Photocatalytic water splitting uses solar energy to simultaneously produce hydrogen and oxygen. Organic polymers are increasingly being explored as photocatalysts for this process as they are cheap and Earth-abundant. Indeed, photocatalysts based on linear conjugated polymers can produce hydrogen from water in the presence of a sacrificial hole scavenger. But overall water splitting without sacrificial scavengers also requires a photocatalyst for water oxidation. Now, writing in Angewandte Chemie International Edition, Reiner Sebastian Sprick et al. show that a linear conjugated polymer loaded with a cobalt co-catalyst can produce oxygen from water using sunlight.
To achieve high energy-conversion efficiencies, photocatalysts for water splitting must operate effectively under sunlight, a significant proportion of which is in the visible region. Traditionally, photocatalysts explored for water splitting have been based on inorganic semiconductors, but tuning the properties of these materials is challenging. “Possible advantages of organic polymer photocatalysts include the tunability of their light-absorption properties and their solution processability, which could make it possible to produce large-area devices,” explains Andrew Cooper, one of the principal investigators.
The team first synthesized ten linear conjugated polymers theoretically predicted to have the thermodynamic driving force for water oxidation, before loading each polymer with a cobalt-based co-catalyst. Under broadband irradiation and in the presence of AgNO3 as an electron scavenger, eight of the cobalt-loaded polymers show some photoactivity, with oxygen evolution rates spanning 0.2–16.6 μmol h–1. “The key is to use the right co-catalyst, without which, these materials cannot do overall water splitting,” says Cooper. “Cobalt works well, though it should be possible to discover better alternatives.”
The best-performing material — the homopolymer poly(dibenzo[b,d]thipophene sulfone) (P10) — has the deepest ionization potential and thus the greatest driving force for water oxidation. However, this alone does not explain the disparity in the photoactivities. The team gained further insight into the relative performance of the polymers by comparing their aqueous dispersibilities using light obscuration measurements. Of the various polymers, the suspension of P10 is the most opaque and has the lowest transmission, indicating that it forms the most stable dispersions in water. This is important because the oxygen production reaction is heterogeneous and occurs at the catalyst–water interface.
Under visible-light irradiation, the oxygen evolution rate of P10 decreases to 5.2 μmol h–1. Although this value is much lower than benchmarks for inorganic photocatalysts, it is considerably higher than that of another type of organic photocatalyst, a cobalt-loaded triazine-based framework, as measured under the same conditions.
“advantages of organic polymer photocatalysts include the tunability of their light-absorption properties and their solution processability”
The demonstration of water oxidation with an organic polymer is a vital step towards the development of an all-organic system — comprising an organic water oxidation catalyst and an organic proton reduction catalyst — for overall water splitting. Challenges include evaluating the long-term stability of organic polymer photocatalysts and increasing the performance to reach the levels of inorganic materials, but Cooper and colleagues are optimistic: “The bar is very high, but we plan to use autonomous robots to locate what might be complex, multicomponent all-organic or organic–inorganic hybrid systems.”
Sprick, R. S. et al. Water oxidation with cobalt-loaded linear conjugated polymer photocatalysts. Angew. Chem. Int. Ed. https://doi.org/10.1002/anie.202008000 (2020)