Electrospun Fuel Cell Cathode Catalyst Layers Under Low Humidity Conditions
Authors: Kallina Valentina , Oezaslan Mehtap , Hasché Frédéric 
Published: December 2024

Electrospinning has emerged as a promising approach to prepare porous cathode catalyst layers (CCLs) with low loading of platinum group metals (PGM) for PEM fuel cells. In particular, electrospun nanometer-sized fibers can be used as a backbone to increase the utilization and accessibility of catalytically active sites at low PGM loading as well as improve water management at high current density. [1, 2] Beside the low PGM loading, the absence of external humidifier is important for practical applications, because it can further decrease the parasitic power losses as well as complexity, weight, volume, and cost of the PEMFC system. [3] In this context, electrospun nanofiber CCLs show a pronounced humidity-dependent performance loss, which hinders their operation range compared to conventional CCLs prepared by decal transfer process. For example, Brodt et al. show a performance decrease for electrospun CCL of around 75 % at relative humidity (RH) of 40 % compared to 100 % RH. [4] In contrast, the electrosprayed CCL exhibits only a loss of around 30 % under the same conditions. [4] Therefore, the development of highly active and robust CCLs operating at high current density under low humidity remain a great challenge to date.

In this work, we systematically investigated the effects of the humidity on the performance of the nanofiber-based CCLs. To prepare the CCLs with a geometric surface area of 50 cm2, a nozzle-free electrospinning machine by Elmarco S.R.O. (Liberec, Czech Republic) was employed. The ink containing Pt/C catalyst, ionomer, and polyacrylic acid (PAA) was mixed using a disperser. Thereby, several highly homogeneous CCLs deposited on a gas diffusion layer (GDL) with controlled platinum loading of up to 0.1 mg cm-2
geo were prepared and characterized using micro X-ray fluorescence spectroscopy (μXRF). The morphology and chemical distribution of the as-prepared CCLs were evaluated from the scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (SEM-EDX). We then correlated the structural information (Pt and Nafion distribution, size of the nanofibers, etc.) of the CCLs with the electrochemical performance (U-I curve, ECSA, HFR, proton conductivity, etc.) at single cell level. Additionally, the impact of the humidity variations (20 – 100 % RH) for the nanofiber-based CCLs was compared to the conventional CCLs prepared by decal-transfer method. Moreover, high resolution (S)TEM-EDX was employed to uncover the distribution of the PFSA ionomer, Pt/C catalyst and PAA. These data are correlated with the ECSA and indicate the utilization and accessibility of Pt nanoparticles linked with the spatial distribution of PFSA ionomer.

Altogether, we provide fundamental insights into the relationship between humidity and performance for nanofiber-based PEMFC CCLs prepared by electrospinning. New strategies will be presented to overcome the structure-humidity-sensitivity behavior of these novel three-dimensional nanofiber-based CCLs. [5]

References

[1] Brodt M, Wycisk R, Pintauro P N: Nanofiber Electrodes for High Power PEM Fuel Cells. J. Electrochem. Soc. 2013, 160 (8), F744-F749. DOI: https://doi.org/10.1149/2.008308jes.

[2] Zhang W, Pintauro P N: High-Performance Nanofiber Fuel Cell Electrodes. ChemSusChem 2011, 4 (12), 1753-1757. DOI: https://doi.org/10.1002/cssc.201100245.

[3] Ren G, Qu Z, Wang X et al.: Electrospun fabrication and experimental characterization of highly porous microporous layers for PEM fuel cells. International Journal of Hydrogen Energy 2024, 55, 455 - 463. DOI: https://doi.org/10.1016/j.ijhydene.2023.11.226.

[4] Brodt M, Han T, Dale N et al.: Fabrication, In-Situ Performance, and Durability of Nanofiber Fuel Cell Electrodes. J Electrochem Soc 2015, 162, F84-F91. DOI: https://doi.org/10.1149/2.0651501jes.

[5] Kallina V, Hasché F, Oezaslan M: Advanced Design of Electrospun Nanofiber Cathode Catalyst Layers for PEM Fuel Cells at Low Humidity. Current Opinion in Electrochemistry 2024, submitted.