Electrochemical performance of boron- and nitrogen-doped tetrahedral amorphous carbon

Tetrahedral amorphous carbon (ta-C) is one of the most versatile carbon electrode materials available. The incorporation of various dopants in ta-C alters the structural, electrochemical, physical, and chemical properties which are tunable for various applications. Furthermore, doped ta-C (ta-C:X) can be deposited on many different substrate materials (metals, ceramics, plastics) and geometries (micro-electrodes to ultra large areas at continuous coating lines). In this work, we present a comparison in electrochemical performance of ta-C electrodes prepared by filtered laser-arc (a physical vapor deposition process), doped with nitrogen (ta-C:N) at various levels, doped by boron (ta-C:B), and doped by boron and nitrogen together (ta-C:B:N). The ta-C:N films were doped via N₂ gas in the deposition chamber, while ta-C:B and ta-C:B:N were deposited via incorporation of the dopant into the graphite cathode used for film synthesis. Films coated on silicon (Si) and titanium (Ti) substrates were compared through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The Fe(CN)₆^3-/4− and Ru(NH₃)₆^3+/2+redox systems were used to investigate the electrochemical characteristics of each ta-C:X film type. Boron-doped diamond (BDD), glassy carbon (GC), and undoped ta-C were used as control electrodes. It was found that all ta-C:X films exhibited appropriate rates of electron transfer, wide working potential window (ca. 3.7 V in KCl), and low capacitance. Specifically, the boronated films produced high heterogenous rate constant (k), the lowest double layer capacitance (C_dl), and a consequentially smaller RC time constant than BDD and GC. Incorporating boron into undoped and nitrogenated ta-C films stabilizes the formation of sp³ bonded content during deposition leading to films with a smoother surface and increased hardness. It is shown that the B-incorporated films exhibit no compromise in their electrochemical performance despite the increase in sp³ content. Due to its versatile nature, ta-C stands out as a material with a wide range of tunable properties for various applications.