Porous polymers have joined the ranks of light-activated catalysts that split water into hydrogen, a carbon-free alternative to fossil fuels. Their properties are easily tuned — a big plus for the development of practically useful catalysts.

The Sun could be harnessed as an unlimited source of energy by exploiting another naturally abundant resource: water. The light-induced splitting of water into oxygen and hydrogen generates storable chemical fuels that have no carbon footprint, potentially solving the world's ever-increasing energy demands. The seemingly simple task of absorbing sunlight to split water requires a semiconductor catalyst, and inorganic catalysts are leading the field. Writing in the Journal of the American Chemical Society, Sprick et al.1 demonstrate that organic photocatalysts (light-activated catalysts) may become just as useful as their inorganic counterparts, and offer intriguing opportunities for future research because the catalysts' physical responses to light can be tailored.

Designing photocatalytic materials for water splitting is far from easy: not only should they absorb light efficiently to form photoexcited states, but also the excitations should be long-lived and effectively lead to separation of charges at the catalyst's surface, where the redox reactions needed for water splitting occur. Particulate photocatalysts often require additional 'sacrificial' agents with a larger thermodynamic driving force than water to accept a light-generated charge carrier. This can dramatically facilitate charge separation, the largest bottleneck in the photocatalytic process. Moreover, the vast majority of photocatalysts are unlikely to work efficiently alone — a co-catalyst such as platinum or another noble metal is needed to lower the energy losses associated with hydrogen or oxygen evolution.

Substantial research efforts are therefore focused on finding materials that offer better light harvesting, charge transport and conversion of water to hydrogen and oxygen. Most of these materials are inorganic semiconductors2, which are highly robust, but whose properties are often barely tunable. Although metal-free photocatalysts such as carbon nitrides have been reported3, soft organic polymers have not yet found their place in the race. Sprick and colleagues now highlight the potential of porous organic polymers for producing hydrogen. These soft materials offer ample opportunities for systematic engineering of their bandgap, a property that governs what part of the solar spectrum is absorbed, and that can be manipulated to improve the effectiveness of photocatalysis.

Organic materials formed from layers of two-dimensional atomic networks, which include crystalline covalent organic frameworks4 and amorphous conjugated microporous polymers (CMPs)5, are chemically inert, thermally stable and have potentially useful optoelectronic properties combined with high surface areas. They have therefore been widely used for gas adsorption, chemical sensing and catalysis6. These materials can be made from a broad range of organic building blocks and bond-forming reactions, thus providing an extensive toolbox for the systematic fine-tuning of their structural and physical properties.

Sprick et al. prepared a series of 15 different CMPs from phenylene and pyrene building blocks, using palladium-catalysed reactions (Fig. 1). The polymers exhibit a continuous variation in optical properties that depends on the ratio of phenylene to pyrene units: increasing the pyrene content causes a gradual decrease in the optical bandgap from 2.95 electronvolts to 1.94 eV, an effect that allows the polymers to absorb increased amounts of the solar spectrum. The researchers rationalize this trend by proposing that low-energy optical excitations in the polymers become dominated by contributions from pyrene's molecular orbitals as the pyrene content rises. They also suggest that structural effects — such as the formation of cyclic substructures (rings) and the strain within them — become more dominant as the pyrene component increases. Figure 1: Optical and photocatalytic properties of a series of porous polymers. a, Sprick et al.1 have prepared 15 conjugated microporous polymers (CMP1 to CMP15) from a mixture of phenylene-containing (red) and pyrene-containing (blue) building blocks, increasing the ratio of pyrene to phenylene across the series. b, The optical bandgap of the polymers decreases as the pyrene content increases, implying that the polymers' ability to catalyse hydrogen production from water when irradiated with visible light should increase across the series. In fact, the rate of hydrogen evolution peaked for CMP10, possibly because of mechanisms that reduce the availability of electrons to take part in the hydrogen-producing reaction in CMP11 to CMP15. Full size image

The authors tested the porous polymers for their ability to catalyse hydrogen evolution from water in visible light, using the organic compound diethylamine as a sacrificial electron donor. Remarkably, all of the polymers promoted stable hydrogen evolution for at least 6 hours, and the best polymer was shown to work for more than 100 hours without much decline in activity. This behaviour was predicted by the researchers' theoretical calculations, which showed that all the CMPs have a strong thermodynamic driving force to promote hydrogen evolution. Sprick et al. observed no signs of light-induced degradation of the CMPs in their experiments; such stability is a key prerequisite for any catalyst if it is to be more than just a laboratory curiosity.

On the basis of the CMPs' optical properties, the rate of hydrogen evolution would be expected to increase across the series of polymers (that is, as the bandgap decreases). But the authors observed that hydrogen evolution reaches a peak for the CMP that has a bandgap of 2.33 eV (Fig. 1); it then declines for the remaining polymers that have smaller bandgaps. The authors suggest that recombination of separated charge carriers (which prevents electrons from being transferred from the polymer) becomes dominant in CMPs that have smaller bandgaps, or that the kinetic barrier to electron transfer increases, both of which would reduce hydrogen evolution.

Remarkably, the polymers are active in the absence of any deliberately added noble metals, providing a possible solution to the long-standing and much-researched question of how to minimize the amount of these expensive co-catalysts that is required. But as the authors point out, traces of palladium trapped in the CMPs during their synthesis might act as masked co-catalysts. To rule out the possibility that residual noble metals appreciably affect the rate of hydrogen evolution, Sprick and colleagues devised a palladium-free synthesis for their photocatalysts, deliberately added platinum to some of their experiments, and performed other tests in which carbon monoxide was added to 'poison' any traces of palladium. All of these studies suggested that the rate of hydrogen evolution correlates more strongly with the optical bandgap than with noble-metal content.

Another notable feature of the CMP photocatalysts is their selective visible-light activity, with almost no activity observed in ultraviolet light. This unusual bias renders them 'true' visible-light photocatalysts, boding well for the future design of photocatalysts that absorb a large fraction of the visible-light spectrum with maximum light-harvesting efficiency.

Against the background of inorganic semiconductor photocatalysis, Sprick and co-workers' findings highlight the power of organic photocatalysis, with its armoury of molecular-engineering protocols suitable for producing complex catalysts that have improved bandgaps. But as with all breakthroughs, there is more to be done: greater insight into charge-carrier dynamics and the nature of the catalytically active sites will be required to further increase the efficiency of polymer photocatalysts and make them practically viable.

Tuning the catalytic activity will necessitate subtle control of the polymers' composition, and especially of their crystallinity and microstructure. This poses grand synthetic challenges, and will probably require the development of alternative cheap, reversible polymerization protocols. Finally, a single porous polymer that catalyses 'complete' water splitting — which does not require sacrificial electron donors — has yet to be realized. The race is on.Footnote 1

Notes

References 1 Sprick, R. S. et al. J. Am. Chem. Soc. 137, 3265–3270 (2015). 2 Simon, T. et al. Nature Mater. 13, 1013–1018 (2014). 3 Wang, X. et al. Nature Mater. 8, 76–80 (2009). 4 Côté, A. P. et al. Science 310, 1166–1170 (2005). 5 Xu, Y., Jin, S., Xu, H., Nagai, A. & Jiang, D. Chem. Soc. Rev. 42, 8012–8031 (2013). 6 Ding, S.-Y. & Wang, W. Chem. Soc. Rev. 42, 548–568 (2013). Download references

Author information Affiliations and the Chemistry Department, Vijay S. Vyas and Bettina V. Lotsch are at the Max Planck Institute for Solid State Research, Stuttgart 70569, Germany, University of Munich (LMU), Germany. Vijay S. Vyas & Bettina V. Lotsch Authors Vijay S. Vyas View author publications You can also search for this author in PubMed Google Scholar Bettina V. Lotsch View author publications You can also search for this author in PubMed Google Scholar Corresponding author Correspondence to Bettina V. Lotsch.

Rights and permissions Reprints and Permissions