Within the first year of the discovery of thermoionic energy convertors (TEC) in 1957, the application of this method in space programs was flourishing, however its restricted ability to be applied within the industrial setting limited its practicality. Of the challenges researchers faced in their commercialization of TECs included its high work function of conventional anode materials, which can directly reduce the output power (Pout) supply of the convertor, as well as the space charge barrier within the device that can similarly reduce Pout1.
TECs function by directing a heat-to-electricity conversion through the absorption of a fraction of electrons that are capable of overcoming the work of the cathode1. As electrons are emission as a result of this overcome work, the inter-electrode gap is traversed and the emitted electrons are collected at a lower-functioning anode. While most thermoelectric devices require the presence of a solid material to exist between the anode and the cathode, TECs instead supply a vacuum gap between the two anodes1. This vacuum gap shows a radical improvement in the conversion efficiency and scalability, as it is capable of reducing parasitic heat conduction in conjunction with enabling ballistic electron transport.
While the use of this technology was abandoned decades ago, a team of researchers led by Dr. Hongyuan Yuan of Stanford University revisited this technology in order to explore its potential application in conjunction with a graphene anode. The prototype used in this experiment was designed by combining a black-gated graphene anode, a dispenser cathode, lt a controllable inter-electrode gap whose diameter measured at 17μm.
Graphene’s Fermi level at this back-gate voltage is increased following capacitive charge accumulation of the TEC, leading to a reduced work function of the anode. As most of the function of the TEC device is dependent upon the work function of the anode, the ability to reduce the anode’s work load greatly improves the efficiency of the device by an additional 67%.
The team also found that a reduction in the size of the inter-electrode gap will exhibit a dramatic increase in Pout as a result of the mitigation of the space charge barrier, making this technology of particular interest for potential industrial purposes. By these two advances alone, the new TEC prototype attacks the two previously existing challenges of reduced voltage output voltage and reduced output current simultaneously.
The findings of this study proved that an electronic efficiency in the energy conversion of the revolutionary TEC measured at 9.8 %, which is the highest efficiency that has ever been found at 1000 °C. As a direct result of the addition of a barium (Ba) coating to the graphene surface, the workload of graphene is reduced, which further enhances the efficiency of the TEC device. This sub-monolayer of Ba deposited onto the graphene surface forms strong dipoles onto the surface of the dispenser cathode, leading to a great reduction in the work function of the TEC.
While the experiment performed in this study was conducted in a controlled laboratory setting, in which a vacuum chamber tested the TEC device, this group of researchers is now looking towards developing a vacuum packaged TEC to further test the reliability and effiency of the device for future real-life applications. Researchers are not only hopeful that this new technology increase the efficiency in converting heat to electricity, thereby reducing the need for burning fossil fuels to produce energy. The proposed and tested porototype represents the first step in revamping the field of thermionic energy conversion that could be potentially used in power stations or to provide energy for people’s homes.
- Hongyuan Yuan, Daniel C. Riley, Zhi-XunShen, Piero A. Pianetta, Nicholas A. Melosh, Roger T. Howe. Back-gated graphene anode for more efficient thermionic energy converters. Nano Energy, 2017; 32: 67
- "Space Energy Technology Restored to Make Power Stations More Efficient." ScienceDaily. ScienceDaily, 6 Mar. 2017. Web. https://www.sciencedaily.com/releases/2017/03/170306110349.htm.