Role of Side Reaction Involving PbOx in Soluble Lead Redox Flow Battery: Effect on Cyclability

Abstract

Soluble lead redox flow battery (SLRFB) has been an active subject of investigation in the past two decades because of its high energy efficiency (~70%) and charge efficiency (~90%) [1]. During charging, Pb2+ ions deposit as solid lead dioxide (PbO2 ) on anode and solid lead (Pb) on cathode, respectively. These features permit a membrane-less single-compartment design, making SLRFB a cost-effective electrochemical energy storage device. Discharge occurs by electro-dissolution of these solids, regenerating the electrolyte. The limited cycle-life of SLRFB is its current deployment challenge (best achieved about 100 cycles [2]). The shedding of solid lead dioxide over cycling is an indirect irreversible loss of active material from the electrolyte, the reason for SLRFB’s limited cyclability.Mathematical models link the deposit formation and the two-step charging potential profile through a side reaction involving PbO and PbO2 [3-5]. Morphological changes in deposit during cycling have been reported and presented as an alternate justification to the two-step potential profile [6]. To clarify these, we tested the role and feasibility of re-oxidation of solid PbO on cycling under varied conditions using an in-house prepared Nafion membrane-divided cell. We show that the charging through side reaction does not occur without the availability of Pb2+ near the electrode. We also show that the observations are independent of the electrodes’ age, the amounts of PbO and PbO2 present, and the applied current density. We obtained FESEM images of anode after short re-charging in a single-compartment design. We observed the formation of a new phase, plausibly lead dioxide from Pb2+ ions, leaving the solid residue almost unaffected. These findings challenge the idea of charging sustained in part by the oxidation of PbO through the side reaction. The deposit, constituted by whisker-like particles (thickness~200 nm), appears weak and layered. Gas evolution reactions known to occur under the limited availability of Pb2+ ions may promote residue disintegration. These observations lead us to hypothesize that cycling leads to irrecoverable residue accumulation, which may negligibly participate in further re-oxidation through the side reaction. These cumulative effects, accompanied by formation of a new phase at every charging, appear to contribute significantly to restricting SLRFB to a short cycle-life. These features raise concerns about the prevailing hypotheses about SLRFB dynamics and suggest need for strict design and operational controls to avoid residue buildup.