Ph.D Thesis Colloquium : Shaswat Srivastava

Electrochemical-Morphological Evolution of the Positive Electrode in Autonomous Soluble Lead Redox Flow Batteries.

The world is witnessing an increasing reliance on renewable energy. According to the Global Energy Outlook, as of 2025, the renewable electricity capacity has risen to roughly 5000 GW (from 2000 GW in 2010). This progressive growth is to be supported through durable energy storage systems to overcome the intermittency and geographic limitations of the renewable energy systems. Redox flow batteries (RFBs) adequately fill this gap between energy generation and utilization. Installations of the most commercialized all-vanadium RFB (VRFB) up to 1GWh capacity and 200 MW power rating are now functional. However, the VRFBs with sluggish kinetics, divided half-cells, and requiring frequent replacement of the ion-exchange membrane pose operational and monetary difficulties.

The soluble lead redox flow battery (SLRFB) offers advantages such as an order of magnitude faster kinetics and cost-effectiveness. The Pb²⁺ ions are the common species for the redox couple. Hence, SLRFB’s architecture simplifies to a single-compartment single-electrolyte flow loop with no ion-exchange membrane. During charging, Pb²⁺ ions are deposited as PbO₂ and Pb on the positive and negative electrodes, respectively. Stored energy is recovered by dissolution of these solids and reduction of PbO2 solid to PbO solid through a reversible ‘side’ reaction on the electrode. These cyclic deposition-dissolution reactions involving the heavy Pb²⁺ ions induce strong concentration gradient driven natural convection flow in the electrolyte near the electrodes.

In this work, we have investigated the closed ‘standard’ cell and a modification of it, previously proposed by our group, called the ‘lift’ cell with electrodes mounted off the cell walls. The full-scale simulations of the SLRFB cells reveal compositional segregation in the former, leading to underutilization of the electrolyte and thereby operating with an adverse chemical environment around the electrodes. On the other hand, lift cells do better by circulatory flow around the electrodes, but only slightly. The present work shows through an analytical model that even with periodically well-mixed regions in the battery on account of natural convection, the active species concentration in the electrode region cycles between two steady states with time-invariant highly segregated regions above and below. We modify the lift cell design to induce natural convection flow that maintains high mass transfer coefficients at the electrodes and autonomously keeps the contents homogenized without any pumps and mixers.

By utilizing electrolyte additives, engineered electrode, or both, a life of 1000+ cycles has been demonstrated in the literature. However, a deeper understanding of the electrochemical and morphological features of the performance-determining positive electrode is necessary to propel the technology to commercial scales. Our investigations of the positive electrode show that the discharge is accompanied by the formation of interspersed, impervious, and poorly conducting morphology over an open 3-D porous structure. The electrode at this stage offers an electrochemically active surface area equivalent to the fresh electrode. The top layer is likely the PbO formed through the side reaction over the reduced PbO₂ slab. The subsequent charging, known to proceed with a potential spike and a low-potential phase of about 120 mV less than the potential recorded during the first charging, occurs through simultaneous oxidation of PbO and deposition of fresh PbO₂. The charging potential then climbs up mid-way to match the conditions of first charging once the side reaction is over. Interestingly, the side reaction pathway does not activate without Pb²⁺ ions at the electrode surface.

The potential peak recurring in the galvanostatic characteristics at the onset of every charge cycle is accompanied by the prolific nucleation of needle-like crystals on the residue-laden electrodes. Charging thus advances with further growth of PbO₂ on a continuously evolving morphology. Further, on subjecting the electrodes to short and systematic discharges, the potential peak at the onset recurs following a discharge of any duration. The duration of the subsequent (initial) low-potential charging phase correlates with the duration of preceding discharge. The observations suggest that the growth of PbO₂ leads to trapping of porous domains by preferred deposition at the outer periphery of a growing 3-dimensional morphology, starting every time with a freshly nucleated deposit. Overall, the cycling of the electrode results in forming a layered and mechanically weak structure. Thus, the residue cleaves as flakes and sheds due to its weight. These intermittently falling solids are a sink for the Pb²⁺ ions, and drain the electrolyte of its active ingredients.