Rasmi Ranjan Tripathy

Oxygen reduction reaction (ORR) is considered a key electrochemical reaction, the kinetics of which are complex and challenging to quantify, even more at such buried metal/polymer interface. Here in this work, a novel approach independent of the polymer barrier property has been developed to quantitively characterize ORR kinetics using a combined hydrogen potentiometry (HP) and electrochemical impedance spectroscopy (EIS) approach. For the ORR measured using EIS on the front side of a bare Pd membrane exposed to an alkaline NaOH electrolyte, a 5-fold decrease in the charge transfer resistance (R_CT) indicated the progress of ORR, in stark contrast to a corresponding 2-fold increase in inert N₂ atmosphere. For a polymethyl methacrylate (PMMA)/Pd interface, a 30-fold decrease in R_CT as compared to bare Pd correlated well with a cathodic shift of around 50 mV (1 pH unit) in the current-potential I(U) curve. At a molecularly tailored octane-thiol/Pd interface, ORR kinetics was highly inhibited, with the current-potential I(U) curve shifted in the cathodic direction by 190 mV, as compared to the Pd/PMMA interface at a charging (ORR) current of −25 μA cm⁻². This could be successfully correlated to a 100-fold decrease in R_CT value indicating interface sensitivity of this HP-EIS combined technique.