nanomaterials Article Effect of Graphene Concentration on the Electrochemical Properties of Cobalt Ferrite Nanocomposite Materials Firas S. Alruwashid 1,2,† , Mushtaq A. Dar 2, * ,† , Nabeel H. Alharthi 1 and Hany S. Abdo 2,3   Citation: Alruwashid, F.S.; Dar, M.A.; Alharthi, N.H.; Abdo, H.S. Effect of Graphene Concentration on the Electrochemical Properties of Cobalt Ferrite Nanocomposite Materials. Nanomaterials 2021, 11, 2523. https://doi.org/10.3390/ nano11102523 Academic Editor: Jin Suk Chung Received: 5 August 2021 Accepted: 22 September 2021 Published: 27 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Department of Mechanical Engineering, College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia; alruwashidf@gmail.com (F.S.A.); alharthy@ksu.edu.sa (N.H.A.) 2 Center of Excellence for Research in Engineering Materials (CEREM), Deanship of Scientific Research (DSR), King Saudi University, Riyadh 11421, Saudi Arabia; habdo@ksu.edu.sa or hany.abdo@aswu.edu.eg 3 Mechanical Design and Materials Department, Faculty of Energy Engineering, Aswan University, Aswan 81521, Egypt * Correspondence: mdar@ksu.edu.sa † Contributed equally to the present work and are co-first authors. Abstract: A two-step process was applied to synthesize the cobalt ferrite-graphene composite materials in a one-pot hydrothermal reaction process. Graphene Oxide (GO) was synthesized by a modified Hummer’s method. The synthesized composite materials were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and Fourier-transform infrared spectroscopy (FTIR). The XRD and FTIR results were in good agreement with the TGA/DTG observations. SEM and TEM disclosed the spherical shape of the nanoparticles in 4–10 nm. The optimized CoFe 2 O 4 -G (1– 5 wt.%) composite materials samples were tried for their conductivity, supercapacity, and corrosion properties. The CV results demonstrated a distinctive behavior of the supercapacitor, while the modified CoFe 2 O 4 -G (5 wt.%) electrode demonstrated a strong reduction in the R ct value (~94 Ω). The highest corrosion current density valves and corrosion rates were attained in the CoFe 2 O 4 -G (5 wt.%) composite materials as 5.53 and 0.20, respectively. The high conductivity of graphene that initiated the poor corrosion rate of the CoFe 2 O 4 -graphene composite materials could be accredited to the high conductivity and reactivity. Keywords: nanocomposite; hydrothermal process; CoFe 2 O 4 nanoparticles; graphene; electrochemi- cal properties 1. Introduction Nanostructured materials have caught the attention of the materials’ community over the past few decades. It was discovered that materials could be involved in multiple applications, specifically the ones related to batteries and sensors [1,2]. Nanostructured materials can be found in many forms, such as metal oxides. Metal oxide is known to be a common nanomaterial that is considered a powerful option as a catalyst or an enhancer to another material [3]. Multiple studies have been done on metal oxides due to their superior properties that they exhibit in more than one application. Therefore, the focus of this study was on a metal oxide, namely, the cobalt ferrite nanostructured material. Cobalt ferrite is known for its magnetic properties, in addition to its outstanding stability, both structurally and chemically [4]. Because it is a ferrite material, it is assumed to exhibit a high hardness [5,6]. However, it can go through thorough mechanical testing to prove if its hardness is high or low. Cobalt ferrite is an interesting material for electro- chemical experimentation because of the conductivity it poses. Nevertheless, a thorough electrochemical characterization on cobalt ferrite has not yet been investigated. Therefore, cobalt ferrite is one of those materials that is capable to be in the biosensors field because of its properties. It was used in an application for detecting mercury ions, which are known for their toxicity on human bodies [7]. Nanomaterials 2021, 11, 2523. https://doi.org/10.3390/nano11102523 https://www.mdpi.com/journal/nanomaterials