2
\$\begingroup\$

I am running an experiment using graphene at the moment to create energy from different chemical solutions. A link to what I am building on is below:

https://phys.org/news/2014-04-team-electricity-saltwater-graphene.html

Amazingly, this does actually work; it is possible to get intermittent pulses of voltage anywhere from 40mV to about 100mV with a period of a few tens of ms when a drop of solution is dropped onto a small sheet of graphene. The reason I say intermittent is that I am dropping the drops of solution intermittently onto the graphene itself, which would be analogous to a real world situation.

However, I am struggling to harness this voltage/energy.

I have tried to put different capacitors in series with the graphene sheet, such that the voltage pulses could be stored in this. However, I think what happens in this case is that as soon as a voltage pulse (presumably) goes into the capacitor, it is just leaked away completely back into the graphene sheet, which is effectively just a resistor when not generating small pulses.

The fact that the pulses are so small means that I can't use a diode. And I don't want to use a transistor switch if I can get away with it, because having to apply an external voltage to this circuit defeats the point of what I am trying to do.

Any comments much appreciated!

Thanks, Jon

\$\endgroup\$
  • \$\begingroup\$ 100mV is very small, i will look at this tonight. \$\endgroup\$ – Sclrx Aug 17 '17 at 9:10
  • \$\begingroup\$ hm, I think you're not getting around a diode (or equivalent transistor circuit) completely, but I do think "defeats the purpose" is not necessarily true – might be possible to bootstrap this with very little secondary energy storage. \$\endgroup\$ – Marcus Müller Aug 17 '17 at 10:35
  • \$\begingroup\$ What do you have access to? university lab or small hobbyist shack? \$\endgroup\$ – Sclrx Aug 17 '17 at 11:20
  • 1
    \$\begingroup\$ What energy can be liberated from a single pulse - to answer this you need to understand what the voltage will drop to under load conditions. Load conditions are the conditions needed to extract energy so please be precise about this. If you don't know, find out. If the energy per drop is X then this can liberate a power of X multiplied by the number of pulses per second. \$\endgroup\$ – Andy aka Aug 17 '17 at 11:45
  • 2
    \$\begingroup\$ Check to see if anyone has cited the original paper(s) or if the authors have published other material on the subject. If neither of these then they too may not only have struggled but given up. Such an amazing idea would either be developed if it worked or abandoned with sorrow if it proved impractical. Sorrow is the norm, alas. Original Chinese reference here \$\endgroup\$ – Russell McMahon Aug 17 '17 at 13:24
1
\$\begingroup\$

Disclaimer : i am no physicist, but here is my best guess :

Build a battery

I think that harnessing 40mV would not be very efficient, you should try generating more than that before.

From the paper you linked, i understand that a drop moving on graphene behaves like a current source in parallel with a resistor :

image taken from http://www.nature.com/nnano/journal/v9/n5/full/nnano.2014.56.html?foxtrotcallback=true

For clarity, I will from now on call the resistance in parallel with the drop the intra-cell resistance.

To allow/improve the energy harnessing, you have two angles of attack : You can either increase the intra-cell resistance, generating a higher voltage (Ohm's law) and wasting less energy, and add more sources in serie, effectively increasing the voltage, making it easier to harvest.

Increase the resistance

From Wikipedia, we know that for a given material, resistance is inversely proportional to the cross-section, which means that if you use a thin strip of graphene instead of a sheet, you should obtain better results (higher voltage and longer pulses) around the drop, but worse results between drops, which brings us to my second point :

Connect your "cells" is series

From the same paper (and intuitively), we learn that drops following each other behave like such source-resistor pairs in serie (adding the resistance of the graphene between the drops, which we will call inter-cell resistance).

Of course, if we want to improve our battery's performance, we have to minimize the inter-cell resistance, which is directly proportional to the distance between drops and inversely proportional to the graphene strip's width.

There is some experimentation/optimization work here, to find the best combination between graphene strip width, strip inclination (and thus drop speed), drop frequency, etc.

Using medical devices, you can control your drop frequency, and conduct experiments to find your optimum.

Additional hypothesis

This is a wild guess, but what if instead of a graphene sheet, you used horizontal strip? or even dots? this way, the intra-cell resistance would be much higher.

\$\endgroup\$
  • \$\begingroup\$ Thanks Sclrx, this sounds like a very sensible idea. I will give it a go! Your help is very much appreciated here, thanks for the excellent answer. \$\endgroup\$ – Jon Magdelof Aug 21 '17 at 14:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.