Chlorophyll-based Bio-Photovoltaic Cells

Robert Murray-Smith. 2014. Making a Solar Cell from a Leaf. Video. Retrieved from youtube.com/watch?v=w0i_Gf55uiE.

• 0:32–1:46: On its use of Cu and Al strips and an electrolyte-impregnated paper towel, and how it's not a battery despite this arrangement; to demonstrate this, a version with significantly larger electrodes was constructed that sandwiched the electrolyte (in a way that left no part of it exposed to light), and which showed 0uA
• 1:50–: In contrast, the 'solar cell' version / geometry (which had an active area of, perhaps, 20cm²) produces 40–50uA under (ambient) fluorescent lighting, even after three days
• 2:44–3:32: On the lack of corrosion of the electrodes, and the use of disparate metals to provide some 'current direction'. “But we're getting constant supplies of small amounts [of current] from that structure, where we've got leaf extract between two different metals. I used these two different metals to do a bit of current direction. If you have two same metals then you still get an output from it, but the output is much, much lower because, I think, the electrons don't know where to go. And in this setup, they know where to go, so you get a much better capture of it.”
• 3:46–4:33: A rather unconvincing demonstration of its increased output under direct exposure to a halogen light. (Spoilers: it didn't seem to make a difference, at least within the few seconds that the exposures were changed.)

Robert Murray-Smith. 2014. Making a Solar Cell from a Leaf: Extracting Chlorophyll into a Deep Eutectic Solvent. Video. Retrieved from youtube.com/watch?v=rhUnFYNbC_k.

In this video, approx. 10g of birch leaves are placed into 250mL of deionised water and blended for 20 mins; the resulting liquid is double-vacuum filtered. The resulting filtrate will ultimately be added to a deep eutectic solvent made from a 2:1 (molar ratio) mixture of Mono-ethylene(?) glycol and (?) chloride. (The chemical names were not clear from the audio, but the molecular masses were given as 62 and 139g/mol, respectively, suggesting they are, in fact, Ethylene glycol (62.07g/mol and Choline chloride (139.6g/mol).)

To prepare the solvent, the mixture needs to be heated to approx. 80°C (whereupon its consistency will change from that of a more sugary molasses to a liquid). This is then mixed, 1:1 with the chlorophyll extract, and kept at approx. 95°C until its volume has reduced back down to a single unit of volume, by which time virtually all of the water will have evaporated (given the much higher boiling points of the other solvents).

Robert Murray-Smith. 2014. Solar Cell from a Leaf: Update. Video. Retrieved from youtube.com/watch?v=fir5uZpJU74.

In this video, an electrolyte made with twice the starting leaf material is presented, and shown to produce ~75uA (compared to ~40uA from an equally-sized 20cm² cell); its response upon exposure to a halogen light is also noticeably quicker.

“What that's suggesting to me, and should be suggesting to you, I guess, is that it's the chlorophyll that's actually responsible for the effect that we're seeing.”

Robert Murray-Smith. 2014. Working on the Leaf Based Solar Cell: A Lab Log. Video. Retrieved from youtube.com/watch?v=0E3R1edpQ6U.

The design, as shown at the end of the video, was reported to produce approx. 0.5W/sqm. While still a lot less than the approx. 30 to 50W/sqm that is typically produced by commercial silicon-based PV cells under real-world situations, this is also much better than the 1.2mW/sqm generated by Copper(I) oxide-based cells. And, when the environmental and financial costs of production (and the feasibility of refurbishment) are taken into account, this type of cell becomes even more interesting.

TBC

Robert Murray-Smith. 2014. Solar Cell from a Leaf Update: In The Sun. Video. Retrieved from youtube.com/watch?v=fsEabyztqhY.

Robert Murray-Smith. 2014. Solar Cell from a Leaf: Review and Direction. Video. Retrieved from youtube.com/watch?v=fFRf_p7M1_g.

1. Exactly why does this work the way it does? (Note to self: look into the liberation / use of electrons in plant-based light-dependent reactions; e.g., see this)
2. Could algae be used instead (as they're almost entirely chlorophyll, at least when compared to leaves)?
3. How quickly does chlorophyll degrade / decompose? Given that it's not actually alive (rather an organic compound produced by living organisms), can it be sterilised (in a way that doesn't break it down) to extend its useful lifespan?