Energy Harvesting Breakthrough

Molecular thermal motion has been studied but yet never been utilized as an energy source. In this work, researchers demonstrate that the energy of liquid molecular thermal motion can be converted into electrical energy by a novel harvesting device, the molecular thermal motion harvester (MTMH). The MTMH was made by using two ZnO-based nano-arrays and one of which was gold coated to form a Schottky junction. The assembled electrodes were immersed in different liquid phase environments. The device was demonstrated to convert the molecule thermal energy of the liquid into a continuous and stable electric current. The output voltage and current can achieve 2.28 mV and 2.47 nA, respectively, and increase with the liquid temperatures. This strategy opens new insights into the development of mini- and micro-scale energy sources, and it can be expected the MTMH will have broad applications in the future.

APL Materials – Molecular thermal motion harvester for electricity conversion

In the era of the Internet of Things (IoT) and 5G, energy demands are decentralized, mobile, and ubiquitous. Some mini- and micro-scale energy sources, such as airflow, human movement blood flow, ultrasound, etc., have already been explored and converted into electricity by various nano-energy generator technologies based on different schemes/mechanisms. Most of these conversions are based on mechanical energy.

Molecular thermal motion is a special kind of dynamic motion that is essentially different from ordinary mechanical motion. It is a component of the internal energy of the physical system, which means that the molecules of all substances are in constant and random movement above absolute zero temperature. Brownian motion of particles is one example that is caused by the molecule thermal motion of the surrounding liquid or gaseous molecules. Molecule thermal motion contains an enormous amount of energy, taking an ideal gas as an example, the average kinetic energy of thermal motion per mole of gas molecules at room temperature (27 °C) is 3.7 kiloJoules. If this form of energy could be utilized from the huge amounts of liquids and gases on the planet effectively, this would provide a new source of energy on an enormous scale.

Preparation of ZnO nanoarray

The Zn substance was ultrasonic cleaned in ethanol, acetone, and distilled water for 10 min, and dried with nitrogen. Then it was covered with polytetrafluoroethylene membrane (Taizhou Aoke Filter Paper Factory, ϕ50, 0.45 µm) and filter paper (Taizhou Aoke Filter Paper Factory, quantitative, slow) in sequence.

The Zn substance was suspended horizontally at a certain distance above the beaker containing the ethylenediamine-water solution with a concentration of 3.75 mol/l. Sealing and leaving it at room temperature for 48 h. Then it was taken out, rinsed with distilled water, and dried with nitrogen. The ZnO nanosheet array was grown on the Zn surface as piezoelectric materials. The other Zn substance was covered with filter paper, forming a ZnO nanosheet/rod hybrid array under the same growth environment.

Device packaging

Then octane was added dropwise to ensure that the gaps between nanoarrays were filled with octane and covered by the top electrode, which was a gold coated ZnO hybrid nanoarray that was also filled with octane. The Au coating of the upper electrode forms a Schottky barrier with ZnO underneath. The Schottky barrier can prevent electrons from escaping from ZnO nanosheets into the top electrode. The gold-coated ZnO hybrid nanoarray surface is used as the negative electrode, and Zn is used as the positive electrode of TMH. After connecting the wires, the entire device is packaged and sealed with epoxy to prevent liquid leakage.

The results reported here indicated that the energy of the thermal motion of octane can be converted into electrical energy through the device based on the piezoelectric properties of ZnO and a nano-array structure. Its output voltage and current can reach 2.28 mV and 2.47 nA at room temperature, respectively. With the increase in temperature, the output currents and voltages of the MTMH also increased. Two kinds of liquids n-octane, cyclohexane, and n-heptane can be used to drive this MTMH. The advantages of MTMH are obvious; for example, after the proper solvent is packaged, no additional energy source is required as long as the ambient temperature is above absolute zero and there is a constant thermal exchange with the surrounding. The generated electrical energy of MTMH is continuous, steady, and clean without any negative impact on the environment compared to fossil and nuclear energy. With the advancement of this technology, we expect, with a single device, not only to generate micro-watt level energy but also to provide novel thoughts for the watt- or even kilowatt-scale energy supply by making large-sized generators. Larger levels of energy supply can be solved by for example, electrolytic hydrogen production. The MTMH technology can be applied potentially to many fields such as home, personal care, outdoor sports, etc.