The increasing global energy demand and the transition toward a more sustainable energy system necessitate the integration of renewable sources, emphasizing the need for effective energy storage systems. Redox flow batteries (RFBs) are particularly suitable due to their efficiency and unique ability to decouple energy and power density. However, their widespread adoption is hindered by the high costs of ion-selective membranes and vanadium-based electrolytes currently used in commercial vanadium RFBs. This study analyzes an alternative membrane-free (membraneless) flow battery technology that relies on immiscible electrolytes, which spontaneously separate into two distinct liquid phases, eliminating the need for an ion-selective membrane or any other kind of physical separator. This approach promises to address key hurdles in advancing RFB technology by reducing overall costs while enhancing sustainability and overall performance. Here, we examine the fundamentals, evolution, and development needed for market implementation of this innovative technology.

The escalating global energy demand, besides environmentally detrimental production methods, highlights the importance of integrating renewable energies into our energy infrastructure. However, renewable energy sources, such as wind and solar, suffer from intermittent production, making effective energy storage systems, particularly batteries, crucial for ensuring a steady supply and balancing production and demand. In particular, Redox Flow Batteries (RFBs) stand out as one of the most suitable candidates. Unlike other types of batteries, RFBs are a unique type of batteries able to decouple energy and power density.1 This distinctive feature stems from their configuration, wherein the active species are dissolved in electrolytes (catholyte and anolyte) stored in two external tanks (energy unit) and pumped through the electrochemical reactor (power unit) where the electrochemical reactions take place. As a result, to increase the energy capacity, larger storage tanks can be used without modifying the electrochemical cell stack. Conversely, to enhance the power output, more or larger power units can be added without changing the volume of the electrolyte storage. This decoupling allows for tailored energy solutions that can meet specific application requirements, ranging from small-scale residential energy storage to large-scale grid support.

State of the art. Commercially available vanadium RFBs utilize acid aqueous vanadium solutions as electrolytes and ion exchange membranes, e.g., Nafion®, as separators. These ion exchange membranes are crucial for the battery performance and efficiency as they prevent the mixing of electrolytes and the short-circuiting of the cell while facilitating the movement of ions necessary for electricity generation.

Problem. The high cost and poor performance of membranes, along with electrolyte issues such as vanadium toxicity, fluctuating prices, and availability, are hindering the widespread adoption of these batteries in the market.

Solution. Membrane-free or membraneless redox flow batteries are a promising class of systems that overcome the drawbacks associated with the use of membranes. They replace the use of the ion-selective membrane with the native liquid–liquid interface of immiscible/biphasic electrolytes. Moreover, in these systems, vanadium species are replaced by organic redox compounds that are abundant and cost-effective and present tunable properties (solubility, redox potential, etc.). These characteristics could enable a significant cost reduction, as the membrane accounts for ∼30% of the overall cost, and enhance the sustainability of RFBs (Fig. 1).

FIG. 1.

Schematic comparison of the commercial vanadium RFBs and the membrane-free flow battery technology.

FIG. 1.

Schematic comparison of the commercial vanadium RFBs and the membrane-free flow battery technology.

Close modal

Figure 2 represents the timeline of the most significant milestones in the development of membrane-free redox flow battery technology based on immiscible electrolytes.2–4 The membrane-free battery concept was first demonstrated in 2017 using a combination of immiscible aqueous (HCl solution)/non-aqueous (ionic liquid) electrolytes, containing dissolved (0.1M) parabenzoquinone active species.5,6 Subsequently, several studies have explored other combinations of aqueous/non-aqueous7,8 using conventional organic solvents, such as propylene carbonate, glymes, and acetonitrile (orange in Fig. 2), and even non-aqueous/non-aqueous immiscible electrolytes9 (pink in Fig. 2) with different dissolved active species [mainly quinones, viologens, and 2,2,6,6-Tetramethylpiperidine 1-oxyl (TEMPO) derivatives], demonstrating the versatility of this membrane-free concept.

FIG. 2.

Timeline of the most relevant advancements in the development of membrane-free flow batteries based on immiscible electrolytes.

FIG. 2.

Timeline of the most relevant advancements in the development of membrane-free flow batteries based on immiscible electrolytes.

Close modal

In 2020, as a step further toward sustainability, more sustainable, safer, and cost-effective batteries were developed. These batteries leveraged the salting-out effect of salts in water to produce and utilize two immiscible neutral aqueous electrolytes (blue in Fig. 2). In addition, a study conducted at the same time identified the inherent self-discharge process as a crucial aspect of this technology.10–12 

Interestingly, the membrane-free concept has also been expanded to hybrid battery configurations (gray dashed lines in Fig. 2), where one of the active species is in the solid state, such as a metallic Zn anode13–16 and, more recently, a metallic Li anode.9,17 This hybrid configuration solved the major drawback of self-discharge, leading to highly efficient batteries with Coulombic efficiencies >97%14–16,18 and reaching high values of energy efficiency (80%–92%).14,16

It is important to note that the vast majority of reported examples have involved static batteries and a combination of aqueous and non-aqueous electrolytes. These configurations face important mass transport limitations due to the static conditions and the lower diffusion rates of non-aqueous electrolytes; consequently, these systems suffer from low current density (<10 mA/cm2) and low capacity utilization, typically between 1% and 55%.7,8,13 Meng et al.14 improved upon this configuration by incorporating continuous stirring of the non-aqueous catholyte, which led to a significant improvement in capacity utilization, reaching 94%, which is particularly notable given the high active species concentration (1.5M; 40.2 Ah/L) in that system.

It was in 2023 when liquid–liquid membrane-free batteries operating under real flow conditions were reported.12,18 The implementation of flowing conditions allows us to enhance by twofold the peak power density in an aqueous-based membrane-free battery.12 This advancement was made possible by the development of a flow-through reactor, patented in 2021,19 specifically designed to operate with any combination of immiscible electrolytes. In such a reactor, immiscible electrolytes are pumped into the horizontal cell perpendicular to the liquid–liquid interface, which promotes the formation of a stable interphase under a flowing regime. This invention restores the ability to decouple energy and power in membrane-free batteries under real operating conditions, marking a pivotal advancement in this technology.

In addition to biphasic batteries, microfluidic RFBs constitute another strategy to eliminate membranes in RFBs. These systems use hydrodynamic engineering to regulate the laminar flow of electrolytes through parallel micro-channels, thereby minimizing electrolyte mixing.20–22 However, their performance is mainly constrained by the low flow rates required to ensure laminar flow, low electrolyte conversion (<50%), and low efficiencies. Furthermore, most studies have employed vanadium as the charged fuel, operating microfluidic reactors as single-pass fuel cells to generate energy. Only recently has a closed-loop microfluidic RFB been developed, capable of operating in recirculation mode, constituting the first steps toward rechargeability of a micro-RFB.23 It is important to note that these microdevices operate at a much smaller scale, with power outputs typically in the range of 10 mW and low electrolyte utilization. As a result, their scalability is limited, making them suitable for small-scale applications but impractical for high-power systems, such as conventional RFBs, which use multi-cell stacks with power outputs ranging from 1 kW to tens of kW.

Intensive research efforts are currently directed toward crafting cost-effective RFBs to overcome the main drawbacks of vanadium technology. The following different approaches are being pursued to replace the problematic membrane and substitute metallic redox species by lower-cost compounds (either organic or inorganic):24–26 

  • Symmetric RFBs. These batteries rely on a single parent molecule as the charge storage species in both the positive and negative electrode reactions. It requires redox materials with at least two redox reversible processes (bipolar/ambipolar active species) with a significant potential difference.27 The most significant benefit is that the physical crossover of electro-active species does not result in mixing of different chemical compounds. This feature simplifies charge rebalancing due to electrolyte crossover, as it eliminates the need for chemical separations. Moreover, crossover losses can be reduced simply by reversing the cell polarity, which diminishes the impact of physical crossover, paving the way for the use of simple porous separators that do not prevent crossover but are very cost-effective (Nafion® ≈ 500$/m2; porous separators <50$/m2).28 Unfortunately, this strategy is limited by the scarcity of suitable ambipolar active species, which are often synthesized through complex routes involving two different redox moieties, reducing its specific capacity.29,30

  • Use of large/voluminous active species. This strategy involves using redox materials with larger sizes, such as redox-active polymers, to prevent electrolyte cross-mixing via a size exclusion (porous) separator.31–33 However, this approach is limited by the low concentration of redox groups and, consequently, low battery capacity, besides the complexity of the synthesis processes and the high viscosity of polymer-based electrolytes.

  • Use of semi-solid electrolytes. This concept relies on dispersions of non-soluble redox materials instead of soluble species. This enables the use of a porous separator instead of a membrane, without significantly increasing the crossover and capacity losses.34–37 However, the main challenges lie in the stability of the dispersions and issues with flowability issues, which can lead to a higher pumping cost.

Unlike the aforementioned strategies that still require some type of membrane (such as a size-exclusion membrane), the membrane-free redox flow battery using liquid–liquid interface eliminates the need for any kind of separator. This is achieved through the use of immiscible electrolytes, where thermodynamics prevents their mixing. This innovative concept tackles the urgent limitations of the state-of-the-art constituting a more cost-effective solution while using sustainable organic-based active materials.

The new membrane-free RFB concept invented at IMDEA Energy was conceived to feature the following value-added aspects:

  • Highly efficient separation of electrolytes. Instead of opposing thermodynamics by using a physical barrier between the electrolytes, this concept leverages thermodynamics as a critical ally. The mutual immiscibility of the two electrolytes, governed by thermodynamics, keeps them naturally separated.

  • Broad variety of active species. Unlike other approaches, this concept is not limited to specific active species, such as bipolar molecules or large macromolecules (e.g., polyoxometalates and redox-active polymers). Instead, commercially available redox materials can be effectively applied in membrane-free RFBs, as long as they fulfill the partitioning behavior requirement. This criterion ensures that each active species exhibits a high affinity for only one of the immiscible electrolytes, thereby maintaining effective separation.

  • Versatility: This concept can be applied to any type of immiscible configuration, including aqueous/non-aqueous, non-aqueous/non-aqueous, and aqueous/aqueous immiscible electrolytes. This provides membrane-free RFBs with varying characteristics in terms of energy, power, operating voltage, cost, sustainability, etc.

  • Constant operation mode. This battery can be operated continuously without the need to schedule routine stops for electrolyte rebalancing, as the thermodynamic separation prevents the migration of active materials from one compartment to the other within the reactor.

  • Affordable and cost-effective. The elimination of separator and the use of organic compound significantly reduce the cost of the battery system while maintaining the existing pumping systems for redox electrolytes. Costly maintenance operations to substitute damaged membranes are not needed anymore.

The feasibility of this membrane-free redox flow battery technology relies on the development of immiscible redox electrolytes that effectively maintain the required separation of active species through thermodynamics. The selection of the redox-active species is based on their specific solubility in each phase (determined by their partitioning behavior) and their electrochemical properties (redox potential, stability, and kinetics). The complete elimination of the membrane, which separates catholyte and anolyte in commercially available RFBs, is a key enabling feature of this technology, reducing costs, as successfully demonstrated in static and flowing operation modes.12 

The membrane-free redox flow battery technology developed at IMDEA Energy is currently at the Technology Readiness Level (TRL) of 3–4, as the proof of concept has been successfully demonstrated on a laboratory scale for both static and flowing operation modes. Transitioning to the next TRLs is crucial for the success of the technology, which involves developing a small-scale prototype and demonstrating the feasibility of the technology in its intended environment. Unbound Potential GmbH, a Swiss start-up company, has licensed the technology with the goal of developing a scalable hardware platform to enable the use of membrane-free redox flow batteries in industrial applications. The innovative approach of Unbound Potential focuses on drastically minimizing the number of active components required for durable operation over years, enabling the usage of multiple cells or reactors, without compromising the electrochemical performance of the system. Unbound Potential is carrying out this work in collaboration with industrialization partners to ensure the highest quality, optimal performance, and seamless compatibility with the designed hardware. The current TRL of the hardware platform developed by Unbound Potential GmbH is 4/5.

Designing a suitable flow reactor that stabilizes the interface of liquid–liquid electrolytes while effectively distributing active species within the carbon electrodes constitutes a significant challenge. The lab-scale reactor patented at IMDEA Energy represents a breakthrough in this technology, demonstrating improved mass transport and reduced diffusion limitation resistance within the reactor. This was achieved through the implementation of a counter-flow design, which resulted in higher capacity utilization and lower resistance of the battery, compared to the flow-by configuration.19 

However, the overall resistance of the reactor tested in the lab remains high, limiting the maximum applied current compared to the reactors employed in vanadium RFBs. This is primarily due to the significant distance between the electrodes in the existing reactor, which ultimately restricts the power output of the membrane-free redox flow battery. Therefore, achieving successful operation of membrane-free batteries under practical current conditions is crucial for advancing this technology further. Consequently, further developments are being implemented to reduce this internal resistance, primarily by bringing the electrodes closer together.

Likely, the most important challenge of the membrane-free battery technology is the low Coulombic efficiency resulting from the inherent self-discharge phenomenon at the liquid–liquid interface when the charged electrolytes come into contact. Currently, further developments in reactor design, specifically aimed at reducing the contact area between the two immiscible liquids, are being investigated. Other approaches have been proposed by the scientific community. One of them is the use of a hybrid membrane-free configuration, where one of the active species is a solid in its charged state (e.g., Zn and Li metals).14,15 Recently, the patented reactor with an aqueous–aqueous biphasic system has been employed, demonstrating the first hybrid Zn membrane-free battery operating under real flow conditions. This system successfully mitigated self-discharge, achieving Coulombic efficiency values near 100%.18 

However, more imaginative solutions are required to overcome the low efficiency in non-hybrid membrane-free batteries. In this regard, IMDEA Energy has recently patented a novel triphasic immiscible system, in which an intermediate liquid layer acts as a “liquid” membrane, preventing direct contact between the charged active species contained within the other two immiscible phases.38 Ongoing efforts by both IMDEA Energy and Unbound Potential are focused on demonstrating the scalability of the triphasic membrane-free battery.

Securing IP rights is essential for commercializing innovations and ensuring economic gains for both the industry and end-users. IMDEA Energy secured the first patent on the membrane-free RFBs in 2016 (ES2633601B1, priority date: March 21, 2016, status: granted), secured a patent on the flow-through reactor designed for continuous operation in 2020 (WO2021209585A1, priority date: April 17, 2020), and, more recently, filed a patent on a system to reduce self-discharge between phases (EP23382722, priority date: July 14, 2023). Unbound Potential GmbH has filed a patent on a scalable cell design to enable the use of the systems at larger scales (EP23020172.5), which is compatible with the chemical systems developed by IMDEA. In addition, Non-Disclosure Agreements (NDAs) are in places between IMDEA and Unbound Potential to ensure secure dissemination of information to relevant stakeholders.

Redox flow batteries provide a versatile solution with scalable storage capacity and power output, making them adaptable to various applications. Different cell designs and chemistries affect power and energy densities: power density is influenced by the cell design and selection of components, while energy density is primarily determined by the chemistry of the electrolytes.

Currently, RFBs are primarily utilized for stationary storage applications, offering adaptable power-to-capacity ratios for medium- to long-term energy storage needs. They excel in three main markets: front-of-the-meter (FTM), behind-the-meter (BTM), and off-grid or remote applications.1,39 FTM applications, such as renewable energy shifting and deferral of transmission and distribution, leverage the high storage capacity, large depth of discharge, and long cycle stability of RFBs. BTM applications, such as power management, although less common, still benefit from the advantages of flow batteries, enabling the efficient use of renewables.

The membrane-free redox flow battery, using immiscible electrolytes, shows promise for various applications similar to conventional redox flow batteries. Once the technology reaches a TRL of 9, indicating commercial viability, it will compete with both vanadium and other non-vanadium RFBs that are currently under development.

The increasing share of renewables, coupled with the global drive to become carbon-neutral, highlights the clear need for installing long duration energy storage (LDES) systems to support the current power grid. Projections suggest that the global LDES capacity could reach from 85 TWh to 140 TWh by 2030, equating to ∼10% of global energy consumption.40 LDES systems must deliver energy for 4 to 12 even 24 hours to align with intermittent renewable energy patterns, creating a strong demand for sustainable, efficient, and cost-effective LDES solutions.

The innovation of scalable hardware concept for membrane-free redox flow battery needs to go in parallel with exploring business models and market entry strategies for membrane-free RFB systems. The current TRL of the hardware platform developed by Unbound Potential GmbH is 4/5.

For pilot systems, BTM applications are more suitable as they are less demanding and less regulated. Industrial-scale pilot systems for such application scenarios are in planning and will represent TRL 6–7. These systems will provide insights into operation under realistic conditions, while requiring simplified power control units and battery management systems. Such RFBs typically have less than 1 MWh of energy storage, with 4–8 h duration. BTM applications include the optimization of self-consumption for SMEs, e-mobility hubs in regions with insufficient grid connection, and back-up power applications.

However, the target market is infrastructure-relevant storage, requiring larger systems and higher maturity (TRL 8–9). Such batteries could be used to service green power purchase agreements (PPAs) and to provide ancillary services, as well as for energy arbitrage and trading on other energy-related markets.

To achieve the EU’s carbon neutrality goals by 2030 and 2050, LDES systems must support the energy transition with a minimal environmental impact. The use of organic aqueous RFBs is therefore key to ensuring the production of electrolytes, which is independent of raw material availability, avoiding critical mined materials, and being able to rely on local supply chains.

Conventional VRFBs are struggling with competing against the price of currently available Li-ion battery systems. In the cost structure of VRFBs, roughly 40% of the costs are associated with electrolytes, about 30% with the membranes, and about 30% with peripherals.41,42 The current membraneless RFB technology is not yet cost-competitive compared to vanadium redox flow batteries or lithium-ion cells. However, cost projections at larger scales are promisingly low thanks to the unique design concept. At Unbound Potential, we are confident that with proper scaling, we can outperform the current levelized cost of storage for conventional systems such as lithium-ion and vanadium. Membrane-free flow batteries designed by Unbound Potential benefit from at least 30% lower CAPEX merely by not requiring membranes. By developing a simplified design with passive control systems and fewer sealing interfaces, battery CAPEX is further drastically reduced.

The timeline for LDES systems to generate net profits currently is similar to the one of photovoltaic (PV) systems. This may change with different boundary conditions (dynamic energy prices and carbon pricing). The payback time for BTM applications might be much shorter, as it is not linked to the energy market but to specific use-cases that would not be feasible without a battery system (e.g., charging of electric cars in places with insufficient grid connection).

The existing competition includes VRFB systems, which have a fully scaled supply chain but vanadium-related issues with costs and supply chain, as well as technical challenges (e.g., changes in density, precipitation, crossover, and leakage). For 4- to 6-h storage, Li-ion systems are currently the most cost-effective option but are not ideal, requiring more cells for reaching extended discharge times and facing temperature stability concerns, especially in emerging countries such as Africa. Hydropower still is one of the most competitive LDES technologies but suffers from a severe environmental impact and its restriction to certain regions. There is a clear need to develop a scalable and robust LDES technology that is free from spatial and supply chain constraints.

The membrane-free redox flow battery based on immiscible electrolytes presents important features to overcome the drawbacks of conventional RFB to be widely implemented. This innovative technology has undergone patenting, demonstration, and extensive study since 2017, exploring various electrolyte combinations, such as aqueous/non-aqueous, non-aqueous/non-aqueous, and aqueous/aqueous, as well as hybrid configurations. Feasibility studies conducted in laboratory environments have successfully validated the concept under both static and flow conditions, paving the way for its development and scaling. Currently, the technology is in the development stage, with several stakeholders expressing interest in further advancing it toward the creation of a commercial demonstrator within the next few years.

The authors acknowledge the financial support from the Spanish Government (Grant Nos. PID2021-124974OB-C21 and TED2021-129378 B–C22) as well as the European Innovation Council through the MeBattery project (No. 101046742) and the European Research Council (ERC) through the MFreeB project (No. 726217). F.P. acknowledges the financial support from the Swiss National Science Foundation (Ambizione Grant No. 209056).

A.P.-M., D.P.T., E.D.R., P.G.R., and F.P. are the founding team of Unbound Potential GmbH, a start-up company developing a scalable hardware platform for membrane-free redox chemistries.

Paula Navalpotro: Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (equal). Jesus Palma: Funding acquisition (supporting); Project administration (supporting); Writing – original draft (supporting). Vanesa Muñoz-Perales: Investigation (supporting). Antonio Martínez-Bejarano: Investigation (equal). María Victoria Martín-Arroyo: Investigation (supporting). David P. Taylor: Funding acquisition (equal); Project administration (equal); Writing – review & editing (equal). Anetta Platek-Mielczarek: Investigation (supporting); Supervision (supporting); Writing – review & editing (supporting). Pier Giuseppe Rivano: Writing – review & editing (equal). Federico Paratore: Writing – review & editing (equal). Emilio Dal Re: Writing – review & editing (equal). Rebeca Marcilla: Conceptualization (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Supervision (equal); Writing – review & editing (equal).

The data are accessible in Zenodo repository  https://doi.org/10.5281/zenodo.14638523, Ref. 43.

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