CLEAN ENERGY & ENVIRONMENT

Powering modern-day devices 

In the quest for high-energy storage devices, stretchable supercapacitors and rechargeable batteries will likely emerge as the primary power source for modern-day portable electronic devices, satellites and even electric vehicles. At UD, faculty members in mechanical engineering are prompting new technologies to meet this growing demand. 

Deformable energy storage devices 

Advances in flexible electronic technology are inspiring UD researchers to create novel electronic devices that can bend, stretch and deform for a broad range of applications, impeded only by the lack of available flexible power sources. Supercapacitors are attracting attention for their ability to generate energy and power densities higher than conventional dielectric capacitors and lithium ion batteries, respectively. Lightweight and flexible carbon nanotubes, in the form of free-standing thin films, are favored for their high surface area, efficient charge storage and excellent electrical conductivity for high rate operation. Professor Bingqing Wei and his research team, in collaboration with Professor Hanqing Jiang of Arizona State University, are developing a reversibly stretchable supercapacitor capable of integrating into any flexible and stretchable electronic device, and able to withstand a tensile strain of up to 30 percent. The design involves impressing single-walled carbon nanotube (SWNT) macro-films onto pre-strained polydimethylsiloxane films. Once the strain is released, the SWNT film assumes a reversibly stretchable periodically sinusoidal shape. Electrochemical characterizations performed under normal and strained conditions indicate energy and power density values comparable to supercapacitors assembled with pristine SWNT films. 

High-temperature supercapacitors 

Electrochemical power sources with high energy and power densities, that can withstand harsh temperatures, are desirable for applications ranging from civilian portable electronic devices to military weapons. One promising application, when coupled with supercapacitors, is for hybrid electric vehicles. Together, they deliver the high power and long cycle life needed for vehicle start-up or acceleration, and for energy recovery during braking. Wide-temperature-withstanding supercapacitors developed by Wei’s team can be stably operated between room temperature and 100 degrees C and within a voltage window of -2 V to 2 V. This ultra-light power source can withstand current densities as high as 100 A/g, yielding specific power density values on the order of 55 kW/kg. An ultra-long galvanostatic charge-discharge cycling up to 200,000 cycles at both room temperature and 100 degrees C, shows excellent stability in capacitance with high efficiency. 

High-performance li-ion batteries 

On another front, Silicon (Si) is now being considered for Li-ion batteries based on its low discharge potential and high theoretical charge capacity (4,200 mAh/g). The second most abundant element on earth, it’s also recyclable, suggesting a sustainable solution to our energy needs. Yet development of Si-anode Li-ion batteries lags due to large volumetric change (400 percent) upon insertion and extraction of lithium, resulting in pulverization and early capacity fading. Wei’s team is tackling this hurdle by depositing silicon films on “soft substrates,” such as carbon nanotube films or elastomeric polydimethylsiloxane layers, capable of releasing the high stress induced during lithium ion insertion and extraction. Collaborating with UD Prof. Joshua Hertz and researchers at ASU, the team is realizing long-cycle life batteries with exceptionally high discharging capacity and better capacity retention. These National Science Foundation-funded research projects demonstrate new concepts in creating different types of stretchable power sources, including rechargeable lithium ion batteries with flexible electrode materials. 

Powering electric vehicles 

In the automotive industry, lithium-titanate batteries show promise as a power source due to their long lifetime, good energy density and ability to withstand large charge/discharge currents. But an active thermal management system is required to maintain a safe operating temperature and prevent battery degradation. Professors Ajay Prasad and Suresh Advani’s team has designed two battery cooling systems -- one water-cooled, the other cooled by air. While both perform effectively, the air-cooled system, which consumes less parasitic power, is more efficient, indicating that ambient air can be used as the active fluid to cool the cells through proper heat exchanger design. 

Metal hydride-based hydrogen storage 

Prasad and Advani are also exploring the use of metal hydrides to safely and cost-effectively overcome on-board storage challenges associated with hydrogen as an automotive fuel source. Typically, hydrogen is stored in tanks at pressures of 5,000 or 10,000 psi, or as a cryogenic liquid. Even at such extreme pressures and temperatures, the fuel occupies too much volume. A promising alternative is solid-state hydrogen storage, which uses metal hydrides to absorb/desorb hydrogen at relatively low pressure, offering safety and cost advantages with potentially unparalleled hydrogen storage density. Hydrogen storage in porous metal hydrides beds is a complex problem involving compressible gas flow in porous media, heat transfer and reaction kinetics. Heat transfer is the key obstacle because the charging time of hydrogen in metal hydride tanks is strongly influenced by the rate at which heat can be removed from the reaction bed. Through numerical simulations, the researchers continue to optimize system design