Studies on Advanced Soluble Lead Redox Flow Batteries: Concepts to Contexts

Abstract

Redox flow batteries (RFBs) offer to make renewable energy storage more affordable, alongside accelerating opportunities for utility-scale development of solar/wind energy storage. Redox flow batteries (RFBs) have many advantages, namely flexibility in design and size in addition to meeting the desired quality of power and energy with the required response time to connect with grids. RFBs can be built and installed anywhere and are not site-centric, like hydropower stations. Soluble-lead-redox-flow-battery (SLRFB) represents a promising solution for large-scale energy storage. In relation to other redox flow batteries, soluble-leadredox-flow-batteries (SLRFBs) present several advantages, namely a single electrolyte tank and pump, no cost-intensive ion exchange membrane separator, and widely abundant, low-cost active materials. SLRFBs make use of variable oxidation states of lead wherein Pb²⁺-ions are dissolved in aqueous methane-sulfonic acid electrolyte that flows through the cells. During charging lead and lead dioxide are deposited at anode and cathode, respectively, which dissolve back into the electrolyte during discharge. Ironically, the development of SLRFBs is hindered due to Pb dendrite formation at anode, oxygen evolution at cathode, sludge formation due to passivation, sluggish kinetics of cathode, stack engineering of SLRFB and mitigation of shunt current. Studies in this thesis are primarily directed to enhance the energy storage capacity of SLRFBs and propel them from laboratory scale to a practical system by addressing key scaling-up issues relating to materials, fabrication, and engineering. The areal specific capacity of SLRFBs in most of the studies have been limited to only about 10 – 80 mAh/cm². The reported cycle life of SLRFBs is about 1350 cycles at 20 mAh/cm² and only about 10 cycles at 80 mAh/cm². Studies in the thesis attempt to address problems related to anode, cathode, single cell, and stack to achieve a functional 8V-3Ah SLRFB with 100 mAh/cm² areal specific capacity. Firstly, efforts are made to solve the problem associated with cathodes of SLRFBs, namely (i) oxygen evaluation and (ii) PbO₂ sludge formation due to passivation (iii) sluggish kinetics and (iv) most importantly, a low coulombic efficiency in initial cycles on increasing the specific capacity, which severely limits the performance of SLRFBs. The concept of using an activated cathode to reduce the number of cycles to achieve high coulombic efficiency in initial cycles, to prevent premature cell failure with low cycle-life, is introduced. To this end, graphite felt is activated by electrochemical and chemical methods to improve its performance as a cathode substrate for SLRFB. The problem at the anode associated with high specific capacity electrodes of SLRFBs is dendrite formation due to the preferential growth of metal at localised sites owing to the high conductivity of metal and the electrodeposition of Pb on the graphite felt surface, which causes (i) an increase in dendrite formation and (ii) inefficient space utilization of the graphite felt. Physico-chemical modification is introduced to realize a high capacity anode for SLRFBs. In this context, graphite felt is physico-chemically modified (PCGF) by means of perforations and chemical activation while collating with reticulated vitreous carbon (RVC) as the anode substrate for SLRFB to achieve a high areal specific capacity of 100 mAh/cm². Single cells of soluble-lead-redox flow-batteries are developed with a high areal specific capacity of 100 mAh/cm² employing pristine graphite felts and various physicochemically modified graphite felts. Studies on single cells are focused on rationalizing the passivation of cathodes and factors influencing the cycle life of the SLRFB. A mechanism for the passivation of cathodes is proposed based on the aforesaid findings. Finally, efforts are made to convert the concepts developed in the thesis into contexts by developing an 8 V / 3 Ah 5-cell SLRFB stack.