  Citation: Chauhan, D.; Pujari, A.; Zhang, G.; Dasgupta, K.; Shanov, V.N.; Schulz, M.J. Effect of a Metallocene Catalyst Mixture on CNT Yield Using the FC-CVD Process. Catalysts 2022, 12, 287. https://doi.org/10.3390/ catal12030287 Academic Editors: Thurid Gspann and Adarsh Kaniyoor Received: 7 October 2021 Accepted: 28 February 2022 Published: 3 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). catalysts Article Effect of a Metallocene Catalyst Mixture on CNT Yield Using the FC-CVD Process Devika Chauhan 1, * , Anuptha Pujari 1 , Guangqi Zhang 1 , Kinshuk Dasgupta 2 , Vesselin N. Shanov 1 and Mark J. Schulz 1, * 1 Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221, USA; pujariaa@mail.uc.edu (A.P.); zhangg5@mail.uc.edu (G.Z.); shanovvn@ucmail.uc.edu (V.N.S.) 2 Materials Group, Bhabha Atomic Research Centre, Mumbai 400085, India; kdg@barc.gov.in * Correspondence: chauhadk@mail.uc.edu (D.C.); schulzmk@ucmail.uc.edu (M.J.S.) Abstract: This work studies synthesis of carbon nanotube (CNT) sheet using the high temperature (1400 ◦ C) floating catalyst chemical vapor deposition (FC-CVD) method. Three metallocenes—ferrocene, nickelocene, cobaltocene—and their combinations are used as precursors for metal catalysts in the synthesis process. For the carbon source, an alcohol fuel, a combination of methanol and n-hexane (9:1), is used. First, the metallocenes were dissolved in the alcohol fuel. Then, the fuel mixture was injected into a tube furnace using an ultrasonic atomizer with Ar/H 2 carrier gas in a ratio of about 12/1. The synthesis of CNTs from a combination of two or three metallocenes reduces the percentage of metal catalyst impurity in the CNT sheet. However, there is an increase in structural defects in the CNTs when using mixtures of two or three metallocenes as catalysts. Furthermore, the specific electrical conductivity of the CNT sheet was highest when using a mixture of ferrocene and cobaltocene as the catalyst. Overall, the multi-catalyst method described enables tailoring certain properties of the CNT sheet. However, the standard ferrocene catalyst seems most appropriate for large-scale manufacturing at the lowest cost. Keywords: FC-CVD; floating catalyst chemical vapor deposition; carbon nanotubes; aerogel; direct spinning; yield; metallocene; ferrocene; nickelocene; cobaltocene; sulfur; iron; nickel; cobalt 1. Introduction Since the discovery of carbon nanotubes (CNTs) by Iijima in 1991 [1], they have drawn the attention of researchers across the world. The physical and chemical properties of CNTs enable their use in applications including supercapacitors [2,3], patch antennas [4,5], and fiber-reinforced composites [6–11], to mention a few among the different industrial, commercial, and defense applications. With the growing list of applications, there is a proportional increase in the global demand for CNTs. As an estimation, the global market value will be about 15 billion US dollars by 2026 [12]. However, the low production rate of CNTs at a reasonable cost is the most important limiting factor in meeting this global demand. Among the three primary methods of CNT production, which are laser ablation [13], chemical vapor deposition (CVD) [14], and arc discharge [15], the CVD method is the most efficient [14] way to meet the estimated global CNT demand. The CVD method can potentially produce high-quality CNT at a large-scale at a comparatively lower cost than the other two methods. The best method for large-scale CNT synthesis is the fluidized bed CVD method [16] although it has limitations. The CNTs produced in the fluidized bed method are in the form of agglomerates or powder and thus are challenging to integrate into different applications. It is very difficult to translate the exotic properties of individual CNTs into a product when powder CNTs are added as fillers to different matrices, such as polymer. The compatibility of the binding element becomes a limitation of powder CNT in many applications. Moreover, loose CNT agglomerates possess a health risk of inhalation, limiting powder CNT use in applications pertaining to human use [17]. Catalysts 2022, 12, 287. https://doi.org/10.3390/catal12030287 https://www.mdpi.com/journal/catalysts