Degradation Characteristics of Electrospun Gas Diffusion Layers with Custom Pore Structures for Polymer Electrolyte Membrane Fuel Cells

Electrospinning has been demonstrated to be a versatile technique for producing hydrophobic gas diffusion layers (GDLs) with customized pore structures for the enhanced performance of polymer electrolyte membrane (PEM) fuel cells. However, the degradation characteristics of custom hydrophobic electrospun GDLs (eGDLs) have not yet been explored. Here, for the first time, we investigate the degradation characteristics of custom hydrophobic eGDLs via an ex situ accelerated degradation protocol using H₂O₂. The surface contact angle of degraded eGDLs (44 ± 12°) was lower than that of pristine eGDLs (137 ± 6°). The loss of hydrophobicity was attributed to the degradation (via hydrolysis) of the fluorinated monolayers (formed via a direct fluorination treatment) on the electrospun carbon fiber surfaces as evidenced by the reduction in surface fluorine content. Degradation of the surface monolayers affected fuel cell performance under multiple operating conditions. At 100% relative humidity (RH), the loss of monolayers led to higher liquid water content and lower cell voltages compared to the pristine eGDL. At 50% RH, the degraded eGDL led to lower cell voltages due to the lower electrical conductivity of the degraded materials. The lower electrical conductivity was attributed to the oxidation of carbon fibers upon loss of the monolayers. Our results indicate the importance of designing robust hydrophobic surface treatments for the advancement of customized GDLs for effective long-term fuel cell operation.

Enhanced Water Management in PEMFCs: Perforated Catalyst Layer and Microporous Layers

An experimental in situ study was performed to investigate the effects of the catalyst layer (CL) and cathode microporous layer (MPL_c ) perforation on the water management and performance of polymer electrolyte membrane fuel cells (PEMFCs). Polymeric pore formers were utilized to produce perforated CL and MPL structures. High-resolution neutron tomography was employed to visualize the liquid water content and distribution within different components of the cell under channel and land regions. The results revealed that at humid conditions, the perforated layers enhanced the liquid water transport under the channel regions. Moreover, at high current densities, the performance was improved for the cells with perforated layers compared to a baseline cell with non-perforated layers, owing to reduced mass transport losses.

In Operando Quantification of Three-Dimensional Water Distribution in Nanoporous Carbon-Based Layers in Polymer Electrolyte Membrane Fuel Cells

Understanding the function of nanoporous materials employed in polymer electrolyte membrane fuel cells (PEMFCs) is crucial to improve their performance, durability, and cost efficiency. Up to now, the water distribution in the nm-sized pore structures was hardly accessible during operation of the cells. Here we demonstrate that phase contrast synchrotron X-ray tomography allows for an in operando quantification of the three-dimensional water distribution within the nm-sized pores of carbon-based microporous layers (MPLs). For this purpose, a fuel cell design optimized for tomographic phase contrast measurements was realized. Water in the pores of the entire MPL was detected and quantified. We found an inhomogeneous distribution of the local water saturation and a sharp boundary between mostly filled MPL and almost empty areas. We attribute the latter observation to the two-phase boundary created because condensation takes place predominantly on one side of the boundary. Furthermore, high water saturation in large areas hints at gas diffusion or transport along preferred three-dimensional paths through the material, therefore bypassing most of the MPL volume. Our approach may contribute significantly to future investigations of nanoporous fuel cell materials under realistic operating conditions.