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2 Vanadium Redox Flow Battery Review Mass Transfer Mass transfer relates to the movement of the ions from the solution to the electrode surface. The rate of mass transfer to the surface can limit the current and hence power density. The current density becomes limited by mass transfer when the electron transfer kinetics are fast, occuring at large overpotentials. As reactants are quickly consumed at the surface the concentration of reactants at the surface reduces, reducing the current as defined in Eq 2.9 [58]. In a stagnant solution the only form of mass transfer is from diffusion, with the rate of mass transfer dependent on multiple variables, including the diffusion coefficient of the ions in solution, the concentration of ions and the solution temperature and viscosity [59]. When the solution is forced through an electrode, convection is the dominant source of mass transfer and is dependent on the flow rate of solution and the cell geometry [60]. During charging and discharging of VRFB the vanadium reactant species must be transported to the carbon felt electrode surface to enable high power densities in the cell. This becomes very challenging at both high and low SOC, as the concentration of the reactant is reduced significantly. If the mass-transport becomes too limited it can cause electrode degradation from oxidation, gas formation and a reduction in efficiency. Battery management systems are often used to control the current and electrolyte flow rate, managing pumping losses from increased flowrate whilst maintaining sufficient power output. At these extreme SOC conditions the power density of VRFB systems becomes limited and is termed concentration polarization [31, 61]. As a result, the concentration overpotential, ππΆ becomes the main source of energy loss within the cell, which is typically dominated by kinetic and contact resistance losses. The limiting current density ππΏ, is defined as [61]: ππΏ = ππΉπππΆπ (2.10) Where πΆπ is the bulk reactant concentration and ππ is the local mass transfer coefficient, which is a function of the convective fluid velocity, π£ [61]: ππ = 1.6 Γ 10β4π£0.4 (2.11) 18PDF Image | Electron Transfer Kinetics in Redox Flow Batteries
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