1. Department of Energy Conversion and Storage, Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark
2. Department of Chemistry, Biochemistry and Pharmaceutical Sciences, University of Bern, Bern CH-3012, Switzerland
3. Departments of Materials and Physics, and the Thomas Young Centre for Theory and Simulation of Materials, Imperial College London, London SW7 2AZ, U.K.