New Alternative Chemistries For Redox Flow Batteries

Energy storage is considered a crucial enabler for the introduction of Renewable Energy sources, especially intermittent and non-programmable ones (e.g., the sun and the wind) in the energy mix. Among the available emerging technologies, Vanadium Flow Batteries (VFBs) stand out in terms of efficiency, long-term storage and ease of deployment. However, there is still room for improving their performance, scalability, and environmental impact. Thus, this thesis addresses some of the most compelling issues of VFBs, namely: (i) the side reactions in the electrolytes, which might degrade the performance; (ii) the high cost, high toxicity, and low availability of vanadium; (iii) the crossover of active species through the membrane and (iv) the low energy density associated with the low cell voltage and the low solubility of vanadium species. Firstly, a formalism is implemented to study how the coordination species and their equilibria can influence the operation of a single-cell VFB. The formalism suggests that some of the reactions considered by the formalism can negatively contribute to the charge retention and to the charging of the battery. In particular, during charge, the estimate of the total energy wasted can account for ca. 22% of the total. Secondly, attempts to substitute vanadium with zinc and iodine (as anodic and cathodic active species, respectively) are pursued to cut costs and to increase the energy density of the device. The main problem faced by Zn-I2 FBs is the uneven and partially irreversible plating of zinc at the anode. In this thesis we explore the development of two sets of innovative electrolyte feeds, wherein both the pH and the chemistry of the redox species are modulated by the introduction of suitable buffers. In the presence of the acetic acid/sodium acetate buffer, battery performance is significantly improved: the energy efficiency rises to 78% from a 70% baseline without the buffer. In the same conditions, the energy efficiency rises to 82% upon the introduction of a tris(hydroxymethyl)aminomethane (Tris) hydrochloric acid buffer. The crossover of active species through the membrane is among the main causes of VFB performance degradation. To address this issue, “Zip-like” Ion Exchange Membranes (ZIEMs) are obtained by a macromolecular reaction between the cationic and anionic moieties of two different ionomers. It is shown that the resulting formation of ionic interactions between the ionomers cuts by a factor of 50 the crossover of vanadium species in comparison with the pristine ionomers taken separately. Single-cell VFB tests show a retained capacity of 82% after 100 cycles at 50 mA⸱cm-2 (in the same conditions, baseline Nafion 212 retains only 21% of the initial capacity). Hence, ZIEMs can be identified among the best performing IEMs for VFBs. The last chapter of this thesis covers the implementation of ion-exchange membranes (IEMs) for Non-aqueous Flow Batteries (NFBs). NFBs are considered an interesting alternative to VFBs as organic solvents in principle enable higher cell voltages, thus improving the energy density. However, the currently available IEMs often face an excessive swelling upon immersion in organic solvents. After a first screening, a family of ionomers based on polybenzimidazole (PBI) and Nafion is proposed. It is revealed that the interactions between PBI and Nafion significantly modulate the microstructure of the resulting IEMs, raising the conductivity and curtailing the swelling upon immersion in organic solvents. The best membrane of this family swells by only 7% after 900 hours in a 1:1 v/v ethylene carbonate/propylene carbonate solution. Conductivity tests in the same solvent dissolving a reference tetraethylammonium salt show conductivities in the order of 0.5-0.9∙10-3 S∙cm-1. On these bases, these IEMs can be considered suitable for implementation in NFBs.