Behavior of molybdenum nitrides as materials for electrochemical capacitors - Comparison with ruthenium oxide

Ruthenium oxide (RuO2), formed as a thin film on a Ru or Ti metal substrate, exhibits a large specific (cm(-2)) and almost constant, electrochemical capacitance over a 1.35 V range in aqueous H2SO4. This behavior has led to its investigation and use as a material for fabrication of supercapacitor devices. However, its cost has encouraged search for other materials exhibiting similar behavior. Work reported in the present paper evaluates two nitrides of Mo, Mo2N and MoN, as substitutes for RuO2. It is shown that very similar capacitance behavior to that of RuO2 films arises, e.g., in cyclic voltammetry and dc charging curves; in the former, almost mirror-image anodic and cathodic current-response profiles, characteristic of a capacitor, arise. However, the nitride materials have a substantially smaller voltage operating range of only some 0.7 V due to electrochemical decomposition above ca. 0.7 V vs. RHE. This limits their usefulness as a substitute for RuO2. Of interest is that the nitride films exhibit potential-decay and potential-recovery on open circuit after respective charge and forced discharge. The decay and recovery processes are logarithmic in time, indicating the role of internal faradaic charge redistribution processes.