Hydrogen Storage: Potential Solutions for a Carbon-Free Transportation Future

 

Hydrogen Storage

Introduction to Hydrogen Storage

With mounting concerns over climate change and global dependence on fossil fuels, hydrogen has emerged as a promising carbon-free alternative fuel source. As an energy carrier, hydrogen offers nearly three times more energy per unit mass than gasoline. When utilized in a fuel cell, hydrogen produces only water, offering a clean solution for powering vehicles, homes, and industries. For hydrogen to become a practical energy carrier, safe and efficient methods must be developed to store it onboard vehicles or in stationary installations. This article explores the current state of hydrogen storage technologies and their potential to enable a hydrogen economy.

Physical Storage Methods

The simplest method of physically storing hydrogen is to compress it into high-pressure gas cylinders or tanks. Pressurizing hydrogen to pressures from 5,000 to 10,000 pounds per square inch (psi) allows sufficient energy densities for mobile applications like fuel cell vehicles. However, high-pressure tanks require heavier and more voluminous tank structures to withstand such pressures, reducing overall vehicle range. Liquid Hydrogen Storage offers higher densities but requires complex cryogenic systems to keep hydrogen at temperatures below -252°C. Besides costly refrigeration needs, phase changes during fueling and use result in significant energy losses.

Chemical Storage in Metal Hydrides

Metal hydrides chemically bond hydrogen within a solid-state crystalline material. These materials can reversibly absorb and desorb hydrogen via exothermic and endothermic reactions. This enables hydrogen storage at moderate pressures and temperatures compared to physical methods. Common hydride-forming metals investigated include magnesium, sodium, and lithium alloys. While able to store more hydrogen per unit volume than compressed gas, typical hydride gravimetric densities remain insufficient for widespread vehicle adoption. Researchers continue optimizing hydrides with lighter-weight elements and nanostructuring to improve densities.

Liquid Organic Hydrogen Carriers

Certain liquid organic compounds canChemically store hydrogen through reversible hydrogenation reactions. These liquid organic hydrogen carriers (LOHCs) offer volumetric hydrogen densities comparable to liquefied hydrogen but remain stable as liquids at moderate temperatures and pressures. Promising LOHC molecules feature benzene or longer alkane backbones that can absorb up to 8% of their weight in molecular hydrogen. The carrier is dehydrogenated to release hydrogen for fuel cells. Challenges include developing efficient catalysts for rapid fueling/discharging and recycling the carrier liquid for reuse. LOHCs show promise as an alternative to high-pressure tanks, especially for stationary power generation refueling.

Novel Carbon-Based Storage Materials

Advanced carbon nanomaterials like fullerenes, carbon nanotubes, and graphene demonstrate exceptional potential for safely storing hydrogen through surface adsorption and spillover mechanisms. These mechanisms involve dissociating hydrogen molecules which are then stored as atomic hydrogen within the material’s surface cavities and outer layers. Gravimetric capacities approach Department of Energy targets of 5.5 weight% when optimized. Further work engineering defects, doping elements, and controlling aggregations could lead to superior system-level storage at ambient conditions. However, challenges remain to develop scalable synthesis methods and realize practical storage densities at reasonable costs.

Each hydrogen storage method exhibits advantages and limitations based on technical maturity, energy density, regulatory challenges, and cost. While compressed gas tanks remain the dominant approach today, alternative storage solutions may better enable a sustainable hydrogen economy. Metal hydrides address density shortfalls but need lighter hydride-forming compositions. LOHCs represent a promising intermediate solution, but more efficient refueling processes must be developed. Novel carbon nanomaterials could potentially exceed all targets if material optimization and scale-up issues are overcome. Going forward, technology advancements integrating several storage approaches may be required, along with complementary efforts in hydrogen production, transportation infrastructure, and fuel cell commercialization. With further progress, hydrogen storage has the potential to play a pivotal role in decarbonizing transportation networks worldwide