Redox flow batteries (RFBs) are one of the strong candidates for large-scale energy storage needed for effective use of solar and wind energy [1]. A redox couple in association with vanadium ions has been utilized for industrial applications and further improvement of cell performance that can lead to reduction of the system cost is of great importance for market penetration. Recently, cell performance has been drastically improved by replacing conventional thick carbon felt electrodes with thin carbon paper electrodes [2-4]. Heat and/or acid treatments applied to carbon electrode materials has been also demonstrated to improve reaction kinetics in all vanadium redox flow batteries (VRFBs) [2-4]. To develop carbon electrodes with higher catalytic activity for VRFB application, clarifying reaction kinetics [5], as well as performance limiting electrode under cell operation are strongly needed. Agar et al. utilized an asymmetric configuration to identify performance limiting electrode and showed the negative electrode limit the cell performance in VRFBs [3]. Aaron et al. applied a dynamic hydrogen reference electrode to an operating VRFB and found larger kinetic polarization at the negative electrode [6]. In these literature, cell polarization at high current density conditions was not fully examined.

In this study, we examined cell polarization of a VRFB with an interdigitated flow filed that was applied to achieve high current density operation [7]. We investigated electrode losses in the VRFB cell operated up to 1A/cm²by using a dynamic hydrogen reference electrode inserted into the cell [6].

We used heat-treated and untreated (as-received) carbon porous materials. Heat treatment to the electrodes was performed in the air or nitrogen atmosphere. Figure 1 shows IR-free overpotentials assigned to the negative (Fig.1(a)) and the positive (Fig.1(b)) electrodes, respectively. As reported in the literature [3,6], the negative electrode showed slow kinetics, but was intensively improved by heat treatment in the air. This suggests that catalytic activity of the electrode was enhanced by oxygen-enriched functional groups that also causes better wettability of the electrolyte. This was also supported by X-ray photoelectron spectroscopy we performed to the electrodes.

On the other hand, the positive electrode showed less performance gain especially in low current density condition, suggesting that reaction kinetics in the positive electrode is faster. In high current density operation, heat treatment both in the air and in the nitrogen improved cell performance and this can be attributed to concentration overpotential. In the positive electrode, vanadium-ion (VO₂⁺) concentration adjacent to the electrode surface should decrease due to vanadium consumption and water production in high current density operation during the discharge. This gave rise to concentration overpotentials in the untreated electrode. But, by applying heat to the electrodes, an electrochemical surface area increased due to shrinkage of polymer binders impregnated in the untreated electrode, effectively suppressing concentration overpotentials.

Acknowledgements

This research was supported by Japan Science and Technology Agency (JST), Precursory Research for Embryonic Science and Technology (PRESTO), Grant No. JPMJPR12C6.

References

[1] Nguyen, T. V. et al., Electrochem. Soc. Interface, 19, (2010), 54. [2] Liu, Q. et al., J. Electrochem. Soc., 159(8), (2012), A1246. [3] Agar, E. et al., J. Power Sources, 225, (2013), 89. [4] Manahan, M. P. et al., J. Power Sources, 222, (2013), 498. [5] Maruyama, J. et al., J. Electrochem. Soc., 160(8), (2013), A1293. [6] Sun, C-N. et al., ECS Electrochemistry Lett., 2(5), (2013), A43. [7] Tsushima, S. et al., Proc. the 15th Int. Heat Trans. Conf., (2014), IHTC15-9326.