International audienceThe stability of carbon-supported electrocatalysts has been largely investigated in acidic electrolytes, but the literature is much scarcer regarding similar stability studies in alkaline medium. Herein, the degradation of Vulcan XC-72-supported platinum nanoparticles (noted Pt/C), a state-of-the-art proton exchange membrane fuel cell electrocatalyst, is investigated in alkaline medium by combining electrochemical measurements and identical location transmission electron microscopy; electrochemical surface area (ECSA) losses were bridged to electrocatalyst morphological changes. The results demonstrate that the degradation in 0.1 M NaOH at 25 degrees C is severe (60% of ECSA loss after only 150 cycles between 0.1 and 1.23 V vs RHE), which is about 3 times worse than in acidic media for this soft accelerated stress test. Severe carbon corrosion has been ruled out according to Raman spectroscopy and X-ray photoelectron spectroscopy measurements, and it seems that the chemistry of the carbon support (in particular, the interface (chemical bounding)) between the Pt nanoparticles and their carbon substrate does play a significant role in the observed degradations
International audienceThe electrochemical oxidation of carbon is a pivotal problem for low-temperature electrochemical generators, among which are proton-exchange membrane fuel cells (PEMFCs), and (non)aqueous-electrolyte Li-air batteries. In this contribution, the structure-sensitivity of the electrochemical corrosion of high-surface area carbon (HSAC) used to support catalytic materials in PEMFC electrodes is investigated in model (liquid electrolyte, 96 h potentiostatic holds at different electrode potentials ranging from 0.40 to 1.40 V at T = 330 K) and real PEMFC operating conditions (solid polymer electrolyte, 12,860 h of operation at constant current). Characterizations from Raman spectroscopy demonstrate that the disordered domains of HSAC supports (amorphous carbon and defective graphite crystallites) are preferentially oxidized at voltages related to the PEMFC cathode (0.40 < E < 1.00 V). Excursions to high electrode potential E > 1.00 V, witnessed during start-up and shut-down of PEMFC systems, accelerate this phenomenon and propagate the electrochemical oxidation to the graphitic domains of the HSAC. Thanks to X-ray photoelectron spectroscopy, a better understanding of the relationships existing between structural changes and carbon surface oxides coverage is also emerging for the first time
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.
Determining the degradation mechanisms of oxygen evolution reaction (OER) catalysts is fundamental to design improved proton-exchange membrane water electrolyzer (PEMWE) devices but remains challenging under the demanding conditions of PEMWE anodes. To address this issue, we introduce a methodology combining identical-location transmission electron microscopy (IL-TEM), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements, and apply it to iridium nanoparticles (NPs) covered by a thin oxide layer (IrO x ) in OER conditions. The results show that, whatever the initial OER activity of the IrO x nanocatalysts, it gradually declines and reaches similar values after 30 000 potential cycles between 1.20 and 1.60 V versus the reversible hydrogen electrode (RHE). This drop in OER activity was ascribed to the progressive increase of the Ir oxidation state (fast change during electrochemical conditioning, milder change during accelerated stress testing) along with the increased concentrations of hydroxyl groups and water molecules. In contrast, no change in the mean oxidation state, no change in the hydroxyl/water coverage, and constant OER activity were noticed on the benchmark micrometer-sized IrO 2 particles. In addition to chemical changes, Ir dissolution/redeposition and IrO x nanoparticle migration/agglomeration/detachment were made evident during the conditioning stage and in OER conditions, respectively. By combining the information derived from IL-TEM images and XPS measurements, we show that Ir(III) and Ir(V) are the best performing Ir valencies for the OER. These findings provide insights into the long-term OER activity of IrO x nanocatalysts as well as practical guidelines for the development of more active and more stable PEMWE anodes.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.