Water-soluble pH-switchable cobalt complexes for aqueous symmetric redox flow batteries

Area of Science:

• Electrochemistry
• Materials Science
• Sustainable Energy

Background:

• Redox flow batteries (RFBs) are promising for grid-scale energy storage.
• Development of efficient and stable electrolytes is crucial for RFB performance.
• Symmetric RFBs, using the same active species in both half-cells, offer potential cost and operational advantages.

Purpose of the Study:

• To investigate a water-soluble octahedral cobalt(II) complex as a symmetric electrolyte for aqueous redox flow batteries.
• To evaluate the electrochemical performance and cycling stability of the proposed symmetric RFB system.

Main Methods:

• Synthesis and characterization of a novel water-soluble octahedral Co(II) complex with four appended carboxylic groups (BCPIP-Co(II)).
• Assembly and testing of an aqueous, symmetric redox flow battery utilizing BCPIP-Co(II) as both the positive (posolyte) and negative (negolyte) electrolyte.
• Electrochemical evaluation including cyclic voltammetry, charge-discharge cycling, and long-term cycling stability tests.

Main Results:

• The BCPIP-Co(II) complex is water-soluble and forms a stable octahedral Co(II) coordination environment.
• The symmetric RFB demonstrated high coulombic efficiencies exceeding 99%.
• The system maintained high efficiency for up to 100 cycles, indicating good electrochemical stability.

Conclusions:

• A water-soluble octahedral Co(II) complex can effectively function as both the positive and negative electrolyte in a symmetric aqueous RFB.
• The demonstrated high coulombic efficiency and cycling stability highlight the potential of this Co(II) complex for practical energy storage applications.
• This work presents a viable strategy for developing cost-effective and high-performance symmetric redox flow batteries.