A Comprehensive Investigation into the Structural Modification of Carbon-based Materials for Hydrogen Storage Applications
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Energy has played a critical role in the industrial and societal development of humanity. Historically, fossil fuels, namely coal, oil, and natural gas, have dominated the global energy supply. According to the International Energy Agency (IEA), these valuable resources account for approximately 81% of primary energy consumption as of 2023. However, this heavy dependence has resulted in severe environmental consequences, negative health effects, and the rapid depletion of natural resources. With the continued growth of the global population and industrial activity, energy demand is expected to rise further. These challenges highlight the urgent need to transition toward a more sustainable energy system centered on green energy sources. Achieving this transition requires the comprehensive utilization of all available renewable energy resources and green energy carriers to ensure a reliable and sufficient energy supply. In this context, hydrogen can be considered as one of the potential green energy sources. Hydrogen is widely recognized as a promising green energy carrier due to its high gravimetric energy density (143 MJ/kg) and zero-emission combustion, which produces only water vapor and heat. It enables efficient storage of thermal and electrical energy with minimal losses, offering a viable solution for balancing energy supply during periods when renewable sources like solar power are less effective. Despite its advantages, a major obstacle to the widespread use of hydrogen is the challenge of developing safe and cost-effective storage solutions. Its low molecular weight and high flammability, coupled with a very low density at ambient conditions (0.0824 kg/m³ compared to 1.184 kg/m³ for air), result in poor volumetric energy density. Addressing these issues requires the development of storage technologies that improve both gravimetric and volumetric energy densities while operating under practical conditions. Various hydrogen storage methods have been proposed, including compressed gas, liquid hydrogen, underground storage, ammonia-based solutions, and liquid organic hydrogen carriers (LOHCs). However, none of these methods are fully satisfactory or cost-effective, primarily due to the high energy demands, challenging storage conditions, geographic constraints, and the production of harmful by-products during hydrogen release. In response, physical adsorption on porous materials has emerged as a promising solution, offering high storage densities at lower pressures and temperatures. The U.S. Department of Energy (DOE) aims to achieve hydrogen–nanostructure binding energies between -0.15 and -0.6 eV, with gravimetric densities exceeding 5.5 wt% by 2025 and reaching an ultimate target of 6.5 wt%, along with a volumetric capacity goal of 50 g H₂/L for efficient hydrogen storage. Among the vast number of available nanostructures, which ones have the potential to be effectively used for achieving this goal? How can the hydrogen storage capacity of existing nanostructures be enhanced through targeted modifications? Answering these question is the key scientific objective driving the present PhD project. In the present project, fundamental work was conducted on improving the hydrogen storage capacity of various nanostructures. A particular focus was placed on carbon-based nanostructures as representative candidates for hydrogen storage applications. Both pristine and chemically or structurally modified forms of these nanostructures were studied in detail to assess their suitability. Special attention was given to understanding the mechanisms and effects of hydrogen adsorption on these materials, in both their unmodified and modified states. Rational multi-step strategies are proposed to systematically assess modified structures-based on the nature of the modification (interstitial or substitution)- by defining assessment parameters, boundaries, and final performance evaluation, with the goal of developing a comprehensive, transferable, and practically applicable framework for evaluating a broad range of potential nanostructures. Through this detailed analysis, several promising nanostructures were identified that demonstrate potential for further development toward real-world applications. Moreover, successful collaboration was undertaken on the modification of a newly synthesized material for hydrogen storage applications. The entire investigation was carried out using Density Functional Theory (DFT)- based computational methods, which enabled a thorough and fundamental exploration of the electronic, structural, and energetic properties of the materials.
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