Uncovering the role of flow rate in redox-active polymer flow batteries: simulation of reaction distributions with simultaneous mixing in tanks

Abstract

Redox flow batteries (RFBs) are potential solutions for grid-scale energy storage, and deeper understanding of the effect of flow rate on RFB performance is needed to develop efficient, lowcost designs. In this study we highlight the importance of modeling tanks, which can limit the charge/discharge capacity of redox-active polymer (RAP) based RFBs. The losses due to tank mixing dominate over the polarization-induced capacity losses that arise due to resistive processes in the reactor. A porous electrode model is used to separate these effects by predicting the time variation of active species concentration in electrodes and tanks. A simple transient model based on species conservation laws developed in this study reveals that charge utilization and polarization are affected by two dimensionless numbers quantifying (1) flow rate relative to stoichiometric flow and (2) size of flow battery tanks relative to the reactor. The RFB's utilization is shown to increase monotonically with flow rate, reaching 90% of the theoretical value only when flow rate exceeds twenty-fold of the stoichiometric value. We also identify polarization due to irreversibilities inherent to RFB architecture as a result of tank mixing and current distribution internal to the reactor, and this polarization dominates over that resulting from ohmic resistances particularly when cycling RFBs at low flow rates and currents. These findings are summarized in a map of utilization and polarization that can be used to select adequate flow rate for a given tank size.

Polymeric Active Materials for Redox Flow Battery Application

“…Advances of characterization work for redox-active polymers in electrolyte solution are pivotal to understand the polymer RFB system and provide rational design of new redox-active polymers . (IV) Although the PRFBs are still in their early stage, reports for polymer RFB devices concerning operation parameters such as viscosity, electrode design, and flow rate have emerged. Those works will become more significant with the successful development of polymeric redox-active materials.…”

Spatially Resolved Analysis of an Organic Alkaline RFB to Investigate the Influence of the Operating Conditions

“…According to the model of Chen et al [21], the experiments of You et al [19] and of Milshtein et al [22], this phenomena is also explained by the bulk supply of active species and by the mass transfer coefficient variations with the flow rate. Nemani and Smith [16] emphasized the effect of the flow rate on the through plane reaction distribution, showing that regions of low reaction rate appear at low flow rate leading to high polarization losses. The polarization effect of the flow rate is also illustrated by the cycle efficiency EE.…”

“…Varying temperature 10 ≤ ≤ 40°C = 40 mA cm -2 ̇=100 mL min -1 As it was discussed by Nemani and Smith [16], it is important to compare the actual flow rate to the stoichiometric value, noted ̇. The flow factor or stoichiometric coefficient is the ratio between the electrolyte flow rate ̇ and ̇:…”

Organic Flow Batteries: Recent Progress and Perspectives

“…In addition, as the solubility and volume specific capacity of organic materials are also affected by the molecular weight, the rational molecular engineering may be applied in the future work to increase the relative content of redox-active submolecular units by deleting nonessential structural fragments. The reaction mechanism study for AORFBs is required to acquire excellent redox-active molecules/polymers. Although the charge transport mechanism research of organic small molecules and conjugated polymers has made great progress in the past few decades, new models of charge transport for organic materials, especially for nonconjugated polymers, still need to be developed to improve the power density and rate capability due to the structural diversity and complexity of organic materials. The operation parameters including flow rate, , pH, ,, viscosity, temperature, and the range of states of charge (SOC) as well as the design of battery structure, etc., need to be optimized because they are of importance to improve cell performance, to reduce operating costs, and to understand the working mechanism of AORFB systems, although the AORFB is still in its early stage. A recent work has first identified that the lifetime of the alkaline 2,6-DHAQ//K 4 Fe­(CN) 6 cell depends on quinone stability because 2,6-DHAQ is prone to undergo an irreversible dimerization to form an anthrone intermediate during the charge–discharge operation .…”

“…The operation parameters including flow rate, , pH, ,, viscosity, temperature, and the range of states of charge (SOC) as well as the design of battery structure, etc., need to be optimized because they are of importance to improve cell performance, to reduce operating costs, and to understand the working mechanism of AORFB systems, although the AORFB is still in its early stage. A recent work has first identified that the lifetime of the alkaline 2,6-DHAQ//K 4 Fe­(CN) 6 cell depends on quinone stability because 2,6-DHAQ is prone to undergo an irreversible dimerization to form an anthrone intermediate during the charge–discharge operation .…”