Effect of Ce element on hydrogen storage property of TiMn2-based alloys

Abstract

This study investigates the effect of Ce content on the hydrogen storage properties of Ti0.98Zr0.02Mn1.5Cr0.05V0.43Fe0.09Cex (x = 0, 0.02, 0.04, and 0.06, at%) alloys. Microstructural analysis of these alloys revealed dendritic microstructures without the segregation of chemical elements, with the C14 Laves phase identified as the dominant phase. After two activation cycles at 4 MPa and 293 K, the alloys exhibited excellent hydrogen absorption properties. The addition of Ce significantly improved the kinetics of the alloys. At x = 0.02, the hydrogen absorption capacity reached 90% of its maximum within 137 s at 293 K. Pressure–composition–temperature curves indicated that hydrogen absorption capacity initially increased first and then decreased with increasing Ce content, reaching a maximum value of 1.85wt% at x = 0.04. Thermodynamic results demonstrated that the enthalpy and entropy of hydrogen absorption followed a similar trend, which was consistent with the variation in hydrogen storage capacity. Thus, the improvement in hydrogen absorption capacity due to the addition of Ce is attributed to the increase in enthalpy. The increase of the lattice constant in the C14 Laves phase and the deoxidization effect of Ce are expected to be beneficial for the improvement of hydrogen absorption kinetics.

1. Introduction

Hydrogen has emerged as a new generation of clean energy carriers because of its high energy density, recyclability, and zero carbon emissions [1–5]. However, its low volumetric density (0.0899 g/L) and the tendency of gaseous hydrogen to leak render efficient and safe storage essential [6]. Developing solid hydrogen storage alloys is a promising solution, as they can absorb and desorb hydrogen at moderate temperatures and pressures with high volumetric density (higher than that of liquid H2) and appropriate gravimetric density [7]. Among various solid hydrogen storage alloys, TiMn2-based alloys have garnered significant attention because of their rapid hydrogen absorption/desorption kinetics at room temperature and high volumetric hydrogen storage density. TiMn2-based alloys have been applied in outdoor portable power sources, hydrogen fuel cell bicycles, fixed hydrogen storage systems, and other fields, demonstrating promising development and market potential [8].

TiMn2-based alloys are AB2-type intermetallic compounds with C14 Laves phase structures. Generally, the A-side consists of elements that form stable hydrides, including Ti and other substituted elements such as Zr and Sc. The B-side includes elements that do not easily form hydrides but exhibit hydrogen catalytic activity, including the main Mn element and other substituted elements such as Cr, V, Fe, and Ni. By adjusting the ratio of elements on the A and B sides, the hydrogen storage properties of these alloys can be optimized [9–10].

Element substitution on the A-side primarily affects the hydrogen storage capacity. Kandavel et al. [11] improved the hydrogen storage properties of Ti1.1CrMn alloys by replacing Ti with Zr, increasing capacity from 1.9 wt% to 2.2 wt%. Substitution of elements on the B-side affects activation behavior, kinetics, and cycling performance. Zhou et al. [12] reported that Cr substitution in Ti0.95Zr0.05Mn1.8−yCryV0.2 alloys improves pressure hysteresis and cycling stability. Vanadium substitution can also lower plateau pressure and hysteresis, as demonstrated by Tu et al. [13]. These studies illustrate the potential of multi-element substitution to tailor hydrogen storage performance.