High-Voltage Aqueous Redox Flow Batteries Enabled by Catalyzed Water Dissociation and Acid–Base Neutralization in Bipolar Membranes

Abstract

Aqueous redox flow batteries that employ organic molecules as redox couples hold great promise for mitigating the intermittency of renewable electricity through efficient, low-cost diurnal storage. However, low cell potentials and sluggish ion transport often limit the achievable power density. Here, we explore bipolar membrane (BPM)-enabled acid–base redox flow batteries in which the positive and negative electrodes operate in the alkaline and acidic electrolytes, respectively. This new configuration adds the potential arising from the pH difference across the membrane and enables an open circuit voltage of ∼1.6 V. In contrast, the same redox molecules operating at a single pH generate ∼0.9 V. Ion transport in the BPM is coupled to the water dissociation and acid–base neutralization reactions. Interestingly, experiments and numerical modeling show that both of these processes must be catalyzed in order for the battery to function efficiently. The acid–base concept provides a potentially powerful approach to increase the energy storage capacity of aqueous redox flow batteries, and insights into the catalysis of the water dissociation and neutralization reactions in BPMs may be applicable to related electrochemical energy conversion devices.

Bipolar Membranes for Ion Management in (Photo)Electrochemical Energy Conversion

“…53 Alternatively, we experimentally observed a dramatic increase in the acid− base neutralization rate in the BPM by modifying the interface with a graphite oxide (GO) catalyst. 51 This effect was confirmed by comparing the experimental data to numerical models, in which the observed potential−current density curves could be fully reproduced only if both the water dissociation and acid−base neutralization reactions were enhanced by interfacial catalysts (Figure 6b). Understanding the mechanism at the microscopic level would further inform the design of BPMs under forward bias.…”

“…Comparison of the experimental cross-membrane potential vs current density of BPMs with and without interfacial catalysts under reverse and forward bias (lower right) . Reproduced with permission from ref . Copyright 2021 American Chemistry Society.…”

“…For example, the formal potential of sulfonated propylviologen (SPV) (∼0.58 V, vs Ag/AgCl) is unaffected by the electrolyte pH between pH 0 and 14 (0 mV/pH slope). Coupling SPV in an acid with ferrocyanide in a base, with the two electrolytes separated by a BPM, leads to a redox flow battery with an open circuit voltage (OCV) of ∼1.6 V, an increase of ∼0.7 V compared to the same cell operating with both molecules under alkaline conditions (Figure b) . The Pourbaix diagram of 1,7-disulfonate anthraquinone (DSAQ), on the other hand, shows slopes of −59, −29, and ∼0 mV/pH within the pH ranges of 0–7, 7–10, and 10–14, resulting in an OCV increase of ∼350 mV, close to the theoretical value (59 mV/pH × 4 pH units = 236 mV for pH 10–14, plus 29 mV/pH × 3 pH units = 87 mV for pH 7–10).…”

“…The acid–base configuration gave a higher open circuit potential (dashed lines), even though both used the same redox molecules, SPV and ferrocyanide (lower left). Comparison of the experimental cross-membrane potential vs current density of BPMs with and without interfacial catalysts under reverse and forward bias (lower right) . Reproduced with permission from ref .…”

Design principles for water dissociation catalysts in high-performance bipolar membranes

“…1a ). BPMs were first conceived as an ionic counterpart to current-rectifying semiconductor pn junctions 3 – 5 , and now are used in electrodialysis devices for desalination and acid/base production from brine 6 , fuel cells 7 , water and CO 2 electrolysis 8 , 9 , flow batteries 10 , and protonic diodes 11 . They provide distinct alkaline and acidic environments that are ideal for water oxidation and water reduction, respectively, offering new pathways for increasing electrolyzer performance while reducing or eliminating precious metals use 12 .…”