The energy of ionic thermal motion presents universally, which is as high as 4 kJ\bullet kg-1\bullet K-1 in aqueous solution, where thermal velocity of ions is in the order of hundreds of meters per second at room temperature1,2. Moreover, the thermal velocity of ions can be maintained by the external environment, which means it is unlimited. However, little study has been reported on converting the ionic thermal energy into electricity. Here we present a graphene device with asymmetric electrodes configuration to capture such ionic thermal energy and convert it into electricity. An output voltage around 0.35 V was generated when the device was dipped into saturated CuCl2 solution, in which this value lasted over twenty days. A positive correlation between the open-circuit voltage and the temperature, as well as the cation concentration, was observed. Furthermore, we demonstrated that this finding is of practical value by lighting a commercial light-emitting diode up with six of such graphene devices connected in series. This finding provides a new way to understand the behavior of graphene at molecular scale and represents a huge breakthrough for the research of self-powered technology. Moreover, the finding will benefit quite a few applications, such as artificial organs, clean renewable energy and portable electronics.