Oxygen Reduction Kinetics on a Platinum RDE Coated with a Recast Nafion Film

Abstract

Measurements of currents of the oxygen reduction reaction (ORR) at a Pt RDE coated with a recast Nafion film yield the permeability of O2 through such films. Combined with results from linear potential scanning experiments on similar electrodes, these measurements also allow the separate evaluation of the concentration of O2 and its diffusion coefficient in such films. Similar recast Nafion films have been used successfully to promote protonic access to catalyst sites in fuel cells. Accordingly, the maximum film thickness which would allow adequate oxygen flux through such a layer is evaluated. Measurements in 85% H3PO4 at room temperature show that a very significant enrichment of O2 (e.g., by a factor of 20) in a region close to the electrode can be achieved by coating the electrode with a recast Nafion film. This result is expected from the high solubility of O2 in Nafion. However, the measured resulting enhancement of the kinetic ORR currents was found to be only a factor of two. The interpretation suggested for the lack of direct proportionality between average O2 concentration near the electrode and ORR kinetic current density is that enhanced local ORR rates are possible only at a limited number of free Pt sites that are immediately adjacent to a surface region covered by the hydrophobic O2‐normalrich phase. Contamination by organics of lower molecular weight originating from the Nafion film may contribute to site‐blocking. With increase in temperature the O2 enrichment effect becomes less pronounced, because the solubility of O2 in 85% H3PO4 increases with temperature while in Nafion it decreases.

Oxygen Reduction Reaction Measurements on Platinum Electrocatalysts Utilizing Rotating Disk Electrode Technique

“…Early half-cell electrochemical studies on Pt wires in electrolytes included cycling the working electrode repeatedly to various potentials or even holding at O 2 evolution potentials to obtain a clean reproducible CV and high ORR activity. 12,69,[85][86][87] Conditioning procedures are also commonly implemented in PEMFCs; the conditioning protocol for PEMFCs involves high/low potentials holds and potential cycling under operating conditions where humidified air flows over the cathode and hydrogen over the anode. 88 RDE literature typically do not elaborate on the details of conditioning protocols or justification for the selected protocol but merely recommend repetitive potential cycling until the CV under N 2 or O 2 becomes stable.…”

“…88 RDE literature typically do not elaborate on the details of conditioning protocols or justification for the selected protocol but merely recommend repetitive potential cycling until the CV under N 2 or O 2 becomes stable. 86,89 Figure 11a illustrates the gradual evolution of the CV profile as the number of 500 mV/s cycles advances to 50 (4 min). As we approach the 50th cycle, we observe the stabilization of the signature H UPD and oxide features indicating completion of conditioning.…”

Shape‐Dependent Electrocatalysis: Oxygen Reduction on Carbon‐Supported Gold Nanoparticles

“…where j is the measured current density, j k and j d are the kinetic and diffusion-limited current densities, respectively, n is the number of electrons transferred per O 2 molecule, k is the rate constant for O 2 reduction, F is the Faraday constant (96 485 C mol À1 ), w is the electrode rotation rate, C b O 2 is the concentration of O 2 in the bulk (1.13 10 À3 mol dm À3 ), [74] D O 2 is the diffusion coefficient of O 2 (1.8 10 À5 cm 2 s À1 ) [74] and n is the kinematic viscosity of the solution (0.01 cm 2 s À1 ). [75] The corresponding K-L plots are shown in Figure 6, inset.…”

Effects of Ethylene Glycol and Caprolactam on the ORR and HOR Performances of Pt/C Catalysts

“…This hysteresis in the Pt ORR polarization curve was observed previously. 12,40,44,45 As explained in the literature, 40,44,45 the coverage of inert adsorbates on the Pt surface is responsible for the lower ORR activity in the negative scan and the strong hysteresis in Pt surface oxide formation/reduction. Fig.…”