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Membrane-Free Redox Flow Batteries

Periodic Reporting for period 2 - MFreeB (Membrane-Free Redox Flow Batteries)

Reporting period: 2018-12-01 to 2020-05-31

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.

One of the main barriers for worldwide deployment of energy storage systems coupled to renewable generation plants and power distribution systems is related to their high investment cost. Membrane-Free RFBs using organic couples may lead to dramatically decrease investment costs because they will spare the cost of membranes, will make use of low cost organic compounds and will not need expensive materials to stand highly corrosive environments. Additionally, Membrane-Free batteries will not require membrane cleaning and substitution, or electrolyte re-balance due to crossover after prolonged use, thus making operation and maintenance significantly simpler and more appropriate for domestic applications. Based on these features, Membrane-Free batteries proposed in this ERC project may boost the market of redox flow batteries for stationary energy storage applications.

One of the main barriers for worldwide deployment of energy storage systems coupled to renewable generation plants and power distribution systems is related to their high investment cost. Membrane-Free RFBs using organic couples may lead to dramatically decrease investment costs because they will spare the cost of membranes, will make use of low cost organic compounds and will not need expensive materials to stand highly corrosive environments. Additionally, Membrane-Free batteries will not require membrane cleaning and substitution, or electrolyte re-balance due to crossover after prolonged use, thus making operation and maintenance significantly simpler and more appropriate for domestic applications. Based on these features, Membrane-Free batteries proposed in this ERC project may boost the market of redox flow batteries for stationary energy storage applications.
During the first 36 months, the project has been focus on three important activities that are detailed below:

1. Proof of concept of Static Membrane-Free battery
The main objective of MFreeB ERC project is to develop a Membrane-Free battery that relies on the immiscibility of redox electrolytes that spontaneously form a biphasic system whose interphase functions as a “natural” barrier making the use of any membrane superfluous and where the vanadium species are replaced by organic molecules. 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. On the other hand, the partition coefficient of redox molecules between the two immiscible phases and the inherent self-discharge occurring at the interphase are revealed as intrinsic features affecting the performance of this type of Membrane-Free Battery. It was successfully demonstrated that the functionalization of redox molecules is an interesting strategy to tune the partition coefficients mitigating the crossover that provokes low battery efficiency. As a result, cycling-life of a battery having OH-TEMPO as active species in the catholyte and containing propylene carbonate-based anolyte was evaluated over 300 cycles achieving 85% capacity retention. These results demonstrated the huge versatility and countless possibilities of this new Membrane-Free Battery concept.

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. This concept is feasible due to the selective enrichment of redox organic molecules in each aqueous phase of ABS, which spontaneously form two liquid-phases (membrane-free) above given concentrations of salt and IL. Therefore, in this concept, the required separation of electrolytes in the battery is not driven by an expensive membrane that hampers mass transfer, but instead, by the intrinsic immiscibility of the two phases. Moreover, the cross-contamination typically occurring through the ineffective membranes is determined by the partition coefficients of the active molecules between the two phases. 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
Unlike the results discussed in previous work related to ionic liquid-based ABS (Advanced Science), the ABS used in the present work was based on polyethylenglycol (PEG) which is a polymer widely used in our daily products and approved by REACH. This PEG-based ABS presents several advantages such as their higher environmentally-friendly properties, non-corrosive, non-flammable, lower cost and large scale production. 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.
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.
The very large number of possible organic redox active molecules and electrolytes forces the need for a better theoretical guidance in order to screen and select the most attractive systems for the design of future Membrane-Free RFBs. In this sense, we will implement a high-throughput computational screening method to establish a database of properties of electrolytes to identify the most promising electrolyte molecular candidates. I propose to use numerical modelling as an efficient method to provide an initial perspective regarding some properties such as solubility, stability, redox potential, partition constants, phase diagrams, transport properties, etc, that are fundamental for the successful development of high performance Membrane-Free RFBs. 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.
During the first 18 months of the project we have demonstrated that it is possible to assemble a Static Membrane-Free Redox Battery making use of immiscible electrolytes. However, moving from a Static design to a Flowing design, in which the two immiscible phases are stored in separated tanks and need to be pumped into the reactor during operation, is one of the more challenging aspects of the proposal. In the future, we will design 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. We anticipate that the design of this prototype will need to be supported by the use of computational fluid dynamics (CFD), which simulates the flow distributions using different flow fields over optimized scaling and dimensions. Single flowing cells will be assembled with those pairs of immiscible electrolytes already demonstrated in Static configurations and will be tested using standard electrochemical methods to define the figures of merit, including cycling ability, battery efficiency and electrical resistance, under different flow rates.

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. Some design aspects that are unique from of this battery concept are; (i) the impact of turbulent flows on the smoothness of the interphase, on pumping consumption and on the overall battery polarization, (ii) the influence of series or parallel flow between cells with respect to the mixing of phases, pumping power, state-of-charge and shunt currents. A Computing Fluid Dynamic tool such as COMSOL Multiphysics will used to assess and compare design alternatives. 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.
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