Actually that comment was justified, because the article says that while the quantum efficiency is around 96%, the water splitting worked with ultraviolet light having wavelengths between 350 nm and 360 nm.
Only a very small fraction of the solar light is ultraviolet light, so the usable efficiency of such a device is quite low.
While this research result is very interesting as a demonstration of what can be done, it is very unlikely that this is a path that can lead to the best way of capturing solar energy.
Either multijunction photovoltaic cells or thermal devices can capture around half of the total solar energy.
If splitting water is desired, that problem can be solved separately, using electrical or thermal energy, and it should be able to reach similarly high efficiencies as with these photocatalysts.
The combined efficiency of the 2 processes should be still far greater than the efficiency of direct water splitting, which is constrained by the lack of enough ultraviolet light in the solar spectrum.
Is the wavelength restriction a blocker though? There's already use of quantum dot film to shift light wavelengths into spectrums more suitable for growing plants[1]. Theoretically shouldn't this be able to be used to shift a larger percentage of the light spectrum into the 350-360nm range, resulting in a far higher actual yield?
Your example converts high-energy light (ultraviolet or blue) to low energy light (red).
This process is easy and it can be done with quantum dots, as you say.
The reverse process of converting red light to ultraviolet light, as it would be needed for this water splitting, is far more difficult and it has much lower efficiencies, especially when the input light has a low intensity. (The common green-light pointers convert infrared to green, but that works because the input is a laser beam with high intensity, so that optical media can behave non-linearly, and it still has a low efficiency).
So no, wavelength conversion would not work.
The only thing that would work would be to replace their semiconductor crystal with one having a much lower bandgap, like the silicon, gallium arsenide or cadmium telluride that are used in photovoltaic devices.
However any such non-oxide semiconductors would be chemically unstable in contact with the nascent oxygen, so that would still not work.
Actually that comment [focusing on efficiency, asserting lack of environmental conditions, and saying it would be useless under hyperbolic conditions] was justified...While this research result is very interesting as a demonstration of what can be done, it is very unlikely that this is a path that can lead to the best way of capturing solar energy.
I think it wasn't justified, because of its 3 complaints, 2 were incorrect (the paper did include reasonable environmental conditions, and the process is useful), and the third omitted that the key to the paper was not its solar to hydrogen efficiency but a novel process that could show how to increase STH in more viable catalysts.
So in the contrary i think my description of the comment as lazy and skeptical was both justified and totally correct.
Only a very small fraction of the solar light is ultraviolet light, so the usable efficiency of such a device is quite low.
While this research result is very interesting as a demonstration of what can be done, it is very unlikely that this is a path that can lead to the best way of capturing solar energy.
Either multijunction photovoltaic cells or thermal devices can capture around half of the total solar energy.
If splitting water is desired, that problem can be solved separately, using electrical or thermal energy, and it should be able to reach similarly high efficiencies as with these photocatalysts.
The combined efficiency of the 2 processes should be still far greater than the efficiency of direct water splitting, which is constrained by the lack of enough ultraviolet light in the solar spectrum.