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This study focuses on the characterization of gas permeation through Nafion, the most commonly used polymer electrolyte membrane (PEMs) for low-temperature fuel cells and water electrolyzers. In the first part of this study, novel modifications of the electrochemical monitoring technique to precisely measure the hydrogen and oxygen permeabilities of Nafion are presented. With these techniques, the gas permeabilities of Nafion were observed to be independent of pressures, which was ascribed to a solely diffusive process. Moreover, the temperature dependence of the hydrogen and oxygen permeabilities through Nafion in the fully hydrated state (where the water content is independent of the temperature) were measured in order to determine the activation energies of the permeation mechanisms. On the basis of the measured influence of temperature and relative humidity on the gas permeabilities of Nafion, the pathways for gas permeation through its aqueous and solid phase are qualitatively discussed. The second part of this study presents a resistor network model to quantitatively correlate the microscopic structure of Nafion and its gas permeabilities.

Core-shell particles with earth-abundant cores represent an effective design strategy for improving the performance of noble metal catalysts, while simultaneously reducing the content of expensive noble metals 1-4. However, the structural and catalytic stabilities of these materials often suffer during the harsh conditions encountered in important reactions, such as the oxygen reduction reaction (ORR) 3-5. Here, we demonstrate that atomically thin Pt shells stabilize titanium tungsten carbide cores, even at highly oxidizing potentials. In situ, timeresolved experiments showed how the Pt coating protects the normally labile core against oxidation and dissolution, and detailed microscopy studies revealed the dynamics of partially and fully coated core-shell nanoparticles during potential cycling. Particles with complete Pt coverage precisely maintained their core-shell structure and atomic composition during accelerated electrochemical ageing studies consisting of over 10,000 potential cycles. The exceptional durability of fully coated materials highlights the potential of core-shell architectures using earth-abundant transition metal carbide (TMC) and nitride (TMN) cores for future catalytic applications. The development of core-shell nanostructures with controllable size, shell thickness, surface facets and composition vastly expands the possibility for engineering noble metal catalysts with enhanced performance 4. Numerous core-shell systems have been synthesized for catalytic applications, very often with other noble metal cores, such as Ru (refs. 6,7), Pd (refs. 5,8), Ag (ref. 9) and Ir (ref. 10) with marginal cost benefits. Critically, these core-shell materials along with those comprising earth-abundant, metallic cores (for example, Fe, Co and Ni) form intrinsically metastable structures with miscible core and shell elements that degenerate during electrochemical cycling or annealing due to metal leaching or migration 5,7,8,11. These instabilities result in considerable losses in active surface area and performance over time, imposing major barriers for the broad usage of core-shell architectures in industrial applications, where stability is essential. This is especially true for fuel cell technologies, the commercialization of which has been hindered by the poor durability of ORR catalysts 3,12-14. Early TMCs and TMNs are ideal core materials due to their thermal and chemical stability, electrical conductivity, low cost and intrinsic ability to bind strongly to noble metals while still being immiscible with them 15,16. Unfortunately, the formation of surface oxides or carbon on TMCs and TMNs presents a difficult synthetic

For the design of a beneficial device structure, in which both electrodes are exposed to the same medium, and considering that the hydrogen evolution is most efficiently carried out in acidic electrolyte and the advantages of the proton exchange membrane, a robust photoanode would be highly desirable. [10][11][12][13][14][15] Nonetheless the development of an efficient and affordable photoanode, which is stable in acidic electrolyte, imposes a great challenge and limits the large-scale implementation of economically viable PEC water-splitting. In light of this challenge, much attention has been drawn to the development of efficient and affordable photoanode systems adapted to acidic electrolytes.Hematite is arguably the most desirable photoanode material. On one hand, its relatively small bandgap of 1.9-2.1 eV and its suitably aligned valence band level perfectly match the thermodynamic energy requirements needed to drive water oxidation. [4,10] On the other hand, it is made from the most abundant transition metal on Earth crust, iron. Unfortunately, the bare hematite surface is catalytically very poor, and therefore requires modification with water-oxidation catalysts (WOCs) in order to extract the thermodynamic power stored when light is absorbed. State-of-the-art water-oxidation catalysts (WOCs) in acidic electrolytesusually contain expensive noble metals such as ruthenium and iridium. However, they too expensive to be implemented broadly in semiconductor photoanodes for photoelectrochemical (PEC) water splitting devices. Here, an Earth-abundant CoFe Prussian blue analogue (CoFe-PBA) is incorporated with core-shell Fe 2 O 3 /Fe 2 TiO 5 type II heterojunction nanowires as composite photoanodes for PEC water splitting. Those deliver a high photocurrent of 1.25 mA cm −2 at 1.23 V versus reversible reference electrode in acidic electrolytes (pH = 1). The enhancement arises from the synergic behavior between the successive decoration of the hematite surface with nanolayers of Fe 2 TiO 5 and then, CoFe-PBA. The underlying physical mechanism of performance enhancement through formation of the Fe 2 O 3 /Fe 2 TiO 5 / CoFe-PBA heterostructure reveals that the surface states' electronic levels of hematite are modified such that an interfacial charge transfer becomes kinetically favorable. These findings open new pathways for the future design of cheap and efficient hematite-based photoanodes in acidic electrolytes.

A variety of synthesis protocols for octahedral PtNi nanocatalysts have led to remarkable improvements in platinum mass and specific activities for the oxygen reduction reaction. Nevertheless, the values achieved are still only one tenth of the activity measured from Pt 3 Ni single-crystal (111) surfaces. These particles lose activity during potential cycling, primarily because of Ni leaching and subsequent loss of shape. Here, we present the syntheses and high catalytic oxygen reduction reaction activities of molybdenum-doped PtNi octahedral catalysts with different sizes (6−14 nm) and compositions. We show that the Mo-doped, Nirich, PtNi octahedral catalysts exhibit enhanced stability over their undoped counterpart. Scanning transmission electron microscopy with energy-dispersive Xray analysis reveals the particular elemental distribution for the size and composition of the different catalysts. By combining high-resolution compositional analysis with electrochemical measurements and online inductively coupled plasma mass spectrometry, it was possible to correlate the size, morphology, and composition with the oxygen reduction reaction activities before and after accelerated stress tests. The octahedral catalysts show high electrochemical surface areas and increasing specific activity with increasing surface area of the (111) facets and Ni content, leading to high mass activities. These results demonstrate the advantages of increasing the (111) surface area and Ni content of PtNi nano-octahedral catalysts to improve the performance and stability for the oxygen reduction reaction.

Direct methanol fuel cells (DMFCs) have the major advantage of the high energy density of the methanol (4.33 kWh/l) they use as a liquid fuel, although their costs remain too high due to the high quantity of Pt needed as a catalyst for oxygen reduction in the presence of methanol. Pt−Ni core−shell catalysts are promising candidates for improved oxygen reduction kinetics as shown in hydrogen fuel cells. The novelty in this work is due to the fact that we studied these catalysts in DMFC cathodes where oxygen must be reduced and membrane-permeating methanol oxidized at the same time. In spite of many attempts to overcome these problems, high amounts of Pt are still required for DMFC cathodes. During measurements over more than 3000 operating hours, the performance of the core−shell catalysts increased so substantially that a similar performance to that obtained with five times the amount of commercial platinum catalyst was achieved. While catalyst degradation has been thoroughly studied before, we showed here that these catalysts exhibit a self-protection mechanism in the DMFC cathode environment and prolonged operation is actually beneficial for performance and further stability due to the formation of a distinct Pt-rich shell on a PtNi core. The catalyst was analyzed by transition electron microscopy to show how the catalyst structure had changed during activation of the core−shell catalyst.

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Physically mixing distinct Ni(OH)2and Fe(OOH) particles leads to formation of highly active “physically mixed” Ni + Fe catalysts with atomically intermixed Ni–(O)–Fe sites.