The 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 °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.
We report an interesting new class of bifunctional electrocatalysts, Pd/C-CeO2, with excellent activity and stability for the hydrogen oxidation reaction (HOR) under alkaline conditions. The unique ...structure of palladium deposited onto a mixed support of Vulcan XC-72 carbon and CeO2 consists of Pd metal preferable deposited on the ceria regions of the catalyst. The CeO2-Pd interaction leads to enhanced HOR kinetics and increased stability. Here we compare catalysts with three different Pd loadings and show that the 10wt% Pd sample has optimized activity. Hydrogen pumping and fuel cell experiments based on this catalyst show higher activities as compared to a Pd/C sample without ceria. Metal dissolution tests and identical location transmission microscopy experiments show that the catalyst stability under harsh potential cycling experiments in alkaline medium is significantly improved as compared to Pd/C, making this material one of the best options for use as highly active and highly stable electrocatalysts for the HOR in anion exchange membrane fuel cells.
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•A new class of electrocatalyst for H2 oxidation in alkaline media is described.•Unique structure of Pd deposited on a mixed support of carbon and CeO2.•Hydrogen pump and fuel cell tests show enhanced activity.•Identical location transmission electron microscopy show improved stability.
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.
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.
Due to their interesting electrocatalytic properties for the oxygen reduction reaction (ORR), hollow Pt‐alloy nanoparticles (NPs) supported on high‐surface‐area carbon attract growing interest. ...However, the suitable synthesis methods and associated mechanisms of formation, the reasons for their enhanced specific activity for the ORR, and the nature of adequate alloying elements and carbon supports for this type of nanocatalysts remain open questions. This Review aims at shedding light on these topics with a special emphasis on hollow PtNi NPs supported onto Vulcan C (PtNi/C). We first show how hollow Pt‐alloy/C NPs can be synthesized by a mechanism involving galvanic replacement and the nanoscale Kirkendall effect. Nickel, cobalt, copper, zinc, and iron (Ni, Co, Cu, Zn, and Fe, respectively) were tested for the formation of Pt‐alloy/C hollow nanostructures. Our results indicate that metals with standard potential −0.4<E<0.4 V (vs. the normal hydrogen electrode) and propensity to spontaneously form metal borides in the presence of sodium borohydride are adequate sacrificial templates. As they lead to smaller hollow Pt‐alloy/C NPs, mesoporous carbon supports are also best suited for this type of synthesis. A comparison of the electrocatalytic activity towards the ORR or the electrooxidation of a COads monolayer, methanol or ethanol of hollow and solid Pt‐alloy/C NPs underlines the pivotal role of the structural disorder of the metal lattice, and is supported by ab initio calculations. As evidenced by accelerated stress tests simulating proton‐exchange membrane fuel cell cathode operating conditions, the beneficial effect of structural disorder is maintained on the long term, thereby bringing promises for the synthesis of highly active and robust ORR electrocatalysts.
Defective is the new chic: Taking advantage of their highly defective structure (disordered surface, high polycrystallinity, contracted lattice, and open porosities), hollow Pt‐alloy/C nanoparticles can be used to achieve a high catalytic enhancement for both the oxygen reduction reaction and oxidation reactions such as CO oxidation or alcohol oxidation, thus opening the path to a generation of electrocatalysts designed around structural defects.
The internal migration of zinc within the thickness of porous composite negative electrodes for rechargeable zinc batteries causes detrimental shape change of the zinc electrode. It depends on ...various parameters linked to the electron conduction in the electrode: electronic percolation, conductive additives, primary current collector. This study highlights a simple solution, that drastically enhances the electrochemical performance of thick and porous zinc electrodes, by rethinking their architecture. Using a multiplicity of thin current collectors enables to create a 3D network of electronic percolation and a better repartition of the current lines within the electrode; as a result, the formation of multiple active zinc cores in the electrode volume is better controlled. This strategy allows to improve the management of zinc migration within the electrode structure, leading to minimal densification of each zinc core, and overall enables higher reversibility of zinc oxidation/reduction upon discharge/charge. It finally mitigates the initial drastic capacity fading observed on standard type calcium zincate negative electrode.
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•Simple and industrially-scalable way to improve thick zinc-porous electrodes.•Overcome drastic capacity fading by using layered primary conductive architecture.•Multi-layer copper grid as alternative replacement to expensive copper foam.•Controlling zinc core formation.
Gaining fundamental insights into the formation and the stability of surface oxides on iridium (Ir) surfaces is pivotal to oxygen evolution reaction (OER) electrocatalysis. Herein, we examined the ...potential-dependent structural and chemical changes occurring on planar Ir(111), Ir(210), and nanofaceted Ir(210) single-crystal surfaces using electrochemistry, scanning probe microscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma mass spectrometry. We show that, after polarization in OER conditions, Ir surface atoms feature mixed oxidation states(0), (+III), and (+IV)and then enrich into Ir(+IV) due to the dissolution of Ir(+III) species. The rate of surface and near-surface layer enrichment in Ir(+IV) species depends on the modulation mode of the potential (linear potential sweeps vs. potential steps) and is faster on opened surfaces. By combining fits derived from the XPS spectra and OER activity measurements, we found that the OER specific activity varies with the Ir oxidation state and is closely related to the fraction of Ir(+III) species.
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