No abstract

The review is devoted to key problems in the development of modern proton‐conducting membranes for hydrogen power assuming its progress for using in fuel cells working at enhanced temperatures without catalysts poisoning and providing stable high proton conductivity and improved mechanical properties. Modern trends in the synthesis approaches such as application of emulsion polymerization and novel efforts for the modification of polymer membranes by chemically stable nanoparticles, carrying protons, are discussed as compared to commercially used membrane materials such as Nafion and Aquivion. The crucial role of advanced structural methods to recognize subtle features of molecular ordering and formation of conducting channels in membranes is considered, focusing on neutron scattering as the most powerful instrument for the analysis of ionomers and other nanoscale structures by means of selective isotopic contrasting structural elements in membrane materials. The integration of novel methods of emulsion polymerization and use of nanodiamonds and other nanoparticles embedded into polymer matrices is prospective in the creation of new generations of membrane materials with higher functional properties.

Poly(N‐vinylcaprolactam) (PVCL) is a synthetic analogue of biomolecules (enzymes, proteins). It demonstrates a specific hydration and undergoes a coil–globule transition. The PVCL–D2O system (PVCL mass M = 106) has been investigated by small‐angle neutron scattering (SANS) at T = 296–316 K to identify the structural features of the collapse at concentration C = 0.5 wt% near the threshold of the coil overlap. (The collapse leads to the segregation of the phase enriched with polymer at T > 305 K). The SANS experiments at q = 0.1–5 nm−1 (scales from monomer unit to globule gyration radius RG≃ 16 nm) have revealed a stretched coil–globule transformation in the range 305–309 K. Using high‐resolution SANS (q = 0.002–0.02 nm−1) the globule association to form fractal structures (sponge‐like) of surface dimension DF≃ 2.4–2.6 was examined. The coexistence of globules and disordered chains (regions ∼5–10 nm) was found. The growth of the content of globular phase was induced by the conformational transition in disordered molecular fragments from coiled (dimension D≃ 1.8) to stretched chains (D≃ 1.2).

No abstract

Compositional proton-conducting membranes based on perfluorinated Aquivion®-type copolymers modified by detonation nanodiamonds (DND) with positively charged surfaces were prepared to improve the performance of hydrogen fuel cells. Small-angle neutron scattering (SANS) experiments demonstrated the fine structure in such membranes filled with DND (0–5 wt.%), where the conducting channels typical for Aquivion® membranes are mostly preserved while DND particles (4–5 nm in size) decorated the polymer domains on a submicron scale, according to scanning electron microscopy (SEM) data. With the increase in DND content (0, 0.5, and 2.6 wt.%) the thermogravimetric analysis, potentiometry, potentiodynamic, and potentiotatic curves showed a stabilizing effect of the DNDs on the operational characteristics of the membranes. Membrane–electrode assemblies (MEA), working in the O2/H2 system with the membranes of different compositions, demonstrated improved functional properties of the modified membranes, such as larger operational stability, lower proton resistance, and higher current densities at elevated temperatures in the extended temperature range (22–120 °C) compared to pure membranes without additives.

Perfluorinated short side chain membranes synthesized by novel aqueous emulsion method demonstrate specific structure dependent on the chemical composition.

Abstract:The structure of a hydrogel consisting of diamond nanoparticles formed by the explosion method has been studied. Small angle neutron scattering has been used as a method for characterization of the gel. Joint approaches for data analysis in reciprocal and direct space have been developed to restore a multilevel structure. The pristine hydrogel of positively charged diamond particles (~5 nm in size, concentration~5 wt %), even by four-fold dilution below its formation critical point, (C*~4 wt %) retains practically the original structure where single particles are joined into small groups integrated into chain fractal-type aggregates creating a network. This indicates a local stability of the gel and means a transformation of continuous gel into a system of micro-domains suspended in water. A perfection of the diamond crystals' facets was revealed that is of principal importance for the configuration of potentials, inducing the diamonds' electrostatic attraction due to different electric charges of facets. It is distinguished from the results for the suspensions of diamonds in graphene shells that showed a deviation of scattering from Porod's law.

Stretching of membranes with low molecular weight makes structure rearrangement according to neutron scattering data on D2O-filled membranes.