Multi-scale experimental and numerical analysis of thermal conductivity improvements in metal hydride composite systems

Abstract

The inherently low thermal conductivity of metal hydrides significantly hinders heat transfer and, consequently, the kinetics of hydrogen absorption and desorption processes. To address this limitation, strategies such as adding high-thermal-conductivity materials have been explored to enhance the effective thermal conductivity of hydride beds. In this work, we investigate the effects of expanded natural graphite (ENG) addition on the thermal and kinetic performance of composite metal hydride materials. The used alloy is a commercially available AB 2 intermetallic compound, Hydralloy®C5. Materials composed of this AB 2 alloy, ethylene-vinyl acetate copolymer (EVA) used as binder, and ENG are investigated here. Experimental tests are conducted in conjunction with multi-scale finite element method (FEM) simulations to evaluate the effects of EVA on the hydrogen storage performance, hydride bed temperature development, and the effective thermal conductivity. Among the tested compositions, the mixture C5 + 10 wt% ENG + 2.5 wt% EVA exhibited the best balance of thermal behavior and storage properties, achieving a capacity close to that of the pristine AB 2 (about 1.7 wt%, calculated on the basis of the active material), showing an improvement of the kinetics around 35–40% and reducing the peak temperature upon hydrogenation in the milligram scale by approximately 8 °C. The proposed model introduces several novel elements, including a modified approach for the evaluation of the effective thermal conductivity and a mathematical model for the porosity of metal hydride composite beds. Validated FEM simulations at different scales (mg, g, and hundreds of grams), incorporating an effective thermal-conductivity model modified for composite materials, provide valuable input for the development and design of hydride-based vessels containing pure and composite hydride-forming materials.