Redox Flow Batteries (RFB) stand out as a promising energy storage technology to mitigate the irregular energy generation from renewable sources. Some technical merits stand out the Redox Flow Batteries (RFBs) when compared to conventional batteries such as their scalability and the total decoupling of energy and power. However, one of the most important limitations that obstruct their massive penetration into the market is the high price of some of the current components of RFBs, specifically, the scarce and, in some cases also toxic, metallic compounds that are used as active species (V, Cr, etc) and the poor-performing ion-exchange membranes that are necessary to avoid the electrolytes cross-mixing. Different approaches have been proposed in the last years to overcome these problems. Among them, the substitution of metallic redox species by sustainable, abundant and tunable organic redox molecules is attracting an increasing interest among the scientific community. Regarding the problematic membrane, the mere substitution of ion-selective membranes by inexpensive porous separators is emerging as a new trend in RFBs. In those systems, the huge and unavoidable crossover of active species though the separator is mitigated, in some extent, by using mixed electrolytes. However, this approach causes a large and implicit loss of material utilization because half of the active species is unusable.
Making use of a disruptive approach, Dr. Rebeca Marcilla, senior researcher in IMDEA Energy, is leading a ERC Consolidator Grant project (https://cordis.europa.eu/project/id/726217/es) aiming to eliminate any type of membrane or separator in RFBs. Her research group is developing a novel concept of Membrane-Free Battery based on the immiscibility of two electrolytes and in which the metallic active compounds were substituted by organic redox molecules.
In their last article*, in collaboration with the group of Prof. Joao Coutinho (University of Aveiro, Portugal), they report the first example of Total Aqueous Membrane-Free Battery. There, they develop an aqueous biphasic system (ABS) containing organic redox compounds with proper redox potentials and suitable partitioning coefficients. They demonstrate the feasible application of this novel membrane-free concept in an Aqueous Biphasic Systems (ABS) which behaves as an unprecedented Total Aqueous Membrane-Free Battery.
(*) P. Navalpotro, C. Trujillo, I. Montes, C.M.S.S. Neves, J. Palma, M.G. Freire, J.A.P. Coutinho, R. Marcilla* "Critical aspects of Membrane-Free Aqueous Battery based on two immiscible neutral electrolytes" Energy Storage Materials, 2020, 26, 400-407. https://doi.org/10.1016/j.ensm.2019.11.011