Electrochemical water
oxidation is a key counter reaction in obtaining
value-added chemicals by reduction in aqueous solution. However, slow
kinetics is a problem in this process, so the quantitative analysis
of kinetic parameters is necessary to design film-type electrocatalysts.
Although electrochemical impedance spectroscopy (EIS) has been proven
to be a powerful tool in analyzing sparsely loaded catalysts on electrically
conducting supporters, it turned out that film-type catalysts above
100 nm thickness are challenging to analyze with conventional models.
Here, we propose a new transmission line model that was implemented
with a Havriliak–Negami (H–N) capacitor and Warburg
element. We successfully extracted meaningful kinetic parameters,
such as the reaction rate constant at active sites and transport parameters
across the film. We utilized this model to analyze monodisperse sub-10
nm partially oxidized MnO nanoparticles (p-MnO NPs) operating with
superb activity under neutral pH. From this analysis, we revealed
that protons are involved in transport on the surface of p-MnO NPs,
explained the rationale for the optimum thickness, and correlated
the reaction rate constant (22.1 s–1 for a 300 nm-thick
film at 1.35 V vs NHE) with the kinetic parameters obtained from electrokinetic
analysis.
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