The performances of all-vanadium redox flow batteries (VRB) can be improved by increasing the electrode kinetics and/or by increasing the solubility of the electroactive species involved in the process [1]. The concentrations of vanadium species in the electrolyte determine the total electrolyte volume and energy density of the system, albeit relatively low for VRBs, between 30-50 Wh/kg [2]. Higher vanadium concentrations are desirable to achieve reasonable energy densities. It has been shown that the solubilities of V(II), V(III) and V(IV) species in H₂SO₄ increase with increasing temperature and decreasing acid concentration. Precipitation of V(II), V(III) and V(IV) have been reported at temperature below 10°C. V(V) however, does not precipitate as a sulphate but undergoes thermal precipitation by an endothermic reaction to form V₂O₅ at temperatures > 40°C. The maximum ion concentration is therefore set by the operating temperature of the VRB which lies between 10 and 40°C. The preferred electrolyte composition for this temperature range is a total vanadium ion concentration between 1.6-2 mol/L and a total sulfate concentration of 4-5 mol/L [3]. It is critical to stabilize V(V) species against thermal precipitation. One solution to increase both thermal stability and solubility of V(V) electrolyte is the use of a mixture of H₂SO₄ and HCl acids as supporting electrolyte [4], [5]. It has been reported that by using this electrolyte the V(V) solubility is increased to 2.5 mol/L while the electrolyte is stable in a temperature range between -5 and 50°C.

Initially we investigated the kinetics of V(V)/V(IV) redox couple in sulfuric acid and mixed electrolyte (H₂SO₄+HCl) using oxidized glassy carbon electrode surface [6]. Cyclic voltammetry and electrochemical impedance spectroscopy techniques were used for the electrochemical investigation. Additionally, the thermal and chemical stability of the V(V) and V(IV) electrolytes in H₂SO₄ and mixed H₂SO₄ + HCl were investigated using X-Ray absorption spectroscopy. Detailed knowledge regarding the V(V) coordination sphere in H₂SO₄ and mixed acids are of significant interest for understanding electrolyte stability but also for elucidating the V(V) electrochemical reduction mechanism. Finally, the change in atomic coordination sphere of V(V) and V(IV) electrolytes in H₂SO₄ and H₂SO₄/HCl were investigated in fresh and aged samples using K-edge EXAFS, which is sensitive to both the electronic configuration and the atomic structure around the V, Cl and S atoms.

References

[1] O. Nibel, S.M. Taylor, A. Patru, E. Fabbri, L. Gubler, T.J. Schmidt, Journal of Electrochemical Society, 164 (7), A1608 (2016).

[2] H.Chen et al., Prog. Nat. Sci. 291, 19 (2009).

[3] M. Skyllas-Kazacos, L. Cao, M. Kazacos, N. Kausar, A. Mousa, ChemSusChem, 9, 1521 (2016).

[4] Vijayakumar et al., Journal of Power Sources, 214, 173 (2013)

[5] M. Bon, T. Laino, A. Curioni, M. Parrinello, The Journal of Physical Chemistry C, 120, 10791 (2016)

[6] S.M. Taylor, A. Patru, D. Streich, M. ElKasi, E. Fabbri, T.J. Schmidt, Carbon, 109, 472 (2016)

Acknowledgements

Financial support of this work by the Commission of Technology and Innovation Switzerland (CTI) and the Swiss Competence Center for Energy Research Heat and Electricity Storage are greatly acknowledged. We acknowledge beam time at the PHOENIX beamline of the Swiss Light Source (Villigen, Switzerland).