Resistance Breakdown of a Membraneless Hydrogen-Bromine Redox Flow Battery

Recent advancements in membrane-free redox flow batteries

Membrane-free redox flow batteries (RFBs) are promising energy-storage technologies that present an innovative solution to address the critical need for sustainable and efficient energy systems. This review provides a detailed examination of membrane-free RFBs, focusing on recent technological advances and design optimization. Moreover, it highlights the growing importance of membrane-free designs for achieving higher efficiency and scalability in energy-storage systems. These designs offer significant improvements in terms of electrolyte concentration, Coulombic efficiency, and flow management, underscoring the potential of these systems for advanced energy-storage solutions. We explore the utilization of immiscible electrolyte solvents and the engineering of laminar flow dynamics to achieve efficient electrolyte separation without traditional ion-exchange membranes. The article discusses metal-free and metal-phase aqueous/nonaqueous and nonaqueous/nonaqueous immiscible solvent-based RFBs; laminar flow-based RFBs; single-phase co-laminar flow batteries; liquid/solid membrane-free RFBs; and triphasic membrane-free RFBs, highlighting their unique design features and operational benefits, as well as their potential and challenges in energy-storage applications. Key parameters such as the coulombic efficiency, self-discharge, flow dynamics, and impedance are analyzed to provide a comprehensive understanding of the performance metrics critical for the development of next-generation membrane-free RFBs. We provide valuable references for developing membrane-free RFBs and highlight their significance, technological advancements, and implications for future energy-storage applications. In the context of global energy transitions, the research and development of membrane-free batteries will provide crucial technical support for achieving sustainable energy development.

Balancing pH and Pressure Allows Boosting Voltage and Power Density for a H2-I2 Redox Flow Battery

The decoupled power and energy output of a redox flow battery (RFB) offers a key advantage in long-duration energy storage, crucial for a successful energy transition. Iodide/iodine and hydrogen/water, owing to their fast reaction kinetics, benign nature, and high solubility, provide promising battery chemistry. However, H2-I2 RFBs suffer from low open circuit potentials, iodine crossover, and their multiphase nature. We demonstrate a H2-I2 operation with a combined neutral-pH catholyte (I3 -/I-) and an alkaline anolyte (KOH), producing an open circuit cell voltage of 1.28 V. Additionally, we incorporate a pressure-balanced gas diffusion electrode (GDE) to mitigate mass transport limitations at the anode. These improvements result in a maximum power density of 230 W/m2 when allowing a mild breakthrough of H2 through the GDE. While minimal crossover occurs, side reactions of permeating active species were found reversible, enabling long-term operation. Future work should address the stability of the GDE and optimization of the electrolyte thickness and concentration to fully leverage the potential unlocked by balancing the pressure and pH in the H2-I2 RFB.

Practical potential of suspension electrodes for enhanced limiting currents in electrochemical CO2 reduction

CO2 conversion is an important part of the transition towards clean fuels and chemicals. However, low solubility of CO2 in water and its slow diffusion cause mass transfer limitations in aqueous electrochemical CO2 reduction. This significantly limits the partial current densities towards any desired CO2-reduction product. We propose using flowable suspension electrodes to spread the current over a larger volume and alleviate mass transfer limitations, which could allow high partial current densities for CO2 conversion even in aqueous environments. To identify the requirements for a well-performing suspension electrode, we use a transmission line model to simulate the local electric and ionic current distributions throughout a channel and show that the electrocatalysis is best distributed over the catholyte volume when the electric, ionic and charge transfer resistances are balanced. In addition, we used electrochemical impedance spectroscopy to measure the different resistance contributions and correlated the results with rheology measurements to show that particle size and shape impact the ever-present trade-off between conductivity and flowability. We combine the modelling and experimental results to evaluate which carbon type is most suitable for use in a suspension electrode for CO2 reduction, and predict a good reaction distribution throughout activated carbon and carbon black suspensions. Finally, we tested several suspension electrodes in a CO2 electrolyzer. Even though mass transport limitations should be reduced, the CO partial current densities are capped at 2.8 mA cm-2, which may be due to engineering limitations. We conclude that using suspension electrodes is challenging for sensitive reactions like CO2 reduction, and may be more suitable for use in other electrochemical conversion reactions suffering from mass transfer limitations that are less affected by competing reactions and contaminations.