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

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.

Pt/Carbon Catalyst Layer Microstructural Effects on Measured and Predicted Tafel Slopes for the Oxygen Reduction Reaction

“…It was predicted by others 12,13,15 and confirmed in our previous work 7 that, under these high current conditions, a doubled TS should be obtained. However, these predictions have never been verified by the controlled experimental variation of these four porous, electroactive layer parameters.…”

“…12,13,15 Thus, the measurement of Tafel slopes at porous electrodes can lead to an incorrect inference about the reaction mechanism or the operative adsorption isotherm.…”

New Interpretation of the Performance of Nickel-Based Air Electrodes for Rechargeable Zinc–Air Batteries

“…Figure b shows the Tafel region of the ORR polarization curves measured after different cumulative times of oxidation at 2.2 V. Tafel slopes observed for the ORR on spinel catalysts in alkaline electrolytes have a minimum value of approximately 40 mV dec –1 . − This is consistent with a theoretical value of 39 mV dec –1 (2.303RT/αF, α = 1.5), which is predicted for the ORR where the second electron-transfer step of the 4-electron pathway is rate-determining . Doubled Tafel slopes are often observed on porous air electrodes due to oxygen transport limitations within porous aggregates, ,,, which would explain the initial value of 76 mV dec –1 observed in Figure b. The shift from the normal Tafel slope to the double Tafel slope likely occurred at a potential above the polarization range.…”

“…79−81 This is consistent with a theoretical value of 39 mV dec −1 (2.303RT/ αF, α = 1.5), 82 which is predicted for the ORR where the second electron-transfer step of the 4-electron pathway is ratedetermining. 83 Doubled Tafel slopes are often observed on porous air electrodes due to oxygen transport limitations within porous aggregates, 54,55,82,84 which would explain the initial value of 76 mV dec −1 observed in Figure 9b. The shift from the normal Tafel slope to the double Tafel slope likely occurred at a potential above the polarization range.…”

Effect of PTFE content in microporous layer on water management in PEM fuel cells

“…The simulation analyses performed in the intermediate potential/current range [29][30][31][32][33][34] to study the effect of oxygen transport kinetics (or water saturation) in the catalyst layer on the polarization characteristics (current-potential relationship) of PEMFC indicated that the slope of the Tafel plot (logarithmic current versus cathodic overpotential) becomes two times larger than the kinetic Tafel slope measured in the activation control region. In other words, in the intermediate potential/current range the oxygen concentration polarization is significant in the catalyst layer and controls the performance of the fuel cell.…”