Dimitar Antonov1, Tsvetelina Toleva2, Blagoy Burdin3, Svetoslav V. Georgiev1, Jürgen Grötsch4 1 Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 24, Sofia 1113, Bulgaria; E-mails: dimia@geology.bas.bg; s_georgiev@geology.bas.bg 2 TITAN Group, Cherni Vrah, blvd., 33E, Sofia 1421, Bulgaria; E-mail: tsvetelina.toleva@abv.bg 3 Institute of electrochemistry and energy system, Bulgarian Academy of Sciences, Acad. Georgi […]