1. Centre for Materials Science and Nanotechnology and Department of Chemistry, University of Oslo, P.O. Box 1033, N-0315 Oslo, Norway, Materials Research Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, P.O. Box 49, DK-4000 Roskilde, Denmark, and Laboratory of Inorganic Chemistry, ETH Zurich, Wolfgang-Pauli-Strasse 10 and Collegium Helveticum Schmelzbergstrasse 30, 8093 Zürich, Switzerland