Enhanced thermionic energy conversion with a graphene anode
- Thermionic energy converters (TECs) are a direct heat-to-electricity conversion technology with the potential to outperform the current state-of-the-art solid-state energy conversion technologies in a wide range of working temperatures. TECs could not only be applied to large-scale power plants in a tandem cycle, but could also be deployed in distributed power generation systems at small scales. In a TEC, electrons evaporate from a hot electrode (the cathode) into a vacuum gap and are collected by a cooler electrode (the anode) to generate electric energy. However, the published TECs suffer from low output voltage as well as low output current because the work function of the anode is too high and the space charge barrier in the inter-electrode is too strong, which ultimately lead to low conversion efficiency. In this thesis, I will first present a phenomenological model to calculate a material's work function. This model shows that graphene has great potential to yield an ultra-low work function. I will then experimentally demonstrate such a graphene anode in an ultra-high vacuum system. The graphene is grown on a piece of copper foil by chemical vapor deposition. It is then transferred onto a 20 nm thick HfO2 layer prepared by atomic layer deposition on silicon. After in-situ deposition of a monolayer of Cs/O, the work function of the graphene anode is dramatically reduced from 4.62 eV to 1.25 eV. Furthermore, I will show that electrostatic gating graphene through the HfO¬2 dielectric layer can further reduce the work function of graphene. Due to the absence of surface Fermi level pinning and low surface state density according to graphene's linear dispersion relation, the Fermi level of graphene can by effectively raised through a DC bias with negligible energy consumption. I will show that combining Cs/O surface coating and electrostatic gating would lead to a world-record low work function for graphene of 1.0 eV. Finally, I will demonstrate a TEC prototype based on this graphene anode, together with a commercial dispenser cathode. At the cathode working temperature of 1000 ˚C, the work functions of both of the electrodes are reduced by Ba. In addition, this TEC prototype deploys a nano-manipulator system that can reduce the inter-electrode gap to 17 μm, leading to a much reduced space charge barrier. By addressing the two challenges simultaneously, a low work function anode and minimizing the space charge barrier, the overall conversion efficiency of this TEC prototype is improved by a factor of over 50, with an estimated maximum electronic conversion efficiency of 9.8 %, which is the highest reported by far at this relatively low working temperature.
|Type of resource
|electronic; electronic resource; remote
|1 online resource.
|Stanford University, Department of Physics.
|Howe, Roger Thomas
|Howe, Roger Thomas
|Statement of responsibility
|Submitted to the Department of Physics.
|Thesis (Ph.D.)--Stanford University, 2016.
- © 2016 by Hongyuan Yuan
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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