Electro-oxidation of hydrazine shows marcusian electron transfer kinetics
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ectro-oxidation of hydrazine shows marcusian electron transfer kinetics *
Ruiyang Miao, Lifu Chen & Richard G Compton
Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, UK Received August 20, 2020; accepted October 15, 2020; published online December 2, 2020
Although hydrazine (N2H4) oxidation in an electrochemical environment has been of great interest for years, its intrinsic electron transfer kinetics remain uncertain. We report that the phenomenological Butler-Volmer (BV) theory is not appropriate for interpreting the process of hydrazine oxidation for which an astonishingly wide range of transfer coefficients, Tafel slopes and diffusion coefficient have been previously reported. Rather Tafel analysis for voltammetry recorded at Glassy Carbon (GC) electrodes reveals a strong potential dependence of the anodic transfer coefficient, consistent with the symmetric Marcus-Hush (sMH) theory. According to the relationship
=
+FE 0f 2
F , the reorganization energy (0.35±0.07 eV) and an approximate E 2
formal potential of the rate-determining first electron transfer were successfully extracted from the voltammetric responses. hydrazine; electrode kinetics; transfer coefficient; Butler-Volmer theory; Marcus-Hush theory Citation:
Miao R, Chen L, Compton RG. Electro-oxidation of hydrazine shows marcusian electron transfer kinetics. Sci China Chem, 2020, 63, https://doi.org/ 10.1007/s11426-020-9889-1
1 Introduction Whilst the chemical oxidation of hydrazine, N2H4, historically helped to launch the V-2 rockets developed in Peenemunde by Wernher von Braun [1], the electro-oxidation of hydrazine has considerable modern interest in the form of a valuable anodic reaction for use in some practical fuel cells [2–4]. Accordingly, the latter has received extensive attention especially from the applications perspective. Of underpinning fundamental interest and importance is the rate of the heterogeneous electron transfer involved in the electro-oxidation, thought to display a high over-potential, and hence electrochemically irreversible kinetics implying that the latter are slow compared to prevailing rates of mass transport on many electrodes made of diverse materials. This has led to the design and use of surfaces that catalyse the oxidation *Corresponding author (email: [email protected])
notably through the adsorptive stabilisation of reactive intermediates on the formation of di-nitrogen from hydrazine, involving the loss of four electrons and four protons from the latter. In this context it is of interest to consider the intrinsic electron transfer behaviour of N2H4 on surfaces where adsorption effects are reported to be minimal [5]. Accordingly, we have investigated the electron transfer kinetics of hydrazine at carbon electrodes as an essential prerequisite underpinning any mechanistic study of active electro-catalysts. The oxidation of hydrazine at carbon electrodes has mostly, been studied from the perspective of developing the c
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