The present disclosure is directed to materials, devices, and methods for resonant ambient thermal energy harvesting. Thermal energy can be harvested using thermoelectric resonators that capture and store ambient thermal fluctuations and convert the fluctuations to energy. The thermal resonators can include heat engines disposed between masses of varying sizes or diodes. The masses or diodes can be made of high and ultra-high effusivity materials to transfer thermal energy through the resonator and optimize power output. The masses or diodes of the resonator can be tuned to the dominant frequency of the temperature waveform to maximize the amount of energy being converted. The resonators can be added to existing structures to supply or generate power, and, in some embodiments, the structures themselves can be a mass of the thermal resonator. Methods for constructing and/or using such devices are also provided, as are methods for formulating ultra-high effusivity materials.