Reliability studies of vanadium redox flow batteries: upper limit voltage effect

Area of Science:

• Electrochemistry
• Materials Science
• Energy Storage

Background:

• All-vanadium redox flow batteries (VRFBs) are promising for long-duration energy storage (LDES).
• Performance fading over time is a major challenge for VRFB reliability.
• Understanding degradation mechanisms is key to improving VRFB lifespan.

Purpose of the Study:

• To systematically investigate the impact of increased upper limit voltages on VRFB reliability and degradation.
• To identify the primary degradation contributors (anode/cathode) at elevated voltages.
• To correlate electrochemical performance with electrode material changes.

Main Methods:

• Long-term cycling tests (500+ cycles) of a scaled VRFB cell (49 cm²).
• Varied upper limit voltages (1.6 V, 1.7 V, 1.8 V).
• Electrochemical impedance spectroscopy (EIS), polarization curves, and electrode characterization (morphological, surficial).

Main Results:

• Higher upper voltages significantly decrease capacity and voltage efficiencies.
• Electrolyte remixing partially restores voltage efficiency at 1.7 V and 1.8 V, indicating degradation.
• Anode degradation during charging and cathode degradation during discharging are amplified by higher voltages, with the anode being more affected.
• Elevated voltages increase resistance, primarily due to charge transfer resistance from the anode.
• The cathode showed severe surface degradation at 1.8 V.

Conclusions:

• Optimizing voltage limits is essential for enhancing VRFB lifetime.
• Degradation is voltage-dependent, with specific anode and cathode contributions.
• Accelerated stressor lifetime testing (ASLT) protocols can inform predictive models for VRFBs.