Carbon Materials as High Performance Positive Electrode for Vanadium Redox Flow Battery


Student thesis: Doctoral Thesis

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Award date17 Jul 2019


Renewable energy sources have gained much attention due to the crisis of global warming and increasing energy demand. However, renewable energy is inherently unstable and unpredictable. Thus, to develop more stable power supply and usage of renewable energy as well as reduction of greenhouse gas emission, the development of efficient battery systems has been the primary focus for battery research in recent years. Redox flow battery (RFB) is considered as the most promising energy storage technology in terms of its unlimited capacity, design flexibility and safety. The electroactive species, stored externally to the battery system itself, can reversibly conduct the conversion to store electrical energy by chemical reactions. The electrolytes containing different redox couples are pumped through porous electrodes in a cell stack, where the electrochemical redox reactions occur on the surface of the electrodes.

The all-vanadium redox flow battery (VRFB) which employs both vanadium electroactive species in both half cells has shown notable progress towards the successful stationary applications. A significant advantage of VRFB compared with other types of RFB which employ two different electroactive species is that undesirable performance fading, mainly due to the inevitable ion cross-over through the membrane, can be effectively suppressed during operation. The capacity fade can be reversed by simply remixing and electrolysis of the electrolytes periodically. However, the key limitations in current VRFB research are still critical challenges for commercial application. Firstly, the energy density is limited due to the vanadium ion solubility and stability in an electrolyte solution over a narrow operational temperature range (10 ºC - 40 ºC ). Secondly, the power density is limited due to the relatively low activity and reversibility of the commonly used graphite felt electrode. To promote the performance of VRFB, the above critical challenges need to be addressed to enable VRFB more competitive in the battery industry.

This research work provides some insights to address the limitation of power density resulting from the graphite felt electrodes. An effective modification approach is studied and a number of promising metal candidates (In, Cr, Co) are investigated and compared through cell performance in terms of energy efficiency and storage capacity. The metal oxide modification not only contributes to the electrode conductivity, but also demonstrates excellent catalytic activity towards the redox reactions. In particular, modification with binary metal oxide NiCoO2 displays superior modification effect as the cell performance can be greatly enhanced. The findings are very promising and of great significance in promoting the power density of VRFB, providing guidelines for the electrode study in future VRFB research.