A working, rechargeable proton battery was developed that could store more energy than currently available lithium-ion batteries. Potential applications include household storage of electricity from solar photovoltaic panels. With some modifications and scaling up, the proton battery technology may also be used for medium-scale storage on electricity grids as well as powering electric vehicles.
The proton battery uses a carbon electrode as a hydrogen store, coupled with a reversible fuel cell to produce electricity. The carbon electrode plus protons from water give the proton battery its environmental and energy advantages. Carbon is abundant and inexpensive compared to both metal hydrogen-storage alloys, and the lithium needed for rechargeable lithium ion batteries.
During charging, the carbon in the electrode bonds with protons generated by splitting water with the help of electrons from the power supply. The protons are released again and pass back through the reversible fuel cell to form water with oxygen from air to generate power. Unlike fossil fuels, the carbon does not burn or cause emissions in the process.
The small proton battery, with an active inside surface area of only 5.5 square centimeters, was able to store as much energy per unit mass as commercially available lithium-ion batteries; this was before the battery had been optimized. Future work will focus on further improving performance and energy density through use of atomically thin, layered carbon-based materials such as graphene.
During charging, protons produced by water splitting in a reversible fuel cell are conducted through the cell membrane and directly bond with the storage material with the aid of electrons supplied by the applied voltage, without forming hydrogen gas. In electricity supply mode, this process is reversed; hydrogen atoms are released from the storage and lose an electron to become protons once again. These protons then pass back through the cell membrane where they combine with oxygen and electrons from the external circuit to re-form water.
A major potential advantage of the proton battery is much higher energy efficiency than conventional hydrogen systems, making it comparable to lithium-ion batteries. The losses associated with hydrogen gas evolution and splitting back into protons are eliminated. A porous activated-carbon electrode made from phenolic resin was able to store about 1 wt% hydrogen in the electrode. This is an energy per unit mass already comparable with commercially available lithium-ion batteries. The maximum cell voltage was 1.2 Volts.