Effect of Non-ionic Surfactant on Fe-N-C Catalyst Layers under HT-PEMFC Conditions
Authors: Zierdt Tanja , Knake Jonas , Müller-Hülstede Julia , Schonvogel Dana , Wagner Peter , Wark Michael , Friedrich K. Andreas 
Published: December 2024

The replacement of Pt-based catalysts by Fe-N-Cs in cathodes of high temperature polymer electrolyte membrane fuel cells (HT-PEMFC) can significantly reduce material costs.[1] In a previous study, we revealed the feasibility of Fe-N-C-based gas diffusion electrodes (GDEs) for oxygen reduction reaction under HT-PEMFC conditions (conc. H3PO4, 160 °C) and showed the demand for improving the catalyst layer (CL) morphology.[1] We discovered that Fe-N-C catalysts are feasible to maintain the cell performance within a wide range of PTFE contents from 20 to 50 wt% in the CL.[2] Non-ionic surfactants can enhance the wetting of the CL with electrolyte in HT-PEMFC, as Mack et al. revealed by a reduced break-in time of Triton™ complemented Pt/C electrodes.[3] Furthermore, Lee et al. reported improved dispersion of PTFE binder within the catalyst ink due to employed non-ionic surfactant and increased the HT-PEMFC performance of the Pt-based electrode.[4]

Our actual study firstly reveals that the non-ionic surfactants Triton™ X-100 and as alternative the less-hazardous Tergitol™ 15-S-9 similarly affect the Fe-N-C ink sedimentation. Therefore, Fe-N-C-based (PMF-0011904, Pajarito Powder) GDEs are fabricated through ultrasonic spray coating with Tergitol™ contents of 0, 1, 20 and 50 wt% in the CL and a constant PTFE amount of 50 wt%.

The H2O contact angle decreases with increasing Tergitol™ content in the Fe-N-C CL, revealing an increased wettability. Furthermore, contact angles with conc. H3PO4 are determined at room temperature (RT) and 160 °C. With this approach the wetting behaviors of the CL at HT-PEMFC conditions are imitated. The contact angle of H3PO4 of the 0 wt% Tergitol™ GDE unveils a decrease of 39 % from RT to 160 °C, for the 1 wt% Tergitol™ GDE by only 4 % and by 15 % for the 20 wt% Tergitol™ GDE. No significant change is observed for the 50 wt% Tergitol™ GDE. Tergitol™ provides a sufficiently wetted surface at RT, whereas without Tergitol™ the Fe-N-C CL must be moistened at higher temperatures. Furthermore, the GDE performance is accessed by hot pressing a polybenzimidazole-based membrane onto the CL and subsequent electrochemically characterization in a half-cell setup under HT-PEMFC condition at 160 °C in conc. H3PO4 electrolyte. Figure 1 reveals the effect of Tergitol™ on the GDE performance. A surfactant content above 1 wt% leads to electrode flooding by the electrolyte and thus a performance decay, as the surfactant increased the CL wetting.

The results are complemented by detection of the iron content of the GDEs by inductively coupled plasma mass spectrometry (ICP-MS) and scanning electron microscopy coupled to energy-dispersive X-ray spectroscopy (SEM/EDS) to compare CL morphologies and compositions along all GDEs.

References:

[1] J. Müller-Hülstede, T. Zierdt, H. Schmies, D. Schonvogel, Q. Meyer, C. Zhao, P. Wagner, and M. Wark, J. Power Sources, 537 231529 (2022).

[2] T. Zierdt, J. Müller‐Hülstede, H. Schmies, D. Schonvogel, P. Wagner, and K. A. Friedrich, ChemElectroChem, 11 e202300583 (2024).

[3] F. Mack, T. Morawietz, R. Hiesgen, D. Kramer, V. Gogel, and R. Zeis, Int J Hydrogen Energy, 41 (18), 7475-7483 (2016).

[4] W. J. Lee, J. S. Lee, H. Y. Park, H. S. Park, S. Y. Lee, K. H. Song, and H. J. Kim, Int J Hydrogen Energy, 45 (57), 32825-32833 (2020).

Figure 1