In a flow battery, the salient impact of the electrolyte velocity on the mass transfer coefficient in carbon felt electrodes is demonstrated and quantified. A lab-scale flow battery, fed with identical electrolyte solutions containing Fe2+/Fe3+ as active substances in both the anode and the cathode, is used to realize stable tests free from side reactions in a broad range of current densities. The electrolyte velocities ranging from 2.5 to 15 mm s(-1) are selected in this work, which are typical in flow through electrodes in most flow batteries. By measuring limiting currents at various flow rates, a correlation between the mass transfer coefficient and the velocity in dimensionless form is obtained as Sh = 1.68 Re-0.9. Meanwhile, a 2-D numerical model incorporating this correlation and the experimentally measured electrolyte conductivity is proposed. Voltage losses of the battery fed with adequate reactants at different velocities are both experimentally measured and numerically simulated. The agreement between simulated results and experimental data verifies the applicability of this correlation under normal operating conditions below limiting currents. (C) The Author(s) 2017. Published by ECS. All rights reserved.

Lab-scale redox flow batteries (RFBs) employing thinner electrodes have achieved outstandingly high power densities. When these high-performance thinner electrodes are scaled up to larger sizes required for kW-scale stacks, adding interdigitated flow fields is a simple solution in maintaining low pressure drops. A 3-D model of a half-battery with an active area of 900cm2 was developed to explore the design rules of flow fields. Optimizing the number and size of channels is essentially striking a balance between the pressure drop and the electrolyte velocity in the electrode, which have important effects on the pumping loss and mass transport loss respectively. In addition to the magnitude of the average velocity, the uniformity of velocity distribution should also be paid attention to in designing flow fields, which is determined by the ratio of flow resistance in the electrode to that in the channels. Acceptably thicker channels are recommended to improve uniformity of velocity distribution.

A symmetric cell using Fe2+/Fe3+ as a redox couple at both sides was fabricated to investigate the impact of electrolyte velocity on mass transport in porous electrodes. Polarization behaviors were measured at different velocities ranging from 1.7 to 20 mm s(-1). The significant mass transport loss is observed at velocities lower than 5 mm s(-1) even though the reactant stoichiometry is high enough. Starting from the lowest velocity of 1.7 mm s(-1), the improvement of cell performance with the increase of velocity is initially significant, which becomes minor as the velocity reaches a high value above 10 mm s(-1). According to the experimental results, velocities lower than 5 mm s(-1) should be avoided in designing flow batteries to prevent significant mass transport loss, while those higher than 20 mm s(-1) are not recommended because the slight benefit might be overweighed by the increment of pumping loss.

A 2-D steady state model was developed to investigate the impact of positive electrode thickness on the performance of a hydrogen-bromine flow battery (HBFB). With this model, the voltage loss is resolved into activation, ohmic and mass transport parts, which provides quantitative assessment of the effect of electrode thickness on the cell performance. Since hydrogen bromide solution serves as both reactant/product and supporting electrolyte, the ionic conductivity varies with the change of state-of-charge (SOC), which in turn affect the trade-off among activation, ohmic and mass transport loss. The impact of electrode thickness on mass transport is also complicated due to the variable distributions of local current and reactant concentration at different operating conditions. Since thinner electrode appeals for stringent mass transport rate in pore scale while thicker electrode leads to appreciable mismatch in the distribution between reactant concentration and reaction rate, the optimal electrode thickness are suggested to be in the range of 2-3 mm on the positive side in a HBFB. (C) 2015 The Electrochemical Society. All rights reserved.

In the porous electrodes of a flow battery operating under high current densities, the mass transport resistance between the bulk solution in the pore and the electrode surface contributes a significant proportion to the voltage loss, especially at the end of charging/discharging process. It is generally recognized that high flow rate of electrolyte is beneficial in reducing voltage loss [1, 2], since the mass transport resistance is closely correlated with the electrolyte velocity. The aim of this work is to investigate the effect of electrolyte velocity on the mass transport in a typical flow battery structure by using a symmetric cell configuration with identical redox couples on both sides. The redox couple Fe2+/Fe3+, owning fast kinetics, is utilized to minimize the activation loss. The electrolyte containing equivalent concentrations of ferrous and ferric ions was stored in a single reservoir and circulated through both sides of the cell in parallel, which can keep the inlet concentrations invariant all the time. The first experiment for investigating mass transport loss was conducted by feeding electrolyte containing 0.2M reactant at a low flow rate of 30 mL min-1, and the stoichiometry of feeding reactant is about 1.9 at 200 mA cm-2. As illustrated in Fig.1, the cell with thinner electrodes has significantly lower overpotential, partially due to its lower ohmic resistance. Operating at 200 mA cm-2, the voltage difference between cells with 2 mm and 6 mm thick electrodes is reduced from 133 to 70 mV by increasing flow rate from 30 to 120 mL min-1. These results suggest that in addition to the ohmic loss, mass transport loss also has significant disparity between different electrode thicknesses, especially at low flow rates. Compared with altering electrode thickness, adjusting flow rate is a favored approach to investigating the effect of electrolyte velocity on mass transport due to the negligible variation of ohmic resistance. However, the stoichiometry of feeding reactant varies with the flow rate and this influence on mass transport loss should be minimized. In the following test, the cell with the 6 mm thick electrodes was fed with the electrolyte at fixed flow rates but different concentrations. It is worth noting that at 30 mL min-1, increasing reactant concentration from 0.4 to 0.8 M improves the cell performance only a little and no improvements are found at 60mL min-1. Thus, the improvement of cell performance by increasing flow rate is mainly attributed to the resulting increment of electrolyte velocity, if the inlet electrolyte concentration exceeds 0.8 M. The decreases of overpotential caused by doubling the velocity are listed in Table 1, where the largest difference exists at the lowest velocity range and the higher current density. At a velocity lower than 5 mm s-1, significant mass transport loss was observed even though the stoichiometry of the reactant was high enough. However, extremely high velocity, higher than 20 mm s-1for instance, is not recommended, since the reduction of voltage loss is limited and this slight benefit might be overweighed by the increment of pumping loss. References [1] A. Tang, J. Bao, M. Skyllas-Kazacos, Studies on pressure losses and flow rate optimization in vanadium redox flow battery, Journal of Power Sources, 248 (2014) 154-162. [2] X.K. Ma, H.M. Zhang, C.X. Sun, Y. Zou, T. Zhang, An optimal strategy of electrolyte flow rate for vanadium redox flow battery, Journal of Power Sources, 203 (2012) 153-158. Figure 1