Low-grade thermal energy is an abundantly available, sustainable and clean energy source. Its source includes waste heat from power plants, ocean thermal energy, geo-thermal energy, distributed solar energy among others. Conventional solid-state thermoelectric devices are limited in application due to their high cost, intrinsically low Seebeck coefficient (10-100 μV/K) and inflexible nature. Thermogalvanic systems, also known as thermocells have recently been found to be a promising alternative. These devices using the temperature dependence of electrochemical potential of redox electrolytes such as aqueous ferri/ferrocyanide possess the advantages of simple design, direct thermal-to-electric energy conversion, continuous operation, low mainte­nance, zero carbon emissions and low cost. However, these devices need to be connected to energy storage devices (battery or capacitor) by a power management circuit in order to store energy. This leads to complexity in system design and operation. Here, we demonstrate an integrated thermal energy harvesting supercapacitor device that can convert a temperature gradient applied over a short period of time, into stored charge. The stored electrical energy can be used to power an external load long after the temperature gradient is disengaged. Furthermore, it is flexible, thin, lightweight and all-solid-state which makes it ideal for powering portable and wearable devices. Electrodes consisting of graphene petals (GP) grown on conductive carbon cloth by microwave plasma chemical vapor deposition (MPCVD) are sandwiched onto a polyvinyl alcohol (PVA)/potassium ferrocyanide/ferricyanide gel electrolyte film to build a supercapacitor device. The device exhibits a Seebeck coefficient of -1.21 mV/K, a high areal capacitance near 9 F cm^-2 at a current density of 0.1 mA cm^-2 and excellent charge retention capability.