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.
Highlights
The intermittent nature of renewable energy sources such as solar and wind presents a barrier to their large-scale implementation, a problem that can potentially be addressed by an efficient and cost-effective means of electrical energy storage over periods of days
The bipolar membranes (BPM) is composed of a sulfonated poly(ether ether ketone) (SPEEK) cation exchange layer (CEL), graphene oxide (GO) as interfacial catalyst, and anion exchange layer (AEL) (30 μm, Fumasep FAS-30)
To study the BPM in isolation, we monitored the cross-membrane potential by using a four-electrode setup (Figure 2a), which eliminates the influence of the redox processes on the electrodes.17 0.1 M KOH and 0.1 M HCl solutions were placed on the AEL and CEL sides of the BPM, respectively, creating a pH gradient with an open circuit potential of ∼710
Summary
The intermittent nature of renewable energy sources such as solar and wind presents a barrier to their large-scale implementation, a problem that can potentially be addressed by an efficient and cost-effective means of electrical energy storage over periods of days. Asymmetric pH conditions have been studied using designs similar to desalination electrodialysis in which the AEM and CEM are separated by macroscopic electrolyte layers.[35,36] In this configuration, the charge carriers are the counterions of the acid and base (K+ and Cl− for KOH and HCl), and the interfacial electric field vanishes due to the large separation between the two membranes
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