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

Abstract

The important effect of the physical and morphological properties of porous cathodes on the oxygen reduction reaction (ORR) Tafel slope, which is normally viewed as a mechanistic parameter, was investigated using a variety of Pt/carbon (Nafion) catalyst layers in room temperature O 2 -saturated sulfuric acid solutions. Consistent with previous theoretical predictions, it was found experimentally that increasing the pore length, the catalyst layer resistance, and the exchange current density, and decreasing the pore diameter, all serve to cause the Tafel slope to be larger than its mechanistically predicted value, thus leading to performance loss. Theoretical Tafel slopes, calculated using a model for the migration-induced distribution of potentials in an electroactive porous layer and employing the catalyst layer properties examined experimentally, showed very good agreement with the measured Tafel slopes. These results reveal the importance of minimizing the ohmic resistance of porous electrocatalyst layers, of relevance in a wide range of applications, e.g., in fuel cells, electrolysis processes, batteries, etc.

Novel Mesoporous Carbon Supports for PEMFC Catalysts

“…As further evidence supporting the role of carbon support wall thickness on the observed ORR activity trends (Figure 9), porous electrode theory [46,[48][49][50][51] was used to calculate theoretical Tafel slopes for the ORR at the four Pt/CIC catalysts and then compare them with the experimentally obtained results (Figure 8). Only the limiting case of ohmic (migration control) overpotentials was considered here, neglecting the contributions from diffusion (i.e., concentration overpotentials) [46].…”

“…This shows that the ORR activity is significantly compromised when the walls of the carbon support become thinner than 10 nm. This would explain the higher Tafel slope observed for the Pt/CIC-26 catalyst in Figure 8b [48]. The results in Figure 9b clearly show a precipitous drop in ORR activity when the CIC wall thickness is <10 nm, but control of the CIC wall thickness lacks the precision to further study the effect of wall thicknesses between 1.5 and 10 nm.…”

“…The average film thickness (3.5 µm) was obtained for three different catalyst layers (on the RDE surface) at 15 different locations using an optical microscope. The conductivity (κ) of the solution in the pores was assumed to be 10 S/m (based on previous measurements of 0.5 M H2SO4) [48]. The same io for the ORR was assumed, as all four CICs have the same 10 wt.…”

Improved stability of mesoporous carbon fuel cell catalyst support through incorporation of TiO2

“…The Tafel slopes in the high current density regime were in the range of 170-200 mV dec −1 (not shown). The high values (theoretical value: 120 mV dec −1 [35]) are most likely due to the same effects that are described above. The shift in the Tafel slope due to the transition from the low to the high current density regime is commonly observed, and can be explained by different effects: (i) A change of the charge-transfer coefficient [37,38], (ii) different reactant adsorption mechanisms (Temkin vs. Langmuir mechanism for low and high current regimes, respectively) [35,39], (iii) a change in surface coverage with oxygen-containing species [40], and (iv) further porosity related diffusion effects [41,42].…”

“…The Tafel slopes are high compared to the theoretical value (60 mV dec −1 [35]) and selected experimental data. For example, measurements conducted by Kucernak et al yielded a Tafel slope of 58 mV dec −1 for mesoporous Pt attached to a microelectrode [36].…”