Plasma-enabled growth of vertically oriented carbon nanostructures for AC line filtering capacitors

• Direct microwave plasma deposition of self-standing VCNs on nickel foils, analyzed by SEM, Raman and XPS. • Optimal plasma growth parameters P=800 W, Q CH4 = 10 sccm, U= − 400 V, t = 60 min. • N-doping (2.3 at% N) of VCNs using an Ar-N 2 plasma treatment. • VCNs integrated as electrodes in high-frequency AC filtering capacitors. • Overall capacitance 481 and 477 µF at 100 Hz, for doped and pure VCN. Self-standing vertically oriented carbon nanostructures (VCNs) were synthesized using a large-scale microwave plasma under low-pressure conditions, employing methane as a carbon precursor. The influence of plasma operational and substrate conditions on nanostructure growth and morphology were systematically studied. Furthermore, post-synthesis N-doping of VCNs with nitrogen content of 2.4 at% N was achieved using an Ar-N 2 microwave plasma. Plasma-enabled direct deposition of VCNs, both doped and un-doped, onto nickel foils has been accomplished. The assessment of the developed nanostructures as electrodes in high-frequency AC filtering capacitors, has demonstrated an overall capacitance of approximately 480 µF at 100 Hz, with a cut-off frequency of 4 kHz for a phase angle of −45°. The excellent electrochemical performance can be attributed to the appropriate structural and morphological properties peculiar for the directly deposited on nickel foil VCNs providing binder-free electrode fabrication, thus enhancing the electrode's conductivity and charge transfer kinetics. This plasma-enabled approach for electrode design on a large scale, coupled with excellent filtering performance, paves the way for many applications in high-frequency scenarios, offering an environmentally friendly alternative to conventional electrolytic capacitors.