Design and development of an electrolyte regeneration process for vanadium redox flow batteries

Vanadium redox flow batteries (VRFBs) represent an alternative solution to store the energy from an intermittent renewable source. Vanadium has four oxidation state: V2+, V3+, V4+, V5+ and these ions can be oxidized and reduced through electrochemical reactions. Thanks to these reactions the battery can accumulate or release energy causing them to be charged or discharged. Redox flow batteries are composed of two tanks, where an electrolyte solution is stored, and one stack of cells, where the reactions take place. Each cell is divided in two halves, where the electrode is located: one represents the positive pole and the other one the negative pole. These two half-cells are separated by a membrane which is permeable only for specific ions, such as hydrogen, and it doesn’t allow the diffusion of the vanadium ions from one half cell to the other. Already commercially exploited, the VRFBs exhibit fast response to the power requested from the grid, high efficiency, room temperature operation and an independent sizing between the power released and the energy stored. But their infiltration in the market is made difficult by their high price and their low energy density. Moreover, even though this battery can perform long charge and discharge cycle, its energy capacity can be affected by side reactions which take place into the electrolyte. Therefore, during long-term performance, the VRFB electrolyte is reported to become unbalanced. Imbalance implies an inequality in the amount of oxidized and reduced species in the two tanks and it is usually caused either by vanadium cross-over through the membrane, or by side reactions such as hydrogen evolution or air oxidation of V2+ ions in the negative side. While any capacity loss caused by a crossover effect can be readily corrected by simple periodic electrolyte remixing, that rebalances the electrolyte compositions and liquid levels in each reservoir, capacity losses by side reactions can only be corrected by chemical or electrochemical rebalancing of the oxidation state of the two half-cell electrolytes. The Fraunhofer Institut für Chemische Technologie in Pfinztal, which has been working on vanadium redox flow battery for years, has developed an electrochemical process which allows to regenerate the electrolyte. This process requires the use of a special reactor which, from a design point of view, is similar to the cell of a VRFB. In one half cell of this reactor, a special electrode, a dimensional stable anode, is used to promote the formation of oxygen in order to facilitate the reduction of the vanadium ions in the other half cell, which is important to rebalance the electrolyte. The goal of this thesis is to run experiments to characterize the new material used, to find a suitable end-point criterion for the recovering process and to calculate the cost of it. The work is organized in three chapters: in the first a comparison between different kind of energy technologies is reported and a detailed description of the vanadium redox flow battery is provided; in the second experimental results for the characterization of the dimensional stable anode are described and discussed and a mathematical model is developed in order to provide an end-point criterion of the recovery process; in the third chapter an economic analysis shows how to calculate the costs both for the vanadium redox flow batteries and the regeneration process by using also a numerical example