Ewan McQueen

A common strategy to improve the efficiency of organic photocatalysts for hydrogen production from aqueous mixtures is to create bulk heterojunction nanoparticles comprised of intermixed donor and acceptor phases that allow for efficient charge separation after photoexcitation. However, many of these systems possess poor stability due to aggregation of these nanoparticles under operating conditions. Moreover, the use of surfactants, that inhibit aggregation and promote donor–acceptor phase intermixing, can form an insulating barrier that reduces the photocatalytic efficiency of these nanoparticles. Here, these issues are bypassed by preparing a single‐component organic heterojunction‐type polymer, P40 , in which a molecular donor, pyrene, is tethered to poly(fluorene‐ co ‐dibenzo[ b , d ]thiophene sulfone), a conjugated polymer acceptor. By tethering the donor and acceptor together, phase intermixing is guaranteed without the need for costly post‐synthesis processing or insulating surfactants. Moreover, the influence of pyrene in P40 is determined to be multifaceted, as it influences the dynamics of the excited state, the aggregate microstructure, and the local solvent environment. P40 is found to have an exceptional external quantum efficiency of 38% at 420 nm in the presence of triethylamine as a hole scavenger, the highest value reported for any linear conjugated polymer to date for sacrificial hydrogen production.

Organic polymer photocatalysts have gained much interest in recent years, largely because of their photocatalytic activity toward sacrificial hydrogen production from water. Time-resolved electronic absorption spectroscopy is commonly employed to understand the photophysical processes occurring following photon absorption, which in turn is used to rationalize photocatalytic activities. The homopolymer of dibenzo[b,d]thiophene sulfone (P10) is a well-studied and high performing photocatalyst for sacrificial hydrogen evolution from water. While sacrificial reagents are well documented as a prerequisite for this reaction, their roles in the picosecond–nanosecond photodynamics have yet to be determined using transient electronic signatures. By employing lifetime density analysis of time-resolved electronic absorption spectra of P10 in a variety of solvent mixtures, we show that the electron polaron (the required charge for hydrogen evolution) is produced on the 0.5–100 and 50–800 ps timescales via excitonic quenching by triethylamine and methanol, respectively, two common sacrificial electron donors. We conclude that there is significant pre-association of triethylamine with the P10 polymer, resulting in efficient excitonic quenching. This mechanism competes effectively with radiative excitonic relaxation, which occurs on similar timescales, reducing exciton losses and improving polaron yields.

Visible-light-driven conversion of CO2 to useful products is a promising process for a more sustainable energy system in a circular economy. Whilst highly efficient, durable, and selective systems have been reported in organic media, the photocatalytic process performing efficiently in aqueous media is highly desirable for the purpose of its practical application. However, efficiency and selectivity for CO2 reduction versus proton reduction in aqueous solution have proven challenging to date. Herein, this study demonstrates the enablement of highly efficient and durable visible-light-driven photocatalytic CO2 reduction to formate in aqueous media by a hybrid photocatalyst consisting of a silver-loaded conjugated polymer and a binuclear Ru(II) complex. The hybrid photocatalyst exhibited high activity and durability for formate production, with an apparent quantum yield of 4.2% at 460 nm and a turnover number of 38,000 (based on the amount of binuclear complexes adsorbed), both of which are the highest values reported amongst hybrid photocatalysts in aqueous media. Even though the conjugated polymer retains residual amounts of palladium from synthesis (which is an active site for H2 production in aqueous media), the loading of Ag nanoparticles onto the conjugated polymer enhanced the activity and selectivity for photocatalytic CO2 reduction by suppressing H2 production.

This highly efficient hybrid photocatalytic system can quantitatively convert CO₂ to formate and produce 0.4 M of formate solution without additional concentration enrichment.

Photocatalyst systems combining donor polymers with acceptor molecules have shown the highest evolution rates for sacrificial hydrogen production from water for organic systems to date. Here, new donor molecules have been designed and synthesised taking inspiration from the structure–performance relationships which have been established in the development of non-fullerene acceptors. While a conventional bulk heterojunction (BHJ) pairing consists of a donor polymer and acceptor small molecule, here we have successfully reversed this approach by using new p-type small molecules in combination with a n-type conjugated polymer to produce non-conventional BHJ (ncBHJ) nanoparticles. We have applied these ncBHJs as photocatalysts in the sacrificial hydrogen evolution from water, and the best performing heterojunction displayed high activity for sacrificial hydrogen production from water with a hydrogen evolution rate of 22 321 μmol h⁻¹ g⁻¹ which compares well with the state-of-the-art for conventional BHJ photocatalyst systems.

Photocatalyst systems combining donor polymers with acceptor molecules have shown the highest evolution rates for sacrificial hydrogen production from water for organic systems to date. Here, new donor molecules have been designed and synthesised taking inspiration from the structure-performance relationships which have been established in the development of non-fullerene acceptors. While a conventional bulk heterojunction (BHJ) pairing consists of a donor polymer and acceptor small molecules, here we have successfully reversed this approach by using our new molecules in combination with a n-type conjugated polymer to produce non-conventional BHJ nanoparticles and applied these blends to the sacrificial hydrogen evolution from water. The best performing heterojunction displayed high activity for sacrificial hydrogen production from water with a hydrogen evolution rate of 22,321 µmol h−1 g−1 which compares well with the state-of-the-art for conventional BHJ photocatalyst systems.

The direct conversion of sunlight into hydrogen through water splitting, and by converting carbon dioxide into useful chemical building blocks and fuels, has been an active area of research since early reports in the 1970s. Most of the semiconductors that drive these photocatalytic processes have been inorganic semiconductors, but since the first report of carbon nitride organic semiconductors have also been considered. Conjugated materials have been relatively extensively studied as photocatalysts for solar fuels generation over the last 5 years due to the synthetic control over composition and properties. The understanding of materials’ properties, its impact on performance and underlying factors is still in its infancy. Here, we focus on the impact of interfaces, and nanostructure on fundamental processes which significantly contribute to performance in these organic photocatalysts. In particular, we focus on presenting explicit examples in understanding the interface of polymer photocatalysts with water and how it affects performance. Wetting has been shown to be a clear factor and we present strategies for increased wettability in conjugated polymer photocatalysts through modifications of the material. Furthermore, the limited exciton diffusion length in organic polymers has also been identified to affect the performance of these materials. Addressing this, we also discuss how increased internal and external surface areas increase the activity of organic polymer photocatalysts for hydrogen production from water.