Finding the Way to Solar Fuels

October 23, 2015

Professor Thomas Meyer

The key elements in a successful solar fuel device are understood: light absorption, excited state electron transfer, vectorial electron/energy/proton transfer driven by free energy gradients, PCET activation of catalysis, rapid rates for key solar fuel half reactions, device design and scale-up. Dye Sensitized Photoelectrosynthesis Cells (DSPEC) offer a structural paradigm for the integration of these functions. As in Dye Sensitized Solar Cells (DSSC), DSPECs are driven by interfacial light absorption and excited state electron or hole injection at the surfaces of oxide semiconductors. However, DSPECs aim 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 DSPECs are “keep it simple” and “let the molecules do the work”. Molecules, clusters, and molecular assemblies are used to absorb light, carry out catalytic half reactions, etc. based on a “modular” approach. In this approach the separate components are evaluated separately and integrated into device configurations. Significant progress has been made in water oxidation and CO2 reduction catalysis and in integrating them in surface stabilized, interfacial structures. Application of transient absorption, photocurrent, and device measurements on the resulting assemblies on TiO2 and other high band gap semiconductors is revealing the underlying factors that control device efficiency and stability.

Success in visible water splitting has been achieved in DSPECs with a major advance recently by using atomic layer deposition (ALD) to prepare core-shell structures with nanoparticle, mesoporous films of the transparent conducting oxide, tin-doped indium oxide, coated by thin 2-4 nm films of TiO2 and ALD stabilization of surface binding. When photolyzed with visible light, core-shell structures derivatized by a chromophore-catalyst assembly have resulted in water splitting with per photon absorbed efficiencies of 4-5% and solar efficiencies for H2 production of ~1%.