Batteries and Fuel Cells

I’ve been getting interested in energy storage lately; specifically how it can be used to make renewable energy sources more viable. In looking around the Internet, I found out about Lithium-Oxygen batteries. These are batteries that have a carbon anode that allows oxygen from the atmosphere to diffuse into the battery and react with lithium products to produce electricity. These batteries have the potential to have  a much higher energy density than Li-ion batteries because half of their reactant is gathered from the atmosphere.

This is kind of along the lines of how hydrogen fuel cells work. In fact, it turns out that both batteries and fuel cells work through the same chemical processes (called redox reactions). Chemically, there’s no difference between batteries and fuel cells. So if they’re chemically similar, what’s the difference between batteries and fuel cells?

It turns out that fuel cells are just batteries with externally stored reactants. In a battery, all of the chemicals are stored internally and never released. A fuel cell may undergo the same reactions but with reactants that are pumped in from external storage tanks. Fuel cells thus have the benefit of being easy to recharge just by replacing the reactants, but they are more of a hassle to use.

So those Lithium-Oxygen batteries that I was so interested in aren’t technically batteries. The lithium for the reaction is stored internally, but the oxygen is gathered from an external source (usually the atmosphere). This means that half of the device is a battery and the other half is a fuel cell. It has the convenience of a battery with the energy density of a fuel cell; the best of both worlds.