Membrane-free Battery Research Articles

Towards Totally Aqueous Membrane-Free Flow Batteries: Fundamentals and Challenges

The implementation of renewable energies (solar, wind, etc) is driven the development of new energy storage systems that can mitigate their inherent intermittency solving the mismatch between the energy demand and production. Among different systems, electrochemical energy storage devices stand out due to their efficiency, flexibility and modularity. Specifically, Redox Flow Batteries (RFB) are the unique type of battery able to decouple energy and power densities which makes them versatile and adaptable to different applications. Nevertheless, although they are commercially available, their implementation is limited by the high cost and issues associated with some components such as the vanadium-based electrolytes and Nafion ion-exchange membranes. In this regard, our approach to overcome these limitations is the development of a membrane-free flow battery based on immiscible/biphasic electrolytes with organic active species1,2. In this talk, we will present a totally aqueous membrane-free battery based on a sustainable and safe aqueous biphasic system with near neutral pH and highly soluble organic active species. The aqueous biphasic electrolytes allow a selective thermodynamic separation of the active species instead of using any membrane or physical barrier. Thanks to a new flow-cell design3, it becomes the first example of aqueous membrane-free flow battery (Figure 1). The results show high coulombic efficiency (94%), capacity utilization (98%), and stable long-term performance over 250 cycles4. The fundamental aspects and the last advances in this innovative concept will be discussed.Figure 1 Schematic representation of the development of a membrane-free flow battery based on two aqueous biphasic electrolytes with organic active species.

A neutral pH aqueous biphasic system applied to both static and flow membrane-free battery

Here, we present a new aqueous biphasic system (ABS) with near neutral pH and containing highly soluble organic/organometallic active species that avoids crossover due to the thermodynamic separation of active species. This ABS become the best example of “Static” Membrane-Free Battery with the highest maximum theoretical energy density (21.7 Wh L−1), enhanced coulombic efficiency (96%) and stable performance over 250 cycles. Moreover, this work assesses the process of the self-discharge stablishing for the first time a protocol to quantify this phenomenon that results in low coulombic efficiencies in membrane-free batteries. Remarkably, a radically new membrane-free flow-reactor was specifically designed to maintain a stable liquid-liquid interphase that allows the battery to operate under flowing conditions. This resulted in flow battery with a two-fold increase of power density, high coulombic efficiencies and excellent capacity retention over 100 cycles. This work demonstrates for the first time the feasibility of this biphasic electrolyte concept in a truly Membrane-free “Flow” Battery.

Close Contact without Mixing: All-Aqueous Membrane-Free Flow Battery

The accumulation of energy produced by renewable sources to be used when demand is high is a key aspect of a truly decarbonized grid. Among all energy accumulation technologies, Redox Flow Batteries (RFBs) are specially promising due to its safety, flexibility of design and the ability to decouple capacity and power of the system. In a standard configuration, both redox electrolytes that are stored in external tanks are pumped in the electrochemical reactor separated only by an ion-selective membrane. This membrane is a critical part of the battery as it allows the selective pass of certain ions that compensate the charge imbalances while maintaining separated the active species in their respective compartments. Membranes used in RFB are expensive (up to 25% of the total cost in vanadium RFB), contribute greatly to the ohmic resistance of the battery, and the undesired crossover through it is never fully avoided which produces a performance decay of the battery over time. These issues are addressed by membrane-free RFB designs developed in our group.The idea behind MFreeB ERC project is to take advantage of the immiscibility of certain liquids to naturally separate redox active species in a membrane-free battery design [1]. Aqueous Biphasic Systems (ABSs) are a special example of all-aqueous natural occurring immiscible phases made by mixing water with two compounds that can be either polymers, inorganic salts or both. The two redox active species added to the biphasic system can be rationally selected and optimized so that they partition specifically to one of the two phases used as anolyte and catholyte [2]. These types of batteries have been proposed by several groups, including us, mostly as “static batteries” because filter-press reactors, typically used in conventional RFB, cannot be easily adapted to this membrane-free concept. In order to develop truly flowing systems, a totally different electrochemical reactor should be designed.Here, we report on the development of an electrochemical reactor that allows the formation of the two immiscible phases making possible the testing of immiscible batteries under flowing conditions for the first time. A counterflow flow-through configuration was used since it reduces flow instabilities while at the same time it allows high conversion rates in the carbon felt electrodes avoiding mass transport issues [3]. Several tests were performed in the battery; e.g. polarization curves, charge and discharge cycling and efficiency over time were measured. Moreover, the self-discharge phenomena, occurring near the interface when charged species enter in contact and provoking a loss of efficiency, was evaluated as a function of operating conditions. Special attention was given to the study of the different processes occurring in the region near the interface in order to mitigate losses and further optimize the design. Acknowledgments This work has been partially funded by project MFreeB which have received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 726217).

A membrane-free, aqueous/nonaqueous hybrid redox flow battery

While membrane-free batteries have been successfully demonstrated in static batteries, membrane-free batteries in authentic flow modes with high energy capacity and high cyclability are rarely reported. Here, we present a biphasic flow battery with high capacity employing organic compound in organic phase and zinc in aqueous phase. Under ambient flow testing conditions, a capacity retention of 94.5% is obtained over 190 charging/discharging cycles with a Coulombic efficiency of > 99% at a current density of 8.54 mA cm−2. Self-discharge at full state-of-charge of the membrane-free RFB is negligible (potential drop = 0.78 mV h−1). DFT calculation and operando UV-visible and FT-IR spectroscopies are performed to probe minor side reactions during cycling and monitor the water content in the organic electrolyte in real time. The successful demonstration of the prototypical membrane-free battery under flow conditions, together with the developed operando spectroscopic techniques, will open a new avenue towards detailed mechanistic studies and practical applications of redox flow batteries for cost-effective energy storage.

Challenges and Opportunities of Membrane-Free Redox Flow Batteries

Redox Flow Batteries (RFB) have been recognized as a viable technology for the integration of renewable energies into the electricity grids due to their ability to decouple power and energy and the possibility of storing large amount of energy at competitive cost. However, wide-spread implementation of conventional RFBs is limited by some obstacles related to low abundance, toxicity and high cost of vanadium redox compounds, and the poor-performing and expensive ion exchange membranes used to separate catholyte and anolyte compartments. This key element can account for more than 30 % of the cost of the battery (2) and may limit the long-term battery performance due to unavoidable cross-contamination and insufficient mechanical, chemical and electrochemical stability.Here, we present an innovative concept of Membrane-Free Battery which proposes to eliminate any separator or membrane by using immiscible redox electrolytes (3). These electrolytes spontaneously form a biphasic system whose interphase functions as a “natural” barrier making the use of any membrane superfluous. Moreover, the vanadium redox species are replaced by cheap and abundant organic redox molecules that can be specifically designed to exhibit high solubility, adequate redox potentials and suitable partition coefficient between the two immiscible phases. It will be demonstrated that this disruptive technology is hugely versatile with respect to the type of organic redox species as well as the nature of immiscible electrolytes forming the biphasic system. We report on the electrochemical performance of Membrane Free batteries based on aqueous/non-aqueous immiscible electrolytes (4) but also a Total Aqueous Membrane-free Battery in which the two phases are aqueous (5), which brings some advantages in terms of cost, environmental issues and battery performance. New challenges such as the inherent selfdischarge in the liquid-liquid interface, the need of developing new reactors designs to keep the two immiscible liquids separated, and future opportunities of this innovative technology making use of thermodynamics and computational chemistry will be also discussed.

Critical aspects of membrane-free aqueous battery based on two immiscible neutral electrolytes

Redox Flow Batteries (RFB) stand out as a promising energy storage technology to mitigate the irregular energy generation from renewable sources. However, some hurdles limit their massive implementation including high cost of vanadium and the poor-performance of ion-selective membranes. Recently, we presented a revolutionary Membrane-Free Battery based on organic aqueous/nonaqueous immiscible electrolytes that eludes both separators and vanadium compounds. Here, we demonstrate the feasible application of this archetype in Aqueous Biphasic Systems (ABS) acting as an unprecedented Total Aqueous Membrane-Free Battery. After evaluating several organic molecules, methylviologen (MV) and 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO) were selected as active species due to their optimum electrochemical behavior and selective partitioning between the phases. When connected electrically, this redox-active ABS becomes a Membrane-Free Battery with an open circuit voltage (OCV) of 1.23 V, high peak power density (23 mWcm−2) and excellent long-cycling performance (99.99% capacity retention over 550 cycles). Moreover, essential aspects of this technology such as the crossover, controlled here by partition coefficients, and the inherent self-discharge phenomena were addressed for the first time. These results point out the potential of this pioneering Total Aqueous Membrane-Free Battery as a new energy storage technology.

Exploring the Versatility of Membrane-Free Battery Concept Using Different Combinations of Immiscible Redox Electrolytes.

Lately, the field of redox flow batteries is flourishing because of the emergence of new redox chemistries, including organic compounds, new electrolytes, and innovative designs. Recently, we reported an original membrane-free battery concept based on the mutual immiscibility of an aqueous catholyte containing hydroquinone and an ionic liquid anolyte containing para-benzoquinone as redox species. Here, we investigate 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 2,2,6,6-tetramethylpiperidine-1-oxyl, 4-hydroxy-2,2,6,6-tetramethylpiperidine1-oxyl (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 open-circuit voltage (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, the 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.

A Membrane-Free Redox Flow Battery with Two Immiscible Electrolytes

Electrochemical energy storage devices are destined to be key technologies to alleviating the intermittent and fluctuating nature of renewable energy sources (1). In particular, Redox Flow Batteries (RFB) have been recognized as a viable technology for the integration and balancing electricity grids due to their ability to decouple power and energy. However, wide-spread implementation of conventional RFBs is limited by some obstacles related to low abundance, toxicity and high cost of vanadium redox compounds, and the poor-performing and expensive ion exchange membranes used to separate catholyte and anolyte compartments (2). Here, we present an innovative concept of Membrane-Free Battery which proposes to eliminate any separator or membrane in the cell by using immiscible redox electrolytes (Fig. 1). Moreover, the metal-based redox species are replaced by cheap and abundant organic redox molecules that can be specifically designed to provide high solubility and adequate redox potentials. As result, this disruptive technology is hugely versatile with respect to the nature of the electrolytes (aqueous / non-aqueous) as well as the type of organic redox species dissolved in each electrolyte (Fig. 1). To demonstrate the feasibility of this concept, we report here a spontaneous biphasic system formed by an acidic solution and an ionic liquid both containing dissolved quinoyl species that behaves as a reversible battery without any membrane or separator (3). The electrochemical performance of this Membrane-Free battery and the new challenges and future opportunities of the technology will be discussed. Figure 1 . a) Scheme of our Membrane-Free Battery concept. b) Versatility of this concept by using different combinations of immiscible electrolytes and their corresponding characteristics.

A Membrane-Free Redox Flow Battery with Two Immiscible Redox Electrolytes.

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 %).

Membrane-free battery for harvesting low-grade thermal energy.

Efficient and low-cost systems are desired to harvest the tremendous amount of energy stored in low-grade heat sources (<100 °C). An attractive approach is the thermally regenerative electrochemical cycle (TREC), which uses the dependence of electrode potential on temperature to construct a thermodynamic cycle for direct heat-to-electricity conversion. By varying the temperature, an electrochemical cell is charged at a lower voltage than discharged; thus, thermal energy is converted to electricity. Recently, a Prussian blue analog-based system with high efficiency has been demonstrated. However, the use of an ion-selective membrane in this system raises concerns about the overall cost, which is crucial for waste heat harvesting. Here, we report on a new membrane-free battery with a nickel hexacyanoferrate (NiHCF) cathode and a silver/silver chloride anode. The system has a temperature coefficient of -0.74 mV K(-1). When the battery is discharged at 15 °C and recharged at 55 °C, thermal-to-electricity conversion efficiencies of 2.6% and 3.5% are achieved with assumed heat recuperation of 50% and 70%, respctively. This work opens new opportunities for using membrane-free electrochemical systems to harvest waste heat.