Shaswat Srivastava

Re-oxidation of lead dioxide residue in Soluble Lead Redox Flow Battery (SLRFB): effect on cyclability

Soluble lead redox flow battery (SLRFB) has been an actively studied system in the past two decades, given its high energy efficiency of about 70% and charge efficiency of about 90% [1]. It is a cost-effective energy storage device with a similar capacity to other large-scale electrochemical energy storage devices. This is due to common electrochemical active species for both electrodes and the cell’s membrane-less single-compartment physical design. However, the number of cycles with stable output parameters achieved is less than 100 [2]. The degradation observables are the solid deposit shedding from the lead dioxide electrode and the gas evolution reaction. On the other hand, the two-step charging potential profile seen in the cycling of soluble lead redox flow battery (SLRFB) is still not fully understood in its dynamics. Several mathematical models predict the feature relying on the solid-solid side reaction within the lead dioxide residue [1,3].

The two-step charging has been envisioned as advantageous in terms of the voltage efficiency of the system. Contrastingly, cyclability is enhanced when the charging happens at the higher potential of the two steps [4]. Hence, an investigation was done to investigate the changes in deposition dynamics and deposit properties due to the recursive low-charging potential region. Using a modified electrolyte in a two-compartment Nafion-divided cell, we show that the side reaction does not occur without Pb2+ in the electrolyte. FESEM images of deposits formed on re-charging under normal conditions show the growth of lead dioxide from Pb2+ ions in the electrolyte on the residual deposit/electrode surface, challenging the idea of charging current sustained through the side reaction. These features raise concerns about the prevailing hypothesis about SLRFB dynamics. The observed deposit was layered, structurally weak, and constituted of whisker-like particles (thickness- 200 nm). We hypothesize the accumulation of such alterations over cycling to restrict SLRFB to a limited number of cycles. The observations could prove vital in designing efficient operating conditions and protocols for performance optimization.

References:
1. M. N. Nandanwar. Modelling and experimental investigations into soluble lead redox flow battery: new mechanisms. Ph.D. Thesis, Indian Institute of Science (2015).
2. J. Collins, G. Kear, X. Li, C. T. J. Low, D. Pletcher, R. Tangirala, D. Stratton-Campbell, F. C. Walsh, and C. Zhang, J. Power Sources, 195, 1731 (2010)
3. A. A. Shah, X. Li, R. G. A. Wills, and F. C. Walsh, J. Electrochem. Soc., 157(5), A589 (2010).
4. M. G. Verde, K. J. Carroll, Z. Wang, A. Sathrum, and Y. S. Meng. Energy & Environmental Science, 6(5), pp.1573-1581 (2013).