Electrochemical capacitive properties of carbon nanotube arrays directly grown on glassy carbon and tantalum foils

Abstract

Many potential applications have been proposed for carbon nanotube arrays (CNTAs), including field emission [1], energy storage [2–5], actuators [6], and sensors [7]. Several groups [2– 4] have reported that CNTA electrodes present superior electrochemical capacitive properties as CNTAs possess regular pore structures and conductive paths, and high specific surface areas. Compared with connecting the CNTAs to the current collectors indirectly (transfer techniques) [3,4], growing CNTAs on current collectors directly is more attractive due to the high-quality carbon nanotube (CNT)-metal contact and one-step process [2]. However, CNTs are known to grow predominantly on nonconductive substrates, posing restrictions on applications where conductive substrates/contacts are required. Recently, direct growths of CNTAs on Ni based [2,8,9] and Fe [10] foils were reported. Compared with Ni based and Fe foils, glassy carbon (GC) and Ta are much more stable in acidic solutions. Therefore, the unique advantage of the direct growth of CNTAs on GC and Ta substrates will be that individual nanotubes are electrically connected with the conductive acid-resisting substrates, overcoming several obstacles to applications in chemical and biological sensing [11], implantable medical devices [5], and supercapacitors using acidic electrolytes. In this letter we present the direct growth of well-aligned CNTAs on GC and Ta foils to fabricate supercapacitor electrodes. CNTA electrodes exhibit low contact resistances, superior rate capability, and stable electrochemical performance in sulfuric acid electrolyte. GC and Ta foils were coated with 20 nm-thick Al2O3 buffer layers, which were used to enhance the CNTA growth efficiency, by e-beam evaporation. CNTAs were synthesized in a 1 in. quartz tube furnace chemical vapor deposition (CVD) system. First, the substrates were put at the center of the quartz tube. A mixture gas of 300 sccm Ar and 50 sccm H2