In this study, we examined cell polarization of a VRFB with an interdigitated flow filed that was applied to achieve high current density operation . We investigated electrode losses in the VRFB cell operated up to 1A/cm2by using a dynamic hydrogen reference electrode inserted into the cell .
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.
This research was supported by Japan Science and Technology Agency (JST), Precursory Research for Embryonic Science and Technology (PRESTO), Grant No. JPMJPR12C6.
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