The electrical performance of a proton exchange membrane fuel cell is limited by the slow oxygen reduction reaction (ORR) kinetics. Catalytic improvements for the ORR have been obtained on alloyed ...PtM/C or M-rich-core@Pt-rich-shell/C catalysts (where M is an early or late transition metal) in comparison to pure Pt/C, due to a combination of strain and ligand effects. However, the effect of the fine nanostructure of the nanomaterials on the ORR kinetics remains underinvestigated. Here, nanometer-sized PtNi/C electrocatalysts with low Ni content (∼15 atom %) but different nanostructures and different densities of grain boundary were synthesized: solid, hollow, or “sea sponge” PtNi/C nanoalloys, and solid Ni-core@Pt-shell/C nanoparticles. These nanostructures were characterized by transmission and scanning transmission electron microscopy, X-ray energy dispersive spectroscopy, synchrotron wide-angle X-ray scattering (WAXS), atomic absorption spectroscopy, and electrochemical techniques. Their electrocatalytic activities for the ORR were determined and structure–activity relationships established. The results showed the following: (i) The compression of the Pt lattice by ca. 15 atom % Ni provides mild ORR activity enhancement in comparison to pure Pt/C. (ii) Highly defective PtNi/C nanostructures feature up to 9.3-fold enhancement of the ORR specific activity over a commercial Pt/C material with similar crystallite size. (iii) The enhancement of the ORR kinetics can be ascribed to the presence of structural defects, as shown by two independent parameters: the microstrain determined from WAXS and the average COads electrooxidation potential (μ1 CO) determined from COads stripping measurements. This work indicates that, at fixed Ni content, ORR activity can be tuned by nanostructuring and suggests that targeting structural disorder is a promising approach to improve the electrocatalytic properties of mono- or bimetallic nanocatalysts.
Simultaneous in situ X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) of hydrogen fuel cell electrocatalyst layers allows nanoparticle degradation mechanisms of aggregation, ...coalescence and ripening to be independently decoupled during cell operation. The ratio of particle size to crystallite size is proposed as a direct measurement of catalyst particle–particle interactions. This metric is applied to track the aggregation of a practical industrial fuel cell catalyst in situ, during accelerated degradation. The dominant catalyst degradation mode changes over the course of the device’s life cycle, passing through distinct phases of aggregation, coalescence and ripening. Understanding the relative contribution of each degradation mode is necessary for engineering next-generation catalysts with commercially acceptable durability.
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•Operando X-ray diffraction of Pt/C catalysts inside fuel cells during stress test.•Combining diffraction and small angle scattering probes catalyst aggregation state.•Degradation mechanism of Pt/C passes through multiple clear phases.•Catalyst aggregation, ripening, and coalescence processes are deconvoluted.
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 theoretical design of effective metal electrocatalysts for energy conversion and storage devices relies greatly on supposed unilateral effects of catalysts structure on electrocatalyzed ...reactions. Here, by using high-energy X-ray diffraction from the new Extremely Brilliant Source of the European Synchrotron Radiation Facility (ESRF-EBS) on device-relevant Pd and Pt nanocatalysts during cyclic voltammetry experiments in liquid electrolytes, we reveal the near ubiquitous feedback from various electrochemical processes on nanocatalyst strain. Beyond challenging and extending the current understanding of practical nanocatalysts behavior in electrochemical environment, the reported electrochemical strain provides experimental access to nanocatalysts absorption and adsorption trends (i.e., reactivity and stability descriptors) operando. The ease and power in monitoring such key catalyst properties at new and future beamlines is foreseen to provide a discovery platform toward the study of nanocatalysts encompassing a large variety of applications, from model environments to the device level.
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 HClO4 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”).
The catalytic performance of extended and nanometer-sized surfaces strongly depends on the amount and the nature of structural defects that they exhibit. However, whereas the effect of steps or ...adatoms may be unraveled with single crystals (“surface science approach”), implementing reproducibly in a controlled manner structural defects on nanomaterials remains hardly feasible. A case that deserves particular attention is that of bimetallic nanomaterials, which are used to catalyze the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC). Point defects (vacancies), planar defects (dislocations and grain boundaries), and bulk defects (voids, pores) are likely to be generated in alloy or core@shell nanomaterials based on Pt and a transition metal due to the high lattice mismatch between the two elements. Here, we report the morphological and structural trajectories of hollow PtNi/C nanoparticles during thermal annealing under vacuum, N2, H2, or air atmosphere by in situ transmission electron microscopy and synchrotron X-ray diffraction. We evidence atmosphere-dependent restructuring kinetics, which enabled us to synthesize a set of catalysts with identical chemical compositions and elemental distributions but different morphologies, crystallite sizes, and lattice strain. By combining the results of Rietveld and pair-distribution function analyses and electrochemical measurements, we demonstrate that the structurally disordered areas located at the interface between individual crystallites are highly active for two reactions of interest for PEMFC devices: the electrochemical COads oxidation and the ORR. These results shed fundamental light on the effect of structural defects on the catalytic performance of bimetallic nanomaterials and should aid in the rational design of more efficient ORR electrocatalysts.
Because of their enhanced oxygen reduction reaction (ORR) kinetics (9 and 6-fold) enhancement of specific activity and mass activity for the ORR relative to those of a commercial Pt/C catalyst, ...respectively), hollow PtNi/C nanoparticles are attracting a growing level of interest. This catalytic enhancement arises from the synergetic combination of strain and ligand effects, the presence of structural defects, and their hollow morphology producing a convex and concave catalytic sites. However, preventing a loss of catalytic activity under practical proton exchange membrane fuel cell (PEMFC) cathode operating conditions on alloys of platinum with other transition metals (PtM alloys, M being a transition metal) or M-rich core@Pt-rich shell nanoparticles remains highly challenging. A loss of performance is usually observed because of the dissolution of Pt and M under the harsh operating conditions of a PEMFC cathode, but the question remains unanswered for nanomaterials in which catalytic activity is not solely due to alloying effects. Herein, we have carefully investigated the changes in the ORR activity of solid and hollow PtNi/C nanoparticles with identical chemical compositions but different nanostructures during an accelerated stress test simulating PEMFC cathode operation. By combining chemical, physical, and electrochemical techniques, we show that the dissolution of Ni atoms constitutes the primary reason for the loss of ORR catalytic activity but that the initial catalytic advantage of hollow over solid PtNi/C nanoparticles is maintained in the long term. Hence, implementing structural disorder in PEMFC cathode electrocatalysts represents a promising direction for sustainably improving ORR kinetics.
The influence of the texture, structure, and chemistry of different carbon supports on the morphological properties, oxygen reduction reaction (ORR) activity, and stability of porous hollow PtNi ...nanoparticles (NPs) was investigated. The carbon nanomaterials included carbon blacks, carbon nanotubes, graphene nanosheets, and carbon xerogel and featured different specific surface areas, degrees of graphitization, and extent of surface functionalization. The external and inner diameters of the supported porous hollow PtNi/C NPs were found to decrease with an increase in the carbon mesopore surface area. Despite these differences, similar morphological properties and electrocatalytic activities for the ORR were reported. The stability of the synthesized electrocatalysts was assessed by simulating electrochemical potential variations occurring at a proton exchange membrane fuel cell (PEMFC) cathode during startup/shutdown events. Identical location transmission electron microscopy (IL-TEM) and electrochemical methods revealed the occurrence of a carbon-specific degradation mechanism: carbon corrosion into CO2 and particle detachment were noticed on carbon xerogels and graphene nanosheets while, on carbon blacks, surface oxidation prevailed (C → COsurf) and did not result in modified electrical resistance of the catalytic layers, rendering these carbon supports better suited to prepare highly active and stable ORR electrocatalysts.
Palladium hydrides (PdH x ) present a model system of both fundamental and applied importance: solute-induced phase transition affects evolution and electrooxidation of molecular hydrogen in water ...electrolyzers and fuel cells, respectively, as well as hydrogen storage, its sensing and catalysis of many hydrogenation reactions. It is well documented that hydrogen (H) atoms get progressively trapped in Pd under various sorption–desorption modes, leading to embrittlement and influencing its bulk and interfacial properties. However, the intensity and progressiveness of this phenomenon remain little explored. Herein, by combining in situ X-ray scattering and electrochemistry, we provide evidence of continuous expansion of the lattice of 3.6 nm carbon-supported Pd nanoparticles during repeated H insertion/de-insertion cycles, resulting in a progressive loss of their reversible H sorption capacity.
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