A thermionic energy converter using single-layer graphene

Heat energy can be converted into electricity with very high efficiency through a temperature-induced electron flow process known as thermionic emission. The basic operating principle of thermionic energy converters is that electrons are evaporated from a heated cathode into a vacuum and then condensed at a cooler anode. After the first practical demonstration in 1957, thermionic energy converters have been used with different heat sources, all of them requiring operation at high temperatures – above 1500 K.

While operation of thermionic energy converters at relatively low temperature (700 K - 900 K) is possible, the efficiency of the process – which normally is > 50% – is greatly reduced since the number of emitted electrons per unit area from the cathode depends on the heating temperature.

For traditional cathode materials – such as metals and semiconductors – the number of emitted electrons is proportional to the square of cathode temperature. However, a recent study demonstrates that the 'square' is revised to 'cubic' for a single-layer graphene, which means that heat temperature can be lowered by an order of magnitude if using graphene as hot cathode. The findings indicate that a graphene-based cathode thermionic converter operating at 900 K could reach an efficiency of 45%.