Flexible Nanocellulose/Lignosulfonates Ion-Conducting Separators for Polymer Electrolyte Fuel Cells.

The utilization of biobased materials for the fabrication of naturally derived ion-exchange membranes is breezing a path to sustainable separators for polymer electrolyte fuel cells (PEFCs). In this investigation, bacterial nanocellulose (BNC, a bacterial polysaccharide) and lignosulfonates (LS, a by-product of the sulfite pulping process), were blended by diffusion of an aqueous solution of the lignin derivative and of the natural-based cross-linker tannic acid into the wet BNC nanofibrous three-dimensional structure, to produce fully biobased ion-exchange membranes. These freestanding separators exhibited good thermal-oxidative stability of up to about 200 °C, in both inert and oxidative atmospheres (N2 and O2, respectively), high mechanical properties with a maximum Young's modulus of around 8.2 GPa, as well as good moisture-uptake capacity with a maximum value of ca. 78% after 48 h for the membrane with the higher LS content. Moreover, the combination of the conducting LS with the mechanically robust BNC conveyed ionic conductivity to the membranes, namely a maximum of 23 mS cm-1 at 94 °C and 98% relative humidity (RH) (in-plane configuration), that increased with increasing RH. Hence, these robust water-mediated ion conductors represent an environmentally friendly alternative to the conventional ion-exchange membranes for application in PEFCs.

Efficient Water Self-Diffusion in Diphenylalanine Peptide Nanotubes.

Nanotubes of self-assembled dipeptides exemplified by diphenylalanine (FF) demonstrate a wide range of useful functional properties, such as high Young's moduli, strong photoluminescence, remarkable piezoelectricity and pyroelectricity, optical waveguiding, etc., and became the object of intensive research due to their ability to combine electronic and biological functions in the same material. Two types of nanoconfined water molecules (bound water directly interacting with the peptide backbone and free water located inside nanochannels) are known to play a key role in the self-assembly of FF. Bound water provides its structural integrity, whereas movable free water influences its functional response. However, the intrinsic mechanism of water motion in FF nanotubes remained elusive. In this work, we study the sorption properties of FF nanotubes directly considering them as a microporous material and analyze the free water self-diffusion at different temperatures. We found a change in the regime of free water diffusion, which is attributed to water cluster size in the nanochannels. Small clusters of less than five molecules per unit cell exhibit ballistic diffusion, whereas, for larger clusters, Fickian diffusion occurs. External conditions of around 40% relative humidity at 30 °C enable the formation of such large clusters, for which the diffusion coefficient reaches 1.3 × 10-10 m2 s-1 with an activation energy of 20 kJ mol-1, which increases to attain 3 × 10-10 m2 s-1 at 65 °C. The observed peculiarities of water self-diffusion along the narrow FF nanochannels endow this class of materials with a new functionality. Possible applications of FF nanotubes in nanofluidic devices are discussed.

Conductive polysaccharides-based proton-exchange membranes for fuel cell applications: The case of bacterial cellulose and fucoidan.

Conductive natural-based separators for application in polymer electrolyte fuel cells (PEFCs) were fabricated by combining a bacterial polysaccharide, i.e. bacterial cellulose (BC), and an algae sulphated polysaccharide, i.e. fucoidan (Fuc). The diffusion of fucoidan aqueous solution containing a natural-based cross-linker, viz. tannic acid, into the wet BC nanofibrous three-dimensional network, followed by thermal cross-linking, originated fully bio-based proton exchange membranes (PEMs). The PEMs present thermal-oxidative stability in the range of 180-200 °C and good dynamic mechanical performance (storage modulus ≥ 460 MPa). Additionally, the BC/Fuc membranes exhibit protonic conductivity that increases with increasing relative humidity (RH), which is a typical feature for numerous water-mediated proton conductors. The traditional Arrhenius-type plots demonstrate a linear behaviour with a maximum protonic conductivity of 1.6 mS cm-1 at 94 °C and 98 % RH. The results showed that these fully bio-based conductive membranes have potential as eco-friendly alternatives to other PEMs for application in PEFCs.

Carbon Nanocomposite Membrane Electrolytes for Direct Methanol Fuel Cells-A Concise Review.

A membrane electrolyte that restricts the methanol cross-over while retaining proton conductivity is essential for better electrochemical selectivity in direct methanol fuel cells (DMFCs). Extensive research carried out to explore numerous blends and composites for application as polymer electrolyte membranes (PEMs) revealed promising electrochemical selectivity in DMFCs of carbon nanomaterial-based polymer composites. The present review covers important literature on different carbon nanomaterial-based PEMs reported during the last decade. The review emphasises the proton conductivity and methanol permeability of nanocomposite membranes with carbon nanotubes, graphene oxide and fullerene as additives, assessing critically the impact of each type of filler on those properties.

High Surface Proton Conduction in Nanostructured ZIF-8.

The zeolitic imidazolate framework-8 (ZIF-8) combines a significantly high microporosity with an excellent thermal, chemical, and hydrothermal stability. Here, we demonstrated that ZIF-8 can display significant levels of protonic conductivity through a water-mediated surface transport mechanism associated to the presence of di-coordinated Zn ions revealed by X-ray photoelectron spectroscopy. A set of powders with particle sizes from 2.8 µm down to 80 nm studied by dynamic water vapour sorption analysis was used to demonstrate that water adsorbs predominantly in the micropore cavities of microcrystalline ZIF-8, whereas adsorption on the external surface becomes the dominant contribution for the nanostructured material. Impedance spectroscopy in turn revealed that the protonic conductivity of the nanocrystalline ZIF-8 was two orders of magnitude higher than that of the micron-sized powders, reaching approximately 0.5 mS·cm-1 at 94 °C and 98% relative humidity. Simple relations were derived in order to estimate the potential gains in water uptake and conductivity as a function of the particle size. This new strategy combining particle nanostructuring with surface defects, demonstrated here for one of the most know metal organic framework, is of general application to potentially boost the conductivity of other materials avoiding chemical functionalization strategies that in most if not all cases compromise their chemical stability, particularly under high humidity and high temperature conditions.

Antimicrobial and Conductive Nanocellulose-Based Films for Active and Intelligent Food Packaging.

Bacterial nanocellulose (BNC) is becoming an important substrate for engineering multifunctional nanomaterials with singular and tunable properties for application in several domains. Here, antimicrobial conductive nanocomposites composed of poly(sulfobetaine methacrylate) (PSBMA) and BNC were fabricated as freestanding films for application in food packaging. The nanocomposite films were prepared through the one-pot polymerization of sulfobetaine methacrylate (SBMA) inside the BNC nanofibrous network and in the presence of poly(ethylene glycol) diacrylate as cross-linking agent. The ensuing films are macroscopically homogeneous, more transparent than pristine BNC, and present thermal stability up to 265 °C in a nitrogen atmosphere. Furthermore, the films have good mechanical performance (Young's modulus ≥ 3.1 GPa), high water-uptake capacity (450-559%) and UV-blocking properties. The zwitterion film with 62 wt.% cross-linked PSBMA showed bactericidal activity against Staphylococcus aureus (4.3-log CFU mL-1 reduction) and Escherichia coli (1.1-log CFU mL-1 reduction), and proton conductivity ranging between 1.5 × 10-4 mS cm-1 (40 °C, 60% relative humidity (RH)) and 1.5 mS cm-1 (94 °C, 98% RH). Considering the current set of properties, PSBMA/BNC nanocomposites disclose potential as films for active food packaging, due to their UV-barrier properties, moisture scavenging ability, and antimicrobial activity towards pathogenic microorganisms responsible for food spoilage and foodborne illness; and also for intelligent food packaging, due to the proton motion relevant for protonic-conduction humidity sensors that monitor food humidity levels.

Robust Multifunctional Yttrium-Based Metal-Organic Frameworks with Breathing Effect.

Phosphonate- and yttrium-based metal-organic frameworks (MOFs), formulated as [Y(H5btp)]·5.5H2O (1), [Y(H5btp)]·2.5H2O (2), (H3O)[Y2(H5btp)(H4btp)]·H2O (3), and [Y(H5btp)]·H2O·0.5(MeOH) (4), were prepared using a "green" microwave-assisted synthesis methodology which promoted the self-assembly of the tetraphosphonic organic linker [1,1'-biphenyl]-3,3',5,5'-tetrayltetrakis(phosphonic acid) (H8btp) with Y3+ cations. This new family of functional materials, isolated in bulk quantities, exhibits a remarkable breathing effect. Structural flexibility was thoroughly studied by means of X-ray crystallography, thermogravimetry, variable-temperature X-ray diffraction, and dehydration and rehydration processes, ultimately evidencing a remarkable reversible single-crystal to single-crystal (SC-SC) transformation solely through the loss and gain of crystallization solvent molecules. Topologically, frameworks remained unaltered throughout this interconversion mechanism, with all compounds being binodal 6,6-connected network with a Schäfli symbol of {413.62}{48.66.8}. Results show that this is one of the most stable and thermally robust families of tetraphosphonate-based MOFs synthesized reported to date. Porous materials 2 and 3 were further studied to ascertain their performance as heterogeneous catalysts and proton conductors, respectively, with outstanding results being registered for both materials. Compound 2 showed a 94% conversion of benzaldehyde into (dimethoxymethyl)benzene after just 1 h of reaction, among the best results registered to date for MOF materials. On the other hand, the protonic conductivity of compound 3 at 98% of relative humidity (2.58 × 10-2 S cm-1) was among the highest registered among MOFs, with the great advantage of the material to be prepared using a simpler and sustainable synthesis methodology, as well as exhibiting a good stability at ambient conditions (temperature and humidity) over time when compared to others.

Nanostructured bacterial cellulose-poly(4-styrene sulfonic acid) composite membranes with high storage modulus and protonic conductivity.

The present study reports the development of a new generation of bio-based nanocomposite proton exchange membranes based on bacterial cellulose (BC) and poly(4-styrene sulfonic acid) (PSSA), produced by in situ free radical polymerization of sodium 4-styrenesulfonate using poly(ethylene glycol) diacrylate (PEGDA) as cross-linker, followed by conversion of the ensuing polymer into the acidic form. The BC nanofibrilar network endows the composite membranes with excellent mechanical properties at least up to 140 °C, a temperature where either pure PSSA or Nafion are soft, as shown by dynamic mechanical analysis. The large concentration of sulfonic acid groups in PSSA is responsible for the high ionic exchange capacity of the composite membranes, reaching 2.25 mmol g(-1) for a composite with 83 wt % PSSA/PEGDA. The through-plane protonic conductivity of the best membrane is in excess of 0.1 S cm(-1) at 94 °C and 98% relative humidity (RH), decreasing to 0.042 S cm(-1) at 60% RH. These values are comparable or even higher than those of ionomers such as Nafion or polyelectrolytes such as PSSA. This combination of electric and viscoelastic properties with low cost underlines the potential of these nanocomposites as a bio-based alternative to other polymer membranes for application in fuel cells, redox flow batteries, or other devices requiring functional proton conducting elements, such as sensors and actuators.

Strontium superstoichiometry and defect structure of SrCeO3 perovskite.

Strontium cerate (SrCeO(3)) is the parent phase of a family of prototype proton-conducting perovskites with important potential applications as electrolytes in protonic ceramic fuel cells, hydrogen-separation membranes, and sensors for hydrogen and humidity. Apparent nonstoichiometric behavior and the microstructure of SrCeO(3) have been investigated. Phase analysis by X-ray diffraction indicates that single-phase material in the system Sr(1+x)CeO(3+)delta is obtained for compositions x = 0.02-0.03 and that nominally stoichiometric SrCeO(3) (x = 0) synthesized by either solid-state reaction or the citrate method is Sr-rich. Selected area electron diffraction confirms that the system crystallizes with the GdFeO(3)-type orthorhombic perovskite structure (space group Pnma). Structural defects characterized by high-resolution transmission electron microscopy include twin domain boundaries and SrO-rich, Ruddlesden-Popper-type planar defects. Magnetic susceptibility measurements down to 2 K indicate that the Ce(3+) content is minor ( approximately 0.01 mol per formula unit for slow-cooled material) and does not influence the observed nonstoichiometry.