Hydrogen is considered as one of the cleanest and most abundant energy resources available on Earth. Being the lightest element, hydrogen has the highest energy content per unit of weight. However, hydrogen has low energy density per unit volume as a gas which makes its storage challenging. Advancements in storage is crucial for developing a hydrogen economy and enabling its use as a renewable transportation fuel.

Physical vs Chemical Storage Methods

Physical storage methods involve compressing or liquefying hydrogen into a liquid or dense gas while chemical storage incorporates hydrogen molecules into chemical compounds. Both approaches have their advantages and limitations.

Compressed Gas Storage

Storing hydrogen gas at high pressures up to 700 bar is currently the most developed storage method in use. Compressed hydrogen gas cylinders allow quick refueling but have relatively low gravimetric and volumetric densities. Additionally, high pressure vessels require heavy reinforcement increasing system weight. Ongoing research focuses on developing low cost high strength lightweight composite tanks to improve storage capacities.

Liquid Hydrogen Storage

Cooling hydrogen to below -252.8°C is another physical method to condense it into a liquid with increased density. However, cryogenic Hydrogen Storage requires considerable energy input for liquefaction and insulation to prevent boil off losses. Special cryogenic tanks must also withstand temperature fluctuations. Researchers have managed to reduce boil off rates to acceptable levels but further cost reductions are needed.

Metal Hydrides for Storage

Metal hydrides are among the most promising chemical storage methods. They involve hydrogen reacting reversibly with metals or alloys to form metal hydrides. Various hydrides demonstrate reasonable storage capacities, fast reaction kinetics for refueling and ability to operate under moderate temperatures and pressures. However, hydrides often have high material costs and weight penalties limiting practical gravimetric storage densities. Ongoing R&D focuses on exploring new low cost high capacity hydride materials.

Complex Hydrides

Complex hydrides containing light elements like boron and nitrogen in addition to metals show enhanced hydrogen capacities exceeding typical metal hydrides. Examples include sodium and magnesium borohydrides (NaBH4, Mg(BH4)2). Although these have high theoretical hydrogen densities, current materials release hydrogen only at elevated temperatures above 200°C limiting practical use. Understanding decomposition pathways and developing destabilized derivatives remains an active area of complex hydride research.

Nanoparticle Storage

Downsizing metal hydride storage materials to the nanoscale using nanoengineering techniques has shown promise for improving hydrogen sorption kinetics and thermodynamics. Reduced particle size increases surface to volume ratio facilitating faster hydrogen absorption/desorption rates especially important for onboard refueling. Lowering the onset temperatures and pressures needed for hydrogen uptake also enhances usability. However, challenges in synthesizing cost effective nanoparticles at scale still need addressing.

Hydrogen Adsorption Applications

Porous materials that strongly adsorb hydrogen through Van der Waals forces and chemical bonding within their inner surfaces and cavities comprise another class of storages. Leading candidates are metal organic frameworks (MOFs), carbon nanotubes, graphene, activated carbons and zeolites. They demonstrate high surface areas and porosities helpful for efficient hydrogen uptake. Optimal designs now aim to tailor framework properties for enhanced affinity, capacities and reversibility under varied temperature-pressure conditions. Scale up production while maintaining adsorption qualities is a critical barrier.

Fuel Cell Vehicle Hydrogen Storage

Onboard vehicular storage is uniquely constrained by limited space and payload capacities in addition to refueling durations. Leading fuel cell electric vehicle (FCEV) technologies currently employ compressed hydrogen gas tanks capable of delivering an operational range over 300 miles. Extensive testing has validated the viability, durability and safety of current generation high-pressure tanks. However, meeting DOE targets of 5.5 kWh/L or 0.081 kg/L system level gravimetric capacities will require major technology breakthroughs. Liquid hydrogen cryo-tanks and metal hydride beds offer potential but cost competitive solutions meeting performance needs are yet to materialize.

Significant progress has been achieved in developing physical and chemical storage systems. However, material costs, low gravimetric/volumetric capacities, thermodynamic limitations and lifecycle performance continue hindering widespread commercialization. Collaborative research focusing on high throughput computational screening, nanoscale engineering, low entropy complex hydrides and advanced adsorption frameworks holds promise for overcoming existing barriers. Success in optimizing affordable, safe onboard vehicular storage with properties matching petrol/diesel alternatives will be key to enabling the hydrogen economy transition. National programs promoting technology demonstrations and scaling pilot projects can help accelerate applied developments.

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