Power efficiency optimization of vanadium redox batteries based on experimental analysis of electrolyte flow through carbon felt of electrodes

The Vanadium Redox Flow Battery (VRB) represents a significant opportunity for future Energy Storage Systems (ESS), which will be the crucial element in Renewable Power Plants. Main expectations of VRB relate to its prolonged service life, high-energy efficiency, outstanding dynamic response and flexible controllability during charge/discharge processes. The typical cell of VRB consists of two compartments (positive and negative) divided by a proton exchange membrane (PEM). The carbon electrodes in each compartment provide the electrochemical reduction-oxidation reactions in electrolyte. Carbon felt material as a rule is chosen for electrodes development due to its ability to provide intensive electrochemical reaction owing enlarged external surface and thus a sufficient current (power). The electrolyte on the base of sulfuric acid includes two pairs of vanadium ions with valences: (2+, 3+) in the negative compartment and (4+, 5+) in the positive one. The main volume of electrolyte is stored in two separate tanks and is pumped through both cell's compartments. There are two main reasons for electrolyte pumping. The first one is the restricted solubility of active vanadium species in sulfuric acid that leads to have an enlarged amount of electrolyte volume, which may be located outside of the cells only. The second reason is the need to decrease concentration polarization effects on the electrode surface. Electric current creates the layer of inactive ions on the electrode surface that increases internal electrical resistance, reduces electromotive force and the battery power. Electrolyte circulation eliminates the effect of polarization but causes hydrodynamic losses. They may be diminished by the optimization of electrolyte flow rate based on correct description of hydrodynamic properties of a carbon felt and on accurate depiction of battery electrical losses. The present research proposes a novel approach to optimization of electrolyte pumping with the purpose to obtain maximum VRB efficiency.