Effect of Carbon Corrosion on Wettability of PEM Fuel Cell Electrodes

Abstract

Large transport resistances at high current densities hinder the proton-exchange-membrane fuel cells from reaching performancecost-durability targets. Extensive carbon corrosion in the cathode catalyst layers (CCL) has been shown to correlate to pore closures and increase in surface oxides of carbon support, which has been associated with the increase in wettability of CCLs. This study investigated effects of microstructure and surface oxides on the change of wetting properties of CCLs after different accelerated stress tests. The apparent contact angles were measured over a broad range of carbon mass losses at 0.3 mg Pt /cm 2 . The apparent contact angles fell sharply after small extent of carbon loss followed by a plateau, and decreased again after 123 μg/cm 2 (35% carbon support loss). The microstructural changes were characterized by surface roughness (atomic force microscopy) and porosity (focus-ion-beam scanning electron microscopy and thermogravimetric analysis). The surface roughness and porosity were used to parameterize Wenzel's, Cassie-Baxter's, and combined models to elucidate the changes in Young's contact angle. The fittings of the models revealed that the CCLs sustained their wettability after up to 35% of carbon support loss due to the increases in the proportion of hydrophobic fluorocarbon chains on the surface of the CCLs measured from XPS.

Understanding the performance losses and “invasiveness” of in situ characterization steps during carbon corrosion experiments in polymer electrolyte membrane fuel cells

“…This initial stronger decrease of mass activities was also observed elsewhere [27] and could be attributed to the opening of carbon porous structure and exposing of internal Pt particles to the ionomer (note that 70% of the Pt particles were originally located within the carbon primary pores, see Experimental). When Pt is in contact with the ionomer, it is well-known that ionomer sulfonic groups adsorb onto the Pt particle and exert a poisoning effect, leading to an overall reduction of mass activity [ 11 , 47 , 58 ].…”

“…The fact that the discrepancy of polarization curves at >2A cm -2 is more significant after carbon corrosion (compare Fig. 6 c to b) could possibly be explained by the decrease of cathode hydrophobicity [27] and thus the more severe flooding under high current outputs. At this point, we would like to notice that an extension of the simplified model to account for the liquid water effect would additionally require detailed empirical information, e.g.…”

“…Nonetheless, only a few studies have reported the amount of carbon lost during their applied AST cycles. For instance, Mittermeier et al reported carbon corrosion rate (in A per gram carbon) in dependence of the applied potential (between 1.1 and 1.5V vs. RHE) [13] , while Fang et al showed the carbon loss (in μg cm -2 or in wt%) after a variety of ASTs [27] . An evaluation of fuel cell performance losses combined with the measurement of actual carbon loss was performed by Hashimasa et al (on three different AST protocols) [34] and Yu et al (on potential holding) [ 16 , 44 ].…”

Engineering Catalyst Layers for Next‐Generation Polymer Electrolyte Fuel Cells: A Review of Design, Materials, and Methods

“…Furthermore, ionomer is also lost from the CL during this carbon corrosion mechanism, which, along with increased cracking, thinning and collapse of the CL pore structure, can significantly reduce the water management abilities of the CL during operation. [ 187 ] Although the exact structure changes that occur during ASTs are beyond the scope of this review, we recommend other reviews on this topic. [ 188,189 ]…”

Simulative Investigation on Local Hydrogen Starvation in PEMFCs: Influence of Water Transport and Humidity Conditions

“…[9][10][11] Besides, mass transfer in the PEMFC worsens due to porosity decay, ionomer redistribution and decreasing hydrophobicity within the CLs. 6,[12][13][14] Possible collapse of CL porous structure further exacerbates the issue. 1,15,16 As a result, small extent of carbon weight loss (in the order of 5%-10%) can already lead to deterioration of the fuel cell performance and the failure in meeting the PEMFC durability target.…”