Amorphous carbon impurities play an active role in redox processes of carbon nanotubes

Carbon nanotubes (CNTs) produced via chemical vapor deposition (CVD) generally contain significant amounts of impurities ranging from residual metallic catalysts to nanographitic and amorphous carbon debris. Here, we show that in addition to the graphitic portion of impurities, also the amorphous carbon impurities significantly alter the electrochemical behavior of CNTs. We use a particular type of amorphous carbon black to specifically simulate amorphous carbon impurities and cyclic voltammetry with a ferro/ferricyanide redox probe to measure the heterogeneous electron transfer (HET) rate. The presence of about 30 wt % of amorphous carbon impurities resulted in an HET rate increased about 16 times compared to that of impurity-free CNTs. Electrochemical investigations of CNTs should, therefore, be performed only after a careful and complete purification from residual metal catalysts, nanographitic, and also amorphous carbon impurities. Our findings have direct implications for the construction of CNT-based electrochemical devices, such as batteries, supercapacitors, and sensors.