Structural and Charge Transport Properties of Composites of Phosphate-Silicate Protonic Glass with Uranyl Hydroxy-Phosphate and Hydroxy-Arsenate Obtained by Mechano-Chemical Synthesis Undergoing Hydration Changes

The introduction of the hydrogen economy, despite its obvious technological problems, creates a need for a significant number of niche-focused solutions, such as small-sized (10-100 W) fuel cells able to run on hydrogen of lesser purity than what is considered a standard in the case of PEMFCs. One of the solutions can be derived from the fact that an increase in the operational temperature of a cell significantly decreases its susceptibility to catalyst poisoning. Electrolytes suitable for the so-called medium temperature operational range of 120-400 °C, hence developed, are neither commercialized nor standardized. Among them, phosphate silicate protonically conductive glasses were found not only to reveal interestingly high levels of operational parameters, but also, to exhibit superior chemical and electrochemical stability over their polymeric counterparts. On the other hand, their mechanical properties, including cracking fragility, still need elaboration. Initial studies of the composite phosphate silicate glasses with uranyl-based protonic conductors, presented here, proved their value both in terms of application in fuel cell systems, and in terms of understanding the mechanism governing the charge transport mechanism in these and similar systems. It was found that whereas systems containing 10-20 wt% of the crystalline additive suffer from significant instability, materials containing 45-80 wt% (with an optimum at 60%) should be examined more thoughtfully. Moreover, the uranyl hydrogen phosphate was found to surpass its arsenate counterpart as an interesting self-healing behavior of the phase structure of the derived composite was proved.

Properties and Applications of Metal Phosphates and Pyrophosphates as Proton Conductors

We review the progress in metal phosphate structural chemistry focused on proton conductivity properties and applications. Attention is paid to structure-property relationships, which ultimately determine the potential use of metal phosphates and derivatives in devices relying on proton conduction. The origin of their conducting properties, including both intrinsic and extrinsic conductivity, is rationalized in terms of distinctive structural features and the presence of specific proton carriers or the factors involved in the formation of extended hydrogen-bond networks. To make the exposition of this large class of proton conductor materials more comprehensive, we group/combine metal phosphates by their metal oxidation state, starting with metal (IV) phosphates and pyrophosphates, considering historical rationales and taking into account the accumulated body of knowledge of these compounds. We highlight the main characteristics of super protonic CsH2PO4, its applicability, as well as the affordance of its composite derivatives. We finish by discussing relevant structure-conducting property correlations for divalent and trivalent metal phosphates. Overall, emphasis is placed on materials exhibiting outstanding properties for applications as electrolyte components or single electrolytes in Polymer Electrolyte Membrane Fuel Cells and Intermediate Temperature Fuel Cells.

Ionic Transport Properties of P2O5-SiO2 Glassy Protonic Composites Doped with Polymer and Inorganic Titanium-based Fillers

This paper is focused on the determination of the physicochemical properties of a composite inorganic–organic modified membrane. The electrical conductivity of a family of glassy protonic electrolytes defined by the general formula (P₂O₅)_x(SiO₂)_y, where x/y is 3/7 are studied by Alternating Current electrochemical impedance spectroscopy (AC EIS) method. The reference glass was doped with polymeric additives—poly(ethylene oxide) (PEO) and poly(vinyl alcohol) (PVA), and additionally with a titanium-oxide-based filler. Special attention was paid to determination of the transport properties of the materials thus modified in relation to the charge transfer phenomena occurring within them. The electrical conductivities of the ‘dry’ material ranged from 10⁻⁴ to 10⁻⁹ S/cm, whereas for ‘wet’ samples the values were ~10⁻³ S/cm. The additives also modified the pore space of the samples. The pore distribution and specific surface of the modified glassy systems exhibited variation with changes in electrolyte chemical composition. The mechanical properties of the samples were also examined. The Young’s modulus and Poisson’s ratio were determined by the continuous wave technique (CWT). Based on analysis of the dispersion of the dielectric losses, it was found that the composite samples exhibit mixed-type proton mobility with contributions related to both the bulk of the material and the surface of the pore space.

The proton conductivity and mechanical properties of Nafion®/ ZrP nanocomposite membrane

Zirconium phosphates (ZrP) were incorporated into Nafion® 117 membrane by impregnating method to obtain a reduced methanol permeation and improved proton conductivity for fuel cell application. The mechanical properties and water uptake of Nafion® membrane incorporated with zirconium phosphates nanoparticles was more improvement when compared to the commercial Nafion® 117, due to the presence of phosphoric acid within the nanoparticles. The effect of ZrP nano filler on the membrane structural morphology and thermal properties were investigated by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Thermal gravimetric analysis (TGA) and Scanning Electron Microscopy (SEM). The improved ion conductivity and decreased methanol permeability on the nanocomposite membranes showed a great potential for fuel cell applications. The nanocomposite membrane with high tensile strength was obtained due to the well dispersed zirconium phosphates within the Nafion® matrix.

Composite Membranes for High Temperature PEM Fuel Cells and Electrolysers: A Critical Review

Polymer electrolyte membrane (PEM) fuel cells and electrolysers offer efficient use and production of hydrogen for emission-free transport and sustainable energy systems. Perfluorosulfonic acid (PFSA) membranes like Nafion® and Aquivion® are the state-of-the-art PEMs, but there is a need to increase the operating temperature to improve mass transport, avoid catalyst poisoning and electrode flooding, increase efficiency, and reduce the cost and complexity of the system. However, PSFAs-based membranes exhibit lower mechanical and chemical stability, as well as proton conductivity at lower relative humidities and temperatures above 80 °C. One approach to sustain performance is to introduce inorganic fillers and improve water retention due to their hydrophilicity. Alternatively, polymers where protons are not conducted as hydrated H3O+ ions through liquid-like water channels as in the PSFAs, but as free protons (H+) via Brønsted acid sites on the polymer backbone, can be developed. Polybenzimidazole (PBI) and sulfonated polyetheretherketone (SPEEK) are such materials, but need considerable acid doping. Different composites are being investigated to solve some of the accompanying problems and reach sufficient conductivities. Herein, we critically discuss a few representative investigations of composite PEMs and evaluate their significance. Moreover, we present advances in introducing electronic conductivity in the polymer binder in the catalyst layers.

A Review on Sulfonated Polymer Composite/Organic-Inorganic Hybrid Membranes to Address Methanol Barrier Issue for Methanol Fuel Cells

This paper focuses on a literature analysis and review of sulfonated polymer (s-Poly) composites, sulfonated organic, inorganic, and organic-inorganic hybrid membranes for polymer electrolyte membrane fuel cell (PEM) systems, particularly for methanol fuel cell applications. In this review, we focused mainly on the detailed analysis of the distinct segment of s-Poly composites/organic-inorganic hybrid membranes, the relationship between composite/organic- inorganic materials, structure, and performance. The ion exchange membrane, their size distribution and interfacial adhesion between the s-Poly composites, nanofillers, and functionalized nanofillers are also discussed. The paper emphasizes the enhancement of the s-Poly composites/organic-inorganic hybrid membrane properties such as low electronic conductivity, high proton conductivity, high mechanical properties, thermal stability, and water uptake are evaluated and compared with commercially available Nafion^® membrane.

Tetrazole-based, anhydrous proton exchange membranes for fuel cells

A tetrazole-based polymer electrolyte membrane showed high conductivity at 20-120°C under dry conditions, offering the potential to dramatically simplify fuel cells for many applications over a wide temperature range without the need for cumbersome humidification and pressurization.

NMR and Electrochemical Investigation of the Transport Properties of Methanol and Water in Nafion and Clay-Nanocomposites Membranes for DMFCs

Water and methanol transport behavior, solvents adsorption and electrochemical properties of filler-free Nafion and nanocomposites based on two smectite clays, were investigated using impedance spectroscopy, DMFC tests and NMR methods, including spin-lattice relaxation and pulsed-gradient spin-echo (PGSE) diffusion under variable temperature conditions. Synthetic (Laponite) and natural (Swy-2) smectite clays, with different structural and physical parameters, were incorporated into the Nafion for the creation of exfoliated nanocomposites. Transport mechanism of water and methanol appears to be influenced from the dimensions of the dispersed platelike silicate layers as well as from their cation exchange capacity (CEC). The details of the NMR results and the effect of the methanol solution concentration are discussed. Clays particles, and in particular Swy-2, demonstrate to be a potential physical barrier for methanol cross-over, reducing the methanol diffusion with an evident blocking effect yet nevertheless ensuring a high water mobility up to 130 °C and for several hours, proving the exceptional water retention property of these materials and their possible use in the DMFCs applications. Electrochemical behavior is investigated by cell resistance and polarization measurements. From these analyses it is derived that the addition of clay materials to recast Nafion decreases the ohmic losses at high temperatures extending in this way the operating range of a direct methanol fuel cell.

Nafion-zirconium phosphate composite membrane: a new approach to prepare (32)P patches for superficial brachytherapy applications

This article describes a method for the preparation of (32)P patch for the treatment of skin cancer. It is based on the surface modification of a Nafion film by treatment with ZrOCl(2) solution, impregnation of a predicted quantity of (32)P into the film, and its subsequent immobilization into a nonleachable matrix by lamination. The effect of variations of critical parameters on the incorporation of (32)P into the membrane, such as solution pH, contact time, reaction volume, inactive carrier concentration of the feed, reaction temperature, and so on, was investigated to arrive at the conditions resulting in optimum retention of (32)P activity. The morphology of the membrane was evaluated by scanning electron microscope and energy dispersive spectral analyses. Quality control tests were carried out to ensure nonleachability, uniform distribution of activity, and stability of the patches.

SPEEK–zirconium hydrogen phosphate composite membranes with low methanol permeability prepared by electro-migration and in situ precipitation

Sulfonated poly(ether ether ketone) (SPK)-zirconium hydrogen phosphate (ZrP) composite membranes were prepared by electro-driven migration of Zr(4+) and simultaneous in situ precipitation of ZrP using phosphoric acid under different electrical gradient, in order to avoid loss in its mechanical stability. Degree of sulfonation was estimated from (1)H NMR and ion-exchange capacity study that was found to be 61% and 57%, respectively. In this method Zr(4+) and HPO(4)(2-) were allowed to diffuse within the pores/channels of the preformed SPK membrane under given electrical gradient and ZrP was precipitated within the membrane matrix. ZrP loading density was measured as a function of applied electrical gradient for a definite reaction time (4 h) and electrolytic environment. Membranes with varied ZrP loading densities were characterized for their thermal and mechanical stabilities, physicochemical and electrochemical properties using thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), water content, proton conductivity and methanol permeability. No loss in thermal and mechanical stability of membranes was observed due to incorporation of inorganic component (ZrP) in the membrane matrix. Although the composite membranes exhibited low proton conductivity in comparison to SPK membrane at room temperature, but the presence of the inorganic particles led to an improvement in high temperature conductivity. Selectivity parameter of these composite membranes was estimated at two temperatures namely 30 and 70 degrees C, in latter case it was found significantly higher than that for Nafion membrane (0.79 x 10(5) S s cm(-3)) under similar experimental conditions.