Skip to main content

Membrane-Free Redox Flow Batteries

Periodic Reporting for period 3 - MFreeB (Membrane-Free Redox Flow Batteries)

Reporting period: 2020-06-01 to 2021-11-30

The environmental concerns over the use of fossil fuels have promoted great interest in generating electric energy from renewable sources such as solar and wind. However, the intermittent nature of those resources demands high performing and cost-effective energy storage systems such as Redox Flow Batteries (RFBs). The major issues of the current Vanadium RFBs are the high price and toxicity of vanadium components and the high cost and low performance of the ion-selective membranes.

The main objective of MFreeB project is to completely remove the problematic membrane of RFBs by developing a disruptive, versatile and scalable Membrane-Free RFB implementing immiscible catholytes and anolytes in which the metallic redox pairs are replaced by cheap, abundant, environmental-friendly molecules. Specific objectives are described below:

- Development of Metal-free redox couples with high solubility, optimum redox potential, fast kinetics and multiple exchange electrons.
- Development of both aqueous and non-aqueous electrolytes that will be used together forming a Membrane-Free RFB.
- Investigation of the Immiscibility and Partition Coefficients of catholyte and anolyte that will determine the spontaneous separation of the two electrolytes endowing the development of Membrane-Free RFB.
- Increase the energy density of RFB due to: increased solubility of organic species, enhanced operating voltage, of multi-electron transfer using compounds such as anthraquinones.
- Increase the power density of RFB by removing the membrane that provokes high internal resistance.
- Reduce the cost of RFB due to the use of organic couples based on cheap, abundant and non-corrosive materials. Innovative Membrane-Free RFB will reduce both operational and maintenance cost related to membranes.
During the first 54 months, the project has developed several important activities that are detailed below:

1. Proof of concept of Static Membrane-Free battery
We demonstrated that the biphasic system formed by one acidic solution and one ionic liquid, both containing dissolved quinoyl species, behaves as a reversible battery without any membrane or separator. This proof-of-concept of Membrane-Free battery exhibits an open circuit voltage of 1.4 V with a high theoretical energy density of 22.5 Wh/L and it is able to deliver the 90% of its theoretical capacity. Moreover, this battery shows excellent long-term performance with coulombic efficiency close to 100% and energy efficiency of 70% upon repeated cycling.

2. Versatility of the Membrane-Free concept
Here, we investigated the versatility of this concept exploring the electrochemical performance of 10 redox electrolytes based on different solvents such as propylene carbonate, 2-butanone or neutral-pH media, and containing different redox organic molecules such as TEMPO, OH-TEMPO or substituted anthraquinones. The most representative electrolytes were paired and used as immiscible anolyte/catholyte in 5 different Membrane-Free Batteries. Those batteries with substituted anthraquinones in the anolyte exhibited up to 50% improved OCV (2.1 V), an operating voltage of 1.75 V and 62% higher power density compared with our previous work.

3. Feasibility of aqueous biphasic system as Static Membrane-Free Batteries
Here, Aqueous Biphasic Systems (ABS) formed by water, ionic liquids (ILs) and salts, in which the two phases are water-rich, are here demonstrated for the first time to act as potential Membrane-Free Batteries. We characterized the phase diagrams of a series of IL-based ABS, determined the partition coefficients of several redox organic molecules, and evaluated the electrochemistry of these redox-active immiscible phases, allowing to appraise the battery performance. Several redox ABS that might be used in Total Aqueous Membrane-Free Batteries with theoretical battery voltages as high as 1.6 V were identified. The viability of a Membrane-Free Battery composed of an IL-based ABS containing methyl viologen and TEMPO as active species was demonstrated.

4. Identification of critical aspects in aqueous Membrane-Free Batteries
By combining thermodynamics and electrochemistry we selected methyl viologen (MV) and TEMPO as active species in the first example of Total Aqueous Membrane-Free Battery. This Aqueous Membrane-Free Battery exhibited a theoretical battery voltage as high as 1.23V much higher peak power density (23 mWcm-2) and excellent long-cycling performance (99.99 % capacity retention over 550 cycles). Moreover, essential aspects of this Membrane-Free Battery concept such as the crossover, controlled here by partition coefficients, and the inherent self-discharge phenomena, were addressed for the first time.

5. Development of Molecular modelling tools to predict key properties of new compounds as follows:
i) Computational prediction of electrochemical-related properties of organic redox compounds: Database of calculated properties (redox potentials, structural reorganization energies, etc.) of functionalized Phenazines as anolytes and catholytes in non-aqueous and aqueous solvents.
ii) Computational investigation of the interactions between redox-active organic molecules and the supporting salts in both non-aqueous an aqueous media by using Molecular Dynamics simulations.
From now until the end of the project the expected results are related with the following aspects that will be investigated in detail.

1. Computational modelling as a tool to accelerate the development of new immiscible electrolytes.
We have implemented a high-throughput computational screening method to establish a database of properties of electrolytes such as solubility, stability, redox potential, partition constants, phase diagrams that will be optimized until the end of the project. The cut-off criteria for such different aspects will be flexible and inclusive to provide an optimum compromise in the final properties.

2. To understand the mechanism of Self-discharge at the liquid-liquid interface. To develop new actions to mitigate this inherent aspect of our Membrane-Free Battery concept that might cause low columbic efficiency of the battery.

3. Developing of Membrane Free Redox Battery operating under flowing conditions.
Moving from a Static design to a Flowing design is one of the more challenging aspects of the proposal. We have designed a home-made single flow cell considering that the compartment for each electrolyte within the battery will not be delimited by a membrane but by spontaneous electrolyte separation under flow conditions. The optimization of this prototype will be supported by the use of computational fluid dynamics (CFD), which simulates the flow distributions using different flow fields over optimized scaling and dimensions.

4. Design Considerations for a Membrane-Free Redox Flow Battery.
The design of a cell stack composed of several cells connected in series/parallel will be addressed in the last period of the project. The outcome of this task will be a list of specifications and design concepts that might be used for future assembly of Membrane-Free RFBs with certain energy/power requirements.