Catalyst splits water with sunlight: efficient, durable, and cheap

Some insight into the physics of this all:

In order to split water, you want to add enough energy to at least cover the formation enthalpy of the reagents, which is 1.23V. This means that, without any other losses, an incoming photon needs to be at least 1.23V. This is the absolute minimum to make this reaction work, but in order to make it energy-neutral (i.e. the reaction products emerge at the same temperature as the reagent - water) this needs to be 1.48V. This is equivalent to a photon of 837nm (deep red, almost IR). This is great, because the majority of the Sun's energy is above this energy - about 55%. image for clarification

However, the energy of higher-energy photons (bright red, green, blue, UV) doesn't get converted into 'more' reactions: the surplus energy of each photon just gets converted to heat. One photon = one redox reaction, that's it. This means that a big portion of the energy in the visible light goes lost; if you do the math you can only use about 45% of the energy above 837nm. So far, we can use about 25% of the sun's energy for useful water splitting.

Then, we get to quantum efficiency (and this is the reason why this article is interesting). Quantum efficiency tells you which proportion of photons actually contribute to the reaction. A QE of 100% means all incoming photons will produce an electron that helps the reaction along. A QE of 200% means every photon, on average, creates TWO electrons. In typical water splitting setups, the best QE that can be attained is about 1%. This article outlines a new organic catalyst that attains a QE up to 16% (green/blue) down to about 5% (orange-red). They claim a total energy conversion efficiency (solar->hydrogen+oxygen) of about 2%. That is good news!

Well... it's still horrible on the whole. As I said before, the highest physically possible efficiency is about 25%, give or take, with 100% QE. Using photovoltaic modules and modern platinum-group catalyst electrolyzers, the best efficiency we get is about 60% for the electrolyzer and 20% for the PV panels, i.e. 10% total efficiency from solar to H2+O2.

The trick here is that theoretically, a cheap organic direct solar catalyst can just be 'dumped' into a big greenhouse with a hydrogen accumulator. You'd need a lot of surface area (at least 5x as many as PV panels), but the installation would be so much cheaper that it's more cost effective in the end.

As for whether this is truly a breakthrough: no. Absolutely not. It is one of many incremental steps towards trying to make photocatalytic water conversion a viable technology for producing hydrogen. As it is now, this relatively new type of research has been going for a decade or so and produced this as their best result, which is still not even near good enough for large-scale commercial deployment (aside from the fact that it's a pure lab result). It will take a long time to even be competitive with (PV-driven) electrolyzers, which by themselves are still about half an order of magnitude more expensive than gas reforming plants. Long way to go still.

/r/science Thread Link - sciencemag.org