Potential and pH dependent pseudocapacitance of Mo/Mo oxides - An impedance study

Pseudocapacitance (Cp) of molybdenum with thin passive surface oxides was assessed over a wide potential (E) window (−0.05 to −1.0 V vs. Ag/AgCl) at various pH levels in 0.6 M Na2SO4 by means of impedance spectroscopy. The E and pH - dependent variation of Cp was correlated with the surface oxides’ stability and H+/Na+ surface/near-surface reversible redox processes. Cp showed a rapid increase in acidic solutions from −0.05 to ∼ −0.55 V owing to the H+ electrosorption/intercalation which was followed by a sharp decrease in the hydrogen evolution region. A rise in Cp at E < ∼ −0.80 V was suggested to be associated with either a strong involvement of water at the interface and/or a continuous ingression of Na+ to the porous surface oxides. In neutral and basic electrolytes, the E - dependent Cp remained to be high without a marked maximum. An associated high oxide film resistance indicated that the thermodynamic instability of the surface oxides in neutral/basic electrolytes resulted in an outer dissolute-product's layer, and that the Na+ electrosorption/intercalation occurred primarily with an inner layer of stable surface oxides. Also presented was a minimum potential in the Cpvs. E plot, whose variation was correlated with the open circuit potential.