Materials science has made progress in maximizing or minimizing the thermal conductivity of materials; however, the thermal effusivity—related to the product of conductivity and capacity—has received limited attention, despite its importance in the coupling of thermal energy to the environment. Herein, we design materials that maximize the thermal effusivity by impregnating copper and nickel foams with conformal, chemical-vapor-deposited graphene and octadecane as a phase change material. These materials are ideal for ambient energy harvesting in the form of what we call thermal resonators to generate persistent electrical power from thermal fluctuations over large ranges of frequencies. Theory and experiment demonstrate that the harvestable power for these devices is proportional to the thermal effusivity of the dominant thermal mass. To illustrate, we measure persistent energy harvesting from diurnal frequencies, extracting as high as 350 mV and 1.3 mW from approximately 10 °C diurnal temperature differences.
Next generation off-the-grid electronic systems call for alternative modes of energy harvesting. The past two decades have witnessed the evolution of a wide spectrum of low dimensional carbon materials with extraordinary physical and chemical properties, ideal for micro-scale electrical energy storage and generation. Tremendous progress has been made in harnessing the energy associated with the interactions between these nano-structured carbon substrates and the surrounding molecular phases, subsequently converting them into useful electricity. This review summarizes the important theoretical and experimental milestones the field has reached to date, and further classifies these energy harvesting processes based on underlying physics, into five mechanistically distinct classesphonon coupling, Coulombic scattering, electrokinetic streaming, asymmetric doping, and capacitive discharging. With a special mechanistic focus, the authors hope to resolve the fundamental attributes shared by this diverse array of molecular scale energy harvesting schemes, offer perspectives on key challenges, and ultimately establish design principles that guide further device optimization.
Thermal diodes, or devices that transport thermal energy asymmetrically, analogous to electrical diodes, hold promise for thermal energy harvesting and conservation, as well as for phononics or information processing. The junction of a phase change material and phase invariant material can form a thermal diode; however, there are limited constituent materials available for a given target temperature, particularly near ambient. In this work, we demonstrate that a micro and nanoporous polystyrene foam can house a paraffin-based phase change material, fused to PMMA, to produce mechanically robust, solid-state thermal diodes capable of ambient operation with Young's moduli larger than 11.5 MPa and 55.2 MPa above and below the melting transition point, respectively. Moreover, the composites show significant changes in thermal conductivity above and below the melting point of the constituent paraffin and rectification that is well-described by our previous theory and the Maxwell-Eucken model. Maximum thermal rectifications range from 1.18 to 1.34. We show that such devices perform reliably enough to operate in thermal diode bridges, dynamic thermal circuits capable of transforming oscillating temperature inputs into single polarity temperature differences - analogous to an electrical diode bridge with widespread implications for transient thermal energy harvesting and conservation. Overall, our approach yields mechanically robust, solid-state thermal diodes capable of engineering design from a mathematical model of phase change and thermal transport, with implications for energy harvesting.
Chemically modified carbon nanotube fibers enable unique power sources driven entirely by a chemical potential gradient. Electrical current (11.9 μA mg ) and potential (525 mV) are reversibly produced by localized acetonitrile doping under ambient conditions. An inverse length-scaling of the maximum power as L that creates specific powers as large as 30.0 kW kg highlights the potential for microscale energy generation.
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