Investigation of the microstructure and the thermodynamic and kinetic properties of ball-milled CeMg12-type composite materials as hydrogen storage materials

Abstract

In order to improve the electrochemical discharge and kinetics of CeMg12-type hydrogen storage materials, ball-milling was used and Ni was added to fabricate alloying materials with a nanocrystalline and amorphous structure. The phase composition and crystalline structure were characterized by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). The electrochemical discharge performances were characterized by determining the discharge capacity and electrochemical cycling stability. High-rate dischargeability (HRD), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization were utilized to investigate the changes in electrochemical kinetics. The results indicate that the addition of Ni significantly enhances the glass-forming ability of the alloy samples during ball milling, which significantly improves the electrochemical cycling stability of the alloy samples. The maximum discharge capacity of the ball-milled composites monotonously increases from 41.7mAh/g to 485.5 mAh/g with an increment in the Ni content from 50 wt>% to 200 wt%. An increase in the Ni content results in a considerable reduction in the enthalpy (ΔH) from −106.2 kJ/molto −43.1 kJ/mol and in the entropy (ΔS) from −350.73 kJ/mol/K to −22.49 kJ/mol/K. The addition of Ni reduces the stability of the hydride and improves the electrochemical discharge capacity. With regard to the electrochemical kinetics, the addition of Ni can diminish the activation energy of the alloy samples from74.02 kJ/mol to 33.28 kJ/mol with an increase in the Ni content from 50 wt% to 200 wt%, which speeds up the charge-transfer reaction on the surface. The addition of Ni accelerates the hydrogen diffusion rate(which is related to the limiting current density IL) inside the alloy particles. These effects improve the electrochemical kinetics.