Enhancing Mass Transport in Redox Flow Batteries by Tailoring Flow Field and Electrode Design

Abstract

In this study, we investigate the mass transport effects of various flow field designs paired with raw and laser perforated carbon paper electrodes in redox flow batteries (RFBs). Previously, we observed significant increases in peak power density and limiting current density when perforated electrodes were used in conjunction with the serpentine flow field. In this work, we expand on our earlier findings by investigating various flow field designs (e.g., serpentine, parallel, interdigitated, and spiral), and continuously measuring pressure drop in each configuration. In all cases, these perforated electrodes are found to be associated with a reduction in pressure drop from 4% to 18%. Flow field designs with a continuous path from inlet to outlet (i.e., serpentine, parallel, spiral) are observed to exhibit improved performance (up to 31%) when paired with perforated electrodes, as a result of more facile reactant delivery and resulting greater utilization of the available surface area. Conversely, flow fields with discontinuous paths which force electrolyte to travel through the electrode (e.g. interdigitated), are adversely affected by the creation of perforations due to the high permeability 'channels' in the electrode. These results demonstrate that mass transport can significantly limit the performance of RFBs with carbon paper electrodes. Redox flow batteries (RFBs) are widely considered to be a promising technology for grid-scale electrical energy storage, in applications such as buffering renewable energy sources and time-shifting energy from periods of high supply to periods of high demand (known as peak shaving).1-5 The unique aspect of RFBs, as compared with conventional secondary batteries, is that energy capacity and power output are decoupled from each other, such that the energy capacity depends on the volume of the electrolyte storage tanks and the concentration of active species in solution, while the power capacity depends on the total cell area and the number of cells in a stack. Although numerous redox chemistries have been proposed and studied, the all-vanadium chemistry is perhaps the most thoroughly characterized. [6][7][8] Regardless of the redox chemistry, the costs associated with these flow battery systems should be minimized to enable their widespread implementation. Previously, it has been shown that the costs associated with the cell stack constitute a substantial portion of the overall system cost.9 This has prompted numerous efforts to increase the power density, thereby decreasing the stack size needed for a given power output, and thus lowering the cost of the stack.10-24 Previous works have primarily focused on cell geometry, 19-24 electrode materials and functionalization, 10-24 and membrane selection. 20In 2012, significant improvements in power density were demonstrated by Mench and co-workers by utilizing carbon paper electrodes instead of more conventional carbon felt electrodes.19-22 Carbon papers tend to be much thinner than conventional carbon felt electrodes, and thus a...

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.…”

“…17,18,22,28 For example, Darling and Perry observed that increasing flow rate with the IDFF yielded a diminishing rate of return on reactor performance enhancement, hinting at complicated relationships between overall cell performance and flow rate. 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.…”

Towards Low Resistance Nonaqueous Redox Flow Batteries

“…Thus, electrolyte design for NAqRFBs must consider viscosity as a key materials optimization parameter, and, in the case of this work, MeCN vastly outperforms PC as a base solvent for a low viscosity and high conductivity electrolyte. Figure 4d hint that mass transfer rates are critical in determining the total ASR for RFBs, but only a handful of prior reports have systematically studied mass transfer effects in AqRFBs, 32,34,[60][61][62] with no reports directly relating to NAqRFBs. To begin addressing this knowledge gap, Figure 5 highlights nonaqueous flow cell performance for the PC-and MeCN-based electrolytes, with a Daramic separator, at 4 flow rates spanning greater than an order of magnitude.…”