Progress and Problems in Visible Light Water Splitting

Future solar energy conversion systems (solar cells and solar water splitting catalysts) must be both efficient and inexpensive in order to be cost-competitive with fossil fuels.  The challenges of high efficiency and device stability at low cost present some interesting technical problems.  Inexpensive polycrystalline semiconductor devices and photocatalysts are generally inefficient because of losses due to charge carrier recombination. In principle, higher efficiency is achievable by exploiting the high quantum yield of charge separation at the dye-oxide semiconductor interface, or using semiconductor p-n junctions.   At the system level, one can couple these photochemical charge separation “devices” to catalysts for water splitting or, in principle, CO₂ reduction.  There are some important system-level issues in both cases.  These include the need for membranes can conduct protons at the pH where the electrocatalysts operate and where the other components of the system are stable, as well as the problem of product crossover and catalytic fuel oxidation.

This talk will describe strategies for addressing these problems.  Nanocrystals and nanocrystal assemblies offer new ways to control the flow of light and the transport of electrons in photocatalysts and photoelectrochemical cells. By coupling molecular photosensitizers to nanoparticulate oxygen evolution catalysts, it is now possible to make dye-sensitized solar cells that split water with visible light, albeit with low efficiency.  Photoelectrochemical cells based on “bed of nails” arrays of semiconductor wires allow one to separately control the length scales of light absorption and photochemical charge separation, and in principle to make multi-junction micro-cells for visible light water photolysis.