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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

Tuning the surface structure at the atomic level is of primary importance to simultaneously meet the electrocatalytic performance and stability criteria required for the development of low-temperature proton-exchange membrane fuel cells (PEMFCs). However, transposing the knowledge acquired on extended, model surfaces to practical nanomaterials remains highly challenging. Here, we propose 'surface distortion' as a novel structural descriptor, which is able to reconciliate and unify seemingly opposing notions and contradictory experimental observations in regards to the electrocatalytic oxygen reduction reaction (ORR) reactivity. Beyond its unifying character, we show that surface distortion is pivotal to rationalize the electrocatalytic properties of state-of-the-art of PtNi/C nanocatalysts with distinct atomic composition, size, shape and degree of surface defectiveness under a simulated PEMFC cathode environment. Our study brings fundamental and practical insights into the role of surface defects in electrocatalysis and highlights strategies to design more durable ORR nanocatalysts.

The impact of the carbon structure, the aging protocol, and the gas atmosphere on the degradation of Pt/C electrocatalysts were studied by electrochemical and spectroscopic methods. Pt nanocrystallites loaded onto high-surface area carbon (HSAC), Vulcan XC72, or reinforced-graphite (RG) with identical Pt weight fraction (40 wt %) were submitted to two accelerated stress test (AST) protocols from the Fuel Cell Commercialization Conference of Japan (FCCJ) mimicking load-cycling or start-up/shutdown events in a proton-exchange membrane fuel cell (PEMFC). The load-cycling protocol essentially caused dissolution/redeposition and migration/aggregation/coalescence of the Pt nanocrystallites but led to similar electrochemically active surface area (ECSA) losses for the three Pt/C electrocatalysts. This suggests that the nature of the carbon support plays a minor role in the potential range 0.60 < E < 1.0 V versus RHE. In contrast, the carbon support was strongly corroded under the start-up/shutdown protocol (1.0 < E < 1.5 V versus RHE), resulting in pronounced detachment of the Pt nanocrystallites and massive ECSA losses. Raman spectroscopy and differential electrochemical mass spectrometry were used to shed light on the underlying corrosion mechanisms of structurally ordered and disordered carbon supports in this potential region. Although for Pt/HSAC the start-up/shutdown protocol resulted into preferential oxidation of the more disorganized domains of the carbon support, new structural defects were generated at quasi-graphitic crystallites for Pt/RG. Pt/Vulcan represented an intermediate case. Finally, we show that oxygen affects the surface chemistry of the carbon supports but negligibly influences the ECSA losses for both aging protocols.

Through a tight collaboration between chemical engineers, polymer scientists, and electrochemists, we address the degradation mechanisms of membrane electrode assemblies (MEAs) during proton exchange membrane fuel cell (PEMFC) operation in real life (industrial stacks). A special attention is paid to the heterogeneous nature of the aging and performances degradation in view of the hardware geometry of the stack and MEA. Macroscopically, the MEA is not fuelled evenly by the bipolar plates and severe degradations occur during start‐up and shut‐down events in the region that remains/becomes transiently starved in hydrogen. Such transients are dramatic to the cathode catalyst layer, especially for the carbon substrate supporting the Pt‐based nanoparticles. Another level of heterogeneity is observed between the channel and land areas of the cathode catalyst layer. The degradation of Pt3Co/C nanocrystallites employed at the cathode cannot be avoided in stationary operation either. In addition to the electrochemical Ostwald ripening and to crystallite migration, these nanomaterials undergo severe corrosion of their high surface area carbon support. The mother Pt3Co/C nanocrystallites are continuously depleted in Co, generating Co2+ cations that pollute the ionomer and depreciate the performance of the cathode. Such cationic pollution has also a negative effect on the physicochemical properties of the proton‐exchange membrane (proton conductivity and resistance to fracture), eventually leading to hole formation. These defects were localized with the help of an infrared camera. The mechanical fracture‐resistance of various perfluorosulfonated membranes further demonstrated that polytetrafluoroethylene‐reinforced membranes better resist hole formation, due to their high resistance to crack initiation and propagation. WIREs Energy Environ 2014, 3:540–560. doi: 10.1002/wene.113 This article is categorized under: Fuel Cells and Hydrogen > Science and Materials Fuel Cells and Hydrogen > Systems and Infrastructure Energy Research & Innovation > Science and Materials

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

Due to their increased surface area to volume ratio and molecular accessibility, microporous and mesoporous materials are a promising strategy to electrocatalyze the cathodic oxygen reduction reaction (ORR), the key reaction in proton-exchange membrane fuel cells (PEMFC). Here, we synthesized and provided atomically resolved pictures of porous hollow PtNi/C nanocatalysts, investigated the elemental distribution of Ni and Pt atoms, measured the Pt lattice contraction, and correlated these observations to their ORR activity. The best porous hollow PtNi/C nanocatalyst achieved 6 and 9-fold enhancement in mass and specific activity for the ORR, respectively over standard solid Pt/C nanocrystallites of the same size. The catalytic enhancement was 4 and 3-fold in mass and specific activity, respectively, over solid PtNi/C nanocrystallites with similar chemical composition, Pt lattice contraction, and crystallite size. Furthermore, 100% of the initial mass activity at E = 0.90 V vs RHE (0.56 A mg −1 Pt) of the best electrocatalyst was retained after an accelerated stress test composed of 30 000 potential cycles between 0.60 and 1.00 V vs RHE (0.1 M HClO 4 T = 298 K), therefore meeting the American Department of Energy targets for 2017−2020 both in terms of mass activity and durability (0.44 A mg −1 Pt, mass activity losses < 40%). The better catalytic activity for the ORR of hollow PtNi/C nanocatalysts is ascribed to (i) their opened porosity, (ii) their preferential crystallographic orientation ("ensemble effect"), and (iii) the weakened oxygen binding energy induced by the contracted Pt lattice parameter ("strain effect").

Recent non-precious-metal catalysts (NPMCs) show promise to replace in the future platinum-based catalysts currently needed for the electroreduction of oxygen (ORR) in proton-exchange membrane fuel cells (PEMFCs). Among NPMCs, the most mature subclass of materials is prepared via the pyrolysis of metal (Fe and Co), nitrogen, and carbon precursors (labeled as metal–NC). Such materials often comprise different types of nitrogen groups and metal species, from atomically dispersed metal ions coordinated to nitrogen to metallic or metal–carbide particles, partially or completely embedded in graphene shells. While disentangling the different contributions of these species to the initial ORR activity of metal–NC catalysts with multidunous active sites is complex, following the fate of these different active sites during electrochemical aging is even more difficult. To shed light onto this, herein, six metal–NC catalysts were synthesized and characterized before/after aging with two different accelerated stress tests (AST) simulating PEMFC cathode operating conditions either in steady-state or transient conditions. The samples differed from each other by the nature of the metal (Fe or Co), the metal content, and the heating mode applied during pyrolysis. Catalysts featuring either only atomically dispersed metal-ion sites (metal–N x C y ) or only metal nanoparticles encapsulated in the carbon matrix (metal@N–C) were obtained after pyrolysis of catalyst precursors containing 0.5 or 5.0 wt % of metal, respectively. All six catalysts showed high beginning-of-life ORR mass activity, but the ASTs revealed marked differences in their ORR activity at end-of-life. After the load-cycling AST (10000 cycles), metal–NC catalysts with metal–N x C y sites retained most of their initial activity at 0.8 V (60–100%), while those with metal@N–C particles retained only a small fraction of initial activity (10–20%). Metal–NC catalysts with metal–N x C y sites lost only 25% of their initial ORR activity after 30000 load cycles at 80 °C, thereby reaching the 2020 stability target defined by US Department of Energy. After 10000 start-up/shut-down cycles, no catalyst showed measurable ORR activity at 0.8 V. However, after 1000 start-up/shut-down cycles, most of the metal–NC catalysts initially comprising metal–N x C y sites showed measurable ORR activity at 0.8 V, while those initially comprising metal@N–C particles did not. Energy-dispersive X-ray spectroscopy and Raman spectroscopy measurements of the cycled rotating disk electrodes revealed that demetalation of the catalytic centers and corrosion of the carbon matrix are the main causes of ORR activity decay during load-cycling and start-up/shut-down cycling, respectively. In contrast to what could have been intuitively expected, the metal–N x C y sites are more robust to both demetalation and carbon corrosion than metal@N–C sites.