Electron Harvesting

Sorry, there's no free lunch.

You're probably referring to a cold-cathode type of device.

It's the same kind of situation that you have in a solid-state diode, a P-N junction. An n-type semiconductor has a lot of loose electrons and a p-type semiconductor had a lot of vacancies or holes. When you join the two, electrons migrate across the junction from the n-type to the p-type, creating an electric field which opposes further migration.

You can take a diode and short the leads. No current flows unless you have a means of adding energy to the electrons such as light in a solar cell or a battery in the circuit.

It's the same situation with a cold-cathode device. You need to add energy for a current flow.

A different approach...

Thermoelectric generator pulls energy from room temperature heat....

..."Scientists in Japan have developed a new organic device that can harvest energy from heat. Unlike other thermoelectric generators, this one works at room temperature without a heat gradient.

Thermoelectric devices are designed to tap into a simple law of physics: heat energy moves from hotter regions to colder ones. In these devices, electrons move from the warmer surface to the cooler one, which produces an electric current. In theory, thermoelectric generators, materials and paints could produce electricity from small temperature differences in engines, power plants, even body heat.

Usually, the bigger the temperature gradient, the better the thermoelectric generator, but now scientists from Kyushu University in Japan have found a way to harness the relatively low energy available from room temperature, without a gradient at all.

Instead, the new device works on a principle called charge separation. Heat from the ambient air causes negative electrons and positive electron “holes” in the material to separate and move in different directions, generating a current.

The materials in question are organic compounds, which can easily transfer electrons between each other. Different types of these compounds are stacked in thin layers like stairs, and the heat gives the electrons or holes enough energy to jump up to the next “step.”

After much trial and error of different compound combinations, the team settled on a device with a 180-nanometer layer of copper phthhalocyanine, 320 nm of copper hexadecafluoro phthalocyanine, 20 nm of fullerene, and 20 nm of bathocuproine.

The end result boasted an open-circuit voltage of 384 millivolts, a short-circuit current density of 1.1 μA/cm², and a maximum output of 94 nW/cm². That’s a tiny amount of electricity, of course, but considering it’s coming from room temperature, it could make for simpler generators.

“We would like to continue working on this new device and see if we can optimize it further with different materials,” said Professor Chihaya Adachi, lead author of the study. “We can even likely achieve a higher current density if we increase the device’s area, which is unusual even for organic materials. It just goes to show you that organic materials hold amazing potential.”

The research was published in the journal Nature Communications.

https://newatlas.com/energy/thermoelectric-generator-room-temperature-heat/

"without a gradient at all" is a pretty strong statement.

Are we sure the second law of thermodynamics is violated, or is there the possibility of energy seeping in from somewhere?

This was probably taken out of context...here is the article in full...

https://www.nature.com/articles/s41467-024-52047-5

This guy will dip until the water's gone. The temperature of the water is less than the ambient temperature. The most energetic molecules leave the liquid and depart the vicinity, leaving the less energetic (cooler) molecules in the liquid. It works because it's not a closed system.

This is a problem that my employer faces every day. We make devices that have a hard vacuum inside & voltages anywhere from 200V-25kV at various parts. Any sharp point or edge causes emissions of electrons. We take great care to avoid any sharpness in manufacture but still have to age it away by running the device bodies at increased voltages to burn back the spikes.

Do you use a "getter" to absorb any remaining molecules,as in high voltage vacuum tubes?

A getter is a reactive material use to complete and maintain the vacuum.

We do use getters, either cartridge or wire type which are activated by applying voltage or the sheet type which are activated by temperature. They are used after the devices are sealed off to mop up any stray molecules in the vacuum envelope.

Thanks for this.

What is the material that has the spikes, that were formed during manufacture presumably ?

What is the container holding the vacuum? Is it glass?

Presumably by running the device at higher voltages the material is warmed sufficiently to smooth out the surfaces.

Clearly the electrons could be harnessed at an anode in close proximity to provide a large value electric field, and drawn off in an external circuit.

The question is e.g. if the anode potential were provided by a solar cell, suitably connected in its own circuit through a large value resistor, would that be sufficient to provide a much larger current in the device circuit, than that which would be obtained from the 25% efficient solar cell ?

The spikes are at a microscopic level, small areas of roughness in a metal surface or in a braze joint. The vacuum envelope is generally a brazed metal/ceramic body with an optic at either end. When run at voltages higher than will be seen in normal use, the body, in vacuum, will spark & burn back high spots.

https://scholar.lib.vt.edu/ejournals/JOTS/v35/v35n1/pdf/yildiz.pdf