Functionalized graphene materials for hydrogen storage

With growing demands of energy and enormous consumption of fossil fuels, the world is in dire need of a clean and renewable source of energy. Hydrogen (H-2) is the best alternative, owing to its high calorific value (144 MJ/kg) and exceptional mass-energy density. Being an energy carrier rather than an energy source, it has an edge over other alternate sources of energy like solar energy, wind energy, and tidal energy, which require a constant energy source dependent upon weather conditions. However, its utilization as an energy carrier has not yet been commercialized due to its poor storage performance, which is attributed to low gravimetric and volumetric densities of adsorbed hydrogen at ambient temperature and technological limitations in meeting the stringent parameters set by Department of Energy, USA. With exceptionally large surface area (2630 m(2)/g), porous nature, lightweight, and high chemical and thermal stability (melting point similar to 4510 K) along with the possibility of economical and scalable production, graphene-based solid-state porous materials have shown promising applications in efficient hydrogen storage. In this context, the present review discusses the recent advances and progress on the utilization of functionalized graphene, graphene oxide, and its derivatives for effective storage of hydrogen, along with important theoretical advancements via DFT calculations, first-principle calculations, and Monte Carlo Simulations, etc. Pristine graphene has poor hydrogen storage characteristics, and addition of dopants like boron and nitrogen or decoration by transition metals significantly improves the performance. In addition, graphene allows the tuning of surface curvature which can help in achieving a reversible hydrogen storage system with fast kinetics. The impact of external stimuli like electric field and strain on electronic structure of graphene is discussed with the applicability in achieving a highly controllable adsorption-desorption system. Finally, the review concludes with life cycle assessment of graphene-engineered composites for effective hydrogen storage applications, along with their energy and environmental implications.