of redox electrolytes and where vanadium is replaced by organic molecules. We show that the biphasic system formed by one acidic solution and one ionic liquid, both containing quinoyl species, behaves as a reversible battery without any membrane. This proof‐of‐concept of a membrane‐free battery has an open circuit voltage of 1.4 V with a high theoretical energy density of 22.5 Wh L−1, and is able to deliver 90 % of its theoretical capacity while showing excellent long‐term performance (coulombic efficiency of 100 % and energy efficiency of 70 %).

Pathways to Widespread Applications: Development of Redox Flow Batteries Based on New Chemistries

“…Moreover, Marcilla and coworkers reported a membrane-free RFB by using redox species dissolved in two immiscible electrolytes (Figure 10E). 99 They found that an interphase barrier was formed between a hydroquinone aqueous solution and a hydrophobic ionic liquid solution of BQ (Figure 10F). In contrast to hybrid RFBs using one solid electrode based on stripping-deposition reactions, such a system with all redox species dissolved in solutions maintains higher scalability.…”

“…(E) A horizontal design of the membrane-free RFB with the formation of two immiscible phases. Reprinted with permission from Navalpotro et al 99 Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA. (F) Schematic representation of the membrane-free battery concept based on immiscible electrolytes in which a hydroquinone aqueous solution acts as the catholyte on top and a BQ in a hydrophobic ionic liquid solution acts as the anolyte at the bottom.…”

“…(F) Schematic representation of the membrane-free battery concept based on immiscible electrolytes in which a hydroquinone aqueous solution acts as the catholyte on top and a BQ in a hydrophobic ionic liquid solution acts as the anolyte at the bottom. Reprinted with permission from Navalpotro et al 99 Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA.…”

Redox Flow Batteries: Materials, Design and Prospects

“…Finally, since one of the major concerns about laminar flow membraneless cells is the reactant flow control, most of works were performed in microfluidic fuel cells. Despite this having a positive impact on the efficiency and current density of the device, it also increases the challenge to upscale this technology [217,220,221,[223][224][225][226].…”

“…The major advantage of this kind of configuration in comparison with the laminar flow strategy is the reduction of reactant cross-over effect since the electrolytes are spontaneously separated due to their partition coefficients [227,228]. Additionally, it is easier to recirculate the electrolytes to operate in regenerative mode (charge-discharge cycles) [226][227][228][229][230].…”