Jung-Min You

Highly Stretchable Fully-Printed CNT-Based Electrochemical Sensors and Biofuel Cells: Combining Intrinsic and Design-Induced Stretchability

We present the first example of an all-printed, inexpensive, highly stretchable CNT-based electrochemical sensor and biofuel cell array. The synergistic effect of utilizing specially tailored screen printable stretchable inks that combine the attractive electrical and mechanical properties of CNTs with the elastomeric properties of polyurethane as a binder along with a judiciously designed free-standing serpentine pattern enables the printed device to possess two degrees of stretchability. Owing to these synergistic design and nanomaterial-based ink effects, the device withstands extremely large levels of strains (up to 500% strain) with negligible effect on its structural integrity and performance. This represents the highest stretchability offered by a printed device reported to date. Extensive electrochemical characterization of the printed device reveal that repeated stretching, torsional twisting, and indenting stress has negligible impact on its electrochemical properties. The wide-range applicability of this platform to realize highly stretchable CNT-based electrochemical sensors and biofuel cells has been demonstrated by fabricating and characterizing potentiometric ammonium sensor, amperometric enzyme-based glucose sensor, enzymatic glucose biofuel cell, and self-powered biosensor. Highly stretchable printable multianalyte sensor, multifuel biofuel cell, or any combination thereof can thus be realized using the printed CNT array. Such combination of intrinsically stretchable printed nanomaterial-based electrodes and strain-enduring design patterns holds considerable promise for creating an attractive class of inexpensive multifunctional, highly stretchable printed devices that satisfy the requirements of diverse healthcare and energy fields wherein resilience toward extreme mechanical deformations is mandatory.

Soft, stretchable, high power density electronic skin-based biofuel cells for scavenging energy from human sweat

This article describes the fabrication, characterization, and real-life application of a soft, stretchable electronic-skin-based biofuel cell (E-BFC) that exhibits an open circuit voltage of 0.5 V and a power density of nearly 1.2 mW cm−2 at 0.2 V, representing the highest power density recorded by a wearable biofuel cell to date. High power density is achieved via a unique combination of lithographically-patterned stretchable electronic framework together with screen-printed, densely-packed three-dimensional carbon-nanotube-based bioanode and cathode array arranged in a stretchable “island-bridge” configuration. The E-BFC maintains its performance even under repeated strains of 50%, and is stable for two days. When applied directly to the skin of human subjects, the E-BFC generates ∼1 mW during exercise. The E-BFC is able to power conventional electronic devices, such as a light emitting diode and a Bluetooth Low Energy (BLE) radio. This is the first example of powering a BLE radio by a wearable biofuel cell. Successful generation of high power density under practical conditions and powering of conventional energy-intense electronic devices represents a major step forward in the field of soft, stretchable, wearable energy harvesting devices.

Deposition of ZnO on bismuth species towards a rechargeable Zn-based aqueous battery

Zn aqueous batteries typically suffer from poor cycle life because water soluble zincate ions are formed during the oxidation of Zn. When Zn is oxidized, most of the Zn2+ ions detach from the current collector and become electrochemically inactive, leaving the battery non-rechargeable. Numerous reports demonstrate the use of Bi2O3 as an electrode additive to enhance electrochemical performance and they attribute this phenomenon to the improvement in electrical conductivity. However, conductivity does not have an effect on the intrinsic solubility of the zincate ion. We conduct a series of characterizations to provide a comprehensive mechanistic role of Bi2O3 in the Zn electrode. We find that upon oxidation, zincate ions are formed but they relax into ZnO on the surface of the bismuth species. This work proposes that the reason for the prolonged cycle life is due to the deposition of ZnO through relaxation and this prevents losing electrochemically active materials. This finding paves the way for further improving the cycle life and understanding the mechanism of the Zn based rechargeable aqueous batteries and possibly other conversion types of rechargeable batteries.

High-Performance Screen-Printed Thermoelectric Films on Fabrics

Printing techniques could offer a scalable approach to fabricate thermoelectric (TE) devices on flexible substrates for power generation used in wearable devices and personalized thermo-regulation. However, typical printing processes need a large concentration of binder additives, which often render a detrimental effect on electrical transport of the printed TE layers. Here, we report scalable screen-printing of TE layers on flexible fiber glass fabrics, by rationally optimizing the printing inks consisting of TE particles (p-type Bi0.5Sb1.5Te3 or n-type Bi2Te2.7Se0.3), binders, and organic solvents. We identified a suitable binder additive, methyl cellulose, which offers suitable viscosity for printability at a very small concentration (0.45–0.60 wt.%), thus minimizing its negative impact on electrical transport. Following printing, the binders were subsequently burnt off via sintering and hot pressing. We found that the nanoscale defects left behind after the binder burnt off became effective phonon scattering centers, leading to low lattice thermal conductivity in the printed n-type material. With the high electrical conductivity and low thermal conductivity, the screen-printed TE layers showed high room-temperature ZT values of 0.65 and 0.81 for p-type and n-type, respectively.

Enzymatic glucose/oxygen biofuel cells: Use of oxygen-rich cathodes for operation under severe oxygen-deficit conditions

A glucose/oxygen biofuel cell (BFC) that can operate continuously under oxygen-free conditions is described. The oxygen-deficit limitations of metabolite/oxygen enzymatic BFCs have been addressed by using an oxygen-rich cathode binder material, polychlorotrifluoroethylene (PCTFE), which provides an internal oxygen supply for the BFC reduction reaction. This oxygen-rich cathode component mitigates the potential power loss in oxygen-free medium or during external oxygen fluctuations through internal supply of oxygen, while the bioanode employs glucose oxidase-mediated reactions. The internal oxygen supply leads to a prolonged energy-harvesting in oxygen-free solutions, e.g., maintaining over 90% and 70% of its initial power during 10- and 24-h operations, respectively, in the absence of oxygen. The new strategy holds considerable promise for energy-harvesting and self-powered biosensing applications in oxygen-deficient conditions.