Billions of years ago, our planet did not have the deep blue oceans we admire today. Instead, they were actually […] ...

chloride on the cathode where iron is reduced to iron hydroxide. The result is a cell with a low potential of only abut 0.6V, but at a claimed material cost of only $0.10 per kWh Wh of stored energy.

Namely, during the planet’s first few billion years the oceans had lots of iron hydroxide, a compound that absorbs blue light. Related: Geophysicists: "Say Goodbye to Your Beaches, SoCal" This ...

which meant it would have given the ocean water a green hue because iron hydroxide absorbs blue light and water absorbs red light—leaving green light to be reflected back into the atmosphere.