Abstract
The deterioration of hydrogen charging performances resulting from the surface chemical action of electrophilic gases such as CO2 is one of the prevailing drawbacks of TiMn1.52 materials. In this study, we report the effect of autocatalytic Pd deposition on the morphology, structure, and hydrogenation kinetics of TiMn1.52 alloy. Both the uncoated and Pd-coated materials were characterized using scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD). XRD analyses indicated that TiMn1.52 alloy contains C14-type Laves phase without any second phase, while the SEM images, together with a particle size distribution histogram, showed a smooth non-porous surface with irregular-shaped particles ranging in size from 1 to 8 µm. The XRD pattern of Pd-coated alloy revealed that C14-type Laves phase was still maintained upon Pd deposition. This was further supported by calculated crystallite size of 29 nm for both materials. Furthermore, a Sieverts-type apparatus was used to study the kinetics of the alloys after pre-exposure to air and upon vacuum heating at 300 °C. The Pd-coated AB2 alloy exhibited good coating quality as confirmed by EDS with enhanced hydrogen absorption kinetics, even without activation. This is attributed to improved surface tolerance and a hydrogen spillover mechanism, facilitated by Pd nanoparticles. Vacuum heating at 300 °C resulted in removal of surface barriers and showed improved hydrogen absorption performances for both coated and uncoated alloys.
Highlights
AB2-type Laves phase alloys are an attractive class of metal hydrides due to their better reversible absorption and desorption of hydrogen, good activation property, and low cost [1,2]
The X-ray diffraction (XRD) analyses indicate that TiMn1.52 alloy exhibits a disordered structure and C14-type Laves phase without any second phase
The most interesting feature about C14-type Laves hydrogen storage materials is that they have favourable hydrogen absorption/desorption kinetics, exhibiting easy penetration of hydrogen atoms [20]
Summary
AB2-type Laves phase alloys are an attractive class of metal hydrides due to their better reversible absorption and desorption of hydrogen, good activation property, and low cost [1,2]. Because of their light weight, Ti–Mn binary alloys possess a large hydrogen absorption capacity of more than 1.0 hydrogen to metal ratio (H/M) and moderate equilibrium plateau pressure (reported to be 0.7 MPa) under near ambient temperatures as compared to other AB2 alloys [6] Regardless of these superior properties, deterioration of hydrogen charging performances resulting from the surface chemical action of poisonous electrophilic gases is still a concern and activation prior to hydrogen absorption is required [1,7]. An example of element substitution includes a study by Liu et al [1], where Ti and Zr comprised the A site, while Mn, Cr, V, Ni, Fe, and Cu metals occupied the B site to produce a (Ti0.85Zr0.15)1.05Mn1.2Cr0.6V0.1M0.1 alloy (where M=Ni, Fe, Cu) This material showed a great improvement in cyclability but poor hydrogenation kinetics due to its poor poisoning-resistance. To the best of our knowledge, such studies on hydrogenation kinetics of Pd-coated TiMn1.52 alloy do not appear to have been reported yet
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