Electrochemical Kinetic Energy Harvesting Mediated by Ion Solvation Switching in Two-Immiscible Liquid Electrolyte

Kinetic energy harvesting (KEH) is the process of converting mechanical energy, typically in the form of motion or vibrations, into electrical energy using specialized devices or systems.[1] KEH is of significance in the realm of renewable energy, as it enables the harnessing of otherwise wasted or underutilized kinetic energy sources, thereby contributing to the development of sustainable power generation and self-sustaining electronic devices.[2] Nonetheless, presently existing techniques, like those relying on friction and deformation, require high-frequency kinetic energy and call for materials possessing exceptional durability.[3] Also, those methods have extremely high impedance, leading to reduced current and power outputs, thus constraining their practical applications.[4]<br/>Herein, we propose an electrochemical KEH system that employs a two-phase immiscible liquid electrolyte, specifically combining aqueous and ionic liquid components, with Prussian blue analogue (PBA) electrodes. This system is designed to capture and harness kinetic energy from gentle and low-frequency mechanical motions while reducing impedance of the system. Starting from the state of equilibrium, where both PBA electrodes are submerged in distinct phases of the immiscible electrolyte, the open-circuit potential difference between these electrodes experiences an increment. This increase in potential difference enables the conversion of kinetic energy, associated with the movement of the electrodes into their respective phases, into electrical energy. The system generated 6.4 μW cm^-2 of peak electrical power, accompanied by 96 mV of peak voltage and 183 μA cm^-2 of peak current density when connected to a load resistor of 300 Ω. The applied load is three orders of magnitude smaller than what is conventional KEH methods. Furthermore, the proposed method demonstrated a continuous current output of around 5 μA cm^-2 over one hundred seconds, at the frequency of 0.005 Hz for 23 cycles without any degradation in performance. Through computational simulations, we have determined that voltage between PBA electrodes originates from the difference in solvation Gibbs free energy within each phase of the two-phase electrolyte. The elimination and subsequent reformation of solvation shells surrounding solvated cations serve as the driving force for both the generation of voltage and the flow of electrons within the system. Moreover, we have effectively demonstrated the capability of our system within a microfluidic device, thereby paving the way for diverse applications. The microfluidic kinetic energy harvester is composed of identical PBA thin-film electrodes and the two-phase electrolyte. By utilizing the conversion of kinetic energy to drive the two-phase electrolyte through the microfluidic channel, our system attained a peak power density of 200 nW cm^-2.<br/>In conclusion, we recognized the possibility of harvesting kinetic energy from solvation Gibbs free energies in the two-phase electrolyte mediated by ion-hosting materials and furnished persuasive evidence of its viability as a power source for self-powered devices. The incorporation of our system into the microfluidic harvester endows it with advantages for powering wearable electronics and Internet of Things (IoT) applications.<br/><br/>Abstract original from, <i>[Preprint] Research Square,</i> DOI: 10.21203/rs.3.rs-3296359/v1<br/>[1] <i>Applied Energy</i>. 2021, 286, 116518.<br/>[2] <i>Nano Energy</i>. 2017, 39, 9-23.<br/>[3] <i>Energy & Environmental Science</i>, 2022, 15, 82.<br/>[4] <i>Nature,</i> 2020, 578, 392-396.