No abstract

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