Strain-dependent modeling of a mechano-electrochemical energy harvester based on carbon nanotube yarn

This paper introduces the electrochemical equivalent circuit model for a carbon nanotube (CNT) coiled yarn subjected to external mechanical deformation. The mechanically coiled CNT yarn electrode, with average coil diameters of 83 μm and 112 μm, operates in an electrode-electrolyte configuration in which applied strain induces mechano-electrochemical energy conversion. Electrochemical impedance spectroscopy (EIS) was employed to characterize the impedance behavior of the coiled CNT electrode under various applied strain conditions, while the scaling behavior associated with different numbers of CNT sheets was anticipated based on physical and geometric considerations. The measured impedance spectrum was analyzed and fitted using ZView software (Scribner Associates) to extract lumped electrochemical parameters, including both fixed and strain-dependent elements grounded in the electrochemical characteristics of the mechano-electrochemical energy harvester. Analysis of the Nyquist spectra reveals that the output impedance increases as the coiled CNT electrode is stretched and decreases with scaling of the device through additional CNT sheets, while the peak-to-peak open-circuit voltage increases with increasing stretch of the yarn. Based on these observations, a lumped-element equivalent circuit with strain-dependent electrochemical parameters was developed. The proposed model enables SPICE-based circuit simulations and facilitates impedance matching between the MEEH and external loads, thereby maximizing power transfer to the load. The equivalent circuit accurately reproduces the measured electrical behavior with a minimum error below 5% for both 3-sheet and 6-sheet MEEH configurations. These results demonstrate that the proposed modeling approach effectively anticipates impedance and power performance as functions of mechanical strain and device scaling.