Phase structure evolution and performance divergence in AB2-type alloys induced by Al and Fe elemental substitution

Abstract

This work investigated the crystal structure, hydrogen storage, and electrochemical properties of the Ti0.2Zr0.8(V0.2Mn0.8)1−xMxNi1.0 (M = Al, Fe; x = 0, 0.05, 0.1) Zr-rich AB2 alloys. Rietveld refinement of X-ray diffraction (XRD) revealed that C14 phase abundance increased with Al content, while Fe promoted C15 phase formation, accompanied by a variation in the lattice constants. Hydrogen storage experiments showed C15 phase abundance positively correlated with maximum adsorption capacity, while plateau pressures were negatively correlated with lattice constants. The Fe0.1 alloy exhibited the largest adsorption capacity and the highest plateau pressure, whereas the Al0.1 alloy displayed opposite characteristics. All alloys demonstrated rapid hydrogen adsorption kinetics, reaching 98% capacity within 1 min after 5 activation cycles, retaining no obvious capacity decay after 20 cycles. Electrochemical studies indicated that Fe doping enhanced discharge capacity and high-rate discharge (HRD) performance due to increased C15 phase abundance. Electrochemical kinetics revealed that the improved HRD performance can be attributed to the enhanced electrocatalytic performance and hydrogen diffusion rate in Fe-doped alloys. This work provides a systematic analysis of how Al and Fe doping influences the AB2-type Laves phase alloys, offering theoretical and experimental evidence for alloy design and optimization.

1. Introduction

Hydrogen energy is one of the most promising new energy sources of this century, renowned for its high energy density and exceptionally abundant reserves [1]. Hydrogen storage alloy serves as a safe and efficient material for hydrogen storage, with emerging applications in nickel–metal hydride (Ni/MH) batteries [2]. Ni/MH batteries are characterized by a broad operating temperature range [3], low heat generation, high cost-effectiveness, and excellent safety [4]. Currently, these batteries continue to be widely utilized in energy storage, consumer electronics, vehicles, and so on [5–7].

A wide variety of hydrogen storage materials have been reported and initially applied to date, such as coordination compounds (LiBH4 [8], LiAlH4 [9], NaBH4 [10], etc.), MgH2 [11], AlH3 [12], AB-type alloys [13], V-based alloys [14], and high-entropy alloys [15]. Nevertheless, their practical application remains constrained by low capacity, high cost, poor activation, sluggish kinetics, and susceptibility to impurities. To address these challenges, researchers have made significant efforts to improve material performance. Huang et al. [14] eliminated activation incubation period of V70Ti10Cr20Ce1-0.3 alloy via Ce doping and cold-rolling, providing valuable insights into the optimization of V-based alloys. Shi et al. [13] elucidated impurity gas poisoning mechanisms in TiFe0.9 alloys through experiments and DFT simulations, advancing interfacial interaction studies. Wang et al. [16] reported amorphous scaly high-entropy boride catalysts with abundant electron traps, which significantly improved the reaction kinetics of MgH2.

Notably, AB2-type alloys demonstrate concurrent gaseous and electrochemical hydrogen storage capability and have emerged as a research focus due to their high energy storage density and cost-effectiveness [7]. In hydrogen storage applications, AB2-type ZrMn2 is a well-established hydrogen storage alloy with approximately 2wt% hydrogen storage capacity [17]. When utilized as an anode for Ni/MH batteries, AB2 alloys serves as a hydrogen storage alloy with high energy storage density and low raw material costs, presenting the potential to replace the commercially available rare earth-based AB5 hydrogen storage alloys [18–19]. However, AB2 alloys exhibit relatively slow electrochemical reaction rates and activation efficiencies, along with poor cycling stability [20], which limits their broader application. Currently, more and more researchers are exploring various approaches to address the challenges faced by Laves phase alloys.