Oak Ridge Scientist Uses Nanomaterials to Make Thermoelectric Converter That Is Much More Efficient Than Fuel Cells, Batteries or Gas Engines

Dr. Zhiyu “Jerry” Hu, a staff scientist at the Nanoscale Science and Device group of Oak Ridge National Laboratory (ORNL), has developed a solid state thermoelectric converter using nanomaterials that could provide power for personal digital assistants (PDA), cellular phones or laptop computers.  The thermoelectric converter is more efficient and more powerful than fuel cells or batteries.  The thermoelectric converter can be fueled by hydrogen or liquid hydrocarbon fuels such as alcohol or methanol.  

Thermoelectricity is the conversion from temperature differentials to electricity or vice versa. Thermoelectricity can be accomplished based on the Peltier-Seebeck effect, thermionic emission, or indirectly through magnetohydrodynamics. 

Hu’s thermoelectric converter earned U.S. Patent 7,696,668 which is assigned to UT-Battelle, LLC (Oak Ridge, TN).  Prior art thermionic emission were exclusively solid-to-vacuum emissions, however, in the case of the Hu's converter thermionic emissions are solid-to-solid emissions. 

Advantages of Hu’s thermionic converter are numerous. Converters are environmentally friendly during both operation and disposal. Since the combustion products are generally carbon dioxide and water, the device can be used indoors. Such converters are more powerful, lighter and smaller than other existing technologies. Using nanoscale heterogeneous catalytic heating rather than conventional gas phase combustion and ignition removes the need for an ignition source or heating to initiate the reaction. Highly localized nano-/micro-scale heating zone and ultrahigh thermal gradients are generated, within a macroscale environment that is essentially cold.

 Converters are solid state structures with no moving parts, thus minimizing maintenance requirements. Since there are no moving parts, the device have no frictional losses when used for creating electrical power. Because heating zones are localized in very small volumes where the heat is needed, the efficiency of the device can be very high.

Power generators can be scaled from micropower supply to even industrial scale, as there is no theoretical upscale limit. Significantly, the light weight and small size of converters makes such converters well suited for use as a portable power source for cell phones, laptop computers and other portable electronic devices.

In 2005, Hu discovered a novel nanoscale energy conversion method. This world-reported method has led toward the development of a new class of ultra-high efficiency solid-state thermoelectric generators that can convert chemical energy in fuels directly to electric power at room-temperature avoiding conventional high-temperature fuel burning and external ignition.

Hu’s solid state thermoelectric converter includes a thermally insulating separator layer, and an electron emitter comprised of a metal nanocatalytic layer or dispersed metal nanocatalyst particles disposed on one side of the separator layer. A first electrically conductive lead is electrically coupled to the electron emitter. A collector layer is disposed on the other side, opposite the emitter, of the separator layer, wherein the thickness of the separator layer is less than 1 micron. A second conductive lead is electrically coupled to the collector layer.

Fuel in the form of hydrogen, alcohol or methanol and oxidant spontaneously react on the surface of the electron emitter generating heat sufficient to cause thermionic emission of electrons from the electron emitter, and solid-to-solid emitting of electrons through the separator layer to collector layer where the  emitter and collector layer are connected by a conductor. 

It is believed that the unexpectedly high electrical conductivity and low thermal conductivity provided by separators and converters can be explained as follows. Conventionally, materials are classified electrically into one of conductors, semiconductors and insulators/dielectrics, which are measured in macroscale, such as in bulk form. However, in the nanoscale such classifications are insufficient to define the electrical properties. It has been found that when the physical size of a given structure is on the same order of the mean-free-path of electrons in the material, the material behaves quite differently from that of its macroscale/bulk properties. The nature of the interface between materials can define the electrical properties. For example, a bulk dielectric can provide semiconducting or even electrically conducting properties at nanoscale.

Figure 7

Devices according to Hu’s invention have a significant advantage over other existing energy storage systems, such as batteries, and power generation systems, such as combustion engines and fuel cells. The relative location of different energy systems are identified in terms of specific power and a specific energy in two Regone Energy Plots in FIG. 7 and FIG. 8. These plots demonstrate converters according to the Hu's design are more powerful, while being smaller than other existing power systems.  (click images to enlarge) 

Figure 8

The metal nanocatalyst particles can have an average size of less than 200 nm and the metal can be selected from the group consisting of Au, Pt, Ru, Rh, Pd, Ag, Cu, Zn, Fe, Ni, and Mn, and alloys of these metals.  The collector layer can be a semiconductor, such as Si or Ge. The solid-to-solid electron emissions occur through a separator layer that is thinner than the mean free path of an electron and can have a thickness of 1 micron to 1 nanometer. The separator layer can be a nanostructured layer which can be a crystalline metal oxide. An alkaline earth oxide can be used as the separator layer with a Si collector layer which can have an alkaline earth silicide at the interface of the collector and separator layers.

Thermionic emission refers to the flow of electrons from a metal or metal oxide surface, caused by thermal vibrational energy overcoming the electrostatic forces holding electrons to atoms or molecules at the surface. This effect increases dramatically with increasing temperature (1000-3000 K), but is always present at temperatures above absolute zero. The science dealing with this phenomenon is thermionics. The charged particles are referred to as thermions.

Owen Willans Richardson worked with thermionic emission and received a Nobel prize in 1928 for his work on the thermionic phenomenon. Regarding Richardson's Law, in any metal, there are generally one or two electrons per atom that are free to move from atom to atom. This is sometimes referred to as a "sea of electrons". Their velocities follow a statistical distribution, rather than being uniform, and occasionally an electron will have enough velocity to exit the metal without being pulled back in.

The minimum amount of energy needed for an electron to leave the surface is referred to as the work function, and varies from metal to metal. A thin oxide coating is often applied to metal surfaces in vacuum tubes to provide a lower work function, as it is generally easier for electrons to leave the surface of the oxide

Hu is also a joint Research Assistant Professor at University of Tennessee, Knoxville and is holding an Honorary Professor position at the Institute of Physics of Chinese Academy of Sciences.