Nonlinear frequency response analysis of oxygen reduction reaction on silver in strong alkaline media

Abstract

Understanding the kinetics of the oxygen reduction reaction on silver in highly concentrated alkaline solutions (e.g., 11 M NaOH) and elevated temperature is of great interest both in terms of technical applications of this reaction (e.g., in chloralkali electrolysis) and in fundamental terms. To gain a better understanding of this process, a nonlinear frequency response analysis was performed in combination with a rotating ring disk analysis. Firstly, the ORR was analyzed under steady-state conditions. The number of exchanged electrons was determined based on rotating ring disk experiments. Secondly, two different dynamic ORR kinetic models have been formulated. These models were used to derive the theoretical first- and second-order frequency response functions (FRFs). Furthermore, the first and second-order FRFs were determined experimentally for both 0.1 M NaOH and 11 M NaOH at different temperatures. In the next step, the kinetic model parameters were numerically estimated from the experimental frequency-domain data. Despite its simplicity, the so-called 1-step model was able to reproduce experimental first- and second-order FRFs in 0.1 M NaOH under different conditions (steady-state potentials and rotation rates). At the same time, this model was able to reproduce the first-order FRF in 11 M NaOH at 60 °C, but not the features observed in the second-order FRF spectra. The more complex 4-step model has shown a better capability in explaining experimental findings, like the first and second-order FRFs, but also experimentally observed change of the Tafel slope as well as the influence of mass transport on the number of exchanged electrons. However, it was also not able to reproduce all experimental features. Possible reasons for these discrepancies are discussed.