Devices for the Internet-of-Things (IoT) are often placed in remote locations or are embedded in vehicles or machines and thus need to be fully wireless, lightweight, and energy-autonomous. The project FOXES aims to provide a clean, compact, low-cost and scalable high energy density solution for powering IoT devices such as wireless sensor nodes. The energy supply system developed by FOXES is constituted by the combination of a lead-free perovskite solid cell and a multilayer relaxor thin film capacitor with high energy density. Coupling these two devices allows solar energy surplus to be stored in the capacitor and being used for periods of time when solar light is not available. The energy balance (intake/discharge) is regulated by an electronic circuit, ensuring a positive energy balance for powering the sensor node. The FOXES system is constituted by: -Fully lead-free perovskite solar cell with > 10% efficiency. -Lead-free perovskite multilayer thin film capacitor with high energy density (> 50 J/cm3). -Graphene and metal-oxide based electronics for energy management circuit. These components will be fully 3D monolithically integrated using low-cost and sustainable processes (e.g. spin coating, spray pyrolysis) minimising the use of harmful chemicals or critical raw materials. This will also improve recycling and end-of-life disposability of the FOXES system. The targeted energy generation of the FOXES system is > 250 mJ/day. The developed system will be then coupled with low-power light-activated gas sensors (as use case) – giving less than 3 mJ/day energy consumption – and the necessary ASIC/data transmission devices for sensor operation. For the latter, commercial low-power solutions will be adopted, so that a positive energy balance will be maintained. The combined energy supply – sensor system will be tested in the lab against gas mixtures during variable irradiation conditions. A roadmap for scaling up the FOXES technology will be also defined.