Polyoxometalate-based Redox Flow Batteries operating under mild conditions. From electrolyte formulation to cell performance

This PhD Thesis involves the development of a new chemistry for the electrolyte of Redox Flow Batteries (RFBs). Polyoxometalates (POMs) have been used as electroactive species due to their versatility has allowed the design of the properties of the RFB, with the main aim of this work being to develop an I) aqueous, II) highly efficient, III) symmetric RFB, IV) with high energy density and V) long-term stability, VI) operating under mild pH conditions. Within this work, a stepwise methodology for the design and construction of a POM-RFB has been established. 

Two cobalt-containing Keggin-type polyoxotungstates have been incorporated as redox active species. After the systematic study of their synthesis, the compositional optimisation of the electrolytes and the detailed study of their physicochemical and electrochemical properties has been carried out. Followed by the implementation of the developed electrolytes in a laboratory scale RFB, showcasing the high efficiency and durability of the developed POM-RFBs. Additionally, different membranes have been explored to boost the performance of the POM-RFBs and reveal the necessities of new chemistries for the electrolyte.

Therefore, this work demonstrates the potential of POMs for the electrolyte of RFBs. The developed POM-RFBs not only contribute to the development of new ESSs to address the current energy crisis, but also shed light on the methodology to build a new RFB and, more specifically, on the influence of the composition and POM structure on the cell performance.