Kompleks Hidrit Nanoparçacıkların Sentezi ve Hidrojen Kinematiğinin Araştırılması
Özet
Today, also in conjunction with the advancements in technology, demand for energy is rapidly increasing. Fossil fuels are the most commonly used energy sources. However, we are running out of fossil fuel sources and fossil fuels have hazardous effects on nature, as well. Therefore, clean and renewable energy sources must be discovered. Hydrogen is a possible candidate that would fulfill our requirements in terms of energy. However, hydrogen must be stored to make it possible to use as an energy source. Storing hydrogen in solid state is the best way to store it in a safe way and with high capacity. Hydrogen can be stored in solid state as hydride structures. Hydride is a chemical compound, formed by hydrogen that bind chemically to the atoms of storage material. Solid state hdrogen storage materials must have a capacity of wt 5.5% and they must work at temperatures less than 85 oC (USDOE, 2009). There is a wide range of materials that scientists research for solid state hydrogen storage. However, none of these materials have displayed a satisfying performance up to now. So it?s still not possible to use hydrogen as an energy source for daily appllications. LiNH2/MgH2 mixture is an attractive material in the last years with it?s better storage performance compared to other materials. Researches show that there are materials with storage capacity more than wt 5.5%, but these materials must be heated up to temperatures more than 300 oC in order to desorb all of the stored hydroge. On the other hand, there are hydrides that desorb hydrogen at room temperatures, but their storage capacities are less than wt 5,5%. So, LiNH2/MgH2 mixture is a favorable material with it?s wt 5.6% storage capacity and hydrogen desorpion temperature about 200 oC. In this study, different catalysts are added to the LiNH2/MgH2 mixture and the effects of these catalysts on the hydrogen storage properties of the samples are investigated. Samples were synthesized via ball milling technique. Structural analysis of the samples were made by utilizing x-ray powder diffraction (XRD) and Fourier transform infrared (FTIR) methods. For thermodynamic analysis, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) methods were used. Kinetic analysis were conducted with a volumetric Sievert?s type apparatus. In the first part of the study, LiNH2/MgH2 mixture is milled with different milling parameters and according to the FTIR measurements of the milled samples, ideal milling parameters were determined. Then LiNH2/MgH2 were mixed with a stoichiometric ratio of 2/1.1 and wt 5% Ca(BH4)2 was added to mixture. This mixture was milled with ideal milling parameters. Beside this, the same mixture was milled with same parameters without adding any catalyst. When we compare the samples with and without catalyst, we have observed that the sample milled with catalyst desorbs hydrogen at a temperature about 16 oC lower than the sample without catalyst. Kinetic measurements were also performed for the samples. Measurements for hydrogen desorption were conducted at 180 oC under 1 bar hydrogen pressure and for hydrogen absorption, measurements were conducted at 150 oC and under 30 bar hydrogen pressure. After kinetic measurements, it was observed that, under same conditions, sample doped with catalyst manifest faster both hydrogen absorption and desorption than the undoped sample. Doped sample desorbed 90% of its total desorbed hydrogen in 45 minutes. However, undoped sample desorbed only 65% of its total desorbed hydrogen in 45 minutes. Regarding absorption, doped sample absorbed 90% of its total absorbed hydrogen in 4.3 hours. On the other hand, undoped sample absorbed only 50% of it?s total absorbed amount of hydrogen in 4.3 hours. After kinetic measurements, it was determined that doped sample desorbed wt 3.45% hydrogen at 180 oC under 1 bar pressure. Undoped sample desorbed only wt 3.10% hydrogen under same conditions. Doped sample absorbed wt 2.72% hydrogen at 150 oC, under 30 bar hydrogen pressure. Undoped sample absorbed only wt 1% hydrogen under same conditions. Theoretical hydrogen storage capacity of the LiNH2/MgH2 mixture is wt 5.6%. However both doped and undoped samples couldn?t absorb or desorb hydrogen in this amount. Moreover, it was observed that doped sample could absorb and desorb hydrogen with higher capacities than undoped sample. In addition, after recycling both hydrogen absorption and desorption capacity of the doped sample increased. On the other hand, sorption kinetics of the doped sample slowed down after recycling. During this study, CaH2 was also added to LiNH2/MgH2 mixture as another catalyst, but this catalyst didn?t drastically effect the performance of the samples. Structural analysis of the samples after milling, desorption and absorption of hydrogen were made by XRD and FTIR measurements. According to these measurements, it was determined that the phases in the mixture were consistent with those provided in the litrature.
