Ion Beam Implantation of Graphite Felt for Redox Flow Batteries: Role of Defects Versus Nitrogen Groups
Authors: McArdle Sophie , Fiedler Holger , Leveneur Jérôme , Kennedy John , Marshall Aaron Timothy 
Published: December 2024

Vanadium flow batteries (VFBs) are a promising solution to the growing demand for large-scale energy storage [1] . A critical component of VFBs are the electrodes, commonly manufactured from highly porous, carbon materials [2] . Carbon-based electrodes have good conductivity, high surface area, and are low-cost, however, they often exhibit poor activity for the vanadium redox reactions. The reactivity of electrode materials is of great interest, with previous research on carbon felt ascribing enhanced electrode reaction kinetics to the presence of oxygen functional groups (OFGs), nitrogen functional groups (NFGs), and/or carbon defects [3–6].

In this study, catalytic sites were introduced into graphite felt using ion beam implantation to probe the effectiveness of specific active sites for the VFB. Specifically, N2
+- and Ne+-ion implantation are used to investigate the influence of introducing both NFGs and carbon defects versus carbon defects only. Polarisation curves and impedance measurements revealed that defects and edges sites introduced onto the electrode surface appear to be the relevant catalytic sites for facilitating the positive vanadium redox reaction. Raman spectroscopy and XPS helped to establish correlations between the surface chemistry and structure of graphite felt to electrode activity. Lower implantation fluences (< 1016 ion cm-2) were found to improve performance compared to pristine electrodes. In contrast, implantation at higher fluences resulted in the formation of amorphous carbon at the electrode surface, which was thought to result in a lower density of active defect sites and lower conductivity. Furthermore, this electrode modification technique was investigated on a scaled-up electrode geometry to assess its effectiveness for larger electrodes. Full- and half-cell measurements allowed for full-cell, positive half-cell, and negative half-cell performance to be independently investigated, whilst providing insight into performance-limiting factors.

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