Redox Flow Batteries are rechargeable energy storage devices. They use two redox couple solutions (electrolytes) in each half-cell to store the energy. The electrolytes are held in external tanks, this means that the capacity of the system is determined by the volume of said electrolyte, while the power is determined by the size of the cell stacks. The ability to scale energy and power independently is one of the main advantages of RFBs.
For more information on Redox Flow Batteries (RFB) here is a comprehensive review on the topic.
Given the size of the electrolyte tanks, these installations have been traditionally aimed for larger capacity systems. However, there has recently been a growing interest in the translation of these principles into micro-scale batteries.
By scaling the technology down to micro-scale, researches hope to achieve a superior performance by increasing the control over the system’s performance, thus resulting in better utilization. There is a great opportunity in scaling down the size of this technology, transforming it into stackable and versatile storage units and saving costs.
Despite the difficulties that this field entails, membraneless micro redox flow batteries (mmRFBs) are an exciting topic with lots of potential.These systems are not yet available on the market, but significant progress towards their maturity has been achieved in the last 10 years.
The transition to micro-scale has faced researchers with many different challenges in the last decade:
The first and most important challenge was the lack of commercial micro-scale pumping devices that complied with the technical needs and requirements. There used to be no option available in the market that was precise enough and allowed for closed-loop recirculation.
The second major issue was the control of the miscible interface in the micro-scale reactor. To achieve precise interface control, the sensitivity and the resolution of the whole hydraulic setup needed to be very accurate.
A third relevant challenge is microfabrication of the necessary components. These processes are complex and prototyping at micro-scale incorporates many difficulties.
Implementation of a mmRFB with B5tec’s microfluidic devices.
B5tec’s implementations of different versions of micro RFB incorporate all our sensors and actuators.
Is used for systems where the electrodes are more compressed and a more powerful pump is needed, or where higher flow rates are needed. PDF systems incorporate pressurized gases as part of their functioning, they can be implemented using air or also inert gases like argon, meaning they don’t interact with the electrolyte and discharge it. The PDF system is used in combination with the flow controller.
Is used in the recirculation versions for a simpler, more accurate and adaptable operation.
Is used for systems where the electrodes are more compressed and a more powerful pump is needed, or where higher flow rates are needed. PDF systems incorporate pressurized gases as part of their functioning, they can be implemented using air or also inert gases like argon, meaning they don’t interact with the electrolyte and discharge it. The PDF system is used in combination with the flow controller.
Is used to monitor the state of the system. It provides information regarding asymmetrical charge loss or blockages in the microhydraulic system. This sensor allows for preventive maintenance and management of the system, it also optimizes the operation.
Can be implemented as an in-line measurement of battery operation given that vanadium’s viscosity is directly related to its state of charge. Therefore, viscosity measurements can give essential information for battery operation optimization.
Are used in membraneless designs. Microvalves are critical to ensure adequate flow distribution to control the interface between fluids.
Here we list some of the more relevant peer-reviewed research works of the last years:
“Electrical Model of a Membraneless Micro Redox Flow Battery – Fluid Dynamics Influence” by Bernaldo de Quirós et al (http://dx.doi.org/10.1109/ACCESS.2023.3273927 , 2023)
This work enhances the incorporation of the influence of microfluidic dynamics in the electrical response of membraneless micro redox flow batteries (mmRFBs) by introducing a modified equivalent electrical circuit with fluid dynamics parameters. The proposed models are validated with experimental data, showing accurate descriptions of the interplay between fluid dynamics and electrical response. This work provides relevant tools that push ahead in operation optimization for mmRFB, and the setup, including flow controllers and microvalves, has been crucial for this research advancement.
“Adaptive Microfluidic Modeling of a Membraneless Micro Redox Flow Battery Using Extended Kalman Filter” by Bernaldo de Quirós et al. (http://dx.doi.org/10.1109/ACCESS.2023.3313416 , 2023)
This article presents and validates an adaptative microfluidic model for the operation of a membraneless micro redox flow batteries (mmRFBs) . The model is developed with dynamic equations, real-time correction factors using a Kalmar-filter and careful instrumentation, that integrates the flow controller and the microvalves. Based on the presented model and the setup, for the first time, a closed loop control is implemented on a mmRFB. The implementation is validated through simulations and experiments, demonstrating pioneering advances in closed-loop optimization and adaptative microfluidic modeling. The setup used in this work has been crucial in obtaining the results presented in the article, since the instrumentation allows for high accuracy in the operation and control of fluids and their interfaces.
“Preconditioning Operation of Membraneless Vanadium Micro Redox Flow Batteries” by Oraá-Poblete et al. (https://doi.org/10.1002/batt.202300367 , 2023)”
This article presents a novel strategy for preconditioning operation in a membraneless vanadium micro redox flow batteries (mvmRFB) with automated closed-loop control. It lays the groundwork for successful charge-discharge cycling in mvmRFB. To do so, the preconditioning process is carefully studied, enhancing the importance of optimizing the flow rate ratio, the effects of low internal resistance and emphasizing the necessity of real-time monitoring of electrolyte concentrations to prevent charge imbalance. To implement the preconditioning process described, the instrumentation and hydraulic setup used has been crucial. The use of a flow controller and microvalves has been essential for the optimization of the preconditioning operation process.
“Deflection and control of the mixing region in membraneless vanadium micro redox flow batteries: Modeling and experimental validation” by de las Heras et al. (https://doi.org/10.1016/j.ijheatmasstransfer.2024.125921 , 2024)”
This article presents a new control system method that implements the PDF, the flow controller and the microvalve. With the suggested setup the article demonstrates the mitigation of undesired disturbances in the hydraulic circuit, thereby minimizing crossover losses. This work sets an important precedent in microfluidic device research by achieving a feasible control method that goes beyond anything previously suggested in literature. All this has been possible thanks to the use of B5tec’s devices.
“Membraneless Micro Redox Flow Battery: From Vanadium to Alkaline Quinone” by Torres et al. (https://doi.org/10.1002/batt.202400331 , 2024)”
This article presents the first proof-of-concept of a membraneless micro redox flow battery (MRFB) with automated closed-loop control, successfully achieving charge-discharge cycling using both vanadium and alkaline quinone electrolytes. The setup used to achieve this groundbreaking advance in mmRFB development incorporates B5tec’s flow controller and microvalves. The high precision of these devices has allowed to obtain the highest energy efficiency reported for such systems up to date, with the alkaline quinone version reaching 28.9% efficiency.
