Water Splitting for Energy Storage
(Simone Piccinin, Changru Ma and Stefano Fabris)

The simplest conceivable mechanism for storing solar energy via artificial photosynthesis is based on splitting water molecules and rearranging their bonds into molecular oxygen and hydrogen. The photocatalytic reaction can be divided in two fundamental steps: the light adsorption by a light-harvesting device, that generates electron-hole pairs, and the catalytic action of specific sites where molecular oxygen (half cell oxidation) and hydrogen (half-cell reduction) are formed starting from water. The key bottleneck for the technological exploitation of this process is the oxidation of water [1,2]. Describing the reaction mechanism and the thermodynamics of water oxidation is extremely challenging, since to create an oxygen molecule from two water molecules four proton-coupled electron-transfer reactions are required. The standard reduction potential for this reaction is 1.23 V (pH=0, NHE) but real systems yield large energy losses (overpotential). Finding efficient and stable catalysts capable of minimizing the overpotential is a major technological and scientific challenge.

[1] N.S. Lewis and D.G. Nocera, PNAS 103, 43 (2006).
[2] N. Armaroli and V. Balzani, Angew. Chem. Int. Ed. 46, 52 (2007).

Ru-based Polyoxometalates
Several heterogeneous (metal oxide surfaces) and homogeneous (organometallic molecular complexes) catalysts have been proposed in the past three decades, most of them based on Ru or Mn. Although capable of catalyzing water oxidation, the homogeneous compounds show however limited turnover numbers and stability, due to the oxidation of the organic ligands. The recent synthesis of fully inorganic homogeneous complexes represents a major breakthrough in this field, since they combine the stability of inorganic compounds with the high activity of homogeneous catalysts. In particular, tetraruthenium polyoxometallate homogeneous catalysts [1,2] have been shown to oxidize water at a low overpotential (~ 0.2 V) with high turnover numbers (>400 h^-1). The working mechanisms of this novel class of complexes remain unresolved. By means of first-principles DFT simulations we study the properties of these catalysts and the mechanisms they promote. In particular we focus on the analysis of the thermodynamics of the water oxidation cycle, considering the relative stability of different candidate intermediates as a function of the external bias that drives the reaction [3]. By means of metadynamics we also investigate the atomistic mechanism of the O-O bond formation.

[1] Y.V. Geletti, D.A. Hillesheim, C.L. Hill, B. Botar, K. Korgeler, D.G. Musaev, Ang. Chem. Int. Ed. 47, 3896 (2008).
[2] A. Sartorel, M. Carraro, G. Scorrano, R. De Zorzi, S. Geremia, N.D. McDaniel, S. Bernhard, M. Bonchio, J. Am. Che. Soc. 130, 5006 (2008).
[3] S. Piccinin and S. Fabris, First principles study of water oxidation catalyzed by a tetraruthenium-oxo core embedded in polyoxometalate ligands, Phys. Chem. Chem. Phys. (2011), DOI: 10.1039/c0cp01915a

Mononuclear Ru complexes

Water splitting leading to (thermal driven) hydrogen formation and (light driven) oxygen evolution has been recently demonstrated to be promoted by a mononuclear Ru-based homogeneous catalyst. This step-wise intramolecular process shows that a single metal center suffices for the complete water splitting cycle, opening a new approach toward the generation of fuels from artificial photosynthesis. By using ab-initio molecular dynamics with an explict description of the solvent, we study the mechanism of these reactions.

[1] S. W. Kohl et al., Science 324, 74 (2009)