Performance comparison of titanium–iron flow batteries using sulfuric acid and ethylenediaminetetraacetate as complexing agents

In this paper, we study titanium and iron electrolytes complexed with Ethylenediaminetetraacetate (EDTA) to be used in Redox flow batteries (RFBs) and compare their performance to that of the sulfuric acid-based system. RFBs represent a promising technology for stationary energy storage due to their ability to decouple power and energy, long cycle life, and safety. Vanadium RFBs are well established but limited by the high costs and resource scarcity of V. Alternative electrolytes based on Ti and Fe have shown promising potential in sulfuric acidic environment. However, the high corrosivity remains a significant drawback in terms of material compatibility and safety. The stability and solubility of Ti-and Fe-EDTA complexes are investigated as a function of pH and concentration of supporting electrolyte (KCl), identifying optimal operating conditions capable of maintaining the species in solution and ensuring a reproducible electrochemical response. Cyclic voltammetry is used to characterize both diffusional and kinetic parameters, confirming the reversibility of the complexed Ti (IV)/Ti (III) and Fe (III)/Fe (II) redox couples. Cell tests reveals that the Ti Fe system in H₂SO₄ achieves high efficiencies, while the Ti and Fe EDTA system displays superior cycling stability. Overall, the results indicate that, though system optimization is still required, Ti-and Fe-EDTA can represent a safer and more sustainable alternative to H₂SO₄-based electrolytes, opening new perspectives for the development more affordable and environmentally friendly RFBs. • Ti and Fe electrolytes were studied using EDTA and sulfuric acid as complexing agents for redox flow batteries. • Ti and Fe EDTA complexes show good stability and reversible redox behavior at pH 4–6.3. • Electrochemical performance and kinetic parameters were characterized by CV and cell tests. • EDTA-based systems have higher stability but lower energy efficiency compared to H₂SO₄-based systems. • Higher temperatures enhance the energy efficiency of EDTA-based system, reaching over 75%.