Mass transport and active area of porous Pt/Ti electrodes for the Zn-Ce redox flow battery determined from limiting current measurements

Abstract

The conversion of soluble cerium redox species in the zinc-cerium redox flow battery and other electrochemical processes can be carried out at planar and porous platinised titanium electrodes.The active area, current density, mass transfer coefficient and linear electrolyte flow velocity through these structures have a direct influence on the reaction yield and the relationship between cell potential and operational current density during charge and discharge of a flow battery. A quantitative and practical characterization of the reaction environment at these electrodes is required. The volumetric mass transfer coefficient, " $ has been calculated from limiting current measurements for Ce(IV) ion reduction in a laboratory, rectangular channel flow cell. This factor can be used to predict fractional conversion and required electrode dimensions. Highly porous platinised titanium felt shows superior " $ values and is wellsuited as a high performance electrode material.

Quantifying Mass Transfer Rates in Redox Flow Batteries

“…Additionally, increasing flow rate increases the limiting current density for the cell, again due to improved mass transfer in the porous electrode. This behavior is anticipated from prior experimental reports, 17,18,23,28 as well as our model, which indicates that faster mass transfer rates (smaller θ) lead to improved electrochemical performance. Figure 6b illustrates another trend previously reported in literature, 45 where increasing the active material concentration for one field (SFF) at a fixed flow rate (2 mL min −1 ) alleviates cell polarization despite a decrease in electrolyte conductivity ( Figure 5).…”

“…Several recent studies have shown that large performance gains are possible in RFBs through changes in the flow field type, as well as the electrode geometry or morphology. 15,17,18,[20][21][22][23][24] While such reports represent excellent engineering efforts to improve RFB power density, increases in mass transfer rates are rarely quantified, arguably due to insufficient knowledge of relevant transport processes within the ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address.…”

“…14. Several recent studies have shown that large performance gains are possible in RFBs through changes in the flow field type, as well as the electrode geometry or morphology. 15,17,18,[20][21][22][23][24] While such reports represent excellent engineering efforts to improve RFB power density, increases in mass transfer rates are rarely quantified, arguably due to insufficient knowledge of relevant transport processes within the specific electrode material. 14 The porous electrodes used for RFBs are typically comprised of loose cylindrical fiber beds where the porosity is ∼ 70-90%, 25 but prior electrochemical modeling efforts have described the mass transport rates in RFBs as a function of either mass transfer coefficients for packed bed reactors 18,26 or Bruggemancorrected diffusion coefficients.…”

“…17 While several reports have quantified mass transfer rates in electrochemical reactors containing parallel plate electrodes, with and without turbulence promoters, 29,30 only a limited number of studies have investigated mass transfer rates for RFBs with porous electrodes. 23,28 Quantifying and validating mass transfer rates in a RFB electrode requires a model of sufficient complexity to capture the underlying physics, but of sufficient simplicity to be coupled with experimental studies. 31 Specifically, in the case of extracting mass transfer coefficients, the model must be flexible enough to apply across a broad range of flow cell operating conditions and configurations, but also must not require knowledge of many experimental parameters.…”

“…Several studies have engaged finite element analysis to offer a detailed description of RFB operation, but these models are computationally intensive and require significant detail. 27,[31][32][33] Specific studies on mass transfer in flow cells with porous electrodes have utilized dimensionless correlations to quantify mass transfer in all-vanadium 28 or zinc-cerium 23 RFBs, but these models can only be applied to direct measurements of the cell's limiting current, which restricts mass transfer coefficient data collection to operating conditions with low flow rates and low active species concentrations. Prior dimensionless porous electrode models by Newman and co-workers are of interest for extracting values from experimental data due to their mathematic reduction to a small number of dimensionless parameters.…”