[blackbird123]

Hi everyone, I am trying to do a science fair project figuring out the best way to split water using energy from the sun. The two ways that I have found to be the most reasonable are photocatalytic water splitting and photobiological water splitting. I was wondering which of these methods are the most efficient and if anyone could help me build this system. Also is there anything that I can do research on that would increase the efficiency? Any comments or ideas would be appreciated!

Thanks in advance!

[Arkcon]

The most efficient way is to use solar energy that has been stored and concentrated millennia ago in fossil fuels to generate power to split water.  Hey, you're the one who wrote:

Any comments or ideas would be appreciated!

Here's an interesting way to work it -- compare the efficiency of photo-catalytic and biological water splitting with fossil fuel derived water splitting.  And see where those procedures stand compared to each other.

[Enthalpy]

The "photo" processes aren't necessarily the best ones. Often, they need a minimum photon energy, which makes them inefficient.

If you run a thermal engine from sun energy and electrolyze water, this will give you a reference, hard to exceed.

Other good processes are known; they use several hot chemistry steps, like
- react water with a metal
- decompose the metal oxide at heat
- use several metals in between
(sorry I've forgotten the details)

You might also first define what "best way" means. Most hydrogen per joule of light? Per hectare*year? Per dollar? Biological processes waste light and ground area but are cheap if land is.

[blackbird123]

I really want to try and make a system that could continuously produce hydrogen to power anything such as a small engine. Sunlight was the best thing that I thought would be a continuous power source to power the production of hydrogen. I want to try and create the most efficient way of producing hydrogen, meaning highest Liters per minute using the least amount of energy.

[Arkcon]

Sunlight was the best thing that I thought would be a continuous power source to power the production of hydrogen.

Really?  Its 4am here, and I'm typing away at my computer, and at this latitude, we do not yet have sunlight.  If I depended on the sun, I would not have power, right now.  Nighttime, cloud cover, surface area required are all things you will have to consider in your report.  I can burn fossil fuels at any time of day, y'know.

[Enthalpy]

From a limited collecting area, forget all biological processes.

Is hydrogen a definitive choice? You have to store energy to run the engine under clouds, and while a hydrogen bottle is possible, batteries are easier. In addition, feeding an engine with hydrogen is not trivial and the technology isn't available from the stores. Fuels cells would be more or less available.

For a science fair, you probably want to assemble existing parts, of which I can only imagine solar cells and electrolysis. Many other processes, often more efficient, have been investigated - things like photodissociation in several steps - but for a fair, would you invest months and years to reproduce a setup with uncertain results?

If using solar cells, the conversion to and from hydrogen is a serious roundabout.

[PhotoElectroMaterials]

if you are looking for the simplest way to demonstrate solar powered water splitting, using commercial multijunction silicon cells is the cheapest option.  they are available for less than ten dollars.  you can choose desired output voltage and area.  3V and atleast 10cm square will be enough if you use direct sunlight.  you can show visible hydrogen and oxygen evoltion for demonstration.