Investigation of Reaction Distribution in Carbon Electrodes in Vanadium Redox Flow Batteries with an Interdigitated Flow Field
To identify performance-limiting electrode, asymmetric cell configuration was used in a VRFB  and showed the negative electrode limit the cell performance in VRFBs. A dynamic hydrogen electrode was applied to an operating VRFB and larger kinetic polarization was found at the negative electrode compared to the positive electrode . In these literature, cell polarization at low current density operation where the reaction kinetics determine overall cell performance was intensively examined. It is also noteworthy that the reaction distribution in the carbon electrode has not been fully explored yet.
In this study, we performed polarization experiments by using the symmetric cell geometry in a VRFB. The VRFB in which an interdigitated flow field was embedded  was used to achieve high current density operation. To examine the performance limiting electrode, we used heat-treated and raw (as-received) carbon porous materials as the better- and poor- kinetics electrode as reported . Furthermore, we applied two sheets of electrode material to each the negative and positive electrode in order to investigate reaction distribution in the electrodes. In the experiments, one of the two sheets of the heat-treated electrodes was replaced by the raw electrode to determine the performance-limiting side of the electrode. Accordingly, we performed polarization experiments in the following five cases:
Base case: (−) HT|HT|PEM|HT|HT (+)
Case 1: (−) Raw|HT|PEM|HT|HT (+)
Case 2: (−) HT|Raw|PEM|HT|HT (+)
Case 3: (−) HT|HT|PEM|HT|Raw (+)
Case 4: (−) HT|HT|PEM|Raw|HT (+)
Figure 1 shows polarization curves obtained in case 1 and 2 where the negative electrode was partially replaced by the raw electrode. Both polarization curves corresponding to case 1 and 2 shows larger overpotentials than the base case in entire operational condition. This indicates that both the current collector side and the membrane side of the negative electrode contributed to the V(II)/V(III) reaction. However, polarization curves at low current density condition (<0.5A/cm2) indicate that cell performance was more deteriorated in case 2. Case 2 corresponds to the raw carbon material placed in the membrane side of the negative electrode. This suggests that V(II)/V(III) reaction was slightly concentrated in the membrane side of the negative electrode during the discharging process of the VRFB at low current density condition. On the other hand, case 1 showed less cell performance at high current density condition (>0.5A/cm2), indicating more reaction in the current collector side of the negative electrode at high current density condition. These observations can be attributed to the transport of proton (H+) and V(II) ion in the negative electrode. At low current density condition (<0.5A/cm2), sufficient amount of V(II) ion was supplied to the electrode and thus proton transport resistance in the electrode can affect the cell performance, resulting in the more reaction in the membrane side of the electrode. However, at high current density condition (>0.5A/cm2), reaction distribution was spatially shifted to the collector side possibly due to an insufficient supply of V(II) ion to the negative electrode. This further causes the concentration overpotential in the negative electrode at high current density operation.
This research was supported by Japan Science and Technology Agency (JST), Precursory Research for Embryonic Science and Technology (PRESTO). The authors acknowledge Prof. Mench for his fruitful communication on high performance VFRBs. The authors appreciate Dr. Kumbur for his useful suggestion on VFRB experiments. Prof. Nguyen is greatly acknowledged for his valuable comments on improving cell performance.
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