The main results are presented as follow:
1) Synthesis of new polyoxometalates (POMs) :ELECTRA project has been mostly focused on the following POMs: PV14; Fe4(PW9)2 and SiV3W9.
2) Evaluation of commertial POMs in flow cell:For that commercial POMs has been implemented in 0.5 M H2SO4 (typical electrolyte for those POMs allowing stability of the POM) SiMo12 / SiW12 and PMo12/OW12. Indeed, to maximize the success of the ELECTRA goals, substitution of W by more environmentally friendly atoms like Fe has been performed. Here, Fe4(PW9)2 can be an intriguing choice since a total of 8e- (4 e- from Fe and 4e- from W) can be involved in the redox process with the instability problems inherent in the structure. Under a performance perspective, PV14 and SiV3W9 show many benefits like easy to prepare in contrast to substituted POMs and, additionally, the synthesis yield is notable.
3) Evaluation of PV14; Fe4(PW9)2 and SiV3W9. In deep investigation has been carried out in order to implement the POMs in electrolytes using characterization techniques like FTIR and NMR. The stability of the POMs clusters is quite dependence of the pH. Thus, strong changes in the pH values could provoke a precipitation of the POMs and, in consequence, failure in the battery. ELECTRA project took a new approach with the implementation of the POM in to electrolyte using buffers instead of salts. In that approach, the H+ concentration changes can be mitigated for the action of the buffer. In this framework, it has been found that 1M acetate/acetic pH 3.7 for PV14 and pH 5 for Fe4(PW9)2 was the best one, provinding highly stability. For the case of SiV3W9 only the 3 atoms of V can operate under 1M acetate/acetic pH 5. Finally, the battery with PV14 and Fe4(PW9)2 has been demonstrated, achieving a capacity of 20 mAh. Particularly, the stability of the Fe4(PW9)2 is remarkable allowing the operation up to -1.2V (without the production of hydrogen evolution reaction) and more than 44 hours.
4) Implementation of bipolar electrochemistry into RFB for the first time, providing decrement of the resistance of the cell and allowing higher power densities as well as capacity . It is important to remar that the capacity obtained even surpasses the theoretical capacity of the electrolyte. This fact only can be explained by assuming that the potential induced in the BPE are enough to provide a contrary reaction of the driving electrodes, allowing the regeneration of the vanadium electrolyte.
5) Interfaces characterization and validation of components by employing new methodologies like Synchrotron x-ray absorption spectroscopy: i) Design of the cell specially for measuring in operando synchrotron radiation since it was measure for the first time in Alba facilities, ii) In operando measurements of Fe4(PW9)2..As a conclusion, the FePOM synthesised in ELECTRA project has been evaluated allowing insights of the stability and reversibility as well their structure.
in terms of exploitation and dissemination:The project results will be disseminated in scientific publications, which are under preparation. CF will first and corresponding author of both of them and will be published in Open Access, containing the EU funding.
In parallel, CF has been publishing 4 articles articles taking account her expertise in RFB in collaboration with her network. All publications are Open Access and contain the EU funding.
Conferences: DeCarbon International Conference 2023; EMRS 2022. In addtion it is planed :CF is going to the International Forum Flow Battery (Glasgow, 2024) where she has an accepted talk. In parallel, CF has an invited talk in the 75th Annual Meeting of the International Society of Electrochemistry" (2024, Montréal, Canada) within the symposium Electrochemistry - science and technology for a sustainable and better planet"
