RESUMO
This work describes an attempt to improve the physical and electrochemical parameters of PEM fuel cells that have electrodes modified by titanium and silicon dioxides. A customized design of membrane electrode assemblies was proposed which is characterized to have an around 6 times higher concentration of catalyst at the cathode side (2.0 mgPt cm-2) in order to investigate the influence of anode catalyst support treatment. Anode catalyst support materials were modified using pristine TiO2 and TiO2-SiO2-VTMS - the composite was crosslinked with the aid of vinyltrimethoxysilane. Surface area and porosity analysis was carried out with the aid of BET, BJH, t-plot and Horvath-Kawazoe (H-K) theories for particular components of the support materials and their catalyst mixtures. The experiment revealed a positive influence of TiO2-SiO2-VTMS (BET 321.9 m2 g-1, BJH 3.7 nm) on the anode catalyst layer in terms of surface area (3-times increase, 75 m2 g-1) and average pore size (decrease from 25.3 to 15.7 nm). Additionally, favourable microporosity (pores less than 2 nm) was introduced to the material according to the H-K analysis results (10.3 m2 g-1, 0.65 nm). Electrochemical experiments, which include polarization curves, electrochemical impedance spectroscopy and cyclic voltammetry, have demonstrated the change of behaviour for the fabricated fuel cells with modified anodes against the reference sample. The mitigation of charge and mass transfer resistance (by 15-20%, 50 mV at 200 mA cm-2), the improvement of power density (up to 72%, 217 mW cm-2) and a better exposure of the catalyst to the reactants of an electrochemical reaction were observed for fuel cells modified by both pristine TiO2 and the hybrid TiO2-SiO2-VTMS-based compound.
RESUMO
Theoretical studies of polyazomethines (PAZs) with various chemical structures designated for photovoltaic applications are presented. PAZ energy levels and optical properties were calculated within density-functional theory (DFT and TDDFT) framework for 28 oligomers (monomer, dimer and trimer) of PAZs. The correlations between chemical structure of PAZ and location of its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were examined. It turned out that the presence of triaminophenylene, dimethoxydiphenylene and fluorine group raises the orbital energies. As a consequence, it is a factor which improves the photovoltaic efficiency of solar cell built on the base of the corresponding PAZ and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). On the contrary, quinone, 1,3,5-triazine and perfluorophenylene groups lower orbital energies and have negative influence on the photovoltaic efficiency. Moreover, calculations for methyl, ethyl and butyl analogs of P3HT as well as polythiophenes were performed and compared with the results obtained for PAZs. In addition experimental data are presented, which cover optical, electrochemical and electrical transport properties of the studied PAZs, allowing to determine HOMO and LUMO energies of the polymers and their conductivity. Finally, comparison between calculated and experimental results were made and discussed.
