Ultimately, Form chose a more traditional battery concept, but the idea stuck with Chiang, who decided to explore it with other metals and landed on the idea of a sodium-based fuel cell. In this fuel cell format, the device takes in chemicals and runs reactions that generate electricity, after which the products get removed. (You might recognize this concept from hydrogen fuel cell vehicles, like the Toyota Mirai.) GRETCHEN ERTL/MITTRThe research team built small test cells to try out the concept and ran them to show that they could use the sodium-metal-based system to generate electricity. Hydrogen fuel cells can achieve high energy density, but that requires the hydrogen to be stored at high pressures and often ultra-low temperatures.