GENERATION OF HYDROGEN FROM METAL HYDRIDE BY THERMOLYSIS USING VARIOUS CATALYSTS

Abstract

The depletion of fossil fuels demands a transition in the fuel economy towards hydrogen. Hydrogen, being the least heavy element, has the most significant ratio of energy to weight, about three times more than the energy content of gasoline and diesel. It reduces dependency on fossil fuels and is recognized as a clean and efficient energy transporter. The reason for this is that the combustion of hydrogen to generate electricity results in the production of just water as a by-product. When hydrogen is burned in an internal combustion engine as a fuel for vehicles, it leads to the generation of NOx and a decrease in power output. This is mainly due to the low volumetric density of hydrogen. Conversely, when transformed into electrical energy by utilizing a fuel cell, the efficiency experiences a 30% enhancement. Hydrogen possesses a significant amount of energy per unit volume. However, challenges arise when it comes to storing hydrogen in a condensed manner, generating it, and using it in a regulated manner. It enables electricity generation while minimizing the release of CO2 into the environment. The goal of hydrogen fuel as an energy carrier is to optimize its storage and transmission. Hydrogen has been stored in compressed cylinders or as a liquid in cryogenic containers. However, its low volumetric density presents a safety concern. Moreover, compressing hydrogen gas from 5,000 psi to the necessary pressure of 10,000 psi for liquefaction is quite difficult. Hydrogen can be kept as metal hydrides or as a cryogenic liquid. In addition, storing liquid hydrogen at extremely low temperatures for automobile use results in substantial losses, with 10-25% of the fuel evaporating during refueling. These factors result in significant initial investment costs, a limited amount of energy per unit of volume, heavy storage tank weights, and high pressures within the storage vessels. The production of hydrogen on board appears to offer a viable storage method. Metal hydride is the main key component for the on-board hydrogen generators. Metal hydrides are hydrogen-absorbing metallic alloys. Due to their capacity to absorb and release hydrogen, these alloys can be a storage mechanism. The liberation of hydrogen is directly correlated with the temperature of the hydride. Metal hydrides generally have a hydrogen storage capacity of approximately 1% to 2% of their total weight. Applying active heating to eliminate the hydrogen can cause it to rise to 5% to 7% of the weight of the hydride.Using a metal hydride tank allows for repeated hydrogen storage and release. However, the accumulation of impurities within the tanks constrains their hydrogen storage capacity. The presence of impurities obstructs the areas designated for hydrogen storage, diminishing the tank's capacity. The heat transfer, hydrogen absorption, and hydrogen desorption in a reversible metal hydride operating at room temperature and atmospheric pressure are primarily determined by the metal alloy used for hydrogen storage. The main obstacles to effectively using metal hydride storage systems are twofold: firstly, determining if there is enough heat produced to extract hydrogen from the hydride, and secondly, addressing the delay between the initial heating and the release of the hydrogen gas. When considering hydrogen extraction through heat generation, it is necessary to have a heat generation and transfer system on board, which can be achieved through either an internal combustion engine or a fuel cell using thermal components. Utilizing a metal hydride storage system for hydrogen fuel imposes external design complexities and energy penalties on the general system, which is not typical of other hydrogen storage options. Heavy metal hydrides, such as those made from vanadium, niobium, and iron-titanium, release hydrogen at ordinary temperatures, reducing the time delay caused by metal hydrides. Regrettably, this also results in a higher system weight due to the inherent physical properties of these dense metals. Lighter hydrides do not incur the same weight disadvantages as heavy-metal hydrides; however, they require external heating until the temperature is high enough to release hydrogen. Metal hydride storage materials can store hydrogen at high densities, surpassing the densities achievable with compressed gas hydrogen storage and liquid hydrogen. The density of hydrogen within the material plays a crucial role in achieving this high-density storage. Nevertheless, the impact of tank weight on the specific application's system of interest is not considered. To improve future transportation applications, the utilization of lighter metal hydrides can significantly decrease this discrepancy. This thesis aims to find suitable additives/catalysts for hydrogen generation from NaBH4/NaH at low temperatures using different material preparation techniques and the effect of system pressure. The additives/catalyst was added, and the composite materials were synthesized by various methods such as facile solution (FS), wet ball milling (WBM), and dry ball milling (DBM) techniques to achieve the high hydrogen generation from a composite material (catalyst/Metal hydride). The prepared composite materials are characterized by X-ray diffraction (XRD), Raman Spectroscopy, Fourier Transform Infrared spectroscopy (FTIR), Particle Size Distribution (PSD), FE-SEM, and Thermal gravimetric analysis (TGA). Using different material preparation techniques, the focus is to generate the highest (maximum) hydrogen from the metal hydride at a low temperature. The thermal decomposition of the composite materials was monitored using the flow reactor (HVC-DRM-5 and CCR-1000), considering the in-situ DRIFTS and in-situ Raman spectroscopy studies at low temperatures. A high-pressure (batch) reactor was also used to monitor the catalytic hydrogen generation from the composite materials by varying the system pressure from 1 bar to 30 bar to understand the effect of system pressure on metal hydride while it was in a storage device. A series of composite materials, xCaCl2/NaBH4, were synthesized by using FS, WBM, and DBM methods. Also, a series of other additives (MgCl2, MnCl2, FeCl2, NiCl2, ZnCl2) were used to prepare the composite materials. The most suitable materials were studied for the in-situ FTIR and in-situ Raman spectroscopy analysis during the material decomposition and hydrogen generation. The research indicated that the synthesized material CaCl2/NaBH4 prepared by using CaCl2 as an additive generated a higher amount of hydrogen than the other additives. The order of hydrogen generation at low temperatures (100 °C) was CaCl2/NaBH4 > MgCl2/ NaBH4 > MnCl2/ NaBH4 >FeCl2/ NaBH4 ≈ NiCl2/NaBH4 ≈ ZnCl2/NaBH4 > Pure-NaBH4. CaCl2 was considered a suitable additive, and the WBM method was the most effective method for synthesizing composite material CaCl2/NaBH4. The optimum loading of additive/catalyst was found to be 30 wt%, 50 wt%, and 50 wt% of CaCl2 for hydrogen generation from sodium-borohydride for the FS, WBM, and DBM methods, respectively. Adding additives, CaCl2 decreased the activation energy for the thermolysis of NaBH4 at 100 °C. This resulted in increased generation of hydrogen at 100 °C. The generation of hydrogen was ~3.68 wt% with 50CaCl2/NaBH4 (WBM) > ~3.63 wt% 50CaCl2/NaBH4 (DBM) > ~1.74 wt% with 30CaCl2/NaBH4(FS) at 100 °C. The time on stream in situ FTIR and Raman study suggested that the rapid decomposition of the material was observed in the first 15 min of decomposition. Then it decomposed very slowly at 100 °C. Moreover, the study also suggested that the decomposition was incomplete even after 60 min of the run. A higher amount of hydrogen generation was observed using the reactor (CCR-1000) compared to the other reactor (HVC-DRM-5); it may be due to the variation of decomposition temperature in both the sample holders. Furthermore, the particle size distribution studies suggested that more than 90% of the particles (d90) were in the size range of 18.20 μm (30CaCl2/NaBH4:FS), 4.45 μm (50CaCl2/NaBH4:WBM) and 10.20 μm (50CaCl2/NaBH4:DBM). The smaller particle size in the case of WBM indicated a better particle size distribution. This resulted in smaller additives/catalyst particles forming in the composite materials and, thus, more solid-solid interaction during the thermolysis reaction. The FE-SEM results were consistent with the PSD results. The TGA analysis indicated that the order of percent weight loss and the decomposition of the composite materials were reported as Pure NaBH4 (MG) < 30CaCl2/NaBH4 (FS) < 50CaCl2/NaBH4 (DBM) < 50CaCl2/NaBH4 (WBM) and the activation energy for the decomposition of prepared material was also reduced. The activation energy of the synthesized composite materials was followed as Pure NaBH4 (MG):267 kJ/mol > 30CaCl2/NaBH4 (FS):209 kJ/mol > 50CaCl2/NaBH4 (DBM):172 kJ/mol > 50CaCl2/NaBH4 (WBM):155 kJ/mol respectively. Due to the reduction in activation energy, there is a high amount of hydrogen generation at low temperatures. In addition, the composite materials (xCaCl2/NaBH4) were synthesized using the facile solution method. The addition of CaCl2 proved advantageous in lowering the decomposition temperature of NaBH4. The generation of hydrogen at 100 °C and 1 bar pressure increased from ~0.11 wt% to ~1.41 wt% after adding 30 wt% of CaCl2. The percentage of hydrogen generation was followed as 30CaCl2/NaBH4 ˃ 20CaCl2/NaBH4 ˃ 10CaCl2/NaBH4 ˃ 40CaCl2/NaBH4 ˃ 50CaCl2/NaBH4 ˃ Pure NaBH4 at 100 °C and 1 bar pressure. The percentage of hydrogen generation decreased with increasing the system pressure from 1 bar to 20 bar. Thus, the system pressure was crucial during the thermal decomposition of the composite material stored in a storage system. Moreover, NaBH4 and the composite material 30CaCl2/NaBH4 at high pressure (30 bar/150 °C /1 h) had a very low and limited reversibility nature. The composite material phase transitions from a powder phase (α-phase) to a semi-solid phase (α-phase and β-phase) was observed by increasing the system pressure from 1 bar to 20 bar. The TGA analysis of the composite materials was consistent, and the mass loss was followed as 30CaCl2/NaBH4 ˃ 20CaCl2/NaBH4 ˃ 10CaCl2/NaBH4 ˃ 40CaCl2/NaBH4 ˃ 50CaCl2/NaBH4 ˃ Pure NaBH4. Using TG-DTA confirmed that the catalyst's effect over the sodium borohydride increased and generated more hydrogen at low temperature (100 °C) and low pressure (1 bar). The initial temperature at which the first endothermic event occurred was lower for the composite material (30CaCl2/NaBH4) than for pure NaBH4 and other composite materials. The composite material 30CaCl2/NaBH4 exhibited the lowest activation energy (~210 kJ/mole) for hydrogen production compared to the other materials. Furthermore, Sodium hydride requires a higher temperature for thermal decomposition, and a significant portion of NaH remains undecomposed. So, a suitable catalyst is required to lower the decomposition temperature and increase the hydrogen generation rate. A series of catalyst/NaH hydrogen storage materials were systematically investigated. The facile solution method synthesized the composite material with CaO, CaF2, and CaCl2 as catalysts. Hydrogen generation via thermolysis from the composite material was studied using in-situ flow reactors. The generated hydrogen was quantified using a gas chromatograph (GC). Adding CaO (catalyst) was beneficial to lower the thermolysis temperature of NaH, and the hydrogen generation increased from ~ 0.31 wt% to ~1.94 wt% at 100 oC. The CaO assisted in the dispersion of NaH and lowered the activation energy to decompose and generate more hydrogen by thermolysis at low temperatures. The generation of hydrogen was ~1.94 wt% with 10CaO/NaH > 1.68 wt% with 5CaO/NaH > 1.61 wt% with 15CaO/NaH > 1.41 wt% with 20CaO/NaH > 0.31 wt% with pure NaH.