I love simple and elegant systems and that is exactly what Andreas Luz and Claus Feldmann have reported in the Journal of Materials Chemistry yesterday.^[JMaterChem] Luz & Feldmann found that when exposing a capped vial of a colorless solution of bismuth chloride dissolved in diethylene glycol to sunlight a black suspension forms. When this black suspension is exposed to air and shook a few times it turns colorless again. The process is shown below.

The chemistry involved is expected to be thus: light causes the solvent to reduce the bismuth to the metallic form, and subsequent exposure to air reoxidizes the system back to BiCl₃,

First Step

• Reduction:
• Oxidation:

In essence they have a solar and oxygen driven battery, a fact not lost on them because they go on to make a battery. They measure a few hundred millivolts out of their device, but without other data like current and power it is too difficult to judge the system wholistically.

There are many potential uses for the system, one off the top my head would be a visual method of detecting when oxygen has leached into your solvent or precious air sensitive materials.

Link to paper: Reversible photochromic effect and electrochemical voltage driven by light-induced Bi⁰-formation

Mitch