Materials Engineering for Advanced Redox Flow Batteries

The demand for cheap, safe, reliable and large scale energy storage has accelerated in the past years due to the the increased implementation of intermittent renewable energy sources. Aqueous redox flow batteries have the potential of meeting the aforementioned demands. In order to achieve that, various chemistries can be employed to optimize the reactor of a redox flow battery (RFB). This work focused on developing and analysing novel materials for use as electrolyte and separator in aqueous redox flow batteries.

Investigating the electrochemical properties of organic electroactive materials has the potential to accelerate the development of organic aqueous redox flow batteries. In that context, the electrochemical properties of pyrazines and quinoxalines was investigated. Different functionalized quinoxalines and pyrazines were were employed to elucidate the relationship between electrochemical properties and functional groups. Moreover, the effect of pH in the electrochemical response was also investigated. It was seen that the effective number of electrons during electron transfer in quinoxaline is affected by aggregation in concentrated solution. This relationship was further examined in anthraquinones. They are amongst the most well studied group of compounds used in RFBs. In the present study it was shown that the effective number of electrons was not affected by aggregation, despite signs of aggregation being observed in concentrated solutions. The reduction of electrons was only observed in the case of Anthraquinone Disulfonate (AQDS) in carbonate solution, and to a much lesser extent in the case of Dihydroxyanthraquinone (DHAQ) in carbonate solution. Lastly, a purely biosynthesized molecule (phoenicin) was employed investigated for use in RFB. It was shown that it can store up to 4 electrons per molecule. A capacity decay was observed during cycling, which led to a post cycling study identifying the main degradation pathways.

Currently, the most widespread separators used in redox flow batteries are based on perfluorosulfonated polymeric materials. A new non-fluorinated membrane consiting of poly[2,2’(4,4’oxybis(1,4phenylene))5,5’-bibenzimidazole] (OPBI) and Poly(vinylbenzylchloride) (PVBC) was investigated for use in VRFB’s. Three different amines were grafted onto the polymer blend and were systematically investigated. The results were correlated with most recent knowledge on ion exchange membrane (IEM) behaviour in high ionic strength solutions. The optimal OPBI/PVBC ratio with the grafted group was found and compared with a benchmark 10 μm mPBI membrane.