13.4 Fuel cells and hydrogen storage nanomaterials

Fuel cells and hydrogen storage are crucial for clean energy. Nanotech enhances their performance through advanced catalysts and materials. These innovations boost efficiency, reduce costs, and improve storage capacity for hydrogen-powered systems.

Proton exchange membrane and solid oxide fuel cells use nanostructured catalysts for better reactions. Metal-organic frameworks, advanced metal hydrides, and carbon-based materials offer promising solutions for hydrogen storage. These advancements bring us closer to a hydrogen economy.

Types of Fuel Cells

Proton Exchange Membrane Fuel Cells (PEMFCs)

• PEMFCs operate at low temperatures (typically 80°C) making them suitable for portable and automotive applications
• Utilize a polymer electrolyte membrane to conduct protons from anode to cathode
• Require pure hydrogen as fuel and oxygen (usually from air) as oxidant
• Produce water and heat as byproducts of the electrochemical reaction
• Offer high power density and quick start-up times compared to other fuel cell types
• Face challenges in catalyst cost reduction and durability improvement

Solid Oxide Fuel Cells (SOFCs)

• SOFCs operate at high temperatures (600-1000°C) enabling efficient electricity generation
• Use a solid ceramic electrolyte to conduct oxygen ions from cathode to anode
• Can utilize various fuels including hydrogen, natural gas, and biogas
• Produce water, heat, and carbon dioxide (when using hydrocarbon fuels) as byproducts
• Offer high electrical efficiency (up to 60%) and potential for cogeneration applications
• Face challenges in material durability and thermal management due to high operating temperatures

Nanostructured Catalysts for Fuel Cells

Advanced Catalyst Designs

• Nanostructured catalysts increase surface area-to-volume ratio enhancing catalytic activity
• Platinum nanoparticles serve as primary catalysts for hydrogen oxidation and oxygen reduction reactions
• Typical platinum nanoparticle size ranges from 2-5 nm for optimal performance
• Carbon-supported catalysts provide high surface area and improved electrical conductivity
• Carbon supports include carbon black, carbon nanotubes, and graphene
• Core-shell nanostructures combine platinum outer layer with less expensive core materials (nickel, cobalt)

Catalyst Performance Optimization

• Alloying platinum with other metals (ruthenium, palladium) improves catalyst stability and activity
• Shape-controlled nanoparticles (cubes, octahedra) expose specific crystal facets for enhanced catalytic performance
• Dealloyed catalysts create nanoporous structures with high surface area and improved activity
• Non-precious metal catalysts (iron-nitrogen-carbon complexes) aim to reduce fuel cell costs
• Atomic layer deposition techniques enable precise control of catalyst loading and distribution
• In-situ characterization methods (X-ray absorption spectroscopy) provide insights into catalyst behavior during operation

Nanomaterials for Hydrogen Storage

Metal-Organic Frameworks (MOFs) for H2 Storage

• MOFs consist of metal ions or clusters coordinated with organic ligands forming porous structures
• Offer exceptionally high surface areas (up to 7000 m²/g) for hydrogen adsorption
• Tunable pore sizes and functionalities allow optimization for hydrogen storage
• Gravimetric hydrogen storage capacities reach up to 10 wt% at cryogenic temperatures
• Face challenges in room temperature storage capacity and thermal management during adsorption/desorption

Advanced Metal Hydrides and Composites

• Nanostructured metal hydrides provide improved kinetics for hydrogen absorption and desorption
• Magnesium-based nanocomposites offer high theoretical storage capacity (7.6 wt% for MgH2)
• Alloying and catalytic doping enhance reaction rates and lower operating temperatures
• Nanoconfinement in porous scaffolds improves thermodynamics and kinetics of metal hydrides
• Reactive ball milling techniques create nanostructured hydrides with enhanced properties
• Core-shell nanostructures protect metal hydrides from oxidation while maintaining fast kinetics

Carbon-Based Nanomaterials for Hydrogen Storage

• Graphene provides large surface area (2630 m²/g) for hydrogen adsorption
• Functionalized graphene (with metal atoms or organic groups) enhances hydrogen binding energy
• Carbon nanotubes offer tunable pore sizes for optimized hydrogen storage
• Activated carbon materials provide high surface area and low cost for practical applications
• Graphene aerogels combine ultra-low density with high surface area for improved storage capacity
• Challenges include increasing room temperature storage capacity and managing thermal effects during adsorption