Progress Toward High Power Output in Thermionic Energy Converters

Enhanced Electron Emission Performance and Air-Surface Stability in ScO-Terminated Diamond for Thermionic Energy Converters

Diamond with negative electron affinity (NEA) and low work function surfaces are suggested as a suitable material for electron-generation applications in vacuum, in particular, as the emitter electrode in thermionic energy converters. Such NEA surfaces can be fabricated by evaporating and then annealing submonolayers of a suitable metal in vacuo onto bare or oxidized diamond. Among the metals studied, scandium termination of bare diamond (100) and (111) surfaces is recently reported to give the largest NEA values reported to date for a metal-diamond system, as well as being thermally stable to 900 °C. It is now shown that preoxidized (100) diamond functionalized with 0.25 monolayers of Sc also produces a large NEA value of -1.02 eV with low work functions (<3.63 eV). Moreover, this surface is thermally stable to 700 °C and can withstand exposure to air for extended periods. Here, the structural and electronic properties of this Sc─O-functionalized diamond surface are characterized in detail using a variety of surface-science techniques. The results suggest that this material may be the ideal candidate for the fabrication of commercial thermionic energy conversion devices, e.g., for solar-power generation, as well as for various other electronic devices that rely upon electron emission.

Solar thermionic energy converters with micro-gap spacers

We present a solar thermionic energy converter (STEC) incorporating interelectrode spacer structures for solar energy harvesting. We demonstrate the impact of spacer loss on the optimal performance and gap size of STECs with and without interelectrode spacers. Under a solar concentration factor of 100, we show that the maximum efficiency of STECs with a spacer (5.3%) is significantly lower than that of STECs without a spacer (8.7%) at different optimal gaps due to the interelectrode spacer that conducts heat parasitically. The optimal gap ranges from a few tenths of a micron to a few microns for STECs without spacers, while the optimal gap is around 8μm for STECs with spacers. Furthermore, the device operation with an internal support structure offers new, to the best of our knowledge, insights into the resulting behavior of different energy exchange mechanisms in STECs, which distinguishes them from devices without spacers.

Hybrid thermionic-photovoltaic converter with graphene-on-semiconductor heterojunction anode for efficient electricity generation

Thermionic energy converters are solid-state heat engines to produce electricity with significant potential, whereas the output voltage is constrained by the work function difference between cathode and anode. In this work, we originally apply a graphene-on-semiconductor heterojunction anode to a thermionic-photovoltaic (TIPV) converter to output additional voltage. Thermionic electrons are injected into the graphene layer and then recombined with photogenerated holes. Photogenerated electrons are extracted from the conduction band and reinjected into the cathode through an external load. A proof-of-concept demonstration of the TIPV converter is developed with barium surface-engineered cathode and anode. Open-circuit voltage is increased from ∼0.9 to ∼1.9 V by comparing with an identical configuration without graphene layer. The TIPV converter yields a power generation density of 2.7 kW/m² with an electronic efficiency of ∼27%. This work paves the way for the development of TIPV converter toward high power density.

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A novel thermionic energy converter is originally proposed and measured

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Open-circuit voltage is increased from 0.9 to 1.9 V by the photovoltaic effect

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A power density of 2.7 kW/m² with an electronic efficiency of ∼27% is measured

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Ultralow work function is obtained by forming barium surface dipoles

Concentrated thermionic solar cells using graphene as the collector: theoretical efficiency limit and design rules

We propose an updated design on concentrated thermionic emission solar cells, which demonstrates a high solar-to-electricity energy conversion efficiency larger than 10% under 600 suns, by harnessing the exceptional electrical, thermal, and radiative properties of the graphene as a collector electrode. By constructing an analytical model that explicitly takes into account the non-Richardson behavior of the thermionic emission current from graphene, space charge effect in vacuum gap, and the various irreversible energy losses within the subcomponents, we perform detailed characterizations on the conversion efficiency limit and parametric optimum design of the proposed system. Under 800 suns, a maximum efficiency of 12.8% has been revealed, where current density is 3.87 A cm^-2, output voltage is 1.76 V, emitter temperature is 1707 K, and collector temperature is 352 K. Moreover, we systematically compare the peak efficiencies of various configurations combining diamond or graphene, and show that utilizing diamond films as an emitter and graphene as a collector offers the highest conversion efficiency, thus revealing the important role of graphene in achieving high-performance thermionic emission solar cells. This work thus opens up new avenues to advance the efficiency limit of thermionic solar energy conversion and the development of next-generation novel-nanomaterial-based solar energy harvesting technology.

Three-dimensional Dirac material anode enables concentrated solar thermionic converters

Concentrated solar thermionic converters (CSTCs) are proposed by using three-dimensional (3D) Dirac material (DM) as the novel anode, significantly improving device performance. A theoretical model is developed to investigate the optimal performance of CSTCs. Under a solar concentration of 500, the CSTC reaches a maximum conversion efficiency of 11.8%. Furthermore, the optimal working condition and parametric selection criteria of the CSTC parameters under different solar concentrations are determined. Importantly, we demonstrate that 3D DM as an anode has more advantages over metal and graphene in CSTCs. This work thus offers a theoretical foundation for the exploration of solar thermionic application in using 3D DMs as novel electrodes.