Electrochimica Acta 452 (2023) 142340 Available online 31 March 2023 0013-4686/© 2023 Elsevier Ltd. All rights reserved. Functionalization of graphene-based nanomaterials for energy and hydrogen storage Emmanuel Boateng a , Antony R. Thiruppathi a , Chi-Kai Hung a , Darren Chow a , Deepak Sridhar b , Aicheng Chen a, * a Electrochemical Technology Centre, Department of Chemistry, University of Guelph, 50 Stone Road East, Guelph, ON N1G 2W1, Canada b Zentek Ltd., 24 Corporate Court, Guelph, Ontario N1G 5G5, Canada A R T I C L E INFO Keywords: Functionalization Graphene Nanomaterials Energy storage Hydrogen storage Heteroatom doping ABSTRACT Graphene and its derivatives are attractive solid-state candidates to meet the needs of next-generation energy and hydrogen storage technologies. Although impressive storage capacities have been demonstrated, materials meeting all targets established by the United States Department of Energy have yet to be identifed. In this re- view, we provide an overview of recent developments in functionalized graphene-based nanomaterials for en- ergy and hydrogen storage systems. First, considerations for the most common synthetic approaches to graphene- based nanomaterials are briefy summarized. Then, the effects of traditional functionalization strategies on the structural characteristics and properties of pristine graphene are discussed. Lastly, recent advances and progress in energy applications using functionalized graphene-based nanomaterials including supercapacitors, batteries and hydrogen storage are highlighted, and new directions are discussed. 1. Introduction The increasing global population and advancement of the modern industry have led to a growing societal dependence on fossil fuels such as coal, petroleum, and natural gas for energy. By continuing to meet the growing energy demand through accelerated exploitation of these re- sources, humanity contributes greatly to environmental pollution and climate change [1–3]. To circumvent these challenges, the development of high-performance, sustainable, and low-cost energy production, storage, and consumption systems is essential. Current research suggests that the effciency of electrochemical energy storage applications (e.g., supercapacitors and batteries) and hydrogen storage technologies de- pends on a range of factors; however, their overall performance strongly relies on the structure and properties of the materials employed [4–6]. Various carbon-based nanomaterials such as carbon nanotubes, fuller- enes and graphene have emerged as strong candidates for a wide range of energy-related applications [7–10]. In particular, graphene-based nanomaterials have received great attention due to their numerous and extraordinary intrinsic properties. Graphene, known to be the basic building block of other carbon nanomaterials, is a single-atom thick planar sheet of graphite with a perfect two-dimensional (2D) crystal structure of sp 2 bonded carbon atoms packed in a honeycomb lattice [11,12]. Graphene has been extensively studied in the felds of chemistry, physics, and materials science due to its unique and superior properties such as large surface-to-volume ratio and specifc surface area, high electron mobility, robust mechanical strength, and exceptional electrical and thermal conductivity [9,11-14]. Due to these properties, graphene has emerged as an attractive material for diverse applications, including electro- chemical energy storage applications; for example, supercapacitors and batteries, catalysts, and hydrogen storage systems. Even though pristine graphene possesses outstanding theoretical characteristics and proper- ties, research has shown that its versatility is signifcantly increased by functionalization. Functionalization of graphene contributes to tuning its properties including chemical reactivity, electronic structure, and porosity, resulting in enhanced electrocatalytic activity, electrochemical reaction kinetics, and adsorption capacities [9,13,15–20]. Therefore, rationally modifying these properties is seen as a promising pathway to electrochemical devices with enhanced performance. In super- capacitors, functionalization of graphene-based nanomaterials can produce large surface areas and favourable pseudocapacitive properties [21–23]; in alkali metal ion batteries, functionalized graphene de- rivatives can improve intercalation kinetics [24] and support fragile conversion/alloying nanostructures [25–28]; in metal-air batteries, * Corresponding author. E-mail address: aicheng@uoguelph.ca (A. Chen). Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.journals.elsevier.com/electrochimica-acta https://doi.org/10.1016/j.electacta.2023.142340 Received 29 September 2022; Received in revised form 22 March 2023; Accepted 31 March 2023