ABSTRACT
Vanadium oxides exhibit promising characteristics for electrochemical energy storage, owing to their capability to switch between different oxidation states, in combination with the incorporation of alkali metals. Here, we report on a systematic investigation of the mechanochemical reduction of V2O5 with NaH. In contrast to conventional high-temperature synthesis methods, the mechanochemical reaction occurs already after a few minutes. We observed a mixture of different (sodium) vanadium oxides with vanadium oxidation states ranging from +III to +V. Remarkably, these highly exothermic self-propagating reactions occur even within a rudimentary pistil-mortar setup. Hereby, the hydride concentration has a greater effect on the final sample composition than the milling time. In general, higher percentages of sodium vanadates are formed instead of vanadium oxides, and the lower oxidation states of vanadium are accessible with increasing amounts of NaH. Theoretical calculations confirm these experimental observations and emphasize the central role of sodium vanadates, especially with vanadium in the +V oxidation state, in carrying out the observed exothermic reactions. This comprehensive study sheds light on the mechanochemical reduction of vanadium oxides and underlines their potential for further development of electrochemical energy storage systems.
ABSTRACT
Partially or fully reduced transition metal oxides show extraordinary electronic and catalytic properties but are usually prepared by high temperature reduction reactions. This study reports the systematic investigation of the fast mechanochemical reduction of rutile-type TiO2 and H-Nb2 O5 to their partially reduced black counterparts applying NaH and LiH as reducing agents. Milling time and oxide to reducing agent ratio show a large influence on the final amount of reduced metal ions in the materials. For both oxides LiH shows a higher reducing potential than NaH. An intercalation of Li+ into the structure of the oxides was proven by PXRD and subsequent Rietveld refinements as well as 6 Li solid-state NMR spectroscopy. The products showed a decreased band gap and the presence of unpaired electrons as observed by EPR spectroscopy, proving the successful reduction of Ti4+ and Nb5+ . Furthermore, the developed material exhibits a significantly enhanced photocatalytic performance towards the degradation of methylene blue compared to the pristine oxides. The presented method is a general, time efficient and simple method to obtain reduced transition metal oxides.
