Polarization Characteristics of Porous Electrode Systems with Adsorbed Intermediates Participating in the Electrode Reaction: A Study of Limiting Tafel Slopes

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Abstract
Current‐potential relationships are derived for porous electrode systems following a homogeneous model and when adsorbed intermediates participate in the electrode reaction. Limiting Tafel slopes were deduced and compared with the corresponding behavior on planar electrode systems. The theoretical results showed doubling of Tafel slopes when the slow‐step is a charge‐transfer reaction and a nonlogarithmic current‐voltage behavior when the slow‐step is a chemical reaction. Comparison of the experimental results with theory for the case of oxygen reduction on carbon surfaces in alkaline media indicates that a slow chemical reaction following the initial charge‐transfer reaction to be the likely rate‐controlling step. Theoretical relationships are utilized to determine the exchange current density and the surface coverage by the adsorbed intermediates during the course of oxygen reduction from alkaline solutions on “carbon.” Tafel slope measurements on planar and porous electrodes for the same reaction are suggested as one of the diagnostic criteria for elucidating the mechanistic pathways of electrochemical reactions.

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