In recent years, the key principles of the electric power industry have radically changed and the number of studies on the development of devices for electrical energy storage in a different form such as mechanical or chemical energy has rapidly increased. This review gives a brief description of these devices. The attention is focused on redox flow batteries (RFBs), a promising type of energy storage devices capable of efficiently operating in distributed power grids, in order to eliminate the imbalance between the time-varying electricity production by ‘unconventional sources’ and electricity consumption. At the design level, RFBs combine the principles of fuel cells and chemical energy sources with solid electroactive materials: transitions between electrical and chemical forms of energy in these devices occur upon oxidation and reduction of redox-active electrolytes, which are stored in separate tanks and pumped into the electrode compartments of the membrane electrode assembly (MEA) separated by a semi-permeable membrane. This approach ensures an important advantage of these devices over other types of chemical energy sources, that is, the possibility of independent scaling of the energy storage capacity and power characteristics of the system. This review provides a systematic description of the main types of RFBs and analysis of their fundamental benefits and drawbacks, which determine the prospects for practical applications of RFBs. The bibliography includes 149 references.

A great deal of research has been dedicated to improving the performance of vanadium redox flow battery (VRFB). In this work, we propose the design of a cell for testing membrane electrode assembly of VRFB, which enables the optimization of the flow field, conditions of charge‐discharge tests, and the nature of components (electrodes, membrane) with minimal time and material expenses. The essence of the proposed cell is that the system of channels distributing the electrolyte is made by cutting shaped holes in the sheets of graphite foil (GF). This manner allows easy modification of the flow field configurations. Polarization curves for serpentine, interdigitated, and flow‐through systems were measured according to procedures used in such studies. Cell with GF plates being tested with vanadium‐sulfuric acid electrolyte, outperforms the cell with conventional graphite plates with the same parameters of the flow field. It demonstrates 734 mW cm−2 of peak power density at SOC 50 and 84.3 % of energy efficiency at 84.5 % of electrolyte utilization under galvanostatic charge/discharge cycling with 75 mA cm−2.

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Summary Renewable energy in recent years plays an increasingly important role in the energy industry. Therefore, the problem of inventing efficient and affordable energy storage devices is of current importance. Vanadium redox flow battery stands out between a wide range of various chemical sources of electric energy. Discovering the way for optimizing the price of energy storage is one of the most important questions of the mentioned battery. New graphite‐polymer composite materials study shows that they may become a replacement for commonly used brittle graphite. The current article presents the study of graphite foil (or flexible graphite) filling with a copolymer of tetrafluoroethylene and vinylidene fluoride F‐42 properties. The article uses an implementation approach of putting the fluoropolymer into the pores of graphite foil, which shows a slight decrease in electrical conductivity, while a significant increase in its electrochemical stability in comparison with untreated graphite foil. Testing results of such material as current collectors (or bipolar plates) in a single membrane‐electrode assembly of vanadium redox flow battery cell show peak value of discharge power density of 843 mW/cm2, energy efficiency of 78%‐79% during cycling at 200 mA/cm2 allowing us to assume the possibility of its successful application in redox flow battery stacks.

Acid‐base flow batteries (ABFB) are a promising and environmentally benign class of flow batteries that utilize neutralization energy. Among the other flow batteries, ABFBs stand out with low cost and high solubility of the electrolytes and the possibility to harvest neutralization energy of acidic and alkaline wastewaters. However, the main ABFB issues, such as low power caused by discharge current limitation and low energy density, are limiting the possibility of their implementation. In this work, a novel two‐membrane ABFB with two hydrogen electrodes was developed to overcome main ABFB issues. The proposed concept demonstrated high power density up to 6.1 mW cm−2 at 13 mA cm−2. It was shown that battery performance was greatly limited by negative electrode overvoltage. Analysis of the voltage losses allowed to estimate main power losses and highlight the possible ways to its minimization.

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This review aims to highlight the current advances in hybrid redox flow battery (HRFB) technology, encompassing one of the best combinations of efficiency, cost and flexibility due to its module construction, which offers independent scaling of power density and energy capacity. This work emphasizes the interest of the scientific community both in (i) studying the properties and principles of HRFB operation in order to improve commonly proposed systems, and in (ii) the development of energy storage devices with new reagent types or RFB concepts. The data provided enhances the reader to conclude whether novel concepts in halogen oxidizers utilization could help to overcome the problem of insufficient power and energy densities of common RFB.

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