Richa Agrawal

Carbon microelectromechanical systems (C-MEMS) based microsupercapacitors

The rapid development in miniaturized electronic devices has led to an ever increasing demand for high-performance rechargeable micropower scources. Microsupercapacitors in particular have gained much attention in recent years owing to their ability to provide high pulse power while maintaining long cycle lives. Carbon microelectromechanical systems (C-MEMS) is a powerful approach to fabricate high aspect ratio carbon microelectrode arrays, which has been proved to hold great promise as a platform for energy storage. C-MEMS is a versatile technique to create carbon structures by pyrolyzing a patterned photoresist. Furthermore, different active materials can be loaded onto these microelectrode platforms for further enhancement of the electrochemical performance of the C-MEMS platform. In this article, different techniques and methods in order to enhance C-MEMS based various electrochemical capacitor systems have been discussed, including electrochemical activation of C-MEMS structures for miniaturized supercapacitor applications, integration of carbon nanostructures like carbon nanotubes onto C-MEMS structures and also integration of pseudocapacitive materials such as polypyrrole onto C-MEMS structures.

Hybridization of lithium-ion batteries and electrochemical capacitors: fabrication and challenges

Conventional electrochemical double-layer capacitors (EDLCs) are well suited as power sources for devices that require large bursts of energy in short time periods. However, when compared to their battery counterparts, EDLCs suffer from low energy densities. The low energy density of EDLCs hinders their applications in devices that require a simultaneous supply of high power and high energy. In order to improve the energy density of EDLCs, the concept of hybridization of lithium-ion batteries (LIBs) and EDLCs has gathered much attention in past years. Such a hybrid is typically referred to as “lithium-ion capacitor” (LIC) or “lithium capacitor” and essentially utilizes a lithium intercalating anode (such as graphite or Li4Ti5O12) and a fast charging-discharging EDLC electrode (such as activated carbon, carbon nanostructures) in a lithium-salt based electrolyte. Although such a system sounds quite ideal in theory, there are major challenges that need to be addressed in order to fully realize the benefits of LIB and EDLC electrodes in conjunction. Most of these challenges stem from the mismatch in capacity of the electrodes and also the charging-discharging times of the electrodes. For instance, the EDLC electrode acts as the limiting factor for the capacity of the system while the LIB electrode limits the power of the system. Here we have fabricated a hybrid capacitor that utilizes a Li4Ti5O12 (LTO) based anode and an activated carbon (AC) composite based cathode. Half-cell testing for both LTO and AC have been shown along with full cell evaluation.

Micro enzymatic biofuel cells: from theoretical to experimental aspect

Miniaturized enzymatic biofuel cells (EBFCs) that convert biological energy into electrical energy by using enzymemodified electrodes are considered as one of the promising candidates to power the implantable medical devices and portable electronics. However, their low power density and insufficient cell lifetime are two big obstacles to need to be tackled before EBFCs become viable for practical application. In this study, the theoretical simulation of this EBFC system is conducted using finite element analysis from COMSOL 4.3a in terms of cell performance, efficiency and optimum cell configurations. We optimized the electrodes design in steady state based on a three dimensional EBFC chip and studied the effect of orientation of the microelectrode arrays in blood artery. In the experimental part, we demonstrated an EBFC system that used 3D micropillar arrays integrated with graphene/enzyme composites. The fabrication process of this system combined top-down carbon microelectromechanical system (CMEMS) technology to fabricate the 3D micropillar arrays platform and bottom-up electrophoretic deposition (EPD) to deposit graphene/enzyme composite onto the 3D micropillar arrays. The amperometric response of the graphene based bioelectrodes exhibited excellent electrochemical performance and the 3D graphene/enzyme based EBFC generated a maximum power density of 136.3 μWcm-2 at 0.59 V, which is about 7 times of the maximum power density of the bare 3D carbon based EBFC.

C-MEMS for Bio-sensing Applications

Developing highly sensitive, selective, and reproducible miniaturized bio-sensing platforms require reliable biointerface which should be compatible with microfabrication techniques. In this study, we have fabricated pyrolyzed carbon arrays with high surface area as a bio-sensing electrode, and developed the surface functionalization methods to increase biomolecules immobilization efficiency and further understand electrochemical phenomena at biointerfaces. The carbon microelectrode arrays with high aspect ratio have been fabricated by carbon microelectromechanical systems (C-MEMS) and nanomaterials such as graphene have been integrated to further increase surface area. To achieve the efficient covalent immobilization of biomolecules, various oxidation and reduction functionalization methods have been investigated. The oxidation treatment in this study includes vacuum ultraviolet, electrochemical activation, UV/Ozone and oxygen RIE. The reduction treatment includes direct amination and diazonium grafting. The developed bio-sensing platform was then applied for several applications, such as: DNA sensor; H2O2 sensor; aptamer sensor and HIV sensor.

Electrostatic spray deposition of Li4Ti5O12 based anode with enhanced rate capability and energy density for lithium-ion batteries

Li4Ti5O12 (LTO) is one of the most promising anode materials for lithium-ion batteries (LIBs) due to its excellent cyclability and extraordinary structure stability during lithium-ion intercalation and deintercalation. However, LTO suffers from the low electronic conductivity and low theoretical capacity, which results in poor rate capability and low energy density. The present work reviews the latest achievement on improving both energy and power density of LTO based anode materials for LIBs. In addition, our recent results on electrostatic spray deposition (ESD) derived LTO electrode is also discussed. Electrochemical test shows that the resulting LTO has a large specific capacity of 293 mAh g-1 under a current density of 0.15 A g-1 and high rate capacity of 73 mAh g-1 under 3 A g-1. As compared with commercial LTO nano-particle electrode, the improved electrochemical performance of ESD-LTO could be attributed to the structure advantages generate from ESD which could lead to reduced diffusion length for lithium ions and electrons.