Roger T. Howe, Department of Electrical Engineering; Jens K. Nørskov, Departments of Chemical Engineering and of Photon Science (SLAC); Piero A. Pianetta, Departments of Electrical Engineering and of Photon Science (SLAC), Stanford University

Objective

The objective of this research program is to discover new nanostructured materials with ultra-low work functions for achieving high-efficiency thermionic energy conversion. This research could help identify a range of new, greener options for energy conversion, from solar-thermal electricity to low-noise, low-maintenance residential combined heat and power systems (micro-CHP).

Background

The work function is the interfacial parameter of a solid surface that determines how easily electrons can escape into a vacuum or gas environment, with lower work functions facilitating electron emission. Subnanometer coatings have long been known to modify the work function. For example, coatings of cesium and oxygen are routinely used to make the low-work function photocathodes used in photomultipliers and night vision devices. However, the theory of work function reductions in such multilayer coatings is still poorly understood. Known coatings were discovered largely by trial and error.

Figure 1: Left: Schematic of a traditional thermionic energy converter. The incoming heat makes the emitter (cathode) hot enough to evaporate electrons off its surface (thermionic effect). The electrons then traverse the vacuum gap, are absorbed by the collector (anode) and drive the electric current through the external load. Right: Calculated efficiency limit for a thermionic energy converter as a function of the cathode temperature for three values of the collector (anode) work function: 1.5, 1.0 and 0.5 eV (based on [Hatsopoulos1973]). For comparison, the dashed curves show the Carnot efficiency limit and the efficiency limits for a thermoelectric converter with a figure of merit of ZT=2, which roughly corresponds to the best existing thermoelectric materials, and ZT = 10, which is much better than the current state of the art. The heat sink is assumed to be at room temperature (300 K) in all cases.

While nanostructured surfaces with ultra-low work functions could broadly impact technologies based on electron emission, this project is focused on high-temperature energy converters. Thermionic energy converters (TECs), which are seeing renewed interest [Moyzhes 2005, Lee 2009], and photon enhanced thermionic emission (PETE) converters, recently developed at Stanford [Schwede 2010], require elements with low work functions (see Figure 1). TEC and PETE converters transform heat or solar energy directly to electricity by utilizing thermal evaporation of electrons from solid surfaces at high temperatures. Discovery of thermally stable materials with work functions of less than 1.0 eV implies that the direct thermionic conversion of high-temperature (> 500° C) heat or solar radiation to electricity can have efficiencies exceeding 50%. If stable materials with work functions of about 0.5 eV can be found, thermionic energy conversion can achieve efficiencies of around 50% of the Carnot limit, even at moderate temperatures (<500° C) - far exceeding the efficiencies of current thermoelectric converters and comparable to the efficiencies of the best mechanical heat engines.

The interdisciplinary research team will: (1) apply density functional theory (DFT) - a leading method of first-principles calculations in solid-state physics - to predict the work functions of materials with multilayer atomic coatings; (2) nanofabricate the most promising coatings and characterize their performance, in order to validate the DFT models and motivate new strategies for achieving lower work functions; and (3) characterize electron emission, thermal stability and other surface properties of the best candidates to assess their applicability in thermionic energy converters.

DFT has been remarkably successful when modeling various properties of solid-state systems [Marzari 2006, Hafner 2006]. The research will harness the predictive power of DFT to guide the discovery of new surfaces with the lowest possible work functions.  The best candidate material combinations from the DFT calculations will be tested experimentally by measuring work functions, using both thermionic emission and photoemission.  In the initial phase, DFT will be used and compared to well-known systems, such as cesium-coated tungsten surfaces. By constructing a variety of periodic supercells to represent various coverage levels and relaxing their atomic structures, the research team will generate an ensemble of simple crystalline surfaces that will provide a realistic description of the actual physical polycrystalline surface. Predictions obtained from the DFT simulations will be compared to the known experimental data for cesium coatings on tungsten [Langmuir 1933, Hatsopoulos 1973]. 

The next phase will focus on systems that have not been studied in detail, in particular, several low-index metal surfaces (tungsten, molybdenum and titanium) with 1-3 layers of coating. Alkali metals, layered oxides and various alkali-dimers will be considered as coatings. Non-metallic surfaces, such as un-doped and doped grapheme, will also be studied (see Figure 2). The calculations will provide quantitative information about the electronic structure, stability, charge transfer, dipole effects, work functions and emission barriers for the different systems. Those with sufficiently low barriers and work function will be fabricated and tested.