A Low-Permeability TEMPO-Phosphate as an Anionic Posolyte for Aqueous Redox-Flow Batteries.

Area of Science:

• Electrochemistry
• Materials Science
• Energy Storage

Background:

• TEMPO (2,2,6,6-tetramethylpiperidinyloxyl) is a promising redox-active material for aqueous redox-flow batteries.
• Improving TEMPO solubility and preventing membrane crossover are critical for practical applications.
• Existing anionic solubilizing groups for TEMPO are limited, hindering further development.

Purpose of the Study:

• To introduce a novel dianionic phosphate solubilizing group for TEMPO.
• To investigate the impact of this phosphate group on TEMPO solubility and membrane permeability.
• To assemble and evaluate a redox-flow battery utilizing the new TEMPO derivative.

Main Methods:

• Synthesis of a novel TEMPO derivative functionalized with a dianionic phosphate group.
• Characterization of the TEMPO-phosphate material's solubility and electrochemical properties.
• Assembly and testing of a redox-flow battery using TEMPO-phosphate and a viologen-based negolyte.
• Measurement of TEMPO permeability through a cation exchange membrane.

Main Results:

• The dianionic phosphate group significantly enhances TEMPO solubility.
• TEMPO-phosphate exhibits much lower permeability through a cation exchange membrane compared to sulfate-based TEMPO.
• The assembled redox-flow battery achieved a power density of 110 mW cm⁻² and a volumetric capacity of 10.6 Ah l⁻¹.
• The -2 charge of the phosphate group is key to reduced crossover.

Conclusions:

• The novel dianionic phosphate group is an effective strategy for designing water-soluble TEMPO derivatives with low crossover.
• This approach offers a straightforward method for improving redox-flow battery performance.
• The phosphate group can be adapted for other redox-active materials, broadening its applicability.