Density functional theory study of solid-state structure and optoelectronic properties of fluorene-based conjugated polymers

Using the dispersion corrected density functional theory (DFT-D/B97D) approach, we have performed molecular and solid-state calculations to study the influence of intra- and inter-molecular interactions on the bulk structure and the electronic and optical properties of fluorene-based conjugated polymers. In particular, we investigate the role of side-chain length on the molecular parking and optical properties of poly (9.9-di-n-alkylfluorene-alt-benzothiadiazole) or FnBT's where n is the number of CH₂ units in the alkyl side-chains. The results indicate that for the FnBT with longer side-chains, due to the significant inter-molecular interactions between the side-chains, the packing of these polymers forms a lamellar structure. On the other hand, for the FnBT with shorter side-chains, the cylindrical phase is more favorable and the corresponding crystals are almost hexagonal. These different packing structures can be attributed to the microphase separations between the flexible side-chains and the rigid backbones and are in agreement with previous investigations for other hairy-rod polymers [1, 2]. In addition, as a result of the efficient inter-chain interactions for the lamellar structures, the dihedral angle between the F and BT units is reduced by about 30ﾟ providing a more planar backbone which in turn leads to a decrease of about 0.2eV and 0.3 eV, in the band gap of the lamellar structure relative to its value for the gas and cylindrical phases respectively. Time-dependent DFT is also used to study the excited states of the monomer of FnBT with various lengths of side chains

Surface Configuration and Wettability of Nickel (Oxy)Hydroxides: A First-Principles Investigation

This article explores the wetting behavior of b-type nickel hydroxide, b-Ni(OH)2, and nickel oxyhydroxide, b-NiOOH, by means of first-principles calculations. Water is found to interact weakly with b-Ni(OH)2(001), but strongly with b-NiOOH(001). As unveiled with the use of ab initio molecular dynamics simulations, surface water layers at b-NiOOH(001) show a high degree of ordering correlated with a large surface polarization effect. In comparison, interfacial water at b-Ni(OH)2(001) exhibits enhanced disorder and higher mobility. The weak interaction of water with b-Ni(OH)2(001) is consistent with the small dipole moment of this surface. On the surface of b-NiOOH(001), in addition to the significantly increased surface dipole moment, unsaturated O atoms increase the number of hydrogen bonds between water molecules and the surface, resulting in strong water binding. The wettability trends found in this simulation study are consistent with experimental observations. Another theoretical observation is the increased work function of b-NiOOH(001) relative to b-Ni(OH)2(001) that agrees with experimental results reported in the literature

Platinum electrocatalysis: Novel insights into the dissolution mechanism and oxygen reduction reaction

Formation of hydrogen peroxide and oxygenated radical species are the leading cause of chemical degradation observed in polymer electrolyte membranes (PEM) in polymer electrolyte fuel cells. Recent experimental studies have shown that Pt nano-deposits in the PEM, which originate from Pt dissolution in the catalyst layer, play an important role in radical-initiated membrane degradation. Surface reactions at Pt particles facilitate the formation of reactive oxygen species. The net effect of Pt surface processes on membrane degradation depends on the local equilibrium conditions around the Pt nano-deposits, specifically, their equivalent local electrode potential. In this thesis, we first present a multi-step theoretical approach, validated by a collaborative experimental study, to understand the impact of environmental conditions around the Pt nanodeposits on membrane chemical degradation. In the first step, we developed a physical analytical model for the potential distribution at Pt nanodeposits in the PEM. Given the local potential, we identify the surface adsorption state of Pt. Thereafter, density functional theory (DFT) was used to investigate the influence of the Pt adsorption state on the mechanism of oxygen reduction reaction (ORR), particularly the formation of hydrogen peroxide and hydroxyl radical as the two important reactive oxygen species for membrane degradation. In a separate work, we employed DFT to study the atomistic mechanism for interfacial place-exchange between surface Pt atom and chemisorbed oxygen at oxidized Pt (111)-water interfaces. Understanding the criteria for Pt oxide growth is a crucial step to comprehend the mechanisms of Pt dissolution during electrochemical processes

Electronic Structure and Conformational Properties of Polybenzimidazole-Based Ionenes—A Density Functional Theory Investigation

Polybenzimidazole-based ionenes are explored for use in both alkaline anion-exchange membrane fuel cells and alkaline polymer electrolyzers. Poly-(hexamethyl-p-terphenylbenzimidazolium) (HMT-PMBI), the material of interest in this article, is exceptionally hydroxide-stable and water-insoluble. The impact of the degree of methylation on conformations and electronic structure properties of HMT-PMBI oligomers, from the monomer to the pentamer, is studied with density functional theory calculations. Optimization studies are presented for both the gas phase and in the presence of implicit water. In addition, time-dependent density functional theory is employed to generate the UV–vis absorption spectra of the studied systems. Results are insightful for experimentalists and theorists investigating the impact of synthetic and environmental conditions on the conformation and electronic properties of polybenzimidazole-based membranes