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 pKa and kinetics of the N2H4/N2H5+ equilibrium in excellent agreement with experimental voltammetric wave shapes.