Harvesting useful electric energy from ambient thermal gradients and/or fluctuations is immensely important. Usually, direct thermoelectric energy conversion is based on the Seebeck effect. However, thermal shorting limits the energy conversion efficiency
To develop advanced energy harvesting systems, a novel concept using nanoporous materials, which was recently developed in our lab, will be investigated. If two nanoporous electrodes are placed at different temperatures, they absorb different amounts of ions, generating a net output voltage. The thermally driven ion motion causes a transient current. The two electrodes are isolated; that is, the direct heat loss between them is minimized.
The proposed study will not only lead to the development of high-performance thermal energy harvesting systems, but also shed light on fundamentals of electrophysics. The thermal effect on surface electrification in nanopores is a new scientific area. The research will lead to the establishment of counterparts of classic electrophysics theories for nanoenvironment.
Broader Impacts
This grant will also provide an important support to the author?s pedagogical efforts. Video of experiments and visualized computer simulation modules will be used in a number of undergraduate and graduate courses, as well as in a series of seminars, having considerable impact on the curriculum. Female students and under-represented minorities will be encouraged to attend the seminars and take the courses. Both undergraduate and graduate students will be actively involved in the project and acquire comprehensive hands-on research experience.
When temperature increases, the capacity of a capacitor may decrease, as a part of the stored electric energy is "lost". While this phenomenon is often regarded as detrimental, its reverse process may be employed to harvest electricity from heat – This is a novel concept of energy conversion and storage, investigated systematically in this NSF project. For instance, if two identical half-capacitors are placed at different temperatures, since electrode potential is thermally dependent, there would be a net output voltage between the two half-capacitors. That is, the high-potential half-capacitor, often the one at the low-temperature side, could charge the low-potential half-capacitor, often the one at the high-temperature side, until a new equilibrium is reached. This procedure, when amplified by the ultrahigh surface areas of nanoporous electrodes, can lead to a high thermal sensitivity, i.e. a high voltage, and a high energy conversion efficiency, i.e. a high energy density. The technique of thermally chargeable supercapacitors (TCS) is particularly attractive for harvesting and storing low-grade heat (LGH), the ubiquitous thermal energy with temperature < 200 oC. Examples of LGH include wasted heat in power plants and vehicles, solar and geo thermal energy, etc. Efficient harvesting and storage of LGH can significantly impact all economic sectors and enhance energy security. It is still a blank area of today’s technology. Our results showed that the energy density of TCS, with a small temperature difference of only 50 oC, can be as high as about 5 mJ/g, hundreds of times higher than that of conventional direct thermal conversion materials, such as thermoelectrics. The temperature sensitivity, which can be regarded as the counterpart to the Seebeck coefficient, is on the scale of a few mV/oC, at least one order of magnitude higher than that of thermoelectrics. With the high temperature sensitivity and the high energy density, the TCS technique may open a new area of energy harvesting and storage of LGH, e.g. solar thermal energy in desert areas. According to a preliminary market analysis, if the energy density of TCS system can reach 10 mJ/g, the power cost of TCS may meet that of grid power. This will be a major direction of our future work.
