Towards Long Cycle Life of Soluble Lead-Acid Redox Flow Batteries

A redox flow battery (RFB), with the freedom to design its power and energy capacity separately, is one of the most promising candidates for grid power management. The soluble lead acid RFB, which possesses characteristics of simple structure and inexpensive comprising materials, stands out for large-scale stationary storage. However, state-of-the-art lead-acid RFBs have limited cycle life and are not applicable to power grid applications. This study focuses on developing a single-flow, membrane-less, soluble lead acid RFB for grid-scale energy storage.

First, a pilot RFB based on soluble lead-acid chemistry and with a 2 cm² electrode area is designed and constructed. A parasitic pump with controllable pumping rate is utilized to circulate the electrolyte through the cell. Composite electrodes comprised of carbon and adhesive polymers are fabricated through hot pressing and slurry drying techniques. Different routes have been tried to prepare methanesulfonic acid solution, which serves to dissolve lead, to achieve desired concentrations. Various materials, including 316 stainless steel and graphite, are tested for their long-term stability when they serve as positive and negative current collectors. The performance of lead-acid RFBs with different combination of components is compared. We have achieved voltage efficiency and columbic efficiency both of over 80%, and energy efficiency of over 60% for over 100 cycles of deep charge and discharge.

In addition, activation of electrodes at low current density before cycling is found to be necessary for higher charge efficiency and longer cycle life. Although the mechanism is not completely understood, we find that the activation procedure able to enhance the attachment of lead dioxide on the positive electrode surface, which in turn leads to improved electrode stability. Electrochemical impedance spectroscopy is conducted and the charge transfer resistance after activation is found to be significantly reduced. Other pertinent thermophysical properties are acquired through electrochemical measurements.

Based on our optimization experience of the pilot cell, a scaled-up, soluble lead acid RFB with a 36 cm² area electrode is built. A magnetic drive pump is used to circulate the electrolyte through the electrolyte storage tank and the battery. The attached figure demonstrates the performance of lead acid redox flow battery with a 36cm² of electrode area, under an operation current density of 12mA/cm². New challenges facing scaled up lead-acid RFBs are investigated. For example, after the scaled-up lead-acid RFB is cycled for a long time, we often find certain yellow color byproduct resides at the corner of the electrode, which was rarely seen in the 2 cm² pilot cell. We believe the byproduct is induced by non-uniform current density distribution in a larger cell. Other factors that could facilitate extension of cycle life and improvement of charge/discharge efficiency will be discussed.