Eventually, you might not need a battery or a conspicuous external power source to keep a device running for years on end. A team at MIT has created a device that produces energy by exploiting the temperature swings that occur between day and night. Known as a thermal resonator, it uses a hybrid of materials that produce both high heat conduction and capacity. A copper or nickel foam at its core is coated with graphene to boost its conductivity, and is infused with a phase-changing material (octadecane) that serves as storage. Effectively, one side of the device is always capturing heat while the other is storing it -- you just have to harvest that energy with conventional techniques.