High-Energy-Density Chelated Chromium Flow Battery Electrolyte at Neutral pH

Area of Science:

• Electrochemistry
• Materials Science
• Energy Storage

Background:

• High-concentration electrolytes are crucial for increasing energy storage capacity in redox flow batteries (RFBs).
• Neutral pH electrolytes face challenges in dissolving metal ions compared to acidic ones, hindering energy-dense electrolyte development.
• Chromium 1,3-propylenediaminetetraacetate (CrPDTA) is a promising material for non-acidic RFBs.

Purpose of the Study:

• To achieve high-concentration, neutral pH operation of CrPDTA-based RFBs.
• To investigate the impact of buffer selection and pH on membrane performance during extended cycling.
• To expand the operational pH range of CrPDTA and evaluate its performance against a ferrocyanide posolyte.

Main Methods:

• Operating CrPDTA RFBs at neutral pH with concentrations of 1.2 M at room temperature and 1.6 M at 40°C.
• Conducting extended full cell cycling tests to assess membrane compatibility and buffer effects.
• Cycling CrPDTA at pH 7 against a ferrocyanide posolyte at 100 mA cm⁻².

Main Results:

• Achieved 60% higher negolyte capacity (up to 42.9 Ah L⁻¹) for CrPDTA at neutral pH compared to previous non-additive solutions.
• Demonstrated the critical role of buffer selection and pH management with the Fumasep E-620(K) membrane.
• Obtained excellent coulombic efficiencies (>99.7%) and energy efficiencies (>87%) at pH 7, operating significantly below the hydrogen evolution window.

Conclusions:

• High-concentration, neutral pH operation of CrPDTA RFBs is feasible, significantly boosting energy density.
• Optimized buffer selection and pH control are essential for stable membrane performance in CrPDTA RFBs.
• CrPDTA demonstrates robust performance across an expanded pH range, offering a promising avenue for efficient and safe energy storage.