High-Resolution Analysis of Ionomer Loss in Catalytic Layers after Operation

Abstract

The function of catalytic layers in fuel cells and electrolyzers depends on the properties of the ionically conductive phase, which are most commonly perfluorinated ionomers based on Nafion and Aquivion. An analysis by atomic force microscopy reveals that the ultrathin ionomer films around Pt/C agglomerates have a thickness distribution ranging from 3.5 nm to 20 nm. Their conductivity and gas permeation properties determine the fuel cell performance to a large extend. For electrodes in Aquivion-based membraneelectrode-assemblies operation-induced structure changes were investigated by means of material-and conductivity-sensitive atomic force microscopy, infrared spectroscopy and electron-dispersive X-ray analysis. The observed thinning of the ultrathin ionomer films was mainly caused by polymer degradation deduced from reduced swelling after long-time operation and a significant loss of ionomer with operation time detected by infrared spectroscopy. From the linear thickness increase of the ultrathin films with rising humidity, a mainly layered structure of the ionomer was deduced. An influence of thickness of such ultrathin ionomer films on fuel cell lifetime was found by analysis of differently prepared membrane-electrode-assemblies, where a linear increase of irreversible degradation rate with ionomer film thickness in the electrodes of unused membrane-electrode-assemblies was found.

Effects of PEMFC Operational History under Dry/Wet Conditions on Additional Voltage Losses due to Ionomer Migration

“…The above stated observations agree with our hypothesis of the ionomer "displacement," as one would except that this would be more prominent with a higher ionomer content in the CL. The ionomer displacement and redistribution, as well as loss of total ionomer fraction in the CL with operation time, were also observed by Morawietz et al 55 They reported that the ionomer displacement and redistribution resulted in a growth of bigger ionomer agglomerates after prolonged operation. Similar in our case, after operation, we observe the increase in amount and size of the ionomer fibers that are present in the CL/GDL interface.…”

“…Furthermore, during the recovery step (> 120% RH), the presence of liquid water in the CL likely facilitates ionomer solving while creating micelles. 55 It is widely known that short side chain low EW ionomer have the propensity for dissolution and flow, compared to the high EW ionomers, which has been rationalized by the comparable higher ionomer water uptake. 58 Thus, the ionomer variant in the present study (725 EW), together with the ink composition and catalyst layer drying protocol used in this work, is prone to such ionomer flows and would irreversibly alter the ionomer distribution in the CL.…”

Possible scenario of forming a catalyst layer for proton exchange membrane fuel cells

“…The material-sensitive and conductive atomic force microscopy (AFM) was used to show the ionomer distribution in CL. [13][14][15][16][17][18][19][20][21] The adhesion force image overlaid topography was used to distinguish the ionomer and the aggregated Pt/C. The ionically conductive regions, which implied the ionomer region in CL, could be recognized by their low adhesion signal.…”

“…Cullen 23 showed the ionomer coating lm thickness is about 7 nm using AFM technology. Hiesgen's group [15][16][17][18][19][20][21] had done a series of works on ionomer distribution in the CL by AFM. The AFM tip of nominal radius of less than 1 nm was used and an image pixel size was less than 1 nm (<0.36 nm) for high-resolution image.…”

“…17 The ionomer lm thickness in CL was thinning and the ionomer was redistributed and aggregated in terms of operation time. 21 This irreversible degradation of the ionomer in CL may result in performance degradation of MEAs. Stiffness mapping and current mapping were also used along with the adhesion force mapping to discern Pt, C and the ionomer.…”