The Electro-Oxidation of Hydrazine: A Self-Inhibiting Reaction

The electro-oxidation of hydrazine to form dinitrogen is reported over a wide range of both pH and unbuffered conditions at glassy carbon electrodes. It is shown that hydrazine molecules are only electro-active in their unprotonated form, N₂H₄, whereas the protonated species N₂H₅⁺ is electro-inactive. The oxidation of N₂H₄ releases four protons per molecule which are diffusing away from the electrode to rapidly (on the voltammetric time scale) protonate unreacted N₂H₄ molecules diffusing to the electrode converting them into the electro-inactive form, N₂H₅⁺; the reaction is self-inhibiting, and the currents flowing are significantly reduced compared to those expected for a simple electrolytic conversion to an extent reflecting the pH and buffer content of the solution local to the electrode. The local pH in turn is controlled partly by the quantity of protons released electrolytically. The self-inhibition is modeled by solving the relevant transport equations with coupled homogeneous chemical kinetics, utilizing Marcus-Hush electron transfer, giving predicted reduced currents reflecting the pK_a and kinetics of the N₂H₄/N₂H₅⁺ equilibrium in excellent agreement with experimental voltammetric wave shapes.