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SummaryAn increasing fraction of energy is generated by intermittent sources such as wind and sun. A straightforward solution to keep the electricity grid reliable is the connection of large-scale electricity storage to this grid. Current battery storage technologies, while providing promising energy and power densities, suffer from a large environmental footprint, safety issues, and technological challenges. In this paper, the acid base flow battery is re-established as an environmental friendly means of storing electricity using electrolyte consisting of NaCl salt. To achieve a high specific energy, we have performed charge and discharge cycles over the entire pH range (0-14) at several current densities. We demonstrate stable performance at high energy density (2.9 Wh L −1 ). Main energy dissipation occurs by unwanted proton and hydroxyl ion transport and leads to low coulombic efficiencies (13%-27%). Although the CGFB has low environmental impact, energy density and power density are too low to be attractive for practical application. KEYWORDSIn this work, we show that the energy density and power density of the CGFB can be improved by implementing a bipolar membrane. The studied system is an energy storage system based on a reversible acid-base reaction. In this system called acid base flow battery (AB-FB), energy is being stored in acid and base solutions created by the bipolar membrane. The charge step of the AB-FB is similar to the well-known bipolar membrane electro dialysis (BPM-ED). BPM-ED converts NaCl solutions into NaOH and HCl solutions by spending electric power to separate protons and hydroxyl ions from water dissociation from the bipolar ----------------------------------------------------This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

The increasing share of renewables in electric grids nowadays causes a growing daily and seasonal mismatch between electricity generation and demand. In this regard, novel energy storage systems need to be developed, to allow large-scale storage of the excess electricity during low-demand time, and its distribution during peak demand time. Acid–base flow battery (ABFB) is a novel and environmentally friendly technology based on the reversible water dissociation by bipolar membranes, and it stores electricity in the form of chemical energy in acid and base solutions. The technology has already been demonstrated at the laboratory scale, and the experimental testing of the first 1 kW pilot plant is currently ongoing. This work aims to describe the current development and the perspectives of the ABFB technology. In particular, we discuss the main technical challenges related to the development of battery components (membranes, electrolyte solutions, and stack design), as well as simulated scenarios, to demonstrate the technology at the kW–MW scale. Finally, we present an economic analysis for a first 100 kW commercial unit and suggest future directions for further technology scale-up and commercial deployment.

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