Aashish Thakur

Quantifying Internal Regeneration Mechanism In Soluble Lead Redox Flow Battery To Support High Cycle Life

The integration of renewable energy sources like wind, solar, and tidal into the grid supply requires the deployment of high-capacity energy storage systems, to absorb the varying rate of energy generation through the day. The soluble lead redox flow battery (SLRFB) [1] is an attractive alternative to the established lithium ion and vanadium flow battery technologies, due to the lower cost of installation and membrane-less operation. Operation of the battery involves deposition and dissolution of solid deposits on the electrodes during charge and discharge, respectively from an electrolyte consisting of lead methanesulfonate dissolved in methanesulfonic acid. Verde et al. [2] have reported performance data for 2000 charge-discharge cycles in a batch setup, with continuous intense stirring. The measurements showed sustained charge and energy efficiencies. The battery performance in a flow-cell for scaled up operation however shows less than 100 cycles of stable operations [3]. Progressive buildup of sludge and flakes blocks the flow pathways. The inability to achieve a scalable configuration for SLRFB remains an unsolved challenge.

The lead dioxide deposit on the positive electrode is known to have poor adhesion as it peels off from the substrate, leading to the loss of active species [1]. Reversible conversion of lead dioxide solid to lead oxide solid through a side reaction is also reported [1]. The calculations show that the reported charge efficiency of 95% over 2000 cycles in batch setup should be accompanied by the loss of 5% active material in each cycle, either as residue on the electrodes or disintegrated material into the bulk with no electrical contact with the electrodes. Under these conditions, the battery can sustain no more than 300 charge-discharge cycles. An analysis of the solid residue collected from the electrodes and in the bulk electrolyte for a similar stirred batch vessel shows that from a large amount of residue collected and dissolved in methansulfonic acid, only a negligibly small amount of lead survives. These measurements establish that any undischarged lead and lead dioxide present in stoichiometric amounts replenish the active species, and the battery continues to cycle with 5% loss in charge efficiency.

We propose that flaking and disintegration of lead, lead dioxide, and lead oxide continues during discharge under high agitation. An internal regeneration mechanism transforms the lead and lead dioxide to lead ions when they come together in the bulk. The chemical dissolution of lead oxide recovers lead ions from it under acidic conditions through a parallel route. Various pathways eventually replenish the lead ion. We propose a three-step cyclic-steady-state operation—deposition during charge, degradation and electro dissolution during discharge, and continuous regeneration of active ions from the flakes and degraded deposits.

We have designed detailed experiments and characterization to independently validate and quantify the regeneration mechanism and determine the factors that influence it, in an operate to arrive at a flow cell design at similar charge and energy efficiencies. The concentration of the active species (Pb2+) at all stages stages of close circuit loop—electrodeposition, electro-dissolution, deposit degradation, dissolution of degraded flakes and powder, and conversion of PbO to lead ion, and back is measured. The measurements involve physical and material characterization of electrodes, structural strength of the deposit, shedding of deposits, and constant monitoring of electrolyte concentration. The mass balance established between deposits on the electrode, deposits at the bottom and the electrolyte concentration, obtained from these results support the proposed mechanism.

References
1. Hazza et al, Physical Chemistry Chemical Physics, 2004, 6, 1773-1778
2. Verde et al, Energy & Environmental Science, 2013, 6, 1573
3. Collins et al, Journal of Power Sources, 2010, 195,