1. a National Institut for Energy, Environment and New Technologies (ENEA) , Applied Physics Department, Via Fermi 45, 00044 Frascati, Rome, Italy
2. b University of Bologna , Department of Chemistry , via Selmi, 2, Bologna, Italy
3. c Italian Electrotecnic Experimental Centre (CESI) Spa , via Rubattino, 54, Milan, Italy
4. d European Community-Joint Research Centre, Ispra, Varese, Italy