Mistry, Priyen C.
(2016)
Coated metal hydrides for stationary energy storage applications.
PhD thesis, University of Nottingham.
Abstract
This thesis explores suitable materials for energy stores for stationary applications, specifically a prototype hydrogen store, domestic thermal store operating between 25-100 C and a moderate thermal store for a concentrated solar power (CSP) plant operating at 400 C. The approach incorporated a unique coating technique to deliver prototype hydrogen and thermal storage media, where the coating could offer commercial advantages, for example, in the form of hydride activation and enhanced kinetics during successive cycling.
The highly reversible Mg-MgH2 system is particularly promising for thermal storage, obtaining an enthalpy of reaction of 74.5 kJ/mol H2 that translates to a thermal energy capacity of approximately 2800 kJ/kg of MgH2. Nevertheless, magnesium is hindered by slow activation and poor kinetics of (de)hydrogenation, even when approaching temperatures ideal for concentrated solar power applications (in the region 400 C). Elevated temperature cycling studies were performed on commercial atomised Mg powder with magnetron sputtered catalysts (chromium, iron, vanadium and stainless steel) applied to their surfaces; the aim of which was to fabricate hydrogen storage materials that possess (de)hydrogenation characteristics equal to or even bettering their nanocrystalline equivalents, yet in a potentially economic and scalable manner.
Following 50 cycles at 400 C, the coatings were found to have little to no positive impact on the behaviour of the atomised Mg powders. In addition, for both uncoated and coated samples the effects of an activation process at 400 C are matched by cycling the material 5 times from the outset, after which identical behaviour is observed during subsequent cycles. At 350 C, the benefits of catalyst coatings on the hydrogen storage properties of atomised Mg powders are evident during activation and successive cycling up to 90 times. The material undergoes different microstructural evolution during cycling when in the presence of a surface catalyst, causing an enhancement of the `nucleation and growth' stage of (de)hydrogenation. This was attributed to particle reorientation dominating particle sintering, whereas the opposite occurs for the uncoated material.
For the domestic thermal and prototype hydrogen stores a selection of AB and AB2 intermetallic hydrides enhanced through catalysis or thermodynamic modification were investigated. TiFe produced via powder atomisation obtained thermodynamic properties (dehydrogenation H = 28.9 kJ/mol H2 and S = 105 J/K.mol H2) in line with published results. The minor substitution of Ni into TiFe1-xNix resulted in different hydrogenation characteristics to TiFe, for example, TiFe0:96Ni0:04 possessed a dehydrogenation of H = 29.9 kJ/mol H2 and S = 107 J/K.mol H2. Discrepancies between maximum achieved and theoretical capacities were observed for both atomised TiFe and TiFe0:96Ni0:04 and a range of possible contributing factors are discussed. A minor addition of Pd (1.17 wt.%) magnetron sputtered to the surface of TiFe0:96Ni0:04 enabled successful room temperature hydrogenation with no activation treatment required. Characterisation (SEM and TEM) confirmed it is not necessary to have complete Pd coverage in the form of a uniform coating and XPS was utilised to derive a theory for the activation mechanism.
The AB2 alloy comparison between the commercially available Hydralloy C5 and in house fabricated Ti0:9Zr0:2Mn1:5V0:2Cr0:3 showed that Hydralloy C5 was the most promising alloy for the hydrogen store application with the higher working capacity (ca. 0.96 wt.%) in the pressure range of 4-15 bar at 22 C, despite Ti0:9Zr0:2Mn1:5V0:2Cr0:3 obtaining a higher maximum storage capacity (1.82 wt.%). The hydrogenation kinetics of both alloys were studied with corresponding activation energies and hydrogen diffusion coefficients determined. The kinetics of hydrogenation for both alloys is sufficiently fast that only the heat transfer of the storage system is the rate limiting parameter for hydrogen exchange for most technical applications.
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