Organic Semiconducting Polymer Photocatalysis for Water and CO2 Reduction

Green hydrogen, produced from water using renewable energy, is expected to become a prominent renewable fuel of the future, providing clean, carbon neutral energy for a wide range of industrial applications. It can also provide complementary energy storage in combination with intermittent solar energy. However, competitive economic solar generated hydrogen production on a large scale remains challenging. One promising approach is photochemical water splitting, using lght absorbing nanoparticle semiconductors that can drive redox reactions on their surface. The more light the photocatalytic nanoparticle absorbs, the more efficiently it can split water into hydrogen and oxygen. Traditionally, wide bandgap inorganic semiconductors have been used for photocatalytic applications. However, these materials almost exclusively absorb UV light which only carries a small fraction (<5%) of solar energy, limiting their efficiency. In this presentation, the development of photocatalysts fabricated from organic semiconductors, chemically tuned to absorb strongly throughout the UV-visible spectrum will be discussed. We demonstrate a larger solar to hydrogen efficiency than traditional inorganic photocatalysts with the potential to achieve solar hydrogen production at a lower levelized cost. We have developed organic semiconductor nanoparticles that contain an internal donor/acceptor heterojunction between two organic semiconductors with a type II energy level offset. The donor/acceptor heterojunction greatly improves charge generation within the nanoparticles, which in turn greatly improves their hydrogen production efficiency. We demonstrate a substantial increase in the H2 production efficiency by tuning the nanoparticle composition. We also observe that the high efficiency of these nanoparticles originates from their ability to generate exceptionally long-lived reactive charges upon illumination, increasing their likelihood to participate in a photocatalytic reaction. In addition, we will discuss solution-processable, linear conjugated polymers of intrinsic porosity for gas phase carbon dioxide photoreduction. The polymers’ photoreduction efficiency is investigated as a function of their porosity, optical properties, energy levels and photoluminescence. All polymers successfully form carbon monoxide as the main product, without the addition of metal co-catalysts. The best performing single component polymer yields a rate of 66 μmol h−1m−2, which we attribute to the polymer exhibiting macroporosity and the longest exciton lifetimes. The addition of copper iodide, as a source of a copper co-catalyst in the polymers shows an increase in rate, with pTA-Ph (the most active) achieving a rate of 175 μmol h−1m−2. The polymers are active for over 100 hours under operating conditions. This work shows the potential of processable polymers of intrinsic porosity for use in the gas phase photoreduction of carbon dioxide towards solar fuels.