Current Distribution in the Discharge Unit of a 10-Cell Vanadium Redox Flow Battery: Comparison of the Computational Model with Experiment.

Shunting currents are among the main problems of all-vanadium redox flow battery stacks since, in addition to capacity losses, they cause negative effects associated with the local destruction of electrodes and bipolar plates. The values of both the shunting currents and their destructive effects on materials can be reduced at the battery development stage by adjusting the resistance of the electrolyte supply channels. The solution to this problem can be found using a calculation model for current distribution based on the current balance in the nodes as well as voltage drops and electromotive force in internal circuits according to Kirchhoff's laws. This paper presents the verification of the model of current distribution in an all-vanadium redox flow battery stack of an original design that allows for the determination of membrane-electrode assembly resistances and electrolyte supply channels via direct measurements. Based on a comparison of the calculated and experimental values of the coulombic efficiency of charge-discharge cycles, the capacity fade associated with the crossover of vanadium compounds through the membrane has been determined.

Mixture of Anthraquinone Sulfo-Derivatives as an Inexpensive Organic Flow Battery Negolyte: Optimization of Battery Cell.

Anthraquinone-2,7-disulfonic acid (2,7-AQDS) is a promising organic compound, which is considered as a negolyte for redox flow batteries as well as for other applications. In this work we carried out a well-known reaction of anthraquinone sulfonation to synthesize 2,7-AQDS in mixture with other sulfo-derivatives, namely 2,6-AQDS and 2-AQS. Redox behavior of this mixture was evaluated with cyclic voltammetry and was almost identical to 2,7-AQDS. Mixture was then assessed as a potential negolyte of anthraquinone-bromine redox flow battery. After adjusting membrane-electrode assembly composition (membrane material and flow field)), the cell demonstrated peak power density of 335 mW cm-2 (at SOC 90%) and capacity utilization, capacity retention and energy efficiency of 87.9, 99.6 and 64.2%, respectively. These values are almost identical or even higher than similar values for flow battery with 2,7-AQDS as a negolyte, while the price of mixture is significantly lower. Therefore, this work unveils the promising possibility of using a mixture of crude sulfonated anthraquinone derivatives mixture as an inexpensive negolyte of RFB.

Analysis of the Composition of Bromide Anion Oxidation Products in Aqueous Solutions with Different pH via Rotating Ring-Disk Electrode Method.

We measured the ring collection coefficient of bromide anion oxidation products in a neutral and slightly alkaline medium on a rotating ring-disk electrode (glassy carbon disk, platinum ring) varying the following parameters: disk electrode rotation velocity, sodium bromide concentration, pH of the medium (in the range of 6−12), anode current on the disk, and the electroreduction potential of the bromide anion oxidation products on the ring. The data obtained are presented via dependences of the cathode ring current on the disk current ratio vs. the ring electrode potential. The analysis of the results was carried out by comparing the experimental polarization curves of the ring electrode with the data of cyclic voltammetry in model solutions to determine the electrical activities of various bromine compounds in positive oxidation states. We claim that the RRDE method could be used to obtain quantitative and qualitative data on the electrooxidation of bromide ions in neutral and alkaline solutions. For the most effective regeneration of the spent oxidizer, the values of pH > 10 and moderate concentrations of NaBr should be used.

Bromine Crossover in Operando Analysis of Proton Exchange Membranes in Hydrogen-Bromate Flow Batteries.

The manuscript deals with the fundamental problem of platinum hydrogen oxidation catalyst poisoning of the hybrid chemical power source based on bromate electroreduction and hydrogen electro-oxidation reactions. The poisoning is caused by the crossover of bromine-containing species through the proton exchange membrane separating compartments of the flow cell. Poisoning results in a drastic decrease in the flow cell performance. This paper describes the results of the direct measurement of bromine-containing species' crossover through perfluorosulfonic acid membranes of popular vendors in a hydrogen-bromate flow cell and proposes corresponding scenarios for the flow battery charge-discharge operation based on the electrolyte's control of the pH value. The rate of the crossover of the bromine-containing species through the membrane is found to be inversely proportional to the membrane thickness.

Two-Membrane Acid-Base Flow Battery with Hydrogen Electrodes for Neutralization-to-Electrical Energy Conversion.

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.

Electrolyte Flow Field Variation: A Cell for Testing and Optimization of Membrane Electrode Assembly for Vanadium Redox Flow Batteries.

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 .

Dataset of a vanadium redox flow battery 10 membrane-electrode assembly stack.

This paper contains a vanadium redox flow battery stack with an electrode surface area 40 cm2 test data. The aim of the study was to characterize the performance of the stack of the original design. The dataset include three series of galvanostatic charge-discharge cycling in the potential region 8-16 V with current densities 75, 150 and 200 mA/cm2 for 100 cycles. Coulomb, voltaic, energy efficiencies and capacity utilization coefficient are also provided for all three series.