Modeling the Effect of Low Pt loading Cathode Catalyst Layer in Polymer Electrolyte Fuel Cells: Part I. Model Formulation and Validation

Abstract

A model for the cathode catalyst layer (CL) is presented, which is validated with previous experimental data in terms of both performance and oxygen transport resistance. The model includes a 1D macroscopic description of proton, electron and oxygen transport across the thickness, which is locally coupled to a 1D microscopic model that describes oxygen transport toward Pt sites. Oxygen transport from the channel to the CL and ionic transport across the membrane are incorporated through integral boundary conditions. The model is complemented with data of effective transport and electrochemical properties extracted from multiple experimental works. The results show that the contribution of the thin ionomer film and Pt/ionomer interface increases with the inverse of the roughness factor. Whereas the contribution of the water film and the water/ionomer interface increases with the ratio between the geometric area and the surface area of active ionomer. Moreover, it is found that CLs diluted with bare carbon provide lower performance than non-diluted samples due to their lower electrochemical surface area and larger local oxygen transport resistance. Optimized design of non-diluted samples, with a good distribution of the overall oxygen flux among Pt sites, is critical to reduce mass transport losses at low Pt loading.

On the optimal cathode catalyst layer for polymer electrolyte fuel cells: Bimodal pore size distributions with functionalized microstructures

“…where p sat H 2 O (T) is the saturation pressure of water (Sánchez-Ramos et al, 2021). This expression shows that although CLs are almost isotherms (temperature variation, ΔT ≲ 10 -3 − 10 -2 K), the temperature gradient created by generated heat is similar to that found in GDLs and cannot be neglected.…”

“…≈ 40 mol m −3 the reference oxygen concentration, and η c the cathode overpotential. i o,c was set somewhat higher than the mean value reported in (Sánchez-Ramos et al, 2021)…”

“…The oxygen diffusive resistance from the channel to the CL and the ohmic resistance of the PEM and its interfaces are incorporated through integral mixed boundary conditions into the 1D cathode CL model. Further details of the implementation can be found in (Sánchez-Ramos et al, 2021). The formulations of the macroscopic and microscopic models are presented in the next two sections.…”

“…The effective oxygen diffusivity is decomposed into a dry component due to the obstruction of the dry CL microstructure and the Knudsen effect, f(ɛ, R v ), and the relative effective diffusivity due to the relative blockage of water saturation, g(s avg ) 10.3389/fenrg.2022.1058913 (Sánchez-Ramos et al, 2021;García-Salaberri et al, 2015b,a; García-Salaberri, 2021)…”

“…A key issue for decreasing Pt loading (mainly at the cathode) is caused by local mass transport resistance introduced by thin ionomer films surrounding Pt nanoparticles (Weber and Kusoglu, 2014;Sánchez-Ramos et al, 2021, 2022. The local oxygen transport resistance (per unit of geometric area) is inversely proportional to the roughness factor, the ratio between the electrochemically active surface area and the cell geometric area, r f = A Pt A −1 geo .…”