Novel ion beam implantation of felt electrodes for the vanadium flow battery: Role of defects versus nitrogen groups for the VO2+/VO2+ redox reaction

A novel approach to introduce catalytic sites to carbon and graphite felts in order to improve electrode activity and investigate the effectiveness of specific active sites for the vanadium flow battery (VFB) has been achieved via ion beam implantation. Here, both N-2(+) and Ne+ implantation is used to show that the positive vanadium (VO2+/VO2+) redox reaction preferentially occurs at the introduced defect sites, and not nitrogen-containing groups. This is shown through short charge and discharge tests and electrochemical impedance spectroscopy, with the electrodes characterised via X-ray photoelectron and Raman spectroscopies. Lower implantation fluences (1 x 10(14)-4 x 10(15) ion cm(-2)) were shown to improve the pristine electrodes due to the formation of defect sites. However, implantation at higher fluences (above 4 x 10(15) ion cm(-2)) resulted in a decrease in defect density, and a layer of amorphous carbon at the electrode surface was thought to hinder the redox reaction due to lower surface conductivity and less available active sites. This new approach of felt electrode modification shows that nitrogen-containing groups are not necessary for the positive vanadium redox reaction and shows the importance of defect engineering of electrodes for the VFB.