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Question: What might serve as an initial starting photocatalyst for this large water-splitting solar simulator? Surely there must have been some planned experiments!

The Gizmodo article Insane Light System Blasts the Energy of 10,000 Suns and Phys.org's Let there be light: German scientists test 'artificial sun' both describe the large, powerful concentrated light source called Synlight which has been built by the German Aerospace Center (DLR).

This large (~350 kW!) solar simulator - built from 149 high intensity xenon arc lamps, all collimated/focused to a single 20x20cm area - will be used to test and develop materials and catalysts that will allow photochemical splitting of water to yield hydrogen gas, to be scaled up later and used with a solar concentrator instead of electrically powered lamps. Hydrogen could be used as fuel directly, or to synthesize hydrocarbon fuels when combined with carbon dioxide or monoxide.

Water is transparent over most of the sun's useful spectrum, and this is for the obvious reason that the wavelengths reaching the ground are those not already absorbed in the atmosphere. Thus a photocatalyst will be necessary for a high yield of hydrogen gas from concentrated solar light.

From the Gizmodo article:

When the array is focused onto a single spot, it heats metal to 1,475 degrees Fahrenheit (800 degrees Celsius), which is then sprayed with water vapor. The metal reacts with the oxygen in water, and hydrogen remains. With further heating, the oxygen is once again separated from the metal.

There are certainly easier ways to heat a metal plate than building a giant solar simulator, so I assume the point is to provide an intense source of photons with a somewhat similar spectrum to sunlight as a test platform for various catalysts. But the quote suggests the surface will react with the oxygen released, and need to then be cycled to higher temperature to release it. This does not sound like a very practical photocatalytic process.

So far I have not found anything clearer about what kinds of catalysts are likely to be investigated, but there must be some potential candidates. Based on known photocatalytic water splitting mechanisms, or if someone can find a discussion by DLR, what would be a starting point, or proof-of-concept photocatalyst that might work here?

below: Images from here, photo credit: DLR (top), DPA (bottom). The shape of each reflector is likely to be close to an ellipsoid with the xenon arc at one focus and the sample near the other. This way a ~100 cm diameter reflector can illuminate a much smaller 20x20cm sample.

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    $\begingroup$ One possible candidate: sciencedirect.com/science/article/pii/S1364032114009265 $\endgroup$
    – Tyberius ♦
    Commented Mar 24, 2017 at 16:34
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    $\begingroup$ @Tyberius Thanks! That looks like a very helpful review of this subject, exactly what I need to read. $\endgroup$
    – uhoh
    Commented Mar 24, 2017 at 23:46
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    $\begingroup$ According to wiki, the cerium (IV) oxide - cerium (III) oxide cycle is "a two step thermochemical water splitting process based on cerium (IV) oxide and cerium (III) oxide for hydrogen production." Maybe this helps. $\endgroup$
    – Ed V
    Commented Jul 13, 2019 at 22:59
  • $\begingroup$ possibly helpful; Phys.org's Water-splitting module a source of perpetual energy $\endgroup$
    – uhoh
    Commented May 4, 2020 at 22:46

2 Answers 2

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Efficient light-harvesting relies on two primary factors: an efficient absorption of solar radiation, and an effective means for charge separation. The absorption of solar radiation by a photovoltaic material induces the creation of an exciton, a quasiparticle comprised of an excited electron that is Coulombically attracted to the positive electron-hole formed in its excitation. The formation of an exciton is essentially the same as simply exciting an electron, but when this motif can move through a bulk material it is often useful to frame it this way. The higher efficiency of a photovoltaic, the more excitons that are generated. The better the charge separation, the longer the excitons last and thus the more likely that their potential energy can be converted into work.

So long as the photocatalyst can 1) absorb light in the solar spectrum 2) stabilize excitons long enough for them to reach water molecules and 3) incubate excitons with energies high enough to activate water-splitting, then it would be a suitable candidate for testing with the Synlight.

The characteristics of a good photocatalyst for water-splitting would optimize the above traits (maximize efficiency of light absorption to maximize exciton formation, maximize charge separation as to increase exciton lifespan, and maximize excitons of sufficient energy to ignite the water-splitting process). There are many many candidates, but some promising candidates in the literature in re water-splitting are cerium dioxide heterostructures (CeO$_2$/___).

Cerium dioxide has promising properties for photovoltaics: cerium dioxide has good charge mobility and oxygen-species mobility due to the high reversibility of the cerium ion between its Ce$^{3+}$ and Ce$^{4+}$ oxidation states; the material is generally non-toxic (it is used in photocatalysis currently for cleaning of wastewater at wastewater treatment plants); and it is generally resistant to corrosion. It has the issue, however, that it is only efficient at absorbing in the ultraviolet range of the solar spectrum; this is due to the material's large band gap, the energy separation between ground-state electrons and excited-state electrons.

In order to ameliorate this, most research on CeO$_2$ photovoltaics focuses on perturbing the crystal structure, surface shape/topology, and purity of the material to lower the band gap and increase efficiency across more of the spectrum. Doping the material with other semiconductors or charge-transferring materials such as cadmium sulfides (CdS), zinc oxides (ZnO), and silver phosphates (Ag$_3$PO$_4$) has led to many shifts in efficiency across the spectrum. Changing the morphology of the structure—pure or doped—has profound changes as well, such as the creation of CeO$_2$/CdS nanoparticles for photovoltaics. Here is a link to a 2021 review article related to this subject by Mekonnen, An Overview on the Photocatalytic Degradation of Organic Pollutants in the Presence of Cerium Oxide (CeO2) Based Nanoparticles: A Review if you would like to learn more.

A more complex photocatalyst that could take two excitons of lower energy and combine them into an exciton of higher energy would also be ideal, as it would be able to access parts of the solar spectrum below the threshold of water-splitting activation—this combination process is called up-conversion, and it is highly technical.

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Vitamin B-12 is a photocatalyst, see, for example, https://pubs.rsc.org/en/content/articlelanding/2018/ra/c7ra13037f#!divAbstract and also https://www.ncbi.nlm.nih.gov/pubmed/29271445 , but it has not been applied to water splitting!

What has been used is TiO2, see this source to split H2O into H2 and O2.

Mechanics: The action of light on TiO2 creates electrons (e-) and electron holes (h+). Some reactions:

$\ce{H+ + e- -> .H}$

$\ce{.H + .H -> H2}$

$\ce{OH- + h+ -> .HO}$

$\ce{.HO -> H+ + .O-}$

$\ce{.O- + h+ -> .O}$

$\ce{.O + .O -> O2}$

One of the less desirable reaction is:

$\ce{.H + .HO -> H2O}$

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