Simulation Of Factors Influencingon The Capacity Decay In All Vanadium Redox Flow Batteries

The all vanadium redox flow battery (VRB) is a promising technology for the middle to large scale stationary energy storage because of its cost-efficiency, long cycling life time and decoupling of its storage capacity and power. In VRB, the imperfections of the ion exchange membrane is the core reason which leads to the penetration of vanadium ions in the positive or negative electrolytes through the membrane to another side. The unfavorable penetration of active vanadium ions through the membrane leads to a series of side reactions, and finally causes imbalance of concentrations and quantities of vanadium ions between positive and negative electrolytes, in turns resulting in the capacity decay of VRB during the cyclic process. Therefore it is of great importance to study influence of the operation conditions and parameters of key materials in batteries on such unfavorable penetration, as well as the correlation of the penetration with the capacity decay of the batteries. In practiced experiments, it is difficult to optimize these parameters in high through-put manner. Therefore, this thesis describes our studies on the influence of velocity of electrolytes, porosity of the electrode and thickness of ion exchange membrane on the capacity decay during cycling of VRB.At first, a two-dimensional isothermal transient model of VRB was established. Quantitative studies the capacity decay during the cycles were carried out regarding the vanadium ions’ penetration of differences of concentrations of vanadium ions between two electrolytes, and capacity decay during the cycling. On the basis of this model, the influences of the operating conditions (velocity of electrolyte) and the material properties (porosity of electrode and thickness of the ion exchange membrane) on the capacity decay were studied respectively. The modeling results show that, as the flow rate changes from 20 to 40 ml · min-1, the battery capacity decay is slower, probably because the different flow rate changes the concentration distribution in battery. As the porosity of the graphite felt changes from 0.5 to 0.9, battery capacity decay is inhibited, because the different porosity will cause different electrode reaction current distribution and different effective ion diffusion coefficient, thus affecting ion concentration distribution in the electrolyte. As the thickness of the ion exchange membrane changes from 100 to 250 μm, the battery shows decreased capacity decay, the ability to block the ions penetration with thicker membranes is due to the longer diffusion time of ion penetration through the membrane.The above modelling results on capacity decay analysis of VRB can provide useful information on the design of stacks and optimization of operation conditions of VRB.