Shaswat Srivastava

Diagnosing optimum current density and electrolyte flow field for extended cyclability of soluble lead redox flow cells

Literature on the soluble lead redox flow battery (SLRFB) shows that high discharge capacity per cycle and extended cycle life could be attained under two exclusive operating conditions: (i) the highest possible operating cell potential (within range of no parasitic gas evolution), and (ii) intense agitation of electrolyte solution, respectively. Both these variations affect the particulate structure and crystallographic forms of PbO2 in deposits formed on the cycling of the PbO2/Pb2+ electrode. Our experiments with combinations of operating current density and agitation conditions attempt to diagnose the optimum operating conditions for stable extended cycling periods. As a rule of thumb, we observe that long cyclability is accompanied by a short span of low-potential charging in the two-step charging potential profile and a constant potential discharge terminating with a precipitous drop in the cell potential. Contrarily, a broad low-potential charging region (usually) accompanied by a gradually decaying discharge potential is a signature of deterioration. With careful testing across various combinations of testing conditions, we find that the variations (i) and (ii) could not surmount deterioration effects in the cell alone. The experiments suggest a double requirement of high (averaged) charging current density during both charge and discharge and simultaneous intense agitation of the electrolyte to attain stable round-trip coulombic efficiency for an extended cycling period. The experiments also show that cells with declining efficiency were accompanied by uncontrolled and profound variability across independent runs, given the start conditions were identical.