Redox Solid Energy Boosters for Flow Batteries: Polyaniline as a Case Study

Abstract

Keywords:redox flow battery redox targeting redox mediator solid charge storage materials polyaniline A B S T R A C TIn this work, the viability of polyaniline as a solid charge storage material in an aqueous redox-mediated flow battery is investigated. Fe(III/II) and V(IV/III) were identified as potential redox mediators to target the emeraldine-pernigraniline and leucoemeraldine-emeraldine redox transitions of the polymer. An indirect chemical cycling method was developed and used to investigate the charging/discharging of the polymer by the selected redox mediators. With Fe(III/II) as a redox mediator, respectable specific capacity and cycling stability were demonstrated over 25 cycles. V(IV/III) was deemed unsuitable as a redox mediator, due to rather poor kinetics. When added to the electrolyte tanks of a complete flow battery, a conductive composite of polyaniline and carbon black provided a significant improvement in capacity, exhibiting a specific capacity of 64.8 mA h g À1 at a current density of 38.5 mA cm À2. This represents a three-fold improvement in volumetric capacity, compared with the electrolyte alone. Moreover, the addition of the polyaniline composite was observed to lower the average potential at the positive electrode, providing a considerable improvement in voltage efficiency. This work demonstrates the potential of utilizing redox mediators to enable bulk solid-phase charge storage in the tanks of aqueous redox flow batteries.

Redox‐Targeting‐Based Flow Batteries for Large‐Scale Energy Storage

“…Recently, polyaniline (PANI), a polymer material that is stable and chemically active in acidic media, has been investigated as a solid charge‐storage material in an aqueous redox‐targeting‐based flow battery . As PANI has three well‐defined oxidation states and exhibits two reversible redox transitions in acidic environment, it was utilized as both the anode and cathode material to form an all RFB.…”

“…The ultimate goal for a lean electrolyte is no doubt resorting to single‐molecule redox‐targeting (SMRT) reactions, which not only reduces the redox targeting of energy‐storage material into “one molecule, one material”, but also “one molecule, one reaction”. In the SMRT reaction, the redox molecule is required to have an identical redox potential to that of the targeted solid material.…”

“…Therefore, the charge/discharge curve of an SMRT cell each exhibits only one voltage plateau, which greatly reduces the voltage hysteresis and boosts the energy efficiency. For instance, in the first reported SMRT‐based flow battery, the catholyte and anolyte only employed one redox species, Fe 3+ /Fe 2+ and V 3+ /V 2+ , respectively, to target PANI as the energy‐storage material . In another non‐aqueous system, a ferrocene derivative (FcIL) with nearly identical redox potential to that of LiFePO 4 was judiciously designed and synthesized .…”

Electrochemical Properties of the Ultrasonically Activated Thermally Expanded Graphite–Polyaniline Hybrid Material

“…The TEG CV was typical for carbon materials and had insignificant peaks at 500 mV on the charging curve and 400 mV on the discharging curve, which were associated with reversible oxidation and reduction of oxygen‐containing functional groups . On the CV curves of hybrid materials, a significant contribution was made by reversible oxidation‐reduction processes, which corresponded to the peaks at 600 mV on the charging curve and 300–400 mV on the discharging curve . These peaks were due to change in the oxidation state of the PANI.…”

Na3V2(PO4)3 as the Sole Solid Energy Storage Material for Redox Flow Sodium‐Ion Battery

“…[21] To take the advantage of high energy density of solid materials while without coating them on electrode sheets, redox targeting concept has been introduced and demonstrated in a variety of systems. [23][24][25] Despite the drastically enhanced energy density, the first-generation redox targeting-based RFBs which employed multiple redox species have relatively low voltage efficiency resulting from the potential difference of the redox molecules. Thus, by loading solid energy storage materials in the tanks, the energy density of RFBs can be boosted without sacrificing the merits of flow battery.…”