Investigation of Pd-Ti and Ni-Ti Multilayer Thin Films for Enhanced Hydrogen Storage Capacity

Abstract

The development of compact and secure storage solutions for hydrogen in solid-state materials presents significant challenges and demands. High-capacity storage technologies that operate effectively at low pressures and exhibit favourable kinetics in absorption and desorption are essential for hydrogen storage solutions. The slow kinetics of hydrogen adsorption and desorption present limitations in metal hydride storage. This investigation utilised advanced materials, specifically palladium-coated and nickel-coated metals, to explore potential enhancements in hydrogen adsorption and desorption. Palladium and nickel act as adsorption catalysts, thereby enhancing the kinetics of hydrogen diffusion into metal interstitial sites. This study examined the multilayers of Pd-Ti and Ni-Ti, which were synthesised and evaluated for hydrogen storage and the kinetics of absorption and desorption. The Pd/Ti/Pd/Ti and Ni/Ti/Ni/Ti multilayers were fabricated using an e-beam evaporator and annealed at different temperatures. Rutherford backscattering spectroscopy confirmed the formation of multilayers, consisting of pure palladium and nickel layers. The titanium layers in both systems exhibited a significant amount of oxygen contamination (up to 63 at.% in the Pd-based system and 61 at.% in the Ni-based system), which was picked from the deposition chamber as residual gases. Hydrogen profiling performed at iThemba LABS revealed a strong temperature dependence of the hydrogen absorption in the multilayers. For both the Pd-Ti and the Ni-Ti based systems, the hydrogen absorption peaked at 200 ◦C. For the Pd-Ti system, the average hydrogen concentration was 3.72 at.% and a total concentration of 51.34 at.%, while Ni-Ti multilayers showed a maximum absorption of 2.58 at.% and a total hydrogen uptake of 46.42 at.%. The hydrogen absorption declined at elevated temperatures, which was likely due to hydrogen embrittlement and structural degradation. X-ray diffraction confirmed the formation of titanium hydrides and tracked the phase transformation with temperature. Atomic force microscopy revealed changes in the surface roughness and morphology. The surface roughness showed the structural response to the hydrogenation temperature. The root mean square roughness for both samples showed a correlation with the total hydrogen content absorbed.