Nature looks at what it takes to make a 100 percent efficient catalyst for getting clean fuel from water:
The largest potential source of renewable energy is the Sun: about 0.02% of the solar energy absorbed by Earth’s surface annually would be enough to cover current global energy consumption. Many approaches for converting solar energy into the chemical energy stored in hydrogen are therefore being investigated, using ‘water-splitting’ reactions in which water is broken down into hydrogen and oxygen.
A key measure of the effectiveness of a photocatalyst is the fraction of absorbed photons that it can use to produce hydrogen, a quantity called the internal quantum efficiency (IQE). A perfect photocatalyst that converts all of the absorbed photons to hydrogen would have an IQE of 1 (or 100%). However, IQE cannot be determined from experiments.
A related quantity that can be experimentally determined for a reaction is the external quantum efficiency (EQE): the fraction of photons illuminating the reaction vessel that the photocatalyst can use to produce hydrogen. This value is always lower than the IQE, because an unknown portion of the illuminating photons will not be absorbed by the photocatalyst, but will instead be lost to other processes, such as scattering. If similar photocatalyst-particle suspensions are investigated using the same experimental set-up, ensuring that the same fraction of light is absorbed, then EQE can be used as an indirect measure of IQE. But EQEs determined using different set-ups cannot be used as a way of comparing IQEs of photocatalytic systems, because the relationship between EQE and IQE is different for each set-up — therefore making it difficult for different research groups to compare results.
Takata et al. focus on strontium titanate (SrTiO3) — one of the first materials found to split water photocatalytically, as reported in 1977. Strontium titanate produces electron–hole pairs by absorbing ultraviolet light. Because the Sun’s intensity is highest in the visible-light range, it is unlikely that UV-driven catalysts will enable sustainable hydrogen production on a large scale. However, strontium titanate is an excellent model system for studying the influence of photocatalyst parameters on quantum efficiency (both EQE and IQE), because the mechanisms that cause efficiency losses in this material are well understood, and strategies for mitigating the losses have been proposed.
This combination of complex mitigation strategies proved highly successful: the authors reported EQEs of up to 96% when their photocatalysts were irradiated with light in the wavelength range of 350–360 nanometres. This is excellent news, because it means they have designed an almost perfect photocatalyst — the IQE must be between 96% and 100%.
This is a spectacular result for several reasons, even though strontium titanate is ‘just’ a model system for visible-light photocatalysts. First, it demonstrates that experiments can be designed in which EQEs come close to IQEs within an acceptable error margin of less than 4%. Improved experimental set-ups in which measured EQEs are very near to IQEs should facilitate the comparison of photocatalysts and therefore accelerate progress in this field.
Second, it proves that the combination of design strategies used by the authors can indeed eliminate efficiency losses associated with recombination.