Introduction to Hydrogen
Hydrogen is the lightest and most abundant element in the universe and has the potential to play a key role in the future of transportation as a zero-emission energy source. As concerns over climate change rise and countries work towards reducing their carbon footprint, hydrogen is gaining attention as a potentially greener alternative to fossil fuels. Storing hydrogen however poses technological challenges that researchers are striving to overcome.
Current Methods of Hydrogen Storage
There are currently three main methods used for hydrogen storage - compression, liquefaction and solid-state storage. Compression is the most developed technology but comes with safety issues at high pressures. Liquefying hydrogen requires cooling it to -253°C, consuming nearly 30% of its energy content just to liquefy it. Solid-state storage using materials like metal hydrides or chemical hydrogen storage shows promise but further development is still needed to meet performance targets.
Metal Hydrides for Storage
Metal hydrides are among the most promising solid-state storage methods thanks to their ability to absorb and release hydrogen reversibly. Hydrides form when hydrogen gas dissociates and is absorbed by a metal usually leading to a change in crystal structure. Properties like Hydrogen Storage capacity, kinetics of adsorption/desorption and thermodynamics of hydride formation/decomposition determine a material's suitability. Researchers experiment with different alloy compositions and nanosizing to improve storage characteristics. Some promising hydrides demonstrated include magnesium and boron-based hydrides but achieving sufficient gravimetric or volumetric densities at modest temperatures/pressures remains a challenge.
Chemical Hydrogen Storage
Chemically storing hydrogen using reversible chemical reactions offers another potential approach. A number of liquid organic and inorganic carriers that undergo hydrogenation/dehydrogenation reactions are being investigated. Ammonia, alcohols and hydrocarbons have drawn early interest due to their high hydrogen contents. However, significant efforts are still required around developing efficient catalysts for reversible hydrogen release/uptake, improving thermodynamic affordability and addressing volatility/toxicity concerns of the carriers. Nanoconfinement and tuning of electronic structures are strategies adopted to address these issues.
Outlook for Onboard Hydrogen Storage
While none of the current technologies fully meet the US Department of Energy's (DOE) targets for onboard vehicular storage, notable progress is being made. Metal hydrides have seen density improvements through intensified cycling and nanostructuring. Novel complex hydrides offer higher capacities but require destabilizing to optimize thermodynamics. Liquid organic carriers offer alternatives to liquefaction but low gravimetric densities remain a hurdle. The hydrogen economy's future relies on a storage method with high energy density, fast refueling, economic viability and safety - researchers worldwide are engaged in multidisciplinary efforts to solve the complex storage challenges. Overcoming thermodynamic barriers through innovative materials design holds promise to accelerate commercialization of hydrogen fuel cell vehicles and infrastructure. Concerted efforts across academic and industrial labs will be crucial to realize the full potential of hydrogen as an emissions-free energy carrier.
New Storage Approaches on the Horizon
A number of new approaches drawing inspiration from nature are also under consideration. Hydrogen adsorption on high-surface-area materials like metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and hypercrosslinked polymers (HCPs) leverages their microporosity but faces capacity/kinetics-related challenges. Imine-linked organic polymers (ILPs) and polyimides show promise as lightweight solid-state hydrogen stores. Photoactive organic polymers that catalyze hydrogen generation under visible light offer potential for onboard solar refueling. Biomimetic catalyst designs and integration of metal-organics with redox-active components could help optimize thermodynamics. Given the diverse interdisciplinary skillsets required, collaborations between chemists, materials scientists, and engineers will play a vital role in translating these conceptually exciting storage paradigms into practical reality.
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