Modeling Materials and Processes in Dye-Sensitized Photolectrochemical Cells for Visible Light Water Oxidation

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Oral, Mariachiara Pastore, presentation 075
Publication date: 5th February 2015

One of the greatest scientific and technological challenges facing humanity is to capture and convert solar energy into electricity¹ or to store it in chemical fuels, producing hydrogen (or other reduced fuels) and oxygen from water.² Solar hydrogen generation from water is a very attractive field of research, allowing the production of inexhaustible renewable fuel without emission of pollutants and greenhouse gases. A possible approach to visible-light water splitting, relies on the use of dye-sensitized semiconductor nanoparticles, linked to catalysts for oxygen evolution (Figure 1a).³ In such a dye-sensitized photoelectrochemical cell (DSPEC) oxygen is produced at the dye-sensitized semiconductor photo-anode while hydrogen is produced by a catalyst at the cathode, where photogenerated electrons are collected.The first DSPEC³ was assembled employing a bifunctional heteroleptic Ru(II) dye, showing phosphonate groups for TiO₂ anchoring, and a malonate group to bind to hydrated iridium oxide (IrO₂•nH₂O) nanoparticles (Figure 1a). Under light irradiation, this device produced both oxygen and hydrogen, but poor internal quantum yield and coulombic efficiency (about 0.9 % and 20 % respectively) were reported.³ The low device performances were mainly attributed to a slow hole transfer from the oxidized dye to the catalyst, which favoured a high back recombination from TiO₂ to the oxidized dye; a fast dye’s excited state quenching by IrO₂•nH₂O has also been envisioned⁴ as an additional deactivation channel.  With the aim of enhancing the rate of the forward electron/hole transfer processes, and effectively suppress the undesired parasitic recombination reactions, here we present a fully first principles modeling of this DSPEC photoanode (Figure 1b). We investigate all the relevant interfacial electron/hole transfer, determining the interplay of structural and electronic factors affecting the DSPEC efficiency. Based on the obtained information, we design a novel class of Ru(II) sensitizers rooted onto sound and documented synthetic chemical strategies, and we computationally investigate and discuss their performance in a DSPEC architecture against the parent  Ru(II)  dye employed the reported DSPEC.
Figure 1. a) Scheme of a prototypical DSPEC, along with main energy levels and relevant electron transfer processes; b) Optimized molecular structure of the Ru(II) dye tethered across the TiO2 and IrO2 systems.
1. O'Regan, B.; Grätzel, M., A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737-740. 2. (a) Young, K. J.; Martini, L. A.; Milot, R. L.; Snoeberger Iii, R. C.; Batista, V. S.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W., Light-driven water oxidation for solar fuels. Coord. Chem. Rev 2012, 256, 2503-2520; (b) Hamann, T. W., Water splitting: An adaptive junction. Nat Mater 2014, 13, 3-4. 3. Youngblood, W. J.; Lee, S.-H. A.; Kobayashi, Y.; Hernandez-Pagan, E. A.; Hoertz, P. G.; Moore, T. A.; Moore, A. L.; Gust, D.; Mallouk, T. E., Photoassisted Overall Water Splitting in a Visible Light-Absorbing Dye-Sensitized Photoelectrochemical Cell. J. Am. Chem Soc. 2009, 131, 926-927. 4. Hoertz, P. G.; Kim, Y.-I.; Youngblood, W. J.; Mallouk, T. E., Bidentate Dicarboxylate Capping Groups and Photosensitizers Control the Size of IrO2 Nanoparticle Catalysts for Water Oxidation. J. Phys Chem. B 2007, 111, 6845-6856.

© FUNDACIO DE LA COMUNITAT VALENCIANA SCITO