Tuning the Frequency Response of Fractional-Order Microsupercapacitors

Electric double-layer capacitors (EDLC) are a subset of electrochemical capacitors that can store and deliver electrical energy at dc and relatively far-from-dc frequencies with effective capacitance between that of aluminum electrolytic capacitors and secondary batteries. They are mostly employed in conventional energy storage applications as secondary power source, such as microprocessors and solar batteries. They have also been demonstrated as efficient energy devices in oscillators and filters circuits, fractional-order controllers, and fractional-order resonators. However, because of the nature and porous structure of their electrodes and the interfacial electrochemistry of their electrodes/electrolyte phase, many fundamental aspects of their performance metrics are still not well understood, and rational design is practically nonexistent. In particular, EDLCs exhibit a dissipative, resistive-capacitive behavior when operating away from dc with an impedance angle anywhere between -90 and 0 deg. In this project, miniaturized EDLCs based on structured 2D and 3D electrode arrays will be designed and fabricated with the objective of understanding and controlling their non-ideal, fractional-order behavior. We will develop and study the effect of doped electrolytes in order to tune the electric-field-induced ionic transport in the presence of physical obstacles. The expected outcome is a general procedure and design rules to apply in order to fine-tune and control the impedance phase shift of EDLCs and their energy-power performance. Modeling and simulation using mean-field Poisson-Nernst-Plank model will be carried out in order to provide a fundamental understanding of the frequency response of the devices. System-level modeling using fractional-order mathematical tools and equivalent circuit models will also be developed in connection with RC-based circuitry. The controllable fractional-order behavior of the EDLCs will be verified and their frequency-domain application will be demonstrated. This project will contribute to the research, education, and diversity goals of Florida International University. The objectives of this project are to tackle the lack of knowledge on the frequency-domain metrics and performance of factional-order capacitors using both experimental and modeling approaches. We aim to investigate the following: (1) electrode-electrolyte interface specifications and electrolyte parameters that enable the tuning of the electrical characteristics of an EDLC over an extended frequency bandwidth; (2) the electro-kinetic effects taking place in the supporting electrolyte of an EDLC, and how they affect the frequency-domain metrics of the device; (3) modeling using 3D-circuit interconnects and finite-element methods to understand the overall electric characteristics; and (4) the frequency response of the EDLCs and their application in (frequency-domain) filtering and (time-domain) memory applications. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.