A One-Dimensional Model of a PEM Fuel Cell with the Cathode Catalyst Layer Hydrophobically Treated for Water Management

Abstract

A proton exchange membrane (PEM) fuel cell with a hydrophobically treated cathode catalyst layer (CL) demonstrates ~220% peak power increase with humidified air at 70°C. To understand the reasons for the increase, a mathematical model was developed focusing on the oxygen-water two-phase transport phenomena. The model suggests the treatment affects the CL in two ways. First, the interface of the ionomer layer exposed to the gas pores becomes more hydrophobic, facilitating less coverage and faster water drainage from the CL and resulting in better performance at high current densities. Second, it also affects the hydration level in the ionomer phase resulting in higher oxygen concentration on and in the catalyst agglomerates, leading to higher performance over the whole polarization curve. The properties having significant influence on the model fitting the experimental data are the capillary pressure property of the CL, the hydrophobic ionomer ratio in the agglomerate, and the oxygen solubility/diffusivity in the Nafion® phases. With this experimentally verified model, additional case studies combining the hydrophobic gas diffusion material with the hydrophobic CL demonstrate that the membrane’s self-humidification (zero-net-water flux) and peak power enhancement (~15%) can be reached simultaneously, providing direction for the future materials development.

Modification of Nafion's nanostructure for the water management of PEM fuel cells

“…Furthermore, our in-situ ECSA measurement reveals that the hydrophobic treatment has minimal influence on the cathode's ECSA (a Pt in Equation ( 2)). 10 Therefore, Equation (2) shows that the only possible reason for the fuel cell improvement in the kinetic-controlled region is related to the oxygen concentration in the ionomer near the catalyst surface (C O ), which is supposed to be determined by the oxygen solubility in the Nafion ionomer phase. The oxygen solubility in the ionomer phase is found to be inversely affected by the hydration level of the ionomer phase.…”

“…One method is to mix hydrophobic PTFE in the catalyst ink, of which details, advantages, and disadvantages have been discussed in the previous publications. 9,10 In this work, we would like to discuss another approach, the direct modification of Nafion ionomer' structure. 11,12 Prior works 12,13 have shown that the surface hydrophobicity of the PFSAbased ionomer phase can be increased by modifying its surface composition to have more of the hydrophobic fluorocarbon backbone and less of the hydrophilic sulfonic acid groups.…”