Enhancing the activation and cycling properties of V-based alloys by trace Ce doping

Abstract

This study focused on improving the activation property and cycling stability of V78Ti6Cr16 alloy through trace Ce doping. V78Ti6Cr16Cex (x = 0, 0.2, 0.4) alloys were prepared by arc melting. The activation property, the kinetic and thermodynamic properties, the cycling stability and the cycling stability mechanism of the prepared alloys were investigated. The results show that trace Ce doping significantly improves the activation performance of the alloy. The kinetics changed little and the thermodynamics changed a little by trace Ce doping. Crucially, trace Ce doping remarkably improved cycling stability of the alloy. V78Ti6Cr16Ce0.2 exhibited a capacity retention rate of 97.43% after 400 cycles, substantially higher than the 93.06% of undoped alloy. Even after 1000 cycles, V78Ti6Cr16Ce0.2 maintained higher than 90% retention, demonstrating excellent cycling stability for practical applications. X-ray diffraction and compressing test reveal that Ce doping effectively improves the crystal structure of the alloys by increasing the cell volume and enhancing the mechanical properties of the alloy, thereby improving the structure stability of the alloy during cycling. Transmission electron microscope analysis indicated that the defect density progressively increases with cycling in undoped alloy, which is the main reason for the capacity decay. But the defect density is much less in V78Ti6Cr16Ce0.2 alloy compared with undoped alloy, which contributes to its superior capacity retention rate. This work provides a new strategy for enhancing hydrogen storage properties via trace rare-earth doping.

1. Introduction

Hydrogen energy is regarded as one of the most promising clean energy sources due to its abundant reserves, low weight, and pollution-free nature [1–3]. However, achieving environmentally friendly, safe, efficient, and economical hydrogen storage and transportation remains a critical challenge [4–5]. Under ambient temperature and pressure, solid-state hydrogen storage outperform liquid and gaseous storage in terms of safety and storage density [6–7], making solid-state storage the most viable pathway forward. Among solid-state hydrogen storage materials, metal hydrides stand out as a promising class due to their high hydrogen capacity, low operating pressure, and favorable safety profile [8].

For instance, Mg-based hydrogen storage materials offer a theoretical capacity of 7.6wt%, yet their practical application is limited by sluggish kinetics and high dehydrogenation temperatures [9–11]. Recently, significant progress has been made to overcome these issues through hierarchical interface engineering [12]. Advanced strategies, such as single-atom catalysts supported on carbon nanotubes [13] and nanoconfinement within porous scaffolds [14], have proven effective in optimizing mass transport and reducing activation barriers. Despite these promising developments, the road to realizing ambient-temperature applications of Mg-based hydrogen storage materials remains arduous.

In contrast, V-based solid-solution alloys have become highly attractive candidates due to their theoretical capacity of 3.8wt% and demonstrate reversible hydrogen absorption/desorption under ambient conditions [15]. However, V-based alloys have some drawbacks hinder their application, such as the high cost of raw pure vanadium, activation issues under mild conditions, and poor cyclic stability during prolonged hydrogen absorption/desorption cycles [16–18]. To reduce costs, researchers have attempted to replace pure vanadium with low-cost ferrovanadium or to lower the vanadium content in the alloys [19–21]. However, these approaches often lead to a notable decrease in hydrogen storage capacity and a deterioration in cycling performance [20,22]. Increasing the vanadium content in the alloy has been demonstrated as an effective way to enhance cycling durability [23–26]. It is found that V-based alloys with a high V content over 75at% showed good durability, hydrogen desorption capability at low temperature and relatively high effective hydrogen capacity simultaneously [24], which are suitable for metal hydride (MH) tank use.

Additionally, the activation properties of the alloys are also very important for MH tank use. Various rare-earth elements, such as Y [27], La [28], and Ce [19,29–31], have been proved to enhance the activation behavior, hydrogen absorption kinetics, and cycling stability of V-based alloys. Among these rare earth elements, Ce is particularly promising because Ce has strong reactivity with oxygen, which would helpfully decrease the oxygen concentration dissolved in the alloy, causing the i