NANOSTRUCTURED PHOTOCATALYTIC WATER SPLITTING SYSTEMS

This project will investigate fundamental electron and proton transfer processes that are key to the development of molecule-based systems for the photoelectrochemical production of fuels from water and carbon dioxide. The efficient conversion of sunlight to fuel is one of the most challenging problems in the field of chemistry, and is the motivation for much current research in artificial photosynthesis. This project builds on recent advances in solar water splitting systems that are based on dye-sensitized semiconductors. New architectures for dye-sensitized photoanodes and photocathodes that will improve their efficiency and stability, as well as membranes that will enable their use in solar fuel generators, will be developed. The principal goals of this project are to develop (1) a full kinetic model for light-driven water oxidation at dye-sensitized photoanodes, including nanowire and core-shell structures, (2) new architectures for hydrogen-evolving photocathodes based on acceptor quenching of dye excited states, and (3) a quantitative understanding of ion and molecule transport in bipolar membranes that can be used with photoanodes and photocathodes to make a complete system for solar fuel generation.

In the photoanode study, dynamic electrochemical methods, spectroelectrochemistry, flash photolysis, and time-resolved terahertz spectroscopy will be combined to elucidate the details of charge separation and recombination in nanowire and core-shell electrode architectures. New oligomeric photosensitizers and single-site water oxidation catalysts wiil be investigated for controlling the kinetics of the water oxidation reaction. 'Mummy' electrode architectures will be synthesized to improve the stability of photosensitizers and to enable operation of the photoanode at higher pH, where higher efficiency is anticipated.

The photocathode study will investigate photoinduced electron transfer in multilayer thin films in which the primary photoprocess is electron transfer from a dye molecule to an oxide semiconductor. This is an alternative architecture to well-studied electron donor quenching by p-type semiconductor oxides. The multilayer films will be grown on planar supports and on transparent conductor nanowire arrays by means of layer-by-layer polyelectrolyte adsorption techniques. This aspect of the project will seek to answer the question of whether such molecule-based assemblies on electrode surfaces can function as good diodes and efficient photodiodes. A combination of electrochemical and spectroscopic methods will be used to address this question through detailed kinetic characterization of the photocathodes.

This project will also investigate system-level issues connected with proton balance in solar fuel generation. In (photo)electrolysis cells that produce fuel and oxygen, protons are consumed at the cathode and are generated at the anode. Recent studies show that bipolar membranes enable the efficient electrolysis of water and CO2 without losses that arise from electrogenerated pH gradients. This project will investigate the catalysis of water autodissociation, ion transport, and crossover of electrolysis products in bipolar membrane cells, the synthesis of new bipolar membranes based on weak acid cation exchange layers, and the integration of membranes with photoelectrodes.