Electrochemical Insertion of Hydrogen into Diniobium Pentaoxide

Abstract The electrical behavior of the Nb2O5 electrode in Na2SO4 solutions was characterized by the impedance spectra. The pH dependence of the flat band potential could be explained in terms of the acid-dissociating equilibrium of hydroxyl groups on the oxide surface. The plot of equilibrium potential of the oxide vs. pH gave a straight line with a slope of −0.06 V. The equilibrium potential became monotonously more negative with increasing extent of the reduction of the oxide. The cyclic voltammograms and chronopotentiograms of the partially reduced oxide indicated that the diffusion of an electroactive species in the oxide controlled the rate of the electrode process. These results suggest that HxNbO2.5 or NbO2.5−x(OH)x can be formed by the reduction of the oxide. The conductivities of the oxide increased about two orders of magnitude by the insertion of a small amount of hydrogen. In conclusion, a blue coloration of the oxide can occur as a result of the diffusion of H+ from the electrolyte into the Nb2O5 lattice and a simultaneous injection of electrons to d-block bands of Nb in the energy band model.