Cell Performance Improvement By Surface Modification of Porous Carbon Electrodes in Vanadium Redox Flow Battery

Redox flow batteries (RFBs) have been gathering much attention as massive energy storage devices which are vital not only for improvement of electricity utilization in the grid with distributed power plants, but also for implementation of renewable energy devices, e.g. solar cells and wind turbines [1]. For practical use of RFBs in our society, one of the remaining challenges is further improvement of cell performance, which brings higher energy conversion efficiency as well as lower production cost of the system. In recent years, intensive efforts have been made to enhance cell performance by improving flow field design [2-3] and materials used in RFBs [4-6]. Introducing an interdigitated flow field gave rise to better cell performance due to efficient supply of active material to the electrodes [3]. Pretreatment and modification of the electrodes are also reported as promising processes to improve cell performance [4-6].

In this study, we investigated an effect of surface treatment of porous carbon electrodes on cell performance in all vanadium redox flow battery with an interdigitated flow field. Thermal pretreatment was applied to the carbon electrode and electrochemical properties and single cell performance were evaluated. We also applied an arc plasma technique [7] to modify surface of the carbon fibers in the electrode by depositing nano-carbon particles and investigated its effect on cell performance.

Figure 1 shows cell performance with heat treated carbon electrodes under discharge condition. Porous carbon electrodes (Sigracet^®10AA, SGL carbon)  which were thermally pre-treated in the air at 500°C and 600°C, respectively, shows better performance than as-received electrodes. As shown in SEM images (Fig.2), applying heat exposed surface of the carbon fibers by removing impregnated materials in the electrode. This treatment possibly increased effective surface area of the electrodes for electrochemical reaction. Higher cell performance with increased heat treatment temperature at 600°C was attributed to better catalytic activity of the carbon electrode, which was confirmed by cyclic voltammetry.  

Figure 3 shows an effect of nano-carbon particles deposited on the carbon fiber in the electrode on polarization curves.  Cell performance was improved by the heat treated carbon electrode with 1,000 arc plasma gun (APG) shots. This can be attributed to enhancement of electrochemical surface area due to nano-carbon deposition as shown in SEM micrograph (Fig.4(b)). On the other hand, 10,000 APG shots applied to the electrode deteriorated cell performance. This was possibly explained by carbon layer formed on the carbon fiber. We could not identify nano-carbon particles on the fiber surface (Fig.4(c)), but observed carbon layer that was formed by accumulation of deposited carbon particles. This carbon layer possibly has less electrical contact, resulting in poor cell performance.

Acknowledgements

This research was supported by Japan Science and Technology Agency (JST), Precursory Research for Embryonic Science and Technology (PRESTO). The authors acknowledge Prof. Matthew M. Mench for his fruitful communication on high performance VFRBs. The authors appreciate Dr. Caglan E. Kumbur for his useful suggestion on VFRB experiments. Prof. Trung V. Nguyen is greatly acknowledged for his valuable comments on improving cell performance.

References

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