Computational Approaches to Alkaline Anion-Exchange Membranes for Fuel Cell Applications.

Anion-exchange membranes (AEMs) are key components in relatively novel technologies such as alkaline exchange-based membrane fuel cells and AEM-based water electrolyzers. The application of AEMs in these processes is made possible in an alkaline environment, where hydroxide ions (OH-) play the role of charge carriers in the presence of an electrocatalyst and an AEM acts as an electrical insulator blocking the transport of electrons, thereby preventing circuit break. Thus, a good AEM would allow the selective transport of OH- while preventing fuel (e.g., hydrogen, alcohol) crossover. These issues are the subjects of in-depth studies of AEMs-both experimental and theoretical studies-with particular emphasis on the ionic conductivity, ion exchange capacity, fuel crossover, durability, stability, and cell performance properties of AEMs. In this review article, the computational approaches used to investigate the properties of AEMs are discussed. The different modeling length scales are microscopic, mesoscopic, and macroscopic. The microscopic scale entails the ab initio and quantum mechanical modeling of alkaline AEMs. The mesoscopic scale entails using molecular dynamics simulations and other techniques to assess the alkaline electrolyte diffusion in AEMs, OH- transport and chemical degradation in AEMs, ion exchange capacity of an AEM, as well as morphological microstructures. This review shows that computational approaches can be used to investigate different properties of AEMs and sheds light on how the different computational domains can be deployed to investigate AEM properties.

Thermal Expansion of 3C-SiC Obtained from In-Situ X-ray Diffraction at High Temperature and First-Principal Calculations.

In situ X-ray crystallography powder diffraction studies on beta silicon carbide (3C-SiC) in the temperature range 25-800 °C at the maximum peak (111) are reported. At 25 °C, it was found that the lattice parameter is 4.596 Å, and coefficient thermal expansion (CTE) is 2.4 ×10-6/°C. The coefficient of thermal expansion along a-direction was established to follow a second order polynomial relationship with temperature (α11=-1.423×10-12T2+4.973×10-9T+2.269×10-6). CASTEP codes were utilized to calculate the phonon frequency of 3C-SiC at various pressures using density function theory. Using the Gruneisen formalism, the computational coefficient of thermal expansion was found to be 2.2 ×10-6/°C. The novelty of this work lies in the adoption of two-step thermal expansion determination for 3C-SiC using both experimental and computational techniques.

Correction: Removal of fluoride ions using a polypyrrole magnetic nanocomposite influenced by a rotating magnetic field.

[This corrects the article DOI: 10.1039/C9RA07379E.].

Removal of fluoride ions using a polypyrrole magnetic nanocomposite influenced by a rotating magnetic field.

The impact of a varying rotating magnetic field in stimulating adsorption of fluoride ions onto a polypyrrole magnetic nanocomposite synthesized via in situ a polymerization process was evaluated. Under the effect of a rotating magnetic field, improved removal of adsorbate (10 mg L-1) from aqueous solution using the polypyrrole magnetic nanocomposite was observed, with a maximum removal of 78.2% observed at a magnetic field intensity of 0.019 T. Particle aggregation resulting from the force owing to the gradient on the particles as the magnetic field was increased resulted in improved fluoride removal. This aggregation of particles leads to an improved chain collision and expanse of particle interaction with the fluoride solution. The process of adsorption of fluoride by the PPy/Fe3O4 nanocomposite followed both the Freudlich isotherm and the Temkin isotherm. Interestingly, under the effect of the rotating magnetic field, the adsorption process was best described by the Freundlich isotherm.

Influence of transition metal doping on the electronic and optical properties of ReS2 and ReSe2 monolayers.

We investigate the structural, electronic and optical properties of transition metal doped triclinic monolayered rhenium disulfide and diselenide (ReS2 and ReSe2) by means of quantum mechanical calculations. The calculated electronic band gaps for ReS2 and ReSe2 monolayers are 1.43 eV and 1.23 eV, respectively, with both having a non-magnetic ground state. The calculated dopant substitutional energies under both Re-rich and X(S or Se)-rich conditions show that it is possible to experimentally synthesize transition metal doped ReX2 (where X is S or Se) monolayer systems. We found that the presence of dopant ions (such as V, Cr, Mn, Fe Co, Nb, Mo, Ta and W) in the ReS2 and ReSe2 monolayers significantly modifies their electronic ground states with consequent introduction of defect levels and modification of the density of states profile. However, it was found that Mn doped structures show a very minute reduction of the electronic band gap. We found that a ferro- or a non-magnetic ground state configuration was obtained depending on the choice of dopant ions in ReS2 and ReSe2 monolayers. Cr, Fe and Co doping result in a ferro-magnetic ground state configuration of the ReX2 structures. The calculated absorption and reflectivity spectra show that this class of dopants causes a general increase in the absorption spectral peaks but only a minute influence on the reflectivity. Optical anisotropy was observed depending on whether the direction of polarization in the xy-plane is either parallel or perpendicular.