Hydrogen is of major interest as a carbon-neutral solar fuel that offers key opportunities to confront issues such as climate change and declining fossil fuel reserves. Integration of all the necessary molecular components into an artificial photocatalytic system for solar hydrogen production represents a promising strategy to address this challenge.

In nature, self-assembly plays a critical role in photosynthesis where complex organelles accurately control the size, spatial location, and proportion of relevant motifs. Natural macromolecules such as proteins and also lipids act as natural scaffolds in photosynthetic organelles which elegantly integrate light harvesting, exciton/charge transport, and catalysis in a single location. Using synthetic polymers to cost-effectively mimic the natural behavior of light harvesting complexes that precisely manipulate the photocatalytic process is a major challenge. We have developed a polymer-based photocatalytic nanofiber system for visible-light driven hydrogen production from water which offers a conceptually new approach to solar fuel production.

The tailored photocatalytic nanofibers were formed from the solution self-assembly of a blend of block copolymers bearing either a pendent photosensitizer (Ps) or cobalt catalyst (Cat) (see for Ps and Cat structures see figure, left). The composition and dimensions, and thereby the catalytic activity of the nanofibers, were controlled via a seeded growth strategy termed “crystallization-driven self-assembly” (for nanofiber structure with seed in the center see figure, left). The tailored blend nanofibers with the photosensitizer and catalyst moieties held in close proximity within a random structure (see super-resolution fluorescence microscopy image, figure, center) function as an integrated, highly-active precious metal-free photocatalytic system.

The resulting artificial assemblies powered by visible-light were found to be durable and remarkably efficient over 300 h (see figure, right), and recyclable (over 30 cycles) at ambient temperature and pressure. The photocatalytic nanofibers stabilize a long life-time Co(I) catalytic reaction intermediate and, under optimized conditions, exhibit a very high turnover number ( >7000 over 5 h) and frequency (>1400 h^-1) and exhibit an exceptional H₂ production rate of 244,314 μmol.h^-1 per gram of catalytic polymer for hydrogen evolution reactions. The overall quantum yield of solar energy to hydrogen gas was 4.0%.

The polymer-based photocatalytic nanofibers exhibit dramatically higher catalytic activities than homogeneous solutions of the photosensitizer (PS) and catalyst (Cat) due to the close proximity of the latter within the corona/shell. They represent a promising new platform for artificial photocatalysis with the potential to be generalized to numerous related photocatalytic components.

To read more about our work at Nature Chemistry: https://doi.org/10.1038/s41557-020-00580-3