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During the operation of an all-vanadium redox flow battery (VRFB), the electrolyte flow of vanadium is a crucial operating parameter, affecting both the system performance and operational costs. Thus, this study aims to develop an on-line optimal operational strategy of the VRFB. A dynamic model of the VRFB based on the mass transport equation coupled with electrochemical kinetics and a vanadium ionic diffusion is adopted to determine the optimal flow rate of the vanadium electrolyte by solving an on-line dynamic optimization problem, taking into account the battery capacity degradation due to electrolyte imbalance. Based on the measurement of modified open-circuit voltage, the extended Kalman filter (EKF) is implemented to estimate a change in the concentration of vanadium, which is used to on-line update the optimal vanadium flow rate. The results show that the on-line optimization of the vanadium flow rate incorporated with the EKF estimator can enhance the system efficiency (7.4% increase in state of charge) when the VRFB is operated under the intermittent current density. Also, it can prevent the battery voltage from reaching the limiting voltage before the battery achieves the desired state of charge.

An all-vanadium redox flow battery (VRFB) system, with multiple stacks, is typically used for large-scale electrical energy storage applications. In a VRFB system, pumps deliver positive and negative electrolytes, through a piping system, to each stack. Because the electrolytes are electrically conductive, shunt currents can occur within a multicell stack and within the piping system, connecting the stacks due to the voltage differences between cells and between stacks. Shunt currents cause energy loss and are affected by the number of cells in a single stack, the number of stacks, and the piping system dimensions. In this study, we develop a mathematical model, based on Kirchhoff's law, to locate shunt currents in a multistack system. Using this model, we estimate the charge efficiency with various numbers of stacks. The results show that the shunt currents in the central stacks are larger than the currents in other stacks. In addition, the piping system dominates the distribution of the electrolytes, and the shunt currents gradually shift from inside the stack to the piping system.

A proton exchange membrane fuel cell (PEMFC) system for the application of unmanned aerial vehicles is equipped without humidifiers and the cathode channels of the stack are open to the environment due to limited weight available for power sources. As a result, the PEMFC is operated under low humidity conditions, causing membrane dehydration, low performance, and degradation. To keep the generated water within the fuel cell to humidify the membrane, in this study, polyvinyl alcohol (PVA) is employed in the fabrication of membrane electrode assemblies (MEAs). The effect of PVA content, either sprayed on the gas diffusion layer (GDL) or mixed in the catalyst layer (CL), on the MEA performance is compared under various humidity conditions. The results show that MEA performance is increased with the addition of PVA either on the GDL or in the CL, especially for non-humidified anode conditions. The result suggested that 0.03% PVA in the anode CL and 0.1% PVA on the GDL can improve the MEA performance by approximately 30%, under conditions of a non-humidified anode and a room-temperature-humidified cathode. However, MEAs with PVA in the anode CL show better durability than those with PVA on the GDL according to measurement with electrochemical impedance spectroscopy.