Frequency Response of Piezoelectrochemical Energy Harvesting

Abstract

Mechanical energy is a readily available and often wasted source of ambient energy, so mechanical energy harvesters are particularly appealing for powering microelectronic systems that cannot be connected to traditional power sources. A promising and recently discovered type of mechanical energy harvester uses the piezoelectrochemical (PEC) effect to convert mechanical energy into electrochemical energy1-6. In these harvesters, an applied mechanical stress changes the redox potential of the active material, which drives ion flux to return the system to equilibrium. Recent literature1-6 has demonstrated that these harvesters have high theoretical energy densities and can harvest lower mechanical frequencies than piezoelectric materials and other traditional mechanical energy harvesters. This indicates that PEC harvesters could fulfill energy harvesting applications where other mechanical harvesters would not be viable. However, despite its promise, research on PEC harvesters is still a relatively new field and there is much to be learned. Recent work6 aimed to standardize the way this new class of harvesters is compared, and this paper noted that the peak current output of a PEC harvester is affected by the input mechanical vibration frequency. However, an in-depth study and explanation of this frequency dependence was left for future studies. In this work, we aim to address this frequency dependence and the fundamental ion kinetics that drive it.Commercially-available lithium ion pouch cells are a convenient system to study the PEC effect1,6. In these pouch cells, both the lithium cobalt oxide and graphite electrodes are PEC materials, and the current and voltage outputs of the harvester are proportional to the applied mechanical stress1. We mechanically cycle these pouch cells in an Instron compressive testing machine and measure the corresponding short-circuit current output. We find the value of the peak current output increases with a slower mechanical cycling frequency. We speculate that the value of the peak current output is the convolution of the change in redox potential due to the applied mechanical stress and how far away the system was from equilibrium before. The initial mechanical cycle primes the system and affects the current output of the succeeding cycles, acting as a kind of mechanical memory effect. We model this as a capacitor being charged: the more time passes, the more ions build up. This insight into the frequency dependence of PEC harvesters allows us to better understand the mechanism behind the PEC effect, which will enable the design of better harvesters.