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

Pseudocapacitance (C_p) 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 C_p was correlated with the surface oxides' stability and H⁺/Na⁺ surface/near-surface reversible redox processes. C_p 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 C_p 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 C_p 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 C_p vs. E plot, whose variation was correlated with the open circuit potential.