Effects of carbonaceous impurities on the electrochemical activity of multiwalled carbon nanotube electrodes for vanadium redox flow batteries Ibrahim Mustafa a , Asma Al Shehhi a , Ayoob Al Hammadi a , Rahmat Susantyoko a , Giovanni Palmisano b , Saif Almheiri a, c, * a Department of Mechanical Engineering, Khalifa University of Science and Technology, Masdar Institute, Masdar City, P.O. Box 54224, Abu Dhabi, United Arab Emirates b Departmentof Chemical Engineering, Khalifa University of Science and Technology, Masdar Institute, Masdar City, P.O. Box 54224, Abu Dhabi, United Arab Emirates c Division of Engineering Technology and Science, Higher Colleges of Technology, United Arab Emirates article info Article history: Received 17 October 2017 Accepted 16 January 2018 Available online 30 January 2018 Keywords: Multiwalled carbon nanotubes Vanadium redox flow battery Carbonaceous impurities Buckypaper Functionalization Electrochemistry abstract The presence of impurities within multiwalled carbon nanotubes (MWCNTs) have major, but as-yet- unrecognized, effects on the electrochemical behavior of MWCNTs in vanadium redox flow batteries. We show that MWCNTs constitute a high-surface-area, electrically conductive, inactive carbon matrix that acts as a bridge between and a support for carbonaceous impurities that are highly active toward the VO 2þ /VO 2 þ redox couple. We use freestanding sheets of MWCNTs, either purified and subsequently not modified with impurities (pure) or purified and subsequently contaminated with carbonaceous impu- rities (impure), as electrodes to eliminate the need for a substrate and thus undesired mixed kinetics. Electrodes fabricated with pure MWCNTs show low activities; attributed to relatively inactive basal sites. In contrast, impure electrodes show high activities due to the increased number of defects and edge sites resulting from the presence of the impurities. Functionalization of pure MWCNTs surface with oxygen- containing groups substantially improve the activity of the electrodes. However, functionalization is not as effective when carbonaceous impurities are present. The carbonaceous impurities form a continuous debris layer over the nanotube fibers, which reduces the effect of the oxidation treatment. Our results indicate that the impurity content of MWCNTs should be considered in studies of their activity and functionalization. © 2018 Elsevier Ltd. All rights reserved. 1. Introduction The ability of carbon to undergo catenation allows it to form several types of bonds and thus to form distinct allotropes, including diamond, graphite, glassy carbon, carbon fibers, carbon black (CB), carbon nanotubes (CNTs), and fullerenes. Varying in the degree of crystallinity, carbon allotropes exhibit a wide range of physicochemical properties (e.g., surface area, electrical conduc- tivity, porosity, and electrochemical activity), which makes them interesting materials for use as electrodes in electrochemical con- version devices such as redox flow batteries [1e4]. All-vanadium redox flow batteries (VRFBs), first proposed by Skyllas-Kazacos et al. [5,6], stand out among redox flow batteries owing to their reasonably good energy densities (20e35 Wh L 1 ) [7 ,8] and cell peak power densities (1340 mW cm 2 )[9e11], low crossover, and high energy efficiencies (>80%) [12e15]. VRFBs consist of a cell (membrane, electrodes, current collectors, gaskets, flow fields, and fixtures), pumps, and two external tanks (con- taining vanadium in aqueous sulfuric acid solutions), all of which are designed to facilitate the following half-cell redox reactions [16]: * Corresponding author. Department of Mechanical Engineering, Khalifa University of Science and Technology, Masdar Institute, Masdar City, P.O. Box 54224, Abu Dhabi, United Arab Emirates. E-mail address: salmheiri@masdar.ac.ae (S. Almheiri). Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon https://doi.org/10.1016/j.carbon.2018.01.069 0008-6223/© 2018 Elsevier Ltd. All rights reserved. Carbon 131 (2018) 47e59