We report a thorough, multitechnique investigation of the structure and transport properties of a UV-cross-linked polymer electrolyte based on poly(ethylene oxide), tetra(ethylene glycol)dimethyl ether (G4), and lithium bis(trifluoromethane)sulfonimide. The properties of the cross-linked polymer electrolyte are compared to those of a non-cross-linked sample of same composition. The effect of UV-induced cross-linking on the physico/chemical characteristics is evaluated by X-ray diffraction, differential scanning calorimetry, shear rheology, 1H and 7Li magic angle spinning nuclear magnetic resonance (NMR) spectroscopy, 19F and 7Li pulsed field gradient stimulated echo NMR analyses, electrochemical impedance spectroscopy, and Fourier transform Raman spectroscopy. Comprehensive analysis confirms that UV-induced cross-linking is an effective technique to suppress the crystallinity of the polymer matrix and reduce ion aggregation, yielding improved Li+ transport number (>0.5) and ionic conductivity (>0.1 mS cm–1) at ambient temperature, by tailoring the structural/morphological characteristics of the polymer matrix. Finally, the polymer electrolyte allows reversible operation with stable profile for hundreds of cycles upon galvanostatic test at ambient temperature of LiFePO4-based lithium-metal cells, which deliver full capacity at 0.05 or 0.1C current rate and keep high rate capabilities up to 1C. This enforces the role of UV-induced cross-linking in achieving excellent electrochemical characteristics, exploiting a practical, easy up-scalable process.
Recent studies suggest that operating anion exchange membrane (AEM) fuel cells at high temperatures has enormous technological potential. However, obtaining a fundamental understanding of the effect of temperature on hydroxide conductivity and membrane stability remains a key hurdle to realizing the full potential of high-temperature AEM fuel cells. In this work, we present a combined theoretical and experimental study to explore the effect of temperature on hydroxide ion and water diffusivities in AEMs. Both fully atomistic ab initio molecular dynamics simulations and 1H pulsed field gradient NMR measurements confirm that the OH– diffusion changes non-monotonically with increasing temperature. Specifically, the D OH– versus T curve exhibits a region in which dD OH– /dT < 0, indicating the presence of a kink in the curve, which we refer to as a “diffusion kink”. The simulations show that the underlying causes of this behavior vary with the hydration level. Furthermore, we were able to rationalize the conditions underlying this counterintuitive behavior and to suggest ways to identify the optimal operating temperature for each model AEM system. We expect that the discovery of this unusual temperature dependence of the diffusivity will play an important role in the design of new, stable, and highly conductive AEM-based devices such as electrolyzers, redox flow batteries, and fuel cells.
Graphene oxide (GO) is well known as an excellent amphiphilic material due to its oxygen-containing functional groups and its chemical tunability. By intercalation chemistry, organo-modified GO containing sulfonilic terminal groups were prepared and used as nanoadditive in Nafion polymer for the creation of hybrid exfoliated composites. The incorporation of hydrophilic 2D platelike layers in the Nafion membranes is expected to induce advantages in terms of thermal stability and mechanical and barrier properties (limitation of the methanol crossover by increased tortuosity and obstruction effect), although it may negatively affect the proton conductivity. In this work, we show how different preparation methods of the nanocomposites influence morphology, transport properties, and barrier effect to methanol. The hybrid membranes are characterized by powder X-ray diffraction and microscopies (SEM, TEM, and AFM). Water and methanol transport properties inside the nanocomposites are investigated by NMR spectroscopy (diffusivity and relaxation times), unveiling a reduction of the methanol diffusion and, nevertheless, an increase in the proton mobility and water retention at high temperatures. Finally, the electrochemical properties are investigated by direct methanol fuel cell (DMFC) tests, showing a significant reduction of the ohmic losses at high temperatures, extending in this way the operating range of a DMFC.
Autonomic self-healing (SH), namely, the ability to repair damages from mechanical stress spontaneously, is polarizing attention in the field of new-generation electrochemical devices. This property is highly attractive to enhance the durability of rechargeable Li-ion batteries (LIBs) or Na-ion batteries (SIBs), where high-performing anode active materials (silicon, phosphorus, etc. ) are strongly affected by volume expansion and phase changes upon ion insertion. Here, we applied a SH strategy, based on the dynamic quadruple hydrogen bonding, to nanosized black phosphorus (BP) anodes for Na-ion cells. The goal is to overcome drastic capacity decay and short lifetime, resulting from mechanical damages induced by the volumetric expansion/contraction upon sodiation/desodiation. Specifically, we developed novel ureidopyrimidinone (UPy)-telechelic systems and related blends with poly(ethylene oxide) as novel and green binders alternative to the more conventional ones, such as polyacrylic acid and carboxymethylcellulose, which are typically used in SIBs. BP anodes show impressively improved (more than 6 times) capacity retention when employing the new SH polymeric blend. In particular, the SH electrode still works at a current density higher than 3.5 A g –1 , whereas the standard BP electrode exhibits very poor performances already at current densities lower than 0.5 A g –1 . This is the result of better adhesion, buffering properties, and spontaneous damage reparation.
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