Interested in doing the project "Water to Fuel to Water"

[candy4me]

Hello,

As the title states, I am interested in doing the project, "Water to Fuel to Water: The Fuel Cycle of the Future". I have a few questions about this project.

Under "Make It Your Own" it suggests using catalysts from metals salts other than cobalt. Can you use titana for this experiement? I understand that titania is used in most photocatalytic water splitting devices, but it is not a metal salt.

Taking it a step further, one of the problems with titania is that it can only absorb UV light, not visible light. What if you used the idea behind multijunction solar cells, and added layers of catalysts with different band gaps? Could you test that in this experiment?

Also, can you connect this photoelectrochemical water splitting device to a fuel cell?

Thanks in advance for all your help.

[kgudger]

Hello and welcome to the forums:

I've tried investigating "titana", and can only find references to it as a moth ??? Did you possibly mean TiO2 (Titanium di-Oxide?) Here's what Wikipedia says about it:

A sample semiconductor with the proper band structure is TiO2. However, due to the relatively positive conduction band of TiO2, there is little driving force for H2 production, so TiO2 is typically used with a co-catalyst such as Pt to increase the rate of H2 production. It is routine to add co-catalysts to spur H2 evolution in most photocatalysts due to the conduction band placement. Most semiconductors with suitable band structures to split water absorb mostly UV light; in order to absorb visible light, it is necessary to narrow the band gap.

My understanding is that in this experiment, the energy to split the water is coming from the batteries, not from light. The Cobalt catalyst speeds up this type of reaction. In the TiO2 case, light energy is used (ultraviolet light). It seems to me that these 2 catalysts work in different ways on different reactions? In other words, I'm not sure whether TiO2 would work in this case, but you could test it out.

With regards to using the ideas behind multi-junction solar cells, I think what's happening there is that you have junctions that respond to various wavelengths of light to give you more efficiency (each junction has a low efficiency, but the sum is higher.) It sounds like TiO2 needs its band gap moved down into the visible spectrum, which is a different problem. What we're hoping to find is a catalyst that works in the visible light spectrum.

Let us know how your experimentation works out!
Best, Keith

[candy4me]

Yes, I did mean TiO2. Titania is another word for it but I guess not as common. Sorry for the confusion.

With regards to using the ideas behind multi-junction solar cells, I think what's happening there is that you have junctions that respond to various wavelengths of light to give you more efficiency (each junction has a low efficiency, but the sum is higher.) It sounds like TiO2 needs its band gap moved down into the visible spectrum, which is a different problem. What we're hoping to find is a catalyst that works in the visible light spectrum.

I might be thinking of this in the wrong context, but with solar cells, if you lower the band gap to increase the light absorption, then the voltage will drop, and I was thinking a similar effect might happen with water splitting. Otherwise, couldn't we just find a photocatalyst with a lower band gap than TiO2, and then its efficiency would be improved?

[kgudger]

Hello:

Yes, I think if there was a catalyst with a band gap associated with visible light, then photocatalytic water splitting should work at visible light frequencies. I haven't seen where anyone has found this type of catalyst yet (at a reasonable cost.)

This Science Buddies experiment, however, uses electrolysis for splitting water, so I wouldn't expect the same catalysts to work for both types of water splitting (necessarily).

Best, Keith

[candy4me]

So if the catalyst worked for visible light, it wouldn't work for UV light?

Under the "Make It Your Own" tab it suggests changing the experiment to get energy from sunlight instead of batteries, so that's why I thought it might work. I can try it and see what happens, or perhaps find a different project that is more similar to what I had in mind. The reason I was asking is because the kit is expensive so I didn't want to purchase it if the experiment wouldn't work!

[candy4me]

I guess what I'm trying to ask is, is there a Shockley-Queisser theoretical efficiency limit for photocatalysts?

[kgudger]

Hello:

I'm not an expert on this, and it's been a while since I took classes about semiconductors, but here's what I've found out.

First, it would appear that TiO2 has a bandgap of 1.23V, which means that light with wavelengths shorter than UV would not work with this catalyst.

Second, it looks like the 3 components that control the amount of electrical energy that is extracted per photon are the same for a solar cell and a Photocatalytic reaction, so yes, I would think there is a Shockley-Queisser theoretical efficiency limit.

Best, Keith