The Dependence of Mass Transfer Coefficient on the Electrolyte Velocity in Carbon Felt Electrodes: Determination and Validation

Abstract

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 Fe 2+ /Fe 3+ 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. Owing to the exclusive advantage of decoupling power generation and energy storage, redox flow batteries (RFBs) have been considered as a critical candidate for large-scale electrical energy storage (EES). [1][2][3][4] Unlike the electrodes of conventional secondary batteries, the electrodes of RFBs do not participate in reactions but only provide electrochemical reaction sites for active ions and support electrons transfer. For numerous kinds of developed RFBs, the porous carbon felts, composed of randomly interlaced carbon fibers, are generally selected as electrodes. [5][6][7] Obtaining high voltage efficiency at high current densities is one of the major challenges in the commercialization of RFBs. Much efforts have been devoted to diminishing the activation loss by modifying the carbon fiber surface. [8][9][10][11][12] In addition, ohmic loss and mass transfer loss also lead to the reduction of the voltage efficiency. It is generally recognized that supplying high flow rate of electrolyte for the RFB is the most feasible method to minimizing mass transfer loss, but more pumping power is inevitably required for increasing flow rates. Tang et al. 6 and Ma et al. 7 carried out special research on optimizing the operating strategy of electrolyte flow rate for kilowatts class allvanadium flow battery (VRFB) systems, in which the significant impact of flow rate on cell voltage was detected during the whole charge/discharge process. In a carbon felt electrode, the mass transfer loss is caused by the species transport between the bulk solution in the pore and the carbon fiber surface, which is usually quantified by the mass transfer coefficient k m . 13 For cells operating under high current densities, the mass transfer loss also contributes a significant proportion to the voltage loss, especially when approaching the end of charge/discharge process. Considerin...

Exploring the Role of Electrode Microstructure on the Performance of Non-Aqueous Redox Flow Batteries

“…We note that some prior reports have chosen to describe electrolyte velocity as an average interstitial velocity by accounting for the electrode porosity in the equation above. 37,51 We elect not to do so here due to the uncertainty associated with porosity and its variation under compression for the different electrode materials. 53 Electrode performances are evaluated at four different electrolyte velocities, 0.5, 1.5, 5.0 and 20 cm s −1 , which correspond to a two order of magnitude range of volumetric flow rates, 0.87 mL min −1 to 75.60 mL min −1 .…”

“…29,33 Moreover, many emerging redox chemistries, particularly those based on organic couples, [34][35][36] have high kinetic rate constants (k 0 > 10 −3 cm s −1 ), and thus reduced activation overpotentials which may obviate the need for modifications of the electrode surface, at least for the purpose of enhancing redox reaction rates. To date, most of the published literature on this topic has focused on understanding and optimizing flow field design, [28][29][30][37][38][39] though there have been some efforts to describe mass transport within given electrode materials 29,40 as well as to engineer electrode structure post-process. For example, Maryhuber et al used a CO 2 -laser to perforate SGL 10AA carbon papers and demonstrated improvements (up to 30%) in the power density of all-vanadium RFBs, which they ascribed to enhancements in electrolyte accessibility.…”

A One-Dimensional Stack Model for Redox Flow Battery Analysis and Operation

“…Mass-Transfer Coefficients This electrode model depends on the value of the mass transfer coefficient for each redox-active species, and estimation of these values is an ongoing area of research [72,73,91]. In light of this, we opted to code in five different correlations from the literature [58,73,[91][92][93], as well as to provide the option for a custom mass-transfer correlation of the general form shown in Equation 10, where π i are dimensionless, empirical, user-defined constants, and Sh, Re, and Sc are the Sherwood (Sh = k m d f /D), Reynolds (Re = ρv e d f /µ), and Schmidt numbers (Sc = µ/ρD). The default fiber diameter (d f ) is 7 µm [94], and the viscosity (µ) and density (ρ) values for both electrolytes are taken to be 5 mPa s and 1.5 g mL −1 , respectively [8].…”

“…To start, Figure 5a shows the impact of mass transfer coefficient on the individual cell performance. The figure itself shows polarization curves at 50% SoC using three different mass transfer coefficient correlations: Schmal et al [92], Barton et al [73], and You et al [91]. While the actual conditions of the cell (1.5 M V, 2.12 L min −1 ) are the same, there is a large variation in the predicted mass transfer coefficients from these empirical correlations.…”

“…While the actual conditions of the cell (1.5 M V, 2.12 L min −1 ) are the same, there is a large variation in the predicted mass transfer coefficients from these empirical correlations. In recognition of the large variation and its impact on estimates of cell performance, we incorporated several empirical correlations [58,73,[91][92][93] directly within the application as well as the option to enter a user-specified correlation in the format of Equation (10). The more recent correlations predict smaller mass-transfer coefficients, and this should be kept in mind throughout the subsequent analysis in which the correlation from Barton et al [73] is selected as it has been demonstrated specifically with interdigitated flow fields.…”