Finding the Way to Solar Fuels with Dye Sensitized Photoelectrosynthesis Cells

T.J. Meyer
University of North Carolina at Chapel Hill, US

Keywords: solar fuels

Summary:

The sun is an unlimited energy source but of low intensity requiring vast collection areas. It is also intermittent with a requirement for massive energy storage at night. The only practical solution to the energy storage problem at the required scale is “Artificial Photosynthesis” with “solar fuels” as the product. Solar fuels are high energy molecules with energy stored in the chemical bonds of hydrogen from water splitting or from reduction of CO2 to CO, other oxygenates, or hydrocarbons. Key elements in artificial photosynthesis are understood: light absorption, excited state electron transfer, vectorial electron/energy/proton transfer driven by free energy gradients, activation of catalysis, rapid rates for key solar fuel half reactions and integration in device architectures. Dye Sensitized Photoelectrosynthesis Cells (DSPEC) offer a general approach. As for Dye Sensitized Solar Cells (DSSC), they are driven by interfacial light absorption followed by excited state electron or hole injection at the surfaces of nano-structured oxide semiconductors. However, the aim of DSPECs is to produce oxygen and a fuel in the separate cell compartments of a photoelectrochemical cell rather than a photopotential and photocurrent. Key guiding principles to this approach are “keep it simple” and “let the molecules do the work”. Molecules, clusters, and molecular assemblies are used to absorb light and carry out the catalytic half reactions, etc based on a “modular” approach. In this approach the various DSPEC components are evaluated separately, and integrated into device configurations. Significant progress has been made in this area in water oxidation and CO2 reduction catalysis and in integrating them in surface-stabilized chromophore-catalyst assemblies. Application of transient absorption, photocurrent, and device measurements on TiO2 and other high band gap, nanostructured semiconductors is revealing the underlying factors that control device efficiency and stability.