Thermodynamic Interactions as a Descriptor of Cross-Over in Nonaqueous Redox Flow Battery Membranes

Abstract

Grid-scale energy storage is increasingly needed as wind, solar, and other intermittent renewable energy sources become more prevalent. Redox flow batteries (RFBs) are well suited to this application because of the advantages in scalability and modularity over competing technologies. Commercial aqueous flow batteries often have low energy density, but nonaqueous RFBs can offer higher energy density. Nonaqueous RFBs have not been studied as extensively as aqueous RFBs, and the use of organic solvents and organic active materials in nonaqueous RFBs presents unique membrane separator challenges compared to aqueous systems. Specifically, organic active material cross-over, which degrades battery performance, may be affected by membrane/active material thermodynamic interactions in a fundamentally different way than ionic active material cross-over in aqueous RFB membranes. Hansen solubility parameters (HSPs) were used to quantify these interactions and explain differences in organic active material permeability properties. Probe molecules with a more unfavorable HSP-determined enthalpy of mixing with the membrane polymer exhibited lower permeability or cross-over properties. The HSP approach, which accounts for the uncharged polymer backbone and the charged side chain, revealed that interactions between the uncharged organic probe molecule and the hydrophobic polymer backbone were more important for determining permeability or cross-over properties than interactions between the probe molecule and the hydrophilic side chain. This result is significant for nonaqueous RFBs because it suggests a decoupling of ionic conduction expected to predominantly occur in charged polymer regions and cross-over of organic molecules via hydrophobic or uncharged polymer regions. Such decoupling is not expected in aqueous systems where active materials are often polar or ionic and both cross-over and conduction occur predominantly in charged polymer regions. For nonaqueous RFBs, or other membrane applications where selective organic molecule transport is important, HSP analysis can guide the co-design of the polymer separator materials and soluble organic molecules.

Transport Property Modulation via Solvent-Specific Behavior in Crosslinked Nonaqueous Membranes

“…In general, ferrocene permeability was greater than that of 4hydroxy-TEMPO, which was consistent with previous reports, 34 but the use of different de-swelling solvents (and the resulting differences in solvent uptake) appeared to affect the magnitude of this difference. For example, the ratios of ferrocene permeability to 4-hydroxy-TEMPO permeability for high swelling membranes (i.e., membranes made using ethanol as the de-swelling solvent) were 2.0, 1.8, and 3.4 for PC, DMC, and ACN measurement solvents, respectively.…”

“…RFB membranes need to resist active material crossover to prevent capacity fade as the battery cycles. Ferrocene and 4-hydroxy-TEMPO were used as representative active materials because they have been used in nonaqueous RFBs, and in our previous work, these compounds had different thermodynamic interactions with the backbone polymer and different permeability through POATS-PPO membranes . Permeability properties describe the tendency of the membrane to permit or block active material crossover.…”

“…Ferrocene and 4-hydroxy-TEMPO were used as representative active materials because they have been used in nonaqueous RFBs, and in our previous work, these compounds had different thermodynamic interactions with the backbone polymer and different permeability through POATS-PPO membranes. 34 Permeability properties describe the tendency of the membrane to permit or block active material crossover. Ferrocene and 4-hydroxy-TEMPO permeability properties for the crosslinked membranes are reported in Figure 3.…”

High-capacity polysulfide–polyiodide nonaqueous redox flow batteries with a ceramic membrane

“…However, the battery performance is still limited by the water stable potential window. Nowadays, nonaqueous redox flow batteries (NRFBs) become more and more attractive considering the apparent advantages for promoting the application of RFBs [27][28][29] . Firstly, more active materials can be reversibly adopted in NRFBs due to the various types of nonaqueous solvents.…”

“…Nowadays, nonaqueous redox ow batteries (NRFBs) have become more and more attractive considering the apparent advantages for promoting the application of RFBs. [27][28][29] Firstly, more active materials can be reversibly adopted in NRFBs due to the various types of nonaqueous solvents. For example, TEMPO in ethylene carbonate (EC) and diethyl carbonate (DEC) solvent, 30 and ferrocene (FeCp 2 ) and cobaltocene (CoCp 2 ) in N,N-dimethylformamide and 1,3-dioxolane (DOL) solvents 31 showed good reversibility and stability, which implies the diversity of NRFB solvents.…”