The possibility of increasing the working temperature of fuel cells using Nafion as ionomer membranes by a new annealing procedure in the presence of dimethylsulfoxide or tributylphosphate as annealing agents has been investigated. The extent of annealing was derived by the shift of nc/T plots of annealed membranes (nc is the index of counter‐elastic force) relatively to the plot of as received membranes. nc/T plots can be used as a powerful analytical method useful for deriving quantitative information on the annealing extent and for obtaining information on the melting temperature of the semi‐crystalline phase grown during annealing. It was found that, by using an annealing temperature of 140 °C in the presence of an annealing agent, an important effect on the thermal stability of Nafion membranes was obtained. The increased thermal stability was related to the increase of the original semi‐crystalline phase which acts as a physical cross‐linker so that the mechanical properties are no longer lost at the glass transition temperature but at the melting point of the semi‐crystalline phase. The use of nc/T plots as analytical tools for detecting and understanding important ionomer properties was denominated as “Ionomer nc Analysis” (INCA).

No abstract

No abstract

In order to have a better understanding of Nafion behavior in fuel cells operating at temperatures higher than 80 °C, the preparation of membranes containing a large amount of layered morphologies prevalently oriented in the direction parallel to the membrane surface (hence of low through-plane conductivity) was attempted. Successful in-plane oriented samples were obtained by forced swelling of membranes between rigid planar constraints. Other than the expected low through-plane conductivity, a first characterization of these modified membranes clearly showed that the dimension change during processes of dehydration and successive hydration takes place essentially in the direction perpendicular to the membrane surface. It was furthermore found that the forced swelling was accompanied by a strong reduction of ionomer density (from an initial value of 2 to about 1.4 g/cm 3 ). Finally, evident changes of the n c /T plots were also found. A discussion on the formation of these in-plane oriented layered morphologies is reported, giving emphasis to the fact that their formation in working fuel cells is particularly dangerous when they are prevalently oriented in the direction parallel to the membrane surface (large extent of through-plane conductivity decay). Some practical expedients for avoiding the formation of these dangerous in-plane "oriented layered morphologies" under the operative conditions of relative humidity and temperature are also reported. The inter-relations between spectroscopic investigations, recent stochastic simulation processes, and our experimental results are finally discussed.

No abstract

Microwave heating holds all the aces regarding development of effective and environmentally friendly methods to perform chemical transformations. Coupling the benefits of microwave-enhanced chemistry with highly reliable copper-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry paves the way for a rapid and efficient synthesis procedure to afford high performance thermoplastic materials. We describe herein fast and high yielding synthesis of 1,2,3-triazole-functionalized polysulfone through microwave-assisted CuAAC as well as explore their potential as phosphoric acid doped polymer electrolyte membranes (PEM) for high temperature PEM fuel cells. Polymers with various degrees of substitution of the side-chain functionality of 1,4-substituted 1,2,3-triazole with alkyl and aryl pendant structures are prepared by sequential chloromethylation, azidation, and microwave-assisted CuAAC using a range of alkynes (1-pentyne, 1-nonyne, and phenylacetylene). The completeness of reaction at each step and the purity of the clicked polymers were confirmed by (1)H-(13)C NMR, DOSY-NMR and FTIR-ATR spectroscopies. The thermal and thermochemical properties of the modified polymers were characterized by differential scanning calorimetry and thermogravimetric analysis coupled with mass spectroscopy (TG-MS), respectively. TG-MS analysis demonstrated that the commencement of the thermal degradation takes place with the decomposition of the triazole ring while its substituents have critical influence on the initiation temperature. Polysulfone functionalized with 4-phenyl-1,2,3-triazole demonstrates significantly higher Tg, Td, and elastic modulus than the ones bearing 4-propyl-1,2,3-triazole and 4-heptyl-1,2,3-triazole groups. After doping with phosphoric acid, the functionalized polymers with acid doping level of 5 show promising performance with high proton conductivity in anhydrous conditions (in the range of 27-35 mS/cm) and satisfactorily high elastic modulus (in the range of 332-349 MPa).

Herein we report the selectivity, stability, and electrochemical characterization of cobalt hexacyanoferrate, the Co-Fe Prussian Blue derivative (CoFePB), as a formate/formic acid oxidation electrocatalyst in aqueous media. CoFePB is able to quantitatively catalyze (100% Faradaic efficiency within less than 8% standard error at pH 5) the electrochemical oxidation of formate to CO2 over a pH range of 1-13. This quantitative formate elecrooxidation is possible due to the exclusive selectivity of the catalyst in a wide potential window (from c.a. 0.8 to 1.3 V vs. NHE at pH 7), where no other substrate in aqueous conditions is activated: neither other organic molecules, such as alcohols or acids, nor water itself. CoFePB is one of the first heterogeneous noble-metal-free catalysts reported for the electrooxidation of small hydrocarbon molecules. Importantly, the catalyst showed a very high tolerance against surface poisoning during the reaction, as supported by the cyclic voltammetry and electrochemical impedance spectroscopy data, thereby allowing CoFePB to operate at greater current density than state-of-the-art noble metal catalysts. For example, we observed that CoFePB is able to achieve a formate oxidation current ~10 mA cm -2 at pH 5, 0.4 M formate at 1.1 V vs NHE, whereas a Pt disk and Pd(5%)/C electrodes had a current of 0.4 and 1.4 mA cm -2 , respectively, under identical conditions. The remarkable selectivity, stability, and high current density of CoFePB, in contrast to state-of-the-art catalysts based on platinum-group metals (PGM), is an important step in the search for inexpensive earth-abundant materials for oxidation of organic molecules for use in liquid fuel cells or for selective organic molecule sensors. Furthermore, because CoFePB is not poisoned by intermediates and can achieve higher current density than Pt or Pd, improvement of the catalyst onset potential can lead to higher power density formate oxidation fuel cells using earth abundant metals than with Pt or Pd.

We investigated the possibility to increase the working temperature and endurance of proton exchange membranes for fuel cells and water electrolyzers by thermal annealing of short side chain perfluorosulfonic acid (SSC-PFSA) Aquivion® membranes. The Ionomer nc Analysis (INCA method), based on nc/T plots where nc is a counter elastic force index, was applied to SSC-PFSA in order to evaluate ionomer thermo-mechanical properties and to probe the increase of crystallinity during the annealing procedure. The enhanced thermal and mechanical stability of extruded Aquivion® 870 (equivalent weight, EW = 870 g·mol−1) was related to an increase of long-range order. Complementary differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) measurements confirmed the increase of polymer stiffness by the annealing treatment with an enhancement of the storage modulus over the whole range of temperature. The main thermomechanical relaxation temperature is also enhanced. DSC measurements showed slight base line changes after annealing, attributable to the glass transition and melting of a small amount of crystalline phase. The difference between the glass transition and melting temperatures derived from INCA plots and the ionic-cluster transition temperature derived from DMA measurements is consistent with the different experimental conditions, especially the dry atmosphere in DMA. Finally, the annealing procedure was also successfully applied for the first time to an un-crystallized cast membrane (EW = 830 g·mol−1) resulting in a remarkable mechanical and thermal stabilization.