Membrane and Active Layer Degradation upon PEMFC Steady-State Operation

Abstract

We studied proton exchange membrane fuel cell membrane electrode assemblies ͑MEAs͒ degradation after fuel-cell operation. Anode and cathode pronounced degradation was monitored by chemical ͓energy dispersive spectrometry ͑EDS͒, X-ray photoelectron spectroscopy ͑XPS͔͒, physical ͓scanning electron microscopy ͑SEM͒, transmission electron microscopy͔, and electrochemical ͑ultramicroelectrode with cavity͒ techniques. Aged MEAs underwent severe redistribution of most elements ͑Pt, C, F͒, coupled to a dramatic change of Pt particles shape, mean particle size and density over the carbon substrate. Among the various scenarios for Pt redistribution, Pt dissolution into Pt z+ species and transport in the ionomer or the proton exchange membrane play important roles. The Pt z+ dissolution/transport is likely favored by activators/ligands ͑For SO x -containing species͒ originating from the alteration of the polymers contained in the MEA. From SEM observations, the source of Pt z+ species is the cathode, while EDS and XPS show some SO x -and F-containing species origin from the anode. Local chemical analyses ͑SEM-EDS and XPS͒ revealed the excess Pt monitored in aged MEAs is associated with F excess. For instrumental limitation concerns, we could not detect the S element, but SO x -containing species could also act as counter ions during Pt z+ transport within the MEA. Pt corrosion/ redistribution is associated with the decrease of Pt-active area as revealed by CO ad -stripping voltammograms.

Review: Durability and Degradation Issues of PEM Fuel Cell Components

“…In this mechanism Brownian motion is the driving force, causing surface diffusion of particles with random collisions leading to coalescence [85]. Usually the fact that sintering does not occur significantly in catalysts in the gas phase at temperatures below 500°C is considered to be an indication that coalescence is not the prevailing mechanism [10,83,84]. In recent PEMFC studies it was found that both potential and increased humidity enhance the particle growth [86,87], which favours the dissolution/redeposition mechanism.…”

“…Still, although both mechanisms lead to an increase in the average particle size with an asymptotic particle size distribution (PSD) the coalescence mechanism has a log-normal distribution (tail at large sizes) and the Ostwald ripening has a tail at the smaller particle sizes but with a maximal particle size cutoff [88]. In many studies a log-normal distribution of Pt particle sizes was found in virgin and used electrodes [10,67,83,86,88]. However, it must be noted that this requires a good sampling also of small particles and results can be affected by the fact that several mechanisms are active at the same time [10,83].…”

“…Microscopy techniques like (FEG)-SEM and TEM are commonly used to establish Pt particle growth, Pt deposition in the ionomer and electrode thinning [10,67,68,83]. Pt particle size growth can also be determined by X-ray diffraction [86].…”

Accelerated Membrane Durability Testing of Heavy Duty Fuel Cells

“…[43][44][45][46][47] Synthetic methods of planting Pt in the membrane have been shown to have a detrimental effect on the membrane, 48,49 where platinum ions can act as a Fenton's reaction promoter. 50,51 However, in our group, field operated MEAs with a naturally grown metallic Pt band in the membrane were tested under COCV AST conditions,…”

The Stability of Platinum–Carbon Aerogel Catalysts upon Repeated Potential Cycles

“…[10][11][12][13][14][15][16] According to the report of Rand and Woods, [9] platinum is dissolved above 1.08 V and the evolution of molecular oxygen occurs above 1.46 V, whereas gold is not dissolved at 1.4 V. Dissolved platinum can then either deposit on existing platinum particles to form larger particles (Ostwald ripening) or diffuse into an electrochemically inaccessible portion of the membrane electrode assembly (e.g., into the gas diffusion layer), and the resulting electrochemical active surface area will accordingly decrease. [13,[17][18][19] Shao-Horn and co-workers described a potential mechanism for the loss of electrochemical active surface area. [17] Recently, Adzic and co-workers reported a stabilized platinum catalyst that uses gold clusters for the oxygen reduction reaction.…”