Electrochemical carbon-based storage: proton batteries

Technology

What is it?

Proton battery – A proton battery is a reversible PEM fuel cell, or a hybrid between a fuel cell and battery-based system, that allows solid-state storage and extraction of hydrogen in atomic form. During charging, protons produced by water splitting on the oxygen-side electrode of the reversible cell pass through the membrane and are neutralised by excess electrons on the solid negatively-charged storage electrode.

A metallic electrode was first used, in which case hydrogen is stored as a metal hydride. Later work employed a porous activated carbon electrode, with the hydrogen stored by a weak bond to the carbon surfaces within pores. During discharging, protons are directly released back into the membrane as electrons flow in the external circuit, with the usual water formation reaction on the oxygen-side electrode driving the electricity-generation process.

Why is it important?

Proton batteries allow the solid-state storage of atomic hydrogen, without requiring the formation or splitting of hydrogen gas, thus potentially increasing the roundtrip energy efficiency of the standard electrolyser–hydrogen gas storage–fuel cell system.

Characteristics

• Volumetric energy density:
  • Theoretical upper ranges
    • electrode material: 0.9 – 1.9 kWhe/L (assuming 0.7 nm spacing between graphene layers, and 1 H stored per 2C or 1C)
    • system density – to be determined
• Gravimetric energy density :
  • Theoretical upper ranges
    • electrode material: 4.0 -7.7 wt%H,  or 0.8 -1.5 kWhe/kg (corresponding to 1 H stored per 2C or 1C atoms)
    • system density – to be determined
• Storage conditions: ambient temperature and pressure
• Roundtrip Energy efficiency: similar in theory to that of a lithium-ion battery

Benefits

• Hydrogen atoms are generated directly from water and stored in atomic form, avoiding energy losses in hydrogen gas formation, compression, and H2 splitting in fuel-cell mode
• High roundtrip energy efficiency
• Near atmospheric pressure operation
• High level of safety, since no high-pressure flammable hydrogen gas
• Carbon is abundant and lightweight, and therefore storage electrodes are cheap and have high internal surface area
• Can be recharged directly with electricity

Limitations

• Does not store hydrogen gas, makes use of water feedstock only. Cannot be used to produce a gaseous hydrogen output

RD&D priorities

• Select and test hydrogen storage electrodes specially synthesised for this application from layered graphene and carbon-nitride materials
• Increase current densities during charging and discharging
• Test multiple cycles
• Understand reactions between hydronium/protons and carbon surfaces
• Increase volumetric and gravimetric energy densities to nearer theoretical maximum

Known active organisations

• Deakin University
• RMIT University
• The University of Technology Sydney