Simulation of the Oxygen Reduction Reaction at an RDE in 0.5 M H 2 SO 4 Including an Adsorption Mechanism

Oxygen reduction on the surface of a rotating disk electrode RDE in 0.5 M H2SO4 is simulated by including mass transfer, adsorption, and charge transfer. A generalized model for the adsorption and reaction of several species is introduced. The oxygen reduction reaction is simulated as a limiting case where oxygen is the only species adsorbed, and oxygen reduction is the only reaction that takes place on the surface of the electrode. The model is based on the Nernst–Planck equations for mass transfer and the Butler–Volmer equation for electrochemical kinetics. The simulated polarization curves capture the change in the Tafel slopes, which are observed experimentally but cannot be explained by the normal four-electron-transfer mechanism. The adsorption model is compared with the four-electron-transfer model by fitting experimental data to both models using a nonlinear parameter estimation technique. The effects of changes in some important kinetic parameters are demonstrated. The oxygen reduction reaction ORR has been studied by using a rotating disk electrode RDE in acidic electrolytes such as sulfuric, perchloric, hydrochloric, and organic acid solutions. 1-4 Several reaction pathways have been proposed for the ORR based on the RDE experimental data, of which the four-electron pathway is primarily used to characterize the behavior of this reaction at a platinum electrode or a glassy carbon electrode coated with platinumbased catalyst. 5,6 The overall ORR is given by O2 +4 H + +4 e � 2H2O 1 where a mechanism that consists of four separate single-electrontransfer steps is implicitly assumed. A linear Tafel plot is expected for this four-electron reaction mechanism when the potential is lower than the standard electrode potential of 1.229 V vs the standard hydrogen electrode SHE. Note that all the potentials mentioned in this work are with respect to the SHE. The potential region of 1.0–0.6 V is where the polymer electrolyte membrane fuel cell PEMFC is operated practically. Important kinetic information such as the exchange current density and the transfer coefficient can be extracted by studying this voltage window for the ORR occurring at metal electrodes or carbonsupported platinum nanoparticle catalyst-coated electrodes. Transitions in the Tafel slopes for the ORR are observed when experimental data is analyzed. 2-4,7-11 The Tafel slope is just about doubled in the higher current density HCD region the potential region below about 0.8 V compared to the lower current density LCD region the potential region above 0.8 V. Table I presents a collection of examples from the literature which present changing Tafel slopes. When the temperature is around 298 K, the Tafel slope in the LCD region is around 60 mV/dec, which is close to 2.303RT/F, whereas this value is about 120 mV/dec or 2 2.303RT/F in the HCD region. The double Tafel slope phenomenon is observed at smooth surfaces such as electrodes made of polycrystalline Pt, Pt 111 ,P t100, or Pt alloy, etc., and at inert electrodes coated with nanosized carbon-supported Pt catalyst powder. 3 Various explanations have been suggested to explain the change in the Tafel slope, 12-17 and one of them is the oxygen adsorption mechanism, 14,16 which relates the change in the Tafel slope to the change in the applied potential. The adsorption of oxygen on the platinum surface is further complicated by the presence of other competing species and intermediates. 18,19 The adsorption of the anion HSO4 can compete with the molecular oxygen adsorption for the Pt site. 19 Reversible dissolution of water at the surface of the electrode can lead to PtOH formation in the potential region of 0.6–1.0 V vs SHE, and this process may interfere with the electrochemical reactions occurring at the surface of the electrode. 18 In this work, a generalized model is presented for the adsorption of multiple species Rl,sur. Each adsorbing species is assumed to be transported to the surface of the electrode and become adsorbed to the electrode surface to form Rl,ads Rl,sur Rl,ads